The  UV-visible absorption spectrum of the molecule is due to the absorption  of UV-visible radiation from certain groups in the molecule, resulting  in an electron energy level transition resulting from the absorption  spectrum. Because  the various substances have their own different molecules, atoms and  different molecular space structure, the absorption of light energy will  not be the same, so each substance has its own unique, fixed absorption  spectrum curve, according to the absorption The absorbance at some characteristic wavelengths on the spectrum  determines or measures the content of the substance, which is the basis  for qualitative and quantitative analysis of spectrophotometry.

Spectrophotometric  analysis is an effective means of studying the composition, structure,  and intermolecular interactions of matter based on the absorption  spectrum of matter. It is a banded spectrum that reflects the information of certain groups in the molecule. Qualitative analysis can be performed using standard light patterns in conjunction with other means.
According  to Lambert-Beer's law, the absorption of light is proportional to the  thickness of the absorptive layer. Beer's law shows that the absorption  of light is proportional to the concentration of the solution. If the  effect of absorption layer thickness and solution concentration on light  absorption is taken into account, Beer's Law. That is, A = εbc, (A for the absorbance, ε for the molar absorption  coefficient, b for the pool thickness, c for the solution concentration)  can be a quantitative analysis of the solution.
The  analytical samples and standard samples were prepared in the same  solvent at the same concentration, and the UV-visible absorption spectra  were measured under the same conditions. If the two are the same material, then the two spectra should be exactly the same. If there is no standard, it can also be compared with the ready-made standard spectrum comparison. This method requires accurate instruments, high precision, and the determination of the same conditions.