Atomic absorption spectroscopy (AAS) is a technique used to analyze elements by measuring electromagnetic radiation (EMR) absorbed by atoms, which causes them to transition to a higher-energy orbit. The most crucial step in AAS is atomization, where the analyte is converted into gas-phase atoms, typically through a flame or furnace. Some of these atoms become thermally excited in the flame, while most remain in the ground state.

When irradiated by EMR of a particular wavelength, these ground-state gas-phase atoms absorb the radiation only if it provides the energy required for their electronic excitation. The difference between incident and transmitted radiant power of EMR is the measure of absorbed radiation, which quantifies the analyte.

Atomic absorption lines are highly narrow, as they generate from characteristic electronic transitions unaccompanied by rotational and vibrational transitions. AAS follows the Beer-Lambert law, which states that the amount of light absorbed is directly proportional to the concentration of the absorbing atoms, assuming a constant path length. AAS is a selective and sensitive technique with detection limits in the nanogram range per milliliter. It is widely used for trace metal analysis in clinical, pharmaceutical, food, mining, environmental, and agricultural fields.

AAS's limitations include the need for solution-phase or volatile solid samples for analysis. In addition, the radiation sources for AAS should either be high-resolution continuum sources or separate line sources for every elemental analysis.