AU2020312536B2

AU2020312536B2 - Pretreatment method for nitrogen and oxygen isotope measurement with chemical conversion method and measurement method

Info

Publication number: AU2020312536B2
Application number: AU2020312536A
Authority: AU
Inventors: Ning AN; Rui CEN; Weiying FENG; Jianyang GUO; Haiqing LIAO; Wenjuan MA; Fengchang WU; Fang Yang; Runyu Zhang
Original assignee: Chinese Research Academy of Environmental Sciences
Current assignee: Chinese Research Academy of Environmental Sciences
Priority date: 2019-07-18
Filing date: 2020-07-17
Publication date: 2023-02-02
Anticipated expiration: 2040-07-17
Also published as: AU2020312536A1; WO2021008617A1; CN110274949A; CN110274949B

Abstract

A pretreatment method for nitrogen and oxygen isotope measurement with a chemical conversion method and a measurement method, relating to the technical field of environmental protection and monitoring. The pretreatment method comprises the following step: performing low-temperature treatment on a sodium azide-acetic acid buffer solution at a temperature lower than or equal to 15°C. According to the method, low-temperature treatment is performed on a sodium azide-acetic acid buffer solution at a temperature lower than or equal to 15°C, so that the interference of the sodium azide-acetic acid buffer solution on N2O obtained from the reaction of a nitrate in a sample during test can be greatly reduced, and the precision of a test result is improved.

Description

Pretreatment Method for Nitrogen and Oxygen Isotope Measurement with Chemical Conversion Method and Measurement Method

Cross-reference to Related Application The present disclosure claims the priority to the Chinese patent application with the filing number 2019106493698 filed on July 18, 2019 with the Chinese Patent Office, and entitled "Pretreatment Method for Determining Nitrogen Oxygen Isotope by Chemical Conversion Method and Measurement Method", the contents of which are incorporated herein by reference in entirety.

Technical Field The present disclosure relates to the technical field of environmental protection and monitoring, in particular to a pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method and a method for determining nitrogen-oxygen isotope by a chemical conversion method.

Background Art Nitrate is a major existing form of nitrogen-containing contaminants in a fresh water system. Too high nitrate content will cause potential health threats to humans, and high concentration of nitrate will cause methemoglobinemia. Water body nitrate contamination has become one of the important water environmental problems. Recognizing the nitrate source is a primary condition for solving the problem. Currently, a method for tracking a source of nitrogen contamination using the technology of nitrogen and oxygen double isotope in soluble nitrate in a water body has been widely applied in researches of food tracing, ecosystem cycling and contaminant migration process, and how to reduce the fractionation effect of nitrogen elements in pretreatment is a difficulty and hot spot in analytical detection of nitrogen isotope all the time. At present, typical methods for determining nitrogen isotope in a fresh water sample mainly include three methods, namely, silver nitrate technology, denitrification bacteria technology, and cadmium reduction method/azide method (chemical conversion method), and the three methods have their own advantages and disadvantages. The silver nitrate method is currently considered to be a more accurate pretreatment method for nitrogen isotope. The probability of fractionation during the whole process is quite low and no impurities are brought in, but the ion exchange process thereof requires 2-3 L of water sample to obtain a sufficient silver nitrate sample to be tested. A single batch of samples consumes 24-48 h, while the chemical conversion method only needs 15-40 mL of water sample for each sample. Moreover, the pretreatment time of a single batch of samples is only 2-3 h. Compared with the silver nitrate method, the chemical conversion method greatly reduces the water sample demand and difficulty of collection and transportation, and saves a large amount of pretreatment time. The advantage of the denitrification bacteria method lies in that once the bacteria are cultured mature, the nitrogen isotope pretreatment work may be performed in large amounts, and the test requirements of 200-400 samples may be satisfied in a single time, thus the denitrification bacteria method is widely adopted by scientific research units and analytical testing organizations, but the denitrification bacteria method has defects in both success rate and accuracy of culture, wherein in a first aspect, the bacteria must be cultured in large amounts, resulting in a relatively low rate of survival, and the culture cycle is around 7-10 days, which is quite time-consuming; and in a second aspect, nitrogen elements in N20 produced by the bacteria through the denitrification process may not come from the cultured water sample, but are produced by self-catabolism, and the interference of the extraneous nitrogen source is imperceptibly added in the pretreated water sample, so there is still controversy, and the denitrification bacteria method has not become a mainstream pretreatment method. In contrast, the chemical conversion method avoids the deficiency of the denitrification bacteria method and the silver nitrate technology, saves the water sample (each sample only needs 15-40 mL of water sample), and consumes a short period of time (the pretreatment work for a single batch of -60 samples can be completed with only 3.5 hours). At present, however, the pretreatment technology of the chemical conversion method is not mature enough yet, and the test stability is low. In view of this, the present disclosure is specifically proposed.

