One-dimensional DOSY.

A new NMR experiment for correlating diffusion coefficients and chemical shifts is presented. This experiment provides the same information as the conventional DOSY experiment, but only requires a single dimension because a nonuniform magnetic field gradient is used to encode the diffusion information into the lineshapes of the peaks in the chemical shift dimension. By fitting the resulting lineshapes, the diffusion coefficient for each peak in the spectrum can be extracted. Using this experiment, a qualitative DOSY spectrum can be generated using the results from a single one-dimensional experiment. Quantitative results can be determined with the use of reference experiments.     

Two-dimensional nuclear magnetic resonance petrophysics

Two-dimensional nuclear magnetic resonance (2D NMR) opens a wide area for exploration in petrophysics and has significant impact to petroleum logging technology. When there are multiple fluids with different diffusion coefficients saturated in a porous medium, this information can be extracted and clearly delineated from CPMG measurements of such a system either using regular pulsing sequences or modified two window sequences. The 2D NMR plot with independent variables of T2 relaxation time and diffusion coefficient allows clear separation of oil and water signals in the rocks. This 2D concept can be extended to general studies of fluid-saturated porous media involving other combinations of two or more independent variables, such as chemical shift and T1/T2 relaxation time (reflecting pore size), proton population and diffusion contrast, etc.     

High resolution 13C DOSY: the DEPTSE experiment

High Resolution Diffusion-ordered Spectroscopy (HR-DOSY) is a valuable tool for mixture analysis by NMR. It separates the signals from different components according to their diffusion behavior, and can provide exquisite diffusion resolution when there is no signal overlap. In HR-DOSY experiments on (1)H (by far the most common nucleus used for DOSY) there is frequent signal overlap that confuses interpretation. In contrast, a (13)C spectrum usually has little overlap, and is in this respect a much better option for a DOSY experiment. The low signal-to-noise ratio is a critical limiting factor, but with recent technical advances such as cryogenic probes this problem is now less acute. The most widely-used pulse sequences for (13)C DOSY perform diffusion encoding with (1)H, using a stimulated echo in which half of the signal is lost. This signal loss can be avoided by encoding diffusion with (13)C in a spin echo experiment such as the DEPTSE pulse sequence described here.     

Xenon porometry: a novel method for the derivation of pore size distributions

Xenon porometry is a novel method used for characterizing porous materials by the (129)Xe nuclear magnetic resonance of xenon gas. With the method, the diffusion of gas is slowed down by immersing the material in a medium, which can be in liquid or solid state during measurements. Because of slow diffusion, the signal of a xenon atom is characteristic of the properties of only one pore, and the composite signal of all atoms represents the distribution of properties. The method is especially applicable for determining pore size distribution because the chemical shifts of two different xenon signals (one from liquid and the other from gas pockets in solid) are dependent on pore size. Therefore, the shapes of these signals represent pore size distribution function. In addition, the porosity of the material can be determined by comparing the intensities of two signals. This article focuses on describing xenon signals observed from gas pockets in a solid medium, which has turned out to be most convenient for pore size determination.     

13C solid-state NMR chromatography by magic angle spinning 1H T1 relaxation ordered spectroscopy

An efficient method to separate the ¹³C NMR spectra of solid mixtures is introduced. The ¹H longitudinal (T₁) relaxation time is used to separate the overlapping ¹³C chemical shift spectra of solid mixtures via an inverse Laplace transform (ILT) of the relaxation dimension. The resulting 2D spectrum of the mixture contains separate ¹³C spectra for each component of the mixture that are identical to ¹³C spectra of the isolated materials. The separation is based on the equalization of ¹H T₁ values in a single domain by rapid ¹H spin diffusion and on the ¹H T₁ value differences between different domains. The introduction of a general ILT scheme enables efficient and reduced data acquisition time. The method is demonstrated on a mixture of two disaccharides and on a commercial drug containing several compounds.     

