Determination of Passive Ammonia Concentrations by the Method of Flow Injection Analysis at a Network of Air Monitoring Sites
Central Analytical Laboratory,
National Atmospheric Deposition Program,
Champaign, IL USA
The determination of ambient ammonia concentrations at a network of air monitoring sites is carried out by Flow Injection Analysis. Field samples for this study are collected from 21 sites in the USA and Canada using Radiello passive diffusion gas samplers, which cores, made of microporous polyethylene coated with phosphoric acid, adsorb ammonia as ammonium ions. After 14 days of exposure at field sites samplers with related field blanks are shipped to Central Analytical Laboratory (CAL).
CAL analysts perform the extraction of ammonium from the samplers with high purity deionised water and then determine the concentration of ammonium in extracts. Ammonium ion is quantified by flow injection analysis as indophenol - the adsorbance of its blue color, which is directly proportional to the ammonium concentration, is measured at 630 nm. The method detection limit is 0.005 mg NH4/L.
Quality Control includes several levels of checks to assure accurate data readings. The deviations from standard procedure of determination NH4 on FIA include storing samples at 40C, stirring samples before the analysis and using a modified buffer.