Molecular ion reaction cavity for chemical ionization mass spectrometry

Abstract

The invention discloses a molecular ion reaction chamber for chemical ionization mass spectrometry, which comprises a sampling module, a gas inlet module, a reaction module and a power supply module, and is characterized in that the sampling module is provided with a main pipeline and a drainage bypass; the center of the air inlet module is provided with a hole for fixing the capillary; an ion source air inlet channel is arranged close to the side surface of the top end of the reaction module, and an air outlet hole is formed in the center of the bottom surface of the reaction module; sampling gas enters from the top end of the conical cavity from the main pipeline through the capillary tube and reacts with ions entering from the ion source gas inlet channel in the conical cavity, and the reacted gas enters the mass spectrum detector from the gas outlet hole; the power supply module provides a radial focusing electric field for the conical cavity. The drainage bypass effectively reduces the response time of the reaction cavity to organic matters, the capillary tube can be replaced according to different experimental conditions, the conical reaction cavity structure effectively reduces ion loss and improves the ionization efficiency in a laminar flow state, and the molecular ion reaction process is effectively controlled through an external temperature adjusting device and a vacuum pump.

Description

Technical Field

[0001] The invention relates to the technical field of chemical ionization mass spectrometry detection, and in particular to a molecular ion reaction chamber for chemical ionization mass spectrometry, which can be applied to chemical ionization mass spectrometry for detecting low and medium volatile organic compound concentrations using iodine as a reagent ion. Background Art

[0002] In order to more effectively control secondary pollution, it is necessary to systematically study and master the theoretical chemical mechanism of atmospheric complex pollution with atmospheric oxidation as the core. The prerequisite is to accurately measure the key intermediates in the atmospheric oxidation process, which puts forward requirements for high-accuracy and high-selectivity online measurement technology.

[0003] At present, the measurement technology for atmospheric oxidation intermediates can be divided into mass spectrometry and spectroscopy. Among them, mass spectrometry represented by chemical ionization mass spectrometry has attracted widespread attention due to its high selectivity and the advantage of being able to measure species. The principle of chemical ionization mass spectrometry is to convert the molecules to be measured into charged ions through chemical reactions with reagent ions, and use mass spectrometry to detect the charged ions to obtain their concentration information. Chemical ionization mass spectrometry usually consists of four modules, namely, ion source, molecular ion reaction chamber, ion transmission system and mass analyzer. The ion source is the place where reagent ions are generated. High-concentration reagent ions enter the reaction chamber with the gas to be measured to react with molecular ions (i.e., ionization process). The ionization efficiency is directly related to the detection efficiency of the instrument. This process is related to the properties of the substance to be measured and the reagent ions themselves, and will also be significantly affected by operating conditions, such as the structure of the reaction chamber, reaction temperature, pressure, etc. The ion transmission system is designed to enable the product ions generated by the reaction to reach the mass spectrometry system for detection, and the mass analyzer is the module for detecting product ions. It should be pointed out here that the chemical ionization mass spectrometers currently used in the field of atmospheric chemistry are basically produced by foreign mass spectrometry instrument companies, and there are few reports on the application of domestic chemical ionization mass spectrometers.

[0004] Due to the high reactivity and low concentration of oxidative intermediate species in the actual atmosphere, the existing mass spectrometry detection methods are still insufficient to meet the measurement needs of these species, and there are problems such as low sensitivity and high detection limit. Taking a reaction chamber designed by the Swiss Tofwerk Instrument Company (hereinafter referred to as the traditional reaction chamber) as an example, the results of flow field simulation and experimental testing show that its internal flow field is mainly turbulent, and gas molecules are very easy to collide or hit the wall. Loss, resulting in its measurement effect on medium and low-level volatile organic compounds in the actual atmosphere is not obvious, and it cannot meet the needs of exploring the theoretical mechanism of atmospheric chemistry. Therefore, it is of great practical significance to design molecular ion reaction chambers in a targeted manner and explore its reaction conditions to effectively promote the molecular ion reaction, which is of great practical significance for realizing the measurement of oxidative intermediate species at the ppt level in the atmosphere. Summary of the invention

[0005] The purpose of the present invention is to provide an efficient molecular ion reaction chamber for chemical ionization mass spectrometry, which solves the problems of large wall loss and poor detection effect caused by internal flow field turbulence in traditional reaction chambers, effectively controls the molecular ion reaction process, and improves ionization efficiency, so as to achieve efficient measurement of medium and low volatile organic compounds.

