EP3292561A1

EP3292561A1 - Method for examining a gas by mass spectrometry and mass spectrometer

Info

Publication number: EP3292561A1
Application number: EP16710230.0A
Authority: EP
Inventors: Michel Aliman; Alexander Laue; Hin Yiu Anthony Chung; Gennady Fedosenko; Rüdiger Reuter; Leonid Gorkhover; Martin Antoni; Andreas GORUS; Valerie Derpmann
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Leybold GmbH
Priority date: 2015-05-04
Filing date: 2016-03-17
Publication date: 2018-03-14
Anticipated expiration: 2036-03-17
Also published as: EP3292561B1; JP6734872B2; DE102015208188A1; JP2020188007A; JP6948500B2; US10141174B2; US20180068842A1; WO2016177503A1; JP6948499B2; JP2020188008A; JP2018518019A

Abstract

The invention relates to a method for examining a gas (4) by mass spectrometry, comprising: ionizing the gas (4) for producing ions (4a, 4b), and storing, exciting and detecting at least some of the produced ions (4a, 4b) in an FT ion trap (2). In the method, producing and storing the ions (4a, 4b) in the FT ion trap (2) and/or exciting 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), which is dependent on the mass-to-charge ratio (m/z) of the ions (4a, 4b). The invention further relates to a mass spectrometer (1).

Description

Method for mass spectrometric analysis of a gas and mass spectrometer

Reference to related application

This application claims the benefit of priority from German patent application DE 10 2015 208 188.5 dated 04 May 2015, the entire disclosure content of which is incorporated herein by reference.

Background of the invention

The invention relates to a method for mass spectrometry

Examining 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 FT electric trap The invention also relates to a mass spectrometer comprising: an FT ion trap, and a

Excitation device for storage, excitation and detection of ions in the FT ion trap. Ion storage, separation and detection are the main functions of conventional mass spectrometers, which are generally housed in different assemblies. This has the consequence that typically expensive interfaces between the modules must be used, which on the one hand makes a compact and efficient solution and on the other hand a fast manipulation of the ion populations difficult. With the transfer In addition, ions through the interfaces can cause signal loss, which degrades the performance and sensitivity of mass spectrometers. In an electrical or possibly a magnetic Fourier transform ion trap (short: FT ion trap), however, many functions (eg ion generation, storage and detection) can be combined "in situ" in the same ion trap and very compact.

In such an FT ion trap, ions or ionized gas components can be measured without retroaction and without interruption and can be detected or detected according to their mass-to-charge ratio, as is

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.

An example of a mass spectrometer with an electric FT ion trap is described in DE 10 2013 208 959 A. The FT ion trap has a

Ring electrode and two other electrodes (cover electrodes) on. The ions stored in the FT ion trap are excited in situ and the detection of the excited ions takes place by recording and evaluating

Mirror charges that induce the stored ions on the cover electrodes of the FT ion trap. For mirror charge measurement, the ions stored in the FT ion trap are excited in broadband (stimulated) in situ and vibrate with characteristic resonance frequencies in the ion trap, depending on the mass-charge ratio. This procedure differs fundamentally from the conventional destructive detection methods in which the ions are no longer available after the measurement.

From WO 2015/003819 A1 it is known that in an FT-ICR ("Fourier

Transform Ion Cyclotron Resonance ") - Trapped by an IFT excitation in the form of a so-called" SWIFT "(Storage Wave-Form Inverse Fourier Transform), Suggestion to remove individual ion populations from the ion trap or to suppress these if their number of particles exceeds a predetermined threshold value at a given mass-to-charge ratio. In this way, large ion populations can be removed from the ion trap so that specific subsets of ion populations can be measured more accurately.

Object of the invention

The object of the invention is to develop a method for mass spectrometric analysis of a gas and an associated mass spectrometer such that the performance of the mass spectrometric analysis is increased.

Subject of the invention

This object is achieved according to a first aspect by a method of the type mentioned, in which the generation and storage of the ions in the FT ion trap and / or the exciting of the ions (immediately) before the

Detecting the ions in the FT ion trap at least one selective IFT ("Inverse Fourier Transform") excitation, in particular a SWIFT ("Storage Wave Form Inverse Fourier Transform"), dependent on the mass-to-charge ratio or the ion resonance frequencies of the ions. ) Stimulation.

According to this aspect, it is proposed that 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 a selective ion excitation (hereinafter also: stimulation), for example, perform a broadband-selective ion stimulation. Such

Stimulation is typically done by means of a powerful IFT excitation, in particular by means of a SWIFT suggestion, which enables the

Significantly increase the performance of the mass spectrometer into which the FT ion trap is integrated. In this way, complex ion manipulations can be carried out, which fundamentally new

Features of the FT ion trap, as described in the following

Individual is described. Broadband-selective stimulation is understood to mean excitation in a large ion resonance frequency band. For such a broadband-selective excitation, for example: (ITI / Z) MAX / (m / z) _M iN> 5, possibly> 10, where (m / z) _M Ax is the maximum mass-to-charge ratio of the IFT Excitation and (m / z) _M iN designate the minimum mass-to-charge ratio of the IFT excitation. It is understood that IFT excitations with a smaller ionic resonance frequency band are also possible.

In one variant, during generation of the ions in the FT ion trap and / or during storage of the ions in the FT ion trap, at least one IFT excitation is performed to select ions to be stored in the FT ion trap. In particular, in an FT electrical ion trap, undesired ions not to be stored in the FT ion trap, which are in a given interval of the mass-to-charge ratio (the interval may have multiple non-contiguous subintervals), can already do so by means of a continuous SWIFT excitation be excessively excited 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 stored there.

In this variant, the ions are generated in the FT ion trap, ie the gas to be investigated is introduced in the charge-neutral state into the FT ion trap. The ionization in the FT ion trap can be carried out, for example, as described in the cited WO 2015/003819 A1, ie it is possible to introduce ions and / or metastable particles of an ionization gas and / or electrons into the FT ion trap which ionize the gas or gas mixture to be investigated 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 supply the FT ion trap with the gas to be examined in the form of gas ions. In this case too, during the storage of the ions in the FT ion trap, a selection of ions to be stored or of ions to be accumulated in the FT ion trap can take place.

In a further development of this variant, only ions for storage or for accumulation are selected whose mass-to-charge ratio is outside an interval of the mass-to-charge ratios of a main gas component of the gas to be investigated. For the purposes of this application, a main gas component is understood to be a gas constituent whose volume fraction is more than 50% by volume, in many applications more than 90% by volume of the gas to be investigated. The main gas component is typically only a single gas constituent, eg, N ₂ or H ₂ , ie, a single substance that generally corresponds to only a mass to charge ratio in the mass spectrum. Optionally, the main gas component, the volume fraction is greater than 50% by volume, possibly more than 90% by volume, also of several gas components

be composed. In this case, each of the gas components of the main gas component has more than 20% by volume or possibly more than 30% by volume of the gas to be analyzed.

