Improvements in and relating to ion analysis

Abstract

A method for Ion Mobility Spectrometry (IMS) comprising, generating ions from a sample in an ion source, and providing a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along a direction transverse to the analytical gap. The method includes establishing a flow of gas entrained with ions generated from the sample, the flow of gas being directed along the drift region. A dispersion voltage waveform comprising a dispersion voltage portion, V D , is applied to one or more of the electrodes thereby to generate a dispersion electric field, E D , across the analytical gap containing the flow of gas. The drift region is irradiated with light from a light source thereby to irradiate the ions entrained within the flow of gas contained therein such that the entrained ions are subject to the dispersion electric field and the light from the light source.

Description

[0001] IMPROVEMENTS IN AND RELATING TO ION ANALYSIS

[0002] Field of the Invention

[0003] The present invention relates to ion mobility spectrometry (IMS) and particularly, although not exclusively, to Field asymmetric IMS (FAIMS), differential mobility spectrometry (DMS).

[0004] Background

[0005] The term ion mobility spectrometry (IMS) refers to the methods and apparatus used to characterise ions from sample substances in terms of the speed at which ensembles of those ions progress through a supporting gas atmosphere when urged through it by an applied electric field. Ion mobility measurements involve injecting an ion ensemble into a “drift region”. An ion detector, or simply an ion outlet, may be provided at the output end of the drift region. While within the drift region, the ion ensemble moves longitudinally towards an output end of the drift region e.g., within a flow of purified neutral support gas (e.g., molecular Nitrogen), also known as a “buffer gas”, in which it is entrained or through which it is driven in a direction along the drift region by an electric field. Simultaneously, in one IMS technique for example, the ion ensemble moves transversely to the buffer gas flow direction under the urging force of an applied electric field, E, generated by an appropriate voltage gradient applied transversely to the buffer gas flow direction.

[0006] Notably, ion mobility measurements pertain only to ion ensembles and not to individual ions for which the speeds can be comparatively large. For example, the median speed between collisions for molecular Nitrogen ions at ambient pressure and at a temperature of 25 degrees Celsius is about 450 metres per second. By comparison, as an example, an ion ensemble may typically be urged by the applied electric field, E, to move transversely to the buffer gas flow direction with a velocity of, say, v = 4 m / s.

[0007] It is established practice to normalise such an ion ensemble velocity value, v, by dividing it with the value of the electric field strength, E, applied transversely to the buffer gas flow direction. This normalisation produces an ion mobility coefficient, K = v / E, which is the measure of the ion ensemble velocity per unit field strength. The relationship between ion ensemble velocity, v, and electric field strength, E, is valid for an ion ensemble at thermal energies measured in a buffer gas atmosphere of constant composition, pressure, and temperature. The value of an ion mobility coefficient is dependent upon buffer gas temperature, T, and pressure, P, inside the drift region. Importantly, the value of an ion mobility coefficient is dependent upon the collision cross-section of the ion under study and an ion mobility coefficient, K, for a given ion is sensitive to fluctuations in this quantity.

[0008] A distinction of IMS in contrast to mass spectrometry, MS, is that ions are characterised in a supporting buffer gas atmosphere, also called a drift gas, that is refreshed continuously. A main practical purpose of this gas is to maintain a purified and constant atmosphere for collision-based movement of the ion ensemble. Field asymmetric IMS, differential mobility spectrometry, or ion drift spectrometry are different names given to the same process, which is a type of IMS. An ion mobility measurement begins when ions formed from components in a sample, called product ions, are injected into the drift region. These methods are based on ions undergoing changes in ion mobility coefficients, K, as a result of changes in the applied transverse electric field, E, at constant buffer gas particle number density, N, i.e., the number of buffer gas particles per unit volume. An approach in IMS technology has enabled studies of field dependence using a Field Asymmetric Ion Mobility Spectrometer (FAIMS) or DMS. The method of high field asymmetric IMS for ion separations is based on a non-linear, high-field dependence of ion mobility coefficients. In particular:

[0009] K(E / N) = l + aCW)

[0010] Here, a(E / N) = <z2x (E / / V)2+ <z4x (E / / V)4+ - + a2nx (E / N)2”

[0011] This function describes the non-linear electric field dependence of ion mobility of an ion. The terms a2n(n = 1,2, ...) are constant coefficients the values of which are particular to a given combination of ion and buffer gas set-up. The function a E / N') is a function describing the dependence of ion mobility on the ratio, E / N, of the electric field strength to neutral gas density. The units of E / N are Townsends (Td) where 1 Td = 10'17Vcm2.

[0012] In this method, termed variously Field Asymmetric IMS (FAIMS), or Differential Mobility Spectrometry (DMS), ions are contained in a gas, e.g., entrained by a gas flow, extending along a drift space between conducting surfaces (e.g., electrodes). The space between the electrodes defines an “analytical gap”. The drift space can be defined between parallel curved or flat electrodes (e.g., plates). A transverse electric field, E, is applied across this analytical gap using an asymmetric voltage waveform known as a “dispersion voltage” (FD), which generates a corresponding dispersion electric field, EDe.g., of ED= +20,000 V I cm or greater in the positive amplitude part of the asymmetric wave cycle and ED= -1,000 V I cm in the negative amplitude part of the asymmetric wave cycle. Ion ensembles move with a speed v within the electric field E according to equations:

[0013] Of course, the value of EDchanges in magnitude and polarity as the wave cycle switches between its positive and negative amplitude parts. As a result, the value of K(ED / NJ' is different during these two different parts of a wave cycle for ions for which a(ED / Nj' + 0. The asymmetric voltage waveform of the dispersion voltage (FD) is designed so that the time integrals of these two parts of the wave cycle are equal. Notably, ions with mobility coefficients, K(ED / NJ' , that are independent of ED, (i.e., such that a(ED / Nj' = 0 even at high field values) are able to pass through the drift region and emerge from it to be detected. In contrast, ions with a dependence of K on ED(i.e., a(ED / Nj' + 0) undergo a net displacement towards a surface of an electrode with repeated exposure of the ion ensemble to the periodic changes in direction and strength of the electric field, ED. The magnitude of displacement depends on the differences in mobility, K(ED / N) = K0(l + a(ED / Ny), at electric field extremes i.e., the “high-field” amplitude, E^Hand “low-field” amplitude, E^Lparts of the asymmetric wave cycle. In other words:

[0014] The “high-field” amplitude, EDHi, and “low-field” amplitude parts of the waveform have opposite polarity with the “high-field” amplitude, EDHi, conventionally being denoted as having positive polarity. The immediate effects of dispersion electric field, ED, in DMS or FAIMS are revealed in the dependence of the mobilities, K(ED / Nj', of ions on dispersion electric field strength, ED, at the two extremes of the asymmetric waveform. The waveform is designed in field strength, ED, and duty cycle (as between durations of positive polarity and negative polarity parts) so that an ion with little or no dependence of mobility on dispersion electric field strength, ED, will pass through the centre of the analyser by being carried by a flow of buffer gas, or driven by an electric field. Ions that do have a dependence of mobility on dispersion electric field strength, ED, will undergo with each complete cycle of the dispersion voltage waveform, successive net displacements from the central ion axis of ion flow. Eventually, the ion ensemble will collide with an electrode defining the analytical gap and will be discharged and removed from the measurement process. When a direct current (DC) “compensation voltage” (Fc) is applied to the electrodes of the analyser, the effects of the dispersion electric field may be compensated, and ion motion can be restored to the centre of the analyser.

[0015] A comparatively low direct current (DC) “compensation voltage” (Fc) may be added to the electrodes or plates defining the analytical gap so as to superimpose that DC electric field (compensation field, Ec) upon the dispersion electric field, ED, to enable control or “compensation” of ion motion towards an electrode. Ions restored to the centre of the analytical gap will be made able to pass through the drift region. A sweep of this compensation voltage, often 10V to 40V in size (producing compensation electric fields of typically 100V / cm to 500V / cm), provides a means to measure mobility of all ions in the analyser for a given dispersion voltage wave form. This method provides ion mobility filtering, and ion separations are based on differences in ion mobility, leading to the name “Differential Mobility Spectrometry”, also known as FAIMS.

[0016] To achieve high quality measurements, the appropriate control of the dispersion electric field, ED, and the compensation electric field, Ec, in the analytical gap is necessary. However, there is a limit to the amount and quality of the information about an ion that can be extracted using DMS or FAIMS methods which relying on dispersion and compensation electric field control alone. The present invention has been devised in light of the above considerations. Summary of the Invention

[0017] In a first aspect, the invention may provide a method for Ion Mobility Spectrometry (IMS) comprising: generating ions from a sample in an ion source; providing a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along a direction transverse to the analytical gap; providing a gas containing ions generated from the sample, the gas being provided along the drift region; applying dispersion voltage waveform comprising a dispersion voltage portion, VD, to one or more of the electrodes thereby to generate a dispersion electric field, ED, across the analytical gap containing the gas; and, irradiating the drift region with light from a light source thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and the light from the light source.

[0018] The method may include providing the gas by establishing a flow of gas entrained with ions generated from the sample, the flow of gas being directed along the drift region. Alternatively, the gas may be substantially static. The ions may be caused to move along the drift region by being entrained within a flow of the gas containing them, and / or by action of an electric field configured to urge the ions in a direction along the drift region, the direction of urging being either along the direction of gas flow or against that flow direction.

[0019] The method may comprise concurrently applying the dispersion voltage waveform and irradiating the drift region with light from a light source thereby to irradiate the ions contained within the gas or entrained within a flow of gas contained therein such that the contained / entrained ions are simultaneously subject to both the dispersion electric field and the light from the light source. The light from the light source may be substantially monochromatic light. Alternatively, non-monochromatic light (e.g., broadband light) may be used if desired.

[0020] Optionally, the method may include providing a vacuum region containing the differential ion mobility assembly. The gas containing ions, or a flow of gas entrained with ions, generated from the sample, may be established in the vacuum region. Optionally, the drift region extends along an ion optical axis. The gas may extend along, or a flow of gas may be directed along, the ion optical axis.

[0021] References herein to a dispersion voltage waveform, comprising a dispersion voltage portion, VD, and to a dispersion electric field, ED, (e.g., when in the context of a repeating dispersion voltage waveform) may include, where the context provides, a reference to a waveform comprising both:

[0022] (a) A first portion comprising a “higher-field” amplitude portion of a given cycle of the dispersion voltage waveform, and to the corresponding “higher-field” part, ED!Hinoted above, of a dispersion electric field generated by use of that dispersion voltage waveform and, (b) A second portion comprising a “lower-field” amplitude portion of a given cycle of the dispersion voltage waveform, and to the corresponding “lower-field” part, EDLinoted above, of a dispersion electric field generated by use of that dispersion voltage waveform, as the context requires. The terms “higher” and “lower” may be interpreted as being described in this way relative to each other. An advantage of the invention is that, in preferred implementations for example, the method / system need not require the “higher-field” portion of the dispersion waveform to be as high in value as would otherwise be required. This is because that portion of the waveform may be accompanied by simultaneous irradiation by the light source. This therefore allows lower power supply values / ratings to be used to supply power for the dispersion voltage waveform and this permits the use of lower cost system.

[0023] An asymmetric dispersion waveform may apply a “higher-field” part, generally referred to in the art as the dispersion voltage (DV) or dispersion electric field, between the electrodes for a short period of time, and then may apply a “lower field” part for a longer time. In other examples, where the context provides, a symmetric dispersion waveform may comprise a first waveform portion serving the function and purpose of a dispersion voltage (DV) or dispersion electric field, and a separate second waveform portion. The first waveform portion may define a first voltage amplitude and the second waveform portion may define a second voltage amplitude which may have substantially the same magnitude (and time duration, optionally) as the magnitude (and time duration, optionally) of the first voltage amplitude. In other words, a dispersion voltage waveform or dispersion electric field may oscillate / swap / flip between both such values or waveform portions cyclically. For example, one cycle (e.g., each cycle) of the dispersion voltage waveform may comprise one single instance of such a first waveform portion and one single instance of such a second waveform portion, such that successive such instances occur only in successive cycles of the waveform.

[0024] References herein to a waveform preferably include a reference to the waveform of a signal, or of a sequence of measurements, in terms of the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time. Preferably the waveform is periodic and of a constant period or frequency. Preferably the waveform is defined according to one repeating waveform shape (e.g., the shape in Fig.1 ) that repeats periodically, i.e. , every successive cycle.

[0025] Preferably, the dispersion voltage waveform defines a voltage which changes in time according to a repeating waveform pattern (e.g., a pattern of successive portions of alternating magnitude and / or polarity). The method may include irradiating the drift region with light from the light source according to a light intensity that changes in time in synchrony with the changes in time defined by the dispersion voltage waveform. The changes in the light intensity may be repeating changes that repeat in synchrony with repeating changes in the polarity (and / or magnitude) of the dispersion voltage waveform. For example, the light intensity may change from a first value to a second value (different to the first value) whenever the dispersion voltage waveform changes polarity (and / or value / magnitude). The first value may be a non-zero value (i.e., photons are output from the light source), whereas the second value may be zero (i.e., no output from the light source). The first value may be implemented to occur only when the dispersion voltage waveform is in a first (e.g., positive or negative) polarity state (and / or specified value / magnitude), and the second value may be implemented to occur only when the dispersion voltage waveform is in a second polarity state (and / or specified value / magnitude) that preferably is opposite in polarity to the first polarity state (e.g., negative or positive).

[0026] Either one or, of both of, the first value and the second value is preferably substantially steady and unchanging (in time) during its application - i.e., until the dispersion voltage waveform changes in such a way as to cause the first value to change to the second value, or vice versa.

[0027] The method may include irradiating the drift region with light of a first light intensity from the light source during the application of a first portion of the dispersion voltage waveform and irradiating the drift region with light of a second light intensity from the light source during the application of a separate second portion of the dispersion voltage waveform, wherein the first intensity differs from the second intensity.

