Electron impact ion source

Abstract

The invention provides an electron impact ion source that is adapted for use in a Time- of-Flight mass spectrometer device, and in particular for use in a Multiple-Reflection TOF mass spectrometer device. The proposed solution avoids background noise in captured spectra and ensures high sensitivity and mass resolution.

Description

[0001] Electron Impact Ion Source

[0002] Background

[0003] In known electron impact storage ion sources, which are for example used in time-of- flight mass spectrometers, a continuous electron beam is injected into an ionisation volume of the ion source, where the electron beam interacts with sample molecules. Electron impact ionisation of the sample molecules occurs thus continuously in the ionisation volume. During a trapping phase, generated ions are accumulated in the confined volume. During a subsequent extraction phase, the accumulated ions are extracted from this volume, while the ionisation continues, and is ongoing in the ionisation volume. An extraction pulse is typically used to trigger the extraction.

[0004] In time-of-flight, TOF, mass spectrometers, the ions extracted from a sample are analysed with respect to their atomic mass-to-charge ratio (m / z) based on the dispersion of their respective ion flight time. After acceleration to a uniform kinetic energy by electrostatic fields and pulsed extraction to a field-free drift region, the ions are spatially separated along the drift path based on their individual constant drift velocities, dependent on the respective m / z. The drift regions acts as the mass separator. Ions with high m / z are thereby travelling slower than ions with low m / z. The recording of the ion arrival times, with a detector positioned distally along the drift path, provides the acquisition of an ion TOF spectrum by setting the time of the release pulse from the accelerator as the baseline. Calibration with ions of known m / z allows to convert the TOF spectrum to a mass spectrum.

[0005] The main performance criteria for TOF mass spectrometers are the sensitivity, the mass resolving power and the signal-to noise ratio, SNR, of the obtained mass spectrum.

[0006] The sensitivity is given by the minimum detectable concentration of a given analytes in the sample analyte. Therefore, high ion extraction efficiency from the sample into the mass analyserionis, the performance of the acceleration and pulsing apparatus, as well as a high ion transmission in the mass analyser are factors that may enhance the sensitivity of a TOF mass spectrometer. High sensitivity measurements are desirable to reliably detect even very low analyte concentrations in a sample.

[0007] The mass resolving power M / AM indicates the capability of a mass spectrometer to resolve a peak for a given m / z as an individual signal in the spectrum. For a TOF mass spectrometer, mass resolving power is given by M / AM = t / (2*At), where t denotes the total mean flight time of the respective m / z and where At denotes the recorded time spread for this iso-mass peak.

[0008] High mass resolving power is relevant in many mass spectrometer applications, as in life-science, to allow even isotopic measurements, which require the distinction between very close peaks in the spectrum. The mass resolving power is limited by the achievable total flight time and the time spread, which is mainly caused by effects in the ion source and in the acceleration and pulsing system, as well as in the optics of the mass analyser.

[0009] Typically, for the iso-mass peak (constant m / z) a Gaussian-shaped distribution can be observed in the TOF spectrum. An initial time spread for the iso-mass ions, that cannot be corrected afterward, is already given, when the ions enter the mass analyser. Thereby, the duration of the extraction pulse is decisive for the initial time spread, and short pulses are required. Other effects determining the initial time spread are dependent on the type of the ion source, acceleration and pulsing method. Furthermore, effects determining the particle properties, such as the initial ion energy and the distribution of the ion motion vector, lead to significant contributions to the final flight time error. Small inevitable distributions of the ion energies after acceleration lead to slight variations of the drift velocity of iso-mass ions and thus to a TOF distribution.

[0010] The signal-to-noise ratio, SNR, indicates how strong the actual signal in the spectrum is in the presence of noise. High performance mass spectrometer instruments demand a low noise level and thus high SNR, which allows for capturing a clean mass spectrum. With a high SNR, peaks are more clearly distinguishable, and even low-intensity peaks become detectable, which improves the accuracy of qualitative and quantitative analysis of captured mass spectrum data. This enhanced clarity is crucial in applications such as proteomics, metabolomics, and trace analysis, where detecting weak signals can provide critical insights. Furthermore, a higher SNR reduces the likelihood of false positives and improves the reliability of data interpretation, making it essential for precise analytical measurements. The spectral noise can be caused by the electronics, such as the detector, amplifiers and data acquisition system, as well as the ion source and acceleration and pulsing system by inconsistency in the creation of the pulsed ion bunches. In a multiple-reflection time-of-flight mass spectrometer, MRTOF, the flight path and thus the total flight time of the ions within the mass analyser compartment is effectively extended by folding the ion trajectory. This is done by integrating an arrangement of at least two opposing ion mirrors. Both open-path and closed-path configurations are known in the art and will not be further detailed. Each mirror provides a set of electrodes with different applied voltage, such that an ion reflecting electrostatic field within the mirror is shaped.

