Gas sensor

A gas sensor including an electrochemical sensor cell which has a anion-exchange solid polymer electrolyte membrane with three attached electrodes is provided. The sensor cell contains no liquid electrolyte and is operated in the potentiostatic as well as the potentiodynamic modes to detect alkaline reactive gases, including vapors, such as hydrazines and derivatives thereof and ammonia. These sensor cells together with electronic circuitry, a pump and a power supply, fit into a compact, pocket-sized container to define the gas sensor of the invention which can detect traces of the above gases including 10 ppb of hydrazine and its derivatives and 10 ppm of ammonia. The invention includes the above gas sensor and the methods of operating same.