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However, the practical experience of one of the present authors in using this technique in the advanced Earth Science laboratories, Oxford, would suggest that Pollard and Heron () were perhaps over optimistic when they wrote “…widespread use of high-resolution ICP-MS machines could herald a new age of rapid and relatively cheap isotopic and chemical studies of archaeological material.” The capital and running costs of an MC-ICPMS instrument are if anything more than for a TIMS machine, and many owners of such MC-ICPMS machines are unwilling to allow the direct introduction of directly dissolved samples into the ICP source (for good economic and analytical reasons), so that the time-consuming step of chemical separation of the element to be analysed in an expensive low blank laboratory is usually still required.This is not only a question of practicality and economics but also because lack of chemical separation can expose one to problems arising from the fact that large amounts of matrix included with the sample may cause differential mass bias behaviour between samples and standards, as well as the possibility of introducing isobaric interferences (e.g. Care should be taken not to confuse MC-ICPMS with an ICP source fitted to a quadrupole mass spectrometer (ICP-QMS) which, though very useful for the chemical analysis of samples taken from archaeological objects, does not possess the accuracy or precision needed for lead isotope analyses applied to archaeological provenancing studies and if misused for this purpose can lead to incorrect conclusions.The authors discuss in detail the basic restrictions and advantages of using the lead isotope compositions of ores in mineral deposits for finding the origin of the raw materials used for making ancient artefacts.Methods for the scientific interpretation of the data are discussed, including attempts to use statistical methods.This paper reviews the research into the methodology of lead isotope provenance studies carried out at the University of Oxford between 19, at first in the Department of Geology (Geological Age and Isotope Research Laboratory), later in the Isotrace Laboratory based in the Department of Nuclear Physics, and eventually part of the Research Laboratory of Archaeology and the History of Art.
A further very significant advance in sample throughput and accuracy of isotopic analyses using TIMS was the routine introduction of magnetic sector mass spectrometers using multi-collectors to collect all isotopes simultaneously in different collectors.The classic method for the measurement of lead isotope ratios, developed largely for isotope geochemistry (Faure ), is thermal ionisation mass spectrometry with a magnetic sector mass spectrometer (TIMS).It was usual before, around 1974, to mount the sample as lead sulphide on a rhenium strip filament, which, on heating emitted ions into the mass spectrometer. devised a thermal ionisation method based on the use of a silica gel/phosphoric acid emitter on a rhenium filament which allowed the sample size to be reduced by an order of magnitude and the accuracy of lead isotope ratio measurement to be greater than ±0.1% (Cameron et al.However, it is well known that there are often matrix effects to consider when using laser ablation (e.g.Rehkämper and Mezger ) did not investigate whether they were measuring correct lead isotope ratios when using laser ablation as the sampling method, for which they did not run an external standard.
The routine use of small samples necessitated also the development of low blank chemical separative methods carried out in special laboratories over-pressured by highly filtered air to reduce contamination to very low levels.