An efficient and accurate method for determining nickel content in soy milk powder

Abstract

The present invention relates to the technical field of testing or analyzing materials by electrical, electrochemical or magnetic methods, and discloses a method for determining the nickel content in soymilk powder efficiently and accurately. The soymilk powder sample is digested by a super microwave digestion instrument, and an inductively coupled plasma mass spectrometer is used to measure the content of the isotope to be measured in the soymilk powder sample by the internal standard method. In the present invention, by selecting <supgt;60< / supgt;Ni which is not easily affected by iron and calcium in soymilk powder as the isotope to be measured, the measurement result being too high is avoided; by screening out the internal standard element rhodium with a high recovery rate when measuring the nickel content in soymilk powder, and then measuring the nickel content in soymilk powder by the internal standard method, the non-mass spectrometry interference is overcome, and the problems of poor linearity and large fluctuations in the results are avoided; by using nickel-free rice paper as the weighing paper to weigh the soymilk powder sample and then digesting it together with the rice paper, not only is the sample easily placed into the digestion tube, but also there is no situation where the soymilk powder sample is not digested because it adheres to the digestion tube wall or the weighing paper.

Description

Technical Field

[0001] The present invention relates to the technical field of testing or analyzing materials by electrical, electrochemical or magnetic methods, and particularly relates to a method for efficiently and accurately determining the nickel content in soy milk powder. Background Art

[0002] Soy milk powder is a powdered food made mainly from soybeans and dairy products through processes such as grinding, heat inactivation of enzymes, concentration, and spray drying. However, the raw materials and nutritional additives of soy milk powder may be contaminated with nickel, and nickel elements may also be introduced during the production and processing process (mainly because a large amount of Monel alloy is used in the grinding and spray drying equipment). Excessive nickel elements can cause cardiovascular and kidney diseases and affect human health. Therefore, it is necessary to monitor the nickel content in soy milk powder.

[0003] GB 5009.138-2017 - Determination of Nickel in Foods stipulates that graphite furnace atomic absorption spectrometry is used for the detection of nickel elements in foods. This method has a narrow linear range (that is, the detection results can only be ensured to be accurate within a very narrow range) and the sensitivity cannot meet the detection requirements of export commodities (which need to meet the detection standards of importing countries, and the requirements of some importing countries are higher, exceeding the linear range of graphite furnace atomic absorption spectrometry).

[0004] ICP-MS (Inductively Coupled Plasma Mass Spectrometry) has been applied to the field of food quality safety and detection, and its accuracy and linear range are generally better than those of graphite furnace atomic absorption spectrometry. However, due to various difficulties, no one has successfully implemented the use of ICP-MS for the determination of nickel content in soy milk powder.

[0005] The inventors found in their research that when using ICP-MS for the determination of nickel content in soy milk powder, the following problems exist, and the reasons for the problems and solutions were found

[0006] 1. The results are on the high side, and the reasons are 58 Fe + and 40 Ca 18 O + interference.

[0007] Milk has a high calcium content while beans have a high iron content. Mass spectrometry usually detects the isotope with the highest natural abundance and then obtains the element content by weighting with the natural abundance (weighting is not required when using the internal standard method). After the soy milk sample is ionized, iron and calcium will form 58 Fe + and 40 Ca 18 O + These two ions and the 58 Ni with the highest natural abundance form the 58 Ni +Having the same mass-to-charge ratio and a relatively large content, exceeding the filtering capacity of ICP-MS, it is identified by ICP-MS (which identifies substances by mass-to-charge ratio) as 58 Ni + 。

[0008] 2. The linearity of the results is poor and the fluctuation is large. The reason is non-mass spectrometry interference.

[0009] Non-mass spectrometry interference is mainly manifested as the signal drift of the element to be measured caused by changes in the experimental environment and deposition of salts at the instrument cone, as well as signal enhancement or signal suppression and other offsets of certain components in the sample matrix to the element to be measured.

[0010] 3. It is difficult to put the sample into the digestion tube. The reason is that the digestion tube is too thin and the soy milk powder is easy to stick to the wall of the digestion tube and the weighing paper.

