Luminol

For the barbiturate drug Luminal, see phenobarbital.
Luminol[1]
Ball-and-stick model of luminol
Names
Preferred IUPAC name
5-Amino-2,3-dihydrophthalazine-1,4-dione
Other names
5-Amino-2,3-dihydro-1,4-phthalazinedione
o-Aminophthaloyl hydrazide
o-Aminophthalyl hydrazide
3-Aminophthalhydrazide
3-Aminophthalic hydrazide
Identifiers
521-31-3 YesY
3D model (Jmol) Interactive image
ChEMBL ChEMBL442329 YesY
ChemSpider 10192 YesY
ECHA InfoCard 100.007.556
EC Number 208-309-4
PubChem 10638
Properties
C8H7N3O2
Molar mass 177.16 g/mol
Melting point 319 °C (606 °F; 592 K)
Hazards
Safety data sheet MSDS for luminol
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
1
2
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

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 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 haemoglobin. Biologists use it in cellular assays to detect copper, iron, and cyanides, as well as specific proteins by western blot.

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.

Synthesis

Luminol may be synthesized by reverse-phosphorescence, two-step process. It begins 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. An acyl substitution condensation reaction occurs, with loss of water, forming 3-nitrophthalhydrazide. Reduction of the nitro group to an amino group with sodium dithionite (Na2S2O4), via a transient hydroxylamine intermediate, produces luminol.

The compound was first synthesized in Germany in 1902,[4] but was not named "luminol" until 1934.[2][5]

Chemiluminescence

Chemiluminescence of luminol

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:

2 H2O2 O2 + 2 H2O
H2O2 + KIO4 → KIO3 + O2 + H2O

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.[6] 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 unstable organic peroxide—is made by the loss of a nitrogen molecule, the change of electrons from excited state to ground state, and the emission of energy as a photon. This emission produces the blue glow.

Use by crime scene investigators

History

In 1928, the German chemist H. O. Albrecht found that blood, among other substances, enhanced the luminescence of luminol in an alkaline solution of hydrogen peroxide.[7][8] In 1936, Karl Gleu and Karl Pfannstiel confirmed this enhancement in the presence of haematin, a component of blood.[9] In 1937, the German forensic scientist Walter Specht made extensive studies of luminol's application to the detection of blood at crime scenes.[10] In 1939, San Francisco pathologists Frederick Proescher and A.M. Moody made three important observations about luminol: (1) although the test is presumptive, large areas of suspected material can be examined rapidly; (2) dried and decomposed blood gave a stronger and more lasting reaction than fresh blood; and (3) 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.[11][12]

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

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

Commercial Formulations

Since its introduction in the early 2000s, a luminol-based proprietary composition[15] trademarked Bluestar is being increasingly used instead of generic formulations.[16] Bluestar luminescence is brighter and lasts longer than carbonate/perborate luminol compositions, so it is possible to use it when complete darkness cannot be achieved. Variations in color, intensity and duration of luminescence with different trace catalysts provide for fewer false positives.[17]

See also

References

  1. Merck Index, 11th Edition, 5470.
  2. 1 2 Ernest Huntress; Lester Stanley; Almon Parker (1934). "The preparation of 3-aminophthalhydrazide for use in the demonstration of chemiluminescence". Journal of the American Chemical Society. 56 (1): 241–242. doi:10.1021/ja01316a077.
  3. Synthesis of luminol
  4. See:
    • Aloys Josef Schmitz, "Ueber das Hydrazid der Trimesinsäure und der Hemimellithsäure" (On the hydrazide of trimesic acid [1,3,5-benzenetricarboxylic acid] and of hemimellitic acid [1,2,3-benzenetricarboxylic acid]), Inaugural Dissertation, Heidelberg University, 1902; see pages 17 and 39-43. Schmitz calls luminol "1-amino-2,3-phtalsäurehydrazid".
    • Note: Gill states that luminol was prepared as early as 1853. See: Steven K. Gill (1983) "New developments in chemiluminescence research," Aldrichimica Acta 16 (3) : 59-61; see footnote 2 on page 60. Available at: Aldrichimica Acta. However, the sources Gill cites don't mention the preparation of luminol before 1902. Gill probably confused luminol with lophine (2,4,5-triphenyl-1H -imidazole), which the sources he cites do mention. Lophine is also chemiluminescent, and was first prepared in 1844 by Auguste Laurent. (See: Auguste Laurent (1844) "Sur un nouvel alcali organique, la lophine" (On a new organic alkali, lophine), Revue scientifique et industrielle, 18: 272-278.) The chemiluminescence of lophine was first observed by: Radziszewski, Bronisław L. (1877) "Untersuchungen über Hydrobenzamid, Amarin und Lophin" (Investigations of hydrobenzamide, amarine, and lophine), Berichte der Deutschen chemischen Gesellschaft zu Berlin, 10 : 70-75. In 1853, Ludwig Teichmann developed a test for blood, but it did not rely on chemiluminescence. See: L Teichmann (1853) "Ueber die Krystallisation der organischen Bestandtheile des Bluts" (On the crystallization of the organic components of blood), Zeitschrift für rationelle Medicin, new series, 3 : 375-388.
  5. See also: Ernest H. Huntress, Lester N. Stanley, and Almon S. Parker (March 1934) "The oxidation of 3-aminophthalhydrazide ("luminol") as a lecture demonstration of chemiluminescence," Journal of Chemical Education, 11 (3) : 142-145.
  6. Ples, Marek. "Chemiluminescence activated by blood". www.weirdscience.eu (in Polish). Retrieved 2014-12-23.
  7. H. O. Albrecht (1928) "Über die Chemiluminescenz des Aminophthalsäurehydrazids" (On the chemiluminescence of aminophthalic acid hydrazide) Zeitschrift für Physikalische Chemie 136: 321-330.
  8. 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.
  9. Gleu, Karl ; Pfannstiel, Karl (1936) "Über 3-aminophthalsaure-hydrazid" Journal für Praktische Chemie 146: 137-150.
  10. 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.
  11. Proescher F. and Moody A.M. (1939) "Detection of blood by means of chemiluminescence," Journal of Laboratory and Clinical Medicine, 24 : 1183–1189.
  12. 1 2 James, Stuart; Kish, Paul E.; Sutton, Paulette Sutton (2005). Principles of Bloodstsain Pattern Analysis. Boca Raton, London, New York, Singapore: Taylor and Francis Group. p. 376. ISBN 0-8493-2014-3.
  13. Ples, Marek. "Chemiluminescence of luminol activated by copper compound". www.weirdscience.eu (in Polish). Retrieved 2014-12-23.
  14. Technical note about Hemaglow Archived January 2, 2015, at the Wayback Machine.
  15. "From luminol to BLUESTAR® FORENSIC : a brief history".
  16. “Use of Bluestar Forensic in Lieu of Luminol at Crime Scenes”, Journal of Forensic Identification, 2006, 56(5), 706-720. Abstract online
  17. "With a little practice, BLUESTAR® FORENSIC makes it impossible to get confused between blood and false positives since the luminescence differs in color, intensity and duration"
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