Underestimation of the total arsenic concentration by hydride generation techniques as a consequence of the incomplete mineralization of arsenobetaine in acid digestion procedures

Individual arsenobetaine (AsB) decomposition products obtained with five different acid digestion procedures (nitric acid/200°C/10 or 30 min, nitric acid/sulphuric acid/300°C/10 or 30 min and nitric acid/sulphuric acid/hydrogen peroxide/300°C/30 min) were determined by high-performance liquid chromatography-(ultraviolet digestion)-hydride generation atomic fluorescence spectrometry (HPLC-(UV)-HGAFS). It was found that AsB was converted to mainly inorganic arsenic, trimethylarsine oxide (TMAO) and dimethylarsinic acid (DMAA), depending on acid digestion procedure; the "stronger" the digestion procedure the more mineralization occurred. The sensitivity of the flow injection-hydride generation atomic fluorescence spectrometry (FI-HGAFS) for each of the decomposition products was also measured which allowed us to calculate the FI-HGAFS response, and thus the total arsenic concentration using arsenate as a quantification standard. Total arsenic concentrations calculated were between 56 and 100% of the initially present AsB, depending on acid digestion procedure. This was completely in accordance with total arsenic concentrations measured so that underestimation of the total arsenic concentration in AsB digests measured by the FI-HGAFS can be fully explained in terms of incomplete mineralization and the lower response of the partly degraded products.