Enrichment and purification device and method for testing stable isotope of trace dissolved nitrogen in water body

Abstract

The invention discloses an enrichment and purification device and method for testing stable isotopes of trace dissolved nitrogen in a water body, and relates to the field of stable isotope analysis and testing. Comprising a device for oxidizing nitrogen elements dissolved in water, a gas purification device, a nitrogen oxide freezing enrichment device, a micro nitrogen oxide reduction device, a nitrogen separation and purification device and a stable isotope ratio mass spectrometer which are arranged in sequence. According to the method, trace dissolved nitrogen in the water body is oxidized into nitrogen oxide at high temperature, the generated nitrogen oxide is subjected to liquid nitrogen freezing enrichment, enriched sample gas is reduced into nitrogen gas through a miniature reduction tube, and then the nitrogen gas is introduced into a stable isotope ratio mass spectrometer, so that the trace dissolved nitrogen isotope in the water body is tested.

Description

Technical Field

[0001] The invention relates to the field of stable isotope analysis and testing, and in particular to an enrichment and purification device and method for testing stable isotopes of trace dissolved nitrogen in water. Background Art

[0002] Nitrogen is a key biogenic element in the ecosystem and is essential for the growth and reproduction of organisms. 2 NH 4+ 、NO 3- 、NO 2- There are several forms of nitrogen, such as organic nitrogen and organic nitrogen. The transformation between these different forms of nitrogen constitutes a complex nitrogen cycle process in water bodies. In recent years, with the continuous growth of population and the development of industrial and agricultural production, the input of nitrogen into the water environment has shown a gradual increase, causing damage to many water environments. Stable isotope technology has received extensive attention and application due to its low interference, stability, and non-toxicity, and can be used to indicate the source of substances and analyze biogeochemical processes. In particular, it provides a more accurate technical method for people to fully explore the transmission, migration and transformation process of nitrogen in water bodies, in the identification and analysis of nitrogen pollution sources in ecosystems, and in the study of ecosystem nitrogen cycles.

[0003] At present, the research on dissolved nitrogen isotopes in water bodies mainly focuses on dissolved inorganic nitrogen, including nitrate nitrogen and ammonium nitrogen, and there are few tests and studies on total dissolved nitrogen isotopes in water bodies. On the one hand, the nitrogen content in general water samples is low and cannot directly meet the isotope testing requirements. In recent years, some instruments have occasionally used direct sampling to analyze dissolved nitrogen isotopes in water bodies, but due to the low nitrogen content in water bodies, large water vapor interference, and air N 2 The strong background influence and other reasons lead to inaccurate test results, and it is impossible to analyze the isotopes of trace dissolved nitrogen in water. On the other hand, the composition of nitrogen in water is complex, and only a single form of nitrogen can be tested for isotope pretreatment each time, such as nitrate nitrogen (NO 3- ) mainly transforms inorganic nitrogen in water into N through chemical conversion and bacterial denitrification. 2 O, and then δ 15 Analysis of N. To achieve the conversion of all forms of nitrogen, complex pre-treatment is required, which is time-consuming, cumbersome, and easily causes isotopic fractionation of nitrogen. Summary of the invention

[0004] The purpose of the present invention is to solve the shortcomings of the above-mentioned existing methods for testing trace dissolved nitrogen isotopes in water bodies, and to provide an enrichment and purification device and method for testing trace dissolved nitrogen stable isotopes in water bodies, so as to realize direct sampling of water bodies, and after high-temperature oxidation, gas purification and freezing enrichment technology are used to increase the nitrogen content, reduce the interference of water vapor and impurity gases, and perform online analysis of trace dissolved nitrogen isotopes in water bodies.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] The invention discloses an enrichment and purification device for testing stable isotopes of trace dissolved nitrogen in water, comprising a dissolved nitrogen oxidation device in water, a gas purification device, a nitrogen oxide freezing enrichment device, a micro nitrogen oxide reduction device, a nitrogen separation and purification device and a stable isotope ratio mass spectrometer which are arranged in sequence; the dissolved nitrogen oxidation device in water comprises an injector and an oxidation tube which are connected to each other; the gas purification device and the nitrogen oxide freezing enrichment device are used to purify and enrich nitrogen oxides after high-temperature oxidation; wherein, the trace dissolved nitrogen in the water is oxidized to nitrogen oxides at high temperature, and the generated nitrogen oxides are frozen and enriched with liquid nitrogen, the enriched sample gas is reduced to nitrogen by a micro reduction device, and then passed through the nitrogen separation and purification device and introduced into the stable isotope ratio mass spectrometer to realize the testing of trace dissolved nitrogen isotopes in water.

