MadSci Network: Genetics

Re: If you touch a surface do you leave a fingerprint?

Date: Wed Oct 3 12:56:02 2007
Posted By: Dale L. Laux, Staff, Serology/DNA, Ohio Bureau of Criminal Identification
Area of science: Genetics
ID: 1191438839.Ge

Hello Callie,

CSI makes everything look magical, doesn't it? Unfortunately, in the real world, things are not as wonderful as in Hollywood.

People leave behind fingerprints on items that they touch because the water containing salts and the oils from our skin are transferred onto the object. But a lot depends on how much the person is sweating and the type of object that is touched. Rub your finger against your forehead, then touch the mirror in your bathroom. You'll leave a nice fingerprint. Now wash your hands real well with soap and water and dry them. Touch the mirror again and you may not see a fingerprint at all. It all depends on the person's activities and the material touched. Fingerprints aren't transferred well onto fabric, molded textured plastic, wood and other substances like that. Fingerprints are transferred very well onto glass, smooth metal surfaces, and smooth plastic surfaces. Surprisingly, fingerprints can be obtained very nicely from paper that is touched. A process using a chemical called ninhydrin that reacts with proteins in the fingerprint turns invisible prints a purple color. And a chemical called super glue, which you may have used to fix something at home, reacts with the oils in a fingerprint and produces a white material that adheres to the fingerprint.

There are many other techniques that we use at the crime laboratory to develop fingerprints including liquid dyes, fluorescent dyes and ultraviolet lights. But to answer your question, you are correct, sometimes the person just doesn't leave a fingerprint.

Best regards,

Dale L. Laux
Forensic Scientist

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