Ion mobility spectrometer for switching ion species by controlling gas pressure

Abstract

An ion mobility spectrometry method that switches the type of photoionized reagent ions by controlling gas pressure includes a photoionization ion mobility spectrometer, a reagent ion generator, and a gas pressure control device. The reagent molecule acetone evaporates in the headspace of the reagent bottle and is carried into the ionization region by a carrier gas. An external mechanical pump connected to the gas outlet of the migration tube reduces the working gas pressure inside the migration tube, and the pressure is read by a barometer, thus reducing the carbon dioxide content and the probability of collisions between ions within the migration tube. At atmospheric pressure, the main reagent ion in negative ion mode is CO3. ‑ (H2O) n Low pressure inhibits CO3 ‑ (H2O) n It is produced, and the main reagent ion is O2. ‑ (H2O) n O2 ‑ (H2O) n Within a photoionization system, these are active ions, which can enhance reactivity and significantly improve the detection sensitivity of target analytes and product ions. For example, for methyl salicylate, which only reacts with O2... ‑ (H2O) n The target substance undergoes an addition reaction, thereby ionizing. Furthermore, this method only requires an external mechanical pump for pressure control, making it simple to operate and eliminating the need for redesigning the migration spectrum. Changing the pressure also allows for a faster response speed, enabling rapid switching of the primary reagent ion in the negative ion mode.

Description

Technical Field

[0001] This invention belongs to the field of analytical chemistry instruments, specifically relating to an ion mobility spectrometer that switches the types of photoionizing reagent ions by controlling gas pressure. Under reduced gas pressure, O2 is obtained. - (H2O) n Reagent ions can enhance reaction activity and greatly improve the detection sensitivity of target product ions. Background Technology

[0002] Vacuum ultraviolet photoionization sources are currently the most widely researched and applied non-radiative ionization sources. Combined with time-of-migration ion mobility spectrometry (IMS), they can enable qualitative and quantitative analysis of most volatile organic compounds (VOCs). Currently, the most reported technologies in the literature are commercially available vacuum ultraviolet Kr lamps that provide 10.0 and 10.6 eV photons. However, most direct photoionization methods have relatively low ionization efficiency, and some substances cannot be detected by direct photoionization. To address this issue, the sensitivity and selectivity of sample detection can be improved by introducing reagent molecules to assist photoionization. Different reagent ions typically lead to different ionization channels, resulting in analyte product ions with different reduced mobility. Furthermore, switching the types of reagent ions can also enhance the detection sensitivity of specific compounds.

[0003] Cheng et al. (Anal Chem, 2014, 86:2687) developed a reagent molecule-assisted photoionization mobility spectrometry (DANP-IMS) capable of rapidly switching reagent ions. By changing the gas flow pattern inside the IMS, they were able to obtain O2 with an ion yield of 89% under unidirectional flow. - (H2O) n CO3 with an ion yield of 88% can be obtained under bidirectional gas flow. - (H2O) n Through O2 - (H2O) n With CO3 - (H2O) n Switching between different analyte types can improve the sensitivity of the analyte and provide more information for the detection of sample molecules. Jiang et al. (Anal Chem, 2018, 90(8): 5280) designed a side-mounted photoionization source, which not only reduced the ozone generation area but also increased the rate at which ozone was blown out of the ionization region, thereby limiting the CO3 emission. - (H2O) n The yield. Li et al. (CN201911288763.X) formed reactive reagent ions H by changing the gas flow field structure and dopant composition in the ionization region.+ (H2O) n This reduced the detection limit of ammonia by nearly an order of magnitude, reaching 0.3 ppbv. Wang et al. (CN114624326A) effectively suppressed CO32 by using isopropanol as the sample solvent, filling the ionization region with isopropanol vapor. - (H2O) n The production of CO4 can increase the amount of CO4 produced. - (H2O) n The concentration of reagent ions can be increased, thereby greatly improving the detection sensitivity of target product ions.

