Device and detection method for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotope in solid

Abstract

The invention discloses a device and a detection method for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in a solid, and belongs to the technical field of nitrogen isotope analysis. The device comprises an oxidation system, a nitrogen oxide enrichment system, a nitrogen reduction system, a mass spectrum interface system, a stable isotope ratio mass spectrometer and pipelines for connecting all parts of the device. The device provided by the invention greatly reduces the quantitation limit of the nitrogen isotope in the solid sample, can realize high-precision determination of the nanogram-level nitrogen isotope in the solid sample, and has the advantages of simplicity and convenience in operation, high accuracy and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nitrogen isotope analysis testing, in particular to a device and method for on-line oxidation-enrichment-reduction determination of nanogram-level nitrogen isotope in solid. BACKGROUND

[0002] Nitrogen is an indispensable element in all living things and plays an important role in the function of living things and even the entire ecological system. Nitrogen isotope plays an important role in the study of nitrogen cycle and can accurately detect the isotope ratio characteristics of nitrogen of different sources, thereby distinguishing and determining the source, providing a direct means for source identification of nitrogen cycle, and thus showing broad application prospects in the fields of geology, environment and ecology.

[0003] There are many methods for determining the nitrogen content and isotope in solid samples. The commonly used methods include Kjeldahl distillation method and high-temperature combustion method of elemental analyzer, which convert the nitrogen element in the solid sample into pure nitrogen gas to test the nitrogen content, and then use gas stable isotope ratio mass spectrometry to test the nitrogen isotope composition. For nitrogen isotope determination, the elemental analysis on-line high-temperature combustion method has the advantages of high efficiency, convenience, high accuracy, etc. compared with the traditional off-line method such as vacuum pyrolysis method or hypobromous acid oxidation method, and has become the mainstream method for determining the nitrogen content and nitrogen isotope in solid samples.

[0004] The elemental analysis and gas stable isotope mass spectrometry combined technology determines the nitrogen isotope by high-temperature (950℃) flash combustion (temperature reaches about 1700℃) of the solid sample wrapped in a tin cup in an oxygen-rich environment to generate a mixed gas of multiple components. The water and carbon dioxide are removed by a chemical trap, the nitrogen oxides are reduced to nitrogen gas in a 600℃ reduction furnace, the generated nitrogen gas and other impurity gases are carried by the carrier gas and separated by a chromatographic column, and then enter the thermal conductivity detector and the gas stable isotope ratio mass spectrometer for determination of the nitrogen content and isotope composition.

[0005] The elemental analysis and gas stable isotope mass spectrometry combined technology (EA-IRMS) accurately determines the nitrogen isotope by ensuring that the sample is fully combusted and oxidized in the flash combustion moment without isotopic fractionation. However, for low-nitrogen-content samples such as sedimentary rocks and tree cores, the sample amount needs to be increased to meet the nitrogen content requirement of the instrument. In this case, the flash combustion may not be sufficient and isotopic fractionation may occur, resulting in abnormal peak shape and unstable test results. The nitrogen isotope of low-nitrogen-content samples has important value in indicating biogeochemical nitrogen cycle, but the related testing technology is relatively weak.

[0006] Therefore, there is an urgent need to design an analysis testing device for solid low-nitrogen-content samples with small sample requirement and accurate determination of nitrogen isotope. SUMMARY

[0007] The present application aims to solve the problems of large sample size, easy fractionation, high cost of reaction tube filling materials, low precision and instability in the process of testing nitrogen isotopes in low nitrogen content solid samples in the prior art, and provides a device and a detection method for on-line oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0009] A device for on-line oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids, comprising an oxidation system, a nitrogen oxide enrichment system, a nitrogen gas reduction system, a mass spectrum interface system and a stable isotope ratio mass spectrometer connected in sequence through pipelines:

[0010] The oxidation system comprises a solid automatic sampler, a variable-diameter oxidation tube and a chemical trap connected in sequence through pipelines, the lower half of the variable-diameter oxidation tube is filled with reaction fillers and the inner diameter of the lower half is smaller than that of the sample combustion space of the upper half, and the inner diameter of the chemical trap is less than or equal to the inner diameter of the lower half of the variable-diameter oxidation tube;

[0011] The nitrogen gas reduction system is internally provided with a reduction tube, and the inner diameter of the reduction tube is smaller than the inner diameter of the chemical trap;

[0012] The nitrogen oxide enrichment system comprises a six-way valve, a cold trap and a liquid nitrogen barrel, which is used for low-temperature freezing enrichment and purification of the nitrogen oxide gas generated by the oxidation system, and by switching the working mode of the six-way valve, the purified nitrogen oxide gas is sent into the nitrogen gas reduction system by using the backflushing carrier gas;

[0013] The mass spectrum interface system comprises a compressible gas trapping device and three-way valves and four-way valves for controlling flow switching, which is used for trapping and saving nitrogen gas, and providing tuning nitrogen gas and sample nitrogen gas to the stable isotope ratio mass spectrometer, respectively.

[0014] In the oxidation system, one end of the variable-diameter oxidation tube is connected to the solid automatic sampler, the other end is connected to the chemical trap, one end of the chemical trap is connected to the variable-diameter oxidation tube, and the other end is connected to the nitrogen oxide enrichment system.

[0015] The variable-diameter oxidation tube is a variable-diameter quartz glass tube, and the lower half of the variable-diameter oxidation tube is sequentially filled with quartz wool, silver-plated cobalt oxide, quartz wool, an oxidizing agent and quartz wool from bottom to top, and the upper half is reserved for a sample combustion space to place an ash tube.

[0016] The chemical trap is sequentially filled with quartz wool, a water removal agent, quartz wool, a carbon dioxide removal filler and quartz wool.

[0017] The reduction tube is internally filled with high-purity reduction copper wire.

