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Who Love to Watch the CSI Fingerprint Capture?

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In old-fashion detective stories, no matter the story comes from which country, there is always a “blood fingerprint” to show up. However, in current movies, such scenarios become less and less, probably because there are many methods to capture and identify the fingerprints and nobody is going to commit such stupid mistakes any more. In fact, even if the evidence is not so obvious like a “blood fingerprint”, CSI can still capture the fingerprints in the crime scene as much as possible, no matter whether visible or not. Or, to be more professional, no matter which type of the fingerprint is—patent, plastic or latent.

 Patent prints are normally referred to those that are caused by transferring of a foreign material from a finger onto a surface, which is obvious to human eyes. A good example is when a man has blood on his hands and touches somewhere such as door knob, handle or steering wheel, he will leave patent prints onto these subjects.

 Plastic prints are friction ridge impressions left on soft substances. For example, a careless murderer left some obvious print details on clay, soap or candle wax.

 Latent prints refer to the invisible prints by naked eyes and they are formed by the secretions from the tiny sweat glands on the fingers. These secretions mostly consist of water (98.5 %) while the rest is solid substances which are small in amount (1.5 %) but wide in variety. Some solid substances are very important since they are the targets to probe with certain chemical reactions, which can help reveal the existence of fingerprints.

Between the three types of prints, latent print is the most difficult to identify, but also the most common one in CSI. Fortunately, people have developed plenty of approaches to treat the prints and visualize them. Three commonly used approaches are powder test, chemical test and optic test.

Power test is the most often used method, just as you see in the CSI series. The CSI professionals attach fine powders onto a brush with fine fibers and then gently brush on the surface to reveal the latent prints. But this procedure requires extra caution in case the prints are damaged due to large forces.

To clearly identify the prints from the background, powders with different colors such as black, white, grey, red, etc., are employed but in most cases CSI choose black and grey.

In addition to normal powder, magnetic powders are also utilized. In this method, fine iron powder are thoroughly ground and scattered onto the prints. With the help of a moving magnet, the iron powders gradually form the shape of the prints.

If the prints are on a subject that is easy to take away, for example a gun or a knife, then CSI will directly bring it to the lab as physical evidence. However, if the prints are on a surface that is hard to be removed, such as wall or window, the only way is to copy it with tape–you may have seen this scene for a hundred times in CSI, right?

 All the above mentioned techniques are good enough for prints on dry surfaces, but how about the wet ones? If the potential prints are on a wet surface, then the traditional approaches won’t work and people need other analyzing method.

Among all currently existing powder tests, the most sensitive one is called vacuum metal deposition (VMD). In this method, you will need to put the subject that is going to be analyzed, such as a gun, into a sealed container and vacuum it. Then, evaporate small amount of gold into the container so that the gold can form a thin layer which can completely cover the surface of the gun. In the next step, small amount of zinc will be also evaporated in the container so zinc also forms a thin layer. However, the secretions from sweater glands on fingers reject zinc but accept gold, hence zinc can’t attach to the prints. The result is obvious—what you’ll get is a gold fingerprints on a layer of white zinc powders.

You could expect that a buddy who commits a crime but doesn’t want to wear gloves must be a careless and mentally weak guy, and there is large change that his sweaty fingerprints are left here and there.

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