TW202336813A - Method and ion trap for detecting ions - Google Patents

Abstract

Method and ion trap for detecting ions, wherein the method comprises the steps of Ionizing ions in the ion trap; Creating an RF storage field by applying a first RF signal to a first electrode of the ion trap; Applying an excitation signal to the ions in the ion trap; Detecting an image current signal induced to a second electrode, a third electrode or differentially between the second and third electrodes by oscillation of the ions excited by the excitation signal. Therein, the excitation signal is a parametric excitation.

Description

Methods for detecting ions and ion traps

An object of the present invention is to provide a method for detecting ions in an ion trap, and further to provide an ion trap for capturing and detecting ions.

Ion trap mass spectrometry utilizes semiconductor manufacturing technology and related fields to monitor plasma material processing. The ion trap is attached to a vacuum device that collects the ions and mass-selectively detects the ions in the capture volume. Detected ions can be identified by their mass and are indicative of gas species, such as those present in vacuum equipment as part of a manufacturing process. Can detect and monitor program information, including product failures, equipment failures, etc.

A radio frequency (RF) ion storage mass spectrometer can be designed to operate using a variety of different technologies and detection algorithms. Most early tools worked by directing ions to charged particle detectors, such as electron multipliers for destructive detection. Ions can be transferred to the detector via a number of different techniques, such as by applying additional monopolar, dipole or quadrupole excitation scanning instabilities or resonance excitations. Later tools used very sensitive electronics for image current detection via induced ion motion.

What these technologies have in common is the loss of the ability to dynamically measure ion responses to external stimuli. Destructive particle detector technology must provide enough energy to the ions to expel them from the capture volume, but does not measure the ion signal before the ions collide with the detector. FFT image current techniques typically use the same electronics for stimulation and detection. However, stimulation and detection must be time-multiplexed because the inevitable crosstalk of applying stimulation signals at the same frequency as the stored ions can saturate the sensitive electronics used for detection.

The object of the present invention is to overcome the disadvantages of the prior art and to provide a detection solution that allows for non-destructive simultaneous stimulation and detection of ions in an ion trap.

This technical problem is solved by a method for detecting ions in an ion trap as in technical solution 1, and is further specified in the accompanying technical solution. Furthermore, a solution to a given technical problem is provided by an ion trap for capturing and detecting ions as in technical solution 14, and is further specified by subsidiary technical solutions.

In a first aspect, a method for detecting ions in an ion trap is provided. The method includes the following steps: a) Ionize ions in the ion trap or transfer them from an external ion source into the ion trap; b) creating an RF storage field by applying a first RF signal to a first electrode of the ion trap; c) applying an excitation signal to the plasma in the ion trap; d) Detect an image current signal induced to a second electrode, a third electrode or a difference between the second electrode and the third electrode through the oscillation of the plasma excited by the excitation signal; Wherein, the excitation signal is a parameter excitation. Wherein, according to the present invention, the excitation signal is a parameter excitation. Therefore, preferably the excitation signal has a frequency different from the oscillation frequency of the respective ions in the ion trap. In step a), ions in the ion trap are ionized in response to the applied electric field. Among them, ionization is a common technology and is not part of the present invention. Any suitable solution that meets the requirements of this method may be used for ionization. Example ionization technologies include electron impact ionization, photoionization, chemical ionization, etc. In step b), an RF storage field is created to store the ionized ions. Preferably, the RF storage field is created by a first RF signal source. The storage field is created by applying a first RF signal to a first electrode of the ion trap. Among them, steps a) and step b) can be performed in the current order, in the reverse order (step b) before (step a), or at the same time. In step c), an excitation signal is applied to the plasma in the ion trap. The excitation signal may be provided by a second RF signal source, or may be provided by modulation of the first RF signal that provides the RF storage field. Wherein, the excitation signal may also be applied to the first electrode of the ion trap. Wherein, according to the present invention, the excitation signal is a parameter excitation. A conventional monopolar or dipole stimulus from the end cap electrode can also be applied to the plasma before detection and before or simultaneously with the parametric excitation signal to seed coherent motion for parametric amplification. In step d), an image current signal is preferably detected by a detector, wherein the image current signal is sensed to a second electrode, a third electrode, or between the second electrode and the third electrode. Electrodes are alternately detected, wherein the image current signal is caused by oscillations of the plasma excited by the excitation signal. Wherein, the excitation of the plasma by the excitation signal is mass selective, and only the plasma is excited by the respective excitation signal having a specific and matching mass-to-charge ratio. Therefore, according to the present invention, ion excitation is promoted by parametric excitation. The effect of parametric oscillation is used to separate the frequency of the excitation signal and the frequency of the image current signal induced to the second and/or third electrode. Therefore, there is no need for time multiplexing of excitation and detection due to different frequencies. With this, the flexibility of the detection scheme is enhanced, and improved signal recovery options such as synchronous demodulation are now possible. Synchronous demodulation or lock-in detection relies on comparison of the detected signal with a reference excitation signal or a modulated excitation signal. Possible implementations include the use of low-frequency modulation (chopping) of the excitation signal, direct multiplication of the ion signal with a calculated I-Q reference signal at half the parametric excitation frequency, or for detailed tracking of specific peaks, later Chapters describe various FM modulation techniques.