Summary Objectives of the present disclosure include, for example, providing a pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method, so as to solve the technical problem of poor test stability existing in the prior art. The present disclosure provides a pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method, including the following step: performing low-temperature treatment on a sodium azide-acetic acid buffer solution under a condition of a temperature less than or equal to 15 °C. By performing an investigation test on reagents and the like which may affect an experimental result when determining the isotope by the chemical conversion method, the present disclosure proves that the sodium azide acetic acid buffer solution is a main reagent interfering with blank. By performing the low-temperature treatment on the sodium azide-acetic acid buffer solution under the condition of a temperature less than or equal to °C, the present disclosure can greatly reduce the interference of the sodium azide-acetic acid buffer solution on N20 obtained from the nitrate reaction in the sample in the test, and improve the accuracy of test result.

In one or more embodiments, the sodium azide-acetic acid buffer solution is formulated by the following method: weighing a proper amount of sodium azide solid (GR) to be dissolved in ultrapure water to a constant volume, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; In the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution.

In one or more embodiments, the sodium azide-acetic acid buffer solution is subjected to low-temperature treatment under a condition of a temperature less than or equal to 10 °C, optionally, under a condition of a temperature less than or equal to 4 °C, and optionally, under a condition of a temperature less than or equal to0 O°C. In one or more embodiments, the sodium azide-acetic acid buffer solution is subjected to the low-temperature treatment under a condition of -5 °C - 4 °C. In one or more embodiments, a method for the low-temperature treatment includes: placing the buffer solution in a system with a temperature less than or equal to 15 °C, or less than or equal to 10 °C, or less than or equal to 4 °C, or less than or equal to0 O°C. In one or more embodiments, the duration of the low-temperature treatment is more than or equal to 2 h. The time of the low-temperature treatment being within the above range may reduce the interference of the buffer solution on the test result. The concentration of the sodium azide-acetic acid buffer solution may be formulated according to actual needs, and generally, for example, the concentration 2 mol/L may be adopted. In one or more embodiments, the duration of the low-temperature treatment may be2 h,3h,4 h,5h,6h,7h,8 h,9h,10 h,11h,12 h,13h,14 h,15h, 16h,17h,18h,19h,20h,21h,22h,23h,24h,25h,26h,27h,28h,29h, h, 31 h, 32 h, etc. In order to take into account both the accuracy of test result and the test efficiency, it is preferable that the duration of the low temperature treatment is 2-30 h, and more preferably 4-24 h. In one or more embodiments, the sodium azide-acetic acid buffer solution may be subjected to low-temperature treatment under a condition of 0-4 °C for -28 h, for example, 24 h. Alternatively, the sodium azide-acetic acid buffer solution may be subjected to the low-temperature treatment under a condition of -5 °C - 0 °C for 2-6 h, for example, 4 h. Through the above processing conditions, the interference of the reagent on the test result may be substantially excluded, and the accuracy of the test result may be improved. In one or more embodiments, the sodium azide-acetic acid buffer solution is purged with a gas before the low-temperature treatment.

Before the low-temperature treatment, the purging with a gas can further reduce the interference of N20 in the buffer solution on the test result.

