Method for detecting volatile organic compounds in water

Abstract

This invention discloses a method for detecting volatile organic compounds (VOCs) in water based on a miniature ion trap mass spectrometer. The detection method includes a membrane sample introduction system 6 and a detection system 10. A polydimethylsiloxane (PDMS) tubular membrane 4 is immersed in the water sample to be tested. An air pump draws air as a carrier gas to carry the VOCs, such as tetrachloroethylene, enriched by the PDMS tubular membrane into the detection system, achieving rapid detection of VOCs in water. This method has advantages such as fast detection speed and high sensitivity, and the instrument is portable and easy to operate, making it suitable for on-site detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of water quality monitoring, in particular to a method for detecting volatile organic compounds in water based on a miniature ion trap mass spectrometer. The method combines a membrane sampling system with a miniature ion trap mass spectrometer, eliminating the need for complex pretreatment and enabling on-site detection and analysis of volatile organic compounds in water. The method is simple to operate and highly sensitive, and can meet the requirements of the national standard for the content of volatile organic compounds in water (0.04 mg / l) using tetrachloroethylene as an example. BACKGROUND

[0002] Many volatile organic compounds are important chemical raw materials and are widely used in the chemical, pharmaceutical, and pesticide industries. Tetrachloroethylene, for example, is commonly used in dry cleaning and metal cleaning industries. Currently, volatile organic compounds have been detected in the water sources of many cities in China. Volatile organic compounds in water are harmful to human health, and long-term consumption of water containing tetrachloroethylene may harm the liver and kidneys. The Water Hygiene Standard (GB 5749-2022) stipulates that the content of tetrachloroethylene in water should not exceed 0.04 mg / l.

[0003] Currently, the main method for detecting volatile organic compounds in water in China is gas chromatography analysis. Before the sample is analyzed by gas chromatography, it must undergo complex sample pretreatment processes such as headspace sampling, purge enrichment, and liquid-liquid extraction, which is not conducive to on-site detection and rapid screening of tetrachloroethylene in water. The membrane sampling system does not require sample pretreatment and can be directly used for enrichment and sampling of volatile organic compounds in gas and liquid samples, with fast analysis speed and ease of long-term online monitoring. The miniature ion trap mass spectrometer with a continuous atmospheric pressure interface is robust and highly sensitive, making it suitable for on-site detection. SUMMARY

[0004] The present application discloses a method for detecting volatile organic compounds in water based on a miniature ion trap mass spectrometer. This method uses a miniature ion trap mass spectrometer equipped with a continuous atmospheric pressure interface as the detection instrument, and uses a PDMS tubular membrane to rapidly extract and analyze volatile organic compounds in water samples. Through direct mass spectrometric analysis, rapid and highly sensitive detection of volatile organic compounds in water is achieved, with a single sample detection time of less than 3 seconds. Using tetrachloroethylene as an example, the detection limit is less than 0.04 mg / L, meeting the requirements of the national standard for the detection of volatile organic compounds in water.

[0005] To achieve the above purpose, the present application adopts the following technical solutions:

[0006] The present application aims to provide a simple method for on-site detection of volatile organic compounds in water.

[0007] To achieve the above purpose, the present application adopts the following technical solutions:

[0008] A method for detecting volatile organic compounds in water, characterized in that:

[0009] The device comprises a membrane sampling system 6 and an ion trap mass spectrometer 20, the membrane sampling system 6 comprises a sampling pump 1, two metal capillary tubes 3 and a polydimethylsiloxane (PDMS) tubular membrane 4, and the ion trap mass spectrometer 20 comprises a continuous atmospheric pressure interface 8, an ionization zone cavity 10 and an ion trap cavity 14.

[0010] The two ends of the PDMS tubular membrane 4 are respectively connected to one end of the two metal capillary tubes 3; the gas outlet end of the sampling pump 1 is connected to the other end of one metal capillary tube 3 through a two-way valve 2, and the other end of the other metal capillary tube 3 is connected to the sampling inlet of the ion trap mass spectrometer 20 through a metal three-way valve 7, and the third interface of the metal three-way valve 7 is a tail gas interface.