[0011] Currently, in sample digestion, the most efficient is the super microwave digestion instrument. It uses less acid, does not require acid removal after digestion, and has many steps that take a long time less. Its digestion tube is a slender tube. When pouring the soy milk sample, it is easy to stick to the wall of the digestion tube, resulting in the soy milk sample on the wall of the digestion tube (which cannot be soaked by the digestion solution) not being digested. In addition, part of the soy milk powder sticks to the weighing paper, resulting in inaccurate results. In addition, the soy milk powder sticking to the wall of the digestion tube is very difficult to clean. Summary of the Invention

[0012] The present invention provides a method for accurately determining the nickel content in soy milk powder with high efficiency.

[0013] The technical problem to be solved is that when measuring the nickel content in soy milk powder by graphite furnace atomic absorption spectrometry, the measurement accuracy and linear range cannot meet the export requirements of soy milk powder. When measuring the nickel content in soy milk powder by ICP-MS, the results are on the high side, the linearity is poor, the fluctuation is large, and it is difficult to put the sample into the digestion tube.

[0014] To solve the above technical problems, the present invention adopts the following technical solution: A method for accurately determining the nickel content in soy milk powder with high efficiency, comprising the following steps:

[0015] Step 1: Weigh the sample and put it into the digestion tube, digest the soy milk powder sample with a super microwave digestion instrument, and then dilute and make up the volume of the digested sample to obtain a test solution; Take the same amount of digestion solution as that during digestion, dilute and make up the volume to the same volume as the test solution, and record it as a blank solution;

[0016] Step 2: Select a nickel isotope and record it as the isotope to be measured;

[0017] Step 3: Use an inductively coupled plasma mass spectrometer to measure the content of the isotope to be measured in the soy milk powder sample by the internal standard method.

[0018] Further, in step two, ions with the same mass number as the isotope to be measured are denoted as interfering ions, and the isotope to be measured needs to meet the following conditions: elements in the nutritional ingredient list of soy milk powder shall not generate interfering ions after ionization.

[0019] Further, the isotope to be measured is 60 Ni; concentrated nitric acid is used as the digestion solution in step one, and ultrapure water is used for dilution and volume fixation; rhodium is used as the internal standard element in step three.

[0020] Further, step three includes the following sub-steps:

[0021] Step 3.1: Prepare a nickel standard working solution and an internal standard solution, and measure the nickel standard curve of nickel with an inductively coupled plasma mass spectrometer.

[0022] Step 3.2: Measure the CPS values of the isotope to be measured in the test solution and the blank solution respectively with an inductively coupled plasma mass spectrometer, and then find out the mass concentrations of nickel elements in the test solution and the blank solution on the nickel standard curve respectively according to the CPS values.

[0023] Step 3.3: Obtain the content of nickel element in the soy milk powder sample.

[0024] Further, in step 3.2, after the test solution or the blank solution is ionized, first use helium gas to remove polyatomic ions in the octapole collision reaction cell, then filter out ions with mass-to-charge ratios different from the isotope to be measured with a quadrupole mass analyzer, and finally obtain the CPS value of the isotope to be measured. The deflection voltage in the octapole collision reaction cell is -18 V and the cut-off voltage is 180 V.

[0025] Further, in step one, first use the orthogonal test method to obtain the combination of digestion parameters that maximizes the nickel recovery rate, and then carry out digestion; the digestion parameters include the dosage of the digestion solution, the digestion temperature, and the digestion time.

[0026] Further, before using the inductively coupled plasma mass spectrometer in step three, it is debugged with a tuning solution to optimize the operating parameters, and the operating parameters include the position of the torch, the inductively coupled plasma parameters, the mass spectrometer parameters, and the measurement parameters.

[0027] Further, the following method is adopted in step one to weigh the sample and put it into the digestion tube: prepare rice paper without nickel, cut it into square pieces of the same size for standby - weigh the soy milk powder sample with the rice paper square as the weighing paper - wrap the soy milk powder sample with the rice paper square and roll it into a slender small packet, and throw the whole small packet into the digestion tube;

[0028] In Step 1, the blank solution is prepared as follows: While preparing the digestion solution, take an equal amount of the digestion solution as that during digestion and a square of rice paper and put them into the digestion tube for digestion. Then, dilute and make up the volume of the digested rice paper to the same volume as the test solution to be detected.