[0018] The length of the upper half of the variable diameter oxidation tube is not less than 260 mm, the inner diameter is not greater than 14 mm, the inner diameter of the lower half is not greater than 6 mm, the transition section of the upper and lower two parts is 5-10 mm long, and the total length is not greater than 440 mm; the inner diameter of the chemical trap is not greater than 4 mm; and the inner diameter of the reduction tube is not greater than 0.5 mm.

[0019] The six-way valve is provided with a first valve port, a second valve port, a third valve port, a fourth valve port, a fifth valve port and a sixth valve port; the first valve port is an air inlet of a nitrogen oxide enrichment system and is connected with an outlet of an oxidation system; the second valve port is connected with an air inlet pipeline of a cold trap and is an air inlet of the cold trap; the fifth valve port is connected with an air outlet pipeline of the cold trap and is an air outlet of the cold trap; the third valve port is connected with a helium source and is a helium back flushing port; the fourth valve port is connected with a nitrogen reduction system and is an air outlet of the nitrogen oxide enrichment system; the sixth valve port is connected with an exhaust pipeline and is an exhaust port; and the six-way valve has two working modes in a test process: a gas enrichment and concentration mode and a helium back flushing mode.

[0020] Further, when the six-way valve is in the gas enrichment mode, the first valve port is in communication with the second valve port, the fifth valve port is in communication with the sixth valve port, and the third valve port is in communication with the fourth valve port; and when the six-way valve is in the helium back flushing mode, the second valve port is in communication with the third valve port, the fourth valve port is in communication with the fifth valve port, the first valve port is in communication with the sixth valve port, and back flushing helium flows in from the third valve port.

[0021] The mass spectrum interface system comprises a three-way valve, a compressible gas trapping device, a four-way valve and pipelines connected with each other; the three-way valve is provided with a first valve port, a second valve port and a third valve port; the four-way valve is provided with a first valve port, a second valve port, a third valve port and a fourth valve port; the first valve port of the three-way valve is an air inlet of the mass spectrum interface system and is connected with an outlet of a nitrogen reduction system; the second valve port is connected with the compressible gas trapping device and is an air inlet of the compressible gas trapping device; the third valve port is connected with the fourth valve port of the four-way valve; the first valve port of the four-way valve is connected with an exhaust pipeline and is an exhaust port, and a switch valve is arranged on the exhaust pipeline; the second valve port is connected with an outlet of the compressible gas trapping device and is an air outlet of the compressible gas trapping device; the third valve port is connected with a stable isotope ratio mass spectrometer through a pipeline and is an air inlet of the stable isotope ratio mass spectrometer; and the fourth valve port is connected with the third valve port of the three-way valve.

[0022] The compressible gas trapping device is a telescopic gas trapping bag.

[0023] A manual switch valve is arranged on an exhaust pipeline of the stable isotope ratio mass spectrometer and works in cooperation with three working modes of the mass spectrum interface system; and the mass spectrum interface system has three working modes in a test process: a nitrogen gas trapping mode, a tuning mode and a sample testing mode.

[0024] The stable isotope ratio mass spectrometer has a manually operated valve on its inlet line, which is open in all three operating modes of the mass spectrometry interface system.

[0025] When the mass spectrometry interface system is in nitrogen capture mode, the first and second ports of the three-way valve are connected, the first and fourth ports of the four-way valve are connected, the valve on the exhaust line of the first port is open, the second and third ports are connected, and the valve on the exhaust line of the stable isotope ratio mass spectrometer is open. When the mass spectrometry interface system is in tuning mode, the first and third ports of the three-way valve are connected, the first and fourth ports of the four-way valve are connected, the valve on the exhaust line of the first port is open, the second and third ports are connected, and the valve on the exhaust line of the stable isotope ratio mass spectrometer is closed. When the mass spectrometry interface system is in sample measurement mode, the first and third ports of the three-way valve are connected, the first and second ports of the four-way valve are connected, the valve on the exhaust line of the first port is closed, the third and fourth ports are connected, and the valve on the exhaust line of the stable isotope ratio mass spectrometer is open.

[0026] The nitrogen reduction system is a miniature reduction furnace device, in which the reduction tube is built-in.

[0027] The testing method based on the aforementioned online oxidation-enrichment-reduction determination device for nanogram-level nitrogen isotopes in solids includes the following steps:

[0028] 1) Wrap the evenly ground sample in a tin cup and place it in a solid autosampler. The sample enters the oxygen-filled variable diameter oxidation tube through the solid autosampler.

[0029] 2) Nitrogen-containing samples are oxidized to nitrogen oxides in a high-temperature variable-diameter oxidation tube;

[0030] 3) The generated sample gas enters the chemical trap to remove water and carbon dioxide;

[0031] 4) The nitrogen oxide gas generated in the reaction is enriched and purified using a nitrogen oxide enrichment system to obtain purified nitrogen oxide solid frozen product;

[0032] 5) The purified nitrogen oxide solid frozen material is sublimated to obtain pure nitrogen oxide gas, which is then reduced to nitrogen gas in a nitrogen reduction system equipped with temperature control.

[0033] 6) Use the compressible gas trapping device in the mass spectrometry interface system to trap the generated nitrogen gas;

[0034] 7) Nitrogen gas from the compressible gas trap is introduced into a stable isotope ratio mass spectrometer for nitrogen ion source file tuning;

[0035] 8) Repeat steps 1) to 5).

[0036] 9) The generated nitrogen gas is introduced into a stable isotope ratio mass spectrometer through a mass spectrometry interface system to test the nitrogen isotope composition;

[0037] Steps 1) to 7) are nitrogen capture mode and tuning mode, and steps 8) to 9) are sample measurement mode.

[0038] The nitrogen trapping mode and tuning mode are used for tuning the nitrogen ion source file, and only need to be performed once before the batch sample testing process.