[0006] To achieve the above-mentioned purpose, the present invention designs a molecular ion reaction chamber for chemical ionization mass spectrometry, comprising a sampling module, an air intake module, a reaction module and a power supply module, wherein the sampling module is provided with a main line for the sampling gas to pass through and a bypass for drainage; the air intake module is a sheet structure with a capillary fixing hole in the center, and the capillary is fixed in the hole through a connecting piece; the interior of the reaction module is a conical cavity, and an ion source air intake channel is opened on the side close to the top of the conical cavity, and an air outlet is provided in the center of the bottom surface of the conical cavity; the sampling module, the air intake module and the reaction module are assembled together in sequence, and the main line of the sampling module and the conical cavity of the reaction module are connected through a capillary fixed in the air intake module; the sampling gas enters the conical cavity from the top of the conical cavity through the capillary from the main line of the sampling module, reacts with the ions entering from the ion source air intake channel in the conical cavity, and the reacted gas enters the mass spectrometer detector from the air outlet on the bottom surface of the conical cavity; the power supply module provides a radial focusing electric field for the conical cavity.

[0007] Furthermore, the sampling module of the molecular ion reaction chamber is a columnar structure, and a main pipeline interface is provided on the top of the sampling module, which is a tapered internal thread structure. Standard parts can be used to connect to the sampling pipeline in the form of a threaded knob, and maintain good air tightness. A drainage bypass interface is provided on the side of the sampling module, which is a tapered internal thread hole, and an external vacuum pump can be connected to drain during sampling to achieve a faster response time of the instrument. The sampling module is provided with a circle of U-shaped grooves on the bottom surface for placing O-rings to ensure the sealing of the connection between the sampling module and the air intake module.

[0008] Furthermore, the air inlet module of the molecular ion reaction chamber is a disc or other shaped sheet with a capillary fixing hole in the center, and the capillary fixing hole is connected to the capillary connector in a threaded knob manner; the capillary is preferably made of Peek material (polyetheretherketone), and a ferrule is installed at the bottom of the capillary to prevent air leakage.

[0009] Furthermore, the reaction module of the molecular ion reaction chamber is a columnar structure, with a conical cavity inside and a smooth inner wall. It can be coated with a Teflon film or silanized to reduce the adsorption effect of organic matter. An ion source air intake channel is provided on the side of the conical cavity near the top for connecting the ion source. A circle of U-shaped grooves is also provided on the top surface of the reaction module for placing an O-ring to ensure the sealing of the connection between the air intake module and the reaction module.

[0010] Furthermore, multiple through holes are opened on the sampling module and the air intake module (such as three, four or more through holes evenly distributed on the periphery of the module), and multiple screw holes are opened at corresponding positions on the top of the reaction module. Screws are passed through the through holes and inserted into the screw holes and tightened to fix the sampling module, the air intake module and the reaction module.

[0011] Furthermore, the side of the reaction module has a protruding outer edge portion, on which a plurality of threaded holes are provided for being fixedly connected to the molecular ion reaction chamber fixing device of the mass spectrometry system by means of screws.

[0012] Furthermore, there are multiple choices of materials for the sampling module, the air intake module and the reaction module of the molecular ion reaction chamber, such as 304 stainless steel, 316 stainless steel, Teflon, Peek, etc., among which the reaction module is best made of 304 stainless steel or 316 stainless steel.

