Will Wright
Effect of magnification on precision of EDS analysis of Native American stone tool source materials from the Great Basin area of Nevada
The Great Basin area of Utah and Nevada was once inhabited by many different groups of people.
These ancient civilizations no longer exist, but remnants of their past cultures do in the form
of archeological artifacts. These artifacts are the only clues left that can tell us about these
people. Hamilton CollegeÕs Tom Jones and Charlotte Beck both study the Great Basin people. The
two, along with David Bailey have devised a technique using X-ray fluorescence (XRF) spectroscopy
to analyze both the artifacts and the possible source rocks for the artifacts. The goal of this
study is to match the chemical signatures of the artifacts with the chemical signatures of their
source rocks. Knowing where the Great Basin people collected the materials for their stone tools
can help archeologists reconstruct migration patterns, trade routes, and many other aspects to the ancient societies.
The XRF system analyzes wavelengths of X-rays that come off of a sample when it is bombarded
with radiation. These X-rays are characteristic of individual atoms. The system analyzes for
twenty individual elements, one at a time. Analysis of a sample is, therefore, extremely time
consuming. In addition, the conventional XRF technique requires that the sample be powdered prior
to analysis. Therefore, it is necessary to develop a new technique that does not involve
destroying the artifact. The energy dispersive X-ray system (EDS) on the scanning electron
microscope (SEM) allows researchers to quickly determine the chemical composition of inorganic
samples. The EDS system analyzes all the elements in a sample at once. Thus, the EDS system has
time advantages over wavelength dispersive spectrometry. The EDS system is also advantageous
because it does not require that the specimen be powdered and destroyed. The final advantage of
EDS analysis over XRF analysis is that much smaller samples can be analyzed. The purpose of this
research was to determine the minimum size sample that could be analyzed and still yield precise
chemical information. Eight source rocks were cut and polished and analyzed for eight major elements using the EDAX EDS system. Spectra were collected for 100 seconds, and quantified using the EDAX software. Spectra were collected from each sample at nine different magnifications: 25x, 50x, 100x, 200x, 400x, 800x, 1600x, 3200x and 6400x. For all samples, the data show a decline in precision around magnification 800x (Figure 1, below). Analyses done at 800x or lower give consistent results. However, at a magnification of 800x the area being analyzed is extremely small (~0.02mm2) and grain size effects become significant.
On the basis of the data collected in this study, it is concluded that precise chemical analyses
can be obtained from quite small ( smaller than 0.5mm2) pieces of stone artifacts and source rocks. It is
recommended that analysis be conducted at a magnification of 200x, on pieces that are larger
than 0.35mm2.
