Lead is a common trace constituent found in all rocks. It occurs as the radiogenic daughter product of U and Th decay, but it also forms its own minerals where radiogenic Pb is not found, due to the exclusion of U and Th in the mineral. As a result, its isotopic ratios vary widely as a function of age and the ratio of U and Th. Aside from its use in geochronometry, this variability makes it a useful tracer in geologic, archaeological and forensic studies, as it records the chemical environment on which the Pb resided. Since these parameters can vary widely between different geological environments, and because various geological processes including the generation and fractionation of magmas; hydrothermal and metamorphic processes; and surficial processes including weathering can change U/Pb and U/Th ratios, Pb isotopic ratios serve as tracers regarding the origins of rocks.
Archeologists use the isotope ratios of Pb much as they do those of Sr; in fact, the two systems are often used in conjunction with each other to determine residential origins and migration patterns of ancestral humans. As with Sr, the assumption is that the isotope ratios for Pb in bone and tooth enamel will reflect those in the portion of the biosphere in which an individual lived, thus providing information about the land where food was grown or grazed.
Depending on the needs of the researcher, our lab extracts Pb by acid digestion; we can digest materials completely, or we can leach readily available material via partial extraction. Pb is then purified by standard cation exchange methods using small-volume columns, thus reducing the volume of reagents used, and minimizing the reagent contribution to blank levels. We use only high-purity reagents and all work is done in a controlled cleanroom environment.
All Pb isotope measurements are done on a nuPlasma II mass spectrometer fitted with 16 faraday cups and 5 ion counters. We run NBS 981 during all sample runs as a reference check, using sample-standard bracketing to monitor instrument drift. We use masses 205 and 203 of Tl to correct for mass discrimination and to improve the stability of measured ratios within a sample run.
We currently only offer solution analyses, using a CETAC dry desolvating system to increase sensitivity; we are in the process of acquiring a laser-ablation setup, and we will announce when this is up and running.