[0028] The method may include irradiating the drift region with light from the light source during the application of a first portion of the dispersion voltage waveform and during none of the duration of a separate second portion of the dispersion voltage waveform.

[0029] Desirably, the dispersion voltage waveform comprises a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the method includes irradiating the drift region with light from the light source during application of at least one of the first waveform portion and the second waveform portion. For example, irradiation may apply during some but not all of the duration of the first waveform portion and during none of the duration of the second waveform portion, or vice versa. For example, irradiation may apply during all of the duration of the first waveform portion and during none of the duration of the second waveform portion, or vice versa. For example, irradiation may apply during all of the duration of the first waveform portion and some (or all) of the duration of the second waveform portion, or vice versa.

[0030] Preferably, the dispersion voltage waveform comprising a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

[0031] Desirably, the dispersion voltage waveform is an asymmetric waveform in which the first waveform portion defines a first voltage amplitude and the second waveform portion defines a second voltage amplitude which is different in magnitude to the magnitude of the first voltage amplitude, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion. The duration of the first waveform portion may differ from the duration of the second waveform portion. The second voltage amplitude may be smaller in magnitude than the magnitude of the first voltage amplitude. The second voltage amplitude may be greater in magnitude than the magnitude of the first voltage amplitude.

[0032] Desirably, the dispersion voltage waveform is a symmetric waveform in which the first waveform portion defines a first voltage amplitude and the second waveform portion defines a second voltage amplitude which has substantially the same magnitude as the magnitude of the first voltage amplitude, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion. In this case, the duration of the first waveform portion is preferably substantially the same as the duration of the second waveform portion.

[0033] The first voltage polarity may be opposite (e.g., positive or negative) in value and polarity of the second voltage polarity (e.g., which, respectively, may be negative or positive in value).

[0034] The method may include irradiating the drift region with polarized light from the light source configured to output said light in a pre-selected state of linear polarization or circular polarization.

[0035] Desirably, the first intensity and the second intensity are each substantially constant during the respective first portion and second portion of the dispersion voltage waveform.

[0036] Preferably, the first intensity and the second intensity are each substantially constant for the whole duration of the respective first portion and second portion of the dispersion voltage waveform.

[0037] The irradiating of the drift region with light may comprise reflecting light from the light source in a direction transverse to the direction in which the drift region extends (e.g., transverse to the ion optical axis) from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced longitudinally in a direction along the drift region, thereby to generate a zig-zag pattern of reflected light along the drift region. Light from the light source may be directed to be incident obliquely on, and reflected obliquely from, each of the plurality of longitudinally spaced reflection points. Multiple reflections may be used in this way to increase ion irradiation efficiency. The cross section for ion - photon collision is typically low, and therefore multiple reflections reduce the requirement for light source brightness.

[0038] The irradiating of the drift region with light may comprise reflecting light from the light source in a direction transverse to the direction in which the drift region extends (e.g., transverse to the ion optical axis) from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced laterally in a direction across the direction in which the drift region extends (e.g., across the ion optical axis), thereby to generate a zig-zag pattern of reflected light across the ion optical axis. Light from the light source may be directed to be incident obliquely on, and reflected obliquely from, each of the plurality of laterally spaced reflection points. The light source may comprise a plurality of separate light sub-sources that are located along at least one side of the drift region and are spaced longitudinally in a direction along the drift region, wherein said irradiating the drift region with light from the light source comprises generating light simultaneously from the plurality of separate light sub-sources.

[0039] In a second of its aspects, the invention may provide an apparatus for Ion Mobility Spectrometry (IMS) comprising: an ion source for generating ions from a sample; a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along a direction transverse to the analytical gap, and configured to contain therein a gas containing ions generated from the sample, the gas being provided along the drift region; a voltage source configured to apply a dispersion voltage waveform comprising a dispersion voltage, VD, to one or more of the electrodes; a light source configured to irradiate the drift region with light; a controller configured to: control the voltage source to apply the dispersion voltage waveform to the one or more electrodes thereby to generate a dispersion electric field, ED, across the analytical gap when containing the flow of gas; and, control the light source to irradiate the drift region with light thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and the light from the light source.

[0040] The differential ion mobility assembly may be configured to establish within the drift region a flow of gas entrained with ions generated from the sample, the flow of gas being directed along the drift region. Alternatively, the differential ion mobility assembly may be configured to provide the gas as a substantially static gas. The differential ion mobility assembly may be configured to cause ions to move along the drift region by being entrained within a flow of the gas containing them, and / or by action of an electric field configured to urge the ions in a direction along the drift region, the direction of urging being either along the direction of gas flow or against that flow direction.

[0041] The controller may be configured to control the voltage source and the light source to concurrently irradiate the drift region with light from the light source and apply the dispersion voltage waveform thereby to irradiate the ions contained within the gas, or entrained within a flow of gas, contained therein such that the contained / entrained ions are simultaneously subject to both the dispersion electric field and the light from the light source.

[0042] Optionally, the apparatus may include a vacuum region containing the differential ion mobility assembly. The gas containing ions, or a flow of gas entrained with ions, generated from the sample, may be established in the vacuum region. Optionally, the drift region extends along an ion optical axis. The gas may extend along, or a flow of gas may be directed along, the ion optical axis. The voltage source is preferably configured to apply a dispersion voltage waveform that defines a voltage which changes in time according to a repeating waveform, and the controller is configured to control the light source to irradiate the drift region with light from the light source according to a light intensity that changes in time in synchrony with the changes in time defined by the dispersion voltage waveform.

[0043] The controller is preferably configured to control the light source to irradiate the drift region with light of a first light intensity during the application of a first portion of the dispersion voltage waveform and to irradiate the drift region with light of a second light intensity during the application of a separate second portion of the dispersion voltage waveform, wherein the first intensity differs from the second intensity.

[0044] Desirably, the controller is configured to control the light source to irradiate the drift region with light from the light source during the application of a first portion of the dispersion voltage waveform and during none of the duration of a separate second portion of the dispersion voltage waveform.

[0045] Preferably, the voltage source is configured to apply a dispersion voltage waveform comprising a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity. Desirably, the controller is configured to control the light source to irradiate the drift region with light during application of at least one of the first waveform portion and the second waveform portion.

[0046] Preferably, the voltage source is configured to apply a dispersion voltage waveform comprising a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity. Preferably, the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

[0047] Preferably, the voltage source is configured to apply an asymmetric dispersion voltage waveform in which the first waveform portion defines a first voltage amplitude and the second waveform portion defines a second voltage amplitude which is different in magnitude to the magnitude of the first voltage amplitude. Desirably, the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

[0048] Desirably, the second voltage amplitude is smaller in magnitude than the magnitude of the first voltage amplitude. The second voltage amplitude may be greater in magnitude than the magnitude of the first voltage amplitude.

[0049] Preferably, the dispersion voltage waveform may comprise a symmetric waveform in which the first waveform portion defines a first voltage amplitude and the second waveform portion defines a second voltage amplitude which has substantially the same magnitude as the magnitude of the first voltage amplitude. Desirably, the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion. In this case, the duration of the first waveform portion is preferably substantially the same as the duration of the second waveform portion.

[0050] Desirably, the first voltage polarity is opposite (e.g., positive) in value and the second voltage polarity (e.g., negative).

[0051] The controller may be configured to control the light source to irradiate the drift region with polarized light from the light source configured to output said light in a pre-selected state of linear polarization or circular polarization.

[0052] Preferably, the first intensity and the second intensity are each substantially constant during the respective first portion and second portion of the dispersion voltage waveform.

[0053] Desirably, the first intensity and the second intensity are each substantially constant for the whole duration of the respective first portion and second portion of the dispersion voltage waveform.

[0054] The apparatus may comprise an optical reflector assembly configured for reflecting light from the light source in a direction transverse to the direction in which the drift region extends (e.g., drift region may extend in a direction along the ion optical axis) from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced longitudinally in a direction along the drift region, thereby to generate a zig-zag pattern of reflected light along the drift region. The light source may be configured to direct light to be incident obliquely on, and reflected obliquely from, each of the plurality of longitudinally spaced reflection points.

[0055] The apparatus may comprise an optical reflector assembly configured for reflecting light from the light source in a direction transverse to the direction in which the drift region extends (e.g., drift region may extend in a direction along the ion optical axis) from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced laterally in a direction across the drift region (e.g., across the ion optical axis), thereby to generate a zig-zag pattern of reflected light across the drift region (e.g., across the ion optical axis). The light source may be configured to direct light to be incident obliquely on, and reflected obliquely from, each of the plurality of laterally spaced reflection points.

[0056] Desirably, the light source comprises a plurality of separate light sub-sources that are located along at least one side of the drift region and are spaced longitudinally in a direction along the drift region, the controller is configured to control the light source to generate light simultaneously from the plurality of separate light sub-sources thereby to irradiate the drift region with light. The apparatus may comprise an ion detector for receiving ions output from the drift region and for generating an ion detection signal in response thereto.

[0057] In a third aspect, the invention may provide an apparatus for ion analysis comprising an apparatus for Ion Mobility Spectrometry (IMS) as described above, and a mass spectrometer configured to receiving ions output from the drift region and for generating an ion mass spectrum in response thereto.

[0058] In a fourth aspect, the invention may provide a method for analysing ions according to the method for Ion Mobility Spectrometry (IMS) as described above, the method for analysing ions comprising: applying said dispersion voltage waveform comprising a dispersion voltage to said one or more of the electrodes thereby to generate said dispersion electric field, ED, across the analytical gap containing the gas (e.g., flow of gas, etc.); and, irradiating the drift region with said light from said light source thereby to irradiate the ions in the gas (e.g., entrained within the flow of gas, etc.) contained therein such that the ions are subject to the dispersion electric field and said light from the light source, wherein said light is substantially monochromatic light comprising photons of a corresponding photon energy; detecting an ion output signal from the differential ion mobility assembly; changing either the magnitude of the dispersion voltage or the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output undergoes a transition to a depressed signal value; identifying an occurrence of a transition of a quantum state of ions or of a fragmentation of ions either according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage or according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

[0059] The method may comprise concurrently applying the dispersion voltage waveform and irradiating the drift region with light from a light source thereby to irradiate the ions within the gas (e.g., entrained within a flow of gas, etc.) contained therein such that the ions are simultaneously subject to both the dispersion electric field and the light from the light source.

[0060] Optionally, the method may include providing a vacuum region containing the differential ion mobility assembly. The gas (e.g, flow of gas entrained with ions) containing ions generated from the sample, may be established in the vacuum region. Optionally, the drift region extends along an ion optical axis. The gas (e.g, flow of gas) may be provided (or directed) along the ion optical axis.

[0061] Preferably, the step of applying the dispersion voltage, and of changing the magnitude of the dispersion voltage includes concurrently applying a compensation voltage, Vc, to the analytical gap without irradiation of the analytical gap by the light source such that an ion output signal corresponding to a selected ion mobility spectral peak is continuously detected for each of the changing values of the dispersion voltage, VD. The step of concurrently irradiating the drift region with light from the light source may be performed at a plurality of (e.g., each of) the different values of the dispersion voltage, VD, as repeated detection of an ion output signal from the differential ion mobility assembly. If the ion output undergoes a transition to a depressed signal value, this may be identified as an occurrence of a transition of a quantum state of the ions (e.g., entrained ions) at a certain value of the dispersion voltage, or across a narrow range of dispersion voltage values. The method may include identifying a relative energy, ER(VD'), associated with the given ion and a given dispersion voltage value VD, where: / ^(Vp) = ETh- Et= ho, i = 0, 1, 2 ... etc. Here, EThis either a discrete energy level of the ion associated with a quantum state or is a threshold energy level associated with the onset of ion fragmentation, and Etis a discrete energy level of the ion associated with a quantum state. The value ho is an energy of a photon output by the light source to irradiate the drift region, where o is the frequency of the light output by the light source and h is Plank’s Constant. The value of ER(VD') may be diagnostic of different ions and their internal structures.

[0062] In the apparatus described above, the controller may be configured to: change either the magnitude of the dispersion voltage or the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output undergoes a transition to a depressed signal value; identify an occurrence of a transition of a quantum state of ions or of a fragmentation of ions either according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage or according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

[0063] References herein to a depressed signal value include a reference to a signal value that has been caused to sink to a lower position.

[0064] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0065] Summary of the Figures

[0066] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0067] Figure 1 schematically shows a differential ion mobility assembly and the dispersion voltage waveform, and compensation voltage, applied to it in use.

[0068] Figure 2 schematically shows a differential ion mobility assembly with an ion output in communication with an ion detector or with a mass spectrometer.

[0069] Figures 3a and 3b show examples of (3a) the time-variation of the light intensity with which a light source irradiates the analytical gap of the differential ion mobility assembly of Figure 2 according to five different duty cycles of light output by the light source, and (3b) the time-variation of the waveform of a dispersion voltage applied across the analytical gap of the differential ion mobility assembly of Figure 2 according to five different duty cycles, and in synchrony with a corresponding duty cycle of light (Figure 3a) output by the light source.

[0070] Figures 4a and 4b show examples of (4a) the time-variation of the light intensity with which a light source irradiates the analytical gap of the differential ion mobility assembly of Figure 2 according to five different duty cycles of light output by the light source, and (4b) the time-variation of the waveform of a dispersion voltage applied across the analytical gap of the differential ion mobility assembly of Figure 2 according to five different duty cycles, and in synchrony with a corresponding duty cycle of light (Figure 4a) output by the light source.

[0071] Figure 5 schematically show parts of a differential ion mobility apparatus.

[0072] Figure 6 schematically show parts of a differential ion mobility apparatus.

[0073] Figure 7 schematically show parts of a differential ion mobility apparatus.

