Method for improving detection sensitivity of photoionization ion mobility spectrometry

Abstract

The method comprises the following steps: firstly, externally connecting a mechanical pump at a tail gas outlet of a migration tube to reduce the working air pressure in the migration tube, and increasing the optical path distance of an ultraviolet lamp in an ionization region by reducing the interferent content of background gas in the migration tube; secondly, the ion composite reaction probability is reduced by increasing the field intensity of the ionization region of the migration tube; the combination of the two can improve the ion number density of the target analyte detected by the detector while improving the ion migration speed, thereby significantly improving the detection sensitivity of the ion mobility spectrometry. The requirements for regulating and controlling the air pressure and the electric field are low, the detection performance of the ion mobility spectrometry can be remarkably improved under the easily-achieved experiment condition, high-throughput sample analysis and detection can still be achieved under the low-air-pressure condition, and the method has universality for the ion mobility spectrometry.

Description

Technical Field

[0001] The invention is based on the photoionization ion mobility spectrometry, and specifically improves the intensity of the photoionization ion mobility spectrometry detection signal from two aspects, namely, lowering the working gas pressure condition in the migration tube and increasing the field intensity of the axial electric field in the ionization region. Background Art

[0002] Ion Mobility Spectrometry (IMS) is usually carried out under atmospheric pressure conditions. The target ion group generated in the ionization zone is injected into the IMS migration zone through ion gating. The ion group is transmitted and separated in the axial DC electric field. The total amount of ions in the ion group directly determines the IMS detection sensitivity. Commonly used ion sources for IMS include radioionization source, ultraviolet photoionization source, field ionization, corona discharge, dielectric barrier discharge, electrospray ionization and other methods. Among them, the ultraviolet photoionization source is the most widely studied and applied non-radioactive ionization source.

[0003] The main methods to improve the detection sensitivity of the photoionization source IMS are: increasing the optical path length in the ionization region; reducing the competitive ionization and recombination reaction process of ions in the ionization region; improving the IMS structure, ion gate gating method and adding dopants.

[0004] Jiang Dandan et al. (CN202011382439.7) improved the detection sensitivity by mixing floating gases with different oxygen concentrations and reducing the oxygen concentration in the floating gases. Yu Jianna et al. (CN202010647868.6) separated the ionization zone and migration zone of IMS by two screens to eliminate the influence of the ion gate on the electric field of the ionization zone and migration zone when the ion gate is closed, thereby ensuring the working stability of the ion source and enhancing the detection sensitivity of IMS. Huang Chaoqun et al. (CN201611121527.5) applied double pulses and asymmetric voltages to BNG to control the opening and closing of the ion gate to simultaneously improve the sensitivity and resolution of IMS. Cang Huaiwen et al. (CN202110747913.X) proposed a BNG control method to compress the ion clusters in the compression zone and the diffusion zone to improve the resolution and sensitivity of the ion mobility spectrum. Xia Lei et al. (CN201711312704.2) improved the IMS detection sensitivity by binding and storing the ions generated during the ion gate closing time. Bai Xueying et al. (CN202111444100.X) improved the detection sensitivity of ammonia in the air by doping butanone reagent in the migration spectrum. Professor Li Haiyang's team (Anal Chem, 2019, 91(14):9138) constructed three ion gate grids to compress the ion clusters multiple times to achieve the improvement of IMS sensitivity and resolution.

[0005] Professor Zimmermann's team (Anal Chem, 2014, 86:11841; Anal Chem, 2014, 86:7023) increased the E / N of the migration tube to about 120Td to reduce the chemical competitive ionization of product ions in the migration tube and improve the detection sensitivity of ions with low PA values ​​and high ionization energy. et al. (Anal Bioanal Chem. 2020, 412: 7899) found that the detection sensitivity of ions with lower proton affinity can be improved by changing the gas pressure.

[0006] However, the above method may increase the difficulty of IMS assembly, or because of the harsh working conditions of IMS, the airtightness of the platform is high and the universality of ion mobility spectrometry is low.

[0007] Since the photoionization ion source is a molecular ion and an electron obtained by ionizing molecules after absorbing photons, when there are interferences that compete with the photons of the ion source for absorption and ionization, the detection sensitivity will be significantly reduced. In addition, due to the recombination reaction of ions in the ionization zone, the ion number density in the ion group is further consumed, reducing the detection sensitivity.

