Luminol is a versatile chemical that exhibits chemiluminescence, with a
striking 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 is insoluble in water. Luminol is used by forensic
investigators to detect trace amounts of blood left at crime scenes, as it reacts
with iron found in hemoglobin. It is used by biologists in cellular assays
for the detection of copper, iron, and cyanides, as well as of specific
proteins by western blot. Luminol can be sprayed evenly across the
area, and trace amounts of an activating oxidant will cause the luminol to emit a
blue glow that can be seen in a darkened room. The glow lasts for about 30
seconds, but the effect can be documented by a long-exposure
photograph. It is important that the spraying be evenly applied to avoid a
biased impression, such as blood traces appearing to be more concentrated in
areas that received more spray. The intensity of the glow does not indicate
the original amount present but indicates only the distribution of trace
amounts of substances left in the area. Synthesis
Luminol may be synthesized by a reverse-phosphorescence, two-step
process. It begins from 3-nitrophthalic acid. First, hydrazine 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, via a transient
hydroxylamine intermediate, produces luminol.
Luminol was first synthesized in Germany in 1902, but the compound 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 and hydroxide ions in water is
used as the activator. In the presence of a catalyst such as an iron or
periodate compound, the hydrogen peroxide is decomposed to form oxygen
and water: 2 H2O2 → O2 + 2 H2O
H2O2 + KIO4 → KIO3 + O2 + H2O In a laboratory setting, the catalyst
used is often potassium ferricyanide or potassium periodate. 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
ion, 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 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. In 1936, Karl Gleu and Karl Pfannstiel confirmed this
enhancement in the presence of hematin, a component of blood. In 1937, the
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 =
Luminol is used by crime scene investigators to locate traces of blood,
even if it has been cleaned or removed. The investigator sprays a solution of
luminol and the oxidant. The iron present in blood catalyzes the
luminescence. 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 blue glow lasts for about 30 seconds. 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 chemiluminescence can also be
triggered by a number of substances such as copper or copper-containing chemical
compounds, and certain bleaches; and, as a result, if a crime scene is thoroughly
cleaned with a bleach solution, residual cleaner will cause the entire crime
scene to produce the typical blue glow, effectively camouflaging any organic
evidence, such as blood. Horseradish sauce, via the enzyme
horseradish peroxidase, catalyses the oxidation of luminol, emitting light at
428 nm, which may result in a false positive.
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.
excessive smoke in an enclosed space can result in positive results with Luminol.
For example, an excessive cigarette smoker who smokes in his or her vehicle.
Related molecules Luminol:
5-amino-2,3-dihydro-1,4-phthalazinedione ; 3-amino-phthalhydrazide ;
1,4-phthalazinedione, 5-amino-2,3-dihydro ; CAS: [521-31-3]
C8H7N3O2 — MW: 177.16 λabs λmax 1 : 347 nm & λmax 2 : 300 nm;
EC: 7650 L/mol × cm λabs / λem: 355/413 nm
Luminol, hemihydrate: 3-amino-phthalhydrazide Na salt; CAS:
[206658-90-4] C8H6N3O2Na · 1⁄2H2O — MW: 217.16
Luminol, Na salt: 3-amino-phthalhydrazide Na salt; CAS:
[20666-12-0] C8H6N3O2Na — MW: 199.12
Luminol, HCl: 3-amino-phthalhydrazide HydroChloride; CAS: [74165-64-3]
C8H7N3O2 · HCl — MW: 213.62 Isoluminol: 4-aminophthalhydrazide; CAS:
[3682-14-1] C8H7N3O2 – MW: 117.16
Isoluminol, monohydrate: 4-aminophthalhydrazide monohydrate
C8H7N3O2 · H2O — MW: 195.15 Isoluminol ABEI: 4-aminophthalhydrazide
monohydrate; CAS: [66612-29-1] See also
Fluorescein Diphenyl oxalate
References External links
HowStuffWorks "How Luminol Works" Luminol demonstration in Turin