In situ monitoring of mechanochemical MOF formation by NMR relaxation time correlation.

In this paper, we present a new approach to monitoring mechanochemical transformations, based on a magnetic resonance (MR) method in which relaxation time correlation maps are used to track the formation of the popular metal-organic framework (MOF) materials Zn-MOF-74 and ZIF-8. The two-dimensional (2D) relaxation correlation measurement employed yields a spectrum which visually and analytically identifies different 1H environments in the sample of interest. The measurement is well-suited to analyzing solid mixtures, and liquids, in complex systems. Application in this work to monitoring MOF formation shows changes in signal amplitudes, and their MR lifetime coordinates, within the 2D plots as the reaction progresses, confirming reaction completion. This new measurement provides a simple way to analyse solid-state reactions without dissolution, and there is a logical pathway to benchtop measurement with a new generation of permanent magnet-based MR instruments. The methodology described permits measurement in an MR compatible milling container, which may be directly transferred from the shaker assembly to the MR magnet for in situ measurement of the entire reaction mixture.

The Global Characterisation of a Drug-Dendrimer Conjugate - PEGylated poly-lysine Dendrimer.

The recent emergence of drug-dendrimer conjugates within pharmaceutical industry research and development introduces a range of challenges for analytical and measurement science. These molecules are very high molecular weight (100-200kDa) with a significant degree of structural complexity. The characteristics and quality attributes that require understanding and definition, and impact efficacy and safety, are diverse. They relate to the intact conjugate, the various building blocks of these complex systems and the level of the free and bound active pharmaceutical ingredient (API). From an analytical and measurement science perspective, this necessitates the measurement of the molecular weight, impurity characterisation, the quantitation of the number of conjugated versus free API molecules, the determination of the impurity profiles of the building blocks, primary structure and both particle size and morphology. Here we report the first example of a global characterisation of a drug-dendrimer conjugate - PEGylated poly-lysine dendrimer currently under development (AZD0466). The impact of the wide variety of analytical and measurement techniques on the overall understanding of this complex molecular entity is discussed, with the relative capabilities of the various approaches compared. The results of this study are an essential platform for the research and development of the future generations of related dendrimer-based medicines.

Studies of periodic seawater spray icing with unilateral NMR.

Sea water ice has a complicated pore structure consisting of brine-filled pockets within a solid ice matrix. In this work, an unilateral Nuclear Magnetic Resonance instrument was used to characterize the evolution of sea-ice brine inclusions in two types of ice: stationary seawater ice and seawater spray ice formed by periodic spraying on horizontal and vertical surfaces. The portable unilateral NMR was capable of measuring very low amounts of brine (<10% of the water-filled volume). CPMG experiments were performed to extract the information on brine content and T2 distribution at temperatures between -6 °C and -16 °C. 1D imaging was used to spatially resolve the ice brine accumulation. The results show that the seawater spray ice growth, brine content (greater for the horizontal orientation than for the vertical one), and T2 distribution (unimodal for stationary ice and bimodal for spray ice) depend on temperature and surface orientation.

Magnetic resonance T1-T2* and T1ρ-T2* relaxation correlation measurements in solid-like materials with non-exponential decays.

Magnetic resonance T1-T2* relaxation correlation is a newly emerging and powerful tool to study the structure and dynamics of materials. However, the T1-T2* of solid-like materials may consist of a linear combination of exponential decays and non-exponential decays, and the traditional methods for processing T1-T2 data would be not applicable. In this paper, a method of processing T1-T2* data with non-exponential decays was proposed. The critical idea is to decompose the data into two sub-datasets, exponential decays and non-exponential decays, employing a non-linear fitting method, and then to invert the sub-datasets and to combine the inversion results. We also introduce a related relaxation correlation measurement, T1ρ-T2*, for examination of solid-like materials. The same data processing strategy as for T1-T2* was implemented. The effectiveness of the proposed method for processing non-exponential data, Sinc Gaussian and Gaussian decay, was validated with simulation and experiment. The results showed that the proposed method recovers T1-T2* and T1ρ-T2* spectra with accurate relative signal intensities. The proposed method provides a platform for further development of MR methods applied to solid-like materials. These relaxation correlations are well suited to measuring composition of mixtures, with solid components in the mixture.

