Abstract: Thermopile-based detection and monitoring/control systems are described, in applications such as inferring concentration of a multicomponent gas by sensing a tracer gas therein, utilizing fiber optic cables to provide multiple sensing paths in a cell, utilizing a modulated IR source switched in on/off cycles, verifying chemical reagent identities, and sensing of effluent following discharge from a contamination removal 8 element or cold trap. A thermopile infrared (TPIR) detector of highly compact character is described for such applications, and permits monitoring of species that may be problematic or otherwise deleterious in such environments. In one implementation, light source modulation and signal processing techniques are employed to improve signal to noise ratio and minimize calibration and complexity of the TPIR detector.