Journal of Magnetic Resonance Open最新文献

Chemical exchange saturation transfer (CEST) NMR is widely used for enhancing the sensitivity of low-abundance exchanging sites in general, and for the water-based detection of labile metabolite protons under in vivo conditions in particular. CEST, however, faces a number of limitations when targeting multiple metabolites, including a radiofrequency (RF)-induced broadening of the detected peaks, and relatively long acquisition times deriving from its continuous-wave nature. Methods have been proposed to overcome these limitations, including a Fourier-encoded version of CEST –the Frequency-Labeled EXchange (FLEX) experiment– and the incorporation of background gradients during the RF saturation time. This work explores an alternative avenue, based on spatiotemporally encoded ultrafast (UF) 2D NMR. UF NMR can compress the time-domain indirect-dimension encoding of 2D NMR into a single shot; to exploit these potential time savings, an UF version of the FLEX experiment was taken as starting point, and the multiple t₁-incremented amplitude modulation cycles that the FLEX experiment normally requires were replaced by a single-shot spatiotemporal encoding. The ensuing UF 2D FLEX experiment was then used to monitor the spectral signatures of multiple moieties as they exchange with the solvent, by imprinting these onto the water resonance as in the original experiment –but now all within a single shot. Upon incorporating two-scan phase cycling and quadrature detection, the resulting method showed an experimental performance similar to t₁-encoded FLEX, while providing significant time savings plus imaging information that could be of further use in in vivo studies. The main advantages, features and drawbacks observed for UF 2D FLEX are briefly discussed.

Nuclear magnetic resonance is widely used to probe the dynamics of fluids confined in porous media, where structural and functional properties of complex systems can be determined. The application includes a large variety of research or industrial areas such as medicine, gas and oil extraction, soil studies, the development of materials for drug delivery, and catalysis, among many other applications. This review covers the use of different NMR experiments applied to study the liquid/surface interaction, mobility, tortuosity, pore connectivity, and exchange phenomena in different porous matrixes. This article, included in the special issue devoted to NMR in Latin America, provides a review of the most applied techniques and a summary of different applications carried out at the Universidad Nacional de Córdoba in Argentina.

In this manuscript we deal with recent advances in low-field Magnetic Resonance Imaging (MRI). The development of low-cost MRI solutions allowing portability and trustable diagnosis is a hot topic worldwide by these days. We analyze basic technical issues of recent examples of fixed-field instruments operating at low-field. Then we discuss pros and cons of the pre-polarized approach, from both physical and technical perspectives. Permanent magnet and electromagnet technology are confronted. Finally, magnetic field-cycling is introduced as an alternative MRI technique, where field-dependent experiments can be exploded for the development of new contrast mechanisms that are not feasible for fixed-field MRI instruments. As field cycled machines usually deals with switched currents in electromagnets, magnetic field instability and inhomogeneity are the main limiting factors affecting image quality. We finalize this manuscript discussing how it turns possible to overcome these limitations, thus opening new possibilities for the development of cost effective MRI technology.

Nanoparticles of LiNbO₃ and NaNbO₃ were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO₃, there are two polimorphs which are stable at room temperature, space groups P2₁ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state ²³Na and ⁹³Nb Nuclear Magnetic Resonance (NMR) techniques. ²³Na results reveal the presence of both phases, Pbma and P2₁ma, for samples prepared using NaNO₃ precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P2₁ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO₃ nanoparticles display the usual rhombohedral structure R3c. ⁷Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li⁺ ions are found at the surface of the nanoparticles, while bulk Li⁺ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.

Multivariate Curve Resolution (MCR) is a multivariate analysis procedure commonly used to analyze spectroscopic data providing the number of components coexisting in a chemical system, the pure spectra of the components as well as their concentration profiles. Usually, this procedure relies on the existence of distinct systematic variability among spectra of the different samples, which is provided by different sources of variation associated to differences in samples origin, composition, physical chemical treatment, etc. In solid-state NMR, MCR has been also used as a post-processing method for spectral denoising or editing based on a given NMR property. In this type of use, the variability is induced by the incrementation of a given parameter in the pulse-sequence, which encodes the separation property in the acquired spectra. In this article we further explore the idea of using a specific pulse sequence to induce a controlled variability in the ¹³C solid-state NMR spectra and then apply MCR to separate the pure spectra of the components according to the properties associated to the induced variability. We build upon a previous study of sugarcane bagasse where a series of ¹³C solid-state NMR spectra acquired with the Torchia-T₁ CPMAS pulse sequence, with varying relaxation periods, was combined with different sample treatments, to estimate individual ¹³C solid-state NMR spectra of different molecular components (cellulose, xylan and lignin). Using the same pulse sequence, we show other application examples to demonstrate the potentiality, parameter optimization and/or establish the limitations of the procedure. As a first proof of principle, we apply the approach to commercial semicrystalline medium density polyethylene (MDPE) and polyether ether ketone (PEEK) providing the estimation of the individual ¹³C ssNMR spectra of the polymer chains in the amorphous (short $T_1$) and crystalline (long $T_1$) domains. The analysis also provided the relative intensities of each estimated pure spectra, which are related to the characteristic $T_1$ decays of the amorphous and crystalline domain fractions. We also apply the analysis to isotactic poly (1-butene) (iPB-I) as an example in which the induced $T_1$ variability occurs due to the mobility difference between the polymer backbone and side-chains. A jack-knifing procedure and a student t text allow us to stablish the minimum number of spectra and the range of relaxation periods that need to be used to achieve a precise estimation of the individual pure spectra and their relative intensities. A detail discussion about possible drawbacks, applications to more complex systems, and potential extensions to other type of ind