CHEMILUMINESCENCE IN BLOOD STAIN

DETECTION

In the media…

Blood and other body fluids found at the scene of a crime may give important physical evidence in three ways Blood typing analysis can be used to eliminate

whole groups of people as suspects. Shape, position, size or intensity of a bloodstain

may support a particular sequence of events. Occurrence of a blood strain in a certain place,

e.g. on a weapon may substantiate an account of a crime.

It is thus obviously important to be able to identify a particular stain as blood or not, or maybe even to reveal "hidden" bloodstains on dark materials or where attempts have been made to wash the blood away.

What is Luminol? Luminol is an organic compound When oxidized, emits light — a

phenomenon known as chemiluminescence

Chemiluminscence is a property in which a substance is undergoing a chemical reaction and releasing energy in the form of lights Example: firefly and glow sticks Produce chemical that, when exposed to

other substances that catalyze the reaction, produce a glow

Formula: C8H7N3O2

Discovered in the late 19th century

Scientists modified the method of using luminol over the yearsWhen placed in alkaline or basic solution

(hydrogen peroxide or permanganate), luminol emits a faint glow

When luminol is exposed to certain trace metals (copper, manganese, or iron), the metals act as a catalyst to create a stronger, more efficient reaction

○Stronger glow

What produces the glow? Oxidation Reaction When Exposed to alkaline conditions, Nitrogen

atoms in Luminol are replaced with oxygen atoms. This causes release of nitrogen gas Energy generated from this reaction is released in

the form of light Research showed that hydrogen peroxide served

as the best base for luminal and Cobalt was the best metal catalyst for a strong reaction…however, there is another metal that was most significant to forensic scientists when using luminol…

The Luminol Reaction(* indicates an electronically excited state)

Suspected bloodstain on cloth? Reveal by spraying with luminol in the dark

•In this reaction, a small amount of luminol (3-aminophthalhydrazide or 5-amino-2,3-dihydro- 1,4-phthalazinedione) is dissolved in a basic aqueous solution, which also contains a small amount of copper(II) sulfate.  •Then, a mild oxidizing agent, which is 0.3% hydrogen peroxide, is added to this solution (Bleach is also used in some recipes as the oxidizing agent.)  The reaction is believed to occur by the following mechanism:

•The luminol is converted by the basic solution into the resonance-stabilized dianion 1, which is oxidized by the hydrogen peroxide into the dicarboxylate ion 2, accompanied by the loss of molecular nitrogen, N2.  •When the molecule 2 is formed, it is in an excited (higher energy) electronic state, and sheds its "extra" energy by emitting a photon of light (hn), allowing the molecule to go to its ground state form (3).•In aqueous solutions, the luminol oxidation is catalyzed by the presence of a metal ion, such as iron(II) or copper(II). •For this reason, luminol can be used in the detection of blood, since it can be activated by the iron in hemoglobin.

Depending on how well the solution is buffered, the blue glow produced by this reaction can persist for a couple of minutes.

Remember, this is an oxidation reaction: luminol gains oxygen atoms while losing nitrogen and hydrogen New compound is 3-aminophthalate The electrons in the 3-aminophthalate are in an excited state and

blue light is emitted as energy is released when the electrons return to the ground state

Forensic Application of Luminol

1930s: German forensic scientist discovered luminol could be used to test for presence of blood

Blood is naturally alkaline (pH is slightly basic…7.35 to 7.45)

Blood contains Hemoglobin, responsible for carrying oxygen throughout the body

Hemoglobin contains the trace metal Iron (Fe) Iron in hemoglobin catches oxygen atoms

so RBC can carry them throughout the body Iron in blood acts as a catalyst, oxidizing

luminol and creating a faint glow

Luminol at the Crime Scene Investigators spray a luminol/hydrogen

peroxide solution at a scene They examine the scene for a reaction of

components of blood with the luminol Even if an attempt is made to clean blood

from the scene, traces will still be visible and react with the luminol

Hemoglobin is very stableLuminol can be used to detect blood at a

scene that is several years old

Drawbacks of Luminol Forensics in the media Luminol may react with other chemicals and produce

a glow, not just blood Reacts with cleaning reagents, such as bleach What can this tell us about a crime scene

Luminol can also destroy genetic evidence in blood Makes DNA testing less efficient or impossible Should not be used where there is little blood evidence

present○ You don’t want to further dilute the blood with luminol or

compromise the little genetic evidence available

Luminol often is used as a last resort to support trace evidence from the scene