Control method of BN type ion gate in ion mobility spectrometry

Abstract

The invention discloses a novel method for regulating and controlling a Bradbury-Nielsen type ion gate in an ion mobility spectrometry. According to the method, by changing the control voltage of the ion gate, the performance of the Bradbury-Nielsen type ion gate in two aspects is improved. Firstly, by increasing the voltage difference between two groups of grid wires when an ion gate is closed, ions in an ionization region of the ion mobility spectrometry are affected by a non-uniform electric field to realize compression of spatial density, and the sensitivity of the ion mobility spectrometry is improved; and on the other hand, the loss of ions with different migration rates in a clearing area when the ion gate is closed is reduced through the voltage matching control of two groups of grid wires of the Bradbury-Nielsen type ion gate, so that the discrimination on the migration rates is reduced, and the enriched ions are fully utilized to improve the sensitivity of an instrument.

Description

Technical Field

[0001] The present invention is a working control method for a BN-type ion gate of an ion mobility spectrometer component. By changing the conventional working mode, control pulses are added to two groups of grids of the BN-type ion gate to cooperate with each other to control and adjust the ion gate, thereby achieving improved mobility discrimination and ion enrichment to enhance sensitivity. Background Art

[0002] The ion gate in ion mobility spectrometry (IMS) is one of the key components that affect the performance of the instrument. The ion gate controls the product ions to enter the drift region by periodically opening and closing, thereby achieving the separation and detection of ions. The on-off time, voltage setting and structure of the ion gate have a significant impact on the sensitivity and resolution of the IMS instrument. The Bradbury-Nielsen ion gate (BNG) is one of the commonly used ion gate structures in IMS. It is composed of parallel thin metal wires or spiral wire electrodes, which are separated in the form of interdigitated fingers to form independent electrodes in the same plane. Under the action of the electric field, the ion gate controls the passage of ions by adjusting the electrode voltage. When the potentials of the two electrodes are the same, the ions can pass through the electrode plane area; when there is a certain potential difference between the two electrodes, the ions are deflected, thereby achieving the control of the ions. The significant advantage of the BNG ion gate is its wide applicability. It is not limited by the type of ionization source and can seamlessly connect to various migration tubes. In addition, the gate potential difference is small and the control circuit design is relatively simple, so it is favored by commercial ion mobility spectrometers. However, the disadvantage of BNG is that the introduction of the vertical gate electric field will cause the electric field around the ion gate to be distorted, and the higher the gate voltage, the more serious the electric field distortion phenomenon. This electric field distortion will not only cause the morphology of the ion cluster to change, but may also lead to mobility discrimination, thus adversely affecting the overall performance of the ion mobility spectrometry.

[0003] In a study in 2012, Du Yongzhai et al. observed that when BNG is closed, the gate-off electric field generated by it will penetrate into the adjacent ion migration zone and reaction zone. The penetration effect of this electric field into the migration zone causes the electric field in the migration zone near the ion gate to be instantly strengthened, thereby compressing the ion group passing through the BNG. As a result, the half-peak width of the ion peak detected by the IMS is reduced, thereby improving the resolution of the IMS. At the same time, the penetration of the gate-off electric field also leads to the formation of obvious ion clearance zones on both sides of the BNG. The axial depth of this zone is significantly greater than the wire diameter of the BNG and is roughly equivalent to the wire spacing. This phenomenon has two effects: first, during the BNG gate opening time, only ions that cross the ion clearance zone can reach the ion detection electrode for analysis, which means that the actual time width of the ion group entering the migration zone is much shorter than the BNG opening time, which weakens the detection sensitivity of the IMS; second, after the BNG opening time ends, the ion clearance zone formed by its closing electric field on the side of the IMS migration zone will attract some ions that have entered the migration zone back to the BNG electrode and consume them, which further reduces the detection sensitivity of the IMS. This effect becomes more serious as the voltage difference (Gating Voltage Difference, GVD) between the two sets of grids increases when the gate is closed. Eiceman et al. also found that there is a potential well in front of the BNG. When the ion gate is closed, ions will be enriched in this area. And there is evidence that the larger the GVD, the stronger the enrichment effect of this potential well.