[0011] The PDMS tubular membrane 4 is completely immersed in the water sample to be detected for detection of volatile organic compounds in water.

[0012] The water sample to be detected is added to a container, and the end ports of the two metal capillary tubes 3 connected to the PDMS tubular membrane 4 are inserted into the water sample to be detected below the liquid level by 1-5 cm (preferably 2-3 cm), so that the PDMS tubular membrane 4 is completely immersed in the water sample to be detected.

[0013] The other end of the other metal capillary tube 3 is connected to the continuous atmospheric pressure interface 8 through the metal three-way valve 7, and the remaining interface of the metal three-way valve 7 is connected to the atmosphere as a tail gas interface to ensure the vacuum degree of the mass spectrometer.

[0014] The ion trap mass spectrometer uses a vacuum ultraviolet lamp 11 as a photoionization source.

[0015] The other end of the continuous atmospheric pressure interface 8 extends into the ionization zone cavity.

[0016] The light emission direction of the vacuum ultraviolet lamp 11 is perpendicular to the axial direction of the other end port of the continuous atmospheric pressure interface 8, and the other end port of the continuous atmospheric pressure interface 8 is located on the light path of the vacuum ultraviolet lamp 11.

[0017] The connection process of the PDMS tubular membrane 4 and the two metal capillary tubes 3 is as follows: the PDMS tubular membrane 4 is soaked in n-hexane for 2-3 minutes, and after the tubular membrane is swelled, the metal capillary tube is inserted into one end of the tubular membrane, and the insertion length of the metal capillary tube at one end of the tubular membrane is 1-2 cm, and after the n-hexane is completely volatilized, a good seal is formed between one end of the PDMS tubular membrane 4 and one end of the metal capillary tube 3.

[0018] The membrane sampling system 6 is subjected to leak detection before use, and the leak detection operation is as follows: in the case of not being connected with the membrane sampling system 6, the continuous atmospheric pressure interface 8 of the ion trap mass spectrometer 20 is blocked at one end connected with the metal three-way valve 7, and the gas pressure in the ionization zone cavity 10 is recorded; the membrane sampling system 6 is connected with the detection system and the sampling pump is not started, and the gas pressure in the ionization zone cavity 10 is recorded, if the gas pressure in the ionization zone cavity 10 at this time is the same as that when the continuous atmospheric pressure interface 8 of the ion trap mass spectrometer 20 is blocked, it is considered that the membrane sampling system 6 does not leak.

[0019] The length of the PDMS tubular membrane (4) can be selected, the inner diameter is 0.4-0.8mm, and the wall thickness is 0.15-0.4mm; the temperature of the water sample to be measured is kept at 60-85℃; the temperature of the metal capillary (3) is kept at 80-120℃, and the inner diameter is 0.5-1.5mm. The detection mode of the ion trap mass spectrometer 20 can be freely switched between full spectrum scanning and target ion monitoring mode, and the sampling time can be adjusted within 10ms-5s; the on-site detection of volatile organic compounds in water can be carried out;

[0020] The pumping speed of the sampling pump can be adjusted within 100ml / min-500ml / min.

[0021] The continuous atmospheric pressure interface is one or more of a metal capillary, a quartz capillary or a peek capillary, and the length and inner diameter can be adjusted to maintain the vacuum degree of the ion trap mass spectrometer 20;

[0022] The continuous atmospheric pressure interface is preferably a metal capillary, the length is 10cm, and the inner diameter is 250um; the sampling time of the ion trap mass spectrometer is preferably 2s.

[0023] The length of the PDMS tubular membrane is preferably 90-100cm; the temperature of the water sample to be measured is preferably 72-75℃; and the temperature of the metal capillary is preferably 100-105℃.