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 examined, for example a process gas, with high total pressure is required. The ratio of these partial pressures to the total pressure is, for example, in the order of magnitude of ppm volume (10 ^"6 ppmV) to pptV (10 ^{" 12} ) per volume. By the above described SWIFT excitation can be the main gas component or the

Filtered main gas components of the gas to be examined, so that only the ions of the gas traces of interest or gas components are stored in the FT ion trap accumulating for subsequent detection. In this way, it is already ensured in the ionization time of the gas trace ions to be measured that the FT ion trap of the

Charge carriers of the main gas components is not flooded. As a result, a dynamic D of more than eight or possibly more than nine orders of magnitude (D> 10 ⁸ or 10 ⁹ ) can be achieved in the subsequent measurement. In addition, the sensitivity (absolute concentration) of the FT ion trap and, consequently, the signal-to-noise ratio (SNR) increases with the accumulation time, thereby allowing the detection limit for individual gas components to be up to 10 ^"16 mbar or less. The dynamics of the necessary for this proof

(electrical) FT ion trap is above the efficiency of

conventional residual gas mass spectrometers.

In a further variant, the excitation degree and / or the. Between a first excitation frequency and a second excitation frequency

Phase angle of the IFT excitation varies, with both the first

Excitation frequency and the second excitation frequency by not more than 10%, preferably not more than 5%, in particular by not more than 1% differ from a predetermined excitation frequency. The degree of excitation refers to the amplitude of the IFT excitation relative to a given maximum amplitude and is typically expressed as a percentage.

When detecting ions in the (electric) FT ion trap is

provided that the high-frequency alternating field E acts solely on the ions. This is practically true as long as there is only a limited amount of charge carriers of the same sign in the FT ion trap. The total number of charge carriers is referred to as "space charge" or "ion cloud". The above the Laplace equation writable and from the high-frequency

Alternating field E derived potential φ (E = - grad ^)) is determined by the

Space charge influenced. This influence of the storage potential by the space charge in a given volume within the FT ion trap is greater, the greater the space charge density p in this volume and the weaker the resulting from the high-frequency alternating field mean restoring force in the associated sub-volume. It follows from the Laplace equation (1) for the high-frequency alternating field: div (grad (c |))) = Δφ = -ρ / ε ₀ (1), where ε _{0 is} the dielectric constant in a vacuum and φ is that associated with the alternating field E. describe high-frequency alternating potential (see above).

In particular, in the excitation of different types of ions that have close to each other ion resonance frequencies can result from the synchronous oscillation of the ion packets that partially large space charge densities arise in "space charge-prone" areas in the FT ion trap, causing whole ion packets strongly disturbed in their resonance frequencies This can lead to a large scatter in the measured ion resonance frequencies, which results in a significantly lower mass resolution.

It therefore proves to be advantageous if the charge carrier packages or

ions (populations) with adjacent ion resonance frequencies do not simultaneously fly through the same trajectory (orbit). By means of suitable orbital (orbital derived), phase-shifted IFT excitations of the ions (e.g., slight orbital separation of the ion packets by appropriate SWIFT excitation), one can obtain predominantly one

sufficiently low space charge density during the measurement or detection results. Alternatively or additionally, a change in the degree of excitation or the amplitude of the SWIFT excitation, which can also cause the interaction between neighboring ion populations to be greatly reduced or to allow them to be different

Move motion paths or orbits.

The variation of the phase position and / or the excitation degree of the SWIFT excitation takes place here in a coherent interval between a first excitation _frequency f _i0 n, i and a second excitation _frequency f _i0 n, 2 (fion.i <fion, 2), both comparatively close to each other, ie both the first and the second (ion) excitation frequency deviate from a predetermined excitation _frequency f _i0 n, a by not more than 10% or 5%, in particular by not more than 1% down or up , that is to say: f _ion , i ^ 0.9 f _ion , a and f _ion , 2 ^ 1, 1 fion.a or correspondingly f _ion , i ^ 0.95 f _ion , a and f _ion , 2 ^ 1, 05 f _ion , a or f _ion , i ^ 0.99 f _ion , a and f _ion , 2 ^ 1, 01 f _ion , a ■ The given

Excitation _frequency f _i0 n, a typically corresponds to the mass-to-charge ratio of the ions of interest or ion population. By the above-described variation of the phase position and / or the

Excitation levels can exist within this interval

ion populations are brought to different orbits, thereby increasing the mass resolution in the study of the species of interest

increased ion population (s).

In a further variant, the phase angle and / or the phase difference between the first excitation frequency and the second excitation frequency vary

Excitation degree as a function of the excitation frequency stepwise. The frequency width of the stages can be chosen to be equal in particular, i. the interval between the first and the second excitation frequency is divided into equally large subintervals or stages, between which the

Phase position and / or the degree of excitation can be changed. It goes without saying, however, that the frequency width of the subintervals does not necessarily have to be chosen to be the same. Ideally, the transition occurs in each case a change in the excitation degree and / or the phase position between in each case two adjacent subintervals, in order to steer the ions assigned to the adjacent subintervals to different orbits.

In a development, between the first excitation frequency and the second excitation frequency, the excitation degree and / or the phase position increase either stepwise or stepwise as a function of the excitation frequency. In this way, the ions assigned to adjacent subintervals can be distributed to different orbits in a particularly simple manner. The increase or the decrease of the excitation degree and / or the phase position between adjacent stages or sub-intervals can each be the same size, but it is also possible for the increase or the

Decrease in the degree of excitation between adjacent sub-intervals to choose or vary differently.

The excitation of the ion packets or the ions with an adjacent mass-to-charge ratio is effected by briefly acting on the ion packets by means of a short-time excitation pulse in the corresponding ion resonance band. Will the ions be in different

excited ion resonant bands with different amplitudes and phases, it is possible to either greatly minimize the interaction between the ion packets, as shown above, or intentionally amplify. Also, such intentional enhancement of interactions between the ion populations may prove beneficial under certain circumstances. In any case, the above-described SWIFT excitation can adaptively affect the interactions between ion populations.

In a further variant, the same ions in the FT ion trap are selectively excited several times (possibly broadband) by IFT excitations, with detection of the ions being carried out after a respective IFT excitation. In the Detection after each IFT excitation, the number of excited ions (or the partial pressure of the excited gas component) is determined. By averaging over the number of ions determined in the detections, the signal-to-noise ratio (SNR) of the excited ions of interest can be significantly increased without the remaining ions being affected by the excitation. Particularly at very low partial pressures (eg 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 ). It is assumed here that the ions can be stably stored over the entire measurement period and retain their characteristic chemical properties.

In a further development, a time interval which is greater than a mean free flight time of the ions in the FT ion trap lies between two IFT excitations which follow one another directly in time. The mean free flight time t _M is linked to the mean free path L _M by the mean velocity v _M , where t _M = L _M / v _M. The mean free flight time t _M is typically greater than about one millisecond (> 1 ms). The IFT excitations, especially the SWIFT excitations, are not repeated until the ions have traversed a multiple of the mean free flight time, eg, more than 3 xt _M , more than 5 xt _M, or more than 10 xt _M.

After ionization, collisions between the neutral gas parts and the ionized gas particles can lead to chemical reactions, e.g.