Preferably, the ion oscillation frequency is related to the long-term frequency of the ions in the RF storage field and depends on the respective masses of the ions. system excitation frequency one half, that is .

Preferably, applying the excitation signal and detecting the image current signal are performed simultaneously. Temporal multiplexing of excitation and detection is no longer necessary, so simultaneous detections can be performed without unintended interactions.

Preferably, the application of the excitation signal is sequentially repeated at different frequencies. Thus, by applying an excitation signal with a frequency, different ion species can be addressed, excited and subsequently detected. Alternatively, more than one excitation signal with one of different frequencies is applied. Here, the excitation signals are superimposed and applied simultaneously to address different ion species simultaneously for simultaneous detection.

Preferably, after applying a first excitation signal before applying a second excitation signal with a different frequency, the ions excited by the first excitation signal are discharged from the ion trap. Thus, detected ions can be removed from the ion trap, for example, by increasing the amplitude of the excitation signal beyond a certain stabilization point, causing the respective ions to collide with the surface of the ion trap or be squeezed out of the trapping volume. removed from the capture volume. Therefore, the remaining ion species in the capture volume can be detected more reliably, and the superposition of response signals of different species can be reduced.

Preferably, the frequency of the excitation signal is scanned within a predetermined range. Scanning can be performed continuously or in discrete steps by changing the frequency of the excitation signal or changing the amplitude of the RF capture field relative to a fixed parameter excitation frequency. Alternatively, the scan may be performed step by step or in a predetermined sequence to address specific ion species of interest, or in a dynamically programmed manner based on peak algorithm tracking and preprogrammed logic rules. Therefore, a range of ions with different masses can be detected by scanning. Furthermore, by scanning the excitation signal, the response spectrum of the ions can now be sampled over a frequency range independently of the response signal of the excited ions.

Preferably, as a means of automatic gain control, the signal level from the selected peak can be tuned to a pre-selected or dynamically calculated level by instantaneously adjusting the excitation level applied at its associated frequency and overall RF capture voltage. . This can be used, for example, to dynamically increase sensitivity during a single measurement process.

Preferably, by scanning the excitation signal on the ion response signal of the excited ions, the sampling of the first derivative of the response signal of the excited ions can be determined. Based on the derivative signal, the shape of the ion response signal can be calculated to determine the shape of the response signal, which can identify overlapping response signals from different ion species (called isobaric species).

Preferably, when tracking a single m/z range, the frequency of the excitation signal is modulated at a low frequency between a non-resonant frequency and a positive resonant frequency, where the positive resonant frequency corresponds to a frequency of the ion response signal (due to parameter excitation). like Indicates a frequency corresponding to one of the excitations of a specific ion or ionic species. The positive resonant excitation frequency is generated as . During various parts of the modulation cycle, namely positive and non-resonant frequencies, the composite waveform formed from the average response is proportional to the peak height and can be demodulated simultaneously with the original low-frequency modulation pattern to further reduce noise.

