NMR applications to low porosity carbonate stones

The purpose of this paper is to investigate NMR applications to porous materials widely employed in artistic and historical monuments and largely studied in the Cultural Heritage conservation field. Carrara marble, Candoglia marble and travertine samples were studied and data from relaxation times measurements were compared. Very interesting results from treated samples are reported and explained under the structure related spin lattice relaxation time point of view. Images of Carrara marble aged sample (XIX century), coming from the Florence Cathedral obtained for short absorption time of water by capillary rise and for relatively small thickness slices together show the fluid's spatial distribution within the stone. Comparative images showing untreated sample with the treated ones were obtained suggesting very useful applications for the determination of treatment effectiveness.     

Effects of hydrophobic treatments of stone on pore water studied by continuous distribution analysis of NMR relaxation times.

The effects of protective hydrophobic products applied to porous media such as stone or mortar vary greatly with the product, the porous medium, and the mode of application. Nuclear Magnetic Resonance (NMR) measurements on fluids in the pore spaces of both treated and untreated samples can give information on the contact of the fluid with the internal surfaces, which is affected by all the above factors. Continuous distributions of relaxation times T(1) and T(2) of water in the pores of both synthetic and natural porous media were obtained before and after hydrophobic treatment. The synthetic porous media are ceramic filter materials characterized by narrow distributions of pore dimensions and show that the treatment does not produce large changes in the relaxation times of the water. For three travertine samples most of a long relaxation time component, presumably from the largest pores, remains after treatment, while the amplitude of an intermediate component is greatly reduced. For three pudding-stone samples, treatment leads to a substantial loss from the long component and an even greater loss from the intermediate component.     

Paramagnetic relaxation in sandstones: distinguishing T1 and T2 dependence on surface relaxation, internal gradients and dependence on echo spacing

This work provides a generalized theory of proton relaxation in inhomogeneous magnetic fields. Three asymptotic regimes of relaxation are identified depending on the shortest characteristic time scale. Numerical simulations illustrate that the relaxation characteristics in the regimes such as the T(1)/T(2) ratio and echo spacing dependence are determined by the time scales. The theoretical interpretation is validated for fluid relaxation in porous media in which field inhomogeneity is induced due to susceptibility contrast of fluids and paramagnetic sites on pore surfaces. From a set of measurements on model porous media, we conclude that when the sites are small enough, no dependence on echo spacing is observed with conventional low-field NMR spectrometers. Echo spacing dependence is observed when the paramagnetic materials become large enough or form a 'shell' around each grain such that the length scale of the region of induced magnetic gradients is large compared to the diffusion length during the time of the echo spacing. The theory can aid in interpretation of diffusion measurements in porous media as well as imaging experiments in presence of contrast agents used in MRI.     

Pore surface exploration by NMR

A carefully chosen set of experimental techniques applied to porous media characterization provides results that can be much greater than the sum of the individual parts. The inter-relation and complementarity of a number of techniques will be considered. NMR cryoporometry provides a valuable method of pore size measurement. An NMR method that is more widely used to assess pore dimensions relies on relaxation time analysis of a liquid that fills the pores and the enhanced relaxation that occurs in a liquid at the solid/liquid interface. Thermoporometry, a method based on the application of Differential Scanning Calorimetry (DSC), is closely related to cryoporometry, but employs a different set of assumptions to evaluate pore size distributions. Comparison of the results obtained on the same samples using all these methods together with gas adsorption serves to validate the methods and provide significantly more information about surface-fluid interaction and the behavior of nano-scale material within pores than each method employed in isolation. Technique developments will be discussed and applications of these methods to ideal silicas will be used to illustrate their power, particularly in combination.     

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.     

储油岩芯二相液体吸附特性的NMR研究及应用

为研究储油岩芯对液体的吸附机理,我们特地用同一岩芯制成以下几种样品:(1)吸附不同量的煤油;(2)吸附不同量的水;(3)先吸附一定量的水再吸附一定量的煤油.对以上三组样品,做了NMR自旋—晶格弛豫时间测定;考查了岩芯样品中各弛豫成分量与液体吸附量的变化规律;得出了岩芯内水和煤油受束缚越紧其弛豫时间越短的结论.并依据大量石油地质学的结论建立了砂岩孔隙中油水二相系统的核磁共振模型.     

