Pretreatment method for determination of silver in soil and sediment by inductively coupled plasma mass spectrometry

Abstract

The invention discloses a pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry, comprising: S1. performing acid microwave digestion on the sample to be tested and performing pretreatment to remove interference; S2. detecting the digested solution by inductively coupled plasma mass spectrometry; dripping sodium hydroxide solution into the digested solution formed in S1 after removing the acid until a brick-red precipitate is generated, dripping 0.5 ml of concentrated ammonia water, standing for reaction, filtering and taking the filtrate to adjust the pH, and measuring on a machine. The invention aims to find a method for simply, quickly and efficiently determining the content of trace silver in soil and sediment. By utilizing the different solubility products of the hydroxides of Ag, Zr and Fe in an alkaline environment and the chemical properties of silver complexing with ammonia water in an alkaline environment, Zr is completely precipitated in the form of zirconium hydroxide, the silver is complexed with ammonia water to maintain a dissolved state, and the brick-red iron hydroxide precipitate is used as an indicator of the complete precipitation of zirconium, thereby achieving complete separation of zirconium and silver.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical analysis methods, and in particular to a pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry. Background Art

[0002] There are few reports on the determination of trace silver in soil and sediments. When determining silver in water samples, the matrix is ​​very simple, and the corresponding matrix interference and mass spectrometry interference are very small, which is easy to accurately determine. The soil and sediment matrix is ​​relatively complex, and there are many interference factors. At present, there is no systematic study on the test methods for silver in soil and sediments, and there are few related literatures. The methods for laboratory detection of trace silver in the environment include spectrophotometry, emission spectroscopy, graphite furnace atomic absorption spectrometry, flame atomic absorption spectrometry, inductively coupled plasma mass spectrometry, etc. Among them, inductively coupled plasma mass spectrometry has higher sensitivity and relatively less interference, which is more suitable for the determination of trace silver in soil and sediments.

[0003] The interference of inductively coupled plasma mass spectrometry in the determination of silver in soil and sediment is mainly caused by mass spectrum interference of elements such as Zr. According to the interference element database and existing literature, soil and sediment contain a large amount of zirconium (Zr). When using ICP-MS to determine the soil digestion solution, 90 Zr 16 O 1 H. 90 Zr 17 O. 91 Zr 16 O ion groups will 107 Ag and 109 The determination of Ag produces a great positive interference, which makes the determination result different from the actual Ag content in the digestion solution by an order of magnitude. At present, there are three main methods for removing Zr interference in the determination of silver in soil by inductively coupled plasma mass spectrometry: establishing an interference calibration equation, using collision mode for determination, and using adsorbents to enrich and separate silver.

[0004] For example, in a patent with application number 2020113766546 and application date 2020.11.30, entitled A method for determining total calcium in soil with high aluminum and iron content, a determination method is disclosed. After the soil to be tested is pre-treated - digested - prepared with a digestion solution - and a release agent is added to prepare a test solution, an inductively coupled plasma emission spectrometer is used to determine the calcium element in the soil. In the present invention, a mixture of boric acid, nitric acid, strontium chloride and triethanolamine is added to the test solution of the soil sample after digestion as a release agent, which effectively avoids the interference of aluminum and iron elements in the soil on the detection of calcium; and the digestion conditions are optimized, the digestion time is shortened, and a simple, fast and accurate test effect can be achieved.

[0005] For example, the patent with application number 2017102668618, application date 2017.08.08, and titled "A method for determining lead isotope ratios in environmental samples using inductively coupled plasma mass spectrometry" discloses a method for determining lead isotope ratios in environmental samples using inductively coupled plasma mass spectrometry. The invention includes three steps: (1) pretreatment of environmental samples, (2) determining ICP-MS analysis conditions, and (3) detecting the sample liquid using an inductively coupled plasma mass spectrometer. The invention has high sensitivity, accuracy, good stability, and small error, and can achieve the purpose of measuring lead isotope ratios in environmental samples.

[0006] Another example is the application number 2018102964940, the application date is 2018.10.23, and the name is a method for simultaneously determining selenium and cadmium in soil, which discloses a method for simultaneously determining selenium and cadmium in soil using a dynamic reaction cell-inductively coupled plasma mass spectrometry, specifically comprising: (1) digesting the sample to be tested; (2) detecting the digested solution using a dynamic reaction cell-inductively coupled plasma mass spectrometry. The present invention digests the sample to be tested, and uses a dynamic reaction cell-inductively coupled plasma mass spectrometry to simultaneously determine selenium and cadmium in soil. By optimizing the reaction gas flow rate and RPq value of the dynamic reaction cell, interference is effectively reduced and the detection limit is improved, thereby improving the accuracy and sensitivity of the detection.

