A rapid and novel alternative to conventional sample treatment for arsenic speciation in rice using enzymatic ultrasonic probe

Abstract

This work proposes an arsenic speciation method, which reduces the sample treatment time from several hours, required by other extraction procedures, to only a few minutes. No organic solvents have to be used and several extraction steps can be avoided, thus diminishing possible sources of error. Total As was determined by ICP–MS and major arsenic species present in rice (As (III), As (V), methylarsonic acid (MMA) and dimethyarsinic acid (DMA)) were quantified by LC–ICP–MS. Different treatments for extraction were evaluated: aqueous, methanol and tetramethyl ammonium hydroxide (TMAH) extraction as well as enzymatic hydrolysis. Several parameters were optimised, such as sonication time, amplitude, immersion depth of the probe into the solution, extraction temperature, etc. Quantitative extraction for total arsenic (>95%) and 90% extraction for the sum of the arsenic species without species transformation were obtained by applying an enzymatic treatment using an aqueous mixture of protease XIV and α-amylase in only 3 min. A disruption of the cell membranes due to the focused sonication energy, which alleviates enzyme attack, seems to be the responsible of such extraction enhancement. The method was applied to a rice sample used in an inter-comparison exercise (SEAS G6RD-CT2001-00473) and to two commercial rice samples. The method was validated for total arsenic extraction by analysing a certified reference material from NIST (SRM 1568a).