Optimizing [ 13 N]N 2 radiochemistry for nitrogen‐fixation in root nodules of legumes

Here we explored the conditions for synthesizing [ 13 N]N 2 in a state that is suitable for the administration to plant root nodules enabling studies of nitrogen fixation. [ 13 N]N 2 was prepared batchwise, starting with [ 13 N]NO   3 −from the 16 O(p,α) 13 N nuclear reaction on a liquid water target. [ 13 N]NO   − 3was first reduced to [ 13 N]NH 3 using Devarda's alloy, and then the [ 13 N]NH 3 was oxidized to [ 13 N]N 2 by hypobromite using carrier‐added NH 4 Cl. The amounts of carrier NH 4 Cl and hypobromite were varied to determine the effects these parameters had on the radiochemical yield, and on the radiotracer specific activity. As expected, increasing the amount of carrier NH 4 Cl improved the radiochemical yield. Unexpectedly, increasing the amount of excess hypobromite from 1.6‐fold to 6‐fold molar equivalents (relative to NH 4 Cl) improved the radiochemical yield and radiotracer specific activity under all conditions of carrier NH 4 Cl. As a comparison, we measured [ 13 N]N 2 specific activity derived from in‐target production based on a 50 µA min irradiation driving the 14 N(p,pn) 13 N reaction on a gaseous N 2 target. The ‘wet’ radiochemistry approach afforded two advantages over the in‐target approach with a ∼600‐fold improvement in specific activity, and the ability to collect the tracer in a small volume of gas (∼20 mL at STP). Copyright © 2010 John Wiley & Sons, Ltd.