Hydride generation, trace metal analysis

In analogy to sample introduction by hydride generation, mercury trace analysis is possible by reducing Hg compounds to the metal using the cold vapour technique or the determination of iodine at the ultratrace level (after oxidation with 70 % perchloric acid of iodide to iodine) via the gas phase. 

On-line solid-phase extraction (SPE) by ion-exchange and preconcentration using FIA techniques are becoming increasingly popular for trace analysis of heavy metals both by flame AAS and by hydride generation and cold vapor AAS45-49. 

In addition to the well-characterized arsenic species discussed above, there are also reports of unidentified arsenic species in water (37,41,46). Careful analysis of a reference material for trace metals revealed that 22% of the total arsenic was not identified (46). The water samples used to prepare the reference material were from the Ottawa River, Canada. The certified value for total arsenic in this reference water is 0.55 0.08 (ig/L. In coastal waters of southern England, 25% of total arsenic was not detected by hydride generation procedures (37). These hidden arsenic species were unidentified, although they appeared to contain di-methylarsenic moieties. Targus estuary water in Portugal contained hidden arsenic up to 25% of the total (47). 

Hydride generation increases the power of detection of atomic spectrometric methods for the determination of certain elements, and allows their matrix-free determination. However, the technique is prone to a number of systematic errors. First, the hydride-forming elements must be present as inorganic compounds in a well-defined valence state. This may require sample decom x>sition prior to analysis. In water analysis, treatment with H2SO4/H2O2 may be effective [106]. Traces of heavy metals such as Cu" may have a catalytic influence on the formation and dissociation of the hydrides, as investigated by Welz et al. [107] in atomic absorption with quartz cuvettes. These in-terferents can be masked by complexation with tartaric acid or coprecipitated with La(OH>3. Calibration by standard addition is advisable. 

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