Separation of nitrogen and oxygen isotopes by liquid chromatography

Abstract

ISOTOPIC enrichment of organic compounds in the stable minor isotopes ¹⁵N, ¹⁷O and ¹⁸O, which are often used as tracers, is commonly achieved by syntheses that involve isotopically enriched simple precursors such as NH₃, H₂O and CO₂. But such labelled compounds are not always commercially available or readily synthesized. Here we report the direct enrichment of nitrogen and oxygen isotopes in dissociable compounds such as amines, phenols and carboxylic acids, using a high-performance liquid-chromatography technique. The separation method relies on the isotope effects on the dissociation equilibria of weak acids and bases that occur in protonation or deprotonation of the relevant isotopes. Enrichment of ¹⁵N to 5–10% isotopic purity in N,N-dimethylaniline was achieved from the natural abundance of 0.37% in less than 14 hours; subsequent runs brought the isotopic purity to 90% or more. Enrichment of 1¹⁷O and ¹⁸O to similar high purities inp-nitrophenol was also obtained. Thus, where suitable dissociation eqilibria exist, direct isotopic enrichment is possible using standard techniques without the need to devise complex synthetic pathways.