A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS

Many recent studies of isotope ratio determination by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) have highlighted the excellent levels of analytical precision obtainable using this newly emergent technology. Unfortunately, it is often a considerably more difficult matter to obtain highly accurate analyses by MC-ICP-MS. This is particularly the case for isotope systems in which internal normalisation for instrumental mass bias effects is not possible, e.g., Pb, Cu, Zn, B, Li. Given the fact that extremely accurate Pb isotope data can now be obtained by double-spike thermal ionisation mass spectrometry (TIMS), this is seemingly a major impediment to the widespread adoption of MC-ICP-MS methods for the analysis of Pb. It is demonstrated in this contribution, however, that a few simple modifications to existing protocols can produce MC-ICP-MS Pb isotope data of comparable accuracy to the best double spike TIMS results. This method also preserves many of the inherent advantages of ICP-MS analysis, such as rapid sample processing, and avoids many of the stringent analytical requirements of double spiking. The method is highly reliable and a summary of results from our laboratory over the past two years is presented, together with suggestions for further development.