[0004] The present invention proposes a new method for controlling BNG. While increasing the enrichment effect of the BNG front region by increasing the GVD, the consumption of ion clusters in the IMS migration region by the ion clearance region formed by the BNG gate voltage is reduced, so that the enriched ions with different mobility can be efficiently injected into the migration region, thereby improving the IMS detection sensitivity. Summary of the invention

[0005] The present invention provides a control method for a BN type ion gate in an ion mobility spectrometer, which aims to utilize the BNG front potential well to enrich ions while weakening the ion loss caused by the mobility discrimination caused by the BNG clearance zone through a pulse waveform, thereby improving the IMS sensitivity.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for controlling a BN-type ion gate in an ion mobility spectrometer, wherein the ion mobility spectrometer comprises an ion migration tube, and the ion migration tube comprises an ionization source, an ionization region, a Bradbury-Nielsen-type ion gate, a migration region and an ion receiving electrode arranged in sequence along an ion migration direction;

[0008] The Bradbury-Nielsen type ion gate is composed of more than four strip electrodes arranged in parallel from top to bottom, from top to bottom, the odd-numbered strip electrodes serve as the first gate electrode, and the even-numbered strip electrodes serve as the second gate electrode; or, it is composed of ring electrodes with the same geometric center arranged in parallel from inside to outside; from inside to outside, the odd-numbered ring electrodes serve as the first gate electrode, and the even-numbered ring electrodes serve as the second gate electrode;

[0009] The first gate electrode and the second gate electrode are respectively connected to independent pulsed DC high voltage power supplies; and the first gate electrode and the second gate electrode are insulated from each other;

[0010] applying a pulse voltage to the first gate electrode and the second gate electrode in sequence according to a first preset time period t1, a second preset time period t2 and a third preset time period t3;

[0011] In a first preset time period t1, a first voltage V1 is applied to the first gate electrode and the second gate electrode at the same time, thereby generating a direct current electric field in the ion transfer tube, the direction of which is from the ion source to the ion receiving electrode, under which the ions generated in the ionization region pass through the ion gate and enter the transfer region;

[0012] In the second preset time period t2, a second voltage V2 higher than the first voltage V1 is simultaneously applied to the first gate electrode and the second gate electrode, a direct current electric field is formed in the ionization region of the ion transfer tube along the ion gate pointing to the ion source direction, and the ions in the ionization region move toward the ion source direction, and a direct current electric field is formed in the migration region of the ion transfer tube along the ion gate pointing to the ion receiving electrode direction, and the ions entering the migration region move toward the ion receiving electrode;

[0013] In the third preset time period t3, the first gate electrode maintains the second voltage V2 which is higher than the first voltage V1 in the second preset time period t2, while the voltage applied by the second gate electrode returns to the first voltage V1. A DC electric field is formed in the ionization region of the ion migration tube, which is directed from the ion source to the ion gate, prompting the ions in the ionization region to move toward the ion gate. At the same time, an electric field perpendicular to the axis of the ion migration tube is generated between the first gate electrode and the second gate electrode, which effectively prevents the ions in the ionization region from passing through the ion gate into the migration region. In the migration region, a DC electric field is formed in the direction from the ion gate to the ion receiving electrode. Under the action of this electric field, the ions in the migration region will successively reach the ion receiving electrode and be detected.

[0014] The strip electrode is a metal wire or a spiral wire electrode wound on a cylinder or a metal sheet or a metal mesh; the ring electrode is a circular ring electrode or a square ring electrode.

[0015] The value of the first preset time period t1 is between 0.001ms and 1ms, the value of the second preset time period t2 is between 0.001ms and 1ms, and the value of the third preset time period t3 is between 1ms and 50ms. The sum of the first preset time period t1, the second preset time period t2 and the third preset time period t3 constitutes a complete cycle of the ion gate operation. When the ion mobility spectrometer is working, the voltage applied to the first gate electrode and the second gate electrode is periodically adjusted and changed according to the first preset time period t1, the second preset time period t2 and the third preset time period t3.

[0016] The first preset time period t1 is the opening time of the ion gate, and ions are injected from the ionization region into the migration region through the ion gate.

[0017] The sum of the second preset time period t2 and the third preset time period t3 is the closing time of the ion gate, and ions cannot enter the migration region.

[0018] The first preset time period t1 and the second preset time period t2 must be closely connected in time. There must be no delayed operation or time overlap between the two time periods, and a strict front-to-back connection relationship must be maintained.

[0019] The first voltage V1 is the position voltage of the ion transfer tube. Taking the first voltage V1 as a reference, the difference between the second voltage V2 and the first voltage is generally 10-700V, and V2>V1.

[0020] Beneficial Effects

[0021] The pulse voltage waveform applied to BNG described in the present invention can improve GVD to obtain a stronger potential well enrichment effect when the ion gate is closed, and can increase the sensitivity of IMS. However, the increase in GVD is accompanied by a stronger clearing zone effect, which causes the enriched ions to be lost. The newly applied pulse waveform can keep the ion gate closed within the second preset time interval while making the potentials of the two sets of grids equal, eliminating the problem of the clearing zone, thereby weakening the mobility discrimination problem caused by it, and allowing the IMS to obtain higher sensitivity.