[0013] Furthermore, the inner diameter of the main pipeline of the sampling module is 20 mm, and a 1 / 8 to 1 / 2 inch Teflon tube can be connected externally as a sampling tube. The best choice is a 1 / 2 inch tube to reduce the wall loss of organic matter.

[0014] Further, the ion source, such as iodine ions, ammonium ions, etc., applies voltage to the reaction module through the power module, so that it is negatively charged or positively charged, and a radial focusing electric field is formed in the conical cavity of the reaction module, so that the product ions are concentrated to flow to the outlet hole at the bottom of the conical cavity. For example, in order to reduce the loss of negative ions (such as iodine ions) in the reaction chamber, the power module provides a negative voltage of -20 to 0V, and the power module is connected to the reaction module (or the sampling module and the air intake module connected to the reaction module) through a cable. After power is turned on, the reaction chamber is negatively charged as a whole, which can effectively reduce the loss of product ions on the wall. The length of the conical cavity of the reaction module is preferably 50 to 53 mm, and the cone angle range is optional 13 ° to 16 °. In one embodiment of the present invention, the length of the conical cavity is 52 mm and the cone angle is 15 °.

[0015] Furthermore, the outer diameter of the capillary used in the air inlet module is 1 / 16 inch, and the inner diameter can range from 0.01mm to 1mm. The best is to use a capillary with an inner diameter of 1mm, which can not only meet the low vacuum conditions of the subsequent mass spectrometry system, but also increase the concentration of the molecules to be tested in the reaction chamber. In order to meet different test requirements and improve selectivity, capillaries of different inner diameters can be prepared in advance for easy replacement.

[0016] Furthermore, when performing chemical ionization mass spectrometry detection, the reaction module body of the molecular ion reaction chamber will be placed in a pre-designed vacuum chamber, which is connected to the rear-end mass spectrometer detector and equipped with a special vacuum pump to provide the pressure conditions required for the reaction. The optimal pressure conditions are 150-400 mbar.

[0017] Furthermore, due to the large pressure difference between the molecular ion reaction chamber and the ambient atmosphere, and the small inner diameter of the capillary, the gas enters the reaction module in the form of a high-speed jet and quickly reaches a laminar flow state; at the same time, adjusting the side ion source air intake flow rate can achieve sufficient mixing of the reagent ions and the sampling gas, effectively improving the ionization efficiency.

[0018] Furthermore, a temperature control device can be installed on the periphery of the reaction module to control the temperature of the molecular ion reaction. On the one hand, the molecular ion reaction is an exothermic reaction, and a lower temperature is conducive to the positive movement of the chemical equilibrium to promote the ionization process; on the other hand, the medium and low volatile organic species concerned by the present invention need to react in the reaction chamber in a gaseous form at a higher temperature. Finally, based on experimental tests, the optimal temperature condition is 40-45°C.

[0019] The main body of the molecular ion reaction chamber designed by the present invention is composed of three modules: sampling, air intake, and reaction. The modules are fixedly connected and the air tightness of the device is ensured by an O-ring. Among them, the sampling module is equipped with a drainage bypass, which effectively reduces the instrument's response time to organic matter; the air inlet of the air intake module is designed as a replaceable capillary line, which can be replaced according to different experimental conditions; the interior of the reaction module is a conical cavity structure, which can effectively reduce ion loss and improve ionization efficiency in a laminar state; the reaction chamber is equipped with an external temperature adjustment device and a vacuum pump, which can artificially adjust the temperature and pressure conditions to achieve effective control of the molecular ion reaction process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the connection between the molecular ion reaction chamber and the mass spectrometer detector of the present invention;

[0021] Figure 2 The figure (a) and the cross-sectional view (b) of the sampling module, the air intake module and the reaction module of the molecular ion reaction chamber of the present invention;

[0022] Figure 1 and Figure 2 Middle: 1-sampling module, 2-air inlet module, 3-reaction module, 4-main line interface, 5-drainage bypass interface, 6-capillary fixing hole, 7-ion source air inlet channel, 8-conical reaction area, 9-power module, 10-molecular ion reaction chamber fixing device, 11-vacuum exhaust port, 12-mass spectrometer detector.

