Toxic arsenic compounds in environmental samples: Speciation and validation

Speciation of arsenic (As) compounds in environmental samples requires rigorous analytical procedures at each stage of sample collection, treatment and measurement. In view of the very high number of As species occurring in environmental samples, this article deals with only those that are considered toxic, including As-containing warfare agents and arsenobetaine (AB), because this is the most abundant As species in marine fish. With the exception of the tetramethylarsonium ion, acute toxicity generally decreases with increasing degree of methylation. The warfare agents considered are mostly phenylarsenic compounds. During storage, events, such as changes in oxidation state or induced by microbial activity or losses by volatilization or adsorption, have to be avoided. Extraction of arsenicals from soils and sediments is still a critical step that is not very well controlled. As long as marine animals are considered, extraction efficiencies >90% are commonly obtained [K.A. Francesconi, D. Kuehnelt, Analyst (Cambridge, UK) 129 (2004) 373]. This is logical, since marine animals are rich in AB, and this small molecule is soluble in water and methanol and in mixtures of them. In terrestrial organisms and plants, extraction efficiencies can be much lower and vary according to the extraction conditions. Since most environmental As species are generally present in soluble forms, liquid-separation techniques, first high-performance liquid chromatography (HPLC) but also capillary electrophoresis (CE), are most frequently used for separating As species. Fast separations can be performed with narrow-bore reversed-phase HPLC columns. Pre-column derivatizations with hydride generation [e.g., As(III), As(V), MA, DMA] and mercaptans/dimercaptans (e.g., Clarks I and II and Pfiffikus) allow separation of the arsenicals by gas chromatography (GC). The most popular detectors are inductively coupled mass spectrometry (ICP-MS) and atomic fluorescence spectrometry (AFS), especially after HPLC separation and hydride formation, increasingly replacing atomic absorption spectrometry (AAS), while HPLC-MS or HPLC-MS-MS (HPLC-MS2) is the detector of choice for (less toxic) arsenosugars and arsenolipids. For phenylarsenic warfare species, ultraviolet (UV) detection is also still used frequently. Finally, as other measurement fields, speciation analysis requires the availability of suitable reference materials (RMs) for verification of accuracy and quality assurance. We address these important aspects although the number of certified RMs is rather limited. (C) 2005 Elsevier Ltd. All rights reserved.