Method for detecting lead in high-salt food

Abstract

The invention belongs to the technical field of food detection, and provides a method for detecting lead in high-salt food. The method comprises the following steps: mixing a sample solution, an extracting solvent and a complexing agent DDTP to complete extraction; separating the extract, and detecting the lead content by graphite furnace atomic absorption spectrometry; the extraction solvent is undecanol. The method disclosed by the invention can be used for pre-enriching and extracting lead in high-salt foods such as hotpot condiments, chicken essence, soy sauce and seasonings, and the extraction solvents are all green solvents, so that the method has good ecological friendliness. The determination of the lead is not interfered and influenced under the coexistence of metal cations such as potassium, sodium, calcium, magnesium, iron, zinc, copper and manganese, and the method has very high tolerance limit and can accurately determine the lead in the high-salt food. The relative deviation of the determination results of the method and the ICP-MS method has no significant difference, and the detection limit is lower.

Description

Technical Field

[0001] The invention belongs to the technical field of food detection, and in particular relates to a method for detecting lead in high-salt food. Background Art

[0002] The disclosure of this background information is intended to enhance understanding of the general background of the invention and should not necessarily be taken as an acknowledgment or any form of suggestion that this information constitutes the prior art already known to a person skilled in the art.

[0003] Heavy metals enter the environment and organisms through natural sources and human activities. Due to their non-biodegradable nature, high solubility in water, and potential for accumulation in different parts of the human body, they seriously affect human health. Lead is one of the most toxic heavy metals, which can affect the nervous system, blood system, and reproductive system, leading to growth retardation, kidney cancer, intellectual impairment, and behavioral changes. Therefore, the determination of heavy metal lead in food is of great significance. The commonly used analytical techniques for the determination of heavy metal lead are graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS). ICP-MS is not widely used due to its high price and high cost. GFAAS has the advantages of low price, low cost consumption, good selectivity, high sensitivity, good accuracy, and a wide range of applications. It is the most commonly used method for the determination of lead in food. However, the food matrix is ​​complex, especially high-salt food. The presence of sodium chloride will cause serious interference to the determination of lead. On the one hand, in a high temperature and high salt system, lead chloride with a melting point of 501°C and a boiling point of 950°C is easily formed, causing the loss of the target analyte. On the other hand, the absorption of sodium chloride molecules will overlap with the 283.3nm wavelength of lead measured by graphite furnace, forming a positive deviation. Therefore, the presence of sodium chloride in the matrix will have a great impact on the accuracy of lead determination. The determination of lead in high-salt foods using GFAAS has always been a difficult problem in the analytical testing industry. The determination of lead in high-salt foods often uses pre-treatment methods such as adding matrix modifiers, matrix matching, direct dilution, and solid phase extraction desalting. However, although relying solely on matrix modifiers can increase the atomization temperature of the graphite furnace, the interference caused by the molecular absorption spectrum of chloride still exists, and the interference of sodium chloride is not fundamentally eliminated; the matrix matching method pollutes the instrument and is not suitable for batch sample detection; the direct dilution method will dilute the content of heavy metal elements in the sample while reducing the background interference of high-salt samples, affecting the accuracy of the determination results, and the solid phase extraction desalting operation is cumbersome, time-consuming, and easy to cause secondary contamination of the solution. Therefore, there are great difficulties and disadvantages in removing sodium chloride during instrument determination, so the best interference removal method should be to remove sodium chloride during the pre-treatment process to ensure efficient, accurate and complete extraction of lead.

[0004] Common methods for pre-enrichment and extraction of heavy metals in food include solid phase extraction (SPE), liquid-liquid extraction (LLE), cloud point extraction (CPE), solidified floating organic drop microextraction (SFODME), etc. Traditional extraction methods of elements such as solid phase extraction, liquid-liquid extraction, and cloud point extraction mostly use hazardous chemical reagents (such as concentrated sulfuric acid, hydrofluoric acid, hydrochloric acid, organic solvents, etc.), which pose a serious threat to the health of analysts and may also cause damage to the environment. In order to achieve the goal of developing green analytical chemistry, developing more cost-effective and green element extraction methods is an important task for analytical chemistry researchers. In recent years, the research direction of analytical chemistry has been moving towards economy, simplicity, reducing sample preparation steps, and reducing or replacing harmful organic solvents.

