Pretreatment method of nitrate nitrogen stable isotope sample

Abstract

The invention discloses a pretreatment method of a nitrate nitrogen stable isotope sample, and belongs to the technical field of environmental science, agriculture and chemical analysis. The pretreatment method of the nitrate nitrogen stable isotope sample comprises the following steps: adding an excessive stannous chloride hydrochloric acid aqueous solution into a nitrate-containing water sample to reduce nitrate in the water sample into nitrite, and continuously adding excessive metal zinc to reduce the nitrite into N2O to obtain a nitrate nitrogen stable isotope sample to be detected. The pretreatment method of the nitrate nitrogen stable isotope sample provided by the invention is efficient, simple, convenient and accurate. And determining the nitrogen isotope ratio of the treated N2O by using a stable isotope ratio mass spectrometer which only responds to the N2O, and deducing the composition of the nitrogen isotope in the nitrate sample.

Description

Technical Field

[0001] The invention belongs to the technical fields of environmental science, agriculture and chemical analysis, and particularly relates to a pretreatment method for nitrate nitrogen stable isotope samples. Background Art

[0002] Nitrogen cycle is one of the important biogeochemical processes in nature. 3- ) is a key compound in the nitrogen cycle, and the source and transformation pathway of nitrate are of great significance in agricultural pollution control and environmental protection. Studying the isotopic composition of nitrate nitrogen can help scientists reveal the source of nitrate and its migration and transformation process in the environment. Nitrous oxide (N2O) is a greenhouse gas and an intermediate product produced during the nitrogen cycle. By measuring the isotopic composition of nitrogen in nitrous oxide, the isotopic information of nitrate can be indirectly inferred. Therefore, the development of an efficient and stable nitrate nitrogen reduction method for subsequent measurement of its isotopic information has important practical application value. Existing sample pretreatment methods usually use bacterial denitrification and chemical sodium azide methods to convert nitrate nitrogen into nitrous oxide gas, but the bacterial denitrification method has high requirements for operators and harsh experimental conditions. Sodium azide is a flammable and explosive controlled drug, and improper operation can easily cause danger to experimenters. Summary of the invention

[0003] To solve the above problems, the present invention provides a pretreatment method for nitrate nitrogen stable isotope samples. Through a simple and safe method, all nitrogen in nitrate is reduced to N2O gas, the composition of nitrogen isotopes generated by N2O is measured, and the nitrogen isotope information of nitrate is inferred.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] One of the technical solutions of the present invention is to provide a method for pre-treating a nitrate nitrogen stable isotope sample, comprising the following steps:

[0006] Add an excess of stannous chloride hydrochloric acid aqueous solution to a water sample containing nitrate to reduce the nitrate in the water sample to nitrite, and continue to add excess metallic zinc to reduce the nitrite to N2O to obtain a nitrate nitrogen stable isotope sample to be tested.

[0007] The reaction process of converting nitrate into N2O is as follows:

[0008] NO3 - +2SnCl2+4HCl→NO2 - +2SnCl4+2H2O;

[0009] 2NO2 - +3Zn+8H+ →2N2O+3Zn 2+ +4H2O.

[0010] The second technical solution of the present invention is to provide an application of the above-mentioned nitrate nitrogen stable isotope sample pretreatment method in determining the nitrogen isotope composition in nitrate.

[0011] The third technical solution of the present invention is to provide a method for determining the nitrogen isotope composition in nitrate, comprising the following steps:

[0012] The nitrate sample to be tested is treated by the above-mentioned pretreatment method for nitrate nitrogen stable isotope samples, and the nitrogen isotope ratio of the generated N2O is measured by a stable isotope ratio mass spectrometer (IRMS) to obtain the nitrogen isotope composition in the nitrate sample to be tested.

[0013] The present invention uses stannous chloride hydrochloric acid aqueous solution and metallic zinc as reducing substances to completely reduce a nitrate sample into N2O gas, and then uses IRMS to measure the nitrogen isotope ratio of the generated N2O to infer the composition of nitrogen isotopes in the nitrate sample. The method is efficient, simple and accurate.

