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4 pages, 164 KiB

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Editorial on Special Issue “Applications of Low Field Magnetic Resonance”

by Robert H. Morris and Michael I. Newton

Appl. Sci. 2021, 11(18), 8471; https://doi.org/10.3390/app11188471 - 13 Sep 2021

Viewed by 1363

Abstract

This Special Issue presents the latest advances in the applications of low field magnetic resonance [...] Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

Research

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25 pages, 5367 KiB

Open AccessArticle

Chemical Analysis of Fluorobenzenes via Multinuclear Detection in the Strong Heteronuclear J-Coupling Regime

by Derrick C. Kaseman, Michael T. Janicke, Rachel K. Frankle, Tammie Nelson, Gary Angles-Tamayo, Rami J. Batrice, Per E. Magnelind, Michelle A. Espy and Robert F. Williams

Appl. Sci. 2020, 10(11), 3836; https://doi.org/10.3390/app10113836 - 31 May 2020

Cited by 7 | Viewed by 7187

Abstract

Chemical analysis via nuclear magnetic resonance (NMR) spectroscopy using permanent magnets, rather than superconducting magnets, is a rapidly developing field. Performing the NMR measurement in the strong heteronuclear J-coupling regime has shown considerable promise for the chemical analysis of small molecules. Typically, [...] Read more.

Chemical analysis via nuclear magnetic resonance (NMR) spectroscopy using permanent magnets, rather than superconducting magnets, is a rapidly developing field. Performing the NMR measurement in the strong heteronuclear J-coupling regime has shown considerable promise for the chemical analysis of small molecules. Typically, the condition for the strong heteronuclear J-coupling regime is satisfied at µT magnetic field strengths and enables high resolution J-coupled spectra (JCS) to be acquired. However, the JCS response to systematic chemical structural changes has largely not been investigated. In this report, we investigate the JCS of C₆H_6−xF_x (x = 0, 1, 2, …, 6) fluorobenzene compounds via simultaneous excitation and detection of ¹⁹F and ¹H at 51.5 µT. The results demonstrate that JCS are quantitative, and the common NMR observables, including Larmor frequency, heteronuclear and homonuclear J-couplings, relative signs of the J-coupling, chemical shift, and relaxation, are all measurable and are differentiable between molecules at low magnetic fields. The results, corroborated by ab initio calculations, provide new insights into the impact of chemical structure and their corresponding spin systems on JCS. In several instances, the JCS provided more chemical information than traditional high field NMR, demonstrating that JCS can be used for robust chemical analysis. Full article

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17 pages, 10675 KiB

Open AccessFeature PaperArticle

Some Applications of a Field Programmable Gate Array Based Time-Domain Spectrometer for NMR Relaxation and NMR Cryoporometry

by J. Beau W. Webber

Appl. Sci. 2020, 10(8), 2714; https://doi.org/10.3390/app10082714 - 15 Apr 2020

Cited by 5 | Viewed by 3255

Abstract

NMR Relaxation (NMRR) is an extremely useful quantitative technique for material science, particularly for studying polymers and porous materials. NMR Cryoporometry (NMRC) is a powerful technique for the measurement of pore-size distributions and total porosities. This paper discusses the use, capabilities and application [...] Read more.

NMR Relaxation (NMRR) is an extremely useful quantitative technique for material science, particularly for studying polymers and porous materials. NMR Cryoporometry (NMRC) is a powerful technique for the measurement of pore-size distributions and total porosities. This paper discusses the use, capabilities and application of a newly available compact NMR time-domain relaxation spectrometer, the Lab-Tools Mk3 NMR Relaxometer & Cryoporometer [Lab-Tools (nano-science), Ramsgate, Kent, UK (2019)]. Being Field Programmable Gate Array based means that it is unusually compact, which makes it particularly suitable for the lab bench-top, in the field and also mobile use. Its use with a variable-temperature NMR probe such as the Lab-Tools Peltier thermo-electrically cooled variable-temperature (V-T) probe is also discussed. This enables the NMRC measurement of pore-size distributions in porous materials, from sub-nano- to over 1 micron sized pores. These techniques are suitable for a wide range of porous materials and also polymers. This instrument comes with a Graphical User Interface (GUI) for control, which also enables both online and offline analysis of the measured data. This makes it is easy to use for material science studies both in the field and in university, research institute, company and even school laboratories. The Peltier cooling gives the precision temperature control and smoothness needed by NMR Cryoporometry, particularly near the probe liquid bulk melting point. Results from example NMR Relaxation and NMR Cryoporometric measurements are given. Full article

