Method for mass spectrometric analysis of a gas

Abstract

Method for the mass spectrometric analysis of a gas (4), comprising: Ionizing the gas (4) to generate ions (4a, 4b), storing, exciting and detecting at least a portion of the generated ions (4a, 4b) in an FT ion trap (2), wherein the generation and storage of the ions (4a, 4b) in the FT ion trap (2) and / or the excitation of the ions (4a, 4b) prior to the detection of the ions (4a, 4b) in the FT ion trap (2) comprises at least one selective IFT excitation, in particular a SWIFT excitation (10), dependent on the mass-to-charge ratio m / z of the ions (4a, 4b), characterized by the steps: Determining a starting phase position α 0 a trajectory B of ions (4a, 4b) at a given ion resonance frequency f ion after an IFT excitation (10) based on a time-dependent ion signal u recorded during the detection of the ions (4a, 4b) ion (t), and Determining a charge polarity of the ions (4a, 4b) based on the starting phase position α 0 of the ions (4a, 4b) after IFT excitation (10).

Description

Background of the invention

[0001] The invention relates to a method for the mass spectrometric analysis of a gas, comprising: ionizing the gas to generate ions, and storing, exciting and detecting at least a portion of the generated ions in an FT (“Fourier transform”) ion trap, in particular an electrical FT ion trap.

[0002] Ion storage, separation, and detection are the main functions of conventional mass spectrometers, which are generally housed in separate modules. This typically requires complex interfaces between the modules, which complicates both a compact and efficient solution and rapid manipulation of the ion populations. The transfer of ions through the interfaces also leads to signal loss, which reduces the performance and sensitivity of mass spectrometers. In an electrical or, if necessary, a magnetic Fourier transform ion trap (FT ion trap for short), however, many functions (e.g., ion generation, storage, and detection) can be combined "in situ" in the same ion trap and in a very compact manner.

[0003] In such an FT ion trap, ions or ionized gas components can be measured without feedback or interruption and detected according to their mass-to-charge ratio, as described, for example, in the article "A novel electric ion resonance cell design with high signal-to-noise ratio and low distortion for Fourier transform mass spectrometry" by M. Aliman and A. Glasmacher, Journal of The American Society for Mass Spectrometry; Vol. 10, No. 10, October 1999.

[0004] An example of a mass spectrometer with an electric FT ion trap is described in DE 10 2013 208 959 A1. The FT ion trap has a ring electrode and two additional electrodes (lid electrodes). The ions stored in the FT ion trap are excited in situ, and the excited ions are detected by recording and evaluating image charges that the stored ions induce onto the lid electrodes of the FT ion trap. For image charge measurement, the ions stored in the FT ion trap are broadband excited (stimulated) in situ and oscillate in the ion trap at characteristic resonance frequencies depending on the mass-to-charge ratio. This procedure differs fundamentally from conventional destructive detection methods, in which the ions are no longer available after the measurement.

[0005] Likewise, DE 102013213501 A1 discloses a mass spectrometer for the mass spectrometric analysis of gas mixtures, which comprises an ionization device and an ion trap for the storage and mass spectrometric analysis of the gas mixture.

[0006] From WO 2015 / 003819 A1, it is known that in an FT-ICR (Fourier Transform Ion Cyclotron Resonance) trap, IFT excitation in the form of a so-called SWIFT (Storage Wave-Form Inverse Fourier Transform) excitation can be used to remove or suppress individual ion populations from the ion trap if their particle number exceeds a predetermined threshold at a given mass-to-charge ratio. In this way, large ion populations can be removed from the ion trap, allowing specific subsets of ion populations to be measured more precisely.

[0007] DE 103 93 404 T5 describes a detector for chemical substances. The ionized chemical substance to be detected is enclosed in an ion trap in which a high-frequency electric field can be generated. The ions present in the ion trap are excited by SWIFT waveforms containing frequency components other than the frequency corresponding to the orbital resonance frequency of the ion of the chemical substance to be detected, thereby removing impurities. Subsequently, the aforementioned ions are excited with a frequency component corresponding to the orbital resonance frequency of the ion of the chemical substance to be detected, thereby fragmenting the ion of the chemical substance to be detected. The masses of the resulting fragments are measured using a mass spectrometer, thereby identifying the chemical substance to be detected.

[0008] GB 2 470 664 A describes an RF ion guide comprising a plurality of axial arrays of radially segmented electrodes. Object of the invention

[0009] The object of the invention is to develop a method for the mass spectrometric analysis of a gas in such a way that the performance of the mass spectrometric analysis is increased. Subject of the invention

[0010] According to a first aspect, this object is achieved by a method of the type mentioned at the outset, in which the generation and storage of the ions in the FT ion trap and / or the excitation of the ions (immediately) before the detection of the ions in the FT ion trap comprises at least one selective IFT (Inverse Fourier Transform) excitation, in particular a SWIFT (Storage Wave Form Inverse Fourier Transform) excitation, which is dependent on the mass-to-charge ratio or on the ion resonance frequencies of the ions.

[0011] According to this aspect, it is proposed to perform selective ion excitation (hereinafter also referred to as stimulation), for example, broadband selective ion stimulation, in the same FT ion trap during the generation and storage of the ions and / or immediately before the detection of the ions or the ion signals generated in the FT ion trap. Such stimulation is typically carried out using powerful IFT excitation, in particular SWIFT excitation, which makes it possible to significantly increase the performance of the mass spectrometer into which the FT ion trap is integrated. In this way, complex ion manipulations can also be carried out, enabling fundamentally new performance characteristics of the FT ion trap, as described in detail below. Broadband selective stimulation is understood to mean excitation in a large ion resonance frequency band.For such a broadband selective excitation, for example, the following can apply: (m / z). MAX / (m / z) MIN > 5, possibly > 10, where (m / z) MAX the maximum mass-to-charge ratio of the IFT excitation and (m / z) MIN denotes the minimum mass-to-charge ratio of the IFT excitation. It is understood that IFT excitations with a smaller ion resonance frequency band are also possible.

[0012] In one variant, at least one IFT excitation is performed during the generation of ions in the FT ion trap and / or during the storage of ions in the FT ion trap to select ions to be stored in the FT ion trap. Particularly in an electrical FT ion trap, unwanted ions that are not to be stored in the FT ion trap and that lie within a predetermined mass-to-charge ratio interval (where the interval may have several non-contiguous sub-intervals) can be excessively excited by continuous SWIFT excitation during ionization or during the storage process, so that these ions or charge carriers are lost to the surrounding electrodes of the FT ion trap, and only the ions to be stored with the desired mass-to-charge ratios remain in the FT ion trap and are stored there.

