CN108593835A - The assay method of the oxygen isotope ratio of nitrate granules object in a kind of atmospheric aerosol - Google Patents

Abstract

The invention discloses a method for measuring the oxygen isotope ratio of nitrate particles in atmospheric aerosol. The method of the present invention fully converts nitrate in the aerosol into nitrite first and then fully reacts to generate _N2O gas. _The oxygen atoms of the generated _N2O gas come from NO2- in the reaction process ^, the nitrogen of _NO2- ^, The oxygen isotope ratio is the same as that of the original NO ₃ ^‑ , and the slope of δ ¹⁸ O from N ₂ O to δ ¹⁸ O from nitrate should be 1 if no oxygen exchange and isotopic fractionation with water had occurred; therefore, by establishing the known nitric acid The correlation curve of the salt oxygen isotope value and the oxygen isotope value of the nitrous oxide gas produced by its conversion, and the slope and intercept of the curve can be used to calculate the nitrate oxygen isotope value in the sample. The method of the invention has high accuracy.

Description

A method for measuring the oxygen isotope ratio of nitrate particles in atmospheric aerosol

technical field

The invention relates to a method for measuring the oxygen isotope ratio of nitrate particles in atmospheric aerosol.

Background technique

With the development of urbanization and industrialization, the degree of air pollution is increasing, and the composition of atmospheric particles is becoming more and more complex. Studies have shown that atmospheric particles have serious adverse effects on traffic, humans, animals and plants [Chen Yongzhong, Xiao Huayun. Analysis of the types of atmospheric aerosols and their impact on humans [J]. Jiangxi Science, 2009,27(06):912-915]. The composition of atmospheric aerosol is complex and diverse, and nitrate particles are an important part of it, so it has become an important research object in the field of aerosol. With the development of stable isotope technology, the application of this technology to analyze the source of atmospheric aerosols has become an important direction of aerosol research. Different sources and transformation processes of atmospheric aerosols have different δ ¹⁸ O, and the application of stable isotope technology to analyze oxygen isotopes can study the source and formation process of nitrate particulate matter [Li Qinkai, Yang Zhou, Huang Jun, Cui Gaoyang, Li Xiaodong. Stable isotope of atmospheric particulate matter Research progress on composition[J].Journal of Ecology,2016,35(04):1063-1071; Qin Yan, Fan Changfu, Hu Bin, Li Yanhe. Nitrogen and oxygen isotope composition of aerosols in Turpan-Hami region of Xinjiang and genesis of nitrate deposits Indicative significance of [J]. Acta Geologica Sinica, 2015, 89(S1): 97-98; Elliott E, Kendall C, WankelS, et al. 2007. Nitrogen isotopes as indicators of NOx source contributions to atmospheric nitrate deposition across the midwestern and northeastern United States. Environmental Science&Technology, 41:7661-7667; Liu Dongwei, Tuying, Fang Yunting. Determination method and application case of ammonium salt and nitrate stable isotope abundance[J]. Journal of Applied Ecology, 2017,28(07):2353-2360; Wang Xi, Cao Yacheng, Han Yong, Tang Haoye, Wang Ruhai, Sun Xiaoli, Sun Yufang. Determination of Nitrogen and Oxygen Isotope Ratio of Nitrate in Water by Chemical Transformation Method[J]. Acta Soil Science, 2015,52(03):558-566], In turn, it can solve complex problems such as the source and transformation process of atmospheric aerosol nitrate particles, and finally contribute to solving the problem of environmental pollution.

At present, there are few domestic studies on the oxygen isotope of nitrate particles in atmospheric aerosol. The pretreatment analysis methods for the determination of nitrate oxygen isotope mainly include: ion exchange method, azide chemical reduction method and bacterial denitrification method [Xu Zhiwei, Zhang Xinyu ,Yu Guirui,Sun Xiaomin,Wen Xuefa.Research progress on nitrate nitrogen and oxygen dual stable isotope traceability in water bodies in China[J].Environmental Science,2014,35(08):3230-3238]. Due to the large amount of samples required by the ion exchange method, the cumbersome pretreatment methods, and the easy contamination of the samples, the chemical reduction method of azide acid and the bacterial denitrification method are currently the most used. However, there is a complex biological reaction mechanism in the denitrification method, which is not easy to control, and the activity of denitrifying bacteria needs to be maintained [Xu Zhiwei, Zhang Xinyu, Yu Guirui, Sun Xiaomin, Wen Xuefa. Research progress on the traceability of nitrogen and oxygen dual stable isotopes of nitrate in water in China[J] .Environmental Science, 2014,35(08):3230-3238]; while the chemical reduction method has a clear mechanism and the reaction process is simple and easy to operate. It is mainly used in the analysis of nitrate isotopes in water bodies in China. During the determination of nitrate, it was found that the reduction rate of nitrate was low, resulting in unstable isotope values, so we improved the chemical conversion method to accurately determine the oxygen isotope of aerosol nitrate.

