Method for rapidly measuring low-concentration dissolved phosphorus by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) reaction tank technology

Abstract

The invention discloses a method for rapidly measuring low-concentration dissolved phosphorus by an ICP-MS (Inductively Coupled Plasma Mass Spectrometry) reaction tank technology, which can realize ppb-level detection limit by adopting the ICP-MS technology, is suitable for detection of trace and ultra-trace elements, especially has effective interference elimination capability in environmental samples, and can realize rapid determination of low-concentration dissolved phosphorus by introducing a collision / reaction tank technology. According to the method, polyatomic ion interference can be effectively removed, and the determination accuracy is improved; the ICP-MS can simultaneously detect the phosphorus element in a short time, the analysis efficiency is improved, the technical problem of trace phosphorus detection in surface water is solved, the detection precision is greatly improved while the detection efficiency is ensured, and a stable and reliable technical means is provided for environment monitoring, water quality evaluation and scientific research; the method is easy and convenient to operate and moderate in cost and has good application and popularization value.

Description

Technical Field

[0001] The present invention relates to the field of environmental monitoring and analysis technology, and in particular to a method for rapidly determining low-concentration dissolved phosphorus using ICP-MS reaction cell technology. The method solves the technical difficulties in detecting trace phosphorus in surface water, significantly improves detection efficiency while ensuring accuracy, and provides a stable and reliable technical means for environmental monitoring, water quality evaluation, and scientific research. The method is simple to operate, moderately cost-effective, and has good promotion and application value. Background Art

[0002] In water environment monitoring and geochemical research, accurately measuring phosphorus content in surface water is crucial for assessing the nutrient status of water bodies, preventing and controlling eutrophication, and formulating scientific environmental management policies. In freshwater ecosystems, phosphorus is often a key nutrient limiting primary productivity, and changes in its concentration have profound impacts on ecosystem structure and function.

[0003] Currently, commonly used methods for phosphorus analysis in water include ammonium molybdate spectrophotometry and ion chromatography. Although these traditional methods are widely used in water quality analysis, they still have many technical bottlenecks and limitations in actual operation.

[0004] Taking ammonium molybdate spectrophotometry as an example, this method has the advantages of low cost and simple equipment, and is suitable for routine monitoring. However, its measurement results are highly dependent on the operator's technical proficiency. The colorimetric color development reaction is easily affected by factors such as temperature, time and light, making it difficult to achieve high repeatability. In addition, the pre-treatment process of this method is cumbersome, such as the digestion, reduction and color development steps are complicated, which increases the difficulty of operation and human error. More importantly, the detection limit of this method is relatively high, which makes it difficult to meet the needs of accurate determination of low-concentration phosphorus, especially trace phosphorus in low-nutrient surface water bodies.

[0005] Ion chromatography theoretically offers lower detection limits and is suitable for analyzing certain low-concentration inorganic anions. However, its application also faces challenges. Its analysis cycle is relatively long, sample throughput is low, and it places high demands on the cleanliness and stability of the chromatographic system. In complex water samples, organic matter (such as natural organic matter and proteins) can easily interfere with phosphate detection, affecting analytical accuracy and potentially causing column contamination or loss, increasing maintenance frequency and operating costs.

[0006] ICP-MS technology has the advantages of wide linear range, low detection limit, fast analysis speed and simple operation, and has become an important means of micro and trace element analysis. However, due to the high first ionization energy of phosphorus (10.48eV), the ionization efficiency is low in conventional ICP-MS analysis, which limits its detection sensitivity. In addition, the main isotopes of phosphorus ( 31P) has a mass number of 31 and is very susceptible to interference from nitrogen-containing polyatomic groups, especially when there are elements such as nitrogen, carbon, hydrogen, and oxygen in the sample. Polyatomic ions (such as 14 N 16 OH + 、 15 N 16 O + These interferents will significantly affect the accuracy of phosphorus detection.

