Surface-Enhanced NMR Using Continuous-Flow Laser-Polarized Xenon

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density

The extraordinary sensitivity of ¹²⁹Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized ¹²⁹Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and ¹²⁹Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of ¹²⁹Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Surface NMR using quantum sensors in diamond

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method's capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid-liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy

Hyperpolarization is a technique that can increase nuclear spin polarization with the corresponding gains in nuclear magnetic resonance (NMR) signals by 4-8 orders of magnitude. When this process is applied to biologically relevant samples, the hyperpolarized molecules can be used as exogenous magnetic resonance imaging (MRI) contrast agents. A technique called spin-exchange optical pumping (SEOP) can be applied to hyperpolarize noble gases such as 129 Xe. Techniques based on hyperpolarized 129 Xe are poised to revolutionize clinical lung imaging, offering a non-ionizing, high-contrast alternative to computed tomography (CT) imaging and conventional proton MRI. Moreover, CT and conventional proton MRI report on lung tissue structure but provide little functional information. On the other hand, when a subject breathes hyperpolarized 129 Xe gas, functional lung images reporting on lung ventilation, perfusion and diffusion with 3D readout can be obtained in seconds. In this Review, the physics of SEOP is discussed and the different production modalities are explained in the context of their clinical application. We also briefly compare SEOP to other hyperpolarization methods and conclude this paper with the outlook for biomedical applications of hyperpolarized 129 Xe to lung imaging and beyond.

Automated Low-Cost In Situ IR and NMR Spectroscopy Characterization of Clinical-Scale 129Xe Spin-Exchange Optical Pumping

We present on the utility of in situ nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopic techniques for automated advanced analysis of the ¹²⁹Xe hyperpolarization process during spin-exchange optical pumping (SEOP). The developed software protocol, written in the MATLAB programming language, facilitates detailed characterization of hyperpolarized contrast agent production efficiency based on determination of key performance indicators, including the maximum achievable ¹²⁹Xe polarization, steady-state Rb-¹²⁹Xe spin-exchange and ¹²⁹Xe polarization build-up rates, ¹²⁹Xe spin-relaxation rates, and estimates of steady-state Rb electron polarization. Mapping the dynamics of ¹²⁹Xe polarization and relaxation as a function of SEOP temperature enables systematic optimization of the batch-mode SEOP process. The automated analysis of a typical experimental data set, encompassing ∼300 raw NMR and NIR spectra combined across six different SEOP temperatures, can be performed in under 5 min on a laptop computer. The protocol is designed to be robust in operation on any batch-mode SEOP hyperpolarizer device. In particular, we demonstrate the implementation of a combination of low-cost NIR and low-frequency NMR spectrometers (∼$1,100 and ∼$300 respectively, ca. 2020) for use in the described protocols. The demonstrated methodology will aid in the characterization of NMR hyperpolarization hardware in the context of SEOP and other hyperpolarization techniques for more robust and less expensive clinical production of HP ¹²⁹Xe and other contrast agents.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Stability Enhancement of 129Xe Hyperpolarizing System Using Alkali Metal Vapor in Spin-Exchange Optical Pumping Cell to Achieve High NMR Sensitivity

Hyperpolarized (HP) ¹²⁹Xe NMR and MRI have enabled ¹²⁹Xe studies with extraordinarily enhanced sensitivity, stimulating new developments in magnetic resonance in chemistry, physics, biology and medicine. However, the standard method of HP ¹²⁹Xe production inevitably demands Rb vapor for the excitation, which has made the method very sensitive to impurities such as water or oxygen. This is the case especially in the recirculating system. In the present study, stability of the hyperpolarizing system is discussed by proposing the "cell decay constant", which symbolizes the decay rate of the NMR signal obtained from the system. The cell decay constant is effectively decreased to 1/3 by introducing separated chambers and mechanical stirring of the alkali metals used in the system, making it effective for accumulating FIDs over 30 to 100 h. The newly developed hyperpolarizing system has been successfully applied for newly detecting a broad signal at 190 ppm with an industrial material Nanofiber.

