Some standard content:
ICS67.040
National Standard of the People's Republic of China
GB/T5009.12—2003
Replaces GB/T5009.12—1996
Determination of lead in foods
Determination of lead infoods
Issued on August 11, 2003
Ministry of Health of the People's Republic of China
Standardization Administration of the People's Republic of China
Implementation on January 1, 2004
This standard replaces GB/T5009.12—1996 "Determination of lead in foods". Compared with GB/T5009.12-1996, this standard has been modified as follows: the Chinese name of the standard has been modified, and the Chinese name of the standard has been changed to "Determination of Lead in Food"; GB/T5009.12-2003
modifies the structure of the original standard in accordance with GB/T20001.4-2001 "Standard Preparation Rules Part 4: Chemical Analysis Methods";
adds oxide atomic fluorescence spectrometry as the second method and single scan polarography as the fifth method. This standard is proposed and managed by the Ministry of Health of the People's Republic of China. The first method of this standard was drafted by Shanghai Food Sanitation Supervision and Inspection Institute, Institute of Nutrition and Food Hygiene, Chinese Academy of Preventive Medicine, Zhejiang Academy of Medical Sciences, Beijing Municipal Health and Epidemic Prevention Station, and Jilin Provincial Food Sanitation Supervision and Inspection Institute. The second method of this standard was drafted by Beijing Imported Food Sanitation Supervision and Inspection Institute and Food Sanitation Supervision and Inspection Institute of the Ministry of Health, and Sichuan Provincial Food Sanitation Supervision and Inspection Institute and Beijing Municipal Health and Epidemic Prevention Station participated in the drafting. The third method of this standard was drafted by Shanxi Provincial Health and Epidemic Prevention Station and Hunan Provincial Health and Epidemic Prevention Station. The fourth method of this standard was drafted by the Food Sanitation Supervision and Inspection Institute of the Ministry of Health. The fifth method of this standard was drafted by the Sichuan Provincial Health and Epidemic Prevention Station, and the Food Sanitation Supervision and Inspection Institute of the Ministry of Health, Fuzhou City Health and Epidemic Prevention Station, and Panzhihua City Health and Epidemic Prevention Station participated in the drafting.
The main drafters of the second method of this standard are Yan Jun, Yang Huifen, Qiang Weiguo, and Mao Hong. The main drafters of the fifth method of this standard are Xiang Shixue, Tang Xiaoqin, Han Hongwei, Lu Mingzhang, and Li Wenzui. This standard was first issued in 1985 and revised for the first time in 1996. This is the second revision. 87
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Lead is a harmful element with accumulation. The Joint Expert Committee on Food Additives and Contaminants (JECFA) of the Food and Agriculture Organization/World Health Organization (FAO/WHO) and the Codex Alimentarius Commission (CAC) in 1993 recommended a per capita weekly intake (PTWI) of 25μg/kg·bw. Based on a body weight of 60kg, the per capita daily intake is 214μg. In order to control the human body lead intake, it is listed as an important monitoring project in the field of food supervision. GB14935-1994 "Hygienic Standard for Lead Limits in Food" stipulates that the permissible limit of lead is ≤0.05mg/kg for milk (fresh): ≤0.2mg/kg for eggs, vegetables and fruits. The sensitivity of the flame atomic absorption spectrometry and disulfide colorimetry in the current national standard GB/T5009.12 does not meet the requirements of this hygiene standard. Graphite furnace atomic absorption spectrometry has high sensitivity, but the instrument is expensive and has serious interference in the determination of complex matrix samples, which often affects the accuracy of the analysis results. This revision proposes the determination of lead in food by hydride atomic fluorescence spectrometry to supplement the current national standard method. This method has high sensitivity and uses domestic instruments, which is easy to promote and apply.
1 Scope
Determination of lead in food
This standard specifies the determination method of lead in food. This standard is applicable to the determination of lead in food. GB/T5009.12—2003
The detection limit of this method is: 5ug/kg for graphite furnace atomic absorption spectrometry: 5μg/kg for solid samples and 1g/kg for liquid samples by hydride atomic fluorescence spectrometry; 0.1mg/kg for flame atomic absorption spectrometry; 0.25mg/kg for colorimetry; 0.085mg/kg for single scan polarography.
Method 1: Graphite furnace atomic absorption spectrometry
2 Principle
After the sample is ashed or acid-digested, it is injected into the graphite furnace of the atomic absorption spectrophotometer. After electrothermal atomization, it absorbs the 283.3nm resonance line. Within a certain concentration range, its absorption value is proportional to the lead content and is quantitatively compared with the standard series. 3 Reagents