[0005] As a green, environmentally friendly, simple and economical heavy metal extraction and enrichment method, SFODME has attracted more and more attention and has been reasonably applied. The technology is based on the use of trace amounts of low-density and water-immiscible organic extraction solvents to extract analytes from aqueous media. Since the melting point of the extraction solvent is near room temperature (10-30°C), it can be solidified by cooling, and the solidified solvent is separated and dissolved for the quantification of the analyte. SFODME has the advantages of low cost, simple operation, large enrichment coefficient, short extraction time, and low extraction solvent usage, and has been widely used in the determination of various heavy metals. At present, there are few studies on the determination of heavy metals in food using SFODME technology, and there is no research on the determination of lead in high-salt foods. Summary of the invention

[0006] In view of the problems in the prior art, the present invention provides a method for detecting lead in high-salt food, which is simple to operate, has good accuracy, high extraction efficiency, low cost and high enrichment coefficient.

[0007] To achieve the above purpose, the present invention adopts the following technical solution.

[0008] A method for detecting Pb in food 2+ The method comprises the following steps: (1) Add 80 μL of DDTP and 50 μL-100 μL of undecanol to every 10 mL of sample solution, mix and ultrasonicate to obtain a mixed solution; separate the mixed solution into an organic phase and an aqueous phase, cool the organic phase to solidify; dissolve the organic phase with 150 μL of methanol to obtain a test solution; (2) The test solution is subjected to graphite furnace atomic absorption spectrometry. The analytical line of lead is 283.3 nm. The slit width is 0.7 nm. Zeeman effect background correction is used. The heating program of the graphite furnace is: Stage 1: temperature 100-110°C, heating time 1-3s, duration 15-20s; Stage 2: temperature 130-140°C, heating time 15-20s, duration 30-35s; Stage 3: temperature 900-1100°C, heating time 10-15s, duration 20-25s; Stage 4: temperature 2100-2250℃, heating time 1-5s, duration 5-8s; Stage 5: temperature 2250-2450℃, heating time 1-3s, duration 3-5s.

[0009] In step (1), the stratification method can be static or centrifugal, preferably centrifugal. The centrifugal condition is 8000 rpm for 2 min.

[0010] Preferably, in step (1), the sample solution preparation method comprises: accurately weighing 0.5 g of sample, adding 4 mL of nitric acid and 2 mL of water into a digestion tube, microwave digestion at 200° C. for 30 min, transferring to a 25 mL tube and fixing the volume to the mark after cooling, and a selective dilution step.

[0011] Among the foods, K + The content of Ca is not higher than 2500mg / kg, 2+ The content of Mg is not higher than 625mg / kg, 2+ The content of Cu is not higher than 250mg / kg, 2+ The content of Fe is not higher than 125mg / kg, 3+ The content of Mn is not higher than 250mg / kg, 2+ The content of Zn is not higher than 25mg / kg, 2+ The content is not higher than 25mg / kg.

[0012] The above method can also detect Pb in high-salt foods 2+ The Na content of high-salt foods + The content is not higher than 125g / kg.

[0013] Preferably, the high-salt food is table salt, chicken essence, soy sauce, salted kelp, and hot pot base.

