The disclosure relates to a method of detecting a change in a chemical composition by contacting a doped oxide material with a monitored stream, illuminating the doped oxide material with incident light, collecting exiting light, monitoring an optical signal based on a comparison of the incident light and the exiting light, and detecting a shift in the optical signal. The doped metal oxide has a carrier concentration of at least 10.sup.18/cm.sup.3, a bandgap of at least 2 eV, and an electronic conductivity of at least 10.sup.1 S/cm, where parameters are specified at a temperature of 25.degree. C. The optical response of the doped oxide materials results from the high carrier concentration of the doped metal oxide, and the resulting impact of changing gas atmospheres on that relatively high carrier concentration. These changes in effective carrier densities of conducting metal oxide nanoparticles are postulated to be responsible for the change in measured optical absorption associated with free carriers. Exemplary doped metal oxides include but are not limited to Al-doped ZnO, Sn-doped In.sub.2O.sub.3, Nb-doped TiO.sub.2, and F-doped SnO.sub.2.
The United States Government has rights in this invention pursuant to the employer-employee relationship of the Government to the inventors as U.S. Department of Energy employees and site-support contractors at the National Energy Technology Laboratory.