A modified segmented continuous flow analysis method for simultaneous determination of total dissolved nitrogen and phosphorus in marine environments

We present a modified segmented flow analysis method for the simultaneous determination of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in marine environments based on a combination of photooxidation and alkaline persulfate digestion. The modified Bran+Luebbe® Continuous Flow Autoanalyzer III (CFA III) manifold consists of a high-energy UV lamp (1.1 W at 185 nm) and a heated bath (90°C). The analytical precision of the method is within 2.1% for TDN and 1.6% for TDP analyses, and within 4.5% for dissolved organic N and P (DON and DOP); the average standard deviation is ± 0.37 µM for DON and ± 0.02 µM for DOP. The detection limit is 1.23 µM for DON and 0.05 µM for DOP analyses. The linear analytical range of the method is up to 200 µM for TDN and up to 6 µM for TDP. The CFA III method was compared with the autoclave alkaline persulfate oxidation (PO) and the high temperature combustion (HTC-TDN) techniques. All methods, when applied to open ocean and coastal samples, produced similar DON concentrations (PO:CFA III = 102% and HTC:CFA III = 100%, on average), but the CFA III method yielded significantly higher oxidation efficiencies for DOP analyses than the PO technique (PO:CFA III = 71%, on average). For CFA IIITDN/TDP analyses samples with different salinities should be analyzed relative to standard solutions prepared with similar matrix. A throughput of 20 samples per hour is easily achieved. This method is simple, fast, reproducible, economical, and is highly suitable for simultaneous TDN and TDP routine analyses.