[0022] The present invention can reduce the mobility discrimination of the ion mobility spectrum and improve the detection sensitivity, the method is simple, and there is no need to improve the hardware of the ion mobility tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The present invention provides an ion migration tube with a Bradbury-Nielsen type ion gate inside, wherein: 1—waste gas outlet; 2—sample gas inlet; 3—drift gas inlet; 4—ionization source; 5—ion receiving electrode; 6—ionization region; 7—migration region; 8—Bradbury-Nielsen type ion gate; 8-1—first gate electrode; 8-2—second gate electrode.

[0024] Figure 2 The control voltage pulse timing diagram of the Bradbury-Nielsen type ion gate in Example 1 disclosed in the present invention, wherein: the voltage of the first gate electrode varies between V1 and V2, the voltage of the second gate electrode varies between V1 and V2, the difference between V1 and V2 is generally 10-700V, t1=0.1ms, t2=0.1ms, t3=14.8ms, V1=6600V, V2=7300V.

[0025] Figure 3 for Figure 1 Conventional control voltage pulse timing diagram of Bradbury-Nielsen type ion gate, where: the voltage of the first gate electrode varies between V1 and V2, the voltage of the second gate electrode remains unchanged at V1, the difference between V1 and V2 is generally 10-700V, t1=0.1ms, t2=14.9ms, V1=6600V, V2=6700V. Figure 4 Comparison of ion migration spectra of 20 ppb TEP obtained with two different voltage control timings in Example 2 and Example 1. DETAILED DESCRIPTION

[0026] The following are specific embodiments of the present invention.

[0027] Example 1

[0028] like Figure 1 The ion transfer tube of the Bradbury-Nielsen type ion gate provided by the present invention is shown in the figure. The ionization source 4 of the ion transfer tube uses a VUV ultraviolet lamp, the ionization zone 6 is 20 mm long, the electric field strength of the ionization zone is 60 V / mm, the migration zone 7 is 96 mm long, and the electric field strength is 60 V / mm; the Bradbury-Nielsen type ion gate 8 is composed of two sets of mutually insulated wire electrodes, the first gate electrode 8-1 and the second gate electrode 8-2, with a wire diameter of 0.1 mm and a wire spacing of 1 mm.

[0029] In the first preset time period t1=0.1ms, the first voltage V1=6600V is applied to the first gate electrode 8-1 and the second gate electrode 8-2 simultaneously, forming a DC electric field in the ion migration tube from the ion source to the ion receiving electrode, and the ions in the ionization zone enter the migration zone through the ion gate.

[0030] Within the second preset time period t2=0.1ms, a second voltage V2=7300V is simultaneously applied to the first gate electrode 8-1 and the second gate electrode 8-2, and a DC electric field is formed in the ionization zone of the ion migration tube along the direction of the ion gate to the ion source, and the ions in the ionization zone move toward the ion source, and a DC electric field is formed in the migration zone of the ion migration tube along the direction of the ion gate to the ion receiving electrode, and the ions entering the migration zone move toward the ion receiving electrode.

[0031] In the third preset time period t3 = 14.8ms, the second voltage V2 = 7300V is applied to the first gate electrode 8-1, and the first voltage V1 = 6600V is applied to the second gate electrode 8-2. In the ionization region of the ion migration tube, a DC electric field is generated from the ion source to the ion gate. In the ionization region, ions will move in the direction of the ion gate. At the same time, between the first gate electrode 8-1 and the second gate electrode 8-2, an electric field perpendicular to the axial direction of the ion migration tube is generated. The function of this electric field is to prevent ions in the ionization region from passing through the ion gate into the migration region. In the migration region of the ion migration tube, another DC electric field is formed, and its direction is from the ion gate to the ion receiving electrode. Under the action of this DC electric field, the ions in the migration region will successively reach the ion receiving electrode and be detected.

[0032] The first preset time period t1 = 0.1 ms, the second preset time period t2 = 0.1 ms, and the third preset time period t3 = 14.8 ms add up to t = 15 ms, which constitutes a complete time cycle of the ion gate operation.

[0033] When the ion transfer tube is working, the voltage applied to the first gate electrode 8-1 and the second gate electrode 8-2 of the ion gate is in accordance with Figure 2 The voltage change timing given in the figure changes periodically with a period of 15ms.

[0034] Example 2

[0035] Figure 4 The spectrum (a) in Example 1 shows the BNG Figure 2 The 20 ppb TEP spectrum obtained under the voltage control method shown in FIG. The TEP dimer signal intensity corresponding to the ion gate control mode is 510 pA, and the resolution is 90.

[0036] Comparative Example 1

[0037] Figure 4 The spectrum (b) in the figure shows the conventional BNG voltage control method (such as Figure 3 The corresponding TEP dimer signal intensity is only 35pA, with a resolution of 91.