[0007] The shortcomings of the above three methods are: the interference removal method of the calibration equation requires the determination and calculation of the interference coefficient, and because the total zirconium content is three orders of magnitude higher than the silver content to be measured, the test results fluctuate greatly during the actual operation, and it is easy to have inaccurate measurements. Measuring in collision mode will reduce the kinetic energy of the ions, reduce the response intensity of the target element, reduce the test sensitivity, and may not effectively remove the interference; using adsorbents to separate and enrich Ag and then measure to eliminate interference will increase the experimental steps, increase uncertainty, and is extremely dependent on the adsorption and desorption efficiency of the adsorbent. Summary of the invention

[0008] The object of the present invention is to provide a pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry, so as to solve one or more of the above-mentioned problems in the prior art.

[0009] The pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry provided by the present invention is characterized by comprising the following steps:

[0010] S1. Digest the sample to be tested and perform pre-treatment to remove interference;

[0011] S2. detecting the digested solution by inductively coupled plasma mass spectrometry;

[0012] After the digestion solution formed in S1 is driven out of the acid until it is nearly dry, sodium hydroxide solution is added until a brick-red precipitate is formed, 0.5 mL of concentrated ammonia water is added dropwise, the reaction is allowed to stand for 10 min, and filtered with slow quantitative filter paper.

[0013] The present invention aims to find a method for simply, quickly and efficiently determining the content of trace silver in soil and sediment. Inductively coupled plasma mass spectrometry has advantages over other instruments in determining trace heavy metals in soil and sediment, so the present invention uses inductively coupled plasma mass spectrometry to determine trace silver in soil and sediment. In order to remove the mass spectrum interference of Zr on Ag, the main technical problem of the present invention is to completely precipitate Zr in the form of zirconium hydroxide before the determination, using the different solubility products of Ag, Zr and Fe hydroxides in an alkaline environment and the chemical properties of silver complexing with ammonia water in an alkaline environment, and keep the silver in a dissolved state by complexing with ammonia water, and use brick red iron hydroxide precipitate as an indicator of complete precipitation of zirconium, so as to achieve complete separation of zirconium and silver, and remove the mass spectrum interference problem of Ag determination.

[0014] In some embodiments, in step S1, the digestion solution is prepared under an acidic microwave digestion environment, and the acid system is nitric acid-hydrofluoric acid.

[0015] In some embodiments, the preparation is carried out under an acidic environment, and the specific steps are as follows: accurately weigh about 0.25 g (accurate to 0.0001 g) of soil sample and place it in a polytetrafluoroethylene digestion tube, then add 9 mL of nitric acid and 3 mL of hydrofluoric acid, heat to 180° C. for 15 minutes and maintain for 20 minutes.

[0016] In some embodiments, the specific steps of preparation under alkaline environment are as follows: after digestion is completed, take out the digestion tube and transfer it to a graphite digestion instrument to drive out the acid at 150°C until it is almost dry, add experimental ultrapure water to repeat the acid driving once until it is almost dry, add sodium hydroxide solution dropwise until a brick-red iron hydroxide precipitate is generated and exists stably, add 0.5 mL of concentrated ammonia water dropwise, let the reaction stand for 10 minutes, and filter with slow quantitative test paper.

[0017] In some embodiments, the specific steps of preparation are: the filtrate after filtration is adjusted to pH < 2 with nitric acid solution, the volume is fixed to 50 mL, and the measurement is performed on a machine.

[0018] In some embodiments, using a suitable isotope 107 Ag and internal standard element Rh were measured in standard mode.

[0019] In some embodiments, according to the different solubility products Ksp of zirconium, silver, and iron hydroxides, the ion reaction equation and reaction steps in S1 are as follows:

[0020] Ksp(Zr)<Ksp(Fe)<Ksp(Ag), the smaller the solubility product, the earlier the hydroxide precipitates.

[0021] (4) Zr 4+ +4OH - →Zr(OH)4↓(Zirconium element precipitates first)

[0022] (5)Fe 3+ +3OH - →Fe(OH)3↓(A brick-red precipitate is formed, which is the endpoint indicator, indicating that the zirconium element is completely precipitated.)

