We are measuring Oxygenates in C4 Hydrocarbons stream in Liquid phase, on a Gas Chromatograph with LSV. Bubble point is calculated as 3.91 bar and we are keeping the sample pressurized with a Back pressure regulator set at 5 barg. The process pressure is at 12 barg

We observe that the Methanol value reads low and Tert-Butanol reads high when pressure kept at 5 barg whereas the Methanol value increases and Tert-Butanol value decreases, with increase in pressure upto process pressure of 12 barg.

Can you please help understand this phenomenon? I do not have deep understanding of chemistry. But what i expect is that if the liquid is kept above its bubble point, Methanol or even other components should not be vaporizing and thus causing such phenomenon.