Crime Scene Chemistry

Tests in Forensic Chemistry for Body Fluids, Fingerprints and Drugs

Aug 10, 2009

When investigators examine evidence at a crime scene or take evidence back to the laboratory many of the tests involve a bit of chemistry.

A wide variety of biological and chemical evidence can be left at a crime scene and it is up to investigators to determine what is relevant. Many of the tests they use involve chemical reactions or the use of instrumentation back at the laboratory. Tests for blood, fingerprints and the criminal use of drugs utilize these tests when connecting them back to a crime.

Luminol Shows Investigators the Way

An important tool in detecting biological evidence at a crime scene is luminol (5–amino–2,3–dihydro–1,4–phthalazine-dione). This chemi-luminescent compound gives off blue greenish light when it reacts with metal ions. Iron is stored in the protein called hemoglobin, a component of red blood cells and reacts with luminol. The metallic element acts as a catalyst when it comes in contact with a luminol mixture causing an oxidation reaction.

Normally, Investigators prepare the luminol solution by mixing luminol powder with hydrogen peroxide and other chemicals. Luminol also reacts with other substances like rust or saliva so more tests are needed to positively determine the presence of blood. Since luminol changes compounds it reacts with, original samples of suspicious substances are collected prior to luminol contamination, for further testing to identify the substance.

Ninhydrin Lends a Hand

Another substance instrumental in crime scene investigation is ninhydrin(2,2-dihydroxyindane-1,3-dione). When oil is secreted through the skin it can collect in the ridges found on the tips of fingers. After someone touches a surface, the oil is left behind in the unique pattern of the ridges as a fingerprint. A ninhydrin reagent will react with the amino acids in the secreted oils and yield a bluish pattern. Fingerprints developed with ninhydrin may take more than a day to develop.

Investigators use ninhydrin derivatives that are better at drawing out latent prints than straight ninhydrin. The derivative 1,2 IND (1,2-Indanedione) has the same properties as ninhydrin, but also provides a luminescent quality to developed prints.

Dusting for Prints

Dusting is still widely used when checking for prints. A powder is brushed lightly over a suspected area and the oil pattern sticks to the oil pattern of the fingerprint. Newer fluorescent powders help investigators develop weaker prints that might otherwise go undetected. These types of prints need an alternative light source to be viewed. The light source devices used can produce ultraviolet or infrared light. Lasers have also been used to view prints.

Investigators Use a Variety of Instrumentation to Test Crime Scene Evidence

Once the evidence is collected, bagged and tagged it is sent back to a laboratory for testing. Laboratories hold an array of instrumentation such as, infrared spectroscopy, mass spectroscopy, gas chromatography and liquid chromatography. In a May 4, 2009 Chemical & Engineering News article entitled "Investigative Science." Rochelle H. Bohaty talks about the FBI laboratory’s time-of-flight mass spectrometer takes seconds to identify illegal substances. The analytical instrument examines solids or liquids and is very sensitive, detecting trace levels of substances from specimens.

"Luminol." World of Forensic Science. Gale. 2005. HighBeam Research. (August 1, 2009).

Hanson, Doug. "Improved Fingerprint Acquisition." Law & Order. 2006. HighBeam Research. (August 10, 2009).

The copyright of the article Crime Scene Chemistry in Forensic Science is owned by Jennifer Graham. Permission to republish Crime Scene Chemistry in print or online must be granted by the author in writing.