Crime Scene Investigations: Luminol? What is it and how does it work?

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This week: Luminol. What is it and how does it work?

Used on all crime scene shows, sprays clear liquid, lights out, florescent light, shows up blood is, invisible to naked eye, that’s Luminol, but what is it?

Invented in 1928, by German chemist H. O. Albrecht who found that many substances emitted a blue glow, chemical luminescence, when Luminol (which is a white-to-pale-yellow crystalline) is dissolved in a solvent such as hydrogen peroxide. Initially it was used in lab-work by biologists to detect copper, iron and cyanides, but in 1936, Karl Gleu and Karl Pfannstiel discovered that Luminol reacts with the iron in haemoglobin (our red blood cells) and realising its potential in 1937, German forensic scientist Walter Specht first used it for the detection of trace elements of blood at crime scenes.

So what is this blue glow? That’s the chemical luminescence, it happens when Luminol breaks down our red blood cells, rearranges the atoms, and as a short chemical reaction takes place, the excess energy is expelled as visible light photons, the blue glow, same chemical seen in fire-flies and glow sticks.

So what are the positives of using Luminol: it shows blood stains even after they’ve been cleaned-up, it only requires trace elements of blood for a reaction, it is very cheap to make, and (although TV shows love showing Forensics with UV/black light) UV light is not needed to see the blue glow, it’s very faint but it is visible to the naked eye, in-fact using a UV light gives worse results, so a pitch black room shows the luminescence clearer. What do they use in tv shows? Luminous paint.

So what are the negatives of using Luminol: it has to be sprayed evenly (otherwise the results can make trace elements appear more concentrated in one area), (unlike in TV shows) the glow only lasts about 30 seconds but you can re-apply to dried blood or semen. To accurately work out where all the blood is, forensics seal up the windows and doors to make the room pitch black, evenly spray the luminol in small patches, and photograph the entire scene using a long-exposure camera, creating one photo of the entire chemical luminescence. One big down side of using Luminol is the blue glow can be triggered by anything containing copper (including some bleaches, liver and oysters), as well as any blood in urine, most animal urine, faecal matter, excessive cigarette smoke in a confined area, and worst of all, is Horseradish sauce.

Want to murder someone, have a dinner party of oysters and Horseradish sauce with several incontinent dogs and heavy-smoking chimps, engaged in a poo-throwing contest. Case closed.

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If you found this interesting? Check out the Mini Mile episodes of the Murder Mile UK True-Crime Podcast, or click on the link below to listen to an episode.

Michael J Buchanan-Dunne is a writer, crime historian, podcaster and tour-guide who runs Murder Mile Walks, a guided tour of Soho’s most notorious murder cases, hailed as “one of the top ten curious, quirky, unusual and different things to do in London”, nominated "one of the best true-crime  podcasts at the British Podcast Awards 2018", and featuring 12 murderers, including 3 serial killers, across 15 locations, totaling 50 deaths, over just a one mile walk