Preferably, when a limited m/z range needs to be studied in detail, the frequency of the excitation signal can be frequency modulated between two closely spaced frequencies and synchronously demodulated. During a single measurement, the center position Scan the entire mass range of interest. The demodulated response at any center frequency then corresponds to the first derivative of the m/z signal at that point, allowing differentiation of overlapping response signals from different species (so-called isobaric species).

Preferably, the excitation signal at a low frequency and a maximum frequency between twice of , which is used to image the current signal or ion response, which generates ,in It is the upper frequency limit of the detector and the highest frequency that the detector can handle, and It is the cut-off frequency of the detector, which is the upper limit of the detector’s cut-off frequency. therefore, Represents the lowest frequency that is not amplified by the detector. also, It is a response signal related to the image current signal or ion. Therefore, it can be determined based on the amplification range of the detector used and image current signal within the range. Alternatively, the image current signal obtained during detection is filtered , which filters out the signal at the frequency of the excitation signal. Therefore, a notch filter or a bandpass filter can be used dedicated to the frequency of the excitation signal. Since the parameter excitation, excitation signal and ion response signal are at different frequencies, the filtering of the excitation signal has no effect on the measured signal, thereby avoiding the impact of the excitation signal on the detection.

Preferably, the excitation system is smaller than one modulation frequency Modulation, in which the detected image current signal is demodulated at a modulation frequency. Thus, by modulating the excitation signal, a recovery scheme can be implemented from the excitation signal, for example, in a phase-locked manner. Modulation may be applied to the amplitude of the excitation signal and/or the phase of the excitation signal.

Preferably, a phase-sensitive detection of image current signals is applied to reliably detect the response signals of respective ions.

In another aspect, an ion trap for capturing and detecting ions is provided, wherein the ion trap includes a first electrode defining a capture volume, a second electrode, and a third electrode. Additionally, a first RF signal source is connected to the first electrode and configured to generate an RF storage field. Additionally, a second RF signal source is connected to the first electrode and configured to generate an excitation signal. Wherein, the second RF signal source may be implemented as an additional module attached to the first RF signal source, or may be integrated into the first RF signal source to modulate the signal provided by the first RF signal source. The first RF signal is used to apply the excitation signal to the ion trap. Preferably, the first RF signal source and/or the second RF signal source are implemented by one or more DDS modules (direct digital synthesis modules). The excitation is a parameter excitation, which results in different excitation frequencies and ion response signals induced in one or more electrodes. Additionally, the ion trap includes a detector connected to the second electrode and/or the third electrode and configured to detect an image current signal caused by oscillations of ions excited by the excitation signal. Wherein, the detector can detect the image current induced to the second electrode or the third electrode, or can detect a differential signal between the second electrode and the third electrode.

Preferably, the first electrode is a ring electrode, and the second electrode and the third electrode are cover electrodes.

Preferably, the ion trap is further constructed along the features described in connection with the above method.

Preferably, the method is further structured along the features described above in connection with the ion trap.

Therefore, with the present ion trap and method, the excitation of ions in the ion trap through parametric excitation yields the advantage of separating the excitation frequency and the ion response of the excited ions by detecting the induced image current signal. Therefore, the influence of the excitation signal on the detected image current signal does not appear. At the same time, simultaneous excitation and detection become feasible, enabling different detection schemes and improving detection sensitivity. In addition, continuous application of the excitation signal is performed, so that the ion response can be measured over a longer period of time, thereby obtaining a higher signal-to-clutter ratio and better mass resolution.

