Advances in Electronics Prompt a Fresh Look at Continuous Wave (CW) Nuclear Magnetic Resonance (NMR)

Abstract

Continuous Wave Nuclear Magnetic Resonance (CW-NMR) was a popular method for sample interrogation at the birth of magnetic resonance but has since been overlooked by most in favor of the now more popular pulsed techniques. CW-NMR requires relatively simple electronics although, for most designs, the execution is critical to the successful implementation and sensitivity of the system. For decades there have been reports in the literature from academic groups showing the potential of magnetic resonance relaxation time measurements in industrial applications such as the production of food and drink. However, the cost, complexity and power consumption of pulsed techniques have largely consigned these to the literature. Advances in electronics and developments in permanent magnet technology now require a fresh look at CW-NMR to see if it is capable of providing cost effective industrial solutions. In this article, we review the electronics that are needed to undertake a continuous wave NMR experiment starting with early designs and journeying through the literature to understand the basic designs and limitations. We then review the more recent developments in this area and present an outlook for future work in the hope that more of the scientific community will take a fresh look at CW-NMR as a viable and powerful low-cost measurement technique.

A Microcontroller System for the Automation of Transient Effect Determination of the Spin-Lattice Relaxation Time Using Continuous Wave NMR

“…The sample is placed in a coil which forms part of a marginal oscillator tank circuit. For convenience, we use a commercial marginal oscillator operating from around 14 MHz to 21 MHz [11]; however, there have been many different marginal oscillator circuits published, which were reviewed recently [6]. The coil used, also produced by LD Didactic as part of their NMR probe bar (514-606), is placed in the fixed magnetic field with sweep coils that allow the field to be swept about the resonance value.…”

“…With a sampling speed of around 160 kSample/second, this demonstrates that the Teensy ADC is capable of accurately capturing the peak amplitude. Note that the positive going "wiggles'" that are common in this type of system [6] are also visible, but as we are only interested in the peak value, no offset is provided to the amplifier to capture the negative going part of the "wiggles". By triggering the 3.6 to capture partway through the sweep ramp, it is easily possible to capture up to 60 peaks with the available RAM, but only around 25 peaks are required to get a reasonable linear fit to provide a value for Tx; the first five values are also discarded.…”

“…Most applications use pulsed NMR for these measurements; however, continuous wave (CW), which was the predecessor to pulsed, is conceptually simpler and has simpler electronics but is often more difficult to implement and so fell out of favor. Recent advances in permanent magnet technology and speed of low-cost electronics have sparked some interest in the topic and the electronics have been recently reviewed [6]. A simplified view of NMR considers the intrinsic nuclear magnetic moments of certain nuclei to be like microscopic bar magnets that tend to orientate in a generally aligned manner with or opposed to the presence of a strong magnetic field ( 0).…”

1H-NMR Determination of Organic Compounds in Municipal Wastewaters and the Receiving Surface Waters in Eastern Cape Province of South Africa

“…The discovery of chemical shifts as a result of the variation in NMR frequencies facilitates the process of chemical structural elucidation [6]. In the early days, NMR started with a continuous wave, a system whereby the oscillator frequency was constant while the magnetic field change gradually, and signal amplitude measured as a function of frequency [7]. The early NMR has a weak magnetic field; measurements depended on the energy absorbed.…”

A preliminary study of milk powder hydration using TEDSpiL continuous wave NMR

“…We refer to this technique as the Transient Effect Determination of Spin–Lattice (TEDSpiL) relaxation times. The use of inexpensive NdFeB magnets and low‐cost microcontrollers to perform this have also been reported giving the potential of a relatively inexpensive, low power, and portable system. This technique has the advantage of giving a value related to T 1 in under 5∼s compared with several minutes for pulsed determination of T 1 with traditional low‐field hardware.…”