Virginia Department of Forensic Science
Latent Fingerprints & Impression Evidence
Latent print examiners examine items for the presence of impressions of friction skin. In most instances, latent prints are not readily visible and development by chemical and/or physical means is required. Detected prints are collected, preserved, and compared to any submitted known prints of victims and suspects or searched through the Automated Fingerprint Identification System (AFIS). Impression examinations include but are not limited to the analysis of footwear and tire tread on items of evidence and at crime scenes.
- Automated Fingerprint Identification System (AFIS)
The Virginia Division of Forensic Science is one of 26 state, local and federal agencies that have access to nearly 1,200,000 fingerprint cards and more than 34,000 unsolved latent prints on the database of the Automated Fingerprint Identification System.
Latent Print examiners use the following procedures for print detection and collection:
- Cyanoacrylate Ester (Super Glue Fuming)
This procedure, first devised in 1978 by the Criminal Identification Division of Japanese National Police Agency, involves the use of glues containing methyl or ethyl cyanoacrylate esteras the active ingrediednt. The vapors are absorbed at the latent print ridge sites. In the presence of moisture, a white polymer is formed when made visible. Cyanoacrylate fuming has been quite successful in developing prints on surfaces such as plastices, electrical tape, sandwich and garbage bags, styrofoam, carbon paper, foil, finished wood, cellophane, rubber and smooth rocks.
Since this process was introduced in this country in 1982, much progress has been made. Today most cyanoacrylate fuming is followed by dye staining or flourescent powdering and viewed with a light source. Fuming containers vary from small fish aquariums to more sophisticated vacuum chambers.
- Light Sources
Light sources, other than lasers, are proving to be necessary tools in the forensic science community, especially in the detection of latent prints. To excite fluorescence, a light source must emit at a wavelength at or very near the maximum excitation of the fluor. It needs to have sufficient power to allow a dark adapted eye to detect a latent print and the light must be pure enough to avoid contrast reduction. Alternate light sources used in latent print detection produce a continuous wave of light, even if most of the output is in many spectral lines. Filters, particularly at the fluorescent emission wavelength, are required to block this continuum. These wavelengths are measured in nanometers.
Ninhydrin, a process widely recommended for porous surfaces, may be applied by dipping, spraying or swapping the substrate. It reacts with the alpha-amino acids, polypetides and proteins in a latent print; the reaction is evident by the purple coloring. Optimum results have been obtained when ninhydrin items were heated at 26.6°C (80°F) and 80% relative humidity.
Ninhydrin was first introduced as a means of developing latent prints by two Swedish scientists, Oden and von Hofsten, in 1954. Oden patented the process in 1955. Today there are several possible formulas and a number of ninhydrin analogs.
- Blood Enhancers
The following chemicals are recommended to enhance or develop latent prints in blood:
- Amido Black B10
- Coomassie Brilliant Blue R250
- Diaminobenzidine (DAB)
- Crowle's Staining Solution
Note: No serologic analysis can be performed after staining of bloody prints, so those examinations should be done first.
- Powder Processing
The simplest and most common way of developing latent prints on non-porous evidence is the physical enhancement procedure of powder dusting. Most regular powders consist of both a resinous polymer, allowing adhesion and a coloring component for contrast. Although there are a variety of colors of powder available, black powder yields the best results. Black magnetic powder is also quite effective.
- Wet Evidence
Quite often, it is necessary to attempt to develop latent print while they are still wet. The two methods that have proven most effective are as follows:
- Physical Developer (PD)
For use on porous items. Photograph any developed latent prints immediately.
- Small Particles Reagent (SPR)
Best used on non-porous items. Residue is very fine, so immediate photography is recommended.
- Physical Developer (PD)
- Adhesive Surfaces
Developing latent prints on the adhesive side of surfaces such as tape can be a challenge. Some recommended methods include: Sticky Side Powder and Gentian Violet.