Precise isotope-ratio measurements of lead species by capillary gas chromatography hyphenated to hexapole Multicollector ICP-MS

The precision and accuracy of lead isotope-ratio determination on a short t ransient signal has been assessed by coupling capillary gas chromatography to the Isoprobe (Micromass, UK), a single-focusing inductively coupled plas ma mass spectrometer with multicollector detection. A T-piece connecting th e GC transfer line to the torch enabled continuous aspiration of thallium s olution for mass-bias correction. The volatile lead species PbEt4 was deriv atized from NIST isotopic certified lead standard SRM 981 and different amo unts of PbEt4 dissolved in iso-octane were injected into the GC. Chromatogr ams were recorded in multicollection mode by use of Faraday cups; seven iso topes (Pb-204, Pb-206, Pb-207, Pb-208, Tl-203, Tl-205, and Hg-202) were mon itored simultaneously at a transient resolution of 160 ms. PbEt4 peaks were obtained with a half-width of 1.2 s and a base width of 3.5 s. Lead isotop e ratios were calculated from the peak areas integrated for each lead isoto pe, giving precision in the range of 0.02 to 0.07% for ratios of high-abund ant isotopes and injections of 5 and 50 pg absolute amount as lead (five re plicates). Mass bias was found to be about 0.5% per mass unit and was corre cted by using the continuously measured thallium signals at Tl-203 and Tl-2 05. After mass-bias correction, deviation of the certified lead ratio value s was found to be in the range of 0.02 to 0.15% accuracy.