A Raman spectroscopic measurement system for measuring the material composition of a mixed phase fluid having a gas phase dispersed in a liquid phase or vice versa is disclosed, which includes an insert to be inserted into a process. The insert includes a measurement chamber partially defined by a phase separating membrane that enables the gas phase to diffuse into and out of the measurement chamber and facilitates coalescing of the liquid phase which into a collector. A first probe of the measurement system is configured to transmit excitation light into the measurement chamber and to receive a Raman signal emanating from the gas phase therein, and a second probe is configured to transmit excitation light into the drain and to receive a Raman signal emanating from the liquid phase therein. The measurement system further includes a spectrometer to determine the material composition of the fluid from the Raman signals.