Nitrogen Isotope Detection of Nitrate in Sea Water via UV Reduction

Current methods widely used for measuring the nitrogen isotopes of nitrate are labor intensive, time consuming, require environmentally unfriendly reagents, and have a high lower limit of detection. Ultra-violet reduction of nitrate has had limited recognition in the past for concentration detection, but its use for isotopes has not previously been explored. Here we present a new approach for the detection of natural abundance levels of nitrogen in sea water nitrate. This method has the advantages of increased throughput and the potential for analysis in regions of the world­_s oceans with concentrations below typical detection limits. This method involves the reduction of nitrate to nitrite during brief exposure of samples to an intense UV light source, complexation of the nitrite with sulfanilic acid and 2-naphthol to form an azo dye, followed by solid phase extraction of the dye. The dye is then eluted, dried, and transferred to a glass fiber filter in a tin cup and combusted in line with a conventional isotope ratio mass spectrometer. Fractionation and yield are dependent on UV exposure time, pH, and dissolved oxygen. Optimization data has been collected over a pH range of 1 through 12, time series of nitrite production and decay, concentrations between zero and 40uM nitrate, various degassing methods, as well as the use of radical scavengers. Yields up to 85% and d15N values with a standard deviation of less than 0.3 per mil are common. Concentrations down to 5uM have been successfully analyzed for 15N:14N ratio, yet the potential for a much lower concentration limit exists due to low blanks and the convenience of working with relatively large volumes (up to 1 L).