AAS uses the absorption of light to determine the concentration of specific metal atoms in a solid or liquid by vaporizing the sample in a flame (FAAS) or graphite furnace (GFAAS). The ground state free metal atoms are excited by a specific wavelength of light, with the amount of energy absorbed proportional to the number of atoms of that element in the sample. The difference between the sample and the background absorption is then measured, and compared with the absorption of a series of standard solutions.