If you've seen a crime scene detective show like CSI, you are probably already familiar with luminol — that substance that glows greenish-blue on a surface, magically revealing where blood once use to be.
But how luminol works is not magic at all — its biochemistry!
Luminol is used by crime scene investigators to locate blood — even if it has been removed or cleaned up years ago! This is because luminol is able to react with the iron in haemoglobin - an oxygen-carrying and iron-containing protein in red blood cells.
Did you know? Luminol is so sensitive that it can detect blood at 1 part per million.
Luminol will fluoresce if there is one drop of blood within a container of 999,999 drops of water! Luminol is actually a powder that contains nitrogen, hydrogen, oxygen and carbon. Investigators mix the luminol powder with a liquid containing hydrogen peroxide. Crime investigators then spray this solution over a large surface area in almost complete darkness. The glow that results is due to a natural property of luminol called chemiluminescence.
Chemiluminescence is the emission of light that results from a chemical reaction that does not involve the production of heat or a flame. During the reaction, the reactants (in this case luminol and hydrogen peroxide) react with the iron from the blood. The iron acts as a catalyst which means it helps to speed the reaction between the luminol and the hydrogen peroxide.
When luminol and hydrogen peroxide react, the luminol becomes oxidized (i.e. it gains oxygen atoms and loses electrons), creating an excited, high-energy state intermediate product named 3-aminophthalate. As the energy in this intermediate product decays, it returns to a low-energy level by releasing electrons as visible photons of light. Because of the iron, the fluorescence is bright enough to be seen in the dark and can then be photographed by the investigator.
Did you know? Chemiluminescence can take place in living organisms, known as bioluminescence, and is responsible for the glow observed in fireflies, New Zealand glow worms and jelly fish to name a few..
But the glow of luminol doesn't 100% guarantee that what it is detecting is blood. Luminol is known to fluoresce when it reacts with copper, certain bleaches, paints, plant matter and even horseradish. Luckily though, how luminol reacts with each is different, allowing a specialist to tell the difference, which can then prompt additional tests to positively determine if it is human blood.
So criminals, take note! Although you think you may have all your tracks covered, there is a science out there to uncover them!