This 2014 paper seems to contain important revelations regarding contamination in commercially available 15N2 gas:
and potentially calls into question earlier studies using 15N2 to indicate N fixation. The issue seems to involve low levels of ammonia and nitrogen oxide gases that produce assimilable ammonium and nitrate when the gas equilibrates with water, which can result in a false indication of nitrogen fixation.
My question is how one could best purify the 15N2 gas by some kind of trapping to remove these contaminants. Any thoughts would be most appreciated.