One- and Two-Dimensional High-Resolution NMR from Flat Surfaces

Determining atomic-level characteristics of molecules on two-dimensional surfaces is one of the fundamental challenges in chemistry. High-resolution nuclear magnetic resonance (NMR) could deliver rich structural information, but its application to two-dimensional materials has been prevented by intrinsically low sensitivity. Here we obtain high-resolution one- and two-dimensional ³¹P NMR spectra from as little as 160 picomoles of oligonucleotide functionalities deposited onto silicate glass and sapphire wafers. This is enabled by a factor >10⁵ improvement in sensitivity compared to typical NMR approaches from combining dynamic nuclear polarization methods, multiple-echo acquisition, and optimized sample formulation. We demonstrate that, with this ultrahigh NMR sensitivity, ³¹P NMR can be used to observe DNA bound to miRNA, to sense conformational changes due to ion binding, and to follow photochemical degradation reactions.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Squeezing xenon into phenylether bis-urea nanochannels

One-dimensional nanochannels, hundreds of microns in persistence length but with elliptical cross-sectional dimensions of only ∼3.7 Å × 4.8 Å, are formed by the columnar assembly of phenylether bis-urea macrocycles. Hyperpolarized Xe-129 NMR is utilized to investigate the Xe atom packing and Xe diffusion inside the needle shaped crystals. The elliptical channel structure produces a Xe-129 powder pattern characteristic of an asymmetric chemical shift tensor extending to well over 300 ppm with respect to the gas phase, reflecting the highly anisotropic electronic environment and extreme confinement of the atom. Consistent with the simple geometrical criterion, hyperpolarized tracer exchange NMR data reveals single-file diffusion in the bis-urea nanochannels.

Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies

Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales.

Multidimensional mapping of spin-exchange optical pumping in clinical-scale batch-mode 129Xe hyperpolarizers

We present a systematic, multiparameter study of Rb/¹²⁹Xe spin-exchange optical pumping (SEOP) in the regimes of high xenon pressure and photon flux using a 3D-printed, clinical-scale stopped-flow hyperpolarizer. In situ NMR detection was used to study the dynamics of ¹²⁹Xe polarization as a function of SEOP-cell operating temperature, photon flux, and xenon partial pressure to maximize ¹²⁹Xe polarization (P_Xe). P_Xe values of 95 ± 9%, 73 ± 4%, 60 ± 2%, 41 ± 1%, and 31 ± 1% at 275, 515, 1000, 1500, and 2000 Torr Xe partial pressure were achieved. These P_Xe polarization values were separately validated by ejecting the hyperpolarized ¹²⁹Xe gas and performing low-field MRI at 47.5 mT. It is shown that P_Xe in this high-pressure regime can be increased beyond already record levels with higher photon flux and better SEOP thermal management, as well as optimization of the polarization dynamics, pointing the way to further improvements in hyperpolarized ¹²⁹Xe production efficiency.

Perspectives of hyperpolarized noble gas MRI beyond 3He

Nuclear Magnetic Resonance (NMR) studies with hyperpolarized (hp) noble gases are at an exciting interface between physics, chemistry, materials science and biomedical sciences. This paper intends to provide a brief overview and outlook of magnetic resonance imaging (MRI) with hp noble gases other than hp (3)He. A particular focus are the many intriguing experiments with (129)Xe, some of which have already matured to useful MRI protocols, while others display high potential for future MRI applications. Quite naturally for MRI applications the major usage so far has been for biomedical research but perspectives for engineering and materials science studies are also provided. In addition, the prospects for surface sensitive contrast with hp (83)Kr MRI is discussed.

Solid-state NMR: a powerful tool for characterization of metal-organic frameworks

Metal-organic frameworks (MOFs) are a new type of porous materials with numerous current and potential applications in many areas including ion-exchange, catalysis, sensing, separation, molecular recognition, drug delivery and, in particular, gas storage. Solid-state NMR (SSNMR) has played a pivotal role in structural characterization and understanding of host-guest interactions in MOFs. This article provides an overview on application of SSNMR to MOF systems.

