Speciation of selenomethionine and selenourea using living bacterial cells†

Abstract

A novel and reliable method is proposed for speciation of selenomethionine (Se-Met) and selenourea (Se-U) combining a time-controlled solid phase extraction by using living bacterial cells and electrothermal atomic absorption spectrometry (ETAAS) for the determination of selenium. The extraction medium consists of a Pseudomonas putida strain cultivated in a culture medium based on glucose contaminated with an organoseleno compound. Firstly, equilibrium between the analyte in the solution and the retention medium is allowed to be established, and then the concentration of the organoseleno compound is determined directly in the biomass by slurry ETAAS. It is possible to discriminate between different chemical species of selenium by combining the optimization of both the growth conditions and the relative rates of their retention from the sample solution. A theoretical model of the retention process, based on a rate-determining step in reaching the adsorption equilibrium, is proposed to describe the experimental adsorption time profiles of the uptake process by the living bacterial cells. This relationship can also provide a feasible quantification of the extraction process before the adsorption equilibrium is reached, whenever the agitation conditions and the sampling time are under control. The experimental data agreed closely with the theoretical model. The study of the adsorption process was derived to develop an analytical procedure to determine Se-Met and Se-U in matrices containing other selenium species (Se-ethionine, Se-cystamine, Se-cystine and SeVI). The best detection limits for the organoseleno compounds at their optimum extraction times are 1.5 ng ml^–1 for Se-Met and 1.2 ng ml^–1 for Se-U. The relative standard deviations of the retention/determination process are 2.2–5.6%.