Abstract: A method and apparatus for determining the types and amounts of impurities contained in a fluid medium wherein light is projected into a fluid medium and light which is not absorbed by impurities contained in the fluid medium is transmitted to a spectrometer or photomultiplier. The relationship between the light injected into the fluid medium and the light transmitted out of the fluid medium is analyzed to determine the amounts and types of impurities contained in the fluid medium based on the percentage of light of certain wavelengths transmitted out of the fluid. The apparatus comprises a probe which preferably is slip-stream fit to a pipeline through which the fluid medium flows. The probe is coupled to a light source, such as a tunable laser, by a first fiber and to a spectrometer or photomultiplier by a second fiber. Light is transmitted along the first fiber to the probe from the light source and from the probe to a spectrometer or photomultiplier along the second fiber.