[0014] Technical solution 4 of the present invention: provides an application of the above-mentioned nitrate nitrogen stable isotope sample pretreatment method in nitrogen cycle research.

[0015] The fifth technical solution of the present invention is to provide an application of the above-mentioned nitrate nitrogen stable isotope sample pretreatment method in nitrate source tracing in environmental samples.

[0016] Technical solution six of the present invention: Provide an application of the above-mentioned method for determining the nitrogen isotope composition in nitrate in nitrogen cycle research.

[0017] Technical solution seven of the present invention: Provide an application of the above-mentioned method for determining the nitrogen isotope composition in nitrate in nitrate tracing in environmental samples.

[0018] For water samples containing nitrates, the water samples are directly used as samples to be treated or tested; for soil samples containing nitrates, the soil samples are first extracted with water, filtered, and the extract is separated as the sample to be treated or tested.

[0019] The beneficial technical effects of the present invention are as follows:

[0020] The invention provides a pretreatment method for a nitrate nitrogen stable isotope sample, which has high efficiency, simplicity and accuracy.

[0021] High efficiency: The reduction reaction of stannous chloride and metallic zinc is fast and efficient, and can complete the reduction of nitrate nitrogen in a short time. The generated nitrous oxide gas has high purity and is suitable for isotope analysis.

[0022] Simplicity: This method does not require complicated experimental equipment and operating steps, the reaction conditions are mild, and it is suitable for processing batch samples.

[0023] Accuracy: It can completely reduce nitrogen in nitrate to N2O, ensuring that the generated nitrous oxide gas has the same nitrogen isotope ratio as the nitrate sample, with little external interference and small errors.

[0024] The treated N2O is then analyzed using a stable isotope ratio mass spectrometer that only responds to N2O (only responds to N2O among nitrogen oxides) to measure the N2O nitrogen isotope ratio and infer the nitrogen isotope composition of the nitrate sample.

[0025] The method for determining the nitrogen isotope composition in a nitrate sample provided by the present invention can be used to determine the nitrogen isotope composition of nitrate in an environmental sample. Since the nitrogen isotope composition of nitrate from different sources is different, the determination method provided by the present invention can be used to trace the nitrate in the environmental sample and reveal the migration and transformation process of nitrate in the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the nitrate nitrogen isotope standard curve in Example 1. DETAILED DESCRIPTION

[0027] Now, various exemplary embodiments of the present invention are described in detail, and this detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention.

[0028] In addition, for the numerical range in the present invention, it is understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0029] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention pertains. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention.

[0030] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0031] The stannous chloride hydrochloric acid aqueous solution used in the present invention is prepared by taking 22.6 grams of stannous chloride (containing crystal water SnCl2·2H2O) solid, dissolving it with 160 mL of concentrated hydrochloric acid, and then adding distilled water to dilute it to 1 L to obtain a stannous chloride hydrochloric acid aqueous solution (stannous chloride concentration is 0.1 mol / L), and adding several grains of tin to prevent oxidation during storage.

[0032] The stable isotope ratio mass spectrometer used in the present invention is the DeltaVAdvantage stable isotope ratio mass spectrometer produced by Thermo Fisher.

[0033] Example 1

[0034] The accuracy of the method for determining the nitrogen isotope composition in nitrate provided by the present invention was investigated using international standard substances containing nitrate:

[0035] (1) Prepare the test solution (10 μmol / L) with international standard substances USGS-34, USGS-35 and IAEA-N-1 respectively, take 5 mL of the test solution into a 50 mL glass bottle, add 5 mL of stannous chloride hydrochloric acid aqueous solution, and shake for 24 h to reduce the nitrate in the test solution to nitrite;

[0036] (2) Add 0.5 g of metallic zinc powder to a 20 mL headspace bottle, add the test solution (containing 100 nmol of nitrite) after reduction in step (1), seal with a cap, quickly mix, and place in a 30° C. constant temperature water bath to react and generate N2O gas. After reacting for 1 hour, test;

[0037] (3) The N2O gas produced by the reaction in the headspace bottle is carried into a stable isotope ratio mass spectrometer (IRMS) by a high-purity He carrier gas to measure the nitrogen isotope ratio (δ 15 N Air ‰), and then the results were compared with the δ 15 N Air ‰The standard value was compared to examine the accuracy of the method.

