Slurry Sampling for the Determination of Mercury in Rice Using Cold Vapor Atomic Absorption Spectrometry

This paper proposes a procedure for the determination of mercury in rice samples using slurry sampling and cold vapor atomic absorption spectrometry (CV AAS). The first step in the preparation of the slurry is to cook the rice in a closed system for 20 min at 60 °C. The suspension is then sonicated for 30 min at 60 °C in the presence of 6 mol L−1 hydrochloric acid and thiourea. Finally, mercury cold vapor is generated using an aliquot of the acidified slurry. During the quantification step, isoamylic alcohol is added to prevent the formation of bubbles. The experimental conditions were optimized using a full two-level factorial and a Box–Behnken design. The mercury content in rice samples can be determined using external calibration technique that employs aqueous standards; the limits of detection and quantification of this method are 0.95 and 3.18 ng g−1, respectively, and the precision, expressed as relative standard deviations, was 1.98% and 12.10% for rice samples with mercury contents of 10.32 and 4.90 ng g−1, respectively. The accuracy was confirmed by analysis of the NIST standard reference material for rice flour, SRM 1568a. This method was used to determine the mercury content in seven rice samples that were purchased at supermarkets in Salvador City, Bahia State, Brazil. The mercury content varied from 4.10 to 13.72 ng g−1, with an average of 8.36 ng g−1. Some of the samples were also analyzed by CV AAS after a complete microwave-assisted digestion. A statistical test showed that there was no significant difference between the results obtained using slurry sampling and those obtained after a complete digestion of the sample by microwave radiation.