Measurement of As species in rice by HPLC-ICPMS after extraction with sub-critical water and hydrogen peroxide

Abstract

A robust method for As speciation based on water-3% v/v hydrogen peroxide has been developed for use in laboratories undertaking rice analyses that gives equivalent results to the 2% v/v nitric acid extraction procedure that is widely used. Samples (0.2 g dry mass) were extracted with 9 mL of deionised water and 1 mL of 30% v/v hydrogen peroxide at 200 °C under 25 Bar using microwave heating in 100 mL Teflon-PFA vessels This has the advantage that all As(III) is converted to As(V) making quantitation using HPLC-ICPMS easier as the inorganic peak is moved away from any As cations in the void and the DMA peak that may interfere in quantitation of inorganic As. Recoveries of As in five rice reference materials and rice samples were 100 ± 6% (n = 50) and 103 ± 14% (n = 97) respectively. Comparison of the measurements of total As in extracts by ICPMS and ETAAS were also equivalent with no biases (y = 1.015 + 0.001, r² = 0.963, p < 0.0001) indicating that organic carbon extracted in rice extracts doesn't cause an enhancement of ICPMS signals There was no significant differences or biases in As speciation between the water–hydrogen peroxide and 2% v/v nitric acid procedures (y = 1.046 + 0.010, r² = 0.997, p < 0.0001). Typical reproducibility of rice samples (mean ± 95% CI, n = 11) containing As concentrations at legislative limits are total As (0.192 ± 0.004 μg g⁻¹), inorganic As (0.108 ± 0.002 μg g⁻¹) and DMA (0.085 ± 0.003 μg g⁻¹).