[0039] Step 4) involves enriching and purifying nitrogen oxide gas as follows: First, the six-way valve 8 is set to enrichment mode. The gas generated by the sample in the oxidation system enters the cold trap placed in liquid nitrogen through the first port of the six-way valve, where the nitrogen oxide gas is frozen. Then, the six-way valve is set to helium backflush mode, raising the physical height of the cold trap so that it leaves the liquid nitrogen tank and is at room temperature. The nitrogen oxides frozen in the cold trap sublimate to form gas, which is carried by the backflush helium into the nitrogen reduction system to undergo a reduction reaction to generate nitrogen gas.

[0040] Furthermore, the operating parameters include: oxidation furnace temperature 1000~1050℃, micro reduction furnace temperature 600~650℃, carrier gas (helium) flow rate not exceeding 50mL / min, and backflushing helium flow rate 2~3mL / min.

[0041] Compared with the prior art, the beneficial effects achieved by the technical solution of this invention are:

[0042] 1. The present invention provides an online oxidation-enrichment-reduction determination device and detection method for nanogram-level nitrogen isotopes in solids. The carrier gas flow rate is lower than that of conventional devices. The oxidation tube is a variable diameter quartz tube, the inner diameter of the packing part is small, and the inner diameter of the chemical trap and reduction tube is small. The above modifications greatly save on the packing material and carrier gas of the oxidation tube.

[0043] 2. The present invention provides an online oxidation-enrichment-reduction device for determining nanogram-level nitrogen isotopes in solids. A nitrogen oxide enrichment system is provided between the oxidation system and the nitrogen reduction system. The pure nitrogen oxide gas generated by the sample is completely frozen in a cold trap placed in liquid nitrogen, which avoids isotope fractionation and peak tailing caused by incomplete combustion of the sample in the oxidation tube, thereby improving the accuracy of the test.

[0044] 3. The present invention provides an apparatus and method for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids. In backflushing mode, the six-way valve introduces 2-3 mL / min of helium gas to blow the pure nitrogen oxide gas generated from the sample into the nitrogen reduction system. Based on the small inner diameter reaction tube and low carrier gas flow rate, the dilution ratio of the carrier gas to the target gas is further reduced, thereby improving sample utilization, reducing sample consumption, improving sample combustion efficiency, reducing the number of cleaning cycles, and extending the service life of the oxidation and reduction tubes, ultimately achieving improved testing efficiency and accuracy.

[0045] 4. The present invention provides an online oxidation-enrichment-reduction determination device for nanogram-level nitrogen isotopes in solids. The mass spectrometry interface system eliminates the need for nitrogen cylinders, saving laboratory space.

[0046] 5. The present invention provides an apparatus and method for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids, which reduces the limit of quantification of nitrogen to about 100 ng and the analytical accuracy is better than 0.15‰. Attached Figure Description

[0047] Figure 1 A schematic diagram of the structure of an online oxidation-enrichment-reduction device for determining nanogram-level nitrogen isotopes in solids, provided by the present invention, in gas enrichment and concentration mode;

[0048] Figure 2 A schematic diagram of the structure of an online oxidation-enrichment-reduction device for determining nanogram-level nitrogen isotopes in solids provided by the present invention, wherein the mass spectrometry interface system enters the nitrogen trapping mode while the device enters the helium backflush mode.

[0049] Figure 3 A schematic diagram of the mass spectrometry interface system entering nitrogen tuning mode for an online oxidation-enrichment-reduction determination device for nanogram-level nitrogen isotopes in solids provided by the present invention;

[0050] Figure 4 A schematic diagram of the mass spectrometry interface system entering sample measurement mode for an online oxidation-enrichment-reduction determination device for nanogram-level nitrogen isotopes in solids provided by the present invention;

[0051] Figure 5 This is a schematic diagram of the six-way valve of the present invention in the connected state of gas enrichment and concentration mode;

[0052] Figure 6 This is a schematic diagram of the six-way valve of the present invention in the helium backflush mode in the connected state;

[0053] Figure 7 This is a schematic diagram of the connected state of the mass spectrometry interface system of the present invention in nitrogen trapping mode;

[0054] Figure 8 This is a schematic diagram of the mass spectrometry interface system of the present invention in the connected state of nitrogen tuning mode;

[0055] Figure 9 This is a schematic diagram of the mass spectrometry interface system of the present invention in the sample measurement mode, showing the connected state.

[0056] Figure 10 This is a schematic diagram of the apparatus for Comparative Example 1. Detailed Implementation

[0057] To make the technical problems, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0058] Example 1

[0059] Embodiment 1 of this invention discloses an apparatus for testing nanogram-level nitrogen isotopes in solid samples (rocks, sand grains, sediments, trees, etc.). Figure 1 As shown, an online oxidation-enrichment-reduction device for determining nanogram-level nitrogen isotopes in solids includes an oxidation system 22, a nitrogen oxide enrichment system 4, a nitrogen reduction system 23, a mass spectrometry interface system 6, and a stable isotope ratio mass spectrometer 7, which are connected in sequence through pipelines.

[0060] The oxidation system 22 includes a solid-state autosampler 1, a variable-diameter oxidation tube 2, and a chemical trap 3 connected sequentially by pipelines. The solid-state autosampler 1 is used to transfer the sample into the variable-diameter oxidation tube 2, and the solid-state autosampler 1 is equipped with interfaces for inputting helium and oxygen. The variable-diameter oxidation tube 2 is used to convert nitrogen in the sample into nitrogen oxide gas. The chemical trap 3 is located after the variable-diameter oxidation tube 2 and is used to remove water vapor and carbon dioxide generated in the variable-diameter oxidation tube 2. The rear end of the chemical trap 3 is connected to a nitrogen oxide enrichment system 4.