Charge transfer or "protonation" that alter the original ion population, and in many applications it is of interest to know the chemical intermediates of such a process. In one variant, therefore, when the ions are detected, a mass spectrometric examination of an ion signal takes place only in a time-shiftable measuring time interval. After the IFT excitation, in particular after the SWIFT excitation, the time-varying mass spectrum can be calculated and displayed by means of a suitably chosen displaceable, short measurement time interval, which is also referred to below as the FFT time window. The temporally displaceable measuring time interval may have a duration in the order of, for example, several milliseconds, preferably 10 ms or less, particularly preferably 5 ms or less. Stepless or discrete shifting of the FFT time window results in a chronological representation of the chemical behavior of the ion population embedded in the gas matrix or in the gas to be investigated.

For analysis of complex analytes consisting of a variety of

consist of different molecules whose molecular masses are partly far, partly close together, a large mass range and a very high mass resolution is needed. In order to meet this requirement, different mass analysis methods are usually combined, eg two mass analysis methods (so-called MS / MS) or, more generally, mass analysis methods (so-called MS ⁿ ). Typically, quadrupole mass spectrometers or conventional ion traps are used for filtering or fragmentation in the

and the selected mass range is then finely analyzed with another high resolution mass analyzer (e.g., Fourier transform based, location or transit time based methods) to override the analyzers (see

Space charge problem) and to the subsequent analysis

simplify.

From the procedure described above, it can be seen that the underlying FT ion trap is used both as a fragmenting or filtering device, as well as a high-resolution mass analyzer is very well suited. It proves to be advantageous, by IFT or SWIFT excitation only the

quickly switching over the mass ranges of interest and significantly increasing the mass resolution with the measures described above.

As described above, FT-mass spectrometers are used to measure the mass-to-charge ratios of ions by Fourier transformation on the basis of their characteristic oscillations or ion resonance frequencies. The image charge currents occurring here are generally from a few fA (10 ^"15 A). The ion resonance frequencies are typically on the order of kHz to MHz, for example, of about 1 kHz to 200 kHz, and therefore of parasitic interference frequencies be superimposed, which can generate so-called "phantom masses". Although the systematic, that is, the interference frequencies known to the measuring system can be eliminated by means of suitable measures, but parasitic, usually unknown to the measuring system, external interference frequencies can lead to a misinterpretation of the mass spectra.

Another aspect relates to a method of the type mentioned, 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 altering the ionic resonance frequencies of the ions in the FT ion trap, re-exciting the ions in the FT ion trap and receiving a second frequency spectrum of the ions, as well as detecting spurious frequencies in the FT ion trap by comparing the first and second recorded frequency spectrum. The changing of the phase position and / or the oscillation amplitude of the ions in the FT ion trap can be effected in particular during the renewed excitation of the ions in the FT ion trap by an IFT excitation, in particular by a SWIFT excitation. By means of the method described herein, spurious frequencies can be uniquely identified and optionally anninned from the regions of interest of the ionic resonance frequencies of interest. It is exploited here that only the ions stored in the FT ion trap react to the IFT excitation or to the change in the ion resonance frequencies. The rest in the

Frequency spectrum existing frequency components that can not be influenced in this way, can be identified as interference frequencies. The detected as interference frequencies mass-to-charge ratios can from the mass spectrum, which the

Frequency spectrum corresponds, be filtered out or eliminated.

In the case of IFT excitation, the phase position and / or the oscillation amplitude of the ions of interest can be influenced virtually as desired, care being taken that the ions are not removed from the FT ion trap in this case. For example, the amplitude and / or the phase position of the ions stored in the FT ion trap can be changed in such a way that the height of the associated lines in the mass spectrum or in the frequency spectrum changes, while the lines of the interference frequencies do not change in such an action ,

In a development, changing the ion resonance frequencies comprises 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 by an excitation signal (stimulus) to oscillations whose resonance frequencies are dependent on the ion masses and the charges of the ions, the ionic resonance frequencies typically in the frequency domain in the order of magnitude of kHz to MHz, for example from about 1 kHz up to 200 kHz. For a given mass-to-charge ratio, a 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).

For example, by increasing the high-frequency storage voltage VRF, the ion resonance frequencies can be increased and, conversely, by reducing the high-frequency storage voltage V _RF, the ion resonance frequencies can be reduced. Inversely, the ion resonance frequencies behave with a variation of the memory frequency f _RF .

In a further variant, the method comprises determining a start phase position of a trajectory of ions of a predetermined ion resonance frequency (directly) after an IFT excitation on the basis of a time-dependent ion signal recorded during the detection. To determine the start phase position of the orbital motion of ions or of an ion species at the given ion resonance frequency, the time-dependent ion signal Ui _0n (t) can be recorded in a sufficiently long measurement time window T ₀ after the excitation to avoid a discretization error, and the Ion resonance frequencies f _i0 n of the ions can be obtained by Fourier analysis, where: T ₀ »1 / fi _0n or T ₀ = N ₀ x 1 / fi _0n and N ₀ integer» 1. The measuring time window T ₀ is typically less than about 1/10 or 1/50 of the total measuring or detection duration, so that the amplitude _on the envelope of the (oscillating) ion signal Ui _0n (t) in the measuring time _window T ₀ approximately remains constant.

The starting phase position ao of the path movement at the beginning of the measurement or the measuring time interval can in this case be determined according to the following formula: From - cos (a ₀ ) = \ -, - L ° u _ion (t) * cos (2nf _ion * t + φ) dt \ follows: a ₀ = cos ^-1 (2 * - ° u _ion t * cos (2nf _ion * t + φ) dt), (2)

'0 * ^u ion ^u where φ represents a start phase of the IFT excitation of the ions at the ion resonance frequency fion and where ΰ, οη is the maximum of the absolute value of the amplitude or the envelope of the (oscillating) ion signal Ui _0n (t) at the beginning of the measurement (t = 0). The expression given in (2) in square brackets has a maximum amount only if the start phase φ of the IFT excitation with the start phase position ao of the

Orbit movement up to k ^* (φ = αο + k * 7t, k integer). The value of the expression in square brackets in this case corresponds to Vi cos (ao). For a start phase φ = 0 ° of the IFT excitation, the value of the expression in square brackets is logically about + Vi and for a start phase φ = 180 ° of the IFT excitation, the value of the expression in the square brackets is about - Vi. As described above, the starting phase φ at a

mass-dependent phase-shifted orbital IFT excitation can be varied as a function of the ion resonance frequency. In this way, ion packets in the mass spectrum can be correspondingly identified differently. If the start phase φ of the IFT excitation is not known, this and thus the start phase position ao of the path movement can be determined by maximizing the amount of the expression given in square brackets.

In a further development, the method additionally comprises: determining a charge polarity of the ions based on the starting phase position of the trajectory of the ions after the IFT excitation. In an FT electric ion trap, both positively and negatively charged ion species can be trapped simultaneously. By evaluating the start phase position ao the ion motion or the trajectory of the ions after the IFT excitation (for example, a SWIFT excitation), more precisely by evaluating the expression Vi cos (ao), the polarity of the ions can be detected the ions through stimulates uniform broadband excitation, move immediately after the excitation, for example, the positively charged ions to one of the electrodes, while the negatively charged ions move away from this. All ions are detected regardless of their polarity after excitation. For each ion resonance frequency f _{i0n of} an associated ion species, the following formula is used

Polarity = sign [- / ₀ ^T ° _ion (t) * cos (2nf _ion * t + <p) dt] (3)

^L 'o * ^u ion J applied directly from formula (2), the polarity (+ or -) of the associated ion species can be determined.