T1-T2* relaxation correlation measurements.

The majority of low field Magnetic Resonance (MR) analyses rely on T2 lifetime measurements. Modification of the T2 measurement to include a T1 dimension has made the T1-T2 measurement a very powerful analytical technique. The T1-T2 measurement is uniquely well suited to characterization of different spin populations in porous materials, such as fluid bearing reservoir rocks, and in soft biopolymer materials, for example foods. However, the T1-T2 measurement is challenging or impossible if the T2 relaxation lifetime, or a component lifetime, is short-lived and the signal unobservable in an echo measurement. This occurs in many important classes of materials. A short lifetime T2 will not however, in general, preclude observation of a free induction decay with signal decay governed by T2*. As outlined in this paper a T1-T2* measurement is a useful analog to the T1-T2 experiment. T1-T2* measurement enables one to differentiate species as a function of T2* in one dimension and T1 in the other dimension. Monitoring changes of the T1-T2* coordinate, and associated signal intensity changes, has the potential to reveal structural changes in materials evolving in time. These methods may also be employed to discriminate and identify solid-like species present in static samples. The T1-T2* measurement is very general in application, but in this paper we focus on cement-based mortars to develop and illustrate the essential ideas. T1-T2* results show a multi-modal behaviour of the MR signal lifetimes, T1 and T2*, in mortar samples under study, indicating at least two different water populations. The short T2* lifetime was assigned to interlayer water (water between C-S-H layers) where the associated T1 is also short lived. The longer T2* lifetime was assigned to water in the pore space, where T1 is also longer lived. In addition to mortar samples we also show application of the method to a crystalline organic species, o-phenylenediamine, which features Sinc Gaussian and exponential decays of transverse magnetization.

A parallel-plate RF probe and battery cartridge for 7Li ion battery studies.

A new parallel-plate resonator for 7Li ion cell studies is introduced along with a removable cartridge-like electrochemical cell for lithium ion battery studies. This geometry separates the RF probe from the electrochemical cell permitting charge/discharge of the cell outside the magnet and introduces the possibility of multiplexing samples under test. The new cell has a geometry that is similar to that of a real battery, unlike the majority of cells employed for MR/MRI studies to this point. The cell, with electrodes parallel to the B1 magnetic field of the probe, avoids RF attenuation during excitation/reception. The cell and RF probe dramatically increase the sample volume compared to traditional MR compatible battery designs. Ex situ and in situ 1D 7Li profiles of Li ions in the electrolyte solution of a cartridge-like cell were acquired, with a nominal resolution of 35 µm at 38 MHz. The cell and RF probe may be employed for spectroscopy, imaging and relaxation studies. We also report the results of a T1-T2 relaxation correlation experiment on both a pristine and fully charged cell. This study represents the first T1-T2 relaxation correlation experiment performed in a Li ion cell. The T1-T2 correlation maps suggest lithium intercalated into graphite is detected by this methodology in addition to other Li species.

MRI monitoring of sea spray freezing.

Sea spray icing is a common hazard for vessels and offshore structures in cold climates. In this paper, quantitative 3D MRI and T1 - T2 mapping of the formation of sea spray ice were performed. Three different freezing regimes were employed. During freezing, changes in both relaxation times and signal intensity were greater than an order of magnitude. Results show strong differences in brine intensity and distribution for the three freezing regimes. The observed ranges of spin densities and relaxation times during freezing are well suited to measurements with portable NMR devices. There is a considerable potential for the use of MRI in studies of sea spray ice.

Controlling susceptibility mismatch effects, signal lifetimes, and SNR through variation of B0 in MRI of rock core plugs.

