Journal of Oceanography, Vol. 54 (No. 4), pp. 337-342, 1998
Hideki Yakushiji and Jota Kanda
School of Science, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan
(Received 17 April 1998; in revised form 1 June 1998; accepted 1 June 1998)
Abstract: A method has been developed for determination of 15N isotope ratio in nitrate nitrogen, which is a major analytical step in tracer experiments for studies of nitrate metabolism in the marine environment. The method is based on diazotization of nitrite with sulfanilic acid following reduction of nitrate to nitrite by a cadmium-copper column. The diazonium compound is then subject to the azo coupling reaction with 2-naphthol, and the azo dye formed is extracted by a solid phase extraction column. The dye eluted from the column is collected, and total nitrogen and 15N content of the dye are determined by mass spectrometry. Sulfanilic acid can also remove preexisting nitrite by heating the sample under acidic conditions before passing through the cadmium-copper reduction column. The average recovery of nitrate nitrogen was 86%. A procedure for reducing the background nitrogen that derives from the analytical operations has been developed; background nitrogen was limited to about 0.25 mg-atom N. The variation in the background nitrogen levels reflects the range of error in 15N determination of nitrate nitrogen by this method. Application of the present method to a 15NO3- isotope dilution experiment for determination of nitrification rate in sea water is demonstrated.