[0011] The extension of the flight path, and thus the total ion flight time, allows to enhance the mass resolving power of the mass spectrometer. The utilization of reflecting mirror fields further allows to compensate for the iso-mass time spread induced by the initial ion energy spread up to a certain degree. This is facilitated by the energy-dependence of the penetration depth of ions in the reflecting mirror field. Slow and low-energy ions experience lower penetration depth and thus flight path than fast higher-energy ions.

[0012] Precise adjustment of the ion mirror field finally allows to have lowest energy- aberrational time spread at the detector plane, contributing to high mass resolution measurements. In MRTOF mass spectrometers, the mass resolving power becomes dependent on the number of full-turns performed by the extracted ions in the mass analyser. The resolution-limiting effect of the initial time spread created by the ion source, acceleration and pulsing system thereby become less significant with increasing number of ion full-turns, while the aberrational time-spread caused by non-ideal ion mirror optics with each ion reflection becomes the main limiting factor.

[0013] Any TOF mass spectrometer requires the formation of pulsed ion bunches, being injected at a defined start time into the mass analyser. Pulsed ion bunches can be generated either by applying a pulsed ionisation of the sample species and immediate injection into the mass analyser, by modulating a continuous ion beam, or by continuously ionising the analyte and retaining the ions until applying an extraction pulse to release them to the mass analyser. A device that implements this last method, in combination with electron impact, El, ionisation is referred to as a pulsed storage El ion source.

[0014] In a known pulsed storage El ion source, an electron beam continuously ionises particles from a gaseous analyte. The ions are thereby retained and accumulated in a confined volume within the ionisation chamber for a certain trapping time ttrapby maintaining a relatively flat potential distribution until an extraction pulse is applied.

[0015] Thereby, a push or pull electrode is pulsed to generate an accelerating electric field within the ionisation and storage chamber, which effectively extracts particles from this compartment towards the downstream region of the ion source and acceleration system, where the ions are accelerated further by an arrangement of electrodes.

[0016] This extraction pulse is given by a duration text, being much shorter than ttrap. The pulsed storage El ion source allows the accumulation of the continuously generated ions during the trapping phase to have a high overall ion count and thus improves the sensitivity. This is particularly important for high-resolution MRTOF experiments, where transmission losses naturally appear with each reflection.

[0017] During the pulsed extraction phase of the ions, the ionisation process of the analyte particles remains ongoing. These ions continuously created during the extraction phase are extracted together with the ions created and accumulated during the trapping phase.

[0018] While the ions originating from the trapping phase are forming the respective peaks in the spectrum, the continuously ionised particles from the extraction phase contribute to the spectrum as noise, based on their different starting times during the extraction phase. The conventional extraction method with continuous ionisation, thus results in spectra with a certain base noise. This is problematic, as low intensity peaks can get lost in this noise, such that spectral quality, sensitivity and signal-to-noise ratio are restricted.

[0019] In known methods, the background noise emanating from the pulsed electron impact source is removed using post-processing of the obtained mass spectrum data.

[0020] The main limitation of these existing solutions based on post-processing includes: the accuracy of the processed data could be influenced by the background subtraction process. While the process could improve the SNR, it may as well remove the information contained in the raw low SNR spectral peaks, background subtraction may reduce the sensitivity, especially on low spectral intensity, since the actual signal information may be removed together with the background. information on the spectral lines below the background may be completely removed by the process and may therefore become undetectable. Further, the applied pulse length for the extraction phase, is kept small (in the range of several hundred nanoseconds) to limit the influence of continuously created ions. This may be problematic as ions with higher m / z have lower mobility and thus higher extraction times. Short extraction pulses can therefore limit the extractable mass range. To compensate for this effect, in known solutions, the extraction field is typically enhanced by applying extraction voltage gradients of several 100 V. However, this results also in enhanced initial energy spread of the ions, which is critical for MRTOF.

[0021] It is an objective of the present invention to provide an ion source and method of operating an ion source, which alleviates at least some of the shortcomings identified in the prior art.

[0022] Summary

[0023] In accordance with an aspect of the invention, an electron impact ion source is provided. The electron impact ion source comprises an ionisation chamber having an electron beam inlet and an ion beam outlet, ion extraction means configured to selectively extract ions from the ionisation chamber through the ion beam outlet, preferably along a first axis; electron beam emission means configured to selectively emit electrons into the ionisation chamber, preferably along a second axis that is perpendicular to the first axis; controlling means configured to synchronize the electron beam emission means and the ion extractions means, so that when ions are extracted from the ionisation chamber, substantially no electrons are emitted into the ionisation chamber and so that electrons are emitted into the ionisation chamber when no ions are extracted from the ionisation chamber, during the trapping phase.

