Non-specific scatter interference

This causes the enhancement of an analytical result at the ppm or sub-ppm levels due to the solution containing a high concentration of dissolved salts. The effect is due to the presence of dried and semi-dried salt particles in the flame which scatter and absorb the incident radiation from the source. Since the intensity of the transmitted radiation will be decreased, there will be Em increase in the absorption signal. This non-specific scatter effect is wavelength dependent and is more pronounced at shorter wavelengths. It is most significant below 250 nm. 

To understand ionisation interferences, it is necessary to appreciate what is occurring in the flame during the aspiration of a sample. The flame is being used as a source of energy to convert elements in the solution droplets created by the nebuliser into ground-state atoms. 

Many determinations require the use of the nitrous oxide—acetylene flame and it is usually under these conditions that ionisation interferences occur. They arise from the energetic nature of the flame which gives ground-state atoms but also excites some atoms to such an extent that one or more electrons are lost and ionisation occurs. 

This effect will obviously be greatest with elements having low ionisation potentials such as the alkali and alkaline earth metals, e.g. barium is approximately 80% ionised in the nitrous oxide flame. Since the ground state therefore becomes depopulated, the sensitivity will decrease. 

An equally important effect arises when an easily ionised element is being determined in the presence of another. There will be an enhancement of sensitivity compared with pure aqueous standards. This arises from the presence of excess free electrons which suppress further ionisation. 

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