[0023] Ag + +2NH3·H2O→Ag(HN3)2 + +2H2O (forms a complex and remains dissolved). BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a schematic diagram of a pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0026] like Figure 1 As shown in the text, the present invention provides a pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry, which utilizes the principle that in an alkaline environment, silver forms a complex with ammonia without precipitation, while zirconium, iron and other elements precipitate as hydroxides to separate silver from zirconium, and then adjusts the pH before determining it by ICP-MS, thereby achieving the determination of silver in soil under standard mode.

[0027] It includes the following steps:

[0028] S1. Perform acid microwave digestion on the sample to be tested and perform pre-treatment to remove interference;

[0029] S2. detecting the digested solution by inductively coupled plasma mass spectrometry;

[0030] The digestion solution formed in S1 was driven out of the acid on a graphite digester until it was nearly dry, and then sodium hydroxide solution was added dropwise until a brick-red precipitate appeared. Then 0.5 mL of concentrated ammonia water was added and allowed to react for 10 min. The filtrate was filtered with a slow quantitative filter paper and the pH value was adjusted before determination on an instrument.

[0031] The sample to be tested (soil sample) is digested by acid microwave, and after digestion, the acid is driven to nearly dryness, and sodium hydroxide solution is added dropwise until a brick-red precipitate is formed. 0.5 mL of concentrated ammonia water is added dropwise, and the reaction is allowed to stand and then filtered. The filtrate is adjusted for pH and then measured on the machine.

[0032] Specifically, accurately weigh about 0.25g (accurate to 0.0001g) of soil sample and place it in a polytetrafluoroethylene digestion tube, then add 9mL nitric acid and 3mL hydrofluoric acid, heat to 180℃ for 15min and maintain for 20min. After digestion, transfer the digestate to a graphite digester and carry out acid removal at 150℃ until nearly dry, add experimental ultrapure water to repeat the acid removal once until nearly dry, add sodium hydroxide solution dropwise until brick-red iron hydroxide precipitate is formed and exists stably, add 0.5mL of concentrated ammonia water, let stand for 10min, and filter with slow quantitative test paper. The filtrate after filtration is adjusted to pH <2 with nitric acid solution, and the volume is fixed to 50mL before measurement on the machine. At the same time, use experimental water instead of soil sample to prepare laboratory blank. Select during measurement. 107 Ag was the isotope to be determined, and Rh was selected as the internal standard element.

[0033] (1) Separation experiment of Ag and Zr under alkaline conditions:

[0034] Since Zr precipitates in an alkaline environment, and silver is converted into a silver-ammonia solution in the presence of ammonia, the present invention mainly uses sodium hydroxide solution to adjust the pH value of the sample after soil and sediment digestion to completely precipitate Zr, and at the same time adds ammonia water to promote Ag to form a dissolved complex, so as to achieve the purpose of separating silver and Zr, and then adjusts the solution to acidity for machine testing.

[0035] The ionic equations and reaction sequences of the main reactions are as follows:

[0036] (1) Zr 4+ +4OH - →Zr(OH)4↓(Zirconium element precipitates first)

[0037] (2)Fe 3+ +3OH - →Fe(OH)3↓(A brick-red precipitate is formed, which is the endpoint indicator, indicating that the zirconium element is completely precipitated.)

[0038] (3) Ag + +2NH3·H2O→Ag(HN3)2 + +2H2O (silver forms a complex with ammonia and remains dissolved)

[0039] When the digestion solution is nearly dry, remove the acid at 150°C, add experimental ultrapure water to repeat the removal of acid until nearly dry, add sodium hydroxide solution dropwise until brick-red iron hydroxide precipitate is formed and exists stably, add 0.5 mL of concentrated ammonia water dropwise, let it react for 10 minutes, and filter with slow quantitative test paper.

[0040] (2) Selection of Ag isotopes and internal standard elements

[0041] Ag has two isotopes in nature.107 Ag and 109 Ag, element abundances are 51.84% and 48.16% respectively. The main interference of these two isotopes is caused by the element Zr. 107 The main interfering ions of Ag are 90 Zr 16 O 1 H and 90 Zr 17 O, 109 The main interfering ions of Ag are 91 Zr 16 O. When the Zr element is completely precipitated, the isotope is selected according to the abundance. The greater the abundance, the higher the test sensitivity and the lower the detection limit of the test method. 107 Ag is the test isotope. The internal standard element is selected based on the principle that it has a mass number close to the element to be tested and has a very low content in the soil, so Rh is selected as the internal standard element.

