Method for controlling BN type ion gate in ion mobility spectrometry

Abstract

The application discloses a new method for regulating Bradbury-Nielsen ion gate in ion mobility spectrometry. The method realizes the improvement of two aspects of Bradbury-Nielsen ion gate by changing the control voltage of the ion gate. One is that the voltage difference between the two groups of grid wires when the ion gate is closed is improved, so that the ions in the ionization zone of the ion mobility spectrometer are affected by the non-uniform electric field to realize the compression of the spatial density, and the sensitivity of the ion mobility spectrometer is improved. The other is that the voltage of the two groups of grid wires of the Bradbury-Nielsen ion gate is controlled, the loss of the empty area of the ion gate when the ion gate is closed to different mobility ions is reduced, the mobility discrimination is reduced, and the sensitivity of the instrument is improved by fully utilizing the enriched ions.

Description

TECHNICAL FIELD

[0001] The application is a working control method of BN type ion gate of ion mobility spectrometry component. By changing the conventional working mode, the control pulse is added to the two groups of grid wires of the BN type ion gate to control and adjust the ion gate, so as to improve the mobility discrimination and enhance the ion enrichment sensitivity. BACKGROUND

[0002] The ion gate in ion mobility spectrometry (IMS) is one of the key components affecting the performance of the instrument. The ion gate controls the product ions to enter the drift region by periodically opening and closing, so as to realize the separation and detection of ions. The on-off time, voltage setting and structure of the ion gate have a significant influence on the sensitivity and resolution of the IMS instrument. Bradbury-Nielsen type ion gate (BNG) is one of the commonly used ion gate structures in IMS. It is composed of parallel arranged thin metal wires or spiral line electrodes. These electrodes are spaced apart in an interdigital form 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 potential of the two electrodes is the same, the ions can pass through the electrode plane area; when the potential of the two electrodes has a certain potential difference, the ions are deflected, thereby realizing the control of the ions. The significant advantage of the BNG type ion gate is its wide applicability. It is not limited by the type of ionization source and can seamlessly connect various mobility tubes. In addition, the gate closing potential difference is small, and the control circuit design is relatively simple, so it is favored by commercial ion mobility spectrometry instruments. However, the disadvantage of BNG is that the introduction of the vertical gate closing electric field will cause the distortion of the electric field around the ion gate, and the higher the gate closing voltage, the more serious the electric field distortion. This electric field distortion not only causes the shape change of the ion cluster, but also may cause the mobility discrimination phenomenon, thereby adversely affecting the overall performance of the ion mobility spectrum.

[0003] In 2012, Du Yongzai et al. observed that when the BNG was closed, the closing electric field generated by the BNG penetrated into the adjacent ion migration zone and reaction zone. The penetration of the electric field into the migration zone caused the electric field near the ion gate part of the migration zone to be strengthened instantaneously, which in turn compressed the ion groups passing through the BNG, resulting in a decrease in the half-peak width of the ion peak detected by the IMS, thereby improving the resolution capability of the IMS. At the same time, the penetration of the closing electric field also caused the formation of a clear ion depletion zone on both sides of the BNG, the axial depth of the zone being significantly greater than the wire diameter of the BNG and being roughly equivalent to the wire spacing. This phenomenon had two effects: first, during the opening time of the BNG, only the ions that had passed through the ion depletion zone could reach the ion detection electrode for analysis, which meant that the time width of the ion groups actually entering the migration zone was much lower than the opening time of the BNG, thereby weakening the detection sensitivity of the IMS; second, after the opening time of the BNG ended, the ion depletion zone formed by the closing electric field near the IMS migration zone would attract some of the ions that had entered the migration zone back to the BNG electrode and consume them, which further reduced the detection sensitivity of the IMS. Moreover, this effect would become more serious as the GVD (Gating Voltage Difference) between the two sets of grid wires increased. Eiceman et al. also found that there was a potential well in front of the BNG, and when the ion gate was closed, ions would be enriched in this area. There is evidence to show that the greater the GVD, the stronger the enrichment effect of this potential well.

