A formation fluid sample is exposed to a rigidly-supported semi-permeable membrane such as silicone rubber to permit diffusion of gases and vapors from the formation fluid into a vacuum chamber, while at the same time, blocking the passage of any liquids. The membrane-transmitted gas is analyzed in the vacuum chamber by a resonator that reacts with it. The resulting change in resonant frequency of the resonator indicates the presence of a gas that reacts with it. An ion pump or sorbent is associated with the evacuated chamber to maintain the vacuum. The ion pump or sorbent removes gases and vapors from the low-pressure chamber, which have diffused into it from the reservoir sample that is on the opposite (high-pressure) side of the semi-permeable membrane.