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Luminol (C8H7N3O2) is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow Crystal that is soluble in most polar organic but insoluble in water.
Forensic investigators use luminol to detect trace amounts of blood at , as it reacts with the iron in hemoglobin. Biologists use it in cellular assays to detect copper, iron, and as well as specific proteins via .
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 can be documented photographically. The glow is stronger in areas receiving more spray; the intensity of the glow does not indicate the amount of blood or other activator present.
The compound was first synthesized in Germany in 1902See:
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. in a variety of biological systems may also catalyse the decomposition of hydrogen peroxide.
The exact mechanism of luminol chemiluminescence is a complex multi-step reaction, especially in aqueous conditions. A recent theoretical investigation has been able to elucidate the reaction cascade as follows. Luminol is first deprotonated in basic conditions then oxidized to the anionic radical, which in turn has two paths available to give the key intermediate α-hydroxy- peroxide. After cyclization to the endoperoxide, the mono-anion will undergo decomposition without luminescence if the pH is too low (< 8.2) for a second deprotonation. The endoperoxide dianion, however, can give the retro-Diels–Alder product: 1,2-dioxane-3,6-dione dianion which, after chemiexcitation by two single-electron transfers (SET) gives 3-aminophthalate dianion in its first singlet excited state (S1). This highly unstable molecule relaxes to the ground state, emitting light of around 425 nm wavelength (purple-blue) in the process termed chemiluminescence.
but was not named luminol until 1934.
Chemiluminescence
To exhibit its luminescence, the luminol must be activated with an oxidant. Usually, a solution containing hydrogen peroxide (H2O2) 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:
Use in criminal investigation
History
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.H. O. Albrecht (1928) "Über die Chemiluminescenz des Aminophthalsäurehydrazids" (On the chemiluminescence of aminophthalic acid hydrazide) Zeitschrift für Physikalische Chemie 136: 321–330.Stuart H. James and William G. Eckert, Interpretation of Bloodstain Evidence at Crime Scenes, 2nd ed. (Boca Raton, Florida: CRC Press LLC, 1998), page 154 . In 1936, Karl Gleu and Karl Pfannstiel confirmed this enhancement in the presence of haematin, a component of blood.Gleu, Karl; Pfannstiel, Karl (1936) "Über 3-aminophthalsaure-hydrazid" Journal für Praktische Chemie 146: 137–150. In 1937, German forensic scientist Walter Specht made extensive studies of luminol's application to the detection of blood at crime scenes.Specht, Walter (1937) "Die Chemiluminescenz des Hämins, ein Hilfsmittel zur Auffindung und Erkennung forensisch wichtiger Blutspuren" (The chemiluminescence of haemin, an aid to the finding and recognition of forensically significant blood traces), Angewandte Chemie 50 (8): 155–157. In 1939, San Francisco pathologists Frederick Proescher and A. M. Moody made three important observations about luminol:Proescher F. and Moody A.M. (1939) "Detection of blood by means of chemiluminescence," Journal of Laboratory and Clinical Medicine, 24 : 1183–1189.
Theory
Crime scene investigators 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
The usage of luminol in a crime scene investigation is somewhat hampered by the fact that it reacts to iron- and copper-containing compounds, bleaches, horseradish, fecal matter, and cigarette smoke residue. Application of luminol to a piece of evidence may prevent other tests from being performed on it; however, DNA has been successfully extracted from samples exposed to luminol. Technical note about Hemaglow
Related molecules
C8H7N3O2 – MW: 177.16
λabs (in 0.1 N NaOH) λmax 1 : 347 nm & λmax 2 : 300 nm; EC (at λmax 1): 7650 L/mol × cm
λabs / λem (MeOH): 355/413 nm
C8H6N3O2Na – MW: 199.12
C8H6N3O2Na – MW: 217.16
C8H7N3O2 · HCl MW: 213.62
C8H7N3O2 – MW: 117.16 (Xi)
C8H7N3O2 – MW: 195.15
See also
External links
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