[0007] The oxidation tube is filled with quartz wool, copper oxide, quartz wool, tungsten oxide and quartz wool from bottom to top.

[0008] The gas purification device comprises a semiconductor refrigeration dewatering trap, a chemical dewatering trap and a carbon dioxide adsorption trap connected in sequence; the semiconductor refrigeration dewatering trap is used to quickly discharge condensed water; the chemical dewatering trap is filled with magnesium perchlorate to remove water vapor; the carbon dioxide adsorption trap is filled with calcium hydroxide to absorb CO generated by the combustion of water. 2 .

[0009] The nitrogen oxide freezing enrichment device comprises a stainless steel tube cold trap, a micro capillary cold trap and a six-way valve; the six-way valve is used to connect the stainless steel tube cold trap, the micro capillary cold trap and the micro nitrogen oxide reduction device; the stainless steel tube cold trap is used to enrich nitrogen oxides in the sample gas, and the micro capillary cold trap is used to purify the enriched nitrogen oxides.

[0010] A carrier gas three-way valve is added between the gas purification device and the nitrogen oxide freezing enrichment device. After the sample gas enters the nitrogen oxide freezing enrichment device, the three-way valve provides carrier gas, and the front-end gas purification device discharges condensed water at the same time.

[0011] The micro reduction device comprises a micro reduction tube with a diameter of 0.75 mm; the micro reduction tube is filled with three reduction copper wires with a diameter of 0.1 mm, which are used to quickly reduce nitrogen oxides to N 2 .

[0012] The nitrogen separation and purification device comprises a column oven and a capillary chromatographic column, wherein the capillary chromatographic column is arranged in the column oven, and the output end of the capillary chromatographic column is connected to a stable isotope ratio mass spectrometer; wherein the reduced N 2 The low-flow helium enters the nitrogen separation and purification device for further separation and purification.

[0013] A method for enriching and purifying stable isotopes of trace dissolved nitrogen in water is tested, comprising the following steps:

[0014] 1) The injector draws water sample and injects it directly into the oxidation tube. The dissolved trace nitrogen in the water sample is oxidized into nitrogen oxides by high temperature;

[0015] 2) The oxidized sample gas is passed into the gas purification device, first through the semiconductor refrigeration dehydration trap, then through the chemical dehydration trap, and finally through the carbon dioxide adsorption trap;

[0016] 3) The dried and purified sample gas enters the nitrogen oxide freezing enrichment device via the carrier gas;

[0017] 4) The stainless steel tube cold trap of the nitrogen oxide freezing enrichment device is placed in liquid nitrogen, and one end of the stainless steel tube cold trap is connected to the exhaust valve port of the six-way valve. The nitrogen oxides in the sample gas are blown into the stainless steel tube cold trap by the high-flow carrier gas for freezing enrichment, and the remaining impurity gases are emptied and purged with the carrier gas; the micro capillary cold trap is immersed in liquid nitrogen in advance, the six-way valve is switched, the stainless steel tube cold trap is lifted out of the liquid nitrogen, and the nitrogen oxides are vaporized and transferred to the micro capillary cold trap for freezing enrichment again;

[0018] 5) The micro capillary cold trap is lifted out of liquid nitrogen, and the micro capillary cold trap is connected to the micro reduction tube. The nitrogen oxides in the capillary are vaporized and sent into the micro reduction tube by helium for high-temperature reduction. The nitrogen oxide gas is reduced to N 2 ;

[0019] 6) Restored N 2 The water sample is passed through a nafion tube, into a capillary column for separation and purification, and finally into a stable isotope ratio mass spectrometer to measure the amount of nitrogen isotopes (δ 15 N) determination.

