Ion trap and method for detecting ions in an ion trap

Abstract

A method for detecting ions in an ion trap and an ion trap, the method comprising the steps of: providing ionized ions to the ion trap; applying an RF storage signal to a first electrode of the ion trap to form an RF storage field, the RF storage signal having a storage voltage V RF and / or storage frequency Ω RF is altered; applying an excitation signal to ions in the ion trap; detecting an image current signal induced at the second electrode, at the third electrode, or differentially between the second electrode and the third electrode by oscillations of ions excited by the excitation signal; and applying an FFT to the detected image current signal to detect ion oscillations while correctly recovering the altered signal.

Description

[Technical Field]

[0001] The present invention relates to an ion trap and a method for detecting ions in an ion trap. [Background technology]

[0002] FFT mass spectrometers, such as quadrupole ion traps, can trap ions and detect their presence via ion image currents induced in the trap's electrodes by the vibrations of trapped ions. Nondestructive detection of ion image currents is a powerful and robust method option for mass spectrometry. However, compared to destructive methods, image current techniques suffer from the presence of "ghost peaks," which are spurious electrical signals resulting from unavoidable coupling pathways with the external environment. While much effort has been devoted to reducing such coupling and developing trapping techniques for end users, fundamental improvements are urgently needed.

[0003] Ghost peaks are known to remain at a relatively constant frequency during a single measurement session, whereas real ion peaks in an RF ion trap will respond in a consistent manner to changes in the trapping signal. The ion frequency is determined by the oscillation frequency ω z is given by the formula:

number

[0004] where z0 is the characteristic size of the ion trap, V ac is the applied constant AC storage voltage, Ω RF is the angular frequency of the trapping field. Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore an object of the present invention to provide a more reliable and sensitive method for detecting ions in an ion trap. [Means for solving the problem]

[0006] This problem is solved by a method for detecting ions in an ion trap according to claim 1 and an ion trap according to claim 11.

[0007] The method according to the present invention for detecting ions in an ion trap comprises: a. providing ionized ions into an ion trap; b. forming an RF storage field by applying an RF storage signal to a first electrode of the ion trap, wherein the RF storage signal has a storage voltage V RF and / or storage frequency Ω RF is changed, and c. applying an excitation signal to ions in the ion trap; d. detecting an image current signal induced at the second electrode, at the third electrode, or differentially between the second and third electrodes by the vibration of the ions excited by the excitation signal; e. applying FFT to the detected image current signal to detect ion oscillations; Includes:

[0008] Therefore, ions are first fed into the ion trap, where they are ionized either outside the volume of the ion trap or directly within the trapping volume of the ion trap, and any known ionization method can be used to ionize the ions, such as electron impact ionization, plasma ionization, or chemical ionization between an ionized gas and a measured gas, and energy is transferred from the ionized gas due to collisions to the measured gas for ionization.

[0009] Ions are trapped in the trapping volume of the ion trap by applying an RF storage signal to the first electrode of the ion trap to form an RF storage field. An excitation signal is then applied to the ions to generate a transient signal due to the oscillation of the ions in the ion trap. The oscillation of the ions in the ion trap induces an image current in the second and / or third electrodes, and the induced image current signal is detected directly from the second and / or third electrodes or differentially between the second and third electrodes to obtain a raw signal. A Fourier transform, such as a fast Fourier transform (FFT) or a discrete FFT (DFT), is applied to the detected image current signal to identify the ion oscillations and detect the signal due to the ion oscillations in the ion trap. The mass-to-charge ratio (m / z ratio) of the ions trapped in the ion trap can be determined from the results of the Fourier transform.

[0010] According to the present invention, the RF stored signal is varied over time, where the voltage V RF is varied over time. Alternatively or additionally, the storage frequency Ω RF is changed over time. The storage voltage V RF , storage frequency Ω RF , or both, to change the natural frequency ω of the ions trapped in the ion trap. z changes according to the following relationship:

number

[0011] Here, ω z is the ion frequency, z0 is the characteristic trap dimension, Ω RF is the angular frequency of the storage field.

