Method for reducing matrix effect of photoionization ion mobility spectrometry detection complex sample

Abstract

The invention discloses a method for reducing the matrix effect of a photoionization ion mobility spectrometry detection complex sample, and the ion mobility spectrometry comprises an ion migration tube with good air tightness, a photoionization source and an ion receiving electrode which are respectively and oppositely arranged at the left end and the right end of the ion migration tube, and an ion gate located between the photoionization source and the ion receiving electrode. The area between the photoionization source and the ion gate is an ionization area, and the area between the ion gate and the ion receiving electrode is a migration area. According to the method, the air pressure in an ion migration tube is maintained within the range of 10-80 kPa, meanwhile, an isolation high-voltage power supply connected with an ionization region is adjusted, and the electric field in the ionization region is maintained within the range of 300-600 V / cm, so that the charge transfer and proton transfer plasma-molecule reaction of ionized complex target compound ions in the ionization region is inhibited, and the target compound ions can be effectively separated from the ion migration tube. Therefore, the influence of a complex matrix on accurate quantification of the ion mobility spectrometry and detection of a low-proton-affinity or high-ionization-energy target compound can be eliminated.

Description

Technical Field

[0001] The invention belongs to the field of analytical chemical instruments, and in particular relates to a method for reducing matrix effects of complex samples in photoionization ion mobility spectrometry detection. Background Art

[0002] In recent years, with the development of atmospheric pressure ionization technology, the application scope of ion mobility spectrometry has expanded to many chemical analysis fields such as clinical exhaled breath detection, environmental pollutant detection, food safety detection, and process analysis.

[0003] However, as the application scope of IMS continues to expand, the matrix of the sample to be tested has become more and more complex, which has greatly challenged the detection sensitivity, selectivity and quantitative accuracy of IMS for the target, hindering its further development. When the ion mobility spectrometer works under atmospheric pressure conditions, the molecules with higher proton affinity or lower ionization energy in the multi-component sample will inhibit the ionization of the molecules with lower proton affinity or higher ionization energy, so that the latter will be rapidly converted through ion-molecule reaction after ionization, and cannot be detected, which seriously affects the detection sensitivity of the ion mobility spectrometer.

[0004] A method for using an ion migration tube for complex sample separation and analysis invented by Chen Chuang et al. (patent number CN201811412003.0) generates H+(H2O)n with a strong proton affinity in the positive ion mode, thereby suppressing the detection of ions with a lower proton affinity in complex sample components, and only detecting sample molecules with a proton affinity higher than H+(H2O)n, thereby achieving the purpose of highly selective detection of target ions in complex samples. However, while this method suppresses the interference of low proton affinity ions on the detection results, it is also unable to detect ions with low proton affinity. Moreover, when there are multiple compounds with a proton affinity higher than H+(H2O)n in a complex sample, it is also impossible to eliminate the interference of different compound ions on the matrix effect of their respective detection results. A method, kit and application for detecting banned antibiotics in cosmetics based on electrospray ion mobility spectrometry (patent number CN202111613766.3) invented by Sun Jing et al. can effectively eliminate the matrix interference effect of other compounds on the detection of banned antibiotics in cosmetics by optimizing the pretreatment method of cationic solid phase extraction purification and concentration samples, thereby reducing false positive test results. However, most of the commonly used commercial solid phase microextraction columns are selective only for a certain type of component, have a narrow range of application, and cannot avoid non-selective adsorption of blood matrix. In addition, particle-filled solid phase microextraction columns are more prone to clogging and damage, are mostly disposable, and have relatively high analysis costs.

[0005] Ammonia, as a doping agent, has been widely used in the detection of chemical warfare agents in organophosphorus mixtures (Anal. Chem. 2010, 82, 9594–960010), which can effectively eliminate the false positives caused by matrix interference with proton affinity lower than NH4+. This method is also unable to detect target compound ions with low proton affinity. Xiao Yao et al. (Sensor Actuat B-Chem: 2022, 350.) used CCl4 as an extractant to extract etomidate from biological samples separately and then detected it by positive ion mode ion mobility spectrometry, effectively avoiding the interference of complex matrices in biological samples on the detection results of etomidate targets. However, this method is also selective for only a certain type of component and does not have universal detection.

