In research presented last week at the 229th national meeting of the American Chemical Society in San Diego, Calif., Los Alamos scientist Christopher Worley describes the detection of fingerprints based on elemental composition using micro-X-ray fluorescence showing how the salts, such as sodium chloride and potassium chloride, excreted in sweat are sometimes present in detectable quantities in human fingerprints.
MXRF actually detects the sodium, potassium and chlorine elements present in those salts, as well as many other elements, if they are present. The elements are detected as a function of their location on a surface, making it possible to “see” a fingerprint where the salts have been deposited in the patterns of fingerprints, the lines called friction ridges by forensic scientists.
The technique has several advantages over traditional fingerprint detection methods that involve treating the suspect area with powders, liquids, or vapors in order to add color to the fingerprint so that it can be easily seen and photographed. Using this technique, known as contrast enhancement, it is sometimes difficult to detect fingerprints present on certain substances, such as multicolored backgrounds, fibrous papers and textiles, wood, leather, plastic, adhesives and human skin. Children’s fingerprints are particularly difficult to detect due to the absence of sebum, an oily substance on the skin that is secreted by the sebaceous glands, which captures the contrast enhancing agents. Also, coloring a fingerprint with traditional contrast enhancement methods can be an arduous process that sometimes yields only limited success.
Worley warns that MXRF is not a panacea for detecting all fingerprints, since some fingerprints will not contain enough detectable elements to be “seen”. However, it is envisioned as a viable companion to the use of traditional contrast enhancement techniques at crime scenes, since it does not require any chemical treatment steps, which are not only time consuming, but can permanently alter the evidence. Since MXRF is noninvasive, a fingerprint analyzed by the method is left pristine for examination by other methods like DNA extraction.
In addition to Worley, the MXRF development team includes Sara S. Wiltshire, Thomasin C. Miller, George J. Havrilla and Vahid Majidi.
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