[0074] Figure 8 schematically show parts of a differential ion mobility apparatus.

[0075] Figure 9 schematically show parts of a differential ion mobility apparatus.

[0076] Figures 10a and 10b each schematically shows a differential ion mobility assembly in which irradiating light laterally input to the analytical gap and is caused to follow a zig-zag path in a direction along the longitudinal axis of the assembly and concurrently (10a) laterally across the longitudinal axis between opposing plane mirrors separating the electrodes of the assembly to define a 2D path, or (10b) laterally across the longitudinal axis laterally across the longitudinal axis between opposing plane mirrors separating the electrodes of the assembly and also between reflective opposing plane surfaces of the electrodes of the assembly to define a 3D path.

[0077] Figures 11a and 11b each schematically shows a differential ion mobility assembly in which irradiating light is longitudinally input to the analytical gap and caused to follow a zig-zag path in a direction along the longitudinal axis of the assembly and concurrently (11a) laterally across the longitudinal axis between opposing plane mirrors separating the electrodes of the assembly to define a 2D path, or (11b) laterally across the longitudinal axis laterally across the longitudinal axis between opposing plane mirrors separating the electrodes of the assembly and also between reflective opposing plane surfaces of the electrodes of the assembly to define a 3D path.

[0078] Figure 12 schematically shows a differential ion mobility assembly in which a plurality of light sources are disposed along opposite lateral sides of the drift region in two opposing linear arrays extending in parallel to the longitudinal axis of the assembly.

[0079] Figures 13a, 13b and 13c schematically show examples of electrodes for use in a differential ion mobility assembly.

[0080] Figure 14a and 14b schematically show examples of electrodes for use in a differential ion mobility assembly.

[0081] Figure 15 schematically shows parts of a differential ion mobility apparatus.

[0082] Figure 16 schematically shows parts of a differential ion mobility apparatus.

[0083] Figure 17 schematically shows parts of a differential ion mobility apparatus.

[0084] Figure 18 schematically shows a method of differential ion mobility analysis.

[0085] Figure 19 schematically shows quantum energy levels of an ion subject to change in response to a changing applied dispersion voltage.

[0086] Figures 20a and 20b schematically show the magnitude of a differential ion mobility signal corresponding to an ion mobility spectral peak as measured in response to a changing applied dispersion voltage in which an ion undergoes (20a) transitions between two discrete quantum states, and (20b) transitions from an initial discrete quantum states to an ion fragmented (continuum) state, in response to absorption of a photon from the light source, by the ion.

[0087] Figure 21 schematically shows quantum energy levels of an ion subject to change in response to a changing photon energy of photons from a light source. Figure 22 schematically shows the magnitude of a differential ion mobility signal corresponding to an ion mobility spectral peak as measured in response to a changing photon energy of photons from a light source in which an ion undergoes transitions from an initial discrete quantum states to an ion fragmented (continuum) state, in response to absorption of a photon from the light source, by the ion.

[0088] Detailed Description of the Invention

[0089] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0090] In examples described herein, a gas may be provided in the form of a flow of gas entrained with ions generated from a sample, the flow of gas being directed along a drift region. However, it is to be understood that such examples are not intended to be limiting in that, alternatively, the gas may be substantially static. The ions may be caused to move along the drift region by being entrained within a flow of the gas containing them, and / or by action of an electric field configured to urge the ions in a direction along the drift region, the direction of urging being either along the direction of gas flow or against that flow direction.

[0091] Ion mobility spectrometry (IMS), such as differential mobility spectrometry (DMS) [I. A. Buryakov, et al., Int. J. Mass Spectrom. Ion Processes 1993, 128, 143] or field asymmetric waveform ion mobility spectrometry (FAIMS) [R. W. Purves, et al., Rev. Sci. Instrum. 1998, 69, 4094] are established methods to separate different types of ions according to differences of their mobility through a gas in response to the application of a force to the ions via an electric field intensity. These mobility differences depend on the physical and chemical properties of ions and gas particles (e.g., neutral particles such as atoms and / or molecules), but are only weakly correlated with the ion mass. The resulting strong orthogonality of this method relative to mass spectrometry (MS) makes FAIMS / MS a powerful analytical approach.

[0092] Referring to Figure 1 , there is schematically illustrates the basic principles and the mechanism for IMS separation based on the non-linear ion mobility dependence on electric field and pressure. Ions are entrained in a stream 30 of buffer gas directed along the axis of a drift region defined between two (or more) opposing electrodes 32. A high frequency asymmetric AC waveform 34 is applied to one of the two opposing electrodes. This is known as the dispersion voltage, VD. It is responsible for causing spatial separation of ions according to differences in ion mobility through the buffer gas within which the ions are entrained. The dispersion electric field, ED= -(c / yD(x) / c), generated by the dispersion voltage induces a motion in the ions in a direction of the dispersion electric field extending from one of the two electrodes to the other electrode. Combined with the concurrent drift motion of the ions in the direction of buffer gas flow, the resulting path of ions follows a zig-zag shape as the polarity of the dispersion voltage, VD, alternates between positive and negative values such that the dispersion electric field, ED, alternates between opposite directions across the gas drift direction. Superimposed to the waveform is a slow compensation DC voltage waveform 36 comprising a succession of "sawtooth" DC ramps. This is known as the compensation voltage, Vc. The frequency of the asymmetric waveform 34 of the dispersion voltage, VD, usually spans between a few hundreds of KHz to ~1 MHz, while that of the "sawtooth" DC ramp 36 typically repeats at a rate of < 1 Hz. The amplitude of the asymmetric waveform of the dispersion electric field when the IMS is operated at ambient pressure, is limited by the breakdown limit of the gas flowing within a given electrode geometry and for a parallel plate IMS system the electric field does not generally exceed 3 kV mm1.

[0093] Still referring to Figure 1 , separation of ions is possible using waveforms substantially different to the pure rectangular waveform. A family of waveforms based on quasi-sinusoidal variations of the voltage as a function of time are widely used; these are the bi-sinusoidal, the clipped sinusoidal or other substantially rectangular waveforms. Asymmetric waveforms are designed so that the area of the positive pulse portion of one waveform cycle matches that of the negative pulse portion, i.e., Ai = A2. For this particular arrangement of time-dependent electric fields, an ion with no mobility dependence on variations in electric field and pressure will therefore be transmitted at zero compensation voltage. The waveform is characterized by its duty cycle 38, usually defined as the width of the short positive pulse part, TH, of one wave cycle, divided by the full duration of one waveform cycle which defines the waveform period T (i.e., defined as: d = TH / T). The sum of the width of the short positive pulse part, TH, and the width of the long negative pulse part, TL, of the wave cycle is equal to the full duration, T, of one waveform cycle.

[0094] There exist optimum duty cycles for separating certain types of ions. For example, the type A and C ions are best separated in the IMS spectrum when the duty cycle is d ~ 0.33. B-type ions exhibit a more complex behaviour and having the ability to vary the duty cycle during the course of an experiment is essential for enhancing instrument performance.

[0095] Also shown in Figure 1 are a stable ion trajectory 40 transmitted successfully through the drift region and a second ion trajectory hitting the top DMS electrode 42. Successful transportation of the lost ion 42 would require the appropriate compensation voltage, Vc, 36 to be applied to the IMS electrode to compensate for the small average displacement, Ax, 46 introduced per waveform cycle. By scanning the compensation voltage, Vc, ions with different non-linear mobility dependencies on electric field and pressure are successively transported through the drift region gap and can either be detected on a plate connected to an electrometer 90 (not shown) or detected / monitored by a mass spectrometer 48 (not shown).

[0096] Referring to Fig. 2, a Field Asymmetric Ion Mobility Spectrometry (FAIMS) device (also known as a Differential Field Mobility, DMS, device) is illustrated. The apparatus comprises an atmospheric pressure ion source (API) 50 comprising an electrospray ionization (ESI) source configured to provide ions from a sample, and a first vacuum region 52 containing a differential ion mobility assembly comprising a drift region defined between electrodes 54 separated by an analytical gap (g). The drift region extends along an ion optical axis towards a second vacuum region 56 containing either an ion detector or a mass analyzer / spectrometer. The first vacuum region is in ion-flow communication with the second vacuum region via a skimmer 58 defining an ion outlet of the first vacuum region and simultaneously an ion inlet of the second vacuum region. The first vacuum region is located before the second vacuum region on the ion optical axis such that in use ions generated from the sample undergo differential ion mobility analysis by the differential ion mobility assembly before undergoing detection by the ion detector or mass spectral analysis by the mass analyzer.

[0097] A gas flow former 60 comprises a capillary providing gas flow communication from the atmospheric pressure ion source (API) 50 to the first vacuum region. The capillary is configured to establish in the first vacuum region a flow of buffer gas (e.g., N2 molecules) entrained with ions generated from the sample, the flow of buffer gas being directed along the drift region (between electrodes 54). A power supply unit 62 provides a voltage source and is configured to apply a dispersion voltage, VD, to one or more of the electrodes 54 to generate between them a dispersion electric field, ED, across the analytical gap (g).

[0098] The power supply unit 62 comprises switches (not shown) configured to switch to provide the AC dispersion voltage waveform, VD, such as described above with reference to Figure 1 , that alternates between adjustable dispersion voltage amplitude values. The power supply unit is configured such that, within an interval of time not exceeding 50ms, it adjusts the dispersion voltage, VD, amplitude thereby to adjust the value of the dispersion electric field, ED. A light source 64 is configured to irradiate the drift region with light 66. In some aspects of the invention, the light source 64 is configured to irradiate the drift region with light 66 in a pre-selected state of linear polarization or circular polarization.

[0099] A controller 68 is configured to control the voltage source 62 to apply the dispersion voltage, VD, to one or more electrodes 54 of the FAIMS / DMS assembly thereby to generate the dispersion electric field, ED, across the analytical gap (g) when the FAIMS / DMS assembly contains the flow of gas entrained with ions from the ion source. In particular, the controller 68 is configured to control the power supply 62 to supply to the electrodes 54 of the drift region, an asymmetric AC waveform 34 of the dispersion voltage, VD, and a DC compensation voltage, Vc, described above with reference to Figure 1 . The dispersion voltage, VD, consists of high field (HF) and low field (LF) segments. This dispersion voltage, VD, creates a dispersion electric field:

[0100] ED= -(dyD(x) / dx) = -VD / g which spans the analytical gap, g, all along the drift region and extends in a direction perpendicular to the longitudinal axis of the drift region and alternating in polarity. The DC compensation voltage, Vc, also generates a compensation electric field:

[0101] Ec= -(dVc(x) / dx) = -Vc / g which spans the analytical gap, g, all along the drift region and extends in a direction perpendicular to the longitudinal axis of the drift region.

[0102] The controller 68 is configured to control the light source 64 to concurrently irradiate the drift region with light 66 thereby to irradiate the ions entrained within the flow of gas contained therein such that the entrained ions are simultaneously subject to both the dispersion electric field, ED, and the light from the light 66 source 64. The voltage source 62 is configured to apply the dispersion voltage, VD, in a form that changes in time according to a repeating dispersion voltage waveform of the type shown in Figure 1 , and the controller 68 is configured to control the light source 64 to irradiate the drift region with light 66 according to a light intensity that also changes in time in synchrony with the changes in time defined by the dispersion voltage waveform (e.g., see Fig. 3b and Fig. 4b).

[0103] Figures 3a and 3b show an example of changes in the light intensity, as shown in Figure 3a, that are in synchrony with changes in the dispersion voltage waveform shown in Figure 3b. Figures 4a and 4b show another example of changes in the light intensity, as shown in Figure 4a, that are in synchrony with changes in the dispersion voltage waveform shown in Figure 4b. In particular, the controller 68 in each of these two examples, is arranged to control the light source to irradiate the drift region with light of a first light intensity during the application of a first portion of the dispersion voltage waveform where the high- field (HF of Figurel) part of the dispersion voltage, VD, occurs. Subsequently, the light source is controlled to irradiate the drift region with light of a second light intensity during the application of a separate second portion of the dispersion voltage waveform where the low-field (LF of Figurel) part of the dispersion voltage, VD, occurs. The first light intensity differs from the second intensity in both examples.

[0104] In particular, in the first example shown in Figure 3a, the first light intensity is a uniform non-zero intensity during the whole duration of the first portion of the dispersion voltage waveform, and the second light intensity is of zero intensity (i.e. , no light from the light 66 source 64) during the whole duration of the second portion of the dispersion voltage waveform, whatever the duty cycle of the dispersion voltage waveform of Figure 3b happens to be. conversely, in the second example shown in Figure 4a, the first light intensity is of zero intensity (i.e., no light from the light 66 source 64) during the whole duration of the first portion of the dispersion voltage waveform, and the second light intensity is a uniform non-zero intensity during the whole duration of the second portion of the dispersion voltage waveform, whatever the duty cycle of the dispersion voltage waveform of Figure 4b happens to be.

[0105] Of course, other arrangements are possible in which each of the first light intensity and the second light intensity are non-zero but differ in intensity from each other in some desired way.