[0013] In this variant, the ions are generated in the FT ion trap, i.e., the gas to be analyzed is introduced into the FT ion trap in a charge-neutral state. Ionization in the FT ion trap can be carried out, for example, as described in WO 2015 / 003819 A1 cited above, i.e., ions and / or metastable particles of an ionization gas and / or electrons can be introduced into the FT ion trap, which ionize the gas or gas mixture to be analyzed in the FT ion trap. It is understood that it is also possible, in principle, to ionize the ions outside the FT ion trap and to introduce the gas to be analyzed into the FT ion trap in the form of gas ions. In this case, too, a selection of ions to be stored or accumulated in the FT ion trap can be carried out during the storage of the ions in the FT ion trap.

[0014] In a further development of this variant, only ions are selected for storage or accumulation whose mass-to-charge ratio lies outside the range of the mass-to-charge ratios of a main gas component of the gas to be analyzed. For the purposes of this application, a main gas component is understood to be a gas component whose volume fraction is more than 50 vol%, in many applications more than 90 vol%, of the gas to be analyzed. The main gas component is typically only a single gas component, e.g., N 2 or around H 2, i.e., a single substance, which usually corresponds to only one mass-to-charge ratio in the mass spectrum. If necessary, the main gas component, whose volume fraction is more than 50 vol%, or possibly more than 90 vol%, may also be composed of several gas components. In this case, each of the gas components of the main gas component represents more than 20 vol%, or possibly more than 30 vol%, of the gas under investigation.

[0015] In many applications, the detection of gas traces or gas components with very low partial pressures or concentrations in a gas matrix of a gas to be analyzed, for example, a process gas, with a high total pressure is required. The ratio of these partial pressures to the total pressure is, for example, in the order of ppm volume (10 -6 ppmV) to pptV (10 -12) per volume. The SWIFT excitation described above allows the main gas component(s) of the gas under investigation to be filtered, so that only the ions of the gas trace(s) of interest are accumulated in the FT ion trap for subsequent detection. This ensures that the FT ion trap is not flooded by the charge carriers of the main gas components during the ionization time of the gas trace ions to be measured. This allows a dynamic range D of more than eight or possibly more than nine orders of magnitude (D > 10 8 or 10 9 ) can be achieved. In addition, the sensitivity (absolute concentration) of the FT ion trap and, accordingly, the signal-to-noise ratio (SNR) increases with the accumulation time. This allows the detection limit for individual gas components to be increased to an order of magnitude of 10 -16mbar or less. The dynamics of the (electrical) FT ion trap required for this detection exceed the performance of conventional residual gas mass spectrometers.

[0016] In a further variant, the excitation level and / or the phase position of the IFT excitation are varied between a first excitation frequency and a second excitation frequency, whereby both the first excitation frequency and the second excitation frequency deviate from a predetermined excitation frequency by no more than 10%, preferably by no more than 5%, in particular by no more than 1%. The excitation level refers to the amplitude of the IFT excitation relative to a predetermined maximum amplitude and is typically expressed as a percentage.

[0017] When detecting ions in the (electric) FT ion trap, it is assumed that the high-frequency alternating field E acts solely on the ions. This is practically true as long as only a limited number of charge carriers of the same sign are present in the FT ion trap. The total number of charge carriers is referred to as the "space charge" or "ion cloud." The potential ϕ (E = - grad(ϕ)), which can be described using the Laplace equation and is derived from the high-frequency alternating field E, is influenced by the space charge. This influence of the space charge on the storage potential in a given volume within the FT ion trap is greater the larger the space charge density ρ in this volume and the weaker the average restoring force in the corresponding sub-volume resulting from the high-frequency alternating field. The following follows from the Laplace equation (1) for the high-frequency alternating field: div(grad(ϕ))=Δϕ=−ρ / εo where ε odenotes the dielectric constant in vacuum and ϕ the high-frequency alternating potential associated with the alternating field E (see above).

[0018] Particularly when exciting different ion types with closely spaced ion resonance frequencies, the synchronous oscillation of the ion packets can lead to large space charge densities in "space charge-sensitive" regions of the FT ion trap, which can severely disrupt the resonance frequencies of entire ion packets or make them impossible to store. This can result in a large scatter in the measured ion resonance frequencies, resulting in significantly lower mass resolution.

[0019] It is therefore advantageous if the charge carrier packets or ions (populations) with neighboring ion resonance frequencies do not simultaneously travel through the same trajectory (orbit). By applying suitable orbital (derived from the orbit), phase-shifted IFT excitations of the ions (e.g., slight orbital stretching of the ion packets by suitable SWIFT excitation), it is possible to achieve a sufficiently low space charge density during the measurement or detection. Alternatively or additionally, the excitation level or amplitude of the SWIFT excitation can be changed, which can also lead to a significant reduction in the interaction between neighboring ion populations or allow them to move along different trajectories or orbits.

[0020] The variation of the phase position and / or the degree of excitation of the SWIFT excitation takes place in a continuous interval between a first excitation frequency f ion,1 and a second excitation frequency f ion,2 (f ion,1 < f ion.2 ), where both are relatively close to each other, ie both the first and the second (ion) excitation frequency deviate from a given excitation frequency f ion,a by no more than 10% or 5%, in particular by no more than 1% downwards or upwards, ie the following applies: f ion,1 ≥ 0.9 f ion,a and f ion,2 ≤ 1.1 f ion,a or accordingly f ion,1 ≥ 0.95 f ion,a and f ion,2 ≤ 1.05 f ion,a or f ion,1 ≥ 0.99 f ion,a and f ion,2 ≤ 1.01 f ion,a . The specified excitation frequency f ion,aTypically corresponds to the mass-to-charge ratio of the ions or ion population of interest. By varying the phase position and / or the degree of excitation described above, ion populations present within this interval can be brought into different orbits, thereby increasing the mass resolution when investigating the ion population(s) of interest.

[0021] In a further variant, the phase position and / or the excitation level vary in steps between the first excitation frequency and the second excitation frequency depending on the excitation frequency. The frequency width of the steps can in particular be chosen to be the same, i.e. the interval between the first and the second excitation frequency is divided into equally sized sub-intervals or steps, between which the phase position and / or the excitation level can be changed. It is understood, however, that the frequency width of the sub-intervals does not necessarily have to be the same. Ideally, the excitation level and / or the phase position changes during the transition between each two adjacent sub-intervals in order to direct the ions assigned to the adjacent sub-intervals into different orbits.

[0022] In a further development, the excitation level and / or phase position between the first excitation frequency and the second excitation frequency either increase or decrease stepwise depending on the excitation frequency. In this way, the ions assigned to adjacent subintervals can be distributed across different orbits in a particularly simple manner. The increase or decrease in the excitation level and / or phase position between adjacent steps or subintervals can be the same in each case, but it is also possible to select or vary the increase or decrease in the excitation level between adjacent subintervals differently.

[0023] The excitation of ion packets, or ions with adjacent mass-to-charge ratios, occurs by briefly exposing the ion packets to a short-term excitation pulse in the corresponding ion resonance band. If the ions are excited in the different ion resonance bands with different amplitudes and phases, it is possible to either greatly minimize the interaction between the ion packets, as described above, or intentionally enhance it.

