Fingerprints Part II

Types of Fingerprints Visible prints – made by fingers touching a surface after the ridges have been in contact with a colored material such as blood, paint, grease, or ink. Plastic prints – are rigid impressions left in a soft material such as putty, wax, soap, or dust. Latent prints – or invisible prints are impressions caused by the transfer of body perspiration or oils present on finger ridges to the surface of an object.

Methods of Detecting Latent Fingerprints Dusting (black, gray, colored) Magna Brush Reflected Ultraviolet Imaging System (RUVIS) Iodine Fuming Ninhydrin Physical Developer (Silver Nitrate) Super Glue Fuming Laser

Dusting Works well on hard and nonabsorbent surfaces. Gray, black, and colored powders adhere to oils and residues in the latent print. The Magna Brush spreads a magnet sensitive powder over a surface with out touching the surface.

Reflected Ultraviolet Imaging System RUVIS uses ultraviolet light to locate latent fingerprints on most non-absorbent surfaces. The oils in the print reflect the ultraviolet light to the viewer, differentiating the print from the background. The UV light is converted to visible light by an image intensifier.

Iodine Fuming Warmed iodine crystals undergo sublimation to form iodine vapors. These iodine vapors adhere to organic molecules (oils and residues) in the print, thus making the print visible. The print evidence must be recorded by photography since the iodine prints are not permanent. It works on porous and nonporous surfaces.

Ninhydrin Ninhydrin is another chemical used to process latent fingerprints on porous substances. It binds to the amino acids in the print and forms a purple compound. Prints typically appear in 1-2 hours. However, weaker prints can requires 24-48 hours. Heat speeds up the process. The processed prints are permanent. Ninhydrin has been used to process prints that were 15 years old. DFO has replaced ninhydrin in recent years.

Physical Developer Physical Developer is a third chemical mixture used for visualizing latent prints. It is a silver nitrate-based liquid reagent. This technique often is effective when other methods are not. It even works in porous surfaces that have been wet. The silver nitrate reacts with the amino acids in the print forming a brown print.