Method for measuring silicon in soil and sediment by inductively coupled plasma emission spectrometry

Abstract

The invention discloses a method for measuring silicon in soil and sediment by inductively coupled plasma emission spectrometry, which comprises the following steps: decomposing a sample: placing the sample in a silver crucible, adding absolute ethyl alcohol for wetting, and covering with sodium hydroxide particles; heating for melting, cooling, adding hot water for extraction, washing a silver crucible, heating, taking down, adding a nitric acid-hydrochloric acid mixed solution, and fixing the volume to obtain a sample solution; preparing a matrix matching solution; an internal standard solution; drawing a standard curve; and calculating. According to the method, sodium hydroxide is adopted as a melting agent, a sample is subjected to high-temperature melting, hot water extraction and nitrate acid mixed acidification pretreatment, matrix interference is eliminated through optimization of the sample weighing amount and the melting agent proportion and rhodium internal standard correction, and the silicon content in the soil and sediment sample is measured under the 251.611 nm spectral line through inductively coupled plasma emission spectrometry. The method is simple to operate, easy to popularize, low in detection limit, high in measurement result accuracy and precision and suitable for measuring silicon in large-batch soil and sediments.

Description

Technical Field

[0001] The invention belongs to the technical field of analytical chemistry, and in particular relates to a method for determining silicon in soil and sediment by inductively coupled plasma emission spectrometry. Background Art

[0002] Silicon is a key geochemical indicator element, an essential nutrient for plants and a must-measure element in geochemical sample analysis. It plays a crucial role in promoting plant growth and development, enhancing plant resistance to stress and pests, participating in biogeochemical cycles, regulating the global carbon cycle, and mitigating global warming. Measuring soil silicon content can reveal soil silicon fertility, helping to guide fertilization and improve soil quality. Currently, the main analytical methods for silicon (silicon dioxide) include animal gel agglomeration, molybdenum blue spectrophotometry, pyrophosphate analysis, volumetric analysis, gravimetric analysis, X-ray fluorescence spectrometry, inductively coupled plasma spectrometry, and inductively coupled plasma mass spectrometry.

[0003] Animal gel agglomeration, spectrophotometry, pyrophosphate, volumetric, and gravimetric methods are cumbersome, time-consuming, labor-intensive, and have low analytical efficiency, making them difficult to meet the requirements for rapid testing of batch samples in daily production. X-ray fluorescence spectrometry instruments are expensive and rarely found in general laboratories. They require sample particle sizes of 200 mesh or more, have high particle size requirements for soil and sediment samples, and are greatly affected by mineral and particle size effects. In addition, the results of low-content soil samples are unstable and have high detection limits. High-temperature melting plates also require platinum crucibles, resulting in high testing costs. Inductively coupled plasma mass spectrometry is only suitable for the determination of low-content silicon and is not suitable for the determination of high-content silicon in soil and sediments. Inductively coupled plasma emission spectrometers are widely used to determine parameters in soil and sediments due to their advantages such as high resolution, high degree of automation, wide dynamic range, low detection limit, high precision, and accuracy.

[0004] Currently, there is extensive research on the determination of silicon (silicon dioxide) using different sample digestion methods combined with inductively coupled plasma optical emission spectrometry. Acid dissolution methods require large amounts of acid, making it difficult to dissolve oxidizing samples (silicon often exists in a lattice form within oxides and is not easily digested), and they can easily lead to silicon volatilization and loss, resulting in lower results. Alkali fusion is a "wet method" for decomposing inorganic samples. Commonly used fluxes for sample preparation include lithium metaborate, lithium tetraborate, sodium carbonate, and sodium peroxide. These chemicals are relatively expensive, and the sample must be melted at temperatures of 750-1000°C, necessitating the use of expensive platinum or yellow metal crucibles. This results in high testing costs and is unsuitable for batch analysis. Zhan Xilin et al. used sodium hydroxide as a flux in a nickel crucible at 500°C, while Wu Jun et al. used sodium hydroxide as a flux in a silver crucible at 640°C to determine silicon dioxide in polymetallic ores, achieving good results. Sodium hydroxide, as a flux, has a low melting point and can completely decompose the sample at relatively low temperatures. It also has a strong ability to decompose silicon. When melted with the sample at high temperatures, it forms soluble sodium silicate, which has a short melting time and is easily leached. Silver crucibles are less expensive than platinum or yellow platinum crucibles, are less prone to cracking during melting than corundum crucibles, and have excellent alkali resistance. The melt is easier to elute than nickel crucibles, resulting in lower blank values. To date, there have been no reports on the determination of silicon in soil and sediment using silver crucibles using sodium hydroxide as a flux.