[0014] The present invention has the following advantages: The present invention provides a method for detecting lead in high-salt food based on suspended organic droplet solidification microextraction and graphite furnace atomic absorption spectrometry, using undecanol as an extraction solvent and diethyl dithiophosphate (DDTP) as a complexing agent, which has the characteristics of complexing lead and not complexing sodium in a high-salt medium, thereby achieving lead enrichment and removal of sodium chloride interference, and improving the accuracy and stability of detection. At the same time, the SFODME technology has a high enrichment multiple, which can effectively offset the inaccurate detection results caused by dilution. The detection limit of this method is very low, comparable to ICP-MS. Common liquid phase microextraction requires a dispersant, but in this method, since the target complex has good solubility in undecanol and can be better extracted into undecanol, no dispersant is needed in this method, and the method has the advantages of simple operation, good accuracy, high extraction efficiency, low cost, use of green solvents and good eco-friendliness, thereby effectively solving the problem of lead determination in high-salt food, and having wide application in the food industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is the effect of DDTP dosage on extraction recovery; Figure 2 The effect of the amount of extraction solvent on the extraction recovery rate; Figure 3 The effect of pH on extraction recovery. DETAILED DESCRIPTION

[0016] The present invention will be further described below in conjunction with embodiments and drawings, but the present invention is not limited by the following embodiments.

[0017] Example 1 Establishment and verification of detection method 1. Method establishment and optimization (1) SFODME process Prepare 0.5µg / L Pb (II) ion solution, take 10mL of the prepared solution, add 80µL DDTP, shake well and then add a certain volume of extraction solvent. The extraction solvent is dispersed in the sample solution in the form of fine droplets, forming a turbid solution. Ultrasonication is performed at room temperature for 5 minutes, and the lead ions form a complex with DDTP and are extracted into the organic phase. The obtained solution is centrifuged at 8000rpm for 2 minutes. After centrifugation, due to the density difference between the aqueous phase and the extraction solvent, the fine droplets of the organic phase float on the top of the test tube. Place it in an ice bath for 10 minutes, the organic phase solidifies, and the liquid is discarded. 150μL of methanol is added to the solidified organic phase to dissolve it and measure it on the machine.

[0018] (2) Instrument working conditions Lead analysis line: 283.3 nm; slit width: 0.7 nm; Zeeman effect background correction. The heating program of the graphite furnace is shown in Table 1.

[0019] Table 1 Heating program of graphite furnace Based on the above conditions, the selection, composition and dosage of complexing agent and extractant were optimized with extraction recovery (ER) as the index.

[0020] ER is the amount transferred to the DES phase n fin Total amount of analyte n 0 ratio, EF is the analyte concentration C in the DES phase fin and the initial concentration C of the analyte in the aqueous solution 0 The ratio of V aq and V fin and are the volumes of the aqueous phase and the organic phase after solidification, respectively.

[0021] (3) Selection and dosage of complexing agent DDTP is a lead ion ligand with a certain specificity, which can form a stable complex with Pb(II) with a chelation ratio of 2:1. In order to study the effect of DDTP concentration, the effect of DDTP addition in the range of 10-200 μL in 10 mL Pb (II) ion solution on the complexation and extraction of the target ion was investigated. The results are shown in Figure 1 As shown in the figure: In the SFODME procedure, as the amount of DDTP added increases, the extraction efficiency of lead ions increases accordingly. When the amount added is 80 μL, the extraction efficiency of lead ions is the highest. As the concentration of DDTP increases further, the extraction efficiency decreases instead, because the high concentration of the complex will saturate the organic phase, causing it to be extracted into the aqueous phase. Therefore, the optimal amount of DDTP in 10 mL of solution is 80 μL.

[0022] (4) Selection, composition and dosage of extraction solvent The characteristics of the ideal extraction solvent in the SFODME extraction method are: melting point close to room temperature, low volatility, good stability during extraction, density lower than water, immiscible with water, high extraction efficiency, high selectivity for the analyte, and no interference with the extraction process for the determination of the analyte. By comparing undecanol (melting point 13-15℃) and dodecanol (melting point 22-24℃), the effects of the two solvents on the extraction effect were studied in the range of 10-200μL. The results are shown in Figure 2As shown: the extraction efficiency of undecanol is higher than that of dodecanol. Therefore, undecanol is selected as the extraction solvent. The extraction efficiency is the best at 50-100 μL, which is basically unchanged. As the extraction volume increases, the extraction efficiency decreases. Therefore, the volume of undecanol is selected to be 50 μL.

