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Particle Identification

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The key to identifying the source of particle contamination is to identify the types of particles present. Identification almost always reveals the source of the contamination.

Optical Microscopy

The most commonly applied technique in particle identification is optical microscopy. It is simple to do, inexpensive and, when done with a trained eye, identifies the largest number of contaminant particles. With experience, a microscopist can recognize a specific particle on sight. Physical characteristics such as shape, size, color and optical properties are used for identification.

Supplementary properties include particle hardness (assessed by pushing the microscope cover slip above the particle with a needle) and magnetism (detected by rotating a small magnet around the particle and seeing if it behaves like a compass needle).

Often a microscopist can identify minute particles that take major efforts with other analytical techniques. For example, skin cells, a common contaminant, are easily recognized on sight. Other methods might show the particles to be complex organic chemicals with traces of sodium and chloride but still not lead to a useful identification.

To learn more about microscopical particle identification, refer to the Particle Atlas* produced by McCrone Associates** (* The Particle Atlas by McCrone and Delly published by Ann Arbor Science Publishers. **McCrone Research Institute, 3620 S. Michigan Avenue, Chicago, IL 60616.)

Other Methods

If a positive identification is not possible through optical microscopy, other methods used in particle identification include the electron microprobe or a scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDXRA). These methods identify the elements present in a sample. Transmission electron microscopy (TEM) may also identify very small particles by means of shape and size. In addition, TEM can give selected area electron diffraction pictures that depend on the particle's crystal structure. By this method, asbestos fibers such as chrysotile, amosite and crocidolite (blue asbestos) can be distinguished from each other and from other fibers. X-ray diffraction may also be used to identify crystal structures and hence chemical compounds. X-ray fluorescence, like EDXRA, identifies the elements present. Atomic absorption spectroscopy or other spectroscopic methods are used to determine specific metals, especially hazardous particles in air (e.g. beryllium or lead). Infrared spectroscopy is useful for identifying organic compounds but, unlike the methods above, requires a relatively large sample size. When optical microscopy is inconclusive, you can identify most common contaminants by one of these methods.