1H relaxometry measurements of petroleum core plugs are commonly performed on low field magnets (<0.5 Tesla) to reduce the influence of magnetic susceptibility mismatch on measurements of the spin-spin relaxation time, T2. The Signal to Noise Ratio (SNR) of the MR signal, however, generally decreases with lower magnetic fields. Higher magnetic fields (>3 Tesla) are typically employed in small animal MRI studies to improve SNR and image resolution. For many rock core plug samples, susceptibility mismatch effects can be severe at these higher fields leading to decreased T2 and T2*. In this work we seek an answer to the general question of what is the best field for MRI of rock core plugs, anticipating that it will be both sample and measurement method dependent. Free Induction Decay (FID) relaxation time measurements were undertaken to investigate the conditions under which the SNR in Centric Scan SPRITE (Single Point Ramped Imaging with T1 Enhancement) MRI measurements is maximized. The image SNR benefits from greater signal at higher fields, but is negatively impacted by the correspondingly shorter signal lifetimes. Depending on the noise regime of the sample, the maximum SNR may be predicted for Centric Scan SPRITE MRI with T2* being B0 field dependent. In this work we describe a series of simple experimental considerations to determine the optimal B0 field for SPRITE MRI. Selection of the best field is aided by a new generation of superconducting magnets which allows the experimentalist to readily vary the field strength. Such magnets allow one to experimentally control sample magnetization for high sensitivity MRI measurements of core plug samples, while controlling the effect of susceptibility mismatch on the signal lifetimes.

Mapping three-dimensional oil distribution with π-EPI MRI measurements at low magnetic field.

Magnetic resonance imaging (MRI) is a robust tool to image oil saturation distribution in rock cores during oil displacement processes. However, a lengthy measurement time for 3D measurements at low magnetic field can hinder monitoring the displacement. 1D and 2D MRI measurements are instead often undertaken to monitor the oil displacement since they are faster. However, 1D and 2D images may not completely reflect the oil distribution in heterogeneous rock cores. In this work, a high-speed 3D MRI technique, π Echo Planar Imaging (π-EPI), was employed at 0.2T to monitor oil displacement. Centric scan interleaved sampling with view sharing in k-t space was employed to improve the temporal resolution of the π-EPI measurements. A D2O brine was employed to distinguish the hydrocarbon and water phases. A relatively homogenous glass bead pack and a heterogeneous Spynie core plug were employed to show different oil displacement behaviors. High quality 3D images were acquired with π-EPI MRI measurements. Fluid quantification with π-EPI compared favorably with FID, CPMG, 1D-DHK-SPRITE, 3D Fast Spin Echo (FSE) and 3D Conical SPRITE measurements. π-EPI greatly reduced the gradient duty cycle and improved sensitivity, compared to FSE and Conical SPRITE measurements, enabling dynamic monitoring of oil displacement processes. For core plug samples with sufficiently long lived T2, T2(∗), π-EPI is an ideal method for rapid 3D saturation imaging.

High pressure magnetic resonance imaging with metallic vessels.

High pressure measurements in most scientific fields rely on metal vessels given the superior tensile strength of metals. We introduce high pressure magnetic resonance imaging (MRI) measurements with metallic vessels. The developed MRI compatible metallic pressure vessel concept is very general in application. Macroscopic physical systems are now amenable to spatially resolved nuclear magnetic resonance (NMR) study at variable pressure and temperature. Metallic pressure vessels not only provide inherently high tensile strengths and efficient temperature control, they also permit optimization of the MRI RF probe sensitivity. An MRI compatible pressure vessel is demonstrated with a rock core holder fabricated using non-magnetic stainless steel. Water flooding through a porous rock under pressure is shown as an example of its applications. High pressure NMR spectroscopy plays an indispensable role in several science fields. This work will open new vistas of study for high pressure material science MRI and MR.

Spatial and temporal mapping of water content across Nafion membranes under wetting and drying conditions.