Referring to Figure 1, an ion trap is shown in accordance with the present invention. The ion trap 10 includes a first electrode 18 configured as a ring electrode, a second electrode 14 configured as a first cover electrode, and a third electrode 16 configured as a second cover electrode, wherein the first electrode 18, The second electrode 14 and the third electrode 16 define a capture volume 20 for capturing ions. A first RF signal source 24 is connected to electrode 18 to generate an RF signal storage field between first electrode 18 and electrodes 14 and 16 to store ions in capture volume 20 . Furthermore, the signal sources 22, 26 may be connected to the second electrode 14 and/or the third electrode 16 to change the storage field. All supplied signals are usually connected to a control unit 28 in order to control the generation of the respective signals. The signal supply may be implemented as a DDS module or integrated in a DDS module. In conventional FFT image current measurement, before detection, the excitation signal is supplied to the electrodes 14 and 16 as a monopole signal, or is differentiated between the electrode 14 and the electrode 16 . Wherein, the excitation signal may be supplied by a second signal source, or as shown in FIG. 1 and described above. As a result of the excitation signal, if the frequency of the excitation signal matches the oscillating ion frequency of the respective ion , then the trapped ions in the trapping volume 20 are excited to oscillate around their equilibrium position in the ion trap 10 . Among them, the long-term frequency Depends on the mass of the ions, thus promoting mass-selective excitation of ion oscillations. Due to the oscillation of the ions in the capture volume 20, an image current is induced into the second electrode 14 and the third electrode 16. The ion trap 10 further includes a detector 30 , which may be connected to the second electrode 14 or to the third electrode 16 . Alternatively, as shown in FIG. 1 , the detector 30 is connected to the second electrode 14 and the third electrode 16 to detect the differential signal of the image current induced in the second electrode 14 and the third electrode 16 . Therefore, the excitation of ions can be detected when image current signals with respective frequencies are detected.

In the present invention, the excitation signal is provided as a parametric excitation to induce a parametric oscillation of the ions. Thus, the excitation signal has a frequency , which is twice the long-term frequency or oscillation frequency of ions, resulting in . This is depicted in Figure 5, which shows the excitation signal and the image current signal induced in one or both of the cover electrodes of the ion trap 10. It can be seen from Figure 5 and the parameter oscillation program that the excitation signal The frequency is different from the frequency of the excited oscillation of the ions and the resulting image current signal, so the two signals can be easily separated with minimal crosstalk or overlap, thereby reducing detection sensitivity. Therefore, time multiplexing of excitation and detection is no longer required, meaning that excitation and detection can be performed simultaneously. In Figure 5 below, the detected image currents are shown in the entire row of the initial response at the beginning of the parametric excitation of the ions. Thereby, a steady-state development is shown in the dotted line due to the excitation of the oscillation by the construction parameters, and by the continuous excitation of the excitation signal.

Referring to Figure 2, a method for detecting ions in an ion trap 10 is shown, which includes: a) Ion S01 in the ion trap; b) creating an RF storage field S02 by applying a first RF signal to a first electrode of an ion trap; c) applying an excitation signal to the ions S03 in the ion trap; d) Detect an image current signal S04 that is induced to a second electrode, a third electrode or a difference between the second electrode and the third electrode through the oscillation of the plasma excited by the excitation signal; Among them, the excitation signal is a parameter excitation.

In step S01, ionization of ions in the ion trap 10 is performed, wherein known ionization schemes can be used to provide ions to be captured in the ion trap. In step S02, an RF storage field is applied by applying a first RF signal to the first electrode 18 of the ion trap to capture ions into the capture volume 20. In step S03, an excitation signal is applied to the ions in the ion trap, wherein, as discussed above, the excitation signal is a parametric excitation having a frequency ,in It is the oscillation frequency of ions. S03 may also include a dipole stimulus before parameter excitation. In step S04, an image current signal is detected and induced into the second electrode 14 or the third electrode 16, or an image current is detected by detecting the difference between the second electrode 14 and the third electrode 16. signal. Based on the detection signal, the oscillations of the respective ions in the species or sample can be reliably detected. If there is a long-term frequency in the capture volume 20 ions, then the plasma will be The excitation signal is excited, thereby inducing an image current signal to one or both of the second and third electrodes being detected by the detector 30 . If the ion response signal is detected through the detection of the image current signal, it can be confirmed that there are oscillation frequencies or long-term frequencies in the capture volume 20 of ions. Among them, the oscillation frequency of ions Depends on the mass of the ion and therefore provides a mass selective detection scheme. According to arrow 32 in FIG. 2 , steps S03 and S04 can be repeated one or more times using the same or different excitation frequencies, so as to scan within a frequency range of interest and detect ions within this range. Scans may be performed continuously, step by step, or in a predetermined manner or in a dynamically determined manner, where the excitation frequency and amplitude are adjusted during a single measurement in direct response to the measured image current response of the associated ion mass.

