METHOD FOR EXAMINING A GAS BY MASS SPECTROMETRY AND MASS SPECTROMETER

Abstract

A method for examining a gas by mass spectrometry includes: ionizing the gas for producing ions; and storing, exciting and detecting at least some of the produced ions in an FT ion trap. Producing and storing the ions in the FT ion trap and/or exciting the ions prior to the detection of the ions in the FT ion trap includes at least one selective IFT excitation, such as a SWIFT excitation, which is dependent on the mass-to-charge ratio of the ions. The disclosure further relates to a mass spectrometer. A mass spectrometer includes: an FT ion trap; and an excitation device for storing, exciting, and detecting ions in the FT ion trap.

Claims

1. A method, comprising: producing ions by ionizing a gas; storing at least some of the ions in an FT ion trap; and detecting at least some of the ions in the FT ion trap, wherein at least one of the following holds: i) producing the ions comprises exposing the ions to an IFT excitation based on a mass-to-charge ratio of the ions; ii) storing the ions in the FT ion trap comprises exposing the ions to an IFT excitation based on a mass-to-charge ratio of the ions; and iii) before detecting the ions in the FT ion trap, exposing the ions to an IFT excitation based on a mass-to-charge ratio of the ions.

2. The method of claim 1, wherein i) holds, and the IFT excitation is used to select ions to be selecting ions to store in the FT ion trap.

3. The method of claim 2, wherein ii) holds.

4. The method of claim 3, wherein iii) holds.

5. The method of claim 2, wherein iii) holds.

6. The method of claim 1, wherein ii) holds.

7. The method of claim 6, wherein iii) holds.

8. The method of claim 1, wherein iii) holds.

9. The method of claim 1, wherein the IFT excitation comprises a SWIFT excitation.

10. The method of claim 1, wherein: at least one of i) and II) holds; and only ions whose mass-to-charge ratio lies outside of an interval of the mass-to-charge ratios of a main gas component of the gas are selected for storage.

11. The method of claim 1, wherein: a degree of excitation and/or a phase angle of the IFT excitation are varied between a first excitation frequency and a second excitation frequency; and both the first excitation frequency and the second excitation frequency deviate from a predetermined excitation frequency by no more than 10%.

12. The method as claimed in claim 11, wherein the phase angle and/or the degree of excitation vary in steps between the first excitation frequency and the second excitation frequency, depending on the excitation frequency.

13. The method as claimed in claim 12, wherein the degree of excitation and/or the phase angle either increase in steps or decrease in steps between the first excitation frequency and the second excitation frequency, depending on the excitation frequency.

13. The method of claim 1, wherein the same ions are repeatedly selectively excited in the FT ion trap by IFT excitations, and detection of the ions is performed after a respective IFT excitation.

14. The method as claimed in claim 13, wherein there is a time interval between two IFT excitations that immediately follow one another in time, and the time interval is greater than a mean free time of flight of the ions in the FT ion trap.

15. The method of claim 1, further comprising examining an ion signal by mass spectrometry only in a temporally displaceable measurement time interval when detecting the ions.

16. The method of claim 1, further comprising: exciting the ions in the FT ion trap; recording a first frequency spectrum; modifying a phase angle and/or an oscillation amplitude of the ions in the FT ion trap and/or modifying ion resonance frequencies of the ions in the FT ion trap, exciting the ions in the FT ion trap again and recording a second frequency spectrum; and detecting interference frequencies in the FT ion trap by comparing the first recorded frequency spectrum and the second recorded frequency spectrum.

17. The method of claim 16, wherein modifying the ion resonance frequencies comprises modifying a storage voltage and/or a storage frequency of the FT ion trap.

18. The method of claim 1, further comprising determining a start phase angle of a trajectory of ions at a given ion resonance frequency after an IFT excitation on the basis of a time-dependent ion signal recorded when detecting the ions.

19. The method of claim 18, further comprising determining a charge polarity of the ions based on the start phase angle of the ions after the IFT excitation.

