An instrumentation system utilizes a single light source collimated through windows through a gas line in communication with a fuel cell. As each beam passes through each window, the gas stream will attenuate each beam. A diffraction grating disperses each attenuated beam and transmits particular wavelength bands through a focusing system to a detector. The measured concentration in the gas stream may then be utilized by a controller to determine the amount of power produced by the cell, determine potential leaks, or determine incomplete reaction.