3.1 Nitric acid.
3.2 Ammonium persulfate.
3.3 Hydrogen peroxide (30%).
3.4 Perchloric acid.
Nitric acid (1+1): Take 50mL nitric acid and slowly add it to 50mL water. 3.5
3.6 Nitric acid (0.5mol/L): Take 3.2mL nitric acid and add it to 50mL water, dilute to 100mL Nitric acid (1mol/L): Take 6.4mL nitric acid and add it to 50mL water, dilute to 100mL. 3.7
Ammonium phosphate solution (20g/L): Weigh 2.0g of ammonium phosphate and dilute it to 100mL with water. 3.8
Mixed acid: nitric acid + perchloric acid (4+1). Mix 4 parts of nitric acid with 1 part of perchloric acid. 3.9
Lead standard stock solution: Accurately weigh 1.000g of metallic lead (99.99%), add a small amount of nitric acid (1+1) in batches, heat to dissolve, the total amount does not exceed 3.10
more than 37mL, transfer to a 1000mL volumetric flask, add water to the scale, and mix well. This solution contains 1.0mg of lead per milliliter. Lead standard working solution: Pipette 1.0mL of lead standard stock solution into a 100mL volumetric flask each time, add nitric acid (0.5mol/L) or nitric acid (1mol/L) to the scale. After multiple dilutions, the standard working solution containing 10.0, 20.0, 40.0, 60.0, and 80.0 ng of lead per ml is obtained. 4 Instruments
All glass instruments used must be soaked in nitric acid (1+5) overnight, rinsed repeatedly with water, and finally rinsed with deionized water. Atomic absorption spectrophotometer (with graphite furnace and lead hollow cathode lamp). 4.1
Muffle furnace.
Drying thermostat.
Pressure digester, pressure digestion tank or pressure dissolution bomb. 4.6
Adjustable electric heating plate, adjustable electric furnace.
5 Analysis steps
5.1 Sample pretreatment
5.1.1 During sampling and preparation, care should be taken not to contaminate the sample. 89
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5.1.2 Grains and beans, after removing impurities, are ground, passed through a 20-mesh sieve, and stored in a plastic bottle for future use. 5.1.3 Fresh samples with high water content such as vegetables, fruits, fish, meat and eggs are homogenized with a food processor or homogenizer, stored in a plastic bottle for future use.
5.2 Sample digestion (any of the following methods can be used for digestion according to laboratory conditions) 5.2.1 Pressure digestion tank digestion method: Weigh 1.00g~2.00g sample (dry sample, sample with high fat content <1.00g, fresh sample <2.0g or weigh the sample according to the instruction manual of the pressure digestion tank) in a polytetrafluoroethylene inner tank, add 2mL~4mL nitric acid and soak overnight. Then add 2mL~3mL hydrogen peroxide (30%) (the total amount cannot exceed one-third of the tank volume). Cover the inner cover, tighten the stainless steel jacket, put it in a constant temperature drying oven, keep it at 120℃～140℃ for 3h～4h, cool it naturally to room temperature in the oven, use a dropper to wash or filter the digestion solution into a 10mL~25mL volumetric flask (depending on the salt content of the sample after digestion), wash the tank with a small amount of water several times, combine the washing solution in the volumetric flask and make up to the scale, mix well and set aside; at the same time, make a reagent blank.
5.2.2 Dry ashing: weigh 1.00g～5.00g (depending on the lead content) of the sample into a porcelain crucible, first carbonize it on an adjustable electric heating plate with low heat until smokeless, move it into a muffle furnace at 500℃ for 6h～8h, and cool it. If the individual samples are not completely ashed, add 1mL of mixed acid and heat on a low heat on an adjustable electric furnace, repeat several times until the digestion is complete, cool, dissolve the ash with nitric acid (0.5 mol/L), use a dropper to wash or filter the sample digestion solution into a 10mL~25mL volumetric flask (depending on the salt content of the sample after digestion), wash the porcelain crucible with a small amount of water several times, combine the washing solution in the volumetric flask and make up to the scale, mix and set aside; at the same time, make a reagent blank. 5.2.3 Ammonium persulfate ash method: weigh 1.00g5.00g of sample into porcelain, add 2mL~4mL nitric acid to soak for more than 1h, first carbonize on a low heat, add 2.00g~3.00g of ammonium persulfate to cover it after cooling, continue carbonization until no smoke, transfer to a muffle furnace, keep the temperature at 500℃ for 2h, then increase to 800℃, keep it for 20min, cool, add 2mL~3mL nitric acid (1.0mol/L), use a dropper to wash or filter the sample digestion solution into (depending on the salt content of the sample after digestion) a 10mL~25mL volumetric flask, wash the porcelain with a small amount of water several times, combine the washing solution in the volumetric flask and dilute to the mark, mix well and set aside, and make a reagent blank at the same time. 5.2.4 Wet digestion method: weigh 1.00g~5.00g of the sample into a conical flask or a tall beaker, put a few glass beads, add 10mL of mixed acid, cover and soak overnight, add a small funnel and digest on the electric stove. If it turns brown and black, add more mixed acid until white smoke comes out and the digestion solution is colorless and transparent or slightly yellow. Let it cool and use a dropper to wash or filter the sample digestion solution into (depending on the salt content of the sample after digestion) a 10mL~25mL volumetric flask, wash the conical flask or the tall beaker with a small amount of water several times, combine the washing solution in the volumetric flask and dilute to the mark, mix well and set aside, and make a reagent blank at the same time.