20. A mass spectrometer, comprising: an FT ion trap; and an excitation device configured to store, excite, and detect ions in the FT ion trap, wherein the excitation device is configured to produce an IFT excitation that is dependent on a mass-to-charge ratio of the ions during at least one member selected from the group consisting of storage of ions and excitation of ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the drawings:

[0049] FIG. 1 shows a schematic illustration of a mass spectrometer with an electric FT-ICR ion trap,

[0050] FIG. 2 shows a schematic illustration of a degree of excitation that is dependent on the ion resonance frequency during a SWIFT excitation,

[0051] FIG. 3 shows a schematic illustration of a timing during a measurement for recording a mass spectrum with the aid of the mass spectrometer from FIG. 1,

[0052] FIG. 4 shows schematic illustrations of three mass spectra of a gas with a main gas component,

[0053] FIGS. 5a,5b show schematic illustrations of the frequency spectrum and of the time profile of a (broadband-)selective SWIFT excitation,

[0054] FIGS. 6a-6c show a schematic illustration of the frequency spectrum in the case of a uniform SWIFT excitation or in the case of a SWIFT excitation that varies in terms of the degree of excitation and the phase angle in a frequency-dependent manner (FIG. 6a) and the associated trajectories of the excited ions (FIGS. 6b,c),

[0055] FIG. 7 shows a schematic illustration of the time profile of a multiple (broadband-) selective SWIFT excitation and a subsequent detection,

[0056] FIG. 8 shows a schematic illustration of a detected ion signal with a temporally displaceable measurement time interval,

[0057] FIG. 9 shows a schematic illustration of two frequency spectra that are recorded at different storage voltages, and

[0058] FIGS. 10a-10d show schematic illustrations of the frequency spectra of positively charged ions (FIG. 10a) and negatively charged ions (FIG. 10b) stored in the FT-ICR ion trap and of all ions stored in the FT-ICR ion trap (FIG. 10c and FIG. 10d).

DETAILED DESCRIPTION

[0059] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0060] FIG. 1 schematically shows a mass spectrometer 1 which has an electric FT-ICR ion trap 2. The FT-ICR trap 2 has a ring electrode 3, which has applied thereto a high-frequency AC voltage V.sub.RF which, for example, can have a frequency f.sub.RF of the order of kHz to MHz, e.g. 1 MHz, and an amplitude V.sub.RF of several hundred volts. The high-frequency AC voltage V.sub.RF produces a high frequency alternating field within the FT-ICR trap 2, ions 4a, 4b of a gas 4 to be examined being dynamically stored in the field.

[0061] From the high-frequency alternating field (E-field), there results a mean restoring force that acts on the ions 4a, 4b more strongly, the further away the ions 4a, 4b are from the middle or center of the FT-ICR ion trap 2. In order to measure the mass-to-charge ratio (m/z) of the ions 4a, 4b, the latter are excited by an excitation signal S1, S2 (stimulus) to carry out oscillations, the frequency f.sub.ion of which is dependent on the ion mass and the ion charge and is typically in the frequency range of kHz to MHz orders of magnitude, e.g. from approximately 1 kHz to 200 kHz. The respective excitation signal S1, S2 is produced by a second excitation unit 5b and a third excitation unit 5c which forms an excitation device 5 together with a first excitation unit 5a, which serves to produce the high-frequency storage voltage V.sub.RF with the predetermined storage frequency f.sub.RF. The excitation device 5 also has a synchronization device 5d, which synchronizes the three excitation units 5a-c in time. An amplifier is disposed downstream of each excitation unit 5a-c, the amplifiers likewise being part of the excitation device 5.

[0062] For the purposes of a non-reactive, non-destructive detection (i.e. the ions 4a, 4b are still present after the detection), the oscillation signals of the ions 4a, 4b are tapped in the form of induced mirror charges at the measurement electrodes 6a, 6b, as described e.g. in DE 10 2013 208 959 A which was cited at the outset, the entirety of the latter being incorporated into the content of this application by reference. As described in detail therein, the respective measurement electrodes 6a, 6b are respectively connected to a low-noise charge amplifier 8a, 8b, respectively by way of a filter 7a, 7b. The charge amplifiers 8a, 8b, firstly, capture and amplify the ion signals from the two measurement electrodes 6a, 6b and, secondly, keep the measurement electrodes 6a, 6b at virtual ground potential for the storage frequency f.sub.RF. From the signals supplied by the charge amplifiers 8a, 8b, an ion signal u.sub.ion(t) is produced via difference formation, the temporal profile of the ion signal being illustrated at the bottom right in FIG. 1. The ion signal u.sub.ion(t) is fed to a detector 9, which, in the example shown, has an analog-to-digital converter 9a and a spectrometer 9b for fast Fourier analysis (FFT) in order to produce a mass spectrum, which is illustrated at the top right in FIG. 1. In this case, the detector 9 or the spectrometer 9b firstly produce a frequency spectrum of the characteristic ion resonance frequencies f.sub.ion of the ions 4a, 4b stored in the FT-ICR ion trap 2, which frequency spectrum is converted into a mass spectrum on the basis of the dependence of the ion resonance frequencies f.sub.ion on the mass and charge of the respective ions 4a, 4b. The mass spectrum represents the number of detected particles or charges as a function of the mass-to-charge ratio m/z.

