I do a lot of process simulation on oil and gas production operations and am wondering if anyone has experience with pressurized oil or condensate sampling and analysis:

What points in the process have you sampled? Separator, wellhead, etc?
I've had companies explain they have a sample chamber that allows them to see where in the sample they are pulling from so they can essentially take a sample anywhere in the process no matter the water, as they can let it separate and pull only the oil from the chamber. Any idea of the effect this would have on the analytical results?

Sampling methods - Piston-type vs water-filled sample vessels?
With the water filled chambers (used to ensure no air gets into the sample and to maintain pressure) I would imagine some hydrocarbon is dissolved in the aqueous phase and would skew the analytical results?

Analytical methods:
For my purposes I need composition detail to at least C10+ and it's always better to have more detailed data, companies will go to C31+. However, I've read that the ASTM standards are only suitable up to C6 analysis. Heavier components will not elute properly in a GC column (adsorbing to the column walls) which skews the composition to the lighter ends.
Does anyone have experience with this type of analysis, particularly with pressurized samples, that can explain how the heavier components are accurately quantified?

Many thanks