[0024] Such a deliberate enhancement of the interactions between ion populations can also prove advantageous under certain circumstances. In any case, the SWIFT excitation described above can have an adaptive influence on the interactions between ion populations.

[0025] In another variant, the same ions in the FT ion trap are excited multiple times (optionally broadband) selectively by IFT excitations, with the ions being detected after each IFT excitation. During detection after each IFT excitation, the number of excited ions (or the partial pressure of the excited gas component) is determined. By averaging the number of ions determined during each detection, the signal-to-noise ratio (SNR) of the excited ions of interest can be significantly increased without the other ions being affected by the excitation. Especially at very low partial pressures (e.g., in the pptV range or below) of gas traces or gas components of interest, an additional SNR gain of 10*log10(N) in dB is possible and advantageous (N describes the number of multiple IFT excitations of the same ion population).Here it is assumed that the ions can be stored stably over the entire measurement period and retain their characteristic chemical properties.

[0026] In a further development, there is a time interval between two consecutive IFT excitations that is longer than the mean free flight time of the ions in the FT ion trap. The mean free flight time t M is with the mean free path L M by the average speed v M linked, where: t M = L M / v M . The mean free flight time t M is typically more than approximately one millisecond (>1 ms). The IFT excitations, especially the SWIFT excitations, are only repeated when the ions have traveled a multiple of the mean free time of flight, e.g., more than 3 × t M , more than 5 × t M or more than 10 × t M .

[0027] After ionization, collisions between the neutral gas particles and the ionized gas particles can lead to chemical reactions such as charge transfer or protonation, which alter the original ion population. In many applications, it is of interest to identify the chemical intermediates of such a process.

[0028] In one variant, a mass spectrometric analysis of an ion signal is therefore carried out only in a temporally shiftable measurement time interval when detecting the ions. After IFT excitation, in particular after SWIFT excitation, the time-varying mass spectrum can be calculated and displayed using a suitably selected, shiftable, short measurement time interval, which is also referred to below as the FFT time window. The temporally shiftable measurement time interval can have a duration on the order of, for example, several milliseconds, preferably 10 ms or less, particularly preferably 5 ms or less. By continuously or discretely shifting the FFT time window, a temporal representation of the chemical behavior of the ion population embedded in the gas matrix or in the gas to be analyzed is obtained.

[0029] For the analysis of complex analytes consisting of a multitude of different molecules, some of which have molecular masses that are close to each other, a large mass range and a very high mass resolution are required. To meet this requirement, different mass analysis methods are usually combined, e.g., two mass analysis methods (so-called MS / MS) or, more generally, n mass analysis methods (so-called MS n Typically, quadrupole mass spectrometers or conventional ion traps are used for filtering or fragmentation in the mass range of interest, and then the selected mass range is analyzed more finely with another high-resolution mass analyzer (e.g., Fourier transform-based, space-time-based, or time-of-flight-based methods) to prevent analyzer overload (see space charge problem) and to simplify subsequent analysis.

[0030] From the procedure described above, it is clear that the underlying FT ion trap is very well suited both as a fragmentation or filtering device and as a high-resolution mass analyzer. It proves advantageous to quickly switch only the mass ranges of interest using IFT or SWIFT excitation, significantly increasing the mass resolution with the measures described above.

[0031] As described above, in FT mass spectrometers, the mass-to-charge ratios of ions are measured non-interactively by Fourier transformation based on their characteristic oscillations or ion resonance frequencies. The resulting image charge currents are usually only a few fA (10 -15A). Ion resonance frequencies typically range from kHz to MHz, e.g., from approximately 1 kHz to 200 kHz, and can therefore be overlaid by parasitic interference frequencies that can generate so-called "phantom masses." While systematic interference frequencies, i.e., those known to the measurement system, can be eliminated using appropriate measures, parasitic external interference frequencies, usually unknown to the measurement system, can lead to misinterpretation of the mass spectra.

[0032] A further aspect relates to a method of the type mentioned at the outset, in particular a method as described above, comprising: exciting the ions in the FT ion trap and recording a first frequency spectrum of the ions, changing the phase position and / or the oscillation amplitude of the ions in the FT ion trap and / or changing the ion resonance frequencies of the ions in the FT ion trap, re-exciting the ions in the FT ion trap and recording a second frequency spectrum of the ions, and detecting interfering frequencies in the FT ion trap by comparing the first and the second recorded frequency spectrum. Changing the phase position and / or the oscillation amplitude of the ions in the FT ion trap can occur particularly during the re-excitation of the ions in the FT ion trap by an IFT excitation, especially by a SWIFT excitation.

[0033] Using the method described here, interfering frequencies can be clearly identified and optionally eliminated from the ranges of interest of the ion resonance frequencies. This takes advantage of the fact that only the ions trapped in the FT ion trap respond to the IFT excitation or to changes in the ion resonance frequencies. The remaining frequency components present in the frequency spectrum, which cannot be influenced at all in this way, can be identified as interfering frequencies. The mass-to-charge ratios identified as interfering frequencies can be filtered out or eliminated from the mass spectrum, which corresponds to the frequency spectrum.

[0034] During IFT excitation, the phase position and / or oscillation amplitude of the ions of interest can be influenced virtually arbitrarily, although care should be taken to ensure that the ions are not removed from the FT ion trap. For example, the amplitude and / or phase position of the ions stored in the FT ion trap can be altered in such a way that the height of the corresponding lines in the mass spectrum or frequency spectrum changes, while the lines of the interfering frequencies remain unchanged.

[0035] In a further development, changing the ion resonance frequencies involves changing a storage voltage and / or a storage frequency of the FT ion trap. As mentioned above, to measure the mass-to-charge ratio of the ions, they are excited to oscillate by an excitation signal (stimulus). The resonance frequencies of these ions depend on the ion masses and charges. The ion resonance frequencies typically lie in the frequency range in the order of magnitude of kHz to MHz, e.g., from approximately 1 kHz to 200 kHz. For a given mass-to-charge ratio, the respective ion resonance frequency is directly proportional to the high-frequency storage voltage V RF and inversely proportional to the square of the storage frequency f RF of the high-frequency alternating field, so that this behavior can be used to shift the ion resonance frequencies (hereinafter also referred to as frequency SHIFT).

[0036] For example, by increasing the high-frequency storage voltage V RF the ion resonance frequencies are increased and vice versa by reducing the high-frequency storage voltage V RF the ion resonance frequencies are reduced. The ion resonance frequencies behave inversely when the storage frequency f is varied. RF .

