Near-zero-field nuclear magnetic resonance and hyperpolarization technology

Dissolution dynamic nuclear polarization (DNP) is one of the most successful techniques that resolves the insensitivity problem in liquid-state nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) by amplifying the signal by several thousand-fold. One way to further improve the DNP signal is the inclusion of trace amounts of lanthanides in DNP samples doped with trityl OX063 free radical as the polarizing agent. In practice, stable monomeric gadolinium complexes such as Gd-DOTA or Gd-HP-DO3A are used as beneficial additives in DNP samples, further boosting the DNP-enhanced solid-state 13C polarization by a factor of 2 or 3. Herein, we report on the use of a trimeric gadolinium complex as a dopant in 13C DNP samples to improve the 13C DNP signals in the solid-state at 3.35 T and 1.2 K and consequently, in the liquid-state at 9.4 T and 298 K after dissolution. Our results have shown that doping the 13C DNP sample with a complex which holds three Gd3+ ions led to an improvement of DNP-enhanced 13C polarization by a factor of 3.4 in the solid-state, on par with those achieved using monomeric Gd3+ complexes but only requires about one-fifth of the concentration. Upon dissolution, liquid-state 13C NMR signal enhancements close to 20 000-fold, approximately 3-fold the enhancement of the control samples, were recorded in the nearby 9.4 T high resolution NMR magnet at room temperature. Comparable reduction of 13C spin-lattice T1 relaxation time was observed in the liquid-state after dissolution for both the monomeric and trimeric Gd3+ complexes. Moreover, W-band electron paramagnetic resonance (EPR) data have revealed that 3-Gd doping significantly reduces the electron T1 of the trityl OX063 free radical, but produces negligible changes in the EPR spectrum, reminiscent of the results with monomeric Gd3+-complex doping. Our data suggest that the trimeric Gd3+ complex is a highly beneficial additive in 13C DNP samples and that its effect on DNP efficiency can be described in the context of the thermal mixing mechanism.

Hyperpolarized Magnetic Resonance Spectroscopy and Imaging

Review: 194 refs.

Applications of Dissolution-DNP for NMR Screening.

Nuclear magnetic resonance (NMR) spectroscopy is a key technique for molecular structure determination in solution. However, due to its low sensitivity, many efforts have been made to improve signal strengths and reduce the required substrate amounts. In this regard, dissolution dynamic nuclear polarization (DDNP) is a versatile approach as signal enhancements of over 10 000-fold are achievable. Samples are signal-enhanced ex situ by transferring electronic polarization from radicals to nuclear spins before dissolving and shuttling the boosted sample to an NMR spectrometer for detection. However, the applicability of DDNP suffers from one major drawback, namely, paramagnetic relaxation enhancements (PREs) that critically reduce relaxation times due to the codissolved radicals. PREs are the primary source of polarization losses canceling the signal improvements obtained by DNP. We solve this problem by using potassium nitrosodisulfonate (Frémy's salt) as polarization agent (PA), which provides high nuclear spin polarization and allows for rapid scavenging under mild reducing conditions. We demonstrate the potential of Frémy's salt, (i) showing that both 1H and 13C polarization of ∼30% can be achieved and (ii) describing a hybrid sample shuttling system (HySSS) that can be used with any DDNP/NMR combination to remove the PA before NMR detection. This gadget mixes the hyperpolarized solution with a radical scavenger and injects it into an NMR tube, providing, within a few seconds, quantitatively radical-free, highly polarized solutions. The cost efficiency and broad availability of Frémy's salt might facilitate the use of DDNP in many fields of research.