[0007] In summary, the existing phosphorus analysis technology still has shortcomings in sensitivity, ease of operation, anti-interference ability and equipment adaptability. It is urgent to develop an analytical method with high sensitivity, strong anti-interference ability, easy operation and suitable for rapid detection of trace phosphorus to meet the actual needs of current surface water environment monitoring. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings of existing phosphorus determination methods in terms of sensitivity, anti-interference ability and operational efficiency, and provides a method for the determination of dissolved phosphorus in surface water with high efficiency, sensitivity and strong anti-interference ability. The method is based on inductively coupled plasma mass spectrometry (ICP-MS) technology and is combined with a dynamic reaction cell (DRC) system. It is suitable for the rapid detection of trace phosphorus, that is, a rapid, accurate and high-throughput detection method for low-concentration dissolved phosphorus in surface water. The method is widely applicable to the analytical needs of environmental monitoring, water quality management, geochemical research and related fields.

[0009] The present invention introduces dynamic reaction cell technology into the ICP-MS analysis system, and introduces oxygen into the reaction cell as a reaction gas. In the reaction cell, phosphorus ions react with oxygen to generate phosphorus oxide ions ( 31 P 16 O + ), which is an exothermic reaction (ΔHr = –3.17eV), contributes to the stable conversion of signals. The mass-to-charge ratio separation of phosphorus oxide ions in the mass spectrometer is higher, and the corresponding signal interference is significantly reduced, thereby effectively improving the analytical sensitivity and anti-interference ability.

[0010] The technical solutions of the present invention are as follows: A method for rapidly determining low-concentration dissolved phosphorus using ICP-MS reaction cell technology comprises the following steps: (1) Establishing phosphorus oxygen ions 31 P 16 O⁺ standard curve; using ultrapure water as the matrix, add phosphate standard solution (1000μg / L stock solution dilution) to prepare a 10-50g / L concentration gradient.31 P was used as the detection isotope. 89 Y (100 μg / L) was used as the internal standard element, and the internal standard acceptance rate was controlled at 70%-125%. The signal intensity of each standard point was measured by ICP-MS, and the net signal intensity (NetIntensity) after internal standard correction was used for linear regression (R 2 ≥0.999) to establish phosphorus oxygen ions 31 P 16 O + Standard curve of (2) Determine the detection accuracy of the instrument; use ultrapure water as a blank matrix solution (no added phosphorus) for three consecutive days to determine the detection limit of the method and the instrument background value; (3) Verify the accuracy of the test results and the stability of the method; perform spike recovery experiments using a known concentration of phosphorus standard solution and perform multiple parallel measurements on the same sample to examine the stability of the method; (4) Use ICP-MS to measure the signal intensity of phosphorus oxide ions in the sample solution to be tested, and calculate the phosphorus content in the sample based on the signal intensity of the phosphorus element and its standard curve.

[0011] The sample to be tested is surface water, drinking water or treated wastewater. Turbid samples need to be digested with nitric acid-perchloric acid and then filtered (0.22μm filter membrane).

[0012] The internal standard element yttrium has a concentration of 100 μg / L and is mixed online with the sample through a three-way pipe to achieve real-time correction.

[0013] The ICP-MS used a dynamic reaction cell mode, with oxygen as the reaction gas at a flow rate of 2.5 mL / min.

[0014] Compared with the prior art, the present invention has the following significant advantages: (1) Fast and easy sample pretreatment: This invention proposes a detection method that does not require digestion treatment, which enables the direct determination of dissolved phosphorus in surface water samples (non-turbid), significantly simplifying the pretreatment steps; compared with traditional methods, it can save more than 60% of sample processing time and greatly improve detection efficiency.

[0015] (2) Optimized standard detection system: This invention optimizes and establishes a 6-point standard curve (50-500 μg / L) based on GB / T 22554-2010 standard, scientifically covering the common phosphorus content range of surface water; combined with dynamic reaction pool technology, 31 P is converted to 31 P 16 O⁺ detection effectively eliminates mass spectrometry interference through gas-phase chemical reactions, reducing the detection limit to 5μg / L (5ppb), which is more than 10 times more sensitive than traditional photometric methods.

