In absorption spectroscopy, light of a particular single frequency is passed through a sample which must be in solution or in gaseous form. The amount of light absorbed is directly relatable to the amount of the particular species present to absorb it. Conditions are somewhat easier to control in this arrangement and quantitative (1%) precision can be obtained. The use of absorption spectroscopy is more general for the lower-energy vibrational and rotational transitions but can be used for electronic transitions as well, as is done in atomic absorption spectroscopy.
See: absorption spectrum
Compare: emission spectroscopy