[0037] According to the present invention, the method comprises: determining a starting phase position of a trajectory of ions of a predetermined ion resonance frequency (immediately) after an IFT excitation using a time-dependent ion signal recorded during detection. To determine the starting phase position of the trajectory movement of ions or of an ion species at the predetermined ion resonance frequency, the time-dependent ion signal can be ion (t) in a sufficiently long measurement time window T 0recorded to avoid a discretization error, and the ion resonance frequencies f ion of the ions can be obtained by Fourier analysis, where: T 0 >> 1 / f ion or T 0 = N 0 × 1 / f ion and N 0 integer >> 1. The measurement time window T 0 is typically less than approximately 1 / 10 or 1 / 50 of the total measurement or detection time, so that the amplitude û ion the envelope of the (oscillating) ion signal u ion (t) in the measurement time window T 0 remains approximately constant.

[0038] The starting phase position α 0 In this case, the path movement at the beginning of the measurement or the measurement time interval can be determined according to the following formula:

[0039] Out of 12cos(α0)=[1T0∗u^ion∫0T0uion(t)∗cos(2πfion∗t+φ)dt] follows: α0=cos−1(2∗1T0∗u^ion(t)∗cos(2πfion∗t+φ)dt), where φ is a starting phase of the IFT excitation of the ions at the ion resonance frequency f ion represents and where û ion the maximum of the absolute value of the amplitude or envelope of the (oscillating) ion signal u ion (t) at the beginning of the measurement (t=0). The expression given in square brackets in (2) only has a maximum value if the starting phase φ of the IFT excitation coincides with the starting phase position α 0 the orbital motion up to k*π (φ = α 0 + k*π, k is an integer). The value of the expression in square brackets in this case corresponds to ½ cos (α 0). For a starting phase φ=0° of the IFT excitation, the value of the expression in square brackets is approximately + ½ and for a starting phase φ=180° of the IFT excitation, the value of the expression in square brackets is approximately - ½. As described above, the starting phase φ can be varied in a mass-dependent phase-shifted orbital IFT excitation depending on the ion resonance frequency. In this way, ion packets can be characterized differently in the mass spectrum. If the starting phase φ of the IFT excitation is unknown, this and thus the starting phase position α 0 the trajectory motion can be determined by maximizing the magnitude of the expression given in square brackets.

[0040] According to the invention, the method additionally comprises: determining the charge polarity of the ions based on the initial phase position of the ion trajectory after IFT excitation. In an electrical FT ion trap, both positively and negatively charged ion species can be trapped simultaneously. By evaluating the initial phase position α 0 the ion movement or the trajectory of the ions after the IFT excitation (for example, a SWIFT excitation), more precisely by evaluating the expression ½ cos (α 0 ), the polarity of the ions can be detected: If the ions are stimulated by a uniform broadband excitation, immediately after excitation, for example, the positively charged ions move towards one of the electrodes, while the negatively charged ions move away from it. All ions are detected regardless of their polarity after excitation. If for each ion resonance frequency f ionan associated ion type the following formula Polarity=sign[1T0∗u^ion∫0T0uion(t)∗cos(2πfion∗t+φ)dt] which results directly from formula (2), the polarity (+ or -) of the corresponding ion species can be determined.

[0041] If the charge polarity of the ions is known, ion populations can be excited differently depending on their polarity, for example, by SWIFT (possibly broadband) selective excitation. This is achieved by applying different excitation transients to the measuring electrodes depending on the charge polarity. It is understood that the procedure described above is not limited to the electrode geometry of the underlying FT ion trap; this method can be applied to measuring electrodes with different electrode geometries, for example, measuring electrodes in the form of measuring tips in the end caps or in the form of toroidal measuring caps of a toroidal ion trap, etc.

[0042] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention. drawing

[0043] Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows Fig. 1 a schematic representation of a mass spectrometer with an electrical FT-ICR ion trap, Fig. 2 a schematic representation of an excitation level dependent on the ion resonance frequency in a SWIFT excitation, Fig. 3 a schematic representation of a time sequence during a measurement to record a mass spectrum using the mass spectrometer of Fig. 1, Fig. 4 schematic representations of three mass spectra of a gas with a main gas component, Fig. 5a,b schematic representations of the frequency spectrum and the time course of a (broadband) selective SWIFT excitation, Fig. 6a-c a schematic representation of the frequency spectrum for a uniform SWIFT excitation or for a SWIFT excitation with frequency-dependent varying excitation level and phase position ( Fig. 6a) and the corresponding trajectories of the excited ions ( Fig. 6b,c), Fig. 7 a schematic representation of the time course of a multiple (broadband) selective SWIFT excitation and a subsequent detection, Fig. 8 a schematic representation of a detected ion signal with a time-shiftable measurement time interval, Fig. 9 a schematic representation of two frequency spectra recorded at different storage voltages, as well as Fig. 10a-d schematic representations of the frequency spectra of positively charged ions stored in the FT-ICR ion trap ( Fig. 10a), negatively charged ions ( Fig. 10b) as well as all ions stored in the FT-ICR ion trap ( Fig. 10c and Fig. 10d).

[0044] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0045] In Fig. 1 schematically shows a mass spectrometer 1 which has an electrical FT-ICR ion trap 2. The FT-ICR trap 2 has a ring electrode 3 to which a high-frequency alternating voltage V RFwhich, for example, has a frequency f RF in the order of kHz to MHz, e.g. 1 MHz, and an amplitude V RF of several hundred volts. The high-frequency alternating voltage V RF generates a high-frequency alternating field in the FT-ICR trap 2, in which ions 4a, 4b of a gas 4 to be examined are dynamically stored.

[0046] The high-frequency alternating field (E-field) results in a mean restoring force, which acts on the ions 4a, 4b with increasing strength the further the ions 4a, 4b are from the center of the FT-ICR ion trap 2. To measure the mass-to-charge ratio (m / z) of the ions 4a, 4b, they are excited to oscillate by an excitation signal S1, S2 (stimulus) whose frequency f iondepends on the ion mass and the ion charge and is typically in the frequency range in the order of magnitude of kHz to MHz, e.g., from approximately 1 kHz to 200 kHz. The respective excitation signal S1, S2 is generated by a second and third excitation unit 5b, 5c, which, together with a first excitation unit 5a, which is used to generate the high-frequency storage voltage V RF with the specified memory frequency f RF serves, forms an excitation device 5. The excitation device 5 also has a synchronization device 5d, which synchronizes the three excitation units 5a-c in time. Each excitation unit 5a-c is followed by an amplifier, which is also part of the excitation device 5.

[0047] For a non-reactive, non-destructive detection (i.e., the ions 4a, 4b are still present after detection), the oscillation signals of the ions 4a, 4b are tapped in the form of induced image charges at the measuring electrodes 6a, 6b, as described, for example, in DE 10 2013 208 959 A cited at the beginning, which is incorporated by reference in its entirety into this application. As described in detail therein, the respective measuring electrodes 6a, 6b are each connected via a filter 7a, 7b to a low-noise charge amplifier 8a, 8b. The charge amplifiers 8a, 8b, on the one hand, detect and amplify the ion signals from the two measuring electrodes 6a, 6b and, on the other hand, hold the measuring electrodes 6a, 6b for the storage frequency f RF at virtual ground potential.