5.3 Determination
5.3.1 Instrument conditions: Adjust to the best state according to the performance of each instrument. Reference conditions are wavelength 283.3nm, slit 0.2nm~1.0nm, lamp current 5mA~7mA, drying temperature 120℃, 20s; ashing temperature 450℃, lasting 15s~20s, atomization temperature 1700℃~2300℃, lasting 4s~5s, background correction is chlorine lamp or Zeeman effect. 5.3.2 Standard curve drawing: Pipette 10L of the lead standard working solution 10.020.0, 40.0, 60.0, 80.0ng/mL (or μg/L) prepared above, inject into the furnace, measure its absorbance value and obtain the linear regression equation of the relationship between absorbance value and concentration. 5.3.3 Sample determination: Take 10μL of sample solution and reagent blank solution respectively, inject into graphite furnace, measure the absorbance value, and substitute into the linear regression equation of the standard series to obtain the lead content in the sample solution. 5.3.4 Use of matrix modifier: For samples with interference, inject an appropriate amount of matrix modifier ammonium dihydrogen phosphate solution (20g/1) to eliminate interference. Generally, 5μL or the same amount as the sample is injected. When drawing the lead standard curve, the same amount of matrix modifier ammonium dihydrogen phosphate solution as that used in sample determination should also be added.
6 Result calculation
The lead content in the sample is calculated according to formula (1). X(CC.)xVx1000
mx1000
Wherein:
X--lead content in the sample, in micrograms per kilogram or microgram per liter (μg/kg or μg/L); Ci--lead content in the test sample, in nanograms per milliliter (ng/mL); C. Lead content in the blank solution, in nanograms per milliliter (ng/mL); V--total quantitative volume of the sample digestion solution, in milliliters (mL); m--sample mass or volume, in grams or milliliters (g or mL). The calculation result shall retain two significant figures.
7 Precision
GB/T5009.12-2003
The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 20% of the arithmetic mean. Method 2 Hydride Atomic Fluorescence Spectrometry
8 Principle
After the sample is digested by acid and heat, the lead in the sample reacts with sodium borohydride (NaBH.) or potassium borohydride (KBH.) in an acidic medium to generate volatile lead hydride (PbH.). Using argon as the carrier gas, the hydride is introduced into an electrothermal quartz atomizer for atomization. Under the irradiation of a special lead hollow cathode lamp, the ground state lead atoms are excited to a high energy state; when deactivated and returned to the ground state, they emit fluorescence of a characteristic wavelength, and the fluorescence intensity is proportional to the lead content, which is quantified according to the standard series. 9 Reagents
9.1 Nitric acid + perchloric acid (4+1) mixed acid: Measure 400mL of nitric acid and 100mL of perchloric acid respectively and mix well. 9.2 Hydrochloric acid solution (1+1): Measure 250mL of hydrochloric acid and pour it into 250mL of water and mix well. 9.3 Oxalic acid solution (10 g/L): weigh 1.0 g oxalic acid, add to dissolve to 100 mL, mix. 9.4 Potassium ferrocyanide [K, Fe(CN).] solution (100 g/L): weigh 10.0 g potassium ferrocyanide, add water to dissolve and dilute to 100 mL, mix. 9.5 Sodium hydroxide solution (2 g/L): weigh 2.0 g sodium hydroxide, dissolve in 1 L water, mix. 9.6 Sodium borohydride [NaBH] solution (10 g/L): weigh 5.0 g sodium borohydride and dissolve in 500 mL sodium hydroxide solution (2 g/L), mix, and prepare before use.
9.7 Lead standard stock solution (1.0 mg/mL)\. 9.8 Lead standard application solution (1.0 μg/mL): accurately pipette lead standard stock solution (1.0 mg/mL) and dilute to 1.0 μg/mL step by step. 10 Instruments
10.1 Dual-channel fluorescence photometer or similar instrument. 10.2 Computer system and coded lead hollow cathode lamp. 10.3 Electric heating plate.
11 Analysis steps
11.1 Sample digestion
Wet digestion: Weigh 0.20g~2.00g of solid sample and 2.00g (or mL)~10.00g (or mL) of liquid sample, place in a 50mL~100mL digestion container (conical flask), then add 5mL~10mL of nitric acid + perchloric acid (4+1) mixed acid, shake and soak, and place overnight. The next day, place on the electric heating plate to heat and digest until the digestion solution is light yellow or colorless (if the color is dark during the digestion process, add a small amount of nitric acid when it is slightly cool, 1) Lead standard stock solution (1.0mg/mL) is a product provided by the National Center for Standard Materials Research. This information is given for the convenience of users of this standard and does not represent the endorsement of the product. If other products can have the same effect, these equivalent products can be used. 91
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Continue digestion), add 20mL of water after cooling slightly and continue heating to remove acid until the digestion solution is 0.5mL~1.0mL. After cooling, transfer it into a 25mL volumetric flask with a small amount of water, add 0.5mL of hydrochloric acid (1+1), 0.5mL of oxalic acid solution (10g/L), shake well, add 1.0mL of potassium ferrocyanide (100g/L), accurately dilute with water to 25mL, shake well, and measure after standing for 30min. At the same time, make a reagent blank. 11.2 Preparation of standard series
Take 7 25mL volumetric flasks, add 0.00.0.1250.25, 0.50.0.75, 1.00, 1.25mL of lead standard application solution (1.00μg/mL) in sequence (corresponding to lead concentrations of 0.0, 5.0, 10.0, 20.0.30.0, 40.0, 50.0ng/mL respectively), dilute with a small amount of water, add 0.5mL of hydrochloric acid (1+1) and 0.5mL of oxalic acid (10g/L), shake the hook, then add 1.0mL of potassium ferrocyanide solution (100g/L), dilute to the mark with water, shake the hook. Leave for 30 minutes before testing.