[0063] As a consequence, the electric FT-ICR trap 2 facilitates a direct detection or the direct recording of a mass spectrum, as a result of which a quick gas analysis is facilitated. However, the fast recording of a mass spectrum with the aid of Fourier spectrometry can be effectuated not only in the electric FT-ICR trap 10 described above, but also in variations of the trap type shown in FIG. 1, for example in the case of a so-called Orbitrap.

[0064] As was described further above, all ions 4a, 4b in the FT-ICR ion trap 2 have an ion resonance frequency Con with which the stored ions 4a, 4b oscillate in the FT-ICR ion trap 2, the ion resonance frequency being proportional to the mass-charge ratio (m/z) of the ions. If the ions 4a, 4b are excited at their respective ion resonance frequency f.sub.ion, they either can be excited in a targeted manner in this way or be thrown out of the FT-ICR ion trap 2 by way of a resonance overshoot. As a consequence, ions 4a, 4b with certain mass-to-charge ratios m/z can be selectively excited or the storage thereof in the FT-ICR ion trap 2 can be prevented/suppressed.

[0065] 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.sub.ion lies within the respective window can be excited or suppressed in a targeted manner. The inverse transformation of these regions by way of an inverse Fourier transform supplies the time signal for the so-called IFT excitation. This is referred to as a SWIFT excitation 10 if these time profiles are calculated in advance. An example of SWIFT excitation 10 with a broadband-selective excitation spectrum is depicted in FIG. 2, wherein the ion resonance frequencies f.sub.ion are related to the storage frequency f.sub.RF. The desired selective excitation spectrum depends on the ion resonance frequencies f.sub.ion and, as a consequence, on the mass-to-charge ratio (m/z) of the ions 4a, 4b. The associated discrete SWIFT time function (not shown in FIG. 2) is output at the instant of the SWIFT excitation in order to obtain the desired excitation spectrum shown in FIG. 2.

[0066] The measurement electrodes 6a, 6b can be used for the SWIFT excitation 10. By way of the SWIFT excitation 10, the ions 4a, 4b can be deflected in the direction of the measurement electrodes 6a, 6b in such a way that certain ions 4a, 4b are firstly either stored or not stored and secondly excited practically continuously or not excited at all, both during the ion production and ion storage and also immediately before the detection of the ion signals u.sub.ion(t).

[0067] Therefore, a number of options emerge by way of the SWIFT excitation for realizing new performance characteristics of the mass spectrometer 1. A precondition for all measurement tasks is that the excitation time of the ions 4a, 4b within the FT-ICR ion trap 2 is substantially shorter than the mean free time of flight or the mean free path length of the molecules or ions 4a, 4b of interest. It was found to be advantageous to use optimized SWIFT algorithms, as are presented, for example, in the article Stored Waveform Inverse Fourier Transform Axial Excitation/Ejection for Quadrupole Ion Trap Mass Spectrometry by S. Guan and A. G. Marshall, Anal. Chem. 1993, pages 1288-1294 or in U.S. Pat. No. 4,945,234, both of which are incorporated into the content of this application by reference. Optimized SWIFT algorithms firstly produce a SWIFT signal output that is as short as possible and secondly prevent overdriving of the low-noise charge amplifiers 8a, 8b that are connected to the measurement electrodes 6a, 6b.

[0068] A SWIFT excitation 10 can be effectuated immediately before the detection of the ions 4a, 4b, i.e. before recording the (normalized) ion signal, as illustrated in FIG. 3, which only shows the envelope of the (normalized) ion signal u.sub.ion(t). However, a SWIFT excitation 10 can also already be effectuated during the production and storage of the ions 4a, 4b, as likewise indicated by the timing in FIG. 3. In this case, the SWIFT excitation 10 serves to select ions 4a, 4b that are to be stored in the FT-ICR ion trap 2.

