NATURAL ABUNDANCE-LEVEL MEASUREMENT OF THE NITROGEN ISOTOPIC COMPOSITION OF OCEANIC NITRATE - AN ADAPTATION OF THE AMMONIA DIFFUSION METHOD

We have adapted the ''ammonia diffusion'' method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devarda's all oy; and (3) the gas-phase diffusion of ammonia onto an acidified glass fiber disk which is sandwiched between two porous Teflon membranes. W e have investigated the conditions necessary to effect complete ammoni a recovery from natural seawater samples and the use of Devarda's allo y under these conditions. In addition, we have characterized the blank s in this method and designed a protocol to minimize them. Here, we re port our protocol for nitrate extraction from seawater and provide an explanation of the protocol based on our method development work. To d emonstrate the performance of the method, we present nitrate nitrogen isotopic data from nitrate standard additions to Sargasso Sea surface water and from several Southern Ocean depth profiles. The nitrate extr action method gives highly reproducible, complete recovery of nitrate and a standard deviation for isotopic analysis of < 0.2 parts per thou sand down to 5 mu M nitrate (or lower). Replicate extractions of a nit rate standard added to Sargasso Sea surface water demonstrate agreemen t between the isotopic composition of the added and recovered N, with the extraction blank causing less than or equal to 0.3 parts per thous and discrepancy for 5 mu M nitrate. The blanks inherent in the extract ion procedure are from Devarda's alloy and seawater dissolved organic nitrogen (''DON''). The N blank of the Devarda's alloy reagent depends on brand and lot number. The Devarda's alloy which we are currently u sing results in a blank of similar to 0.4 nmol N per 100 mi of seawate r (effectively 0.4 mu M). An isotopic correction is made for this blan k. For standard incubation conditions, stored Woods Hole seawater(with similar to 10 mu M DON) gives a similar to 0.6 mu M DON blank, while stored Sargasso Sea(with similar to 6 mu m DON) surface water gives a DON blank of 0.3-0.5 mu M The DON blank appears to cause the less than or equal to 0.3 parts per thousand difference between the measured an d actual isotopic composition of nitrate added to Sargasso Sea surface water at the 5 mu M nitrate level. We discuss several ways to lower t he DON blank for samples in which the DON concentration is high relati ve to the nitrate concentration. The nitrogen isotopic data from sever al Southern Ocean profiles, in conjunction with the other results pres ented in this paper, demonstrate the consistency of the data produced by the ammonia diffusion method. The ammonia diffusion-based protocol is more reliable and allows for better precision than the nitrate redu ction/ammonia distillation method (Cline and Kaplan, 1975) in our hand s. While the samples have an incubation time of 4 days or longer, we f ind that the diffusion method allows for higher throughput than the di stillation method because samples can be run conveniently in large bat ches.