1. Ashing method (also known as dry method)
Ashing method is to use high temperature to remove organic matter in the sample, and the remaining ash is dissolved with acid as the sample solution to be tested. Dry ashing is suitable for most metal element content analysis, but samples containing more fat and sugar require a longer time, while samples containing more cellulose and protein require a shorter time. According to the type of sample and the nature of the components to be tested, different crucibles and ashing temperatures are selected. Commonly used crucibles are made of quartz, platinum, silver, nickel, iron, porcelain, polytetrafluoroethylene and other materials. Advantages: No or less chemical reagents are used, and a larger sample volume can be processed, so it is conducive to improving the accuracy of trace element determination, simple operation and safety. Disadvantages: The ashing temperature is generally 500-600℃, which is not suitable for processing samples with volatile components. For example, mercury, lead, cadmium, tin, selenium, etc. are easily volatile and lost under high temperature conditions, so this method is not applicable; the increase in temperature will also introduce pollution caused by crucible loss, and the sample volume is limited. Dry samples generally do not exceed 109℃, and fresh samples do not exceed 509℃. If the sample quantity is too large, it will cause difficulty in ashing or take too long, which will inevitably introduce new errors. On the contrary, if the sample quantity is too small, it will also introduce errors due to sample inhomogeneity.
2. Wet digestion is also called wet digestion.
It is a method of destroying organic matter or reducing substances in the sample with acid or alkali solution under heating conditions. Commonly used acid hydrolysis systems include: HN03-H2S04, HN03-HCl04, HF, H202, etc. They can completely destroy organic matter and reducing substances in sewage and sediments such as cyanide, nitrite, sulfide, sulfite, thiosulfate and thermally unstable substances such as thiocyanate. Alkaline hydrolysis often uses caustic soda solution. Digestion can be carried out in a crucible (nickel, polytetrafluoroethylene). Advantages: Wet digestion has the advantages of flexible adjustment of digestion temperature, digestion acid type and dosage and digestion time, and is widely used in sample processing for elemental analysis. Disadvantages: Wet digestion uses a large amount of acid and it is difficult to make the solution clear, which is easy to cause environmental pollution and element loss (such as Ni), and some elements (such as Zn) may be easily contaminated. In addition, for biological samples containing a large amount of organic matter, especially samples with high fat and cellulose content, such as meat, fat, flour, rice, straw, etc., a large amount of foam is easily generated during heating and digestion, which can easily cause the loss of the measured components. If HNO3 is added first and then placed at room temperature for 24 hours before digestion, the generation of foam can be greatly reduced. In some cases, an anti-foaming agent can be added.
3. Microwave digestion
Microwave digestion usually refers to wet digestion (there are also open container microwave digestion) that uses microwaves to heat the digestion solution (various acids, some alkali solutions and salts) and samples in a closed container, so that various samples can be quickly dissolved under high temperature and pressure conditions. In contrast to the traditional conduction heating method, microwaves directly heat the inside of the substance, greatly accelerating the decomposition of the sample. Advantages: It can quickly and effectively decompose the sample and shorten the dissolution time; the amount of reagents used is small, generally only a few milliliters are needed; the loss and cross-contamination of the sample during the digestion process are greatly reduced; energy consumption is reduced, and automatic operation is easy to achieve, while reducing the pollution of the environment by conventional digestion acid mist; the loss of volatile trace elements can be avoided. Disadvantages: The maximum temperature can only reach 2000℃, and the digestion of difficult-to-decompose substances in biological samples is not thorough enough, which will cause negative errors and make the measurement results low; high pressure (up to l×l-1.5×107Pa), high temperature (usually 180~240℃), and strong acid vapor bring psychological pressure on safety to the experimenter; the small amount of digested samples is insufficient.
4. Sample dissolution in a high-pressure sealed tank
In a high-pressure resistant thick-walled stainless steel outer sleeve, place a tightly sealed polytetrafluoroethylene crucible container, add the sample and acid into the crucible and then add them into the sleeve. The method of heating decomposition is usually also called the high-pressure canning method. Advantages: small amount of acid is used, which is a few tenths of the general non-sealed system; due to the small amount of acid, the blank value is low and the reproducibility is good; it is not affected by the laboratory atmosphere; the loss of volatile elements is small; the sample decomposition speed is fast, the operation is simple, and a large number of samples can be decomposed at the same time. Disadvantages: The shell is easily corroded by acid; the decomposition reaction process cannot be seen from the outside, and it can only be judged whether the decomposition is complete after cooling and opening; the amount of decomposed sample is small, generally not exceeding 1.09, so it is subject to certain restrictions when determining ultra-trace elements; when decomposing organic matter or soil containing high organic matter, there is a risk of explosion, especially in the presence of perchloric acid.