Abstract
Simple, straightforward non-chromatographic method for inorganic arsenic (i-As) determination in rice using species-selective hydride generation (HG) combined with inductively coupled plasma optical emission spectrometry (ICP OES) without prior separation of methylated organoarsenicals (o-As) before measurements was developed and fully validated. i-As [As(III,V)] and o-As (DMA, MMA) species were extracted in aqua regia that oxidized As(III) to As(V), while integrity of both o-As forms was preserved. Arsenic hydrides from i-As were generated in reaction with NaBH4 (1%) in strong acidic conditions (10 mol L−1 HCl) after pre-reduction of As(V) to As(III) in a KI (0.5%)-ascorbic acid (2%)-HCl (3 mol L−1) mixture. Reactivity of As species toward HG under conditions of a rice matrix was investigated in order to improve detectability and selectivity of i-As when o-As coexists. Interferences related to contribution of DMA and MMA to the As signal recorded for i-As were pointed out. Limitations of the method for DMA and matrix effects for MMA were found advantageous to selectively measure i-As by HG-ICP OES. No matrix effects on i-As allowed to use external calibration for its quantification. Detection limit of 0.28 ng g−1 (5.6 ng g−1 in original sample), precision < 5%, and adequate accuracy (96.5–103.9%), verified by the analyte recovery study, were achieved. Applicability of the method was demonstrated by i-As determination in nine brown rice samples. Its concentrations found along with the percentage of i-As to total As (obtained after microwave digestion and HG-ICP OES detection) were similar with these reported for this type of rice.
Highlights
Rice constitutes a staple food product for half of the world’s population due to its nutritive properties and health benefits, much interest with this food product comes from a toxicological point of view, because a rice plant can accumulate As into grains (Sommella et al 2013)
aqua regia (AR) led to obtain good results for As(III) and As(V), case of organic As species (DMA and MMA) was not considered in this work. This sample preparation approach, completed with properly selected optimal prereduction and hydride generation (HG) reaction conditions, was tested in the present work for non-chromatographic As speciation in rice by HG-inductively coupled plasma optical emission spectrometry (ICP optical emission spectrometry (OES)) and extended to four As species commonly reported to be present in rice, i.e., inorganic arsenicals (i-As) [As(III,V)] and o-As (DMA, MMA)
Several useful speciation procedures (SPs) using 1.0% NaBH4 (e.g., SP1 for As(III) {citrate buffer in the S solution}, SP2 for As(III) + DMA {acetate buffer in the S solution}, SP3 for As(III) + As(V) + MMA {after prior pre-reduction (0.5% KI-2.0% AA) of As species in the acidified S solution (3 mol L−1 HCl) and with the 10 mol L−1 HCl A solution for HG reaction} or SP4 for DMA + MMA {after prior pre-reduction (0.5% l-cysteine (LC)) of As species in the unacidified S solution, followed by HG in the 2 mol L−1 HCl A solution) for species-selective As determination by HG-ICP OES in one solution were proposed and verified
Summary
Rice constitutes a staple food product for half of the world’s population due to its nutritive properties and health benefits, much interest with this food product comes from a toxicological point of view, because a rice plant can accumulate As into grains (Sommella et al 2013). Hydride generation (HG), related to conversion of As to highly volatile hydrides in reaction with sodium tetrahydroborate (NaBH4) in acid media, in combination with atomic and mass spectrometric detectors is a promising alternative to speciate As instead of chromatographic systems (Cerveira et al 2015; Musil et al 2014; Petursdottir et al 2014; Torres-Escribano et al 2008). This technique, originally developed to separate As from the sample matrix, was found to discriminate between various As species. HG-based differentiation of As(III), As(V), DMA, and MMA is viable because all of them react with NaBH4 with different effectiveness; careful control of HG reaction conditions may result in selective generation of hydrides of individual As species (individual speciation) or species having the same nature, i.e., i-As and o-As (operational speciation) (see, e.g., Bundaleska et al 2005; Karadjova et al 2005; Welna and Pohl 2017)
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