[0029] Further, in Step 1, the nickel-free rice paper is prepared as follows: Weigh 10 parts by mass of soluble starch of analytical reagent grade and 1 - 4 parts by mass of gelatin of analytical reagent grade, add 60 - 100 parts by mass of ultrapure water and mix them into a suspension. Boil the suspension at 60 - 80 °C for 10 - 20 minutes to obtain a paste. Then, brush the paste onto a polytetrafluoroethylene plate, and after drying, peel it off to obtain the nickel-free rice paper.

[0030] Further, in Step 1, the sample is weighed and put into the digestion tube as follows:

[0031] Take a disposable plastic dropper, cut the rubber head of the disposable plastic dropper obliquely, and cut off the tip of the disposable plastic dropper. Place the disposable plastic dropper on the electronic balance and zero the electronic balance. Use the cut rubber head of the disposable plastic dropper to scoop up the soymilk powder sample and put it on the electronic balance for weighing. Then, put the lower end of the disposable plastic dropper into the digestion tube so that the soymilk powder sample falls into the digestion tube.

[0032] Compared with the prior art, the method for determining the nickel content in soymilk powder of the present invention has the following beneficial effects:

[0033] In the present invention, by selecting 60 Ni that is not easily affected by iron and calcium in soymilk powder as the isotope to be measured, first removing polyatomic ions with helium gas in the octopole collision reaction cell, and then filtering out ions with different mass-to-charge ratios from the isotope to be measured with a quadrupole mass analyzer, the measurement result being too high is avoided;

[0034] In the present invention, by screening out the internal standard element rhodium with a high recovery rate when measuring the nickel content in soymilk powder, and then measuring the nickel content in soymilk powder by the internal standard method, the non-mass spectrometry interference is overcome, and the problems of poor linearity and large fluctuations in the results are avoided;

[0035] In the present invention, a super microwave digestion instrument is used for digestion. By using nickel-free rice paper as the weighing paper to weigh the soymilk powder sample and then digesting it together with the rice paper, not only is the sample easily put into the digestion tube, but also there is no situation where the soymilk powder sample is not digested because it adheres to the digestion tube wall or the weighing paper. The advantages of high efficiency of the super microwave digestion instrument are fully exerted. Description of the Drawings

[0036] Figure 1 It is a comparison chart of the recovery rates of alternative internal standard elements in the present invention;

[0037] Figure 2It is the nickel standard curve graph in the embodiments of the present invention; in the graph, the vertical coordinate is the ratio of the CPS value of the isotope to be measured to the CPS value of the internal standard element, and the horizontal coordinate is the mass concentration of nickel element in the liquid to be tested.

[0038] Figure 3 It is a schematic diagram of a cut disposable plastic dropper. Specific embodiments

[0039] In this application, the "content" of a certain substance in a solid refers to the mass of the substance divided by the total mass of the solid.

[0040] An efficient and accurate method for determining the nickel content in soymilk powder includes the following steps:

[0041] Step 1: Weigh the sample and put it into a digestion tube, digest the soymilk powder sample with a super microwave digester, and then dilute and make the volume constant for the digested sample to obtain the liquid to be tested; take the same amount of digestion solution as that during digestion, dilute and make the volume constant to the same volume as the liquid to be tested, and record it as the blank solution.

[0042] Step 2: Select a nickel isotope and record it as the isotope to be measured.

[0043] Step 3: Use an inductively coupled plasma mass spectrometer to measure the content of the isotope to be measured in the soymilk powder sample by the internal standard method.

[0044] In Step 2, the ion with the same mass number as the isotope to be measured is recorded as the interfering ion, and the isotope to be measured needs to meet the following conditions: the known components in the ingredient list of the soymilk powder shall not generate interfering ions after ionization.

[0045] The isotope to be measured is 60 Ni; concentrated nitric acid is used as the digestion solution in Step 1, and ultrapure water is used for dilution and volume constant; rhodium is used as the internal standard element in Step 3.

