Composite ion source ion migration tube

Abstract

The invention discloses a composite ion source ion migration tube. The ion migration tube is provided with a photoionization source and a low-pressure plane self-sustaining discharge ion source at the same time, and the working pressure of the inner cavity is reduced to-0.3 bar. On one hand, the working air pressure of the inner cavity of the ion migration tube is reduced, the influence of factors such as water vapor light absorption and chemical competitive ionization on the single-photon ionization process of a target sample can be effectively inhibited, and generation of sample ions M + is promoted; on the other hand, low-pressure plane self-sustaining discharge can cover the whole radial section of the inner cavity of the ion migration tube, generated hydrated hydrogen positive ions (H2O) n.H < + > can further react with sample molecules which are not ionized by ultraviolet light to generate protonated sample ions M.H < + >, the sample ionization efficiency can be improved, sensitive detection of a target sample is achieved, and the detection sensitivity is improved. A brand-new sample characteristic ion peak can be provided, and accurate recognition of a target sample is achieved.

Description

Technical Field

[0001] The present invention relates to an ion mobility tube, which is a core component of an ion mobility spectrometer. Specifically, it is an ion mobility tube that operates at a low pressure (~0.3 bar) and is equipped with a photoionization source and a low-pressure planar self-sustained discharge ionization source at the same time. Background Art

[0002] Atmospheric pressure photoionization is one of the most commonly used ionization techniques in ion mobility spectrometry and is widely used in the detection fields of volatile organic pollutants, explosives, drugs, chemical agents, etc. In the early stage, laser was generally used as the light source for photoionization technology, which was first introduced into the field of ion mobility spectrometry by Lubman et al. in 1982 (Anal.Chem. 1982, 54:1546). With the emergence of miniaturized commercial vacuum ultraviolet lamps (VUV lamps), Hill, Eiceman et al. successively used Krypton VUV lamps and Hydrogen VUV lamps directly as the photoionization sources of ion mobility spectrometry (Anal.Chem. 1983, 55:1761; Anal.Chem. 1986, 58:2142). In order to improve the detection sensitivity and the range of detected target substances of the photoionization source ion mobility spectrometry, Spangler disclosed a photoionization source ion mobility spectrometry technology with an axially fixed structure in 1992, using a Krypton VUV lamp as the photoionization source (US5338931). Hans-Rudiger et al. disclosed a high-sensitivity photoionization source ion mobility spectrometry technology based on Dopant doping in 1997 (US5968837), realizing the detection of positive and negative polarity target substances by the photoionization source ion mobility spectrometry. Li Haiyang et al. disclosed an array photoionization source ion mobility tube technology in 2012 to enhance its detection sensitivity in the negative ion mode (CN103871828).

[0003] Currently, ion mobility spectrometers generally operate under atmospheric pressure conditions and use purified air as the working support gas. On the one hand, O2 and H2O molecules contained in the air will absorb a large amount of vacuum ultraviolet light, resulting in the effective transmission distance of ultraviolet light in the ionization region of the ion mobility spectrometer being only about 5 mm (Anal.Chem. 2006, 78, 4553). A large number of sample molecules cannot be ionized because they cannot contact the ultraviolet light. On the other hand, under atmospheric pressure conditions, the complex matrix present in the sample will undergo intense chemical competitive ionization with the target sample molecules, resulting in further loss of target sample ions. These all cause insufficient sensitivity of the photoionization ion mobility spectrometry when detecting ultra-trace target substances. How to solve the above problems has become an important direction for the development of current photoionization ion mobility spectrometry technology. Summary of the Invention

[0004] The present invention discloses a composite ion source ion mobility tube. The ion mobility tube is provided with a photoionization source and a low-pressure planar self-sustained discharge ion source at the same time, and the working pressure in the inner cavity is reduced to ~0.3 bar. On the one hand, reducing the working pressure in the inner cavity of the ion mobility tube effectively suppresses the influence of factors such as water vapor light absorption and chemical competitive ionization on the single-photon ionization process of the target sample, and promotes the generation of sample ions M + ; on the other hand, the low-pressure planar self-sustained discharge can cover the entire radial cross-section of the inner cavity of the ion mobility tube. The hydrated hydrogen cations (H 2 O) n ·H + generated by it will further react with the sample molecules that have not been photoionized by ultraviolet light to generate protonated sample ions M·H + , which can not only improve the sample ionization efficiency and realize the sensitive detection of the target sample, but also provide a brand-new sample characteristic ion peak to realize the accurate identification of the target sample.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A composite ion source ion mobility tube, the ion mobility tube is a hollow columnar cavity formed by alternately coaxially stacking a ring electrode and a ring insulator from left to right. A left end plate electrode is provided at the left end of the cavity, and a right end plate electrode is provided at the right end of the cavity. The left and right end plate electrodes are respectively hermetically connected to the circumference of the cavity through the ring insulator; an ion gate is provided between the left and right end plate electrodes inside the cavity, dividing the inner part of the cavity into two regions on the left and right, and each region includes more than 2 ring electrodes. Among them, an ionization region is formed between the left end plate electrode and the ion gate, and a migration region is formed between the ion gate and the right end plate electrode; an ion source is provided on the left end plate electrode, and an ion receiving electrode is provided on the right end plate electrode;

