Chromatography can be used to identify elements of lower atomic mass, such as hydrogen, carbon, nitrogen and oxygen, which are not identifiable using the atomic absorption method, but are abundant in typical propellants.  This technique relies on the fact that molecules of different masses will travel at different speeds.  In gas chromatography, a vaporised sample is passed through a powder packed column with the aid of an inert gas known as a ‘carrier’ gas.  The time taken for the various components of the sample to reach a detector is measured.  The resulting spectrum can be compared with that of known propellants and the company that provided the explosive can be traced.  Liquid chromatography follows the same principles, but a liquid is used as the carrier instead of a gas.  This method can be used to identify certain additives that are often present in small amounts and the company that produced the explosives can then be traced.  In order to carry out liquid chromatography of a sample, the suspect’s hand is first swabbed with acetone to remove fine particles.  It has been shown that even if the swab is not taken for up to three hours after the gun is fired, residues can still be detected. 

Amount of nitroglycerine found on the skin at various times after firing

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