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When processing a crime scene, one of the first steps for law enforcement officials is to dust surfaces for fingerprints. There are no two fingerprints in the world that are exactly the same, and often criminals will leave fingerprints behind on many different types of surfaces, which can help investigators solve crimes. There are some types of surfaces that make it easier to lift fingerprints and also require different techniques for success.
Latent fingerprints are often left visible on porous surfaces, such as paper. To develop these prints, investigators can use the chemical ninhydrin, which reacts with the amino acids in the fingerprint to bring out the ridge surfaces and make them highly visible. The fingerprint then turns purple and can be easily seen by the naked eye or a microscope.
If a surface is nonporous, it means that it is permeable by water or air. These surfaces include plastic and glass; they can easily hold fingerprints that can be obvious or hidden. The best way to treat nonporous surfaces is to use fingerprint dusting powder to bring out the print and then lift it with special tape to preserve the print. Another way to lift these prints is to use fuming methods or luminescent lighting in crime labs.
When guns are involved in a crime, finding a fingerprint on a gun or a metal cartridge can be crucial to investigators. Scientists working with the Northamptonshire Police in England have started using techniques to find fingerprints on metal surfaces even after they have been wiped down or compromised. An electric charge is applied to the metal, which reacts with the corrosion left by the oils from the finger to reveal the fingerprint.
Another technique for lifting fingerprints off of fabric has been developed by scientists in Scotland. Vacuum metal deposition is a highly sensitive process that uses gold and zinc to adhere to fingerprints on fabric. This discovery may make it easier to pull clear fingerprints from fabric, which can often be a difficult task. Fabrics with high-thread counts, such as nylon, polyester and silk, are best for retaining the prints.