[0023] Figure 3 4 is a flow field simulation diagram of an embodiment of the present invention.

[0024] Figure 4 1 and 1. The structural appearance diagram (a) and cross-sectional diagram (b) of a conventional reaction chamber compared with an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The following is a further detailed description of this patent through specific implementation cases in conjunction with the accompanying drawings. The specific implementation cases described here are only for explaining the present invention, not for limiting the present invention.

[0026] It should be noted that the following detailed descriptions are exemplary and are intended to further illustrate the technical solutions of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0027] In addition, the terms “upper”, “lower”, “left”, “right”, “top”, “bottom”, etc. used in the following description are only for the convenience of explaining the structure of the present invention, and are not intended to limit the present invention.

[0028] like Figure 1 As shown, the molecular ion reaction chamber used for chemical ionization mass spectrometry in this embodiment needs to be mounted on a matching molecular ion reaction chamber fixture 10, the molecular ion reaction chamber fixture 10 is connected to a mass spectrometer detector 12 at the rear end, and the fixture is also connected to an external vacuum pump through a vacuum pump port 11 to maintain the vacuum condition for normal operation of the reaction chamber. The molecular ion reaction chamber is divided into a sampling module 1, an air intake module 2, a reaction module 3 and a power module 9, see Figure 2 The sampling module 1 is provided with a main pipeline for the sampling gas to pass through and a bypass for drainage. The main pipeline interface 4 is located at the top of the sampling module 1, and the drainage bypass interface 5 is located on the side of the sampling module 1; the main body of the air intake module 2 is a flat disc, and a columnar capillary fixing hole 6 is provided in the center of the disc; the interior of the reaction module 3 is a conical reaction area 8, and an ion source air intake channel 7 is opened on its side. The sampling module 1, the air intake module 2, and the reaction module 3 are sequentially connected and assembled together to form a columnar structure, see Figure 1 The portion of the columnar structure where the conical reaction area 8 is located is installed in the vacuum chamber of the molecular ion reaction chamber fixture 10, while the ion source air inlet channel 7 and the sampling module 1 and the air inlet module 2 are located outside the vacuum chamber to facilitate the connection of various pipelines. The power module 9 is connected to the reaction module 3 through a cable to provide a radial focusing electric field for the reaction chamber. A heating device and a temperature sensor are arranged in the vacuum chamber of the molecular ion reaction chamber fixture 10 to control the temperature of the reaction module 3.

[0029] It should be noted that the dimension design and application conditions of this embodiment are optimized by combining theoretical calculation and experimental testing.

[0030] The sampling module 1 is composed of two cylinders with a total length of 40mm. The center of the cylinder is hollowed out as the main pipeline with an inner diameter of 20mm. The top is the main pipeline interface 4, which is designed as a 1 / 2NPT threaded hole for connecting the sampling pipeline. The lower side is the drainage bypass interface 5, which is designed as a 1 / 4NPT threaded hole for connecting the drainage pipeline. A circle of U-shaped grooves is designed on the bottom surface of the sampling module 1 for placing O-rings to ensure the air tightness of the device.

[0031] The air intake module 2 is a disc with a cylindrical groove. The outer diameter of the cylindrical groove in the center is 5 mm, and the inner wall of the groove is a 1 / 16 NPT thread for connecting with a capillary. The capillary is placed on a 1 / 16 inch screw connector, and capillaries of different inner diameters can be replaced according to actual needs.

[0032] The reaction module 3 is cylindrical in shape as a whole, and the interior is hollowed out into a conical radial cavity, which is the area for molecular ion reaction (conical reaction area 8). A 1 / 8NPT threaded hole is provided on the side of the reaction module 3 for connecting the ion source air inlet pipeline. A circle of U-shaped grooves is designed on the top surface of the reaction module 3 for placing O-rings to ensure the air tightness of the device; a small hole is provided in the center of the bottom surface of the reaction module 3, and the product ions after the reaction of the molecules to be measured and the reagent ions enter the mass spectrometer detector 12 through the small hole.

