Forensic drug testing. High-pressure liquid chromatography equipment being used to identify drugs in a forensic laboratory. The top rack holds bottles of solvents for particular drugs, such as heroin solvent (centre bottle). A sample of the unknown drug is injected into the sample line. Under high pressure the solvent carries the drug molecules into columns packed with beads (not visible), where different chemical compounds separate out according to their physical and chemical properties. At the end of the column the relative concentration of each fraction is determined by ultraviolet light absorption.