An isotope dilution equation is derived whereby the spike concentration is calibrated through so-called reverse isotope dilution with the primary standard solution, It is also demonstrated that potential systematic errors due to false spike enrichment values and the mass bias effect are minimized provided that spike additions are judiciously controlled so that the isotope ratio of the spiked primary standard solution does not differ from those of the spiked samples by more than 20%. Lithium was used as a chemical tracer to measure flow rates in an experimental test facility. Thirty-two samples were collected downstream of the injection point and analysed by isotope dilution ICP-MS. Six experiments were carried out on September 28 and November 8, 1995, to measure tlow rates by the chemical tracer method. By optimizing the experimental conditions, the chemical tracer method gave flow rates that differed by < 0.4% from the reference value.
