A sensitive flow-batch system for on board determination of ultra-trace ammonium in seawater: Method development and shipboard application

• Fluorescence detection was coupled with flow analysis and solid phase extraction for ammonium determination.
• The solid phase extraction technique has been seldom used to enrich compounds for fluorescence detection.
• Different storage methods for ultra-trace ammonium samples were compared during the South China Sea survey.
• This method was applied on board to analyze seawater samples collected from 65 stations in the South China Sea.
• A detailed ammonium profile was obtained in the SEATS station.
• The proposed method offered the benefits of improved sensitivity, reduced reagent consumption, negligible salinity interference and lower cost.

Combining fluorescence detection with flow analysis and solid phase extraction (SPE), a highly sensitive and automatic flow system for measurement of ultra-trace ammonium in open ocean water was established. Determination was based on fluorescence detection of a typical product of o-phthaldialdehyde and ammonium. In this study, the fluorescence reaction product could be efficiently extracted onto an SPE cartridge (HLB, hydrophilic–lipophilic balance). The extracted fluorescence compounds were rapidly eluted with ethanol and directed into a flow cell for fluorescence detection. Compared with the common used fluorescence method, the proposed one offered the benefits of improved sensitivity, reduced reagent consumption, negligible salinity effect and lower cost. Experimental parameters were optimized using a univariate experimental design. Calibration curves, ranging from 1.67 to 300 nM, were obtained with different reaction times. The recoveries were between 89.5 and 96.5%, and the detection limits in land-based and shipboard laboratories were 0.7 and 1.2 nM, respectively. The relative standard deviation was 3.5% (n = 5) for an aged seawater sample spiked with 20 nM ammonium. Compared with the analytical results obtained using the indophenol blue method coupled to a long-path liquid waveguide capillary cell, the proposed method showed good agreement. The method had been applied on board during a South China Sea cruise in August 2012. A vertical profile of ammonium in the South East Asia Time-Series (SEATS, 18° N, 116° E) station was produced. The distribution of ammonium in the surface seawater of the Qiongdong upwelling in South China Sea is also presented.

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