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How To Clean a Bloody KnifeDoes DNA come off with soap and water?

By Juliet Lapidos
Posted Tuesday, Nov. 20, 2007, at 4:01 PM ET
Amanda Knox. Click image to expand.

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 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 percent solution (one part commercial bleach to nine parts water) to clean workspaces (PDF) so that old samples don't contaminate fresh evidence. Likewise, when examining ancient skeletal remains (PDF), 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.

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Juliet Lapidos is a Slate assistant editor.
Photograph of Amanda Knox by STR/AFP/Getty Images.
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