[0024] The controlling means may preferably be configured to pulse the electron emission means.

[0025] Preferably, the controlling means may be configured to keep the ion extraction means and the ionisation chamber at the same potential during a trapping phase, and to maintain a lower potential of the ion extraction means to establish a potential difference between the ionisation chamber and the ion extraction means during an extraction phase. Further preferably, the electron beam emission means may comprise a filament for emitting electrons, the filament being surrounded by a shield electrode. The controlling means may be configured to keep the shield electrode potential below the potential of the ionisation chamber and the ion extraction means during a trapping phase; and maintain the potential of the ion extraction means below the potential of the ionisation chamber, and maintain an increased potential of the shield electrode to accelerate emitted from the filament towards the shield electrode during an extraction phase the filament, shield electrode and ion extraction means are electrically insulated from each other.

[0026] Preferably, a potential difference between the ionisation chamber and the filament may be in a range from 30 V to 500 V, and preferably of about 70 V.

[0027] Preferably, the electron beam emission means may comprise electron beam steering means, and the controlling means may be configured to steer the electron beam into the ionisation chamber during a trapping phase, and to steer the electron beam away from the ionisation chamber during an extraction phase. The steering means may preferably comprise deflection electrodes.

[0028] The ionisation chamber may preferably comprise a sample inlet for introducing analyte molecules into the ionisation chamber.

[0029] According to another aspect of the invention, a time-of-f light spectrometer having an electron impact ion source in accordance with an aspect of the invention is provided. The spectrometer may preferably be a multi-reflection time-of-flight spectrometer.

[0030] According to yet another aspect of the invention, a method for operating an electron impact ion source in accordance with aspects of the invention is provided. The method generates a sequence of trapping and extraction phases, and comprising the steps of causing electron beam emission beams to emit electrons into an ionisation volume of the electron impact ion source during a trapping phase, so that the electrons interact with an analyte in the ionisation volume, and to emit substantially no electrons into the ionisation volume during an extraction phase; causing ions trapped in the ionisation volume to be extracted from the ionisation volume during an extraction phase only. According to yet another aspect of the invention, a method for operating a time-of-flight spectrometer in accordance with aspects of the invention is provided, wherein the electron impact ion source of the time-of-flight spectrometer is operated in accordance with a method according to aspects of the invention.

[0031] In accordance with aspects of the invention, it is proposed to provide an electron impact ionisation source in which ionisation in the ionisation chamber is prevented before and after the trapping phase. Ionisation only occurs during the trapping phase, when generated ions accumulate in the ionisation chamber. During the extraction phase, no further ions are thus generated. This is achieved by avoiding interaction between the electron beam impacting the sample molecules in the ionisation chamber both before and after the trapping phase. The electron beam may for example be pulsed by an on / off signal which is synchronized to the trapping phase, or equivalently, to the ion extraction pulse signal.

[0032] During the trapping phase, the electron beam is switched on. At the end of the trapping phase, the electron beam is switched off and an extraction pulse is used to extract the ions that have accumulated during the trapping phase, for further analysis thereof.

[0033] Alternatively, the electron beam may remain switched on, but it may be prevented from interacting with sample molecules in the ionisation chamber by appropriately controlling electron beam deviation means, which are synchronized to the trapping phase. During the trapping phase, the electron beam is guided towards the ionisation chamber, and after the trapping phase, the electron beam is guided to avoid entering the ionisation chamber.

[0034] The present invention provides an ion source and method of operating an ion source, which alleviates at least some of the shortcomings identified in the prior art. The invention provides new ways to operate an electron impact, El, ionisation source for time-of-flight, TOF, mass spectrometry, and in particular for multi-reflection time-of-flight, MRTOF, mass spectrometry. By using the improved ion source, mass spectrum quality obtained using a TOF or MRTOF mass spectrometer is improved, as is the analysis sensitivity and precision.

[0035] The invention is based on the synchronization of an electron emitter that is switched or pulsed, to the pulsed ion beam extraction signal. The proposed electron impact arrangement and the switching electronics allow to continuously maintain the electron impact ionisation during the trapping phase of the source, and to switch the ionisation off at the start of the pulsed particle extraction. Once extracted, the charged particles are analysed using TOF spectrometry. The features of the invention enable the obtention of a clean mass spectrum signal with high signal-to-noise ratio, due to the fact that no ionisation occurs during the ion extraction phase from the source. Further this allows to keep high sensitivity due to trapping of the charged particles prior to extraction.

