Composite sampling of a fluid flowing through a conduit includes collecting, in a vessel coupled to the conduit through which the fluid is flowing, a first discrete sample of fluid from the conduit, the first discrete sample having a first selected volume, and collecting, in the vessel and at a first interval from the first sample, a second discrete sample of the fluid from the conduit, the second discrete sample having a second selected volume, thereby forming a composite sample in the vessel while the vessel is coupled to conduit. The composite sample includes the first discrete sample and the second discrete sample, and may include one or more additional discrete samples. An apparatus for collecting the composite sample includes a gas chromatograph, and is arranged such that the composite sample is provided to the gas chromatograph without removing the composite sample from the apparatus or transporting the composite sample.