not be a major issue for sampling studies where the instrument can be frequently calibrated. However, if the ammonium sensor is used in longer-term deployments, drift is likely to occur. The extent of the drift will vary depending on the age of the probe, the flow rate at the site, and the quality of the water. For all monitoring studies using ion-selective electrodes, the user should acquire a few grab samples during the deployment for analysis in the laboratory or with another ammonium sensor that has been recently calibrated. The typical accuracy specifications for the sensors refer to sampling applications where only minimal time has elapsed between calibration and field use.