Effect of impermeable interfaces on apparent diffusion coefficient in heterogeneous media

It is known that the short-time behavior of the diffusion coefficient, which is measurable by nuclear magnetic resonance (NMR), provides an estimate of the specific surface of porous samples filled with an NMR-detectable fluid. This method is not directly applicable to the exploration of structure of impermeable restrictions in mixtures such as living tissues, in which the inherent microscopic structure in the bulk medium results in the same pattern of the temporal dynamics of the diffusion coefficient as the impermeable restrictions studied. Here, an approach to describe diffusion and the boundary effect in heterogeneous media is developed in the framework of a cumulant expansion of the NMR signal. The leading term of this expansion is determined by the velocity autocorrelation function which is expressed in terms of properties of microscopic transport in the medium. Given these properties, the apparent diffusion coefficient as measured by NMR can be found by a straightforward integration. Calculations are performed in one spatial dimension.     

Enhanced spectral resolution by high-dimensional NMR using the filter diagonalization method and "hidden" dimensions

High-dimensional (HD) NMR spectra have poorer digital resolution than low-dimensional (LD) spectra, for a fixed amount of experiment time. This has led to "reduced-dimensionality" strategies, in which several LD projections of the HD NMR spectrum are acquired, each with higher digital resolution; an approximate HD spectrum is then inferred by some means. We propose a strategy that moves in the opposite direction, by adding more time dimensions to increase the information content of the data set, even if only a very sparse time grid is used in each dimension. The full HD time-domain data can be analyzed by the filter diagonalization method (FDM), yielding very narrow resonances along all of the frequency axes, even those with sparse sampling. Integrating over the added dimensions of HD FDM NMR spectra reconstitutes LD spectra with enhanced resolution, often more quickly than direct acquisition of the LD spectrum with a larger number of grid points in each of the fewer dimensions. If the extra-dimensions do not appear in the final spectrum, and are used solely to boost information content, we propose the moniker hidden-dimension NMR. This work shows that HD peaks have unmistakable frequency signatures that can be detected as single HD objects by an appropriate algorithm, even though their patterns would be tricky for a human operator to visualize or recognize, and even if digital resolution in an HD FT spectrum is very coarse compared with natural line widths.     

基于ADI-FDTD模拟的岩石核磁共振横向弛豫特征分析

将复杂的骨架-孔隙系统抽象成等效双孔介质,根据Bloch方程构建数学模型,用交替隐式时域有限差分(ADI-FDTD)和联合反演迭代法(SIRT)进行横向宏观磁化矢量的数值模拟与核磁共振T₂谱的反演,定量研究扩散系数、弛豫速率、孔隙组分比和孔隙宽度对核磁响应的影响.结果表明:横向宏观磁化矢量衰减速率与扩散系数和微孔隙分量成正比,与孔隙宽度成反比,与表面弛豫速率基本无关.当扩散系数较大、孔隙宽度较小时,核磁共振T₂谱难以直观反映孔隙组分及孔隙结构.应用核磁共振评价孔隙结构时需特别注意扩散系数和孔隙尺寸的影响.     

Analysis of protein/ligand interactions with NMR diffusion measurements: the importance of eliminating the protein background

Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) is a well-established method for the determination of translational diffusion coefficients. Recently, this method has found applicability in the combinatorial arena with the introduction of affinity NMR for characterizing protein/ligand interactions. Although affinity NMR has been reported to be an effective method for the identification of active compounds in a complex mixture, there are limitations of this method. We have developed a simple mathematical model to predict optimum concentration ratios of the ligand and protein in order to observe maximum changes in the ligand diffusion coefficient upon protein binding. The ligand/protein systems of L-tryptophan and ibuprofen binding to human serum albumin were chosen to demonstrate the usefulness of this model. However, even when the conditions of the mathematical model are satisfied, the spectral background arising from the protein in proton-detected experiments can be problematic. To this end, we have employed spectral subtraction of the protein spectrum to yield ligand diffusion coefficients that are in agreement with those predicted by simulation.     

Effects of macroscopic spinning upon linewidth of NMR signals of liquid in magnetically inhomogeneous systems

The results of a theoretical analysis of a simple liquid-solid phase system were used to estimate the limiting size of solid particles at which the line width of NMR signals of a liquid begins to be appreciably affected by diffusion of the liquid molecules. The limits of the effectiveness of the MAR-NMR method for removing the effects of magnetic inhomogeneity of the medium upon line width have been determined. The derived relations were verified by the measurement of conventional and MAR-NMR spectra of several real heterogeneous systems.     