[0106] The dispersion voltage waveforms illustrated in figures 3b and 4b, as well as in Figure 1 , each comprises a first waveform portion where the high-field (HF of Figurel) part defines a first voltage polarity and a second waveform portion where the low-field (LF of Figurel) part defines a second voltage polarity which is opposite in polarity to the first voltage polarity. In the examples illustrated in figures 3a and 3b, the controller 68 controls the light source 64 to irradiate the drift region with light 66 during application of only one of the first waveform portion and the second waveform portion of opposite polarity. Of course, the dispersion voltage waveforms illustrated in figures 3b and 4b, as well as in Figure 1 , each defines an asymmetric dispersion voltage waveform in which the first waveform portion defines a first voltage amplitude (HF of Figurel) and the second waveform portion defines a second voltage amplitude (LF of Figurel) which is different (i.e., smaller) in magnitude to the magnitude of the first voltage amplitude. The controller 68 is configured to control the light source 64 to irradiate the drift region with light 66 during application of only one of the first waveform portion and the second waveform portion. It is to be understood that the although the high-amplitude part of the asymmetric dispersion voltage waveform (HF of Figurel) is shown as preceding the low-amplitude part (LF of Figurel) within a given waveform cycle, it is equally possible to arrange the high-amplitude part of the asymmetric dispersion voltage waveform to succeed the low-amplitude part within a given waveform cycle (e.g., by adjusting the duty cycle as appropriate). Furthermore, as is shown in Figure 3b and Figure 4b, the controller may control the dispersion voltage, VD, so that it comprises a symmetric waveform in which the first waveform portion defines a first voltage amplitude and the second waveform portion defines a second voltage amplitude which has substantially the same magnitude as the magnitude of the first voltage amplitude. In this case, the duration of the first waveform portion is substantially the same as the duration of the second waveform portion.

[0107] The apparatus may comprise an optical reflector assembly (not shown in Figure 2) configured to reflect light from the light source in a direction transverse to the ion optical axis. Examples of the optical reflector assembly are shown in Figure 5 (item 22), Figure 6 (item 122), Figure 7 (item 222), Figure 11a (item 602), Figure 11 b (items 602 and 604), Figure 12a (items 702, 704 and 716) and figure 12b (items 702, 704 and 716). As is discussed in more detail below, in each of these arrangements, the light source is arranged such that light from the light source is incident obliquely on, and reflected obliquely from, a plurality of separate reflection points that are located at opposite sides of the drift region. The reflection points may be spaced longitudinally in a direction along the drift region and / or may be spaced laterally in a direction across the ion optical axis. As a result, a zig-zag pattern of reflected light is generated along the drift region and / or across the ion optical axis. Alternatively, or in addition, the light source may comprise a plurality of separate light sub-sources (not shown in Figure 2) that are located along at least one side of the drift region and are spaced longitudinally in a direction along the drift region. Examples of the plurality of separate light sub-sources are shown in Figure 10 (item 506) and this example is discussed in more detail below. The controller 68, in this example, is configured to control the plurality of separate light subsources to generate light simultaneously thereby to irradiate the drift region with light.

[0108] EXAMPLE 1 (Figure 5):

[0109] Figure 5 schematically illustrates a first embodiment of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2.

[0110] The apparatus comprises a high voltage power supply 2 and an RF (radio-frequency) switching unit 4 collectively corresponding to a power supply 62 as schematically illustrated in Figure 2. A laser unit 8, and optical beam-forming assembly (10, 12, 14), collectively provide a light source corresponding to light source 64 of Figure 2, for generating a laser beam 24 corresponding to the light 66 of Figure 1 for irradiating the drift region of a differential mobility Spectrometry (DMS) assembly 20, which extends between two plane parallel opposing DMS electrodes 20 from a DMS ion entrance 26 to a DMS ion exit 28. The laser unit is configured to be configured to emit pulses of laser light with pulsing frequency of, for example, 200kHz or higher. For example, the laser unit may comprise a semiconductor laser.

[0111] A waveform generator unit 6 is configured to generate a desired waveform with a user-selected frequency and a user-selected duty cycle d = TH / T, see Figure 1), and to control the RF switching unit 4 to generate a dispersion voltage according to the selected waveform. As noted above with reference to figures 3b and 4b, the waveform may be symmetric or asymmetric depending on the user-selected duty cycle. In addition to controlling the RF switching unit 4, the waveform generator is configured to control the optical output generated by the laser unit 8 such that the optical output is synchronised with the waveform generated by the waveform generator in the manner shown in any of the examples described above with reference to figures 3a and 4a. Consequently, since the same waveform generator unit 6 controls both the waveform of the dispersion voltage applied between to electrodes of the DMS assembly and the optical output of the laser unit 8, the changes imposed on the optical output of the laser unit are synchronised with the waveform of the dispersion voltage. The waveform generator unit 6 corresponds to the controller 68 of Figure 2.

[0112] Accordingly, the high-voltage power supply, the RF switching unit and the waveform generator (2, 4, 6) are configured to generate a voltage waveform for the DMS assembly 20. The waveform generator 6 controls the pulsing of the laser light source 8 at a frequency and duty cycle corresponding to the waveform applied to the electrodes 20 of the DMS assembly. The laser light source generated a laser beam produced 24 which is directed through beam shaping optics 10 configured, optionally, to expand the cross-sectional shape and / or area of the laser beam, and / or to manipulate the cross-sectional distribution of light energy across the beam profile, for example to render it more evenly distributed. The laser light source is subsequently directed from the output of the beam shaping optics 10 to pass through a polarising filter unit 12. This may comprise an optical polariser configured to transmit light possessing a pre-selected state of linear, circular or elliptical polarisation. If the laser unit is configured to generate linearly polarized laser light, then the polarising filter unit 12 may be unnecessary.

[0113] An adjustable mirror unit 14, which is optional, may be provided to receive the laser beam transmitted by the polarising filter unit 12 and to reflect the received laser beam at a selected, adjustable angle such that the reflected laser beam progresses in a direction that is suitably oblique to the reflecting surfaces of an optical reflector assembly 22 comprising a pair of plane-parallel optical mirrors that extend in a direction parallel to, and at opposite sides of the optical axis of the DMS assembly, so as to oppose each other across the optical axis of the DMS assembly. The adjustable mirror unit 14, or the orientation of the laser beam 24 if the adjustable mirror unit is omitted, is configured such that the laser beam is directed through a beam inlet opening of one of the two opposing plane parallel mirrors 22 into the DMS assembly in a direction close to (but deliberately slightly oblique to) perpendicular to the ion optical axis of the DMS assembly and, therefore, a direction close to (but deliberately slightly oblique to) perpendicular to the plane reflecting surface of each one of the two opposing plane parallel mirrors 22.

[0114] The beam inlet opening 15a is located close to the ion exit 28 of the DMS assembly. The laser beam 24 is reflected from the reflecting surface of the plane-parallel opposing mirrors 22 in such a way that it is cause to follow a zig-zag path reflecting to-and-from from one mirror to the other while being directed along the ion optical axis of the DMS assembly towards the DMS ion entrance 26. In following this zig-zag path, the reflected laser beam irradiates a large area of the drift region within the DMS assembly. The laser beam can in this way interact with ions moving through the DMS assembly. A beam outlet opening 15b of one the other (opposing) one of the two opposing plane parallel mirrors 22 is located at a position close to the DMS ion entrance 26 such that the laser beam 24 may exit the DMS assembly via the beam outlet opening in a direction onto a photon detector 18 to allow detection of laser beam transmission, or for other desired photon-based analytical methods. A further polarising filter unit 16 (optional) may be provided in the path of the laser beam 24 between the photon detector 18 and the beam outlet opening for use in controlling, or filtering / constraining, a state of polarisation of the laser beam output from the DMW assembly.

[0115] EXAMPLE 2 (Figure 6):

[0116] Figure 6 schematically illustrates a second embodiment of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Example 2, like items are assigned like reference symbols.

[0117] Example 2 is similar to the apparatus described above for Example 1 with reference to Figure 5, with the difference that the waveform generator unit 5 (Figure 5) is replaced with a mechanical chopper assembly 106. The mechanical chopper assembly comprises a rotatable chopping disc 106a rotatingly mounted upon the rotation axis 106b of an electrical rotary drive motor 106c. A circumferential array of a plurality of uniformly spaced circular through-openings of equal diameter is formed in the chopping disc. The circular array of through-openings 106d extends around the and is centred upon the rotation axis 106b of the electrical rotary drive motor 106c. In other words, each one of the plurality of uniformly spaced circular through-openings 106d is equidistant, radially, from the rotation axis 106b of the electrical rotary drive motor 106c, and the spacing, in the circumferential direction, between neighbouring circular through- openings is the same for all circular through-openings. An optical emitter-detector unit 106e comprises a light emitter part (e.g., an LED) arranged to emit a continuous light output of uniform light intensity, and a light detector part (e.g., a photodiode) opposing the light emitter part across a gap and configured continuously to monitor the light output from the light emitter part.

[0118] The peripheral edge of the rotatable chopping disc 106a is arranged to reside within the gap such that, depending upon the rotational position of the chopping disc, either the light emitter part is in optical communication with the light detector part when a through-opening of the chopping disc resides between the two, or the light emitter part is not in optical communication with the light detector part when occulted by a portion of the chopping disc defining a circumferential spacing (L) between neighbouring through- openings. The ratio of the diameter (£)) of each through-opening and the circumferential spacing (L) between neighbouring through-openings defines the duty cycle (d = TH / T = D / (D + L), or d = TH / T = L / (D + L), see Figure 1). When the light emitter part is occulted by a portion of the chopping disc defining a circumferential spacing (L) between neighbouring through-openings, such that the light detector part is unable to receive light directly from the light emitter part, then the light detector part is configured to output a first light detection signal to the RF switching unit. Conversely, when the light emitter part is not occulted by a portion of the chopping disc but is able to receive light from the light emitting part directly via a through-opening, then the light detector part is configured to output a second light detection signal to the RF switching unit.

[0119] The RF switching unit is responsive to a first light detection signal to apply the high-amplitude part of an asymmetric dispersion voltage waveform (HF of Figurel) to electrodes of the DMS assembly 120 and is responsive to a second light detection signal to apply the the low-amplitude part (LF of Figurel) to electrodes of the DMS assembly, of opposite polarity to that of the high-amplitude part, thereby providing a given waveform cycle. Of course, alternatively, the RF switching unit may be responsive to a first light detection signal to apply the low-amplitude part of an asymmetric dispersion voltage waveform (LF of Figurel) to electrodes of the DMS assembly 120 and may be responsive to a second light detection signal to apply the the high-amplitude part (LF of Figurel) to electrodes of the DMS assembly. Furthermore, if the duty cycle is such that d = TH / T = D / (D + L) = 1 / 2, or d = TH / T = L / (D + L) = 1 / 2, then the waveform becomes symmetric.

[0120] Consequently, as the chopping disc 106a rotates, the RF switching unit receives a light detection signal from the optical emitter-detector unit 106e that alternates between the first light detection signal and the second light detection signal thereby causing the RF switching unit to apply a dispersion voltage waveform to electrodes of the DMS assembly that alternates in magnitude and polarity between the low- amplitude part and the high-amplitude part.

[0121] The chopping disc 106a is positioned relative to the laser unit 8 such that, depending upon the rotational position of the chopping disc, either the laser beam 24 output by the laser unit 8 is in optical communication with the beam shaping assembly 10 when a through-opening of the chopping disc resides between the two, or the laser beam 24 is not in optical communication with the beam shaping assembly 10 when occulted by a portion of the chopping disc defining a circumferential spacing (L) between neighbouring through-openings. As a result, the laser beam input to the optical beam shaping assembly is chopped in synchrony with the asymmetric (optionally symmetric) dispersion voltage waveform generated by the RF switching unit, and with a duty cycle, dLight= TOut / T = D / (D + L), where T is the duration of a dispersion voltage waveform cycle time, and TOutis the duration for which the laser beam is output to the beam shaping assembly 10, and thence to the DMS assembly 20, within a dispersion voltage waveform cycle. The controllable chopper rotation speed determines the pulsing rate of the laser beam prior to it being directed through the optics (10, 12, 14) and into the DMS assembly. The mechanical chopper assembly 106 corresponds to the controller 68 of Figure 2.

[0122] EXAMPLE 3 (Fiqure 7):

[0123] Figure 7 schematically illustrates a third embodiment of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Example 3, like items are assigned like reference symbols.

[0124] Example 3 is similar to the apparatus described above for Example 1 with reference to Figure 5, with the difference that, instead of controlling the pulsing of the laser unit 8 directly, the pulsing is controlled by a liquid crystal (LC) shutter 230 controlled by a waveform generator 6 connected to the RF switches 4. The LC shutter is situated after the laser beam shaping assembly (e.g. optical expander) 10 to reduce the heating effect of the laser beam 24 on the LC shutter.

[0125] EXAMPLES 4 - Alternative light sources (Figures 8 and 9):

[0126] Figure 8 and Figure 9 each schematically illustrates a fourth embodiment and a fifth embodiment, respectively, of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Examples 4, like items are assigned like reference symbols.

[0127] Examples 4 are each similar to the apparatus described above for Example 1 with reference to Figure 5, with the difference that the light source is not a laser, and the beam shaping optics 10, polarising filter unit 12, and adjustable mirror unit 14 are absent. In the embodiment shown in Figure 8, the light source 308 comprises a light-emitting diode (LED). In the embodiment shown in Figure 9, the light source 408 is an incandescent bulb or arc. When the light source 408 comprises an incandescent bulb or arc, it may be directly pulsed by controlling the power employed to drive the light source. Alternatively, it may be combined with a mechanical chopper as described above with reference to Figure 6, or with a liquid crystal (LC) shutter to modulate the intensity of light input to the DMS assembly 20. When the light source comprises an incandescent bulb or arc, it may also comprise one or more dichroic filters or / and dye filters or / and diffraction grating filters for controlling the optical spectrum of the light it outputs, as desired. When the light source 308 comprises an LED, it may be directly pulsed by controlling the power employed to drive the LED. Whether the light source is an LED or an incandescent bulb or arc, in both cases the light source is controlled by the waveform generator 6 to output light pulses in synchrony with the waveform, as described above with reference to figures 3a, 3b, 4a and 4b. In Figure 8 and Figure 9, the waveform generator is in direct control both of the light source (308, 408) and of the RF switching unit 4, as in Example 9 (Figure 5).

[0128] EXAMPLES 5 - Arrangement of mirrors (Figures 10a and 10b):

[0129] Figure 10a and Figure 10b each schematically illustrates a sixth embodiment and a seventh embodiment, respectively, of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2.

