Spectroscopie atomique : absorption, émission et fluorescence

Atomization is the process of converting solid, liquid, or solution-phase samples into gas-phase atoms and ions. It is the critical first step in all three atomic spectroscopy methods—AAS, AES, and AFS—because the techniques require gaseous atoms to interact with electromagnetic radiation or heat for analysis.

In AAS, gaseous ground-state atoms absorb photons at specific wavelengths that promote electrons to excited states. The detector measures the reduction in transmitted light at those wavelengths. The amount of light absorbed is proportional to the concentration of the element, allowing quantitative elemental analysis.

In AES, heat or electrical discharge excites ground-state gaseous atoms to high-energy states. These excited atoms quickly relax back to the ground state, emitting photons with energies matching the energy gaps. The detector measures the intensity of emitted light, creating a fingerprint spectrum unique to each element.

In AFS, ground-state gaseous atoms are irradiated at a characteristic wavelength to promote them to an excited state. If the atom is present, it fluoresces by emitting light at the same wavelength. The detector, positioned at a right angle to the laser beam, measures only the fluorescence signal without background interference.

Atomic spectra display sharp lines because gas-phase atoms lack the rotational and vibrational energy states present in molecules. These additional energy states in molecules cause peak broadening. The absence of such states in atoms results in discrete, well-defined spectral lines corresponding to specific electronic transitions.

The three main optical atomic spectroscopy methods are atomic absorption spectroscopy (AAS), which measures light absorption; atomic emission spectroscopy (AES), which measures emitted light; and atomic fluorescence spectroscopy (AFS), which measures fluorescence. All three begin with atomization and provide qualitative and quantitative elemental analysis.

Atomic spectroscopy identifies elements qualitatively by detecting characteristic wavelengths unique to each element's electronic transitions. Quantitatively, the intensity or amount of absorbed, emitted, or fluorescent light is directly proportional to elemental concentration, enabling precise measurement of sample composition and elemental identification.