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Optical media and trace evidence

October 19, 2011

From talking with numerous law enforcement folks over the years I have to admit that situations where any sort of trace evidence on a disc is important are quite rare. Most of the time, discs are collected from nice clean rooms and whatever might be on them isn’t important to the case.

So, I am not going to get into the how’s and why’s of collecting and preserving trace evidence on a disc. Let’s just leave it at saying if it is important, it needs to be dealt with in a non-destructive manner long before the disc’s content is examined.

However, there is something that doesn’t really occur to most people, even those that handle evidence every day. Let’s say you encounter a disc with a fine powdery substance on it… What should your first consideration be? Preserving the evidence? How to collect it for analysis? Nope. None of the above. The most important consideration here is officer safety.

One thing that makes optical media interesting is the fact that it is relatively impervious to most chemicals. Things that would have bad effects on paper, cloth or even some metals have little or no effect on the polycarbonate and lacquer that is the outer parts of a CD or DVD. This makes such discs useful for things like a “coke mirror”, and yes, I have seen them used for that. Also, you can place a disc in an environment where the disc becomes coated with materials that can affect your perception of reality – something that really doesn’t matter to the original owner and may in fact be an extra treat.

You see, at some point this disc is going to be handled. There are lots of fun substances out there in the drug-using world that can have effects just by contact. If you put the disc (unmodified) into a drive and spin it up, the disc is spinning at up to 5,000 RPM and materials not firmly attached to the disc are likely to be flung off into the air around the computer.

This means that without clearly understanding what is on a disc you should never, ever put a disc with any sort of foreign material on it into a drive. You have no idea what might be there and in general nobody is terribly interested in what sort of trace evidence might be present. But you may be very interested in what such substances might to do you personally.

My suggestion is that any disc you encounter with any sort of foreign substance on it should be at least dunked in a bucket of water. Plain water isn’t going to affect the disc in any measurable way, other than perhaps to destroy water-color marker writing on the disc. So, maybe it is wisest to first photograph the disc (with foreign materials present) first to document anything written on the disc. There is generally no need to use anything stronger than plain water – tap water – as it will deal with getting materials off the disc that are not strongly stuck there. Dry the disc with a cloth just by patting the disc or wiping in a straight line – never use anything on a disc in a circular motion.

A future article will deal with more intensive cleaning of discs, but that isn’t the point here. Just get anything off the disc that will be in the air around you if you put it in a drive. And, make sure you control contact with the disc before cleaning it.

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