High Resolution Determination of Nanomolar Concentrations of Dissolved Reactive Phosphate in Ocean Surface Waters Using Long Path Liquid Waveguide Capillary Cells (LWCC) and Spectrometric Detection

In the last decade, long path length, low volume, liquid waveguide capillary cells (LWCC) in conjunction with conventional nutrient auto-analyzers have been applied to determinations of nanomolar levels of phosphate, nitrate, and nitrite in oligotrophic waters. This article reports a high resolution, real-time, continuous method for nanomolar dissolved reactive phosphate measurements in ocean surface waters with data logging every 30 seconds for up to 16 consecutive hours. Surface seawater is pumped continuously from a shipboard underway tow-fish unit to a helium gas-segmented, continuous-flow, nutrient auto-analyzer modified with a 250 cm LWCC. To circumvent baseline instability due to reagents, a parallel channel with deionized water (DI) and reagents is run and later subtracted from the sample absorbances. The detection limit is 0.8 nmol/L. The precision (as relative standard deviation) at 5 nmol/L phosphate is 6.1% (n = 5) and 0.8% (n = 5) at 50 nmol/L. We also report an optimized method for discrete samples using a 200 cm LWCC. To minimize any background phosphate concentration in low nutrient seawater used as wash water solution, we use DI water, but increase sample and wash times to achieve plateau-shaped peaks after the transient wash/sample mixing period. The detection limit is 0.5 nmol/L. The precision at 10 nmol/L phosphate is 1.8% (n = 8) and 0.9% (n = 9) at 60 nmol/L. The two systems have successfully been deployed on the U.S. GEOTRACES 2010 cruise, transecting the upwelling area northwest of Africa and the highly stratified, oligotrophic, subtropical North Atlantic gyre