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I can’t say enough about avoiding cross contamination. Put on gloves, use gloves, change gloves. Do that every time you touch a piece of evidence. Likewise, use disposable tweezers, scalpels, etc. Change these each time they are used, as well.

There are some special considerations when you are working to secure fingerprint evidence. Be careful when handling evidence for prints. Remember to pick up or touch the item where someone would normally not touch it—that will be the most likely place for prints. Also be mindful of the fact that gloves can actually ruin prints. Of course, if the gloves are contaminated, you can contaminate the evidence. But a second consideration is that under certain circumstances when you are wearing gloves, you can leave your own prints. Gloves can easily become contaminated with oils and the like. When you push on something while wearing these thin gloves, you can actually push your ridges against the gloves, transferring your prints to whatever you are touching. The use of cotton gloves can help alleviate this problem.

Your job as an investigator is to eliminate the questions on the collection of the evidence. Make sure that the evidence will stand up in court.

A note about being questioned in court. I have found that the easiest way for your work to hold up in court is to be consistent about the way you handle a scene. Every scene—from simple cases to homicides—should be processed the same way.

From: Evidence Handling and Collection by Dick Warrington

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