ADRF-CP Surface-Coil Spectroscopy of Synthetic Calcium Phosphates and Bone Mineral

Proton to phosphorus-31 cross polarization via adiabatic demagnetization in the rotating frame (ADRF-CP) has been used, in conjunction with a surface coil, to detect monohydrogen phosphate (acid phosphate) ions in the presence of a large background of nonprotonated phosphate (orthophosphate) ions in porcine bone and synthetic calcium phosphates. Transient oscillations were observed in the transfer of polarization between the proton dipolar and phosphorus Zeeman nuclear-spin reservoirs at short times after the initiation of thermal contact. The oscillations were observed in all samples, including bone. Orthophosphate suppression was achieved by detecting the signal when the orthophosphate oscillation was passing through zero, and by adjusting the phosphorus RF field to achieve optimal cross polarization with the proton local fields of the acid phosphate ions. ADRF-CP techniques deposit less RF power than traditional spin-lock CP techniques, and are hence compatible within vivoapplication. As the ratio of the protonated to nonprotonated phosphate can be used as a marker for bone-mineral maturity, ADRF-CP spectroscopy creates the possibility of characterizing bone-mineral dynamicsin vivoby solid-state NMR.