[0005] Therefore, a method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry is urgently needed. Summary of the Invention

[0006] To address the deficiencies in the prior art, the present invention provides a method for determining silicon in soil and sediment using inductively coupled plasma emission spectrometry, overcoming the problems of the acid digestion method, which cannot completely dissolve silicon in soil and sediment, the high melting point of other fluxes and the high cost of platinum crucibles or platinum yellow crucibles, and the high particle size requirements of X-ray fluorescence spectrometry for soil and sediment samples, unstable results for low-content soil and sediment samples, and high cost of high-temperature melt testing with platinum yellow crucibles.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] The present invention provides a method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry, comprising the following steps:

[0009] S1. Decomposition of samples:

[0010] S11. Weigh the sample and place it in a silver crucible. Add anhydrous ethanol to moisten the sample, and then add sodium hydroxide pellets to evenly cover the sample.

[0011] S12, then place the crucible in a muffle furnace for heating and melting, take out the silver crucible, cool it, place it in a beaker, add hot water for extraction, and rinse the silver crucible with a nitric acid-hydrochloric acid mixed solution after a vigorous reaction;

[0012] S13, placing the beaker on a hot plate to heat, removing it, adding a nitric acid-hydrochloric acid mixed solution, cooling it, and then diluting the volume to obtain a sample solution;

[0013] S14. Decompose the blank sample in the same manner as the sample to obtain a sample blank solution;

[0014] S2. Preparation of matrix matching solution: Weigh sodium hydroxide into a beaker, add hot water, and add the nitric acid-hydrochloric acid mixture while stirring. After complete dissolution, dilute to volume with water. Use this solution as a blank solution and prepare the standard solution.

[0015] S3, internal standard solution: blank solution, standard solution, sample blank solution, and sample solution are all directly added with internal standard rhodium to make the concentration consistent in each solution, the concentration is 20.0 mg / L;

[0016] S4. Standard curve drawing: Pipette an appropriate amount of silicon standard stock solution and dilute it to prepare a silicon standard series with mass concentrations of 0.00, 5.00, 10.0, 20.0, 30.0, 40.0, and 50.0 mg / L, respectively. Add matrix matching solution to the mark, analyze using inductively coupled plasma optical emission spectrometry, and draw a standard curve; under the same conditions, test the sample solution and sample blank solution prepared in step S1;

[0017] S5. Calculation: Substitute the emission intensities of the sample solution and the sample blank solution obtained by measurement into the standard curve to obtain the concentration of silicon in the sample solution, and then calculate the content of silicon in the sample.

[0018] Preferably, in step S1, the mass ratio of the sample to sodium hydroxide is 1:5.

[0019] Preferably, the temperature of the muffle furnace heating and melting in step S12 is 650-750° C., and the heating and melting time is 15-25 minutes.

[0020] Preferably, the step S12 of cooling and placing the melt in a beaker is cooling until the melt agglomerates and then placing the melt in a beaker, and the temperature of the hot water added for extraction is 90°C.

[0021] Preferably, in step S13, the beaker is placed on a hot plate and heated to boiling, and the volume is fixed after cooling to room temperature.

[0022] Preferably, the mass ratio of nitric acid to hydrochloric acid in the nitric acid-hydrochloric acid mixed solution in step S1 and step S2 is 1:4-6.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This method uses sodium hydroxide as a flux, undergoes a pretreatment process involving high-temperature melting, hot water extraction, and mixed acidification with nitric and hydrochloric acids. By optimizing the sample weight and flux ratio, and using rhodium internal standard correction to eliminate matrix interference, the silicon content in soil and sediment samples is determined using inductively coupled plasma optical emission spectrometry at 251.611 nm. This method features simple operation, easy scalability, a low detection limit, and excellent accuracy and precision, making it suitable for large-scale determination of silicon in soil and sediment. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram showing the effect of the amount of flux added on the test results in Example 2 of the present invention;

[0026] Figure 2 It is the standard curve spectrum and linear equation in Example 3 of the present invention. DETAILED DESCRIPTION

[0027] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0028] Example 1

[0029] Decomposition of the sample: Accurately weigh 0.2000g of sample in a silver crucible, add 3-4 drops of anhydrous ethanol to moisten the sample, and then add sodium hydroxide particles 5 times the mass of the sample to evenly cover the sample. Place it in a muffle furnace heated to 650°C and heat to melt for 15 minutes. Take out the crucible and cool it until the melt agglomerates. Then place it in a 250mL beaker and add 100mL of hot water for extraction. Cover it with a watch glass. After a violent reaction, rinse the crucible with a (1+4) nitric acid-hydrochloric acid mixed solution. Place the beaker on a hot plate and heat it to a boil. Remove it and add 40mL of (1+4) nitric acid-hydrochloric acid mixed solution while stirring. After cooling to room temperature, transfer it to a 500mL volumetric flask, dilute to the mark with water, and shake well to obtain the sample solution. At the same time, decompose the blank sample using the same treatment method as the sample to obtain a sample blank solution.

