These days, forensic scientists can call on an armoury of scientific techniques in their search for vital information to help solve a crime. Where chemical composition offers important clues, gas chromatography, atomic absorption spectroscopy and mass spectroscopy often provide powerful evidence. For identifying individuals from minute traces of blood, semen, saliva, skin or hair, DNA fingerprinting is increasingly the chosen method. Fingerprints are a valuable means of identification because we all have a unique pattern of ridges and valleys on our fingers, palms and toes and soles of our feet. They are such useful clues because prints can be left on almost anything, and they can be preserved for many years.
Prints tend to evaporate quickly from non-porous surfaces in cold, dry weather.
But on a porous surface such as damp paper, they can last for centuries. Fingerprints have even been recovered from ancient Egyptian papyri. Visible prints are made when someone touches a 'transmitting medium', for example a sticky substance such as grease, ink, paint or blood, and later touches a smooth surface leaving some of the transmitting medium behind. Plastic impressions consist of indentations left when a surface that is pliable enough to hold an impression, for example soft soap or drying paint, is touched. Latent prints are formed by perspiration or grease from the skin surface left on a surface.