11.3 Determination
11.3.1 Instrument reference conditions
Negative high voltage: 323V, lead hollow cathode lamp current: 75mA; atomizer: furnace temperature 750℃~800℃, furnace height: 8mm, argon gas flow rate: carrier gas 800mL/min, shielding gas: 1000mL/min, reducing agent addition time: 7.0s: reading time: 15s; delay time: 0.0s; measurement method: standard curve method; reading method: peak area, injection volume: 2.0mL. 11.3.2 Concentration measurement method
Set the best conditions of the instrument, gradually raise the furnace temperature to the required temperature, and start measuring after stabilizing for 10min to 20min. Continue to use the zero tube of the standard series for sample injection. After the reading is stable, switch to the measurement of the standard series, draw the standard curve, and switch to the sample measurement. Measure the sample blank and sample digestion solution separately. Clean the injector before measuring each different sample. The sample measurement results are calculated according to formula (2). 11.3.3. The instrument automatically calculates the results. The measurement method sets the best conditions for the instrument. In the sample parameter screen, enter the following parameters: sample mass or volume (g or mL), dilution volume (mL), and select the concentration unit of the result. Gradually raise the furnace temperature to the required temperature. After stabilization, measure. Continue to inject the sample with the zero tube of the standard series. After the reading is stable, switch to the standard series measurement and draw the standard curve. Before switching to the sample measurement, enter the blank value measurement state and inject the sample blank digestion solution. Let the instrument take its average as the blank value to be deducted. Then, the sample solution can be measured in turn. After the measurement is completed, select "Print Report" to automatically print the measurement results. 12. Result calculation
The lead content in the sample is calculated according to formula (2). (cc) × VX1 000
m×1000×1000
Wherein:
X--lead content in the sample, in milligrams per kilogram or milligrams per liter (mg/kg or mg/L); c--concentration of the sample digestion solution, in nanograms per milliliter (ng/mL); Co--concentration of the reagent blank solution, in nanograms per milliliter (ng/mL); m--mass or volume of the sample, in grams or milliliters (g or mL); V--total volume of the sample digestion solution, in milliliters (mL). The calculation result shall retain three significant figures.
13 Precision
The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of the arithmetic mean. Method 3 Flame atomic absorption spectrometry
14 Principle
·(2)
After the sample is treated, the lead ions form a complex with DDTC under certain pH conditions, which is extracted and separated by 4-methylpentanone-2-yl, and introduced into the atomic absorption spectrometer. After flame atomization, it absorbs the 283.3nm resonance line, and its absorption amount is proportional to the lead content. It is quantitatively compared with the standard series.
15 ReagentswwW.bzxz.Net
15.1 Nitric acid-pernitrogen acid (4+1).
15.2 Ammonium sulfate solution (300g/L): Weigh 30.0g ammonium sulfate [(NH,), SO.J, dissolve it in water and add water to 100mL. 15.3 Ammonium citrate solution (250 g/L): weigh 25.0 g ammonium citrate, dissolve in water and add water to 100 mL. 15.4 Bromothymol blue aqueous solution (1 g/L). 15.5 Sodium diethyldithiocarbamate (DDTC) solution (50 g/L): weigh 5 g sodium diethyldithiocarbamate, dissolve in water and add water to 100 mL.
15.6 Ammonia water (1+1).
15.7 4-Methylpentyl ketone-2 (MIBK).
15.8 Lead standard solution: operate the same as 3.10 and 3.11. Prepare a standard solution of 10 μg/mL lead. 16 Instruments
Atomic absorption spectrophotometer with flame atomizer, the rest is the same as 4.2, 4.3, 4.4 and 4.5. 17 Analysis steps
17.1 Sample treatment
17.1.1 Beverages and alcoholic beverages: Take 10.0g~-20.0g of uniform sample in a beaker. Alcohol should be evaporated on a water bath first, evaporate to a certain volume on a hot plate, add nitric acid-perchloric acid (4+1) and digest completely, then transfer and adjust the volume to a 50mL volumetric flask. 17.1.2 The soaking liquid of packaging materials can be directly absorbed for determination. 17.1.3 Cereals: remove impurities and dust, remove the shell if necessary, crush, pass through a 20-mesh sieve, and mix. Weigh 5.0g to 10.0g, place in a 50mL porcelain crucible, carbonize with low heat, then transfer to a muffle furnace, incinerate at 500℃ or below for 16h, remove the crucible, cool it down, add a small amount of mixed acid, heat it with low heat, do not let it dry out, add a little more mixed acid if necessary, and repeat this process until there is no carbon particles in the residue. When the crucible cools down slightly, add 10mL hydrochloric acid (1+11), dissolve the residue and transfer it to a 50mL volumetric flask, then wash it repeatedly with water, add the washing liquid to the volumetric flask, dilute to the scale, mix well and set aside.
