Abstract: An apparatus is provided for detecting each of the five known anesthetic agents, CO2, and N2O when monitoring a patient's respiratory gas stream during anesthesia. A source of infrared radiation emits the infrared tight through a gas stream and onto the infrared detector/sensor. The detector may contain ten analytical channels, each channel containing an independent infrared detector and a specific bandpass infrared filter which only permits certain wavelengths of the infrared radiation to reach the respective infrared detector. One of the channels is a blocked detector which serves as the reference point. A calibration and processing means is also provided to compensate for factors such as offset voltage, cross coupling, wideband changes and thermal drifts which may affect the accuracy of the gas measurements.