[0046] During the detection process of inductively coupled plasma mass spectrometry, the non-mass spectrometry interferences are mainly manifested as the signal drift of the element to be measured caused by the changes in the experimental environment and the deposition of salts at the instrument cone mouth, as well as the signal enhancement or signal suppression and other offsets of certain components in the sample matrix to the element to be measured. The experiment uses the internal standard correction method to eliminate the non-mass spectrometry interferences. According to the selection principle of the internal standard element, that is, stable in nature, similar ionization energy to the element to be measured, similar mass number to the element to be measured, large abundance, high sensitivity and small interference, three elements of germanium, rhodium and indium are selected as the alternative internal standard elements. Examine the recovery rates of these three elements, and the results are shown in Figure 1 , indicating that the recovery rate of rhodium element is the highest (if the recovery rate of the internal standard element is low, the result is inaccurate), so rhodium is used as the internal standard element to determine nickel element.

[0047] Step 3 includes the following sub-steps:

[0048] Step 3.1: Prepare the nickel standard working solution and the internal standard solution, and measure the nickel standard curve of nickel with an inductively coupled plasma mass spectrometer;

[0049] Step 3.2: Measure the CPS values of the isotopes to be detected in the test solution and the blank solution respectively with an inductively coupled plasma mass spectrometer, and then find out the mass concentrations of nickel elements in the test solution and the blank solution on the nickel standard curve respectively according to the CPS values;

[0050] Step 3.3: Obtain the nickel element content in the soy milk powder sample. That is, subtract the mass concentration of nickel element in the blank solution from the mass concentration of nickel element in the test solution, then multiply by the constant volume of the test solution, and divide by the mass of the soy milk powder sample.

[0051] In Step 3.2, after the test solution or the blank solution is ionized, first use helium gas to remove polyatomic ions in the octupole collision reaction cell, then filter out the ions with mass-to-charge ratios different from the isotopes to be detected with a quadrupole mass analyzer, and finally obtain the CPS value of the isotope to be detected. The deflection voltage in the octupole collision reaction cell is -18 V and the cut-off voltage is 180 V. These two voltages are also obtained by orthogonal experiments, which can ensure that the recovery rate of the isotope to be detected is close to 100%.

[0052] In Step 1, first use the orthogonal experiment method to obtain the digestion parameter combination that maximizes the nickel recovery rate, and then carry out digestion; the digestion parameters include the amount of digestion solution, digestion temperature, and digestion time.

[0053] Considering that concentrated nitric acid can decompose most foods safely and gently under closed conditions and has the advantages of being non-explosive and non-flammable, concentrated nitric acid is selected as the digestion acid in super microwave digestion. At the same time, the amount of acid, digestion temperature, and digestion time have a great influence on the digestion effect. Therefore, the key digestion conditions such as the amount of acid, digestion temperature, and digestion time in super microwave digestion are optimized.

[0054] The orthogonal experiment method is used to solve this multi-factor and multi-level experimental problem, and then optimize the digestion conditions. The orthogonal experiment method utilizes statistical principles to reasonably design the orthogonal experiment table, so as to obtain the optimal experimental scheme with as few experimental times as possible. According to the orthogonal experiment principle, an orthogonal table is selected, and the experiment is arranged according to three factors and four levels (see Table 1). The nickel recovery rate (the ratio of the measured value to the certified value of the standard substance) of the nickel quality control sample in the soymilk powder is used as an objective index to measure the digestion effect. At the same time, 3 repeated experiments are carried out under each digestion condition, and the average value is taken as the final result. The results of the orthogonal experiment are shown in Table 2. By performing the range analysis on the results of the orthogonal experiment, the results are shown in Table 3. It can be found that the influence degrees of the three experimental factors on the experimental results are in the order of acid dosage > digestion time > digestion temperature. Among them, the influence of the acid dosage on the experimental result is significantly greater than that of the digestion time and digestion temperature, and the difference in the influence of the latter two on the experimental result is not significant. Considering the overall influence of the three factors on the experimental results, and taking into account the economy, safety and experimental efficiency of the experiment, the optimal conditions for super microwave digestion are as follows: the dosage of concentrated nitric acid is 3 mL, the digestion temperature is 210 °C, and the digestion time is 30 min.

