NMR study of tomato pericarp tissue by spin-spin relaxation and water self-diffusion

Pericarp tissues of tomato varieties Quest and Cameron were studied by low-field nuclear magnetic resonance (NMR) at a controlled temperature of 20°C. The spin-spin relaxation times and the water diffusion coefficients were measured with Carr-Parcell-Meiboom-Gill and pulsed field gradient multi-spin-echo (PFGMSE) NMR sequences. Four relaxing components were extracted from the spin-spin relaxation. The components withT ₂=11 ms,T ₂=65 ms,T ₂=430 ms andT ₂=1500 ms were related to the nonexchangeable protons and water proton in each cell compartment (i.e., cell wall-extracellular space, cytoplasm and vacuole, respectively). In contrast to the relative intensities, theT ₂ values appeared insensitive to variety and harvest period. The difference in relative intensity was related to the size of the pericarp cell. The water self-diffusion coefficients for each cell compartment were determined simultaneously with the PFGMSE sequence. The water self-diffusion coefficients for the vacuole and cytoplasm were not affected by the harvest date or variety. However, the water self-diffusion in the cell wall-extracellular space was significantly different between the two varieties.     

具有表面弛豫的液体自扩散的Monte Carlo模拟

研究多孔介质中液体的自扩散行为能获得介质的微观结构信息,有助于了解介质中液体的传输性质.以具有不同孔隙大小的无限长圆柱体模型中的液体为对象,采用Monte Carlo随机行走方法,模拟存在表面弛豫时液体的自扩散系数和核自旋磁化强度随时间的变化,导出将NMR弛豫参数和随机行走参数联系在一起的表达式.结果表明:在快扩散区,液体的核自旋磁化强度随弛豫时间呈单指数衰减,且自扩散系数在短时情况下独立于表面弛豫率;在慢扩散区,液体的核自旋磁化强度衰减和自扩散系数在短时情况下均与表面弛豫率无关.模拟结果与理论分析相吻合,可用于求解介质的表面积与体积之比及孔径大小等结构信息.     

Molecular Diffusion and DNP Enhancement in Aqueous Char Suspensions

The heterogeneous¹H dynamic nuclear polarization (DNP) effect is studied at low magnetic fields for a system consisting of several newly synthesized carbon chars suspended in water. By using Fourier Transform pulsed-field-gradient spin–echo NMR spectroscopy, several different self-diffusion coefficients have been observed in aqueous char suspensions, corresponding to regions of differing water mobility in the porous structure. Proton spin–lattice relaxation data generally confirm the results of molecular diffusion measurements. Through utilization of the Torrey model, the influence of “cage effects” on DNP enhancement in porous media is discussed. Results suggest that short-range nuclear–electronic interactions in pores have a dominant effect on DNP enhancement in char suspensions.     

Water behavior in perfluorinated ion-exchange membranes

The water molecules in acid and salt forms of perfluorinated sulfocation membranes (MF-4SK) have been investigated by employing nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) techniques. The mobility parameters, correlation time and activation energy of water molecules were estimated from the results of the temperature dependence of¹H relaxation time and compared with water self-diffusion coefficients obtained with pulsed field gradient NMR. The NMR data showed no frozen unbound water in membranes at low water content with an amount of water molecules per sulfonate groupn being comparable to the cationic hydration numberh _o, whereas DSC thermograms showed peaks which are usually interpreted as a water fusion phenomenon in the membranes. The diffusion mechanism of water molecules below 260 K is different from that above 300 K due to additional hydrogen bonds in water clusters at the low-temperature region.     

Diffusion-weighted spatial information from1H relaxation in restricted geometries

Summary NMR relaxation of water¹H confined in restricted geometries, whatever is the nature of the system (porous media saturated by water as well as biological tissues), exhibits common characteristics. Artificial microporous media saturated by water have been chosen as model systems to study the longitudinal and transverse relaxation of¹H magnetization of water molecules diffusing in restricted geometries. These systems are very stable, easy to prepare, with well-characterized pore size distribution and connections, and with highly homogeneous surface properties. The response was compared with that from more complex natural porous media. Scanning Electron Microscopy techniques demonstrated spatial characteristics and surface properties of the samples. The information content of longitudinal relaxation curves associated with spatial structure and due to restricted diffusion is shown in these samples. The effect on transverse relaxation of self-diffusion in the presence of spatially varying magnetic fields due to susceptibility differences is shown. A simple linear relationship has been found in all samples between the transverse relaxation rate and the interpulse delay in CPMG experiments, in spite of the variety of pore shapes and sizes. In general, one can say that relaxation curves beardiffusion-weighted information on the pore space framework. The role of the investigated relaxation mechanisms is important also in the response of biological tissues, including in the presence of MR Imaging contrast agents inducing microscopic magnetic-field gradients. Work partially supported by CNR and MURST Grants.     