[0006] In order to reduce the matrix effect associated with ion mobility spectrometry detection of complex samples, the present invention adjusts the working gas pressure conditions in the migration tube to suppress the interference of multi-component samples in the ionization zone on the detection results of a single component. In addition, the present invention can simultaneously detect multi-component compounds in complex samples, and can effectively suppress the matrix effect produced by most ions, which has universal applicability. Summary of the invention

[0007] The purpose of the present invention is to provide a method for reducing the matrix effect of complex samples in photoionization ion mobility spectrometry detection. The method utilizes the different collision frequencies between ions and molecules under different gas pressure conditions. Under the reduced gas pressure conditions, the collision frequency between ions is smaller, and reactions such as charge transfer and proton transfer that can occur between ions and molecules are suppressed. At the same time, the electric field strength of the ionization zone of the ion migration tube is increased. The increased electric field strength of the ionization zone shortens the residence time of ions in the ionization zone, thereby shortening the reaction time of ion-molecule reactions, and ultimately greatly suppressing the negative impact of the matrix effect on ion detection, thereby achieving the purpose of greatly reducing the matrix effect of the ion mobility spectrometry.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0009] A method for reducing the matrix effect of complex samples detected by photoionization ion mobility spectrometry, the photoionization ion mobility spectrometer comprising a photoionization source and an ion receiving electrode respectively arranged opposite to each other at the left and right ends, and an ion gate located between the photoionization source and the ion receiving electrode, the area between the photoionization source 1 and the ion gate being the ionization area, and the area between the ion gate and the ion receiving electrode being the migration area; the method is characterized in that: a sample gas inlet is provided on the side of the upper wall surface of the ionization area close to the photoionization source, and the sample gas inlet is connected to the sample gas source through a first mass flowmeter; a gas outlet is provided on the side of the upper wall surface of the ionization area close to the ion gate, and the gas outlet is connected to the atmosphere through a barometer, a needle valve and a mechanical pump arranged in sequence on a connecting pipeline; a floating gas inlet is provided on the right wall surface of the migration area above the ion receiving electrode, and the floating gas inlet is connected to the floating gas source through a second mass flowmeter;

[0010] The ionization zone, ion gate and migration zone are all controlled by independent isolated high-voltage power supplies, so that the electrostatic field in each area can be adjusted independently without affecting the electrostatic field strength in other areas.

[0011] The method is specifically as follows:

[0012] 1) controlling the outlet gas velocity in the ion transfer tube by adjusting the needle valve and the mechanical pump to maintain the gas pressure in the ion transfer tube in the range of 10 to 80 kPa, preferably 10 to 50 kPa, more preferably 10 to 20 kPa;

[0013] At the same time, 2) by adjusting the isolated high voltage power supply connected to the ionization zone, the electric field in the ionization zone is maintained in the range of 300 to 600 V / cm, preferably 300 to 500 V / cm, and most preferably 400 to 500 V / cm.

[0014] A needle valve is provided at the front end of the mechanical pump interface to cooperate with the mechanical pump to control the air pressure conditions in the entire migration tube. Under atmospheric pressure conditions, the sample gas to be tested is carried by clean compressed air into the ionization zone of the migration tube through the sample gas inlet and is first ionized; then, the sample ions of each component undergo further ion-molecule charge transfer and proton transfer reactions in the ionization zone according to the differences in their respective proton affinity or ionization energy to form the final product ions under different air pressure conditions, resulting in the loss of some difficult-to-detect target compound ions. By reducing the working air pressure of the ion migration tube, the mean free path of ions in the ionization zone increases, the frequency of ion-molecule collisions decreases significantly, and the plasma-molecule reactions such as charge transfer and proton transfer are suppressed. The different molecular ions generated in the ionization zone are introduced into the migration zone and detected.

[0015] An isolated high-voltage power supply with adjustable voltage is applied separately to the ionization zone to control the electric field conditions in the ionization zone. The increased electric field strength in the ionization zone will shorten the residence time of ions in the ionization zone, thereby shortening the reaction time for ion-molecule reactions to occur, and ultimately greatly suppressing the negative impact of matrix effects on ion detection.