[0016] (3) Stable detection condition control: The present invention adopts 89 Y was used as an internal standard element for real-time signal correction to keep the method reproducibility RSD within 5%. A standardized tuning procedure was used to ensure instrument stability. The tuning solution optimization indicators included key parameters such as Li, Co, In, and U to ensure that the instrument operated in the best condition.

[0017] (4) Reliable verification results: The standard addition recovery test confirmed that the method recovery rate was 95%-105%. The spiked experiments in various actual samples including surface water, groundwater, sewage, etc. showed good applicability and met the quality control requirements of environmental monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the standard curve. DETAILED DESCRIPTION

[0019] The present invention will be further described below with reference to specific embodiments.

[0020] Example 1

[0021] A method for rapidly determining low-concentration dissolved phosphorus using ICP-MS reaction cell technology specifically comprises the following steps: 1. Sample collection and preservation Surface water samples were collected on-site using 500 mL polytetrafluoroethylene bottles, 0.5 mL of 1 mol / L sulfuric acid was added to stabilize the phosphorus form, the samples were sealed and refrigerated at 4 °C, and then sent to the laboratory for testing within one week. 2. Establish the standard curve solution configuration of the element to be measured (phosphorus) 2.1 First, prepare test solutions of different preset concentrations. Purchase a certified single-label phosphorus solution (1000 μg / L) and dilute it with Milli-Q ultrapure water to prepare 50 μg / L, 100 μg / L, 200 μg / L, 300 μg / L, 400 μg / L, and 500 μg / L. The specific preparation method is as follows: 50 μg / L standard solution: Take 0.50 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well. 100 μg / L standard solution: Take 1.00 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well. 200 μg / L standard solution: Take 2.00 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well. 300 μg / L standard solution: Take 3.00 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well. 400 μg / L standard solution: Take 4.00 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well. 500 μg / L standard solution: Take 5.00 mL of 1000 μg / L phosphorus standard solution, dilute to 10.0 mL with ultrapure water, and mix well.

[0022] 2.2 Determination of phosphorus isotopes: Select the most important stable isotope of phosphorus in the determination 31 P was tested; As shown in Table 1, 31 P 16 O⁺ is the most abundant isotope in phosphorus oxygen ions, accounting for 99.76%. At the same time, its signal intensity is significantly higher than that of other isotopes. Therefore, m / z=46.99 is preferred in the detection process. 31 P 16 O⁺ can be used for quantitative analysis to effectively improve sensitivity and obtain better analytical performance.

[0023] Table 1 Common stable isotopes of phosphorus oxide ions

[0024] 2.3 Select internal standard: choose yttrium ( 89 Yttrium (Y) was used as an internal standard element. The initial concentration of the internal standard solution was 1000 μg / mL and it was diluted to 100 μg / L before use. The test signal intensity was controlled between 100,000 and 200,000 cps. Yttrium (Y) single-label standard solution was purchased from Tanmo Company.

[0025] In ICP-MS analysis, the rational selection of internal standard elements (such as Sc, Y, Rh, etc.) is of great significance for monitoring and correcting short-term fluctuations and long-term drifts of instrument signals. It can also effectively compensate for the influence of matrix effects on measurement results, thereby improving the accuracy and stability of the analysis.

[0026] In this method, yttrium (Y) is selected as the internal standard element for signal calibration. On the one hand, yttrium has a moderate mass-to-charge ratio (m / z=89), and its natural background content in surface water samples is extremely low, with less interference, and can provide a stable internal standard signal; on the other hand, the Y element can react with the oxygen introduced into the reaction cell to generate YO + ions, this reaction is an exothermic process and can be used to monitor the reaction efficiency and status in the reaction cell, further improving the stability and reliability of the method.

