Luminol: What is it? How Does it Work?

Any fan of forensic TV shows know, that when an investigation occurs at the scene of a crime - chances are a substance is sprayed over the ground and moments later blood splatter is revealed. While there is a lot of exaggerated technology in TV shows or movies, this special chemical is entirely real.

Luminol - a powdery compound made up of nitrogen, hydrogen, oxygen, and carbon. When mixed with an appropriate oxidizing agent, luminol exhibits a striking blue glow when it detects trace amounts of blood as it reacts with iron found in hemoglobin. Unlike TV shows however, the glow is much shorter lived - lasting only around thirty seconds.

Photo via Flickr / Jack Spades

To perform a luminol test, the criminalists spray a mixture of luminal powder plus a liquid containing hydrogen peroxide, a hydroxide, and other chemicals, wherever they think blood may be found. If the hemoglobin (even if wiped up) and luminol mixture come in contact, a cyclic peroxide is produced by the reaction, which quickly decomposes to give a chemical called 3-aminophthalate. As explained by Compound Interest:

“[This] reaction releases energy, which is transferred to electrons in the 3-aminophthalate molecules, promoting them to a higher energy level. As the electrons drop down to a more stable energy level, they release their excess energy as photons of light, resulting in blue chemiluminescence. Luminol can detect the presence of blood at dilutions of up to 1:1,000,000, or 1 part per million.”

It is important to note that luminol typically only shows investigators that there might be blood in an area, given that other substances like bleach and urine can cause luminol to glow. When this occurs, other tests must be run to verify it is really human blood.

In itself, luminol will not solve a murder case, however it is a step in the investigative process that can reveal essential information. On the down side, it can also result in a chemical reaction that destroys other evidence from a crime scene - so it is used cautiously and only when necessary.

For more information on luminol and its use in investigations, please click here. 

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