Near-ambient-pressure photoionization mass spectrometry for volatile and semivolatile compounds

Abstract

The present invention provides a near-normal-pressure photoionization mass spectrometer for volatile and semi-volatile compounds, comprising a photoionization chamber, wherein the two axial ends of the photoionization chamber are respectively provided with a light source and a mass spectrometer, wherein the end of the photoionization chamber close to the light source is respectively provided with a sampling capillary and a dopant introduction tube in the radial direction, wherein the dopant introduction tube is connected to a dopant introduction mechanism, and the connection position of the dopant introduction tube and the photoionization chamber is located on the side close to the light source relative to the connection position of the sampling capillary and the photoionization chamber. The present invention requires that the sample be transferred to a high vacuum ionization chamber, and the semi-volatile sample can be directly volatilized for analysis under near-normal-pressure conditions, and there is no problem of capillary clogging; under near-normal-pressure conditions, higher molecular density and ion-molecule collision reaction frequency can significantly improve the sensitivity of the object to be tested, and ionization is promoted by dopants to improve signal strength.

Description

Technical Field

[0001] The invention relates to the technical field of mass spectrometry analysis, and in particular to a near-normal-pressure photoionization mass spectrometer for volatile and semi-volatile compounds. Background Art

[0002] Mass spectrometry ionizes the sample molecules or atoms to be tested, and then analyzes and detects the ions according to the mass-to-charge ratio (m / z). Electron impact ionization (EI) is one of the earliest and most widely used ionization sources in mass spectrometry. EI uses hot electrons generated by electrically heated metal wires (tungsten, rhenium, thorium-iridium alloy or thorium-rhenium alloy) to bombard the molecules to be tested. Since the electron energy is usually 70eV, which is much higher than the ionization energy of organic molecules, it will cause a large number of fragments of the test object.

[0003] Photoionization (PI) is a soft ionization source. Low-pressure photoionization usually uses a vacuum ultraviolet lamp (such as a krypton lamp) as a photoionization source. The energy of the photons emitted by a krypton lamp is 10.0 and 10.6 eV, which exceeds the ionization energy of most organic compounds. Compared with EI, PI ionization can almost avoid or only produce a small number of fragment ions. It has the advantages of high sensitivity, no polarity discrimination, and soft ionization, and is increasingly attracting attention.

[0004] Direct analysis of volatile and semi-volatile gas phase components has always been an important research direction in the field of mass spectrometry. Currently, the most important mass spectrometry analysis technology for gaseous samples is gas chromatography-mass spectrometry. During the process of programmed temperature rise of the chromatographic column, the gaseous samples are first separated and then detected by mass spectrometry.

[0005] With the development of the times, direct and rapid in-situ mass spectrometry analysis technology has emerged for large quantities of samples without pretreatment, and has grown rapidly in the industry. However, in-situ mass spectrometry analysis technology is currently mainly used for direct analysis of solid or gaseous complex matrix samples, and there are relatively few technological innovations for gaseous sample analysis. If the chromatographic separation step in traditional gas chromatography-mass spectrometry technology is discarded and the gaseous sample is directly analyzed by electron impact ionization mass spectrometry, the fragment ions generated by the electron impact ionization of the gas phase mixture will interfere with each other, and the direct mass spectrometry detection results will be unable to be analyzed.

[0006] The utility model with announcement number CN208795694U discloses a photoionization mass spectrometry imaging device combined with desorption electrospray ionization, in which a solid analyte is parsed by a solvent sprayed from an electrospray needle, enters a photoionization chamber through a sampling capillary, and is photoionized under the action of synchrotron radiation or laser, and the ionized analyte enters a mass spectrometer for analysis. Although this scheme is suitable for sampling and detection of analytes with larger particle sizes, it is not a scheme proposed for gas mass spectrometry analysis, and cannot solve the problem of low ionization degree of gas samples during mass spectrometry analysis.