Contents of the invention

The purpose of the present invention is to propose a method for accurately measuring the oxygen isotope ratio of atmospheric aerosol nitrate particles, so as to provide an effective basis for studying air pollution sources and pollutant migration and transformation problems.

Above-mentioned purpose of the present invention is achieved by following technical scheme:

At present, the chemical conversion method is mainly applied to the determination of nitrate isotopes in water bodies in China. We first applied the chemical conversion method described by the predecessors to the determination of aerosols. The specific flow chart is shown in Figure 1.

When using this method to test, it is found that the final reduction rate is low, which leads to the instability of the measured oxygen isotope value. Therefore, on the basis of this method, we determine a condition by improving the conditions of 1-5 steps shown in Figure 1. A method for measuring the oxygen isotope ratio of atmospheric aerosol nitrate particles, the specific improvement and its reduction rate and the measured isotope value are as follows:

The schematic diagram of the improved pretreatment process is shown in Figure 2.

Specifically, a method for measuring the oxygen isotope ratio of nitrate particles in atmospheric aerosols, comprising steps:

(1) First, use ion chromatography to analyze the mass concentration of NO ₃ ^-ions on the atmospheric particulate matter filter membrane collected, calculate the required cut membrane area, water addition and volume according to the measured concentration, and add a certain amount of ultrapure water to make the solution The nitrate concentration is 0.057μmol·ml ^-1 , and the aerosol nitrate particles are fully dissolved in the aqueous solution by vibrating with an ultrasonic oscillator;

(2) transfer step (1) gained nitrate sample solution 5ml in centrifuge tube, add 1.5g sodium chloride again, guarantee that chloride ion concentration reaches 5M;

(3) Add 10 μl of 0.5M hydrochloric acid solution and 20 μl of 1M imidazole solution to the solution obtained in step (2) to adjust the pH;

(4) Add 0.4-0.5 g of cadmium powder washed with 10% hydrochloric acid solution to the solution obtained in step (3), to reduce nitrate to nitrite;

(5) Put the centrifuge tube processed in step (4) into a constant temperature oscillator to vibrate, so that the reaction is fully carried out;

(6) 2M NaN ₃ solution and 20% acetic acid are mixed in equal volume to prepare sodium azide acetate buffer solution, and purged with helium;

(7) Move the solution left still in step (5) to the headspace bottle, inject 0.2ml of the buffer solution prepared in step (6) therein, and reduce nitrite to nitrous oxide gas;

(8) place the headspace bottle treated in step (7) upside down for 2 hours, and then inject 0.4 ml of 10M NaOH solution to terminate the reaction;

(9) Put the headspace bottle treated in step (8) upside down and let it stand for 2 hours, then let the nitrous oxide gas enter the MAT253 stable isotope ratio mass spectrometer to measure the oxygen isotope through the autosampler and the pre-concentration device in the Gas Bench mode ratio;

(10) The oxygen atoms of the generated N ₂ O gas come from NO ₂ ^- in the reaction process, and the nitrogen and oxygen isotope ratios of NO ₂ ^- are the same as those of the original NO ₃ ^- , if there is no oxygen exchange and isotope fractionation with water, The slope of δ ¹⁸ O from N ₂ O to δ ¹⁸ O from nitrate should be 1; therefore, by establishing a correlation curve between the known oxygen isotope values of nitrate and the oxygen isotope values of nitrous oxide gas produced by its conversion, the application The slope and intercept of this curve allow the calculation of the nitrate oxygen isotope value in the sample;

The solutions of 0.057 μmol·ml ^-1 NO ₃ ^- of international standard samples USGS32 (δ ¹⁸ O = 25.7) and USGS34 (δ ¹⁸ O = -27.9) with known isotope values were prepared according to different ratios of 6:0, 5:1 , 4:2, 3:3, 2:4, 1:5, 0:6 to form samples with oxygen isotope values of 25.7, 16.8, 7.8, -1.1, -10, -19, -27.9 respectively; The sample of nitrous oxide is converted according to the above steps, and the curve of the oxygen isotope ratio of nitrous oxide gas and the oxygen isotope ratio of nitrate standard is obtained, and the ratio of nitrogen isotope ratio of nitrous oxide gas in step (9) is original by converting the ratio formula of the curve Nitrogen isotope ratios for nitrate samples.

