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Atomic absorption spectrometry (AAS) is a spectro-analytical procedure for qualitative and quantitative assessment of chemical elements based on the absorption of light(optical radiation) by free atoms in the gaseous state. It is used to determine the concentration of a particular element in a sample. AAS can be used to determine over 70 different elements in a single sample. It was first used as an analytical technique in the second half of the 19th century. The modern form of AAS was largely developed during the 1950s by a team of chemists in Australia.
Atomic absorption spectroscopy is based on standards with known analyte content to establish the relation between the measured absorbance and the analyte concentration and on the Beer-Lambert Law. The electrons in the atomizer can be placed in an excited state for a period of nanoseconds by absorbing the radiation of a given wavelength. This wavelength is specific to a particular electron transition in a particular element, so each wavelength corresponds to only one element, and the width of an absorption line is only a few picometers (pm), which gives the technique its elemental selectivity. The radiation flux with and without a sample in the atomizer is measured and the ratio between the two values is converted to analyte concentration or mass using the Beer-Lambert Law.
Hollow cathode lamps are the most common radiation source in AAS. Inside the sealed lamp, filled with argon or neon gas at low pressure, is a cylindrical metal cathode containing the element being tested and an anode. A high voltage is applied across the anode and cathode, resulting in an ionization of the fill gas. The gas ions are accelerated towards the cathode and the cathode material is excited. This causes the glow discharge to emit the radiation of the element being tested.
The relatively small number of atomic absorption lines and their narrow width make spectral overlap rare. Molecular absorption, in contrast, is much broader, so that it is more likely that some molecular absorption band will overlap with an atomic line. This kind of absorption might be caused by undissassociated molecules of concomitant elements of the sample or by flame gases. Another source of background absorption is scattering of the primary radiation at particles that are generated in the atomization stage. These phenomena can result in artificially high absorption and a high calculation for the concentration or mass of the analyte. This has to be accounted for. In atomic absorption spectroscopy background absorption can only be corrected with instrumental techniques and all of them are based on two sequential measurements. Because of this and the use of additional devices in the spectrometer, the signal-to-noise ratio of background-corrected signals is always significantly inferior compared to uncorrected signals. Once background absorption was accounted for, atomic absorption spectroscopy became an important tool in science.
We’ve also recorded an entire episode of Astronomy Cast all about Energy Levels and Spectra. Listen here, Episode 139: Energy Levels and Spectra.