 <unk> xenon can be used by surface <unk> . Normally , it is difficult to characterize surfaces using NMR , because signals from the surface of a sample will be overwhelmed by signals from the far @-@ more @-@ numerous atomic nuclei in the bulk . However , nuclear spins on solid surfaces can be selectively polarized , by transferring spin polarization to them from <unk> xenon gas . This makes the surface signals strong enough to measure , and distinguishes them from bulk signals . 