Pathway to cryogen free production of hyperpolarized Krypton-83 and Xenon-129

Hyperpolarized (hp) ¹²⁹Xe and hp ⁸³Kr for magnetic resonance imaging (MRI) are typically obtained through spin-exchange optical pumping (SEOP) in gas mixtures with dilute concentrations of the respective noble gas. The usage of dilute noble gases mixtures requires cryogenic gas separation after SEOP, a step that makes clinical and preclinical applications of hp ¹²⁹Xe MRI cumbersome. For hp ⁸³Kr MRI, cryogenic concentration is not practical due to depolarization that is caused by quadrupolar relaxation in the condensed phase. In this work, the concept of stopped flow SEOP with concentrated noble gas mixtures at low pressures was explored using a laser with 23.3 W of output power and 0.25 nm linewidth. For ¹²⁹Xe SEOP without cryogenic separation, the highest obtained MR signal intensity from the hp xenon-nitrogen gas mixture was equivalent to that arising from 15.5±1.9% spin polarized ¹²⁹Xe in pure xenon gas. The production rate of the hp gas mixture, measured at 298 K, was 1.8 cm³/min. For hp ⁸³Kr, the equivalent of 4.4±0.5% spin polarization in pure krypton at a production rate of 2 cm³/min was produced. The general dependency of spin polarization upon gas pressure obtained in stopped flow SEOP is reported for various noble gas concentrations. Aspects of SEOP specific to the two noble gas isotopes are discussed and compared with current theoretical opinions. A non-linear pressure broadening of the Rb D₁ transition was observed and taken into account for the qualitative description of the SEOP process.

Hyperpolarized Xenon Nuclear Magnetic Resonance (NMR) of Building Stone Materials

We have investigated several building stone materials, including minerals and rocks, using continuous flow hyperpolarized xenon (CF-HP) NMR spectroscopy to probe the surface composition and porosity. Chemical shift and line width values are consistent with petrographic information. Rare upfield shifts were measured and attributed to the presence of transition metal cations on the surface. The evolution of freshly cleaved rocks exposed to the atmosphere was also characterized. The CF-HP ¹²⁹Xe NMR technique is non-destructive and it could complement currently used techniques, like porosimetry and microscopy, providing additional information on the chemical nature of the rock surface and its evolution.

In vivo MRI using real-time production of hyperpolarized 129Xe

MR imaging of hyperpolarized (HP) nuclei is challenging because they are typically delivered in a single dose of nonrenewable magnetization, from which the entire image must be derived. This problem can be overcome with HP (129)Xe, which can be produced sufficiently rapidly to deliver in dilute form (1%) continuously and on-demand. We demonstrate a real-time in vivo delivery of HP (129)Xe mixture to rats, a capability we now routinely use for setting frequency, transmitter gain, shimming, testing pulse sequences, scout imaging, and spectroscopy. Compared to images acquired using conventional fully concentrated (129)Xe, real-time (129)Xe images have 26-fold less signal, but clearly depict ventilation abnormalities. Real-time (129)Xe MRI could be useful for time-course studies involving acute injury or response to treatment. Ultimately, real-time (129)Xe MRI could be done with more highly concentrated (129)Xe, which could increase the signal-to-noise ratio by 100 relative to these results to enable a new class of gas imaging applications.

Probing the porosity of cocrystallized MCM-49/ZSM-35 zeolites by hyperpolarized 129Xe NMR

One- and two-dimensional 129Xe NMR spectroscopy has been employed to study the porosity of cocrystallized MCM-49/ZSM-35 zeolites under the continuous flow of hyperpolarized xenon gas. It is found by variable-temperature experiments that Xe atoms can be adsorbed in different domains of MCM-49/ZSM-35 cocrystallized zeolites and the mechanically mixed counterparts. The exchange of Xe atoms in different types of pores is very fast at ambient temperatures. Even at very low temperature two-dimensional exchange spectra (EXSY) show that Xe atoms still undergo much faster exchange between MCM-49 and ZSM-35 analogues in the cocrystallized zeolites than in the mechanical mixture. This demonstrates that the MCM-49 and ZSM-35 analogues in cocrystallized zeolites may be stacked much closer than in the physical mixture, and some parts of intergrowth may be formed due to the partially similar basic structure of MCM-49 and ZSM-35.