[0030] The specific amount of anhydrous ethanol added is not limited; its main function is to wet and gather the sample so that the sample can fully contact the sodium hydroxide and melt at high temperature. Cooling the crucible until the melt agglomerates can prevent splashing after the addition of hot water due to excessive reaction.

[0031] Preparation of matrix matching solution: Weigh 2.0 g of flux sodium hydroxide into a beaker, add 100 mL of hot water, and add 80 mL of (1+4) nitric acid-hydrochloric acid mixed solution while stirring. After complete dissolution, dilute to a 1000 mL volumetric flask with water. Use this solution as a blank solution and to prepare the standard solution.

[0032] Internal standard solution: directly add internal standard rhodium to the blank solution, standard solution, sample blank solution, and sample solution to make the concentration consistent in each solution, which can be 20.0 mg / L.

[0033] Standard curve drawing: Pipette an appropriate amount of silicon standard stock solution and dilute it to prepare a silicon standard series with mass concentrations of 0.00, 5.00, 10.0, 20.0, 30.0, 40.0, and 50.0 mg / L, respectively. Use matrix matching solution to make the volume up to the mark, analyze using inductively coupled plasma optical emission spectrometry, and draw a calibration curve to eliminate the influence of matrix effect.

[0034] Calculation: Substitute the emission intensities of the sample solution and the sample blank solution obtained by measurement into the standard curve to obtain the concentration of silicon in the sample solution, and then calculate the content of silicon in the sample.

[0035] Example 2: Determination of sample weight and flux ratio.

[0036] In this example, the ratios of sample and flux were 1:2, 1:3, 1:4, 1:5, 1:7 and 1:10 respectively for melting test. GSS-27 was used as the sample for analysis. The experimental test results are shown in Fig. Figure 1 .Depend on Figure 1 It can be seen that when the sample weight is 0.2000g and the flux ratio is 1:2 and 1:3, the sample cannot be completely melted at 650°C, indicating that the sample decomposition is incomplete, resulting in a significantly lower measurement result. When the sample weight and flux ratio are 1:7 and 1:10, the sample can be completely melted, but due to the increase in the amount of flux, the matrix effect is significantly enhanced, the atomizer is easily clogged, and the instrument stability is seriously reduced. When the sample weight and flux ratio is 1:5, the sample can be completely melted, the extracted solution is clear and transparent, and the measured value is consistent with the certified value. Therefore, the present embodiment determines that the sample weight and flux ratio is 1:5.

[0037] Example 3: Selection of analysis spectral lines.

[0038] In ICP spectral analysis, the selection of analytical spectral lines has a significant impact on the accuracy and precision of sample analysis results. By consulting the spectral line table in the ICP spectrometer, it is shown that silicon has multiple spectral lines. Three more sensitive silicon analytical lines are selected for measurement, with wavelengths of 212.412nm, 251.611nm, and 288.158nm. A standard curve spectrum is drawn between the instrument signal value and different silicon concentrations, and the corresponding linear equation is obtained. Figure 2 .

[0039] At the same time, soil and sediment standard materials (GSS-7, GSD-2a) were used as samples for analysis. The results of silicon (measured as SiO2) detected by repeated measurement six times under three spectral lines are shown in Table 1.

[0040] Table 1: Detection results of standard substances at different wavelengths (n=6).

[0041]

[0042] By observing the waveforms and instrument signal intensities of the spectral lines at different wavelengths, combined with the results of the standard substance determination, it can be seen that although the standard curves under the three spectral lines all have a good linear relationship, the signal intensity of the 251.611nm spectral line is significantly stronger than that of the 212.412nm and 288.158nm spectral lines at the same concentration. The 251.611nm spectral line also has a good waveform, stable peak shape, low background value, good precision, and high accuracy. Therefore, the 251.611nm spectral line was selected as the analytical line for silicon in this method, and the internal standard rhodium spectral line is 343.489nm.

[0043] Example 4: Detection limit and determination limit of the method.

