Development of an Integrated Syringe-pump-based Environmental-water Analyzer (iSEA) for the Determination of Dissolved Reactive Phosphorus in Natural Waters

Phosphate is an essential limited nutrient in marine environments and its concentration can be as low as nanomolar levels, especially in those oligotrophic surface oceans. Methods for determination of dissolved reactive phosphorus (DRP) have been fully developed. However, there is rare method that can be applied for measure DRP concentration covering nM to tens of μM level with one single instrument. Here, we present a multipurpose integrated syringe-pump-based environmental-water analyzer (iSEA) combined with on-line solid phase extraction, based on the classic phosphomolybdenum blue (PMB) chemistry. The iSEA system consists of a syringe pump equipped with a 9-port valve and programs written by LabVIEW. A 3 cm Z-shaped flow cell and a hydrophilic lipophilic balanced (HLB) solid phase extraction cartridge were equipped to measure different levels of phosphate. The effects of reagent volume, sample reaction time and temperature, salinity and carry-over were evaluated. Without the sample concentration, this automated analyzer had a detection limit of 0.11 μM with sample throughput of 20 h^-1. The relative standard deviation (RSD) (n=11) at different concentrations (2-8 μmol/L) were 0.44%-2.3%. The calibration curve was linear (R²=0.9998) over the ranged of 0 to 12 μM. The iSEA system was connected with the HLB cartridge while determining nanomolar level phosphate samples. The sample volume was less than 25 mL and the analytical time is around 7.5 min. The RSD (n=80) of continuous measurement was 4.9% for samples of 50 nM. The detection limit is 2 nM. The iSEA system had been successfully applied in underway analysis during a cruise in estuarine and coastal area and to measure discrete samples collected from open ocean. More evaluation and applications are under development.