Abstract: A fiber optic sensor system for analyzing fluid systems is described which comprises a sampling probe which transmits tunable ultraviolet light to a sampling location, the probe including an optical fiber terminating in a sampling structure at the distal end of the probe which is inserted into the liquid to be analyzed, a light source for transmitting a selected excitation light beam to the sample location through the fiber, the resulting fluorescence characteristic of the liquid being transmitted back through the fiber structure to an optical detector. The sampling region at the end of the probe is configured to absorb or reflect away all excess excitation light so that only the resulting characteristic fluorescence is transmitted back to the detector, and may have a microporous base to exclude solid particles from the sampling region.