atomic fluorescence spectroscopy
/ uh-TOM-ik flor-ESS-ence /
Atomic fluorescence spectroscopy is like watching something glow in the dark after you shine a light on it, then quickly look away. You hit free atoms with light to excite them, then measure the fresh light they re-emit a moment later — and crucially you look from the side, so you see only the glow the atoms make, not the lamp.
More precisely, a light source excites the analyte atoms; as they relax, they emit light at characteristic wavelengths. The detector is placed off to the side, away from the exciting beam, so against a dark background even a faint fluorescence stands out clearly. Signal grows with concentration, as in the other atomic methods.
It matters because measuring a small signal against darkness, rather than a small dip in a bright beam, gives superb sensitivity for a handful of elements — mercury, arsenic, selenium and the like — often paired with vapour generation. The honest caveat is that it shines for only a limited set of elements, so it is a specialist tool rather than a general one.
To measure trace mercury in fish, the mercury is chemically turned into cold vapour, lit by a mercury lamp, and its sideways fluorescence at 253.7 nm is read against a dark background — detecting mercury at parts-per-trillion levels.
Excite the atoms, then read their re-emitted glow from the side against darkness.
Atomic fluorescence resembles atomic emission in that the atoms emit light, but the atoms are excited by light first, and the detector looks off-axis from that exciting beam. The off-axis dark-field geometry is what buys its low backgrounds and high sensitivity.