[0027] In contrast, scandium (Sc) has a31 P 16 O⁺ (m / z=47) has a mass-to-charge ratio (m / z=45), but in surface water it is easily affected by common elements such as calcium (Ca) and products generated by the reaction of silicon with oxygen (such as SiOH + ) interference, resulting in poor signal stability, so Sc is not suitable as 31 P 16 The internal standard element of O⁺ cannot effectively indicate its signal changes.

[0028] 2.4 ICP-MS test signal intensity: The ICP-MS sampling system introduces the sample and internal standard into the nebulizer synchronously through a three-way pipe, and then measures the signal intensity of the tuning liquid element at each preset concentration. The ICP-MS adopts a dynamic reaction cell mode, oxygen is used as the reaction gas, the flow rate is 2.5mL / min, and the concentration of the internal standard element yttrium is 100μg / L. It is mixed online with the sample through a three-way pipe to achieve real-time calibration. The signal intensity of phosphorus oxide ions in the sample to be tested and the internal standard intensity are measured by ICP-MS to obtain the net signal intensity of phosphorus oxide ions; The instrument tuning steps and test principles are as follows: ICP-MS special tuning solution contains Be, Ce, Fe, In, Li, Mg, Pb, and U at a concentration of 1 μg / L. Place the tuning solution in the ICP-MS injection system and observe the intensity value of each element to ensure that all parameters meet the test requirements.

[0029] Before the actual analysis, the ICPF-MS was optimized and adjusted using a tuning solution to ensure that the instrument was in the best condition. The sample was filtered through a 0.22 μm filter membrane and introduced into the nebulizer simultaneously with the internal standard solution. After atomization, it entered the high-temperature plasma to complete the ionization process and then entered the mass analyzer for mass separation. 31 P 16 O⁺ was used as the target detection ion, which significantly improved the sensitivity and selectivity, and ultimately achieved quantitative analysis of phosphorus through the external standard method.

[0030] After tuning is completed, 31 P 16 O⁺ was used as the determination ion of phosphorus element, and the above-mentioned standard solutions of each concentration were measured. 31 P 16 The phosphorus standard curve was drawn, and a linear fit through the origin was performed with phosphorus concentration as the horizontal axis and net signal intensity as the vertical axis. The fitting result required a correlation coefficient (R 2 ) is not less than 0.999 to ensure the accuracy and reliability of quantitative analysis.

[0031] 2.5 The net signal intensity of the standard point was linearly fitted through the origin to obtain the standard curve corresponding to the phosphorus element; based on different preset concentrations and the phosphorus-oxygen signal intensity corresponding to the phosphorus element to be measured at each preset concentration, a standard curve of the phosphorus element to be measured was constructed, wherein the abscissa represents the concentration of the phosphorus element to be measured, the ordinate represents the phosphorus-oxygen signal intensity, and the correlation coefficient of the linear fit is required to be not less than 0.999. The standard curve and the online acceptance rate of the internal standard of each sample are detailed in Table 2. The results show that the initial response value of the internal standard in the standard solution is set to 100.000%, and all the measured values of the internal standard are kept within the range of 70%-125% of the initial value, which meets the requirements of the relevant quality control standards. Figure 1 Perform data fitting and obtain the standard curve as y=0.00266x R 2 =0.99935, where y is the net intensity and x is the concentration in μg / L.

[0032] Table 2 Standard curve measurement table

[0033] 3. Determine the instrument's detection limit and background value To determine the detection limit of this method, Milli-Q ultrapure water was used as the blank matrix, and multiple repeated measurements were performed on three non-consecutive days. Blank signal data was collected, and its standard deviation was calculated. According to the 3σ method, the detection limit of the method was 3 times the standard deviation of the blank signal. The results showed that the detection limit of the method was 4.29μg / L, which was lower than 5μg / L, indicating that the method had good sensitivity. During the measurement process, the instrument background signal corresponding to the phosphorus oxide ion background concentration instrument value was 28.6μg / L, which was lower than 50μg / L.

