DETERMINATION OF N-15 NITRATE AND NITRITE IN SPIKED NATURAL-WATERS

Dissolved N-15 nitrate and N-15 nitrite were determined by derivatizat ion of (NO2-)-N-15 with 2,4-dinitrophenylhydrazine (2,4-DNPH), followe d by FT-IR spectrophotometric analysis of the resulting 15N azide. Sam ples were derivatized in the aqueous phase and partitioned into CHCl3 with a single extraction with greater than 98% efficiency. The N-15 az ide displays an asymmetric stretching band at 2089 cm(-1) in a region of the infrared spectrum with relatively few interferences. N-15 nitra te can be determined after initial reduction to N-15 nitrite. The desc ribed method is quick and has minimal sample manipulations, undetectab le blanks, and a low limit of detection of 0.5 mu M, with an average p recision in coastal seawater of 6% RSD for (NO2-)-N-15 and 9% RSD for (NO3-)-N-15. Analytical results were verified with a completely indepe ndent standard colorimetric technique, in both deionized and natural w aters. The two methods produced nitrite concentrations that agreed to within a few percent with no statistical differences observed in the d ata up to 60 mu M. The quantification of N-15 was not affected by vari able amounts of N-14 nitrate typically found in natural waters. Additi onally, there was greater than 98% recovery of an added N-15 nitrite s pike from a coastal seawater sample, Application of the analysis to ni trogen uptake studies is also presented. In one such experiment, the a verage uptake of (NO3-)-N-15 in coastal seawater was 13.6 mu M day(-1) between day 3 and day 7. This( 15)N uptake corresponded to maximum ch lorophyll a increases in the cultured seawater, suggesting the biota w ere actively assimilating the added nitrate.