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 done by applying different excitation transients to the measuring electrodes as a function of the charge polarity. It will be appreciated that the procedure described above is not limited to the electrode geometry of the underlying FT ion trap, i. This method can be applied to measuring electrodes with different electrode geometries, for example, in measuring electrodes in the form of measuring tips in the

End caps or shape of toroidal measuring caps of a toroidal ion trap, etc.

Another aspect of the invention relates to a mass spectrometer of the type mentioned, in which the excitation device is formed during storage and / or during the excitation of ions at least one selective, dependent on the mass-to-charge ratio of the ions IFT excitation, in particular a SWIFT suggestion to generate. The mass spectrometer described here is particularly suitable for carrying out the methods described above. It has proved to be advantageous if the FT ion trap is designed as an electrical FT ion trap, ie the mass spectrometer is an electrical ion resonance mass analyzer in which the ions are dynamically stored by a high-frequency alternating field.

In a further development, the mass spectrometer is designed to ionize a gas to be investigated in the FT ion trap, wherein the

Evaluation device is preferably designed to generate during the ionization (and during storage) an IFT excitation, in particular a SWIFT excitation. The mass spectrometer may for this purpose comprise a device for supplying electrons and / or an ionizing gas into the FT ion trap. As described above in connection with the method, a selection of ions which are to be stored in the FT ion trap (accumulating) can already be made during the ionization in this way, as a result of which the dynamics or the sensitivity of the FT ion trap can be increased.

In a further embodiment, the excitation device is formed between a first excitation frequency and a second excitation frequency

Excitation frequency to vary the excitation level (or the amplitude) and / or the phase angle of the IFT excitation, preferably both the first excitation frequency and the second excitation frequency by not more than 10%, more preferably not more than 5%, in particular not more than 1%, deviate from a given excitation frequency. As described above, in this embodiment, the

Mass resolution can be increased by targeted ions or ion populations with closely spaced mass-to-charge ratios are suitably orbital excited so that they do not have the same trajectories

run through. In a development of this embodiment, the excitation device is formed between the first excitation frequency and the second

Excitation frequency, the phase angle and / or the degree of excitation in

Dependent on the excitation frequency to vary gradually, preferably between the first excitation frequency and the second

Excitation frequency of the excitation degree and / or the phase position in

Depending on the excitation frequency either increase gradually or gradually decrease. By gradually, in particular steadily increasing or steadily decreasing change in the excitation degree or the phase position, a sufficient spacing of the orbits, a low local space charge density during the measurement and thus a higher mass resolution can be achieved.

In a further embodiment, the mass spectrometer comprises a detector, which is designed to determine a phase position of a trajectory of ions with a predetermined ion resonance frequency based on a time-dependent ion signal recorded during the detection of the ions after IFT excitation, wherein the detector is preferably formed to determine a charge polarity of the detected ions based on the phase position. As described above, by evaluating the phase position of the ion motion after the IFT excitation, the

Charge polarity of the ions are detected.

Another aspect of the invention relates to a mass spectrometer of the type mentioned, in particular as described above, in which the

Excitation device is formed, a phase angle and / or a

Vibration amplitude of the ions in the FT ion trap and / or ionic resonance frequencies of the ions in the FT ion trap, wherein the mass spectrometer additionally comprises a detector which is formed, based on a comparison of a first, before changing the phase angle and / or the vibration amplitude of the ions in the FT ion trap and / or changing the ionic resonance frequencies of the ions in the FT ion trap recorded frequency spectrum with a second, after changing the phase angle and / or the amplitude of vibration of the ions in the FT ion trap and / or changing the ion resonance frequencies of Ions detected in the FT ion trap frequency spectrum to detect interference frequencies in the FT ion trap. As has been described above, interference frequencies in the recorded spectra can be recognized by the fact that they do not react or possibly only slightly to the changing of the ion resonance frequencies or to the changing of the phase position and / or the oscillation amplitude of the ions.

In a further embodiment, the FT ion trap is designed as an FT-ICR ion trap or as an Orbitrap. Mass spectrometry by means of a Fourier transformation can in principle be carried out with different types of FT ion traps for carrying out rapid measurements, the combination with the so-called ion cyclotron resonance trap (FT-ICR ion trap) being the most common. In the FT-ICR trap, which may be formed as a magnetic or electrical ICR trap is by means of

Cyclotron resonance excitation mass spectrometry operated. The so-called orbitrap has a central, spindle-shaped electrode around which the ions are held by the electrical attraction on circular paths, whereby a decentral injection of the ions creates a vibration along the axis of the central electrode, which generates signals in the detector plates, which can be detected similar to the FT-ICR trap (by FT). It is understood that the mass spectrometer can also be operated in combination with other types of FT ion traps, i. with ion traps, in which an induced by the stored ions on measuring electrodes induction current is detected and amplified time-dependent.

Other features and advantages of the invention will become apparent from the

following description of embodiments of the invention, based the figures of the drawing, the invention essential details show, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

drawing

Embodiments are illustrated in the schematic drawing and will be explained in the following description. It shows

1 is a schematic representation of a mass spectrometer with an electric FT-ICR ion trap,

2 is a schematic representation of one of the ionic

Resonant frequency-dependent excitation in a SWIFT excitation,

Fig. 3 is a schematic representation of a timing at a

Measurement for receiving a mass spectrum using the mass spectrometer of Fig. 1,

Fig. 4 are schematic representations of three mass spectra of a

Gas with a main gas component,

Fig. 5a, b are schematic representations of the frequency spectrum and the

Time course of a (broadband) selective SWIFT excitation,

6a-c is a schematic representation of the frequency spectrum with a uniform SWIFT excitation or with a SWIFT frequency-dependent varying in the excitation level and in the phase relationship Excitation (FIG. 6a) and the associated trajectories of the excited ions (FIGS. 6b, c),

Fig. 7 is a schematic representation of the time course of a multiple

(broadband) selective SWIFT excitation and subsequent detection,

8 is a schematic representation of a detected ion signal with a temporally displaceable measuring time interval,

Fig. 9 is a schematic representation of two at different

Storage voltages recorded frequency spectra, as well

Fig. 10a-d are schematic representations of the frequency spectra of in the

FT-ICR ion trap stored positively charged ions (Figure 10a), negatively charged ions (Figure 10b) as well as all ions stored in the FT-ICR ion trap (Figures 10c and 10d).

In the following description of the drawings are for the same or

functionally identical components identical reference numerals used.

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 on which a high-frequency AC voltage V _{RF is} applied, which may have, for example, a frequency f _RF in the order of kHz to MHz, for example, 1 MHz, and an amplitude V _RF of several hundred volts. In the FT-ICR trap 2, the high-frequency alternating voltage V _RF generates a high-frequency alternating field in which ions 4a, 4b of a gas 4 to be investigated are stored dynamically. From the high-frequency alternating field (E-field) results in a middle

Restoring force, the stronger the effect on the ions 4a, 4b, the further the ions 4a, 4b are removed from the center or 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 by an excitation signal S1, S2 (stimulus) to oscillations whose frequency f _i0 n is dependent on the ion mass and the ion charge and typically in the frequency range in order of magnitude from kHz to MHz, for example from about 1 kHz up to 200 kHz. The respective

Excitation signal S1, S2 is generated by a second and third excitation unit 5b, 5c, which forms an excitation device 5 together with a first excitation unit 5a, which serves to generate the high-frequency storage voltage V _RF with the predetermined storage frequency f _RF . The

Excitation device 5 also has a synchronization device 5d, which synchronizes the three excitation units 5a-c in time. Everyone

Excitation unit 5a-c is an amplifier downstream, which are also part of the exciter 5.

