The principle of atomic absorption spectroscopy: Each atom of an element can not only emit a series of characteristic spectral lines, but also absorb characteristic spectral lines with the same wavelength as the emission line. When a characteristic wavelength of light emitted by a light source passes through atomic vapor, that is, the frequency of incident radiation is equal to the energy frequency required for electrons in the atom to transition from the ground state to a higher energy state (usually the first excited state), the outer electrons in the atom will selectively absorb the characteristic spectral lines emitted by the same element, causing the incident light to weaken. The degree to which the characteristic spectral lines weaken due to absorption is called absorbance A, which is directly proportional to the content of the measured element.

Spectrophotometry: When the thickness l of the solution layer and the absorbance coefficient are fixed, the absorbance A is linearly related to the concentration of the solution. In quantitative analysis, it is necessary to first determine the absorption of different wavelengths of light by the solution (absorption spectrum), determine the maximum absorption wavelength, and then use this wavelength of light as the light source to measure the absorbance A of a series of known concentrations of c solutions, and create A~c working curves. When analyzing unknown solutions, the corresponding concentration can be determined by looking up the working curve based on the measured absorbance A. This is the basic principle of measuring concentration using spectrophotometry.