Latent prints are the kind of finger prints found at crime scenes which are formed by the oil and sweat secretions deposited by a person's fingers when they touch a surface or an object. They are invisible to the naked eye and need some kind of treatment to make them visible.
The most widely used method of detecting latent prints is to dust using a fine powder that adheres to the traces of oil and sweat. Dusting is suitable for hard and/or non-absorbent surfaces. There any many different kinds of powder in use today. For example;
aluminium dust, which is grey and highly visible on dark and mirrored surfaces;
carbon black for white surfaces; and
luminescent powders which fluoresce under ultraviolet light.
N.B. A common every day application of luminescence is washing powder that contains optical brightener. The optical brightener is nothing more than a fluorescent dye that shows a blue luminescence when excited by the ultraviolet radiation present in sunlight. The optical brightener is added to counteract the yellowing of cotton as it ages.
For porous surfaces like paper or cloth, a number of methods have been developed over the years to help detect fingerprints. Examples are as follows:
This is the oldest of the chemical methods for detecting fingerprints. Solid iodine sublimes (becomes a vapour without becoming a liquid) at room temperature. If an object, say a piece of paper, is placed in a chamber which a has had some crystals of iodine placed in it, any fingerprints on the object will appear as brownish prints. What is thought to be happening is that the iodine vapour is dissolving in the skin oils (or may be the traces of water from the perspiration) that make up the print. Whatever is happening the print is temporary and will soon fade. It must therefore either be photographed immediately or "fixed" by spraying with starch solution to give a blue print which will last longer.
Iodine fuming has to a great extent replaced by other methods including spraying with the chemical ninhydrin which reacts with amino acids present in the skin secretions to give purple prints.
The active ingredient in superglues is a cyanoacrylate ester. When vapours of these compounds come into contact with fingerprints the molecules of the cyanoacrylate attach to the print and polymerise. The visible prints produced are white, to improve their detectability they are often treated with a fluorescent dye, such as Rhodamine 6G, before being photographed under an special light source or a laser.
Luminescence and Fingerprinting
Some molecules when irradiated with certain types of light fluoresce, the phenomenon is called luminescence. Researchers discovered some years ago that some components of sweat are luminescent and fluoresce when illuminated with lasers. There are disadvantages in using lasers as they are expensive and there are significant safety issues to consider. With the advent of treatment of fingerprints with fluorescent dyes (see above), the use of so-called alternate light sources which are essentially high intensity lights has increased as they are less expensive, safer and more portable than lasers.