For a non-reactive, non-destructive detection (ie, the ions 4a, 4b are still present after detection), the vibration signals of the ions 4a, 4b in the form of induced mirror charges at the measuring electrodes 6a, 6b tapped, as for example in the initially cited DE 10 2013 208 959 A, which is incorporated herein by reference in its entirety. As described in detail there, the respective measuring electrodes 6a, 6b are connected via a respective filter 7a, 7b to a respective low-noise charge amplifier 8a, 8b. On the one hand, the charge amplifiers 8a, 8b 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 memory frequency f _RF at virtual ground potential. From the signals supplied by the charge amplifiers 8a, 8b, an ion signal Ui _0n (t) is generated by subtraction, whose time course is shown in Fig. 1 bottom right. The ion signal Ui _0n (t) is a detector. 9 supplied in the example shown, an analog-to-digital converter 9a and a spectrometer 9b for fast Fourier analysis (FFT) to a

To produce mass spectrum, which is shown in Fig. 1 top right. In this case, the detector 9 or the spectrometer 9b initially generates a frequency spectrum of the characteristic ion resonance frequencies _{f.sub.o.sub.n of} the ions 4a, 4b stored in the FT-ICR ion trap 2, which due to the

Dependence of the ion resonance frequencies f _i0 n on the mass and charge of the respective ions 4a, 4b is converted into a mass spectrum. The mass spectrum shows the number of particles or charges detected as a function of the mass-to-charge ratio m / z.

The electrical FT-ICR trap 2 thus enables a direct detection or the direct recording of a mass spectrum, whereby a rapid gas analysis is made possible. However, the rapid recording of a mass spectrum using Fourier spectrometry can be done not only in the above-described FT-ICR electric trap 10 but also in modifications of the trap type shown in Fig. 1, for example, in a so-called orbitrap.

As described above, all ions 4a, 4b in the FT-ICR ion trap 2 have an ion resonance frequency fion proportional to their mass-to-charge ratio (m / z), with which the stored ions 4a, 4b in the FT ICR ion trap 2 vibrate. If the ions 4a, 4b are excited with their respective ion resonance frequency fion, they can either be selectively excited in this way or thrown out of the FT-ICR ion trap 2 by a resonance increase. It can thus ions 4a, 4b with

certain mass-to-charge ratios m / z are selectively excited or their storage in the FT-ICR ion trap 2 prevented / suppressed.

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 fion lies within the respective window, are targeted can be stimulated 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, wherein the ion resonance frequencies f _i0 n are related to the memory frequency f _RF . The desired selective excitation spectrum depends on the ion resonance frequencies f _i0 n and thus 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 time of the SWIFT start to obtain the desired excitation spectrum shown in FIG.

For the SWIFT excitation 10, the measuring electrodes 6a, 6b can be used. By means of the SWIFT excitation 10, the ions 4a, 4b can be deflected in the direction of the measuring electrodes 6a, 6b such that specific ions occur both during the ion generation and ion storage and immediately before the detection of the ion signals Ui _0n (t) 4a, 4b, on the one hand either stored or not stored, on the other hand practically continuously excited or not excited at all.

The SWIFT excitation therefore provides several possibilities for realizing new features of the mass spectrometer 1. The 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 flight time or the mean free path of the molecules or ions 4a, 4b of interest. It has proven to be advantageous to use optimized SWIFT algorithms, as described for example 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, pp. 1288-1294 or shown in US 4,945,234, both by reference to the content of this application. On the one hand, optimized SWIFT algorithms generate the shortest possible SWIFT signal output and, on the other hand, prevent overloading of the low-noise charge amplifiers 8a, 8b connected to the measuring electrodes 6a, 6b.

A SWIFT excitation 10 may occur just prior to the detection of the ions 4a, 4b, i. prior to the recording of the (normalized) ion signal, as shown in Fig. 3, in which only the envelope of the (normalized) ion signal Uion (t) is shown. However, a SWIFT excitation 10 can also already take place during the generation and storage of the ions 4a, 4b, as is also indicated in the time sequence of FIG. In this case, the SWIFT excitation 10 is for selecting ions 4a, 4b to be stored in the FT-ICR ion trap 2.

For the generation of the ions 4a, 4b by the ionization of the gas 4, there are basically two possibilities: Either the ions 4a, 4b are generated within the FT-ICR ion trap 2 or the gas 4 becomes the FT-ICR ion trap 2 in charge-neutral form Such ionization in the FT-ICR ion trap 2 can be carried out, for example, in the manner described in the cited WO 2015/003819 A1, which with respect to this aspect is described by reference to FIG Content of this application is made.

If the ionization occurs in the FT-ICR ion trap 2, a continuous SWIFT excitation can already take place during the ionization of the gas 4 (see Fig. 3), whereby undesired gas components are excessively excited; as a result, the charge carriers of the undesired gas components at the surrounding electrodes 3, 6a, 6b are lost, and only the carriers of interest 4a, 4b are accumulatively stored in the FT-ICR ion trap 2 for measurement; As a result, the ionization time of the ions 4a, 4b to be detected ensures that the FT ICR ion trap 2 is not flooded by the unwanted charge carriers. 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 into the FT-ICR ion trap 2.

Such a selection during or before storage is favorable, 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 a high total pressure is required. An example of one

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 in Fig. 4 is shown at the top right, it may be at the unwanted gas components, which should not be stored in the FT-ICR ion trap 2, to a main gas component 1 1 of

acting gas 2 act. 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% by volume, in many applications more than 90% by volume of the gas 2 to be investigated.

In the example shown in FIG. 4, the main gas component 1 1 has two ion populations with different mass-to-charge ratio (m / z) i or (m / z) ₂ , whose volume fraction is in each case more than 30% by volume. of the gas to be examined 2 is such that the volume fraction of

Main gas component 1 1 in total at more than 50% by volume of the

examining gas 2 is located. That of the mass spectrometer 1

recorded mass spectrum of the gas 4 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 actually interesting traces of gas whose mass-to-charge ratio lies outside of an interval I illustrated in FIG. to- Charge ratios (m / z) i and (m / z) _{2 of} the main gas component 1 1 are included.

Due to the broadband-selective SWIFT excitation 10, a selective filtering of those mass-to-charge ratios m / z that are within the interval I can take place or targeted filtering of the first mass-to-charge ratio (m / z) i and of the second mass-to-charge ratio (m / z) _{2 of} the main gas component 1 1. In this way, only those ions 4a, 4b are stored in the FT-ICR ion trap 2, whose mass-to-charge ratios m / z are outside of the interval I, so that they can be detected with high accuracy, as shown in FIG

Mass spectrum in Fig. 4 can be seen in the upper right.