[0004] The present application will propose a new method for controlling the BNG, which can increase the enrichment effect of the front area of the BNG by increasing the GVD while reducing the consumption of ion groups in the IMS migration zone caused by the ion depletion zone formed by the closing voltage of the BNG, so that the enriched ions of different mobilities can be efficiently injected into the migration zone, thereby improving the detection sensitivity of the IMS. SUMMARY

[0005] The present application provides a control method for a BN-type ion gate in ion mobility spectrometry, which aims to utilize the ion enrichment in the potential well in front of the BNG while weakening the ion loss caused by the mobility discrimination of the ion depletion zone of the BNG through pulse waveforms, thereby improving the sensitivity of the IMS.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] A control method for a BN-type ion gate in ion mobility spectrometry, wherein the ion mobility spectrometry comprises an ion mobility tube, and the ion mobility tube comprises, in sequence along the ion migration direction, an ionization source, an ionization zone, a Bradbury-Nielsen type ion gate, a migration zone, and an ion receiving electrode.

[0008] The Bradbury-Nielsen type ion gate is composed of more than four strip electrodes arranged in parallel and spaced apart from top to bottom, from top to bottom, the odd strip electrodes are used as the first gate electrodes, and the even strip electrodes are used as the second gate electrodes; or, the ion gate is composed of ring electrodes with the same geometric center arranged in a spaced-apart manner from inside to outside of the geometric center; from inside to outside, the odd ring electrodes are used as the first gate electrodes, and the even ring electrodes are used as the second gate electrodes.

[0009] The first gate electrodes and the second gate electrodes are respectively connected with independent pulse direct-current high-voltage power supplies; and the first gate electrodes and the second gate electrodes are insulated from each other.

[0010] Pulse voltages are sequentially applied to the first gate electrodes and the second gate electrodes in a first preset time period t1, a second preset time period t2 and a third preset time period t3.

[0011] In the first preset time period t1, the first voltages V1 are simultaneously applied to the first gate electrodes and the second gate electrodes, so that a direct-current electric field is generated in the ion migration tube, the direction of the electric field is from the ion source to the ion receiving electrode, under the action of the electric field, the ions generated in the ionization zone enter the migration zone through the ion gate;

[0012] In the second preset time period t2, the second voltages V2 higher than the first voltages V1 are simultaneously applied to the first gate electrodes and the second gate electrodes, a direct-current electric field is formed in the ionization zone of the ion migration tube, the direction of the electric field is from the ion gate to the ion source, the ions in the ionization zone move toward the ion source, a direct-current electric field is formed in the migration zone of the ion migration tube, the direction of the electric field is from the ion receiving electrode to the ion gate, the ions entering the migration zone move toward the ion receiving electrode;

[0013] In the third preset time period t3, the first gate electrodes maintain the second voltages V2 higher than the first voltages V1 in the second preset time period t2, and the voltages applied to the second gate electrodes return to the first voltages V1. A direct-current electric field is formed in the ionization zone of the ion migration tube, the direction of the electric field is from the ion source to the ion gate, so as to promote the ions in the ionization zone 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 electrodes and the second gate electrodes, the electric field effectively prevents the ions in the ionization zone from entering the migration zone through the ion gate. In the migration zone, a direct-current electric field is formed, the direction of the electric field is from the ion gate to the ion receiving electrode. Under the action of the electric field, the ions in the migration zone will reach the ion receiving electrode in turn and be detected.

[0014] The strip electrodes are metal wires or spiral wire electrodes wound on a cylindrical body or metal sheets or metal meshes; the ring electrodes are circular ring electrodes or square ring electrodes.

[0015] The value of the first preset time period t1 is between 0.001 ms and 1 ms, the value of the second preset time period t2 is between 0.001 ms and 1 ms, and the value of the third preset time period t3 is between 1 ms and 50 ms. 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 spectrometry is in operation, the voltage applied to the first gate electrode and the second gate electrode is periodically and cyclically 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 the ions are injected from the ionization zone into the migration zone 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 the ions cannot enter the migration zone.

[0018] The first preset time period t1 and the second preset time period t2 must be closely connected in time, and there cannot be a delay operation or a time overlap between the two time periods, and a strict front and rear connection relationship must be maintained.

[0019] The first voltage V1 is the position voltage of the ion mobility tube. With the first voltage V1 as the reference, the difference between the value of the second voltage V2 and the value of the first voltage is generally between 10 V and 700 V, and V2>V1.

