Particle Composition By Energy Dispersive X-Ray Analysis
Determining the actual composition of the individual particles in an environmental sample requires an analysis of the energies (or wavelengths) of the X-rays generated when the probe electrons interact with the sample atoms. Two types of X-ray are produced. These are referred to as characteristic X-rays and background X-rays. The characteristic X-rays, when measured with an energy dispersive X-ray detector, have energies that are characteristic of the elements present in the sample. By way of illustration, if calcium (Ca) is present in the sample, then in the X-ray spectrum (a plot of X-ray energy against intensity) an X-ray emission line ("peak") for Ca will appear.
By identifying the emission lines in an acquired spectrum it becomes possible to determine which elements are present in the particulate sample. This particle chemistry information can be portrayed in several ways. A visual representation of particle composition can be created by mapping the occurrence of characteristic X-rays to their point of origin on the particles imaged in the SEM. For instance, the accompanying X ray maps, which were acquired for a sub-set of particles from a sample of mine waste soil from Idaho Springs, CO, demonstrate particle by particle variations in element content (increasing color density indicates increasing concentration) in relation to the backscattered electron image (image A).
An alternative way of expressing information on particle chemistry is to quantify the proportions of each element present in the individual particles. Element concentrations can be determined from the (relative) heights of the peaks in the X-ray spectrum. This is achieved by normalizing the number of X-ray counts in the peaks to a major peak or to the total number of counts after corrections have been made for peak overlap and X-ray background. Background X-rays are the second type of X-rays produced by the sample, however they carry no composition information. The accompanying X-ray spectra for two of the X-ray mapped mine waste particles [6 and 7 in the binary image (above)], demonstrate background subtraction and digital filtering. After background subtraction, it was determined that particle 6 contained 49% Ca, and 51% S, and particle 7 contained 14% Al, 53% Si, 4% S, 22% K and 7% Fe.