Elemental speciation with liquid chromatography–inductively coupled plasma isotope dilution mass spectrometry

Abstract

For the determination of elemental species, which normally exist at low concentrations in the environment, coupling of liquid chromatographic systems with inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method with respect to detection limit. However, accurate results are still a problem in trace analyses but can be obtained by the application of isotope dilution mass spectrometry (IDMS). The developed LC–ICP-IDMS system consists of a high-performance liquid chromatography pump, a sample injection valve, a separation column (different types of chromatographic separation systems, e.g., ion or size-exclusion chromatography, were used depending on the separation problem), a non-destructive detector (e.g. a UV detector) for simultaneous determination of organic substances, and an element-specific ICP mass spectrometer. Isotope dilution is carried out by adding an isotopically enriched species-specific spike solution to the sample prior to the separation step in the case of the determination of well-defined species, or by continuous on-line introduction of a species-unspecific spike solution in the case of species with unknown composition and structure. The species-specific spiking method is demonstrated for the determination of iodide and iodate in mineral water using an ion chromatographic separation column. For example, iodate concentrations in the range of 0.5–20 ng ml–1 could be determined with relative standard deviations of about 2%. The species-unspecific spiking mode is used to determined heavy metal complexes with humic substances at a level of about 1 ng ml–1 as well as organo-iodine species in the concentration range 0.7–1.4 ng ml–1 in natural water systems. The accuracy of speciation could be verified by comparing the total element concentration with the sum of the different elemental species.