The gas chromatograph is essentially a very hot oven holding a hollow coiled column. It separates volatile substances by passing the drugs or chemicals through a long absorbent column. It may be necessary to vaporize a tiny sample of the material to provide volatile gases for analysis. The components of the vapor are then passed through the column and separated into a number of components, each of which can be captured and analyzed. Different substances are retained in the column for different amounts of time. In most cases, the accelerant will be mixed with portions of burned paint or building materials, making simple identification impossible, but the ability of the chromtatograph to separate tiny portions helps in identification.
The chromatograph is often connected to a mass spectrometer. Mass spectrometry breaks samples apart and separates the ionized fragments by mass and charge. Most forensic laboratories have access to a combined gas chromatograph/mass spectrometer. High pressure liquid chromatography separates many types of drugs and may also be combined with Mass spectrometry.
Forensic chemists may use several types of spectrophotometry. A typical spectrophotometer consists of a light source that provides light of a known wavelength; a holder to position solid, liquid, or gaseous samples; and a system of lenses and photocells that compare light shining on the sample with light passing through. A decrease in the intensity of light passing through the substance indicates the presence of materials that absorb light at that wavelength; the absorbance is quantitive and a measure of the concentration of material, and the wavelengths of maximum absorbance show the type of material. Infrared spectrophotometry is especially useful for the identification of organic compounds, as bonds between certain atoms readily absorb infrared radiation.
Ultraviolet (UV) spectrophotometry helps distinguish between samples of proteins and nucleic acids such as deoxyribonucleic acid (DNA). Atomic absorption spectrophotometry provides ways of determining absorption and emission spectra, useful tools in the analysis of metals such as bullet fragments.