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Gas chromatograph being used by a forensic scientist. He is preparing a sample for analysis. The display screen shows the output from the detector, showing the chemical composition of a sample. This allows small traces of both known and unknown materials to be detected. For example, drugs can be detected in hair samples. Agas chromatograph is used to separate mixtures of volatile gases or liquids by passing them through a material on which the components of the mixture travel at different rates. This allows analysis of very small amounts of chemicals. Many kinds of detectors can be used, most commonly a mass spectrometer. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAURO FERMARIELLO) === ===