The ratio of the partial pressures of the gas of interest ingredients to the total pressure, for example, in the order of ppm by volume (10 ^"6 ppmv) to PPTV (10 ^'12) then. In this case, the detection limit for individual gas components can be up to 10 ^"16 mbar in order to achieve a dynamic D of more than eight orders of magnitude (D> 10 ⁸ ) In addition, the sensitivity (absolute concentration) of the ions ^{4 a, 4} b increases the FT-ICR ion trap 2 and, accordingly, the signal-to-noise ratio SNR with the accumulation time during storage.

In an electric FT-ICR ion trap 2, the high frequency alternating field (E field) is affected by the space charge, more specifically by the space charge density, in the FT-ICR ion trap 2, i. There is a retroactive effect of the charges or ions 4a, 4b present in the FT-ICR ion trap 2 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 resulting from the high-frequency alternating field E mean restoring force in the associated sub-volume. In particular, in the excitation of ions 4a, 4b with different but closely related ion resonance frequencies or mass-to-charge ratios, large space charge densities can sometimes arise in regions of the FT-ICR ion trap 2 which are particularly susceptible to the occurrence of large space charge densities , By the big one

Space charge density, whole ion packets can be greatly disturbed in their ionic resonance frequencies, resulting in a significant reduction of the measurement resolution.

The local space charge in the FT-ICR ion trap can be reduced if ions 4a, 4b do not simultaneously travel through the same _trajectory (orbit) with closely adjacent ion resonant frequencies f _i0 n. This can be achieved by varying the frequency between a first ion excitation frequency f _i0 ni and a second _ion excitation frequency f _ion 2 of the excitation degree A of the SWIFT excitation 10

Depending on the mass or the mass-to-charge ratio m / z of the ions 4a, 4b is varied, as shown in Fig. 5a. FIG. 5b shows the associated time-dependent excitation signal (S1 or S2) of the SWIFT excitation.

In the example shown in FIGS. 5a, b, the excitation degree A of the SWIFT excitation varies stepwise as a function of the ion excitation frequency fion, the excitation degree A varying over the entire interval between the first ion excitation _frequency f _i0 ni and the second ion excitation _frequency f _i0 ni. Excitation _frequency f _i0 n2 by no more than about 20% of the maximum

Excitation degree A (ie the maximum amplitude of the SWIFT excitation 10) varies. In the example shown, the excitation level A increases gradually from the first ion excitation _frequency f _i0 ni to the second ion excitation frequency f _ion 2, wherein the step height between adjacent stages of the excitation level A is the same. It is understood that the excitation degree A is from the first ion excitation _frequency f _i0 ni to the second, larger ionic _frequency. Alternatively, excitation frequency f _ion 2 can also decrease. Also, the step height, ie, the difference between the excitation levels of adjacent stages of the SWIFT excitation 10 is not necessarily constant, but may vary from stage to stage. A continuous stepless variation of the

Degree of excitation A between the first ion excitation _frequency f _i0 ni and the second ion excitation frequency f _ion 2 is basically also conceivable.

In addition or as an alternative to the variation of the excitation degree 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 angle φ is also changed stepwise, in each case by a value of 45 °, wherein the phase position φ of the SWIFT excitation 10 in the example shown in Fig. 6a gradually increases with increasing ion excitation _frequencies f _i0 n. It is understood that a stepwise decrease of 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 of the phase angle φ is defined only modulo 360 °, ie in the example shown, a phase angle φ of 0 ° is reached again after eight stages. The phase angle φ corresponds to a time shift or delay of the SWIFT excitation, wherein the phase angle φ is related to a predetermined ion excitation frequency f _ion , a.

The predefined ion excitation _frequency f _i0 n, a can _correspond , for example, to the ion resonance frequency f _i0 n or the mass-to-charge ratio m / z of an ion population to be analyzed. However, the predetermined ion excitation frequency fion.a can also lie in an interval between two ion excitation _frequencies f _i0 ni, fion2 or two associated ion resonance frequencies whose mass-to-charge ratios m / z are dense lie together. For example, the first (smaller) ion excitation _frequency f _i0 ni can deviate from the predetermined ion excitation frequency fion.a by no more than 10%, preferably not more than 5%, in particular not more than 1%. The same applies to the second, larger ion excitation frequency f _i0 n2. In the example shown in FIG. 6 a, the ratio _{f 0n} i fion, a is approximately 0.999 (deviation: 0.1%), while FIG

Ratio fion.a is about 1.009 (deviation: 0.9%), ie, both ion excitation _frequencies f _i0 ni, fion2 are within the value range of less than 1% deviation described above.

Fig. 6b shows the trajectory B of the ions 4a, 4b in the FT-ICR ion trap 2 with a uniform SWIFT excitation, i. a SWIFT An with constant exciting degree A (shown in dashed lines in Fig. 6a), which also takes place synchronously 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 denotes the maximum deflection. The value T denotes the period of the oscillation of the ions 4a, 4b with the predetermined ion excitation _frequency f _i0 n, a- Space charge density arises.

6c shows the trajectories B of the ions 4a, 4b in the case of the orbital SWIFT excitation 10 shown in FIG. 6a with a different one

Degree of excitation A, in which additionally the phase position φ was varied as shown in FIG. 6a, using the example of ten ion packets or

ion populations with adjacent ion resonance frequencies f _i0 n or with adjacent mass-to-charge _ratios m / z. It can be clearly seen in FIG. 6c 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, thereby increasing the mass resolution. The ions 4a, 4b typically undergo the (periodic) trajectories B more than about 100 times - 1000 times before the measurement or detection takes place. In this way, only a very low pressure in the FT-ICR ion trap 2 is required to perform the measurement or detection.

FIG. 7 shows a further application of a SWIFT excitation 10, in which the same ions 4a, 4b in the FT-ICR ion trap 2 are excited in succession by two (broadband) selective SWIFT excitations 10 and in each case

be detected below. Upon detection after a respective SWIFT excitation 10, the number of excited ions 4a, 4b (or the

Partial pressure of the excited gas component) determined. By a

Averaging over the number of ions 4a, 4b determined during the detections can increase the signal-to-noise ratio (SNR) of the

excited ions 4a, 4b are significantly increased without the remaining ions being affected by the excitation.

The prerequisite for such a multiple detection is that between two temporally immediately following IFT excitations 10 there is a time interval τ which is greater than an average free time of flight t _{M of} the ions 4a, 4b in the FT-ICR ion trap 2, ie it holds that τ> t _M , where typically t _M is more than about one millisecond (> 1 ms). The SWIFT suggestions are not repeated until the ions 4a, 4b have traveled a multiple of the mean free time of flight t _M , for example more than 3 xt _M , more than 5 xt _M or more than 10 xt _M.