[0012] Therefore, modifying the RF storage signal will modify the resulting image current signal in a similar manner, which can be detected in the calculated FFT spectrum to distinguish between signals coming from ions in the ion trap and ghost signals or ghost peaks that are the result of unwanted coupling paths between the individual electrodes and the external environment. While the oscillation frequency of ions in the ion trap will be consistently modified with modifications to the RF storage signal, ghost peaks in the FFT spectrum will not be modified and can therefore be identified and easily removed.

[0013] Preferably, the RF storage signal has a modulation frequency ω m Therefore, the detected image current signal is a frequency modulated (FM) signal. In the example of a sinusoidal modulation of the RF storage signal, the oscillation frequency of the ions is

number

[0014] Preferably, the modulation frequency ω m is lower than the lower limit of the passband of a detector or charge amplifier connected to the second electrode and / or the third electrode. The detector or charge amplifier extracts an image current signal from the second electrode and / or the third electrode. Since the modulation frequency is outside the passband of the detector or charge amplifier, adverse effects on the detector or charge amplifier are avoided.

[0015] Preferably, peaks in the FFT spectrum of the detected image current without sidebands are ignored. mWhen a carrier signal is periodically modulated at a frequency of ω, the resulting frequency-modulated image current signal is the original carrier signal and the modulation frequency ω. m The resulting peaks can be expressed as a sum of multiples of the original carrier frequency plus new sidebands corresponding to the mixed carrier frequency. Ghost peaks, however, will have no sidebands at all. By identifying peaks without sidebands, ghost peaks in the FFT spectrum can be identified and ignored, leaving peaks associated with ions in the ion trap in the FFT spectrum.

[0016] Preferably, the method further includes identifying peaks in the FFT spectrum of the detected image current and determining the instantaneous frequency (IF) of each peak. An IQ demodulation scheme (in-phase and quadrature component demodulation scheme) is then applied to the IF of each peak. This allows either the in-phase (I), quadrature (Q), or root mean square (RMS) of I and Q components of each peak to be extracted from the IQ demodulation scheme. The heights of the peaks in the FFT are then weighted by the results of the IQ demodulation scheme, such as I, Q, or RMS. Ion peaks in the FFT spectrum following the IF from the frequency-modulated image current signal are preserved by this process, while noise peaks that are not synchronously demodulated with the original modulation signal are automatically deweighted. This also allows ion signals overlapping with ghost peaks to be separated, since the IQ demodulation results return signals due to actual ion motion and eliminate the contribution of ghost peaks. Thus, in the weak modulation limit, this process can remove spurious signals and clean up the original mass spectrum.

[0017] Preferably, the FFT is applied in a non-stationary or co-moving reference frame. In particular, if the RF stored signal varies over time, such as a periodic modulation, instead of using a traditional FFT, a Fourier transform can be applied in a co-moving time frame, in which the modulation and time frame of the RF stored signal have the same frequency ω mIf the frequency is chosen, the modulation frequency may be fixed. Thus, unmodulated signals, such as ghost peaks, are distributed across multiple different frequencies or frequency bins, while modulated signals, such as the image current signal of ions in an ion trap, follow an evolution in a rotating frame. As a result, the resulting data transform is concentrated at a small number of frequencies, or even at a single frequency or frequency bin, where a frequency bin corresponds to a discrete frequency range in the DFT.

[0018] Preferably, before applying the FFT, the detected image current is multiplied by a coefficient exp(-iω) that is the complex conjugate of a preselected modulation. m t) or a similar function. This term balances the modulation of the RF stored signal so that the Fourier transform can be performed in a frame that rotates at the same rate as the modulation of the RF stored signal.

[0019] Preferably, the FFT is performed at a modulation frequency ω m The parametric Fourier transform (PFT) applies an FFT in a rotating reference frame that rotates at . Viewed in the time-frequency plane, this parametric Fourier transform has the effect of integrating along frequencies or frequency bins that are themselves oscillating at the modulation frequency. This is equivalent to shifting the analysis to a rotating reference frame. In addition to concentrating the modulated signal in a single bin, this transform also has the effect of distributing both unmodulated data and data modulated at different frequencies or phases across multiple bins. As the modulation index increases, these unwanted, unmodulated signals spread across more bins, thus reducing their impact on wanted signals that are, of course, localized in a particular bin.

