Behaviour of various arsenic species in electrothermal atomic absorption spectrometry

Abstract

The behaviour of arsenite (As^III), arsenate (As^V), monomethylarsonate (MMA), dimethylarsinate (DMA), arsenobetaine (AsB) and arsenocholine (AsC) in pyrolytic graphite coated graphite (pyrocoated) tubes was investigated. 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 and DMA are less volatile in pyrocoated tubes; a tungsten carbide coating produces a good thermal stabilization, but a marked ‘dip’ in the pyrolysis curves is observed in aqueous solutions. No pronounced stabilizing effect for the highly volatile AsB and AsC was observed in tungstentreated tubes or in the presence of palladium chloride. The determination of these species requires the addition of palladium nitrate in both pyrocoated and tungsten-treated tubes. A comparison of the stabilzing action of palladium as its chloride and nitrate was made. Palladium nitrate exhibits efficient stabilizing action for each of the species studied, whereas palladium chloride is efficient only for inorganic As species in pyrocoated tubes. The tungsten treatment of the tube and addition of palladium nitrate leads to a further increase in the pyrolysis temperature and better sensitivity for the As species. Tungsten treatment plays an important role in improving the performance of palladium chloride, particularly in the determination of organically bound species. The effective stabilization and relative ‘levelling-off’ of the signal for each of the As species (except for AsC) in methanolic solution was observed in the presence of a palladium modifier in tungsten-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 for As^III determination, complete separation of these species was not possible.