[0048] From the signals supplied by the charge amplifiers 8a, 8b, an ion signal u is generated by subtraction. ion(t) is generated, the temporal course of which is Fig. 1 is shown at the bottom right. The ion signal u ion (t) is fed to a detector 9, which in the example shown has an analog-digital converter 9a and a spectrometer 9b for fast Fourier analysis (FFT) in order to generate a mass spectrum which is Fig. 1 is shown top right. The detector 9 or the spectrometer 9b first generates a frequency spectrum of the characteristic ion resonance frequencies f ion of the ions 4a, 4b stored in the FT-ICR ion trap 2, which is determined due to the dependence of the ion resonance frequencies f ion from the mass and charge of the respective ions 4a, 4b, is converted into a mass spectrum. The mass spectrum shows the number of detected particles or charges as a function of the mass-to-charge ratio m / z.

[0049] The electrical FT-ICR trap 2 thus enables direct detection or the direct recording of a mass spectrum, which enables rapid gas analysis. However, the rapid recording of a mass spectrum using Fourier spectrometry can be used not only with the electrical FT-ICR trap 10 described above, but also with modifications of the Fig. 1, for example a so-called Orbitrap.

[0050] As described above, all ions 4a, 4b in the FT-ICR ion trap 2 have an ion resonance frequency f that is proportional to their mass-to-charge ratio (m / z). ion with which the stored ions 4a, 4b oscillate in the FT-ICR ion trap 2. If the ions 4a, 4b are oscillated with their respective ion resonance frequency f ionexcited, they can either be specifically excited or ejected from the FT-ICR ion trap 2 by a resonance enhancement. Thus, ions 4a, 4b with specific mass-to-charge ratios m / z can be selectively excited or their storage in the FT-ICR ion trap 2 can be prevented / suppressed.

[0051] The generalization of this principle leads to one or more regions (“windows”) in the ion resonance frequency range in which ions 4a, 4b, whose ion resonance frequency f ion within the respective window can be selectively excited or suppressed. The inverse transformation of these regions via an inverse Fourier transformation provides the time signal required for the so-called IFT excitation. If these time profiles are calculated in advance, this is referred to as SWIFT excitation 10. An example of a SWIFT excitation 10 with a broadband selective excitation spectrum is shown in Fig. 2, where the ion resonance frequencies f ion to the memory frequency f RF The desired selective excitation spectrum depends on the ion resonance frequencies f ion and thus on the mass-to-charge ratio (m / z) of the ions 4a, 4b. The corresponding discrete SWIFT time function (in Fig. 2 not shown) is issued at the time of SWIFT excitation to determine the desired Fig. 2 to obtain the excitation spectrum shown.

[0052] The measuring electrodes 6a, 6b can be used for the SWIFT excitation 10. The SWIFT excitation 10 can deflect the ions 4a, 4b towards the measuring electrodes 6a, 6b in such a way that both during ion generation and ion storage, as well as immediately before the detection of the ion signals ion(t) certain ions 4a, 4b are either stored or not stored on the one hand, and on the other hand are excited practically continuously or not excited at all.

[0053] SWIFT excitation therefore opens up several possibilities for realizing new performance features of the mass spectrometer 1. A prerequisite for all measurement tasks is that the excitation time of the ions 4a, 4b within the FT-ICR ion trap 2 is significantly shorter than the mean free time of flight or the mean free path of the molecules or ions 4a, 4b of interest. It has proven advantageous to use optimized SWIFT algorithms, such as those presented in the article "Stored Waveform Inverse Fourier Transform Axial Excitation / Ejection for Quardupole Ion Trap Mass Spectrometry" by S. Guan and AG Marshall, Anal. Chem. 1993, pages 1288-1294, or in US Pat. No. 4,945,234, both of which are incorporated by reference into this application.Optimized SWIFT algorithms generate the shortest possible SWIFT signal output on the one hand and prevent overloading of the low-noise charge amplifiers 8a, 8b connected to the measuring electrodes 6a, 6b on the other.

[0054] A SWIFT excitation 10 can be performed immediately before the detection of the ions 4a, 4b, ie before the recording of the (normalized) ion signal, as shown in Fig. 3, in which only the envelope of the (normalized) ion signal u ion (t). However, a SWIFT excitation 10 can also take place during the generation and storage of the ions 4a, 4b, as is also shown in the time sequence of Fig. 3. In this case, the SWIFT excitation 10 serves to select ions 4a, 4b to be stored in the FT-ICR ion trap 2.

[0055] There are basically two possibilities for generating the ions 4a, 4b by ionizing the gas 4: Either the ions 4a, 4b are generated within the FT-ICR ion trap 2 or the gas 4 is supplied to the FT-ICR ion trap 2 in charge-neutral form and the ionization takes place in the FT-ICR ion trap 2. Such ionization in the FT-ICR ion trap 2 can be carried out, for example, in the manner described in WO 2015 / 003819 A1 cited at the outset, which is incorporated into the content of this application by reference with regard to this aspect.

[0056] If the ionization takes place in the electrical FT-ICR ion trap 2, a continuous SWIFT excitation can occur during the ionization of the gas 4 (cf. Fig. 3), which excessively excites unwanted gas components; as a result, the charge carriers of the unwanted gas components are lost at the surrounding electrodes 3, 6a, 6b, and only the charge carriers or ions 4a, 4b of interest are accumulated in the FT-ICR ion trap 2 for measurement. This ensures that the FT-ICR ion trap 2 is not flooded with unwanted charge carriers during the ionization time of the ions 4a, 4b to be detected. The ions 4a, 4b to be analyzed or detected are stored and accumulated in the FT-ICR ion trap 2 immediately after ionization or after transfer to the FT-ICR ion trap 2.

[0057] Such a selection during or before storage is advantageous, since in many applications the detection of gas traces or gas components with very low partial pressures or concentrations in a gas matrix or a gas 4 with high total pressure is required. An example of a mass spectrum of such a gas is shown in Fig. 4 below. If gas traces with very low partial pressures are to be detected, whose mass spectrum is in Fig. 4 top right, the undesired gas components that are not to be stored in the FT-ICR ion trap 2 may be a main gas component 11 of the gas 2 to be analyzed. For the purposes of this application, a main gas component 11 is understood to be a gas component whose volume fraction is more than 50 vol%, in many applications more than 90 vol%, of the gas 2 to be analyzed.

[0058] In the Fig. In the example shown in Figure 4, the main gas component 11 has two ion populations with different mass-to-charge ratios (m / z) 1 or (m / z) 2 whose volume fraction is more than 30 vol% of the gas 2 to be analyzed, so that the volume fraction of the main gas component 11 is more than 50 vol% of the gas 2 to be analyzed. The mass spectrum of the gas 4 recorded by the mass spectrometer 1 without a mass-selective SWIFT excitation is shown in Fig. 4 top left. In the mass spectrum shown there, only the ion populations of the main gas component 11, for example a majority carrier gas, can be seen, but not the gas traces of interest, whose mass-to-charge ratio lies outside a Fig. 4 shown interval I, in which the mass-to-charge ratios (m / z) 1 or (m / z) 2 the main gas component 11.