[0130] Furthermore, as between Example 1 and Examples 5, like items are assigned like reference symbols.

[0131] Referring to Figure 10a, the apparatus comprises an optical reflector assembly configured for reflecting light from the light source in a direction transverse to the ion optical axis from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced longitudinally in a direction along the drift region. As a result, light from the light source is caused to form / follow a zig-zag pattern of reflected light along the drift region. The light source (not shown) is configured to direct light to be incident obliquely on, and reflected obliquely from, each of the plurality of longitudinally spaced reflection points. In particular, once more refering to Figure 10a, a light beam 610 (e.g., from a laser or other light source) is input to the DMS assembly 20 through beam inlet opening 15a in a direction substantially parallel to the plane of the opposing plane-parallel electrodes 20 of the DMS assembly. Furthermore, the light beam is directed in a direction oriented at a selected angle chosen such that, once within the DMS assembly, the light beam progresses in a direction that is suitably oblique to the reflecting surfaces of the optical reflector assembly 22. As noted above, these reflecting surfaces comprise a pair of plane-parallel optical mirrors that extend in a direction parallel to, and at opposite sides of, the optical axis of the DMS assembly, so as to oppose each other across the optical axis of the DMS assembly. The orientation of the light beam 610 is such that the light beam is directed through a beam inlet opening 15a of one of the two opposing plane parallel mirrors 22 into the DMS assembly in a direction close to (but deliberately slightly oblique to) perpendicular to the ion optical axis of the DMS assembly and, therefore, a direction close to (but deliberately slightly oblique to) perpendicular to the plane reflecting surface of each one of the two opposing plane parallel mirrors 22.

[0132] The light beam 610 is reflected from the reflecting surface of the plane-parallel opposing mirrors 22 in such a way that it is caused to follow a zig-zag path reflecting to-and-from from one mirror to the other, as shown, while being directed along the ion optical axis of the DMS assembly towards the DMS ion entrance. In following this zig-zag path, the reflected laser beam irradiates a large area of the drift region within the DMS assembly. The light beam can in this way interact with ions moving through the DMS assembly. The beam outlet opening 15b of the other (opposing) one of the two opposing plane parallel mirrors 22 is located such that the light beam may exit 612 the DMS assembly via the beam outlet opening.

[0133] Referring to Figure 10b, an alternative optical reflector assembly is shown which is configured for reflecting light from the light source in a direction transverse to the ion optical axis from a plurality of separate reflection points that are located upon opposing plane reflecting surfaces of the two opposing electrodes 20 of the DMS assembly at opposite sides of the drift region. These reflection points are spaced laterally in a direction across the ion optical axis. The result is that the light beam is caused to follow a zig-zag pattern of reflected light across the ion optical axis, from one reflective electrode surface to the other. To achieve this the light source is configured to direct light to be incident obliquely on, and reflected obliquely from, each of the plurality of laterally spaced reflection points on the plane reflecting surfaces of the two opposing electrodes 20. In particular, the light beam 614 (e.g., from a laser or other light source) is input to the DMS assembly 20 through beam inlet opening 15a in a direction transverse / oblique to the plane of the opposing plane- parallel electrodes 20 of the DMS assembly. Furthermore, the light beam is directed in a direction oriented at a selected angle chosen such that, once within the DMS assembly, the light beam progresses in a direction that is suitably oblique to the reflecting surfaces of the optical mirror 22 of the optical reflector assembly.

[0134] Consequently, the optical reflector assembly, in this example, comprises not only the reflecting surfaces of the pair of plane-parallel optical mirrors 22 that extend in a direction parallel to, and at opposite sides of, the optical axis of the DMS assembly, but also the opposing optically reflective surfaces of the electrodes 20 of the DMA assembly, which also oppose each other across the optical axis of the DMS assembly. The opposing optically reflective surfaces of the electrodes 20 may be polished to achieve a desired optically reflecting surface, or may be coated with a suitably reflective optical coating if desired.

[0135] The orientation of the light beam 614 is such that the light beam is directed towards one of the electrodes 20 of the DMS assembly, via the beam inlet opening 15a of one of the two opposing plane parallel mirrors 22 into the DMS assembly. The beam orientation is close to (but deliberately slightly oblique to) perpendicular to the ion optical axis of the DMS assembly and is close to (but deliberately slightly oblique to) perpendicular to the plane of the surface of each of the plane-parallel electrodes of the DMS assembly, and concurrently the beam direction is close to (but deliberately slightly oblique to) perpendicular to the plane reflecting surface of each one of the two opposing plane parallel mirrors 22.

[0136] The light beam 614 is reflected from the reflecting surfaces both of the plane-parallel opposing electrodes 20 of the DMS assembly and of the plane-parallel opposing mirrors 22 in such a way that it is caused to follow a zig-zag path, in three spatial dimensions, reflecting to-and-from from one electrode surface to the other, and to-and-from from one mirror to the other, as shown, while being directed along the ion optical axis of the DMS assembly towards the DMS ion entrance. In following this zig-zag path, the reflected laser beam irradiates a large area of the drift region within the DMS assembly. The light beam can in this way interact with ions moving through the DMS assembly. The beam outlet opening 15b of the other (opposing) one of the two opposing plane parallel mirrors 22 is located such that the light beam may exit 612 the DMS assembly via the beam outlet opening.

[0137] In this way, the light beam can be directed across the ion optical axis of the DMS assembly. In Figure 10a the beam is directed across the DMS in a planar fashion and reflected from the side mirrors 22, in which case the light beam 610 is input to the DMS assembly in a direction across the DMS cell but parallel to the two electrodes 20 of the DMS assembly. Alternatively, in order to increases the vertical coverage of the DMS analytical gap by the beam, the light beam can additionally be reflected from the surface of the electrodes 20 of the DMS assembly, as in Figure 10b, in which case the light beam 614 is input to the DMS assembly in a direction both across the DMS cell and towards one of the two electrodes. Figure 11a and Figure 11 b each schematically illustrates a eighth embodiment and a ninth embodiment, respectively, of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Examples 6, like items are assigned like reference symbols.

[0138] Referring to Figure 11a, the light beam 710 may be introduced from either end of the DMS cell, in a direction parallel (or close to parallel) to the ion optical axis of the DMS assembly 20. This arrangement is shown in Figure 11a and in Figure 11 b. In Figure 11a the light beam 710 is introduced via the analytical gap at the ion entrance (or ion exit) of the DMS assembly, and propagates along the DMS assembly within the analytical gap to the ion exit (or ion entrance) of the DMS assembly. The light beam 710 is then reflected by an internal beam-directing mirror assembly 716 which direct the light beam obliquely towards the reflecting surface of one of the two mutually opposing plane parallel mirrors 22 of the optical reflector assembly such that the light beam executes a zig-zag path between the opposing plane parallel mirrors and along the axis of the DMS assembly, in the manner described above with reference to Figure 10a.

[0139] Referring to Figure 11 b, by employing a further oblique mirror 706 to the internal beam-directing mirror assembly 716 the light beam may be directed within the DMS assembly to follow a 3D zig-zag path along an a manner described above with reference to the arrangement depicted in Figure 10b. In this way, the light beam can also be reflected from the optically reflective opposing surfaces of the electrodes 20 of the DMS assembly as shown in Figure 10b. Once again, this allows the beam to cover a greater vertical area of the DMS analytical gap.

[0140] EXAMPLE 7 - Arrangement of light sources (Figure 12):

[0141] Figure 12 schematically illustrates a tenth embodiment of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Example 7, like items are assigned like reference symbols.

[0142] Whereas in some of the above embodiments, mirrors or other reflective surfaces are used to ensure adequate irradiation of the entire length of the DMS cell, other arrangements to achieve this are also envisaged. In the embodiment illustrated in Figure 12, instead of using mirrors to reflect a single light beam, an array of separate light sub-sources (e.g., LEDs) 506 are positioned on each side of the analytical gap between the electrodes 20. Electrode separators are used to separate the electrodes, and these separators are provided with individual holes for individually housing the separate light sub-sources 506. This configuration may allow substantially uniform irradiation of the length of the DMS cell. The light sources may be controlled to generate light in synchrony with the dispersion voltage waveform according to methods described herein. In more detail, in the present example, the plurality of separate light sub-sources 506 that are arranged with uniform spacing to collectively define two parallel and opposing linear arrays extending along opposite sides of the drift region, between the opposing electrodes 20 of the DMS assembly so as to be in optical communication with the drift region. The plurality of separate light sub-sources are spaced longitudinally in a direction along the drift region, parallel to the ion optical axis.

[0143] Two opposing electrode separators 504, e.g., formed of suitable dielectric material, are configured to hold the two opposing electrodes of the DMS assembly in spaced opposition by a uniform gap defining the analytical gap (g). The electrode separators preferably present to the drift region an optically reflective outer surface to assist in reflecting light from the plurality of separate light sub-sources. Each one of the electrode separators 504 comprises a linear array of holes each one of which is configured for mounting therein a respective one of the plurality of separate light sub-sources 506. Each of the light sub-sources may comprise a light-emitting diode (LED). The controller 68 is configured to control the separate light sub-sources 506 to generate light simultaneously (and preferably each outputting the same optical output intensity, and colour, wavelength or spectral / wavelength distribution) thereby to irradiate the drift region with light.

[0144] It is noted that one side of the upper electrode 20 of the DMS assembly is notionally omitted in Figure 12 purely to aid clarity and reveal an electrode separator and its array of separate light sub-sources in detail.

[0145] EXAMPLE 8 - Arrangements in tandem with other analytical devices or detectors (Figures 15, 16 and 17):

[0146] Figures 15, 16 and 17 each schematically illustrates a eleventh, twelfth and thirteenth embodiment, respectively, of an apparatus according to the invention. It is to be understood that this example illustrates components of an apparatus for Ion Mobility Spectrometry (IMS) in which certain components, such as an ion source (50, 60) and vacuum regions / housings (52, 56, 58), are omitted from the illustration merely to aid clarity but are to be understood as being present as discussed above e.g., with reference to Figure 2. Furthermore, as between Example 1 and Example 8, like items are assigned like reference symbols.

[0147] Referring to Figure 16 and Figure 17, the invention may provide an apparatus for ion analysis comprising an apparatus for Ion Mobility Spectrometry (IMS) as described above, and a mass spectrometer configured to receiving ions output from the drift region and for generating an ion mass spectrum in response thereto. In this way, the invention may comprise a combination, e.g., operable at atmospheric pressure, in which the ion output of the DMS assembly is in communication with the ion input of a mass spectrometer. Ions passing through the DMS assembly (20, 22) would enter the mass spectrometer via an interface 908 that maintains the pressure difference (i.e. low pressure within the MS). The ion beam is then passed through various focussing optics (912, 914) and into one or more mass analysers (in this case a quadrupole mass analyser (916) and then onto an ion detector (918). This format provides a DMS- MS device. The invention may provide a low pressure DMS system. In this arrangement, the DMS assembly (20, 22) is situated after the ambient pressure-MS interface 808 and followed by various ion optics (812, 814) one or more mass analysers 816 and an ion detector 818. The low pressure DMS can be situated in various locations in the MS envelope including prior to, between two or after a mass analyser. Referring to Figure 17, the apparatus may comprise an ion detector for receiving ions output from the drift region of the DMS assembly and for generating an ion detection signal in response thereto. In one embodiment the ion detector may comprise a pair of detector electrodes (1020) configured with detector electronics (not shown) to detect the passage of ions out of the DMS assembly (20, 22). This device may be operated at ambient pressure (P2) e.g., as a stand-alone DMS system.

[0148] EXAMPLE 9 - Arrangements of DMS electrodes (Figures 13a, 13b, 13c, 14a, 14b):

[0149] Figures 13a, 13b, 13c, 14a, 14b show examples of different configurations for the DMS electrode structure, shown in isolation for clarity, that may be employed within the DMS assembly according to the invention.

[0150] Figure 13a shows an arrangement of two substantially identical planar, rectangular electrodes 20 arranged in plane-parallel opposition to each other such that each part of a continuous plane face of one of the two electrodes faces a corresponding part of a continuous plane face the other of the two electrodes across a uniform spacing defining the analytical gap, g. The power supply unit 62 may be arranged to apply a voltage to one or both of the electrodes such that an electrical potential difference exists between them corresponding to the dispersion voltage. This arrangement of electrodes is employed within the DMS assembly according to figures 5 to 12 described above, and is also an arrangement consistent with the DMS assembly according to figures 1 to 2 described above.

[0151] Figure 13b shows an arrangement of two substantially identical planar, rectangular, laterally-segmented electrodes 20b arranged in plane-parallel opposition to each other. Each laterally-segmented electrode comprises a set of substantially identical electrode segments, with the same total number of segments present in each respective set, in which each electrode segment is spaced from its neighbouring electrode segment(s), within the given set, by an inter-segment spacing that is uniform along the length of the spacing, and in which the size of the spacing is common to each pair of neighbouring electrode segments within the respective set. Each electrode segment, and each inter-segment spacing, extends linearly in a direction parallel to the ion optical axis of the DMS assembly along the entire length of the assembly from its ion inlet end to its ion outlet end. Each part of a continuous plane face of one electrode segment of any one set of electrode segments faces a corresponding part of a continuous plane face of one electrode segment of the other of the two sets of electrode segments across a uniform spacing defining the analytical gap, g. The power supply unit 62 may be arranged to apply a voltage to one or both of the laterally-segmented electrodes such that an electrical potential difference exists between them corresponding to the dispersion voltage. This arrangement of electrodes may be employed within the DMS assembly according to figures 5 to 12 described above, and is also an arrangement consistent with the DMS assembly according to figures 1 to 2 described above.