[0008] Therefore, the present invention improves the detection sensitivity of photoionization ion mobility spectrometry from two aspects, and does not require improvement of the migration tube structure, has simple requirements on platform working conditions, and has high universality. Summary of the invention

[0009] The technical problem to be solved by the present invention is: a method for improving the detection sensitivity of photoionization ion mobility spectrometry, which is achieved by reducing the working gas pressure conditions in the migration tube and increasing the field strength of the axial electric field in the ionization zone.

[0010] Specifically:

[0011] 1) Method of reducing the working pressure in the migration tube: The total amount of ions generated by the photoionization source is directly related to its optical path length. After reducing the working pressure in the migration tube, the content of light-absorbing components such as O2 and H2O in the drift gas will drop significantly, which will help increase the optical path length of the photoionization source, thereby increasing the total amount of ions of the analyte obtained by ionization. The specific method is to adjust the needle valve to change the pumping gas flow rate of the mechanical pump, so that the working pressure of the migration tube is stabilized at any pressure condition within the range of 1 to 80 kPa, so as to increase the ultraviolet light path length in the ionization zone to improve the detection sensitivity;

[0012] 2) Method of increasing the electric field in the ionization zone: After ions are ionized by the photoionization source, positive and negative ion recombination reactions will occur. After increasing the electric field in the ionization zone, the residence time of ions in the ionization zone is reduced, which will reduce the probability of ion recombination reactions, thereby significantly improving the detection sensitivity. The specific method is to control the electric field in the ionization zone to be stable under any condition within the range of 700 to 1000 V / cm, so as to reduce the probability of ion recombination reactions and improve the detection sensitivity.

[0013] In order to further improve the sensitivity of IMS detection, the parameters of ion mobility spectrometry are controlled such that the field strength in the migration region is between 400 and 600 V / cm, and the lamp current is adjusted to be between 50 and 100 μA.

[0014] In order to ensure high-throughput detection of ion mobility spectrometry, the flow rate of the carrier gas inlet of the migration tube is in the range of 100 to 300 mL / min, and the flow rate of the drift gas inlet is in the range of 100 to 300 mL / min.

[0015] The ion gate used is a Tyndall-Powell type ion gate. Each TP type ion gate consists of two parallel metal grids, which are separated by a circular insulating ring (polytetrafluoroethylene insulating ring) with a thickness of 0.1 to 1 mm.

[0016] The present invention realizes enhancing the detection sensitivity of photoionization ion mobility spectrometry through two approaches.

[0017] The present invention firstly connects an external mechanical pump at the tail gas outlet of the migration tube to reduce the working air pressure in the migration tube, and increases the optical path distance of the ultraviolet lamp in the ionization zone by reducing the interference content of the background gas in the migration tube; secondly, the probability of ion recombination reaction is reduced by increasing the field strength in the ionization zone of the migration tube. The combination of the two can increase the ion number density of the target analyte detected by the detector while increasing the ion migration speed, thereby significantly improving the detection sensitivity of the ion migration spectrum. The present invention has low requirements for regulating air pressure and electric field, can significantly improve the detection performance of the ion migration spectrum under easily achievable experimental conditions, and can still achieve high-throughput sample analysis and detection under low air pressure conditions, and has universal applicability to the ion migration spectrum.

[0018] The present invention will be further described in detail below in conjunction with the accompanying drawings: BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The schematic diagram of the structure of an ion migration spectrometer for regulating gas pressure includes: 1. ion source; 2. carrier gas inlet; 3. flow meter; 4. tail gas outlet; 5. needle valve; 6. mechanical pump; 7. floating gas inlet; 8. ion receiving electrode; 9. ion gate; 10. ionization zone; 11. migration zone.

[0020] Figure 2 Ion migration spectra of 10 ppm acetone ion signal intensity comparison under the condition of reducing the gas pressure in the migration tube

[0021] Figure 3 Ion mobility spectra of 10 ppm acetone ion signal intensity comparison when the field strength in the ionization region is 700 V / cm (right) and 1000 V / cm (left) at 40 kPa pressure;

[0022] Figure 4 Ion migration spectra comparing the signal intensity of a 10 ppm acetone standard sample at 3 kPa, 300 V / cm (left) and 40 kPa, 1000 V / cm (right). DETAILED DESCRIPTION

