Single-file diffusion behavior is expected for atoms and molecules in one-dimensional gas phases of nanochannels with transverse dimensions that do not allow for the particles to bypass each other. Although single-file diffusion may play an important role in a wide range of industrial catalytic, geologic, and biological processes, experimental evidence is scarce despite the fact that the dynamics differ substantially from ordinary diffusion. We demonstrate the application of continuous-flow laser-polarized 129 Xe NMR spectroscopy for the study of gas transport into the effectively one-dimensional channels of a microporous material. The novel methodology makes it possible to monitor diffusion over a time scale of tens of seconds, often inaccessible by conventional NMR experiments. The technique can also be applied to systems with very small mobility factors or diffusion constants that are difficult to determine by currently available methods for diffusion measurement. Experiments using xenon in nanochannel systems can distinguish between unidirectional diffusion and single-file diffusion. The experimental observations indicate that single-file behavior for xenon in an organic nanochannel is persistent even at long diffusion times of over tens of seconds. Finally, using continuousflow laser-polarized 129 Xe NMR spectroscopy, we describe an intriguing correlation between the observed NMR line shape of xenon within the nanochannels and the gas transport into these channels.
The triclinic form of AlPO 4 -34, a microporous aluminophosphate with the chabazite (CHA) topology, adopts a rhombohedral symmetry upon calcination. The framework structure of this phase remains intact under ambient conditions, but it distorts dramatically, though reversibly, in the presence of water. Following these structural changes in situ by X-ray diffraction revealed that there are actually two stable rehydrated phases, which differ from each other by one water molecule in the channel. Both of these phases have triclinic unit cells that are closely related to that of the calcined rhombohedral phase. The structure of the low-temperature (10 °C), fully rehydrated phase (phase B) was elucidated by combining high-resolution synchrotron powder diffraction with solid-state NMR techniques. Coordination of three of the six Al atoms to water molecules causes the deformation of the framework and the reduction of the symmetry. Rietveld refinement of the structure of phase B in the triclinic space group P1 (a ) 9.026, b ) 9.338, c ) 9.508 Å, R ) 95.1°, β ) 104.1°, and γ ) 96.6°) converged with R F ) 0.079 and R WP ) 0.176 (R exp ) 0.087). Framework connectivities derived from the structure were used to assign 31 P NMR lines as well as part of the 27 Al NMR signal.
Two-dimensional nuclear magnetic resonance (2D NMR) is increasingly explored as a tool for metabolomics because of its superior resolution compared to one-dimensional NMR (1D NMR). However, 2D NMR is characterized by longer acquisition times, which makes it less suitable for high-throughput studies. In this Article, we evaluated two methods for the acceleration of nD NMR, ultrafast (UF) and nonuniform sampling (NUS), in the context of metabolomics. To this end, model samples mimicking the metabolic profile variations in serum from subjects affected by colorectal cancer and controls were analyzed by 1D (1)H NMR along with conventional and accelerated DQF-COSY and HSQC. A statistical analysis (OPLS-DA) yielded similar results for the group separation with all techniques, but biomarker identification from 2D spectra was substantially enhanced, both in terms of number of molecules and easiness of assignment. Most interestingly, fast 2D NMR techniques lead to similar results as conventional 2D NMR, opening the way for high-throughput metabolomics studies using 2D NMR.