Water transport and water management are fundamental to polymer electrolyte membrane fuel cell operation. Accurate measurements of water content within and across the Nafion layer are required to elucidate water transport behavior and validate existing numerical models. We report here a direct measurement of water content profiles across a Nafion layer under wetting and drying conditions, using a novel magnetic resonance imaging methodology developed for this purpose. This method, multi-echo double half k-space spin echo single point imaging, based on a pure phase encode spin echo, is designed for high resolution 1D depth imaging of thin film samples. The method generates high resolution (<8 microm) depth images with an SNR greater than 20, in an image acquisition time of less than 2 min. The high temporal resolution permits water content measurements in the transient states of wetting and drying, in addition to the steady state.

Quantitative discrimination of water and hydrocarbons in porous media by magnetization prepared centric-scan SPRITE.

MRI has considerable potential as a non-destructive probe of porous media, offering the possibility of rapid quantification of local oil and water content. This potential has not yet, however, been completely realized. In this paper, we explore a general magnetization preparation approach to the discrimination of water and oil in a model, representative, porous medium. These measurements have, as a common element, a centric scan pure phase encode readout based on the SPRITE methodology. Magnetization preparation permits facile T1, T2 and diffusion coefficient mapping as the basis for oil and water discrimination. Diffusion coefficient mapping proved to be the most robust approach to discrimination of oil and water. These methods are illustrated through static experiments and a dynamic immiscible fluid displacement experiment.

Phantom materials for single point imaging pulse sequences.

The popularity of pure phase encode MRI techniques, including single point imaging (SPI), is steadily increasing, particularly in instances where the samples of interest are solid-like, or for other reasons possess short effective transverse relaxation times, T2*. As the interest in these techniques grows, so too does the need for a phantom material which is representative of this class of samples. The characteristics of such a phantom should include chemical and physical stability, straightforward preparation, high signal to noise ratio and relaxation times which are both easily manipulated and representative. To this end, we have developed a gelatin/sucrose-based gel which addresses the above criteria and behaves as a very flexible short T2* phantom. An order of magnitude variation in T1 and T2 can be achieved over a reasonable range of sucrose concentration. Even larger changes can be achieved with the addition of further doping agents.

Direct detection of hydrocarbon displacement in a model porous soil with magnetic resonance imaging.

The direct detection of hydrocarbon fluid and the discrimination of water through carbon-13 magnetic resonance imaging (MRI) would be a significant advance in many scientific fields including food, petrogeological, and environmental sciences. Carbon-13 MRI is a noninvasive analytical technique that has great potential for direct detection of hydrocarbons. However, the low natural abundance of carbon-13, low gyromagnetic ratio, and generically short transverse signal lifetimes in realistic porous media all conspire to hinder carbon-13 MRI. A multiple echo pure phase encode MRI technique introduced in this paper helps to overcome these limitations. As a pure phase encode technique, it is immune to artifacts arising from inhomogeneous B0 fields. It is also, by its nature, more quantitative than most MRI methods. Viscous hydrocarbon flow through a sand bed, a simple realistic porous medium, was used as our test system. Flow in this model system was driven by capillary suction. The detection limit, spatially resolved, was determined to be 26 mg.

Centric scan SPRITE magnetic resonance imaging.

Two rapid, pure phase encode, centric scan, Single Point Ramped Imaging with T1-Enhancement (SPRITE) MRI methods are described. Each retains the benefits of the standard SPRITE method, most notably the ability to image short T2* systems, while increasing the sensitivity and generality of the technique. The Spiral-SPRITE method utilizes a modified Archimedean spiral k-space trajectory. The Conical-SPRITE method utilizes a system of spirals mapped to conical surfaces to sample the k-space cube. The sampled k-space points are naturally Cartesian grid points, eliminating the requirement of a re-gridding procedure prior to image reconstruction. The effects of transient state behaviour on image resolution and signal/noise are explored.