Field Nuclear Magnetic Resonance Spectrometer Research Articles

Super-resolution NMR spectroscopy

Spectral resolution is one of the limiting factors in nuclear magnetic resonance (NMR) spectroscopy of biological systems where signal overlap often interferes with chemical shift assignment as well as dynamics and structure analysis. This problem can be addressed in part by using higher magnetic field NMR spectrometers operating at up to 1.2 GHz 1H frequency to enhance the resolution proportionally with the field strength, and by deuteration in combination with transverse relaxation-optimized spectroscopy that reduces the transverse relaxation rate and proportionally the resonance linewidth of the peaks yielding higher spectral resolution. As a complement or alternative to these expensive and often insufficient approaches, we present here a generally applicable method to reduce the linewidth of peaks in indirect dimensions of multi-dimensional NMR spectra by increasing the number of scans per time increment exponentially as a function of time in order to compensate, in part, the decay of the signal caused by transverse relaxation. This enables to achieve a user-defined linewidth of the peaks without undue increase of the noise. Optimization by including in the number of scans also a cosine apodization function as well as processing spectra with an exponential-cosine window function in the direct dimension results typically in a resolution enhancement (linewidth reduction) by a factor of 1.5–2 in comparison to a standard measurement with a constant number of scans per time increment. This is comparable to the 2-fold resolution enhancement that can be obtained by going from a 600 MHz 1H frequency NMR spectrometer to a 1.2 GHz instrument, or from 1.2 GHz to a spectrum measured hypothetically at 2.4 GHz 1H frequency. A factor of two resolution enhancement causes thereby a signal to noise loss of a factor of three. The sensitivity gain by dynamic number of scan sampling is thereby ∼20 % over the use of a digital apodization function.

Review of Recent Developments in NMR Core Analysis

The last 10 years have witnessed numerous new developments and advancements in nuclear magnetic resonance (NMR) rock core analysis both in conventional and unconventional reservoirs. On the hardware side, NMR instruments are now capable of handling hundreds of thousands of radio frequency (RF) pulses, and the pulses can be more closely spaced than ever. This allows measuring samples with very fast relaxation times, which was not possible before. Higher field NMR spectrometers operating at frequencies of ~12 MHz and ~20 MHz (in addition to the standard low-field ~2 MHz instruments) were introduced, which drastically improved the signal-to-noise ratio (SNR) and even allowed two-dimensional (2D) and three-dimensional (3D) NMR imaging to be performed on the samples to complement traditional relaxation measurements. Another consequence of using higher field instruments is that even samples with a very low amount of NMR signal can now provide useful data in a reasonable amount of time. Additional hardware improvements include the development of NMR-compatible high-temperature and high-pressure vessels and faster and more robust electronics and computers, which allow faster data acquisition, processing, and a more user-friendly environment for new NMR pulse sequence design and implementation. Following the hardware improvements, new applications were developed for rock core analysis, including modified Carl-Purcell-Meiboom-Gill (CPMG) pulse sequences for minimizing sample heating. NMR-based wettability, relative permeability, capillary pressure, cuttings, and long core measurements have also been successful. In unconventionals, NMR-based gas isotherm measurements and fluid-typing measurements are now common practices. This review paper sets out to summarize the recent NMR developments in the industry as well as provide references where more details can be found on each topic.

An Inexpensive Earth Field NMR Spectrometer with Improved Shimming.

A simple, cost-effective, do-it-yourself earth field nuclear magnetic resonance spectrometer with shimming is demonstrated. A spectrometer for understanding magnetic resonance physics in an academic environment is presented here along with its coil and system component design. This system was designed with inexpensive and readily available electronic components costing less than US $130. The three signal chains in the system include the polarizer along with the polarization coil, transmitter driver with transceiver coil, and receiver circuit serving as a low-noise amplifier. A microcontroller acting as the signal generator and processor controls the entire system. Switching between the transmitter and the receiver is via a relay circuit. In experiments, the free induction decay obtained from the water sample lasted 2 s with an amplitude of 5 a.u. at 1512 Hz. The line width decreased by 13 Hz after active shimming compared to 3 Hz with passive shimming. The spin echo results of the commercially available Terranova-MRI system with and without sample were used as a benchmark for the low-cost earth field nuclear magnetic resonance spectrometer. The entire system had to be fine-tuned to visualize the free induction decay because the 1.2 kHz and 50 Hz noise was predominant before tuning. Future work will involve the incorporation of gradients and time-shared pulse sequence design.

