Nuclear Magnetic Resonance Probe Head Research Articles

Noncontact In Situ Multidiagnostic NMR/Dielectric Spectroscopy.

Introduction of a dielectric material in a nuclear magnetic resonance (NMR) probe head modifies the frequency response of the probe circuit, a phenomenon revealed by detuning of the probe. For NMR spectroscopy, this detuning is corrected for by tuning and matching the probe head prior to the NMR measurement. The magnitude of the probe detuning, "the dielectric shift", provides direct access to the dielectric properties of the sample, enabling NMR spectrometers to simultaneously perform both dielectric and NMR spectroscopy. By measuring sample dielectric permittivity as a function of frequency, dielectric permittivity spectroscopy can be performed using the new methodology. As a proof of concept, this was evaluated on methanol, ethanol, 1-propanol, 1-pentanol, and 1-octanol using a commercial cross-polarization magic angle spinning (CPMAS) NMR probe head. The results accurately match the literature data collected by standard dielectric spectroscopy techniques. Subsequently, the method was also applied to investigate the solvent-surface interactions of water confined in the micropores of an MFI-type, hydrophilic zeolite with a Si/Al ratio of 11.5. In the micropores, water adsorbs to Bro̷nsted acid sites and defect sites, resulting in a drastically decreased dielectric permittivity of the nanoconfined water. Theoretical background for the new methodology is provided using an effective electric circuit model of a CPMAS probe head with a solenoid coil, describing the detuning resulting from the insertion of dielectric samples in the probe head.

A Portable NMR Spectrometer With a Probe Head Combining RF and DC Capabilities to Generate Pulsed-Field Gradients

Portable nuclear magnetic resonance (NMR) spectrometers are used in various scientific fields. A vast number of applications are suitable for low-field permanent magnets. However, most of these systems lack the ability of pulsed-field gradients as these require cumbersome additional hardware. We herein report an NMR probe head that incorporates a radio frequency (RF)-coil equipped with pulsed-field gradient capabilities in a single center-tapped solenoid. We developed a circuitry allowing for the application of direct currents without disturbing the RF reception capabilities. Furthermore, we present a System-on-Chip-based NMR spectrometer. Combined with the NMR probe head, it represents a versatile low-cost system capable of field gradient generation. The probe head is characterized in terms of scatter parameters, quality factor, and transversal field homogeneity. We exhibit simulation data of the field gradient and present the required external circuitry. Experimental results show further the dephasing capabilities of the gradient and give insights into the spatial shape. Finally, the system is applied for the mapping of J-couplings in 1-butanol via zero-quantum coherences. A theoretical analysis of the pulse sequence is given for a homonuclear two-spin system. The experimental results demonstrate the potential use of this setup.

Nuclear magnetic resonance probe head design for precision strain control.

We present the design and construction of an NMR probe to investigate single crystals under strain at cryogenic temperatures. The probe head incorporates a piezoelectric-based apparatus from Razorbill Instruments that enables both compressive and tensile strain tuning up to strain values on the order of 0.3% with a precision of 0.001%. 75As NMR in BaFe2As2 reveals large changes to the electric field gradient and indicates that the strain is homogeneous to within 16% over the volume of the NMR coil.

Studying Fast Reaction Kinetics with Online NMR Spectroscopy

AbstractA liquid thermostated microreactor Nuclear Magnetic Resonance (NMR) probe head is presented that facilitates the investigation of kinetics of fast reactions. With this setup, reaction kinetics can be studied in a broad temperature (–20 – 100 °C) and pressure range (0 – 60 bar). The setup and NMR probe head are discussed from a reaction engineering standpoint based on experiments and simulations. The microreactor probe head widens significantly the range of applications of online NMR spectroscopy for reaction and process monitoring.

An optimised detector for in-situ high-resolution NMR in microfluidic devices

Integration of high-resolution nuclear magnetic resonance (NMR) spectroscopy with microfluidic lab-on-a-chip devices is challenging due to limited sensitivity and line broadening caused by magnetic susceptibility inhomogeneities. We present a novel double-stripline NMR probe head that accommodates planar microfluidic devices, and obtains the NMR spectrum from a rectangular sample chamber on the chip with a volume of 2μl. Finite element analysis was used to jointly optimise the detector and sample volume geometry for sensitivity and RF homogeneity. A prototype of the optimised design has been built, and its properties have been characterised experimentally. The performance in terms of sensitivity and RF homogeneity closely agrees with the numerical predictions. The system reaches a mass limit of detection of 1.57nmols, comparing very favourably with other micro-NMR systems. The spectral resolution of this chip/probe system is better than 1.75Hz at a magnetic field of 7T, with excellent line shape.

