14.4 : Atomic Absorption Spectroscopy: Instrumentation

An atomic absorption spectrophotometer (AAS) comprises several components: a radiation source, an atomizer, a monochromator, and a detector. The radiation source can be a hollow-cathode lamp (HCL) or an electrodeless-discharge lamp (EDL), both of which provide a narrow emission line of the required wavelength. However, some instruments use continuum sources and high-resolution monochromators to achieve a narrow range of radiation.

The atomizer used in AAS can be either a flame atomizer or an electrothermal atomizer. The gas-phase atoms generated by the atomizer often get thermally excited and emit radiation, causing an interference with the radiation source signal. A circular metal disk, called a chopper, is used to eliminate such interference between the radiation source and the flame atomizer. The metal chopper isolates the radiation source by creating an alternating signal, unlike the continuous signal from flame.

In a single-beam spectrophotometer, the radiation passes directly through the atomized sample, while in a double-beam instrument, the source beam is split, with one beam passing through the atomized sample and the other being the reference beam. The two beams are then recombined using a half-silvered mirror. The combined beam then goes through the monochromator, which allows only a narrow wavelength range to reach the detector. The detector is typically a photomultiplier tube that processes and converts the radiation into amplified electrical signals.