Sensitivity enhancement in multiple-quantum NMR experiments with CPMG detection

We present a modified multiple-quantum (MQ) experiment, which implements the Carr-Purcell-Meiboom-Gill (CPMG) detection scheme in the static MQ NMR experiment proposed by W. S. Warren et al. (1980, J. Chem. Phys.73, 2084-2099) and exploited further by O. N. Antzutkin and R. Tycko (1999, J. Chem. Phys.110, 2749-2752). It is demonstrated that a significant enhancement in the sensitivity can be achieved by acquiring echo trains in the MQ experiments for static powder samples. The modified scheme employing the CPMG detection was superior to the original MQ experiment, in particular for the carbonyl carbon with a very large chemical shift anisotropy.     

CAGEBIRD: improving the GBIRD filter with a CPMG sequence

An improvement of the GBIRD-filter is presented. The current approach utilizes Carr-Purcell-Meiboom-Gill type pulse train during the BIRD delay. The method enables recording of purely absorptive 1D spectrum using only one isotope editing element. In the current method, the parent signal leakage due to JHH evolution during the BIRD delay is considerably smaller than in the conventional approach. As a consequence, the t1-noise is smaller also in 2D applications, such as GBIRD-filtered HSQC.     

Categories of coherence pathways for the CPMG sequence

The CPMG sequence has been extremely useful for efficient measurements of NMR signal, spin-spin relaxation, and diffusion, particularly in inhomogeneous magnetic fields, such as when samples are outside the magnet and RF coil. Due to the inaccuracy of the pulses and the off-resonance effects, the CPMG echoes have contributions from the Hahn echo as well as signals that are similar to stimulated echoes. The systematic understanding of the CPMG pulse sequence requires decomposing the magnetization dynamics into different coherence pathways. In this paper, we describe a method to classify the CPMG coherence pathways and illustrate the nature of these types of pathways. This classification shows that direct echo and stimulated echoes are the major contribution to the CPMG signal. It also provides a clear understanding of the effect of restricted diffusion in porous media.     

29Si NMR in solid state with CPMG acquisition under MAS

A remarkable enhancement of sensitivity can be often achieved in ²⁹Si solid-state NMR by applying the well-known Carr–Purcell–Meiboom–Gill (CPMG) train of rotor-synchronized π pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D ²⁹Si{X} CPMAS spectra and 2D ²⁹Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X = ¹H or ²⁷Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of ²⁹Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40 kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of ²⁹Si–²⁹Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings.     

Spectral characterization of diffusion in porous media by the modulated gradient spin echo with CPMG sequence

Carr-Purcell-Meiboom-Gill train of radiofrequency pulses applied to spins in the constant magnetic field gradient is an efficient variant of the modulated magnetic field gradient spin echo method, which provides information about molecular diffusion in the frequency-domain instead in the time-domain as with the two-pulse gradient spin echo. The frequency range of novel technique is broad enough to sample the power spectrum of displacement fluctuation in water-saturated pulverized silica (SiO(2)) and provides comprehensive information about the molecular restricted motion as well as about the structure of medium.     

Probing short length scales with restricted diffusion in a static gradient using the CPMG sequence

We experimentally verify a new method of extracting the surface-to-volume ratio (S/V) of porous media with diffusion NMR. In contrast to the widely used pulsed field gradient (PFG) technique, which employs the stimulated echo coherence pathway, we use here the direct Carr-Purcell-Meiboom-Gill (CPMG) path. Even for high echoes, which exhibit ample attenuation due to diffusion in the field gradient, the relevant ruler length for the direct pathway is fixed by the diffusion length during a single inter-pulse spacing. The direct path, therefore, is well suited for probing shorter length scales than is possible with the conventional approach. In our experiments in a low-field static-gradient system, the direct CPMG pathway was found to be sensitive to structure an order of magnitude smaller than accessible with the stimulated-echo pathway.     

Feasibility of probing boundary morphology of structured materials by 2D NMR q-space imaging

It is well known that one-dimensional (1D) q-space imaging allows retrieval of structural information at cellular resolution. Here we demonstrate by simulation that boundary morphology of structured materials can be derived from 2D q-space mapping. Based on a finite-difference model for restricted diffusion, 2D q-space maps obtained from water diffusion inside apertures at various levels of asperity were simulated. The results indicate that the observed ring patterns (diffraction minima) reveal the boundary profiles of the apertures but become blurred in the case of significant variation in aperture size. For uniform size distribution of apertures, a quantitative measure of surface roughness can be established by means of spatial autocorrelation analysis. The results suggest that 2D q-space imaging may allow probing of the boundary morphology of structured materials and possibly biological cells.     

Experimental determination of pore shape and size using q-space NMR microscopy in the long diffusion-time limit

The signal obtained with q-space NMR imaging applied to a confined liquid is directly related to the pore shape in the limit where all molecules have sampled the whole pore. We investigate the diffusion of water across a approximately 50 microm thick film formed between planes of glass. The diffusion time t is changed almost three orders of magnitude. For short t, the root-mean-square displacement increases with a rate which is slightly less than for freely diffusing water. At t longer than 0.3 s, the displacement is constant at 24 microm which implies that the water is confined in the measuring direction defined by the applied gradient pulses. Perfectly smooth and aligned planes give rise to sharp diffraction-like features on the echo attenuation curve, i.e., NMR signal vs. the reciprocal space vector q. The experimental data with rather smooth local minima and maxima can be explained in terms of either surface roughness or a misalignment of the planes. We discuss the averaging effect of diffusion along a laterally inhomogeneous film and propose two model-free methods to determine the pore shape from the echo attenuation curve obtained in the long-t limit.     