Take the same amount of mixed acid and hydrochloric acid (1+11) as the sample, and perform a reagent blank test according to the same operation method. 17.1.4 Vegetables, fruits and beans: Wash and dry the edible part, chop it thoroughly and mix well. Weigh 10.00g~20.00g and place it in a porcelain crucible, add 1mL phosphoric acid (1+10), carbonize on low heat, and then transfer to a muffle furnace according to 17.1.3. 17.1.5 Poultry, eggs, aquatic products and dairy products: Take the edible part and mix it thoroughly. Weigh 5.00g~10.00g and place it in a porcelain crucible, carbonize on low heat, and then transfer to a muffle furnace according to 17.1.3. After milk is mixed, measure 50mL, place it in a porcelain crucible, add phosphoric acid (1+10), evaporate to dryness on a water bath, and then carbonize on low heat, and then transfer to a muffle furnace according to 17.1.3. 17.2 Extraction and separation
Depending on the sample, take 25.0mL~50.0mL of the sample solution and reagent blank solution prepared above, respectively, and place them in a 125mL separatory funnel, and add water to 60mL. Add 2mL of ammonium citrate solution, 3~5 drops of bromothymol blue indicator, and adjust the pH with ammonia water (1+1) until the solution changes from yellow to blue, add 10.0mL of ammonium sulfate solution and 10mL of DDTC solution, and shake. Let stand for about 5 minutes, add 10.0mL of MIBK, shake vigorously to extract for 1 minute, and after standing and separating, discard the water layer, and put the MIBK layer into a 10mL stoppered graduated tube for use. Pipette 0.00, 0.25, 0.50, 1.00, 1.50, 2.00mL of lead standard solution (equivalent to 0.0, 2.5, 5.0, 10.0, 15.0, 20.0g of lead) into a 125mL separatory funnel, and the following operations are the same as for the sample. 17.3 Determination
17.3.1 Beverages, alcoholic beverages and packaging materials soaking liquid can be directly sampled and determined after extraction. 93
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17.3.2 When sampling the extract, the flow rate of acetylene gas can be appropriately reduced. 17.3.3 Instrument reference conditions: hollow cathode lamp current 8mA; resonance line 283.3nm; slit 0.4nm; air flow 8L/min; burner height 6mm, BCD mode.
18 Calculation of results
The lead content in the sample is calculated according to formula (3). X=(G1-0)XViX1000
mxVs/V,×i000
Wherein:
X-the content of lead in the sample, in milligrams per kilogram or milligrams per liter (mg/kg or mg/L);-the content of lead in the sample solution for determination, in micrograms per milliliter (\g/mL):Ch
cThe content of lead in the reagent blank solution, in micrograms per milliliter (μg/mL):m-the mass or volume of the sample, in grams or milliliters (g or mL);Vthe volume of the sample extract, in milliliters (mL);V.-the total volume of the sample treatment solution, in milliliters (mL);Vs-the total volume of the sample treatment solution for determination, in milliliters (mL). The calculation result shall be rounded to two significant figures.
19 Precision
The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 20% of the arithmetic mean. Method 4 Disulfide Colorimetric Method
20 Principle
.·(3)
After the sample is digested, at pH 8.5-9.0, lead ions and disulfide form a red complex that dissolves in chloroform. Add ammonium citrate, potassium cyanide, and hydroxylamine hydrochloride to prevent interference from iron, copper, zinc, etc. ions, and compare with the standard series for quantification. 21 Reagents
21.1 Ammonia water (1+1).
21.2 Hydrochloric acid (1+1): Measure 100 mL of hydrochloric acid and add it to 100 mL of water. 21.3 Phenol red indicator solution (1 g/L): Weigh 0.10 g of phenol red, dissolve it in small amounts of ethanol several times, transfer it to a 100 mL volumetric flask and dilute to the mark.