We introduce an analytical method that combines in one pot the advantages of column chromatography separation and NMR structural analysis. The separation of the NMR spectra of the components of a mixture can be achieved according to their apparent diffusion rates [James, T. L. and McDonald, G. G. (1973) J. Magn. Reson. 58, 58 -61]. We show that the separation of the spectral components, corresponding to single molecular species, can be enhanced by order of magnitudes upon addition of a typical stationary phase used in HPLC. The solid phase imbibed by the mixture for analysis is an heterogeneous ensemble, so that solidstate NMR methods (high-resolution magic angle spinning) are necessary to recover high-resolution spectra. We demonstrate applications of this combination of high-resolution magic angle spinning and NMR diffusometry on test mixtures for direct (silica gel) and inverse (C18) columns. However, many common chromatographic supports available for HPLC should be readily adaptable for use with this technique.T he challenge in the analysis of complex mixtures is 2-fold: to achieve an effective separation of the components and provide a proper structural characterization for each of them. The common solution is hyphenation, which is a sequential combination of chromatography and spectroscopic analysis such as mass or UV spectrometry. Because NMR is often the tool of choice for precise structural characterizations of organic molecules, an effort has been done to introduce NMR as a detector in hyphenated techniques (1). An alternative solution for the NMR characterization of mixture components exploits the behavior of the NMR signal of a molecule diffusing in an inhomogeneous magnetic field. In this case, the frequency of the observed target becomes time dependent, with consequent broadening of the resonance line (2). The apparent diffusion rate associated to a molecular resonance can be estimated by performing a series of experiments varying the amplitude of the inhomogeneous field and inverting the corresponding decay curve of the signal amplitude. Diffusion-ordered spectroscopy (DOSY) (3) is a particularly convenient means of displaying this information, organized in a bidimensional array with the NMR spectrum on one dimension and the apparent diffusion rate on the other one. As a collateral aspect, the DOSY display decomposes the overall NMR spectrum of a mixture into those of its components if these latter possess different molecular mobility, without previous collection of separate fractions (4-7). In optimal conditions, the diffusion experiment is capable of resolving contributions from molecules whose diffusion coefficients differ only by a few percent (8). In cases of serious signal overlapping, other methods for processing diffusion data achieve better spectral separation (9-11). We present here an analytical procedure that capitalizes on this idea, but in which the separation properties of the NMR method are enhanced by addition of a solid phase, a typical stationary phase material used for chromato...
The metabo-ring initiative brought together five nuclear magnetic resonance instruments (NMR) and 11 different mass spectrometers with the objective of assessing the reliability of untargeted metabolomics approaches in obtaining comparable metabolomics profiles. This was estimated by measuring the proportion of common spectral information extracted from the different LCMS and NMR platforms. Biological samples obtained from 2 different conditions were analysed by the partners using their own in-house protocols. Test #1 examined urine samples from adult volunteers either spiked or not spiked with 32 metabolite standards. Test #2 involved a low biological contrast situation comparing the plasma of rats fed a diet either supplemented or not with vitamin D. The spectral information from each instrument was assembled into separate statistical blocks. Correlations between blocks (e.g., instruments) were examined (RV coefficients) along with the structure of the common spectral information (common components and specific weights analysis). In addition, in Test #1, an outlier individual was blindly introduced, and its identification by the various platforms was evaluated. Despite large differences in the number of spectral features produced after post-processing and the heterogeneity of the analytical conditions and the data treatment, the spectral information both within (NMR and LCMS) and across methods (NMR vs. LCMS) was highly convergent (from 64 to 91 % on average). No effect of the LCMS instrumentation (TOF, QTOF, LTQ-Orbitrap) was noted. The outlier individual was best detected and characterised by LCMS instruments. In conclusion, untargeted metabolomics analyses report consistent information within and across instruments of various technologies, even without prior standardisation.Electronic supplementary materialThe online version of this article (doi:10.1007/s11306-014-0740-0) contains supplementary material, which is available to authorized users.
The performance of several different two-dimensional NMR methods for the measurement of carbon-proton dipolar couplings in liquid crystalline phases is analyzed. Proton-detected local field spectroscopy allows the measurements of short range C-H couplings, which correspond to directly bond pairs, by direct inspection of the spectra. Off magic angle (OMAS) spinning techniques can be applied to anisotropic phases that can be oriented mechanically at an angle to the magnetic field, such as nematic phases. The consequent scaling of the chemical shift anisotropy and dipolar couplings can be used to resolve otherwise overlapping resonances. Moreover, an estimate of the accuracy of the technique can be achieved by performing a series of OMAS experiments with different sample orientations.
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