[0016] 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 50 to 200 mL / min, and the flow rate of the drift gas inlet is in the range of 50 to 200 mL / min.

[0017] The photoionization source is any one of a laser light source and a vacuum ultraviolet lamp.

[0018] The ion gate is any one of a Tyndall-Powell type ion gate and a Bradbury-Nielsen type ion gate.

[0019] The floating gas source and the carrier gas source are clean compressed air, and the clean compressed air is air filtered through 13X molecular sieve and activated carbon in sequence.

[0020] The present invention achieves the suppression of the influence of the matrix effect in the photoionization ion mobility spectrometry detection process through two approaches.

[0021] The present invention firstly controls the outlet gas velocity in the migration tube by adjusting the needle valve at the tail gas port connected to the mechanical pump, cooperates with the mechanical pump to jointly control the outlet gas flow rate in the migration tube, realizes arbitrarily adjusting the working gas pressure in the migration tube within the gas pressure range of 10-80kPa, reduces the frequency of reactions such as charge transfer and proton transfer that can occur between ions and molecules by adjusting the working gas pressure in the migration tube, thereby achieving the purpose of suppressing the matrix effect; secondly, by increasing the field strength of the migration tube ionization zone within the range of 300-600V / cm, the negative impact of the matrix effect on the ion mobility spectrometry detection is also reduced. The present invention has relatively low requirements for regulating gas pressure and electric field, can significantly improve the detection performance of ion mobility spectrometry under easily achievable experimental conditions, and has universality to ion mobility spectrometry. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 . A schematic diagram of an ion migration spectrometer structure for regulating gas pressure. Including: 1. Photoionization source; 2. Sample gas inlet; 3. First mass flowmeter; 4. Gas outlet; 5. Needle valve; 6. Mechanical pump; 7. Floating gas inlet; 8. Ion receiving electrode; 9. Ion gate; 10. Ionization region; 11. Migration region; 12. Second mass flowmeter; 13. Barometer.

[0023] Figure 2 When the gas pressure is fixed at 100 kPa and the electric field strength in the ionization zone is fixed at 200 V / cm, the ion migration spectrum of the mixed standard gas of benzene, toluene and xylene with a concentration of 1 ppmv is shown.

[0024] Figure 3 When the gas pressure is fixed at 50 kPa and the electric field strength in the ionization zone is fixed at 200 V / cm, the ion migration spectrum of the mixed standard gas of benzene, toluene and xylene with a concentration of 1 ppmv is shown.

[0025] Figure 4 When the gas pressure is fixed at 10 kPa and the electric field strength in the ionization zone is fixed at 200 V / cm, the ion migration spectrum of the mixed standard gas of benzene, toluene and xylene with a concentration of 1 ppmv is shown.

[0026] Figure 5 Ion migration spectrum of a standard gas mixture of benzene, toluene and xylene with a concentration of 1 ppmv when the gas pressure is fixed at 10 kPa and the electric field strength in the ionization zone is fixed at 450 V / cm. DETAILED DESCRIPTION