[0034] 4. Verify the accuracy of test results and the stability of test methods To verify the accuracy and repeatability of this method in the determination of dissolved phosphorus, a spike recovery experiment was carried out using a phosphorus standard solution of known concentration, and multiple parallel determinations were performed on the same sample to investigate the stability of the method.

[0035] In terms of accuracy, a phosphorus standard solution with a concentration of 50.0 μg / L was selected for spike testing. The standard solution was added to a blank water sample with a known background, and its phosphorus content was determined according to this method. The results showed that the spike recovery rate was between 95.4% and 103.2%, indicating that this method has good measurement accuracy and can accurately determine low-concentration phosphorus in environmental samples.

[0036] In terms of stability, the same sample was measured three times under the same instrument conditions, and the relative standard deviation (RSD) was calculated. The results showed that the RSD of the repeated sample measurements was less than 3.0%, indicating that this method has good signal stability and measurement consistency during continuous measurement.

[0037] In summary, the method of this embodiment performs well in terms of accuracy and stability, and is suitable for the analysis of low-concentration dissolved phosphorus in environmental water samples.

[0038] Example 2

[0039] The DRC-ICP-MS method of Example 1 of the present invention was used to measure the dissolved phosphorus concentration in six actual surface water samples. According to the established standard curve, the net signal intensity of the samples was converted into the dissolved phosphorus concentration. The results are shown in Table 3.

[0040] Table 3 Surface water sample measurement data

[0041] In the experiment, the internal standard ( 89 The acceptance rate of sample Y) ranged from 73% to 86%, showing good overall stability, but there were still some fluctuations (sample 3 had the lowest acceptance rate of 73.64%), which may be related to the following factors: (1) Sample matrix interference: High salt or complex matrix may affect plasma stability, resulting in changes in internal standard signal; (2) Instrument fluctuation: The sensitivity of ICP-MS may drift slightly during long-term operation, affecting the internal standard calibration effect; (3) Changes in atomization efficiency: Differences in sample injection method or solution viscosity may cause fluctuations in the internal standard signal.

[0042] It is noteworthy that although the internal standard acceptance rate for some samples was slightly lower than 80% (e.g., samples 2, 3, and 4), the measured phosphorus concentrations still maintained a high degree of accuracy and acceptability (Table 3). This indicates that within the appropriate concentration range (5–500 μg / L), the ICP-MS reaction cell technology of the present invention has strong anti-interference ability and reliability for the rapid determination of low-concentration dissolved phosphorus. Even slight fluctuations in the internal standard signal do not affect the final quantitative results.

[0043] Example 3

[0044] The effect of nitrogen (1-5 mg N / L) on phosphorus determination results was investigated. The specific steps are as follows: 1. Preparation of verification standard solutions containing different concentrations of nitrogen Prepare verification standard solution: Measure 10.0 mL of a 1000 μg / L phosphorus (P) single standard solution, dilute it to 100 mL with pure water in a volumetric flask, and shake thoroughly to prepare a 50 μg / L phosphorus standard solution. Take a certain amount of certified potassium nitrate standard solution (100 mg N / L, calculated as N) as the interfering ion (nitrogen) solution, add a certain amount of 100 μg / L phosphorus standard solution, and dilute to the required volume with ultrapure water to prepare a series of verification solutions with the same phosphorus concentration but different nitrogen concentrations. The specific preparation method is as follows: 1mg N / L verification solution: Take 0.1mL of 100mg N / L potassium nitrate solution, add 5.0mL of 100μg / L phosphorus standard solution, dilute to 10mL with pure water, and mix. The verification solution now has a phosphorus concentration of 50.0μg / L and a nitrogen concentration of 1mg N / L. 2 mg N / L verification solution: Take 0.2 mL of 100 mg N / L potassium nitrate solution, add 5.0 mL of 100 μg / L phosphorus standard solution, dilute to 10 mL with pure water, and mix thoroughly. The verification solution now has a phosphorus concentration of 50.0 μg / L and a nitrogen concentration of 2 mg N / L. 3 mg N / L verification solution: Take 0.3 mL of 100 mg N / L potassium nitrate solution, add 5.0 mL of 100 μg / L phosphorus standard solution, dilute to 10 mL with pure water, and mix thoroughly. The verification solution now has a phosphorus concentration of 50.0 μg / L and a nitrogen concentration of 3 mg N / L. 4 mg N / L verification solution: Take 0.4 mL of 100 mg N / L potassium nitrate solution, add 5.0 mL of 100 μg / L phosphorus standard solution, dilute to 10 mL with pure water, and mix thoroughly. The verification solution now has a phosphorus concentration of 50.0 μg / L and a nitrogen concentration of 4 mg N / L. 5 mg N / L verification solution: Take 0.5 mL of potassium nitrate solution with a concentration of 100 mg N / L, add 5.0 mL of 100 μg / L phosphorus standard solution, dilute to 10 mL with pure water, and mix. At this time, the phosphorus concentration of the verification solution is 50.0 μg / L and the nitrogen concentration is 5 mg N / L.

