Optimization of data acquisition and processing in Carr–Purcell–Meiboom–Gill multiple quantum magic angle spinning nuclear magnetic resonance

Abstract

Data acquisition using the Carr–Purcell–Meiboom–Gill (CPMG) train of π pulses has been recently explored in multiple quantum magic angle spinning (MQMAS) nuclear magnetic resonance of half-integer quadrupolar nuclei [T. Vosegaard, F. H. Larsen, H. J. Jakobsen, P. D. Ellis, and N. C. Nielsen, J. Am. Chem. Soc. 119, 9055 (1997)]. Significant increase of sensitivity can be obtained by using this technique at the expense of spectral definition, as the spectrum transforms into a manifold of narrow sidebands. A detailed analysis of the CPMG method, with emphasis on the MQMAS experiment, is presented. The MQ-QCPMG-MAS approach is adapted to spin-5/2 nuclei. Several numerical methods of data treatment are shown that allow for improvement of the definition of the sideband envelope, reconstruction of the standard line shape, and improvement of S/N ratio via optimized filtering. Theoretical and experimental estimates of signal enhancement through proper acquisition and processing of MQ-QCPMG-MAS data are given.

High Field 33S Solid State NMR and First-Principles Calculations in Potassium Sulfates

“…1 Their applications range from solid electrolytes in fuel cell applications 2 to construction 3 and agro-chemistry. 4 The properties of sulfates depend greatly on the local environment of sulfur, and 33 S solid state (SS) NMR could potentially play a significant role in studies of the chemistry of these materials. The technique, however, has seen very few applications, with just over 10 publications on the subject being produced in the past 25 years.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16] The problem arises mainly from the great difficulty in obtaining the spectra. The only magnetically active isotope, 33 S, has a low natural abundance of 0.75%, and is a spin 3/2 quadrupolar nucleus with rather small magnetogyric ratio γ (absolute resonance frequency Θ ) 7.676 MHz). Low natural abundance and resonance frequency together with a moderate quadrupole moment of -8.17 fm 2 make 33 S SS NMR quite a challenging exercise.…”

“…The only magnetically active isotope, 33 S, has a low natural abundance of 0.75%, and is a spin 3/2 quadrupolar nucleus with rather small magnetogyric ratio γ (absolute resonance frequency Θ ) 7.676 MHz). Low natural abundance and resonance frequency together with a moderate quadrupole moment of -8.17 fm 2 make 33 S SS NMR quite a challenging exercise. When the sulfur is not in a highly symmetric environment, such as the cubic site occupied in many sulfides, 5,7,8 the observed signals of central transitions are commonly broadened by the second-order quadrupolar interaction, 17,18 drastically reducing the intensity of the signals.…”

Rapid estimation of nuclear magnetic resonance experiment time in low‐concentration environmental samples

“…If this delay is shortened to the point where the echoes are applied at a faster rate than the chemical shift can evolve, the result is that the chemical shift envelope is reduced to a single spike within the recorded bandwidth at the transmitter frequency [25]. In addition, to concentrating the signal into a single spike, the spin lock also perturbs T 2 relaxation and removes the effect of nonhomogeneous broadening, which can be significant in complex, heterogeneous samples [26][27][28]. Thus, signal persists for much longer, permitting increased sampling, which leads to increased S/N (see Fig.…”

“…Thus, it can be used not only to indicate the existence of observable nuclei but also to predict the time required to attain a particular S/N in a conventional NMR spectrum. The use of CPMG is well documented in many NMR applications, and various articles describe the pulse sequence in greater detail [28][29][30][31][32].…”