Inductively coupled plasma magnetic sector mass spectrometry method for stable lead isotope tracer studies

Abstract

Studies in our laboratory and elsewhere require the analyses of stable lead (Pb) isotopes at sub-ppb levels as tracers of lead metabolism and therapeutic chelation treatment in animal models and humans. While analytical alternatives such as thermal ionization mass spectrometry (TIMS) provide excellent isotope measurement sensitivity and precision, TIMS is not well-suited for rapid throughput of large numbers of biological samples. Here we report the development of an inductively coupled plasma magnetic sector mass spectrometry (Finnigan MAT Element) method, possessing suitable lead isotope measurement sensitivity and precision for use in biologically stable lead isotope tracer studies in which the natural relative abundance of one or more isotopes (i.e., ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, or ²⁰⁸Pb) has been enriched by >5%. Standard reference materials (SRMs) NBS 981 lead isotope standard and four different NIST biological SRM materials were spiked with bismuth-209 as an internal standard, and analyzed at lead concentrations of 1–10 ppb. The method consisted of 300 passes per sample for masses 201, 204, 206, 207, 208 and 209, with a total analysis time of about five minutes per sample. Concentrations for ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb were determined by comparison to a certified lead concentration standard that had been isotopically characterized by TIMS. Analyses of lead concentrations in the NIST biological SRMs yielded a measurement accuracy (recoveries) of 96–101%, with a detection limit of 0.013 ppb total lead. Accuracy of isotope abundance analyses of NBS SRM 981 (n=12) was 99.92–99.99% of that expected for all four isotopes, with a measurement precision of 0.07–0.22% RSD. Thus, this method is ideally suited for the rapid, accurate, and precise measurements of lead concentrations and stable isotope abundances in lead isotope tracer studies in animals and humans, where one or more lead isotopes are enriched above natural levels.