[0038] δ of each international standard substance in Example 1 15 N Air ‰The determination results and standard values ​​are shown in Table 1.

[0039] Table 1

[0040]

[0041] It can be calculated from Table 1 that the test accuracy of international standard materials USGS-34, USGS-35 and IAEA-N-1 are 0.04‰, 0.07‰ and 0.08‰ respectively.

[0042] The δ of different international standard materials in Table 1 15 N Air ‰ standard value is the horizontal axis, δ 15 N Air The average value of the determination results is used as the vertical axis to draw the nitrate nitrogen isotope standard curve ( Figure 1 ),from Figure 1 It can be seen that the slope of the standard curve is close to the theoretical value of 1, and the correlation coefficient is higher than 0.999, indicating that the nitrogen isotope fractionation effect of the treatment method provided by the present invention is very small, the reduction product of nitrate is all N2O, and there is no other form of nitrogen oxides; the standard curve also proves that the determination method provided by the present invention has good reproducibility and accuracy.

[0043] Example 2

[0044] Determination of laboratory water samples:

[0045] Take laboratory water sample (NO3 - 0.5 mL of stannous chloride solution (content: 96.81 μmol / L) was added to a 50 mL glass bottle, and 5 mL of stannous chloride solution was added. The solution was shaken for 24 h to reduce the nitrate in the water sample to nitrite. 0.5 g of metal zinc powder was added to a 20 mL headspace bottle, and 1 mL of the reduced nitrite sample was added. The bottle was sealed with a pressure cap, mixed quickly, and then placed in a 30 ° C constant temperature water bath to react and produce N2O gas. After reacting for 1 h, the stable isotope ratio mass spectrometry was used for testing. δ 15 N Air ‰ The determination results are shown in Table 2; the nitrogen concentration of the water sample after the reaction in the headspace bottle was further determined using a SAN++ continuous flow analyzer, and the determination results are shown in Table 3.

[0046] Table 2

[0047]

[0048] Table 2 shows that the δ 15 N Air-NO3 ‰The standard deviation is 0.11, which meets the requirements.

[0049] Table 3

[0050]

[0051] Table 3 shows that the pretreatment method provided by the present invention can 3- Completely reduced to N2O gas.

[0052] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for pre-treating nitrate nitrogen stable isotope samples, characterized in that: The following steps are involved: Add an excess of stannous chloride hydrochloric acid aqueous solution to a water sample containing nitrate to reduce the nitrate in the water sample to nitrite, and continue to add excess metallic zinc to reduce the nitrite to N2O to obtain a nitrate nitrogen stable isotope sample to be tested.

2. Application of the pretreatment method of nitrate nitrogen stable isotope sample according to claim 1 in determining the nitrogen isotope composition in nitrate.

3. A method for determining the nitrogen isotope composition in nitrate, characterized in that: The following steps are involved: The nitrate sample to be tested is treated with the pretreatment method of the nitrate nitrogen stable isotope sample described in claim 1, and the nitrogen isotope ratio of the generated N2O is measured using a stable isotope ratio mass spectrometer to obtain the nitrogen isotope composition in the nitrate sample to be tested.

4. Application of the pretreatment method of nitrate nitrogen stable isotope samples according to claim 1 in nitrogen cycle research.

5. Application of the pretreatment method of nitrate nitrogen stable isotope samples according to claim 1 in nitrate source tracing in environmental samples.

6. Application of the method for determining nitrogen isotope composition in nitrate according to claim 3 in nitrogen cycle research.

7. Application of the method for determining the nitrogen isotope composition in nitrate according to claim 3 in tracing the source of nitrate in environmental samples.

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