21.4 Hydroxylamine hydrochloride solution (200g/L): Weigh 20.0g of hydroxylamine hydrochloride, add water to dissolve to 50mL, add 2 drops of phenol red indicator solution, add ammonia water (1+1), adjust the pH to 8.5~9.0 (from yellow to red, add 2 more drops), extract with disulfide-chloroform solution until the chloroform layer remains green, then wash twice with trichloromethane, discard the trichloromethane layer, add hydrochloric acid (1+1) to acidify the water layer, and add water to 100mL. 21.5 Ammonium citrate solution (200g/L): Weigh 50g of ammonium citrate, dissolve in 100mL of water, add 2 drops of phenol red indicator solution, add ammonia water (1+1), and adjust the pH to 8.5~9.0, extract with dithiocarbamide-chloroform solution several times, 10mL20mL each time, until the chloroform layer remains green, discard the chloroform layer, wash twice with chloroform, 5mL each time, discard the chloroform layer, dilute with water to 250mL. 21.6 Potassium cyanide solution (100g/L): weigh 10.0g potassium cyanide, dissolve in water and dilute to 100mL. 21.7 Chloroform: should not contain oxides. 21.7.1 Inspection method: measure 10mL chloroform, add 25mL freshly boiled water, shake for 3min, let stand for stratification, take 10mL water solution, add a few drops of potassium iodide solution (150g/L) and starch indicator solution, and no blue should appear after shaking. 21.7.2 Treatment method: Add one tenth to one twentieth volume of sodium thiosulfate solution (200g/L) to trinitroform for washing, then wash with water, add a small amount of anhydrous calcium chloride for dehydration and distill, discard the first and last tenth of the distillate, collect the intermediate distillate for later use.
21.8 Starch indicator solution: weigh 0.5g of soluble starch, add 5mL of water and stir well, then slowly pour into 100mL of boiling water, stir while pouring, boil, cool and set aside, prepare when needed. 21.9 Nitric acid (1+99): measure 1mL of nitric acid and add it to 99mL of water. 21.10 Disulfide trichloromethane solution (0.5g/L): store in the refrigerator and purify by the following method when necessary. Weigh 0.5g of finely ground disulfide and dissolve it in 50mL chloroform. If it is not completely dissolved, filter it with filter paper in a 250mL separatory funnel and extract it three times with ammonia water (1+99), 100mL each time. Filter the extract with cotton into a 500mL separatory funnel and adjust it to acidity with hydrochloric acid (1+1). Extract the precipitated disulfide with chloroform 2 to 3 times, 20mL each time. Combine the chloroform layers, wash twice with an equal amount of water, discard the washing liquid, and evaporate the chloroform in a 50℃ water bath. Put the refined disulfide in a sulfuric acid desiccator and dry it for use. Or extract the precipitated disulfide with 200, 200, and 100mL chloroform three times, and combine the chloroform layers to form a disulfide solution. 21.11 Disulfide working solution: Take 1.0mL of disulfide solution, add chloroform to 10mL and mix well. Use a 1cm colorimetric cup, adjust the zero point with chloroform, measure the absorbance (A) at a wavelength of 510nm, and use formula (4) to calculate the milliliters (V) of disulfide solution required to prepare 100mL disulfide working solution (70% transmittance).
2112 Nitric acid-sulfuric acid mixture (4+1). V=10×(2-lg70)_1.55
（4）
21.13 Lead standard solution: Accurately weigh 0.1598g lead nitrate, add 10mL nitric acid (1+99), and after all are dissolved, transfer to a 100mL volumetric flask and dilute to the mark with water. This solution is equivalent to 1.0mg lead per milliliter. 21.14 Lead standard working solution: Take 1.0mL of lead standard solution, place it in a 100mL volumetric flask, and dilute to the mark with water. This solution is equivalent to 10.0μg lead per milliliter.
22 Instruments
All glassware used are soaked in nitric acid (10% 20%) for more than 24 hours, rinsed repeatedly with tap water, and finally rinsed with deionized water.
Spectrophotometer.
23 Analysis steps
23.1 Sample pretreatment
Same as the operation in 5.1.
23.2 Sample digestion
23.2.1 Nitric acid-sulfuric acid method
23.2.1.1 Grain, vermicelli, vermicelli, dried bean products, cakes, tea, etc. and other solid foods with little water content: Weigh 5.00g or 10.00g of the crushed sample, place it in a 250mL~500mL nitrogen determination bottle, first add a little water to moisten it, add a few glass beads and 10mL~15mL nitric acid, let it stand for a while, heat slowly over low heat, wait for the effect to ease, and let it cool. Add 5mL or 10mL sulfuric acid along the bottle wall, and heat it again. When the liquid in the bottle begins to turn brown, continue to drip nitric acid along the bottle wall until the organic matter is completely decomposed. Increase the firepower until white smoke is produced. When the white smoke at the bottle mouth is completely gone and the liquid in the bottle produces white smoke again, the digestion is complete. The solution should be clear and colorless or slightly yellow, and let it cool. (Care should be taken to prevent boiling or explosion during the operation) Add 20mL of water and boil, remove the residual nitric acid until white smoke is produced, and do this twice, and let it cool. Transfer the cooled solution to a 50mL or 100mL volumetric flask, wash the nitrogen determination bottle with water, add the washing liquid to the volumetric flask, let it cool, add water to the scale, and mix well. After the solution is fixed to volume, each 10mL is equivalent to 1g of the sample, which is equivalent to the amount of sulfuric acid added 1mL. Take the same amount of nitric acid and sulfuric acid as the digestion sample, and perform a reagent blank test according to the same method. 23.2.1.2 Vegetables and fruits: weigh 25.00g or 50.00g of the washed and homogenized sample, place it in a 250mL~500mL nitrogen determination bottle, add several glass beads, 10mL~15mL nitric acid, and operate according to 23.2.1.1 from "let it stand for a while...", but every 10mL of 95