Referring to FIG. 3 , a different embodiment of the method is shown, in which steps S01 to S04 are equal to the method described with respect to FIG. 2 . In addition, after executing S04, an additional step S031 is executed. In step S031, ions excited by the excitation signal are discharged from the capture volume 20, and further adjustment of the frequency of the excitation signal is performed to provide a second excitation signal. Subsequently, in step S03, a second excitation signal is applied to the remaining ions in the capture volume 20 to excite different ions in the species in response to the second excitation signal (if present). Subsequently, in step S04, the induced image current signal is detected based on the second excitation signal, wherein steps S03, S04 and S031 may be repeated one or more times to cover a range of different excitation signal frequencies and detect respective ions. Kind.

Referring to Figure 8, it is shown that excitation signals for different excitation frequencies scan a respective frequency range to address different ions with different masses, thereby generating different image current signals. The excitation signal is a parametric excitation, which has a frequency that is twice higher than the image current signal shown in FIG. 8 . Wherein, at the beginning of the respective excitation signal, the oscillation of the ions in the capture volume 20 is established until the excitation signal reaches a signal at a set threshold level according to a preset time, or a signal caused by the discharge of ions from the storage volume. Lost and stopped.

Different from scanning a frequency range in a continuous manner or a stepwise manner as shown in Figure 8, it is also possible to address different ions with different masses by superimposing different excitation signals, thereby simultaneously exciting different ions of the ion species. of oscillation.

Referring to Figure 6, a frequency range relevant to the present invention is shown. Among them, curve 34 shows the respective detection profile of the used detector. Indicates the maximum frequency that can be detected by the detector, where Indicates the upper limit cutoff frequency of the respective detector 30. Therefore, since it is possible to detect and The parameter excitation frequency of the image current signal is between, resulting in an excitation frequency between and between. increase will result in a frequency that cannot be detected by the detector 30, where This will result in unnecessary overlap between the excitation signal and the detection range of the detector 30 .

Referring to Figure 7, instead of considering the detection profile 34 itself, a notch filter 38 or a low pass filter 40 can be used to shape the respective detection profile 34 accordingly and separate the image current signal from the excitation signal. Wherein, when using a notch filter, the detected contour is converted into a curve 36a. exist At , when excited, due to the notch filter, there will be no detection of the excitation signal, where, at , the image current signal can be reliably detected. Thus, by means of a notch filter 38, lower frequencies for parametric excitation can be used even within the detection profile 34 of the detector 30. The same applies to the use of a low-pass filter 40, which shapes the detection contour 34 as curve 36b as shown in Figure 7, separating the excitation signal from the image current signal, even for lower frequencies. That's right.

Referring now to FIGS. 9A to 9C , the ion response signal 100 shown in FIG. 9C shows the complete response of an ion species in the frequency domain. If the frequency of the excitation signal now slowly scans the entire frequency range of the ion response, or steps in discrete steps smaller than the width of the ion resonance 100, as shown in Figures 9A and 9B, and during each discrete frequency step or at a In a short scan, if 108 image responses are detected within a sufficient averaging period, a derivative spectrum can be generated. Thus, referring to Figure 11, for ion species with different ions of different masses, the differential signal 106 obtained by the detection scheme described with respect to Figure 9 results in different curves with respect to the ions contained in the species. If only one type of ion is present within the detection range scanned by the excitation signal 102, a single peek 100 is displayed. If there are two or more different ions with slightly different masses within the detection range, the ion response 100' is the sum of the individual ion response signals 101'. In the differential measurement signal 106', a clear second peek can be identified, providing a clear indication of a mixture of ions (also called an isobaric species) containing different ions with slightly different masses.

Referring to FIG. 10A and FIG. 10C , in another detection scheme, the frequency of the excitation signal is switched between two different frequencies within the detection range. The excitation signal 102 switches between positive resonance and non-resonance relative to the ion response signal 100 . The switching frequency can be at a frequency well below one of the amplifier's passbands. Thus, a composite signal is formed between the positively resonant and non-resonant image current signals 110, which is synchronously demodulated with respect to the frequency switching signal to provide a measure of the total peak height.

Therefore, a new method for detecting ions in an ion trap using parametric excitation to separate the excitation signal from the detection signal is provided. Therefore, there is no need for multiple tasks, and stimulation and measurement can be performed at the same time. In addition, new detection schemes can be exploited to gain insights into ion species.

