The Use of the SEM for GSR Detection

The most successful technique to date for the analysis of GSR particles is without a doubt the - canning electron microscope with an energy-dispersive X-ray analyzer (SEM-EDX).

Basically, the SEM is a microscope which uses a beam of electrons to visualize the object under observation rather than visible light as in a conventional optical microscope. As the beam of electrons is focused by a series of magnets rather than glass lenses, the control is infinitely finer. The electron beam scans the sample in a TV-type raster pattern which is picked up, after reflection, by a video camera. The image is then manipulated electronically and the result visualized on a high-definition monitor.

With a depth of field in the region of 200 times greater than an optical microscope and an extremely high resolution, magnifications in excess of 1 000 000x are possible. In addition, on striking the sample, the electrons give up some of their energy to the elements present and this energy is then re-emitted as X-rays, the wavelength of which is particular to the elements present. These X-rays are analyzed via the EDX for wavelength and intensity, and a qualitative and quantitative analysis of the object under examination can be obtained.

With most of the other techniques which have been used for GSR analysis, the sample is destroyed during its examination. With the SEM-EDX, however, the sample is virtually unaffected by the analysis and can be re- examined, if necessary, many times.

Probably the earliest researches into the use of the SEM-EDX for GSR analysis were carried out in the Metropolitan Police Forensic Laboratory, New Scotland Yard, England around 1968. It was not, however, until 1978 that their first paper was published as a Met. Lab. Report by Dr Robin Keeley. This was a general introduction to electron microscopy with GSR examination forming only a small part. It did, however, lay down the basic techniques for the collection, examination and identification of GSR tapings taken from the hands of suspects.

In 1977, Metracardi and Kilty of the FBI laboratory produced an extensive paper (Matricardi and Kilty, 1977) on the subject. Without doubt, the most extensive work on the subject is by Wolton, Nesbitt, Calloway, Loper and Jones (Wolten et al., 1978). This is a contract paper sponsored under the Law Enforcement Administration, and its findings probably did more to advance this subject than any other. The paper is in three parts and covers everything from primer compositions, particle formation, distribution during firing, collection, analysis, and interpretation to environmental considerations.

Other papers followed in profusion (Wallace and Quinlan, 1984; Zeichner et al., 1989; DeGaetano, 1992; Zeichner, Levin and Dvorachek, 1992; Gunarat-nam and Himber, 1994), all of which have added more to the science. The basic techniques for obtaining the samples and examining them on the SEM have, however, remained the same.

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