Crime and Clues

Small Particle Reagent

Prior to the discovery of molybdenum disulfide (MoS2) in suspension as a latent print development agent, latent prints on wet, nonporous surfaces were virtually impossible to recover. Latent prints containing oily or sebaceous material can now be developed dependably on wet surfaces using a suspension known as " Small Particle Reagent" or " SPR" for short.

In a latent fingerprint on a wet surface or one submerged under water, the oily components of the fingerprint residue are held in place by the surface tension of the water. As long as the latent remains wet, the oils retain the shape and details of the fingerprint. However, when the surface is exposed to air and the water drains off or evaporates, the oily residue tends to spread out or run, resulting in a smudged print. Processing with "SPR" before the surface dries will develop and fix the print.

SPR is prepared by mixing approximately an ounce of MoS2 powder in a quart of water. In metric measurements, one would measure around 30 grams of MoS2 per liter of water. In either case, two or three drops of laboratory detergent is added. A good detergent is PhotoFlo. The detergent should be used sparingly, however; too little and the MoS2 will fog the background, too much and the detergent itself will degrade the latent. Normally, the MoS2 and detergent are placed in water in a bottle or jar with a watertight lid, and shaken vigorously to create the suspension. It should look like dirty used motor oil when it is ready to use.

It has been reported that adjusting the pH of the suspension to somewhere between 3 and 4 by the addition of acetic acid increases the effectiveness of the SPR. If you want to try this, mix the suspension first, then add a few drops of acetic acid. Monitor the pH with test paper until the correct range is established.

The SPR suspension must be continuously agitated during use, usually by shaking the bottle from which it is being dispensed. The word "suspension" refers to particles of insoluble material mixed into the water, but not dissolved in it. The word "solution" would refer to a material which actually dissolves in the water.

SPR is normally sprayed on the wet surface being examined for latent prints. For example, if a vehicle were submerged in a lake and there was a request for a latent print examination, SPR could be sprayed onto wet vehicle body and glass surfaces as it was being removed from the lake. After the SPR is sprayed onto the surface, it must be rinsed off with clean water before it begins to dry. All that remains after rinsing is the MoS2 which has adhered to the latent print. The latent may then be photographed and, after it has dried, lifted in normal fashion.

Smaller items may be processed using the SPR in a tray. The items should be submerged in SPR with the side to be processed facing up, then removed and rinsed under clean water. As with SPR in a spray bottle, the suspension must be agitated or stirred frequently to ensure the MoS2 remains in suspension and does not sink to the bottom of the tray.

SPR can also be used successfully where foreign residue would prevent the use of dry powder, such as a soft drink can coated with sticky droplets of the beverage, or on a bar top with spilled drinks on it.

Experiments by the Israel National Police have also found SPR effective at arson scenes in developing latent prints from glass fragments coated with accelerant, even after burning. For a detailed discussion of their tests, see The Journal of Forensic Identification, Vol. 46, No. 5 (Sept./Oct. 1996).

Although the Home Office, in its Scene of Crime Handbook of Fingerprint Development Techniques, says that MoS2 presents no known hazards, molybdenum and its compounds are listed by others as being toxic. Therefore, precautions are recommended in the use of SPR. Read the MSDS on the product. Wear gloves and a dust mask when mixing the MoS2, and again when spraying the SPR. The compound is extremely messy, as well, and clean up may be necessary after use. Soap and water are normally sufficient to clean the residue left by SPR.