Lab # 8 Enhancement Techniques to Develop Latent Fingerprints

Lab # 8 Enhancement Techniques to Develop Latent Fingerprints

Afingerprintin its narrow sense is an impression left by the friction ridges of a human finger. Impressions of fingerprints may be left behind on a surface by the natural secretions of sweat from the eccrine glands of the finger. No two people have the same exact patterns of ridges and this forms the basis of the Henry system of fingerprint analysis used as a staple of matching a suspect to a piece of evidence. More recently the “ridge flow” analysis has replaced Henry benchmarks in NIST database analysis. Whatever analysis is used, the analysis depends on a clean visualization of the ridge pattern.

A fingerprint residue contains a large variety of chemicals. It is typically 98.5% water and 1% organic materials such as lactic acid, fatty acids, riboflavin, pyridoxin, glucose, ammonia, urea, creatine, albumin, peptides, isoagglutinogens and amino acids. Inorganic compounds make up roughly 0.5% and consist of cations Na, Ca, and K+ , and anions: PO43-, Cl- , CO32-, and SO42- . It is the presence of these compounds in the print residue that can allow enhancement of prints that cannot be seen with the naked eye. Many factors including age of the print, humidity, the surface of the object and size of the object will determine which method is most effective in developing a print pattern. Microphotography is almost universally required to document fingerprint evidence obtained through chemical enhancement as this is a partially destructive type of testing.

Method one: ethyl cyanoacrylate fuming.

This method depends on humidity catalyzed polymerization of the cyanoacrylate on fingerprints. The exact chemistry of polymerization inititation is not known. It has been known to work even on old prints whose water and oils would have evaporated. This method works well on glass metal and plastic surfaces. Removal of the residue is problematic and thus is considered a destructive technique.

general method - place item to be analyzed in a closed environment and gently heat a sample of cyanoacrylate glue in a metal container and fume the object. With this method a print will develop in about 15 minutes. If heat is not available, NaOH can be used to initiate fuming.

Method two: iodine fuming.

Iodine fuming has been used since the turn of the 20th century to develop latent fingerprints on porous surfaces, particularly paper. Iodine fuming is still widely used because it is inexpensive and easy, sensitive, and is non-destructive because the stains it produces are ephemeral. If it is used at all, iodine fuming is normally the first processing method attempted. Some forensics texts state that iodine fuming is used less often nowadays than formerly. That may be true in the limited sense that there are now many alternatives, but iodine fuming is still used frequently by many forensics labs. It’s cheap, fast, effective, and completely reversible. What’s not to like? In the lab, iodine fuming is done in a chamber, but the process was adapted to field use quite early. The first iodine fuming wands were simple tubes with a small reservoir for iodine crystals. The operator warmed the tube in his hand and blew gently into one end of the wand. His breath vaporized iodine and expelled iodine vapor from the other end of the tube, which was aimed at the surface to be treated. Modern versions of the iodine fuming wand substitute battery power for body heat and the operator’s breath to avoid the risk of inhaling iodine vapor, but the principle remains the same. Iodine reacts with starches to produce a blue complex.

Method 3: Ninhydrin

This method has an advantage in that it visualizes amino acids and can be used on old prints. This method is typically applied to latent prints found on paper.

In this method a 3-5% fresh solution of ninhydrin in acetone is applied to the paper and allowed to react with gentle heating such as a hot plate or incubation oven set for 600 C. A blue-violet print is usually visualized and should be photographed immediately. A 3 % solution of zinc chloride can be applied to change the color of the print to orange which may allow it to stand out better. This method is generally considered destructive and irreversible.

Method 4: Silver nitrate

This method detects the chloride from salt left in the fingerprint and as such can be used on very old prints. It is also somewhat reversible. A 2% solution of silver nitrate is used . This will slowly react with the chloride to form AgCl. Upon exposure to UV light, or bright sunlight, the silver is reduced and the print may stand out as a black print on a brown background. The visualization is quite labile so photographs should be taken immediately. The procedure can be reversed with sodium thiosulfate (photographic hypo)/sodium bisulfate.

Procedure:

Prepare several fingerprint samples by rolling your fingers over a glass slide (watch instructor) or aluminum foil, or index card. To make sure you are leaving a good print, you may touch your nose to “refresh” the eccrine secretions on your fingertips.