The scanning electron microscope (SEM) - when equipped with one or more x-ray spectrometers - is undoubtedly one of the most powerful tools ever devised for the chemical analysis of solid samples at micrometer and submicrometer length scales. As a finely focused electron beam is scanned across the surface of the specimen it excites characteristic x-rays from the elements in the sample, allowing a quantitative chemical analysis to be performed at each pixel in the scanned raster. The spatial resolution of this chemical information is determined by the sample's composition, the energy of the incident beam electrons, and the excitation threshold energy of the x-ray lines used in the analysis; it varies from tens of nanometers in favorable cases to more than 10 mm when high energy electrons are scattered in low-Z targets [1, 2, 3].The two most common x-ray spectrometers used with SEMs are the wavelength-dispersive spectrometer (WDS) and the energy-dispersive spectrometer (EDS). WDS systems use diffracting crystals for x-ray discrimination and have x-ray energy resolutions on the order of 10 eV. This translates into chemical detection limits of 100 υg/g (100 ppm) or better for many elements. Unfortunately, WDS spectrometers must be scanned serially to acquire an energy spectrum, and multiple crystals are required to cover the usual spectral range of 0-20 keV. This makes qualitative analysis with the WDS tedious and time-consuming. In contrast, EDS systems excel at qualitative analysis since they can acquire spectra covering large energy ranges all at once, but their inferior x-ray energy resolution (approximately 130 eV) limits their use for most applications to analyte concentrations above 1 mg/g (0.1 wt%).After more than three decades of evolutionary change in the field of electron-beam microanalysis, recent advances in the technology of x-ray detectors promise to advance the capabilities of EDS systems by leaps and bounds. Two of the most exciting developments, cryogenic microcalorimeter detectors and silicon drift detectors, are explained below, followed by a brief discussion of electron backscatter diffraction analysis, a technique for determining the chemical phase of submicrometer domains without using x-rays.
Analytical and Bioanalytical Chemistry
EBSD, EDS, electron backscatter, microcalorimeter, SDD, silicon drift detector, ucal
Analytical Advances in the SEM, Analytical and Bioanalytical Chemistry
(Accessed June 5, 2023)