[0007] A sample gas inlet and an air outlet are provided on the left end plate electrode, and a drift gas inlet is provided on the right end plate electrode. The sample gas inlet, the drift gas inlet and the air outlet are respectively communicated with the inner cavity of the ion mobility tube. A path of drift gas enters the inside of the migration region through the drift gas inlet, flows into the ionization region through the ion gate, mixes with the sample gas entering the ionization region through the sample gas inlet, and then flows out of the ion mobility tube through the air outlet. The air outlet is connected to the pumping port of the vacuum pump to control the pressure in the inner cavity of the ion mobility tube to vary between 1 and 0.1 bar;

[0008] The ion source is any light source capable of emitting ultraviolet light with any wavelength in the range of 80-150 nm. The ultraviolet light emitted from the light outlet of the light source enters the inner cavity of the ionization region along the axis of the ion mobility tube, is absorbed by the sample molecules entering the ionization region through the sample gas inlet, and undergoes single-photon ionization to generate positively charged sample molecular ions M + ;

[0009] The first annular electrode and the second annular electrode adjacent to the ion source in the ionization region form a discharge pair of electrodes. Circular sheet metal meshes with a mesh number of 20 - 80 are respectively arranged at the middle through holes of the first annular electrode and the second annular electrode. The circular planes of the two metal meshes are perpendicular to the axis of the ion migration tube, and the distance between the circular planes of the two metal meshes is 1 - 3 mm;

[0010] When the air pressure in the inner cavity of the ion migration tube is reduced to 0.1 - 0.3 bar, the potential difference between the first annular electrode and the second annular electrode is adjusted to be greater than 1000 V, and a low-pressure planar self-sustained discharge covering the entire radial cross-section of the inner cavity of the ion migration tube is formed between the discharge pair of electrodes, constituting the second ion source of the ion migration tube. The hydrated hydrogen positive ions (H 2 O) n ·H + generated by the discharge further react with the sample molecules not ionized by ultraviolet photoionization to generate protonated sample ions M·H + . On the one hand, it improves the sample ionization efficiency and realizes the sensitive detection of the target sample. On the other hand, it provides a brand-new sample characteristic ion peak to realize the accurate identification of the target sample;

[0011] The ion gate is one of the Bradbury-Neilsen type ion gate or the Tyndall-Powell type ion gate;

[0012] The drift gas is compressed air filtered by molecular sieve and activated carbon.

[0013] In the present invention, by reducing the working air pressure in the inner cavity of the ion migration tube and simultaneously setting a photoionization source and a low-pressure planar self-sustained discharge ionization source, on the one hand, it solves the influence of factors such as water vapor light absorption and chemical competitive ionization on the single-photon ionization process of the target sample. Reducing the working air pressure in the inner cavity of the ion migration tube can effectively inhibit the influence of factors such as oxygen and trace water vapor light absorption and chemical competitive ionization on the single-photon ionization process of the target sample, and promote the generation of sample ions M + ; on the other hand, the low-pressure planar self-sustained discharge can cover the entire radial cross-section of the inner cavity of the ion migration tube. The hydrated hydrogen positive ions (H 2 O) n ·H + generated by it will further react with the sample molecules not ionized by ultraviolet photoionization to generate protonated sample ions M·H + . It can not only improve the sample ionization efficiency and realize the sensitive detection of the target sample, but also provide a brand-new sample characteristic ion peak to realize the accurate identification of the target sample; Utilizing the advantage of the wide coverage of the low-pressure planar discharge, it fully reacts with the sample molecules not ionized by ultraviolet photoionization, improves the sample ionization efficiency, and additionally provides one more kind of sample characteristic ion peak.

