Copper and tin isotopic analysis of ancient bronzes for archaeological investigation: development and validation of a suitable analytical methodology

Abstract

Although in many cases Pb isotopic analysis can be relied on for provenance determination of ancient bronzes, sometimes the use of "non-traditional" isotopic systems, such as those of Cu and Sn, is required. The work reported on in this paper aimed at revising the methodology for Cu and Sn isotope ratio measurements in archaeological bronzes via optimization of the analytical procedures in terms of sample pre-treatment, measurement protocol, precision, and analytical uncertainty. For Cu isotopic analysis, both Zn and Ni were investigated for their merit as internal standard (IS) relied on for mass bias correction. The use of Ni as IS seems to be the most robust approach as Ni is less prone to contamination, has a lower abundance in bronzes and an ionization potential similar to that of Cu, and provides slightly better reproducibility values when applied to NIST SRM 976 Cu isotopic reference material. The possibility of carrying out direct isotopic analysis without prior Cu isolation (with AG-MP-1 anion exchange resin) was investigated by analysis of CRM IARM 91D bronze reference material, synthetic solutions, and archaeological bronzes. Both procedures (Cu isolation/no Cu isolation) provide similar δ 65 Cu results with similar uncertainty budgets in all cases (±0.02-0.04 per mil in delta units, k02, n0 4). Direct isotopic analysis of Cu therefore seems feasible, without evidence of spectral interference or matrix-induced effect on the extent of mass bias. For Sn, a separation protocol relying on TRU-Spec anion exchange resin was optimized, providing a recovery close to 100 % without oncolumn fractionation. Cu was recovered quantitatively together with the bronze matrix with this isolation protocol. Isotopic analysis of this Cu fraction provides δ 65 Cu results similar to those obtained upon isolation using AG-MP-1 resin. This means that Cu and Sn isotopic analysis of bronze alloys can therefore be carried out after a single chromatographic separation using TRU-Spec resin. Tin isotopic analysis was performed relying on Sb as an internal standard used for mass bias correction. The reproducibility over a period of 1 month (n042) for the mass bias-corrected Sn isotope ratios is in the range of 0.06-0.16 per mil (2 s), for all the ratios monitored.

Tin isotope characterization of bronze artifacts of the central Balkans

“…Prior to tin isotopic analysis, a selective separation of tin from other elements must be done to avoid isobaric interferences at 112 Cd, 114 Cd, 115 In, 120 Te, 122 Te, and 124 Te Haustein et al, 2010;Balliana et al, 2013). The chromatographic extraction material, TRU-Spec resin from Eichrom Industries Inc., is the most suitable for separation due to large partition coefficients for some actinides including Sn in HCl.…”

“…The chromatographic extraction material, TRU-Spec resin from Eichrom Industries Inc., is the most suitable for separation due to large partition coefficients for some actinides including Sn in HCl. The ability of Sn to be retained on the column decreases in HNO 3 , thus, while interfering elements are released (Cd, In, and Te) in a first elution phase of HCl, Sn is collected when the column is flushed with HNO 3 (Balliana et al, 2013;Yamazaki et al, 2014).…”

“…The very high ionization efficiency of MC-ICP-MS allows for the analysis of high ionization potential elements, such as tin. Several studies using MC-ICP-MS have yielded small but detectable variations in the isotopic composition of tin in bronze and cassiterite (Nowell et al, 2002;Haustein et al, 2010;Nickel et al, 2012;Balliana et al, 2013;and Yamazaki et al, 2013). So far, the datasets produced have been numerically and/or geographically limited, and have reached varying conclusions as to the viability of defining bronze groupings based on tin isotopes.…”

“…However, all samples in this study clustered without any evident subgroups. Balliana et al (2013) noted that the lack of distinct compositions in their study could simply indicate a single tin source due to the limited geographic variation of the analyzed artifacts. They concluded that further testing of the feasibility of tin ore provenancing through isotope analysis was required.…”

“…Furthermore, Pb has a wide natural variation in isotopic composition that is not affected during ore processing and/or the production of objects (Pernicka et al, 1993, 1997, and Pernicka, 2014Gale and Stos-Gale, 2000;Cui and Wu, 2011;Balliana et al, 2013;). Several studies that have used Pb isotopes in bronzes have been successful in tracing copper, lead, and silver-bearing artifacts to their sources (e.g., Gale and Stos-Gale, 2000;Stos-Gale and Gale, 2009).…”