Photométrie de flamme : laboratoire

When a sample solution is aspirated into the flame, the solvent evaporates from fine droplets, leaving a dehydrated sample. The sample then dissociates into free gaseous atoms in their ground state. Some atoms absorb energy from the flame and transition to excited electronic states, while others remain in the ground state.

Excited atoms have short lifetimes and return to their ground state by emitting photons at characteristic wavelengths. Because only electronic transitions are involved, the resulting emission lines are very narrow. The intensity of emitted radiation is directly related to the concentration of analyte atoms undergoing excitation and emission.

At low analyte concentrations, emission intensity is linearly proportional to analyte concentration. However, at higher concentrations, more ground-state atoms accumulate and reabsorb the emitted radiation, causing self-absorption. This reabsorption reduces the emission signal and deviates from the linear relationship observed at lower concentrations.

Side reactions in the flame often result in diminished population of desired emitting species, thereby reducing the emission signal. These competing reactions consume analyte atoms or form non-emitting compounds, lowering the number of atoms available to emit characteristic photons and affecting measurement accuracy.

Flame photometry is primarily used for quantitative determination of alkali and alkaline earth metals, especially sodium, potassium, lithium, and calcium. Applications include clinical laboratory analysis, measurement of residual alkali metals in biodiesel, and determination of sodium, potassium, and calcium in cement samples.

Modern flame photometers can automatically process multianalyte multipoint calibration data and perform measurements without requiring sample dilution, accommodating analyte concentrations up to 1000 mg/L. This capability improves efficiency and reduces sample preparation time while maintaining measurement accuracy across a wide concentration range.

While flame photometry measures emission from excited atoms, atomic absorption spectroscopy measures ground-state atom absorption. Historically, flame emission could measure up to 60 elements in hot nitrous oxide-acetylene flames. Today, atomic absorption spectroscopy is predominantly used for measuring metals other than alkali metals due to superior sensitivity and selectivity.