GB/T5009.12—2003
solution after volume determination is equivalent to 5g of sample, which is equivalent to adding 1mL of sulfuric acid. 23.2.1.3 Sauces, soy sauces, vinegar, cold drinks, tofu, fermented bean curd, pickled vegetables, etc.: weigh 10.00g or 20.00g of sample (or absorb 10.0mL or 20.0mL of liquid sample), place it in a 250mL~500mL nitrogen determination bottle, add several glass beads, 5mL~15mL nitric acid. The following procedures are in accordance with 23.2.1.1 from "Let it stand for a while...", but each 10mL of the solution after the volume is fixed is equivalent to 2g or 2mL of the sample. 23.2.1.4 Alcoholic beverages or beverages containing carbon dioxide: Pipette 10.00mL or 20.00mL of the sample and place it in a 250mL~500mL nitrogen determination bottle, add a few glass beads, first heat over low heat to remove ethanol or carbon dioxide, then add 5mL10mL nitric acid, mix, and the following procedures are in accordance with 23.2.1.1 from "Let it stand for a while...", but each 10mL of the solution after the volume is fixed is equivalent to 2mL of the sample. 23.2 .1.5 Foods with high sugar content: Weigh 5.00g or 10.0g of the sample and place it in a 250mL500mL nitrogen determination bottle. First add a little water to moisten it, add a few glass beads and 5mL10mL nitric acid, and shake well. Slowly add 5mL or 10mL sulfuric acid. After the effect is relieved and the foaming stops, first heat slowly with a low fire (sugar is easy to carbonize), and continue to add nitric acid along the bottle wall. After all the foam disappears, add more fire until the organic matter is completely decomposed and white smoke is generated. The solution should be clear and colorless or slightly yellow, and then cool. The following operations shall be carried out according to 23.2.1.1 starting from "add 20mL water to boil".
23.2.1.6 Aquatic products: Take the edible part of the sample and make it into a homogenate, weigh 5.00g or 10.0g (the sampling amount of marine algae and shellfish can be appropriately reduced), put it in a 250mL~500mL nitrogen determination bottle, add a few glass beads, 5mL10mL nitric acid, mix well, and operate according to 23.2.1.1 starting from "add 5mL or 10mL sulfuric acid along the bottle wall..." 23.2.2 Ashing method
23.2.2.1 Grain and other foods with little water content: weigh 5.00g of sample, put it in a quartz or porcelain crucible, heat until carbonized, and then Then transfer to a muffle furnace, ash at 500℃ for 3h, cool, remove the crucible, add nitric acid (1+1), moisten the ash, evaporate to dryness with low heat, burn at 500℃ for 1h, cool. Remove the crucible. Add 1mL nitric acid (1+1), heat to dissolve the ash, transfer to a 50mL volumetric flask, wash with water, add the washing liquid to the volumetric flask, add water to the scale, mix and set aside. 23.2.2.2 Food or liquid sample with high water content: weigh 5.00g or absorb 5.00mL of the sample, place it in an evaporating dish, evaporate to dryness on a water bath, and then operate according to 23.2.2.1 starting from "heating to carbonization....\". 23.3 Determination
Absorb 10.0mL of the digested fixed volume solution and the same amount of reagent blank solution, place them in 125mL separatory funnels respectively, and add water to 20mL each.
Pipette 0, 0.10, 0.20, 0.30, 0.40, 0.50mL lead standard solution (equivalent to 0.0, 1.0, 2.0, 3.0, 4.0, 5.0μg lead) and place them in 125mL separatory funnels respectively. Add nitric acid (1+99) to 20mL. Add 2.0mL ammonium citrate solution (200g/L), 1.0mL hydroxylamine hydrochloride solution (200g/L) and 2 drops of phenol red indicator solution to the sample digestion solution, reagent blank solution and lead standard solution respectively. Adjust to red with ammonia water (1+1), then add 2.0mL potassium cyanide solution (100g/L) to each and mix. Add 5.0mL disulfide solution, shake vigorously for 1min, let stand for stratification, filter the chloroform layer through absorbent cotton into a 1cm colorimetric cup, adjust the zero point with chloroform and measure the absorbance at a wavelength of 510nm. After subtracting the zero tube absorption value at each point, draw a standard curve or calculate a one-dimensional regression equation, and compare the sample with the curve. 24 Result calculation
The lead content in the sample is calculated according to formula (5). (m-m2)X1000
mxV2/Vix1000
Wherein:
X—the content of lead in the sample, in milligrams per kilogram or milligrams per liter (mg/kg or mg/L); m
the mass of lead in the sample solution for determination, in micrograms (ug); mz---the mass of lead in the reagent blank solution, in micrograms (μg); ms
-the mass or volume of the sample, in grams or milliliters (g or mL); Vi——the total volume of the sample treatment solution, in milliliters (mL); V.---the total volume of the sample treatment solution for determination, in milliliters (mL). 96
·(5)
The calculation result shall retain two significant figures.
