Atomic absorption spectroscopy, or AAS, is an elemental analysis technique based on the principle of absorption of electromagnetic radiation resulting in electronic excitation.

In AAS, the sample is first atomized—converting analytes into gas-phase atoms. Although the flame temperature excites some gas-phase atoms, most remain unexcited.

When electromagnetic radiation of a particular wavelength irradiates an atom, it is absorbed if it provides the exact energy for an electronic transition to a higher-energy orbital.

The difference in the incident and transmitted radiation intensities measures the radiation absorbed, quantifying the analyte.

AAS follows the Beer–Lambert law, where the absorbance directly depends on the concentration of the gas-phase atoms in the flame, assuming a constant pathlength in the flame.

Notably, AAS offers high sensitivity with detection limits in nanogram per milliliter levels and is widely used for trace-metal analysis.

However, AAS necessitates liquid-phase or otherwise volatile solids. Additionally, it requires either a high-resolution continuum-source spectrometer or characteristic radiation sources for every element to be analyzed.