In order to answer the question “what is this stuff”, we began by doing a physical examination using a stereomicroscope. This time we will determine what major elements comprise the sample using x-ray spectroscopy. I prefer x-ray spectroscopy because it’s quick, requires little if any sample preparation, detects most elements in the periodic chart unambiguously and has a wide dynamic range. When sufficient sample volume is available I recommend wavelength dispersive x-ray fluorescence (WDXRF) using a vacuum spectrometer and rhodium x-ray source. The larger sample size ensures that the results are truly representative of the overall elemental composition. The vacuum spectrometer allows detection of very light elements (down to Be) and the rhodium source gives a good compromise in sensitivity between light and heavy elements.
For microscopic samples I recommend energy dispersive x-ray spectroscopy (EDXS) performed in a scanning electron microscope. Samples measuring a few microns in diameter or greater can be analyzed for most elements (C through U) at electron beam accelerating voltages of 5-20 keV. Energy dispersive x-ray spectroscopy lacks the spectral resolution achievable with WDXRF, but most elements can be unambiguously identified by making multiple measurements at different accelerating voltages. EDXS is also useful in identifying the spatial distribution of elements on the surface of larger samples and these distributions can be visualized in the form of elemental maps.
Next time we will use two molecular spectroscopy techniques to answer the question “how are these elements combined to form chemical compounds” in our sample.