10:Ion trap 14: Second electrode/electrode 16:Third electrode/electrode 18: First electrode/electrode 20: Capture volume 22:Signal supply 24:First RF signal source 26:Signal supply 28:Control unit 30: Detector 32:Arrow 34:Curve/Detect Contour 38: Notch filter 40: Low pass filter 100: Ion response signal/ion resonance/single peek 100’: ion response 101: Ion response signal 101’: Individual ion response signal 102: Excitation signal 106: Differential signal 106’: Differential measurement signal 108:Image response 110:Image current signal S01: Steps S02: Steps S03: Steps S04: Steps S031: Steps S032: Steps

The invention is described in more detail below with reference to the accompanying drawings.

Graphic display: Figure 1 A schematic ion trap according to the present invention; Figure 2 is a flow chart of the method according to the present invention; Figure 3 Another embodiment of the method according to the present invention; Figure 4 Another embodiment of the method according to the present invention; Figure 5 Example of signal in ion trap; Figure 6 shows a frequency representation used in the present invention in a first embodiment; Figure 7 shows a frequency representation in another embodiment; Figure 8: One of the excitation schemes of the present invention; Figures 9A to 9C are based on a first detection scheme of the present invention; Figures 10A to 10C are a second detection scheme according to the present invention; and Figure 11 An example signal result obtained from the detection scheme of Figure 9 or Figure 10.

10:Ion trap

14: Second electrode/electrode

16:Third electrode/electrode

18: First electrode/electrode

20: Capture volume

22:Signal supply

24:First RF signal source

26:Signal supply

28:Control unit

30: Detector

Claims (14)

A method for detecting ions in an ion trap, comprising: a) Provide ionized ions to the ion trap; b) creating an RF storage field by applying a first RF signal to a first electrode of the ion trap; c) applying an excitation signal to the plasma in the ion trap; d) Detect an image current signal induced to a second electrode, a third electrode or a difference between the second electrode and the third electrode through the oscillation of the plasma excited by the excitation signal; Wherein, the excitation signal is a parameter excitation. The method of claim 1, wherein the excitation signal has one frequency, where is the oscillation frequency of the plasma. The method of claim 1 or 2, wherein applying the excitation signal and detecting the image current signal are performed simultaneously. The method of any one of claims 1 to 3, wherein the excitation signals are repeatedly applied sequentially at different frequencies, or wherein more than one excitation signals are applied simultaneously at different frequencies. The method of claim 4, wherein after applying a first excitation signal and before applying a second excitation signal with a different frequency, ions excited by the first excitation signal are discharged from the ion trap. The method of any one of claims 1 to 5, wherein the frequency of the excitation signal sweeps across a predetermined range. The method of any one of claims 4 to 5, wherein the frequency of the excitation signal is switched between a non-resonant frequency and a positive resonant frequency. The method of any one of claims 1 to 7, wherein for the excitation signal , which produces ,, and for this image current signal, it generates ,in is the frequency upper limit of the detector and is the cutoff frequency of the detector. The method of any one of claims 1 to 8, wherein the image current signal Filtered, wherein signals at the frequency of the excitation signal are filtered out. The method of any one of claims 1 to 9, wherein the excitation signal is less than one modulation frequency Modulation, wherein the detected image current signal is demodulated at the modulation frequency. A method as claimed in any one of claims 1 to 10, wherein phase-sensitive detection of the image current signal is applied. The method of any one of claims 1 to 11, wherein before applying the excitation signal, a dipole signal is applied. An ion trap for capturing and detecting ions, which includes: a first electrode, a second electrode, and a third electrode defining a capture volume; a first RF signal source connected to the first electrode and configured to generate an RF storage field; a second RF signal source connected to the first electrode and configured to generate an excitation signal; and a detector connected to the second electrode and/or the third electrode and configured to detect an image current induced by the oscillation of the plasma excited by the excitation signal; Wherein, the excitation signal is a parameter excitation. The ion trap of claim 13, wherein the first electrode is a ring electrode, and the second electrode and the third electrode are cover electrodes.

Images