Preferably, in step (1), oscillate for 30 minutes with an ultrasonic oscillator.

Preferably, the sodium chloride in step (2) needs to be wrapped in tin foil and baked in a muffle furnace at 450° C. for 4 hours before use.

Preferably, in step (5), the set speed of the constant temperature oscillator is 120r/min, the temperature is 37°C, and the time is 4h.

Preferably, step (6) is purged with high-purity helium for 10 min at a flow rate of 70 ml/min.

The international standard samples IAEA-NO-3 (δ ¹⁸ O _SMOW ＝25.6) and USGS35 (δ ¹⁸ O _SMOW ＝57.5) with known isotope values were prepared into 0.057μmol·ml ^-1 NO ₃ ^- solution for analysis to verify the method Feasibility and accuracy, the measured values obtained by substituting the results into the standard curve conversion are as follows:

The average standard deviation of the measurement is 0.24‰. It can be seen that the method realizes the accurate determination of the oxygen isotope ratio of nitrate.

Beneficial effect:

(1) The reaction process of the method is simple, easy to operate, and the accuracy of the result is high.

(2) This method is suitable for the case of low sample volume, which solves the problem of large sample demand in general methods.

(3) The method can be processed in batches, saving labor.

Description of drawings

Fig. 1 is the schematic diagram of the chemical conversion method pretreatment process adopted before improvement;

Fig. 2 is the schematic diagram of the pretreatment process of the improved chemical conversion method;

Figure 3 is a schematic diagram of the nitrogen isotope analysis process of atmospheric aerosol nitrate particles;

Figure 4 is the conversion curve of nitrate oxygen isotope results.

Detailed ways

The following describes the substantive content of the present invention in detail in conjunction with the drawings and embodiments, but does not limit the protection scope of the present invention.

A method for measuring the oxygen isotope ratio of nitrate particles in atmospheric aerosol, comprising steps:

The method includes the following steps:

(1) First, use ion chromatography to analyze the mass concentration of NO ₃ ^-ions on the atmospheric particulate matter filter membrane collected, calculate the required cut membrane area, water addition and volume according to the measured concentration, and add a certain amount of ultrapure water to make the solution The nitrate concentration is 0.057umol.ml ^-1 , and the aerosol nitrate particles are fully dissolved in the aqueous solution by vibrating with an ultrasonic oscillator for 30 minutes.

(2) Add 1.5 g of sodium chloride to the nitrate sample solution obtained in step (1) (wrapped in tin foil and baked in a muffle furnace at 450° C. for 4 h), to ensure that the chloride ion concentration reaches 5M.

(3) Add 10 ul of 0.5M HCl solution and 20 ul of 1M imidazole solution to the solution obtained in step (2), and adjust the pH of the solution to about 8.

(4) Add 0.4-0.5 g of cadmium powder washed with 10% HCl solution to the solution obtained in step (3), to reduce nitrate to nitrite.

(5) Put the centrifuge tube treated in step (4) into a constant temperature shaker (shaker), set the rotation speed at 120r/min, and shake for 4 hours at a temperature of 37°C, so that the reaction in step (4) can fully proceed.

(6) Mix 2M NaN ₃ solution and 20% acetic acid in a ratio of 1:1 to prepare sodium azide acetate buffer solution, and purging with high-purity helium for 10 min at a flow rate of 70 ml/min.

(7) Move the solution after standing in step (5) to a headspace bottle, inject 0.2 ml of the buffer solution prepared in step (6) into it, and reduce nitrite to nitrous oxide gas.

(8) The headspace vial treated in step (7) was placed upside down for 2 hours, and then 0.4 ml of 10M NaOH solution was injected to terminate the reaction.

(9) Put the headspace bottle treated in step (8) upside down and let it stand for 2 hours, then let the nitrous oxide gas enter the MAT253 stable isotope ratio mass spectrometer through the autosampler and the preconcentration device (Precon) in the Gas Bench mode Determine the oxygen isotope ratio.

The schematic diagram of the specific pretreatment process and instrument analysis is shown in Figure 3.

(10) The oxygen atoms of the generated N ₂ O gas come from NO ₂ ^- in the reaction process, and the nitrogen and oxygen isotope ratios of NO ₂ ^- are the same as those of the original NO ₃ ^- , if there is no oxygen exchange and isotope fractionation with water, The slope of δ ¹⁸ O for N ₂ O versus δ ¹⁸ O from nitrate should be unity. Therefore, by establishing a correlation curve between the known oxygen isotope value of nitrate and the oxygen isotope value of nitrous oxide gas produced by its conversion, the slope and intercept of the curve can be used to calculate the oxygen isotope value of nitrate in the sample.

