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Atomic absorption spectroscopy

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:

  1. Desolvation the liquid solvent is evaporated, and the dry sample remains
  2. Vaporisation the solid sample vaporises to a gas
  3. 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.

Interferences

Some elements will interfere with the signal obtained. For example, you may have an element which absorbs at the same of a very similar frequency as the one you are measuring. This will interfere with your measurements, but as each element has multiple absorbance frequencies, you can normally avoid this through careful selection of your measurement frequency.

Elements with very low ionisation energies will cause interference by absorbing energy from the radiation beam that is not necessarily associated with the element that you are attempting to measure. Sodium is the most common cause of this interference.

Incomplete volatilisation can also cause interferences. Some compounds bond so strongly that their bonds might not be broken by the amount of energy in the flame. To overcome this a hotter flame may be used, but this will increase the risk of interference from ionisation.

Range of linearity

The machine will only have a certain range of signal strength (i.e. difference between the input and output radiation) that produces a linear signal response. The two main factors influencing the amount of radiation absorbed by the element in the flame are the chosen frequency (different absorbance frequencies for any given element will absorb at different strengths); and the concentration of the element in the sample. If the signal is too strong, you can simply dilute your sample. If it is too weak, you could use a more strongly absorbing frequency if one is available or you could use the method of addition, as described in analytical chemistry.

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.

See also electromagnetic spectroscopy, spectroscopy, spontaneous emission



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