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.     

岩心孔隙介质中流体的核磁共振弛豫

弛豫时间是核磁共振研究中的一个重要参数,岩心孔隙介质流体的弛豫过程是自由流体弛豫机制、表面弛豫机制和流体的扩散弛豫机制共同作用的结果,它包含了丰富的孔隙和流体本身的信息. 弛豫时间和自扩散系数的测量及对弛豫时间的分析是核磁共振技术应用于岩心分析和石油勘测的重要内容.     

Electrophoretic NMR studies of electrical transport in fluid-filled porous systems.

An NMR technique is described which allows the observation of ionic charge carriers moving in the electric field within a porous system saturated with electrolyte solution. This method, which was recently developed in our laboratory, gives experimental access to the study of electric transport in disordered media on a microscopic level and offers new potential for morphology studies. We performed 1H NMR PFG self-diffusion measurements on ions combined with ionic drift velocity measurements by electrophoretic NMR (ENMR), each as a function of observation time Delta. In this way we obtained time-dependent self-diffusion coefficients D(+/-) (Delta) and time-dependent electric mobilities mu(+/-) (Delta) of polyatomic cations and anions in porous media. The porous media used were gels and glass bead packs. From the behaviour of D(+/-) (Delta) and mu(+/-) (Delta) at long observation times the tortuosities T(p) (D(+/-)) and T(p) (mu(+/-)) are derived, allowing a direct experimental check of the validity of the Einstein relation (D(+/-) is proportional to mu(+/-)) in a disordered medium. The tortuosities obtained via the diffusivity of ions are compared with those obtained via the diffusivity of water molecules. We also make a first attempt to derive the specific surface S/V(p) from the time-dependence of the ionic mobility at short observation times and discuss possible advantages of those measurements in morphology studies of porous media.     

Determination of water self-diffusion coefficient in complex food products by low field 1H PFG-NMR: comparison between the standard spin-echo sequence and the T1-weighted spin-echo sequence

In 1990, Van Den Enden et al. proposed a method for the determination of water droplet size distributions in emulsions using a pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) T1-weighted stimulated-echo technique. This paper describes both the T1-weighted spin-echo sequence, an improved method based on this earlier work, and, the standard PFG spin-echo sequence. These two methods were compared for water self-diffusion coefficient measurement in the fatty protein concentrate sample used as a 'cheese model.' The transversal and longitudinal relaxation parameters T1 and T2 were determined according to the temperature and investigated for each sample; fat-free protein concentrate sample, pure anhydrous milk fat, and fatty protein concentrate sample. The water self-diffusion in fat-free protein concentrate samples followed a linear behavior. Consequently, the water self-diffusion coefficient could be easily characterized for fat-free protein concentrate samples. However, it seemed more complicated to obtain accurate water self-diffusion in fatty protein concentrate samples since the diffusion-attenuation data were fitted by a bi-exponential function. This paper demonstrates that the implementation of the T1-weighted spin-echo sequence, using the different T1 properties of water and fat phases, allows the accurate determination of water self-diffusion coefficient in a food product. To minimize the contribution of the 1H nuclei in the fat phase on the NMR echo signal, the fat protons were selectively eliminated by an additional 180 degrees pulse. This new method reduces the standard errors of diffusion data obtained with a basic spin-echo technique, by a factor of 10. The effectiveness of the use of the T1-weighted spin-echo sequence to perform accurate water self-diffusion coefficients measurement in fatty products is thus demonstrated.     

NMR Study of Water Distribution inside Tomato Cells: Effects of Water Stress

Tomato pericarp tissue was studied by low-field nuclear magnetic resonance (NMR) relaxometry. Two kinds of experiments were performed to investigate the correlation between multi-exponential NMR relaxation and the subcellular compartments. The longitudinal (T ₁) versus transverse (T ₂) relaxation times were first measured on fresh samples and then the transverse relaxation time was measured on samples exposed to water stress. Four signal components were found in all experiments. The results showed that all signal components corresponded to the water in different cell compartments, and that no signal from non-exchangeable protons was present. Moreover, we demonstrated that NMR relaxation is suitable for the continuous monitoring of water rebalancing between subcellular compartments of plant tissues.     

Dynamic properties of water in silicalite-1 powder

Self-diffusion of D₂O in partially filled silicalite-1 crystals was studied at 25°C by ²H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10⁻⁷ to ∼10⁻¹⁰ m²/s, which include values much higher and lower than that of bulk water (∼10⁻⁹ m²/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.     

