How CSIs process wet surfaces for latent fingerprints.
It had rained for more than 5 hours when an observant police patrol officer found the car believed to have been used in an armed robbery. The vehicle’s description, color and license tag all match the radio bulletin broadcast earlier during the “graveyard shift.” The vehicle had also been reported stolen the day before.
In short order, the crime scene investigation team arrives at the scene and makes a quick visual survey of the vehicle’s exterior. Of particular interest are areas that may have been touched by the previous occupants. Using flashlights held at different angles, the CSIs examine the exterior for possible latent prints. They may or may not see any. After all, latent print defined means one that is not readily visible.
Processing for latent prints throughout the vehicle’s interior will be a routine matter, but the exterior poses a particular problem—it is soaking wet, and a constant drizzle continues to fall.
Latent fingerprint developing powders are generally the first choice for investigators working crime scenes, and in this case, vehicular crime scenes. To process areas of the exterior, the crime scene technician will apply a specially formulated powder using either a fiberglass filament or animal (camel or squirrel) hair brush. But using this dry powder on a wet surface will only result in a mess—and any fingerprint ridge detail will be obliterated.
The investigators are left with just two options:
- Wait until it stops raining and allow the vehicle to dry out, or
- Find some other means to process the vehicle for latent prints despite the rain
For Option 2. above, many CSI teams will employ a formulation that has become known as Small Particle Reagent or simply SPR.
Unlike many other latent print development chemicals such as Silver Nitrate and Ninhydrin, which originally had other scientific uses, SPR resulted from specific research and experimentation into finding a means of developing latent prints on wet surfaces.
The original SPR formula consisted of a very finely ground black powder—Molybdenum disulfide, along with a liquid detergent and water. This mixture, when sprayed on a vertical or near-vertical surface, tends to run downward. If latent prints are present, the liquid-suspended black powder will attach itself to the moisture residue of latent prints, thus making the ridge structure of the print plainly visible.
SPR Dark works fine on light colored surfaces but is of little value if the surface is painted black. What researchers came up with next was a formula using finely milled white powder—Titanium dioxide—and the same water-detergent mixture. SPR Light and SPR Dark are found in many crime scene investigation kits.
In recent years, a third formula was developed that will aid in latent print recovery from multicolored backgrounds. This mixture is fluorescent—it glows in the dark when longwave UV light is applied. Like its counterparts, SPR UV employs the same detergent-water carrier.
SPR UV then fills an apparent void in formulations providing the CSI with enough variety to cover most wet surface conditions.
Application of a SPR formula is simple-it is applied using a spray bottle. The mixture is sprayed onto the surface above the area suspected of containing latent prints, and it is allowed to run down across the surface. The solution also lends itself to tray development.
While its primary purpose is latent print recovery on wet surfaces, it may be used on virtually non-porous surface—wet or dry.
Once latent prints are developed, they must be immediately photographed, as they are quite fragile and when wet, they may be very difficult to lift using tape, hinge-lifters, etc.
Resourcefulness resulted in the discovery of SPR. Researchers saw a need and took the logical steps to meet it. To learn more about how forensic science uses existing processes as well as inventing new ones in the war against crime, please visit the many other sections in this blog. Use the navigation window to the right labeled Categories.
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