Insert magnet and shim coils design for a 27 T nuclear magnetic resonance spectrometer with hybrid high and low temperature superconductors

An extremely-high field nuclear magnetic resonance (NMR) spectrometer has the potential to significantly augment the resolution and sensitivity of the atomic resonance spectrum, which will significantly boost the development of a range of scientific areas. The success of an extremely-high field NMR spectrometer depends to a great extent on the development of a superconducting magnet system. A high-temperature superconducting (HTS) insert magnet and low-temperature superconducting (LTS) shim coils design scheme for a 27 T NMR spectrometer is proposed in this work. A LTS background magnet was used to generate a 15 T magnetic field in the central area and the designed HTS insert magnet contributed the remaining 12 T magnetic field. The HTS insert magnet adopted notch double-pancake (DP) optimization to improve the rough magnetic field distribution in the central area to a highly-homogeneous level, and shim compensation schemes were proposed for the hybrid magnet to eliminate the inhomogeneous magnetic field components in the practical fabrication. The HTS insert magnet design had a current margin of 45.64% in terms of the perpendicular magnetic field intensity and the maximum hoop stress was reduced to 260 MPa with binding. There were in total eight shim coils, including two Nb3Sn zonal coils located between the insert magnet and the background magnet and six NbTi tesseral coils placed outside the background magnet. The magnet system will be fabricated in the near future.

Low-field 1 H NMR spectroscopy: Factors impacting signal-to-noise ratio and experimental time in the context of mixed microstructure polyisoprenes.

Low-cost, high-accuracy characterization of polymeric materials is critical for satisfying societal demand for high-quality materials with ultra-specific requirements. Low-field nuclear magnetic resonance (NMR) spectroscopy presents an opportunity to replace costlier or destructive methods while utilizing nondeuterated solvents. Many factors play key roles in the ability of low-field NMR spectroscopy to accurately analyze polymer systems. Sample characteristics such as polymer concentration, composition, and molecular weight all directly affect the capability of low-field spectrometers to accurately determine polymer microstructure compositions. In addition to inherent sample properties affecting instrumental accuracy, many choices concerning instrumental parameters (including number of scans, relaxation delay, spectral width, and points per scan) must be made that impact the quality of the resulting NMR spectra. In this work, we benchmark the capability of a 60-MHz low-field NMR spectrometer for analyzing polymer materials using mixed microstructure polyisoprenes as a model polymer system of interest. The aforementioned critical sample and instrumental variables are varied, and we report on the ability to quantitatively characterize polyisoprene microstructure to within 1-2% of a higher field NMR spectrometer (400 MHz). We anticipate our findings to be generally applicable to other low-field spectrometers of similar field strength and other polymer systems.

Recent Contributions of Nuclear Magnetic Resonance in Organocatalysis Mechanism Elucidation

Background: Nuclear Magnetic Resonance (NMR) is one of the most employed techniques in structural elucidation of organic compounds. In addition to its use in structural characterization, it has been widely employed in the investigation of reaction mechanisms, especially those involving catalysis. Objective: In this review, we aim to provide recent examples of the interface of NMR and organocatalysis reaction mechanism. Methods: Selected examples of different approaches for mechanism elucidation will be presented, such as isotopic effect, catalyst labelling and online reaction monitoring. A discussion involving the use of solid-state NMR will also be disclosed. Conclusion: NMR consists of a non-destructive technique, extremely useful in the real-time identification of intermediates in crude reaction mixtures. With the advent of two-dimensional experiments and high field NMR spectrometers, the reports of studies involving mechanistic elucidation were greatly enhanced. In this context, nowadays NMR appears as a powerful tool for the comprehension of reaction mechanisms, including the possibility of the proposal of unknown reaction mechanisms within organocatalysis.

