Finding Fingerprints

Introduction

Do you know that on everything you touch, you leave fingerprints? If your hands are very dirty, this is obvious, because you can actually see them. But even if your hands seem clean, your fingerprints will stay behind on the surfaces you touch—they are just invisible! Do you want proof? Then make them visible in this activity and collect your own fingerprints!

Background

Detecting invisible fingerprints is an important task in forensic science, a branch of science that helps criminal investigations by collecting and analyzing evidence from crime scenes. Fingerprints are the most commonly-collected type of evidence. Because fingerprint patterns are unique to a specific person, they are a very reliable way of identifying a suspect. There are different types of fingerprints that can be left behind: 1) a fingerprint imprint in a soft surface, such as wax or soap; 2) a patent fingerprint, visible to the naked eye, such as fingerprints resulting from dirty hands; and 3) latent fingerprints, which are invisible, but still present.

These invisible latent fingerprints are made of water, fatty acids, amino acids, and triglycerides—in other words, they result from the oil and sweat that your skin produces naturally. To make them visible, you have to find a way to detect one of these substances present in the invisible fingerprint. The easiest method is called dusting, in which you use a very fine powder that can stick to the oil in the fingerprint. Once the fingerprint becomes visible, you can lift it from the surface with clear tape and transfer it to another surface to then take into the laboratory to analyze further. Other methods include using chemicals that react with the amino acids or water in the fingerprint; the chemical reaction results in a colored fingerprint, which you can then analyze easily.

Many factors determine the quality of a fingerprint on a surface. One of the most important factors is the surface texture. Fingerprints are most easily detected on smooth, non-textured, and dry surfaces. The rougher or more porous the material, the more difficult it will be to get good fingerprint evidence. Another factor is the condition of the skin on your fingertips. If it is very sweaty and oily, you are more likely to leave behind prints than if it is nice and clean. Of course, wearing gloves also prevents leaving behind fingerprints. Test it yourself and collect your own fingerprint evidence like a real crime scene investigator in this activity!

Materials

Preparation

Procedure

Observations and Results

Were you able to collect some of your own fingerprints? On a smooth surface like glass or metal, fingerprints tend to stick very well. With your unwashed hands, you should have been able to make your fingerprint visible with either cacao or baby powder. Just a little powder applied with a brush should be enough to reveal your fingerprint. If you apply too much powder, however, the fine details of your print tend to get lost. When you press too hard onto the surface with the brush, the fingerprint will be wiped away, so you have to be careful when treating the surface with the powder.

Your freshly-washed hands have much less oil and sweat on their skin as they have been washed away with the soap and water. This results in a much less pronounced fingerprint. You might have had difficulties in collecting this fingerprint or may not have found one at all. On the other hand, if you apply hand lotion, which contains lots of oil and fat, this will make your fingertips much stickier, which leads to a much more pronounced fingerprint. You should have seen a big fat fingerprint once you applied the powder to the surface where you put your finger. If you compare all the prints you collected, the one with hand lotion should be most visible, whereas the print with your washed hands should be barely visible.

If you did the extra activities, you might have noticed that porous or rough surfaces or materials such as paper or textiles are not very good for collecting fingerprints. Also, when wearing gloves, no fingerprints are left behind. These are all important factors that real crime scene investigators have to take into account when collecting fingerprints at a crime scene. Considering your results, where would you look for fingerprints in your home? Did you find some?

Cleanup

More to Explore

• A Simplified Guide to Fingerprint Analysis, from Forensic Sciences Simplified
• How does fingerprint powder work?, from Scientific American
• How do different surfaces affect the preservation of finger prints?, from UCSB ScienceLine
• Fingerprints give police new clues for solving crime, from BBC News
• Science Activities for All Ages!, from Science Buddies

Credits