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

Abstract

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 10 min reaction at 40°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 5.0 mM of NaOH, and determined with a spectrophotometer at 640 nm. Parameter, including extraction conditions, reagent concentrations, pH, temperature, and reaction time, were optimized. Under the optimized conditions, the detection limit was 3.5 nM and the linear range was 0–428 nM. The relative standard deviations were 5.7% (n = 8) for 44.6 nM standard solution and less than 6.0% (n = 3–5) for samples within concentrations of about 52.4–288.7 nM; the recovery was in the range 93.6 to 108.5%. The sample throughput was 3 h⁻¹. The proposed method provides a simple, cheap, and automatic way to determine ammonium in seawater samples without complicated sample treatment.

Keywords:

• Ammonium
• Seawater
• Sequential injection
• Solid phase extraction

Acknowledgments

This paper was submitted as part of a Special Issue on Flow Injection Analysis.

The work was financially supported by the National High Technology Research and Development Program of China (No. 2006AA09Z174 and No. 2007AA161501) and program for Chiangjiang scholars and lnnovative Research Team in University (No. 40821063). Prof. John Hodgkiss is thanked for his assistance with English.

Notes

The total analysis time was 1245sec.

^a Recovery was defined as the ratio of the slope of the seawater calibration curve to that of Milli-Q water.

Y: the method has been applied to seawater. N: the method has not been reported to apply to seawater; FIA: flow injection analysis; FL: fluorescence; LLE: liquid-liquid extraction; Spec: Spectrophotometry; CZE: capillary zone electrophoresis; UV Spec: Ultraviolet spectrophotometry; FI-GD-IC: flow injection gas diffusion coupled with ion chromatography; CD: conductivity detection; CL: chemiluminescence; SCFA: segmented continuous flow analysis; LWCC: liquid waveguide capillary cell; GD-MSFS: gas diffusion multi-commuted flow system; SI-SPE: sequential injection coupled with solid phase extraction.