Large-volume injections in capillary gas chromatography using a separately heated packed pre-column: application to mercury speciation in natural water

Abstract

Large-volume injection using a system consisting of packed column GC, serially connected to capillary GC, and coupled to a microwave-induced plasma atomic emission spectrometric (MIP-AES) detector is described. The greater sample capacity of the packed column is exploited to allow large-volume injections to be accommodated. Preliminary chromatographic separation of the solvent and mercury species on the packed pre-column permits selective transfer of the analytes to a capillary GC system. There, the mercury species can be focused and further separated on the analytical column, minimizing the risks of extinguishing the plasma due to excess solvent reaching the MIP, and stationary phase or detector fouling, which accompany large-volume injections using alternative methods. Application to the determination of mercury species in natural waters following solid-phase extraction on a dithiocarbamate resin, elution into acidic thiourea, complexometric extraction into hexane and Grignard derivatization is used to illustrate the benefits and limitations of this approach. The relative limit of detection obtained for methylmercury is 8 pg l^–1 utilizing 50 µl injection on to the packed pre-column, compared with 40 pg l^–1 for direct injection of 13 µl into the capillary GC–MIP-AES system. This improved performance has enabled more detailed kinetic studies of the uptake of methylmercury from humic-rich natural waters on the dithiocarbamate resin to be undertaken, and shows that incipient and added analyte behave differently during enrichment, which may have implications for the use of spiking during method development.