A review of nuclear magnetic resonance (NMR) technology applied in the characterization of polymer gels for petroleum reservoir conformance control

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons with large thermal polarization to NMR-active nuclei via microwave (mw) irradiation. The ability to generate arbitrarily shaped mw pulses using arbitrary waveform generators allows for remarkable improvement of the robustness and versatility of DNP. We present here novel design principles based on single-spin vector effective Hamiltonian theory to develop new broadband DNP pulse sequences, namely, an adiabatic version of XiX (X–inverse X)–DNP and a broadband excitation by amplitude modulation (BEAM)–DNP experiment. We demonstrate that the adiabatic BEAM-DNP pulse sequence may achieve a 1H enhancement factor of ∼360, which is better than ramped-amplitude NOVEL (nuclear spin orientation via electron spin locking) at ∼0.35 T and 80 K in static solids doped with trityl radicals. In addition, the bandwidth of the BEAM-DNP experiments (~50 MHz) is about three times the 1H Larmor frequency. The generality of our theoretical approach will be helpful in the development of new pulsed DNP sequences.

With the increasing shortage of timber resources and the advancement of environmental protection projects, many artificial fast-growing forests are planted and used as raw materials in China. There are significant differences in the properties of natural forest wood and artificial fast-growing forest wood, and the properties of wood mainly depend on the change in the status of bound water in the cell wall. In this study, the fiber saturation point (FSP) and pore size distributions within the cell wall of six species of fast-growing forest wood were studied by low-temperature nuclear magnetic resonance (NMR) technology. The effects of species, growth rings, and extractives on the FSP and pore structure were analyzed. The water vapor sorption experiments were performed, and the adsorption isotherms of the samples were fitted through the Guggenheim-Anderson-de Boer (GAB) equation. According to the least-square regression of the adsorption isotherms and combined with the low-temperature NMR experiments, the content and proportion of the different types of bound water were analyzed. The results showed that the average FSP of each Chinese fir was about 40% and that of each poplar was about 35%. There is about a 10% difference between the FSP measured by NMR technology and the adsorption bound water content obtained by adsorption isothermal. The pore size distribution results show that in all samples, the proportion of pores larger than 10.5 nm is very low, about 10%; the proportion of 1.92-10.5 nm pores is about 30%; and the proportion of pores smaller than 1.92nm is more than 50%. This work will be helpful to the study of the wood moisture status and provide reference data for wood modification.

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons to nuclear spins. DNP performance can vary significantly depending on the types of polarizing agents employed, and the criteria for optimum DNP efficiency are not fully understood. Thus, a better understanding of the structure, electron paramagnetic resonance (EPR) line widths, and relaxation properties would aid in designing more efficient DNP polarizing agents. However, EPR characterizations of the polarizing agents are typically performed in different environments (e.g., strength of magnetic field and microwave power) than typical DNP experiments. Here, we demonstrate a low-cost and home-built setup that enables in situ EPR detection in a dual resonance DNP-NMR/EPR probe using an optical chopper. The optical chopper modulates the microwave irradiation, thereby modulating the longitudinal magnetization (Mz) of the electron spins, which results in a LOngitudinal-Detected (LOD) EPR spectrum. The EPR spectrum acquired for TEMPOL at 6.7 T/188 GHz using a mechanical modulation showed a good agreement with the spectrum acquired using an electrical modulation. Hence, our work provides a proof-of-principle setup that enables an optical chopper EPR spectrometer.

Zero- and ultralow-field (ZULF) nuclear magnetic resonance (NMR) is an emerging tool for precision chemical analysis. Unlike conventional (high-field) NMR, which relies on chemical shifts for molecular identification, zero-field analog reports J-spectra that depend on the nuclear spin-spin coupling topology of molecules under investigation. While chemical shifts are usually a small fraction of the resonance frequencies, J-spectra for various spin systems are completely different from each other. In this work, we use zero-field NMR to study dynamic chemical processes and investigate the influence of chemical exchange on ZULF NMR spectra. We develop a computation approach that allows quantitative calculation of ZULF NMR spectra in the presence of chemical exchange and apply it to study aqueous solutions of [15N]ammonium as a model system. In this system, proton exchange rates span more than three orders of magnitude depending on acidity (pH), as monitored by high-field and ZULF NMR. We show that chemical exchange substantially affects the J-coupled NMR spectra and, in some cases, can lead to degradation and complete disappearance of the spectral features. To demonstrate potential applications of ZULF NMR for chemistry and biomedicine, we show a ZULF NMR spectrum of [2-13C]pyruvic acid hyperpolarized via dissolution dynamic nuclear polarization (dDNP). The metabolism of pyruvate provides valuable biochemical information and its monitoring by zero-field NMR could give spectral resolution that is hard to achieve at high magnetic fields. We foresee applications of affordable and scalable ZULF NMR coupled with hyperpolarization modalities to study chemical exchange phenomena in vivo and in situations where high-field NMR detection is not possible to implement.

