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Atomic absorption spectroscopy in analytical chemistry is a technique for determining the concentration of an element within a sample.

The sample is prepared such that it is contained in a liquid. This liquid is fed into the machine. The machine combines the liquid with a stream of gas-based fuel and oxidant by aerosolising it. Baffles remove the larger droplets in the stream of fuel and oxidant. The mixture passes to a burner, which burns the mixture. Whilst the sample is in the flame it passes through several stages:

Desolvation – the liquid solvent is evaporated, and the dry sample remains
Vaporisation – the solid sample vaporises to a gas
Volatilisation – the compounds making up the sample are destroyed, releasing free atoms.

The flame is arranged such that it is laterally long (usually 10cm) and not deep. The height of the flame must also be controlled by controlling the flow of the fuel mixture. A beam of electromagnetic radiation is focussed through this flame at its longest axis (the lateral axis) onto a detector past the flame.

The frequency of the radiation in the beam is pre-set to a frequency that the element to be analysed is known to absorb at. The electrons of the atoms in the flame can be promoted to higher orbitals for an instant by absorbing a set quantity of energy (a quantum). This amount of energy is specific to a particular electron transition in a particular element. As the frequency of the radiation beam can be controlled, this amount of energy can be supplied in abundance. As the quantity of energy put into the flame is known, and the quantity remaining at the other side (at the detector) can be measured, it is possible to calculate how many of these transitions took place, and thus get a signal that is proportional to the concentration of the element being measured.

Fuel / oxidant mixtures

For a low temperature flame, acetylene and air is used. A hotter flame can be produced using acetylene and pure oxygen, and an even hotter flame can be attained using nitrous oxide and acetylene, although this mixture is explosive.

Spectroscopy Analytical chemistry

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Topics: Atomic Absorption Spectroscopy, Atomic Number, Atomic...

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