AUTOMATED ONLINE ISOTOPE-DILUTION ANALYSIS WITH ICP-MS USING SANDWICHFLOW-INJECTION

An automated flow injection (FI) manifold is described to perform the addition of isotopic spikes to aqueous samples on-line with ICP-MS for isotope dilution (ID) analysis. The manifold uses the sandwich techni que (with the nested loop approach) to perform an injection of the iso topic spike solution within a sample (or standard) plug, the resulting sample-spike-sample sequence being pushed toward the nebulizer by a 1 % HNO3 carrier, A standard, which must contain one element not present in the spike solution to allow the determination of the dispersion co efficient, must also be used to allow a reverse isotope dilution analy sis, as well as corrections for mass discrimination and/or spectroscop ic interferences. Indeed, because the signals from the individual isot opes are monitored continuously, only one isotope free of spectroscopi c interference is required for elements whose isotopic distribution do es not vary in nature (two isotopes are still needed for the other ele ments), as a correction for the interference can be made by comparison with the signals from the standard. Furthermore, this automated appro ach makes ID-ICP-MS a faster method and does not require any prelimina ry analysis of the sample because the concentration profile resulting from EI allows the selection of the best isotopic ratio. It was succes sfully applied to the determination of Mo in saline water.