Abstract
Low-field NMR (LF-NMR) relaxation analyzers have been more and more widely used in food science, biomedicine, and petroleum exploration in recent years. An LF-NMR device analyzes various interactions between sample molecules through spin relaxation, diffusion, flow, and imaging experiments. However, temporal field fluctuations in the magnet limit the scope of application. A stable magnetic field is the basic guarantee for detecting weak signals with shorter relaxation time. This paper presents an approach involving a field-frequency lock for a 21.3-MHz high-performance NMR bipolar permanent magnet. The diameter spherical volume (DSV) of the magnet is only 60 mm. The field-frequency lock system uses 19F in a micro coil integrated into the main probe as a lock detector. The required magnetic field compensation can be calculated from the lock free induction decay (FID) signal frequency, which is measured by lock spectroscopy. The compensation coils used to produce the magnetic field were designed based on Helmholtz coils. The system determines the signal frequency by detecting the lock FID signal and calculates the required compensation-coil current to stabilize the main magnetic field of the analyzer. The results of practical locking experiments in a 21.3-MHz high-performance NMR relaxation analyzer showed that this new approach helps to reduce magnetic field fluctuations from 11 ppm/3h (11 × 10–6) to 0.4 ppm/3h (0.4 × 10–6), which meets the application requirements. This approach is especially viable and effective for a permanent magnet with large field fluctuations. This paper also provides observations of the effect of these fluctuations on NMR measurements before and after installation of the field-frequency lock system.
Highlights
Unlike magnetic resonance spectroscopy and magnetic resonance imaging (MRI), Low-field NMR (LF-NMR) relaxation analysis is a newly established direction in the NMR field
This paper has described a direct field-frequency lock approach for a 21.3-MHz high-performance NMR relaxation analyzer
The frequency shift of the 19F lock free induction decay (FID) signal was periodically detected by a lock channel spectrometer and a radio frequency system
Summary
Unlike magnetic resonance spectroscopy and magnetic resonance imaging (MRI), LF-NMR relaxation analysis is a newly established direction in the NMR field. This technique enables direct detection of samples containing hydrogen, such as oil, gas, water, polymer materials, and porous media with fluid-filled pores. Examples include evaluation and study of shale pore structure in unconventional petroleum and gas exploration fields, weak hydrocarbon discovery and reservoir evaluation, low-permeability tight-type reservoir evaluation, quality inspection of meat, identification of adulteration in cooking oil and fake honey in the field of food safety, evaluation of micro-fracture development in geotechnical engineering, and research on live small laboratory animals in bioengineering
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