Urine selenium determination by electrothermal atomic absorption spectrometry: influence of urinary phosphates on the trimethylselenonium ion signal

Abstract

A feasibility study to determine inorganic selenium and its organic metabolites in urine by means of ETAAS with Zeeman-effect background correction is presented in detail. The procedure in itself is rather simple: dilution of the urine prior to digestion with a nitric acid–hydrogen peroxide mixture and direct analysis of the digest by ETAAS using copper (100 µg) and magnesium nitrate (200 µg) as the chemical modifier. Matrix matched calibration is used. There is little emphasis on the various instrumental parameters (charring and atomization curves) and the choice of chemical modifiers. Instead, the focus is on the behaviour of TMSe⁺(trimethylselenonium ion) in urine and on ways of improving its recovery from a urine sample. The effect of the specific gravity, the digestion procedure, the amounts and nature of various inorganic anions, pre-dilution of the urine before testing and modifier concentration on the TMSe⁺ signal is clearly demonstrated. Actual data from real exposed workers show the effect of the concentration of phosphates on the recovery of TMSe⁺(78 ± 21%) in individual spot samples. It can be seen that the suppression effect of phosphates on the TMSe⁺ signal in urine is more pronounced and more correlated in test samples spiked with phosphates (with the same urine matrix) than in various workers' spot samples where the phosphates were experimentally determined. This proves that phosphates are only partly responsible for this suppression effect and that most probably some unknown chemical species are involved.