Determination of structural and dynamic characteristics of biodegradable polymers by the NMR relaxation method

A theory of the NMR spectra of heterogeneous polymer systems is proposed that makes it possible to simulate signals observed over a wide temperature range with account of spectral diffusion. Based on this theory, a technique of rapid analysis of molecular structure parameters, including the degree of crystallinity and NMR line shape, is developed. The degree of crystallinity is demonstrated to be a linear function of the area under the NMR spectrum. It is shown that this dependence, universal for a given substance, makes it possible to reliably determine the degree of crystallinity over a wide temperature range (or the fraction of low-molecular additives in composites). The NMR spectra of chitosan is simulated and compared to experimental data. The universal dependence of the degree of crystallinity on the area under the spectrum of chitosan is calculated. A comparison with experimental signals allowed determining the degree of crystallinity.     

Applications of DOSY NMR in the Chemical IndustryApplications of DOSY NMR in the Chemical Industry

DOSY(diffusion ordered spectroscopy)NMR is a technique that separates NMR signals of different species according to their diffusion coefficients.It is a powerful technique to provide solid characterization evidence for various applications in the chemical industry.Encapsulation and functional polymer grafting are two important capabilities in the chemical industry for new product development with challenging characterization requirements.Model systems of encapsulation and grafted polymer were studied and the characterization methods were set up by DOSY NMR.     

烧结过程中Ni-Al金属间化合物形成的内耗

Ni-Al金属间化合物是一类重要的高温结构材料,在多种领域具有明确的目标需求.粉末冶金技术是制备Ni-Al金属间化合物的一种重要选择.探索烧结过程中Ni-Al金属间化合物形成和转变过程,明确固相扩散反应发生温度和金属间化合物种类对调控烧结工艺和优化产品质量至关重要.本文采用内耗技术系统研究了Ni-Al粉末混合物压坯烧结过程的内耗行为.在内耗-温度谱上观察到一个显著内耗峰,随测量频率的增大而降低,但峰温无明显频率依赖性.同时,内耗峰随升温速率的增大向高温方向移动且峰值增加.分析认为,该峰与升温过程中金属间化合物NiAl3和Ni2Al3的形成有关,属于典型的相变内耗峰.此外,机械球磨可调控Ni-Al粉末混合物的微观结构,内耗峰随球磨时间增加向低温方向移动且峰值降低,表明固相扩散反应可在低温区域以较低速率进行.这与球磨过程中粉末颗粒的细化、粉末混合物的片层化、固溶度和表面能的提高以及缩短的原子扩散路径有关.同时也表明机械球磨可有效降低固相扩散反应起始温度进而降低烧结温度.     

DEPT-(90+45),一个新的谱编辑技术

将DEPT-135序列的135°-脉冲分成一个90°-脉冲和一个45°-脉冲,设计了一个新的谱编辑技术,称为DEPT-(90+45).应用不同的相位循环,可以分别得到只有甲基的¹³C谱和只有亚甲基的¹³C谱.只有甲基的¹³C谱完全没有其他基团的残余干扰信号.但是,只有次甲基的¹³C谱(DEPT-90)和只有亚甲基的¹³C谱均混有所选基团之外的残余干扰信号.因此,DEPT-(90+45)将是一个非常有用的谱编辑技术.     

Restricted diffusion and exchange of water in porous media: average structure determination and size distribution resolved from the effect of local field gradients on the proton NMR spectrum

NMR Pulsed field gradient measurements of the restrained diffusion of confined fluids constitute an efficient method to probe the local geometry in porous media. In most practical cases, the diffusion decay, when limited to its principal part, can be considered as Gaussian leading to an apparent diffusion coefficient. The evolution of the latter as a function of the diffusion interval yields average information on the surface/volume ratio of porosities and on the tortuosity of the network. In this paper, we investigate porous model systems of packed spheres (polystyrene and glass) with known mean diameter and polydispersity, and, in addition, a real porous polystyrene material. Applying an Inverse Laplace Transformation in the second dimension reveals an evolution of the apparent diffusion coefficient as a function of the resonance frequency. This evolution is related to a similar evolution of the transverse relaxation time T2. These results clearly show that each resonance frequency in the water proton spectrum corresponds to a particular magnetic environment produced by a given pore geometry in the porous media. This is due to the presence of local field gradients induced by magnetic susceptibility differences at the liquid/solid interface and to slow exchange rates between different pores as compared to the frequency differences in the spectrum. This interpretation is nicely confirmed by a series of two-dimensional exchange experiments.     

