An apparatus and method of inline measurement of low-concentration hydrocarbons overlaps fluorescence, scatter and absorption spectroscopy devices so as to measure scatter and absorption of fluorescing oil and the excited fluorescence itself. The apparatus includes a fitting, an input port, an output port, and a sapphire tube having a hollow interior in fluid connection with the input port and the output port. Flow medium passes through the input port, the sapphire tube, and the output port. The apparatus also includes a light emitter, a first detector, and a second detector. The light emitter can include a lens, an absorption and scatter wavelength emitter, and a fluorescence wavelength emitter. An incident absorption and scatter beam and an incident fluorescence beam from the light emitter and parallel so as to determine free hydrocarbon, dissolved hydrocarbons, and solids in a sample within the sapphire tube.