Forensic Analysis of Blood using
Luminol
Luminol
• Redox reaction with oxygen in
the presence of catalyst
produces a bright blue glow.
• Used in forensic analysis for
detection of blood
– Iron in hemoglobin acts as
catalyst
• Other common catalysts
– Biological: copper and cyanide
– Laboratory: potassium
ferricyanide
Luminol
5-Amino-2,3-dihydro1,4-phthalazinedione
Chemiluminescence
• Chemical reaction
produces an excited
state compound
• Excited state compound
passes through
intermediate
– Decays to ground state
– Emits a photon
http://en.wikipedia.org/wiki/File:Luminol2006.jpg
Chemical Reactions
• Luminol must be activated by hydroxide salt
forming a dianion:
-2 OH-
Luminol
2 H20 +
Dianion
Dianion
• In the presence of iron (catalyst), hydrogen
peroxide decomposes to form oxygen:
Fe
2H2O2
O2 + 2H2O
• The dianion reacts with oxygen producing the
unstable organic peroxide intermediate:
+O2
Dianion
Triplet Dianion (excited state)
• This intermediate decomposes from higher energy
states to the ground state, emitting a photon.
Intersystem
Crossing
Triplet Dianion
(excited state)
hv
Singlet Dianion
(excited state)
Ground State
*intersystem crossing is radiationless transition between different states of spin multiplicity
Chemiluminescence
Energy
Triplet
Dianion
Singlet
Dianion
Ground State
Dianion
As shown by the red dot, Luminol is excited into the triplet dianion state chemically.
Through intersystem crossing, the molecule is able to transtion into the singlet
dianion state of the same energy. Finally, the molecule fluoresces from the singlet
dianion state to the ground state dianion emitting a photon of energy hv.
Advantages & Disadvantages
• Advantages:
– Fast
– Easy
• Disadvantages:
– Fluoresces in the presence of other compounds,
including bleach & horseradish
– Can prevent other testing
– Possible carcinogen
Application
• Luminol can be used in a healthcare setting
for contamination assessment