Page 65 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Q. Why is ionization interference severe in AAS?

Q. What precautions with regard to both standards and samples would be needed when determining
potassium in sea-water?

2.4.3 Chemical Interferences

Given how easily the two types of interference discussed above can be overcome, this third type
constitutes the biggest source of problems in AAS. A brief discussion is given of solid-phase
interferences centred around the following classification:

(a) depressions caused by the formation of less volatile compounds which are difficult to dissociate;

(b) enhancements caused by the formation of more volatile compounds;

(c) depressions due to occlusion into refractory compounds;

(d) enhancements due to occlusion into more volatile compounds.

(a) Formation of Less Volatile Compounds
The best known of this type of interference is that of phosphate on calcium; sulphate and silicate have
the same effect. Figure 2.18 illustrates the effect of increasing phosphate concentration on the calcium
signal. The graph shows a pronounced 'knee' at ratios of P:Ca variously reported in the range 0.3-1.1.
The effect is less pronounced higher up the flame and is not observed in a hotter flame, such as the
nitrous oxide-acetylene flame. The effects are seen at the 422.7 or 620.0 nm line or the calcium
hydroxide band at 554 nm. The constant level of interference observed above a certain level strongly
suggests formation of a compound (probably a calcium phosphate). This compound is less volatile than
calcium chloride, and hence the formation of calcium atoms is hindered.

There are many examples of this type of interference, several involving aluminates. They all show the
pronounced 'knee' in the graph—this distinguishes them from non-specific occlusions.

Several approaches can be made to reduce such interferences:

(i) use a hotter flame;

(ii) adjust the nebulizer to produce a smaller particle size;

(iii) make observations higher in the flame;

(iv) use a 'releasing agent', an element that will enter into a 'law of mass action competition' with the
analyte to combine with the interferent;