Organic Compounds

Organic compounds are based on carbon and are found in living things. They are thus distinguished from inorganic compounds, which are those containing the other elements such as nitrogen, phosphorus, and metals (such as iron and zinc). In fact, this distinction between organic and inorganic is perhaps a little simplistic. Carbon-based compounds need not always come from living things, synthetic fibers like polyester and nylon are carbon-based, but are not found in plants or animals. As far as forensic science is concerned, both organic and inorganic compounds are found in items of evidence. The techniques used for determination of the chemical composition of such evidence will depend upon whether its component compounds are organic or inorganic.

An important feature of compounds based on carbon is that their chemical bonds—with other carbon atoms or with hydrogen, oxygen, or nitrogen atoms—absorb energy in the infrared, visible, and ultraviolet region of the electromagnetic spectrum. This is the basis of spectroscopy, which involves scanning samples containing organic compounds (such as textile fibers or paint fragments) with light, producing a "fingerprint" that is characteristic of the compound. The fingerprint shows the intensity of absorption in the infrared, visible, and ultraviolet region at each wavelength. This sample fingerprint can be compared to those from a reference database, which can reveal the origin of the paint or textile sample. If samples from a suspect—such as fibers found on their clothing—give the same fingerprint as the evidence, then it can be argued they came from the same source.

Another important technique for analyzing organic compounds in evidence is thin layer chromatography. A colored sample, such as a minute sample of ink from a questioned document, is placed on an absorbent paper that is dipped into a solvent mixture. When the mixture is drawn up the paper, it sweeps the sample with it and separates it into its components as a pattern of spots on the paper. As with spectroscopy, this pattern is characteristic of the compound and can be compared with reference or suspect samples for identification.