Recovery of ethanol from a crude ethanol product obtained from the hydrogenation of acetic acid by hydrolyzing a portion of the crude ethanol product or one or more derivative streams obtained therefrom. An insulator layer is grown on, and removed via a wet etch procedure, from only the portion of the active device region to be used to accommodate the thin silicon dioxide gate insulator component of the dual gate insulator device. Patterned areas may be selectively hydrogenated, such as mesas in Avalanche Photodiode Arrays. The method comprises measuring a concentration of sulphur dioxide in at least a first measuring point corresponding to a first section of the at least two sections of the contact zone to obtain measurement information; and comparing the measurement information to a reference value to determine a local sulphur dioxide removal performance of the first section.