The international standard samples USGS32 (δ ¹⁸ O = 25.7) and USGS34 (δ ¹⁸ O = -27.9) with known isotope values are formulated into a 0.057uM.ml ^-1 NO ₃ ^- solution through 6:0, 5:1, 4:2 , 3:3, 2:4, 1:5, and 0:6 were mixed to form samples with oxygen isotope values of 25.7, 16.8, 7.8, -1.1, -10, -19, and -27.9, respectively. Samples with different abundances are converted according to the above steps to obtain the curve of the oxygen isotope ratio of nitrous oxide gas and the oxygen isotope ratio of nitrate standard, and the nitrogen isotope of nitrous oxide gas in step (9) is converted by the ratio formula of the curve The ratio is the nitrogen isotope ratio of the original nitrate sample. The results of the standard curve are shown in Figure 4.

The purpose of the above embodiments is to specifically introduce the substantive content of the present invention, but those skilled in the art should know that the protection scope of the present invention should not be limited to the specific embodiments.

Claims (5)

1. the assay method of the oxygen isotope ratio of nitrate granules object in a kind of atmospheric aerosol, which is characterized in that including step Suddenly：

(1) first, using the NO on the Atmospheric particulates filter membrane of ion chromatography acquisition₃ ^-The mass concentration of ion, according to institute That surveys concentration calculating needs cuts membrane area, amount of water and volume, adds a certain amount of ultra-pure water that solution nitrate concentration is made to be 0.057 μmol·ml^-1, vibrated with sonic oscillation instrument, aerosol nitrate granules be fully dissolved into aqueous solution；

(2) in nitrate sample solution 5ml to centrifuge tube obtained by transfer step (1), 1.5g sodium chloride is added, ensures chlorion Concentration reaches 5M；

(3) the 1M imidazole solutions of 0.5M hydrochloric acid solutions and 20 μ l that 10 μ l are added into step (2) acquired solution adjust pH；

(4) the washed cadmium powder of 10% hydrochloric acid solution of use of 0.4-0.5g is added into step (3) acquired solution, by nitrate reduction For nitrite；

(5) by step (4), treated that centrifuge tube is put into constant temperature oscillator vibrates, and react fully progress；

(6) by 2M NaN₃Solution and the isometric mixed preparing Sodium azide hac buffer of 20% acetic acid, helium purge；

(7) solution after standing step (5) moves in ml headspace bottle, and it is molten to inject the buffering that 0.2ml steps (6) are prepared thereto Nitrite reduction is nitrous oxide gas by liquid；

(8) the 10M NaOH solutions that step (7) treated ml headspace bottle back-off is placed to injection 0.4ml after 2h terminate reaction；

(9) after step (8) treated ml headspace bottle back-off being stood 2h, by autosampler and pre-concentration device in Gas So that nitrous oxide gas is entered MAT253 stable isotopes under Bench patterns and measures oxygen isotope ratio than mass spectrograph；

(10) by the international standards USGS32 (δ of known isotope value¹⁸) and USGS34 (δ O=25.7¹⁸O=-27.9 0.057 μ) mol·ml^-1NO₃ ^-Solution according to different ratio 6:0、5:1、4:2、3:3、2:4、1:5、0:6 are mixed to form oxygen isotope value point Not Wei 25.7,16.8,7.8, -1.1, -10, -19, -27.9 sample；The sample of different abundance is turned according to above-mentioned steps Change, obtains the curve of the oxygen isotope ratio of nitrous oxide gas and the oxygen isotope ratio of nitrate standard, pass through the curve Ratio formula scales step (9) nitrous oxide gas nitrogen isotope ratio be orthonitric acid salt sample nitrogen isotope ratio.

2. assay method according to claim 1, it is characterised in that：Step (1) sonic oscillation instrument vibrates 30min.

3. assay method according to claim 1, it is characterised in that：Sodium chloride in step (2) is using preceding needing to use tin Paper bag is rolled in 450 DEG C of baking 4h in Muffle furnace.

4. assay method according to claim 1, it is characterised in that：The setting speed of step (5) constant temperature oscillator is 120r/min, temperature are 37 DEG C, time 4h.

5. assay method according to claim 1, it is characterised in that：Step (6) uses high-pure helium under 70ml/min flow velocitys 10min is swept in air-blowing.