[0020] Preferably, the modulation of the RF storage signal is periodically modulated, specifically chopped off. This allows for encoding of the resulting image current signal to more reliably identify peaks associated with ions detected in the ion trap. Specifically, when the modulation level corresponds to the first zero of the Bessel function J0, the signal intensity at the center frequency bin of the nominal peak location will be completely attenuated. In this case, if the modulation is periodically chopped, the intensity measured at its center height from a series of short-time (SF) PFFTs matched to the chopping pattern will follow the chopping pattern. The matched series of conventional ST-FFTs will follow a bit pattern that is the inverse of the chopping signal. The chopped ST-PFFTs and ST-FFTs can be synchronously demodulated with respect to the original chopping signal to further enhance sensitivity.

[0021] Preferably, the FFT is a fractional FFT (FRFT), which corresponds to a fractional rotation between a pure time representation and a pure signal representation, compared to the 90° rotation performed by a conventional Fourier transform (see https: / / en.wikipedia.org / wiki / Fractional_Fourier_transform). The FRFT is applied with a parameter α, which corresponds to a linear change in the RF stored signal. Thus, the stored voltage V of the RF stored signal RF , storage frequency Ω RFA linear increase in either or both of the RF storage signal and the RF signal results in an increase or decrease line in the time-frequency plane of the detected image current signal. The FRFT parameter α corresponds to this increase or decrease, resulting in a series of frequency bins for a discrete FFT in the time-frequency plane that slope / decay at the same angle as the modulation of the RF storage signal, i.e., the detected image current signal. Therefore, the ion signal is concentrated in one or a few frequency bins, and unwanted unmodulated signals in the detected raw data are distributed across multiple bins, reducing their impact on the measurement results. Therefore, when applying the FRFT to a linear change in the RF storage signal, ghost peaks can be removed from the spectrum because they are unmodulated, while simultaneously improving the signal-to-noise ratio of the ion signal.

[0022] In a further aspect, the present invention relates to an ion trap for trapping and detecting ions, the ion trap comprising a first electrode, a second electrode, and a third electrode defining a trapping volume, an RF storage signal supply connected to the first electrode and configured to generate an RF storage field, the RF storage signal having a storage voltage V RF and / or storage frequency Ω RF The RF excitation signal supply is connected to the first electrode and configured to generate an excitation signal, and the detector is connected to the second electrode and / or the third electrode and configured to detect an image current induced by the oscillation of ions excited by the excitation signal.

[0023] Preferably, the first electrode is a ring electrode and the second and third electrodes are cap electrodes that surround the trapping volume of the ion trap.

[0024] Preferably, an evaluation unit is connected to the detector, the evaluation unit being configured to carry out the steps of the method as described above, wherein the evaluation unit can be a separate unit with respect to the detector or can be constructed integrally with the detector.

[0025] The invention will now be explained in more detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is an ion trap according to the present invention. [Figure 2] 1 is a schematic diagram of a method according to the present invention; [Figure 3] 2 shows detailed steps of the method according to the present invention; [Figure 4] FIG. 1 is a frequency diagram. [Figure 5] FIG. 1 illustrates a detection scheme according to the present invention. [Figure 6A] 1 is a detection scheme according to the present invention by fractional Fourier transform. [Figure 6B] 1 is a detection scheme according to the present invention by fractional Fourier transform. [Figure 6C] 1 is a detection scheme according to the present invention by fractional Fourier transform. [Figure 7A] 1 is a detection scheme according to the present invention by parametric Fourier transform. [Figure 7B] 1 is a detection scheme according to the present invention by parametric Fourier transform. [Figure 8a] This is the result of the present invention. [Figure 8b] This is the result of the present invention. [Figure 8c] This is the result of the present invention. [Figure 8d] This is the result of the present invention. [Figure 9A] 1 is a further detection scheme according to the present invention. [Figure 9B(a)] 1 is a further detection scheme according to the present invention. [Figure 9B(b)] 1 is a further detection scheme according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring to FIG. 1 , an ion trap according to the present invention is shown. The ion trap 10 comprises a first electrode 18 constructed as a ring electrode, a second electrode 14 constructed as a first cap electrode, and a third electrode 16 constructed as a second cap electrode, with the first electrode 18, second electrode 14, and third electrode 16 defining a trapping volume 20 for trapping ions. An RF signal supply 22 is connected to the first electrode 18 to generate an RF storage field between the first electrode 18 and the electrodes 14 and 16, storing ions within the trapping volume 20. Additionally, an RF excitation signal supply 26 may be connected to the second electrode 14 and / or the third electrode 16 to generate an excitation signal either as a unipolar signal to either electrode 14, 16 or differentially between electrodes 14 and 16 prior to detection. The excitation signal results in an excitation signal frequency that is proportional to the oscillation ion frequency ω of each ion. z , ions trapped within the trapping volume 20 are excited to oscillate around an equilibrium position within the trapping volume 20, where the frequency ω z Because m / z depends on the mass of the ion, mass-selective excitation of ion vibration can be facilitated. Due to the vibration of ions within the trapping volume 20, an image current is induced at the second electrode 14 and the third electrode 16. The ion trap 10 further includes a detector 24, which can be connected to either the second electrode 14 or the third electrode 16. Alternatively, as illustrated in FIG. 1 , the detector 24 is connected to the third electrode 16 and the second electrode 14 to detect a differential signal of the image currents induced at the second electrode 14 and the third electrode 16. Thus, excitation of ions can be detected at respective frequencies upon detection of the image current signal to selectively determine ions or ion species having specific m / z ratios within the ion trap 10.