[0024] The advantages of the present application are:

[0025] 1. The present application has the following advantages: the membrane sampling system is used for extracting and enriching volatile organic compounds in water, the pretreatment is simple and efficient, and is suitable for on-site analysis. The miniature ion trap mass spectrometer with a continuous atmospheric pressure interface and a vacuum ultraviolet lamp has good robustness and high sensitivity. Taking tetrachloroethylene as an example, the combination of the membrane sampling system and the miniature ion trap mass spectrometer can realize on-site rapid and high-sensitivity detection of volatile organic compounds in water. BRIEF DESCRIPTION OF DRAWINGS

[0026] The present application will be further described in detail in combination with the drawings and examples:

[0027] Figure 1 The structure of the present application is shown in the schematic diagram;

[0028] Wherein 6 is a membrane sampling system, including 1 is a sampling pump, 2 is a two-way valve, 3 is two metal capillary tubes, 4 is a PDMS tubular membrane, 5 is a sample cup. The gas outlet of the sampling pump 1 is connected with one end of one metal capillary tube 3 through a two-way valve, and each of the two metal capillary tubes has one end connected with two ends of the PDMS tubular membrane, and is located 1-5 cm below the liquid level, and the PDMS tubular membrane is immersed in the water sample to be measured. 20 is an ion trap mass spectrometer, and one end of the other metal capillary tube 3 is connected with a continuous atmospheric pressure interface 8 of the ion trap mass spectrometer 20 through a metal two-way valve 7. The ion trap mass spectrometer further comprises, 10 is an ionization zone cavity, 14 is an ion trap cavity, and the ionization zone cavity 10 and the ion trap cavity 14 are connected through a hole electrode 13. The ionization zone cavity 10 comprises 9 as a ring-shaped focusing electrode, 11 as a vacuum ultraviolet lamp, and 12 as a hexapole rod; the ion trap cavity 14 comprises 15 as a ring-shaped extraction electrode, the ring-shaped extraction electrode is coaxial with the center hole of the hole electrode, 16 as an ion trap mass analyzer, and 17 as an electron multiplier. The vacuum system of the ion trap mass spectrometer comprises 18 as a mechanical pump and 19 as a molecular pump;

[0029] Figure 2 The mass spectrum of the 0.04 mg / l tetrachloroethylene water sample detection;

[0030] Figure 3 The mass spectrum of the laboratory tap water sample detection. DETAILED DESCRIPTION

[0031] Figure 1 The structural diagram of the application.

[0032] The application relates to a field detection method for volatile organic compounds in water based on a micro ion trap mass spectrometer,

[0033] The device comprises a membrane sampling system and an ion trap mass spectrometer, the membrane sampling system comprises a sampling pump, two metal capillary tubes 3 and a polydimethylsiloxane (PDMS) tubular membrane, and the ion trap mass spectrometer comprises a continuous atmospheric pressure interface, an ionization zone cavity and an ion trap cavity.

[0034] The two ends of the PDMS tubular membrane are respectively connected with one end of the two metal capillary tubes; the gas outlet end of the sampling pump is connected with the other end of one metal capillary tube through a two-way valve, and the other end of the other metal capillary tube is connected with the sampling inlet of the ion trap mass spectrometer through a metal three-way valve, and the third interface of the metal three-way valve is a tail gas interface;

[0035] The PDMS tubular membrane is completely immersed in the water sample to be measured, and the detection of the volatile organic compounds in water is carried out.

[0036] The sample of the water to be tested is added into the container, and the one end port of the two metal capillary tubes connected with the PDMS tubular membrane is inserted into the water to be tested below 3 cm from the liquid surface, so as to ensure that the PDMS tubular membrane is completely immersed in the water to be tested.

[0037] The other end of the other metal capillary tube is connected with the continuous atmospheric pressure interface, and the remaining one interface of the metal three-way valve is connected with the atmosphere as a tail gas interface, so as to ensure the vacuum degree of the mass spectrometer.

[0038] The ion trap mass spectrometer uses a vacuum ultraviolet lamp as a photoionization source.

