# Non-Destructive pXRF on Prehistoric Obsidian Artifacts from the Central Mediterranean
## 3 Analytical Methods
After about 2005, commercially produced hand-held portable XRF (pXRF) instruments were available from several companies. Their small size makes them transportable in a backpack and as carry-on luggage on planes. \begin{table}
\begin{tabular}{c c c c c} \hline _Years_ & _\(\geq\)10 Per Site_ & _\(\geq\)25 Per Site_ & _Analyses_ & _\% Total_ \\ \hline
1964–1994 & 26 & 7 & 884 & 5\% \\ \hline
1995–2010 & 97 & 33 & 3217 & 19\% \\ \hline
2011–2021 & 158 & 108 & 12,895 & 76\% \\ \hline Total & 281 & 148 & 16,996 & 100\% \\ \hline \end{tabular}
\end{table} 
Table 1: Analyses of obsidian artifacts in the central Mediterranean. In traditional XRF instruments, samples are placed within a vacuum chamber so that secondary X-rays are not absorbed prior to reaching the detector. However, a vacuum is not necessary for quantitative measurements of elements above potassium, so that obsidian objects need not be contained in a vacuum chamber for measurement of K-shell electron replacements of elements from Ca to La. For hand-held XRF instruments, 50 kV is the highest energy setting for primary X-rays, thus limiting K-line energy measurements of only a few elements in periodic table row 6. For pXRF analyses, elements specifically used for obsidian source identification include major elements Ca, Ti, Mn, Fe, and trace elements, including Rb, Sr, Y, Zr, and Nb (Figure 4). The use of a filter (12 mil Al, 1 mil Ti, 6 mil Cu) reduces the background for these elements, with detection limits for trace elements in single digit ppm [27; 28]. Figure 3: Most recent pXRF instrument (Bruker Tracer V\({}_{S}\)) used in these studies. Mounted upright using a home-made plastic stand. As with all XRF instruments, the specific energy peaks for different elements, especially the L-lines for elements in period table rows 6 and 7 (e.g., Cs, Ba, La, Ce, Nd, Th, U) overlap with the K-lines for lower \(Z\) elements, thus limiting precision for those trace elements with similar energy levels to major elements (e.g., Fe and below) when significantly present. Other elements, such as those used for compositional analysis of metals and other materials (Ti, Cr, Fe, Co, Ni, Cu, Zn, As, Pd, Ag, Cd, Sn, Sb, Pt, Au, Hg, and Pb), may also be detected, and other filters used to minimize background effects [29]. In most cases, the highly precise measurements of the K-lines of just a few of these trace elements is sufficient to distinguish obsidian sources in any part of the world, while multi-variate statistics of 5-7 trace elements may be used for identifying pottery production groups [27]. The beam size of the X-rays is typically about 5-8 mm in diameter, with some having options for smaller beams reaching the sample. The length of time necessary for analysis has decreased with newer pXRF models, which use silicon drift (SDD) rather than silicon PIN (Si-PIN) detectors; with the first Bruker Tracer model used in 2007-2012 (III-V+), each spot for trace element analysis was run for 180 s; for 2013-2016 (III-SD), 60-90 s; and for 2017 to the present (Vi, Vg), just 30 s. Running for longer times does not go beyond limitations of the detector and increase the precision or sensitivity limits. For homogenous obsidian, one analysis per artifact was sufficient and only in a few cases were reruns needed to confirm the results and source assignment. In some of those cases, the samples were of irregular shape, or were small bladelets just a few mm in width and fewer in thickness, with lower than usual total counts. Incorrect source assignment is avoided in such cases by using trace element ratios. Museums and government agencies in many countries are open to international collaboration and access to their archaeological collections, with analytical research facilitated when done without necessary permissions for destructive sampling, and movement (of intact artifacts or samples) to laboratories, even within the same country.