[0036] The novel operation method allows improvements in the TOF mass spectrum quality by physically reducing spectral background noise caused by continuously created ions during the extraction phase, improving SNR and enhancing sensitivity and precision. The continuous operation of the electron source itself can be maintained together with the alternating sequence of ion trapping and extraction

[0037] Brief description of the drawings

[0038] Several embodiments of the present invention are illustrated by way of figures, which do not limit the scope of the invention, wherein: figure 1 provides a schematic illustration of a cut-through view of an electron impact source in accordance with a preferred embodiment of the invention; figure 2 provides the time evolution of the electron beam status, the corresponding ionisation and trapping phases, and a synchronized ion extraction signal, as used in accordance with a preferred embodiment of the invention; figure 3 provides a schematic illustration of a cut-through view of an electron impact source in accordance with a preferred embodiment of the invention, during an ion trapping phase; figure 4 provides a schematic illustration of a cut-through view of the electron impact source depicted in figure 3, during an ion extraction phase; figure 5 illustrates the time evolution of the electric potentials applied to the filament shield and extraction optics of the electron impact source shown in figures 3 and 4; figure 6 provides a schematic illustration of a cut-through view of an electron impact source in accordance with a preferred embodiment of the invention, during an ion trapping phase; figure 7 provides a schematic illustration of a cut-through view of the electron impact source depicted in figure 6, during an ion extraction phase; figure 8 illustrates the time evolution of the electric potentials applied to the filament shield and extraction optics of the electron impact source shown in figures 6 and 7; figures 9a and 9b provide a mass spectrum of ambient air, obtained using a TOF mass spectrometer that uses a known electron impact source (a) and an electron impact source in accordance with a preferred embodiment of the invention (b), respectively.

[0039] Detailed description of the invention

[0040] This section describes aspects of the invention in further detail based on preferred embodiments and on the figures. The figures do not limit the scope of the invention. Throughout the description, like numerals will be used to describe like concepts in different embodiments. For example reference numerals 100, 200 and 300 each denote an electron impact source in accordance with the invention, but in three different embodiments. Details that are described in the context of a particular embodiment are applicable to other embodiments, unless otherwise stated.

[0041] The description puts focus on those aspects that are relevant for understanding the invention. It will be clear to the skilled person that an ion source also other commonly known aspects, such as an appropriately dimensioned power supply including converter circuitry, or mechanical holding means for holding the various elements of the device in their respectively required positions, even if those aspects are not explicitly mentioned.

[0042] The evaluation of the operating concept of the conventional storage El ion source has shown that the continuously generated ions during the extraction phase in particular have a decisive influence on the achievable performance in terms of At, AE and the background noise. In order to achieve a clean spectrum with very low or ideally no background noise, the contribution of the continuously generated ions during the extraction phase must be eliminated. This also allows a significant reduction of At due to the elimination of the initial TOF error. By eliminating these continuously generated particles, the energy spread also becomes independent of text and in order to achieve a low energy spread, Uextraction must be kept low. The problem of a low extractable mass range is then circumvented by increasing text, which in turn does not have a negative effect on the remaining characteristics due to the suppressed contribution of continuously generated ions. To allow such an operation concept from a technological perspective, the continuous generation of ions during the extraction phase must be blocked. A simple and stable approach is to maintain the extraction of electrons from the filament but to cut off the electron beam entering the ionisation chamber such that no further ionisation due to El can take place. This cut-off must then be synchronized with the extraction pulse applied to the cap electrode.

[0043] Figure 1 illustrates an electron impact ion source 100 in accordance with a preferred embodiment of the invention. The electron impact source comprises an ionisation chamber 110 that has an inlet 111 for introducing gaseous analyte molecules, an electron beam inlet 112 and an ion beam outlet 114. Ion extraction means 120, which may for example be provided by an optical element such as an electrode surrounding the ion beam outlet are configured to selectively extract ions from the ionisation chamber through the ion beam outlet, preferably along a first axis 122.

[0044] The electron impact ion source further comprises electron beam emission means 130, that are able selectively emit electrons into the ionisation chamber 110, preferably along a second axis 132 that is perpendicular to the first axis 122. Orthogonality between the first and second axis is not a limiting element of the invention.

[0045] Controlling means 140 are electrically connected to the ion extraction means 120 and to the electron beam emission means 130. The controlling means may for example comprise converter circuits and a data processing unit programmed by appropriate software code means, to apply predetermined voltages to the ion extraction means 120 and electron beam emission means through corresponding control signals 142, 143. The controlling means 140 are configured to synchronize the electron beam emission means 130 and the ion extractions means 120, so that when ions are extracted from the ionisation chamber 110, substantially no electrons are emitted into the ionisation chamber and so that electrons are emitted into the ionisation chamber when no ions are extracted from the ionisation chamber, during the trapping phase.

