The purity of metals is relative to impurities, which broadly include chemical impurities (elements) and physical impurities (crystal defects). Physical impurities mainly refer to dislocations and vacancies, while chemical impurities refer to the incorporation of atoms outside the matrix in the form of substitution or filling gaps. But only when the purity of the metal reaches a very high standard (such as metals with a purity of 9N or higher), the concept of physical impurities is meaningful. Therefore, in production, the content of chemical impurities is generally used as the standard for evaluating the purity of the metal, which is expressed as the percentage of the main metal minus the total impurity content. N (the first letter of nine) is commonly used to represent it, such as 99.9999% written as 6N and 99.99999% written as 7N.

At present, there are two main ways to represent the purity of high-purity metals: one is based on the purpose of the material, such as "spectral purity", "electronic grade purity", etc; Another way is to represent it with certain characteristics, for example, semiconductor materials are represented by carrier concentration, which is the number of conductive impurities in a 1 cubic centimeter matrix element, while metals are mainly represented by residual resistivity RRR and purity level R.

With the improvement of material processing technology, the purity of metals is constantly increasing, but it is impossible to prepare absolutely pure metals. The so-called "high-purity" and "ultra pure" have relative meanings, referring to the standards achieved technically. Due to the development of science and technology, the standards for "high-purity" and "ultrapure" are constantly being upgraded. For example, in the past, impurities in high-purity metals were at the ppm level (i.e. parts per million), while impurities in ultra pure semiconductor materials were at the ppb level (parts per billion). However, now it has gradually developed to be expressed at the ppb level (parts per billion) and ppt level (parts per trillion). At the same time, the purification difficulty of each metal is not the same. For example, in semiconductor materials, 9N or above is considered high purity, while refractory metals up to 6N are considered ultra high purity.