Development of a magic-angle spinning nuclear magnetic resonance probe with a cryogenic detection system for sensitivity enhancement

Overcoming challenges in 67Zn NMR: a new strategy of signal enhancement for MOF characterization

67 Zn solid-state NMR suffers from low sensitivity, limiting its ability to probe the Zn2+ surroundings in MOFs. We report a breakthrough in overcoming challenges in 67Zn NMR. Combining new cryogenic MAS probe technology and performing NMR experiments at a high magnetic field results in remarkable signal enhancement, yielding enhanced information for MOF characterization.

A roadmap to high-resolution standard microcoil MAS NMR spectroscopy for metabolomics

Current microcoil probe technology has emerged as a significant advancement in NMR applications to biofluids research. It has continued to excel as a hyphenated tool with other prominent microdevices, opening many new possibilities in multiple omics fields. However, this does not hold for biological samples such as intact tissue or organisms, due to the considerable challenges of incorporating the microcoil in a magic-angle spinning (MAS) probe without relinquishing the high-resolution spectral data. Not until 2012 did a microcoil MAS probe show promise in profiling the metabolome in a submilligram tissue biopsy with spectral resolution on par with conventional high-resolution MAS (HR-MAS) NMR. This result subsequently triggered a great interest in the possibility of NMR analysis with microgram tissues and striving toward the probe development of "high-resolution" capable microcoil MAS NMR spectroscopy. This review gives an overview of the issues and challenges in the probe development and summarizes the advancements toward metabolomics.

Cryogenic signal amplification combined with helium-temperature MAS DNP toward ultimate NMR sensitivity at high field conditions

The low sensitivity of NMR spectroscopy is of historical concern in the field, and various approaches have been developed to mitigate this limitation. On the shoulder of giants, today one can routinely implement, for example, the pulse/Fourier transform NMR with the cross polarization together with the ultra-low temperature MAS DNP under high-field conditions. We show in this work this current opportunity should further be augmented by combining them with the cryogenic signal amplification. Our presented MAS DNP probe operates with the closed-cycle helium MAS system, and cools the internal preamplifier-duplexer module with the "return" helium gas on its way back to the compressor in the loop. The signal-to-noise (S/N) gain relative to the room-temperature measurements of a factor of 4.6 and 2.4 was found for the measurement using the cold- and room-temperature preamplifier, respectively, at the sample temperature of T = 20 K at B₀ = 16.4 T. The ratio of these factors reveals ∼ two-fold sensitivity improvement that results purely from the introduction of the cold signal amplification, i.e., noise reduction. Together with the increase of the thermal Boltzmann polarization at low temperatures, the combined S/N gain of max. ∼70-fold is possible without DNP. The DNP enhancement factor of ∼40 as we found in this work for a microcrystalline MLF sample may be multiplied to this gain. We also demonstrated the sensitivity improvement with a ¹³C-detected 2D NCaCx spectrum, illustrating the generality of the S/N gain from combining DNP with the cold signal amplification.

An electro-mechano-optical NMR probe for 1H–13C double resonance in a superconducting magnet

A compact nanomembrane radiofrequency-to-light transducer brings the emerging Electro-Mechano-Optical (EMO) NMR technique into the realm of practical NMR in chemistry using a superconducting magnet.

Noise Reduction in Solid-State NMR Spectra Using Principal Component Analysis

A noise reduction method was developed for solid-state nuclear magnetic resonance spectroscopy using multivariate analysis. Principal component analysis was first applied for cross-polarization/magic angle spinning and ¹³C spin-lattice relaxation measurements of solid-state nuclear magnetic resonance array spectra. The contact time of cross-polarization/magic angle spinning and the delay time in spin-lattice relaxation measurements were continuously changed to obtain a series of spectra, which were used for noise reduction using principal component analysis. The noise reduction method successfully produced spectra with improved signal-to-noise ratios. This noise reduction method shortens the measurement time and allows for detection of components with minute signals.

Recent developments in MAS DNP-NMR of materials

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B₀ ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B₀ field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Rapid formation of small mixed-valence luminescent silver clusters via cation-coupled electron-transfer in a redox-active porous ionic crystal based on dodecamolybdophosphate

Redox-active porous ionic crystals based on polyoxometalate (POM) were utilized to form and stabilize small mixed-valence luminescent silver clusters via cation-coupled electron-transfer (CCET) reactions. Reduction-induced ion-exchange between Cs+ and Ag+via CCET took less than 1 min to complete and consisted of two steps: electron transfer from reduced POM to Ag+ and the subsequent formation of a silver cluster, and diffusion of the silver cluster and exchange with Cs+. Notably, the simple ion-exchange took more than 24 h. The compound containing the silver cluster showed high affinity toward unsaturated hydrocarbon guests.

Instrumental Platforms for NMR-based Metabolomics

Advances and improvements in the hardware and automation of nuclear magnetic resonance (NMR) technologies have enabled recent progression in strategies and technologies to aid comprehensive structural identification and new perspectives in the chemical sciences. Particularly, these developments have enabled the growing area of metabolomics by NMR. At the centre of the evolution of NMR hardware is the relative size reduction in NMR probes and NMR magnets, and computational support advances. Furthermore, automation advances and technical precision have allowed for epidemiological and clinical population analysis to become a reality by NMR. Metabolic laboratories inherent improvement in spectral deconvolution in areas of chemical and biological sciences also allow for better structural elucidation on a molecular level in a complex environment. This chapter details the modern state-of-the-art hardware and equipment that are currently used in NMR spectrometry and how they are exploited in the area of small-molecule profiling of complex fluid analysis.