[0046] During an ionisation phase, the electron beam emission means are switched on, so that ionisation of the analyte molecules occurs with high probability through electrons impacting the molecules. The generated ions, shown as circles, are trapped within the ionisation chamber, as the potential of the ion extraction means is maintained at a value that is equal to the potential of the ionisation chamber. No acceleration towards the ion beam outlet occurs. The corresponding electron beam control signal 132 and the duration of both the ionisation and trapping phases is illustrated in the first three plots of Figure 2. The fourth plot of Figure 2 illustrates the time evolution of the extraction phase: during a short period, the potential difference between the extraction means 120 and the ionisation chamber is increased, to that the trapped ions are accelerated and directed towards the ion beam outlet. During the extraction phase, the electron beam emission means are switched off, so that the probability of generating new ions while the trapped ions are being extracted, is low.

[0047] Figures 3-5 show a preferred embodiment of an ion source 200, in accordance with the invention, wherein the control of the electron injection into the ionisation chamber 210 is achieved by selectively shielding an integrated electron filament of the ion source. The configuration during the ion trapping phase is illustrated in Fig. 3, while the configuration during the ion extraction phase is illustrated in Fig. 4. The plots in Fig. 5 show the corresponding evolution of voltages applied to the extraction optics and the filament shielding, respectively. Figure 2 illustrates the electron impact ion source 200 in accordance with a preferred embodiment of the invention during an ion generation and trapping phase. The electron impact source comprises an ionisation chamber 210 that has an inlet 211 for introducing gaseous analyte molecules, an electron beam inlet 212 and an ion beam outlet 214. Ion extraction means 220, which may for example be provided by an optical element such as an electrode surrounding the ion beam outlet are configured to selectively extract ions from the ionisation chamber through the ion beam outlet, preferably along a first axis 222.

[0048] The electron impact ion source further comprises electron beam emission means 230, that are able selectively emit electrons into the ionisation chamber 210, preferably along a second axis 232 that is perpendicular to the first axis 222. The electron beam emission means 230 comprise a filament 234 for emitting electrons. Such filaments are well known in the art. The electron emission means 230 further comprise a shielding electrode 236 that preferably surrounds the filament 234. The filament is preferably electrically isolated from the shielding electrode 236, for example by using an interposed ceramic insulator element, which is not depicted. Similarly, the ion beam extraction means 220 are insulated from both the filament and the shielding electrode, by appropriate insulation means.

[0049] Controlling means 240 are electrically connected to the ion extraction means 220 and to the electron beam emission means 230. The controlling means are configured to apply predetermined voltages to the ion extraction means 220 and electron beam emission means 230 through corresponding control signals 242, 243. The continuous ionisation during the trapping phase foresees to keep the filament shield 236 potential 1 / ^.1 below the potential of the ionisation chamber 210 and the cap electrode I ion extraction means 220. The corresponding voltages are depicted in the first time phase (ion generation and trapping) of Figure 5.

[0050] Figure 4 depicts the configuration of the electron impact ion source 200 during the extraction phase. For the pulsed particle extraction, an accelerating electric field is applied in the ionisation chamber by lowering the cap 220 potential from L / Chamber by L / extraction- At the same time, the potential of the filament shield 236 is increased from iis.i to l / fiis.2, which accelerates the electrons towards the filament shield 236 instead of towards the ionisation chamber 210. After the desired extraction time, the potentials of the cap 220 and the filament shield are switched back to the trapping configuration. The corresponding voltages are depicted in the second time phase (ion extraction) of Figure 5.

[0051] In the given design of the ion source, the electron beam cut-off is implemented by applying a higher potential to the filament shield than to the ionisation chamber to reverse the direction of electron movement away from the ionisation chamber, during the extraction phase.

[0052] To allow this operation, within the ionisation and trapping stage, the filament is electrically isolated from the filament shield and the cap electrode with the extraction aperture. This allows to independently apply different pulsed potentials to these electrodes. During the trapping phase the potential of the cap electrode 220 L / capis kept at / chamber such that no electric field can accelerate the generated particles.