[0061] The variable diameter oxidation tube 2 is a variable diameter quartz glass tube with a total length of 440mm. It is divided into upper and lower parts. The upper part is 260mm long with an outer diameter of 18mm and an inner diameter of 14mm. The lower part is 175mm long with an outer diameter of 10mm and an inner diameter of 6mm. The transition section between the upper and lower parts is 5mm long.

[0062] The variable-diameter oxidation tube 2 is filled from bottom to top with 10mm quartz wool 17, 30mm silver-plated cobalt oxide 16, 5mm quartz wool 17, 50mm copper oxide 15, 5mm quartz wool 17, 70mm chromium oxide 14, and 10mm quartz wool 17, with the filler extending to the transition section of the variable-diameter oxidation tube 2. The upper half of the variable-diameter oxidation tube 2 is reserved for sample combustion, where the ash collection tube 13 is placed.

[0063] The chemical trap 3 is a plastic flexible tube, 220mm long, with an outer diameter of 6mm and an inner diameter of 4mm.

[0064] The chemical trap 3 is sequentially filled with 10mm quartz wool 17, 90mm magnesium perchlorate 20, 10mm quartz wool 17, 100mm soda lime 19 and 10mm quartz wool 17.

[0065] Both the variable-diameter oxidation tube and the chemical trap have smaller inner diameters. With the flow rate also decreasing, these changes reduce the dilution ratio of the carrier gas to the target gas, thereby lowering the metering limit of the device.

[0066] In this embodiment, the nitrogen oxide enrichment system 4 is used to enrich and concentrate the nitrogen oxides generated in the variable diameter oxidation tube 2, and includes a six-way valve 8, a cold trap 9, and a liquid nitrogen tank 10.

[0067] like Figure 5 and Figure 6 As shown, the six-way valve 8 is equipped with a first valve port 8-1, a second valve port 8-2, a third valve port 8-3, a fourth valve port 8-4, a fifth valve port 8-5, and a sixth valve port 8-6. The first valve port 8-1 is the inlet of the nitrogen oxide enrichment system 4 and is connected to the outlet of the chemical trap 3. The second valve port 8-2 is connected to the inlet pipe of the cold trap 9 and is the inlet of the cold trap 9. The fifth valve port 8-5 is connected to the outlet pipe of the cold trap 9 and is the outlet of the cold trap 9. The third valve port 8-3 is connected to the helium source and is the helium backflush port 21. The fourth valve port 8-4 is connected to the micro reduction furnace 5 and is the outlet of the nitrogen oxide enrichment system 4. The sixth valve port 8-6 is connected to the exhaust pipe and is the exhaust port.

[0068] The six-way valve 8 has two working modes during the test: gas enrichment and concentration mode and helium backflushing mode.

[0069] The cold trap 9 and liquid nitrogen tank 10 are used to freeze and enrich the nitrogen oxide gas generated in the variable diameter oxidation tube 2. The liquid nitrogen tank 10 is filled with liquid nitrogen.

[0070] The six-way valve 8, cold trap 9, and liquid nitrogen tank 10 work together during the test.

[0071] When the six-way valve 8 enters the gas enrichment and concentration mode ( Figure 1 The connection status of each valve port is as follows: Figure 5 As shown, the first valve port 8-1 is connected to the second valve port 8-2, the fifth valve port 8-5 is connected to the sixth valve port 8-6, and the third valve port 8-3 is connected to the fourth valve port 8-4. The cold trap 9 descends into liquid nitrogen. The nitrogen oxide gas generated in the variable-diameter oxidation tube 2 is carried by helium through the first valve port 8-1 and the second valve port 8-2 of the six-way valve 8 into the cold trap 9 where it is frozen. Helium and other impurity gases that cannot be frozen by liquid nitrogen are discharged through the fifth valve port 8-5 and the sixth valve port 8-6.

[0072] After nitrogen oxide enrichment is complete, when the six-way valve enters the helium backflushing mode ( Figure 2 The connection status of each valve port is as follows: Figure 6 As shown, the second valve port 8-2 is connected to the third valve port 8-3, the fourth valve port 8-4 is connected to the fifth valve port 8-5, and the first valve port 8-1 is connected to the sixth valve port 8-6. The cold trap 9 is raised to leave the liquid nitrogen, where the solid nitrogen oxides revert to a gaseous state at room temperature. Backflush helium gas 21 at a rate of 2-3 mL / min flows in from the third valve port 8-3, passes through the second valve port 8-2, enters the cold trap 9, and carries the enriched and purified nitrogen oxide gas out from the fifth valve port 8-5, passing through the fourth valve port 8-4 into the micro-reduction furnace 5. After enrichment, the backflush helium gas flow rate is reduced to 2-3 mL / min, while the carrier gas flow rate from the solid autosampler 1 into the instrument is 50 mL / min. Therefore, after enrichment, the dilution ratio of the carrier gas to the nitrogen oxide gas is significantly reduced, thereby achieving the purpose of enriching and concentrating nitrogen oxides.

[0073] The liquid nitrogen tank 10 is a 5L Dewar flask.

[0074] The cold trap 9 is made of a stainless steel tube with an outer diameter of 1.5875 mm, an inner diameter of 0.75 mm, and a length of 600 mm.

[0075] The cold trap 9 has three intertwined 0.1mm nickel wires inside its stainless steel tube.

[0076] In this embodiment, the nitrogen reduction system 23 is a device consisting of a miniature reduction furnace 5. The miniature reduction furnace 5 is used to reduce enriched and purified nitrogen oxide gas into pure nitrogen gas, which is then carried by helium gas into the mass spectrometry interface system 6.

[0077] The miniature reduction furnace contains a reduction tube made of ceramic, with an outer diameter of 1.5875 mm, an inner diameter of 0.5 mm, and a length of 320 mm.

[0078] The reduction tube is filled with several high-purity reduced copper wires, each 310 mm in length.