In one or more embodiments, the gas is a non-oxidizing gas. In one or more embodiments, the non-oxidizing gas is helium and/or nitrogen. The purging with helium and/or nitrogen in cooperation with the low temperature treatment can significantly reduce and eliminate the interference.

In one or more embodiments, the buffer solution may be subjected to low temperature treatment, helium purging + low-temperature treatment or nitrogen purging + low-temperature treatment so as to reduce the interference. After the buffer solution is treated by the above method, the buffer solution can be stored in a headspace bottle, the bottle mouth is sealed with a polyethylene cover and a rubber gasket mating with the polyethylene cover, and when the buffer solution is taken to be used, the headspace bottle may be inverted first, then a clean syringe is inserted from the bottle mouth to take the buffer solution for use, thus preventing the outside air from entering and interfering the experiment, etc. The present disclosure further provides a method for determining nitrogen oxygen isotope by a chemical conversion method, and a sample to be tested is tested using the sodium azide-acetic acid buffer solution pretreated by the above method. By performing the foregoing pretreatment on the sodium azide-acetic acid buffer solution, the interference of N20 therein on the test result is substantially excluded, and the test result of the nitrogen-oxygen isotope in the sample to be tested can be greatly improved. In one or more embodiments, before test, the gas isotope mass spectrometer may be checked for stability to ensure that the mass spectrometer is stable and has no interference factors. In one or more embodiments, before test, a detection range of the gas isotope mass spectrometer having undergone the check for stability is determined, wherein 20± 5 pg of N20 is a suitable concentration range for MAT252. In one or more embodiments, a protective gas used in the test is air or helium.

The N20 signal value from the purging under the air condition is not quite different from that under the inert gas condition, for example, in order to reduce the cost, air may be used to replace the inert gas such as helium. In one or more embodiments, a feeding device adopted in the test includes: a cross communication tube, a hose and a feeding needle, wherein respective ports of the cross communication tube are provided with a first valve, a second valve, a third valve and a fourth valve in a clockwise direction, respectively; and the port of the cross communication tube provided with the second valve is communicated with the feeding needle through the hose. The two ports of the cross communication tube provided with the first valve and the third valve are in communication with a test pipeline of the gas isotope mass spectrometer for feeding a gas sample into test instrument. The port of the cross communication tube provided with the fourth valve is in communication with a vacuum pump, and the feeding device can be vacuum evacuated. In one or more embodiments, the hose is provided therein with a drying column. In one or more embodiments, the composition of the drying column is calcium oxide.

In one or more embodiments, the hose is a polyethylene hose. In one or more embodiments, the feeding needle is provided with a rubber plug so as to block the feeding needle. Since the sample fed is a gas-liquid mixture sample, and the content of water is relatively large in the headspace bottle for chemical conversion, providing the drying column can ensure that during the sampling, the contents of water and carbon dioxide are reduced, the content of impurities is reduced, and the interference is reduced, and the accuracy of the measurement result is improved. In one or more embodiments, the feeding needle is provided with a switch for opening or closing the feeding needle.

In practical test, an existing automatic feeding device may be adopted. However, by adopting the feeding device in the present disclosure, N20 generated by a reaction liquid in the sampling headspace bottle can be ensured to enter the mass spectrometer at a stable intensity of pressure and a suitable concentration, thereby improving the operability of accuracy control.

Compared with the prior art, beneficial effects the present disclosure are as follows: (1) by performing the investigation test on reagents and the like which may affect the experimental result when determining the isotope by the chemical conversion method, the present disclosure proves that the sodium azide acetic acid buffer solution is a main reagent interfering with blank; by performing the low-temperature treatment on the sodium azide-acetic acid buffer solution under the condition of a temperature less than or equal to °C, the interference of the sodium azide-acetic acid buffer solution on N20 obtained from the nitrate reaction in the sample in the test can be greatly reduced, and the accuracy of test result is improved, so that the accuracy of 180 is controlledat.1%,andtheaccuracyof 1 5 Nis controlled at 0.12%o;

(2) the present disclosure further optimizes the pretreatment method, can eliminate the interference of the buffer solution on the determination of the sample to be tested, and improves the test stability and accuracy; and (3) the feeding device in the present disclosure can ensure N20 generated by the reaction liquid in the sampling headspace bottle to enter the mass spectrometer at a stable intensity of pressure and a suitable concentration, thereby further improving the operability of accuracy control.

