Analysis of Chemical and Biological Properties

Abstract

Electron Microscopy-Based Microanalysis of Airborne Particles IntroductionElectron microscopy (EM), namely transmission and scanning, is the most common technique used for particle analysis, at both the morphological and chemical levels; it is assumed that the reader is familiar with the basic principles of electron microscope design and operation and only the practical applications will be considered. In terms of particle identification based on morphology, transmission electron microscopy (TEM) is the more robust system, especially when dealing with airborne particles that are submicron in size. TEM will yield information on the ultrastructure, providing a two-dimensional interpretation of a particle. In contrast, scanning electron microscopy (SEM) is more limited to airborne particles that are microns in size and will produce information about a particle's surface structure or a three-dimensional interpretation.The added bonus of EM is its capacity to determine the elemental composition of airborne particles, at both the individual and bulk particle levels. Several analytical techniques are available for the elemental characterization of specimens within the electron microscope (e.g. X-ray analysis, Auger electron spectroscopy, and electron energy loss spectroscopy). For all such techniques, a signal is produced by either direct electron bombardment, or by interaction with auxiliary excitation sources (e.g. X-rays) and collected in appropriate detectors (i.e., an energy dispersive spectrometer [EDS]), to yield specific information about the elemental features of a specimen. In the case of particle analysis, elemental characterization is best achieved by means of X-ray detection using a technique more commonly known as electron probe X-ray microanalysis (EPXMA). X-rays within the electron microscope are generated by the interaction of the electron beam with individual atoms in the specimen (e.g. characteristic X-rays). These X-rays carry information about the elemental composition of the specimen in the region that is being irradiated and consequently provide a means of correlating structural information with elemental content. Therefore, in theory, X-ray microanalysis is possible using any electron microscope fitted with a suitable detector system, and this is generally true for qualitative X-ray microanalysis. Quantitative X-ray microanalysis is a 105 2.4