Exploring hyperpolarized 83Kr by remotely detected NMR relaxometry

For the first time, a hyperpolarized (hp) noble gas with a nuclear electric quadrupole moment is available for high-field nuclear-magnetic-resonance (NMR) spectroscopy and magnetic-resonance imaging. Hp (83)Kr (I=92) is generated by spin-exchange optical pumping and separated from the rubidium vapor used in the pumping process. Optical pumping occurs under the previously unstudied condition of high krypton gas densities. Signal enhancements of more than three orders of magnitude compared to the thermal equilibrium (83)Kr signal at 9.4 T magnetic-field strength are obtained. The spin-lattice relaxation of (83)Kr is caused primarily by quadrupolar couplings during the brief adsorption periods of the krypton atoms on the surrounding container walls and significantly limits the currently obtained spin polarization. Measurements in macroscopic glass containers and in desiccated canine lung tissue at field strengths between 0.05 and 3 T using remotely detected hp (83)Kr NMR spectroscopy reveal that the longitudinal relaxation dramatically accelerates as the magnetic-field strength decreases.

2D Multinuclear NMR, Hyperpolarized Xenon and Gas Storage in Organosilica Nanochannels with Crystalline Order in the Walls

The combination of 2D 1H-13C and 1H-29Si solid state NMR, hyperpolarized 129Xe NMR, synchrotron X-ray diffraction, together with adsorption measurements of vapors and gases for environmental and energetic relevance, was used to investigate the structure and the properties of periodic mesoporous hybrid p-phenylenesilica endowed with crystalline order in the walls. The interplay of 1H, 13C, and 29Si in the 2D heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, revealed the spatial relationships (<5 angstroms) among various spin-active nuclei of the framework. Indeed, the through-space correlations in the 2D experiments evidenced, for the first time, the interfaces of the matrix walls with guest molecules confined in the nanochannels. Organic-inorganic and organic-organic heterogeneous interfaces between the matrix and the guests were identified. The open-pore structure and the easy accessibility of the nanochannels to the gas phase have been demonstrated by highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution. Two-dimensional exchange experiments showed the exchange time to be as short as 2 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the nanochannel space and surface, a physisorption energy of 13.9 kJ mol-1, and the chemical shift value of xenon probing the internal surfaces. These results prompted us to measure the high storage capacity of the matrix towards benzene, hexafluorobenzene, ethanol, and carbon dioxide. Both host-guest, CH...pi, and OH...pi interactions contribute to the stabilization of the aromatic guests (benzene and hexafluorobenzene) on the extended surfaces. The full carbon dioxide loading in the channels could be detected by synchrotron radiation X-ray diffraction experiments. The selective adsorption of carbon dioxide (ca. 90 wt %) vs that of oxygen and hydrogen, together with the permanent porosity, high thermal stability, and high degree of order, makes this a suitable matrix for purifying hydrogen in clean-energy generation.

NMR velocity mapping of gas flow around solid objects

We present experimental visualizations of gas flow around solid blunt bodies by NMR imaging. NMR velocimetry is a model-free and tracer-free experimental means for quantitative and multi-dimensional flow visualization. Hyperpolarization of (129)Xe provided sufficient NMR signal to overcome the low density of the dilute gas phase, and its long coherence time allows for true velocity vector mapping. In this study, the diverging gas flow around and wake patterns immediately behind a sphere could be vectorally visualized and quantified. In a similar experiment, the flow over an aerodynamic model airplane body revealed a less disrupted flow pattern.

Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging

For the first time, magnetic resonance imaging (MRI) with hyperpolarized (hp) krypton-83 (83Kr) has become available. The relaxation of the nuclear spin of 83Kr atoms (I = 9/2) is driven by quadrupolar interactions during brief adsorption periods on surrounding material interfaces. Experiments in model systems reveal that the longitudinal relaxation of hp 83Kr gas strongly depends on the chemical composition of the materials. The relaxation-weighted contrast in hp 83Kr MRI allows for the distinction between hydrophobic and hydrophilic surfaces. The feasibility of hp 83Kr MRI of airways is tested in canine lung tissue by using krypton gas with natural abundance isotopic distribution. Additionally, the influence of magnetic field strength and the presence of a breathable concentration of molecular oxygen on longitudinal relaxation are investigated.

