Luminol in crime scene investigations
Luminol is a chemical substance widely utilized in crime scene investigations to detect and visualize blood evidence that may not be immediately visible. Bloodshed is common in crimes against persons, and while blood can often be seen at the scene, luminol plays a crucial role in enhancing subtle traces. This compound reacts with hemoglobin in blood, producing a brief blue-white luminescence when sprayed on suspected areas. The sensitivity of luminol allows it to detect blood even in highly diluted forms, making it valuable in various situations, such as on dark or patterned surfaces or after attempts to clean bloodstains.
Investigators typically conduct luminol examinations in low-light conditions, often waiting until nighttime to perform tests effectively. However, the application of luminol can distort blood patterns, so it is important to document visible evidence before use. While effective, luminol's nonspecific nature means that other substances, such as bleach or paint, can also generate luminescence, necessitating careful interpretation by crime scene examiners. Overall, luminol is an essential tool that aids in uncovering critical evidence in criminal investigations, while emphasizing the need for meticulous procedure and documentation.
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Luminol in crime scene investigations
DEFINITION: Chemical substance widely used in presumptive or nonspecific tests for blood during crime scene examinations.
SIGNIFICANCE: Blood is often clearly visible at crime scenes, but in some cases crime scene examiners need to be able to detect and visualize blood evidence that is not readily apparent. The chemical substance known as luminol is used for this purpose.
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 conditions, it can detect blood even if the blood is extremely diluted.
Because of the characteristics of the luminescence, examination of a crime scene with luminol is usually conducted in the dark. At outdoor crime scenes, examiners must wait until nighttime and arrange for nearby streetlights and other sources of light to be turned off before the luminol examination begins. For indoor crime scenes, all windows and doors can be screened off with black plastic to block any incoming light. Before applying the luminol, examiners need to wait until their eyes are accustomed to the darkness, so they will be able to see the luminescence. A solution of luminol is then sprayed over the scene and any luminescence observed. Although the glow can be photographed, this is not always easy, so examiners must make full notes to document what they see.
The application of luminol is usually the last step taken by examiners at a crime scene. The liquid spray distorts any blood patterns, so these need to be observed before luminol is applied. This includes bloody shoe prints if shoe-print comparison work is needed. Because any dilution of blood samples can reduce the chances of successful DNA (deoxyribonucleic acid) analysis, samples should be collected before luminol is used whenever this is possible. Samples of any trace evidence of interest should also be collected before luminol is applied, as the spraying of the solution can disturb such evidence.
Luminol is a nonspecific test—that is, substances other than blood can give positive results. Examples of such substances that may be present at crime scenes include bleach, rust, and some paints. The appearance of the luminescence from these is different from that caused by blood, but nevertheless, crime scene examiners need to exercise care in the interpretation of positive results.
Bibliography
Barni, Filippo, et al. “Forensic Application of the Luminol Reaction as a Presumptive Test for Latent Blood Detection.” Talanta 72.3 (2007): 896–913. Print.
Fenoff, Roy, et al. Crime Scene Investigation, 4th ed, Routledge, 2022.
Fisher, Barry A. J. Techniques of Crime Scene Investigation. 9th ed. CRC Press, 2022.
James, Stuart H., Paul E. Kish, and T. Paulette Sutton. Principles of Bloodstain Pattern Analysis: Theory and Practice. Boca Raton: CRC, 2005. Print.
Pfefferli, Peter. Forensic Evidence Field Guide: A Collection of Best Practices. Academic Press, 2015. eBook Collection (EBSCOhost), search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=942215&site=ehost-live. Accessed 30 Dec. 2016.
Siegel, Jay A., Pekka J. Saukko, and Geoffrey C. Knupfer, eds. Encyclopedia of Forensic Sciences. San Diego: Academic, 2000. Print.