Gas Chromatography with Flame Ionization Detection (GC-FID), following ASTM D1945-03(2010), is the standard procedure for determining the purity and composition of butane and other light hydrocarbon gases. The GC system employs a packed or capillary column designed for hydrocarbon separation, typically using a non-polar stationary phase such as Porapak or alumina-based materials. Sample introduction is performed via a gas sampling valve with a fixed loop to ensure consistent injection volume.

Under controlled carrier gas flow and temperature programming, the GC separates individual components by volatility, and the FID provides a linear response to the carbon content of each hydrocarbon species. Calibration with certified natural gas or hydrocarbon reference standards enables precise quantification and calculation of component mole fractions. The resulting chromatogram allows determination of butane purity, detection of minor constituents, and computation of key gas properties in accordance with ASTM D1945. This method delivers high accuracy, repeatability, and reliability for laboratory and refinery gas quality assurance.