NMR-Based Biosensing with Optimized Delivery of Polarized129Xe to Solutions

Laser-enhanced (LE) 129Xe nuclear magnetic resonance (NMR) is an exceptional tool for sensing extremely small physical and chemical changes; however, the difficult mechanics of bringing polarized xenon and samples of interest together have limited applications, particularly to biological molecules. Here we present a method for accomplishing solution 129Xe biosensing based on flow (bubbling) of LE 129Xe gas through a solution in situ in the NMR probe, with pauses for data acquisition. This overcomes fundamental limitations of conventional solution-state LE 129Xe NMR, e.g., the difficulty in transferring hydrophobic xenon into aqueous environments, and the need to handle the sample to refresh LE 129Xe after an observation pulse depletes polarization. With this new method, we gained a factor of >100 in sensitivity due to improved xenon transfer to the solution and the ability to signal average by renewing the polarized xenon. Polarized xenon in biosensors was detected at very low concentrations, </=250 nanomolar, while retaining all the usual information from NMR. This approach can be used to simultaneously detect multiple sensors with different chemical shifts and is also capable of detecting signals from opaque, heterogeneous samples, which is a unique advantage over optical methods. This general approach is adaptable for sensing minute quantities of xenon in heterogeneous in vitro samples, in miniaturized devices and should be applicable to certain in-vivo environments.

Distribution of Gallium Nanocrystals in Ga/MCM-41 Mesocomposites by Continuous-Flow Hyperpolarized 129Xe NMR Spectroscopy

The distribution of gallium nanocrystals in mesoporous MCM-41 host were analyzed by continuous-flow hyperpolarized 129Xe NMR spectroscopy. In contrast to unclear TEM images for the high metal contents, laser-polarized 129Xe probe can detect the whole distribution of gallium in the MCM-41 host. It is found that gallium nanocrystals are included in the mesochannels of MCM-41; a part of them also remains in the interparticle voids. The distribution of gallium metal in MCM-41 is heterogeneous. Not all the mesochannels host metallic gallium even at a high gallium loading of 65.1 wt %. Variable temperature measurements can provide information on the xenon adsorption parameters. This approach opens a sensitive way to probe the distribution of high content species in porous host materials.

Amplification of xenon NMR and MRI by remote detection

A technique is proposed in which an NMR spectrum or MRI is encoded and stored as spin polarization and is then moved to a different physical location to be detected. Remote detection allows the separate optimization of the encoding and detection steps, permitting the independent choice of experimental conditions and excitation and detection methodologies. In the initial experimental demonstration of this technique, we show that taking dilute 129Xe from a porous sample placed inside a large encoding coil and concentrating it into a smaller detection coil can amplify NMR signal. In general, the study of NMR active molecules at low concentration that have low physical filling factor is facilitated by remote detection. In the second experimental demonstration, MRI information encoded in a very low-field magnet (4-7 mT) is transferred to a high-field magnet (4.2 T) to be detected under optimized conditions. Furthermore, remote detection allows the utilization of ultrasensitive optical or superconducting quantum interference device detection techniques, which broadens the horizon of NMR experimentation.

Microscale NMR

NMR spectroscopy is increasingly being used to characterize microliter and smaller-volume samples. Substances at picomole levels have been identified using NMR spectrometers equipped with microcoil-based probes. NMR probes that incorporate multiple sample chambers enable higher-throughput NMR experiments. Hyphenation of capillary-scale separations and microcoil NMR has also decreased analysis time of mixtures. For example, capillary isotachophoresis/NMR allows the highest mass sensitivity nanoliter-volume flow cells to be used with low microliter volume samples because isotachophoresis concentrates the microliter volume sample into the nanoliter volume NMR detection probe. In addition, the diagnostic capabilities of NMR spectroscopy allow the physico-chemical aspects of a capillary separation process to be characterized on-line. Because of such advances, the application of NMR to smaller samples continues to grow.

