Direct isotope ratio measurement of ultra-trace lead in waters by double focusing inductively coupled plasma mass spectrometry with an ultrasonic nebuliser and a desolvation unit

Abstract

A double focusing inductively coupled plasma mass spectrometer coupled with an ultrasonic nebuliser and a desolvation unit was used to measure ²⁰⁶Pb/²⁰⁷Pb, ²⁰⁶Pb/²⁰⁸Pb and ²⁰⁷Pb/²⁰⁸Pb isotope ratios in Pb standard SRM981, diluted to total Pb concentrations ranging from 1 to 1000 ng l^–1. The synthetic water standard SRM1654d, diluted by a factor of 100, and the natural riverine water standard SLRS-3 were also analysed for their Pb isotope ratios. It was found that lead isotope data with good accuracy could be obtained down to 10 ng l^–1. At 1 ng l^–1, the blanks became too problematic. The internal precisions found appear to be well correlated with counting statistics, and range from ca. 3 to 0.2% RSD, depending on the Pb concentration and the instrument sensitivity. The long term reproducibility is even better and may reach values down to 0.02% RSD at concentrations above 100 ng l^–1 total Pb. Lead isotope values are proposed for the SRM1654d and SLRS-3 water standards. It appears that the reproducibility is affected by matrix effects in the latter, natural water. In future work aimed at lead isotope measurements at the sub-ng l^–1 level with this approach, it will be necessary to solve serious instrument contamination and memory effects for Pb.

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