[0023] (5) Effect of pH The pH value of the solution is an important factor affecting the extraction efficiency of metal ions because it is directly related to the formation of metal complexes. In order to study the effect of pH on the results, after adding DDTP to the sample solution, hydrochloric acid and sodium acetate were used to adjust the pH of the solution. The effect of pH on the extraction efficiency was investigated in the range of 1.0-7.0. The extraction solvent and volume were limited to 50 μL of undecanol in the test. The results are shown in Figure 3 As shown: The absorbance of the target analyte in the pH range of 1.0-4.0 is almost constant and reaches a maximum value. As the pH value further increases, the absorbance decreases. Because the DDTP ligand (chelating agent) itself has a strong acidic property, it forms a complex with lead in an acidic solution and retains the molecular form of the complex. As the pH value of the solution increases, the formed complex is ionized, its solubility in the aqueous solution increases, and the extraction efficiency decreases. Since the sample needs to be digested before the microextraction procedure, the pH of the solution is between 1.0-2.0, so in order to prevent contamination, no acid or buffer solution is used to adjust the pH value.

[0024] 2. Specificity Any ion that changes the lead ion extraction recovery rate by more than ±5% is called an interfering ion, and an absorbance error of 5.0% is considered the tolerance limit. The tolerance limit is calculated by the following formula: Tolerance limit = [amount of interfering ions] / [amount of analyte]. The specific detection method is as follows: (1) Preparation of sample solution Preparation containing different concentrations of K + 、Na + , Ca 2+ Mg 2+ , Cu 2+ , Fe 3+ , Mn 2+ 、Zn 2+ The 0.5 µg / L Pb(II) ion solution with the same coexisting ions was analyzed according to the SFODME procedure established in 1.

[0025] (2) Testing process Take 10mL of the prepared sample solution, add 80µL DDTP, shake well and then add 50μL undecyl alcohol. Undecyl alcohol is dispersed in the sample solution in the form of fine droplets to form a turbid solution. Ultrasonication is performed at room temperature for 5min. Lead ions form a complex with DDTP and are extracted into the organic phase. The obtained solution is centrifuged at 8000rpm for 2 minutes. After centrifugation, due to the density difference between the aqueous phase and the extraction solvent, the fine droplets of the organic phase float on the top of the test tube. Place it in an ice bath for 10min, the organic phase solidifies, and the liquid is discarded. The solidified organic phase is dissolved with 150μL methanol and used for measurement on the machine. The analytical line of lead: 283.3nm; slit width: 0.7nm; Zeeman effect background correction is used. The heating program of the graphite furnace is shown in Table 1.

[0026] Table 2 Study of coexisting ions The results are shown in Table 2. + 、Na + , Ca 2+ Mg 2+ , Cu 2+ , Fe 3+ , Mn 2+ 、Zn 2+ The recovery rate in the presence of coexisting ions was maintained at 95%-105%, so this method has a high anti-interference ability. Under the optimal conditions, the interference of coexisting ions on the recovery rate of Pb(II) can be ignored, indicating that the established method is capable of analyzing the target elements in high-salt foods.

[0027] 3. Linear range Prepare Pb (II) ion solutions of different concentrations, take 10mL of the prepared series of concentration solutions, add 80μLDTP respectively, shake well and then add 50μL undecanol, and ultrasonicate for 5min at room temperature. Centrifuge the obtained solution at 8000rpm for 2min, place it in an ice bath for 10min, solidify the organic phase, discard the liquid, add 150μL methanol to the solidified organic phase, dissolve it and measure it on the machine, and the rest of the detection process is as described above.