[0079] In this embodiment, the mass spectrometry interface system 6 is divided into two paths. One path is equipped with a compressible gas trapping device 18, which is used to trap nitrogen generated in the micro reduction furnace 5 for use in the tuning of the stable isotope ratio mass spectrometer 7. The other path is used for sample measurement, that is, the nitrogen generated from the micro reduction furnace 5 is introduced into the stable isotope ratio mass spectrometer 7 through this path to determine the nitrogen isotope ratio in the sample.

[0080] The mass spectrometry interface system 6 includes a three-way valve 11, a compressible gas trapping device 18, a four-way valve 12, and pipelines connecting various parts.

[0081] like Figures 7~9The display shows that the three-way valve 11 is equipped with a first valve port 11-1, a second valve port 11-2, and a third valve port 11-3; the four-way valve is equipped with a first valve port 12-1, a second valve port 12-2, a third valve port 12-3, and a fourth valve port 12-4; the first valve port 11-1 of the three-way valve is the inlet of the mass spectrometry interface system 6 and is connected to the outlet of the reduction tube; the second valve port 11-2 is connected to the compressible gas trap 18 and is the inlet of the compressible gas trap 18; the third valve port 11-3 is connected to the fourth valve port 12-4 of the four-way valve 12; the four-way valve 12-3... The first valve port 12-1 of the three-way valve is connected to the exhaust pipe and serves as the exhaust port. A switch valve 24 is installed on the exhaust pipe. The second valve port 12-2 is connected to the outlet of the compressible gas trap 18 and serves as the exhaust port of the compressible gas trap 18. The third valve port 12-3 is connected to the stable isotope ratio mass spectrometer 7 through a pipeline and serves as the inlet of the stable isotope ratio mass spectrometer 7. A switch valve 25 is installed on the pipeline. The fourth valve port 12-4 is connected to the third valve port 11-3 of the three-way valve and serves as the exhaust port of the third valve port 11-3 of the three-way valve.

[0082] The mass spectrometry interface system 6 has three working modes during the test: nitrogen capture mode, tuning mode, and sample measurement mode.

[0083] The three-way valve 11 and the four-way valve 12 are used to switch the three working modes of the mass spectrometry interface system 6. They are used in conjunction with each other in the three working modes of the mass spectrometry interface system 6. The switching between different working modes is done manually.

[0084] The exhaust pipe 26 of the stable isotope ratio mass spectrometer is equipped with a manual on / off valve 27, which works in conjunction with the three working modes of the mass spectrometry interface system.

[0085] When the mass spectrometry interface system 6 is in nitrogen trapping mode ( Figure 2 The connection status of each valve port of the three-way valve 11 and the four-way valve 12 is as follows: Figure 7 As shown, the first valve port 11-1 of the three-way valve is connected to the second valve port 11-2, the first valve port 12-1 of the four-way valve is connected to the fourth valve port 12-4, and the second valve port 12-2 is connected to the third valve port 12-3. The valve 24 on the exhaust pipe of the first valve port 12-1 is open, and the valve 27 on the exhaust pipe 26 of the stable isotope ratio mass spectrometer is open. Nitrogen gas from the reduction tube enters the compressible gas trap 18 through the first valve port 11-1 and the second valve port 11-2 of the three-way valve 11. The compressible gas trap 18 is used to store nitrogen gas and provides a portion of nitrogen gas to the stable isotope ratio mass spectrometer 7 through the second valve port 12-2 and the third valve port 12-3 of the four-way valve 12. When the ion source signal of the stable isotope ratio mass spectrometer 7 begins to rise, the three-way valve 11, the four-way valve 12, and the switch valve 27 are manually switched to enter the tuning mode.

[0086] When the mass spectrometry interface system 6 is in tuning mode ( Figure 3 The connection status of each valve port of the three-way valve 11 and the four-way valve 12 is as follows: Figure 8 As shown, the first valve port 11-1 of the three-way valve 11 is connected to the third valve port 11-3, the first valve port 12-1 of the four-way valve 12 is connected to the fourth valve port 12-4, and the second valve port 12-2 is connected to the third valve port 12-3. At this time, the connection between the outlet of the reduction tube and the compressible gas trap 18 is disconnected. The nitrogen in the compressible gas trap 18 continues to be supplied to the stable isotope ratio mass spectrometer 7 for ion source tuning through the second valve port 12-2 and the third valve port 12-3 of the four-way valve 12. At the same time, the valve 27 on the exhaust pipe 26 of the stable isotope ratio mass spectrometer is closed to prevent air from entering the stable isotope ratio mass spectrometer 7. The valve 24 on the exhaust pipe of the first valve port 12-1 is opened, and the gas from the reduction tube is discharged from the instrument through the first valve port 11-1 and the third valve port 11-3 of the three-way valve 11 and the first valve port 12-1 and the fourth valve port 12-4 of the four-way valve 12. After tuning, manually switch the three-way valve 11, four-way valve 12, and switch valves 24 and 27 to enter the sample testing mode.

[0087] When the mass spectrometry interface system 6 is in sample measurement mode ( Figure 4 The connection status of each valve port of the three-way valve 11 and the four-way valve 12 is as follows: Figure 9 As shown, the first valve port 11-1 of the three-way valve 11 is connected to the third valve port 11-3, the first valve port 12-1 of the four-way valve 12 is connected to the second valve port 12-2, and the third valve port 12-3 is connected to the fourth valve port 12-4. The valve 27 on the exhaust pipe 26 of the stable isotope ratio mass spectrometer is open. At this time, the connection between the reduction tube outlet and the compressible gas trap 18 is disconnected. The nitrogen in the compressible gas trap 18 is connected to the exhaust port through the first valve port 12-1 and the second valve port 12-2 of the four-way valve 12. The valve 24 on the exhaust pipe of the first valve port 12-1 is closed, and the nitrogen in the compressible gas trap 18 is stored for use in the next tuning. The nitrogen gas from the reduction tube is introduced into the stable isotope ratio mass spectrometer 7 through the first valve port 11-1 and the third valve port 11-3 of the three-way valve 11 and the fourth valve port 12-4 and the third valve port 12-3 of the four-way valve 12 to determine the nitrogen isotope composition in the sample.