[0053] The potential of the filament L / fjiament is kept around 70V below L / Chamber- Electrons emitted from the filament are thus accelerated with sufficient energy towards the ionisation chamber such that electron impact ionisation can occur. At the time of particle extraction, L / capis lowered by the extraction voltage L / extraction below L / Chamberto establish a moderate accelerating field within the ionisation chamber. At the same time, the potential of the filament shield t / ms. is increased from 1 / ^.1 well above L / Chamberto iis.2 while L / fiiamentstays at its original potential. Electrons emitted from the filament are thus much more likely accelerated towards the filament shield than towards the ionisation chamber such that the ionisation rate is effectively reduced to a minimum. Particles contained in the ionisation chamber can now leave the chamber to the downstream region. The duration of this extraction process depends on the m / z. Hence, the minimum pulse duration has to be set in accordance with the extraction time of the highest mass of interest. Without limiting the invention to any of the following values, it should be noted that the durations depend on the applied extraction voltage gradient and on the ion optical design of the ion source. For example, for applying an extraction voltage gradient to cap electrode of 100 V , the effective extraction time may be of 1300 ns for m / z of 500 amu / e (i.e. , extract all m / z from 1 - 500 amu / e, m / z above 500 amu / q are only partially extracted), while the appropriate trapping time can last 80 microseconds. Once, all the desired particles are extracted, the cap electrode switches back to L / Chamberto stop particle extraction and the filament shield electrode is again set to C / fiiS.i to allow electron acceleration towards the ionisation chamber and continuous ionisation until the next extraction pulse is applied.

[0054] Figures 6-8 show another preferred embodiment of an ion source 300, in accordance with the invention, wherein the control of the electron injection into the ionisation chamber 310 is achieved by selectively steering an electron beam into the ionisation chamber, or not. The configuration during the ion trapping phase is illustrated in Fig. 6, while the configuration during the ion extraction phase is illustrated in Fig. 7. The plots in Fig. 8 show the corresponding evolution of voltages applied to the extraction optics 320 and the steering means 336, respectively. Figure 6 illustrates the electron impact ion source 300 in accordance with a preferred embodiment of the invention during an ion generation and trapping phase. The electron impact source comprises an ionisation chamber 310 that has an inlet 311 for introducing gaseous analyte molecules, an electron beam inlet 312 and an ion beam outlet 314. Ion extraction means 320, which may for example be provided by an optical element such as an electrode surrounding the ion beam outlet are configured to selectively extract ions from the ionisation chamber through the ion beam outlet, preferably along a first axis 322.

[0055] The electron impact ion source further comprises electron beam emission means 330, that are able selectively emit electrons into the ionisation chamber 310, preferably along a second axis 332 that is perpendicular to the first axis 322. The electron beam emission means 330 comprise a non-depicted electron source for emitting electrons. The electron emission means 330 further comprise a steering or deflecting means 336 that are arranged downstream of the electron source and upstream of the electron beam inlet 312. By applying a potential difference to the deflection plates, the electrons that are emitted from the electron source are deflected by the generated electric fields. In the depicted example, when the deflection or steering means 336 are grounded, the electron beams evolve freely along axis 332, as shown using the solid arrow, into the ionisation chamber 310.

[0056] Controlling means 340 are electrically connected to the ion extraction means 320 and to the electron beam emission means 330. The controlling means are configured to apply predetermined voltages to the ion extraction means 320 and electron beam emission means 330 through corresponding control signals 342, 343. The continuous ionisation during the trapping phase foresees to keep the deflection means grounded while the potential of the ionisation chamber 310 and the cap electrode I ion extraction means 320 are kept equal. The corresponding voltages are depicted in the first time phase (ion generation and trapping) of Figure 8.

[0057] Figure 7 depicts the configuration of the electron impact ion source 300 during the extraction phase. For the pulsed particle extraction, an accelerating electric field is applied in the ionisation chamber by lowering the cap 320 potential below the potential o the ionisation chamber. At the same time, a deflection potential is applied to the steering means 336, which steers the passing electrons away from the electron beam inlet 312, as shown by the angled solid arrow. After the desired extraction time, the potentials of the cap 320 and the steering means 336 are switched back to the trapping configuration. The corresponding voltages are depicted in the second time phase (ion extraction) of Figure 8.

[0058] Existing TOF or MRTOF mass spectrometers may be upgraded by integrating the electron impact ion source described herein into their design. Remaining components, such as acceleration means and mass analysing means, are arranged downstream of the electron impact ion source’s ion beam outlet 114, 214, 314. After acceleration to a uniform kinetic energy by electrostatic fields and pulsed extraction to a field-free drift region, the ions are spatially separated along the drift path based on their individual constant drift velocities, dependent on the respective m / z. The drift regions acts as the mass analyser. Ions with high m / z are thereby travelling slower than ions with low m / z. The recording of the ion arrival times, with a detector positioned distally along the drift path, provides the acquisition of an ion TOF spectrum by setting the time of the release pulse from the accelerator as the baseline. Calibration with ions of known m / z allows to convert the TOF spectrum to a mass spectrum.

