Luminol Test for Blood

Introduction

The forensic luminol test has long been valued for its ability to detect trace amounts of blood that are invisible to the naked eye.The reaction is used by crime scene investigators to detect traces of blood at crime scenes and is also used by biologists in cellular assays for the detection of copper, iron, and cyanides.

This reaction is also responsible for the glow of light sticks.

What Happens Behind

During the reaction, the reactants (luminol and hydrogen peroxide) react with the iron from the blood. The iron acts as a catalyst so if the iron found in the hemoglobin and the luminol mixture come in contact; the iron in the hemoglobin accelerates a reaction between the hydrogen peroxide and the luminol. (The iron catalysis 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.

So this iron in the hemoglobin catalysis an oxidation reaction in which the luminol gains oxygen atoms while losing nitrogen and hydrogen. This produces a compound called 3-aminophthalate. The reaction leaves the 3-aminophthalate in an energized state, the electrons in the oxygen atoms are boosted to higher orbitals. The electrons quickly fall back to a lower energy level, emitting the extra energy as a light photon (Blue light is emitted as energy is released when the electrons return to the ground state)so because the reactants have more energy than the products, the molecules (products)get rid of the extra energy in the form of visible light photons. With iron accelerating the process, the light is bright enough to see in a dark room.

Materials

Luminol Materials

• luminol stock solution

2 g luminol + 15 g potassium hydroxide + 250 mL water

• 3% hydrogen peroxide in water (common over-the-counter concentration)
• A Hydroxide (KOH)
• A sterile blood lancet and sterile alcohol pad

Procedure

In the procedure, luminol powder (C8H7O3N3) is mixed with hydrogen peroxide (H2O2) and a hydroxide (KOH) in a spray bottle and is sprayed where blood might be found.

The glow is activated when blood is present (by adding a drop of blood). The blood must be on the alcohol pad. The forensic test is for dried or latent blood, so the reaction between the alcohol and fresh blood is necessary.

Luminol Test for Blood

Information

Luminol is actually a powder that contains nitrogen, hydrogen, oxygen and carbon. Investigators mix the luminol powder with a liquid containing hydrogen peroxide as stated. Crime investigators then spray this solution over a large surface area in almost complete darkness. The glow that results is due to a natural property of luminol called chemiluminescence.

• Chemiluminescence is the emission of light that results from a chemical reaction that does not involve the production of heat or a flame.

Hala Madi,Yassmin Odeh and Tala Sawalha