Luminol (Forensic Science)
Crimes against persons often result in the shedding of blood, and blood is usually visible at the scenes of such crimes. Bloodstain pattern analysts can examine visible bloodstain evidence to determine, for example, whether blood on a wall or a floor could have gotten there through impact spatter, arterial spurting, or passive dripping. In some circumstances, however, bloodstains require enhancement because only traces of blood can be seen by the naked eye; in other cases, investigators may believe that blood is likely to be present at a scene even though no blood is immediately visible. Enhancement of bloodstains also might be necessary when blood is on a surface that makes it difficult to see (such as a dark-colored or highly patterned fabric), when attempts have been made to clean bloodstains or they have become diluted in other ways (such as by rain or by traffic through the area of interest), or when investigators need to follow a trail of shoe prints or drag marks.
Crime scene investigators often use the chemical substance 3-aminophthalhydrazide, known as luminol, to make latent bloodstains visible. Blood contains hemoglobin, which transports oxygen in the blood. When a luminol solution is applied—usually sprayed over the area from a spray bottle—hemoglobin catalyzes the oxidation of the luminol to produce a faint, but distinct, blue-white luminescence, or glow. This luminescence is short-lived. Luminol is very sensitive; in ideal...
(The entire section is 531 words.)
Further Reading (Forensic Science)
Barni, Filippo, et al. “Forensic Application of the Luminol Reaction as a Presumptive Test for Latent Blood Detection.” Talanta 72, no. 3 (2007): 896-913. Thorough article covers all aspects of luminol work in some depth.
James, Stuart H., Paul E. Kish, and T. Paulette Sutton. Principles of Bloodstain Pattern Analysis: Theory and Practice. Boca Raton, Fla.: CRC Press, 2005. Informative work on the analysis of blood patterns includes a chapter on the use of luminol.
Siegel, Jay A., Pekka J. Saukko, and Geoffrey C. Knupfer, eds. Encyclopedia of forensic Sciences. San Diego, Calif.: Academic Press, 2000. Includes an informative entry on luminol that relates the successful use of the substance in a homicide case
(The entire section is 109 words.)
Luminol (World of Forensic Science)
When investigating suspected crime scenes where the visible evidence of crime was removed by the perpetrator, nothing is more useful than luminol, a chemi-luminescent compound, which reacts to red blood cells (hemoglobin) and gives off a blue-greenish light. Luminol (5mino,3ihydro,4hthalazine-dione) was accidentally discovered in 1928 by the German chemist H.O. Albrecht, and was first used at a crime scene in 1937 in Germany. Luminol is highly sensitive to bloodstains or residues, even to old stains, in walls, carpets, upholstery, wooden floors, or painted surfaces.
When a luminol solution is sprayed on surfaces, it reacts with metal ions, such as iron, which are stored and transported by hemoglobin cells (red blood cells). Very discrete iron concentrations on a surface, such as 1 part per million, are enough to catalyze luminol's chemi-luminescence (react and cause a glow). However, luminol sensitivity is not blood-specific, and the compound also reacts with other substances, such as saliva, rust, potassium permanganate, animal proteins, vegetable enzymes, and other organic fluids and tissues. Therefore, luminol tests are not conclusive for blood and cannot be admitted for evidence in court.
A biomarker (or a chemical marker) produces conclusive identification when it combines high sensitivity to a particular substance with high specificity, e.g., it is significantly more sensitive to that substance than to others. For instance, 100% sensitivity plus 959% specificity to a given compound number one, compared with 400% sensitivity and 300% specificity to substance number two, would indicate strong evidence of the presence of compound number one. Luminol, however, only meets the first criteria, a high sensitivity to blood and to other protein-containing animal fluids, whether human or not. Another aspect of a luminol reaction is its different degrees of sensitivity from one substance to another. Luminol shows higher sensitivity to animal or human blood, organic tissues and fluids than to other compounds containing metal ions, such as paints, metallic surfaces, household products, or vegetable enzymes. Therefore, the light emitted by luminol has different intensities and time duration, depending on the material of contact. In other words, the lesser the sensitivity, the shorter the period of luminol chemi-luminescence. Such variations constitute useful leads to experts investigating the scene.
In spite of the above-described limitation, luminol is very useful in crime scene investigation. The inside of an apparently clean room in which blood and other crime evidence is not visible can be sprayed with luminol over suspected surfaces. If a reaction occurs in a carpet, for instance, closer examination may reveal bloodstains or residues on the floor beneath it. It can also indicate direction of bloodstains, spatters, and reveal concealed bloody shoeprints.
When biological samples have to be collected for DNA or other tests, luminol should only be used after samples are seized. Luminol's chemical reactions with blood and other body proteins destroy some important genetic markers required for DNA fingerprinting.
SEE ALSO Blood; Blood, presumptive test; Blood spatter; Bloodstain evidence; Chemical and biological detection technologies; Crime scene investigation; DNA fingerprint; Fluids; Genetic code; Hemoglobin.