Optimization of microwave digestion for determination of selenium in human urine by flow injection-hydride generation-atomic absorption spectrometry

Abstract

A microwave digestion program, which completely decomposes and oxidizes selenium compounds in urine to selenate, was developed by monitoring the pressure and the temperature during microwave digestion. The efficient decomposition and quantitative recovery of trimethylselenonium iodide spiked into urine was achieved in 18 min using the optimized microwave program reaching 200 °C and 8 bar. The selenate in the digest was reduced to selenite by hydrochloric acid with the aid of microwave energy. Urea was found useful to eliminate NO_x fumes, which might be absorbed in the digest and interfere in the determination of selenium by flow injection-hydride generation-atomic absorption spectrometry (FI-HG-AAS). The recovery of trimethylselenonium iodide, selenomethionine, and selenoethionine added to urine was 96.5–105%. The whole procedure, FI-HG-AAS determination following microwave digestion of urine sample and microwave reduction of selenate in the digests into selenite, was checked with two Standard Reference Materials 2670 (toxic metals in human urine). The results showed good agreement with the certified values (normal level 30 ± 8 µg Se l^–1 and elevated level 460 ± 30 µg Se l^–1. The detection limit of the whole procedure was 3 µg Se l^–1 urine. Selenomethionine and selenoethionine were found unstable during the microwave heating used to reduce selenate to selenite. Such a microwave reduction procedure should be cautioulsy used to distinguish selenate from selenite in the matrices which might contain organic selenium compounds.

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