Atomic absorption spectrophotometer is an instrument that quantitatively analyzes the phenomenon of atoms absorbing light of a specified wavelength. The core principle involves the energy level transitions of atoms and the absorption characteristics of light.

（1） Atomic Energy Levels and Absorption

Under normal circumstances, atoms are in the ground state, which is a low energy state. When atoms are excited by external energy (such as heat provided by flames, graphite furnaces, etc.), electrons will transition to higher energy levels, forming excited atoms. And atoms of different elements have energy level structures, and the energy required for their electronic transitions is specified. When a beam of light of a specified wavelength passes through atomic vapor, if the energy of the light is exactly equal to the energy required for electrons in the atom to transition from the ground state to a higher energy level, the atom will absorb this part of the light, reducing its intensity. The phenomenon of atoms absorbing light of a specified wavelength forms the basis of atomic absorption spectroscopy.

（2） The function of the spectroscopic system

The spectroscopic system in an atomic absorption spectrophotometer plays a crucial role. It can decompose the continuous spectrum emitted by the light source into a single wavelength beam, and then accurately select the characteristic spectral line wavelength corresponding to the tested element. For example, for the determination of sodium element, the spectroscopic system will screen out the light of a specified wavelength (such as 589.0nm) corresponding to the electronic transition of sodium element when it is excited in the flame, and pass it through the atomic vapor sample. In this way, it is possible to specifically detect the absorption of sodium atoms in the sample at the specified wavelength of light, thereby eliminating interference from other wavelengths of light and improving the accuracy of analysis.

（3） Detection and quantification

When light of a specified wavelength passes through an absorption cell containing atomic vapor of the element under test (such as a flame burner or graphite furnace), the intensity of the light will decay. The degree of attenuation is closely related to the concentration of the element to be tested in the sample. By detecting the intensity of light passing through the sample and comparing it with the absorbance of a standard solution of known concentration under the same conditions, a quantitative relationship between absorbance and concentration can be established based on Lambert Beer's law (A=kc, where A is absorbance, k is molar absorptivity, and c is the concentration of the tested element), thereby achieving accurate determination of the content of the tested element.