Many learned societies, such as the American Chemical Society, the German Chemical Society, and the Royal Society of Chemistry (UK), require not only structural but also compositional information on newly reported materials. Several techniques are available to the solid state chemist: Energy dispersive x-ray spectroscopy (EDX), inductively coupled plasma optical emission spectroscopy (ICP-OES), x-ray wavelength dispersive spectroscopy (WDS), x-ray fluorescence spectroscopy (XRF), x-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) are well known examples. Use of any of these techniques will, however, usually yield the average elemental composition over a large volume of the sample. This is accurate as a measure of material composition if the sample in question is one pure material, but if the sample is a mixture of materials, then the composition analysis will return an average, which may be unrepresentative of the specific material under investigation. In solid state chemistry, this can be a significant problem, as separation of mixtures is much more challenging than in, for example, solution phase chemistry. If a solid state reaction produces a mixture, and none of the components can be easily dissolved, sublimed, or otherwise removed (a very common scenario with oxide chemistry), it might be challenging to accurately measure the elemental composition of the target material. Thus, synthesis of highly phase-pure samples is normally an important part of new materials discovery, as this is the best route to accurate compositional information.