EXAMINING EARTH EVIDENCE: IS ENERGY DISPERSIVE SPECTROSCOPY ENOUGH?

Energy dispersive spectroscopy (EDS) is a common technique in forensic analysis. An EDS detector is readily coupled to a scanning electron microscope (SEM), is relatively easy to operate, and provides simultaneous spectra for the elements it can detect. Investigators routinely use SEM imaging coupled with EDS to analyze evidence and exemplars consisting of metals, ceramics, glasses, other manufactured materials, and minerals. However, can we always be certain of the conclusions we draw from the analysis of minerals? We will present several examples of hypothetical cases demonstrating when the data obtained from EDS is enough to provide negative or positive confirmation of a relationship between two samples, and when additional techniques such as polarized light microscopy (PLM), electron probe microanalysis (EPMA) and mass spectrometry (MS) are warranted. These examples include a comparison between some common feldspars of similar appearance, between several samples of apatite from the same mining district exhibiting similar fluorescence, as well as an actual case involving suspected stolen red feldspar from Oregon, known as sunstone. While glass is not a mineral, it is frequently examined as evidence and exhibits some properties similar to minerals; therefore we will analyze a sample of float glass and describe how EDS can determine the nature of manufacture and from which side of a pane of glass the sample originated. We will also briefly discuss the implications of using rough vs. polished samples, the implications of standardless analysis, and the presence of water in some minerals. EDS is a powerful technique; however, the criminalist must be aware of its limitations and the need to correlate some findings using other analytical methods.