Bottles of 15N2 are nearly always sold at a purity of around 99% (and >97% isotopic enrichment). However, that single remaining per cent of foreign substance can turn out to be detrimental since it was found out that a significant fraction of it is in the form of 15N-labelled ammonia and nitrate \cite{Dabundo_2014}. Ideally, every batch of labelled gas should be tested for potential contamination either by direct measurement of ammonia, or indirectly by incubating a culture of non-diazotrophic microorganism in the presence of the gas as a sole nitrogen source and then testing if label has accumulated in the biomas.