Determination of ¹⁵N Nitrate and Nitrite in Spiked Natural Waters

Dissolved 15 N nitrate and 15 N nitrite were determined by derivatization of 15 NO 2 - with 2,4-dinitrophenylhydrazine (2,4-DNPH), followed by FT-IR spectrophotometric analysis of the resulting 15 N azide. Samples were derivatized in the aqueous phase and partitioned into CHCl 3 with a single extraction with greater than 98% efficiency. The 15 N azide displays an asymmetric stretching band at 2089 cm - 1 in a region of the infrared spectrum with relatively few interferences. 15 N nitrate can be determined after initial reduction to 15 N nitrite. The described method is quick and has minimal sample manipulations, undetectable blanks, and a low limit of detection of 0.5 μM, with an average precision in coastal seawater of 6% RSD for 15 NO 2 - and 9% RSD for 15 NO 3 - . Analytical results were verified with a completely independent standard colorimetric technique, in both deionized and natural waters. The two methods produced nitrite concentrations that agreed to within a few percent with no statistical differences observed in the data up to 60 μM. The quantification of 15 N was not affected by variable amounts of 14 N nitrate typically found in natural waters. Additionally, there was greater than 98% recovery of an added 15 N nitrite spike from a coastal seawater sample. Application of the analysis to nitrogen uptake studies is also presented. In one such experiment, the average uptake of 15 NO 3 - in coastal seawater was 13.6 μM day - 1 between day 3 and day 7. This 15 N uptake corresponded to maximum chlorophyll a increases in the cultured seawater, suggesting the biota were actively assimilating the added nitrate.