[0038] It can be seen from the TEP dimer signal intensity and resolution data obtained under different BNG voltage control methods in Example 2 and Comparative Example 1 that the novel Bradbury-Nielsen type ion gate control method in the present invention significantly improves the TEP dimer signal intensity while maintaining the resolution basically unchanged. For ions with small mobility, the new control mode can reduce mobility discrimination and thus achieve higher sensitivity detection.

Claims

1. A method for controlling a BN type ion gate in an ion mobility spectrometer, wherein the ion mobility spectrometer comprises an ion migration tube, wherein the ion migration tube comprises an ion source (4), an ionization region (6), a Bradbury-Nielsen type ion gate (8), a migration region (7) and an ion receiving electrode (5) arranged in sequence along an ion migration direction; characterized in that: The Bradbury-Nielsen type ion gate (8) is composed of more than four strip electrodes arranged in parallel from top to bottom, from top to bottom, odd-numbered strip electrodes serve as first gate electrodes (8-1), and even-numbered strip electrodes serve as second gate electrodes (8-2); or, it is composed of ring electrodes with the same geometric center arranged in parallel from inside to outside; from inside to outside, odd-numbered ring electrodes serve as first gate electrodes (8-1), and even-numbered ring electrodes serve as second gate electrodes (8-2); The first gate electrode (8-1) and the second gate electrode (8-2) are respectively connected to an independent pulsed DC high voltage power supply; and the first gate electrode (8-1) and the second gate electrode (8-2) are insulated from each other; applying pulse voltages to the first gate electrode (8-1) and the second gate electrode (8-2) in sequence according to a first preset time period t1, a second preset time period t2 and a third preset time period t3; In a first preset time period t1, a first voltage V1 is applied to the first gate electrode (8-1) and the second gate electrode (8-2) at the same time, thereby generating a direct current electric field in the ion transfer tube, the direction of which is from the ion source to the ion receiving electrode. Under this effect, the ions generated in the ionization region enter the transfer region through the ion gate; In a second preset time period t2, a second voltage V2 higher than the first voltage V1 is simultaneously applied to the first gate electrode (8-1) and the second gate electrode (8-2), so that a direct current electric field is formed in the ionization region of the ion transfer tube along the ion gate pointing to the ion source direction, and ions in the ionization region move toward the ion source direction, and a direct current electric field is formed in the migration region of the ion transfer tube along the ion gate pointing to the ion receiving electrode direction, and ions entering the migration region move toward the ion receiving electrode; In the third preset time period t3, the first gate electrode (8-1) maintains a second voltage V2 higher than the first voltage in the second preset time period t2, while the voltage applied by the second gate electrode (8-2) returns to the first voltage V1; a DC electric field is formed in the ionization region of the ion migration tube, pointing from the ion source to the ion gate, causing the ions in the ionization region to move toward the ion gate; at the same time, an electric field perpendicular to the axis of the ion migration tube is generated between the first gate electrode (8-1) and the second gate electrode (8-2), and this electric field effectively prevents the ions in the ionization region from passing through the ion gate into the migration region; a DC electric field is formed in the migration region, pointing from the ion gate to the ion receiving electrode, and under the action of this electric field, the ions in the migration region will successively reach the ion receiving electrode and be detected.

2. The control method according to claim 1, characterized in that: The value of the first preset time period t1 is between 0.001ms-1ms, the value of the second preset time period t2 is between 0.001ms-1ms, and the value of the third preset time period t3 is between 1ms-50ms. The sum of the first preset time period t1, the second preset time period t2 and the third preset time period t3 constitutes a complete cycle of the ion gate operation.

3. The control method according to claim 1, characterized in that: The first voltage V1 is the position voltage of the ion transfer tube, the difference between the second voltage V2 and the first voltage V1 is between 10-700V, and V2>V1.

4. The control method according to claim 1, characterized in that: The first preset time period t1 is the opening time of the ion gate, and the sum of the second preset time period t2 and the third preset time period t3 is the closing time of the ion gate. The first preset time period t1 and the second preset time period t2 are closely connected in time, and there is no delayed operation and no time overlap between the two time periods.

5. The control method according to claim 1, characterized in that: The strip electrode is a metal wire or a spiral wire electrode wound on a cylinder or a metal sheet or a metal mesh; the ring electrode is a circular ring electrode or a square ring electrode.

6. The pulse voltage waveform according to claim 1, characterized in that: When the ion mobility spectrometer is working, the voltage applied to the first gate electrode (8-1) and the second gate electrode (8-2) is periodically adjusted and changed according to the first preset time period t1, the second preset time period t2 and the third preset time period t3.

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