[0088] The switch valve 25 on the pipeline connecting the third valve port 12-3 of the four-way valve 12 to the stable isotope ratio mass spectrometer 7 is opened before the instrument starts to inject samples to check the instrument background. If the background is normal, nitrogen collection, tuning and sample measurement can be performed. When there is no sample injection, the switch valve 27 on the exhaust pipeline is opened and all carrier gas flows out from the exhaust pipeline 26.

[0089] The compressible gas collection device 18 is a retractable gas collection bag with a volume of 200 mL.

[0090] Example 2

[0091] Another embodiment of the present invention discloses a method for testing nanogram-level nitrogen isotopes in solid samples. Based on the online oxidation-enrichment-reduction determination device for nanogram-level nitrogen isotopes in solids described in Example 1, this method enables the testing of nanogram-level nitrogen isotopes in solid samples (rocks, sand grains, sediments, trees, etc.). The method includes the following steps:

[0092] I. Nitrogen capture mode and tuning mode:

[0093] (1) Select USGS40 (glutamic acid, δ 15 (N = -4.5 ± 0.1‰; N% = 9.52%) is used as the standard substance. Weigh out an appropriate amount of USGS40 and wrap it in a tin cup for ion source tuning.

[0094] (2) After correctly filling and installing the variable diameter oxidation tube 2, chemical trap 3 and reduction tube into the instrument according to the instructions in Example 1, a leak test is performed. After the leak test is passed, the carrier gas (helium) is turned on to purge the entire pipeline. The carrier gas flow rate is 50 mL / min, and oxygen is turned on at the same time.

[0095] (3) Heat the instrument: heat the oxidation furnace to 1000℃ and the reduction furnace to 640℃. Increase the temperature by 200℃ each time. When the target temperature is reached, let the instrument stabilize for several minutes before continuing to heat it. This can reduce thermal shock and remove any moisture that may be present. During the heating process, the carrier gas needs to be kept at 50mL / min to continuously purge the pipeline.

[0096] (4) When the instrument is heated, the switch valve 25 on the gas inlet of the stable isotope ratio mass spectrometer 7 should be closed to prevent impurity gases from entering the ion source; manually switch the three-way valve 11 and the four-way valve 12 to the nitrogen trapping mode. Figure 7 );

[0097] (5) After the temperature rise is completed, liquid nitrogen is added to the liquid nitrogen tank 10, and the standard substance used for tuning in (1) is placed in the solid autosampler 1. The sample enters the oxygen-filled variable diameter oxidation tube 2 through the solid autosampler 1. The nitrogen-containing sample is oxidized into nitrogen oxides in the high-temperature variable diameter oxidation tube 2.

[0098] (6) The generated nitrogen oxide gas enters chemical trap 3 to remove water and carbon dioxide;

[0099] (7) Nitrogen oxide gas continues to enter the nitrogen oxide enrichment system 4, and the cold trap 9 enters the liquid nitrogen tank 10 during solid autosampler injection, with a freezing enrichment time of 300s. At this time, the component is in gas enrichment and concentration mode (Figure 1 Nitrogen oxide gas is carried by helium through the first valve port 8-1 and the second valve port 8-2 of the six-way valve 8 and enters the cold trap 9 where it is frozen. Helium and other impurity gases that cannot be frozen by liquid nitrogen are discharged through the fifth valve port 8-5 and the sixth valve port 8-6, thereby obtaining purified solid frozen nitrogen oxides.

[0100] (8) After the cryogenic enrichment is completed, the nitrogen oxide enrichment system 4 automatically switches to helium backflushing mode. Figure 2 The cold trap 9 is raised to leave the liquid nitrogen, and the solid nitrogen oxides in it are restored to the gaseous state at room temperature. 2~3 mL / min of backflush helium 21 flows in from the third valve port 8-3, passes through the second valve port 8-2 and enters the cold trap 9. The gas carrying the enriched and purified nitrogen oxides flows out from the fifth valve port 8-5 and enters the micro reduction furnace 5 through the fourth valve port 8-4.

[0101] (9) The enriched and purified nitrogen oxide gas enters the micro reduction furnace 5 at 600℃ and is reduced to nitrogen gas, which is then carried by helium gas into the mass spectrometry interface system 6.

[0102] (10) such as Figure 2 As shown, the mass spectrometry interface system 6 is in nitrogen trapping mode. Nitrogen gas from the reduction tube enters the retractable trapping gas bag 18 through the first valve port 11-1 and the second valve port 11-2 of the three-way valve 11. Simultaneously, the valve 24 on the exhaust line of the first valve port 12-1 is opened, and the valve 27 on the exhaust line 26 of the stable isotope ratio mass spectrometer is opened. The retractable trapping gas bag 18 is used to store nitrogen gas and provides a portion of nitrogen gas to the stable isotope ratio mass spectrometer 7 through the second valve port 12-2 and the third valve port 12-3 of the four-way valve 12. When the ion source signal of the stable isotope ratio mass spectrometer 7 begins to rise or nitrogen gas begins to emerge in the spectrum, the three-way valve 11, the four-way valve 12, and the switch valve 27 are manually switched to enter the tuning mode.