Brief Description of Drawings

In order to more clearly illustrate the technical solutions in specific embodiments of the present disclosure or in the prior art, drawings which need to be used in the description of specific embodiments or the prior art will be introduced briefly below, and apparently, the drawings in the description below merely show some embodiments of the present disclosure, and a person ordinarily skilled in the art still could obtain other drawings in light of these drawings without creative efforts. FIG. 1 is a structural schematic view of a feeding device provided in an example of the present disclosure; FIG. 2 is a measurement curve of an instrument under vacuum condition for outdoor air bottle-outdoor-1 in one example of the present disclosure; FIG. 3 is a measurement curve of an instrument under vacuum condition for indoor air blank-1 in one example of the present disclosure; FIG. 4 is a measurement curve of dummy bottle 1 when adding N20 gas (about 20 pg) in one example of the present disclosure; FIG. 5 is measurement curve of dummy bottle 2 when adding N20 gas (about 20 pg) in one example of the present disclosure; FIG. 6 is a measurement curve of He under helium condition in one example of the present disclosure; FIG. 7 is a measurement curve of Air under nitrogen condition in one example of the present disclosure; FIG. 8 is a measurement curve of N1-1 under a condition of adding no cadmium chloride in one example of the present disclosure;

FIG. 9 is a measurement curve of N1-2 under the condition of adding no cadmium chloride in one example of the present disclosure; FIG. 10 is a measurement curve of N2-1 under a condition of adding no ammonium chloride in one example of the present disclosure; FIG. 11 is a measurement curve of N2-2 under a condition of adding no ammonium chloride in one example of the present disclosure;

FIG. 12 is a measurement curve of N3-1 under a condition of adding no zinc sheet in one example of the present disclosure; FIG. 13 is a measurement curve of N4-1 under a condition of adding no sodium azide-acetic acid buffer solution in one example of the present disclosure; FIG. 14 is a measurement curve of N4-2 under a condition of adding no sodium azide-acetic acid buffer solution in one example of the present disclosure;

FIG. 15 is a measurement curve of N5-1 under a condition of adding no sodium hydroxide in one example of the present disclosure;

FIG. 16 is a measurement curve obtained by using the sodium azide-acetic acid buffer solution treated in Example 1 of the present disclosure, wherein air2-1 and air2-2 are two parallel experiments, respectively; FIG. 17 shows a measurement curve Airl-2 obtained by using the sodium azide-acetic acid buffer solution treated in Example 3 of the present disclosure and a measurement curve Air1 obtained by using the sodium azide-acetic acid buffer solution treated in Comparative Example 2; FIG. 18 shows a measurement curve Airl-2 obtained by using the sodium azide-acetic acid buffer solution treated in Example 4 of the present disclosure and a measurement curve Airl-1 obtained by using the sodium azide-acetic acid buffer solution treated in Comparative Example 2; FIG. 19 shows a measurement curve air2 obtained by using the sodium azide-acetic acid buffer solution treated in Comparative Example 1 of the present disclosure and a measurement curve air1 obtained by using untreated sodium azide-acetic acid buffer solution; and FIG. 20 shows a measurement curve air4 obtained by using the sodium azide-acetic acid buffer solution treated in Comparative Example 2 of the present disclosure and a measurement curve air3 obtained by using untreated sodium azide-acetic acid buffer solution. Reference signs: 1-cross communication tube; 2-hose; 3-feeding needle; 11-first valve; 12-second valve; 13-third valve; 14-fourth valve; 21-drying column.

