Abstract: An instrument and a method are provided for determining the concentration of at least one species in a substance. The instrument and method can be used to identify and distinguish among various degrees of contamination of motor oil, diesel fuel, and hydraulic fluid by water, ethylene glycol, wear particles, and loss of anti-oxidants. The instrument includes a broad band light source, such as a tungsten filament incandescent lamp, which is very inexpensive and reliable. A fiber optic link is provided to a detector that receives and discriminates among optical spectral transmissions through the fiber optic. Discrimination circuitry is provided for evaluating the transmission and providing a readout that indicates the quality of the fluid. The instrument is suitable for in situ determination of oil quality.