[0039] The other end of the continuous atmospheric pressure interface 8 extends into the ionization zone cavity.

[0040] The light emission direction of the vacuum ultraviolet lamp is perpendicular to the axial direction of the other end port of the continuous atmospheric pressure interface, and the other end port of the continuous atmospheric pressure interface is located on the light emission path of the vacuum ultraviolet lamp.

[0041] The connection process of the PDMS tubular membrane and the two metal capillary tubes is as follows: the PDMS tubular membrane is soaked in n-hexane for 3 minutes, and after the tubular membrane is swelled, the one end of the metal capillary tube is inserted into one end of the tubular membrane, and the insertion length of the one end of the metal capillary tube in the one end of the tubular membrane is 1.5 cm. After the n-hexane is completely volatilized, a good seal is formed between the one end of the PDMS tubular membrane and the one end of the metal capillary tube.

[0042] The membrane sampling system needs to be leak tested before use. The leak testing operation is as follows: without connecting the membrane sampling system, the one end of the continuous atmospheric pressure interface of the ion trap mass spectrometer connected with the metal three-way valve is blocked, and the gas pressure in the ionization zone cavity is recorded as 4.2 Pa; the membrane sampling system is connected with the detection system without starting the sampling pump, and the gas pressure in the ionization zone cavity is recorded as 4.2 Pa, which indicates that the membrane sampling system does not leak.

[0043] The length of the PDMS tubular membrane is 100 cm, the inner diameter is 0.51 mm, and the wall thickness is 0.215 mm; the temperature of the water to be tested is kept at 70°C; the temperature of the metal capillary tube 3 is kept at 100°C, and the inner diameter is 0.75 mm.

[0044] The detection mode of the ion trap mass spectrometer 20 is full-spectrum scanning mode, and the sampling time is 2 s; the on-site detection of volatile organic compounds in water can be carried out.

[0045] The pumping speed of the sampling pump is 250 ml / min.

[0046] The continuous atmospheric pressure interface is a metal capillary tube with a length of 10 cm and an inner diameter of 250 um.

[0047] Example 1

[0048] Prepare a 100 mg / L tetrachloroethylene solution using deionized water and serially dilute it to 1 mg / L. Add 240 ml of deionized water and 10 ml of the 1 mg / L tetrachloroethylene solution to a sample cup, sonicate for 1 minute, and then pour the mixture into the sample cup. Set the sampling pump speed to 250 ml / min, start the sampling pump, and begin continuous mass spectrometry monitoring. The mass spectrum of the 0.04 mg / L tetrachloroethylene aqueous solution is shown below. Figure 2 As shown, the entire analysis process takes no more than 2 minutes, meeting the needs of rapid on-site testing.

[0049] Example 2

[0050] Add laboratory tap water to the sample cup. Set the sampling pump speed to 250 ml / min. After starting the sampling pump, begin continuous mass spectrometry monitoring. The mass spectrum of the laboratory tap water is shown below. Figure 3 As shown in the mass spectrum, trace volatile organic compounds in the water include benzene, toluene, chlorobenzene, and tetrachloroethylene, verifying the detection capability of the membrane sample introduction system combined with the miniature ion trap mass spectrometer for volatile organic compounds in water.