[0042] (3) Determination of digestion acid system

[0043] Experiments were carried out using three systems: nitric acid-hydrofluoric acid (9+3), nitric acid-hydrofluoric acid-hydrogen peroxide (9+3+1), and nitric acid-hydrofluoric acid-perchloric acid (9+3+1). The experimental results showed that the nitric acid-hydrofluoric acid (9+3) system was able to completely decompose the soil and sediment, and the measurement results also met the accuracy requirements.

[0044] Table 1 Test accuracy of different digestion acid systems

[0045]

[0046] (4) Setting of microwave digestion program

[0047] The nitric acid + hydrofluoric acid system was used to test the digestion efficiency at different temperatures and times. The test results showed that when the temperature was maintained at 180°C and the holding time was 20 minutes, the digestion efficiency met the test accuracy requirements. Experiments showed that it takes 15 minutes to heat up to 180°C when the power is 1600W.

[0048] Table 2 Test results of samples digested under different microwave digestion procedures

[0049]

[0050] (5) Experiment on the removal of interference of zirconium on silver

[0051] The effect of the Zr interference test on the silver determination was tested. The present invention is an improvement and enhancement on the previous experimental results. When the brick-red precipitate is generated and stably exists, the pH is between 8 and 10, which is suitable for silver and ammonia to form a dissolved complex, but excessive ammonia water will promote the transformation of the complex. Experiments show that generally 0.5 to 1.0 mL of sodium hydroxide solution is required to ensure the rapid appearance of the brick-red precipitate, and the amount of concentrated ammonia water added is required to be 3.0 mL or more to ensure the rapid generation of the brick-red precipitate. At this time, the ammonia water in the solution is in an excessive state, which may lead to the transformation of the silver ammonia solution, thereby affecting the stability of the determination method. The measurement results show that the relative standard deviation is small when sodium hydroxide is added dropwise, and the error rate is also small, and the accuracy can be guaranteed for soil samples with a concentration of only 0.050 ± 0.006. In addition, Zr will not be precipitated when the volume is directly fixed after the acid is driven out. The measurement results show that the presence of Zr in the digested sample does have a positive interference with silver, so the complete precipitation of zirconium hydroxide greatly improves the accuracy of silver determination.

[0052] Table 3 Sample test results under different interference removal methods

[0053]

[0054] (6) Relationship between sample storage time and pH

[0055] The relationship between different pH values ​​of the test filtrate and the storage time. The test results show that when the pH is acidified to pH < 2, the storage time of the sample increases, and there is basically no significant change within 48 hours. Although the results meet the quality control requirements when the pH is 8-10 and the centrifugal machine is tested immediately, the requirements for the atomizer are relatively high. Therefore, the filtrate should be acidified and then fixed to a fixed volume before testing on the machine.

[0056] Table 4 Relationship between sample storage time and pH value

[0057]

[0058] The present invention mainly uses an acid microwave digestion method to digest the silver in the soil and sediment, and then completely precipitates the interfering element zirconium in the sample in the form of zirconium hydroxide in an alkaline environment, and introduces ammonia water to complex the silver in the soil and sediment to achieve complete separation of silver and zirconium, thereby eliminating the mass spectrum interference of the ICP-MS method for determining silver. The results after optimizing the experimental conditions show that the method can effectively remove the interference of zirconium and ensure the accuracy and precision of the determination value of silver in the soil and sediment. The method meets the quality control requirements and can be used to determine trace silver in soil and sediment.

[0059] The method of the present application effectively removes the mass spectrum interference of the Zr element on silver, and significantly improves the accuracy.

[0060] The invention is supported by the Jiangsu Provincial Environmental Monitoring Research Fund Project "Determination of Tantalum and Silver in Soil and Sediment" (1812). The soil and sediment standard materials determined by the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences were mainly used for research. The method was tested on soil samples from the province during the detailed investigation of soil pollution in agricultural land in Jiangsu Province in 2018, and the test results met the quality control requirements of HJ 168-2020. The method was verified by 4 qualified laboratories, namely: Nantong Environmental Monitoring Center of Jiangsu Province, Nanjing Environmental Monitoring Center of Jiangsu Province, Nanjing Customs Animal and Plant and Food Testing Center, and Nantong Urban Water Supply Water Quality Testing Center.

