Determination of Mercury in Food (2)

4.
Operation steps

(1) Sample digestion

①High pressure digestion method: This method is applicable to the determination of total mercury in foods such as grains, beans, fruits and vegetables, lean meats, fish, eggs, and milk and dairy products
.

a.
Dry samples of grains and beans: Weigh 0.
2～1.
00g of the dry sample that has been crushed and mixed and passed through a 40-mesh sieve, and placed in a polytetrafluoroethylene plastic inner tank.
Add 5 mL of nitric acid , mix well and place overnight, then add 7mL of hydrogen peroxide , cover the inner lid and put it in the stainless steel jacket, screw tightly to seal
.

b.
Fresh samples of vegetables, lean meat, fish and eggs with high moisture content: beat into a homogenate with a masher, weigh 1.
00～5.
00g of the homogenate, place it in a polytetrafluoroethylene plastic inner tank, and cover it.
Leave the seam in a 65°C blast drying oven or a general oven until it is nearly dry, take it out, and follow the steps ①a from "adding 5mL nitric acid.
.
.
" according to the law
.

②Microwave digestion method: Weigh 0.
10～0.
50g sample into the digestion tank, add 1～5mL nitric acid , 1～2mL hydrogen peroxide , after the safety valve is good, put the digestion tank into the microwave digestion system, according to different types of The sample is set to the optimal analysis conditions of the microwave oven digestion system (see Table 4-1 and Table 4-2), until the digestion is complete, after cooling, use nitric acid solution (1+9) to quantitatively transfer and dilute to 25mL (low content sample The volume can be adjusted to 10mL), and mix well to be tested
.

Table 4-1 Microwave analysis conditions of food, vegetables, fish and meat samples

Table 4-2 Microwave analysis conditions of oil and sugar samples

(2) Standard series preparation

① Low-concentration standard series: Draw 100ng/mL mercury standard solution 0.

②High-concentration standard series: Draw 0.
25, 0.
50, 1.
00, 1.
50, 2.
00mL of 500mg/mL mercury standard solution into a 25mL volumetric flask, dilute to the mark with nitric acid solution (1+9), and mix well
.

(3) Determination

①Instrument reference conditions
.

Photomultiplier tube negative high voltage: 240V; mercury hollow cathode lamp current: 30mA; atomizer: temperature 300℃, height 8.
0mm; argon flow rate: carrier gas 500mL/min, shielding gas 1000mL/min; measurement method: standard curve method ; Reading method: peak area, reading delay time: 1.
0s; reading time: 10.
0s; potassium borohydride solution addition time: 8.
0s; standard solution or sample solution addition volume: 2mL
.

Note: AFS series atomic fluorescence instruments such as 230, 230a, 2202, 2202a, 2201 and other instruments are fully automatic or off-sequence flow instruments, all with the operating software of this instrument, and the analysis conditions of the instrument should be set to the analysis prompted by this instrument Condition, after the instrument is stable, test the standard series until the correlation coefficient r>0.
999 of the standard curve is tested
.

②Measurement method (choose one of the following methods according to the situation)
.

a.
Concentration measurement method measurement: Set the optimal conditions of the instrument, gradually increase the furnace temperature to the required temperature, and start measurement after it is stable for 10-20 minutes
.

b.
The instrument automatically calculates the result measurement method: Set the best condition of the instrument, enter the following parameters in the sample parameter screen: sample mass (g or mL), dilution volume (mL), and select the concentration unit of the result, and gradually change The furnace temperature is raised to the required temperature and measured after it has stabilized
.

5.

Where X——the content of mercury in the sample, mg/kg or mg/L

p——The content of mercury in the digestion solution of the sample, ng/mL

p ₀ ——the mercury content in reagent blank, ng/mL

V——Total volume of sample digestion solution, mL

m——mass or volume of sample, g or mL

The calculation result retains three significant figures

The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of the arithmetic mean