[0004] However, the above methods require redesigning the ionization region of the instrument, reducing the universality of the mobility spectrum; while changing the dopant composition may affect the accuracy of the analysis by introducing other ions.

[0005] Therefore, this invention only requires an external mechanical pump for gas pressure control, making it simple to operate and eliminating the need for redesigning the migration spectrum. Furthermore, changing the gas pressure allows for a faster response speed, thus enabling rapid switching of the main reagent ions in negative ion mode. Summary of the Invention

[0006] The technical problem to be solved by this invention is: in negative ion mode, by reducing the working gas pressure within the photoionization ion mobility spectrum, the reagent ions obtained by ionizing acetone reagent molecules are converted from CO3... - (H2O) n Switch to O2 - (H2O) n Furthermore, methyl salicylate only reacts with O2. - (H2O) n Highly sensitive detection of VOCs ionized by reagent ions undergoing an addition reaction.

[0007] The specific content includes:

[0008] Methods for controlling gas pressure to switch reagent ion types: In a normal pressure environment, under negative ion mode, reagent molecules generate a large number of low-energy electrons after vacuum ultraviolet photoionization. Oxygen with a higher affinity captures these low-energy electrons, generating O2. - Furthermore, oxygen can undergo a photochemical reaction under ultraviolet light to produce ozone, and capture low-energy electrons to further generate O3. - Both can react with trace amounts of water molecules in the air to generate corresponding hydrated ions, namely O2. - (H2O) n With O3 - (H2O) n And O3 - (H2O) n It can further combine with CO2 molecules in the air to generate CO3.- (H2O) n Ions. Lowering the air pressure not only reduces the relative content of carbon dioxide, but also makes CO3... - (H2O) n The generation rate of CO3 is slowed down, and the probability of collisions between ions is reduced, allowing for more reaction steps. - (H2O) n The ions were effectively suppressed. Therefore, after the gas pressure decreased from atmospheric pressure to low pressure, the main reagent ions were suppressed by CO32-. - (H2O) n Switch to O2 - (H2O) n ion.

[0009] The method for controlling the working air pressure conditions inside the migration tube is to adjust the exhaust flow rate at the tail gas outlet and read the reading through the barometer to stabilize the working air pressure of the migration tube within any air pressure condition in the range of 10 to 100 kPa.

[0010] To ensure high-throughput detection by ion mobility spectrometry, the flow rate range of the carrier gas inlet of the migration tube is 100–300 mL / min, and the flow rate range of the drift gas inlet is 100–300 mL / min.

[0011] To improve the detection sensitivity of ion mobility spectrometry, the electric field strength in the ionization region was controlled at 500–700 V / cm, and the electric field strength in the migration region was controlled at 300–500 V / cm.

[0012] The ionization source is a commercially available vacuum ultraviolet Kr lamp that can provide photon energies of 10.0 and 10.6 eV, and the lamp current is adjusted to be controlled between 50 and 100 μA; the ion gate used is a Bradbury-Nielsen type ion gate. Attached Figure Description

[0013] Figure 1 A schematic diagram of a photoionization ion mobility spectrometer that controls the switching of reagent ions by controlling gas pressure.

[0014] Figure 2 Ion migration spectrum of O2-(H2O)n as the main reagent ion peak in negative ion mode at 20 kPa.

[0015] Figure 3 Comparison of the sensitivity of methyl salicylate detection under negative ion mode at 20 kPa and 90 kPa. Detailed Implementation

