Antimony is a non-essential element both for plants and animals. Its uptake and binding to thiol-containing enzymes is responsible for toxic effects. Its toxicity depends on the species being present, with inorganic species being more toxic than organic species. Even further, the toxicity of inorganic species also depends on its oxidation state, with Sb(III) compounds being ten times more toxic than those of Sb(V). It is obvious that a meaningful risk assessment of antimony occurrence in the environment calls for speciation analysis.
Most methods for antimony speciation analysis are based on either HPLC or electrophoretic separation followed by detection via atomic spectrometric techniques. In case that the speciation analysis can be limited to the differentiation of only two species such as Sb(III) and Sb(V), also non-chromatographic separation methods have been proposed. However, such methods are time-consuming, laborious, and lack of reproducibility if performed by manual operation.
A solution to circumvent such manual operation is to use flow techniques with an appropriate manifold, which allow automation of the procedures, reduce the consumption of reagents and improve the reproducibility. A group of researchers from Spain and Brazil have now developed a two-step procedure for the speciation of inorganic antimony. The procedure is fully automated by using a multisyringe flow injection (MSFIA) manifold coupled to ICP-MS.
The developed method is based on a simple two-step procedure with a first selective Sb(III) determination followed by a total inorganic antimony determination after the reduction of Sb(V) to Sb(III). All necessary steps such as sample injection, addition of reducing agent are performed by the MSFIA system. The sensitive detection of Sb(III) is done by hydride generation coupled to ICP-MS. A mini-column packed with an cation exchange resin was used to retain organic antimony species, otherwise interfering with the detection of Sb(III).
Following optimization of the whole procedure using a Box Behnken design, the optimized method was validated with respect to different figures of merit.
Using a sample volume of 2 mL, impressive detection power was obtained with detection limits of 0.016 µg/L for Sb(III) and total inorganic Sb and determination limits of 0.053 µg/l for the same species. The linear working range was 0.053 to 5.0 µg/L. The high reproducibility of the procedure is showing up by the good precision of 1.4% RSD at 0.50 µg/L Sb(III). The two-step procedure is also relatively fast, allowing an analysis frequency of 30 samples per hour.
The accuracy of the method was evaluated by spike recovery tests as proposed by IUPAC for cases where CRMs are not available. Recoveries ranged between 93 and 111% for Sb(III) and 90 to 109% for Sb(V). Finally, the method was applied to the analysis of natural water samples, comprising drinking water, ground water and coastal seawater samples from Majorca Island (Spain).
The authors concluded, that their method demonstrated high selectivity and sensitivity. The automated on-line procedure was also considered to be fast and in accordance with the premise of green chemistry.
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