[0055] Table 1 Factors and levels of the orthogonal experiment for super microwave digestion

[0056]

[0057] Table 2 Orthogonal experiment scheme and results for super microwave digestion

[0058]

[0059] Table 3 Range analysis of the results of the orthogonal experiment for super microwave digestion

[0060]

[0061] After determining the digestion method and optimizing the digestion conditions, accurately weigh 0.5 g of the sample (accurate to 0.001 g) into a 15 mL digestion tube, add 3 mL of concentrated nitric acid, cover and let stand for 1 h, then put it into the super microwave digester and carry out digestion according to the optimized heating program (see Table 4). After digestion is completed and cooled, take it out, wash the digestion tube with a small amount of ultrapure water several times, transfer it to a 50 mL volumetric flask, dilute it to the mark with ultrapure water, mix well and set aside. At the same time, prepare the reagent blank.

[0062] Table 4 Heating program for super microwave digestion

[0063]

[0064] Before using the inductively coupled plasma mass spectrometer in Step 3, it is debugged with tuning solution to optimize the operating parameters, which include the position of the torch, inductively coupled plasma parameters, mass spectrometer parameters, and measurement parameters. The optimized operating parameters of ICP-MS are shown in Table 5.

[0065] Table 5 Operating Parameters of ICP-MS

[0066]

[0067] In Step 1, the following method can be used to weigh the sample and put it into the digestion tube: Prepare nickel-free rice paper, cut it into square pieces of the same size for standby - use the square rice paper as the weighing paper to weigh the soymilk powder sample - wrap the soymilk powder sample with the square rice paper and roll it into a slender small packet, and throw the whole small packet into the digestion tube;

[0068] If the above-mentioned rice paper is selected to assist in weighing and putting, then in Step 1, the following method is used to prepare the blank solution: While preparing the digestion solution, take the same amount of digestion solution as that during digestion and a square piece of rice paper and put them into the digestion tube for digestion, and then dilute and make the volume constant of the digested rice paper to be the same as that of the test solution.

[0069] In Step 1, the following method can be used to prepare nickel-free rice paper: Weigh 10 parts by mass of analytical reagent grade soluble starch and 1 - 4 parts by mass of analytical reagent grade gelatin, add 60 - 100 parts by mass of ultrapure water and mix them into a suspension, boil the suspension at 60 - 80 °C for 10 - 20 minutes to obtain a paste, then brush the paste onto a polytetrafluoroethylene plate (for easy peeling), and peel it off after drying to obtain nickel-free rice paper.

[0070] The reason for choosing rice paper here is that the rice paper melts immediately and will not affect the digestion process. Conventional paper digests too slowly. Note that metal foil must not be used to replace rice paper. Most metal foils consume a large amount of acid, which will cause serious adverse effects on the digestion results and there is a risk of explosion due to the generation of a large amount of gas. And for a small part of metal foils, such as aluminum / iron / gold foils, they will be passivated or do not react at all in concentrated nitric acid, resulting in inability to digest smoothly.

[0071] The amount of gelatin here cannot be too much or too little. When it is more than 4 parts by mass, the rice paper digests slowly and is difficult to peel off. When it is less than 1 part by mass, the rice paper is very brittle. This method is a makeshift measure with low efficiency, and the formula may not necessarily be the optimal one. In actual use of the film machine, the formula can be changed according to needs or film can be made with the film machine, as long as it is ensured that the rice paper can wrap the sample without breaking and is easy to digest, and the rice paper does not contain nickel.

[0072] In Step 1, the following method is used to weigh the sample and put it into the digestion tube:

[0073] Such asFigure 3 , take a disposable plastic dropper, cut the rubber head of the disposable plastic dropper obliquely, and cut off the tip of the disposable plastic dropper. Place the disposable plastic dropper on an electronic balance and zero the electronic balance. Use the cut rubber head of the disposable plastic dropper to scoop up the soy milk powder sample and place it on the electronic balance for weighing. Then put the lower end of the disposable plastic dropper into the digestion tube so that the soy milk powder sample falls into the digestion tube.

