Summary
A method has been developed for the routine determination of four arsenic species (arsenite, arsenate, monomethylarsenic and dimethylarsenic) in natural water, and has been applied to a study of arsenic cycling in open ocean (Atlantic and Southern Ocean) and estuarine (Huon Estuary, Tasmania) waters. The method uses hydride generation and cold trapping of the hydrides, coupled with atomic fluorescence detection (AFS) at 193.7 nm. With the optimised system, detection limits for As(III), As(V), MMA and DMA are 2.3, 0.9, 2.4 and 3.7 ng L-1, respectively, in a 5-mL sample. The precision for nine sample replicates was better than 3.5% for all the arsenic species. Accuracy of the method was determined by analysis of a seawater certified reference material (NASS-4) and by recovery studies on natural samples. The manual method was semi-automated and modified for shipboard use by automating several of the procedures in the analytical sequence, including addition of NaBH4 to samples, cooling and heating the U-trap used for preconcentration and separation of the arsines, and logging the AFS output. The semi-automated method has a number of advantages over the earlier manual HG-AFS method, namely, shorter sample throughput time, increased precision and, most significantly, ease of use under shipboard conditions. The absence of pure standards for MMA and DMA was identified and methods were developed to purify and characterise suitable standards. Samples were collected from tropical and subtropical regions in the western basin of the Atlantic Ocean during an Intergovernmental Oceanographic Commission (IOC) baseline contaminant survey. The biogeochemical cycling of arsenic in this environment is discussed briefly. In addition, an interlaboratory comparison has been carried out to compare the results determined by HG-AFS analysis of stored samples, with those obtained during the IOC voyage by hydride generation-gas chromatography-photo ionisation detection (HG-GC-PID). In the