The 2D-J-DOSY experiment: resolving diffusion coefficients in mixtures

Many diffusion-ordered spectroscopy (DOSY) NMR techniques have recently been developed to aid in the deconvolution of complex mixtures. Spectroscopic separation based on chemical and physical properties facilitates the identification of mixture components while eliminating time-consuming separation steps and preserving the chemical environment. One way to improve resolution in such experiments is to spread the spectroscopic information into two dimensions. The 2D-J-DOSY experiment has been designed to resolve mixture components in terms of a chemical shift and proton coupling constant as well as distinguishing them on the basis of translational diffusion. Acquiring a series of spectra as a function of gradient amplitude permits the determination of diffusion coefficients for components that cannot be resolved in the one-dimensional (1D) (1)H NMR spectrum. Comparison of the resulting values with those obtained through the traditional 1D diffusion experiment for a mixture of sugars validates The 2D-J-DOSY technique.     

New insights into silica-based NMR "chromatography"

Silica is used as an important component for NMR “chromatography”. In this study the effect of the binding strength to silica of a variety of compounds on their diffusion rate is measured for the first time. Over two orders of magnitude of diffusion difference enhancement was obtained in the presence of silica for some compounds. An explanation of the enhancement is given that also allows one to predict the “chromatographic” behavior of new compounds or mixtures. The binding strength is divided into categories of weakly bound, singly bound and multiply bound. Carboxylates, sulfonates, and diols are found to be particularly strongly bound and to diffuse up to 2½ orders of magnitude more slowly in the presence of silica.     

通过HSQC和1H,13C-COSY叠合增强二维谱分辨率的方法

通常二维谱F₁维分辨率较差,F₂维分辨率较好. 即HSQC实验¹³C维度分辨率较差,¹H维度分辨率较好. 而¹H, ¹³C-COSY实验¹³C维度分辨率较好,¹H维度分辨率较差. 提出一种通过HSQC和¹H, ¹³C-COSY叠合,取较弱的值储存并显示的方法增强分辨率,采用Bruker Topspin软件AU程序实现. 该方法在不明显增加采样时间的情况下,可显著增强分辨率,信噪比也没有降低. 同时抑制了F₁噪声. 处理后,谱图美观清晰.     

基于PDMS的NMR中色谱技术的分离性能研究

以聚二甲基硅氧烷(polydimethylsiloxane,PDMS)作为NMR中色谱技术的虚拟“固定相”,利用高分辨液体NMR中的DOSY技术考察其对正戊烷、1-溴戊烷和1,5-二溴戊烷这3种烷烃的分离性能．结果显示,在没有添加PDMS时,上述3种组分的扩散系数较接近,不能被分离;但当检测体系中含有PDMS时,3种组分的扩散系数相差较大,因而能被很好地分离．此外,还研究了样品温度和溶液粘度对分离性能的影响,其影响的趋势为：升高样品温度能改善混合物的分离;而增加溶液粘度则不利于混合物的分离．     

J-modulation effects in DOSY experiments and their suppression: the Oneshot45 experiment

Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for identifying compounds in mixtures. DOSY experiments are very demanding of spectral quality; even small deviations from expected behaviour in NMR signals can cause significant distortions in the diffusion domain. This is a particular problem when signals overlap, so it is very important to be able to acquire clean data with as little overlap as possible. DOSY experiments all suffer to a greater or lesser extent from multiplet phase distortions caused by J-modulation, requiring a trade-off between such distortions and gradient pulse width. Multiplet distortions increase spectral overlap and may cause unexpected and misleading apparent diffusion coefficients in DOSY spectra. These effects are described here and a simple and effective remedy, the addition of a 45° purging pulse immediately before the onset of acquisition to remove the unwanted anti-phase terms, is demonstrated. As well as affording significantly cleaner results, the new method allows much longer diffusion-encoding pulses to be used without problems from J-modulation, and hence greatly increases the range of molecular sizes that can be studied for coupled spin systems. The sensitivity loss is negligible and the added phase cycling is modest. The new method is illustrated for a widely-used general purpose DOSY pulse sequence, Oneshot.