[0014] The advantages of the present invention are as follows: The ion migration tube disclosed by the present invention combines low-pressure planar discharge with photoionization ion migration spectrometry, which can not only solve the influence of factors such as water vapor photoabsorption and chemical competitive ionization on the single-photon ionization process of target samples, but also utilize the advantage of the wide coverage of low-pressure planar discharge to further improve the sample ionization efficiency and provide a brand-new sample characteristic ion peak. The present invention will be further described in detail below with reference to the accompanying drawings: Description of the Drawings

[0015] Figure 1 . Structural diagram of the composite ion source ion migration tube disclosed by the present invention. Among them: 1. Ultraviolet photoionization source; 2. Ionization region; 3. Ion gate; 4. Migration region; 5. Ion receiving electrode; 6. Ring electrode; 6-1. First ring electrode; 6-2. Second ring electrode; 7. Ring insulator; 8. Sample gas inlet; 9. Drift gas inlet; 10. Gas outlet.

[0016] Figure 2 . Spectrograms obtained by the composite ion source ion migration tube disclosed by the present invention under different working modes. (1) Ion migration spectrogram of 200 ppb anisole when working with single photoionization, (2) Ion migration spectrogram of 200 ppb anisole when working with photoionization and planar self-sustained discharge simultaneously. Detailed Description of the Invention

[0017] Example 1

[0018] The composite ion source ion migration tube disclosed by the present invention is as Figure 1 shown in

[0019] The ion migration tube is a hollow cylindrical cavity formed by alternately coaxially stacking a circular ring electrode 6 and a circular ring insulator 7 from left to right in sequence. A left end plate electrode is provided at the left end of the cavity, and a right end plate electrode is provided at the right end of the cavity. The left and right end plate electrodes are respectively hermetically connected to the left and right opening ends of the cylindrical cavity through the circular ring insulator 7 to form a sealed cylindrical chamber; An ion gate 3 is provided between the left and right end plate electrodes inside the cavity, dividing the cavity into two regions on the left and right, and each region includes more than 2 ring electrodes 6. Among them, an ionization region 2 is formed between the left end plate electrode and the ion gate 3, and a migration region 4 is formed between the ion gate 3 and the right end plate electrode; An ion source 1 is provided on the left end plate electrode, and an ion receiving electrode 5 is provided on the right end plate electrode inside the cavity;

[0020] The first ion source of the ion mobility tube is an ultraviolet photoionization source 1 composed of a 10.6 eV VUV Kr lamp, which is fixed on the left end plate electrode; the ionization region 2 and the migration region 4 are both composed of annular electrodes with an axial length of 3.5 mm, an outer diameter of 30 mm, and an inner diameter of 20 mm and annular insulators with an axial length of 1.5 mm, an outer diameter of 30 mm, and an inner diameter of 20 mm stacked coaxially and alternately. The axial length of the ionization region 2 is 36.5 mm, and the axial length of the migration region 4 is 35 mm; the ion gate 3 is a Bradbury-Nielsen type ion gate with a wire diameter of 0.1 mm and a wire spacing of 1 mm; the ion receiving electrode 5 is a Faraday disk with a diameter of 6 mm, which is fixed on the right end plate electrode with an outer diameter of 30 mm by insulating sealing; circular metal grids with a thickness of 0.05 mm and an outer diameter of 20 mm are arranged in the inner through holes of the first annular electrode 6-1 and the second annular electrode 6-2, and the plane spacing of the metal grids is 1.5 mm;

[0021] A sample gas inlet 8 and an air outlet 10 are provided on the left end plate electrode. A drift gas inlet 9 is provided on the left end plate electrode. A stream of drift gas at 500 mL / min enters the ion mobility tube through the drift gas inlet 9, flows through the ion gate 3 into the ionization region 2, and mixes with a stream of sample gas at 100 mL / min entering the ionization region 2 through the sample gas inlet 8, and then flows out of the ion mobility tube through the air outlet 10. The drift gas is clean air filtered by activated carbon and 13X molecular sieve in sequence, and the sample gas is clean air containing a specific concentration of target analyte; the air outlet 10 is connected to the suction port of the vacuum pump. By controlling the pumping speed of the suction pump, the air pressure in the inner cavity of the ion mobility tube can be controlled to drop from atmospheric pressure to 0.1 bar; the drift gas is purified air obtained by filtering compressed air through silica gel, molecular sieve, and activated carbon, which contains about 10 ppb of trace water vapor, and the sample gas is purified air carrying the target analyte molecules to be measured.

[0022] When the air pressure in the inner cavity of the ion mobility tube is controlled to be ~0.3 bar, when the potential difference between the first annular electrode 6-1 and the second annular electrode 6-2 is adjusted to 1200 V, a low-pressure planar self-sustained discharge can occur, forming the second ion source of the ion mobility tube.