Detailed Description of Embodiments Technical solutions of the present disclosure will be described clearly and completely below in combination with the accompanying drawings and specific embodiments, while a person skilled in the art would understand that the examples described below are some but not all examples of the present disclosure, and they are merely used for illustrating the present disclosure, but should not be considered as limiting the scope of the present disclosure. Based on the examples in the present disclosure, all of other examples, obtained by a person ordinarily skilled in the art without using creative efforts, shall fall within the scope of protection of the present disclosure. If no specific conditions are specified in the examples, they are carried out under normal conditions or conditions recommended by manufacturers. If manufacturers of reagents or apparatuses used are not specified, they are conventional products commercially available. In the description of the present disclosure, it should be indicated that orientation or positional relationships indicated by terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and so on are based on orientation or positional relationships as shown in the accompanying drawings, merely for facilitating describing the present disclosure and simplifying the description, rather than indicating or suggesting that related devices or elements have to be in the specific orientation or configured and operated in a specific orientation, and therefore, they should not be construed as limitation on the present disclosure. Besides, terms "first", "second", and "third" are merely for descriptive purpose, but should not be construed as indicating or implying importance in the relativity. In the description of the present disclosure, it should be noted that unless otherwise specified and defined clearly, terms "mount", "join", and "connect" should be understood in a broad sense, for example, a connection can be a fixed connection, a detachable connection, or an integrated connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediary, and it also can be an inner communication between two elements. For a person ordinarily skilled in the art, specific meanings of the above-mentioned terms in the present disclosure could be understood according to specific circumstances. FIG. 1 is a structural schematic view of a feeding device provided in an example of the present disclosure. It can be seen from the drawing that the feeding device includes a cross communication tube 1, a hose 2 and a feeding needle 3. Respective ports of the cross communication tube 1 are provided with a first valve 11, a second valve 12, a third valve 13 and a fourth valve 14 in a clockwise direction, respectively. Two ports of the cross communication tube 1 provided with the first valve 11 and the third valve 13 are in communication with a test pipeline of a gas isotope mass spectrometer. The port of the cross communication tube 1 provided with the second valve 12 is communicated with the feeding needle 3 through the hose 2. The port of the cross communication tube 1 provided with the fourth valve 14 is in communication with a vacuum pump. Optionally, the hose 2 is provided therein with a drying column 21. The composition of the drying column 21 preferably is calcium oxide. Optionally, the hose 2 may be a polyethylene hose, but is not limited thereto. Optionally, the feeding needle 3 is provided with a switch that may be used for opening or closing the feeding needle. Alternatively, the feeding needle 3 may blocked by a rubber plug.

Before testing each sample to be tested, the feeding needle 3 is closed, and the feeding needle 3 may also be blocked with the rubber plug. Then the first valve 11 and the third valve 13 are closed, the second valve 12 and the fourth valve 14 are opened, and at the same time, the vacuum pump is turned on. After the feeding device is evacuated for about 3-5 min, the fourth valve 14 is closed, the feeding needle 3 is inserted into a bottle of the sample to be tested through the rubber plug, to obtain a proper amount of sample to be tested, then the second valve 12 is closed, the first valve 11 and the third valve 13 are opened, so that the sample to be tested enters the test pipeline of the gas isotope mass spectrometer through a passage of the first valve 11 and the third valve 13, to perform the test.

Example 1

A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps: (1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and (2) refrigerating the sodium azide-acetic acid buffer solution obtained in step (1) under a condition of 0-4 °C for 4 h, to obtain the treated sodium azide acetic acid buffer solution.

Example 2 A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps:

(1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and (2) refrigerating the sodium azide-acetic acid buffer solution obtained in step (1) under a condition of 5-10 °C for 4 h, to obtain the treated sodium azide acetic acid buffer solution.

Example 3 A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps:

(1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and

(2) purging the sodium azide-acetic acid buffer solution obtained in step (1) with helium for 4 h, then refrigerating the sodium azide-acetic acid buffer solution under a condition of 0-4 °C for 4 h, to obtain the treated sodium azide-acetic acid buffer solution.

Example 4

A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps: (1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and (2) purging the sodium azide-acetic acid buffer solution obtained in step (1) with nitrogen for 4 h, then refrigerating the sodium azide-acetic acid buffer solution under a condition of 0-4 °C for 4 h, to obtain the treated sodium azide-acetic acid buffer solution.

