Monday, December 3, 2007
The sensitivity of NMR spectra for solids containing protons can be increased dramatically by using cross polarization. Cross polarization is a technique where magnetization is transferred from an abundant proton source to a dilute (isotopically of chemically) nucleus, X, during a "contact" period. During the contact time, rf fields for both 1H and X are turned on. The ratio of power levels between 1H and X must be equal to the ratio of gyromagnetic ratios between X and 1H. The enhanced magnetization of the dilute isotope is then detected while the abundant protons are decoupled. The maximum gain in sensitivity is equal to the ratio of gyromagnetic ratios between 1H and X. The technique has the additional advantage in that the relaxation delay can be chosen based on the T1(rho) for 1H rather than the T1 of the dilute isotope, which is often larger by at least an order of magnitude.