Isotope-specific analysis of Ni by ICP-MS: applications of stable isotope tracers to biokinetic studies

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Inductively coupled plasma mass spectrometry (ICP-MS) offers excellent detection limits and isotopic analysis of Ni in aqueous standards, but is prone to interferences — mainly from Ca-containing polyatomics — when biological matrices are analyzed for Ni. We have used multivariate calibration with principal components analysis (PCA) to correct for mass overlaps in serum digests. The resulting detection limit for Ni is below 1 μg/l and the within-run imprecision is 6% at 1.46 μg Ni/l. In urine, the higher Ca content renders routine application of PCA problematic. We evaluated several methods of pre-concentration, and have developed a method of Ca oxalate precipitation that allows direct analysis of Ni in the diluted supernatant. The stable isotope 62Ni and the radioisotope 63Ni were co-administered i.v. to rats and the serum and urinary clearances were determined by liquid scintillation counting and ICP-MS. Ni measurements by both methods were in excellent agreement, and serum clearance fit a double exponential decay consistent with the two-compartment model of Onkelinx et al. [24]. A human volunteer ingested 61Ni (20 μg Ni/kg body wt.) in water after an overnight fast. Identical serum levels, peaking near 35 μg/l at 2 h, were measured by electrothermal atomic absorption spectrometry and ICP-MS with PCA. Urinary excretion of 61Ni measured by ICP-MS demonstrated absorption of 30% of the administered dose. We conclude that Ni isotopes can be measured in body fluids by ICP-MS at levels that allow stable isotope tracer studies in humans.