[0023] As Figure 2 shown, when the air pressure in the inner cavity of the ion mobility tube is controlled to be ~0.3 bar, when only the ultraviolet photoionization source 1 of the ion mobility tube is working, the ion mobility spectrum formed by the sample gas containing 200 ppb anisole is as shown in (1), and only an M + ion peak with a signal intensity of about 200 pA can be observed; when the ion mobility tube works in the composite ion source mode, that is, when the ultraviolet photoionization source 1 and the low-pressure planar self-sustained discharge ion source composed of the first annular electrode 6-1 and the second annular electrode 6-2 work simultaneously, the ion mobility spectrum formed by the sample gas containing 200 ppb anisole is as shown in (2), and an M·H with about 600 pA can be observed +Ion peak and M of about 900 pA + Ion peak. By comparing spectra (1) and (2), the following conclusion can be drawn that the combined ionization source mode can provide higher ion signal intensity and more ion spectral peak information.

Claims

1. A composite ion source ion mobility tube, wherein the ion mobility tube is a hollow cylindrical cavity formed by coaxially stacking a circular electrode (6) and a circular insulator (7) alternately from left to right. A left end plate electrode is provided at the left end of the cavity, and a right end plate electrode is provided at the right end of the cavity. The left and right end plate electrodes are respectively hermetically connected to the left and right open ends of the cylindrical cavity through the circular insulator (7) to form a sealed cylindrical chamber; an ion gate (3) is provided inside the cavity between the left and right end plate electrodes, dividing the interior of the cavity into two regions on the left and right, and each region includes more than 2 circular electrodes (6). Among them, an ionization region (2) is formed between the left end plate electrode and the ion gate (3), and a migration region (4) is formed between the ion gate (3) and the right end plate electrode; an ion source (1) is provided on the left end plate electrode, and an ion receiving electrode (5) is provided on the right end plate electrode inside the cavity. It is characterized in that: a sample gas inlet (8) and an air outlet (10) are provided on the left end plate electrode, and a drift gas inlet (9) is provided on the right end plate electrode. The sample gas inlet (8), the drift gas inlet (9) and the air outlet (10) are respectively communicated with the inner cavity of the ion mobility tube. A drift gas enters the interior of the migration region (4) through the drift gas inlet (9), flows into the ionization region (2) through the ion gate (3), mixes with the sample gas entering the ionization region (2) through the sample gas inlet (8), and then flows out of the ion mobility tube through the air outlet (10). The air outlet (10) is connected to the pumping port of a vacuum pump to control the air pressure in the inner cavity of the ion mobility tube to vary between 1 bar and 0.1 bar; The ion source (1) is any light source capable of emitting ultraviolet light with any wavelength in the range of 80 to 150 nm. The ultraviolet light emitted from the light outlet of the light source enters the inner cavity of the ionization region (2) along the axis of the ion migration tube, is absorbed by the sample molecules entering the ionization region (2) through the sample gas inlet (8), and undergoes single-photon ionization to generate positively charged sample molecular ions M + ; a first circular electrode (6-1) adjacent to the right side of the ion source (1) and a second circular electrode (6-2) adjacent to the right side of the first circular electrode (6-1) in the ionization region (2) form a discharge pair electrode. Circular metal meshes with a mesh number of 20 to 80 are respectively provided at the middle through holes of the first circular electrode (6-1) and the second circular electrode (6-2). The circular planes of the two metal meshes (i.e., the surfaces of the circular metal meshes) are perpendicular to the axis of the ion mobility tube, and the distance between the circular planes of the two metal meshes is 1 to 3 mm.

2. The ion mobility tube according to claim 1, characterized in that: when the air pressure in the inner cavity of the ion mobility tube is reduced to 0.1 to 0.3 bar, adjusting the potential difference between the first circular electrode (6-1) and the second circular electrode (6-2) to be greater than 1000 V can cause a low-pressure planar self-sustained discharge, constituting a second ion source of the ion mobility tube.

3. The ion mobility tube according to claim 1 or 2, characterized in that: The low-pressure planar self-sustained discharge formed between the first annular electrode (6-1) and the second annular electrode (6-2) can cover the entire radial cross-section of the inner cavity of the ion migration tube, and react further with the hydronium ions (H 2 O) n ·H + generated from trace water vapor and the sample molecules that have not been photoionized by ultraviolet light to generate protonated sample ions M·H + . On the one hand, it improves the sample ionization efficiency and enables sensitive detection of the target sample. On the other hand, it provides a brand-new sample characteristic ion peak for accurate identification of the target sample.

4. The ion mobility tube according to claim 1, characterized in that: the ion gate (3) is one of a Bradbury-Neilsen type ion gate or a Tyndall-Powell type ion gate; the drift gas is compressed air filtered by molecular sieve and / or activated carbon.

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