In situ IR, NMR, EPR, and UV/Vis Spectroscopy: Tools for New Insight into the Mechanisms of Heterogeneous Catalysis

The development of new solid catalysts for use in industrial chemistry has hitherto been based to a large extent upon the empirical testing of a wide range of different materials. In only a few exceptional cases has success been achieved in understanding the overall, usually very complex mechanism of the chemical reaction through the elucidation of individual intermediate aspects of a heterogeneously catalyzed reaction. With the modern approach of combinatorial catalysis it is now possible to prepare and test much more rapidly a wide range of different materials within a short time and thus find suitable catalysts or optimize their chemical composition. Our understanding of the mechanisms of reactions catalyzed by these materials must be developed, however, by spectroscopic investigations on working catalysts under conditions that are as close as possible to practice (temperature, partial pressures of the reactants, space velocity). This demands the development and the application of new techniques of in situ spectroscopy. This review will show how this objective is being achieved. By the term in situ (Lat.: in the original position) is meant the investigation of the chemical reactions which are taking place as well as the changes in the working catalysts directly in the spectrometer.

Formation and decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y investigated by in situ stopped-flow MAS NMR spectroscopy

Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ (13)C MAS NMR spectroscopy under flow conditions. The in situ (13)C continuous-flow (CF) MAS NMR experiments were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1 to 4:1, and modified residence times of (13)CH(3)OH between 20 and 100 (g x h)/mol. The methylation reaction was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a (13)C NMR signal at 58 ppm constitute the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH(2)CH(3)](+)) and N,N-dimethylaniline ([PhNH(CH(3))(2)](+)) were also observed at ca. 39 and 48 ppm, respectively. After increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated reaction products strongly increased, accompanied by a decrease of the amount of N,N,N-trimethylanilinium cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase, respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl groups is suggested to play a key role for the products distribution in the gas phase.

Capillary isotachophoresis/NMR: extension to trace impurity analysis and improved instrumental coupling

Building upon its promising initial performance, the online coupling of capillary isotachophoresis (cITP) to nuclear magnetic resonance (NMR) is extended to trace impurity analysis. By simultaneously concentrating and separating dilute charged species on the basis of their electrophoretic mobility, cITP greatly facilitates NMR structural elucidation. cITP/NMR appears particularly attractive for identifying trace charged synthetic and natural organic compounds obscured by large excesses of other components. A 9.4 microL injection of 200 microM (1.9 nmol) atenolol in a 1000-fold excess of sucrose (200 mM) is analyzed by cITP/NMR. A microcoil, the most mass sensitive NMR probe, serves as the detector as it provides optimal NMR observation of the capillary-scale separation. cITP successfully isolates the atenolol from the sucrose while concentrating it 200-fold to 40 mM before presentation to the 30 nL observe volume microcoil, thereby enabling rapid 1H NMR spectral acquisition of atenolol (experimental time of 10 s) without obstruction from sucrose. For this particular probe and sample, the stacking efficiency is near the theoretical limit as 67% of the sample occupies the 1 mm long microcoil during peak maximum. A multiple-coil probe with two serial 1 mm long microcoils arranged 1 cm apart has been developed to facilitate peak trapping and sample band positioning during cITP/NMR.

Nuclear magnetic resonance of laser-polarized noble gases in molecules, materials, and organisms

The sensitivity of conventional nuclear magnetic resonance (NMR) techniques is fundamentally limited by the ordinarily low spin polarization achievable in even the strongest NMR magnets. However, by transferring angular momentum from laser light to electronic and nuclear spins, optical pumping methods can increase the nuclear spin polarization of noble gases by several orders of magnitude, thereby greatly enhancing their NMR sensitivity. This review describes the principles and magnetic resonance applications of laser-polarized noble gases. The enormous sensitivity enhancement afforded by optical pumping can be exploited to permit a variety of novel NMR experiments across numerous disciplines. Many such experiments are reviewed, including the void-space imaging of organisms and materials, NMR and MRI of living tissues, probing structure and dynamics of molecules in solution and on surfaces, NMR sensitivity enhancement via polarization transfer, and low-field NMR and MRI.