[0027] The photoionization ion mobility spectrometer structure used in the method of the present invention is as follows: Figure 1 As shown. The photoionization ion migration spectrometer comprises an ion migration tube composed of annular electrodes and annular insulators alternately coaxially stacked in sequence, a photoionization source 1 and an ion receiving electrode 8 are arranged opposite to each other at the left and right ends of the ion migration tube, and an ion gate 9 is located between the photoionization source 1 and the ion receiving electrode 8, the area between the photoionization source 1 and the ion gate 9 is the ionization zone 10, and the area between the ion gate 9 and the ion receiving electrode 8 is the migration zone 11; a sample gas inlet 2 is arranged on the upper wall of the ionization zone 10 near the photoionization source 1, and the sample gas inlet 2 is connected to the sample gas source through a first mass flowmeter 3; an air outlet 4 is arranged on the upper wall of the ionization zone 10 near the ion gate 9, and the air outlet 4 is connected to the atmosphere through a barometer 13, a needle valve 5 and a mechanical pump 6 arranged in sequence on the connecting pipeline; a floating gas inlet 7 is arranged above the ion receiving electrode 8 on the right side of the migration zone 11, and the floating gas inlet 7 is connected to the sample gas source through a first mass flowmeter 3. The ionization region 10, the ion gate 9 and the migration region 11 are all controlled by independent isolated high-voltage power supplies to achieve independent adjustment of the electrostatic field in each region without affecting the electrostatic field strength in other regions. The ion gate used in the ion migration tube is a Tyndall-Powell type ion gate, and the TP ion gate is composed of two metal grids parallel to each other, and the two metal grids are separated by a circular insulating ring (polytetrafluoroethylene insulating ring), and the thickness of the insulating ring is 0.5mm. The ion source 1 is a 10.6eV VUV photoionization source; the ion receiving electrode 8 is a Faraday disk with a diameter of 6mm, fixed on a metal shielding tube with an outer diameter of 30mm; the ionization region 10 and the migration region 11 are both composed of coaxially alternatingly stacked annular conductive electrode sheets with an axial length of 5mm and an outer diameter of 30mm and annular insulating electrode sheets with an axial length of 5mm and an outer diameter of 30mm.

[0028] Comparative Example 1

[0029] The electric field strength of the fixed ionization zone is 200V / cm, and the electric field strength of the migration zone is 400V / cm. The needle valve is adjusted to stabilize the gas pressure in the migration tube at 100kPa, and the pressure gauge is read. The drift gas flow rate is 200mL / min. A mixed sample standard gas of 1ppmv benzene, toluene and xylene is introduced into the ionization zone at an inlet flow rate of 100mL / min and ionized by the vacuum ultraviolet light. After that, the final stable product ions in the ionization zone are introduced into the migration zone by the electric field and reach the detector for detection. The ion migration spectrum of the mixed sample detection at this time is recorded, such as Figure 2 As shown, it can be found that when the electric field strength in the ionization zone is 200 V / cm and the working gas pressure in the migration tube is 100 kPa, only the xylene molecular ions with the lowest ionization energy can be detected, while the toluene and benzene ions with higher ionization energy will continue to take away the charge of the xylene molecules and convert into toluene and benzene molecules through charge transfer reactions.

[0030] Under atmospheric pressure conditions, the sample gas to be tested is carried by clean compressed air into the ionization zone of the migration tube through the sample gas inlet and is first ionized; then, the sample ions of each component undergo further ion-molecule charge transfer and proton transfer reactions in the ionization zone according to the differences in their respective proton affinities or ionization energies to form final product ions under different gas pressure conditions, resulting in the loss of some difficult-to-detect target compound ions and their inability to be detected.

[0031] Example 1

[0032] The electric field strength of the fixed ionization zone is 200V / cm, and the electric field strength of the migration zone is 400V / cm. The needle valve is adjusted to stabilize the gas pressure in the migration tube at 50kPa and 10kPa respectively, and the reading is read from the barometer. The drift gas flow rate is 200mL / min. The mixed sample standard gas of 1ppmv benzene, toluene and xylene is introduced into the ionization zone at an inlet flow rate of 100mL / min and ionized by the vacuum ultraviolet light. After that, the final stable product ions in the ionization zone are introduced into the migration zone by the electric field and reach the detector for detection. Record the ion migration spectrum of the mixed sample detection at this time, such as Figure 3 and Figure 4 As shown,

[0033] Combination Figures 2 to 4 From the results, we can find that lowering the working gas pressure of the ion transfer tube can suppress the matrix effect of xylene molecules on the detection of benzene ions and toluene ions. When the working gas pressure of the ion transfer tube is 10 kPa, the mean free path of ions in the ionization zone increases, the frequency of ion-molecule collisions decreases significantly, and plasma-molecule reactions such as charge transfer and proton transfer are suppressed. This can effectively suppress the reaction process of charge transfer between benzene, toluene and xylene, and effectively suppress the matrix effect of xylene molecules on the detection of benzene ions and toluene ions.

