In a system and process for selectively purifying various pharmacologically-relevant components of a source plant such as cannabis, an initial step provides a low-temperature, robust essential oil/terpene capture that also dehydrates and decarboxylates the starting product—fresh raw cannabis—by means of a vacuum-assisted microwave distillation process. By doing the terpene capture under vacuum distillation temperature may be kept low. The low distillation temperature maximizes yields of thermally-sensitive components such as terpenes and cannabinoids. The system includes an oil/water separator configured to prevent leakage of ambient air into the system.