[0059] The broadband selective SWIFT excitation 10 can be used to selectively filter those mass-to-charge ratios m / z that lie within the interval I or to specifically filter the first mass-to-charge ratio (m / z) 1 and the second mass-to-charge ratio (m / z) 2 of the main gas component 11. In this way, only those ions 4a, 4b are stored in the FT-ICR ion trap 2 whose mass-to-charge ratios m / z lie outside the interval I, so that they can be detected with high accuracy, as can be seen from the mass spectrum in Fig. 4 can be seen top right.

[0060] The ratio of the partial pressures of the gas components of interest to the total pressure can be measured, for example, in the order of ppm volume (10 -6 ppmV) to pptV (10 -12 ). The detection limit for individual gas components can be up to the order of 10-16 mbar. In this way, a dynamic D of more than eight orders of magnitude (D > 10 8 ) can be achieved. In addition, the sensitivity (absolute concentration) of the ions 4a, 4b in the FT-ICR ion trap 2 and, accordingly, the signal-to-noise ratio (SNR) increases with the accumulation time during storage.

[0061] In an electric FT-ICR ion trap 2, the high-frequency alternating field (E-field) is influenced by the space charge, or more precisely, by the space charge density, in the FT-ICR ion trap 2. This means that the charges or ions 4a, 4b present in the FT-ICR ion trap 2 have a feedback effect on the high-frequency alternating field, which serves to store the ions 4a, 4b. The influence of the alternating field E is greater the greater the space charge density in the respective sub-volume of the FT-ICR ion trap 2 and the weaker the average restoring force resulting from the high-frequency alternating field E in the corresponding sub-volume.

[0062] Particularly when exciting ions 4a and 4b with different but closely spaced ion resonance frequencies or mass-to-charge ratios, large space charge densities can arise in areas of the FT-ICR ion trap 2 that are particularly susceptible to the occurrence of large space charge densities. The large space charge density can severely perturb entire ion packets in their ion resonance frequencies, resulting in a significant reduction in measurement resolution.

[0063] The local space charge in the FT-ICR ion trap can be reduced if ions 4a, 4b with close ion resonance frequencies f ion not simultaneously follow the same trajectory (or orbit). This can be achieved by alternating between a first ion excitation frequency f ion1 and a second ion excitation frequency f ion2the excitation level A of the SWIFT excitation 10 is varied frequency-dependently or depending on the mass or the mass-to-charge ratio m / z of the ions 4a, 4b, as shown in Fig. 5a is shown. Fig. Figure 5b shows the corresponding time-dependent excitation signal (S1 or S2) of the SWIFT excitation.

[0064] In the Fig. In the example shown in Figures 5a,b, the excitation level A of the SWIFT excitation varies stepwise depending on the ion excitation frequency f ion , where the excitation level A over the entire interval between the first ion excitation frequency f ion1 and the second ion excitation frequency f ion2 by no more than approximately 20% of the maximum excitation level A (i.e., the maximum amplitude of the SWIFT excitation 10). In the example shown, the excitation level A decreases from the first ion excitation frequency f ion1 to the second ion excitation frequency f ion2increases stepwise, with the step height between adjacent steps of the excitation level A being equal. It is understood that the excitation level A depends on the first ion excitation frequency f ion1 to the second, larger ion excitation frequency f ion2 Alternatively, it can also decrease. The step height, ie the difference between the excitation levels of adjacent steps of the SWIFT excitation 10, is not necessarily constant, but can vary from step to step. A continuous, stepless variation of the excitation level A between the first ion excitation frequency f ion1 and the second ion excitation frequency f ion2 is also conceivable in principle.

[0065] In addition or alternatively to the variation of the excitation level A or the amplitude of the SWIFT excitation 10, a variation of the phase position φ of the SWIFT excitation 10 can also take place, as shown in Fig. 6a. In the example shown, the phase position φ is also changed in steps, each time by a value of 45°, whereby the phase position φ of the SWIFT excitation 10 in Fig. 6a shown example with increasing ion excitation frequencies f ion increases in steps. It is understood that a stepwise decrease in the phase position φ of the SWIFT excitation 10 is also possible and that the difference between the phase positions φ of adjacent stages can deviate from 45° and, in particular, vary from stage to stage. It is also understood that the stepwise increase or decrease in the phase position φ is only defined modulo 360°, i.e., in the example shown, a phase position φ of 0° is reached again after eight stages. The phase position φ corresponds to a temporal shift or delay of the SWIFT excitation, whereby the phase position φ is set to a predetermined ion excitation frequency f ion,a is related.

[0066] The specified ion excitation frequency f ion,a For example, the ion resonance frequency f ion or the mass-to-charge ratio m / z of an ion population to be analyzed. The specified ion excitation frequency f ion,a but can also be in an interval between two ion excitation frequencies f ion1 , f ion2 or two corresponding ion resonance frequencies whose mass-to-charge ratios m / z are close to each other. The first (smaller) ion excitation frequency f ion1 can, for example, be deviated by no more than 10%, preferably by no more than 5%, in particular by no more than 1% from the specified ion excitation frequency f ion,a The same applies to the second, larger ion excitation frequency f ion2 . In the Fig. In the example shown in Figure 6a, the ratio f ion1 / f ion,a approximately 0.999 (deviation: 0.1%), while the ratio f ion2 / f ion,ais approximately 1.009 (deviation: 0.9%), ie both ion excitation frequencies f ion1 , f ion2 are within the value range of less than 1% deviation described above.

[0067] Fig. Figure 6b shows the trajectory B of the ions 4a, 4b in the FT-ICR ion trap 2 during a uniform SWIFT excitation, ie a SWIFT excitation with a constant excitation level A (in Fig. 6a shown in dashed lines), which is also synchronous or phase-locked. In Fig. 6b, the value z denotes the deflection of the ions 4a, 4b in the z-direction, ie to the measuring electrodes 6a, 6b in the FT-ICR ion trap 2, where z 0 is the maximum deflection. The value T denotes the period of oscillation of the ions 4a, 4b with the given ion excitation frequency f ion,a . In Fig. 6b it is clearly visible that the trajectories B of the ions 4a, 4b overlap, resulting in a high space charge density.

