The chemiluminescence of luminol, which produces light rather than heat when it reacts with an oxidizing agent, has become a valuable tool in crime scene investigations.
Although many chemical reactions release heat as a by-product, some reactions release light energy instead. This process is called chemiluminescence.
Luminol is one such molecule that can undergo chemiluminescence. The luminol process is often known as the firefly reaction because it emits a bright blue glow for about thirty seconds before the reaction goes to completion. This molecule was first synthesized in Germany in 1902, but it gained the name luminol in the 1920s.
Unexpectedly, the chemiluminescence of luminol has found an important practical application in the area of forensic science. Luminol can react with the iron in hemoglobin to produce its firefly glow, so luminol can be used to detect the presence of blood at a crime scene even if the residue is very small or if there has been an effort to clean up the trace evidence. It also works not only on smooth surfaces, but also on carpets, upholstery, and painted surfaces. Luminol was first used at a crime scene in Germany in 1937.
In the forensic detection of blood, a solution of luminol and an activator are sprayed around the crime scene. (It must be an interior space or a situation that can be darkened.) Then the lights are turned off to allow the blue light to shine in the darkness. Investigators then take a long-exposure photograph of the scene to record the findings.
Because luminol is so sensitive, it reacts not only with human blood but also with saliva, rust, and animal blood. As a result, it is not a conclusive test for human blood so it is not admissible in court, but luminol remains an important tool in the forensic scientist’s toolbox.
More information about luminol can be found at http://www.enotes.com/forensic-science/luminol