[0028] Referring to Figure 2, which shows a flow diagram of the method steps, the method for detecting ions in an ion trap comprises: In step S01, ionized ions are supplied to an ion trap; In step S02, an RF storage field is formed by applying an RF storage signal to a first electrode of the ion trap, and a storage voltage V RF and / or storage frequency Ω RF is changed, In step S03, an excitation signal is applied to the ions in the ion trap; In step S04, an image current signal induced in the second electrode, in the third electrode, or differentially between the second electrode and the third electrode by the vibration of the ions excited by the excitation signal is detected; In step S05, an FFT is applied to the detected image current signal to detect ion vibrations. Includes steps.

[0029] Thus, in step S01, ions are provided to the ion trap or the trapping volume of the ion trap, where the ions can be directly ionized inside the trapping volume or first ionized outside the ion trap and then transferred to the trapping volume of the ion trap, where any conventional method for generating ions can be used in conjunction with the present invention.

[0030] In step S02, ions are trapped in the trapping volume of the ion trap by applying an RF storage field formed by an RF storage signal applied to the first electrode 18 of the ion trap, which may be the ring electrode of a quadrupole ion trap, where a storage voltage V RF is varied over time. Preferably, the stored voltage is varied at a frequency ω m Alternatively or additionally, the RF storage signal is periodically modulated at a storage frequency Ω RF is varied over time, also preferably at a frequency ω m where the storage voltage V RF and storage frequency Ω RF When simultaneously modulating different modulation frequencies ω m and ω' mHowever, in a preferred embodiment, the storage voltage V RF or storage frequency Ω RF Only the signal is modulated at the same frequency.

[0031] In step S03, an excitation signal is applied to the ions in the ion trap, causing the ions to oscillate.

[0032] In step S04, an image current signal induced in the second electrode 14 and the third electrode 16 is detected, the image current signal being caused by the oscillation of ions excited by the excitation signal, where the image current signal can be detected directly from the second electrode 14 or the third electrode 16 of the ion trap, or differentially detected between the second and third electrodes 14, 16, where by changing the RF storage signal, the oscillation frequency of the excited ions in the ion trap changes correspondingly.

number

[0033] Therefore, the storage voltage V RF and storage frequency Ω RF Changing either of these will change the oscillation frequency of the ions.

[0034] In step S05, a Fourier transform is applied to the detected image current signal to detect the ion oscillation frequency. Thus, the Fourier transform can determine the ion oscillation frequency, from which the mass-to-charge ratio of each ion in the ion trap can be calculated to identify individual ion species. Here, ghost peaks are independent of the modification of the RF storage signal and can therefore be identified and removed in the FFT spectrum.