In-Vivo NMR Spectroscopy: A Powerful and Complimentary Tool for Understanding Environmental Toxicity.

Part review, part perspective, this article examines the applications and potential of in-vivo Nuclear Magnetic Resonance (NMR) for understanding environmental toxicity. In-vivo NMR can be applied in high field NMR spectrometers using either magic angle spinning based approaches, or flow systems. Solution-state NMR in combination with a flow system provides a low stress approach to monitor dissolved metabolites, while magic angle spinning NMR allows the detection of all components (solutions, gels and solids), albeit with additional stress caused by the rapid sample spinning. With in-vivo NMR it is possible to use the same organisms for control and exposure studies (controls are the same organisms prior to exposure inside the NMR). As such individual variability can be reduced while continual data collection over time provides the temporal resolution required to discern complex interconnected response pathways. When multidimensional NMR is combined with isotopic labelling, a wide range of metabolites can be identified in-vivo providing a unique window into the living metabolome that is highly complementary to more traditional metabolomics studies employing extracts, tissues, or biofluids.

Full Correlations across Broad NMR Spectra by Two-Field Total Correlation Spectroscopy.

Total correlation spectroscopy (TOCSY) is a key experiment to assign nuclear magnetic resonance (NMR) spectra of complex molecules. Carbon-13 TOCSY experiments are essential to assign signals of protein side chains. However, the performance of carbon-13 TOCSY deteriorates at high magnetic fields since the necessarily limited radiofrequency irradiation fails to cover the broad range of carbon-13 frequencies. Here, we introduce a new concept to overcome the limitations of TOCSY by using two-field NMR spectroscopy. In two-field TOCSY experiments, chemical shifts are labelled at high field but isotropic mixing is performed at a much lower magnetic field, where the frequency range of the spectrum is drastically reduced. We obtain complete correlations between all carbon-13 nuclei belonging to amino acids across the entire spectrum: aromatic, aliphatic and carboxylic. Two-field TOCSY should be a robust and general approach for the assignment of uniformly carbon-13 labelled molecules in high-field and ultra-high field NMR spectrometers beyond 1000 MHz.

In situ measurement of liquid-liquid equilibria by medium field nuclear magnetic resonance

A new method for non-invasive measurement of liquid-liquid equilibria (LLE) using a compact medium field nuclear magnetic resonance (NMR) spectrometer is presented. Mixing of all components, phase separation, and analysis of the composition of the coexisting phases is performed in situ in an NMR glass tube. Thus, the experimental effort is reduced and errors caused by sampling are eliminated. Furthermore, calibration of the analysis method is not necessary as quantitative information is obtained directly from the NMR spectra. The proposed method for studying LLE in situ can be swiftly conducted in standard chemical laboratories as medium field NMR spectrometer do not require dedicated laboratory infrastructure but enable convenient handling and fast analysis of the samples. In the present work, four non-reactive ternary solvent systems with closed miscibility gap (toluene + acetone + water, diethyl ether + acetone + water, diethyl ether + methanol + water, and acetonitrile + ethanol + cyclohexane) and one reactive ternary system (water + acetic acid + acetic anhydride) were investigated at a temperature of 22.0 °C using 1H medium field NMR spectroscopic measurements. For comparison, the composition of the coexisting phases is also examined for one non-reactive system (acetonitrile + ethanol + cyclohexane) using 13C medium field NMR spectroscopy as well as spatially resolved spectroscopy in a conventional high field NMR spectrometer. The comparison of the results of the present work to literature data shows that the new proposed method enables swift and reliable investigations of LLE.