[0045] 2. Experimental results The experimental results are shown in Table 4. As the nitrogen concentration increases, 31 P 16 O + The signal intensity was stable (4588.2-4658.9 cps), indicating that DRC-ICP-MS can effectively eliminate nitrogen interference, despite the internal standard 89The Y acceptance rate decreased slightly (100.20%-94.26%) due to the matrix effect caused by the increase in nitrogen concentration, but the variation was within an acceptable range. At the same time, the recovery rate of phosphorus determination in the verification sample was 102.9%~110.4%, which did not significantly affect the determination results.

[0046] Table 4 Verification of the strength of the standard solution at different N contents and the internal standard

[0047] In summary, within the range of 0-5 mgN / L, nitrogen has no significant interference with the determination of phosphorus, which verifies the good anti-interference performance of the method of the present invention. This method is suitable for phosphorus analysis in nitrogen-containing surface water samples and has high accuracy and stability.

[0048] Example 4

[0049] The phosphorus content of six samples was determined using the method of the present invention and a flow analyzer, respectively. The results of the two methods were compared (as shown in Table 5). The method of Example 1 accurately detected phosphorus in all samples (16.47-231.65 μg / L), while the flow analyzer failed to detect phosphorus in low-concentration samples (<25 μg / L) (Samples 2, 4, and 6). This may be related to the higher detection limit of this method. For quantifiable samples, the relative deviation of the two methods was between 1.31% and 6.32%, with the deviation being smaller (≤6.32%) for high-concentration samples (>60 μg / L), indicating that the two methods provide relatively consistent results at higher phosphorus concentrations.

[0050] Table 5 Comparison with flow analyzer data

[0051] In summary, the method of the present invention has a lower method detection limit and a wider linear range, and can accurately detect even low-concentration phosphorus samples. Although the flow analyzer is fast and simple, its applicability to ultra-low concentration samples is limited.

Claims

1. A method for rapidly determining low-concentration dissolved phosphorus using ICP-MS reaction cell technology, characterized in that: The following steps are involved: (1) Selection of yttrium 89 Y was used as the internal standard element, and the phosphorus oxide ion was established based on the net signal intensity after internal standard correction. 31 P 16 O + Standard curve of (2) Using ICP-MS to measure the signal intensity of phosphorus oxide ions and the internal standard intensity in the sample to be tested to obtain the net signal intensity of phosphorus oxide ions; (3) Based on the established standard curve, the net signal intensity is converted into the concentration of dissolved phosphorus in the sample.

2. The method according to claim 1, wherein: The sample to be tested is surface water, drinking water or treated wastewater.

3. The method according to claim 1, wherein: The concentration of the internal standard element yttrium was 100 μg / L.

4. The method according to claim 1, wherein: ICP-MS used a dynamic reaction cell mode with oxygen as the reaction gas at a flow rate of 2.5 mL / min.

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