The determination of aqueous ammonia by ion mobility spectrometry

A novel aqueous ammonia analyzer, based on an ion mobility spectrometer (IMS) is described. Ammonia is thermally purged from an aqueous sample stream into the IMS through a silicone membrane. Studies were undertaken to establish the limit of detection of the unit and evaluate the effect of ammonium ions. Following this work the system was assessed under simulated process monitoring conditions. Data indicate that the IMS response is to aqueous ammonia, and the limit of detection of the configuration used was found be 1.2 mg 1−1, although lower concentrations were clearly attainable. The ability to determine ammonium ion concentration through pH adjustment of the sampled material was also demonstrated; emphasising the importance of accurate definition of the analyteʹs chemical state. The system was shown to be capable of monitoring accurately, changes in aqueous ammonia concentration over a simulated ammonia concentration excursion for a period of approximately 24 h. Further, no measurement artifacts attributable to memory effects within the membrane were observed.