Automated simultaneous determination of total dissolved nitrogen and phosphorus in seawater by persulfate oxidation method

Abstract

This paper describes an automated persulfate oxidation method for the simultaneous determination of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in seawater using a gas-segmented continuous flow analyzer. Automation of simultaneous oxidation and colorimetric determination of TDN and TDP was performed by installing a heating oil bath unit with a long Teflon tube and an air compressor on the analyzer. Blank selection using single and mixed salt solutions dissolving sodium chloride, sodium sulfate, potassium bromide, and/or sodium hydrogen carbonate in pure water indicated that the single sodium chloride solution was most appropriate. The recoveries of various N and P compounds varied from 101% to 108% and 86% to 100%, respectively. Calibration curves showed suitable linearity ranges of 0–80 μM for the TDN analysis and 0–8 μM for the TDP analysis (r2 = 1.00). The detection limits for TDN and TDP analyses were 0.17 and 0.04 μM, respectively. A throughput of 17 samples per hour was achieved. Application to oceanic water samples showed that TDN and TDP concentrations in the upper 300 m varied from 3.8 μM to 11.0 μM and 0.08 μM to 0.41 μM, respectively, with the coefficients of variation of <7%. These concentrations were not significantly different from those obtained from conventional methods. This proposed method has simple, fast, and economical characteristics and is suitable for simultaneous analyses of TDN and TDP, which help our understanding of N and P dynamics in the ocean.