Claims

1. A method for detecting volatile organic compounds in water, characterized in that: The device includes a membrane sample introduction system (6) and an ion trap mass spectrometer (20). The membrane sample introduction system (6) includes a sampling pump (1), two metal capillaries (3) and a polydimethylsiloxane (PDMS) tubular membrane (4). The ion trap mass spectrometer (20) includes a continuous atmospheric pressure interface (8), an ionization chamber (10) and an ion trap chamber (14). The two ends of the PDMS tubular membrane (4) are respectively connected to one end of two metal capillaries (3); the outlet of the sampling pump (1) is connected to the other end of one metal capillary (3) through a two-way valve (2), and the other end of the other metal capillary (3) is connected to the inlet of the ion trap mass spectrometer (20) through a metal three-way valve (7). The third port of the metal three-way valve (7) is the tail gas port. The PDMS tubular membrane (4) was completely immersed in the water sample to be tested for the detection of volatile organic compounds in the water. Add the water sample to be tested into the container, and insert one end of the two metal capillaries (3) connected to the PDMS tubular membrane (4) into the water sample 1-5 cm below the liquid surface, so as to ensure that the PDMS tubular membrane (4) is completely immersed in the water sample. The other end of the other metal capillary tube (3) and the continuous atmospheric pressure interface (8) are connected through a metal three-way valve (7). The remaining interface of the metal three-way valve (7) is the tail gas interface connected to the atmosphere to ensure the vacuum degree of the mass spectrometer. The ion trap mass spectrometer uses a vacuum ultraviolet lamp (11) as the photoionization source; The other end of the continuous atmospheric pressure interface (8) extends into the ionization cavity.

2. The method according to claim 1, characterized in that: Add the water sample to be tested into the container, and insert one end of the two metal capillaries (3) connected to the PDMS tubular membrane (4) into the water sample 2-3 cm below the surface of the sample.

3. The method according to claim 1, characterized in that: The light output direction of the vacuum ultraviolet lamp (11) is perpendicular to the axial direction of the other end port of the continuous atmospheric pressure interface (8), and the other end port of the continuous atmospheric pressure interface (8) is located in the light output path of the vacuum ultraviolet lamp (11).

4. The method according to claim 1, characterized in that: The connection process between the PDMS tubular membrane (4) and the two metal capillaries (3) is as follows: the PDMS tubular membrane (4) is immersed in n-hexane for 2-3 minutes. After the tubular membrane swells, it is taken out and one end of the metal capillary is inserted into one end of the tubular membrane. The insertion length of one end of the metal capillary into one end of the tubular membrane is 1-2 cm. After the n-hexane completely evaporates, a good seal is formed between one end of the PDMS tubular membrane (4) and one end of the metal capillary (3).

5. The method according to claim 1, characterized in that: Before use, the membrane sample introduction system (6) must be leak-tested. The leak-testing operation is as follows: without connecting to the membrane sample introduction system (6), block the end of the continuous atmospheric pressure interface (8) of the ion trap mass spectrometer (20) connected to the metal three-way valve (7) and record the gas pressure in the ionization chamber (10). Connect the membrane sample introduction system (6) to the detection system without turning on the sampling pump, and record the gas pressure in the ionization chamber (10) of the ion trap mass spectrometer (20). If the gas pressure in the ionization chamber (10) is the same as when the continuous atmospheric pressure interface (8) of the ion trap mass spectrometer (20) is blocked, then the membrane sample introduction system (6) is considered to be leak-free.

6. The method according to claim 1, characterized in that: The length of the PDMS tubular membrane (4) is optional, the inner diameter is 0.4-0.8 mm, and the wall thickness is 0.15-0.4 mm; the temperature of the water sample to be tested is maintained at 60-85℃; the temperature of the metal capillary (3) is maintained at 80-120℃, and the inner diameter is 0.5-1.5 mm.

7. The method according to claim 1, characterized in that: The ion trap mass spectrometer (20) can freely switch between full-spectrum scanning and target ion monitoring modes, and the injection time is adjustable within 10ms-5s; it can perform on-site detection of volatile organic compounds in water. The pumping speed of the sampling pump is adjustable from 100ml / min to 500ml / min.

8. The method according to claim 1, characterized in that: The continuous atmospheric pressure interface is one or more of a metal capillary, a quartz capillary, or a PEEK capillary, and its length and inner diameter are adjustable to maintain the vacuum of the ion trap mass spectrometer (20). The continuous atmospheric pressure interface is a metal capillary tube with a length of 10 cm and an inner diameter of 250 μm; the injection time is 2 s.

9. The method according to claim 6, characterized in that: The PDMS tubular membrane has a length of 90-100 cm; the temperature of the water sample to be tested is 72-75℃; and the temperature of the metal capillary is 100-105℃.

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