Luminol

OVERVIEW

Luminol (LOO-min-ol) is a substance that glows when it come in contact with blood. It was discovered in the late nineteenth century, but chemists found little use for the compound for half a century. Then, in 1928, the German chemist H. O. Albrecht found that the addition of hydrogen peroxide to luminol produces a bluish-green glow, an example of the phenomenon known as chemiluminescence. Chemiluminescence is the process by which light is emitted as the result of a chemical reaction. Albrecht found that the reaction between hydrogen peroxide and luminol required a catalyst, a small amount of a metal such as copper or iron.

KEY FACTS

OTHER NAMES:

5-amino-2,3-dihydro-1,4-phthalazinedione; 3-aminophthalhydrazide

FORMULA:

C₈H₇N₃O₂

ELEMENTS:

Carbon, hydrogen, nitrogen, oxygen

COMPOUND TYPE:

Heterocyclic ring (organic)

STATE:

Solid

MOLECULAR WEIGHT:

177.16 g/mol

MELTING POINT:

319°C to 320°C (606°F to 608°F)

BOILING POINT:

Not applicable; decomposes

SOLUBILITY:

Slightly soluble in water; soluble in alcohol

The most important application for luminol was discovered in 1937 by the German forensic scientist Walter Specht (1907–1977), at the University Institute for Legal Medicine and Scientific Criminalistics in Jena, Germany. Specht found that blood itself could act as the catalyst needed to produce chemiluminescence with luminol. He simply sprayed a mixture of luminol and hydrogen peroxide on a drop of blood, and the blood emitted a bluish-green glow. Specht later determined the explanation for this reaction. Blood contains a protein called hemoglobin that carries oxygen from the lungs to cells. Hemoglobin is a complex molecule with a single iron atom at its center. The small amount of iron in hemoglobin is sufficient to initiate the chemiluminescent reaction between luminol and hydrogen peroxide.

HOW IT IS MADE

Luminol is prepared commercially by treating 3-nitrophthalic acid (C₈H₅NO₆) with hydrazine (NH₂NH₂), resulting in the formation of nitrophthalhydrazide. Nitrophthalhydrazide is then treated with sodium bisulfite (NaHSO₃) to obtain luminol.

COMMON USES AND POTENTIAL HAZARDS

The most common application of luminol is to find traces of blood at crime scenes. When a violent crime is committed, a certain amount of blood is often spilled on the floor, walls, furniture, and other objects at the crime scene. The perpetrator of the crime may attempt to clean up after the crime, but it is virtually impossible to remove all traces of blood. Forensic scientists who investigate a crime scene often assume that blood is present, even if it is not obvious. They check for the blood by spraying a mixture of luminol and hydrogen peroxide around the crime scene. If blood is present, it glows with a bluish-green color. The distribution of the blood can provide information as to where the crime was committed, whether the injured or murdered person was moved, and, if so, in what direction. Investigators typically take photographs of the illuminated crime scene for study at a later date.

Interesting Facts

• Luminol can be used to detect bloodstains that are many years old.
• One disadvantage of using luminol in testing for blood is that it destroys the sample being investigated, making further tests on the same sample impossible.

False positive results are possible with a luminol test. A false positive test is a test in which the results seem to indicate the presence of blood even if it is not actually there. Some metals, plants, paints, cleaning materials, and other substances may act as catalysts for the reaction between luminol and hydrogen peroxide and give a false positive test. For this reason, positive tests obtained by the luminol reagent are always subjected to further tests to confirm the results.

Luminol does have applications beyond criminal investigations. It is the active ingredient in glow sticks, the plastic sticks that glow green when broken. Chemists use the compound in chromatography, a process by which the components of a mixture are separated from each other, as well as in studies of DNA patterns and other biochemical tests.

Words to Know

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

FOR FURTHER INFORMATION

Genge, Ngaire E. The Forensic Casebook: The Science of Crime Scene Investigation. New York: Ballantine, 2002.

"Material Safety Data Sheet for Luminol = 3-Aminophthalhydrazide, 98%." Department of Chemistry, Iowa State University. http://avogadro.chem.iastate.edu/MSDS/luminol.htm (accessed on October 14, 2005).

"Nitric Oxide/NOS Detection (incl. Kits/Sets)." Axxora. http://www.axxora.com/nitric_oxide-nos_detection_(incl._kits-sets)-ALX-610-002/opfa.1.1.ALX-610-002.169.4.1.html (accessed on October 14, 2005).

"Technical Note: Hemaglow™." Lightning Powder Company. http://www.redwop.com/technotes.asp?ID=118 (accessed on October 14, 2005).