X-ray Fluorescence is a technique for chemical compositional measurement in which X-rays of a known energy are directed towards a target or sample, causing the atoms within the material to emit "fluorescent" X-rays at energies characteristic of its elemental composition. Recent advances in x-ray tube and detector technology have allowed for the development of portable X-ray Fluorescence devices (PXRF), units about the size of a hair-dryer that can be transported with ease.
The Elemental Analysis Facility houses two portable X-ray Fluorescence devices, a Bruker Tracer IV, and an Innov-X Alpha series portable XRF. With both instruments, it is possible to conduct surface analysis at the ppm level totally non-destructively on a wide range of materials including metals, ceramics, obsidian, and glass. The Bruker Tracer is primarily utilized by our conservation staff, who use it to identify the composition of pigments and other colarants on collections objects, aiding them in understanding the best ways to conserve these objects. Additionally, our conservationists use XRF to determine the concentration of arsenic and other toxic metals on older collections objects. These toxic substances were used early in the history of the muesum to prevent insects and rodents from damaging collections.
Portable X-ray Fluorescence analysis of soil chemistry at Cerro Baul, Peru (left) and of an obsidian axe from the Maya site of San Jose, Belize on exhibit in the Ancient Americas hall (right). Photos courtesy of James Meierhoff. The Innov-X Alpha device is used by our curators and other research scientists in their field and laboratory projects. As a portable analytical technique, PXRF can be used to conduct on site analysis of soils and architectural materials that may be difficult or impossible to return to the laboratory for analysis. Additionally, PXRF allows for the analysis of materials such as metals or obsidian in countries where it is not possible or difficult to export archaeological artifacts for research purposes. For example, curator Ryan Williams has used PXRF to study soil chemistry at the Wari provincial center of Cerro Baul, southern Peru. Differences in soil chemistry can be related to room use, for instance metal working, cooking, and brewing that were conducted at Cerro Baul. The PXRF is also a valuable tool for on site research at the museum, as it is rapid, non-destructive, and can in many cases be applied directly even to objects actively on display at the museum. One such project involved the analysis of obsidian objects excavated by former Field Museum curator J. Eric Thompson at the Maya site of San Jose, Belize. Using PXRF, it was possible to chemically link obsidian objects from San Jose, including a large ceremonial obsidian axe, to distant obsidian sources in highland Guatemala and central Mexico.