25 Precision
GB/T5009.12—2003
The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of the arithmetic mean. Method 5 Single Sweep Polarography
26 Principle
After the sample is digested, lead exists in the form of ions. In an acidic medium, the PbI- complex ion formed by Pb2+ and I- is electrically active and generates a reduction current on the dropping mercury electrode. The peak current is linearly related to the lead content and is quantitatively compared with the standard series. 27 Reagents
27.1 Base solution: Weigh 5.0g potassium iodide, 8.0g potassium sodium tartrate, and 0.5g ascorbic acid in a 500mL beaker, add 300mL water to dissolve, then add 10mL hydrochloric acid, transfer to a 500mL volumetric flask, and add water to the scale (store in a refrigerator and can be stored for two months). 27.2 Lead standard stock solution: Accurately weigh 0.1000g of metallic lead (99.99% content) in a beaker, add 2mL (1+1) nitric acid solution, heat to dissolve, cool and quantitatively transfer to a 100mL volumetric flask and add water to the mark, mix well (this solution contains 1.0mg/mL lead). 27.3 Lead standard working solution: Before use, pipette 1.00mL of lead standard stock solution into a 100mL volumetric flask, add water to the mark, mix well (this solution contains 10.0pg/mL lead).
27.4 Mixed acid: nitric acid-perchloric acid (4+1). Measure 80mL of nitric acid, add 20mL of perchloric acid, mix well. 28 Instruments
All glass instruments used should be soaked in 10% nitric acid solution overnight, rinsed repeatedly with water, and finally cleaned with distilled water, dried and set aside. 28.1 Polarographic analyzer.
28.2 Universal electric furnace with electronic regulator. 29 Analysis steps
29.1 Polarographic analysis reference conditions
Single sweep polarography (SSP method). Select starting potential: -350mV, end potential: -850mV, scanning speed 300mV/s, three electrodes, secondary derivative, static time: 5s and appropriate range. At the peak potential of -470mV, record the peak current of lead. 29.2 Standard curve drawing
Accurately pipette 0, 0.05, 0.10, 0.200.30, 0.40mL of lead standard solution (equivalent to 00.5, 1.0, 2.0, 3.0, 4.0g of lead) into 6 10mL colorimetric tubes, add base solution to 10.0mL, and mix well. Transfer the solution in each tube to the electrolytic cell in turn and place it in the three-electrode system. Determine under the above polarographic analysis reference conditions and record the peak current of lead respectively. Draw a standard curve with the content as the horizontal axis and the corresponding peak current as the vertical axis.
29.3 Sample treatment
Samples with low moisture content such as grains and beans are ground and passed through a 20-mesh sieve after removing impurities; fresh samples with high moisture content such as vegetables, fruits, fish, and meat are made into homogenate using a pulping machine and stored in plastic bottles. 29.3.1 Sample treatment (except salt and sugar, such as grains, beans, cakes, tea, meat, etc.): Weigh 1.0g~2.0g of sample in a 50mL triangular flask, add 10mL~20mL of mixed acid, cover and soak overnight. Heat at a low setting on a multi-purpose electric stove with an electronic regulator. If the color of the digestion solution gradually deepens and becomes brown-black, remove the multi-purpose electric stove, cool it, add an appropriate amount of nitric acid, and continue heating and digestion. When the solution color no longer deepens and becomes colorless and transparent or slightly yellow with white smoke, the remaining acid can be driven out at a high level until it is close to 100%. Then heat it at a low level to obtain a white residue for testing. At the same time, make a reagent blank. 29.3.2 Salt and sugar: weigh 2.0g of the sample into a beaker for testing.0g sample in a 50mL conical flask, add 10mL~20mL mixed acid, cover and soak overnight. Heat at low setting on a universal electric stove with an electronic regulator. If the color of the digestion solution gradually deepens and turns brown-black, remove the universal electric stove, cool it, add an appropriate amount of nitric acid, and continue heating to digest. When the color of the solution no longer deepens and becomes colorless and transparent or slightly yellow, and emits white smoke, the remaining acid can be driven out at high setting until it is close to , and then heat at low setting to obtain a white residue to be tested. At the same time, make a reagent blank. 29.3.2 Salt and sugar: Weigh 2.0g of the sample in a beaker and test it. 970g sample in a 50mL conical flask, add 10mL~20mL mixed acid, cover and soak overnight. Heat at low setting on a universal electric stove with an electronic regulator. If the color of the digestion solution gradually deepens and turns brown-black, remove the universal electric stove, cool it, add an appropriate amount of nitric acid, and continue heating to digest. When the color of the solution no longer deepens and becomes colorless and transparent or slightly yellow, and emits white smoke, the remaining acid can be driven out at high setting until it is close to , and then heat at low setting to obtain a white residue to be tested. At the same time, make a reagent blank. 29.3.2 Salt and sugar: Weigh 2.0g of the sample in a beaker and test it. 97
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