21 November 2007

How To Clean a Bloody Knife; for entertainment purposes only

With the holidays right around the corner some of us may be lunging at relatives so I thought this article might come in handy. Enjoy.

Does DNA come off with soap and water?
By Juliet Lapidos for Slate

“Investigators in Perugia, Italy, have found new evidence linking a 20-year-old American exchange student, Amanda Knox, to the brutal stabbing death of her roommate, British student Meredith Kercher.

According to the latest reports, Knox and her Italian boyfriend, Raphael Sollecito, cleaned the alleged murder weapon—an 8-inch black-handled kitchen knife—with bleach. Nevertheless, police discovered Kercher's DNA on the tip and Knox's DNA by the handle.

Is it even possible to clean DNA off a knife?

Yes, if you know what you're doing. Knox and Sollecito were on the right track: Bleach contains sodium hypochlorite, an extremely corrosive chemical that can break the hydrogen bonds between DNA base pairs and thus degrade or "denature" a DNA sample. In fact, bleach is so effective that crime labs use a 10% solution (one part commercial bleach to nine parts water) to clean workspaces so that old samples don't contaminate fresh evidence. Likewise, when examining ancient skeletal remains, researchers first douse the remains in diluted bleach to eliminate modern DNA from the surface of bones or teeth.

So, why did Knox and Sollecito's bleaching gambit fail?

It's difficult to swab a knife thoroughly. Dried blood can stick to the nooks and crannies in a wood handle, to the serrated edge of a blade, or become lodged in the slit between the blade and the hilt.

With help from a Q-tip, it's possible to eliminate most stains, but what's not visible to the naked eye might still be visible to a microscope, and sophisticated crime labs need only about 10 cells to build a DNA profile.

Bleach is perhaps the most effective DNA-remover (though evidently no methodology is failsafe), but it's not the only option. Deoxyribonuclease enzymes, available at biological supply houses, and certain harsh chemicals, like hydrochloric acid, also degrade DNA strands.

It's even possible to wipe a knife clean of DNA-laden hair follicles, saliva, and white blood cells with generic soap and warm water. The drawback to this last method is that the tell-tale cells don't just disappear once off the knife. They linger on sponges, in drains, and even in sink traps, where wily investigators search for trace evidence.”
I've, uh, heard mace also works well. Certain elements within the lachrymatory agent seem to render blood and fluid information useless by degrading the DNA strands. The original formulation of mace consisted of 1% chloroacetophenone gas in a solvent of 2-butanol, propylene glycol, cyclohexene, and dipropylene glycol methyl ether. Some formulations now also include Oleoresin Capsicum which is the active ingredient in pepper spray; but that's neither here nor there.

I think they did this in Boondock Saints actually.

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