[0068] Fig. Figure 6c shows the trajectories B of the ions 4a, 4b in the Fig. 6a shown orbital SWIFT excitation 10 with different excitation levels A, in which the phase position φ is also shown as in Fig. 6a was varied, using the example of ten ion packets or ion populations with neighboring ion resonance frequencies f ion or with neighboring mass-to-charge ratios m / z. In Fig. Figure 6c clearly shows that the trajectories B of the ten ion packets are spatially separated by the SWIFT excitation 10, reducing the local space charge density in the FT-ICR ion trap 2 and thereby increasing the mass resolution. The ions 4a, 4b typically traverse the (periodic) trajectories B more than approximately 100 to 1000 times before the measurement or detection occurs. Thus, only a very low pressure is required in the FT-ICR ion trap 2 to perform the measurement or detection.

[0069] Fig. Figure 7 shows another application of SWIFT excitation 10, in which the same ions 4a, 4b in the FT-ICR ion trap 2 are excited successively by two (broadband) selective SWIFT excitations 10 and subsequently detected. During detection after each SWIFT excitation 10, the number of excited ions 4a, 4b (or the partial pressure of the excited gas component) is determined. By averaging the number of ions 4a, 4b determined during each detection, the signal-to-noise ratio (SNR) of the excited ions 4a, 4b of interest can be significantly increased without affecting the remaining ions.

[0070] The prerequisite for such multiple detection is that between two consecutive IFT excitations 10 there is a time interval τ that is longer than a mean free flight time t Mof the ions 4a, 4b in the FT-ICR ion trap 2, ie τ > t M , where typically t M is more than approximately one millisecond (>1 ms). The SWIFT excitations are only repeated when the ions 4a, 4b have reached a multiple of the mean free flight time t M have traveled, e.g. more than 3 × t M , more than 5 × t M or more than 10 × t M .

[0071] Fig. 8 shows a time-dependent ion signal u ion (t) after a SWIFT excitation 10 and a dashed, time-shiftable measurement time interval 12 (FFT time window), which has a time duration t Iin the order of magnitude of, for example, several milliseconds, preferably 10 ms or less, particularly preferably 5 ms or less. By continuously or discretely shifting the measurement time interval 12, a time-resolved representation of the chemical behavior of the ion population embedded in the gas matrix or in the gas to be analyzed can be achieved. In this case, the mass spectrometric analysis is carried out only on the basis of the values ​​of the ion signal u ion (t) is carried out during the measurement time interval 12, ie an evaluation is only carried out in the measurement time interval 12. This is particularly advantageous if chemical reactions such as charge transfer or "protonation" occur during the detection of the ions 4a, 4b, which change the originally present ion population during the detection period. By evaluating only in the measurement time interval 12, for example, a reaction such as the transition from H 2 O + to H3 O + can be observed practically in real time, meaning that even intermediate products of chemical reactions can be detected. In particular, this allows for verification of whether the selected ions 4a, 4b stored in the FT-ICR ion trap 2 actually correspond to the ion population intended for the chemical reaction. If necessary, the selection or selection process of the ions 4a, 4b to be accumulated in the FT-ICR ion trap 2 can be appropriately adjusted.

[0072] When recording mass spectra using the mass spectrometer 1, parasitic interference frequencies f R occur, which lead to lines in the recorded mass spectrum that are not generated by the ions 4a, 4b stored in the FT-ICR ion trap 2. Such interference frequencies f R can lead to misinterpretation of the mass spectrum.

[0073] To avoid interference frequencies R In order to identify and, if necessary, eliminate the ions in the mass spectrum, a method can be used which is described below: In a first step, the ions 4a, 4b are excited in the FT-ICR ion trap 2 by means of a SWIFT excitation and subsequently detected in order to obtain a first frequency spectrum 13a of the ion resonance frequencies f ion to record (in Fig. 9 shown dashed). In a second step, the ion resonance frequencies f ion of the ions 4a, 4b in the FT-ICR ion trap 2 is changed and in a third step the ions 4a, 4b are excited again by means of a SWIFT excitation 10 and subsequently detected, whereby a second frequency spectrum 13b is recorded, which in Fig. 9 is shown with solid lines.

[0074] When comparing the two in Fig. 9, it is clearly visible that the first and second frequency spectra 13a, 13b have lines whose frequencies change with the change of the ion resonance frequencies f ion in the FT-ICR ion trap 2 have practically not shifted, so that their position in both frequency spectra 13a, 13b is practically identical. These lines can be interpreted as interference frequencies f R identified or determined. Those lines in the two frequency spectra 13a, 13b that are caused by the change in the ion resonance frequencies f ion can be systematically shifted, can be assigned to the ions 4a, 4b stored in the FT-ICR ion trap 2, ie they are lines at “real” ion resonance frequencies f ion .

[0075] As in Fig. 9, was used to change the ion resonance frequencies f ion the storage voltage V RFof the FT-ICR ion trap 2 is changed from a first value Vrf1 to a second value Vrf2. Since, for a given mass-to-charge ratio m / z, the ion resonance frequency f ion directly proportional to the storage voltage V RF can be achieved by changing the storage voltage V RF the ion resonance frequencies f ion Since, for a given mass-to-charge ratio m / z, the ion resonance frequency f ion inversely proportional to the square of the storage frequency f RF a change in the ion resonance frequencies f ion alternatively or additionally by changing the memory frequency f RF take place.

[0076] Alternatively or in addition to a change in the ion resonance frequencies f ion In the second step, a change in the phase position φ and / or the oscillation amplitude z / z 0The trajectories B of the ions 4a, 4b in the FT-ICR ion trap 2 can be determined, for example, by means of a mass-dependent SWIFT excitation 10, as described for example in Fig. 6a-c. During such a SWIFT excitation, the trajectories of the ions 4a, 4b change, which is noticeable, for example, by a change in the heights of the lines of the second frequency spectrum 13b compared to the first frequency spectrum 13a. The interference frequencies f R However, the SWIFT excitation 10 has practically no influence, so that the interference frequencies f R can also be detected or identified in this variant by comparing the two frequency spectra 13a, 13b.

[0077] A further application of SWIFT excitation 10 is the determination of the charge polarities (pos. / neg.) of the ions 4a, 4b stored in the electrical FT-ICR ion trap 2. For the determination or identification of the positively charged ions 4a or the negatively charged ions 4b in the FT-ICR ion trap 2, a phase position α 0 of the orbital motion B at the beginning of the detection, ie immediately after the SWIFT excitation 10, at a given ion resonance frequency f ion determined according to formula (2) given above, which is reproduced below:

[0078] Out of 12cos(α0)=[1T0∗u^ion∫0T0uion(t)∗cos(2πfion∗t+φ) dt] follows: α0=cos−1(2∗1T0∗u^ion∫0T0uion(t)∗cos(2πfion∗t+φ)dt) where φ is a starting phase of the SWIFT excitation 10 of the ions 4a, 4b at the ion resonance frequency f ion represents, û ion the maximum of the absolute value of the ion signal u ion(t) at the beginning of the measurement, and where: T 0 >> 1 / f ion or T 0 = N 0 × 1 / f ion and N 0 integer >> 1. The value of the amplitude or envelope of the oscillating ion signal û ion changes during the measurement time interval T 0 typically only slightly, ie the duration of the measurement interval T 0 is significantly smaller than the mean free flight time of the ions.