[0074] Example 1:

[0075] The main reagents and reference materials involved in this method are as follows:

[0076] 1) Concentrated nitric acid: Trace Metal grade, Fisher Chemical Company;

[0077] 2) Nickel standard stock solution: GSBG62022 - 90, 1000 μg / mL, National Center for Steel Materials Testing;

[0078] 3) Multi - element internal standard stock solution: Scandium, Germanium, Indium, Bismuth, Yttrium, Terbium, Rhodium, with a concentration of 100 μg / mL each, National Center for Analysis and Testing of Non - ferrous Metals and Electronic Materials;

[0079] 4) Mass spectrometry tuning solution: Cerium, Cobalt, Lithium, Titanium, Yttrium, with a concentration of 10 μg / mL each, Agilent Technologies;

[0080] 5) Nickel standard working solution (1000 μg / L): Accurately pipette 1 mL of nickel standard stock solution, dilute and make up to 100 mL with 5% nitric acid, then pipette 10 mL of the above solution and dilute and make up to 100 mL with 5% nitric acid;

[0081] 6) Mixed internal standard working solution (1 μg / mL): Accurately pipette 1 mL of multi - element internal standard stock solution, dilute and make up to 100 mL with 5% nitric acid;

[0082] 7) The test water used is ultrapure water;

[0083] 8) Commercially available soy milk powder samples and nickel quality control samples in soy milk powder (ZKQC5210, Henan R & D Center for Reference Materials);

[0084] The main instruments and equipment involved in this method are as follows:

[0085] 5) Inductively coupled plasma mass spectrometer: 7700 X, Agilent Technologies, USA;

[0086] 6) Super microwave digestion instrument: Ultrawave, LabTech;

[0087] 7) Ultrapure water treatment system: Milli - Q type, USA;

[0088] 8) Electronic balance: Sartorius 1601MP8 type made in Germany.

[0089] Weigh 0.5 g of the sample into a 15 mL digestion tube, add 3 mL of concentrated nitric acid, cover it and let it stand for 1 h, then put it into a super microwave digestion instrument and carry out digestion according to the heating program shown in Table 4. After digestion is completed and cooled, take it out, wash the digestion tube with a small amount of ultrapure water several times, transfer it to a 50 mL volumetric flask, dilute it to the mark with ultrapure water, mix well and reserve for use. At the same time, prepare a blank solution.

[0090] Prepare nickel standard working solutions with concentrations of 0, 10.0, 20.0, 30.0, 40.0, 50.0 μg / L and an internal standard solution with a concentration of 20 ppb, and measure the nickel standard curve of nickel with an inductively coupled plasma mass spectrometer; the nickel standard curve is shown in Figure 2 ; among which the correlation coefficient is 0.9999, and the linear relationship is good in the range of 0 - 50 μg / L.

[0091] Measure the CPS values of the isotopes to be detected in the test solution and the blank solution respectively with an inductively coupled plasma mass spectrometer, and then find out the mass concentrations of nickel elements in the test solution and the blank solution on the nickel standard curve respectively according to the CPS values.

[0092] Sensitivity of measurement results:

[0093] Continuously measure the reagent blank solution 11 times and calculate the standard deviation. The concentration value corresponding to 3 times the standard deviation is 0.02 μg / L. Combining the sample weighing amount and the constant volume volume, calculate the detection limit of this method to be 0.002 mg / kg, which is far better than the detection limit of 0.02 mg / kg for the determination of nickel in foods by graphite furnace atomic absorption spectrometry in GB5009.138 - 2017. Therefore, this method has higher sensitivity.

[0094] Precision of measurement results:

[0095] Randomly select 1 soymilk powder sample to measure the background value of nickel element. Then add nickel element standard solutions at 3 concentration levels of low, medium and high. Each concentration is done in 6 parallels, and ICP - MS is used for the determination of nickel. Calculate the spike recovery rate and precision. The results are shown in Table 6. The spike recovery rate of ICP - MS is between 94.5% - 100.6%, and the RSD is between 1.8% - 2.3%. The precision and recovery rate of the method both meet the detection requirements of GB / T 27404 - 2008 "Laboratory Quality Control Specification for Physical and Chemical Testing of Foods". Note that for inorganic mass spectrometry, the recovery rate is between 90 - 110%, and 100% is the best.

[0096] Table 6 Spike Recovery Rate and Precision Results (n = 6)

[0097]

[0098] Accuracy of measurement results:

[0099] Using the nickel quality control sample in soymilk powder, digestion and determination were carried out in the same method as the soymilk powder sample. The calibrated value and measured value of nickel element in the quality control sample are shown in Table 7. The measured value is within the calibrated range, indicating its high accuracy.

[0100] Table 7 Measurement results of nickel quality control sample in soymilk powder

[0101]

[0102] It can be seen from the result analysis that this method has the advantages of high sensitivity, wide linear range, high precision, high accuracy, etc.