[0059] In a multiple-reflection time-of-flight mass spectrometer, MRTOF, the flight path and thus the total flight time of the ions within the mass analyser compartment is effectively extended by folding the ion trajectory. This is done by integrating an arrangement of at least two opposing ion mirrors. Both open-path and closed-path configurations are known in the art and will not be further detailed. Each mirror provides a set of electrodes with different applied voltage, such that an ion reflecting electrostatic field within the mirror is shaped.

[0060] To directly compare the effects of the novel and the conventional ion extraction, the TOF spectrum for air as a representative gas mixture, was recorded for both methods on the primary time focus plane, as shown in Fig. 9a and Fig9b. These figures show a comparison of the simulation TOF spectra for air recorded at the measurement plane for a) the conventional known extraction method and b) the novel extraction method in accordance with embodiments of the invention. The conventional extraction method involved an ongoing continuous ion generation during the extraction phase and a back switching of the cap potential to the trapping potential after 1750 ns. For the novel extraction method, the ion generation was switched off during the extraction phase and the cap electrode was kept at the extraction potential. The noise, caused by particles being generated and extracted during the extraction phase, limits the signal-to-noise ratio, SNR, and thus the dynamic range and sensitivity for the conventional extraction method. The novel extraction method eliminates this noise and allows high SNR and improved dynamic range.

[0061] To model the ion generation in SIMION™, the generation rates were set according to the typical relative abundances of the m / z ratios contained in air. The ionisation and accumulation of the particles took place over a period of 5 ps. For both methods, the particles were then accelerated towards the primary time focus plane by applying Uextraction = 60V. The primary time focus is suitable as the measurement plane for direct comparison of the spectra, as the flight time is largely independent of the energy distribution of the particles at this point. The voltages applied to all electrodes were identical to the previously described measurements for both methods. For the conventional extraction method, continuous ongoing ionisation was assumed during extraction. The cap electrode was pulsed by decreasing the potential by the extraction voltage with an assumed rise and fall time of 50 ns each and a static duration at the extraction potential of 1750 ns. This extraction pulse width was chosen because it allows the extraction of the mass range for m / z from 1 amu / e to 500 amu / e at given voltage values and thus covers the common spectra. For the novel operation method the continuous creation of ions was switched off at t = 5 ps when the extraction pulse was applied. The rise time for the extraction pulse was also set to 50 ns, however there was not applied a back switching of the cap electrode to the trapping potential, instead it was kept at the extraction potential. The comparison of the obtained spectra in Figure 9 reveals that there is an intensive spectral baseline for the conventional operation method a). This baseline is caused by the continuously generated particles during the extraction phase and overlaps with the actual spectrum from the trapped ions leading to noise in the signal. It can be seen that this baseline increases up to a certain level with increasing time in the recorded spectrum, which is due to the mass-dependent mobility of the continuously generated ions. Light and fast ions with high mobility thus contribute to the baseline relatively early on, while heavy and slow ions with low mobility only become visible after a delay. In the air spectrum shown, the relatively weak peaks for m / z at 40 amu / e and 44 amu / e are completely overlaid by the baseline noise. The spectrum obtained with the novel extraction method b) reveals that there is no such baseline, as only particles created during the trapping phase contribute to the spectrum. The weak peaks for m / z at 40 amu / e and 44 amu / e are thus still distinguishable. The novel extraction method allows an improved SNR for the recorded spectrum and thus increased dynamic range.

[0062] The storage El ion source, is a commonly applied option for the instrumentation in TOF mass spectrometers and was thus evaluated by simulation for its feasibility as a pulsed ion source for high-performance MRTOF experiments. The results have shown, that by applying a flat potential distribution within the ionisation chamber during the trapping phase, particles can be retained up to a certain degree, which allows the accumulation of ions. This is beneficial for MRTOF measurements, as an initially high injected count of ions is decisive for the sensitivity of the obtained spectrum. However, the evaluations of the conventional operation principle for the pulsed ion extraction indicated, that a trade-off between different performance characteristics has to be made. Most importantly, continuous ionisation during the extraction phase is very problematic, as it generates considerable background noise in the recorded spectrum, which ultimately leads to very weak peaks being lost in this noise and therefore no longer being reliably detected. Therefore, in the conventional method, the extraction phase must not last too long, which in turn has an impact on the choice of the extraction voltage.