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17 pages, 4803 KiB

Open AccessArticle

Image Quality Improvement and Memory-Saving in a Permanent-Magnet-Array-Based MRI System

by Jia Gong, Wenwei Yu and Shao Ying Huang

Appl. Sci. 2020, 10(6), 2177; https://doi.org/10.3390/app10062177 - 23 Mar 2020

Cited by 3 | Viewed by 3137

Abstract

Point-of-care magnetic resonance imaging (MRI) requires clear images within a short scanning time, a small footprint of the scanner, and relatively low memory required for image reconstruction. A permanent magnet array (PMA)-based MRI system is a good candidate to supply a magnetic field [...] Read more.

Point-of-care magnetic resonance imaging (MRI) requires clear images within a short scanning time, a small footprint of the scanner, and relatively low memory required for image reconstruction. A permanent magnet array (PMA)-based MRI system is a good candidate to supply a magnetic field due to its compactness and low power consumption. However, it has relatively inhomogeneous magnetic field and thus non-linear gradients, which results in location-dependent k-spaces (so called local k-spaces) and uneven signal point populations in the local k-spaces, compromising the image quality. Moreover, owing to the non-linearity, imaging reconstruction using Fourier transform does not work, which leads to an increase in the required computation memory. In this study, in order to improve the image quality, the approaches of compensating the uneven signal point population by increasing the numbers of sampling points or rotation angles are investigated in terms of their impacts on image quality improvement, acquisition time, image reconstruction time, and memory consumption. Both methods give a significant improvement on image image quality although they result in a large and dense encoding matrix and thus a large memory consumption. To lower the memory consumption, it is further proposed to transform such a matrix to frequency domain where the matrix could be sparse. Moreover, a row-wise truncation to the transformed encoding matrix is applied to further reduce the memory consumption. Through the results of numerical experiments, it is shown that the required memory for calculation can effectively be reduced by 71.6% while the image becomes clearer by increasing the number of sampling point and/or the number of rotation angles. With the successful demonstration where improved image quality and a lowered memory required can be obtained simultaneously, the proposed study is one step forward for a PMA-based MRI system towards its targeted point-of-care application scenario. Full article

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23 pages, 9718 KiB

Open AccessArticle

Permeability Evaluation of Clay-quartz Mixtures Based on Low-Field NMR and Fractal Analysis

by Zhen Lu, Aimin Sha and Wentong Wang

Appl. Sci. 2020, 10(5), 1585; https://doi.org/10.3390/app10051585 - 27 Feb 2020

Cited by 6 | Viewed by 2747

Abstract

Nuclear magnetic resonance (NMR) technology has been widely used for predicting permeability coefficients of porous medium, such as shales, sandstones, and coals. However, there have been limited studies on the prediction model of clay-quartz mixtures based on NMR technology. In this study, evaporation [...] Read more.

Nuclear magnetic resonance (NMR) technology has been widely used for predicting permeability coefficients of porous medium, such as shales, sandstones, and coals. However, there have been limited studies on the prediction model of clay-quartz mixtures based on NMR technology. In this study, evaporation tests at 40 °C and NMR tests were simultaneously performed on eight clay-quartz mixtures with different mineral compositions. The results show that during the evaporation process, the decay rate of T₂ total signal amplitudes was constant at first, and then decreased to 0 after a period of time. Based on the decay rate, the evaporation process was divided into two stages: the constant rate stage and the falling rate stage. Based on the two stages, the T₂ cut-offs of eight mixtures were determined. The water in the mixture was divided into two parts by the T₂ cut-off: the free water and the bound water. The prediction model of permeability coefficients of clay-quartz mixtures was established based on the Timur-Coates model. In order to simplify the process of predicting the permeability coefficient, fractal analysis was used to develop the relationship between the T₂ cut-off and fractal dimension of the T₂ spectrum of saturated mixture. A simplified method for predicting permeability coefficients of clay-quartz mixtures based on NMR technology without centrifugal and evaporation experiments was also proposed. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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22 pages, 3884 KiB

Open AccessArticle

Time-Domain Nuclear Magnetic Resonance Determination of Wettability Alteration: Analysis for Low-Salinity Water

by Bryan X. Medina-Rodriguez, Teresa Reilly, Heng Wang, Erik Robert Smith, Griselda Garcia-Olvera, Vladimir Alvarado and Saman Aryana

Appl. Sci. 2020, 10(3), 1017; https://doi.org/10.3390/app10031017 - 4 Feb 2020

Cited by 8 | Viewed by 4706

Abstract

Wettability has been shown to influence oil recovery. This property has become central to low-salinity (LSW) and smart (SWF) water flooding recovery mechanisms research. The challenge lies in the fact that oil recovery results from the combined effects of solid-liquid and liquid-liquid interactions. [...] Read more.

