The search for the presence of blood at a crime scene is normally done by close visual examination.  The possibility exists, however, that blood may be present in amounts too little to see with the unaided eye, or that the blood at the scene had been “cleaned up” prior to arrival of the crime scene team.  Scientists can take advantage of the luminol reaction to locate potential blood evidence that would be undetectable through visual examination.

The light, or luminescence, emitted in the luminol reaction is thought to result when an oxidizing agent, such as blood, catalyzes the oxidation of luminol by hydrogen peroxide in a basic solution.  The reaction is not specific to blood, however, as other oxidizing agents such as sodium hypoclorite (bleach), certain metals, and plant peroxidases may also cause luminescence with luminol.  Because the reaction is not specific to blood, a follow up presumptive test, such as phenolphthalein, is typically run on potential samples prior to collection.

Besides being useful in locating minute amounts of blood, the luminescent pattern observed on surfaces could indicate such things as; the route of exit from the crime scene, drag marks in blood, or an attempt to clean up blood.

Above left: a footwear impression in blood that has been enhanced with luminol reagent.  Above right: by applying luminol to a linoleum floor, an attempt to clean up blood is apparent.