[0069] In principle, there are two options for producing the ions 4a, 4b by ionizing the gas 4: either the ions 4a, 4b are produced within the FT-ICR ion trap 2 or the gas 4 is supplied to the FT-ICR ion trap 2 in a charge-neutral form and the ionization is effectuated in the FT-ICR ion trap 2. By way of example, such an ionization in the FT-ICR ion trap 2 can be carried out in the way described in WO 2015/003819 A1, which is cited at the outset and incorporated into the content of this application in respect of this aspect by reference.

[0070] If the ionization is effectuated in the electric FT-ICR ion trap 2, a continuous SWIFT excitation can already be effectuated during the ionization of the gas 4 (cf. FIG. 3), as a result of which unwanted gas components are excessively excited; 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 stored in accumulating fashion in the FT-ICR ion trap 2 for measurement purposes; as a result, this ensures that the FT-ICR ion trap 2 is not flooded by the unwanted charge carriers during the ionization time of the ions 4a, 4b to be detected. The ions 4a, 4b to be analyzed or to be detected are stored and accumulated in the FT-ICR ion trap 2 immediately after the ionization or after the transfer into the FT-ICR ion trap 2.

[0071] Such a selection during or before storage is advantageous since many applications involve the detection of gas traces or gas components with very low partial pressures or concentrations in a gas matrix or a gas 4 with a high overall pressure. An example of a mass spectrum of such a gas is illustrated at the bottom of FIG. 4. If gas traces with very low partial pressures, the mass spectrum of which is illustrated top right in FIG. 4, should be detected, the unwanted gas components that should not be stored in the FT-ICR ion trap 2 may be a main gas component 11 of the gas 2 to be examined. Within the meaning of this application, a main gas component 11 is understood to mean a gas constituent, the volume fraction of which lies at more than 50% by volume, in many applications at more than 90% by volume, of the gas 2 to be examined.

[0072] In the example shown in FIG. 4, the main gas component 11 has two ion populations with different mass-to-charge ratio (m/z).sub.1 and (m/z).sub.2, the volume fraction of which lies at more than 30% by volume of the gas 2 to be examined in each case such that the overall volume fraction of the main gas component 11 lies at more than 50% by volume of the gas 2 to be examined. The mass spectrum of the gas 4 recorded by the mass spectrometer 1 without a mass-selective SWIFT excitation is illustrated top left in FIG. 4. In the mass spectrum presented there, it is only possible to identify the ion populations of the main gas component 11, for example of a majority carrier gas, but not the gas traces actually of interest, the mass-to charge ratios of which lie outside of an interval I illustrated in FIG. 4, in which the mass-to-charge ratios (m/z).sub.1 and (m/z).sub.2 of the main gas component 11 are contained.

[0073] As a result of the broadband-selective SWIFT excitation 10, there can be selective filtering of those mass-to-charge ratios m/z which lie within the interval I or there can be targeted filtering of the first mass-to-charge ratio (m/z).sub.1 and of the second mass-to-charge ratio (m/z).sub.2 of the main gas component 11. In this way, only the ions 4a, 4b whose mass-to-charge ratios m/z lie outside of the interval I are stored in the FT-ICR ion trap 2, and so these can be detected with a high accuracy, as may be identified on the basis of the mass spectrum top right in FIG. 4.

[0074] The ratio of the partial pressures of the gas constituents of interest to the overall pressure may be, for example, of the orders of ppm volume (10.sup.6 ppmV) to pptV (10.sup.12). Here, the detection limit for individual gas components may be of the order of 10.sup.16 mbar. In this way, a dynamic range D of more than eight orders of magnitude (D>10.sup.8) can be achieved. Additionally, 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 the storage.

[0075] In the case of an electric FT-ICR ion trap 2, the high-frequency alternating field (E-field) is influenced by the space charge, more precisely by the space charge density, in the FT-ICR ion trap 2, i.e. there is feedback of the charges or ions 4a, 4b present in the FT-ICR ion trap 2 on the high-frequency alternating field that serves to store the ions 4a, 4b. The alternating field E is influenced more strongly the greater the space charge density is in the respective partial volume of the FT-ICR ion trap 2 and the weaker the mean restoring force arising from the high-frequency alternating field E is in the associated partial volume.