Wettability has been shown to influence oil recovery. This property has become central to low-salinity (LSW) and smart (SWF) water flooding recovery mechanisms research. The challenge lies in the fact that oil recovery results from the combined effects of solid-liquid and liquid-liquid interactions. This demands methods that allow an independent interpretation of wettability alteration contributions. The primary objective of this work is to assess changes in wettability through the application of Time-Domain Nuclear Magnetic Resonance (TD-NMR) T

_{2}

distribution and diffusion coefficient, starting with a well-controlled porous system, that is, glass beads, and then a model rock (Berea), in the presence of one phase, either oil or brine exclusively. Subsequently, two-phase fluid saturation was tested. For the glass beads, dimethyldichlorosilane was used to induce a hydrophobic response, as confirmed by contact angle experiments on slides of the same material. Sodium sulfate was used for its known positive influence on oil recovery during LSW and SWF. In cases where alteration of surface properties was expected, a leftward shift of the average T

_{2}

distribution curve modes, accompanied by a reduction on the diffusion coefficient during the aging process was observed. The results of this work confirm that fluid-solid interactions during LSW and SWF, namely a shift in wettability, take place after the injection of low-salinity water. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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8 pages, 2049 KiB

Open AccessFeature PaperArticle

NMR CAPIBarA: Proof of Principle of a Low-Field Unilateral Magnetic Resonance System for Monitoring of the Placenta during Pregnancy

by Robert H. Morris, Najlaa K. Almazrouei, Christophe L. Trabi and Michael I. Newton

Appl. Sci. 2020, 10(1), 162; https://doi.org/10.3390/app10010162 - 24 Dec 2019

Cited by 3 | Viewed by 4049

Abstract

A growing body of literature shows that the transverse relaxation times of the placenta change during pregnancy and may be an early indicator of disease. Magnetic resonance imaging (MRI) of pregnant women is not, however, currently used frequently despite this evidence. One significant [...] Read more.

A growing body of literature shows that the transverse relaxation times of the placenta change during pregnancy and may be an early indicator of disease. Magnetic resonance imaging (MRI) of pregnant women is not, however, currently used frequently despite this evidence. One significant barrier to adoption is the cost of undertaking an MRI scan and the over utilization of existing equipment. Low-field nuclear magnetic resonance (NMR) offers a low-cost alternative, capable of measuring transverse relaxation in a single point in space. Ultrasound imaging (US) is routinely used at several points during pregnancy but is not capable of early detection of pre-eclampsia, for example. It does, however, provide a technique that is capable of locating the placenta with ease. In combination with a single point low-field measurement, localised with ultrasound imaging allows access to this exciting technique without the need for an expensive traditional MRI. In this work, we present a unilateral system (NMR CAPIBarA), operating at a magnetic field of only 18mT, which measures transverse relaxation times at distances from its surface equivalent to the positioning of a human placenta. Data are presented to characterise the system using relation time standards covering the full transverse relaxation time range relevant for the developing placenta, which are also measured on a 1.5 T clinical MRI scanner. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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10 pages, 1192 KiB

Open AccessArticle

Solids Content of Black Liquor Measured by Online Time-Domain NMR

by Ekaterina Nikolskaya, Petri Janhunen, Mikko Haapalainen and Yrjö Hiltunen

Appl. Sci. 2019, 9(10), 2169; https://doi.org/10.3390/app9102169 - 27 May 2019

Cited by 16 | Viewed by 4831

Abstract

Black liquor, a valuable by-product of the pulp production process, is used for the recovery of chemicals and serves as an energy source for the pulp mill. Before entering the recovery unit, black liquor runs through several stages of evaporation, wherein the solids [...] Read more.

