Isotopic analysis of antimony using multi-collector ICP-mass spectrometry for provenance determination of Roman glass

Abstract

Based on promising results obtained in earlier work on antimony ores, we have investigated the use of natural variation in the isotopic composition of Sb as determined using multi-collector ICP-mass spectrometry (MC-ICP-MS) for provenancing Roman glasses. In antiquity, Sb was used either as a decolourizer or as an opacifier and thus, colourless and opaque coloured glasses from different regions and periods were selected for investigation. Both, the sample digestion and isolation of Sb preceeding isotopic analysis via MC-ICP-MS have been optimized and validated for this type of materials. For this purpose, NIST SRM 610 glass reference material and an in-house standard were employed. Isotope ratio determinations were carried out by MC-ICP-MS, relying on In as an internal standard for correction for instrumental mass discrimination. The merits of external calibration in a sample-standard bracketing approach and the so-called “revised Russell's law” for mass bias correction were compared, with the latter providing higher precision and better reproducibility. The ¹²³Sb/¹²¹Sb isotope ratio in glass could be determined with a reproducibility (or external precision) of ca. 0.4ε, while in Roman colourless and Hellenistic opaque glass samples, a variation of 3 to 4ε units was found. The results seem to suggest the use of similar Sb sources for both types of glasses, and the existence of at least two different Sb sources for glass production in the Roman era.