Earth's Field NMR Research Articles

Modelling of T1 dispersion effects on fluid polarization in oil flow

In this article we use numerical simulations to study the effect of T1 dispersion on fluid polarization buildup in oil flow to characterize the sensitivity of both a conventional NMR concept (ROI located inside the polarization magnet) and a Earth's field NMR concept (ROI outside and downstream of the polarization magnet) to T1 dispersion of flowing samples. As a polarization field in both concepts we use a 90 cm long Halbach magnet. The T1 dispersion behavior of the oils is based on a set of crude oils that span a viscosity range of 0.7 cP up to 2·104 cP and T1 relaxation measurements for Larmor frequencies between 10 kHz and 20 MHz. Numerical simulations based on solving the Bloch-Torrey equation for the longitudinal magnetization component show that fluid polarization levels in a ROI of a Earth's field NMR system concept are much more strongly affected by T1 dispersion than in the conventional NMR system concept. As a result, we may conclude that the Earth's field NMR system design is less robust for measuring flowing samples that show strong T1 dispersion behavior. In comparison, the conventional NMR system design is relatively insensitive to the effect of T1 dispersion, as T1 dispersion effects were found to form a relatively small correction to the magnetization buildup. The conventional NMR system design consequently is the preferred implementation of a NMR system that operates on fluids with strong T1 dispersion behavior. We show that in the presence of T1 dispersion s = vT1(0)/Lm* may be used as a governing parameter for fluid polarization buildup, where T1(0) is the T1 relaxation time in the center of the polarization magnet, and we show how an modified analytical uniform field model can be used to describe fluid polarization for a uniform flow velocity distribution in the presence of T1 dispersion with an accuracy within 1% for the samples and field distribution considered in this study at industrially relevant flow velocities.

Earth's field NMR relaxation of pre-polarised water protons for real-time detection of free-radical formation.

Real-time imaging of free-radical formation is important in physical chemistry, biochemistry, and radiobiology, especially for the study of radiation dose-rate effects. Herein, we show for the first time that the formation of free radicals during the time course of a chemical reaction can be imaged through NMR relaxation measurements of water protons in the Earth's magnetic field, in an open-coil spectrometer. The relaxation rate constants of water magnetisation are enhanced as reactions leading to the formation of hydroxyl radicals and oxygen proceed on the timescale of tens of minutes. The reaction rate of iodide-catalysed H2O2 decay was followed by Earth-field 1H NMR relaxation in real time. The relaxivities of the reaction product and several other paramagnetic compounds were determined. Spin-trap molecules were then used to capture ˙OH radical species, thus altering the reaction rate in proportion to the formation of new paramagnetic compounds. Thereby, a new experimental method for magnetic resonance imaging of the formation of intermediate and stable radical species in water is proposed.

Physical methods and techniques: NMR spectroscopy

NMR spectroscopy continues to evolve, with publications in 2012 providing an eclectic collection of applications, advances and incremental improvements. Publication highlights include: the “Prodigy” probe from Bruker, a 60 MHz benchtop NMR spectrometer, Earth-field NMR, dual receivers and dual acquisition methods, para-hydrogen enhancement by chemical-association and high-field DNP. I am indebted to the great and the good whose work has been reported here, all credit is theirs; errors, omissions and blame are all mine.

Earth field NMR with chemical shift spectral resolution: Theory and proof of concept

A new method for obtaining an NMR signal in the Earth’s magnetic field (EF) is presented. The method makes use of a simple pulse sequence with only DC fields which is much less demanding than previous approaches in terms of the pulses’ rise and fall times. Furthermore, it offers the possibility of obtaining NMR data with enough spectral resolution to allow retrieving high resolution molecular chemical shift (CS) information – a capability that was not considered possible in EF NMR until now. Details of the pulse sequence, the experimental system, and our specially tailored EF NMR probe are provided. The experimental results demonstrate the capability to differentiate between three types of samples made of common fluorine compounds, based on their CS data.