Comparative Example 1 A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps: (1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and

(2) purging the sodium azide-acetic acid buffer solution obtained in step (1) with nitrogen for 4 h, without refrigeration or low-temperature treatment, to obtain the treated sodium azide-acetic acid buffer solution.

Comparative Example 2 A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method in the present example includes the following steps: (1) formulating a sodium azide-acetic acid buffer solution: weighing 6.501 g of sodium azide solid (GR) to be dissolved in 25 mL of ultrapure water to a constant volume of 50 mL, adding 20% acetic acid solution in the same volume, and mixing the resultant uniformly to obtain the sodium azide-acetic acid buffer solution; in the above, a method for preparing the 20% acetic acid solution is: taking glacial acetic acid (GR), and mixing the glacial acetic acid with ultrapure water in a volume ratio of 1:4 to obtain the 20% acetic acid solution; and (2) purging the sodium azide-acetic acid buffer solution obtained in step (1) with helium for 4 h, without refrigeration or low-temperature treatment, to obtain the treated sodium azide-acetic acid buffer solution.

Experimental Example 1

(1) Checking the stability of instrument before test: To confirm MAT252 gas isotope mass spectrometer used to have better stability and the degree of interference of ambient gas to the instrument, outdoor air bottle-outdoor-1 and indoor air blank-1 were tested under vacuum condition, as shown in FIGS. 2 and 3, after four groups of comparative gases (3 minutes 48 seconds), the curve shows only one peak of C02 at 5 minutes -15 seconds, within a normal range, then it can be ignored, demonstrating that the instrument used was relatively stable and there was no interference factor. (2) Determining an appropriate detection range of the instrument:

A quantitative amount of N20 gas (about 20 pg) was added to two 60 mL headspace bottles, respectively, numbered as dummy bottle 1-1 and dummy bottle 2-1, respectively, and upon test on a machine, the test result is as shown in FIGS. 4-5, and the result shows that the N20 signal value was relatively stable, indicating that about 20 pg of N20 was a relatively suitable concentration range. (3) Various protective gas accuracy control: 5 bottles (60 mL headspace bottle) of each sample were prepared under air condition and helium condition, 40 mL of ultrapure water was added to each sample, the bottles numbered as Air1, Air2, Air3, Air4 and Air5 were all located under air condition, the bottles numbered as Hel, He2, He3, He4 and He5 were all located under helium condition, and under both conditions, the newly formulated sodium azide-acetic acid buffer solution (hereinafter referred to as buffer solution for short) was used. The test (refer to the test steps in Experimental Example 2) result shows that the signal values of N20 under the air condition and under the helium condition are not quite different, and air may be used to replace helium in later stage, but it can be seen from the graph that two blank samples contained a large amount of nitrous oxide gas, and cannot meet the blank requirement. FIGS. 6-7 show test results corresponding to Hel and Air1, respectively.

Experimental Example 2 In order to verify the influence factors which might interfere with the test results and cause the decrease in accuracy when the nitrogen-oxygen isotope was determined by the chemical conversion method, the following experiment was performed, and conditions are as follows. Two parallel samples were set under each condition, as shown in Table 1 below, respectively N1-1, N1-2, N2-1, N2-2, N3-1, N3-2, N4-1, N4-2, N5-1, N5-2; and the experiment was carried out under air condition. Table 1 Influence Condition Check List