[0016] The ion migration tube structure disclosed in this invention, which allows for controllable switching of reagent ion types based on gas pressure, is as follows: Figure 1As shown in the figure. The ion mobility tube used in this method includes an ion mobility tube formed by coaxially stacking a circular electrode and a circular insulator alternately in sequence. Inside the ion mobility tube, an ion source 1, an ion gate 14, and an ion receiving electrode 11 are arranged in sequence along the direction of ion mobility; the ionization region 12 and the migration region 13 are separated by the ion gate 14. A carrier gas inlet 2 is provided in the ionization region 12 near the ion source 1, a tail gas outlet 7 is provided in the ionization region near the ion gate 14, and a drift gas inlet 6 is provided in the migration region 13 near the ion receiving electrode 11; the ion gate used inside the migration tube is a Bradbury-Nielsen type ion gate; the ion source 1 is a VUV vacuum ultraviolet Kr lamp with 10.0 and 10.6 eV; the ion receiving electrode 11 is a Faraday disk with a diameter of 6 mm, fixed on a metal shielding cylinder with an outer diameter of 30 mm; both the ionization region 12 and the migration region 13 are composed of annular conductive electrode sheets with an axial length of 5 mm and an outer diameter of 30 mm and annular insulating electrode sheets with an axial length of 5 mm and an outer diameter of 30 mm coaxially stacked alternately.

[0017] The method for controlling the working pressure inside the migration tube is as follows: Configure a 10 ppm acetone standard gas, fix the carrier gas flow rate carrying the acetone standard gas at 100 mL / min through the gas mass flowmeter 3, the drift gas flow rate is 200 mL / min, change the pumping flow rate of the mechanical pump 10 by adjusting the needle valve 9 provided at the tail gas outlet, and read the working pressure value of the migration tube through the pressure gauge 8 to make it stable at any pressure condition within the range of 10 - 100 kPa. This can not only reduce the relative content of carbon dioxide to slow down the generation rate of CO3 - (H2O) n , but also reduce the collision probability between ions, effectively inhibit the CO3 - (H2O) n ions with more reaction steps, and make the O2 - (H2O) n ions become the main reagent ions.

[0018] 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, and the temperature of the migration tube is 100 °C.

[0019] The tail gas outlet 7 is connected to the intake port of the mechanical pump 10 through the pressure gauge 8 and the needle valve 9 in sequence.

[0020] The reagent molecule headspace purge device 4 includes a reagent bottle with an open upper end and a closed container. Reagent molecules are filled in the reagent bottle, and the reagent bottle is placed inside the closed container; air intake ports and reagent molecule outlet ports are respectively provided on the left and right sides of the closed container opposite to the reagent bottle; the air intake port is connected to a clean compressed air source through a pipeline, and the reagent molecule outlet port is connected to the carrier gas inlet 2 through a pipeline;

[0021] Clean compressed air passes through the mass flow controller 3 at a fixed flow rate, carrying out reagent molecules through the reagent molecule headspace purge device, and then enters the ionization region 12 through the carrier gas inlet 2 for ionization.

[0022] Both the carrier gas and the drift gas are air, and the drift gas inlet is connected to a clean compressed air source through a pipeline;

[0023] Example 1

[0024] The pumping speed of the mechanical pump is adjusted at the exhaust outlet using a needle valve to maintain the working gas pressure of the migration tube at 20 kPa. A 10 ppmv acetone standard gas is introduced into the ionization region through the carrier gas inlet at a flow rate of 100 mL / min for detection and recording. Figure 2 As shown; it can be observed that in negative ion mode, the main reagent ion is O2. - (H2O) n CO3 - (H2O) n The ionic strength is low.

[0025] Example 2

[0026] The pumping speed of the mechanical pump is adjusted at the exhaust outlet using a needle valve to fix the working gas pressure of the migration tube at 20 and 90 kPa, respectively. 10 ppmv acetone standard gas and 10 ppmv methyl salicylate standard gas are introduced into the ionization region through the carrier gas inlet at a flow rate of 100 mL / min for detection and recording. Figure 3 As shown. It can be observed that at 90 kPa, CO3... - (H2O) n Using O2 as the main reagent ion, the detection sensitivity of methyl salicylate was low, with a current intensity of approximately 50 pA; at 20 kPa, O2... - (H2O) n Using methyl salicylate as the main reagent ion, the detection of methyl salicylate showed high sensitivity, with a current intensity of approximately 350 pA, representing a 700% improvement in detection sensitivity compared to 90 kPa.