[0007] The invention patent application with the publication number CN113092567A discloses an automatic and rapid measuring device for tobacco chemical components, in which a liquid or solid extract is subjected to photoionization mass spectrometry detection at atmospheric pressure, wherein a dopant bubbler is provided, and gas is delivered to the dopant bubbler containing the liquid dopant through the second carrier gas delivery pipe to make it bubble, so that the dopant volatilizes into a gaseous state and enters the glass atomization chamber, and then participates in the ionization process of the sample, and improves the ionization efficiency. However, the scheme cannot control the outlet flow rate of the dopant bubbler tube, and it is possible that only the gas provided by the second carrier gas delivery pipe enters the glass atomization chamber, resulting in the substance to be detected not being effectively ionized, affecting the test results. Moreover, in the scheme, back-blowing is required before the ions enter the mass spectrometer, affecting the sampling efficiency. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a photoionization mass spectrometer for direct and rapid analysis of volatile and semi-volatile compounds. The device can not only overcome the problem of pipeline clogging by semi-volatile and difficult-to-volatile compounds in high vacuum photoionization technology, but also increase the sensitivity of the object to be measured by increasing the molecular density of the gas phase components to be measured and the molecular ion collision reaction in a near-normal pressure environment.

[0009] The present invention solves the above technical problems through the following technical solutions: A near-normal-pressure photoionization mass spectrometer for volatile and semi-volatile compounds comprises a photoionization chamber, wherein two axial ends of the photoionization chamber are respectively provided with a light source and a mass spectrometer, and one end of the photoionization chamber close to the light source is respectively provided with a sampling capillary and a dopant introduction tube in the radial direction, wherein the dopant introduction tube is connected to a dopant introduction mechanism, and the connection position of the dopant introduction tube and the photoionization chamber is located on the side close to the light source relative to the connection position of the sampling capillary and the photoionization chamber.

[0010] Preferably, the dopant introduction mechanism includes a bubbling tube, a first conduit and a second conduit, the bubbling tube is used to accommodate liquid dopant; the inlet end of the first conduit is connected to an external carrier gas supply mechanism, and the outlet end of the first conduit is inserted into the liquid surface of the bubbling tube; the inlet end of the second conduit is inserted into the bubbling tube and is above the liquid surface, and the outlet end of the second conduit is connected to the dopant introduction tube.

[0011] Preferably, the first conduit and the second conduit are respectively provided with a first flow control meter and a second flow control meter for adjusting the flow rate, and the flow rate of the first flow control meter is smaller than the flow rate of the second flow control meter.

[0012] Preferably, the dopant is selected from one of toluene, acetone, anisole, chlorobenzene, bromobenzene and carbon disulfide.

[0013] Preferably, the carrier gas supply mechanism provides an inert gas to the first conduit.

[0014] Preferably, the light source is a vacuum ultraviolet lamp, the end of the light source has a flange protruding radially outward, the end of the photoionization chamber that cooperates with the light source is provided with an end cover, the end cover is detachably provided with a cover plate that can be sleeved on the light source, and the flange can be clamped and fixed between the end cover and the cover plate.

[0015] Preferably, the photoionization chamber is coaxially fixed with a transmission capillary of a mass spectrometer.

[0016] Preferably, a vacuum extraction tube is provided at one end of the photoionization chamber close to the mass spectrometer, a guide tube connected to the mass spectrometer is provided at one end of the photoionization chamber close to the mass spectrometer, an inner tube of the photoionization chamber is coaxially connected to the guide tube through a tapered bell mouth, a transition tube is provided on the outside of the guide tube, the vacuum extraction tube is connected to the transition tube, the transition tube is connected to one end of the guide tube facing the mass spectrometer, and the transition tube is annular and coaxially arranged on the outside of the guide tube.

[0017] Preferably, the inner diameter of the photoionization chamber is 4-8 mm, the inner diameter of the flow guide tube is 1.2-1.6 mm, the distance between the outer diameter of the flow guide tube and the inner diameter of the transition tube is 1-2 mm, the inner diameter of the sampling capillary is 0.5-2 mm, and the axial distance between the sampling capillary and the light source is less than 8 mm.

[0018] Preferably, the temperature of the photoionization chamber, the sampling capillary and the dopant introduction tube during operation is 200-400°C, and the vacuum degree of the photoionization chamber during operation is 2×10 2 ~5×10 4 Pa.

[0019] The advantages of the near-normal-pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided by the present invention are: a light source is used to photoionize a sample without chromatographic separation, a complex gas phase mixture can be directly and quickly analyzed, and the analysis time of the gas phase mixture can be shortened from tens of minutes to a few seconds; the molecular ion peaks of the object to be tested are mainly generated by photoionization, which is cleaner than the mass spectrum generated by electron bombardment ionization, has almost no fragments, and is easier to attribute the mass spectrum peaks; compared with high-vacuum ultraviolet single-photon ionization, there is no need to transfer the sample to a high-vacuum ionization chamber, and the semi-volatile sample can be directly volatilized for analysis under near-normal-pressure conditions, thereby realizing the ionization of the sample to be tested, and there is no problem of capillary clogging; by introducing a dopant, the ionization effect is improved, and sample analysis under near-normal pressure is realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided by an embodiment of the present invention;