8 shows a time-dependent ion signal Ui _0n (t) after a SWIFT excitation 10 as well as a dashed shown, time-displaceable measuring time interval 12 (FFT time window), which has a time duration ti in the order of, for example, several milliseconds, preferably 10 ms or less, more preferably 5 ms or less. By a stepless or discrete shifting of the measuring time interval 12, a time-resolved representation of the chemical behavior of the in the gas matrix or in the to be examined gas embedded ion population. The

Mass spectrometric analysis in this case only on the basis of the values of the ion signal Ui _0n (t) during the measuring time _interval 12

made, i. only in the measuring time interval 12 is an evaluation. This is particularly favorable if, during the detection of the ions 4a, 4b, chemical reactions such as e.g. Charge transfer or "protonation" occur, which replaces the originally present ion population during the

Change detection period. By the evaluation only in the

Measuring time interval 12, for example, a reaction such as the transition from H ₂ O ⁺ to H ₃ O ⁺ can be observed practically in real time, ie it can also intermediates of chemical reactions are detected.

In particular, it can be checked in this way 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. Optionally, the selection or selection process of the ions 4a, 4b to be accumulated in the FT-ICR ion trap 2 may be suitably adjusted.

When mass spectra are recorded by means of the mass spectrometer 1, it is possible for parasitic interference frequencies f _{R to} occur which lead to lines in the recorded mass spectrum which are not generated by the ions 4 a, 4 b stored in the FT-ICR ion trap 2. such

Interference frequencies f _R can lead to a misinterpretation of the mass spectrum.

In order to identify and possibly eliminate interference frequencies f _R 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 to receive a first frequency spectrum 13a of the ion resonance frequencies fion (shown in phantom in FIG. 9). In In a second step, the ion resonance frequencies f _i0 n of the ions 4a, 4b in the FT-ICR ion trap 2 are changed and in a third step, the ions 4a, 4b are excited again by means of a SWIFT excitation 10 and

subsequently detected, wherein a second frequency spectrum 13b

is recorded, which is shown in Fig. 9 in solid lines.

When comparing the two frequency spectra 13a, 13b shown in FIG. 9, it can be clearly seen that the first and second frequency spectrum 13a, 13b have lines whose frequencies change as the ion resonance frequencies fion in the FT-ICR system change. Have practically not shifted ion trap 2, so that their position in both frequency spectra 13a, 13b practically matches. These lines can be identified or determined as interference frequencies f _R. Those lines in the two frequency spectra 13a, 13b, which are due to the change of the ion resonance frequencies f _i0 n

can be systematically shifted, however, the ions 4a, 4b stored in the FT-ICR ion trap 2 can be assigned, ie they are lines at "true" ion resonance frequencies f _i0 n.

As indicated in FIG. 9, in order to change the ion resonance frequencies fion, the storage voltage V _{RF of} the FT-ICR ion trap 2 has been changed from a first value Vrfl to a second value Vrf2. Since, for a given mass-to-charge ratio m / z, the ion resonance frequency fion is directly proportional to the storage voltage V _RF , the

Changing the memory voltage V _RF, the ion resonance frequencies are shifted f _ion . Since for a given mass-to-charge ratio m / z, the ion resonance frequency f _{ion is} inversely proportional to the square of the

Memory frequency f _RF is a change in ionic resonance frequencies fion alternatively or additionally by a

Change the memory frequency f _RF done. Alternatively or in addition to a change in the ion resonance frequencies fion, in the second step, a change in the phase position φ and / or the oscillation amplitude z / z _{0 of} the trajectories B of the ions 4a, 4b in the FT-ICR ion trap 2, for example by means of a mass-dependent SWIFT excitation 10 take place, as shown by way of example in Fig. 6a-c. With such a SWIFT excitation, the trajectories of the ions 4a, 4b change, which is manifested, for example, by a change in the heights of the lines of the second frequency spectrum 13b in comparison to the first frequency spectrum 13a. By contrast, the SWIFT excitation 10 has practically no influence on the interference frequencies f _R , so that the interference frequencies f _{R can} also be detected or identified in this variant by comparing the two frequency spectra 13 a, 13 b.

Another application of a SWIFT excitation 10 is to determine 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 4 a or the negatively charged ions 4 b in the FT-ICR ion trap 2, a phase position a o of the path movement B at the beginning of the detection, ie immediately after the SWIFT excitation 10, at a predetermined ion resonance frequency f _i0 n according to the above

determined formula (2), which is reproduced below:

1 Γ 1 T 1

From - cos (a ₀ ) = - / ^O _ion (t) * cos (27r / _ion * t + ^) dt follows:

Δ LJ ₀ * ^u ion ^{υ J}

a ₀ = cos ^_1 (2 * - / ° _ion (t) * cos (2nf _ion * t + φ) dt) (2)

'o * ^u ion ^u where φ represents a start phase of the SWIFT excitation 10 of the ions 4a, 4b at the ion resonance frequency f _i0 n, u _i0 n the maximum of the absolute value of the ion signal Ui _0n (t) at the beginning of the measurement and where: T ₀ »1 / fion or T ₀ = N ₀ × 1 / fion and N ₀ integer» 1. The value of the amplitude or the envelope of the oscillating ion signal ₀ typically varies during the measurement time interval T slightly, ie, the duration of the measurement interval T ₀ is significantly less than the mean free flight time of the ions.

In the FT-ICR electric ion trap 2, both positively charged ions 4a and negatively charged ions 4b can be simultaneously captured. All ions 4a, 4b are detected independently of their charge polarity after the SWIFT excitation 10, which may result in a frequency spectrum, for example, which is shown in FIG. 10c. The frequency spectrum of all the ions 4a, 4b stored in the FT-ICR ion trap shown in Fig. 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 negative charged ions 4b, which is shown in Fig. 10b.

By evaluating the phase position ao the ion movement or the trajectory B of the ions 4a, 4b after the SWIFT excitation, the charge polarity of the ions 4a, 4b can be detected: If the ions 4a, 4b stimulated by a uniform broadband excitation, move immediately after SWIFT excitation 10, for example, the positively charged ions 4a to the first measuring electrode 6a, while the negatively charged ions 4b move away from this.

For each ion resonance frequency f _i0n corresponding to a frequency spectrum shown in FIG. 10c, for example, the following formula

Polarity = sign [ ^L -'o * ^u - / ₀ ^T ° _ion (t) * cos (2nf _ion * t + <p) dt] (3)

If ion J is applied, the positive ions 4a can be identified, for example, by means of a positive sign (ao = 0 °, polarity +1) and the negative ions 4b by means of a negative sign (ao = 180 °, polarity -1). The positive ions 4a and the negative ions 4b can in this way in Frequency spectrum of all ions 4a, 4b are identified, as shown in Fig. 10d.

In the example described above, it was assumed that the

SWIFT excitation 10 with a start phase φ = 0 is performed. As described above, the start phase φ can be varied in the case of a mass-dependent phase-shifted orbital SWIFT excitation 10, but also as a function of the ion resonance frequency f _i0 n. In this way, ion packets in the frequency spectrum or in the mass spectrum can be marked correspondingly differently.

If the charge polarity (pos. / Neg. Or + / -) of the ions 4a, 4b is known, 10 ion populations can be excited differently depending on their charge polarity, for example by SWIFT (broadband) selective excitation. This can be done by depending on the

Charge polarity at the respectively associated ion resonance frequencies f _i0 n different excitation _transients are applied to the measuring electrodes 6a, 6b. It is understood that the procedure described above is not limited to the electrode geometry of the FT-ICR ion trap shown in FIG. 1, ie this method can be used with measuring electrodes

different electrode geometries are used, for example, in the case of 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.

