To perform absorption spectroscopy: 

• The solvent liquid is put in a cuvette or transport vessel and placed in a spectrometer to determine the loss of light caused due to absorbance and scattering by the solvent. The absorbance of the solvent is noted. 
• The sample solution is put in another cuvette and placed in the spectrometer. The absorbance of the sample is noted at various frequencies, typically ranging from 200 nm to 800 nm. 
• Different concentrations of the samples are used and a spectrum is formed, noting the absorbance of each concentration. 
• A graph is plotted marking the absorbance measured against the concentration of the solvent. This is used to determine the unknown concentration of the sample.