Inorganic Compounds

There are two main classes of chemical compounds, organic and inorganic. Organic compounds are based on carbon (containing the element carbon as a structural backbone) and are found in living things. Inorganic compounds are those based on other elements. This distinction is a generalization. Some important inorganic compounds, like calcium carbonate, or lime, contain carbon. Carbon-based compounds need not come from living things; synthetic fibers like nylon and polyester are carbon based but not found in plants or animals. From the point of view of forensic science, both organic and inorganic compounds are found in items of evidence. The techniques used for determination of chemical composition of such evidence often depend upon whether the component compounds are organic (derived form living tissue or material) or inorganic.

Glass and paint are probably the most important types of evidence containing inorganic compounds. Glass is composed of silica, soda, lime and impurities that are usually mineral salts. The chemical composition of glass is often determined with the scanning electron microscope (SEM) in conjunction with energy dispersive x-ray analysis (EDX). The microscope ensures magnification of ten thousand times or more of the sample, using electrons rather than light. The item under the microscope will emit radiation that is characteristic of the elements of which it is made. The presence and proportions of these elements can be determined and then compared with reference types of glass. This allows not only the type of glass found at the crime scene to be identified, but also a comparison with glass fragments found on a suspect.

Paint is the other type of evidence that contains inorganic compounds, as pigments or extenders. The red, yellow, and white pigments of paint tend to be inorganic in nature. As with glass, XRD (x-ray diffraction) and EDX together are used to look at the inorganic content of a paint flake or smear. These are crossed against paint references or comparison samples. Atomic absorption spectroscopy is another technique that can be used to identify inorganic components of evidence. This relies on the emission of specific wavelengths of light when an inorganic element, such as a metal, is heated. Atomic absorption is used in a forensic context to determine the chemical nature of soil or mud samples found at the scene of a crime