Luminol is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow crystalline solid that is soluble in most polar organic solvents, but insoluble in water. Forensic investigators use luminol to detect trace amounts of blood at crime scenes, as it reacts with the iron in hemoglobin. Biologists use it in cellular assays to detect copper, iron, cyanides, as well as specific proteins via western blotting. When luminol is sprayed evenly across an area, trace amounts of an activating oxidant make the luminol emit a blue glow that can be seen in a darkened room. The glow only lasts about 30 seconds, but investigators can document the effect with a long-exposure photograph. Crime scene investigators must apply it evenly to avoid misleading results, as blood traces appear more concentrated in areas that receive more spray. The intensity of the glow does not indicate the amount of blood or other activator present, but only shows the distribution of trace amounts in the area.
To exhibit its luminescence, the luminol must be activated with an oxidant. Usually, a solution containing hydrogen peroxide and hydroxide ions in water is the activator. In the presence of a catalyst such as an iron or periodate compound, the hydrogen peroxide decomposes to form oxygen and water: Laboratory settings often use potassium ferricyanide or potassium periodate for the catalyst. In the forensic detection of blood, the catalyst is the iron present in haemoglobin. Enzymes in a variety of biological systems may also catalyse the decomposition of hydrogen peroxide. Luminol reacts with the hydroxide ion, forming a dianion. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction, an organic peroxide, is very unstable and immediately decomposes with the loss of nitrogen to produce 5-aminophthalic acid with electrons in an excited state. As the excited state relaxes to the ground state, the excess energy is liberated as a photon visible as blue light.
In 1928, German chemist H. O. Albrecht found that blood, among other substances, enhanced the luminescence of luminol in an alkaline solution of hydrogen peroxide. In 1936, Karl Gleu and Karl Pfannstiel confirmed this enhancement in the presence of haematin, a component of blood. In 1937, German forensic scientist Walter Specht made extensive studies of luminol's application to the detection of blood at crime scenes. In 1939, San Francisco pathologists Frederick Proescher and A. M. Moody made three important observations about luminol:
although the test is presumptive, large areas of suspected material can be examined rapidly;
dried and decomposed blood gave a stronger and more lasting reaction than fresh blood; and
if the luminescence disappears, it may be reproduced by the application of a fresh luminol-hydrogen peroxide solution; dried bloodstains may thus be made luminescent repeatedly.
Theory
s use luminol to find traces of blood, even if someone has cleaned or removed it. The investigator sprays a solution of luminol and the oxidant. The iron in blood catalyses the luminescence. The amount of catalyst necessary to cause the reaction is very small relative to the amount of luminol, allowing detection of even trace amounts of blood. The blue glow lasts for about 30 seconds per application. Detecting the glow requires a fairly dark room. Any glow detected may be documented by a long-exposure photograph.
Drawbacks
Luminol has drawbacks that can limit its use in a crime scene investigation:
Luminol chemiluminescence can also be triggered by a number of substances such as copper or copper-containing chemical compounds, and certain bleaches. As a result, if someone cleans a crime scene thoroughly with a bleach solution, residual cleaner makes the entire crime scene produce the typical blue glow, which effectively camouflages organic evidence such as blood.
Luminol can detect the small amount of blood present in urine, and can be distorted if animals' urine is present in the room being tested.
Luminol reacts with faecal matter, causing the same glow as if it were blood.
Luminol's presence may prevent other tests from being performed on a piece of evidence. However, it has been shown that DNA can be successfully extracted from samples treated with luminol reagent.
Excessive smoke in an enclosed space—e.g., a car that someone frequently smokes in—can cause positive results with Luminol.
