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Digestion of pre-treatment methods, special applications need to be kept in mind----Nanjing binglab

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Digestion of pre-treatment methods, special applications need to be kept in mind----Nanjing binglab

heating plate is a digestion or acid driven equipment commonly used in elemental analysis laboratories


In the inspection of heavy metals in food, sample pretreatment is one of the most critical steps in food inspection, which directly affects the precision and accuracy of the analysis results. Choosing an appropriate pretreatment method and shortening the sample pretreatment time is the key to ensuring the quality of the inspection. It is an important method to improve the inspection efficiency at the same time. Based on the four commonly used sample pretreatment methods combined with the inspection experience of metal elements in food, the application and precautions of the four methods in food metal inspection were analyzed, and some references were provided for food inspectors to choose appropriate sample pretreatment methods.


Food is the basic element of human survival. Due to the development of industrialization, food may contain or be contaminated with substances that are harmful to human health. With the improvement of people’s living standards, food safety issues have been paid more and more attention, and the state has increased the supervision of food. At 

the same time, it also increases the workload of food inspectors, which requires food inspectors to improve work efficiency while ensuring the quality of inspection.


WET DIGESTION

The wet digestion method is to add an oxidative strong acid to an appropriate amount of food samples, heat to destroy the organic matter, release the inorganic components to be tested, and form non-volatile inorganic compounds for analysis and determination.


Wet digestion is a widely used pretreatment method for food samples. This method is practical, and almost all foods can be digested by this method.


Advantages of wet digestion:
First of all, the reagents used in the pretreatment, that is, acids, can be found with high purity, and the matrix components are relatively simple (occasionally some sulfates are produced);


Secondly, during the experiment, as long as the digestion temperature is well controlled, most of the elements are generally little or almost no loss. For example, when determining the content of arsenic in soy sauce, wet digestion was used to add mixed acid of nitric acid, perchloric acid and sulfuric acid, and the recovery rate of standard addition was more than 95%. Even for highly volatile elements such as “mercury”, as long as the digestion temperature is correctly controlled, there will be no loss.


Wet Digestion Disadvantages:
First, since the reaction is an oxidation reaction, the oxidation time of the sample is relatively long. It takes about an hour (depending on the composition of the sample), and too many samples cannot be digested at one time during the experiment, so the labor intensity of the method is relatively high.


Secondly, nitric acid, perchloric acid, hydrogen peroxide, and sulfuric acid, which are commonly used in sample digestion, are corrosive and dangerous. The acid mist and smoke generated when nitric acid and perchloric acid are used are also very corrosive to the fume hood. Special attention should be paid to the fact that when using perchloric acid to digest samples, the operating procedures should be strictly followed, the liquid in the beaker should not be boiled dry, and it must be ensured that only a small amount of organic components exist when the temperature reaches 200 degrees Celsius, otherwise the oxidation potential of perchloric acid will rise rapidly at this temperature, which will cause a violent explosion! Therefore, it is recommended that when using perchloric acid, it is best to first Use nitric acid to oxidize part of the organic matter, or add a mixture of nitric acid and perchloric acid to soak overnight, and the experiment should be carried out in a fume hood. Digestive juice can not be evaporated to prevent the loss of some elements such as selenium and lead.


In addition, due to the addition of concentrated acid during the oxidation reaction, these acids may cause damage to the instrument and affect the test results, so it is necessary to discharge the acid after the digestion is completed.


For example, if atomic fluorescence is used to measure total arsenic, the presence of nitric acid will hinder the production of arsenic and interfere with the determination. After the digestion is complete, the nitric acid should be removed by heating as much as possible. In the national standard experiment, nitric acid-sulfuric acid was used to digest samples. Since the boiling point of sulfuric acid is higher than that of nitric acid, there is basically no nitric acid in the final digestion solution. However, it should be noted that when the sample is digested with nitric acid-sulfuric acid to avoid carbonization, arsenic will be seriously lost during the digestion process. Therefore, if the color of the digestion solution becomes darker during the digestion process, nitric acid should be added appropriately. It is worth noting Even in the standard curve, it is also necessary to ensure the same acid concentration as in the sample digestion solution, that is, to match the matrix.