[0103] (13) such as Figure 3As shown, the mass spectrometry interface system 6 is in nitrogen tuning mode. The first port 11-1 and the third port 11-3 of the three-way valve 11 are connected, and the valve 27 on the exhaust line 26 of the stable isotope ratio mass spectrometer is closed. The connection between the outlet of the reduction tube and the retractable gas trap 18 is disconnected. The nitrogen in the retractable gas trap 18 continues to be supplied to the stable isotope ratio mass spectrometer 7 for ion source tuning through the second port 12-2 and the third port 12-3 of the four-way valve 12. The valve 24 on the exhaust line of the first port 12-1 is opened, and the gas from the reduction tube is discharged from the instrument through the first port 11-1 and the third port 11-3 of the three-way valve 11 and the first port 12-1 and the fourth port 12-4 of the four-way valve 12. After tuning, the three-way valve 11, the four-way valve 12, and the switching valves 24 and 27 are manually switched to enter the sample measurement mode. Typically, the nitrogen trapping and tuning mode of the mass spectrometry interface system 6 is used for nitrogen ion source file tuning, and only needs to be performed once before batch testing of samples.

[0104] II. Sample Testing Mode:

[0105] (1) Weigh about 1 μg of USGS40, wrap it in a tin cup, and put it into the solid autosampler 1. The sample enters the oxygen-filled variable diameter oxidation tube 2 through the solid autosampler 1. The nitrogen-containing sample is oxidized to nitrogen oxides in the high-temperature variable diameter oxidation tube 2.

[0106] (2) Repeat steps (6) to (9) in the nitrogen capture mode and the tuning mode;

[0107] (3) such as Figure 4 As shown, the mass spectrometry interface system 6 is in sample measurement mode. Valve 27 on the exhaust line 26 of the stable isotope ratio mass spectrometer is open. Nitrogen gas from the reduction tube is introduced into the stable isotope ratio mass spectrometer 7 through the first valve port 11-1 and the third valve port 11-3 of the three-way valve 11, and the fourth valve port 12-4 and the third valve port 12-3 of the four-way valve 12 for determination of nitrogen isotope composition in the sample. At this time, the outlet of the reduction tube is disconnected from the retractable gas trap 18. Nitrogen gas in the retractable gas trap 18 is connected to the exhaust port through the first valve port 12-1 and the second valve port 12-2 of the four-way valve 12. Valve 24 on the exhaust line of the first valve port 12-1 is closed, and nitrogen gas in the retractable gas trap 18 is stored for use in the next tuning.

[0108] Table 1 shows the statistical results of USGS40 nitrogen isotope values. The average value of the nitrogen isotope values ​​from the six USGS40 samples is -4.5‰, and the standard deviation is 0.12‰. This indicates that when the nitrogen sample amount is around 100 ng, the nanogram-level nitrogen isotope testing technology for solid samples provided by this invention yields good results and meets the testing accuracy requirements.

[0109] Table 2 is a statistical table of nitrogen isotope test results for actual sedimentary rock samples. When the nitrogen isotope mass of the six samples (parallel samples) is in the range of 88~118ng, the standard deviation of the test value is 0.15‰, indicating that the nanogram-level nitrogen isotope testing technology in solid samples provided by this invention can be used for actual sample testing.

[0110] Table 1. Results of nanogram-level nitrogen isotope testing in solid samples using USGS40.

[0111]

[0112] Table 2. Results of nanogram-level nitrogen isotope testing in solid samples on sedimentary rock samples.

[0113]

[0114] Comparative Example 1

[0115] like Figure 10 As shown, compared to Example 1, the apparatus of Comparative Example 1 is connected in sequence to a reduction tube, a chemical trap, and a GC column 29 after the variable-diameter oxidation tube. The total length of the variable-diameter oxidation tube is 440 mm, with the upper half being 260 mm long, having an outer diameter of 18 mm and an inner diameter of 14 mm, and the lower half being 175 mm long, having an outer diameter of 10 mm and an inner diameter of 6 mm. The transition section between the upper and lower parts is 5 mm long, and the ash tube placed in it has the same specifications as in Example 1. The chemical trap is 220 mm long, with an outer diameter of 6 mm and an inner diameter of 4 mm. The total length of the reduction tube is 440 mm, with an outer diameter of 10 mm and an inner diameter of 6 mm. The total length of the chromatographic column is 500 mm, with an inner diameter of 2 mm. The stable isotope ratio mass spectrometer is equipped with nitrogen reference gas. The reduction tube is filled with linear reduced copper 28. The packing method of the variable-diameter oxidation tube and the chemical trap, the carrier gas flow rate, and the temperatures of the oxidation furnace and the reduction furnace are the same as in Examples 1 and 2. The temperature of the GC column is 60°C.

[0116] Table 3 Results of the comparative test on USGS40

[0117]

[0118] Table 4 Results of sediment sample testing using a comparative apparatus

[0119]

[0120] The comparative example used the same test as Example 2. Table 3 shows the results of the comparative example device testing the standard material USGS40, and Table 4 shows the results of the comparative example device testing actual sediment samples (6 parallel samples). The results show that the limit of quantification for nitrogen element of the comparative example device is about 2 μg, and the test accuracy is 0.2‰.

Claims

1. An apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids, characterized in that: This includes an oxidation system, a nitrogen oxide enrichment system, a nitrogen reduction system, a mass spectrometry interface system, and a stable isotope ratio mass spectrometer, all connected sequentially via piping. The oxidation system includes a solid autosampler, a variable-diameter oxidation tube, and a chemical trap connected in sequence by pipelines. The lower half of the variable-diameter oxidation tube is filled with reaction filler and its inner diameter is smaller than the inner diameter of the sample combustion space in the upper half. The inner diameter of the chemical trap is less than or equal to the inner diameter of the lower half of the variable-diameter oxidation tube. The nitrogen reduction system has a built-in reduction tube, the inner diameter of which is smaller than the inner diameter of the chemical trap. The nitrogen oxide enrichment system includes a six-way valve, a cold trap and a liquid nitrogen tank, which are used to enrich and purify nitrogen oxide gas generated by the oxidation system at low temperature. By switching the working mode of the six-way valve, the purified nitrogen oxide gas is sent into the nitrogen reduction system using backflushing carrier gas. The mass spectrometry interface system includes a compressible gas trapping device and a three-way valve and a four-way valve for controlling flow path switching, used for nitrogen trapping and storage, and providing tuning nitrogen and sample nitrogen to the stable isotope ratio mass spectrometer, respectively.