A single coil radio frequency gradient probe for nuclear magnetic resonance applications

A single coil nuclear magnetic resonance (NMR) probehead and associated electronics capable of asynchronously applying both homogeneous and inhomogeneous radio frequency (rf) pulses to solid, liquid, or gaseous samples is described. This equipment can be interfaced with a conventional single channel NMR spectrometer. Carefully placed PIN diodes on the NMR probehead are used to switch the coil between a homogeneous end tapped configuration and an inhomogeneous center tapped rf gradient configuration. This approach dramatically improves channel isolation in comparison to existing two coil designs. Descriptions of the new probehead, the transistor-transistor logic (TTL) controlled dc pulser for PIN diode gating, and the high power rf switch are provided. Several NMR pulse sequences are used to test the channel isolation and probe performance. Finally an application to liquid phase solvent suppression is provided.

Exceeding the limit of dynamics studies on biomolecules using high spin-lock field strengths with a cryogenically cooled probehead

Internal motions in the microsecond timescale have been proposed to play an active part in a protein’s biological function. Nuclear magnetic resonance (NMR) relaxation dispersion is a robust method sensitive to this timescale with atomic resolution. However, due to technical limitations, the observation of motions faster than ∼40μs for 15N nuclei was not possible. We show that with a cryogenically cooled NMR probehead, a high spin-lock field strength can be generated that is able to detect motions as fast as 25μs. We apply this high spin-lock field strength in an NMR experiment used for characterizing dynamical processes. An on-resonance rotating-frame transverse relaxation experiment was implemented that allows for the detection of a 25μs process from a dispersion curve, and transverse relaxation rates were compared at low and high spin-lock field strengths showing that at high field strengths contributions from chemical exchange with lifetimes up to 25μs can be removed. Due to the increase in sensitivity towards fast motion, relaxation dispersion for a residue that undergoes smaller chemical shift variations due to dynamics was identified. This technique reduces the previously inaccessible window between the correlation time and the relaxation dispersion window that covers four orders of magnitude by a factor of 2.

Hardware modification of a 7 mm MAS NMR probe to a single-crystal goniometer

Tensorial terms of the Hamiltonian can be measured by solid-state single-crystal nuclear magnetic resonance (NMR) spectroscopy which requires a goniometer NMR probehead. Goniometer probes; however, are not standard parts of solid NMR spectrometers and are available only at a much higher price than magic-angle spinning (MAS) probeheads widely used in research. Due to requirements of MAS experiments, modern probeheads are designed for small ceramic rotors, which are 1–4 mm in diameter, to reach very high angular frequencies, so there are several older 7 mm MAS probeheads used rarely todays in NMR laboratories. In this paper, a simple method is presented how to rebuild step-by-step a 7 mm Bruker MAS probehead to be suitable for single-crystal spectroscopy. In the second part 31P chemical shift tensors of Na 4 P 2 O 7 · 10 H 2 O are determined to demonstrate the functionality of the rebuilt probehead.

Low-cost high-temperature NMR probe head

The design of a simple high-temperature nuclear magnetic resonance (NMR) probe head for narrow-bore magnets is presented. It covers the temperature range from 20 to 1300°C, necessitating a heating power of below 100 W. Several probe heads of this design, manufactured for NMR solenoids with bores from 30 to 54 mm have shown good stability and long life times.

Director reorientation processes in a monodomain thin nematic liquid crystal film: A deuterium NMR spectroscopy study

Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the director dynamics in the nematic liquid crystal, 4-pentyl-4′-cyanobiphenyl (5CB), confined between two glass plates and subject to magnetic and electric fields. The nematic cell was held in the NMR probe head so that the electric field, whose direction is normal to the substrate surface, makes an angle of about 45° with the magnetic field. This experimental geometry avoids the degeneracy in the field-induced alignment pathway for the director found for larger angles. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of time. When the electric field, whose intensity is controlled so that the director makes an angle with the magnetic field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to the magnetic field because Δε̃ and Δχ̃ are both positive for 5CB. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-on and the turn-off alignment processes as a function of time. The realignment pathway of the director was monitored by measuring Δν̃, the deuterium quadrupolar splitting. We have studied the time dependence of the director orientation for the turn-on and turn-off processes at different temperatures in the nematic phase. The temperature independence of Δχ̃/Δε̃ is also discussed on the basis of experiment and theory. The diamagnetic anisotropy and the rotational viscosity coefficient were also determined as a function of temperature. The deuterium NMR spectra corresponding to the field-induced director dynamics were predicted by an analysis based on hydrodynamic theory.

Measurement of dilute 29Si species in solution using a large volume coil and DEFT NMR

A large-sample-volume nuclear magnetic resonance (NMR) spectroscopy probehead has been developed for the detection and characterization of low concentrations of 29Si species in aqueous solution. The approach described entails the use of a large-diameter radio frequency solenoid coil that permits substantially larger sample volumes to be investigated at moderate magnetic field strengths, compared with conventional NMR probehead configurations. In addition, difficulties presented by long 29Si T1 relaxation times have been circumvented by using the DEFT NMR pulse sequence, which permits more rapid signal averaging. Through a combination of these hardware and methodological improvements, high-resolution 29Si NMR spectra have been obtained at 4.2 T (29Si resonance frequency = 36.8 MHz) for an 800 microM solution of 96% 29Si-enriched silicic acid, H4SiO4 (pH approximately 8), with a signal-to-noise ratio of 16 and a line width of 31 Hz after 3 h of total measurement time.