Operation of a 400 MHz NMR magnet using a (RE:Rare Earth)Ba2Cu3O7−x high-temperature superconducting coil: Towards an ultra-compact super-high field NMR spectrometer operated beyond 1 GHz

High-temperature superconductors (HTS) are the key technology to achieve super-high magnetic field nuclear magnetic resonance (NMR) spectrometers with an operating frequency far beyond 1GHz (23.5T). (RE)Ba2Cu3O7−x (REBCO, RE: rare earth) conductors have an advantage over Bi2Sr2Ca2Cu3O10−x (Bi-2223) and Bi2Sr2CaCu2O8−x (Bi-2212) conductors in that they have very high tensile strengths and tolerate strong electromagnetic hoop stress, thereby having the potential to act as an ultra-compact super-high field NMR magnet. As a first step, we developed the world’s first NMR magnet comprising an inner REBCO coil and outer low-temperature superconducting (LTS) coils. The magnet was successfully charged without degradation and mainly operated at 400MHz (9.39T). Technical problems for the NMR magnet due to screening current in the REBCO coil were clarified and solved as follows: (i) A remarkable temporal drift of the central magnetic field was suppressed by a current sweep reversal method utilizing ∼10% of the peak current. (ii) A Z2 field error harmonic of the main coil cannot be compensated by an outer correction coil and therefore an additional ferromagnetic shim was used. (iii) Large tesseral harmonics emerged that could not be corrected by cryoshim coils. Due to those harmonics, the resolution and sensitivity of NMR spectra are ten-fold lower than those for a conventional LTS NMR magnet. As a result, a HSQC spectrum could be achieved for a protein sample, while a NOESY spectrum could not be obtained. An ultra-compact 1.2GHz NMR magnet could be realized if we effectively take advantage of REBCO conductors, although this will require further research to suppress the effect of the screening current.

Ultralow field NMR spectrometer with an atomic magnetometer near room temperature

We present a Cs atomic magnetometer with a sensitivity of 150fT/Hz1/2 operating near room temperature. The nuclear magnetic resonance (NMR) signal of 125μL tap water was detected at an ultralow magnetic field down to 47nT, with the signal-to-noise ratio (SNR) of the NMR signal approaching 50 after eight averages. Relaxivity experiments with a Gd(DTPA) contrast agent in zero field were performed, in order to show the magnetometer’s ability to measure spin–lattice relaxation time with high accuracy. This demonstrates the feasibility of an ultralow field NMR spectrometer based on a Cs atomic magnetometer, which has a low working temperature, short data acquisition time and high sensitivity. This kind of NMR spectrometer has great potential in applications such as chemical analysis and magnetic relaxometry detection in ultralow or zero fields.

Characterizing longitudinal and transverse relaxation rates of ferrofluids in microtesla magnetic fields

Shortening spin-lattice relaxation rates (1/T1) or spin-spin relaxation rates (1/T2) is the purpose of magnetic resonance imaging contrast agents. In this work, an ultralow field nuclear magnetic resonance spectrometer and imager are set up to characterize the spin relaxation rates of Fe3O4 superparamagnetic iron oxide (SPIO) for image contrast. It was found that both 1/T1 and 1/T2 increase linearly when the magnetic susceptibility χ of SPIO increases by increasing the concentration of SPIO dispersed in water. In an applied field, magnetic moments of SPIO generate microscopic field gradients that weaken the field homogeneity, in turn de-phasing the proton’s nuclear spin and enhancing the relaxation rates. A T1-contrast image is demonstrated, using SPIO as the contrast agent and high-Tc superconducting quantum interference devices as the detector. T1-contrast imaging in microtesla fields might provide a potential modality for discriminating cancer.