[0023] The ion transfer tube structure disclosed in the present invention can regulate the gas pressure. Figure 1 The ion transfer tube used in the method includes an ion transfer tube composed of annular electrodes and annular insulators alternately coaxially stacked in sequence, and an ion source 1, an ion gate 9 and an ion receiving electrode 8 are arranged in sequence along the direction of ion migration inside the ion transfer tube; the ionization region 10 and the migration region 11 are separated by the ion gate 9, a carrier gas inlet 2 is provided in the ionization region 10 close to the ion source 1, an exhaust gas outlet 4 is provided in the ionization region close to the ion gate 9, and a drift gas inlet 7 is provided in the migration region 11 close to the ion receiving electrode 8; the ion gate used inside the migration tube is a Tyndall-Powell Type ion gate, the TP ion gate is composed of two parallel metal grids, which are separated by a circular insulating ring (polytetrafluoroethylene insulating ring) with a thickness of 0.5 mm; the ion source 1 is a 10.6 eV VUV photoionization source; the ion receiving electrode 8 is a Faraday disk with a diameter of 6 mm, fixed on a metal shielding tube with an outer diameter of 30 mm; the ionization zone 10 and the migration zone 11 are both composed of coaxially alternatingly stacked annular conductive electrode pieces with an axial length of 5 mm and an outer diameter of 30 mm and annular insulating electrode pieces with an axial length of 5 mm and an outer diameter of 30 mm.

[0024] The method of reducing the working pressure in the migration tube is as follows: prepare 10ppm acetone standard gas, fix the carrier gas flow rate of the acetone standard gas at 100mL / min through the flowmeter 3, and the drift gas flow rate is 200mL / min, and change the pumping gas flow rate of the mechanical pump 6 by adjusting the needle valve 5 set at the tail gas port, so that the working pressure of the migration tube is stabilized at any pressure condition within the range of 10 to 100kPa. This greatly reduces the content of light-absorbing components such as O2 and H20 in the drift gas, which helps to increase the optical path length of the photoionization source, thereby increasing the total amount of ions of the analyte obtained by ionization.

[0025] The method of increasing the electric field of the ionization region is to control the output power of the ionization region 10 so that the electric field can be stabilized under any condition within the range of 700 to 1000 V / cm. After increasing the electric field of the ionization region, the residence time of ions in the ionization region is reduced, which will reduce the probability of ion recombination reaction, thereby significantly improving the detection sensitivity.

[0026] In order to further improve the IMS detection sensitivity, the lamp current of the VUV ionization source used in the ionization source is controlled at 100 μA, the field strength in the migration region is controlled at 500 V / cm, the closing voltage of the ion gate 9 is 400 V, and the temperature of the migration tube is 100° C.

[0027] Example 1

[0028] The electric field strength of the migration zone of the fixed ion mobility spectrometer is 700V / cm. The mechanical pump gas flow rate is adjusted by a needle valve at the tail gas outlet to stabilize the working pressure of the migration tube under five pressure conditions: 20, 40, 60, 80 and 100kPa. The 10ppmv acetone standard gas is passed through the carrier gas inlet into the ionization zone for detection at a flow rate of 100mL / min. The corresponding acetone product ion peak height and migration time are recorded under five pressure conditions: 20, 40, 60, 80 and 100kPa. Figure 2 As shown in the figure, under atmospheric pressure, the peak height of acetone is 700mV, while under 40kPa pressure, the peak height of acetone is 1470mV, which indicates that the detection sensitivity under low pressure is at least increased by 2 times; however, under 20kPa, the peak height of acetone is reduced to 1100mV, because as the pumping gas flow rate of the mechanical pump increases, and the carrier gas flow rate of the injection is fixed at 100mL / min through the flow meter 3, in a closed migration tube cavity, according to the ideal gas equation PVc=NkT, where P is the pressure, V is the migration tube volume, c is the acetone sample gas volume concentration, N is the number of molecules, k is the Boltzmann constant, and T is the migration tube temperature. When the pumping gas flow rate of the mechanical pump 6 is increased to reduce P, the acetone molecular content per unit volume N / V in the migration tube is bound to decrease, and the acetone molecular content corresponding to each pressure value at this concentration can be calculated by this formula.

