Methods of Detecting Latent Prints 

Methods of Detecting Latent Prints
Latent prints- Sometimes called hidden prints, are caused by the transfer of oils and other body secretions onto a surface.
They can be made visible by dusting with powders or by using a chemical reaction.
Surface Determines Method
-Hard/Non-absorbent surfaces (Metal, plastic, glass):
Contrasting or magnetic powder
UV Light sources
Superglue
-Soft/Porous surfaces (Paper, cloth, cardboard):
Iodine Fuming
Ninhydrin
Gentian violet
Fingerprint Powder Basics
Come in more colors than just black to allow improved contrast for photographing purposes depending on the color of the surface.
Goal is to gently apply a fine coating to a non-absorbing surface
Do NOT press hard or over apply the power
The powder will stick to the perspiration residues and body oils
Fingerprint Powder Options
Fluorescent- The use of UV light can help create a stark contrast when the print surface may not be a solid color (ex. Plaid) to allow for a quality image to be taken.
Magnetic- Best utilized on surfaces that are glossy such as plastic, but can be used the same as regular powders.
UV Light Sources
UV light can allow for quick identification of a prints location so other methods of detection can be apply to the specific location of the prints.
This method works on nonabsorbent surfaces and does not alter the prints since the light is simply reflected back causing a visual contrast.
Superglue (Cyanoacrylate)
This method works best on nonporous surfaces. (Metal, plastic, leather)
The superglue is put on a hot plate to create vapors that will polymerize on the print.
However, this does require an enclosed environment to isolate the toxic vapors.
The resulting latent print will be white in color, but enhancing powders can be added for improved documentation and analysis.
Iodine Fuming
Iodine is a solid crystal that when heated, will sublime
(turn into a vapor without passing through a liquid phase)
Enclosed environment is required to contain (toxic) vapors combine with oil in latent prints to produce a visible brownish print.
However, the print fade quickly (so immediately photograph) unless sprayed with a fixing solution which is often a 1% starch solution and will turn the prints blue allowing them to potentially last for several weeks.
Ninhydrin
Ninhydrin is effective at binding to the amino acids in fingerprint residues.
The goal is to spray a mist on the evidence and the fingerprints will turn a permanent pink/purple color.
However, take precautions as the substance is toxic.
Gentian (Crystal) Violet
Crystal violet is a dye that will transfer visible color to the fatty components of sebaceous sweat residue in latent prints
A highly concentrated, intense, purple image is produced.
It is very effective on items having a sticky adhesive quality (Ex. tape)
Crystal Violet is also effective on grease- and oil-contaminated surfaces. Instructions are included.
Silver Nitrate
Objects can be dipped or sprayed with a silver nitrate solution.
The chlorides from salt in perspiration on the print will combine with silver nitrate and produce a black or red print that can be viewed under UV light.
Is used to lift prints from Styrofoam or wood surfaces.
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