Chapter 15. Reproducibility and Validity of Portable ED-XRF Instruments: A Comparison of Spectral and Quantitative Results from Belizean and Ethiopian Obsidian

Abstract

Handheld or portable XRF (pXRF) instruments enable archaeologists with little or no training in physics or chemistry to conduct obsidian studies worldwide. During the last decade we have witnessed a rapid increase in their use, alongside an accompanying concern over data accuracy and precision, and by extension repeatability. Two issues plague research: the use of different makes and models of instruments without accounting for their differing equipment capabilities and a current absence of standardized obsidian calibration and operational procedures. In this chapter, we use an obsidian calibration that is factory installed in all Bruker Tracer Series instruments, derived from the use of multiple trace-element analysis techniques (INAA, LA-ICP-MS, and MD-ICP-MS) on a set of 40 specific obsidian source samples at the University of Missouri Research Reactor (MURR) (Glascock and Ferguson 2012). Here, we report on a comparative study of multiple calibrated instruments using effectively identical analytical and operational procedures to analyze disparate obsidian sets from an extensive quarry site in SW Ethiopia and the Maya site of Caracol in Belize. Our results demonstrate that variation between instruments is negligible within the constraints of this study. We also suggest that analysts follow specific assay procedures and disclose all instrument outputs (i.e., photon counts and ppm) if future research is to become more repeatable and comparable.