Determination of Lead in Instant Noodles (2)

5.
Operation steps

(1) The instant noodles in the pretreatment bag (barrel) of the sample are all crushed, mixed, and sampled by quarter method
.

Then choose any of the following methods to process

①Wet digestion: Weigh 1.
00～5.
00g (accurate to 0.
001g, depending on the lead content) of the sample in a conical flask or a tall beaker, put a few glass beads, add 10mL of mixed acid, cover and soak overnight, add Dissolve in a small funnel on an electric stove.
If it turns brown-black, add mixed acid until white smoke appears.
The digestive juice is colorless and transparent or slightly yellow.
Let it cool.
Use a dropper to wash or filter the sample solution.
Into (depending on the salt content of the sample after digestion) 10-25mL volumetric flask, wash the Erlenmeyer flask or tall beaker several times with a small amount of water, combine the lotion in the volumetric flask and dilute to the mark, mix well for later use; Reagent blank
.

②Dry ashing: Weigh 1～5g sample (accurate to 0.
001g, depending on the lead content) in a porcelain crucible, first carbonize it on an adjustable electric heating plate until it is smokeless, and move it into a muffle furnace (500 When ashing at ±25)℃ for 6～8h, cool down
.

If individual samples are not completely ashed, add 1 mL of mixed acid and heat it on a small fire on an adjustable electric furnace.

(2) Determination

① Graphite furnace heating program:

Background correction: Deuterium lamp or Zeeman effect
.

② Standard curve drawing: draw 10uL each of the lead standard solution 10.

0, 20.

③Sample measurement: draw 10uL each of the sample solution and reagent blank solution, pour into the graphite furnace, measure the absorbance value, and substitute it into the standard series of unary linear regression equations to obtain the lead content in the sample solution
.

④Use of matrix modifier: For samples with interference, inject an appropriate amount of matrix modifier ammonium phosphate solution (20g/1, generally 5uL or the same amount as the sample) to eliminate interference
.

When drawing the lead standard curve, add the same amount of matrix modifier ammonium phosphate solution as the sample measurement

6.
Result calculation

Where x-lead content in the sample, mg/kg (mg/L)

p ₁ ——determine the lead content in the sample solution, ng/mL

p ₀ ——Lead content in blank solution, ng/mL

V——Quantitative total volume of sample digestion solution, mL

m——mass or volume of sample, g or mL

Expression of results: report the two significant digits of the arithmetic mean
.

7.

Tips

(1) The detection limit of this method is 0.
005mg/kg
.

(2) Lead detection is a trace analysis that is easy to contaminate and has a low limit.
The lower the blank, the higher the accuracy
.

Strictly control pollution during the detection process, the reagents and water used should meet the requirements, and the utensils used should be soaked in acid

(3) The acid concentration in the digestion solution should not be too high
.

Because too much acidity has a great influence on the determination of elements by the graphite furnace method, especially the damage to the graphite tube, it is generally necessary to add water to dissolve the salt and to drive the acid after the digestive juice is clear and transparent.

(4) When digesting samples with perchloric acid , the operating procedures should be strictly followed, and only a small amount of organic components should be present when the temperature reaches 200°C
.

Otherwise, the oxidation potential of perchloric acid will rise rapidly at this temperature and cause a violent explosion

(5) When measuring with atomic absorption spectrophotometer, attention should be paid to the appropriate depth and left and right position of the sample
.

The injection must be accurate and stable, which determines the linearity of the standard curve and the reproducibility of the experiment
.