Use of an alternative method for monitoring total nitrogen concentrations in Florida lakes

A recently described method for the determination of total nitrogen in natural waters involving sample oxidation with persulfate and subsequent determination of nitrate-nitrogen with second derivative spectroscopy was compared with a U.S. EPA approved method involving the sum of nitrate-nitrogen and Kjeldahl-nitrogen as measured with automated analyser. Two kinds of samples were used. Five of them were standard organic-nitrogen samples that had certified concentrations of 0.1, 0.5, 1.0, 2.0 and 3.0 mg nitrogen per litre. The other samples consisted of 21 lake water samples collected from 7 lakes in north-central Florida chosen to range from oligotrophic to hypereutrophic. The new method had an instrument detection limit of 0.07 mg per litre and the standard deviation of 20 sets of lake water samples averaged 0.03 mg per litre. Concentrations were equivalent to those found with the U.S. EPA method but was more sensitive and had a higher level of precision. Addition of acid to lake water samples stored under refrigeration was not necessary for preservation. Freezing was an effective method of sample preservation for 90 d.