[0076] Particularly when exciting ions 4a, 4b with different but close together ion resonance frequencies or mass-to-charge ratios, high space charge densities may arise in parts of regions of the FT-ICR ion trap 2 which are particularly susceptible to the occurrence of large space charge densities. The ion resonance frequencies of whole ion packets can be strongly interfered with by the large space charge density, having a significant reduction in the measurement resolution as a consequence.

[0077] The local space charge in the FT-ICR ion trap can be reduced if ions 4a, 4b with close-together ion resonance frequencies f.sub.ion do not simultaneously pass over the same path of motion (or the same orbit). This can be achieved by virtue of the degree of excitation A of the SWIFT excitation 10 being varied in a frequency-dependent manner or depending on the mass or the mass-to-charge ratio m/z of the ions 4a, 4b between a first ion excitation frequency f.sub.ion1 and a second ion excitation frequency f.sub.ion2, as illustrated in FIG. 5a. FIG. 5b shows the associated time-dependent excitation signal (S1 and S2) of the SWIFT excitation.

[0078] In the example shown in FIGS. 5a,b, the degree of excitation A of the SWIFT excitation varies in steps depending on the ion excitation frequency f.sub.ion, wherein the degree of excitation A varies by no more than approximately 20% of the maximum degree of excitation A (i.e. the maximum amplitude of the SWIFT excitation 10) over the whole interval between the first ion excitation frequency f.sub.ion1 and the second ion excitation frequency f.sub.ion2. In the shown example, the degree of excitation A increases in steps from the first ion excitation frequency f.sub.ion1 to the second ion excitation frequency f.sub.ion2, with the step height between adjacent steps of the degree of excitation A being of equal size. It is understood that, alternatively, the degree of excitation A also may decrease from the first ion excitation frequency f.sub.ion1 to the second, higher ion excitation frequency f.sub.ion2. Moreover, the step height, i.e. the difference between the degrees of excitation of adjacent steps of the SWIFT excitation 10, is not necessarily constant but may vary from step to step. A continuous, step-free variation of the degree of excitation A between the first ion excitation frequency f.sub.ion1 and the second ion excitation frequency f.sub.ion2 likewise is possible here as a matter of principle.

[0079] In addition or as an alternative to varying the degree of excitation A or the amplitude of the SWIFT excitation 10, there may also be a variation of the phase angle of the SWIFT excitation 10, as illustrated in FIG. 6a. In the shown example, the phase angle is likewise modified step-by-step, to be precise by a value of 45 in each case, with the phase angle of the SWIFT excitation 10 increasing in steps with increasing ion excitation frequencies f.sub.ion in the example shown in FIG. 6a. It is understood that a step-by-step decrease in the phase angle of the SWIFT excitation 10 is likewise possible and that the difference between the phase angles of adjacent steps may deviate from 45 and, in particular, may vary from step to step. It is likewise understood that the step-by-step increase or decrease of the phase angle is only defined modulo 360, i.e. a phase angle of 0 is attained again after eight steps in the shown example. Here, the phase angle corresponds to a temporal shift or retardation of the SWIFT excitation, with the phase angle being related to a predetermined ion excitation frequency f.sub.ion,a.

[0080] By way of example, the predetermined ion excitation frequency f.sub.ion,a may correspond to the ion resonance frequency f.sub.ion or the mass-to-charge ratio m/z of an ion population to be analyzed. However, the predetermined ion excitation frequency f.sub.ion,a may also lie in an interval between two ion excitation frequencies f.sub.ion1, f.sub.ion2 or two associated ion resonance frequencies, the mass-to-charge ratios m/z of which lie close together. By way of example, the first (smaller) ion excitation frequency f.sub.ion1 can deviate from the predetermined ion excitation frequency f.sub.ion,a by no more than 10%, preferably by no more than 5%, in particular by no more than 1%. The same applies to the second, higher ion excitation frequency f.sub.ion2. In the example shown in FIG. 6a, the ratio f.sub.ion1/f.sub.ion,a is approximately 0.999 (deviation: 0.1%) while the ratio f.sub.ion2/f.sub.ion,a lies at approximately 1.009 (deviation: 0.9%), i.e. both ion excitation frequencies f.sub.ion1, f.sub.ion2 lie within the value range, described further above, of less than 1% deviation.