Precautions for wet digestion of some special foods

Foods with high oil content, such as vegetable oil, peach cake, etc., after adding mixed acid, because the sample floats on the surface of the mixed acid, it is easy to form a complete film, and there is a violent reaction on the liquid surface when heated, which is likely to cause bumping or boiling. Splash, so it is recommended that the sample weight should not be higher than 1g(Vegetable oil is preferably 0.1-0.2g), at the same time, add nitric acid at any time during the digestion process. Generally speaking, add 15ml of nitric acid perchloric acid mixture, let it slowly oxidize overnight, and add about 10ml of mixed acid in the middle of digestion the next day.


Alcoholic samples such as wine and fruit wine contain a large amount of ethanol, so they must be heated to evaporate the ethanol before adding mixed acid for digestion.(Be careful not to dry up) After the ethanol has evaporated, add acid for digestion. Someone once added mixed acid directly to ethanol without ethanol when digesting a wine sample. As a result, after a few minutes, the reaction was very violent, a large number of bubbles were generated and the sample overflowed, resulting in the failure of the test.


Precautions when digesting heavy metals

Heavy metals originally refer to metals with a density greater than 4.5g/cm3, including gold, silver, copper, iron, mercury, lead, cadmium, etc. Heavy metals accumulate in the human body to a certain extent and can cause chronic poisoning. However, in terms of environmental pollution, heavy metals mainly refer to heavy elements with significant biological toxicity such as mercury (mercury), cadmium, lead, chromium, and metalloid arsenic.


Heavy metals are very difficult to be biodegraded. On the contrary, they can be enriched by hundreds of times under the biomagnification of the food chain, and finally enter the human body. Heavy metals can strongly interact with proteins and enzymes in the human body, making them inactive, and may also accumulate in certain organs of the human body, causing chronic poisoning.


Since heavy metals are non-degradable in the human body, all walks of life now attach great importance to the detection of heavy metals, and hereby introduce the pre-treatment precautions for the detection of heavy metals in food:

Note: This method is mainly described for the first method of GB 5009.268-2016 Determination of multi-elements in food.


Precautions:

1. All pre-treatment tests except sample weighing must be carried out in a fume hood. All cleaning must be protected in advance. It is best to wear complete gloves and masks.

3. The digestion tube cannot be cleaned with a brush.

4. Before using the digestion tube, it needs to be dried on the acid cleaner or shake off the residual water on the inner wall of the tube, and then use it after drying the outer wall of the digestive tube with paper; note: the outer wall needs to be dried with paper, and the inner wall should be shaken as much as possible No water droplets flow out, otherwise it will have a certain impact on the results.

5. Gloves are required for weighing.

6. The sample should be placed at least one-third of the bottom of the sample digestion tube and two-thirds of the top of the digestion tube, otherwise it will stick to the wall and rinse unevenly.

7. Static electricity will be generated only when powder samples are weighed directly. It is best to weigh with weighing paper.

8. When adding reagents to the digestive tube after weighing, you need to wear a double-layer book cover to prevent the reagents from corroding your hands.

9. If there is nitric acid in the reagent, it needs to be of superior purity or higher.

10. Do not insert the tip of the pipette directly into the inner wall of the digestive tube when the reagent is flushing the sample digestive tube, so as not to scratch the inner wall and damage the digestive tube and affect the test results.

11. Before adding the reagent, it is best not to draw directly into the original reagent bottle, it is best to pour it into an uncontaminated reagent bottle, and pour it as you use it.

12. After adding the reagents, shake well and let stand for about 5 minutes.

13. After adding the inner cover and outer cover, let it stand for about 10-15 minutes to react.

14. The microwave digestion instrument needs regular maintenance to keep the inside dry and clean.

15. After the digestion is completed, the transferred sample must be in a transparent state, otherwise it is considered as incomplete digestion.


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