A Metrological Approach to Analytical Biases for MC-ICP-MS Isotope Ratio Measurements - Application to the Certification of New Pb Isotopic Reference Materials (Candidate ERM-38xx Series)

Lead (PB) is known to be a particularly toxic chemical element since several centuries. Mining and smelting of Pb and its domestic use have contaminated the surface of Earth and jeopardized the health of humans, domestic animals and wildlife. Consequently, regulations were progressively introduced to control, restrict or ban the use and dissemination of lead compounds. Currently, Pb is one of the few metals covered by the legislation (European directives EC-82/884/EEC, 1999/30/EC and 200/60/EC). Omnipresence of PB has however and advantage. It can be used as an isotopic tracer of pollution sources and pathways in the environment. Pb isotopic Certified Reference Materials (iCRM) with undisputed characteristics are then required to validate Pb isotope ratio measurements. The materials currently available worldwide were produced in the 1960¿s by the National Bureau of Standards in the USA and are now the object of polemics in the scientific literature regarding the accuracy of some of the accompanying certified values. Moreover, new materials with lower relative uncertainty statements (below 0.01%) are demanded by users. This PhD work was centred on the production and the certification of a new series of Pb iCRM (candidate ERM-38xx series). The production included sic Pb gravimetric isotope mixtures, a common Pb material dedicated to routine calibration work and a series of four 207Pb slightly enriched natural-like Pb materials for the validation of the ¿-scale method below 0.1%o. The gravimetric isotope mixtures method, originally supported by mass spectrometry measurements performed on GS-MS or TI-MS instruments, was applied successfully to MC-ICP-MS. This study demonstrated that this isotopic measurement technique is a portential powerful tool for the certification of new iCRM. It necessitated in certain cases methodological adjustments which are discussed hereafter. MC-ICp-MS instrumental biases and especially mass discrimination effects, which are still not well understood, were investigated to guaranty the reliability of the produced iCRM. Our measurements methods were validate by several ways, including a systematic investigation on possible significant sources of uncertainty, several comparisons of results obtained by different methods and different laboratories and of course an extensive use of reference materials. Relative uncertainties on isotope ratios obtained for the newly produced Pb iCRM are as low as 0.017%, which is between 2 and 4 times smaller than the uncertainties carried by the NIST-981 material. Additionally, we propose new reference values for this material that was measured agains our independent Pb iCRM. It was not possible to produce iCRM carrying reference values with relative uncertainties below 0.01% as demanded by the Pb community, mainly because of uncompressible uncertainties coming from the weighing stage. We suggest that the newly produced 207Pb series, for which the ¿-values span from 0.001%to 0.013%, could be used as validation tool for isotopic variations below 0.01% (sub-epsilon scale measurements). Results obtained from an inter-laboratory comparison (involving 5 laboratories) on ¿-scale measurements show that relative isotopic variations below 0.005% are hardly measurable accurately.

PONZEVERA Emmanuel; 

2009-01-22

University of Antwerpen

JRC49704