A Possible Difference in the Surface Relaxivity of Costal and Inland Sands

The 1 H nuclear magnetic resonance (NMR) spin-lattice relaxation rate of hydrated sands is related to the sur- face-to-volume ratio of the voids or pores between the hydrated sand grains and the surface relaxivity of the grains. The electron spin resonance (ESR) signal is often used to predict the relative surface relaxivity as the surface relaxivity is thought to be proportional to the concentration of paramagnetic species in the sand grains. We have identified a discrep- ancy in the surface relaxivity and ESR signal of an ocean beach sand compared to two sands of similar diameter from in- land deposits. This difference can either be due to more surface weathering of the inland sand or more paramagnetic mate- rial from seawater adhering to the ocean sand. tion of these metals on the surface layer to differ from that within the grain. The comparison made to exemplify this difference is between an active beach sand from Asilomar, CA USA, and two inland sands from Illinois USA. We pre- sent the details of these findings in this paper. In the previous paper, we reported R1 values at proton resonance frequencies ( ) between 30 MHz and 10 kHz. This data was used to extrapolate a value for R1 and  at  = 2.5 kHz, the approximate resonance frequency of protons in the magnetic field of the Earth. This extrapolation was risky because the dispersion in R1 with  could either continue at  < 10 kHz or stop and R1 level off. In the current presentation, we have added a datum point at 1.9 kHz using an Earth's field NMR spectrometer that eliminated the need to extrapo- late. BACKGROUND

Evaluation of Composites Miscibility by Low Field NMR

Solid state nuclear magnetic resonance (NMR) is a powerful technique to analyze polymer composites. The use of proton relaxation data to evaluate the dispersity of the dispersion phase in the composite and its homogeneity are very wide. In this work PVC/silica composites were prepared using different ratios to obtain a new material to be used in chromatograph column for separation of plant extracts. Thus, this study used proton spin–lattice and spin-spin nuclear magnetic resonance relaxation data, determined in a low field NMR spectrometer as a method to characterize those composites in relation to silica dispersion as well as PVC/silica compatibility. From the relaxation data analyses, the best ratio of PVC/silica, according to the components dispersion and consequently composite compatibility, was 80/20, according to the determined T1 values.

Weak field NMR and ESR spectrometers and magnetometers

Weak field magnetic resonance is usually limited by the signal-to-noise ratio, which decreases drastically with the field. The authors present high-resolution NMR (nuclear magnetic resonance) and ESR (electron spin resonance) sensors developed for Earth field magnetometry and spectrometry. For some of them the resonance signal is amplified by the Overhauser effect or laser optical pumping. The spectrometers and magnetometers are described. The features of magnetometers and many application fields are summarized. The performance of the low field spectrometers and their potential use in spin dynamic studies are also presented. >

3] 13C NMR Analysis of Complex Carbohydrates

This chapter describes the applications of 13C nuclear magnetic resonance (NMR) using higher field NMR spectrometers to the study of oligosaccharides containing a number of different monosaccharide units with special attention given to the examination of solution conformations. Typically, 13C NMR spectra are obtained using broadband decoupling of protons. Spectra are relatively simple, with a single sharp resonance for each carbon in the compound. At natural abundance levels, 13C–13C coupling is not observable. Proton-coupled 13C spectra are much more complex and show the number of hydrogens covalently bonded to each carbon because each 13C resonance is split by covalently bonded hydrogens into n + 1 lines. Additional splittings or line-broadening because of 2- and 3-bond and long-range coupling also add to the complexity of 1H-coupled 13C spectra. Several experimental options are available on most high-field FT spectrometers that facilitate assignment of 13C resonances and of 13C–1H coupling constants. These include: gated 1H decoupling, off-resonance 1H decoupling, selective 13C saturation combined with gated 1H decoupling and FT difference spectroscopy, and two-dimensional J spectroscopy. The latter may be particularly valuable for complex molecules because it permits 13C chemical shifts and 13C–1H coupling of each 13C nucleus to be displayed separately.

Magnet system of the 500 MHz NMR spectrometer at the Francis Bitter National Magnet Laboratory: I. Design and development of the magnet

A high field nuclear magnetic resonance spectrometer has been put into operation at the MIT, Francis Bitter National Magnet Laboratory. The spectrometer is built around a high homogeneity magnet, the design field of which was 11.74 T, and which in fact operates in persistent mode at a field of 11.71 T. The magnet is wound using niobium–titanium superconductor cooled to 2.3 K and is housed in a special dewar designed to permit continuous subatmospheric cryogenic operation and to minimize helium consumption. This and a companion paper describe the analysis, design, and construction of the magnet.