Estimation of pore size in a microstructure phantom using the optimised gradient waveform diffusion weighted NMR sequence

There has been increasing interest in nuclear magnetic resonance (NMR) techniques that are sensitive to diffusion of molecules containing NMR visible nuclei for the estimation of microstructure parameters. A microstructure parameter of particular interest is pore radius distribution. A recent in silico study optimised the shape of the gradient waveform in diffusion weighted spin-echo experiments for estimating pore size. The study demonstrated that optimised gradient waveform (GEN) protocols improve pore radius estimates compared to optimised pulse gradient spin-echo (PGSE) protocols, particularly at shorter length scales. This study assesses the feasibility of implementing GEN protocols on a small bore 9.4 T scanner and verifies their additional sensitivity to pore radius. We implement GEN and PGSE protocols optimised for pore radii of 1, 2.5, 5, 7.5, 10 μm and constrained to maximum gradient strengths of 40, 80, 200 mT m(-1). We construct microstructure phantoms, which have a single pore radius for each phantom, using microcapillary fibres. The measured signal shows good agreement with simulated signal, strongly indicating that the GEN waveforms can be implemented on a 9.4 T system. We also demonstrate that GEN protocols provide improved sensitivity to the smaller pore radii when compared to optimised PGSE protocols, particularly at the lower gradient amplitudes investigated in this study. Our results suggest that this improved sensitivity of GEN protocols would be reflected in clinical scenarios.     

Assignment of coherence features in NMR q-space plots to particular diffusion modes in erythrocyte suspensions.

NMR q-space plots derived from water diffusing inside and around erythrocytes in a suspension display reproducible and characteristic coherence features. The aim of the present work was to determine which water population gives rise to the respective features. The central experimental strategy was to use choline and choline phosphate which are virtually membrane impermeant on the time scale of the experiment; the former was incorporated into erythrocytes by a lysis-resealing method and the latter was simply added to the suspensions. Dimethyl sulfoxide, which readily but more slowly exchanges across the cell membranes than water, also yielded q-space plots which were similar to those of water, but the differences were able to be accounted for on the basis of its slower transmembrane exchange rate. Random walk simulations using a Monte Carlo procedure, together with a model of an array of biconcave discocytes, helped verify the interpretations of the assignment of the features of the plots to molecules diffusing in the two regions. In addition, the simulations revealed how the presence or absence of transmembrane exchange affects the form of q-space plots.     

Magnetic studies with zero field NMR

A review is given of the information about a ferromagnetic system obtainable from swept frequency NMR. The distinction between signals from nuclei in domains and domain walls is stressed and recent advances in instrumentation described.     

Velocity autocorrelation spectra of fluid in porous media measured by the CPMG sequence and constant magnetic field gradient

Carr-Purcell-Meiboom-Gill (CPMG) train of radiofrequency pulses applied to spins in the constant magnetic field gradient is an efficient variant of the modulated magnetic field gradient spin echo method, which provides information about molecular diffusion in the frequency domain instead of in the time domain as with the two-pulse gradient spin echo. The frequency range of this novel technique is broad enough to sample the power spectrum of displacement fluctuation in water-saturated pulverized silica (SiO2) and provides comprehensive information about the molecular restricted motion as well as about the structure of the medium.     

Application of k- and q-space encoding NMR techniques on granular media in a 3D model fluidized bed reactor

A combination of PFG-NMR imaging and velocity encoding methods was applied to investigate the dynamic behavior of a bed of poppy seeds subjected to air flow, representing a model setup for fluidized bed reactors. The particle motion is described both from a statistical point of view, by determining propagators and dispersion coefficients representing an average over the whole bed volume, as well as combined with spatial resolution by generating velocity maps. Velocity images of different horizontal slices in the bed confirm the notion of a toroidal particle flow pattern inside the shallow granular bed. Despite the need of considerable averaging due to the random motion of the relatively few particles in the bed, quantitative velocity images and statistical information about the random particle motion can be obtained from monitoring the fluid component in the seeds by conventional spin-echo techniques.     

Combining PFG and CPMG NMR measurements for separate characterization of oil and water simultaneously present in a heterogeneous system

When analyzingT ₂ relaxation time curves from an ordinary Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information regarding the mobility of the components and on the type of environment experienced by each component. Compared to a standard CPMG experiment, a combination of pulsed field gradient (PFG) methods with the CPMG experiment will increase the amount of information that is obtainable from the nuclear magnetic resonance (NMR) experiment on a system of components differing significantly in molecular mobility. We propose a method for achieving separate measurements of theT ₂ attenuation of two components simultaneously present within a sample, and we believe it to be generally valid for any system in which the components differ significantly in molecular mobility. The two components could be oil and water in porous rock, or fat and water in a biological tissue, where a separation of theT ₂ attenuations for the two components will add insight to the study of the systems. In order to verify the method we made use of a sample containing a mixture of oil and water in two separate bulk phases, and compared the results with PFG-CPMG experiments performed on samples containing oil or water only, respectively. The method was applied to systems containing glass spheres immersed in water and oil, and it was possible to obtain information about the physical environment of the components which otherwise is not easily obtainable. The method presented here is therefore presumably applicable to whole rock cores or tissue samples.     

Solid-state NMR studies of supercapacitors

Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.