Atoms or molecules that are excited to high energy levels can decay to lower levels by emitting radiation (emission or luminescence). For atoms excited by a high-temperature energy source this light emission is commonly called atomic or optical emission (atomic-emission spectroscopy) and for atoms excited with light it is called atomic fluorescence (atomic-fluorescence spectroscopy.)
Atomic fluorescence is the optical emission from gas-phase atoms that have been excited to higher energy levels by absorption of electromagnetic radiation. Atomic fluorescence spectroscopy (AFS) is a multi-element technique used for analysis of trace metals in e.g., in seawater, biological substances and agricultural samples. It has its greatest sensitivity for zinc, mercury and selenium.
Fluoresence Spectrometry - Instrumentation
Analysis of solutions or solids requires that the analyte atoms be desolvated, vaporized, and atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace. A hollow-cathode lamp or laser provides the resonant excitation to promote the atoms to higher energy levels. The atomic fluorescence is dispersed and detected by monochromators and photomultiplier tubes, similar to atomic-emission spectroscopy instrumentation.
There are two types of fluorescence instruments, dispersive and nondispersive. A dispersive instrument is made up of a light source, an atomizer, an analyzer, a detector and processor and display. The ideal source for atomic fluorescence are lasers but the most common source is the electrodeless discharge lamp. A non-dispersive instrument is built up of a light source, an atomizer and a detector. There is no need for an analyzer when an electrodless discharge lamp serves as excitation source, the emitted radiation is that of a single element.
A flame atomizer consist of a nebulizer which converts the sample into an aerosol that is fed into the burner. The best burner system for AFS is a combination of acetylene/ nitrous oxide and hydrogen/oxygen/argon using a rectangular flame. To isolate a narrow band of wavelength, a monochromator or an interference filter system is used. A photomultiplier converts radiant energy to electrical signals.