Determination of selenocystamine by slurry sampling electrothermal atomic absorption spectrometry after a selective preconcentration by living Pseudomonas putida

Abstract

A method to determine selenocystamine (Se-Cystm) in water samples, containing other selenium species, is proposed. This novel method makes use of living bacterial cells for preconcentration of Se-Cystm prior to the determination of selenium by slurry sampling electrothermal atomic absorption spectrometry (ETAAS). The extraction–preconcentration procedure consists of putting aPseudomonas putida strain in contact with a glucose culture medium supplemented with Se-Cystm. By allowing maximum growth of the bacterial cells to be reached, an equilibrium between the analyte in the solution and in the extractive solid phase is established. Then, discarding the supernatant, the concentration of the organoselenium compound is determined directly in the biomass pellet by slurry ETAAS. The retention procedure was optimized by controlling the growth conditions (amount of glucose, seeding density and growth time). A theoretical model intended for the mathematical modelling of the uptake of Se-Cystm by the living bacterial cells was developed. The theoretical model fits well with the experimental data. This relationship also provides a feasible quantification of the extraction process before the uptake equilibrium is achieved. The determination stage was also optimized by controlling the amount of the chemical modifier used (palladium), as well as the pyrolysis and atomization temperatures. The interference effects from other selenium species [selenoethionine (Se-Eth), Se^IV, selenomethionine (Se-Met), selenourea (Se-U), selenocystine (Se-Cyst), Se^VI] were also studied. The method is selective for Se-Cystm and it can be used for speciation of this organoselenium compound in the presence of the others. The analytical performances of the developed procedure were established. The detection limit (three times the standard deviation of the blank) and the sensitivity (analyte concentration for an integrated absorbance of 0.0044 s) of the developed approach were of 0.10 and 2.29 ng ml^–1 of Se-Cystm, respectively.