[0021] Figure 2 A cross-sectional view of a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided by an embodiment of the present invention;

[0022] Figure 3 A cross-sectional view of a dopant introduction mechanism for a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided by an embodiment of the present invention;

[0023] Figure 4 A schematic diagram of a smoke analysis using a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided by an embodiment of the present invention;

[0024] Figure 5 A mass spectrum of smoke analysis using a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided in an embodiment of the present invention;

[0025] Figure 6 A schematic diagram of a near-ambient pressure photoionization mass spectrometer for analyzing semi-volatile components of volatile and semi-volatile compounds provided by an embodiment of the present invention;

[0026] Figure 7 A schematic diagram of analyzing volatile components using a near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the technical solution of the present invention is clearly and completely described below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] like Figure 1 and Figure 2 As shown, this embodiment provides a normal-pressure photoionization mass spectrometer for volatile and semi-volatile compounds, including a photoionization chamber 2, wherein a light source 1 and a mass spectrometer 4 are respectively disposed at two axial ends of the photoionization chamber 2, and a sampling capillary 2-1 and a dopant introduction tube 2-5 are respectively disposed radially at one end of the photoionization chamber 2 close to the light source 1, wherein the dopant introduction tube 2-5 is connected to a dopant introduction mechanism 3, and a connection position of the dopant introduction tube 2-5 and the photoionization chamber 2 is located on a side close to the light source 1 relative to a connection position of the sampling capillary 2-1 and the photoionization chamber 2.

[0029] This embodiment uses light source 1 to photoionize the sample without chromatographic separation, and can directly and quickly analyze complex gas phase mixtures, and can shorten the analysis time of gas phase mixtures from tens of minutes to seconds; photoionization mainly produces molecular ion peaks of the object to be tested, which is cleaner than the mass spectrum produced by electron bombardment ionization, with almost no fragments, and it is easier to attribute mass spectrum peaks. Compared with high vacuum ultraviolet single photon ionization, there is no need to transfer the sample to a high vacuum ionization chamber, and semi-volatile samples can be directly volatilized for analysis under near-normal pressure, thereby realizing ionization of the sample to be tested, and there is no problem of capillary clogging; by introducing dopants, the ionization effect is improved, and sample analysis under near-normal pressure is realized.

[0030] For high vacuum ionization, a quartz capillary with an inner diameter less than 250 μm is required, which is prone to clogging. However, the vacuum degree requirement of this embodiment is lower than that of the prior art, and a stainless steel capillary with an inner diameter greater than 500 μm is used as the sampling capillary 2-1. Therefore, when performing direct mass spectrometry analysis on semi-volatile and non-volatile compounds, it is not easy to be clogged. Specifically, the inner diameter of the sampling capillary 2-1 in this embodiment is set to 0.5 to 2 mm.

[0031] Due to the high vacuum degree, the molecular concentration of the analyte is greatly increased during the detection in this embodiment. In addition to being directly ionized by vacuum ultraviolet light, the analyte can also be ionized by molecular ion collision reaction when mass spectrometry is performed at near normal pressure. The sensitivity is greatly increased. When performing mass spectrometry analysis, the vacuum degree of the photoionization chamber 2 is set to 2×10 2 ~5×10 4 Pa.

[0032] Combination Figure 3 The dopant introduction mechanism 3 includes a bubbling tube 3-4, a first conduit 3-3 and a second conduit 3-6. The bubbling tube 3-4 is used to contain liquid dopants. The inlet end of the first conduit 3-3 is connected to an external carrier gas supply mechanism, and the outlet section of the first conduit 3-3 is inserted into the liquid surface of the bubbling tube 3-4. The inlet end of the second conduit 3-6 is inserted into the bubbling tube 3-4 and is above the liquid surface. The outlet end of the second conduit 3-6 is connected to the dopant introduction tube 2-5.

[0033] Gas is introduced through the first conduit 3-3 and bubbled in the bubbling tube 3-4 to volatilize the liquid dopant into gas and enter the second conduit 3-6, so that the dopant can enter the photoionization chamber 2 through the dopant introduction tube 2-5 to react with the test object to promote ionization. The dopant introduction tube 2-5 is located between the sampling capillary 2-1 and the light source 1, thereby ensuring that the sample can be ionized under the catalysis of the dopant.

