NH4-Digiscan: an in situ and laboratory ammonium analyzer for estuarine, coastal and shelf waters

The NH4-Digiscan is an in situ analyzer designed for measuring ammonium in estuarine, coastal, and shelf waters at depths of less than 3 m. This wet chemical analyzer uses micro-solenoid pumps to propel sample and reagents, a gas diffusion cell to isolate the analyte from the matrix, and a conductivity detector for analyte detection. Instrument measurements are stable for deployments of at least 30 d. In estuarine and coastal waters, the analyzer is capable of sampling hourly and has a detection limit of 0.2 µM. In shelf waters, the NH4-Digiscan can be configured to have a detection limit of 0.014 µM. The simple chemistry, in situ capability, and high resolution sampling minimizes the use of toxic reagents, minimizes many of the problems plaguing ammonium analyses, and adequately captures the high temporal variability of coastal waters, which is often undersampled. The analyzer has been successfully deployed on coastal moorings, benthic flux chambers, and on a drifter 500 km west of Monterey Bay, California. The system can also be easily configured for laboratory bench top analysis of discrete samples.