Agent Removed Parallel Sample Nos. with Agent Removed

adding no cadmium chloride N1-1, N1-2

adding no ammonium chloride N2-1, N2-2

adding no zinc sheet N3-1, N3-2

adding no sodium azide-acetic N4-1, N4-2 acid buffer solution

adding no sodium hydroxide N5-1, N5-2

Test steps included: taking 40 mL of the sample to be tested in a 60 mL headspace bottle, adding 0.8 mL of cadmium chloride solution (20 g/L), adding 0.8 mL of ammonium chloride solution (250 g/L), finally adding 3x10 cm 4N (or 3N) clean zinc sheets (wiped clean with alcohol), and oscillating on a shaker at a rotational speed of 220 r/min for 20 min; taking out the zinc sheets, and closing the headspace bottle, thus completing the nitrate reduction step; then adding 2 mL of sodium azide-acetic acid buffer solution to the headspace bottle, and vigorously oscillating to mix the sample and the reagent well; subsequently, oscillating at 220 r/min for 30 min, and finally adding 1 mL of NaOH solution (10 mol/L) as a stopper, thus ending the azidation reaction; then testing the sample on a machine. For the "Agent Removed" in Table 1, referring to the above test steps, at corresponding positions, no cadmium chloride, no ammonium chloride, no zinc sheets, no sodium azide-acetic acid buffer solution or no sodium hydroxide is added, and after being treated, the sample to be tested (the sample to be tested is distilled water) was tested on machine. The test results are as shown in FIGS. 8-15. From FIGS. 8-15, it can be seen that the sample measurement curve without the addition of the sodium azide-acetic acid buffer solution does not have a nitrous oxide signal value, and all the remaining sample measurement curves have nitrous oxide, and the signal values are not quite different, demonstrating that the sodium azide-acetic acid buffer solution is a main reagent which interferes with the blank.

Experimental Example 3

In order to compare the influence of pretreatment of sodium azide-acetic acid buffer solutions on the test results in various examples and comparative examples of the present disclosure, sodium azide-acetate buffer solutions treated in Examples 1, 3 and 4 and Comparative Examples 1 and 2 were adopted, respectively, referring to the test steps in Experiment Example 2, each of the examples and the comparative examples adopted distilled water to obtain two blank parallel samples to be tested on machine, and test results are as shown in FIGS. 16-18. The measurement curves obtained with the sodium azide-acetic acid buffer solution treated in Comparative Example 1 and the sodium azide-acetic acid buffer solution untreated are as shown in FIG. 19, wherein air2 is nitrogen purging in Comparative Example 1, and air1 is an untreated measurement curve.

The measurement curves obtained with the sodium azide-acetic acid buffer solution treated in Comparative Example 2 and the sodium azide-acetic acid buffer solution untreated are as shown in FIG. 20, wherein air4 is helium purging in Comparative Example 2, and air3 is an untreated measurement curve.

It can be seen from the above figures that the effect is optimal after the formulated sodium azide-acetic acid buffer solution is subjected to helium purging and then low-temperature refrigeration treatment; and the effect is inferior after the formulated sodium azide-acetic acid buffer solution is subjected to nitrogen purging and then low-temperature refrigeration treatment. Meanwhile, through experimental verification, the longer the purging period is, the better the effect is, and the smaller the experimental interference is. Moreover, the refrigeration treatment takes effect more quickly than the purging process, the longer the refrigeration period is, the smaller the experimental interference is, and even the experimental interference may be eliminated, indicating that the generation of impurity gases may be suppressed under refrigeration conditions. Finally, it should be explained that the various examples above are merely used for illustrating the technical solutions of the present disclosure, rather than limiting the present disclosure; although the detailed description is made to the present disclosure with reference to various preceding examples, those ordinarily skilled in the art should understand that they still could modify the technical solutions recited in various preceding examples, or make equivalent substitutions to some or all of the technical features therein; and these modifications or substitutions do not make the corresponding technical solutions essentially depart from the scope of the technical solutions of various examples of the present disclosure.

Industrial Applicability (1) By performing the investigation test on reagents and the like which may affect the experimental result when determining the isotope by the chemical conversion method, the present disclosure proves that the sodium azide acetic acid buffer solution is a main reagent interfering with blank; by performing the low-temperature treatment on the sodium azide-acetic acid buffer solution under the condition of a temperature less than or equal to °C, the interference of the sodium azide-acetic acid buffer solution on N20 obtained from the nitrate reaction in the sample in the test can be greatly reduced, and the accuracy of test result is improved, so that the accuracy of 180 iscontrolled at .1%,andtheaccuracyof 1 5 Nis controlled at 0.12%o;

(2) the present disclosure further optimizes the pretreatment method, can eliminate the interference of the buffer solution on the determination of the sample to be tested, and improves the test stability and accuracy; and

(3) the feeding device in the present disclosure can ensure N20 generated by the reaction liquid in the sampling headspace bottle to enter the mass spectrometer at a stable intensity of pressure and a suitable concentration, thereby further improving the operability of accuracy control.

