On-Line Solid Phase Extraction and Spectrophotometric Detection with Flow Technique for the Determination of Nanomolar Level Ammonium in Seawater Samples

The determination of low-level ammonium in seawaters suffers from low sensitivity and high contamination; therefore, it is desirable to develop highly sensitive methods for automatic measurements. A highly sensitive and automated flow technique system for nanomolar level ammonium measurement is described. Reagents for Berthelot reaction were automatically added into seawater samples. After a 10min reaction at 40 degrees C, the formed indophenol blue compound was on-line extracted onto an Hydrophilic-Lipophilic Balance (HLB) cartridge. The enriched compound was rinsed with water and ethanol solution (30%, v/v) and, in turn, eluted with an eluent containing 30% (v/v) ethanol and 5.0mM of NaOH, and determined with a spectrophotometer at 640nm. Parameter, including extraction conditions, reagent concentrations, pH, temperature, and reaction time, were optimized. Under the optimized conditions, the detection limit was 3.5nM and the linear range was 0-428nM. The relative standard deviations were 5.7% (n=8) for 44.6nM standard solution and less than 6.0% (n=3-5) for samples within concentrations of about 52.4-288.7nM; the recovery was in the range 93.6 to 108.5%. The sample throughput was 3h-1. The proposed method provides a simple, cheap, and automatic way to determine ammonium in seawater samples without complicated sample treatment.