Selenium speciation by high-performance liquid chromatography–fraction collection–electrothermal atomic absorption spectrometry: optimization of critical parameters

Abstract

Chromatographic effluents were analysed by electrothermal atomic absorption spectrometry (ETAAS) using a sampling procedure based on fraction collection and hot injection onto a graphite furnace. In order to reduce analyte dilution and to attain the maximum sensitivity of the combined system, special attention was paid to the volume of fraction collected and the volume of each fraction injected onto the furnace. Total volumes of 60 µl (50 µl of sample + 10 µl of modifier) were introduced into the graphite furnace by hot injection, using a pre-heating temperature of 120 °C and an injection rate of 3.3 µl s^–1. Under these conditions, the detection limit for determination of selenium was 1.21 µg l^–1 and the repeatability 2.4% for 40 µg l^–1. When effluent fractions were analysed with the hot injection programme, the use of fraction volumes of 0.50 ml or lower provided a detection limit below 1 ng of selenium with respect to the sample injected into the chromatograph. This procedure was applied to the speciation of selenium. The separation of trimethylselenonium, selenite and selenate was performed by anion-exchange chromatography using 0.01 mol l^–1 ammonium citrate at pH 3.0 and 7.0 as eluent. The presence of such a concentration of citrate produced a negative interference on selenium determination and this signal suppression was reduced by 17% on increasing the amount of nickel (200 µg) and also adding magnesium nitrate (50 µg) and overcome by using matrix-matched standards. Detection limits were 1.67, 1.27 and 0.76 ng of Se for trimethylselenonium, selenite and selenate, respectively.