CN109001290A - The measuring method of the nitrogen isotope ratio of nitrate granules object in a kind of atmospheric aerosol - Google Patents

Abstract

The invention discloses a kind of measuring methods of the nitrogen isotope ratio of nitrate granules object in atmospheric aerosol.Nitrate in aerosol is first fully converted into nitrite by the method for the present invention, and sufficiently reaction generates N again₂O gas, the N of generation₂The molecule of O gas is made of two nitrogen-atoms and an oxygen atom, and two nitrogen-atoms one from NO₂ ^‑, one from N₃ ^‑, NO in reaction process₂ ^‑Nitrogen isotope ratio and original NO₃ ^‑It is identical, therefore, the case where determining fully reacting and N₃ ^‑Nitrogen isotope ratio stablize in the case where, product N₂The δ of O¹⁵N value should be the average value of nitrate and azide, standard nitrate δ¹⁵The δ of N and the nitrous oxide of generation¹⁵The expection slope of N will be 0.5.Therefore, by establishing the correlation curve of the isotope value of the nitrous oxide gas of known nitrate isotope value and its conversion generation, the nitrate isotope value in sample can be calculated using the slope of a curve and intercept.The method of the present invention accuracy is high.

Description

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

technical field

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

Background technique

With the development of society and economy, the degree of air pollution has intensified, and many air pollution sources have had important adverse effects on human production, life and health[Chen Yongzhong, Xiao Huayun. Analysis of the types of atmospheric aerosols and their impact on humans[J] . Jiangxi Science, 2009,27(06):912-915.], which also makes the composition of atmospheric aerosols more complex and diverse. Nitrate particulate matter is an important component of atmospheric aerosols, so it has become an important research object. 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. The nitrogen isotope of nitrate is an important tool to study the source of nitrate. Different sources have different δ15N [Li Qinkai, Yang Zhou, Huang Jun, Cui Gaoyang, Li Xiaodong. Research progress on the stable isotope composition of atmospheric particulate matter[J]. Ecology Journal, 2016 , 35(04):1063-1071]. Therefore, the source of aerosol nitrate can be analyzed by measuring the nitrogen isotope in atmospheric aerosol nitrate [Qin Yan, Fan Changfu, Hu Bin, Li Yanhe. Nitrogen and oxygen isotope composition of aerosol in Turpan-Hami region of Xinjiang and genesis of nitrate deposits Indicative significance [J]. Acta Geological Sinica, 2015, 89(S1): 97-98; Elliott E, Kendall C, Wankel S, et al. Nitrogen isotopes as indicators of NOx source contributions to atmospheric nitrate deposition across the midwestern and northeastern United States. Environmental Science&Technology ,2007.41:7661-7667], and then contribute to solving the problem of air pollution.

At present, domestic studies on the nitrogen isotope of nitrate particles in atmospheric aerosols are less, and the pretreatment analysis methods for the determination of nitrate nitrogen isotope mainly include: distillation method, diffusion method, ion exchange method, azide chemical reduction method and bacterial reaction. Nitrification method [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 Conversion Method[J]. Acta Soil Science, 2015,52(03):558-566]. Because the first three methods have problems such as large sample requirements, cumbersome pretreatment methods, and easy contamination of samples, the most commonly used methods are the chemical reduction method of azide acid and the bacterial denitrification method. However, there is a complex biological reaction mechanism in the denitrification method, which is not easy to control, and it is necessary to maintain the activity of denitrifying bacteria [Liu Dongwei, Tuying, Fang Yunting. Ammonium salt and nitrate stable isotope abundance determination method and its application cases [J]. Application Acta Ecologica Sinica, 2017, 28(07): 2353-2360; Xu Zhiwei, Zhang Xinyu, Yu Guirui, Sun Xiaomin, Wen Xuefa. Research progress on traceability of nitrate, nitrogen and oxygen dual stable isotopes in water bodies in China[J].Environmental Science, 2014,35(08 ):3230-3238]; while the chemical reduction method has a clear mechanism and a simple and easy-to-operate reaction process, and is currently mainly used for the detection of environmental samples such as water bodies. When this method was used in the determination of atmospheric aerosols, it was found that the reduction rate of nitrate was low, which led to the instability of the test results of nitrogen isotope values. Therefore, we improved the existing chemical conversion method to make it suitable for the analysis of nitrogen isotopes of nitrate in atmospheric aerosol.

Contents of the invention

The purpose of the invention is to propose a method for accurately measuring the nitrogen 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.

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 applying this method to test, it is found that the final reduction rate is low, which causes the measured nitrogen isotope value to be negative. Therefore, on the basis of this method, we determine a certain condition by improving the conditions of 1-5 steps shown in Figure 1. A method for measuring the nitrogen 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 nitrogen isotope ratio of nitrate particles in atmospheric aerosols, comprising the steps of:

(1) Use ion chromatography to analyze the mass concentration of NO ₃ ^-ions on the collected atmospheric particulate matter filter membrane, 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 contain nitrogen The concentration is 0.2μgN·ml ^-1 , and the aerosol nitrate particles are fully dissolved in the aqueous solution by shaking 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) After the headspace bottle treated in step (8) was turned upside down and left standing for 2 hours, the nitrous oxide gas entered the MAT253 stable isotope ratio mass spectrometer through the autosampler and pre-concentration device in Gas Bench mode to determine the nitrogen isotope ratio;

(10) The molecule of N ₂ O gas produced is composed of two nitrogen atoms and one oxygen atom, one of the two nitrogen atoms comes from NO ₂ ^- , and the other comes from N ₃ ^- , the nitrogen isotope of NO ₂ ^- in the reaction process The ratio is the same as that of the original NO ₃ ^- , therefore, under the condition that the reaction is complete and the nitrogen isotope ratio of N ₃ ^- is stable, the δ ¹⁵ N value of the product N ₂ O should be the average of nitrate and azide value, the expected slope of δ ¹⁵ N of standard nitrate and δ ¹⁵ N of produced nitrous oxide would be 0.5; therefore, by establishing a correlation curve , the slope and intercept of the curve can be used to calculate the nitrate isotope value in the sample;

