References to Small Particle Reagent (SPR) began appearing in the literature dealing with latent print development during the late 1980s. The odd-sounding name is derived from the fact that the prime ingredient in SPR is a very finely-ground form of molybdenum disulfide. The popularity of this reagent draws from the fact that it is an ideal medium for developing latent fingerprints on wet, non-porous surfaces.
Molybdenum disulfide produces dark colored prints on light backgrounds. Since it was originally introduced, this process resulted in other formulations for use on dark surfaces (titanium dioxide) and a UV fluorescent formula works on virtually any background.
Most fingerprint powders and chemical methods are ineffective on wet surfaces. SPR, on the other hand, has proven its value time and again. SPR is available commercially in a pre-mixed liquid form. The reagent works best when sprayed on vertical surfaces, but it may also be used with tray development.
The molybdenum sulfide is mixed with a surfactant. One popular method of getting the powder into solution was the use of Kodak Photoflo®, but this product was discontinued.
SPR seems to perform equally well on dry and wet surfaces and residue is easily washed off most surfaces. The following procedure is recommended:
SPR works well on oily windows, oxidized metals, galvanized surfaces and salt-sprayed surfaces. Use No. SPR100 Dark SPR for light-colored surfaces, SPR200 White SPR for dark-colored surfaces and SPR400UV for multicolored surfaces. Latent prints developed using this method may be lifted using standard lifting mediums once the surface has dried.
1. Select the proper reagent based upon background contrast.
2. Shake the spray bottle well to get the particles into solution. On vertical surfaces, spray above the area suspected of containing latent prints and allow it to drain down over the area.
3. For tray-development, shake the reagent well and pour into a suitable developing tray. Place the evidence in the tray. Rock the tray back and forth to permit adequate contact between the
4. Immediately after prints appear, rinse the surface with water to remove excess reagent. Do not allow the water to flow directly onto the developed prints. On vertical surfaces, apply water directly above the prints and allow it to flow across them. For tray development, remove the evidence from the developing tray and place it in a clean tray. Add running water but do not allow it to fall directly onto the developed prints
5. Photograph any developed prints as soon as possible. Be certain to include a scale.
For more information on various latent print development methods, download the Latent Print Overview Manual.
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