Page 67 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
P. 67
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if an excess of the releasing agent is added, the analyte is released from the interfering anion (e.g.
excess lanthanum or strontium releases calcium from phosphate interference);
(v) use a protective chelating agent which preferentially complexes the analyte, protecting it from the
'grasp' of the interferent (e.g. excess EDTA protects calcium from phosphate interference).
(b) Formation of More Volatile Compounds
These interference effects are far less common. Under this heading, some authors classify the
enhancement of signals from several, otherwise refractory, elements by fluoride. The use of protective
agents (e.g. EDTA for calcium or 8-hydroxyquinoline for aluminium or chromium) are also examples
of this type of effect.
(c) Occlusion into Refractory Compounds
Such depressions can be encountered when the matrix is refractory (e.g. zirconium, uranium or a rare
earth element), and the small amount of analyte can be physically trapped in clotlets of matrix oxide in
the flame. Such systems do not show a 'knee' [see type (a)] and can be minimized by higher flame
temperature.
(d) Occlusion into Volatile Compounds.
Some compounds (e.g. ammonium chloride) explosively sublime in the flame, thus enhancing
atomization. By adding excess ammonium chloride to all samples and standards, this effect can be used
to minimize interferences of types (a) and (c).
Q. How can specific and non-specific depressions [types (a) and (c)] be distinguished?
Q. List several ways in which the interference of phosphate on calcium may be minimized?
Q. How might aluminium interfere with magnesium during AAS?
Q. Iron depresses atomic absorption by chromium in the air-acetylene flame. For what reasons do
workers add 8-hydroxyquinoline and/or ammonium chloride to minimize this interference?