The 0.2 μgN ml ^-1 solution of international standard samples UGSS32 (δ ¹⁵ N=180) and USGS34 (δ ¹⁵ N=-1.8) with known isotope values were prepared according to different ratios of 6:0, 5:1, 4: 2. Samples with nitrogen isotope values of 180, 149.7, 119.4, 89.1, 58.8, 28.5, and -1.8 were formed at 3:3, 2:4, 1:5, and 0:6, and the nitrogen content of nitrous oxide gas generated by the standard was established. The correlation curve of the isotope ratio and the nitrogen isotope theoretical value of nitrate, the nitrogen isotope ratio of step (9) nitrous oxide gas is converted into the original nitrate sample nitrogen isotope ratio by the ratio formula.

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 8h.

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

Prepare the international standard samples IAEA-NO-3 (δ15NAir=4.7) and USGS35 (δ15NAir=2.7) with known isotope values into 0.2ugN/ml solution for analysis to verify the feasibility and accuracy of the method, and substitute the results into the standard curve The converted measured values are as follows:

The average standard deviation of the measurement is 0.08‰. It can be seen that the method realizes the accurate determination of the nitrate nitrogen isotope ratio.

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 nitrogen 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 nitrogen isotope ratio of nitrate particles in atmospheric aerosol, comprising 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 nitrogen concentration is 0.2ugN.ml ^-1 , and the aerosol nitrate particles are fully dissolved in the aqueous solution by vibrating with an ultrasonic oscillator for 30 minutes.

(2) 5ml of the obtained nitrate sample solution in transfer step (1) is put into a 13ml centrifuge tube, and 1.5g of sodium chloride is added thereto (wrapped in tin foil in a muffle furnace and baked at 450°C for 4h), ensuring 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 8 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 Determination of nitrogen isotope ratios. .

The specific pretreatment process and instrumental analysis are shown in Figure 3.

(10) The molecule of N ₂ O gas produced is composed of two nitrogen atoms and one oxygen atom, one of the two nitrogen atoms comes from NO ₂ ^- , and the other comes from N ₃ ^- , the nitrogen isotope of NO ₂ ^- in the reaction process The ratio is the same as that of the original NO ₃ ^- , therefore, under the condition that the reaction is complete and the nitrogen isotope ratio of N ₃ ^- is stable, the δ ¹⁵ N value of the product N ₂ O should be the average of nitrate and azide value, the expected slope of δ ¹⁵ N for standard nitrate versus δ ¹⁵ N for generated nitrous oxide would be 0.5. Therefore, by establishing a correlation curve between the known nitrate isotope value and the isotope value of nitrous oxide gas produced by its conversion, the slope and intercept of the curve can be used to calculate the nitrate isotope value in the sample.

The solutions of 0.2ugN.ml ^-1 NO ₃ ^- of the international standard samples UGSS32 (δ ¹⁵ N = 180) and USGS34 (δ ¹⁵ N = -1.8) with known isotope values were prepared according to different ratios of 6:0, 5:1 , 4:2, 3:3, 2:4, 1:5, and 0:6 form samples with nitrogen isotope values of 180, 149.7, 119.4, 89.1, 58.8, 28.5, and -1.8, respectively, and establish standards to produce nitrous oxide The correlation curve of the nitrogen isotope ratio of the gas and the nitrogen isotope theoretical value of the nitrate is converted into the nitrogen isotope ratio of the original nitrate sample in step (9) by a ratio formula. 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. a method for measuring the nitrogen isotope ratio of nitrate particulate matter in atmospheric aerosol, is characterized in that, comprises steps: (1) Use ion chromatography to analyze the mass concentration of NO ₃ ^-ions on the collected atmospheric particulate matter filter membrane, 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 contain nitrogen The concentration is 0.2μgN·ml ^-1 , and the aerosol nitrate particles are fully dissolved in the aqueous solution by shaking 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) After the headspace bottle treated in step (8) was turned upside down and left to stand for 2 hours, the nitrous oxide gas entered the MAT253 stable isotope ratio mass spectrometer through the autosampler and pre-concentration device in GasBench mode to measure the nitrogen isotope ratio ; (10) The 0.2μgN·ml ^-1 solution of the international standard samples UGSS32 (δ ¹⁵ N = 180) and USGS34 (δ ¹⁵ N = -1.8) with known isotope values were mixed according to different ratios of 6:0, 5:1 , 4:2, 3:3, 2:4, 1:5, and 0:6 form samples with nitrogen isotope values of 180, 149.7, 119.4, 89.1, 58.8, 28.5, and -1.8, respectively, and establish standards to produce nitrous oxide The correlation curve of the nitrogen isotope ratio of the gas and the nitrogen isotope theoretical value of the nitrate is converted into the nitrogen isotope ratio of the original nitrate sample in step (9) by a ratio formula. 2. The assay method according to claim 1, characterized in that: step (1) oscillates with an ultrasonic oscillator for 30min. 3. The assay method according to claim 1, characterized in that: the sodium chloride in the step (2) needs to be wrapped in tin foil and baked at 450°C for 4h in a muffle furnace before use. 4. The assay method according to claim 1, characterized in that: in step (5), the set speed of the constant temperature oscillator is 120r/min, the temperature is 37°C, and the time is 8h. 5. The assay method according to claim 1, characterized in that: step (6) is purged with high-purity helium for 10 min at a flow rate of 70 ml/min.