A system for separating and analyzing a discrete sample of multiphase fluid includes a separation vessel having a first inner chamber containing a discrete sample of multiphase fluid, and an analytical cell in fluid communication with the separation vessel. The analytical cell has a second inner chamber containing a diluted aqueous liquid phase sample for analysis. The system further includes probes disposed in the second inner chamber, each probe having a sensing area at a distal end, and being oriented in the second inner chamber such that the sensing area is immersed in the diluted aqueous liquid phase sample contained in the second inner chamber. The plurality of probes include a first probe whose sensing area surface is coated with a first ion-exchange membrane; and a second probe whose sensing area surface is coated with a second ion-exchange membrane, the second ion-exchange membrane being different from the first ion-exchange membrane.