26 April, 2012

Mystery Musings: How to lift that fingerprint?

There's more than one way to lift a print. And did you know that there are three types of prints?  Latent, patent or visible, and plastic or impression. 

Visible fingerprints can be photographed directly, and impression fingerprints can usually be photographed under special lighting conditions. It is only the invisible latent fingerprints that are difficult to photograph. They must first be made visible, and the fact that they are easily wiped away with a cloth or Clorox only makes it harder.

The old stand by standard fingerprint powder is still used, but it's no longer the method of choice.  One reason being that the volcanic ash it is made is extremely fine and almost impossible to remove if dumped on a rug or upholstered furniture. Regular powder can be applied smooth, shiny surfaces such as windows, televisions, kitchen counter tops, painted surfaces, and vehicles—either the painted surfaces on the exterior or on glass. Prints are then lifted using adhesive film.

The Magna Brush uses standard powder with pieces of iron metal added and also works best on shiny surfaces. When dusting for fingerprints with magnetic powder, crime scene investigators must use a magnetic applicator which has a magnet. Besides being available in the colors of black, white, silver/gray and biochromatic, magnetic powder is also available in fluorescent magnetic powder colors like red and green which are useful for objects like soda cans. The fluorescent powder will reveal the print under black light or with a specially coated camera lens. Since it is applied with a magnetic brush, there is less diffusion and less mess, making it the preferred method in most cases.

But what about some of the gee whiz stuff we see on TV?  Turns out NCIS's lab guru Abby Sciutto is right: super glue is effective.  Actually it's one of the glue's components—cyanoacrylate—which, when mixed with heat and humidity, adheres to a print. The print can then be dusted using one of the available powders.  Don't try this at home! Cyanoacrylate contains cyanide and must be used in a closed container.

Finger prints consist primarily of ordinary sweat and other organic components exuded through the fingertips. Sweat is mostly water, and will dry after a fairly short period of time, making them difficult or impossible to lift with powders.  In this case, a ninhydrin solution can be sprayed, swabbed or dripped onto the surface. Ninhydrin reacts with the amino acids in the prints, forming a purple or pink compound.

Iodine crystals can also be used for card stock. The crystals are placed in a glass tube known as a fumer. The CSI then blows into the fumer, causing the transformation from solid to gas. The iodine vapours are emitted from the other end, and if the tube is aimed at a latent print, it will become visible for a short time. Do not try this at home! If inhaled, the fumes will convert back to crystals in your lungs and they are highly carcinogenic.

Silver Nitrate is a less toxic way of detecting prints on paper. Silver chloride turns black in light, and one of the components of sweat is sodium chloride. The silver nitrate is placed with distilled water and applied to the paper. The paper is exposed to light, and any prints will turn black.

On wet surfaces, if there is no time to allow it to dry, police use a small particle reagent (SPR) . The SPR can be sprayed on  wet surface, for instance a car, then hosed off.  Fingerprints will stay clear.

Fingerprints on duct tape, sticky tape, or labels can be lifted using crystal violet which is absorbed by the fatty constituents in a fingerprint, thereby coloring it purple. Again, don't try it at home, as it is highly poisonous and easily absorbed through the skin.

And finally, there is Amido Black, a chemical used to develop fingerprints in blood. A fixing agent is first applied to the blood stain, and then amido black is used, resulting in a dark blue to black staining of the protein that can enhance the contrast and visibility of patterns and impressions and allow for easier documentation.

Fingerprints will not adhere to unfinished wood or rock, although there is a solution which can be used to enhance prints on these surfaces.  Unfortunately it costs $500 per ounce!


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