In situ fluid typing and quantification with 1D and 2D NMR logging

In situ nuclear magnetic resonance (NMR) fluid typing has recently gained momentum due to data acquisition and inversion algorithm enhancement of NMR logging tools. T(2) distributions derived from NMR logging contain information on bulk fluids and pore size distributions. However, the accuracy of fluid typing is greatly overshadowed by the overlap between T(2) peaks arising from different fluids with similar apparent T(2) relaxation times. Nevertheless, the shapes of T(2) distributions from different fluid components are often different and can be predetermined. Inversion with predetermined T(2) distributions allows us to perform fluid component decomposition to yield individual fluid volume ratios. Another effective method for in situ fluid typing is two-dimensional (2D) NMR logging, which results in proton population distribution as a function of T(2) relaxation time and fluid diffusion coefficient (or T(1) relaxation time). Since diffusion coefficients (or T(1) relaxation time) for different fluid components can be very different, it is relatively easy to separate oil (especially heavy oil) from water signal in a 2D NMR map and to perform accurate fluid typing. Combining NMR logging with resistivity and/or neutron/density logs provides a third method for in situ fluid typing. We shall describe these techniques with field examples.     

水驱油过程的核磁共振二维谱研究

岩心水驱油过程中油水分布状况是岩心多孔介质的重要性质. 水驱油过程的研究是进一步进行提高采收率研究的基础. 核磁共振扩散-弛豫二维谱提供了岩心中流体性质的多方面信息,与核磁共振一维弛豫谱相比极大地提高了区分油水的能力. 该文通过2组岩心水驱油实验,从不同含油饱和度的扩散-弛豫二维谱中提取出水的一维弛豫谱,在原油粘度比较高的情况下获得了驱替过程中油水在不同孔隙中的分布状况以及润湿性等信息, 解决了单独用一维弛豫谱方法难以区分油水的问题. 该文的研究方法对油田提高采收率的研究有比较大的参考价值.     

NMR relaxometry study of gelatin based low-calorie soft candies

ABSTRACT

Soft candies are popular confectionery products. The most significant concern on the consumption of these products is the high amount of sugar and thus the high calories. The use of low-calorie sweeteners is a desirable trend in confectionery research. In this study, gelatin-based soft candies were formulated by using different sweeteners and their characterisation was performed using high and low field nuclear magnetic resonance (NMR) relaxation experiments. To complement the information obtained by NMR experiments, moisture content, water activity, texture analysis and differential scanning calorimeter experiments were also conducted. T₁ and T₂ relaxation times were measured at both low and high fields and also temperature-dependent measurements were conducted at the high field system. Candies were formulated by substitution of sucrose with maltitol, isomalt and stevia at 30%, 50% and 70% ratios. Significant difference was observed on relaxation times. T₁ values were best described by a mono-exponential model, whereas for T₂ relaxation times a bi-exponential model gave better results at both fields.     

Diffusion measurements with the aid of nutation spin echoes appearing after two inhomogeneous radiofrequency pulses in inhomogeneous magnetic fields

Nutation echoes are generated by radiofrequency (RF) pulses with an inhomogeneous amplitude, B(1) = B(1)(r), in inhomogeneous magnetic fields, B(0) = B(0)(r). The two gradients of strengths G(1) and G(0), respectively, must be aligned in parallel for a maximum echo signal. After two RF pulses, two echoes appear at times tau(a) = 2 tau(1) + tau(2) + (G(1)/G(0))tau(1) and tau(b) = 2 tau(1) + tau(2) + 2(G(1)/G(0))tau(1), where tau(1) is the RF pulse duration and tau(2) the interpulse interval. It is shown that these echoes can favorably be employed for the determination of self-diffusion coefficients even in the poor experimental situation one often faces in low-resolution or low-field NMR. The signal intensity is comparable to that of ordinary Hahn echoes. Diffusion coefficients and spin-lattice relaxation times can be evaluated from the same experimental data set if both nutation echoes are recorded. Test experiments are in good agreement with literature data. Applications of the technique to "inside out" NMR, well logging NMR, surface coil NMR, toroid cavity NMR, etc., are suggested.     

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurement of the relaxation times T₁ and T₂ provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T₁ was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T₂ was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times τ_c for different types of intracellular water were calculated according to a simple two-phase model. The values for T₁, T₂, and proton spin density (i.e., water content) are: nucleus ⪢ animal cytoplasm > vegetal cytoplasm. Based on the estimation of τ_c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.     

