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

Abstract

The precision and accuracy of lead isotope-ratio determination on a short transient signal has been assessed by coupling capillary gas chromatography to the Isoprobe (Micromass, UK), a single-focusing inductively coupled plasma mass spectrometer with multicollector detection. A T-piece connecting the GC transfer line to the torch enabled continuous aspiration of thallium solution for mass-bias correction. The volatile lead species PbEt₄ was derivatized from NIST isotopic certified lead standard SRM 981 and different amounts of PbEt₄ dissolved in iso-octane were injected into the GC. Chromatograms were recorded in multicollection mode by use of Faraday cups; seven isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²⁰³Tl, ²⁰⁵Tl, and ²⁰²Hg) were monitored 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 isotope ratios were calculated from the peak areas integrated for each lead isotope, giving precision in the range of 0.02 to 0.07% for ratios of high-abundant isotopes and injections of 5 and 50 pg absolute amount as lead (five replicates). Mass bias was found to be about 0.5% per mass unit and was corrected by using the continuously measured thallium signals at ²⁰³Tl and ²⁰⁵Tl. After mass-bias correction, deviation of the certified lead ratio values was found to be in the range of 0.02 to 0.15% accuracy.