[0035] 4, which illustrates the situation of the present invention in the frequency regime. An ion signal 30 is within the passband 32 of the detector 24 and can therefore be detected by the detector 24. Here, the RF storage signal is modulated by a frequency 34, which is outside the passband and preferably below the lower limit of the passband 32 of the detector 24. Therefore, changes in the RF storage signal do not affect the detector 24, and saturation of the detector due to changes in the RF storage signal can be avoided.

[0036] In particular, the RF storage signal has a low amplitude frequency ω m When periodically modulated with ω z -ω m and ω z +ω m Only two sidebands appear. In the high modulation regime, multiple sidebands appear. Because ghost peaks are not affected by the modulation of the RF storage signal, these ghost peaks do not show sidebands in the FFT spectrum. Therefore, peaks in the FFT spectrum for ions in the ion trap can be identified by their sidebands instead of ghost peaks, which consequently have no sidebands. Therefore, peaks in the FFT spectrum without sidebands can be ignored.

[0037] 3 and 5, an additional detection scheme is shown for identifying and removing ghost peaks in the acquired FFT spectrum.

[0038] In step S51, peaks in the FFT spectrum of the detected image current are identified.

[0039] In step S52, the instantaneous frequency (IF) of each peak is determined.

[0040] In step S53, an IQ demodulation scheme is applied to the instantaneous frequency of each peak, where the result of the IQ demodulation scheme can be in-phase (I) and quadrature (Q) components, or the RMS of I and Q.

[0041] In step S54, the height of each peak is weighted by the result of the IQ demodulation scheme.

[0042] Following arrow 100 in FIG. 3, an IQ demodulation scheme S53 and a weighting step S54 are applied successively to each peak of the FFT spectrum.

[0043] See FIG. 5, which illustrates an example of the steps in FIG. 3. In FIG. 5a, the instantaneous frequencies of exemplary real ion and ghost signals are shown. In FIG. 5b, the FFT spectrum is shown, revealing that the real ion signal generates sidebands as described above, while the ghost peaks do not exhibit any sidebands. FIG. 5c shows that the RF storage signal is sinusoidally modulated. FIG. 5d shows the result of point-by-point multiplication between the signal's IF and the applied modulation. An IF signal that is not frequency-locked and phase-locked to the modulation will average to zero, while an IF signal that remains locked will generate a cosine or sine-squared signal that does not average to zero. An IQ demodulation scheme is applied to each of the individual instantaneous frequencies determined according to FIG. 5a. In the example of FIG. 5, the IQ RMS is applied as a weighting to the individual peaks, and signals that do not respond to the impost modulation are deweighted from the spectrum. As shown in FIG. 5e, synchronous demodulation of the real ion signal is achieved, resulting in significant noise reduction due to the reduction of ghost peaks. Therefore, the resulting FFT spectrum, shown in Figure 5e, enhances the peaks due to ions in the ion trap and removes signals not affected by the modulation of the RF storage signal, thus eliminating ghost peaks and improving the signal-to-noise ratio.

[0044] 6A to 6C illustrate another detection scheme. Referring to FIG. 6A, as shown in image a) of FIG. 6A, a slope line of the detected image current signal is obtained in the time-frequency plane with respect to the linear change of the RF storage signal. When a conventional FFT is applied according to image b) of FIG. 6A, which shows the horizontal frequency bins, the ion signal spreads across multiple frequency bins. As a result, the ion signal is not weighted in the spectrum shown in image c) of FIG. 6A. According to the present invention, instead of using a conventional FFT, a fractional FFT (FRFT) is applied to the detected image current signal. The FRFT is an arbitrary rotation in the time-frequency plane, as shown in FIG. 6B. Here, the FRFT can be considered as an n-th power Fourier transform, where n does not need to be an integer. In this regard, please refer to "https: / / en.wikipedia.org / wiki / fractional_fourier_transform" and L. Stankovic et al., "Time-frequency signal analysis with applications," 2013, Artech House. There, the angles of the frequency bins are tilted by an angle α=nπ / 2 as shown in images b) of FIGS. 6A and 6C.

