Lead isotope analysis accuracy

Early workers had to strnggle with serious lithium fractionation effects during mass spectrometric analysis. Today most workers use the multicollector sector ICP-MS technique first described by Tomascak et al. (1999). Improvements of the analytical techniques in recent years have lead to an accuracy better than 0.3%o. Unfortunately, there are no internationally accepted Li isotope values for rocks or waters. James and Palmer (2000) have determined nine international rock standards ranging from basalt to shale relative to the so-called NIST L-S VEC standard. 

Lead has four stable isotopes, ° Pb, Pb, Pb, and ° Pb, but three of these are radiogenic, such that there is no invariant isotope that can be used for internal correction of instrumental mass fractionation. This poses a significant analytical challenge for precise Pb isotopic analysis. This problem is most readily overcome with MC-ICP-MS, because the Pb isotope ratio measurements can utilize the ratio of added T1 for external normalization of the Pb isotope ratio data (see also Chapter 5). The simplicity of this procedure is the main reason why MC-ICP-MS Pb isotopic analysis is now commonly used in geo- and cosmochemistry. This approach needs to be applied with care, however, and it has been argued that Pb isotopic analysis that utilizes the double spike methodolc (either in conjunction with TIMS or MC-ICP-MS) is typically superior in accuracy and precision [86, 87]. 

To date, there are three publications demonstrating the on-line determination of the Pb isotopic composition in lead species by means of GC-MC-ICP-MS [11, 16,17]. Tetraethyllead species were obtained by Krupp and co-workers (11,16, 17] by derivatization of Pb " " from an isotopically certified lead solution, by dissolving NIST SRM 981 Lead Wire (Table 17.1). Tetraethyllead was dissolved in isooctane and introduced into an MC-ICP-MS instrument through a gas chromatograph for isotopic analysis based on a short transient signal. The authors discussed the precision, accuracy, and other analytical parameters for optimization of the measurement. 

Of the various analytical techniques, methods using mass spectrometry with isotope dilution play a special role. Initially they were used for determination of inorganic analytes, but later, in the last decades of twentieth century, they were also used for the analysis of organic species. These methods require the use of isotope-labeled compounds, but lead to very good precision and accuracy, as required in definitive procedures [14]. 

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