You may have seen Crime Scene Investigators on TV using a chemical called Luminol to find traces of blood at a crime scence. The investigator prepares a solution of luminol and sprays in throughout the area under investigation. The lights are then turned off and any traces of blood show up as a bright blue glow. Let's look at how Luminol works.

The luminol reacts with iron in blood to produce something called chemiluminescence (the bright blue glow). Chemiluminescence is the emission of light during a chemical reaction. The amount of iron needed to react with the luminol to produce the blue glow, is very small and this allows the detection of even trace amounts of blood.

Luminol has some disadvantages which can give the CSI guys a bit of a headache. The disadvantages are:

• Luminol chemiluminescence can be triggered by a number of substances such as copper metal and bleach. This means that if a crime scene has been cleaned with bleach, the bleach residue will cause the entire crime scene to glow blue. This camoflages any traces of blood.
• Luminol can also detect tiny traces of blood in urine and even feces and reacts the same to animal blood as to human blood.

For a video of luminol in the lab here at NUI, Galway, check out our videos page.

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