BEHAVIOR OF VARIOUS ARSENIC SPECIES IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

The behaviour of arsenite (As-III), arsenate (As-V), monomethylarsonat e (MMA), dimethylarsinate (DMA), arsenobetaine (AsB) and arsenocholine (AsC) in pyrolytic graphite coated graphite (pyrocoated) tubes was in vestigated, The influence of a tungsten carbide coating on the thermal pretreatment losses of the analytes and on the analytical signals in aqueous and methanolic solutions was studied. Inorganic species, MMA a nd DMA are less volatile in pyrocoated tubes; a tungsten carbide coati ng produces a good thermal stabilization, but a marked 'dip' in the py rolysis curves is observed in aqueous solutions. No pronounced stabili zing effect for the highly volatile AsB and AsC was observed in tungst en-treated tubes or in the presence of palladium chloride, The determi nation of these species requires the addition of palladium nitrate in both pyrocoated and tungsten-treated tubes, A comparison of the stabil izing action of palladium as its chloride and nitrate was made, Pallad ium nitrate exhibits efficient stabilizing action for each of the spec ies studied, whereas palladium chloride is efficient only for inorgani c As species in pyrocoated tubes, The tungsten treatment of the tube a nd addition of palladium nitrate leads to a further increase in the py rolysis temperature and better sensitivity for the As species. Tungste n treatment plays an important role in improving the performance of pa lladium chloride, particularly in the determination of organically bou nd species, The effective stabilization and relative 'levelling-off' o f the signal for each of the As species (except for AsC) in methanolic solution was observed in the presence of a palladium modifier in tung sten-treated tubes. The in situ separation and determination of As-III and AsB in tungsten-treated tubes was attempted, However, because of the presence of significant amounts of AsB under the conditions used f or As-III determination, complete separation of these species was not possible.