In summary, in the manner described above the

Performance characteristics of a mass spectrometer 1 with a FT ion trap 2 can be significantly increased.

Claims

claims

1 . Method for the mass spectrometric analysis of a gas (4), comprising:

Ionizing the gas (4) to produce ions (4a, 4b),

Storing, exciting and detecting at least part of the generated ions (4a, 4b) in an FT ion trap (2),

characterized,

generating and storing the ions (4a, 4b) in the FT ion trap (2) and / or exciting the ions (4a, 4b) before detecting the ions (4a, 4b) in the FT ion trap (2) at least one selective, from the mass-to-charge ratio (m / z) of the ions (4a, 4b) dependent IFT excitation, in particular a SWIFT excitation (10) includes.

2. The method of claim 1, wherein during the generation of the ions (4a, 4b) in the FT ion trap (2) and / or during storage of the ions in the FT ion trap (2) at least one IFT excitation (10 ) for selecting ions (4a, 4b) to be stored in the FT ion trap (2).

3. The method of claim 2, wherein only ions (4a, 4b) for storing

whose mass-to-charge ratio (m / z) is outside an interval (I) of the mass-to-charge ratios ((m / z) i, (m / z) ₂ ) of a main gas component (1 1) of the gas ( 4) is located.

4. Method according to one of the preceding claims, in which the excitation degree (A) and / or the phase position (φ) of the IFT excitation (10) are varied between a first excitation _frequency (f _i0 ni) and a second excitation frequency (fion2), wherein both the first excitation _frequency (f _i0 ni) and the second excitation _frequency (f _ion 2) are not more than 10%, preferably not more than 5%, in particular not more than 1% of a predetermined excitation _frequency (fi _0n , a) differ.

5. The method of claim 4, wherein between the first

Excitation _frequency (f _ion i) and the second excitation _frequency ( _lon2 ), the phase _angle (φ) and / or the excitation degree (A) in _steps vary in dependence on the excitation _frequency (f _i0 n).

6. The method of claim 5, wherein between the first

Excitation _frequency (f _ion i) and the second excitation _frequency ( _lon2 ), the excitation _degree (A) and / or the phase position (φ) depending on the excitation _frequency (f _i0 n) either increase gradually or gradually decrease.

7. The method according to any one of the preceding claims, wherein the same ions (4a, 4b) in the FT ion trap (2) by IFT excitations (10) are several times selectively excited, wherein after each IFT excitation (10) detection the ions (4a, 4b) is performed.

8. The method of claim 7, wherein between two temporally immediately following IFT excitations (10) is a time interval (τ) which is greater than an average free flight time (t _M ) of the ions (4a, 4b) in the FT - ion trap (2).

9. The method according to any one of the preceding claims, wherein the

Detecting the ions (4a, 4b) a mass spectrometric examination of an ion signal (Ui _0n (t)) only in a temporally displaceable

Measuring time interval (12) takes place.

10. Method according to one of 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 (fn) of the ions (4a, 4b) in the FT ion trap (2), re-exciting the ions (4a, 4b) in the FT ion trap (2) and

Recording a second frequency spectrum (13b), as well

Detecting interference frequencies (f _R ) in the FT ion trap (2)

Comparing the first and second recorded frequency spectrum (13a, 13b).

11. The method of claim 10, wherein varying the ionic resonance frequencies (f _ion ) comprises changing a storage voltage (V _RF ) and / or a storage frequency (f _RF ) of the FT ion trap (2).

12. The method according to any one of the preceding claims, further comprising:

Determining a start phase position (ao) of a trajectory (B) of ions (4a, 4b) at a given ion resonance frequency (f _i0 n) after an IFT excitation (10) based on a in the detection of the ions (4a, 4b )

recorded time-dependent ion signal (Ui _0n (t)).

13. The method of claim 12, further comprising: determining a

Charge polarity of the ions (4a, 4b) based on the starting phase position (ao) of the ions (4a, 4b) after the IFT excitation (10).

14. mass spectrometer (1) comprising:

an FT ion trap (2),

an excitation device (5) for storage, excitation and detection of ions (4a, 4b) in the FT ion trap (2),

characterized,

in that the excitation device (5) is formed during storage and / or during the excitation of ions (4a, 4b) at least one selective, from the mass-to-charge ratio (m / z) of the ions (4a, 4b) dependent IFT excitation, in particular a SWIFT excitation (10) to produce.

15. Mass spectrometer according to claim 14, which is designed to ionize a gas to be investigated (4) in the FT ion trap (2), wherein the evaluation device (5) is preferably formed, during the ionization of an IFT excitation, in particular a SWIFT Stimulation (10) to generate.

16. A mass spectrometer according to claim 14 or 15, wherein the

Excitation device (5) is formed between a first

Excitation _frequency (f _ion i) and a second excitation _frequency ( _lon2 ) to vary the excitation level (A) and / or the phase position (φ) of the IFT excitation, preferably both the first excitation _frequency (f _i0 ni) and the second excitation _frequency ( f _ion 2) differ by not more than 10%, particularly preferably by not more than 5%, in particular by not more than 1% from a predetermined excitation _frequency (f _i0 n, a).

17. A mass spectrometer according to claim 16, wherein the

Excitation device (5) is formed between the first

Excitation _frequency (f _ion i) and the second excitation _frequency ( _lon2 ) the phase _angle (φ) and / or the excitation degree (A) in dependence on the excitation _frequency (f _i0 n) to vary stepwise, preferably between the first excitation _frequency (f _i0 ni ) and the second

Excitation _frequency ( _LON2 ), the excitation _degree (A) and / or the phase position (φ) depending on the excitation _frequency (f _i0 n) either increase gradually or gradually decrease.

18. Mass spectrometer according to one of claims 14 to 17, further

full: a detector (9), which is designed, after the IFT excitation, a starting phase position (cio) of a trajectory (B) of ions (4a, 4b) with a predetermined ion resonance frequency (f _i0 n) based on a in the

Detecting the ions (4a, 4b) recorded time-dependent ion signal (Uion (t)) to determine, the detector (9) is preferably formed, based on the starting phase position (cio) a charge polarity (pos., Neg.) Of the detected ions (4a, 4b).

19. mass spectrometer (1) according to the preamble of claim 14,

in particular according to one of claims 14 to 18, in which the

Excitation device (5) is formed, a phase position (φ) and / or a vibration amplitude (z / z ₀ ) of the ions (4a, 4b) in the FT ion trap (2) and / or ion resonance frequencies (f _i0 n ) of the ions (4a, 4b) in the FT ion trap (2), wherein the mass spectrometer (1) additionally comprises a detector (9), which is formed on the basis of a comparison of a first, before changing the phase position (φ ) and / or the

Oscillation amplitude (z / z ₀ ) and / or the frequency spectrum (13a) of the ion resonance frequencies (fion) with a second, after changing the phase angle (φ) and / or the oscillation amplitude (z / zo) ) and / or the ion resonance frequencies (f _ion ) of the ions (4a, 4b) in the FT ion trap (2) recorded frequency spectrum (13b)

Detect interference frequencies (f _R ) in the FT ion trap (2).

20. Mass spectrometer according to one of claims 14 to 19, wherein the FT ion trap (2) is designed as FT-ICR ion trap or as Orbitrap.