“…These metal oxides (mostly SnO2, Ga2O3 and TiO2) can also be doped with metals such as Cu (Zhang and Liu 2000), Pt plus Nb (Comini et al 2000) and Sb, Bi, Fe, Au, Cd (Morimitsu et al 2000), or metal oxides of Co, Cr, La, Ce (Fukui and Katsuki 2000), Mg, Al , Ru, Rh and Ir (Lang et al 2000) among others. These doping metals help to diminish the humidity and temperature dependence (Fukui and Katsuki 2000;Morimitsu et al 2000), and to improve the sensitivity and the selectivity (Comini et al 2000;Mielle 1996). …”