[0034] Furthermore, the first conduit 3-3 and the second conduit 3-6 are respectively provided with a first flow controller 3-2 and a second flow controller 3-7 for adjusting the flow rate. The flow rate of the first flow controller 3-2 is smaller than the flow rate of the second flow controller 3-7. Specifically, in use, the flow rate of the second flow meter 3-7 is 2 to 20 ml / min greater than the flow rate of the first flow meter 3-2, thereby ensuring that the dopant volatilized in the bubbling tube can follow the inert gas through the second flow meter 3-7 and enter the photoionization chamber 2.

[0035] When performing sample analysis, the ion molecule reaction can be participated in by means of a dopant as required, and the ionization of the sample is realized under ultraviolet light, thereby improving the effect of sample ionization under near normal pressure. The dopant selects one of toluene, acetone, anisole, chlorobenzene, bromobenzene, and carbon disulfide, and a certain proportion of organic solvents such as methanol, acetonitrile, etc. can be added to the dopant. An external carrier gas supply mechanism provides an inert gas to the first conduit 3-3, where the inert gas refers to a gas that does not participate in the reaction, including but not limited to nitrogen or argon. The inlet of the second flow meter 3-7 is communicated with the outlet of the second conduit 3-6, and the outlet of the second flow meter 3-7 is communicated with the dopant introduction pipe 2-5.

[0036] During analysis, the temperatures of the photoionization chamber 2, the sample injection capillary 2-1 and the dopant introduction tube 2-5 are all controlled at 200-400°C, and heating can be performed by arranging heating wires on the outer surfaces of the components.

[0037] Refer to Figure 2 The light source 1 is a vacuum ultraviolet lamp, and the end of the light source 1 has a flange 1-1 protruding radially, and the end of the photoionization chamber 2 that cooperates with the light source 1 is provided with an end cap 1-3, and the end cap 1-3 is detachably provided with a cover plate 1-2 that can be sleeved on the light source 1, and the flange 1-1 can be clamped and fixed between the end cap 1-3 and the cover plate 1-2, and the light source 1 is locked on the photoionization chamber 2 through the cover plate 1-2. Different rare gases can be filled in the light source 1 to obtain vacuum ultraviolet light of different energies, and specifically one of helium, neon, krypton, xenon, and nitrogen can be selected according to needs.

[0038] The photoionization chamber 2 is coaxially arranged with the transmission capillary 4-1 on the mass spectrometer 4, and can be specifically connected via a threaded connector; a flow guide tube 2-2 connected to the mass spectrometer 4 is arranged at one end of the photoionization chamber 2 close to the mass spectrometer 4, and the inner tube of the photoionization chamber 2 is connected to the flow guide tube 2-2 via a tapered bell mouth.

[0039] During analysis, due to the vacuum operating environment inside the mass spectrometer 4, the semi-volatile gas phase components enter the photoionization chamber 2 through the sampling capillary 2-1 under the action of the vacuum pressure difference, and are directly ionized by the light source 1. The ionized gas phase products enter the transmission capillary 4-1 at the entrance of the mass spectrometer 4 through the flow guide tube 2-2, and are then detected and analyzed by the mass analyzer in the mass spectrometer 4.

[0040] Furthermore, a transition tube 2-3 is provided on the outside of the guide tube 2-2, and a vacuum extraction tube 2-4 is connected to the transition tube 2-3. The transition tube 2-3 is connected to one end of the guide tube 2-2 facing the mass spectrometer 4. The transition tube 2-3 is an annular structure and is coaxially arranged on the outside of the guide tube 2-2 with the guide tube 2-2, so that the vacuum extraction tube 2-4 is covered by the tube wall between the transition tube 2-3 and the guide tube 2-2, thereby preventing the substance to be tested from entering the vacuum extraction tube 2-4.

[0041] The mass spectrometer 4 itself is a vacuum system. Due to the internal vacuum environment, gas phase ions are vacuum-absorbed into the mass spectrometer 4 through the transmission capillary 4-1. Under the joint action of the vacuum pump 2-4, the guide tube 2-2 and the transition tube 2-3, the pressure difference at both ends of the photoionization chamber 2 can be increased, the sample extraction rate can be increased, and the ions are guided to the transmission capillary 4-1 and then enter the mass spectrometer 4.

