Fact or fiction: What is in your 15N2 and 15NH3 cylinders for sustainable ammonia and urea research?

Attaining reliable performance indicators via ¹⁵N-labeling is a daunting research endeavor for sustainable ammonia and urea synthesis processes, as currently obtained reaction rates and yields are often low. However, artifact and misinterpretation can be induced by ppm-level impurities in commercial ¹⁵N₂ and ¹⁵NH₃ cylinders, which are not well understood. Here, we report quantitative in-line gas chromatography-mass spectrometry (GC-MS) analysis of impurities in commercial ¹⁵N₂ and ¹⁵NH₃ cylinders at ppm level, in conjunction with ¹H-NMR methods, exemplified by 808-16,252 ppm of ¹⁴N¹⁵N, 0-4,891 ppm ¹⁵NH₃, 3-319 ppm ¹⁴NH₃, 0-231 ppm of ¹⁴NO₂, 0-176 ppm ¹⁵NO₂, and 0-566 ppm ¹⁵N¹⁶O impurities, some of which were not reported in cylinder certification nor previous studies. The collected results have formed recommended protocols applicable to all future ¹⁵N labeling experiments intended to eliminate false positives.