CSI teams beware — an increasingly common household product cleans up blood thoroughly enough to make it undetectable by three of the most common forensic tests, scientists are reporting in an upcoming Naturwissenschaften.
These “presumptive tests” are a quick and dirty means of identifying important stains — such as blood — at a crime scene. “They tell you what to collect,” says Walter Rowe, chair of the forensic sciences department at George Washington University, who was not involved with this study. The tests rely on the blood protein hemoglobin’s love of oxygen.
But the various “oxy” cleaners that have arrived on the scene in recent years appear to drown the hemoglobin in so much oxygen that the protein’s got no love left for the tests.
While the research suggests another avenue for immaculate killers to clean the scene, most people who commit murder are not in a position or frame of mind to think over which detergent they should use to cover their tracks, Rowe points out.“People committing violent crimes often don’t have time to clean up; they leave a lot of stuff behind,” Rowe says. Killing another human is generally a very disturbing and stressful experience, says Rowe. The phrase “blood simple” refers to the addled state that occurs when a killer spills blood, he says. “You shed blood and you become stupid – you are in an emotionally overwrought state, not in control of yourself.”
Human blood has to carry about 600 liters of oxygen from the lungs to the body’s tissues everyday — a task carried out by hemoglobin, a sort of donut-shaped protein made of four globular subunits. Each subunit has its own heme group, which contains a bit of iron.
It’s the iron that’s mad about oxygen. Existing forensic blood tests involve swabbing the stain with hydrogen peroxide — H2O2. If the stain is blood, the hemoglobin tears the oxygen atoms away from the hydrogen peroxide. (The same thing happens when you clean a cut with hydrogen peroxide — the foam that arises is oxygen.) With some of the tests an indicator is added, such as a chemical that changes color when it reacts with the oxygen (thus the telltale “pink” swab from crime scenes).
To investigate the possible doctoring of evidence by the oxygen-rich cleaning detergents, a team of scientists led by Fernando Verdú of the University of Valencia in Spain took samples of their own blood and stained several fabrics including a soft cotton cloth, jeans and a towel. Each stain was made with five drops of blood. The fabrics were washed with a product called Neutrex, which contains “active oxygen,” or sodium percarbonate, that releases hydrogen peroxide when it is dissolved in water. The clothes were then left to soak in soapy hot water for two hours. The team also ran iterations with cold water and with and without oxygen-rich detergent.
The researchers then conducted three different tests on each cloth. One, the Kastle-Meyer test, uses a form of phenolphthalein that turns pink when it reacts with oxygen that is released from administered hydrogen peroxide. The team also tried the luminol test, in which luminol binds to the free oxygen and emits blue light as energy (the same action that happens in party glow sticks). The scientists also tested for human hemoglobin.
The two presumptive tests and the hemoglobin test were negative on stains washed in warm water with the oxygen cleaner. Stains were tested after several days and still were negative. Controls all tested positive. When washed in hot water alone, the fabrics tested positive for blood. Fabrics cleaned in oxygen cleaner and cold water were negative, except for a light luminescence in a quarter of the samples that appeared after several minutes, and a faint hemoglobin positive after several minutes. Lab protocol considers these delayed reactions a negative result.
The sodium percarbonate “oxidizes the daylights out of iron,” says Rowe. “It probably is then not going to react with anything.” And these cleaners also probably “tear up the protein molecule,” he says, perhaps destroying DNA as well.
Verdú says the team was not surprised by the results. “It is very difficult for something to surprise us about criminal investigations,” he says. “It is a forensic saying: ‘Seldom say never, seldom say always.’ ”