[0033] The sampling module 1 and the air intake module 2 are provided with through holes, and the two can be fixed to the threaded holes on the reaction module 3 by passing M4 screws through the through holes. A heating device and a temperature sensor are installed on the outside of the reaction module 3 to realize the reaction temperature control, so as to promote the volatilization of organic matter and minimize its wall loss.

[0034] Table 1

[0035]

[0036] In this embodiment, the reaction module 3 serves as the site for molecular ion reaction. Its design parameters and application conditions are key factors affecting the efficiency of molecular ion reaction. Table 1 summarizes the key parameters and application conditions of this embodiment based on experimental tests.

[0037] Figure 3 The following is a flow field simulation diagram of the conical reaction area 8 of this embodiment based on the relevant parameters described in Table 1. Figure 3As shown, after the sampling gas flow enters the reaction chamber through the capillary, it is disturbed by the ion source gas flow from the side. The gas molecules in the sampling gas flow react quickly with the reagent ions in the ion source gas flow. This part is the mixed reaction area. After the mixed gas flow is transmitted for a distance, it gradually reaches a laminar flow state and hardly collides with the inner wall of the reaction chamber. Part of the gas flow enters the mass spectrometer detector from the axial outlet for detection, and the rest of the gas flow is pumped out by the vacuum pump. Figure 3 It shows that the molecular ion reaction chamber designed in the present invention effectively realizes the laminar flow state in the reaction area, and better avoids the loss caused by the collision between the airflow and the wall. The calculated inlet flow rate of the sampling gas in this embodiment is about 360m / s, and the average residence time is 0.3 milliseconds.

[0038] To more intuitively demonstrate the advantages of this embodiment in reducing molecular ion loss and improving molecular ion reaction efficiency, the following compares the measurement effects of the conventional reaction chamber and the reaction chamber of this embodiment when iodine ions are used as reagent ions for measuring low- and medium-level volatile organic compounds. Figure 4 The basic structure of the traditional reaction chamber is shown. The flow field simulation results of the traditional reaction chamber show that the internal airflow is relatively turbulent, and most of the airflow will directly contact the wall, which will cause a large amount of ion loss, but its advantage is that it has a contraction outlet, which enables it to focus the airflow. This embodiment optimizes the structure of the reaction chamber to reduce ion loss, and focuses the airflow through an external DC power supply.

[0039] Table 2

[0040]

[0041] Table 2 shows the measurement results of the present embodiment and the traditional reaction chamber for medium and low volatile organic compounds. The specific implementation method of the comparative experiment is: use the same set of standard gas generation system to provide standard substance gas containing the same concentration, this group of gases enters the chemical ionization mass spectrometer equipped with the present embodiment and the traditional reaction chamber respectively through the same pipeline, and the performance indicators of the mass spectrometer detector at the back end are completely consistent, and the detection signal intensity (i.e., sensitivity) of the present embodiment and the traditional reaction chamber for the same substance are compared. The test species selected are mainly the products generated during the gas phase oxidation of aromatic hydrocarbons. These species are substances that need to be focused on in the field of atmospheric chemistry research, such as 2,4-hydroxytoluene (C7H8O2), 4-nitrophenol (C6H5NO3), 2-methyl-4-nitrophenol (C7H7NO3), products generated by the oxidation of natural organic matter, such as troperidine acid (C 10 H 16 O3), and formic acid (CH2O2) with relatively high volatility. The experimental results show that the detection sensitivity of this embodiment for these species is significantly better than that of the traditional reaction chamber. 10 H16 O3 is a low-volatility organic compound with a high relative molecular mass and special functional groups, which is very easy to sink to the wall and cause gas phase loss. However, this embodiment reduces the loss of these substances in the reaction and transmission stages by optimizing the reaction chamber structure, temperature regulation, electric field focusing and other measures, and the optimization effect of its detection sensitivity reaches more than 50%; CH2O2 and C7H8O2 are relatively volatile, and it can be found that the detection effect of this embodiment and the traditional reaction chamber is not much different. Overall, this embodiment can better optimize the molecular ion reaction efficiency and improve the detection effect.

