In a flame photometer, when a solution like potassium chloride is aspirated into the flame, the solvent evaporates, leaving behind dehydrated salt. This salt dissociates into free gaseous atoms in their ground state. Some of these atoms absorb energy from the flame, leading to their excitation. The excited atoms return to the ground state, emitting photons at characteristic wavelengths. Because only electronic transitions are involved, the resulting emission lines are very narrow. The intensity of the emitted radiation is linearly proportional to the concentration of the analyte in the solution being aspirated. Still, this linearity is observed mainly at the low end of the calibration curve. At higher analyte concentrations, more atoms in the ground state, which can reabsorb the emitted radiation, deviates from the linear relationship.

Modern flame photometers can automatically process multianalyte multipoint calibration data and perform measurements without the need for sample dilution, accommodating concentrations up to 1000 mg/L. While flame photometry is primarily used for the determination of sodium (Na), potassium (K), and lithium (Li) in clinical laboratories, it finds niche applications in various fields. For example, it measures residual alkali metals in biodiesel and determines sodium, potassium, and calcium in cement. In the past, flame emission spectrometry enabled the measurement of up to 60 elements in hot nitrous oxide, acetylene flames. However, today, atomic absorption spectrometry, or AAS, is predominantly used for measuring metals other than alkali metals.