Ultrasonic Motor Using Two Sector-Shaped Piezoelectric Transducers for Sample Spinning in High Magnetic Field

This paper presents the design, fabrication process, and evaluation of an ultrasonic motor for sample spinning in a high magnetic field of solid-state Nuclear Magnetic Resonance (NMR). To decrease effects of the magnetic field on rotation, all motor components are made of materials that have low magnetic permeability. The motor, including the sample casing, is a maximum 31 mm in diameter and 50 mm high. The motor has two sector-shaped piezoelectric transducers. One transducer generates two different vibration modes, longitudinal and flexural, when two sinusoidal voltages are applied to transducers. To confirm that transducers can be driven in a high magnetic field, the effect of the magnetic field on the transducer was evaluated. The motor was driven at a frequency of 329.0 kHz. The maximum rotation speed and starting torque were 1.50 × 103rpm and 26 µNm when applied voltage was 40 Vp-p. The rotation speed, controlled by a proportional-integral control system, was 1.20 × 103rpm in a 7.0-T magnetic field. The motor was also applied to the sample spinning system of a high-resolution NMR spectrometer. We succeeded in obtaining1H-NMR signals of H2O. The motor can therefore be used for a sample spinning system in a high magnetic field.

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14 T and 30 K

We describe a (1)H polarization enhancement via dynamic nuclear polarization (DNP) at very low sample temperature T≈30 K under magic-angle spinning (MAS) conditions for sensitivity-enhanced solid-state NMR measurement. Experiments were conducted at a high external field strength of 14.1 T. For MAS DNP experiments at T<<90 K, a new probe system using cold helium gas for both sample-cooling and -spinning was developed. The novel system can sustain a low sample temperature between 30 and 90K for a period of time >10 h under MAS at ν(R)≈3 kHz with liquid He consumption of ≈6 L/h. As a microwave source, we employed a high-power, continuously frequency-tunable gyrotron. At T≈34 K, (1)H DNP enhancement factors of 47 and 23 were observed with and without MAS, respectively. On the basis of these observations, a discussion on the total NMR sensitivity that takes into account the effect of sample temperature and external field strength used in DNP experiments is presented. It was determined that the use of low sample temperature and high external field is generally rewarding for the total sensitivity, in spite of the slower polarization buildup at lower temperature and lower DNP efficiency at higher field. These findings highlight the potential of the current continuous-wave DNP technique also at very high field conditions suitable to analyze large and complex systems, such as biological macromolecules.

Total compensation of pulse transients inside a resonator

The profile of rf pulses that nuclear spins experience inside a resonator deviates from that of rf voltage signals generated by a NMR spectrometer according to users' pulse programming, when change of the profile in time is comparable to or shorter than the time constant of the resonator. In our previous work [Takeda et al., J. Magn. Reson. 197 (2009) 242-244], we proposed active compensation of rf pulse transients, in which the amplitude transient of the rf pulse can be suppressed without sacrificing the Q factor of the probe. Here we extend the idea of active compensation toward total compensation of the amplitude as well as phase transients. By measuring the transient response of the probe to a given excitation using a pickup coil, the response function determining the transient behavior of the probe is numerically obtained. Then, by numerically solving the convolution equation with the help of Laplace transformation, one can obtain the amplitude and phase profiles of the pulse that should be programmed in the spectrometer in order to apply the rf pulses to the nuclear spins as intended. Accurate rf pulsing based on this idea is experimentally demonstrated, and prospect and requirements for coping with the receiver dead-time problem are discussed.

Development of a cryogenic duplexer for solid-state nuclear magnetic resonance

To fully appreciate sensitivity enhancement achievable by reducing thermal noise at the detection coil for nuclear magnetic resonance (NMR), a duplexer operational at low temperature is required. In this work, we developed a cryogenic duplexer that can work below 50 K using GaAs diodes and rf-microelectrical mechanical systems switches. As the observed leakage of a rf pulse (100 W and 50 micros) through the duplexer is below 1 mW, it can be used not only for NMR in liquids but also for NMR in solids. We reported recently the cryocoil magic-angle spinning (MAS) probe [T. Mizuno et al., Rev. Sci. Instrum. 79, 044706 (2008)] into which was incorporated with this duplexer and a commercial rf preamplifier, we show that signal-to-noise ratio gain of 4.0 times can be achieved for (1)H MAS-NMR by lowering the detection-coil temperature to 12 K and the temperatures for the rf preamplifier and the duplexer to 43 K.

Active compensation of rf-pulse transients

A new approach to compensate rf-pulse transients is proposed. Based on the idea of the response theory of a linear system, a formula is derived to obtain the required excitation voltage profile back from the intended target rf-pulse shape. The validity of the formula is experimentally confirmed by monitoring the rf-field created inside the sample coil with a pickup coil. Since this approach realizes accurate rf-pulse shapes without reducing the Q-factor of the tank circuit of the probe, it can be used not only to suppress the transient tail of the rf-pulse, but also as a general concept for accurate rf-pulsing.