129Xe

129Xe is a stable isotope of the noble gas xenon with a mass number of 129. It is one of the more abundant spin-active xenon isotopes, comprising roughly a quarter of natural xenon. The nucleus has spin I = 1/2 and a magnetic moment of about −0.778 μN. Its gyromagnetic ratio γ/2π is approximately −11.77 MHz/T, yielding a Larmor frequency around 11.8 MHz per tesla for NMR experiments.

In practice, 129Xe is widely used in magnetic resonance techniques, notably as a hyperpolarized gas for MRI. Hyperpolarization is achieved by spin-exchange optical pumping (SEOP) with alkali metals such as rubidium, which boosts polarization far beyond thermal equilibrium. The resulting hyperpolarized gas provides a strong signal, enabling high-contrast imaging of lungs and airways. Xenon dissolved in tissue or blood also experiences large chemical shifts, producing a broad chemical-shift range that helps differentiate compartments and environmental conditions.

129Xe NMR is also valuable as a chemical and biophysical probe. Its large, environment-dependent chemical shifts make it sensitive to binding in protein cavities and to the properties of solvents and porous media. Dissolved-phase xenon can report on hydration, lipophilicity, and molecular interactions, complementing conventional NMR techniques.

Natural xenon contains multiple isotopes, and 129Xe is typically enriched from atmospheric xenon for research and medical applications. Its inert gas properties, combined with favorable NMR characteristics, have established 129Xe as a versatile tool in both biomedical imaging and fundamental science.