Reference substances can be classified according to the properties to which they are valued. According to the provisions of the Measures for the management of reference materials, the management of reference materials can be divided into chemical composition reference materials (including metallurgical, environmental analysis, chemical and other reference materials), physical or physicochemical properties reference materials (including optical, magnetic, acidity, conductivity and other reference materials). , and engineering characteristics of the standard materials (including particle size, rubber wear resistance, surface roughness and other standard materials).

At present, although the International Organization for Standardization Standard Substances Committee has a classification method for standard substances, it is not fully adopted in the relevant international organizations and national metrology bodies. Therefore, it can be said that the classification of standard substances is varied. In fact, this situation has caused great inconvenience to the development and application of standard substances, especially for users. According to the characteristics of the actual classification of international organizations and countries, they are summarized into the main methods of classifying people according to the subject of characteristic quantity value, according to the application field of standard material and according to the physical form of standard material.

Classification method according to the subject characteristics of the characteristic value:

This classification method is classified according to the characteristics of the subject reflected by the characteristic quantity and the subject applied. Usually divided into:

1, chemical composition or purity standard material.

2, physical (physical chemical) characteristics of standard material.

3, engineering and technical characteristics of standard material.

4. Biochemical standard material.

Typical is the International Laboratory Accreditation Cooperation (ILAC) classification, which divides the properties of reference substances into five categories:

(1) Chemical classification: reference material, pure compound or representative matrix sample, natural or added analyte (e.g., insecticide added animal fat used for pesticide residue analysis), characterized by one or more chemical or physicochemical property values.

(2) Biological and clinical properties Class: Standard substances similar to Catalog A, but characterized by one or more biochemical or clinical property values, such as enzyme activity.

(3) Physical properties class: standard substances characterized by one or more physical property values, such as melting point, viscosity, and density.

(4) Engineering properties class: standard substances characterized by one or more engineering property values, such as hardness, tensile strength, and surface properties.

(5) Other characteristics.

In the chemical composition category, the reference material can be further divided into two categories: single-component reference material and matrix reference material.

The standard material of a single component is a pure substance (element or compound), or a solution of a pure substance whose purity, concentration, melting point, melting enthalpy, viscosity, UVVIS absorptivity, flash point, and other reference values have been precisely determined. One of the important uses of this type of reference material is the verification or calibration of analytical instruments.

The matrix reference material is usually a real material in which the analyte of interest exists in its natural environment in its natural state (natural matrix reference material). The selected matrix reference material should have a similar matrix to the test sample.

In addition, the accurately determined analyte content of the matrix reference material should be as close as possible to the sample under test. One of the most important uses of matrix reference materials is the testing and validation of analytical measurement methods. Unlike single-component reference materials, matrix reference materials are introduced at the beginning of the analysis process.

Therefore, they are used to evaluate the quality of the entire analytical process, including sample extraction, cleaning, concentration and final measurement steps. The matrix reference material can also be synthesized. Synthetic matrix reference materials may have some differences from natural matrix reference materials when used.

The above content is an introduction to the classification method of reference substances. In analytical chemistry, reference substances are the main components of the traceability chain. Therefore, their metrological characteristics, especially the uncertainty of the properties and their position in the traceability hierarchy, are the focus of concern in the analysis and measurement quality assurance.