[0045] In the case of linear frequency modulation, an FRFT is used instead of a conventional FFT. The FRFT corresponds to a partial rotation between the pure time representation and the pure signal representation, compared to the 90-degree rotation performed by a conventional Fourier transform. In this regard, according to the present invention, the tilt angle α of the frequency bins of the FRFT matches the tilt angle of the image current signal, as shown in image a) of Figure 6C, due to the adaptive / linear modulation of the RF storage signal. This coincidence of these angles results in the ion signal concentrating in one of the frequency bins using the FRFT, resulting in the distinct peak shown in image c) of Figure 6C. The resulting spectrum is the integral of the image current signal along each frequency bin. Other peaks unaffected by the modification of the RF storage signal, such as ghost peaks, appear as horizontal lines in the time-frequency plane. In the example of Figure 6A, if a conventional Fourier transform were used, this would lead to a distinct peak in the resulting FFT spectrum. In contrast, when using the FRFT, the unmodified signals are spread across several frequency bins, which deweights and removes them from the resulting FFT spectrum.

[0046] Thus, by using FRFT in conjunction with specific modifications of the RF storage signal, enhancement of the real ion signal can be achieved while simultaneously achieving de-weighting of ghost signal peaks in the resulting FFT spectrum.

[0047] See Figures 7A and 7B, which show a detection scheme using a parametric Fourier transform. In the case of nonlinear modulation (such as frequency modulation - FM), a parametric Fourier transform can be chosen, which is equivalent to performing a conventional Fourier transform in a frame that rotates at the same rate as the modulation (see L. Stankovic et al., "Time-frequency signal analysis with applications," 2013, Artech House).

[0048] When viewed in the time-frequency plane, an unmodulated signal looks like a straight line 40 (see image a in Figure 7B), while a modulated signal 42 of ions in an ion trap looks like a sine wave (see image a in Figure 7A). Performing a conventional FFT is equivalent to calculating the signal intensity found in a specific frequency range or set of frequency bins (which can be thought of as horizontal divisions in the time-frequency plane). As a result, frequency-modulated signals 42, such as those present in Figure 7A, find their intensities divided into multiple bins, resulting in an apparent de-weighted spectrum.

[0049] Before the Fourier transform, the frequency modulated signal 42 (A(t)e iφ(t) , A(t) is the image current signal detected by the detector 24, φ(t) is the modulation frequency ω m ) is a second function of t (e -iφ(t) ), the resulting transformation of the data will be concentrated in a single bin at the original frequency before modulation.

[0050] Viewed in the time-frequency plane, this parametric Fourier transform has the effect of integrating along frequency bins 44 that are themselves oscillating at the modulation frequency, as shown in image b) of Figures 7A and 7B. This is equivalent to shifting the analysis into a rotating reference frame.

[0051] In addition to concentrating the modulated signal into a single bin, this transformation also has the effect of distributing both the unmodulated signal 40 and signals 46 modulated at different frequencies or phases across multiple bins, as shown in FIG. 7B. As the modulation index increases, these unwanted, unmodulated signals 40, 46 will be spread across more bins, thus reducing their impact on wanted signals that are, of course, localized in a particular bin. The unwanted, unmodulated signals 40, 46 will result in a displaced FFT spectrum, as shown in image c) of FIG. 7B, while the modulated, underlying image signal will sum to a clear, distinct peak, as shown in image c) of FIG. 7A.

[0052] The signal-to-noise advantage of this frequency modulation approach is more fully realized as the modulation index increases, but of course this also means that for proper detection the full range of instantaneous frequency changes must remain within the bandwidth of the detector 24. With this technique, even closely spaced frequency ion species can be separated because modulation is applied simultaneously to all peaks in the image current signal, preventing overlap of signals from different ion species.

[0053] The detection scheme therefore uses knowledge of the modulation and its modulation frequency to coherently sum signals that are aligned with the drive signal and to reject signals that are not aligned.