In order to explore the change characteristics of the adsorption and desorption performance of coal under high gas pressure, low-field nuclear magnetic resonance (LFNMR) technology was used to conduct experimental research on coal adsorption and desorption. The results show that (1) the T 2 spectrum distribution diagram shows adsorption peaks ( T 2 = 0.01 ms ~ 1 ms ) and free peaks ( T 2 = 5 ms ~ 1000 ms ); (2) with the increase of equilibrium pressure, the peak area of adsorbed methane gradually increased at first and then tended to equilibrium. The initial increase rate of free methane was slower than that of adsorbed methane, and the increase rate of free methane was faster in the later stage; (3) the relationship between the amount of adsorbed gas in the adsorption state of coal and the gas pressure conforms to the Langmuir equation. Taking the equilibrium pressure P = 8.7 MPa as the critical hysteresis pressure, it can be divided into two stages which are higher gas pressure (0.5~8.7 MPa) and high gas pressure (8.7~10.33 MPa); the amount of adsorbed gas in the free state has a linear relationship with the gas pressure, and there is no obvious hysteresis; (4) comparative analysis under the same experimental conditions, the mass of the adsorbed gas in the desorption process is greater than the mass of the adsorbed gas in the adsorption process, and there is basically no difference in the mass of the free gas.

Characterization of mechanical properties and microstructure of silica mortar and coral mortar based on nuclear magnetic resonance technology

Freeze-thaw damage evolution of fractured rock mass using nuclear magnetic resonance technology

“Fracturing network+imbibition oil production” is a new attempt to effectively develop low-permeability tight reservoirs. Fracturing fluid is not only a carrier for sand carrying but also a tool in the process of imbibition. On the basis of imbibition experiments, combined with nuclear magnetic resonance and pseudo-color processing technology, this paper clarified the dominant forces of different types of pore-throat and quantitatively characterized the contribution of different levels of pore-throat to imbibition oil recovery. The results show that gravity is the main controlling force of imbibition for reservoirs with higher permeability. Fluid replacement mainly occurs in the early period of imbibition. Macropores contribute most of the imbibition recovery, mesopores have a weak contribution, and the contribution of micropores and pinholes can be ignored. For the reservoirs with low permeability, capillary force is the main controlling force of imbibition. Fluid replacement mainly occurs in the later period of imbibition. Macropores contribute most of the imbibition recovery rate, mesopores contribute a small part of the imbibition recovery factor, and the contribution of micropores and pinholes can be ignored. This paper clarified that macropores and mesopores are the main sources of the contribution of imbibition recovery efficiency, and oil content and connectivity are key factors for the imbibition recovery efficiency.

The photoreactions of poly(methyl isopropenyl ketone) (PMIK) and two low‐molecular‐weight model compounds have been investigated in solution with proton NMR spectroscopy. Chemically induced dynamic nuclear polarization (CIDNP) has been observed in all three systems, but the CIDNP intensities have been found to depend on the chain lengths of the reaction products: long‐chain products fail to show CIDNP at room temperature, whereas simultaneously formed small fragments exhibit nuclear spin polarization. The CIDNP originates from the Norrish type I decomposition of the ketones, which for the polymer occurs from the triplet state in all solvents. In the model compounds this cleavage can occur from the triplet or singlet state. The main photoreaction of PMIK, namely, scission of the polymer backbone, does not give rise to CIDNP. The failure of long‐chain products to display CIDNP is attributed in part to a relaxation phenomenon: the short nuclear spin relaxation times of the long‐chain product molecules destroy the CIDNP before it can be detected. In high‐temperature studies above 150°C, however, in which the relaxation times are longer, CIDNP has been detected in long‐chain molecules as well.