DEVELOPING FINGERPRINTS

Constituent Quantity per mm2 Chlorides >10 μg Amino acids 10-100 μg Urea ~1 μg Ammonia <0.5 μg Sebum 5-100 μg





A table showing the typical contents of a fingerprint

Fingerprints are made up of tiny droplets ranging in size from 1-20μm, which consist mainly of sweat and approximately one millionth of a gram of chemical material in total. A ‘hidden’ fingerprint, for example, one left at the scene of a crime, is called a ‘latent’ fingerprint (‘latent’ is the Latin word for ‘hidden’). One way of making these fingerprints visible is called the ‘powder and brush’ technique. In this method, the surface is brushed with a very fine powder that sticks to these droplets. Some surfaces, however, absorb this powder and the fingerprints are not distinguishable. This problem can be overcome by the use of electrostatics.

The article to be analysed is attached to a negatively charged electrode connected to a high voltage. A very fine powder is placed on the adjacent positive electrode, becomes charged and is attracted to the negatively charged specimen. Due to the high voltage, the particles travel quickly and stick firmly to the fingerprint. Particles hitting the electrode lose their positive charge and return to the positive electrode where they are recharged. This process continues until the fingerprint has been sufficiently built-up.

This technique is not appropriate for use on all surfaces, but there are several other methods that can be applied:

· Laser luminescence: Involves the illumination of fingerprints due to fluorescing particles picked up during everyday life from paints, inks and oil. It can be used on painted walls, metals, plastic and rubber, cloth and wood.

· Metal evaporation: The fingerprint is developed by first evaporating a thin layer of gold onto the specimen, followed by a layer of cadmium which fills in the print and provides a contrast.

· Silver nitrate: Used for fingerprints on paper, silver nitrate is sprayed onto the fingerprint where it reacts with the chlorides, to give the insoluble silver salt, silver chloride. Not suitable for fabrics or rough surfaces such as wood.

· Ninhydrin test: Indantrione hydrate reacts with the amino acids in the fingerprint, giving a visible deposit. Also not suitable for fabrics or rough surfaces.

· Iodine vapour: Can be used to develop fingerprints on fabrics and rough surfaces. Iodine vapour alone is useful only for prints up to 24 hours old, however a mixture of the vapour and steam allows this method to be effective for up to two months.

· Bacteria: Certain bacteria, for example acinetobacter calciacatieus, can be used to develop prints on valuable oil paintings, without harming the painting in the process. The bacteria in a nutrient gel are pasted onto the surface of the painting, making the print visible as they multiply. The gel can then simply be wiped off, leaving the painting unaffected.

· Autoradiography: Radioactive atoms are incorporated into the fingerprint by placing the piece of fabric into a container containing radioactive gases, such as iodine or sulphur dioxide, at a humidity of less than 50%. The fabric is then put into contact with photographic film, and the radioactive atoms cause a picture to become clear.

After the prints have been developed they can then be entered into a computer, which allows them to be quickly and easily recalled and compared to the fingerprint of a suspect. It should be noted that this method is as useful for proving the innocence of a suspect as it is for convicting criminals.

...Back Next...