Explosive fingerprints: spectroscopy IDs TNT in dabs

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  • Published: Apr 15, 2011
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Explosive fingerprints: spectroscopy IDs TNT in dabs

Spectroscopic breakdown

Optical catapulting-laser-induced breakdown spectroscopy has been used to successfully detect residues of common explosives in human fingerprints, according to researchers from Egypt and Spain.

Fingerprints are the often fragile impressions left by the friction ridges of human digits, they contain a complex chemical cocktail that is a mixture of natural skin secretions and contaminations from the environment. In dactyloscopy, fingerprints are used for forensic identification of criminal suspects and there is a whole raft of techniques available to crime-scene investigators including photoluminescence and ultraviolet absorption, powdering, small particle reagent and vacuum metal deposition, physical developing, multi-metal deposition, iodine and cyanoacrylate and chemical complexation used to reveal and capture apparent and latent fingerprints and to link the suspect to the crime and to exclude the innocent. The choice of best detection technique depends on various factors such as the nature of the surface on which the fingerprint was dabbed.

In terms of particular crimes, such as terrorism involving explosives, fingerprints can provide critical chemical evidence by exposing specific traces of chemicals associated with the explosives.

Pointing the finger

Now, Fran Fortes and Javier Laserna of the Department of Analytical Chemistry, at the University of Malaga, and Mohamed Abdelhamid and Mohamed Abdel Harith of the National Institute of Laser Enhanced Science, NILES, at Cairo University, Giza have focused on optical catapulting in combination with laser induced breakdown spectroscopy (OC-LIBS) as a method of choice for investigating explosive contaminants in fingerprints.

"The most important feature in this field is the ability for discriminating between explosive and non-explosive materials," the team explains. "The identification of these compounds relies on the detection of C, H, N and O, as well as molecular emissions from CN radicals and C2 molecules."

Standard LIBS has made some head way in detecting explosive and energetic compounds but relies heavily on statistical tools that can easily succumb to spectroscopic interferences, the team says. On the other hand, OC-LIBS is a relatively new and developing technique used for effectively analysing solid aerosols. It exploits advances in laser technology and falling prices to bring an otherwise inaccessible tool to forensic science. The technique works by ejecting particles from the sample using an acoustic pulse or pressure wave generated by a laser beam transmitted along the sample. The particles thus catapulted make no mechanical contact and so are free from interferences from the substrate when subsequently analysed by LIBS in the spectral range 370 to 500 nanometres.

Explosive proof

The researchers have demonstrated proof of principle using fingerprint samples contaminated with one of a set of organic explosives, mononitrotoluene (MNT), dinitrotoluene (DNT), and the more familiar trinitrotoluene (TNT)) as well as non-explosive materials in the form of oil and Bermuda Grass Smut Spores. Less than a microgram per square millimetre of fingerprint was used.

The team was able to discriminate between fingerprints contaminated with explosives and the non-explosive substances. They could also generate chemical images of the fingerprint to determine where in the whorls of the digital grooves and ridges the explosive contaminant had lodged, perhaps hinting at how an explosive material might have been handled or whether contamination was inadvertent and entirely fleeting. Such evidence if supported by additional evidence could mean the difference between convicting a bomber and an accessory to the fact.

"Discrimination between explosive and non-explosive materials has been also demonstrated," the team concludes. "The capability of analyzing explosive residues in human fingerprints without the use of reactives or other treatments is an important outcome of this technology."



Credit: JAAS, Royal Society of Chemistry. Optical catapulting¯laser-induced breakdown spectroscopy has been used to successfully detect residues of common explosives in human fingerprints using, according to researchers from Egypt and Spain.

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