2. The apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids as described in claim 1, characterized in that: The upper half of the variable-diameter oxidation tube has a length of not less than 260 mm and an inner diameter of not more than 14 mm. The lower half has an inner diameter of not more than 6 mm. The transition section between the upper and lower parts is 5-10 mm long, and the total length is not more than 440 mm. The variable-diameter oxidation tube is filled from bottom to top with quartz wool, silver-plated cobalt oxide, quartz wool, copper oxide, quartz wool, chromium oxide, and quartz wool. The filler is filled to the transition section of the variable-diameter oxidation tube. The upper half is reserved for sample combustion space and ash tube placement.

3. The apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids as described in claim 1, characterized in that: The inner diameter of the chemical trap is no more than 4 mm, and the interior of the chemical trap is sequentially filled with quartz wool, dehydrating agent, quartz wool, carbon dioxide removal filler, and quartz wool.

4. The apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids as described in claim 1, characterized in that: The inner diameter of the reduction tube is no greater than 0.5 mm, and the reduction tube is filled with high-purity reduced copper wire.

5. The apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids as described in claim 1, characterized in that: The six-way valve is equipped with a first valve port, a second valve port, a third valve port, a fourth valve port, a fifth valve port, and a sixth valve port. The first valve port is the inlet of the nitrogen oxide enrichment system and is connected to the outlet of the oxidation system. The second valve port is connected to the inlet pipe of the cold trap and is the inlet of the cold trap. The fifth valve port is connected to the outlet pipe of the cold trap and is the outlet of the cold trap. The third valve port is connected to the helium source and is the helium backflush port. The fourth valve port is connected to the nitrogen reduction system and is the outlet of the nitrogen oxide enrichment system. The sixth valve port is connected to the exhaust pipe and is the exhaust port. The six-way valve has two working modes during the test: gas enrichment and concentration mode and helium backflush mode.

6. The apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids as described in claim 1, characterized in that: The mass spectrometry interface system includes a three-way valve, a compressible gas trap, a four-way valve, and connecting pipes. The three-way valve is equipped with a first valve port, a second valve port, and a third valve port. The four-way valve is equipped with a first valve port, a second valve port, a third valve port, and a fourth valve port. The first valve port of the three-way valve is the inlet of the mass spectrometry interface system and is connected to the outlet of the nitrogen reduction system. The second valve port is connected to the compressible gas trap and is the inlet of the compressible gas trap. The third valve port is connected to the fourth valve port of the four-way valve. The first valve port of the four-way valve... The system has three operating modes: a first exhaust port (connected to the exhaust pipe) and a second valve (connected to the outlet of the compressible gas trap). The second valve is connected to the outlet of the compressible gas trap. The third valve is connected to the stable isotope ratio mass spectrometer (SMR) via a pipe (connected to the SMR's inlet). The SMR's exhaust pipe has a valve. The fourth valve is connected to the third valve of a three-way valve. The mass spectrometry interface system operates in three modes during testing: nitrogen trapping mode, tuning mode, and sample measurement mode.

7. A testing method based on the apparatus for online oxidation-enrichment-reduction determination of nanogram-level nitrogen isotopes in solids according to any one of claims 1 to 6, characterized in that... Includes the following steps: 1) Wrap the evenly ground sample in a tin cup and place it in a solid autosampler. The sample enters the oxygen-filled variable diameter oxidation tube through the solid autosampler. 2) Nitrogen-containing samples are oxidized to nitrogen oxides in a high-temperature variable-diameter oxidation tube; 3) The generated sample gas enters the chemical trap to remove water and carbon dioxide; 4) The nitrogen oxide gas generated in the reaction is enriched and purified using a nitrogen oxide enrichment system to obtain purified nitrogen oxide solid frozen product; 5) The purified nitrogen oxide solid frozen material is sublimated to obtain pure nitrogen oxide gas, which is then reduced to nitrogen gas in a nitrogen reduction system equipped with temperature control. 6) Use the compressible gas trapping device in the mass spectrometry interface system to trap the generated nitrogen gas; 7) Nitrogen gas from the compressible gas trap is introduced into a stable isotope ratio mass spectrometer for nitrogen ion source file tuning; 8) Repeat steps 1) to 5). 9) The generated nitrogen gas is introduced into a stable isotope ratio mass spectrometer through a mass spectrometry interface system to test the nitrogen isotope composition; Steps 1) to 7) are nitrogen capture mode and tuning mode, and steps 8) to 9) are sample measurement mode.

8. The test method as described in claim 7, characterized in that: The nitrogen trapping mode and tuning mode are used for tuning the nitrogen ion source file, and only need to be performed once before the batch sample testing process.

9. The test method as described in claim 7, characterized in that: Step 4) involves enriching and purifying nitrogen oxide gas as follows: First, the six-way valve 8 is set to enrichment mode. The gas generated by the sample in the oxidation system enters the cold trap placed in liquid nitrogen through the first port of the six-way valve, where the nitrogen oxide gas is frozen. Then, the six-way valve is set to helium backflush mode, raising the physical height of the cold trap so that it leaves the liquid nitrogen tank and is at room temperature. The nitrogen oxides frozen in the cold trap sublimate to form gas, which is carried by the backflush helium into the nitrogen reduction system to undergo a reduction reaction to generate nitrogen gas.

10. The test method as described in claim 7, characterized in that, The operating parameters include: oxidation furnace temperature 1000~1050℃, micro reduction furnace temperature 600~650℃, carrier gas flow rate not higher than 50mL / min, and backflushing helium flow rate 2~3mL / min.

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