低场核磁共振：弛豫和扩散的多维实验

近年来，低场核磁共振日益展示其在弛豫与扩散测量中的有效应用。在材料研究领域，诸如催化剂的研制、水泥的水化、岩石及土壤中的液体输运等，以及地质勘探和医疗诊断学中的人体组织性质的表征方面，核磁共振的弛豫与扩散测量都十分重要。特别是此技术在多维度方法方面的发展，大大促进了其在多孔材料方面的应用。多孔材料在人们生活的环境中无所不在，其微结构（微米级至毫米级）对于其性质和应用方面，具有决定性的作用。该文概述低场核磁共振的弛豫及扩散的多维度技术的基本原理、核磁共振方法学及其应用的若干关键性进展。     

Fast-field-cycling NMR: Applications and instrumentation

Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magnetic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical “flying time” being a few hundreds of milliseconds. As a natural evolution of the experimental technique and the need to extend its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special electric networks combined with sophisticated air core magnets allowed one to switch magnetic fields between zero and fields of the order of 0.5 T in a few milliseconds. Today we refer to this new generation of instruments as “fast-field-cycling” devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turning to be an important tool for the design of contrast agents for magnetic resonance imaging. Fast field cycling was mainly oriented toT ₁ relaxometry as a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventional applications of the technique that can also provide relevant information concerning molecular dynamics, structure and molecular order. In this article we will briefly deal with basic aspects of the technique, its evolution, present-day relevant applications and the last improvements concerning specialized instrumentation.     

Selective imaging of biofilms in porous media by NMR relaxation.

Nuclear magnetic resonance imaging (NMRI) techniques were employed to identify and selectively image biological films (biofilm) growing in aqueous systems. Biofilms are shown to affect both the longitudinal (T1) and transverse (T2) NMR relaxation time values of proximal water hydrogens. Results are shown for biofilm growth experiments performed in a transparent parallel-plate reactor. A comparison of biofilm distributions by both NMR and optical imaging yielded general agreement for both an open-flow system and an idealized porous system (the reactor without and with packed glass beads, respectively). The selective imaging of biofilm by relaxation NMRI is dependent upon the resolution of relaxation times for the fluid phases, dynamic range, and signal-to-noise ratio. For open-flow systems, the use of a rapid and quantitative T2-sorted NMRI technique was preferred. For porous systems where T2 values are generally more similar, a T1-weighted technique was preferred.     

Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering

The artificial oxalate protection method was analyzed in laboratory experiments in order to achieve an optimum treatment application and concentration giving rise to its most effective protective nature. Spectroscopic (Fourier transform infrared, Micro-Raman and UV-vis colorimetry), microscopic (scanning electron microscope) and contact-angle analyses were carried out to characterize Carrara marble samples before and after application of the treatment to validate its efficiency. The resistance effects against chemical weathering were subsequently observed in a lab-controlled weak acid rain experiment. An acid spray at pH 5.5, representative of normal rain was used to provoke degrade of natural marble, marble treated with the artificial oxalate protective at concentrations of 0.4 and 5% and marble treated with a commercial organic silicon product. Run-off solutions sampled at timely intervals were tested for any change in pH followed by ion chromatography measurements for the presence of calcium ions in solution. The chromatography results of the oxalate treatment applied at a 5% concentration are analogous to an organic commercial product indicating its validity as a method for the conservation of carbonate substrates conferring protection to stone materials against acid environments.     

A nuclear magnetic resonance microscopy study of mass transport in porous materials

The¹H nuclear magnetic resonance (NMR) microimaging is employed to study the mass transport processes in porous materials, including individual catalyst support pellets and beds comprised of porous grains. Drying and adsorption are investigated by detecting the temporal evolution of the one-dimensional spatial profiles or two-dimensional maps of liquid content without interrupting the process under study. The characteristic features of these processes, such as the main mechanisms and the limiting stages of mass transport, and some factors which can alter the efficiency of mass transport are considered. The problems associated with the relaxation weighting of the NMR signals, often unavoidable for liquids permeating mesoporous solids, and the possibilities to overcome these problems are discussed.     

Sodium NMR relaxation in porous materials

The NMR relaxation of hydrogen nuclei of a fluid in a porous material is generally interpreted in terms of the Brownstein-Tarr model, in which the relaxation rate of the signal is inversely proportional to the pore size. We have investigated whether this model can be applied to the relaxation of Na nuclei in a NaCl solution in a porous material. The results indicate that the ion distribution over the pores can be obtained from an analysis of the Na NMR signal decay, if the pore sizes are roughly below 1 microm. This information is very useful for studies of combined moisture and ion transport in porous building materials.     

