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Atomic absorption spectroscopy
Atomic absorption spectroscopy in analytical chemistry is a technique for determining the concentration of a particular metal element within a sample. Atomic Absorbtion Spectroscopy can be used to analyse the concentration of over 62 different metals in a solution.
The technique of Atomic Absorption Spectroscopy was first developed during the 1950's by a team of Australian chemist's, lead by Alan Walsh , working at the CSIRO (Commonwealth Science and Industry Research Organisation) Division of Chemical Physics, in Melbourne Australia.
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, where it is subject to high temperatures. 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 broken into 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 light is focussed through this flame at its longest axis (the lateral axis) onto a detector past the flame.
The light that is focussed into the flame is produced by a hollow cathode tube. Inisde the tube is a cylindrical metal cathode containing the metal for excitation, and an anode. When a high voltage is applied across the anode and cathode, the metal atoms in the cathode are exciting into producing light with a certain emission spectra. The type of hollow cathode tube depends on the metal being analysed. For analysing the concentration of copper in an ore, a copper cathode tube would be used, and likewise for any other metal being analysed. 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 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 flame can be attained using nitrous oxide and acetylene, although this mixture is explosive.
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