[0020] The water sample in step 1) was filtered through a glass fiber filter membrane before injection to remove impurities and particulate matter.

[0021] The method described in the present invention tests the nitrogen isotopes of the total dissolved nitrogen in the water body. The total dissolved nitrogen in the water body is oxidized into nitrogen oxides through high-temperature oxidation. The nitrogen oxides pass through double dehydration to eliminate the interference of impurity gases, then enter the nitrogen oxide freezing enrichment device, are reduced to nitrogen gas through a micro-reduction tube, and finally are introduced into a stable isotope ratio mass spectrometer to determine the stable isotope values ​​of trace dissolved nitrogen in the water body.

[0022] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0023] 1. The enrichment and purification device and method for testing the stable isotopes of trace dissolved nitrogen in water provided by the present invention change the conventional testing process through a gas purification device and a nitrogen oxide freezing enrichment device, and realize direct automatic online testing of trace dissolved nitrogen isotopes in water samples. Specifically, the total dissolved nitrogen in the water body is oxidized into nitrogen oxides at high temperature, and the generated nitrogen oxides are frozen and enriched. The enriched gas is then reduced to nitrogen and passed into a stable isotope ratio mass spectrometer for testing.

[0024] 2. The present invention solves the problem of enrichment and testing of trace dissolved nitrogen in water by freezing and enriching the intermediate product (nitrogen oxides) after high-temperature oxidation.

[0025] 3. The present invention adds a He gas three-way valve between the gas purification device and the nitrogen oxide freezing enrichment device, thereby achieving simultaneous condensate discharge from the front-end gas purification device and freezing enrichment of nitrogen oxides at the back-end, saving test time and improving test efficiency.

[0026] 4. The device of the present invention is simple, the test cost is low, the interference of water vapor is effectively eliminated, the purification and enrichment efficiency is improved, and the trace nitrogen isotopes (δ 15 N) automated online analytical testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the structure of the device for enriching and purifying the stable isotopes of trace dissolved nitrogen in testing water according to the present invention;

[0028] Figure 2 are five nitrogen standard solutions δ 15 N Correlation results between measured value and true value.

[0029] Reference numerals:

[0030] Automatic sampler 1; oxidation tube 2; semiconductor refrigeration dewatering trap 3; chemical dewatering trap 4; carbon dioxide adsorption trap 5; He gas three-way valve 6; six-way valve 7; large-volume stainless steel tube cold trap 8; micro capillary cold trap 9; liquid nitrogen 10; micro reduction tube 11; nafion tube 12; column oven 13; capillary chromatographic column 14; stable isotope ratio mass spectrometer 15. DETAILED DESCRIPTION

[0031] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0032] like Figure 1As shown, this embodiment is an enrichment and purification device for testing the stable isotopes of trace dissolved nitrogen in water, including a dissolved nitrogen element oxidation device in water, a gas purification device, a nitrogen oxide freezing enrichment device, a micro nitrogen oxide reduction device, a nitrogen separation and purification device, and a stable isotope ratio mass spectrometer 15 which are arranged in sequence.

[0033] The device for oxidizing dissolved nitrogen in water comprises an automatic sampler 1 and an oxidation tube 2; the oxidation tube 2 has a combustion temperature of 850°C and is filled with 10 mm quartz wool, 150 mm copper oxide, 5 mm quartz wool, 50 mm tungsten oxide and 10 mm quartz wool from bottom to top.

