apparatus and methods for detecting selected chemical compounds in air or other gas streams at room or ambient temperature includes a liquid - free humidity - resistant amperometric sensor comprising a sensing electrode and a counter and reference electrode separated by a solid electrolyte . the sensing electrode preferably contains a noble metal , such as pt black . the electrolyte is water - free , non - hygroscopic , and substantially water - insoluble , and has a room temperature ionic conductivity ≧ 10 - 4 - 1 , and preferably ≧ 0 . 01 - 1 . the conductivity may be due predominantly to ag + ions , as in ag 2 wo 4 . 4agi , or to f - ions , as in ce 0 . 95 ca 0 . 05 f 2 . 95 . electrical contacts serve to connect the electrodes to potentiostating and detecting circuitry which controls the potential of the sensing electrode relative to the reference electrode , detects the signal generated by the sensor , and indicates the detected signal .