[0103] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. An efficient and accurate method for determining the nickel content in soy milk powder, characterized in that: It includes the following steps: Step 1: Weigh the sample and put it into a digestion tube. Digest the soymilk powder sample with a super microwave digestion instrument, and then dilute and make up the volume of the digested sample to obtain the test solution. Take the same amount of digestion solution as that during digestion, dilute and make up the volume to the same volume as the test solution, and record it as the blank solution. Step 2: Select a nickel isotope and record it as the isotope to be measured. Step 3: Use an inductively coupled plasma mass spectrometer to measure the content of the isotope to be measured in the soymilk powder sample by the internal standard method. In Step 1, the sample is weighed and put into the digestion tube by the following method: Prepare nickel-free rice paper, cut it into small squares of the same size for standby. Weigh the soymilk powder sample with the rice paper square as the weighing paper. Wrap the soymilk powder sample with the rice paper square and roll it into a slender small packet, and then throw the whole small packet into the digestion tube. In Step 1, the blank solution is prepared by the following method: While preparing the digestion solution, take the same amount of digestion solution as that during digestion and a rice paper square and put them into the digestion tube for digestion, and then dilute and make up the volume of the digested rice paper to the same volume as the test solution.

2. The method for determining the nickel content in soy milk powder according to claim 1, which is efficient and accurate, is characterized in that: In Step 2, the ion with the same mass number as the isotope to be measured is recorded as the interfering ion, and the isotope to be measured needs to meet the following conditions: The elements in the nutrition label of the soymilk powder shall not generate interfering ions after ionization.

3. An efficient and accurate method for determining the nickel content in soy milk powder according to claim 1, characterized in that: The isotope to be measured is 60 Ni; in the first step, concentrated nitric acid is used as the digestion solution, and ultrapure water is used for dilution and volume fixation; in the third step, rhodium is used as the internal standard element.

4. An efficient and accurate method for determining the nickel content in soy milk powder according to claim 1, characterized in that: Step 3 includes the following sub-steps: Step 3.1: Prepare a nickel standard working solution and an internal standard solution, and use an inductively coupled plasma mass spectrometer to measure the nickel standard curve of nickel. Step 3.2: Use an inductively coupled plasma mass spectrometer to measure the CPS values of the isotope to be measured in the test solution and the blank solution respectively, and then find out the mass concentrations of nickel elements in the test solution and the blank solution on the nickel standard curve respectively according to the CPS values. Step 3.3: Calculate the nickel element content in the soymilk powder sample.

5. The method for determining the nickel content in soy milk powder according to claim 4, characterized in that: In Step 3.2, after the test solution or the blank solution is ionized, first remove the polyatomic ions with helium gas in the octapole collision reaction cell, then filter out the ions with different mass-to-charge ratios from the isotope to be measured with a quadrupole mass analyzer, and finally calculate the CPS value of the isotope to be measured. The deflection voltage in the octapole collision reaction cell is -18 V and the cut-off voltage is 180 V.

6. The method for determining the nickel content in soy milk powder according to claim 1, which is efficient and accurate, is characterized in that: In Step 1, first use the orthogonal test method to obtain the combination of digestion parameters that maximizes the nickel recovery rate, and then carry out digestion. The digestion parameters include the dosage of the digestion solution, the digestion temperature, and the digestion time.

7. An efficient and accurate method for determining the nickel content in soy milk powder according to claim 1, characterized in that: Before using the inductively coupled plasma mass spectrometer in Step 3, it is debugged with a tuning solution to optimize the operating parameters. The operating parameters include the position of the torch, the inductively coupled plasma parameters, the mass spectrometer parameters, and the measurement parameters.

8. The method for determining the nickel content in soy milk powder according to claim 1, which is efficient and accurate, is characterized in that: In Step 1, the nickel-free rice paper is prepared by the following method: Weigh 10 parts by mass of analytical reagent grade soluble starch and 1 - 4 parts by mass of analytical reagent grade gelatin, add 60 - 100 parts by mass of ultrapure water and mix them into a suspension. Boil the suspension at 60 - 80 °C for 10 - 20 minutes to obtain a paste, and then brush the paste onto a polytetrafluoroethylene plate. After drying, peel it off to obtain the nickel-free rice paper.

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