Measurement of Inorganic Arsenic Species in Rice after Nitric Acid Extraction by HPLC-ICPMS: Verification Using XANES

The measurement of As species in rice is normally accomplished by extraction followed by HPLC-ICPMS analysis. This method, however, has not been comprehensively validated by comparing these speciation results with XANES, which does not require sample extraction, due to the challenge of conducting XANES analysis at very low As concentrations. In this study As speciation data using nitric acid extraction/HPLC-ICPMS and XANES are compared to verify the efficacy of using 2% v/v nitric acid extraction and HPLC-ICPMS to measure inorganic As, DMA, and MA in reference rice materials and common rice varieties obtainable in Australia. Total As and As species (As(III), As(V), DMA, and MA) concentrations measured in 8 reference materials were in agreement with published values. XANES analysis was performed on 5 samples having total As concentrations ranging from 0.198 to 0.335 μg g–¹. XANES results gave similar proportions of total As(III), As(V), and DMA to HPLC-ICPMS. XANES was able to distinguish two forms of As(III): As(III) and As(III)GSH. Total As concentrations in rice samples varied from 0.006 to 0.45 μg g–¹ As (n = 47) with a mean ± std of 0.127 ± 0.112 μg g–¹ As with most As present as inorganic species (63 ± 26%). DMA was found in nearly all the rice samples with the majority of samples containing concentrations below 0.05 μg g–¹ As while MA concentrations were negligible (<0.003 μg g–¹ As). Six rice varieties produced in Australia, China, and Spain all had elevated DMA concentrations (0.170–0.399 μg g–¹ As) that were correlated with total As concentrations (r² = 0.7518). In conclusion, comparison of As speciation by HPLC-ICPMS and XANES showed that similar As species were detected indicating the appropriateness of using 2% v/v nitric acid for extraction of rice prior to speciation. Common rice varieties obtainable in Australia generally have low As concentrations with most As present as inorganic As.