A SEMIAUTOMATIC, WET-OXIDATION METHOD FOR THE DETERMINATION OF PARTICULATE ORGANIC NITROGEN COLLECTED ON FILTERS

A wet-oxidation method with peroxydisulfate digestion at 120-degrees-C is presented for routine analysis of particulate organic N (PON) collected on glass-fiber filters. The technique was adapted from a previous procedure for oxidizing total N in seawater to NO3-. Comparisons with PON determinations by dry combustion at 800-degrees-C indicated high efficiency of the procedure, giving 100% recovery of N with cell cultures and natural seawater samples. Neutralizing buffer and pH adjustment proved to be necessary but were simplified by using a Technicon AutoAnalyzer system. The method was applied to follow NO3- assimilation by phytoplankton cultures and revealed excretion of N compounds by the cells at the end of the exponential phase of growth. The sensitivity of the method makes it possible to filter smaller quantities of seawater than needed for the combustion method (by 1-2 orders of magnitude) and can easily be adapted to shipboard use.