Exploiting flow injection and sequential injection for trace metal determinations in conjunction with detection by electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry

摘要

Despite their excellent analytical chemical capacities, Electrothermal Atomic Absorption Spectrometry (ETAAS) and Inductively Coupled Plasma Mass Spectrometry (ICPMS), nevertheless, often require suitable pretreatment of the sample material in order to obtain the necessary sensitivity and selectivity. Either in order to separate/preconcentrate the analyte material, or because of the presence of potentially interfering matrix constituents. Such pretreatments are advantageously performed in flow injection (FI) or sequential injection (SI) manifolds, where all appropriate unit operations can be effected under enclosed and strictly controlled conditions, thereby ensuring that all samples are subjected to identical and reproducible manipulations and at the same time minimizing the risk for contamination from the environment. Various separation/preconcentration procedures are feasible, such as liquid-liquid extraction, (co)precipitation with collection in knotted reactors, adsorption, hydride generation, or the use of ion-exchange columns. Apart from hydride generation, where the analyte is converted into a gaseous species, the common denominator for these approaches is that the analyte material finally is contained within a well-defined small volume of eluate, which then is introduced into the analytical instruments. While the graphite tube of the ETAAS only can accommodate up to 50 l solution, yet can tolerate inorganic as well as organic eluates, the ICPMS has a larger volumetric capacity yet cannot accept organic solvents as these adversely will impair its performance. This apparent disadvantage can, however, be circumvented by using a combination of extraction and back-extraction, where the analyte material via suitable chemical treatment first is extracted into an organic solvent and secondly is back-extracted into an aqueous phase before being presented to the plasma. Selected examples of separation/preconcentration FI/SI-procedures will be presented, emphasis being placed on the determination of trace levels of metals in elevated salt-containing matrices. Such samples are traditionally very difficult to analyse.