Quantitative solid-state 13C NMR with signal enhancement by multiple cross polarization. Johnson, R. L & Schmidt-Rohr, K. Journal of Magnetic Resonance, 239:44–49, February, 2014.
Quantitative solid-state 13C NMR with signal enhancement by multiple cross polarization [link]Paper  doi  abstract   bibtex   
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 (\textgreater10ms) 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 T1$ρ$ 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.
@article{Johnson2014,
	title = {Quantitative solid-state {13C} {NMR} with signal enhancement by multiple cross polarization},
	volume = {239},
	issn = {10907807},
	url = {http://www.sciencedirect.com/science/article/pii/S1090780713003005},
	doi = {10.1016/j.jmr.2013.11.009},
	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 ({\textgreater}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 T1\$ρ\$ 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.},
	journal = {Journal of Magnetic Resonance},
	author = {Johnson, Robert L and Schmidt-Rohr, Klaus},
	month = feb,
	year = {2014},
	pmid = {24374751},
	keywords = {Aromaticity, Complex organic matter, Cross polarization, MPCP, Quantitative CP, Ramp CP},
	pages = {44--49},
}

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