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How can luminol help investigators detect blood traces at a crime scene?

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French National Police: patrolling — police Officers of the French National Police patrolling a housing project.

In forensic science, investigators use luminol to detect trace amounts of blood that are otherwise invisible to the naked eye, even after attempts have been made to clean a crime scene.1

The process relies on chemiluminescence, a light-emitting chemical reaction. Investigators spray a solution containing luminol and hydrogen peroxide onto a suspected surface. Hemoglobin—specifically the iron-containing heme molecule within blood—acts as a catalyst for the reaction. When the luminol reacts with the heme and hydrogen peroxide, it releases energy in the form of a blue-green glow.12

This method is highly sensitive, capable of detecting blood at a concentration of one part per million, and it does not destroy DNA, which allows for further forensic analysis. However, because other substances like bleach or certain plant enzymes can also trigger the reaction, investigators must perform follow-up tests to confirm the presence of human blood.1