We develop a toy model of a CF-GS device, composed of (i) a catalytic filter (CF), in the form of a tubular reactor packed with catalyst particles, in series with (ii) a solid-state gas sensor (GS). See Fig. . The gas containing analyte and interferent is input to the CF; the effluent of the CF is exposed to the GS. Assume continuous, isothermal, steady-state operation.