Isotopic Measurement of Lead in Nanogram Quantities on Multi-Collector Inductively Coupled Plasma Mass Spectrometry

Lead isotopes have been used as geochemical tracers in Earth Sciences, such as geochemistry, paleoclimatology and chronology, due to the diverse ratios and variable elemental abundance. Determination of Pb isotope ratios, with 2-sigma external precisions of 200 ppm for 207Pb/206Pb and 208Pb/206Pb and 800 ppm for for 206Pb/204Pb, can be performed with a Faraday-cup protocol in static mode on a multi-collector ICP-MS (MC-ICP-MS), Thermo Electron Neptune. The sample size is as low as 3 ng of Pb consumed per measurement. Lead blanks, from acid, labware, and airborne particulate, was effectively reduced to less than 10 pg, which causes an isotopic ratio bias of 30-50 ppm at most. Isobaric interference of 204Hg on 204Pb was corrected by monitoring the ion beam intensity of 202Hg. Mass dependant instrumental fractionation was normalized to 205Tl/203Tl value. A desolvation nebulization system, Cetac Aridus, and an X-skimmer cone were used to enhance signal intensity. With a sample uptake rate of 50 μ L/min, Pb concentration of 5-10 ng/ml offers an ion beam intensity of larger than 1 volt for 208Pb. The measured isotope ratios with 2-sigma external uncertainty of an international standard of NIST- Pb 981 are: 206Pb/204Pb= 16.9419 ± 0.012, 207Pb/206Pb= 0.91475 ± 0.0002 and 208Pb/206Pb= 2.1674 ± 0.00035. The key merit of this technique is to provide a possibility of analyzing Pb isotopic composition in trace-quantity of 1-10 ng, mainly for sample with limited Pb content.