Determination of Ammonium in Seawater by Purge-and-Trap and Flow Injection with Fluorescence Detection

ABSTRACT

An improved method for the determination of ammonium in seawater was established using purge-and-trap pretreatment coupled with flow injection fluorescence. An appropriate volume of sodium hydroxide was added to seawater to convert ammonium to gaseous ammonia that was purged from the solution to the vapor phase by argon at 350 mL/min for 15 min at 80°C and trapped in hydrochloric acid. The solution was analyzed based on the fluorescence detection of the reaction product of o-phthaldialdehyde, sulfite, and ammonium. Parameters affecting the purge-and-trap efficiency and determination were optimized, and the matrix effects of seawater were investigated. The influence of primary amines and amino acids was tested and negligible effects were found. The detection limit was 7.4 nmol/L. The relative standard deviation was 4.4% for a seawater fortified with 200 nmol/L ammonium. Compared to the use of gas diffusion cell, this approach offers higher sensitivity, higher collection efficiency, and negligible effects from salinity. In addition, deionized water was used for standard preparation instead of matrix-matched standards.

KEYWORDS:

• Ammonium
• fluorescence
• purge-and-trap
• seawater

Acknowledgments

Professor John Hodgkiss of The University of Hong Kong is thanked for his assistance with English