Luminol

Luminol is a versatile chemical that exhibits chemiluminescence, with a striking blue glow, when mixed with an appropriate oxidizing agent. It is a white to slightly yellow crystalline solid that is soluble in water and most polar organic solvents.

Luminol is used by forensic investigators to detect trace amounts of blood left at crime scenes. It is also used by biologists in cellular assays for the detection of copper, iron, and cyanides.

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Chemistry

Synthesis

Luminol may be synthesized beginning from 3-nitrophthalic acid.^[2][3] First, hydrazine (N₂H₄) is heated with the 3-nitrophthalic acid in a high-boiling solvent such as triethylene glycol. A condensation reaction occurs, with loss of water, forming 5-nitrophthalhydrazide. Reduction of the nitro group to an amino group with sodium dithionite (Na₂S₂O₄) produces luminol.

Chemiluminescence

  To exhibit its luminescence, the luminol must first be activated with an oxidant. Usually, a solution of hydrogen peroxide (H₂O₂) and a hydroxide salt in water is used as the activator. In the presence of a catalyst such as an iron compound, the hydrogen peroxide is decomposed to form oxygen and water:

2 H₂O₂ → O₂ + 2 H₂O

In a laboratory setting, the catalyst used is often potassium ferricyanide. In the forensic detection of blood, the catalyst is the iron present in hemoglobin. Enzymes in a variety of biological systems may also catalyze the decomposition of hydrogen peroxide.

When luminol reacts with the hydroxide salt, a dianion is formed. 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.

Use by crime scene investigators

Theory

Luminol is used by crime scene investigators to locate traces of blood, even if it has been cleaned or removed. The investigator prepares a solution of luminol and the activator and sprays it throughout the area under investigation. The iron present in any blood in the area catalyzes the chemical reaction that leads to the luminesence revealing the location of the blood. The amount of catalyst necessary for the reaction to occur is very small relative to the amount of luminol, allowing the detection of even trace amounts of blood. The glow lasts for about 30 seconds and is blue. Detecting the glow requires a fairly dark room. Any glow detected may be documented by a long exposure photograph.

Drawbacks

Luminol has some drawbacks that may limit its use in a crime scene investigation:

• Luminol also fluoresces in the presence of copper or an alloy of copper, horseradish, and certain bleaches; and as result if a crime scene is thoroughly cleaned with a bleach solution, residual bleach will cause the entire crime scene to fluoresce, effectively camouflaging any organic evidence, such as blood.
• Luminol will also detect the small amounts of blood present in urine and it can be distorted if animal blood is present in the room that is being tested.
• Luminol reacts with fecal 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.^[4]

References

1. ^ Merck Index, 11th Edition, 5470.
2. ^ Huntress et al., J. Am. Chem. Soc., 1934, 241.
3. ^ Synthesis of luminol
4. ^ Technical note about Hemaglow