[0063] A modified approach for the pulsed extraction of ions for the storage El ion source was developed, which involves a synchronized ionisation with the extraction process. For this purpose, the continuous electron beam is diverted for the duration of the extraction so that no ionisation can take place in the ionisation chamber. The design of a continuously emitting El ion source was modified in such a way that both the cap electrode, which separates the ionisation chamber from the subsequent acceleration electrodes, and the shielding electrode around the filament can be operated with individual potentials and pulsed. This modification in operation and design makes it possible to prevent continuous ionisation during extraction, thus avoiding the noise that would otherwise occur in the spectrum. This allows a significantly improved SNR. In addition, the extraction process is simplified in that the duration of the extraction pulse can be selected without taking the noise behaviour into account and time-of-flight errors due to different start times of the ions during the extraction phase are eliminated. As a result, long extraction pulses can be used to extract a wide mass range while the voltage gradient required for extraction at the cap electrode can be kept small, which in turn results in a narrower distribution of particle energy. The test series with this new extraction method have shown that pulsed ion packets can be generated for the selected parameter values, which allows a very low energy spread of less than 15 eV (FW50) for a considered range for m / z from 2 amu / e to 500 amu / e. For this purpose, a voltage gradient of 60V was applied to the cap electrode for extraction. The duration of the extraction pulse was increased to 1750 ns in order to be able to extract a mass range of up to 500 amu / e even for this low voltage. With these settings, a time spread (FW50) of 25ns could be achieved for m / z of 50 amu / e on the measurement plane. The novel extraction method was compared with the conventional method for recording the TOF spectrum of air, whereby the background noise was effectively reduced with the novel method, so that even very weak peaks for m / z of 40 and 44 amu / e are still recognizable, while these are lost in the noise with the conventional method. With the new modified operating mode, the pulsed El storage ion source thus provides significantly improved starting conditions for MRTOF mass spectrometers.

[0064] It should be noted that features described for a specific embodiment described herein may be combined with the features of other embodiments unless the contrary is explicitly mentioned. It should be understood that the detailed description of specific preferred embodiments is given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to the person skilled in the art. The scope of protection is defined by the following set of claims.

Claims

Claims1. An electron impact ion source (100) comprising- an ionisation chamber (110) having an electron beam inlet (112) and an ion beam outlet (114),- ion extraction means (120) configured to selectively extract ions from the ionisation chamber through the ion beam outlet (114), preferably along a first axis (122);- electron beam emission means (130) configured to selectively emit electrons into the ionisation chamber (110) along a second axis (132);- controlling means (140) configured to synchronize the electron beam emission means (130) and the ion extractions means (120), so that when ions are extracted from the ionisation chamber (110), substantially no electrons are emitted into the ionisation chamber and so that electrons are emitted into the ionisation chamber when no ions are extracted from the ionisation chamber, during the trapping phase.

2. The electron impact ion source according to claim 1 , wherein the controlling means (140) are configured to pulse the electron emission means (140).

3. The electron impact ion source according to any of the preceding claims, wherein the controlling means are configured to- keep the ion extraction means and the ionisation chamber at the same potential during a trapping phase, and- maintain a lower potential of the ion extraction means to establish a potential difference between the ionisation chamber and the ion extraction means during an extraction phase.

4. The electron impact ion source according to any of the preceding claims, wherein the electron beam emission means comprise a filament for emitting electrons, the filament being surrounded by a shield electrode, and wherein the controlling means are configured to- keep the shield electrode potential below the potential of the ionisation chamber and the ion extraction means during a trapping phase;- maintain the potential of the ion extraction means below the potential of the ionisation chamber, and maintain an increased potential of the shield electrode toaccelerate emitted from the filament towards the shield electrode during an extraction phase.

5. The electron impact source according to claim 4, wherein the filament, shield electrode and ion extraction means are electrically insulated from each other.

6. The electron impact source according to any of claims 4 or 5, wherein a potential difference between the ionisation chamber and the filament is in a range from 30 V to 500 V, and preferably of about 70 V.

7. The electron impact source according to any of claims 1 to 3, wherein the electron beam emission means comprise electron beam steering means, and wherein the controlling means are configured to steer the electron beam into the ionisation chamber during a trapping phase, and to steer the electron beam away from the ionisation chamber during an extraction phase.

8. The electron impact source according to any of the preceding claims, wherein the ionisation chamber comprises a sample inlet.

9. A time-of-flight spectrometer having an electron impact ion source in accordance with any of the preceding claims.

10. The time-of-flight spectrometer according to claim 9, wherein the spectrometer is of the multi-reflection time-of-flight type.

11. A method for operating an electron impact ion source in accordance with any of claims 1 to 8, by generating a sequence of trapping and extraction phases, the method comprising the steps of- causing electron beam emission beams to emit electrons into an ionisation volume of the electron impact ion source during a trapping phase, so that the electrons interact with an analyte in the ionisation volume, and to emit substantially no electrons into the ionisation volume during an extraction phase;- causing ions trapped in the ionisation volume to be extracted from the ionisation volume during an extraction phase only.

12. A method for operating a time-of-flight spectrometer in accordance with any of claims 9 or 10, wherein the electron impact ion source of the time-of-flight spectrometer is operated in accordance with the method of claim 11.

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