Luminol is a chemical tool used in forensic investigations to reveal evidence invisible to the naked eye. It allows crime scene investigators to search for and document minute traces of blood that may have been cleaned or are too diluted to see. By reacting with specific components in blood, luminol produces a distinctive glow, acting as a crucial first step in finding biological evidence where hidden blood is suspected.
The Mechanism of Chemiluminescence
Luminol functions through chemiluminescence, the emission of light resulting from a chemical reaction. The powder is prepared in a solution containing an oxidizing agent, typically hydrogen peroxide, and a base to create an alkaline environment. A catalyst is required to speed up the oxidation of the luminol molecule and produce visible light.
In blood, the iron atoms within the heme group of the hemoglobin protein act as this catalyst. When the solution is sprayed, the iron accelerates the reaction between luminol and hydrogen peroxide. This oxidation leaves the luminol molecule in an excited state, which relaxes back to its ground state by releasing energy as a visible blue photon.
Locating Latent Blood Evidence
Forensic scientists utilize luminol to detect latent, or hidden, bloodstains across large surface areas. Because the chemical is sensitive, it can detect blood diluted up to 10,000 times, even if the stain is aged or subjected to cleaning efforts. This allows traces of blood to be found on surfaces that appear spotless or have been thoroughly washed.
Luminol can reveal patterns of blood, such as footwear impressions, drag marks, or transfer smears. The short-lived blue glow, which lasts about 30 seconds, requires the investigative team to work in complete darkness. Specialized long-exposure photography captures the faint luminescence and permanently records the evidence, as spraying the solution can alter or destroy the blood pattern.
Sources of Interference and False Positives
Despite its sensitivity, luminol is not specific to blood and is considered a presumptive test that can produce false positive results. Any substance containing an oxidizing agent or a metallic ion that can catalyze the reaction may cause the solution to glow. A common source of interference is household bleach, a strong oxidizing agent that triggers a reaction similar to blood.
Other substances can also cause a false positive reaction:
- Copper compounds
- Rust
- Various plant peroxidases found in materials like horseradish, turnips, or parsnips
- Some paints or cleaning agents
Because of these potential interferences, a positive luminol test alone is not enough to definitively identify a substance as human blood.
Necessary Confirmation Tests
When a positive luminol reaction is observed and documented, forensic scientists perform follow-up tests to confirm the substance. Since luminol is presumptive, the next step often involves a second, more specific presumptive test for blood, such as the Kastle-Meyer test. This test chemically identifies hemoglobin by producing a distinct pink color change.
After chemical tests indicate the presence of blood, confirmatory analysis is necessary to prove the substance is human blood, not animal blood. Confirmatory tests, such as the Takayama or Teichmann tests, are employed to form characteristic hemoglobin crystals for microscopic identification. Luminol does not destroy the DNA within the bloodstain, allowing the sample to be collected for subsequent DNA analysis to identify the individual source.