An in-Situ Chemical Analyzer for the Determination of Trace Ammonia in Natural Waters

Abstract:

In recent decades chemists have devoted a considerable effort to automating classical wet chemistry. The instruments manufactured for analysis of a large number of samples can be categorized into two main groups—batch and continuous flow analyzers. Our technique, autonomous batch analyzer (ABA), takes advantages of previously described batch analysis and continuous flow analysis. With its simpler design, ABA is robust, flexible, inexpensive, and requires minimal maintenance. ABA achieves complete mixing of sample with reagents using a syringe and a simple mixing chamber. The system can autonomously produce a calibration curve by auto-diluting a single stock standard solution. In addition it incorporates a pre-filtering subsystem enabling measurements in turbid, sediment-laden waters. Over the typical range for ammonia in marine waters (0-10 µM), the response is linear (r² = 0.9930) with a limit of detection (S/N ratio > 3) of 10 nM. The working range for marine waters is 0.05-10 µM. Repeatability is 0.3% (n = 10) at an ammonia level of 2 µM. Results from automated operation in 15 min cycles over 16 days had good overall precision (RSD = 3%, n = 660). The system was field tested at three shallow South Florida sites, a tidal pond and the Indian River Lagoon, FL. Diurnal cycles and possibly a tidal influence were expressed in the concentration variability observed.