Speciation of selenium in aqueous and biological matrices by microbore ion chromatography coupled with electrothermal atomic absorption spectrometry via ultra low volume fraction collection†
A robust, user-friendly, sensitive and affordable speciation method based on microbore anion-exchange chromatography (IC) for the separation of selenium species and Zeeman-effect ETAAS for element specific detection is described. By exploiting the very low flow rates normally employed in microbore chromatography, the analytical usefulness of chromatography coupled to the sensitive but discontinuous ETAAS detector has been increased in comparison with couplings incorporating normal bore liquid chromatography. The flow rate of the mobile phase in this particular IC system was 80 µl min–1. A highly reproducible and automated collection of 20 µl fractions in sampler cups was used for interfacing the chromatographic separation with ETAAS. The IC–ETAAS results were also directly compared with those obtained by IC-direct injection nebulizer ICP-AES for assessing the chromatographic resolution of the proposed method. It is possible to separate selenomethionine, selenite, selenate and selenocystine in 6 min. The trade-off in chromatographic resolution and time consumption in the detection step using IC–ETAAS is compensated for by a high degree of simplicity and the high specificity and sensitivity of the Zeeman-effect ETAAS resulting in relative detection limits of 2.8–4.1 ng ml–1 (42–61 pg absolute). These detection limits are comparable to those for published HPLC–ICP-MS methods. The method was evaluated by injecting aqueous standards, unspiked and spiked sample extracts from the biological certified reference material CRM 402. The eluent was injected along with a palladium–magnesium nitrate modifier since the pyrolysis curves for trimethylselenonium, selenomethionine, selenite, selenate and selenocystine revealed a similar response for all species by using this modifier, i.e, all species are thermally stabilized up to 1000 °C. Without adding the modifier, the species showed very different volatilities during the thermal pre-treatment step.