Abstract: Methods of separating and classifying the relative amounts of two or more classes of hydrocarbon molecules in a sample are disclosed wherein an input stream comprising a mobile phase and the sample is injected into a chromatographic column, and an effluent stream exiting a chromatographic column is split into a first effluent stream and a second effluent stream. The mass of carbon in each of the classes of hydrocarbons in the first effluent stream is determined, preferably using a flame ionization detector, and the second effluent stream is directed through a variable restrictor, allowing the pressure and mass flow rate of the input stream to be independently controlled and improved results obtained. In a preferred embodiment the present invention also includes identifying the hydrocarbon molecules in the second effluent stream in order to verify the analysis or provide additional data, preferably by using an ultraviolet detector.