Underway analysis of nanomolar dissolved reactive phosphorus in oligotrophic seawater with automated on-line solid phase extraction and spectrophotometric system

•On-line solid phase extraction of phosphomolybdenum blue without common interference.•Automated flow analyzer with laboratory-made hardware and software.•2-week continuous underway determination of nanomolar phosphate in the South China Sea.•Accurate and high resolution field seawater analysis with occasional maintenance.

The automated in-field determination of trace phosphate (and other nutrients) is highly valuable for studying nutrient dynamics and cycling in oligotrophic oceans. Here, we report an automated portable analyzer for week-long underway analysis of nanomolar dissolved reactive phosphorus (DRP) in seawater. The method is based on classic phosphomolybdenum blue (PMB) chemistry combined with on-line solid phase extraction (SPE) and flow analysis. Under optimized conditions, the formed PMB from sample is automatically concentrated on a hydrophilic lipophilic balanced (HLB) copolymer SPE. The PMB compound can be eluted with NaOH solution and measured in a flow-through detection system. All the components of the analyzer are computer controlled using laboratory-programmed software based on LabVIEW. The system exhibited advantages of high sensitivity (detection limit of 1.0 nmol L−1) and reproducibility (relative standard deviation of 5.4%, n = 180), insignificant carry-over effect and no interferences from salinity, silicate, arsenate and other P-containing compounds (concentrations at environmental level). The analytical time was 4-7 min/sample, depending on the DRP concentration. The accuracy of the method was validated through the analysis of reference materials and comparison with two other published methods (slope of 0.986 ± 0.027, intercept of 0.39 ± 0.64 nmol L−1, R2 of 0.9608, range of 0-80 nmol L−1, n = 57). The system has been successfully applied for a two-week continuous underway determination of DRP in surface seawater during a cruise in the South China Sea. Based on the laboratory and field evaluations, it is concluded that this system is suitable for accurate and high resolution underway DRP measurements in oligotrophic areas.

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