Novel sampling techniques for trace element quantification in ancient copper artifacts using laser ablation inductively coupled plasma mass spectrometry

https://doi.org/10.1016/j.jas.2017.04.009Get rights and content

Highlights

  • Two portable laser ablation sampling techniques have been developed and evaluated.

  • Laser ablation sampling is uncoupled from ICPMS analysis.

  • Laser ablation sampling is performed at the museum.

  • Trace element quantification in Neolithic copper artifacts was performed.

  • Assignment of Neolithic copper artifacts to a certain artisanship was possible.

Abstract

Elemental analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) have great potential in archaeometric research due to the quasi-nondestructive sampling and excellent sensitivity of the method. However, the application of LA-ICPMS in cultural heritage research is often limited because samples are too large to fit within an ablation cell or cannot be moved to the laboratory. This work reports the development of analytical routines that allow trace element quantification in ancient copper artifacts regardless their mobility, size or geometry.

In this study, the LA sampling step was performed in ambient air using a portable laser ablation device (pLA). The LA module was placed on the object of interest and the laser-generated aerosol was either directly transferred into the ICPMS via a large-capacity gas exchange device (GED) or collected on polycarbonate membrane filters, which were later analyzed by LA-ICPMS. The analytical performances of both approaches were assessed using various copper reference materials. The laboratory-based, ablation-cell-independent pLA-GED-ICPMS method, yielded accuracies comparable to those obtained via conventional LA-ICPMS (±10%). Good performances (±30%) were also obtained with the pLA + filter sampling approach and subsequent LA-ICPMS analysis. Limits of detection for both approaches were in the low μg/g or sub- μg/g range, making these methods interesting for trace element analysis.

After validating these laser-based techniques on an ancient copper object whose elemental composition had previously been determined by graphite furnace atomic absorption spectroscopy (GFAAS), five Neolithic copper artifacts found in Switzerland and France were analyzed using the pLA + filter sampling approach. A copper dagger found in Lattrigen, Switzerland was analyzed using the pLA-GED-ICPMS method. Furthermore, the laser-induced sample damage was investigated.

The trace element profiles of the objects under investigation were compared to those of well-characterized copper artifacts. Thus, the chronological and cultural background of these artifacts could be determined. One group of copper artifacts showed high arsenic concentrations (up to 1% [w/w]) and could be attributed to “Mondsee copper”, which was particularly common in the eastern Alps during the Middle European Late Neolithic. Other objects under investigation showed trace element concentrations, which are typical for the Late Neolithic north of the Alps. One artifact had a composition typical for objects from the Late Neolithic of Southern France.

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.

References (37)

  • H.G. Edwards et al.

    Analytical Archaeometry: Selected Topics

    (2012)
  • C. Frank et al.

    Copper Artefacts of the Mondsee group and their possible sources

  • H.K. Gersch et al.

    PIXE analysis of prehistoric and protohistoric Caborn-Welborn phase copper artifacts from the lower Ohio River Valley

    J. Radioanal. Nucl. Chem. Artic.

    (1998)
  • A. Giumlia-Mair

    Copper and copper alloys in the southeastern alps: an overview

    Archaeometry

    (2005)
  • M.D. Glascock et al.

    Neutron activation analysis and provenance research in archaeology

    Meas. Sci. Technol.

    (2003)
  • R. Glaus et al.

    Isotope ratio determination of objects in the field by portable laser ablation sampling and subsequent multicollector ICPMS

    J. Anal. At. Spectrom.

    (2013)
  • R. Glaus et al.

    Portable laser ablation sampling device for elemental fingerprinting of objects outside the laboratory with laser ablation inductively coupled plasma mass spectrometry

    Anal. Chem.

    (2012)
  • R.S. Houk

    Mass spectrometry of inductively coupled plasmas

    Anal. Chem.

    (1986)
  • Cited by (11)

    • Portable laser ablation sheds light on Early Bronze Age gold treasures in the old world: New insights from Troy, Poliochni, and related finds

      2023, Journal of Archaeological Science
      Citation Excerpt :

      While it is possible to apply laser-induced breakdown spectroscopy to quantify PGE (Rifai et al. 2020; Mohamed et al. 2021), there seems to be no portable version available (pLIBS; Fortes and Laserna 2010) that quantifies PGE in gold artefacts. However, in the last decade, a portable laser ablation device (pLA) has become available for field micro-sampling of elemental and isotopic analyses of archaeological objects, including gold (Glaus et al. 2012, 2013; Born et al. 2015; Käser 2015; Burger et al. 2017; Knaf et al. 2017, 2021; Seman et al. 2021; Merkel et al. 2022). Considering the above-mentioned constraints regarding the analysis of gold artefacts, pLA devices seem to be the best choice as a virtually non-destructive, on-site sampling method for archaeological gold in order to determine their elemental and, potentially, even isotopic composition.

    • Colorimetric assay for determination of Cu (II) ions using L-cysteine functionalized silver nanoplates

      2020, Microchemical Journal
      Citation Excerpt :

      Therefore, accurate detection of copper is critically important to prevent and diagnose these conditions. Nowadays, the detection of copper is performed by one of several methods such as inductively coupled plasma mass spectrometry (ICP-MS) [5], inductively coupled plasma atomic emission spectrometry (ICP-AES) [6], atomic absorption spectrophotometry (AAS) [7], and UV–visible spectrophotometry [8]. These methods have high sensitivity and good accuracy.

    • A combined experimental approach to the study of ancient coins and its application the Venetian “sesino”

      2019, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
      Citation Excerpt :

      The set of XRF data points, concerning trace elements, including lead, can be the starting point for the identification of the raw materials provenance sites [26], for which, in case of the Republic of Venice, a good deal of literature sources are actually available and can provide the relevant analytical comparison [1]. It is worth mentioning the constant interest that these analyses are attracting, also in view of a continuous instrumental development, with the possibility of being conducted both with combined and multi-analytical approaches, [27–29], like the one herewith proposed and with moderately destructive protocols, e.g., using laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS) [30,31] Eventually, our investigation makes sense, on materials science basis, of the name traditionally given to the mistura alloy.

    View all citing articles on Scopus
    View full text