title: Quantitative solid-state 13C NMR with signal enhancement by multiple cross polarization.
abstract: A simple new method is presented that yields quantitative solid-state magic-angle spinning (MAS) (13)C NMR spectra of organic materials with good signal-to-noise ratios. It achieves long (>10ms) cross polarization (CP) from (1)H without significant magnetization losses due to relaxation and with a moderate duty cycle of the radio-frequency irradiation, by multiple 1-ms CP periods alternating with (1)H spin-lattice relaxation periods that repolarize the protons. The new method incorporates previous techniques that yield less distorted CP/MAS spectra, such as a linear variation ("ramp") of the radio-frequency field strength, and it overcomes their main limitation, which is T1ρ relaxation of the spin-locked (1)H magnetization. The ramp of the radio-frequency field strength and the asymptotic limit of cross polarization makes the spectral intensity quite insensitive to the exact field strengths used. The new multiCP pulse sequence is a "drop-in" replacement for previous CP methods and produces no additional data-processing burden. Compared to the only reliable quantitative (13)C NMR method for unlabeled solids previously available, namely direct-polarization NMR, the measuring time is reduced by more than a factor of 50, enabling higher-throughput quantitative NMR studies. The new multiCP technique is validated with 14-kHz MAS on amino-acid derivatives, plant matter, a highly aromatic humic acid, and carbon materials made by low-temperature pyrolysis.
authors.affiliation: Department of Chemistry, Iowa State University, Ames, IA 50011, USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Johnson
authors.affiliation: Department of Chemistry, Iowa State University, Ames, IA 50011, USA. Electronic address: srohr@iastate.edu.
authors.forename: Klaus
authors.initials: K
authors.lastname: Schmidt-Rohr
keywords: Aromaticity; Complex organic matter; Cross polarization; Quantitative CP; Ramp CP