[0034] Example 2

[0035] The working gas pressure of the ion mobility spectrometer is fixed at 10 kPa, and the isolated high-voltage power supply connected to the ionization zone is adjusted to increase the electric field strength of the ionization zone from 200 V / cm to 450 V / cm. At the same time, the electric field strength of the migration zone is still fixed at 400 V / cm. The drift gas flow rate is 200 mL / min. 1 ppmv of benzene, toluene and xylene mixed standard gas is carried into the ionization zone through an intake volume of 100 mL / min and the respective signal response intensities under the condition of 450 V / cm electric field strength in the ionization zone are recorded, such as Figure 5 shown.

[0036] Compared to Figure 4 From the results in the experiment, it can be found that on the basis of a gas pressure of 10 kPa, after increasing the electric field in the ionization zone, the higher electric field strength in the ionization zone can shorten the residence time of ions in the reaction zone, thereby suppressing the occurrence of plasma-molecule reactions such as proton transfer and charge transfer, thereby further effectively suppressing the matrix effect of xylene molecules on the detection of benzene ions and toluene ions, significantly improving the signal intensity of toluene and benzene whose ionization is suppressed by xylene, and improving the detection sensitivity of ion mobility spectrometry.

Claims

1. A method for reducing the matrix effect of complex samples in photoionization ion mobility spectrometry detection, the photoionization ion mobility spectrometer comprising a photoionization source (1) and an ion receiving electrode (8) arranged opposite to each other at the left and right ends, respectively, and an ion gate (9) located between the photoionization source (1) and the ion receiving electrode (8), the area between the photoionization source (1) and the ion gate (9) being an ionization area (10), and the area between the ion gate (9) and the ion receiving electrode (8) being a migration area (11); characterized in that: A sample gas inlet (2) is provided on the upper wall of the ionization region (10) near the photoionization source (1), and the sample gas inlet (2) is connected to the sample gas source through a first mass flowmeter (3); a gas outlet (4) is provided on the upper wall of the ionization region (10) near the ion gate (9), and the gas outlet (4) is connected to the atmosphere through a pressure gauge (13), a needle valve (5) and a mechanical pump (6) arranged in sequence on a connecting pipeline; a drift gas inlet (7) is provided above the ion receiving electrode (8) on the right wall of the migration region (11), and the drift gas inlet (7) is connected to the drift gas source through a second mass flowmeter (12); The ionization region (10), the ion gate (9) and the migration region (11) are all controlled by independent isolated high-voltage power supplies, so that the electrostatic field of each region can be independently adjusted without affecting the electrostatic field strength of other regions; The method is specifically as follows: 1) controlling the outlet gas flow rate in the ion transfer tube by adjusting the needle valve (5) and the mechanical pump (6) to maintain the gas pressure in the ion transfer tube in the range of 10 to 80 kPa, preferably 10 to 50 kPa, more preferably 10 to 20 kPa; At the same time, 2) by adjusting the isolated high voltage power supply connected to the ionization zone, the electric field in the ionization zone is maintained in the range of 300 to 600 V / cm, preferably 300 to 500 V / cm, and most preferably 400 to 500 V / cm.

2. The method for reducing the matrix effect of complex samples in photoionization ion mobility spectrometry detection according to claim 1, characterized in that: The photoionization source includes a laser emitter and a vacuum ultraviolet lamp.

3. The method for reducing the matrix effect of complex samples in photoionization ion mobility spectrometry detection according to claim 1, characterized in that: The flow rate of the migration spectrum sample gas inlet (2) is controlled by a first mass flow meter to be in the range of 50 to 200 mL / min, the flow rate of the drift gas inlet (7) is controlled by a second mass flow meter to be in the range of 50 to 200 mL / min, and the electric field strength in the migration zone is 200 to 500 V / cm.

4. The method for reducing matrix effect of complex samples in photoionization ion mobility spectrometry detection according to claim 1, characterized in that: The ion gates used are Tyndall-Powell type ion gate and Bradbury-Nielsen type ion gate.

5. The method for reducing the matrix effect of complex samples in photoionization ion mobility spectrometry detection according to claim 1, characterized in that: The floating gas source and the carrier gas source are clean compressed air, and the clean compressed air is air filtered through 13X molecular sieve and activated carbon in sequence.

Images