[0152] Figure 13c shows an arrangement of two substantially identical planar, rectangular, longitudinally- segmented electrodes 20c arranged in plane-parallel opposition to each other. Each longitudinally- segmented electrode comprises a set of substantially identical electrode segments, with the same total number of segments present in each respective set, in which each electrode segment is spaced from its neighbouring electrode segment(s), within the given set, by an inter-segment spacing that is uniform along the length of the spacing, and in which the size of the spacing is common to each pair of neighbouring electrode segments within the respective set. Each electrode segment, and each intersegment spacing, extends linearly in a direction perpendicular to the ion optical axis of the DMS assembly along the entire lateral width of the assembly. Each part of a continuous plane face of one electrode segment of any one set of electrode segments faces a corresponding part of a continuous plane face of one electrode segment of the other of the two sets of electrode segments across a uniform spacing defining the analytical gap, g. The power supply unit 62 may be arranged to apply a voltage to one or both of the longitudinally-segmented electrodes such that an electrical potential difference exists between them corresponding to the dispersion voltage. This arrangement of electrodes may be employed within the DMS assembly according to figures 5 to 12 described above, and is also an arrangement consistent with the DMS assembly according to figures 1 to 2 described above.

[0153] Figure 14a shows an arrangement of two substantially identical curved, rectangular electrodes 20 arranged in parallel opposition to each other such that each part of a continuous plane face of one of the two electrodes faces a corresponding part of a continuous plane face the other of the two electrodes across a uniform spacing defining the analytical gap, g. The two curved electrodes share the same radius of curvature which is uniform in value along the entire longitudinal length of the respective electrode, and radiates from a centre of curvature constrained to a locus of points extending linearly in parallel to the longitudinal axis of the electrode in question. The result is that each electrode defines a circularly cylindrical curvature along its length. The power supply unit 62 may be arranged to apply a voltage to one or both of the electrodes such that an electrical potential difference exists between them corresponding to the dispersion voltage. This arrangement of electrodes may be employed within the DMS assembly according to figures 5 to 12 described above, and is also an arrangement consistent with the DMS assembly according to figures 1 to 2 described above.

[0154] Figure 14b shows an arrangement of two substantially identical curved, rectangular electrodes 20 arranged in parallel opposition to each other such that each part of a continuous plane face of one of the two electrodes faces a corresponding part of a continuous plane face the other of the two electrodes across a uniform spacing defining the analytical gap, g. The two curved electrodes share the same radius of curvature which is uniform in value along the entire lateral width of the respective electrode, and radiates from a centre of curvature constrained to a locus of points extending linearly perpendicular to the longitudinal axis of the electrode in question. The result is that each electrode defines a circularly cylindrical curvature along its width. The power supply unit 62 may be arranged to apply a voltage to one or both of the electrodes such that an electrical potential difference exists between them corresponding to the dispersion voltage. This arrangement of electrodes may be employed within the DMS assembly according to figures 5 to 12 described above, and is also an arrangement consistent with the DMS assembly according to figures 1 to 2 described above.

[0155] Figure 18 illustrates steps in a method for Ion Mobility Spectrometry (IMS) which may be implemented using the apparatus according to the invention, e.g., using apparatus as illustrated and described above. This method does not preclude other methods of IMS falling within the scope of the invention, and is provided for illustrative purposed to aid an understanding of the invention. The method for Ion Mobility Spectrometry (IMS) comprises:

[0156] STEP 1 : Generating ions from a sample in an ion source;

[0157] STEP 2: Providing a vacuum region containing a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along an ion optical axis;

[0158] STEP 3: Establishing in the vacuum region a flow of gas entrained with ions generated from the sample, the flow of gas being directed along the ion optical axis;

[0159] STEP 4: Applying a dispersion voltage, VD, to one or more of the electrodes thereby to generate a dispersion electric field, ED, across the analytical gap containing the flow of gas; and, concurrently irradiating the drift region with light from a light source thereby to irradiate the ions entrained within the flow of gas contained therein such that the entrained ions are simultaneously subject to both the dispersion electric field and the light from the light source.

[0160] In STEP 4, the dispersion voltage, VD, defines a voltage which changes in time according to a repeating dispersion voltage waveform, and the method includes irradiating the drift region with light from the light source according to a light intensity that changes in time in synchrony with the changes in time defined by the dispersion voltage waveform. The irradiating of the drift region may include irradiating with light of a first light intensity from the light source during the application of a first portion of the dispersion voltage waveform and irradiating the drift region with light of a second light intensity from the light source during the application of a separate second portion of the dispersion voltage waveform. The first intensity preferably differs from the second intensity. The irradiating the drift region may include irradiating with light from the light source during the application of a first portion of the dispersion voltage waveform and during none of the duration of a separate second portion of the dispersion voltage waveform. The properties, application and control of the irradiating light and the dispersion voltage, in STEP 4, may be as described herein in any aspect or example. STEP 1 to STEP 4 may be implemented by the apparatus described with reference to Figure 2, as also discussed with reference to figures 1 to 17 herein, and STEP 4 may be implemented by the controller 68 described herein with reference to Figure 2.

[0161] APPLICATIONS OF THE INVENTION EXAMPLE 1

[0162] At sufficiently high values of dispersion electric field (i.e., the quantity ED / N) a situation may arise in which ion drift velocities are no longer negligible as compared to the thermal velocities of buffer gas molecules (e.g., N2). As noted in Shvartsburg, A.A. (2008). Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and Fundamentals ofFAIMS (1st ed.), the statistical distribution of ion velocities may then still be approximated by the Maxwell-Boltzmann formula when the temperature parameter, T, input to the formula is replaced by a higher effective temperature, TEF, as follows, noting that v = KED

[0163] Here, k is Boltzmann’s constant, K is the ion mobility, and M is the mass of buffer gas particles. The higher effective temperature arises due to, amongst other things, vibrational excitation of the ions entrained within the buffer gas and is equivalent to an ion temperature that would otherwise be achieved in the absence of the dispersion electric field if the buffer gas had a temperature of TEF. This effect is known as “field heating” and the quantity of heating produced by the dispersion electric field can be quantified as TH= (TEF- T). Molecular collisions with buffer gas particles are often inelastic in that the kinetic energy of a colliding buffer gas particle is transferred not only to translational energy of the ion involved in the collision, but also to the excitation of different electronic, rotational and / or vibrational internal modes of the ion, as well as to the excitation of different conformational states (i.e., different shapes) of the ion. By their very nature, these excited states are defined by quantum eigenstates (I 'i), where i identifies a discrete state) of the Schroedinger Equation defining the ion permitting only certain well-defined quantised internal energies (i.e., Eitthe eigenvalues of the Schroedinger Equation):

[0164] H\^i) = Ei\^i)

[0165] Here, the Hamiltonian is: H = HIon+ H^ED~), and comprises a first component, HIon, defining the internal dynamics of the ion in the absence of the dispersion electric field, ED, and a second component H^ED~) defining a perturbation to the internal dynamics of the ion as caused by the application of the dispersion electric field, ED.

[0166] Referring to Figures 19 to 22 there are schematically illustrated uses of the invention according to any aspect or example herein. In Figure 19 there is shown a graphical representation of a plurality of quantum eigenstates i / ^), where i = 0,1, 2, 3, ... ) of the Schroedinger Equation defining the ion, and their associated eigenvalue energies: E0,E1,E2. These eigenvalue energies change in value according to changes in the dispersion electric field, ED, applied to the ion which change the Hamiltonian: H = Hlon+ H(ED) ' of the ion. An energy fteo is the energy of a photon of light emitted from the light source into the analytical gap of the differential ion mobility assembly, such that the ions entrained within the buffer gas are simultaneously subject to both the dispersion electric field:

[0167] ED= -(dyD(x) / dx) = -VD / g and the light from the light source. The dispersion electric field, ED, causes a “field heating” effect which perturbs the quantised internal eigenvalue energies, Eitthe eigenvalues of the Schroedinger Equation for the entrained ions. For example, three quantum energy levels are shown: E0,E1, E2which each correspond to the eigenvalue associated with a respective quantum eigenstate: |i / >0), I 'iX I’ / 'z), of the ion in the presence of the dispersion electric field, ED. Due to the presence of the second component H(E0) ' of the Hamiltonian defining a perturbation to the internal dynamics of the ion as caused by the application of the dispersion electric field, ED, the three quantum energy levels: E0,E1,E2each change continuously in value in response to continuous changes in the magnitude of the dispersion voltage, VD, that is applied across the analytical gap in order to generate there the dispersion electric field ED= -VD / g. In Figure 19, the continuous changes in the three quantum energy levels: E0,E1,E2are shown as being linear changes as a function of continuous changes in the dispersion voltage, VD, and as being the same changes to all quantum energy levels. It is to be understood that this choice is purely to simplify the illustration to allow clarity for a better understanding, and it is not asserted that, in practice, these changes will be linear and / or that, in practice, these changes will be the same for all eigenvalues. Also shown in Figure 19 is a higher energy value, ETh, which may be either further discrete eigenvalue associated with a further quantum eigenstate of the ion in the presence of the dispersion electric field, ED, or may be a threshold energy value above which the ion is caused to fragment (i.e., not corresponding to a discrete eigenvalue of the Schroedinger Equation associated with a “bound state”).

[0168] In Figure 19, it can be seen that when certain particular values of the dispersion voltage, VD, are applied to the analytical gap, the value of a respective one of the three quantum energy levels: EOI E- , E2is brought into resonance with the higher energy value, ETh, by absorption of one photon of light from the light source such that: Et+ ha> = ETh, i = 0, 1,2. When one of these resonances is brought into existence, by application of an appropriate value of the dispersion voltage, the result is that the respective quantum eigenstate: |i / >0), I 'iX I’ / 'z), 'sable to transition to either one of the following two final states:

[0169] (a) A quantum eigenstate of the ion, having discrete energy level ETh, or

[0170] (b) A state in which the ion is fragmented, having any one of a continuum of energies residing above energy ETh.

[0171] In either case, the result of such a transition may be to change (i.e., cause a transition in) the mobility of the resulting ion, or fragment, in this final state.

[0172] The compensation voltage, Vc, applied to the analytical gap, is controlled such that an ion output signal, corresponding to a selected ion mobility spectral peak, corresponding to a given ion species under scrutiny, is continuously detected from the differential ion mobility assembly for each of the changing values of the dispersion voltage, VD, shown in Figure 19, when no irradiation of the analytical gap by the light source takes place. In this way, the selected ion mobility spectral peak is ‘followed’ as the mobility of the mobility of the given ion species under scrutiny changes in response to the changing values of the dispersion voltage. Changes in the height of this ion mobility spectral peak, in response to irradiation of the analytical gap by the light source, are diagnostic of the quantum eigenstates of the ion in question. Thus, at each of the changing values of the dispersion voltage, VD, or at least a sufficiently large sample of them, a repeated measurement is performed when a given dispersion voltage is applied and with the analytical gap concurrently irradiated by the light source. If a transitional drop in the ion output signal from the differential ion mobility assembly (i.e., a fall in the height of the selected ion mobility spectral peak) is detected to a depressed ion output signal at a certain value of the dispersion voltage, or across a narrow range of dispersion voltage values, then this indicates the occurrence of the resonance condition at which: Et+ UM = ETh, i = 0, 1, 2. The transitional drop in the height of the selected ion mobility spectral peak corresponds to a transition to a depressed ion output signal value when the resonance condition is satisfied.

[0173] One may ‘follow’ a selected ion mobility peak to ensure that a detected signal corresponds to the same ion for all relevant values of dispersion voltage. In this way one may be sure that the resonances seen (e.g., Fig.20a) all correspond to the same ion. The peak could be followed for example by performing what is known in the art as a “CV / DV scan” (CV = compensation voltage; DV = dispersion voltage) such as is well within the skill of the person of ordinary skill in the art.

[0174] Figure 20a shows a schematic representation of the nature of the transitional drop in the ion output signal when the higher energy value, ETh, corresponds to a further quantum eigenstate of the ion, having discrete energy level ETh. This is condition (a) noted above. The result is that the drop in the ion output signal from the differential ion mobility assembly exists at a discrete, narrow range of values of the dispersion voltage, VD, centred upon the resonant voltage value that achieves the relevant resonance condition: Et+ ha> = ETh, i = 0, 1,2. This drop ion output signal corresponds to a transition to a depressed signal value, due to a transition from a discrete quantum state of the ion to a new state causing a transition to a new value of ion mobility. The transition to a depressed ion output signal value may occur rapidly as a function of changing dispersion voltage values.

[0175] Due to the discrete nature of the energy eigenvalues of the quantum states of the ion, there can only be resonance transitions (and associated changes in ion mobility) between allowed, discrete quantum eigenstates of the ion such that, as the dispersion voltage is further increased a resonant quantum energy level is moved out of resonance and a transition of the ion to new state of ion mobility can no longer occur, at least, not until the next resonance is brought into existence.

[0176] Figure 20b shows a schematic representation of the nature of the drop in the ion output signal when the higher energy value, ETh, corresponds to a fragmentation threshold energy of the ion. This is condition (b) noted above. The result is that the drop in the ion output signal from the differential ion mobility assembly exists at the value of the dispersion voltage, VD, that achieves the relevant resonance condition: Et+ ha> = ETh, i = 0, 1, 2, and at all values of the dispersion voltage above the resonance value. This drop in ion output signal corresponds to a transition to a depressed signal value, due to a transition from a discrete quantum state of the ion to a new “continuum” state causing a transition to a new value of ion mobility. The transition to a depressed ion output signal value may occur rapidly as a function of changing dispersion voltage values. Due to the “continuum” of the energy values of the ion fragment(s), there can be a continuum of dispersion voltage values that all result in ion fragmentation above this threshold value and the associated changes in ion mobility between one discrete quantum eigenstate of the ion and a continuum of fragment energies. This means that as the dispersion voltage is further increased a transition of the ion to the new fragmented state of ion mobility can continue to longer occur, and the drop in the ion output signal from the differential ion mobility assembly also continues.