[0079] In the electrical FT-ICR ion trap 2, both positively charged ions 4a and negatively charged ions 4b can be trapped simultaneously. All ions 4a, 4b are detected regardless of their charge polarity after the SWIFT excitation 10, which can result, for example, in a frequency spectrum that Fig. 10c. The Fig. The frequency spectrum of all ions 4a, 4b stored in the FT-ICR ion trap shown in Figure 10c represents a superposition of the frequency spectrum of the positively charged ions 4a, which is shown in Fig. 10a, and the frequency spectrum of the negatively charged ions 4b, which is shown in Fig. 10b is shown.

[0080] By evaluating the phase position α 0 The charge polarity of the ions 4a, 4b can be detected by the ion movement or the trajectory B of the ions 4a, 4b after the SWIFT excitation: If the ions 4a, 4b are stimulated by a uniform broadband excitation, immediately after the SWIFT excitation 10, for example, the positively charged ions 4a move towards the first measuring electrode 6a, while the negatively charged ions 4b move away from it.

[0081] For each ion resonance frequency f ion , which corresponds to a line in Fig. 10c, for example, the following formula Polarity=sign[1T0∗u^ion∫0T0uion(t)∗cos(2πfion∗t+φ)dt] applied, the positive ions 4a can be identified, for example, by a positive sign (α 0 = 0°, polarity +1) and the negative ions 4b by a negative sign (α 0 =180°, polarity -1). The positive ions 4a and the negative ions 4b can be identified in this way in the frequency spectrum of all ions 4a, 4b, as shown in Fig. 10d is shown.

[0082] In the example described above, it was assumed that the SWIFT excitation 10 is carried out with a starting phase φ = 0. As described above, the starting phase φ in a mass-dependent phase-shifted orbital SWIFT excitation 10 can also be dependent on the ion resonance frequency f ionIn this way, ion packets can be characterized differently in the frequency spectrum or the mass spectrum.

[0083] If the charge polarity (positive / negative or + / -) of the ions 4a, 4b is known, ion populations can be excited differently depending on their charge polarity, for example by SWIFT (broadband) selective excitation. This can be achieved by varying the excitation frequency at the corresponding ion resonance frequencies f, depending on the charge polarity. ion different excitation transients are applied to the measuring electrodes 6a, 6b. It is understood that the procedure described above does not apply to the electrode geometry of the Fig.1, i.e. this method can be applied to measuring electrodes with different electrode geometries, for example, measuring electrodes in the form of measuring tips in the end caps or in the form of toroidal measuring caps of a toroidal ion trap, etc.

[0084] In summary, the performance characteristics of a mass spectrometer 1 with an FT ion trap 2 can be significantly improved in the manner described above.

Claims

[1] Method for the mass spectrometric analysis of a gas (4), comprising: Ionizing the gas (4) to generate ions (4a, 4b), storing, exciting and detecting at least a portion of the generated ions (4a, 4b) in an FT ion trap (2), wherein the generation and storage of the ions (4a, 4b) in the FT ion trap (2) and / or the excitation of the ions (4a, 4b) prior to the detection of the ions (4a, 4b) in the FT ion trap (2) comprises at least one selective IFT excitation, in particular a SWIFT excitation (10), dependent on the mass-to-charge ratio m / z of the ions (4a, 4b), characterized by the steps: Determining a starting phase position α 0 a trajectory B of ions (4a, 4b) at a given ion resonance frequency f ion after an IFT excitation (10) based on a time-dependent ion signal u recorded during the detection of the ions (4a, 4b) ion (t), and Determining a charge polarity of the ions (4a, 4b) based on the starting phase position α 0 of the ions (4a, 4b) after IFT excitation (10). [2] Method according to claim 1, wherein during the generation of the ions (4a, 4b) in the FT ion trap (2) and / or during the storage of the ions in the FT ion trap (2) at least one IFT excitation (10) is carried out for selecting ions (4a, 4b) to be stored in the FT ion trap (2). [3] Method according to claim 2, in which only ions (4a, 4b) are selected for storage whose mass-to-charge ratio m / z lies outside an interval I of the mass-to-charge ratios (m / z) 1 , (m / z) 2 a main gas component (11) of the gas (4), wherein the main gas component is a gas component whose volume fraction is more than 50 vol%. [4] Method according to one of the preceding claims, in which in a frequency interval between a first excitation frequency fion1 and a second excitation frequency f ion2 the excitation level A and / or the phase position φ of the IFT excitation (10) are varied, wherein both the first excitation frequency f ion1 as well as the second excitation frequency f ion2 by not more than 10%, preferably by not more than 5%, in particular by not more than 1% of a given excitation frequency f ion,a differ. [5] Method according to claim 4, wherein in the frequency interval between the first excitation frequency f ion1 and the second excitation frequency f ion2 the phase position φ and / or the excitation level A depending on the excitation frequency f ion vary gradually. [6] Method according to claim 5, wherein in the frequency interval between the first excitation frequency f ion1 and the second excitation frequency f ion2 the excitation level A and / or the phase position φ depending on the excitation frequency f ioneither increase gradually or decrease gradually. [7] Method according to one of the preceding claims, in which the same ions (4a, 4b) in the FT ion trap (2) are selectively excited several times by IFT excitations (10), wherein after each IFT excitation (10) a detection of the ions (4a, 4b) is carried out. [8] Method according to claim 7, wherein between two IFT excitations (10) which follow one another immediately in time there is a time interval τ which is greater than a mean free flight time t M of the ions (4a, 4b) in the FT ion trap (2). [9] Method according to one of the preceding claims, in which, when detecting the ions (4a, 4b), a mass spectrometric examination of an ion signal u ion(t) takes place in a measuring time interval (12) which is continuously or discretely shifted in time, wherein only in the measuring time interval (12) an evaluation of a time-dependent ion signal u recorded during the detection of the ions (4a, 4b) ion (t) occurs. [10] Method according to the preamble of claim 1, in particular according to one of the preceding claims, comprising: Exciting the ions (4a, 4b) in the FT ion trap (2) and recording a first frequency spectrum (13a), Changing the phase position (φ) and / or the oscillation amplitude (z / z 0 ) of the ions (4a, 4b) in the FT ion trap (2) and / or changing the ion resonance frequencies (f ion ) of the ions (4a, 4b) in the FT ion trap (2), Re-excitation of the ions (4a, 4b) in the FT ion trap (2) and Recording a second frequency spectrum (13b), as well as detecting interference frequencies (f R) in the FT ion trap (2) by comparing the first and second recorded frequency spectrum (13a, 13b). [11] A method according to claim 10, wherein changing the ion resonance frequencies f ion changing a storage voltage V RF and / or a storage frequency f RF the FT ion trap (2).

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