[0029] Since c in the formula is the volume concentration of the acetone sample, the molecular content of acetone of different concentrations must be different under the same gas pressure condition. When the concentration of the acetone sample changes, for example, when it becomes 5ppm, it is necessary to use a needle valve at the tail gas outlet of the migration tube to adjust the mechanical pump gas flow rate again, so that the working gas pressure of the migration tube is stable under five gas pressure conditions of 20, 40, 60, 80 and 100kPa. The 5ppm acetone standard gas is passed through the carrier gas inlet into the ionization zone for detection through a carrier gas with a flow rate of 100mL / min. The corresponding acetone product ion peak height and migration time are recorded under five gas pressure conditions of 20, 40, 60, 80 and 100kPa to find the corresponding gas pressure condition with the highest sensitivity. That is, when the sample concentration changes, the corresponding gas pressure condition with the best detection sensitivity needs to be optimized within the gas pressure range of 10 to 100kPa.

[0030] Example 2

[0031] The working pressure of the fixed ion mobility spectrometer is 40 kPa, such as Figure 3 As shown, the left figure shows that the 10ppm acetone ion peak height is 1470mV corresponding to the ionization field strength of 700V / cm, and the right figure shows that the 10ppm acetone ion peak height is 2840mv corresponding to the ionization field strength of 1000V / cm; this embodiment can reduce the probability of ion recombination and thus significantly improve the detection sensitivity of photoionization ion mobility spectrometry.

[0032] Comparative Example

[0033] Combining Example 1 with Example 2, it can be seen that the detection sensitivity of the 10ppm acetone standard sample is the highest when the gas pressure is 40kPa and the electric field in the ionization zone is 1000V / cm, and the peak intensity is 2840mV. At this time, the gas pressure is adjusted to 3kPa outside the range of 10-100kPa; the electric field in the ionization zone is adjusted to 300V / cm outside the range of 700-1000V / cm. At this time, the detection sensitivity decreases, and the peak intensity is 158mV, which is about 18 times different from each other. The comparison results are shown in FIG. Figure 4 .

Claims

1. A method for improving the detection sensitivity of photoionization ion mobility spectrometry, characterized in that: The ion mobility spectrometer using a vacuum ultraviolet photoionization source is realized by reducing the working gas pressure conditions in the migration tube and / or increasing the electric field in the ionization zone.

2. The method for improving the detection sensitivity of photoionization ion mobility spectrometry according to claim 1, characterized in that: Specifically: 1) A method for reducing the working pressure condition in the migration tube: by adjusting the exhaust flow rate of the tail gas outlet, the working pressure of the migration tube is stabilized at any pressure condition within the range of 10 to 100 kPa; And / or, 2) a method for increasing the electric field in the ionization zone: by controlling the electric field in the ionization zone (along the ion transmission direction, from the ion source (1) to the ion receiving electrode (8) or in the opposite direction) to stabilize it under any condition within the range of 700 to 1000 V / cm.

3. The method for improving the detection sensitivity of photoionization ion mobility spectrometry according to claim 2, characterized in that: A needle valve (5) and a mechanical pump (6) are provided at the tail gas outlet, and the exhaust gas flow rate of the mechanical pump (6) is changed by adjusting the needle valve (5).

4. The method for improving the detection sensitivity of photoionization ion mobility spectrometry according to claim 1 or 2, characterized in that: A vacuum ultraviolet photoionization source was used. In order to further improve the sensitivity of IMS detection, the parameters of ion mobility spectrometry were controlled such that the field strength in the migration region was between 400 and 600 V / cm, and the lamp current was adjusted to be between 50 and 100 μA.

5. The method for improving the detection sensitivity of photoionization ion mobility spectrometry according to claim 1 or 2, characterized in that: The ion mobility spectrometer comprises an ion source (1), an ion gate (9), an ion receiving electrode (8) which are arranged in sequence, an ionization region (10) and a migration region (11) which are separated by the ion gate (9), a carrier gas inlet (2) being provided in the ionization region (10) close to the ion source (1), an exhaust gas outlet (4) being provided in the ionization region (10) close to the ion gate (9), and a drift gas inlet (7) being provided in the migration region (11) close to the ion receiving electrode (8); In order to ensure high-throughput detection of ion mobility spectrometry, the flow rate of the carrier gas inlet of the migration tube is in the range of 100 to 300 mL / min, and the flow rate of the drift gas inlet is in the range of 100 to 300 mL / min.

6. The method for improving the detection sensitivity of photoionization ion mobility spectrometry according to claim 1, characterized in that: The ion gate used is a Tyndall-Powell type ion gate. Each TP type ion gate consists of two parallel metal grids, which are separated by a circular insulating ring (polytetrafluoroethylene insulating ring) with a thickness of 0.1 to 1 mm.

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