[0177] In both Figure 20a and 20b, the drop on the ion output signal from the differential ion mobility assembly is shown as being partial, in the sense that the ion output signal does not fall to zero. This may be the case when each one of the three respective quantum eigenstate: |i / >0), I 'iX I’ / 'z), 'spopulated such that a drop in the ion output signal is the result of loss of ions from only one of the three populated states, the ions existing in the other states being unable to undergo a transition from their initial quantum state, and therefore are unable to undergo a transition in ion mobility. As a result, ions in those non-transitioning quantum states remain able to provide ions for the ion output signal.

[0178] Consequently, the invention may comprise a method for analysing ions comprising: applying said dispersion voltage, VD, to the one or more of the electrodes of the differential ion mobility assembly thereby to generate the dispersion electric field, ED, across the analytical gap containing the flow of gas; and, concurrently irradiating the drift region with light from the light source thereby to irradiate the ions entrained within the flow of gas contained therein such that the entrained ions are simultaneously subject to both the dispersion electric field and said light from the light source. The light may be substantially monochromatic light (or of suitably narrow spectral band) comprising photons of a corresponding photon energy (or narrow band of energies). The method may include: detecting an ion output signal from the differential ion mobility assembly; changing the magnitude of the dispersion voltage to identify a respective threshold magnitude at which the detecting an ion output signal undergoes a transition to a depressed signal value; identifying an occurrence of a transition of a quantum state of the entrained ions (i.e., as between two discrete quantum states) or of a fragmentation of the entrained ions according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage.

[0179] The step of applying the dispersion voltage, and of changing the magnitude of the dispersion voltage, includes concurrently applying a compensation voltage, Vc, to the analytical gap without irradiation of the analytical gap by the light source such that an ion output signal corresponding to a selected ion mobility spectral peak is continuously detected for each of the changing values of the dispersion voltage, VD. The step of concurrently irradiating the drift region with light from the light source may be performed at a plurality of (e.g., each of) the different values of the dispersion voltage, VD, as repeated detection of an ion output signal from the differential ion mobility assembly. If a transitional drop in the ion output signal from the differential ion mobility assembly identifies an occurrence of a transition of a quantum state of the entrained ions to a depressed ion output signal at a certain value of the dispersion voltage, or across a narrow range of dispersion voltage values. The method may include identifying a relative energy, ER(VDassociated with the given ion and a given dispersion voltage value VD, where: ^(Vp) = ETh- Et= ha>, i = 0, 1, 2. The value of ER(VD') may be diagnostic of different ions and their internal structures. Figure 21 shows an alternative arrangement in which the value of the dispersion voltage, VD, is held substantially constant, and the energy of the photons output by the light source, to irradiate the analytical gap, is changed. Consequently, as the photon energy is changed, a different respective one of the three quantum energy levels: E0,E1,E2,E3is brought into resonance with the higher energy value, ETh, by absorption of one photon of light from the light source such that: Et+ ha> = ETh, i = 0, 1, 2, 3. When one of these resonances is brought into existence, by application of an appropriate value of the dispersion voltage, the result is that the respective quantum eigenstate: > 'sable to transition to either one of the following two final states:

[0180] (a) The further quantum eigenstate of the ion, having discrete energy level ETh, or

[0181] (b) A state in which the ion is fragmented, having any one of a continuum of energies residing above energy ETh.

[0182] In either case, the result if such a transition may be to change the mobility of the resulting ion, or fragment, in its final state. The change in photon energy may be achieved by applying one or more narrow-band optical filters to the output of light from a broadband light generator, within the light source, wherein the pass band of the optical filter is variable such between different pass bands having substantially no spectral overlap. The different pass bands may correspond to different discrete filters having different respective pass bands or may correspond to one tunable optical filter configured to be tuned to possess different selected pass bands. Alternatively, the change in photon energy may be achieved by employing a tunable narrow-band generator light which is configured to output light with a selected narrow spectral band. In Figure 22, the drop on the ion output signal from the differential ion mobility assembly when the drift region is irradiated with photons (as compared to when it is not) is shown as being partial, in the sense that the ion output signal does not fall to zero. This may be the case when each one of the four respective quantum eigenstate: > is populated such that a drop in the ion output signal is the result of loss of ions from only one of the three populated states, the ions existing in the other states being unable to undergo a transition from their initial quantum state, and therefore are unable to undergo a transition in ion mobility. As a result, ions in those non-transitioning quantum states remain able to provide ions for the ion output signal.

[0183] Consequently, the invention may comprise a method for analysing ions comprising: applying said dispersion voltage, VD, to the one or more of the electrodes of the differential ion mobility assembly thereby to generate the dispersion electric field, ED, across the analytical gap containing the flow of gas; and, concurrently irradiating the drift region with light from the light source thereby to irradiate the ions entrained within the flow of gas contained therein such that the entrained ions are simultaneously subject to both the dispersion electric field and said light from the light source. The light may be substantially monochromatic light comprising photons of a corresponding photon energy. The method may include: detecting an ion output signal from the differential ion mobility assembly; changing the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output signal undergoes a transition to a depressed signal value; identifying an occurrence of a transition of a quantum state of the entrained ions (i.e., as between two discrete quantum states) or of a fragmentation of the entrained ions according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

[0184] The step of applying the dispersion voltage preferably includes concurrently applying a compensation voltage, Vc, to the analytical gap without irradiation of the analytical gap by the light source such that an ion output signal corresponding to a selected ion mobility spectral peak is detected for that value of the dispersion voltage, VD. The step of concurrently irradiating the drift region with light from the light source may be performed at a plurality of (e.g., each of) the different values of the photon energy as a repeated detection of an ion output signal from the differential ion mobility assembly. This process may be repeated as desired with a different dispersion voltage, VD, applied across the analytical gap. A change in the values of the photon energy at which the ion output signal, corresponding to the same selected ion mobility spectral peak, transition to the suppressed state, may be diagnostic of the ion under study. In other words, as noted above with reference to Figure 19, the quantum eigenvalues of the ion in question may change in response to the changed dispersion voltage, VD, and different ions may change their quantum eigenvalues in different ways unique to those ions.

[0185] Figure 22 shows a schematic representation of the nature of the drop in the ion output signal when the higher energy value, ETh, corresponds to a fragmentation threshold energy of the ion. This is condition (b) noted above. The result is that the drop in the ion output signal from the differential ion mobility assembly exists at the value of the photon energy, fteo, that achieves the relevant resonance condition: EL+ ha> = ETh, i = 0, 1, 2, 3. This drop in ion output signal corresponds to a transition to a depressed signal value, due to a transition from a discrete quantum state of the ion to a new “continuum” state causing a transition to a new value of ion mobility. The method may include identifying a relative energy, ER(VDassociated with the given ion and a given dispersion voltage value VD, where: ^(VD) = ETh- Et= ha>, i = 0, 1, 2. The value of ER(VD') may be diagnostic of different ions and their internal structures.

[0186] Irradiating ions in one or more segments of the DMS waveform will excite ions as they undergo collisions with photons. This energy will excite the ion, either to a higher electronic energy state or by increasing its internal energy, and the increased energy may result in a conformational change in the ion. All of these conditions may result in the ions mobility changing as is observed for ions in standard DMS as the dispersion field strength is increased. This change in mobility at higher energies is fundamental to the separation of ions by DMS.

[0187] Application of the above system may be performed for the investigation of ion energy levels in low or high field environments.

[0188] Ions in the DMS go through repeated cycles of exposure of high and low electric field. In the high field region, the ions undergo intensive collisional heating followed by a period of collisional cooling in the low field. By altering the frequency of the waveform and / or the duty cycle, the heating / cooling time and magnitude can be modulated. In the DMS device, the following effect may be used to select ions. An ion, accelerating in the high field, takes time to reach its maximum velocity. Higher velocity results in higher collisional heating that leads to change in cross section of the ion. Conversely, when the ion enters a low field segment of the waveform the ions begins to decelerate to a lower velocity. Lower velocity results ion cooling and the cross section reverting to its previous value. Exposing ion to short period of high filed and longer period of low field of reversed polarity will result in ion landing in similar place when duty cycle is properly adjusted. For technical simplicity, preliminary chosen duty cycle is set and compensation voltage (yc) is used to balance response of cross section for different ion species. Ions movement along electrodes which is also separation time is provided by buffer gas flow.

[0189] Given sufficient energy, an ion or molecule will undergo fragmentation. In addition, acceptance of energy causes, heating, electronic excitation and can cause conformational changes and isomerisation. Changes can be divided to short lived and long lived where short and long is with the respect to time for ion staying between electrodes. Long lived changes that includes fragmentation, long lived conformational changes and isomerisation will result in ion loss in the DMS signal.

[0190] An initial basal energy may be provided by the dispersion electric field of the DMS assembly. That energy can be easily regulated by changing: the dispersion voltage amplitude; or / and frequency; or / and duty cycle. Additional quantised energy is be provided by photons from the light source. That energy is well defined by the photon wavelength. If an incident photon is accepted by an ion several outcomes may occur. If the ion undergoes significant structural rearrangement (conformational change, isomerisation) then the ions stability will likely be altered. If the ion undergoes electronic or rotational; or vibrational excitation a similar result will occur with the ion having an altered mobility and compensation field stability. Finally, fragmentation would also likely result in the loss of the ions due to altered stability of the fragment ions.

[0191] In the first application photon is used to probe ion internal energy levels when is in high or low dispersion field. These levels can be very different for different ions species. Ions, due to different cross sections, will have different times to accelerate / deaccelerate, changes in cross section due to collisional heating / cooling can occur at different rates and ions have different internal structures resulting in different energy storage capacity in atomic vibrations / rotations and electronic excitations. Nature also forbids instant reactions, which means when heating / cooling time is short enough ions will not reach thermal equilibrium with collisional heating. As a result, the total response of ion to short periods of heating or cooling can be very specific providing information that can be used in molecular structure studies. Alternatively, this specific response for ion species can be used to identify ions providing orthogonal measurement to mass and mobility spectrometry.

[0192] In a further variation, a DMS waveform with a 50% duty cycle could be used. In this instance the ion that is continuously heated (i.e., with no low field interval) can be investigated.

[0193] In certain conditions ions can show dipole alignment whereby oriented molecules will have different cross section for photons than randomly oriented molecules. In a case in which the light source is configured to irradiate the drift region with polarised light, the cross section of molecular ions with the respect to photon polarization orientation can be studied.

[0194] The invention may be applied for fragmentation of ions in low or high field environments. The effect of rapid heating and cooling for the purpose of modifying molecules could be studied. As with the fragmentation technique described above, the ion fragments could be examined to give information on the conformation and structure of the species present.

[0195] EXAMPLE 2

[0196] The invention may be applied to enhancing differential mobility spectrometry.

[0197] One application of the addition of energy via photons is to increase the energy experienced by the ion without increasing the electronic field. In a conventional DMS device the ion dispersion field is increased by applying a greater voltage across the analytical gap. The maximum voltage that can be applied across a gap is limited by electrical breakdown between the two electrodes. The point at which breakdown occurs is determined by a number of factors including the gap width and gas pressure. The onset of breakdown is often the limiting factor in the separation of ions by DMS. The addition of energy to the ions by other means (i.e. by collision with photons) results in the ions experiencing a higher energy without altering the electrical field and therefore without risk of causing breakdown. This, in turn allows the separation power of the DMS to be extended.

[0198] In many cases, the transmission of ions is lower at higher E / N values (due to various factors including fragmentation, conformational change resulting in self-cleaning etc.). Conversely, the separation of ions is generally better at higher E / N values. These competing factors can limit the performance of the DMS assembly. The additional separation power provided by this invention may allow separation of a given population of ions at a lower E / N value and thus with higher ion transmission increasing sensitivity.

[0199] As high E / N values are often required for separation of populations of ions the power supply requirements for DMS assembly can be high (in terms of both voltage and stability). The high voltage requirements (in particular at higher pressures e.g., atmospheric) can be a limiting factor in DMS design. Using the invention described here to reduce the E / N values and therefore voltage requirements (by using photon excitation separation in place of part or all of the E / N based separation) may reduce the cost, size and technical complexity of the DMS device.

[0200] The above can be implemented also in cases with a symmetric waveform (i.e., Fig. 1 in which duty cycle d = 0.5). Conventional DMS systems and methods make use of waveforms with “f” values of greater than 1 .0 (i.e., duty cycle d > 50%) to generate ion separation. Here, f = ED / ELis known as the “high-to- low” ratio f, where EDis the amplitude of the dispersion electric field in the “high-field (HF)” part of the asymmetric dispersion voltage waveform, and ELis the amplitude of the dispersion electric field in the “low-field (LF)” part of the asymmetric dispersion voltage waveform). A waveform with f = 1 would result in no separation as the ions would experience equal amounts of time in the “high” and “low field” environment, and the “high” and “low field” environments would present the same electric field amplitude to the ion. Thus, any differential effect would cancel out. However, irradiating ions with photons from the light source in a waveform with 50% duty cycle (symmetric waveform, d = 0.5) would cause ions to become photonically excited in one part of the waveform and not the other. The resulting increase in energy, as described above, would give rise to ions having different mobilities in one half of the waveform but not in the other half. This would comprise the application of a compensation voltage, Vc, to allow the ions to traverse the DMS cell. In effect one may generate a differential mobility separation using a symmetric waveform (and therefore distinct from FAIMS). There are several advantages to this system, for example, the system does not require a high field region and can therefore be implemented in a much lower cost system.