[0034] The gas purification device comprises a semiconductor refrigeration dewatering trap 3, a chemical dewatering trap 4, and a carbon dioxide adsorption trap 5 which are arranged in sequence; the semiconductor refrigeration dewatering trap 3 is used to quickly discharge condensed water; the chemical dewatering trap 4 is filled with magnesium perchlorate to remove water vapor; the carbon dioxide adsorption trap 5 is filled with calcium hydroxide to absorb CO generated by the combustion of water. 2 .

[0035] A He gas three-way valve 6 is additionally provided between the gas purification device and the nitrogen oxide freezing enrichment device.

[0036] The nitrogen oxide enrichment device comprises a six-way valve 7, a large-volume stainless steel tube cold trap 8, a micro capillary cold trap 9, and liquid nitrogen 10. The large-volume stainless steel tube cold trap 8 is used to enrich nitrogen oxides in the sample gas, the micro capillary cold trap 9 is used to concentrate and purify the enriched nitrogen oxides, and the six-way valve 7 is used to connect the large-volume stainless steel tube cold trap 8, the micro capillary cold trap 9, and the micro reduction tube 11, and realize the on-off of the large-volume stainless steel tube cold trap 8 and the micro capillary cold trap 9.

[0037] The micro reduction tube 11 has a temperature of 620°C, a tube diameter of 0.75 mm, and is filled with 3 reduction copper wires with a diameter of 0.1 mm, which are used to quickly reduce nitrogen oxides to N 2 .

[0038] The nitrogen separation and purification device comprises a column oven 13 and a capillary chromatographic column 14 arranged in the column oven. 2 The low-flow helium gas enters the capillary chromatography column 14 for further separation and purification.

[0039] The method of this embodiment tests the nitrogen isotopes of the total trace dissolved nitrogen in the water body. The total nitrogen in the water body is oxidized into nitrogen oxides through high-temperature oxidation. After the nitrogen oxides are frozen and enriched in a device, they are reduced to nitrogen gas through a reduction tube and finally introduced into a stable isotope ratio mass spectrometer to measure the isotope values ​​of trace dissolved nitrogen in the water body.

[0040] The device and method for enriching and purifying trace dissolved nitrogen stable isotopes in water comprises the following steps:

[0041] 1) Take 10 ml of water sample and filter it through a 0.45 um glass fiber filter to remove impurities and particulate matter.

[0042] 2) The automatic sampler 1 draws 500 μl of water sample and injects it directly into the oxidation tube 2. The dissolved trace nitrogen in the water sample is oxidized into nitrogen oxides at high temperature.

[0043] 3) The oxidized sample gas is passed into the gas purification device, first passing through the semiconductor refrigeration dehydration trap 3 to quickly remove a large amount of condensed water, then passing through the chemical dehydration trap 4 to remove residual water vapor, and finally passing through the carbon dioxide adsorption trap 5 to remove CO 2 Impact on later enrichment and testing.

[0044] 4) The dried and purified sample gas is provided with carrier gas by the He gas three-way valve 6 and enters the nitrogen oxide freezing enrichment device. At the same time, the front-end gas purification device discharges the condensed water in time, saving 8 minutes of sample testing time.

[0045] 5) The large-volume stainless steel tube cold trap 8 of the nitrogen oxide freezing enrichment device is placed in liquid nitrogen, and one end of it is connected to the exhaust valve port of the six-way valve 7. The nitrogen oxides in the sample gas are blown into the large-volume stainless steel tube cold trap by the high-flow carrier gas for freezing enrichment, and the remaining impurity gases are emptied and purged with the carrier gas. The micro capillary cold trap 9 is immersed in liquid nitrogen in advance, and the six-way valve is switched by the pneumatic control system. At the same time, the large-volume stainless steel tube cold trap 8 is lifted out of the liquid nitrogen, and the nitrogen oxides are vaporized and transferred to the micro capillary cold trap 9 for freezing enrichment and purification again.

[0046] 6) The micro capillary cold trap 9 is lifted out of liquid nitrogen, and the micro capillary cold trap 9 is connected to the micro reduction tube 11. The nitrogen oxides in the capillary are vaporized and sent into the micro reduction tube 11 by helium for high-temperature reduction. The nitrogen oxide gas is reduced to N 2 , wherein the micro reduction tube 11 is placed in the micro reduction furnace.

