Luminol

When investigating suspected crime scenes where the visible evidence of crime was removed by the perpetrator, nothing is more useful than luminol, a chemi-luminescent compound, which reacts to red blood cells (hemoglobin) and gives off a blue-greenish light. Luminol (5–amino–2,3–dihydro–1,4–phthalazine-dione) was accidentally discovered in 1928 by the German chemist H.O. Albrecht, and was first used at a crime scene in 1937 in Germany. Luminol is highly sensitive to bloodstains or residues, even to old stains, in walls, carpets, upholstery, wooden floors, or painted surfaces.

When a luminol solution is sprayed on surfaces, it reacts with metal ions, such as iron, which are stored and transported by hemoglobin cells (red blood cells). Very discrete iron concentrations on a surface, such as 1 part per million, are enough to catalyze luminol's chemi-luminescence (react and cause a glow). However, luminol sensitivity is not blood-specific, and the compound also reacts with other substances, such as saliva, rust, potassium permanganate, animal proteins, vegetable enzymes, and other organic fluids and tissues. Therefore, luminol tests are not conclusive for blood and cannot be admitted for evidence in court.

A biomarker (or a chemical marker) produces conclusive identification when it combines high sensitivity to a particular substance with high specificity, e.g., it is significantly more sensitive to that substance than to others. For instance, 100% sensitivity plus 95–99% specificity to a given compound number one, compared with 40–60% sensitivity and 30–40% specificity to substance number two, would indicate strong evidence of the presence of compound number one. Luminol, however, only meets the first criteria, a high sensitivity to blood and to other protein-containing animal fluids, whether human or not. Another aspect of a luminol reaction is its different degrees of sensitivity from one substance to another. Luminol shows higher sensitivity to animal or human blood, organic tissues and fluids than to other compounds containing metal ions, such as paints, metallic surfaces, household products, or vegetable enzymes. Therefore, the light emitted by luminol has different intensities and time duration, depending on the material of contact. In other words, the lesser the sensitivity, the shorter the period of luminol chemi-luminescence. Such variations constitute useful leads to experts investigating the scene.

In spite of the above-described limitation, luminol is very useful in crime scene investigation. The inside of an apparently clean room in which blood and other crime evidence is not visible can be sprayed with luminol over suspected surfaces. If a reaction occurs in a carpet, for instance, closer examination may reveal bloodstains or residues on the floor beneath it. It can also indicate direction of bloodstains, spatters, and reveal concealed bloody shoeprints.

When biological samples have to be collected for DNA or other tests, luminol should only be used after samples are seized. Luminol's chemical reactions with blood and other body proteins destroy some important genetic markers required for DNA fingerprinting.