Method validation for As speciation in rice using LC-ICP-MS and the inorganic arsenic limit for Brazilian rice

Abstract

A methodology for As speciation in rice using liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS) was optimised and validated. Chromatographic separation and elution of As(III), DMA(V) (dimethylarsenic acid), MMA(V) (monomethylarsenic acid) and As(V) were achieved in less than 8 min using 10 mmol L−1 (NH4)2HPO4 as the mobile phase, at pH 6 and a flow rate of 1.4 mL min−1. The limits of detection for As(III), DMA(V), MMA(V) and As(V) in rice were 1.87, 2.89, 0.54 and 3.00 ng g-1, respectively. The method’s accuracy was assessed through analyte recovery tests and analysis of certified reference materials, which included white and brown rice flours. The validated method was applied for As speciation in 640 rice samples, comprising polished (white) rice, parboiled rice and brown rice collected from different mesoregions in the Rio Grande do Sul State in Brazil. By comparing the total concentration of As (tAs) found in the acid digested samples with the As species sum, the average extraction efficiencies were 98 ± 13%, 93 ± 12% and 86 ± 6%, for white, parboiled, and brown rice, respectively. The average concentrations of inorganic arsenic (iAs, that is As(III) + As(V)), DMA and tAs in brown and parboiled rice were not different but differed from those in white rice; the average concentrations of tAs, iAs and DMA in brown and parboiled rice were higher than those found in white rice were. The iAs concentration in all rice samples was lower than 200 ng g−1, which could be recommended as the maximum iAs concentration for Brazilian rice.