Nitrogen-15's a stable, non-radioactive isotope of nitrogen. It's often used in agricultural and medical research. Nitrogen-15's frequently used in nuclear magnetic resonance spectroscopy (NMR), because unlike the more abundant nitrogen-14, it's a nuclear spin of ½, which simplifies observation by NMR. Proteins can be isotopically labelled by cultivating them in a medium containing only nitrogen-15. In addition, nitrogen-15's used to label proteins in quantitative proteomics (for example S… (More on Nitrogen-15)