Novel sampling techniques for trace element quantification in ancient copper artifacts using laser ablation inductively coupled plasma mass spectrometry
Graphical abstract
Introduction
Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is a versatile and powerful technique for elemental analyses of solid samples in a quasi-nondestructive manner (Koch and Günther, 2011). In particular, LA-ICPMS is interesting for archaeometric research where the elemental composition of an artifact can provide crucial information about its origin, age, way of production, or authenticity (Edwards and Vandenabeele, 2012, Resano et al., 2010a). However, in standard LA-ICPMS analyses, sample sizes are restricted to the dimensions of an enclosing ablation cell. Furthermore, conventional LA-ICPMS is laboratory-based, which prohibits its application in museums or at excavation sites. In order to allow the analysis of arbitrarily sized objects, ablation cells that can be attached to sample surfaces in an airtight manner have been developed (Devos et al., 1999, Wagner and Jedral, 2011). Alternatively, LA can be carried out in ambient air with the laser-generated aerosol guided into the ICPMS via a large-capacity gas exchange device (GED), so that a sealed ablation cell is no longer required (Kovacs et al., 2010). The suitability of this LA-GED-ICPMS setup was demonstrated for spatially resolved analyses of large-scale stalagmites (Tabersky et al., 2013), as well as for isotopic analyses of arbitrarily sized objects (Dorta et al., 2013). A recently published study reported the development of a portable laser ablation device (pLA), which enables mobile LA-sampling and collection of laser-generated aerosols with subsequent quantitative LA-ICPMS analysis of the collected aerosol carried out in the laboratory (Glaus et al., 2012). The suitability of this approach was demonstrated for trace element analyses of glass, ceramics and gold samples. Compared to other portable techniques, such as X-ray fluorescence spectroscopy (XRF) and laser induced breakdown spectroscopy (LIBS), multiple orders of magnitude lower limits of detection can be obtained (Glaus et al., 2012). Additionally, the LA-based method allows accurate isotopic analyses, which was demonstrated for Pb isotope ratio determinations in ancient Chinese ceramics, including a terracotta warrior in Xi'an (Glaus et al., 2013).
Elemental analyses of archaeological copper artifacts can be challenging due to several reasons. Heavy surface corrosion and possible heterogeneity of the metal/alloy make representative sampling difficult. Trace element concentrations are typically low. Apart from copper as matrix element, concentrations commonly vary from 1000 to 0.1 μg/g. XRF allows non-invasive analyses of objects (in cases where an oxidation layer does not have to be removed). However, this technique is not sensitive enough to quantify trace element concentrations below 1 μg/g. Therefore, destructive sampling followed by sample digestion and liquid analysis is frequently carried out (e.g. by graphite furnace atomic absorption spectroscopy (GFAAS) or inductively coupled plasma optical emission spectroscopy (ICPOES) (Giumlia-Mair, 2005)). Apart from the obvious sample damage, these techniques are time consuming. Moreover, drilling samples cannot be taken from objects that are very thin or small. Owing to their non-destructive nature, their multi-element detection capabilities and high sensitivity, methods relying on nuclear physics such as instrumental neutron activation analysis (INAA) and particle induced x-ray emission (PIXE) have been widely used in the field of archaeometry during the last decades (Fleming and Swann, 2000, Gersch et al., 1998, Glascock and Neff, 2003, Moreau and Hancock, 1999). However, these facilities are not readily available and often require a particle accelerator. Therefore LA-ICPMS has been introduced and successfully applied for the analysis of various copper objects in previous studies (Cevey et al., 2006, Dussubieux, 2007, Dussubieux et al., 2008, Lattanzi, 2008).
In this work, the analytical performances of the pLA + filter sampling LA-ICPMS approach and the pLA-GED-ICPMS setup were assessed by performing trace element quantification of copper standard reference materials and an ancient copper artifact. Finally, these two novel sampling techniques were applied to assess the trace element composition of seven copper artifacts found at Neolithic sites in Switzerland and France. An allocation to a certain artisanship was made.
Section snippets
Portable laser ablation sampling
An overview of the two pLA-based elemental quantification techniques applied in this study is presented in Fig. 1. In the first technique, the pLA sampling device was connected directly to the ICPMS instrument using a large-capacity gas exchange device (GED) (Nishiguchi et al., 2008). In the second method, a previously reported pLA + filer sampling technique was optimized and applied for trace element analysis of copper materials. Both methods are described in detail in the following
Evaluation of the analytical performance
Analytical performances of the pLA-GED-ICPMS setup and the offline pLA + filter sampling LA-ICPMS technique were evaluated. Fourteen trace elements with concentrations ranging from 1 to 100 μg/g were quantified in copper reference materials BAM M-385 and BAM M-384. The results were compared to the reference values given in the certificate of analysis. BAM M-385 was also analyzed by means of conventional ns-LA-ICPMS (LSX-213, CETAC Technologies, Omaha, NE, USA, 213 nm wavelength, 5 ns pulse
Conclusion
Two recently developed sampling techniques were for the first time used to quantify trace element concentrations in ancient copper artifacts. One setup, involving a large-capacity gas exchange device (GED), allowed trace element quantification in arbitrarily sized copper artifacts with LA-ICPMS in a laboratory environment without the need of an ablation cell. Its analytical figures of merit were similar to those offered by standard LA-ICPMS, including the capability to perform depth profiling.
Acknowledgements
The authors would like to thank Roland Mäder of the mechanical workshop at ETH Zürich for manufacturing the tripod. Dr. Alexander Gundlach-Graham, Dr. Gunnar Schwarz, Dr. Bodo Hattendorf and Dr. Steffen Allner are gratefully acknowledged for proofreading the manuscript. M. Burger would like to acknowledge financial support by SNF under grant agreement Nr. 200020_141292.
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