Flame, Furnace or Plasma - Which to Choose

Flame AAS is a well established technique which suffers from relatively few interferences and these are well characterised. The instrument is relatively inexpensive, easy to operate and, of the three techniques, requires the least operator experience. For most elements the LODs achieved with FAAS or with ICP-OES are comparable - within a factor of two or three of each other. The ICP approach is advantageous (i) for elements that form refractory oxides and (ii) for elements which are characterised by low ionisation energies. Included in the former are B, V, Ti and W which are only partially dissociated in the flame. Typical of the latter group are elements Mg, Ca, Be, Zr and the rare earth elements whose most sensitive emission lines are from ions rather than atoms. By contrast, the more volatile heavy elements such as Cd and Zn have slightly lower limits of detection by FAAS. 

For the analysis of major elements (rather than trace determinations), AAS has been demonstrated to provide coefficients of variation (CVs) of approximately 0.2 % which is not appreciably different from ICP-OES. Both techniques can be automated to a high degree. For example, a highly automated AAS can determine six elements in 50 samples within 35 min. Other examples in which a larger number of elements are to be determined in a fewer number of samples would favour ICP-OEs in terms of speed of analysis. In both cases the actual analysis time is small relative to the time required for sample preparation. 

The principal advantage of the furnace technique is one of sensitivity which is improved 10- to 100-fold relative to either flame or plasma. The principal disadvantage is probably the time required for each analysis which can be up to several minutes coupled with a somewhat limited linear dynamic range. In practice, the furnace is limited to ultra-trace determinations. 

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