[0061] The test data quality control results of the present invention are summarized as follows

[0062] (1) Method detection limit and determination lower limit

[0063] According to all the steps of sample analysis, the silver in the blank sample was measured in parallel 7 times. The standard deviation of the 7 parallel measurements was calculated, and the method detection limit was calculated. Four times the detection limit was used as the method determination lower limit. When the sampling amount was 0.25g and the fixed volume was 50ml, the maximum detection limit of the obtained data was 0.005mg / kg, so the detection limit of silver in soil and sediment was 0.006mg / kg, and the determination lower limit was 0.020mg / kg.

[0064] (2) Precision

[0065] When using the microwave digestion method, the relative standard deviations within the laboratory ranged from 3.7% to 7.9%, 2.3% to 4.2%, and 1.6% to 2.9%, respectively; the relative standard deviations between laboratories were 3.5%, 3.2%, and 1.1%, respectively; the repeatability limits r were 0.003 mg / kg, 0.004 mg / kg, and 0.005 mg / kg, respectively; the reproducibility limits R were 0.003 mg / kg, 0.005 mg / kg, and 0.005 mg / kg, respectively.

[0066] (3) Accuracy

[0067] Four laboratories conducted spike recovery determinations on three actual samples of silver content. When microwave digestion was used, the spike recovery rates were between 89.0% and 104%, 88.3% and 94.0%, and 92.5% and 96.0%, respectively. The final values ​​of spike recovery rates between laboratories were (95.8±6.7)%, (91.6±2.5)%, and (94.5±1.6)%, respectively.

[0068] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several similar modifications and improvements without departing from the creative concept of the present invention, and these are also considered to be within the scope of invention protection.

Claims

1. A pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry, characterized in that: The steps include: S1, performing acid microwave digestion on the sample to be tested and pre-treatment to remove interference; S2, detecting the digested solution by inductively coupled plasma mass spectrometry; In step S1, the digestion solution is prepared under an acidic microwave digestion environment, and the acid system is nitric acid-hydrofluoric acid; the preparation under the acidic environment is as follows: accurately weigh 0.25g of soil sample and place it in a polytetrafluoroethylene digestion tube, then add 9mL nitric acid and 3mL hydrofluoric acid, and heat to 180°C for 15min and maintain for 20min; The digestion solution formed in S1 was driven out of the acid by a graphite digestion instrument until it was nearly dry, and then sodium hydroxide solution was added dropwise until a brick-red precipitate appeared. Then 0.5 mL of concentrated ammonia water was added, and the reaction was allowed to stand for 10 min. The filtrate was filtered with a slow quantitative filter paper, and the pH value was adjusted before measurement on an instrument. According to the different solubility products Ksp of zirconium, silver and iron hydroxides, the ion reaction equation and reaction steps in S1 are as follows: Ksp(Zr)<Ksp(Fe)<Ksp(Ag), the smaller the solubility product, the earlier the hydroxide precipitates; (1)Zr 4+ +4OH - →Zr(OH)4↓; (2)Fe 3+ + 3OH - →Fe (OH)3↓; <h2 style=";text-align:left;direction:ltr">(3) Ag<h2 style=";text-align:left;direction:ltr"> + <h2 style=";text-align:left;direction:ltr"> +2NH3·H2O→Ag(HN3)2<h2 style=";text-align:left;direction:ltr"> + <h2 style=";text-align:left;direction:ltr"> +2H2O.

2. The pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry according to claim 1, characterized in that: The specific steps for preparation under alkaline environment are as follows: after digestion, take out the digestion tube and transfer it to a graphite digester and carry out acid removal at 150°C until it is almost dry, add experimental ultrapure water to repeat the acid removal once until it is almost dry, add sodium hydroxide solution dropwise until a brick-red iron hydroxide precipitate is formed and exists stably, add 0.5 mL of concentrated ammonia water dropwise, let the reaction stand for 10 minutes, and filter with slow quantitative test paper.

3. The pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry according to claim 1, characterized in that: The specific steps of preparation are as follows: the filtrate after filtration is adjusted to pH < 2 with nitric acid solution, the volume is fixed to 50 mL, and the measurement is performed on a machine.

4. The pretreatment method for determining silver in soil and sediment by inductively coupled plasma mass spectrometry according to claim 1, characterized in that: Using the right isotopes 107 Ag and internal standard element Rh were measured in standard mode.

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