In the analytical work, except for dry analysis (such as spectral analysis, etc.), the test of the sample is basically carried out in the solution, so if the sample is in a non-solution state, it needs to be converted into a solution by appropriate methods. This process is called sample decomposition. Sample decomposition is an important part of the analytical work, which is not only related to the transformation of the components to be tested into a suitable form, but also related to the future separation and determination.
When decomposing the sample, care must be taken to completely decompose the sample, and no fines or powder of the original sample shall remain in the treated solution. During the decomposition process of the sample, the components to be tested should not be volatilized and lost, and the components to be tested and interfering substances should not be introduced. There are many ways to decompose the sample, and the appropriate method can be selected for decomposition according to the composition and characteristics of sample, the properties of the components to be measured and the purpose of analysis. The following are several common decomposition methods.
1. Dissolving Method
Dissolution refers to the use of appropriate solvent to dissolve the sample into a solution, this method is relatively simple and rapid. Water is one of the important solvents to dissolve inorganic substances. Alkali metal salts, ammonium and magnesium salts, inorganic nitrates and most alkaline earth metal salts are easily soluble in water. Acids, bases or mixed acids are usually used as solvents for the decomposition of inorganic substances insoluble in water.
2. Fusion Method
Fusion method refers to mixing the sample with an acidic or alkaline solid solvent, and allowing it to undergo a double decomposition reaction at high temperature, so that the components to be measured are converted into water- or acid-soluble compounds, such as sodium salts, potassium salts, sulfates or chloride, etc. Inorganic samples that are insoluble in water, acid or alkali can generally be decomposed by this method.
3. Semi-melting Method
The semi-melting method, also known as the sintering method, reacts the sample with the flux at a temperature lower than the melting point. Compared with the melting method, the semi-melting method has a lower temperature and a longer heating time, but it is not easy to damage the dry pot, and it can usually be carried out in a porcelain crucible without precious metal utensils.
4. Dry Ashing Method
Dry Ashing is suitable for decomposing organic matter and biological samples in order to determine the content of metal elements, sulfur and halogen elements. In this method, the sample is heated and decomposed in a muffle furnace, and the oxygen in the atmosphere acts as an oxidant, leaving inorganic residues after combustion. The residue is usually leached with a small amount of concentrated hydrochloric acid or hot concentrated nitric acid, and then quantitatively transferred to a glass container. In the dry ashing process, a small amount of certain oxidizing substances (auxiliary agents) can be added to the sample as needed to improve the ashing efficiency. Magnesium nitrate is one of the commonly used additives. For liquid or wet animal and plant cell tissues, they should be dried by steam bath or mild heating method before ashing and decomposition. The muffle furnace should be gradually heated to the desired temperature to prevent ignition or foaming.
5. Wet Ashing Method
In this method, the mixture of nitric acid and sulfuric acid is usually placed in a gram flask together with the sample, and digested at a certain temperature, in which nitric acid can destroy most of the organic matter. During the digestion process, nitric acid is evaporated, and finally sulfuric acid remains. When thick SO3 white smoke begins to emerge, reflux is carried out in the flask until the solution becomes transparent. During digestion, nitric acid oxidizes organic matter to carbon dioxide, water, and other volatile products, leaving inorganic acids or salts.
6. Microwave Digestion
In addition to dissolving at room temperature and heating conditions, microwave heating can also be used to assist dissolution. Microwave digestion uses the sample and appropriate solvent (flux) to absorb microwave to heat the sample. At the same time, the alternating magnetic field produced by microwave polarizes the dielectric molecules. The alternating arrangement of polarized molecules in the high-frequency magnetic field leads to the high-speed oscillation of the molecules, which makes the molecules obtain high energy. As a result of these two actions, the surface layer of the sample is constantly agitated and broken, resulting in rapid dissolution (melting).