[0028] The results showed that there was a good linear relationship in the range of 0.15-100µg / L, with a correlation coefficient of 0.99841. The detection limit (LOD) of the method was 0.031μg / L, which was calculated by dividing 3 times the standard deviation of 11 blank measurements by the slope of the calibration curve. The limit of quantification was 0.093μg / L and the enrichment factor (EF) was 52.3.

[0029] 4. Precision Spiked recovery experiments at different concentration levels and six parallel measurements were carried out on samples such as hot pot soup base, salted kelp, chicken essence, and soy sauce. The recovery rate and relative standard deviation (RSD) were calculated to evaluate the accuracy and precision of the method.

[0030] Prepare the sample solution as follows: weigh 0.5g of sample (accurate to 0.0001g), add 4mL nitric acid and 2mL water to a digestion tube, microwave digest at 200℃ for 30min, transfer to a 25mL centrifuge tube after cooling, and dilute to the mark with ultrapure water; take 2mL of the above solution and dilute to 10mL with ultrapure water to obtain the sample solution. Take 10mL of the prepared sample solution, add 50μL of 100μg / L lead standard solution to the solution, and the rest of the detection process is as described above.

[0031] Table 3 SFODME precision experiment The experimental results are shown in Table 3. The results show that the recovery rate of SFODME is 86.2%-114.0%, and the RSD is 2.33%-4.51%, which are in line with the methodological requirements.

[0032] 5. Accuracy Hot pot base, salted kelp, chicken essence, soy sauce and other samples were determined by GFAAS determination after direct dilution without extraction and ICP-MS, and the results were compared with those by SFODME-GFAAS method.

[0033] Table 4 Comparison of different testing methods for four high-salt foods The results are shown in Table 4. The direct dilution method has significant differences from the other two methods, while the relative deviation (RE) of the SFODME-GFAAS method and the ICP-MS method is 4.32%-6.17%, indicating that both methods have good accuracy and are comparable to the ICP-MS method, and there is no significant difference between the two methods. According to GB 500.12-2023 "Determination of Lead in Food in National Food Safety Standard", the quantitative limit of inductively coupled plasma mass spectrometry (ICP-MS method) is 0.05 mg / kg and the detection limit is 0.02 mg / kg. Due to the presence of pre-enrichment treatment, the detection limit of the SFODME-GFAAS method is lower (the quantitative limit is 0.93 µg / kg and the detection limit is 0.31 µg / kg), which has a very outstanding advantage.

[0034] Example 2 Detection of lead ion content in rice, wheat flour and cakes 1. SFODME process Weigh 0.5 g of rice, wheat flour, and cake samples (accurate to 0.0001 g) respectively, add 2 mL of nitric acid and 3 mL of water into a digestion tube, microwave digest at 200 °C for 30 min, transfer to a 25 mL centrifuge tube after cooling, and dilute to the mark with ultrapure water to obtain a sample solution.

[0035] Take 10mL of the prepared sample solution, add 50μL of 100μg / L lead standard solution to the solution, then add 80μLDTP, shake well and then add 50μL of undecanol. Undecanol is dispersed in the sample solution in the form of fine droplets, forming a turbid solution. Ultrasonication is performed at room temperature for 5min, and lead ions form a complex with DDTP and are extracted into the organic phase. The obtained solution is centrifuged at 8000rpm for 2min. After centrifugation, due to the density difference between the aqueous phase and the extraction solvent, the fine droplets of the organic phase float on the top of the test tube. Place it in an ice bath for 10min, the organic phase solidifies, and the liquid is discarded. The solidified organic phase is dissolved with 150μL methanol and used for measurement on the machine.

[0036] 2. Graphite furnace atomic absorption spectrometry Lead analysis line: 283.3nm; slit width: 0.7nm; Zeeman effect background correction is used. The heating program of the graphite furnace is shown in the following table: Table 5 Heating program The determination results showed that the recoveries of lead in rice, wheat flour and pastries were 92.4%-95.7%, 91.2%-97.1% and 93.0%-102.3%, respectively, indicating that the SFODME-GFAAS method is also applicable to common foods.

