Luminol: The Chemical Illuminating Crime Scenes
Blood is essential evidence in many crime scenes. However, in many cases, this essential evidence has remained right under our noses. It was invisible to us up until 1937. That year, blood revealed itself through one substance in a bright blue glow.
Luminol, or 3-aminopthalhydrazide, is a chemical that reveals blood and is invisible to the naked eye on most surfaces.
But what is luminol really? To accurately describe what it is we have to get scientific for a second. Luminol is a forensic reagent—AKA a chemical used to visualize, detect, or identify blood. Luminol is a cyclic hydrazide compound, which when mixed with a catalyst—for example, the iron found in blood—and then oxidized, produces a blue glow in dark atmospheres. This chemical is used in crime scenes to reveal latent blood—blood that we can not see because it has been cleaned, diluted, or aged.
Luminol can reveal blood that is many years old, and in some cases, even decades old. Forensic scientist Walter Specht found that the older the blood, the brighter the luminol glowed. According to this study by Specht, “Of particular forensic significance is the fact that luminol rarely destroys other evidence (if properly prepared and used) and will not interfere with the future DNA testing of recovered crime scene blood.” It can be used multiple times in the same area, and it would still illuminate the blood just as if it was used for the first time.
However, this reagent is very sensitive. Specht claims, “It can detect the presence of blood in a ratio of one part per million (1:1,000,000). In contrast to its high sensitivity, it has a relatively low rate of specificity: in addition to reacting to the presence of blood, it can also react to chemical oxidants such as chlorine bleach, certain types of chemical cleaners, and detergents.”
Additionally, the issue with luminol picking up bleach is that bleach is a commonly used substance to clean up blood. This makes it hard for forensic analysts to be sure when telling the difference between bleach or blood. Even with its minor issues, the introduction of Luminol into forensic science was revolutionary; however, it took a while to get to that point.
Luminol was first synthesized in 1902 by Aloys J. Schmitz, though the glow it emitted was not yet noticed. In 1928, its chemiluminescence was discovered, and in 1934, it was named luminol; According to this Britannica overview, “Two years later researchers observed that the chemical emitted light when it reacted with blood.” In 1937, a breakthrough was made in the forensic world. Walter Specht thought of using luminol as a forensic investigative tool, and he ran a series of tests on it. He sprayed blood on stone walls, furniture, a garden, and various other areas, and then let it sit for fourteen days. After being exposed to the elements for so long, the luminol proved effective, illuminating that the blood was sprayed for ten-fifteen minutes.
Throughout the mid-20th century, luminol began to gain attention in the scientific community as scientists made it more reliable and sensitive. By the 1970’s, it was standard to utilize luminol in crime laboratories and crime scenes due to its unique capability to keep blood unaltered and reveal cleaned up or aged blood.
Luminol still remains a widely used tool in modern forensic investigations, and it plays an important role in revealing hidden evidence that aids in the process of solving crimes.
About the author: Amelia Gaviglio is a 3rd year student at the University of Oregon, majoring in advertising with a minor in food studies. She enjoys writing about the human condition, fashion, and the intricacies of life. In her free time, she sketches landscapes, goes on peaceful walks in nature, and watches old movies.