[0081] FIG. 6b shows the trajectory B of the ions 4a, 4b in the FT-ICR ion trap 2 in the case of a uniform SWIFT excitation, i.e. a SWIFT excitation with a constant degree of excitation A (represented by a dashed line in FIG. 6a) which, moreover, is effectuated in a synchronous or phase-locked manner. In FIG. 6b, the value z denotes the deflection of the ions 4a, 4b in the z-direction, i.e. toward the measurement electrodes 6a, 6b in the FT-ICR ion trap 2, where z.sub.0 denotes the maximum deflection. The value T denotes the period duration of the oscillations of the ions 4a, 4b with the predetermined ion excitation frequency f.sub.ion,a. What can clearly be recognized in FIG. 6b is that the trajectories B of the ions 4a, 4b superpose on one another such that a high space charge density arises.

[0082] FIG. 6c shows the trajectories B of the ions 4a, 4b in the case of the orbital SWIFT excitation 10 illustrated in FIG. 6a with different degree of excitation A, in which, additionally, the phase angle was also varied as illustrated in FIG. 6a, using the example of ten ion packets or ion populations with adjacent ion resonance frequencies f.sub.ion or with adjacent mass-to-charge ratios m/z. What can clearly be identified in FIG. 6c is that the trajectories B of the ten ion packets are spatially separated by the SWIFT excitation 10, as a result of which the local space charge density in the FT-ICR ion trap 2 is reduced and the mass resolution is increased as a result thereof. Typically, the ions 4a, 4b pass over the (periodic) trajectories B more than approximately 100 times-1000 times before the measurement or detection is effectuated. In this way, only a very low pressure is involved in the FT-ICR ion trap 2 in order to carry out the measurement or detection.

[0083] FIG. 7 shows a further application of a SWIFT excitation 10, in which the same ions 4a, 4b are successively excited in the FT-ICR ion trap 2 by two (broadband-)selective SWIFT excitations 10 and subsequently detected in each case. During the detection after a respective SWIFT excitation 10, the number of excited ions 4a, 4b (or the partial pressure of the excited gas constituent) is determined. By forming a mean over the number of ions 4a, 4b determined during the detections in each case, it is possible to significantly increase the signal-to-noise ratio (SNR) of the excited ions 4a, 4b of interest, without the remaining ions being influenced by the excitation.

[0084] The precondition for such a multiple detection is that there is a time interval between two IFT excitations 10 that immediately follow one another in time, the time interval being longer than a mean free time of flight t.sub.M of the ions 4a, 4b in the FT-ICR ion trap 2, i.e. >t.sub.M applies, where, typically, t.sub.M lies at more than approximately one millisecond (>1 ms). The SWIFT excitations are only repeated once the ions 4a, 4b have traversed a multiple of the mean free time of flight t.sub.M, for example more than 3t.sub.M more than 5t.sub.M or more than 10t.sub.M.

[0085] FIG. 8 shows a time-dependent ion signal u.sub.ion(t) after a SWIFT excitation 10 and a temporally displaceable measurement time interval 12 (FFT time window) represented by dashed lines, the measurement time interval having a time duration ti in the order of e.g. a plurality of milliseconds, preferably of 10 ms or less, particularly preferably of 5 ms or less. A time-resolved representation of the chemical behavior of the ion population that is embedded in the gas matrix or in the gas to be examined can emerge by a continuous or discrete displacement of the measurement time interval 12. In this case, the examination by mass spectrometry is only undertaken on the basis of the values of the ion signal u.sub.ion(t) during the measurement time interval 12, i.e. an evaluation is carried out only in the measurement time interval 12. This is particularly advantageous if chemical reactions such as e.g. charge transfer or protonation, which modify the originally present ion population during the detection time period, occur during the detection of the ions 4a, 4b. By carrying out the evaluation only in the measurement time interval 12 it is possible, for example, to observe a reaction such as a transition from H.sub.2O.sup.+ to H30.sup.+ practically in real time, i.e. intermediate products of chemical reactions can also be detected. In particular, this renders it possible to check whether the selected ions 4a, 4b that are stored in the FT-ICR ion trap 2 in fact correspond to the ion population that is provided for the chemical reaction. Optionally, the selection or the selection process of the ions 4a, 4b that should be accumulated in the FT-ICR ion trap 2 can be adapted in a suitable manner.