[0020] Beneficial effects

[0021] The pulse voltage waveform applied to the BNG can improve the GVD and obtain a stronger potential well enrichment effect when the ion gate is closed, and can increase the IMS sensitivity. However, the increase of the GVD will be accompanied by a stronger emptying zone effect, so that the enriched ions are lost. The newly applied pulse waveform can keep the ion gate closed in the second preset time interval, and at the same time, the potentials of the two groups of grid wires are equal, which eliminates the problem of the emptying zone, thereby weakening the mobility discrimination problem caused thereby, so that the IMS obtains higher sensitivity.

[0022] The present application can realize the reduction of ion mobility discrimination and the improvement of detection sensitivity of the ion mobility spectrometry, and the method is simple and does not need to improve the hardware of the ion mobility tube. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The present application provides an ion mobility 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 zone; 7-migration zone; 8-Bradbury-Nielsen type ion gate; 8-1-first gate electrode; 8-2-second gate electrode.

[0024] Figure 2 A control voltage pulse timing diagram of a Bradbury-Nielsen ion gate in Example 1 of the present application, 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 between 10-700V, t1 = 0.1ms, t2 = 0.1ms, t3 = 14.8ms, V1 = 6600V, V2 = 7300V.

[0025] Figure 3 A control voltage pulse timing diagram of a Bradbury-Nielsen ion gate in Example 1 of the present application, 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 between 10-700V, t1 = 0.1ms, t2 = 0.1ms, t3 = 14.8ms, V1 = 6600V, V2 = 7300V. Figure 1

[0026] Figure 4 A comparison of ion mobility spectra of 20ppb TEP obtained by two different voltage control timing of Example 2 and Example 1.DETAILED DESCRIPTION

[0027] The following is a detailed description of the present application.

[0028] Example 1

[0029] As shown in Example 1 of the present application, an ion mobility tube of a Bradbury-Nielsen ion gate is provided. The ionization source 4 of the ion mobility tube uses a VUV ultraviolet lamp, the ionization region 6 has a length of 20mm, the ionization region has an electric field strength of 60V / mm, the migration region 7 has a length of 96mm, and the electric field strength is 60V / mm; the Bradbury-Nielsen ion gate 8 is composed of two groups of mutually insulated filament electrodes, i.e. the first gate electrode 8-1 and the second gate electrode 8-2, the filament diameter is 0.1mm, and the filament spacing is 1mm. Figure 1 During the first preset period t1 = 0.1ms, the first gate electrode 8-1 and the second gate electrode 8-2 are simultaneously applied with the first voltage V1 = 6600V, a direct current electric field is formed in the ion mobility tube along the direction from the ionization source to the ion receiving electrode, and the ions in the ionization region enter the migration region through the ion gate.

[0030]

[0031] ​​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 region of the ion transfer tube along the direction of the ion gate pointing to the ion source, and the ions in the ionization region move toward the ion source, and a DC electric field is formed in the migration region of the ion transfer tube along the direction of the ion gate pointing to the ion receiving electrode, and the ions entering the migration region move toward the ion receiving electrode.

[0032] During the third preset time period t3 = 14.8 ms, a second voltage V2 = 7300 V is applied to the first gate electrode 8-1, and a first voltage V1 = 6600 V is applied to the second gate electrode 8-2. This generates a DC electric field in the ionization region of the ion transfer tube, directed from the ion source toward the ion gate. In the ionization region, ions migrate toward the ion gate. Simultaneously, an electric field perpendicular to the axis of the ion transfer tube is generated between the first gate electrode 8-1 and the second gate electrode 8-2. This electric field prevents ions in the ionization region from passing through the ion gate and into the transfer region. In the transfer region of the ion transfer tube, another DC electric field is generated, directed from the ion gate toward the ion receiving electrode. Under the influence of this DC electric field, ions in the transfer region sequentially reach the ion receiving electrode and are detected.

[0033] 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 period for the ion gate to operate.

[0034] 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.

[0035] Example 2

[0036] Figure 4 The spectrum (a) shows the BNG in Example 1. Figure 2 The 20 ppb TEP spectrum obtained under the voltage control method shown. The TEP dimer signal intensity corresponding to this ion gating mode is 510 pA, with a resolving power of 90.