[0042] In this embodiment, the inner diameter of the photoionization chamber 2 is 4 to 8 mm, the inner diameter of the transition tube 2-3 is 1 to 2 mm larger than the outer diameter of the guide tube 2-2, the inner diameter of the guide tube 2-2 is 1.2 to 1.6 mm, the outer diameter of the transition tube 2-3 is the same as the inner diameter of the photoionization chamber 2, and the axial distance between the sampling capillary and the light source is less than 8 mm; the photoionization chamber is placed in a negative pressure environment by a vacuum pump 2-4.

[0043] refer to Figure 4 For cigarette 5 or electronic cigarette smoke, it can be directly placed close to the sampling capillary 2-1, and the generated smoke can directly enter the photoionization chamber through the sampling capillary 2-1. The ionization efficiency is improved under the action of the introduced dopant. After ionization, the smoke enters the mass spectrometer through the transmission capillary 4-1 for analysis. The analysis results are shown in Figure 5 , the main component nicotine (m / z 163) can be detected with high sensitivity.

[0044] refer to Figure 6 If it is necessary to analyze the semi-volatile components with poor volatility in the sample, the sample can be placed in the heating element 6, the sample can be heated to resolve the volatile and semi-volatile components, and then directly analyzed by near-normal pressure photoionization mass spectrometry.

[0045] refer to Figure 7If the volatile components in the sample need to be analyzed, the sample can be directly placed at the mouth of the sampling capillary 2-1, and the sample components can be volatilized autonomously into the sampling capillary 2-1 for rapid ionization analysis.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds, characterized in that: The invention comprises a photoionization chamber, wherein two axial ends of the photoionization chamber are respectively provided with a light source and a mass spectrometer, and one end of the photoionization chamber close to the light source is respectively provided with a sampling capillary and a dopant introduction tube in the radial direction, wherein the dopant introduction tube is connected with a dopant introduction mechanism, and the connection position of the dopant introduction tube with the photoionization chamber is located on the side close to the light source relative to the connection position of the sampling capillary with the photoionization chamber; the dopant introduction mechanism comprises a bubbling tube, a first conduit and a second conduit, wherein the bubbling tube is used to contain liquid dopant; the inlet end of the first conduit is communicated with an external carrier gas supply mechanism, and the outlet end of the first conduit is inserted into the liquid surface of the bubbling tube; the inlet end of the second conduit is inserted into the bubbling tube and is located above the liquid surface, and the outlet end of the second conduit is communicated with the dopant introduction tube; the first conduit and the second conduit are also respectively provided with a first flow controller and a second flow controller for adjusting the flow, and the flow of the first flow controller is smaller than the flow of the second flow controller.

2. A near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: The dopant is selected from one of toluene, acetone, anisole, chlorobenzene, bromobenzene and carbon disulfide.

3. The near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: The carrier gas supply mechanism supplies an inert gas to the first conduit.

4. The near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: The light source is a vacuum ultraviolet lamp, and the end of the light source has a flange protruding radially outward. The end of the photoionization chamber that cooperates with the light source is provided with an end cover, and the end cover is detachably provided with a cover plate that can be sleeved on the light source, and the flange can be clamped and fixed between the end cover and the cover plate.

5. The near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: The photoionization chamber is coaxially fixed with a transmission capillary of a mass spectrometer.

6. The near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: A vacuum extraction tube is provided at one end of the photoionization chamber close to the mass spectrometer, and a guide tube connected to the mass spectrometer is provided at one end of the photoionization chamber close to the mass spectrometer. The inner tube of the photoionization chamber is coaxially connected to the guide tube through a tapered bell mouth, and a transition tube is provided on the outside of the guide tube. The vacuum extraction tube is connected to the transition tube, and the transition tube is connected to one end of the guide tube facing the mass spectrometer. The transition tube is an annular structure and is coaxially arranged on the outside of the guide tube.

7. A near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 6, characterized in that: The inner diameter of the photoionization chamber is 4-8 mm, the inner diameter of the flow guide tube is 1.2-1.6 mm, the distance between the outer diameter of the flow guide tube and the inner diameter of the transition tube is 1-2 mm, the inner diameter of the sampling capillary is 0.5-2 mm, and the axial distance between the sampling capillary and the light source is less than 8 mm.

8. The near-ambient pressure photoionization mass spectrometer for volatile and semi-volatile compounds according to claim 1, characterized in that: The temperature of the photoionization chamber, the sampling capillary and the dopant introduction tube during operation is 200-400°C, and the vacuum degree of the photoionization chamber during operation is 2×10 2 ~5×10 4 Pa.

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