Determination of lead, cadmium, arsenic, mercury and copper residues in traditional Chinese medicine (1)

In modern society, with the acceleration of industrialization, environmental pollution has become increasingly serious

The heavy metals and harmful elements that may exist in traditional Chinese medicine are represented by arsenic, mercury, lead, cadmium, and copper

At present, among the analysis methods of trace elements, the instrumental analysis method occupies a dominant position due to its high sensitivity and strong specificity

The detection of lead, cadmium, arsenic, mercury, and copper residues in traditional Chinese medicine belongs to the category of trace analysis, which is different from ordinary macro or micro analysis, and requires high detection sensitivity, and the test environment and reagents often interfere with the determination

The analysis of heavy metals and harmful elements in traditional Chinese medicine mainly includes two steps: sample digestion (pretreatment) and analysis and determination.

1.

The pretreatment of samples in trace element analysis mainly includes three aspects: sample preparation, digestion and preparation of test solution

(1) Preparation of samples

Due to individual differences in Chinese medicinal materials, the repeatability of the analysis results firstly depends on the completeness, homogeneity and representativeness of the samples.

For samples of proprietary Chinese medicines, samples should be taken after preparing a uniformly mixed sample according to the characteristics of the dosage form

(2) Digestion of the sample

The decomposition and dissolution of the sample is an important part of the analysis.

The ideal digestion method should be able to fully destroy the organism without obvious loss of the elements to be tested, easy to operate, safe, pollution-free, fast, economical, and efficient

1.

Generally take 0.

2.

Nitric acid is widely used in wet digestion

Digestion can be carried out using a Kjeldahl flask on an electric heating plate

The sample solution obtained by wet digestion can be used for the trace detection of all five elements specified in the Pharmacopoeia, but when the sample needs to be used for the determination of Hg, in order to prevent the volatilization loss of Hg, the digestion temperature should not be higher than 140℃
.

3.
Microwave Digestion Method Microwave is an electromagnetic wave with a frequency of 300MHz～300GHz, that is, a wavelength of 100cm～1mm, which is located between infrared and radio waves
.
Microwave heating actually uses microwaves to act on polar molecules, causing them to produce more than 2.
5 billion molecular rotations and collisions per second, which quickly raises the reaction temperature, and polar molecules are in a metastable state, which promotes further ionization or oxidation.
Response
.
Engineers designed a microwave digestion instrument by using the three characteristics of reflection, transmission and absorption of microwaves for different substances.
Compared with traditional electric heating plate heating, microwave heating does not require an intermediate heat transfer process, and the reaction is rapid.
Energy can be precisely controlled in real time
.
The microwave digestion method has the advantages of rapid, complete digestion, small reagent usage, low pollution, and low blank value.
It is included in the 2010 edition of the "Chinese Pharmacopoeia" Appendix (XB) "Lead, Cadmium, Arsenic, Mercury, Copper Inspection Method" The preferred method of sample digestion
.
Currently, there are finished microwave digesters available for sale both domestically and internationally
.
The microwave digestion apparatus must be operated in strict accordance with the manufacturer's operating procedures and pay attention to safety
.

When digesting general botanical samples of Chinese medicinal materials, take 0.
5g of the sample powder, add 5-7ml of nitric acid , and install the digestion tank.
Take the Mars5 digestion apparatus (using 14 HP500 medium-pressure digestion tanks) of CEM Corporation in the United States as an example, you can press Digestion by the above procedure: Step 1: Power 600W, increase from room temperature to 110°C in 3 minutes, hold for 3 minutes;

Step 2: Power 1200W, increase from 110°C to 150°C in 3 minutes, keep it for 3 minutes;

The third step: The power is 1200W, the temperature is increased from 150°C to 180°C in 3 minutes, and the temperature is maintained for 12 minutes
.

Step-by-step digestion is conducive to the complete digestion of the sample, and can prevent the pressure in the tank from rising too fast during the digestion, reducing the risk of operation
.
Different Chinese medicine samples may have different digestion procedures.
If necessary, pre-experiments should be done to find the best digestion conditions
.

After the digestion is completed, cool the digestion tank to below 70°C, then slowly release the pressure, open the digestion tank, and place the hot plate to slowly heat below 130°C until the red-brown nitrogen oxide vapor is exhausted and continue to be concentrated to About 2ml, let cool, transfer to a 25ml measuring flask with water, and dilute to the mark
.
The digested solution should be colorless or slightly yellowish green and clear
.

(3) Matters needing attention

1.
The analysis of heavy metal and hazardous element residues belongs to the category of trace analysis.
In the whole process of the experiment, attention must be paid to prevent pollution
.

High-purity reagents must be used for nitric acid, and double distilled water is the best water.
All reagents and solvents should be checked before use to ensure that they meet the analysis requirements
.

2.
Because glassware is easy to absorb or release trace metal ions, it is better to use modified polypropylene and other plastic utensils in the experiment process, and the utensils made of polytetrafluoroethylene material are the best
.
Glassware can only be used for operations such as constant volume in a short time
.
The utensils used can be soaked in nitric acid solution (1→3) overnight before use, and then rinsed with water before use.
It is not recommended to use detergent or conventional lotion to prevent contamination
.

3.
The digestion process requires the use of highly toxic, highly volatile strong acid, and it is also a high-temperature heating process.
Special attention should be paid to the personal safety of the tester.
Protective measures such as gloves, acid-proof clothing, eye masks should be prepared, and exhaust ventilation should be strengthened.

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