Cross Polarization/Magic Angle Spinning Nuclear Magnetic Resonance (CP/MAS NMR) Spectroscopy

Abstract

Standard techniques for solution-state ¹³C NMR seldom yield useful spectra when applied to solid samples as the signals are usually too weak and broad. However, the problems with weak signals can be largely overcome by using cross polarization pulse sequences (Pines et al. 1973), and the broad linewidths can be sharpened by spinning samples at the “magic angle” of 54.7° relative to the applied magnetic field (Schaefer and Stejskal 1976). The first CP/MAS spectrum of solid wood showed a resonance band assigned to lignin (Schaefer and Stejskal 1976), but there was a gap of several years before resolution improved sufficiently to reveal detailed structural information about solid lignin (Bartuska et al. 1980). The number of research groups using the technique then increased rapidly as commercial CP/MAS NMR equipment became available and wood scientists began to recognize the advantages of being able to characterize lignin in situ.