[0042] The above are preferred embodiments of the present invention. Any changes made according to this implementation scheme, as long as the resulting functional effects do not exceed the scope of the technical solution of the present invention, shall fall within the protection scope of the present invention.

[0043] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making any creative work shall fall within the scope of protection of the present invention.

Claims

1. A molecular ion reaction chamber for chemical ionization mass spectrometry, comprising a sampling module, an air intake module, a reaction module and a power supply module, wherein: The sampling module is provided with a main line for the sampling gas to pass through and a bypass for drainage; the air intake module is a sheet structure with a capillary fixing hole in the center, and the capillary is fixed in the hole through a connector; the interior of the reaction module is a conical cavity, and an ion source air intake channel is opened on the side close to the top of the conical cavity, and an air outlet is provided in the center of the bottom surface of the conical cavity; the sampling module, the air intake module and the reaction module are assembled together in sequence, and the main line of the sampling module and the conical cavity of the reaction module are connected through a capillary fixed in the air intake module; the sampling gas enters the conical cavity from the top of the conical cavity through the capillary from the main line of the sampling module, reacts with ions entering from the ion source air intake channel in the conical cavity, and the reacted gas enters the mass spectrometer detector from the air outlet on the bottom surface of the conical cavity; the power module provides a radial focusing electric field for the conical cavity.

2. The molecular ion reaction chamber according to claim 1, characterized in that: The sampling module is a columnar structure, with a main line interface provided on the top of the sampling module, which is a conical internal thread structure and is connected to the sampling pipeline with a threaded knob; a drainage bypass interface is provided on the side of the sampling module, which is a conical internal thread hole, and an external vacuum pump is used for drainage during sampling; the bottom of the sampling module is provided with a circle of U-shaped grooves, in which an O-ring is placed to seal the connection between the sampling module and the air intake module.

3. The molecular ion reaction chamber according to claim 1, characterized in that: The air intake module is a disc with a capillary fixing hole at the center, and the capillary fixing hole is connected to the capillary connector in a threaded knob manner.

4. The molecular ion reaction chamber according to claim 1, characterized in that: The reaction module is a columnar structure with a conical cavity inside and the inner wall is coated with Teflon film or silanized. A circle of U-shaped grooves is provided on the top surface of the reaction module, in which an O-shaped sealing ring is placed to seal the connection between the air intake module and the reaction module.

5. The molecular ion reaction chamber according to claim 1, characterized in that: A plurality of through holes are provided on the sampling module and the air intake module, and a plurality of screw holes are provided at corresponding positions on the top of the reaction module. Screws are passed through the through holes and inserted into the screw holes and tightened to fix the sampling module, the air intake module and the reaction module together.

6. The molecular ion reaction chamber according to claim 1, characterized in that: The side of the reaction module has a protruding outer edge portion, on which a plurality of threaded holes are provided, and screws pass through the threaded holes to fix the molecular ion reaction chamber to the molecular ion reaction chamber fixing device of the mass spectrometry system.

7. The molecular ion reaction chamber according to claim 1, characterized in that: The materials of the sampling module, the air intake module and the reaction module of the molecular ion reaction chamber are selected from 304 stainless steel, 316 stainless steel, Teflon and Peek.

8. The molecular ion reaction chamber according to claim 1, characterized in that: The inner diameter of the capillary is in the range of 0.01 mm to 1 mm; the length of the conical cavity is 50 to 53 mm, and the cone angle is 13° to 16°.

9. The molecular ion reaction chamber according to claim 1, characterized in that: A temperature control device is installed on the periphery of the reaction module to control the temperature of the molecular ion reaction.

10. The molecular ion reaction chamber according to claim 1, characterized in that: The portion where the conical cavity of the reaction module is located is located in a vacuum chamber.

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