[0044] In accordance with the "Technical Guidelines for the Development of Environmental Monitoring Analytical Method Standards" (HJ 168-2020), if the target substance is not detected in the blank test, all sample analysis steps should be followed, and n (n ≥ 7) replicates should be performed on samples with concentrations or contents 3 to 5 times the estimated method detection limit. The standard deviation and method detection limit of the n replicates were calculated. Seven sample blank solutions were prepared according to the test method and measured under optimized instrument conditions. The average value of the solution analysis results was -0.0108 mg / L, and the standard deviation was 0.0092 mg / L. See Table 2. When the sample weight is 0.2000 g and the fixed volume is 500 mL, the detection limit of silicon (calculated as SiO2) in soil and sediment is calculated to be 0.0617%. The lower limit of determination calculated as 4 times the detection limit is 0.2468%. Compared with the detection limit of 0.07% for the determination of silicon (calculated as SiO2) in soil and sediment by the HJ 974-2018 method, the detection limit is lower, indicating that this method is more suitable for the determination of silicon content in soil and sediment at low concentration levels.

[0045] Table 2: Method detection limits (n=7).

[0046]

[0047] Example 5: Calibration analysis of standard substances.

[0048] Six national reference materials for soil and sediment (GSS-7, GSS-20, GSS-27, GSD-10, GSD-22, and GSD-2a) were selected for analysis, and the results of silicon determinations were statistically analyzed. The relative standard deviations and relative errors are shown in Table 3. As can be seen from Table 3, the measured values are generally consistent with the certified values of the national reference materials. The relative standard deviations (RSDs) of the six determinations ranged from 0.81% to 3.59%, and the relative errors ranged from -0.03% to 0.18%, indicating that this method has high accuracy and meets the analytical requirements for precision.

[0049] Table 3: Method precision and accuracy (n=6).

[0050]

[0051] Example 6: Analysis of real samples.

[0052] To further verify the accuracy and feasibility of this method, one actual sample was randomly selected and the spike recovery was determined six times at three different concentrations. The test results are shown in Table 4. As can be seen from Table 4, the spike recovery rates for the actual sample at different concentrations ranged from 93.4% to 97.6%. Compared with the spike recovery range of 65% to 125% required by the HJ 974-2018 method, this method also achieved relatively good spike recovery rates, meeting the accuracy requirements.

[0053] Table 4: Actual sample determination results and spiked recovery (n=6).

[0054]

[0055] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry, characterized in that: The following steps are involved: S1. Decomposition of samples: S11. Weigh the sample and place it in a silver crucible. Add anhydrous ethanol to moisten the sample, and then add sodium hydroxide pellets to evenly cover the sample. S12, then place the crucible in a muffle furnace for heating and melting, take out the silver crucible, cool it, place it in a beaker, add hot water for extraction, and rinse the silver crucible with a nitric acid-hydrochloric acid mixed solution after a vigorous reaction; S13, placing the beaker on a hot plate to heat, removing it, adding a nitric acid-hydrochloric acid mixed solution, cooling it, and then diluting the volume to obtain a sample solution; S14. Decompose the blank sample in the same manner as the sample to obtain a sample blank solution; S2. Preparation of matrix matching solution: Weigh sodium hydroxide into a beaker, add hot water, and add the nitric acid-hydrochloric acid mixture while stirring. After complete dissolution, dilute to volume with water. Use this solution as a blank solution and prepare the standard solution. S3, internal standard solution: blank solution, standard solution, sample blank solution, and sample solution are all directly added with internal standard rhodium to make the concentration consistent in each solution, the concentration is 20.0 mg / L; S4. Standard curve drawing: Pipette an appropriate amount of silicon standard stock solution and dilute it to prepare a silicon standard series with mass concentrations of 0.00, 5.00, 10.0, 20.0, 30.0, 40.0, and 50.0 mg / L, respectively. Add matrix matching solution to the mark, analyze using inductively coupled plasma optical emission spectrometry, and draw a standard curve; Under the same conditions, the sample solution and the sample blank solution prepared in step S1 are tested; S5. Calculation: Substitute the emission intensities of the sample solution and the sample blank solution obtained by measurement into the standard curve to obtain the concentration of silicon in the sample solution, and then calculate the content of silicon in the sample.

2. The method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry according to claim 1, characterized in that: In step S1, the mass ratio of the sample to the sodium hydroxide is 1:

5.

3. The method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry according to claim 1, characterized in that: In step S12, the temperature of the muffle furnace for heating and melting is 650-750° C., and the heating and melting time is 15-25 minutes.

4. The method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry according to claim 1, wherein: In the step S12, the process of cooling and placing the melt in a beaker is to cool the melt until the melt agglomerates and then place the melt in a beaker, and the temperature of the hot water added for extraction is 90°C.

5. The method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry according to claim 1, wherein: In step S13, the beaker is placed on a hot plate and heated to boiling, and the volume is adjusted after cooling to room temperature.

6. The method for determining silicon in soil and sediment by inductively coupled plasma optical emission spectrometry according to claim 1, characterized in that: The mass ratio of nitric acid to hydrochloric acid in the nitric acid-hydrochloric acid mixed solution in step S1 and step S2 is 1:4-6.

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