XRF in the Wild, Wild West
posted March 07th, 2011
On March 3 I continued my Tanjay Sourcing Project. I used the portable X-Ray Fluorescence (XRF) spectrometer to analyze the 33 decorated earthenware samples from the prehispanic coastal polity of Tanjay. Results from my previous (see earlier blog post) LA-ICP-MS analysis will be compared to those obtained using The Field Museum's XRF device in order to contribute to discussions regarding methods of geochemical analysis on archaeological materials. Like LA-ICP-MS, the XRF measures the chemical composition of the ceramics by looking at the amounts (in parts per million) of particular elements in the clay. Unlike LA-ICP-MS, however, there are no high-powered lasers or plama torches operating at temperatures approaching that of the surface of the Sun. The device itself is something out of the Wild West. It resembles a six shooter and researchers at The Field Museum would play a lot more cowboys and aliens if the device was not mounted on its stand!
With XRF a x-ray excitation source from a radioactive source strikes a sample and the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the "photoelectric effect." During this process, if the primary x-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells. Because each element has a unique set of energy levels, each element produces x-rays at a unique set of energies, allowing one to non-destructively measure the elemental composition of a sample.
Running the XRF took only 1.5 hours and training lasted about 15 minutes. Furthermore, processing for each sample was 3 minutes so I was able to read at least a few pages of my romance novel in between x-ray blasts. The entire process was once again dummy-proof and the portability of the XRF device means that it has analytical potential in the field. I hope to bring the portable XRF into the field in Peru next summer (2012) to analyze the floor chemistry on 5,000-year-old temple surfaces (stay tuned!).
The next step in the Tanjay Sourcing Project will be to "clean up" the statistics--remove outliers, standardize values, and compute concentrations. The LA-ICP-MS data will require the most work (the XRF outputs data directly in ppm), but LA-ICP-MS Lab Manager Laure Dussubieux has created an Excel "macro" worksheet to take the thinking out of the numbers. Apparently, Watson defeating Ken Jennings at Jeopardy was only the beginning.