Towards quantitative CP-MAS NMR

Many of the combined cross polarization-magic angle spinning (CP-MAS) experiments that have been developed over the past ten years for structural studies of chemical and biological systems rely on the measurement of quantitative signal intensities. However, one of the drawbacks of the CP technique is that the efficiency of polarization transfer depends on the strength of the heteronuclear dipolar interaction which is generally different for different chemical groups. In this paper, we propose the use of a linear ramp of CP amplitudes in combination with high speed MAS to improve signal quantitation. The linear ramp is centered on one of the sidebands of the Hartmann-Hahn matching profile and the contact time is chosen to maximize the total signal in the CP-MAS spectrum. The approach is illustrated on N-t-Boc-alanine whose six carbons exhibit a range of heteronuclear dipolar couplings and molecular motions.