NMR Investigations of the Protonic Transport Mechanism in Composed Materials on the Basis of Cesium Acid Sulfates and Phosphates

The composite materials Cs(HSO₄)_1?x (H₂PO₄) _x were investigated by X-ray phase analysis, differential scanning calorimetry, nuclear magnetic resonance (NMR) relaxation, pulsed field gradient NMR (PFG-NMR) and impedance spectroscopy. Three composite materials types x = 0.1 ÷ 0.3 mixture CsHSO₄, α-Cs₃(HSO₄)₂(H₂PO₄), β-Cs₃(HSO₄)_2.5(H₂PO₄)_0.5—compositions of area I; x = 0.4 ÷ 0.5 mixture α-Cs₃(HSO₄)₂(H₂PO₄) and Cs₂(HSO₄)(H₂PO₄)—compositions of area II; x = 0.6 ÷ 0.9 mixture Cs₂(HSO₄)(H₂PO₄) and CsH₂PO₄—compositions of area III, were synthesized. The phase transition temperature from the low-to-high conductive phase for obtained composite materials is notably below (about 100 °C) than that for the individual components. The proton self-diffusion coefficients measured by PFG-NMR are lower than the diffusion coefficients calculated from proton conductivities data. The correlation times τ _d controlling the ³¹P–¹H magnetic dipole–dipole interaction were calculated according to data of the spin–lattice relaxation on ³¹P nuclei. The self-diffusion coefficients estimated from the Einstein equation are in good agreement with the experimental self-diffusion coefficients measured by PFG-NMR. It confirms the fact that the proton mobility is caused by the rotation of PO₄ anion tetrahedra.     

Self-diffusion of water and benzene molecules adsorbed in synthetic opal samples

The self-diffusion of adsorbed molecules of water and benzene in porous synthetic opals has been studied by pulsed field gradient nuclear magnetic resonance. The study indicates two “phases” of water molecules differing by the self-diffusion coefficients, indicating two types of porosities existing in the synthetic opals. The smallest self-diffusion coefficient characterizes water in ultramicropores found on the surface of nanospheres. The form of the diffusion decay depends on temperature in the region of high temperatures as a result of exchange between pores of different sizes. The ultramicropores are inaccessible for the largest benzene molecules. Water adsorption and self-diffusion data confirm that annealing of the opal samples at 1293 K caused the collapse of ultramicropores.     

An NMR-Based Analysis of Soil–Water Characteristics

Both direct and indirect methods for determining soil–water characteristic curves rely on determination of some empirical coefficients, which may not necessarily represent real microscopic mechanisms. Proton nuclear magnetic resonance (NMR) is a powerful tool for investigating water content and their interaction with solid particles in porous media. The NMR technique is widely used in food science and petroleum. In the present study, proton NMR spin–spin relaxation time (T ₂) distribution measurement is integrated with a Tempe apparatus to characterize the hydraulic processes of unsaturated soils, shedding insights into the microscopic mechanisms of pore water distribution and migration in the soil during hydraulic cycles. It is revealed that during a drying process the drainage of pore water occurs sequentially from larger pores to smaller pores, whereas in a wetting process the water invades into the soil sequentially from smaller pores to larger pores. A new procedure is developed which can be used to determine the pore size distribution of the soil based on the NMR T ₂ distribution measurements; compared to the traditional methods, the new method is rapid and non-destructive. The new procedure is validated by comparing the new result with the measurement of the mercury intrusion porosimetry.     

Structural and dynamic properties of polyoxyethylene sorbitan monooleate micelle in water dispersion studied by pulsed field gradient NMR

The diffusion phenomenon of a nonionic surfactant, polyoxyethylene sorbitan monooleate (POE-SMO), micelle in aqueous solution was investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR) with a high gradient strength of 17.4 T/m at the diffusion timet _d varied from 3 to 300 ms. This high gradient strength allowed us to measure the slow self-diffusion coefficient of POE-SMO micelle, and the short diffusion time below 10 ms showed the restricted diffusion of the micelle. At the shortt _d the self-diffusion of the micelle was restricted and the restricted sizes were 1.8, 1.5, and 0.8 μm for the POE-SMO concentration of 100, 200 and 300 mM, respectively, and 0.6 μm for the POE-SMO only. The possible reason of this restriction was assumed to be the formation of a spatial network or a micellar clustering. Furthermore, a proton exchange between water molecule and surfactant OH group on the micelle surface was proposed. With respect to this proposal, the residence time of the proton at the micelle surface and the thickness of the surface were investigated from proton self-diffusion coefficients by PFG NMR.