Earth field NMR of porous systems and soils: experimental approach and results

Earth field NMR of porous systems and soils: experimental approach and results

Brine diffusion in first-year sea ice measured by Earth's field PGSE-NMR

Pulsed gradient spin echo (PGSE) measurements on first-year Antarctic sea ice have been performed using a portable Nuclear Magnetic Resonance probe that utilises the terrestrial magnetic field. This probe is a new device that incorporates independent transverse and longitudinal field gradient coils. A depth profile of the self-diffusion coefficient of liquid in brine inclusions has been measured through the 2002 season sea ice sheet in McMurdo Sound, near Cape Evans, Ross Island. The data is comprised of both horizontal and vertical brine diffusivity that have been measured in situ and with reduced invasive impact on the ice sheet, which constitutes a significant refinement of the Earth's field NMR technique. The observation that the depth dependence of the diffusion coefficient matched the expected magnitude with the temperature gradient through the ice sheet both confirmed the validity of our experiment and suggested that the sheet is best modelled as a matrix consisting of liquid inclusions in which the water molecules move in a Brownian manner.

Optimizing Sudden Passage in the Earth's-Field NMR Technique

The equation of motion d M/ dt = γM × B( t ) is solved numerically for the case B( t ) = j B p ( t ) + k B e . The field B e is a small static field, typically the earth's field. The field B p ( t ) is a damped oscillation having frequency greater than, or on the order of, the precession frequency in field B e . Such oscillation inevitably occurs at the end of the rapid cutoff of the coil current used to polarize the sample. It is assumed that B p ( t ) is initially large compared to B e , and that magnetization M is initially along the resultant field B. This is the usual situation in the earth's-field NMR technique when the polarizing field is produced by a coil of moderate to high impedance. It is shown that, when properly damped, the transient can be used to restore the magnetization to the x–y plane, thereby maximizing the amplitude of the subsequent free precession signal. The damping required is close to critical damping, so that the problem of circuit ringing when the coil is switched to receiver mode is also eliminated.

Condition for Sudden Passage in the Earth's-Field NMR Technique

The equation of motiondM/dt= γM × B is solved numerically in order to study the dynamic behavior of the magnetization during the cutoff of the polarizing field in the earth's-field NMR technique. It is assumed that the polarizing field is perpendicular to the earth's field and is reduced to zero linearly in a time short compared to all spin relaxation times. The numerical solution for arbitrary cutoff rates is obtained using the fourth-order Runge–Kutta algorithm and is found to be consistent with a power series solution. It is shown that the magnetization remains within a single octant, and that, for moderate to rapid cutoff, the cutoff rate is related to the cone angle of the subsequent free precession by[formula]Here θcis the complement of the cone angle in radians, γ is the proton gyromagnetic ratio, andB0is the magnitude of the static field (normally, the earth's field) used to detect the free precession.

Instrumentation for the Earth's Field NMR Technique

An instrument utilizing free precession in the earth's magnetic field has been developed for studying the field dependence of nuclear spin relaxation in liquids. Both 1H and 19F nuclei are accessible to measurement. The method requires that a current be switched on in the sample coil for a certain time, reduced quickly to some smaller value, and later reduced suddenly to zero. The resulting sample magnetization then precesses about the earth's magnetic field, inducing a voltage in the coil which is amplified and displayed on an oscilloscope screen. The sample coil, current switching circuit, signal detection circuit, and timing circuits are described. The instrument is capable of measuring spin-lattice relaxation times (T1) of 30 msec and up in fields ranging from 2 to 500 G; the range of measurable spin-spin relaxation times (T2) is 10 msec-1 sec in the earth's field of 0.54 G. The signal-to-noise ratio for a 450 ml water sample is greater than 700:1 at a bandwidth of 20 Hz. Estimated precision of the relaxation time measurements is ±3%.