[0201] In a further variation of this embodiment, the duty cycle of the waveform could be changed such that the waveform is asymmetric, as used in conventional FAIMS. Under these conditions the amount of time spent in either the irradiated or non-irradiated region could be increased to optimise the experiment (e.g., increasing the time for photon acceptance or increasing the time for relaxation of the ion to ground state). While the waveform would be superficially similar to FAIMS (being asymmetric) both parts of the waveform would be in the “low field” regime, in which ion mobility is not changed by application of the “low field” dispersion electric field in any part of the asymmetric waveform. The result is that the separation of ions would be based on the photonic excitation of the ions to higher energy states rather than exposure to high dispersion electric fields.

[0202] In a further application of this invention, in a “low field” regime the absolute mobility of the ion may be derived by comparing the difference in mobility between the ground and excited state conditions. Under “low field” regime conditions, the relationship between collisional cross section (CCS, the effective cross section of the ion considering its orientation and rotation in space) and the compensation voltage value, Vc, is linear and therefore can be simply related to one another. For a compound with a known low field CCS from, for example, drift tube ion mobility, the CCS of the excited state could be extrapolated. The information provided would be analogous to a conventional ion mobility system wherein relative or absolute cross sections can be determined.

[0203] Irradiation with a selectively absorbed wavelengths of light can provide additionally selectivity to the DMS apparatus of the invention. There are many known examples of classes of ions which selectivity absorb a given wavelength or chirality of light. When a mixed population of ions is irradiated with such a form of light (i.e., specific wavelength or chirality) only those ions which can absorb photons with those specific characteristics will be excited. For example, in a co-population of chiral isomers, only one enantiomer will absorb the energy of the photons and so will gain energy and require a different correction voltage to travel through the DMS in effect separating the two enantiomers. A similar experiment could be conceived in which a population of peptide isomers contains one peptide with a tryptophan and so only this peptide will gain energy from irradiation with 280 nm light. Selection of the chirality or wavelength of light has significant advantage over the use of gas dopants or modifiers in that (assuming prior knowledge of the chemical structure) the behaviour of ions with regard to photon absorbance is predictable in advance. Alternatively, the ions can be irradiated with a broadband light source which will be absorbed by all ions.

[0204] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0205] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0206] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0207] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0208] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0209] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0210] References

[0211] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0212] A. Buryakov, et al., Int. J. Mass Spectrom. Ion Processes 1993, 128, p143

[0213] R. W. Purves, et al., Rev. Sci. Instrum. 1998, 69, p4094 Shvartsburg, A.A. (2008). Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and

[0214] Fundamentals of FAIMS (1st ed.). CRC Press, https: / / doi.org / 10.1201 / 9781420051070

Claims

Claims:1 . A method for Ion Mobility Spectrometry (IMS) comprising: generating ions from a sample in an ion source; providing a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along a direction transverse to the analytical gap; providing a gas containing ions generated from the sample, the gas being provided along the drift region; applying a dispersion voltage waveform comprising a dispersion voltage portion, VD, to one or more of the electrodes thereby to generate a dispersion electric field, ED, across the analytical gap containing the gas; and, irradiating the drift region with light from a light source thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and the light from the light source.

2. A method according to any preceding claim wherein the dispersion voltage waveform defines a voltage which changes in time according to a repeating waveform, and the method includes irradiating the drift region with light from the light source according to a light intensity that changes in time in synchrony with the changes in time defined by the dispersion voltage waveform.

3. A method according to any preceding claim including irradiating the drift region with light of a first light intensity from the light source during the application of a first portion of the dispersion voltage waveform and irradiating the drift region with light of a second light intensity from the light source during the application of a separate second portion of the dispersion voltage waveform, wherein the first intensity differs from the second intensity.

4. A method according to any preceding claim wherein the method includes irradiating the drift region with light from the light source during the application of a first portion of the dispersion voltage waveform and during none of the duration of a separate second portion of the dispersion voltage waveform.

5. A method according to any preceding claim wherein the dispersion voltage waveform comprises a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the method includes irradiating the drift region with light from the light source during application of at least one of the first waveform portion and the second waveform portion.

6. A method according to any preceding claim wherein the dispersion voltage waveform comprises a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

7. A method according to any preceding claim wherein the dispersion voltage waveform is an asymmetric waveform in which a first waveform portion defines a first voltage amplitude and a second waveform portion defines a second voltage amplitude which is different in magnitude to the magnitude of the first voltage amplitude, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

8. A method according to claim 7 wherein the second voltage amplitude is smaller in magnitude than the magnitude of the first voltage amplitude.

9. A method according to claim 7 wherein the second voltage amplitude is greater in magnitude than the magnitude of the first voltage amplitude.

10. A method according to any preceding claim wherein the dispersion voltage waveform is a symmetric waveform in which a first waveform portion defines a first voltage amplitude and a second waveform portion defines a second voltage amplitude which has substantially the same magnitude as the magnitude of the first voltage amplitude, wherein the method includes irradiating the drift region with light from the light source during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

11. A method according to any preceding claim including irradiating the drift region with polarized light from the light source configured to output said light in a pre-selected state of linear polarization or circular polarization.

12. A method according to any preceding claim when dependent on claim 3 wherein the first intensity and the second intensity are each substantially constant during the respective first portion and second portion of the dispersion voltage waveform.

13. A method according to claim 12 wherein the first intensity and the second intensity are each substantially constant for the whole duration of the respective first portion and second portion of the dispersion voltage waveform.

14. A method according to any preceding claim wherein said irradiating the drift region with light comprises reflecting light from the light source in a direction transverse to the direction in which the drift region extends from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced longitudinally in a direction along the drift region, thereby to generate a zig-zag pattern of reflected light along the drift region.

15. A method according to any preceding claim wherein said irradiating the drift region with light comprises reflecting light from the light source in a direction transverse to the direction in which the drift region extends from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced laterally in a direction across the drift region, thereby to generate a zig-zag pattern of reflected light across the drift region.

16. A method according to any preceding claim wherein the light source comprises a plurality of separate light sub-sources that are located along at least one side of the drift region and are spaced longitudinally in a direction along the drift region, wherein said irradiating the drift region with light from the light source comprises generating light simultaneously from the plurality of separate light sub-sources.

17. A method according to any preceding claim comprising: applying said dispersion voltage waveform comprising a dispersion voltage portion, VD, to one or more of the electrodes thereby to generate a dispersion electric field, ED, across the analytical gap containing the gas; and, concurrently irradiating the drift region with light from a light source thereby to irradiate the ions within the gas contained therein such that the ions are simultaneously subject to both the dispersion electric field and the light from the light source.

18. A method according to any preceding claim wherein the drift region extends along an ion optical axis and includes providing the gas containing ions generated from the sample, the gas being provided along the ion optical axis.

19. An apparatus for Ion Mobility Spectrometry (IMS) comprising: an ion source for generating ions from a sample; a differential ion mobility assembly comprising a drift region defined between electrodes separated by an analytical gap wherein the drift region extends along a direction transverse to the analytical gap,and configured to provide therein a gas containing ions generated from the sample, the gas being provided along the drift region; a voltage source configured to apply a dispersion voltage waveform comprising a dispersion voltage portion to one or more of the electrodes; a light source configured to irradiate the drift region with light; a controller configured to: control the voltage source to apply the dispersion voltage waveform to the one or more electrodes thereby to generate a dispersion electric field, ED, across the analytical gap when containing the gas; and, control the light source to irradiate the drift region with light thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and the light from the light source.

20. An apparatus according to claim 19 wherein the voltage source is configured to apply a dispersion voltage waveform that defines a voltage which changes in time according to a repeating waveform, and the controller is configured to control the light source to irradiate the drift region with light from the light source according to a light intensity that changes in time in synchrony with the changes in time defined by the dispersion voltage waveform.

21. An apparatus according to any of claims 19 to 20 wherein the controller is configured to control the light source to irradiate the drift region with light of a first light intensity during the application of a first portion of the dispersion voltage waveform and to irradiate the drift region with light of a second light intensity during the application of a separate second portion of the dispersion voltage waveform, wherein the first intensity differs from the second intensity.

22. An apparatus according to any of claims 19 to 21 wherein the controller is configured to control the light source to irradiate the drift region with light from the light source during the application of a first portion of the dispersion voltage waveform and during none of the duration of a separate second portion of the dispersion voltage waveform.

23. An apparatus according to any of claims 19 to 22 wherein the voltage source is configured to apply a dispersion voltage waveform comprising a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the controller is configured to control the light source to irradiate the drift region with light during application of at least one of the first waveform portion and the second waveform portion.

24. An apparatus according to any of claims 19 to 23 wherein the voltage source is configured to apply a dispersion voltage waveform comprising a first waveform portion defining a first voltage polarity and a second waveform portion defining a second voltage polarity which is opposite in polarity to the first voltage polarity, wherein the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

25. An apparatus according to any of claims 19 to 24 wherein the voltage source is configured to apply an asymmetric dispersion voltage waveform in which a first waveform portion defines a first voltage amplitude and a second waveform portion defines a second voltage amplitude which is different in magnitude to the magnitude of the first voltage amplitude, wherein the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

26. An apparatus according to claim 25 wherein the second voltage amplitude is smaller in magnitude than the magnitude of the first voltage amplitude.

27. An apparatus according to claim 25 wherein the second voltage amplitude is greater in magnitude than the magnitude of the first voltage amplitude.

28. An apparatus according to any of claims 19 to 27 wherein the dispersion voltage source is configured to apply a symmetric dispersion voltage waveform in which a first waveform portion defines a first voltage amplitude and a second waveform portion defines a second voltage amplitude which has substantially the same magnitude as the magnitude of the first voltage amplitude, wherein the controller is configured to control the light source to irradiate the drift region with light during application of one of the first waveform portion and the second waveform portion but not during application of the other of the first waveform portion and the second waveform portion.

29. An apparatus according to any of claims 19 to 28 wherein the controller is configured to control the light source to irradiate the drift region with polarized light from the light source configured to output said light in a pre-selected state of linear polarization or circular polarization.

30. An apparatus according to any of claims 19 to 29 when dependent on claim 20 wherein the first intensity and the second intensity are each substantially constant during the respective first portion and second portion of the dispersion voltage waveform.

31. An apparatus according to claim 30 wherein the first intensity and the second intensity are each substantially constant for the whole duration of the respective first portion and second portion of the dispersion voltage waveform.

32. An apparatus according to any of claims 19 to 31 comprising an optical reflector assembly configured for reflecting light from the light source in a direction transverse to the direction in which the drift region extends from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced longitudinally in a direction along the drift region, thereby to generate a zig-zag pattern of reflected light along the drift region.

33. An apparatus according to any of claims 19 to 32 comprising an optical reflector assembly configured for reflecting light from the light source in a direction transverse to the direction in which the drift region extends from a plurality of separate reflection points that are located at opposite sides of the drift region and are spaced laterally in a direction across the drift region, thereby to generate a zigzag pattern of reflected light across the drift region.

34. An apparatus according to any of claims 19 to 33 wherein the light source comprises a plurality of separate light sub-sources that are located along at least one side of the drift region and are spaced longitudinally in a direction along the drift region, the controller is configured to control the light source to generate light simultaneously from the plurality of separate light sub-sources thereby to irradiate the drift region with light.

35. An apparatus according to any of claims 19 to 34 wherein the controller is configured to: apply said dispersion voltage waveform comprising a dispersion voltage portion, VD, to one or more of the electrodes thereby to generate a dispersion electric field, ED, across the analytical gap containing the gas; and, concurrently irradiate the drift region with light from a light source thereby to irradiate the ions within the gas contained therein such that the ions are simultaneously subject to both the dispersion electric field and the light from the light source.

36. An apparatus according to any of claims 19 to 35 wherein wherein the drift region extends along an ion optical axis such that said gas containing ions generated from the sample is provided along the ion optical axis.

37. An apparatus according to any of claims 19 to 36 comprising an ion detector for receiving ions output from the drift region and for generating an ion detection signal in response thereto.

38. An apparatus for ion analysis comprising an apparatus for Ion Mobility Spectrometry (IMS) according to any of claims 19 to 38 and a mass spectrometer configured to receiving ions output from the drift region and for generating an ion mass spectrum in response thereto.

39. A method for analysing ions according to the method for Ion Mobility Spectrometry (IMS) according to any of claims 1 to 18, the method for analysing ions comprising: applying said dispersion voltage waveform comprising a dispersion voltage to said one or more of the electrodes thereby to generate said dispersion electric field, ED, across the analytical gap containing the gas; and, irradiating the drift region with said light from said light source thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and said light from the light source, wherein said light is substantially monochromatic light comprising photons of a corresponding photon energy; detecting an ion output signal from the differential ion mobility assembly; changing either the magnitude of the dispersion voltage or the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output signal undergoes a transition to a depressed signal value; identifying an occurrence of a transition of a quantum state of the ions or of a fragmentation of the ions either according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage or according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

40. An apparatus according to any of claims 19 to 38 wherein the controller is configured to: change either the magnitude of the dispersion voltage or the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output signal undergoes a transition to a depressed signal value; identify an occurrence of a transition of a quantum state of ions or of a fragmentation of ions either according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage or according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

41. A method for analysing ions according to the method for Ion Mobility Spectrometry (IMS) according to any of claims 1 to 18, the method for analysing ions comprising: applying said dispersion voltage waveform to said one or more of the electrodes thereby to generate said dispersion electric field, ED, across the analytical gap containing the gas; and, irradiating the drift region with said light from said light source thereby to irradiate the ions within the gas contained therein such that the ions are subject to the dispersion electric field and said light from the light source, wherein said light is substantially monochromatic light comprising photons of a corresponding photon energy; detecting an ion output signal from the differential ion mobility assembly;changing either the magnitude of the dispersion voltage or the magnitude of the photon energy to identify a respective threshold magnitude at which the detecting an ion output signal undergoes a transition to a depressed signal value; identifying an occurrence of a transition of a quantum state of ions or of a fragmentation of ions either according to the photon energy and the corresponding threshold magnitude of the changing dispersion voltage or according to the dispersion voltage and the corresponding threshold magnitude of the changing photon energy.

Images