Nondestructive Chemical Analysis Laboratory

Lab Directors: Bob Florek, Marvin Rowe, and Eric Blinman

Nondestructive Chemical Analysis: Portable X-ray Fluorescence Spectroscopy

We have the capacity at OAS to determine what metals heavier than potassium are present in an artifact within minutes. Over the past few decades, portable X-ray fluorescence (pXRF) devices have become routine to nondestructively analyze metals in artifacts. These devices produce rapid, semi-qualitative information. In several minutes, we can accumulate measurements that characterize important elements in an artifact.

Principles of pXRF

pXRF instruments consist of an X-ray tube source and a solid-state X-ray detector. Primary (incident) X-rays emitted from the source strike atoms in the artifact with sufficient energy to eject low-energy inner shell electrons, creating an unstable ion. When this occurs, an electron from a higher-energy outer orbital fills the vacancy – releasing energy in the form of secondary (fluorescent) X-rays. The energy of any given secondary X-ray is characteristic of the specific metallic elements that were struck by the incident X-rays, allowing identification of elements present. In addition, the detector also counts the number of these secondary X-rays emitted from the sample, allowing semi-quantitative elemental concentrations to be determined.

This figure shows the basics of pXRF. The X-rays emitted are characteristic of the element impinged by the incident X-ray. Courtesy of Bruker Corporation.

Instrumentation

We use a hand-held Innov-X Systems Alpha Series pXRF device with a silver anode X-ray tube source and a diode detector, powered by lithium-ion rechargeable batteries. The instrument can analyze most elements from potassium to uranium. A Hewlett Packard iPAQ personal digital assistant is used to control the instrument and store data, which is exported into a spreadsheet for data analysis. We have used 30 and up to 300 second analysis times, depending on the circumstances. We have concentrated on pigment analysis of decorated ceramic sherds from the Colorado Plateau and pigment analysis of pictographs, but have also analyzed numerous other artifacts of special interest. For ceramics and pictographs, areas of unpainted matrix adjacent to pigmented areas (backgrounds/controls) are analyzed to ascertain what observed elemental levels were from pigment minerals and not the clay/rock substrate. Elemental concentrations are displayed on the mini-computer screen attached to the instrument as shown in the photograph below. This type of immediate feedback as you conduct the analysis is satisfying as you can consider and discuss results very quickly with collaborators.

Further reading:

2011 American Indian Rock Art–Chemistry as a Criterion for Selecting Pictographs for Radiocarbon Dating

2013 American Chemical Society–pXRF Analysis of Arsenic When Lead Is Present: A Cautionary Tale

2014 Archaeometry–Portable X-ray Fluorescence Spectroscopy of Pictographs: A Case Study from the Lower Pecos Canyonlands, Texas

2015 Antiquity–The Death of a Pterodactyl