[0037] Example 3 Detection of lead ion content in salt 1. SFODME process Weigh 0.5 g of salt sample (accurate to 0.0001 g), add 4 mL of nitric acid and 2 mL of water into a digestion tube, digest at 200 °C for 30 min, transfer to a 25 mL centrifuge tube after cooling, and dilute to the mark with ultrapure water; take 2 mL of the above solution and dilute to 10 mL with ultrapure water to obtain a sample solution.

[0038] Take 10mL of the prepared sample solution, add 50μL of 100μg / L lead standard solution to the solution, then add 80μLDTP, shake well and then add 50μL of undecanol. Undecanol is dispersed in the sample solution in the form of fine droplets, forming a turbid solution. Ultrasonication is performed at room temperature for 5min, and lead ions form a complex with DDTP and are extracted into the organic phase. The obtained solution is centrifuged at 8000rpm for 2min. After centrifugation, due to the density difference between the aqueous phase and the extraction solvent, the fine droplets of the organic phase float on the top of the test tube. Place it in an ice bath for 10min, the organic phase solidifies, and the liquid is discarded. The solidified organic phase is dissolved with 150μL methanol and used for measurement on the machine.

[0039] 2. Graphite furnace atomic absorption spectrometry Lead analysis line: 283.3nm; slit width: 0.7nm; Zeeman effect background correction is used. The heating program of the graphite furnace is shown in the following table: Table 6 Heating program The results showed that the recovery rate of lead in table salt was 86.1-91.3%, indicating that the SFODME-GFAAS method is also applicable to table salt.

[0040] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A method for detecting Pb in food 2+ The method is characterized in that The following steps are involved: (1) Add 80 μL of DDTP and 50 μL-100 μL of undecanol to every 10 mL of sample solution, mix and ultrasonicate to obtain a mixed solution; separate the mixed solution into an organic phase and an aqueous phase, cool the organic phase to solidify; dissolve the organic phase with 150 μL of methanol to obtain a test solution; (2) The test solution is subjected to graphite furnace atomic absorption spectrometry. The analytical line of lead is 283.3 nm. The slit width is 0.7 nm. Zeeman effect background correction is used. The heating program of the graphite furnace is: Stage 1: temperature 100-110°C, heating time 1-3s, duration 15-20s; Stage 2: temperature 130-140°C, heating time 15-20s, duration 30-35s; Stage 3: temperature 900-1100°C, heating time 10-15s, duration 20-25s; Stage 4: temperature 2100-2250℃, heating time 1-5s, duration 5-8s; Stage 5: temperature 2250-2450℃, heating time 1-3s, duration 3-5s.

2. The method according to claim 1, characterized in that In step (1), the stratification method is standing or centrifugation.

3. The method according to claim 2, characterized in that The centrifugation conditions were 8000 rpm and 2 min.

4. The method according to claim 1, characterized in that In step (1), the preparation method of the sample solution includes: accurately weighing 0.5 g of sample, adding 4 mL of nitric acid and 2 mL of water into a digestion tube, microwave digestion at 200° C. for 30 min, transferring to a 25 mL tube and fixing the volume to the mark after cooling, and a selective dilution step.

5. The method according to claim 1, characterized in that K in the food + The content is not higher than 2500mg / kg, Na + The content is not higher than 125g / kg, Ca 2+ The content of Mg is not higher than 625mg / kg, 2+ The content of Cu is not higher than 250mg / kg, 2+ The content of Fe is not higher than 125mg / kg, 3+ The content of Mn is not higher than 250mg / kg, 2+ The content of Zn is not higher than 25mg / kg, 2+ The content is not higher than 25mg / kg.

6. The method according to claim 1, characterized in that The food is selected from rice, wheat flour, cakes, salt, chicken essence, soy sauce, salted kelp and hot pot base.

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