Arsenic speciation in estuarine and oceanic waters by hydride generation-atomic fluorescence spectroscopy

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(11), 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 interlaboratory comparison particular emphasis was placed on the results for the methyl arsenic species, for which no certified reference materials are currently available in seawater. Surface and vertical seawater profiles were obtained in the Subantarctic Zone of the Southern Ocean along a transect between 42¬∞ and 55¬∞ S along 141¬∞ 30' E, south of Australia. The transect line passes through four different oceanic environments: the Subtropical Convergence Zone (STCZ), Subantarctic Zone (SAZ), the Subantarctic Front (SAF) and the Polar Frontal Zone (PFZ). These represent some of the first measurements of arsenic in the Southern Ocean. Cycling of arsenic is interpreted using the chemical and biology data obtained from the cruise, and the shipboard performance of the semi-automated method is also discussed. A detailed study of arsenic cycling in the Huon estuary, in south east Tasmania, was undertaken. Arsenic species data were obtained during eight 3-monthly spatial surveys, and weekly sampling from a single site in the estuary over a six-month period. The data has been correlated with other chemical measurements, including nutrients, salinity and dissolved oxygen, and also with biological information about the phytoplankton species present. Arsenic concentrations in the Huon estuary are very low, even when compared with other pristine systems. The seasonal cycle of arsenic is similar to that found in other temperate estuaries of the Northern Hemisphere. Arsenic cycling is almost entirely biologically influenced and the uptake of arsenic, particularly by the phytoplankton Pseudo-nitzschia, is discussed in detail. Chapter 6 provides a final summary of the work presented in this thesis, and also recommendations for further development of the analytical method and research opportunities revealed in the estuarine study of arsenic.