This method is for the preparation and cleaning of sample cylinders, and for sampling techniques necessary to obtain an air-free sample of a liquid hydrocarbon. The procedures are applicable to liquefied natural gases, high vapor pressure gasolines, various liquefied petroleum gases, air-unstable gasolines or other liquid hydrocarbon products that require rigorous exclusion of air. Sampling for the analysis of sulfur or other heteroatom compounds in C3-C4 fractions requires a special cylinder or a special cylinder preparation. An alternative to the sampling system and procedure is available as "Closed Sampling System," UOP, PIC. If the sample is to be transported offsite, see NOTE 1.