[0086] Parasitic interference frequencies f.sub.R that lead to lines in the recorded mass spectrum that are not produced by the ions 4a, 4b stored in the FT-ICR ion trap 2 may occur when recording mass spectra via the mass spectrometer 1. Such interference frequencies f.sub.R may lead to an incorrect interpretation of the mass spectrum.

[0087] In order to identify and optionally eliminate interference frequencies f.sub.R in the mass spectrum, use can be made of a method that is described below: in a first step, the ions 4a, 4b in the FT-ICR ion trap 2 are excited via a SWIFT excitation and subsequently detected in order to record a first frequency spectrum 13a of the ion resonance frequencies f.sub.ion (illustrated using dashed lines in FIG. 9). In a second step, the ion resonance frequencies f.sub.ion of the ions 4a, 4b in the FT-ICR ion trap 2 are modified and, in a third step, the ions 4a, 4b are again excited via a SWIFT excitation 10 and subsequently detected, with a second frequency spectrum 13b being recorded, the latter being illustrated in FIG. 9 using full lines.

[0088] When comparing the two frequency spectra 13a, 13b shown in FIG. 9, it can be clearly seen that the first frequency spectrum 13a and the second frequency spectrum 13b have lines whose frequencies are practically not displaced when the ion resonance frequencies f.sub.ion in the FT-ICR ion trap 2 are changed such that their position practically corresponds in both frequency spectra 13a, 13b. These lines can be identified or determined as interference frequencies f.sub.R. By contrast, the lines in the two frequency spectra 13a, 13b that can be systematically displaced by modifying the ion resonance frequencies f.sub.ion can be assigned to the ions 4a, 4b stored in the FT-ICR ion trap 2, i.e. the lines are real ion resonance frequencies f.sub.ion.

[0089] As indicated in FIG. 9, the storage voltage V.sub.RF of the FT-ICR ion trap 2 was changed from a first value Vrf1 to a second value Vrf2 for the purposes of modifying the ion resonance frequencies f.sub.ion. Since the ion resonance frequency f.sub.ion is directly proportional to the storage voltage V.sub.RF in the case of the predetermined mass-to-charge ratio m/z, the ion resonance frequencies f.sub.ion can be shifted by modifying the storage voltage V.sub.RF. Since the ion resonance frequency f.sub.ion is inversely proportional to the square of the storage frequency f.sub.RF at a given mass-to-charge ratio m/z, a change in the ion resonance frequencies f.sub.ion can be effectuated, alternatively or additionally, by a change in the storage frequency f.sub.RF as well.

[0090] As an alternative or in addition to a change in the ion resonance frequencies f.sub.ion, there can be a change in the phase angle and/or the oscillation amplitude z/z.sub.0 of the trajectories B of the ions 4a, 4b in the FT-ICR iron trap 2 in the second step, for example via a mass-dependent SWIFT excitation 10, as is illustrated in FIGS. 6a-c in an exemplary manner. The trajectories of the ions 4a, 4b are modified in the case of such a SWIFT excitation; this becomes noticeable, for example, by a change in the heights of the lines of the second frequency spectrum 13b in comparison with the first frequency spectrum 13a. By contrast, the SWIFT excitation 10 has practically no influence on the interference frequencies f.sub.R, and so the interference frequencies f.sub.R also can be detected or identified in this variant by a comparison of the two frequency spectra 13a, 13b.

[0091] A further application of a SWIFT excitation 10 consists in the determination of the charge polarities (positive/negative) of the ions 4a, 4b that are stored in the electric FT-ICR ion trap 2. In order to determine or identify the positively charged ions 4a and the negatively charged ions 4b in the FT-ICR ion trap 2, a phase angle .sub.0 of the trajectory B at the start of the detection, i.e. immediately after the SWIFT excitation 10 is determined initially at a predetermined ion resonance frequency f.sub.ion in accordance with formula (2) specified further above, which is reproduced again below:

[0092] the following emerges from

[00003]$\begin{array}{cc}\frac{1}{2}.Math.\cos \left({}_0\right)=\left[\frac{1}{T_0*{\hat{u}}_{\mathrm{ion}}}.Math.{}_0^{T_0}.Math.u_{\mathrm{ion}}\left(t\right)*\cos \left(2.Math..Math..Math.f_{\mathrm{ion}}*t+\right).Math.\mathrm{dt}\right].Math.\text{:}.Math..Math..Math.{}_0={\cos }^{-1}\left(2*\frac{1}{T_0*{\hat{u}}_{\mathrm{ion}}}.Math.{}_0^{T_0}.Math.u_{\mathrm{ion}}\left(t\right)*\cos \left(2.Math..Math..Math.f_{\mathrm{ion}}*t+\right).Math.\mathrm{dt}\right)& \left(2\right)\end{array}$

where denotes a start phase of the SWIFT excitation 10 of the ions 4a, 4b at the ion resonance frequency f.sub.ion, .sub.ion denotes the maximum of the absolute value of the ion signal u.sub.ion(t) at the start of the measurement and where the following applies: T.sub.0>>1/f.sub.ion and T.sub.0=N.sub.01/f.sub.ion and No integer >>1. Typically, the value of the amplitude or the envelope of the oscillating ion signal .sub.ion changes only slightly during the measurement time interval T.sub.0, i.e. the duration of the measurement interval To is significantly shorter than the mean free time of flight of the ions.

[0093] In the electric FT-ICR ion trap 2, it is possible to capture both positively charged ions 4a and negatively charged ions 4b at the same time. All ions 4a, 4b are detected after the SWIFT excitation 10 independently of their charge polarity, as a result of which e.g. a frequency spectrum which is illustrated in FIG. 10c may arise. The frequency spectrum, shown in FIG. 10c, of all ions 4a, 4b stored in the FT-ICR ion trap represents a superposition of the frequency spectrum of the positively charged ions 4a, which is illustrated in FIG. 10a, and of the frequency spectrum of the negatively charged ions 4b, which is illustrated in FIG. 10b.

[0094] By evaluating the phase angle .sub.0 of the ion movement or of the trajectory B of the ions 4a, 4b after the SWIFT excitation, it is possible to detect the charge polarity of the ions 4a, 4b. If the ions 4a, 4b are stimulated by uniform broadband excitation, e.g. the positively charged ions 4a move toward the first measurement electrode 6a immediately after the SWIFT excitation 10 while the negatively charged ions 4b move away therefrom.

[0095] By way of example, if the following formula is applied for each ion resonance frequency f.sub.ion, which corresponds to a line in the frequency spectrum shown in FIG. 10c,

[00004]$\begin{array}{cc}\mathrm{polarity}=\mathrm{sign}\left[\frac{1}{T_0*{\hat{u}}_{\mathrm{ion}}}.Math.{}_0^{T_0}.Math.u_{\mathrm{ion}}\left(t\right)*\cos \left(2.Math..Math..Math.f_{\mathrm{ion}}*t+\right).Math.\mathrm{dt}\right],& \left(3\right)\end{array}$

the positive ions 4a can be identified e.g. on the basis of a positive sign (.sub.0=0, polarity +1) and the negative ions 4b can be identified on the basis of a negative sign (.sub.0=180, polarity 1). In this way, the positive ions 4a and the negative ions 4b can be identified in the frequency spectrum of all ions 4a, 4b, as illustrated in FIG. 10d.

[0096] In the example described above, the assumption is made that the SWIFT excitation 10 is carried out with a start phase =0. However, as was described further above, the start phase may also be varied depending on the ion resonance frequency f.sub.ion in the case of a mass-dependent phase-shifted orbital SWIFT excitation 10. In this way, ion packets in the frequency spectrum or in the mass spectrum can be accordingly marked differently.

[0097] 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 a SWIFT (broadband-)selective excitation 10. This can be effectuated by virtue of different excitation transients being applied to the measurement electrodes 6a, 6b depending on the charge polarity at the respectively associated ion resonance frequencies f.sub.ion. It is understood that the procedure described above is not restricted to the electrode geometry of the FT-ICR ion trap shown in FIG. 1, i.e. this method can be applied to measurement electrodes with different electrode geometries, for example to measurement electrodes in the form of measurement tips in the end caps or in the form of toroidal measurement caps of a toroidal ion trap, etc.

[0098] In conclusion, the performance characteristics of a mass spectrometer 1 having an FT ion trap 2 can be significantly increased in the manner described further above.