Simple fingerprint collection method

Fingerprints can be divided into three categories:

Obvious lines: lines visible to the naked eye.

Such as paint, blood, ink and other items stained by hands, are usually printed on fingerprint cards as basic information.

Molding pattern: This refers to fingerprints found on soft substances, such as candles and clay.

Latent finger: fingerprint is a fingerprint pattern formed by the transfer of sweat and other natural secretions of human body, which is not easy to be found by naked eyes and is the most common fingerprint at the crime scene.

Fingers first touch grease, sweat or dust, and then touch clean surfaces, often leaving potential fingerprints. Although these fingerprints are invisible to the naked eye, they can be revealed by special methods and special chemical reagents.

The most common fingerprint that forensic doctors come into contact with is latent fingerprint. If fingerprints are left on the surface of non-absorbent objects such as metal, plastic, glass and ceramic tiles, the inspection method will be easier. Usually, powder method can be used to select powder with large color contrast and scatter it on the surface of the object to extract a complete fingerprint; Another method is magnetic powder method, which uses fine iron powder particles as brushes and brushes them back and forth to show fingerprints.

If fingerprints are left on the surface of absorbent articles such as paper, cards, leather and wood, they must be chemically treated before they can appear in the laboratory.

Commonly used chemical methods are:

Iodine fumigation-that is, iodine crystals generate steam when heated, which reacts with the grease of fingerprint residues, and then yellow-brown fingerprints will appear, which must be photographed immediately or cured by chemical methods.

Ninhydrin method-when the reagent is sprayed on the sample, it will react with the amino acids in the body secretions and will show purple fingerprints.

Silver nitrate method-after silver nitrate solution reacts with sodium amino in latent fingerprints, black fingerprints will be produced in the sun.

Fluorescent reagent method-Fluorescent ammonia and phthalaldehyde react with protein or amino acid of fingerprint residue almost immediately, resulting in high fluorescence fingerprint. The reagent can be used on the surface of colored articles.