ARSENIC SPECIATION IN AQUEOUS SAMPLES USING A SELECTIVE AS(III) AS(V)PRECONCENTRATION IN COMBINATION WITH AN AUTOMATABLE CRYOTRAPPING HYDRIDE GENERATION PROCEDURE FOR MONOMETHYLARSONIC ACID AND DIMETHYLARSINIC ACID

Differentiation between As(III) and As(V) is accomplished using earlie r developed selective preconcentration methods (carbamate and molybdat e mediated (co)precipitation of As(III) and As(V) respectively) follew ed by AAS detection of the (co)precipitates. Apart from this, separati on of methylated arsenic species is p-formed by an automatable system comprising a continuous flow hydride generation unit in which monometh ylarsonic acid (MMAA) and dimthylarsinic acid (DMAA) are converted int o their corresponding volatile methylarsines, monomethylarsine (MMA) a nd dimethylarsine (DMA) respectively. These species are cryogenically trapped in a Teflon-lined stainless stell U-tube packed with a gas chr omatographic solid-phase and subsequently separated by selective volat ization. A novel gas drying technique by means of a ''Perma Pure'' dry er was applied successfully prior to trapping. Detection is by atomic absorption spectrometry (AAS). MMAA and DMAA are determined with absol ute limits of detection of 0.2 and 0.5 ng, respectively. Investigation of the behaviour of the methylarsines in the system was conducted wit h synthesized As-73 labeled methylated arsenic species. It was found t hat MMA is taken through the system quantitatively whereas DMA is reco vered for about 85%. The optimized system combined with selective As(I II)/As(V) preconcentration has been tested out for arsenic speciation of sediment interstitial water from the ''Chemiehaven'' at Rotterdam. The obtained concentrations xe 28.5, 26.8 and 0.60 ng . ml-1 for As(II I), As(V) and MMAA, respectively, whereas the DMAA concentration was b elow 0.16 ng . ml-1.