NMR 1D-imaging of water infiltration into mesoporous matrices

It is shown that coupling nuclear magnetic resonance (NMR) 1D-imaging with the measure of NMR relaxation times and self-diffusion coefficients can be a very powerful approach to investigate fluid infiltration into porous media. Such an experimental design was used to study the very slow seeping of pure water into hydrophobic materials. We consider here three model samples of nuclear waste conditioning matrices which consist in a dispersion of NaNO₃ (highly soluble) and/or BaSO₄ (poorly soluble) salt grains embedded in a bitumen matrix. Beyond studying the moisture progression according to the sample depth, we analyze the water NMR relaxation times and self-diffusion coefficients along its 1D-concentration profile to obtain spatially resolved information on the solution properties and on the porous structure at different scales. It is also shown that, when the relaxation or self-diffusion properties are multimodal, the 1D-profile of each water population is recovered. Three main levels of information were disclosed along the depth-profiles. They concern (i) the water uptake kinetics, (ii) the salinity and the molecular dynamics of the infiltrated solutions and (iii) the microstructure of the water-filled porosities: open networks coexisting with closed pores. All these findings were fully validated and enriched by NMR cryoporometry experiments and by performing environmental scanning electronic microscopy observations. Surprisingly, results clearly show that insoluble salts enhance the water progression and thereby increase the capability of the material to uptake water.     

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.     

Probing Structural Compositions of Porous Media with Two-dimensional Nuclear Magnetic Resonance

Combination of two-dimensional nuclear magnetic resonance (2-D NMR) correlation maps and a simple three-component model are proposed here to identify the structural composition of porous media. Homogeneous magnetic field and field with constant gradient of a novel Halbach sensor are employed for respective relaxation and diffusion measurements. NMR results are compared and confirmed with independent measurements on a scanning electron microscope and by energy dispersive spectrometer methods.     

Nuclear magnetic resonance in contemporary art: the case of “Moon Surface” by Turcato

Nuclear Magnetic Resonance (NMR) methodologies were applied to characterize the constitutive materials and the state of degradation of a contemporary painting. The investigation was mandatory to plan a suitable restoration. Noninvasive, portable NMR allowed the detection of degraded regions of the painting based on the measurement of longitudinal relaxation time. A few samples were investigated by high resolution solid state NMR and NMR in solution, which allowed us to identify the polyurethane constituting the artefact, to investigate the microstructure in detail, and to assess that the degradation process mostly affected the ethylene units used to cap the polypropylene oxide polymeric chain. As a matter of fact, a shortening of longitudinal relaxation time was accompanied by a degradation of ethylene units. The degradation of the inorganic loading was investigated by ²⁷Al MAS, which evidenced the absence of penta-coordinated aluminum in degraded samples.     

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.     

Measurement of thermal diffusivity by magnetic resonance imaging

Nuclear magnetic resonance (NMR) may be used for monitoring temperature changes within samples based on measurements of relaxation times, the diffusion coefficient of liquids, proton resonance frequency or phase shifts. Such methods may be extended to the explicit measurement of the thermal diffusivity of materials by NMR imaging. A method based on measuring nuclear spin phase shifts or changes in the equilibrium nuclear magnetization has been developed for measuring transient thermal diffusion effects and thermal diffusivity with potential applications in NMR thermotherapy and materials science. In this method, a thermal pulse is applied to a medium, and the resultant temporal variations of the nuclear spin phase or of the magnitude of the nuclear magnetization produced by the thermal pulse are monitored at a spatial distance. The results obtained on common fluids agree well with the data from other methods.     

Quadrupolar relaxation of hyperpolarized krypton-83 as a probe for surfaces

This work reports the first systematic study of relaxation experienced by the hyperpolarized (hp) noble gas isotope (83)Kr (I=9/2) in contact with surfaces. The spin-lattice relaxation of (83)Kr is found to depend strongly on the chemical composition of the surfaces in the vicinity of the gas. This effect is caused by quadrupolar interactions during brief periods of surface adsorption that are the dominating source of longitudinal spin relaxation in the (83)Kr atoms. Simple model systems of closest packed glass beads with uniform but variable bead sizes are used for the relaxation measurements. The observed relaxation rates depend strongly on the chemical treatment of the glass surfaces and on the surface to volume ratio. Hp (83)Kr NMR relaxation measurements of porous polymers with pore sizes of 70-250 microm demonstrate the potential use of this new technique for material sciences applications.     

Analysis of structure and properties of active carbons and their copolymeric precursors

The relations between chemical structures of BM-DVB copolymers obtained with various monomer molar ratios and their carbonization products were studied. Three porous copolymers 1:4, 1:1, and 4:1 of BM to DVB were the starting materials for preparation of active carbons. Two activation agents were employed: air and phosphoric acid. The carbonization process was performed in the same way in these two cases. To characterize the obtained materials FTIR spectroscopy, thermal and elemental analyses were applied. Porous structure parameters were obtained by means of nitrogen sorption. The results proved that differences in the molar ratio of monomers used in the syntheses of polymeric precursor play a key role for structure and properties of copolymers but have rather small influence on properties of the obtained carbons. Preliminary treatment is more effective during the activation process. The carbons obtained by activation with phosphoric acid are microporous and have well developed porous structures. The air activated carbons are mesoporous with specific surface areas similar to those of polymeric precursors.