Claims

1. An ion mobility spectrometry method for switching the types of photoionizing reagent ions by controlling gas pressure, characterized in that: This includes the migration tube of the vacuum ultraviolet photoionization source, the reagent molecule headspace purge device, and the gas pressure control device; specifically: The migration tube of the vacuum ultraviolet photoionization source includes a vacuum ultraviolet photoionization source (1), an ion gate (14), and an ion receiving electrode (11) arranged from left to right. An ionization region (12) and a migration region (13) are separated by the ion gate (14). A carrier gas inlet (2) is provided in the ionization region (12) near the ion source (1), a tail gas outlet (7) is provided in the ionization region (12) near the ion gate (14), and a drift gas inlet (6) is provided in the migration region (13) near the ion receiving electrode (11). The gas mass flow meter (3) controls clean compressed air with a fixed flow rate as carrier gas to carry the reagent molecules in the reagent molecule headspace purge device (4) into the carrier gas inlet (2). A barometer (8), a needle valve (9), and a mechanical pump (10) are provided at the exhaust outlet (7). The exhaust outlet (7) is connected in sequence to the air inlet of the barometer (8), the needle valve (9), and the mechanical pump (10). The working gas pressure is controlled by adjusting the needle valve (9) to change the pumping speed of the mechanical pump (10). The working gas pressure value in the migration tube under different pumping speeds is read by the barometer (8), thereby realizing the switching of CO3. - (H2O) n reagent ions and O2 - (H2O) n reagent ions; The working gas pressure in the migration tube is controlled by adjusting the needle valve (9) to change the pumping speed of the mechanical pump (10); during the process of reducing the gas pressure from standard atmospheric pressure to 10 kPa, the content of carbon dioxide molecules and the frequency of ion collisions in the ionization region (12) decrease as the gas pressure decreases, resulting in more reaction steps and CO3 that requires the participation of carbon dioxide to be generated. - (H2O) n The production of ions is suppressed, while O2 has fewer reaction steps under low pressure. - (H2O) n Ions become the main reagent ions; ultimately, by reducing the gas pressure, CO3 at normal pressure is converted into CO32-2 ... - (H2O) n Ions switch to O2 under low pressure - (H2O) n ion; The method for controlling the working air pressure conditions inside the migration tube is to adjust the exhaust flow rate at the tail gas outlet and read the reading through the barometer to stabilize the working air pressure of the migration tube within any air pressure condition in the range of 10~100 kPa.

2. The method according to claim 1, characterized in that: To ensure high-throughput detection of ion mobility spectrometry, the flow rate range of the carrier gas inlet of the migration tube is 100~300 mL / min, and the flow rate range of the drift gas inlet is 100~300 mL / min.

3. The method according to claim 1, characterized in that: The reagent molecule headspace purge device (4) includes a reagent bottle with an open top and a sealed container. The reagent bottle is filled with reagent molecules and placed in the sealed container. An air inlet and a reagent molecule outlet are respectively provided on the sealed container on the left and right sides of the reagent bottle. The air inlet is connected to a clean compressed air source through a pipeline, and the reagent molecule outlet is connected to a carrier gas inlet (2) through a pipeline. Clean compressed air passes through the gas mass flow meter (3) at a fixed flow rate through the reagent molecule headspace purge device, carrying out the reagent molecules, and then enters the ionization region (12) through the carrier gas inlet (2) for ionization.

4. The method according to claim 1, characterized in that: The carrier gas and the drift gas are both clean compressed air. The drift gas inlet is connected to the clean compressed air source through a pipeline via the second gas mass flow meter (5). The ionization source is a commercially available vacuum ultraviolet Kr lamp that can provide photon energies of 10.0 and 10.6 eV, and the lamp current is adjusted to be controlled between 50 and 100 μA; The ion gate used is a Bradbury-Nielsen type ion gate.

Images