Excitation by rare gas metastables in a glow discharge afterglow is effective for the sensitive chemiluminescent detection of trace moisture. In the development of a flowing afterglow FLAG system for the study of effluent gases from high explosives, two approaches were taken in the design of a modular reactor. A glass reactor system with a tantalum shell hollow cathode source isolated by a Woods horn was compared to a 3 in. dia. steel reactor in which the reactor entrance port acts as the cathode. The sensitivity of both systems increases linearly with currents up to 15 microaluminum in operation at 1 Torr argon or helium. The steel reactor provides an order of magnitude greater sensitivity for OH than the glass reactor. A continuous moisture background at 60 C over vacuum dried nitrocellulose of 0.9 nmolgm-s 0.02 ppm is demonstrated. With the steel system a photon count sensitivity for OH at 308.9 nm of 30000 countsnmol in argon and 5200 countsnmol in helium is obtained using a 0.4 nm badnpass.