[0054] In order to apply the parametric Fourier transform, the real-world data must be transformed into an analytical form. Fortunately, at least for predominantly oscillatory data, this can be easily generated as the sum of the real-world data and its Hilbert transform. s A (t)=s(t)+is h (t) For a modulated cosine signal, s(t)=cos(ωt+φ(t)) which means s h (t)=sin(ωt+φ(t)) Therefore, s A (t)=s(t)+is h (t)=cos(ωt+φ(t))+i * sin(ωt+φ(t)) (Equation 1) This becomes:

[0055] As can be seen in the example of the modulated cosine function, this analytical signal generation process allows us to recover phase information that would otherwise be unavailable. Refactoring Equation 1, if φ(t) is known, we can use the multiplication function e to remove this term before the Fourier transform. -iφ(t)You can find out how to select. s a (t)=e i(ωt+φ(t)) =e i(ωt) e i(φ(t))

[0056] Figure 8 shows the results of the above detection scheme using a parametric FFT. Figures 8a and 8c relate to the low coupling regime, which produces only a few sidebands, while Figures 8b and 8d relate to the strong coupling regime, which produces many sidebands. Figure 8a relates to the case where the RF storage signal is sine wave modulated, for example as follows:

number

[0057] 9A and 9B, the modulation of the RF storage signal can be modulated, for example, as shown in FIG. 9Aa. In the example of FIG. 9A, the modulation of the RF storage signal is switched on and off to correspond to a chopping scheme. When a parametric FFT is used during detection, as shown in FIG. 9Ab, only during the modulation phase of the RF storage signal is a relevant peak achieved in the resulting FFT spectrum. In contrast, during the modulation-free phase, as shown in FIG. 9Ac, a peak is obtained when a conventional FFT is used. Therefore, by turning the modulation of the RF storage signal on and off, the resulting spectrum alternates between the spectra of FIG. 9Ba and FIG. 9Bb. Therefore, a synchronous demodulation intensity can be obtained, as shown in FIG. 9Ad, which improves the signal-to-noise ratio and allows clear identification of signals due to ion oscillations in the ion trap.

[0058] Thus, by complex modulation of the RF storage signal, an adapted response of the image current signal can be obtained, which can be used to further enhance detection by eliminating ghost peaks or improving the signal-to-noise ratio of the resulting ion signal.

[0059] Thus, in accordance with the present invention, the RF storage signal is modulated to obtain a modulated image current signal. Varying the storage voltage or storage frequency changes the corresponding ion frequency. Knowledge of the modulation can be used in determining the ion oscillation frequency using an IQ demodulation scheme or by applying a fractional order FRFT to a linear variation of the RF storage signal. In general, any modulation scheme can be used as long as the complex conjugate can be calculated for use in the parametric Fourier transform.

Claims

1. 1. A method for detecting ions in an ion trap, comprising: providing ionized ions into the ion trap; b. Applying an RF storage signal to a first electrode of the ion trap to form an RF storage field, wherein the RF storage signal has a storage voltage V RF and / or storage frequency Ω RF is changed, and c. applying an excitation signal to ions in the ion trap; d. detecting an image current signal induced at the second electrode, at the third electrode, or differentially between the second and third electrodes by the vibration of ions excited by the excitation signal; e) applying FFT to the detected image current signal to detect ion oscillations; Including, The RF storage signal is periodically modulated at a modulation frequency ω m ; A method wherein, before applying the FFT, the detected image current signal is multiplied by the complex conjugate of the applied modulation signal.

2. The method of claim 1 , wherein the modulation frequency is below the lower passband limit of a charge amplifier / detector connected to the second electrode and / or the third electrode.

3. The method of claim 1 , wherein peaks in the FFT spectrum of the detected image current signal that are free of sidebands are ignored.

4. 2. The method of claim 1, wherein the FFT is a fractional order FFT (FRFT), and the RF stored signal is modified by a parameter α corresponding to a linear change in the RF stored signal.

5. 1. An ion trap for trapping and detecting ions, comprising: a first electrode, a second electrode, and a third electrode defining a trapping volume; an RF storage signal supply connected to the first electrode and configured to generate an RF storage field; RF and / or storage frequency Ω RF is changed, an RF storage signal supply; an RF excitation signal supply connected to the second electrode and / or the third electrode and configured to generate an excitation signal; a detector connected to the second electrode and / or the third electrode and configured to detect an image current signal induced by the oscillation of ions excited by the excitation signal; Equipped with An ion trap, wherein an evaluation unit is connected to the detector, the evaluation unit being configured to perform the steps of the method according to any one of claims 1 to 4.

6. The ion trap of claim 5 , wherein the first electrode is a ring electrode and the second electrode and the third electrode are cap electrodes.

Images