[0037] Comparative Example 1

[0038] Figure 4 The spectrum (b) shows the conventional BNG voltage control mode (such as Figure 3 The corresponding TEP dimer signal intensity is only 35 pA, with a resolution of 91.

[0039] From the TEP dimer signal intensity and resolution capability data obtained by different BNG voltage control methods in Example 2 and Comparative Example 1, it can be seen that the control method of the new Bradbury-Nielsen type ion gate in the application significantly improves the TEP dimer signal intensity while the resolution capability remains basically unchanged. The new control mode can weaken the mobility discrimination for ions with small mobility, thereby achieving higher sensitivity detection.

Claims

1. A method for controlling a BN-type ion gate in ion mobility spectrometry, the ion mobility spectrometry comprising an ion mobility tube, the ion mobility tube comprising, in sequence along an ion mobility direction, an ionization source (4), an ionization region (6), a Bradbury-Nielsen-type ion gate (8), a mobility region (7), and an ion receiving electrode (5); the method being characterized by: the Bradbury-Nielsen-type ion gate (8) being formed by four or more strip electrodes arranged in parallel and spaced apart from each other from top to bottom, wherein, 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, the Bradbury-Nielsen-type ion gate (8) being formed by ring electrodes with the same geometric center arranged in spaced apart from each other from inside to outside, wherein, 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 electrodes (8-1) and the second gate electrodes (8-2) being respectively connected to independent pulsed direct-current high-voltage power supplies; and the first gate electrodes (8-1) and the second gate electrodes (8-2) being insulated from each other; applying pulsed voltages to the first gate electrodes (8-1) and the second gate electrodes (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 the first preset time period t1, simultaneously applying a first voltage V1 to the first gate electrodes (8-1) and the second gate electrodes (8-2) to generate a direct-current electric field in the ion mobility tube, the electric field being directed from the ionization source to the ion receiving electrode; under the action of the electric field, ions generated in the ionization region pass through the ion gate into the mobility region; in the second preset time period t2, simultaneously applying a second voltage V2 higher than the first voltage V1 to the first gate electrodes (8-1) and the second gate electrodes (8-2) to form a direct-current electric field in the ionization region of the ion mobility tube directed from the ion gate to the ionization source, causing ions in the ionization region to move toward the ionization source, and to form a direct-current electric field in the mobility region of the ion mobility tube directed from the ion gate to the ion receiving electrode, causing ions in the mobility region to move toward the ion receiving electrode; in the third preset time period t3, the first gate electrodes (8-1) maintain the second voltage V2 higher than the first voltage V1 in the second preset time period t2, while the second gate electrodes (8-2) apply the first voltage V1; a direct-current electric field is formed in the ionization region of the ion mobility tube directed from the ionization source to the ion gate, causing 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 mobility tube is generated between the first gate electrodes (8-1) and the second gate electrodes (8-2), which effectively prevents ions in the ionization region from passing through the ion gate into the mobility region; in the mobility region, a direct-current electric field is formed directed from the ion gate to the ion receiving electrode, under the action of which ions in the mobility region will reach the ion receiving electrode in sequence and be detected. The first preset time period t1 is between 0.001 ms and 1 ms, the second preset time period t2 is between 0.001 ms and 1 ms, and the third preset time period t3 is between 1 ms and 50 ms; and 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. The first voltage V1 is the position voltage of the ion mobility tube, the difference between the second voltage V2 and the first voltage V1 is between 10 V and 700 V, and V2>V1.

2. The control method according to claim 1, characterized by: The first preset time period t1 is the opening time of the ion gate, 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, there is no delay operation between the two time periods, and there is no overlap in time.

3. The control method according to claim 1, characterized by: The strip-shaped electrode is a metal wire or a spiral-shaped electrode wound on a cylinder or a metal sheet; and the ring-shaped electrode is a circular ring-shaped electrode or a square ring-shaped electrode.

4. The control method according to claim 1, characterized by: During the operation of the ion mobility spectrometer, the voltages applied to the first gate electrode (8-1) and the second gate electrode (8-2) are periodically and cyclically 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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