Samples prior speciation analysis

The compounds MMA, DMA, and TMAO are reduced in acidic aqueous media by borohydride solutions to methylarsine (MeAsH2, bp 2°C), dimethylarsine (Me2AsH, bp 35°C), and trimethylarsine (Me3As, bp 55°C), respectively. These products are useful derivatives for speciation analysis of arsenic because they are readily separated from complex sample matrices and may be further separated from each other by distillation (41) or by gas chromatography (42) prior to their determination by element-specific detectors. Consequently, arsine generation techniques are the most commonly used methods for determining MMA, DMA, and TMAO in marine samples. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

In terms of methodology, the speciation analysis of Hg has reached its maturity. Because of a convenient conversion of Hg species into volatile compounds, GC has been the dominant separation technique prior to Hg speciPc ICP-MS detection [10, 11, 13, 19]. HPLC methods cannot compete in terms of Pgures of merit with GC. HPLC-ICP-MS is, however, a valuable independent analytical technique that allows the formation of artifacts during derivatization in GC to be controlled. Owing to the availability of a number of CRMs, analytical procedures for the analysis of seafood samples were extensively validated and can be applied in routine analysis. 

Sample Preparation Methods Prior to Se Speciation Analysis 602... 

SAMPLE PREPARATION METHODS PRIOR TO Se SPECIATION ANALYSIS... 

On the basis of the experience accrued so far, the extraction efficiency can be affected by both the nature of the sample under test and the As species actually present in the sample. The lipid content of the sample and the presence of As species with hydrophobic residues play a key role in this context. As a consequence, it may be useful to remove or partition lipids with an organic solvent prior to the methanol-water extraction in order to increase the extraction efficiency. This approach is also viable to prevent emulsification of methanol and lipids, which could otherwise significantly reduce the extraction efficiency [26]. Thus, sample preparation prior to As speciation usually starts with the chloroform-methanol extraction, followed by centrifugation of the sample solution to remove solid particles. Subsequently, after the addition of methanol and water, the supernatant is separated in a funnel. The amount of As remaining in the organic phase is usually quantified as total As by wet digestion after solvent evaporation. Obviously, this entails the loss of all speciation information related to the lipid-soluble As species [2]. Speciation analysis is finally carried out on the methanol-water phase, usually after solvent evaporation (see Table 19.2). 

Microwave-assisted extraction has also been used as a solid sample treatment prior to speciation analysis [264-266], leaving the organometallic compound moiety intact. This is a prerequisite for a successful extraction procedure to be applied prior to speciation analysis and can be met by careful optimization of the conditions of the microwave attack. Open-vessel treatment is preferred to pressurized bomb systems commonly used in the analysis for total metals because it offers milder reaction conditions — the increase in temperature is governed to a great extent by the boiling point of the solvent — and easier control of process variables [266]. 

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