[0047] 7) Restored N 2 The water sample is passed through the nafion tube 12, enters the capillary column 14 for separation and purification, and finally enters the stable isotope mass spectrometer 15 to detect the dissolved trace nitrogen isotopes (δ 15 N) determination.

[0048] The working parameters of the trace dissolved nitrogen stable isotope enrichment and purification device in this embodiment include: oxidation tube temperature 850°C, micro-reduction tube temperature 620°C, chromatographic column temperature 80°C; the carrier gas is helium, the carrier gas pressure is 15psi, and the flow rate is 80ml / min; the oxygen pressure is 15psi, the flow rate is 20ml / min, and the oxygen flow time is 10s.

[0049] In this example, standard substances TN-1 (caffeine, -3.62‰), TN-2 (urea, 20.17‰), IAEA-N 2 (ammonium sulfate, 20.3‰), TN-3 (ammonium chloride, 37.5‰), TN-4 (ammonium chloride, 58.1‰), prepared into a standard solution with a dissolved nitrogen content of 10μg / ml, 500μl was injected each time, and each standard solution was tested in parallel 3 times.

[0050] Table 1 shows TN-1, TN-2, IAEA-N 2 , TN-3, TN-4 nitrogen standard solution isotope test results statistical table, five standard solutions measured δ 15 The average N values ​​were 1.06‰, 13.97‰, 14.80‰, 23.91‰, and 36.77‰, respectively; the standard deviations were 0.2‰, 0.48‰, 0.23‰, 0.32‰, and 0.36‰, respectively.

[0051] The test results of the five nitrogen standard solutions have a certain deviation from the true value. The standard substances TN-1, TN-2, IAEA-N 2 The correlation curves between the measured values ​​and the true values ​​of the nitrogen standard solutions of TN-3 and TN-4 are shown in Figure 2 From the figure, we can see that when the dissolved nitrogen content of the five isotope standard substances is 10μg / ml and the injection volume is 500μl, their δ 15 The measured value of N has a linear response relationship with the true value. The linear equation is y=0.5765x+2.8306, R 2 =0.9987, good linearity.

[0052] The above results show that the device and method for testing the stable isotope enrichment and purification of trace dissolved nitrogen in water can be used to determine the isotope value (δ 15 It also shows that the device and method can meet the test accuracy requirements of trace dissolved nitrogen isotopes in different water bodies.

[0053] Table 1 Statistics of test results of five nitrogen standard solutions

[0054]

[0055] In summary, the present invention can realize direct sampling of water bodies, and after high-temperature oxidation, the nitrogen content can be increased through gas purification and cryoenrichment technology, and the interference of water vapor and impurity gases can be reduced, so as to perform online analysis of trace dissolved nitrogen isotopes in water bodies.

Claims

1. An enrichment and purification device for testing stable isotopes of trace dissolved nitrogen in water, characterized in that: The invention comprises a dissolved nitrogen oxidation device in water, a gas purification device, a nitrogen oxide freezing enrichment device, a micro nitrogen oxide reduction device, a nitrogen separation and purification device and a stable isotope ratio mass spectrometer which are arranged in sequence; the dissolved nitrogen oxidation device in water comprises an injector and an oxidation tube which are connected to each other; the gas purification device and the nitrogen oxide freezing enrichment device are used to purify and enrich nitrogen oxides after high-temperature oxidation; wherein, trace dissolved nitrogen in the water body is oxidized into nitrogen oxides at high temperature, and the generated nitrogen oxides are frozen and enriched with liquid nitrogen, the enriched sample gas is reduced into nitrogen gas by a micro reduction device, and then is introduced into the stable isotope ratio mass spectrometer after passing through the nitrogen separation and purification device to realize the testing of trace dissolved nitrogen isotopes in the water body.

2. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: The oxidation tube is filled with quartz wool, copper oxide, quartz wool, tungsten oxide and quartz wool from bottom to top.

3. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: The gas purification device comprises a semiconductor refrigeration dewatering trap, a chemical dewatering trap and a carbon dioxide adsorption trap connected in sequence; The semiconductor refrigeration dewatering trap is used to quickly discharge condensed water; the chemical dewatering trap is filled with magnesium perchlorate to remove water vapor; the carbon dioxide adsorption trap is filled with calcium hydroxide to adsorb CO2 generated by the combustion of water.

4. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: The nitrogen oxide freezing enrichment device comprises a stainless steel tube cold trap, a micro capillary cold trap and a six-way valve; the six-way valve is used to connect the stainless steel tube cold trap, the micro capillary cold trap and the micro nitrogen oxide reduction device; the stainless steel tube cold trap is used to enrich nitrogen oxides in the sample gas, and the micro capillary cold trap is used to purify the enriched nitrogen oxides.

5. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: A carrier gas three-way valve is added between the gas purification device and the nitrogen oxide freezing enrichment device. After the sample gas enters the nitrogen oxide freezing enrichment device, the carrier gas is provided by the carrier gas three-way valve, and the front-end gas purification device discharges condensed water at the same time.

6. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: The micro reduction device comprises a micro reduction tube with a tube diameter of 0.75 mm; the micro reduction tube is filled with 3 reduction copper wires with a diameter of 0.1 mm, which are used for quickly reducing nitrogen oxides to N2.

7. The enrichment and purification device for testing the stable isotope of trace dissolved nitrogen in water as claimed in claim 1, characterized in that: The nitrogen separation and purification device comprises a column oven and a capillary chromatographic column, wherein the capillary chromatographic column is arranged in the column oven, and the output end of the capillary chromatographic column is connected to a stable isotope ratio mass spectrometer; wherein the reduced N2 enters the nitrogen separation and purification device along with the low-flow rate helium for further separation and purification.

8. A method for enriching and purifying stable isotopes of trace dissolved nitrogen in water, characterized in that: The following steps are involved: 1) The injector draws water sample and injects it directly into the oxidation tube. The dissolved trace nitrogen in the water sample is oxidized into nitrogen oxides by high temperature; 2) The oxidized sample gas is passed into the gas purification device, first through the semiconductor refrigeration dehydration trap, then through the chemical dehydration trap, and finally through the carbon dioxide adsorption trap; 3) The dried and purified sample gas enters the nitrogen oxide freezing enrichment device through the carrier gas three-way valve; 4) The stainless steel tube cold trap of the nitrogen oxide freezing enrichment device is placed in liquid nitrogen, and one end of the stainless steel tube cold trap is connected to the exhaust valve port of the six-way valve. The nitrogen oxides in the sample gas are blown into the stainless steel tube cold trap by the high-flow carrier gas for freezing enrichment, and the remaining impurity gases are emptied and purged with the carrier gas; the micro capillary cold trap is immersed in liquid nitrogen in advance, the six-way valve is switched, the stainless steel tube cold trap is lifted out of the liquid nitrogen, and the nitrogen oxides are vaporized and transferred to the micro capillary cold trap for freezing enrichment again; 5) The micro capillary cold trap is lifted out of liquid nitrogen, the micro capillary cold trap is connected to the micro reduction tube, the nitrogen oxides in the capillary are vaporized, and helium is sent into the micro reduction tube for high temperature reduction, and the nitrogen oxide gas is reduced to N2; 6) The reduced N2 passes through the nafion tube, enters the capillary column for separation and purification, and finally enters the stable isotope ratio mass spectrometer to achieve the measurement of the trace nitrogen isotopes (δ 15 N) determination.

9. A method for enriching and purifying stable isotopes of trace dissolved nitrogen in test water as claimed in claim 8, characterized in that: The water sample in step 1) was filtered through a glass fiber filter membrane before injection to remove impurities and particulate matter.

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