Claims

What is claimed is: 1. A pretreatment method for determining nitrogen-oxygen isotope by a chemical conversion method, comprising a following step: performing a low-temperature treatment on a sodium azide-acetic acid buffer solution under a condition of a temperature less than or equal to 15 °C.
2. The pretreatment method according to claim 1, wherein the sodium azide-acetic acid buffer solution is formulated by a following method: weighing a proper amount of sodium azide solid (GR) and making the sodium azide solid dissolved in ultrapure water to a constant volume, adding 20% acetic acid solution in the same volume, and mixing a resultant uniformly to obtain the sodium azide-acetic acid buffer solution, wherein a method for preparing the 20% acetic acid solution is as follows: mixing glacial acetic acid (GR) with ultrapure water in a volume ratio of 1:4, so as to obtain the 20% acetic acid solution.
3. The pretreatment method according to claim 1 or 2, wherein the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of a temperature less than or equal to 10 °C; preferably, the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of a temperature less than or equal to 4 °C; and preferably, the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of -5 °C - 4 °C.
4. The pretreatment method according to claim 1 or 2, wherein the sodium azide-acetic acid buffer solution is placed in a system with a temperature less than or equal to 15 °C, or less than or equal to 10 °C, or less than or equal to 4 °C, or less than or equal to0 O°C.
5. The pretreatment method according to any one of claims 1-4, wherein a duration of the low-temperature treatment is more than or equal to 2 h; and preferably, the duration of the low-temperature treatment is 2-30 h.
6. The pretreatment method according to any one of claims 1-4, wherein a duration of the low-temperature treatment is 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 21 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, 28 h, 29 h, 30 h, 31 h, or32 h.
7. The pretreatment method according to claim 3, wherein the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of 0-4 °C for 20-28 h.
8. The pretreatment method according to claim 3, wherein the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of -5 °C - 0 °C for 2-6 h.
9. The pretreatment method according to any one of claims 1-8, wherein the sodium azide-acetic acid buffer solution is purged with a non-oxidizing gas, before the sodium azide-acetic acid buffer solution is subjected to a low-temperature treatment under a condition of a temperature less than or equal to 15 °C.
10. The pretreatment method according to claim 9, wherein the non-oxidizing gas is nitrogen and/or helium; and preferably, the non-oxidizing gas is helium.
11. A method for determining nitrogen-oxygen isotope by a chemical conversion method, wherein a sample to be tested is tested by using the sodium azide-acetic acid buffer solution pretreated by the pretreatment method according to any one of claims 1-10; and preferably, a protective gas used in the test is air or helium.
12. The method according to claim 11, wherein before the test, a gas isotope mass spectrometer is checked for stability to ensure that the mass spectrometer is stable and has no interference factors.
13. The method according to claim 12, wherein before the test, a detection range of the gas isotope mass spectrometer having been checked for stability is determined.
14. The method according to any one of claims 11-13, wherein a feeding device adopted in the test comprises: a cross communication tube, a hose and a feeding needle, wherein respective ports of the cross communication tube are provided with a first valve, a second valve, a third valve and a fourth valve in a clockwise direction, respectively; and a port of the cross communication tube provided with the second valve is communicated with the feeding needle through the hose.
15. The method according to claim 14, wherein the hose is provided therein with a drying column; preferably, a composition of the drying column is calcium oxide; and preferably, a port of the cross communication tube provided with the fourth valve is in communication with a vacuum pump.
16. The method according to claim 14 or 15, wherein the hose is a polyethylene hose.
17. The method according to any one of claims 14-16, wherein the feeding needle is provided with a switch for opening or closing the feeding needle.
18. The method according to any one of claims 14-16, wherein the feeding needle is provided with a rubber plug so as to block the feeding needle.