Using MC-ICP-MS in Objective Determination of Provenance

Multicollector inductively coupled plasma-mass spectrometry (MC-ICP-MS) combines an inductively coupled plasma (ICP) source, a double-focusing sector field mass spectrometer of Nier-Johnson geometry, and multiple collectors for the measurement of isotope ratios. Frank Vanhaecke, PhD, of the Department of Analytical Chemistry at Ghent University (Ghent, Belgium), has been using this technique for objective provenance determination in the context of archaeometry. He provided Spectroscopy with further insight into his work.

Spectroscopy: How did you become interested in objective determination of provenance using multicollector inductively coupled plasma–mass spectrometry?

Vanhaecke: When I was carrying out my PhD research on fundamental aspects of ICP-MS in the late 1980s and early 1990s, another research group in the department of Analytical Chemistry at Ghent University was developing various techniques for provenancing of ancient marble. I got sideways involved, as this was the PhD research topic of my then girlfriend (now my wife for 19 years). Later on, I began using single-collector ICP-MS for trace-element fingerprinting and isotopic analysis of lead in the context of archaeometry every so often. Much of this work was carried out in cooperation with Prof. Martín Resano from the University of Zaragoza (for example, Spanish medieval ceramics, Celtiberian coins, rock art paintings). We also paid attention to provenancing of diamonds and of wine in that period. By acquiring multicollector ICP-MS in my research group and by starting to collaborate with colleagues from various other research institutions, primarily the universities of Leuven (Prof. Patrick Degryse) and Brussels (Prof. Philippe Claeys), provenance determination in the context of archaeometry has become one of the major research lines in my research group. Also, further cooperation with the University of Zaragoza is still ongoing.

Spectroscopy: What is innovative about your approach?

Vanhaecke: When deploying ICP-MS for objective provenance determination in an archaeometric context, one can either rely on trace-element fingerprinting or on isotope ratio determination. The isotopic composition of most of the target elements is more robust (that is, not changed upon processing of the raw material or the manufacturing of the final object) and can often be linked to, for example, the raw material used for the manufacturing of an artifact (ore, clay) or the geographical area of a living species (plant or animal) and the material derived there from. We aim at expanding the application range of isotopic analysis in this context by not only relying on elements that show a quite pronounced natural variation in their isotopic composition as a result of one or more of their isotopes being radiogenic (such as strontium or lead), but by also including elements the isotopic composition of which only shows much more subtle natural variation as a result of isotope fractionation processes. Except perhaps for the lightest elements, only multicollector instrumentation provides sufficient precision for successful isotopic analysis of such elements.

Spectroscopy: In doing this work, what obstacles have you had to overcome?

Vanhaecke: A major hurdle is ensuring the financial means to be able to continue our work. Acquiring high-end ICP-MS instrumentation is expensive and also the running costs are high. With the economic crisis, it is getting harder to find the funding for research that cannot immediately be monetized.

But, also from a scientific point of view, there are plenty of challenges. An analytical approach for quantitative isolation from the concomitant matrix needs to be developed every time a new target element or material is considered. Also, the correction for instrumental mass discrimination (an instrument-induced bias of typically ~1% compared to a precision as low as 0.001%) needs to be paid attention to, especially as this phenomenon is not entirely understood yet. Self-evidently, also finding relevant archaeological problems to be studied and obtaining suitable sample material is a great challenge, but fortunately, our existing collaboration with colleagues from that field covers that aspect.

Spectroscopy: What results have you seen so far?

Vanhaecke: We have shown that “panoramic” multielement analysis, combined with chemometric treatment of the multivariate data, opens possibilities for a variety of materials. We have carried out isotopic analysis of the “usual suspects,” strontium and lead, in both traditional and more novel contexts (for example, of strontium for provenance determination of human remains in present-day forensic cases and of lead to study prenatal lead intoxication during the Roman era). We have tailored or are tailoring analytical approaches for isotopic analysis of elements such as B, Cu, Fe, Sb, Sn, and Zn and are evaluating the merits of their isotopic systems for use in archaeometric studies.

Spectroscopy: What are your next steps in this work?

Vanhaecke: In addition to deploying the analytical protocols developed on artifacts of high archaeological relevance, we also aim at deploying these approaches in different contexts. For instance, we are currently investigating whether isotopic analysis of transition metals in human whole blood can be deployed as a means to diagnose diseases affecting the metabolism of these metals, diseases that otherwise can only be diagnosed at a later stage or via more invasive methods. And of course, we will also continue to study the capabilities (and limitations) of other isotopic systems that have not yet been sufficiently characterized so far. Next to more application-oriented research, we also focus on more fundamentally oriented research, and in this context, our systematic studies of the aforementioned instrumental mass discrimination can be mentioned. Finally, we would like to also evaluate the use of laser ablation multicollector ICP-MS in this context, as this would make our approaches less invasive, which is always appreciated by the archaeologists and, especially, by the owners of the artifacts studied.

This interview has been edited for length and clarity.