Black liquor, a valuable by-product of the pulp production process, is used for the recovery of chemicals and serves as an energy source for the pulp mill. Before entering the recovery unit, black liquor runs through several stages of evaporation, wherein the solids content (SC) can be used to control the evaporation effectiveness. In the current study, the time-domain nuclear magnetic resonance (TD-NMR) technique was applied to determine the SC of black liquor. The TD-NMR system was modified for flowing samples, so that the black liquor could be pumped through the system, followed by the measurement of the spin-spin relaxation rate, R₂. A temperature correction was also applied to reduce deviations in the R₂ caused by the sample temperature. The SC was calculated based on a linear model between the R₂ and the SC values determined gravimetrically, where good agreement was shown. The online TD-NMR system was tested at a pulp mill for the SC estimation of weak black liquor over seven days without any fouling, which demonstrated the feasibility of the method in a harsh industrial environment. Therefore, the potential of the TD-NMR technology as a technique for controlling the black liquor evaporation process was demonstrated. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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14 pages, 1503 KiB

Open AccessArticle

Hyperpolarised ¹H–¹³C Benchtop NMR Spectroscopy

by Alastair D. Robinson, Peter M. Richardson and Meghan E. Halse

Appl. Sci. 2019, 9(6), 1173; https://doi.org/10.3390/app9061173 - 20 Mar 2019

Cited by 15 | Viewed by 4756

Abstract

Benchtop NMR spectrometers with sub-ppm spectral resolution have opened up new opportunities for performing NMR outside of the standard laboratory environment. However, the relatively weak magnetic fields of these devices (1–2 T) results in low sensitivity and significant peak overlap in ¹H [...] Read more.

Benchtop NMR spectrometers with sub-ppm spectral resolution have opened up new opportunities for performing NMR outside of the standard laboratory environment. However, the relatively weak magnetic fields of these devices (1–2 T) results in low sensitivity and significant peak overlap in ¹H NMR spectra. Here, we use hyperpolarised ¹³C{¹H} NMR to overcome these challenges. Specifically, we demonstrate the use of the signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarisation technique to enhance the sensitivity of natural abundance 1D and 2D ¹³C{¹H} benchtop NMR spectra. We compare two detection methods for SABRE-enhanced ¹³C NMR and observe an optimal ¹³C{¹H} signal-to-noise ratio (SNR) for a refocused INEPT approach, where hyperpolarisation is transferred from ¹H to ¹³C. In addition, we exemplify SABRE-enhanced 2D ¹³C benchtop NMR through the acquisition of a 2D HETCOR spectrum of 260 mM of 4-methylpyridine at natural isotopic abundance in a total experiment time of 69 min. In theory, signal averaging for over 300 days would be required to achieve a comparable SNR for a thermally polarised benchtop NMR spectrum acquired of a sample of the same concentration at natural abundance. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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8 pages, 1022 KiB

Open AccessArticle

Analysis of Early Performance of Cement Paste by Low Field NMR

by Dan Jin, Zejun Lang and Wu Yao

Appl. Sci. 2019, 9(5), 896; https://doi.org/10.3390/app9050896 - 2 Mar 2019

Cited by 15 | Viewed by 2888

Abstract

The change of the evaporable water content in the early hydration process of cement paste with different water-to-cement ratios was monitored by low field nuclear magnetic resonance (LF-NMR) relaxometry. The hydration degree and gel/space ratio were accordingly calculated and analyzed. The test results [...] Read more.

The change of the evaporable water content in the early hydration process of cement paste with different water-to-cement ratios was monitored by low field nuclear magnetic resonance (LF-NMR) relaxometry. The hydration degree and gel/space ratio were accordingly calculated and analyzed. The test results show that the first derivative curves of the transverse magnetization are in good agreement with the known five stages of cement hydration process at an early age. The initial and final setting times of the cement paste are closely related to the times the gel/space ratio reaches a fixed value. The early compressive strength of the hardened cement paste exhibits a power function relationship with the gel/space ratio. Full article

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14 pages, 4050 KiB

Open AccessArticle

Effect of Heat Treatment on Water Absorption of Chinese fir Using TD-NMR

by Yulei Gao, Kang Xu, Hui Peng, Jiali Jiang, Rongjun Zhao and Jianxiong Lu

Appl. Sci. 2019, 9(1), 78; https://doi.org/10.3390/app9010078 - 26 Dec 2018

Cited by 24 | Viewed by 3893

Abstract

Knowledge of the dynamic changes in the water absorption process of heat-treated wood is important for providing a scientific basis for the reasonable application of heat-treated wood, especially for outdoor applications. Nuclear magnetic resonance (NMR) techniques provide detailed information about the moisture components [...] Read more.

Knowledge of the dynamic changes in the water absorption process of heat-treated wood is important for providing a scientific basis for the reasonable application of heat-treated wood, especially for outdoor applications. Nuclear magnetic resonance (NMR) techniques provide detailed information about the moisture components and moisture transport processes in wood, which are not available with other methods. In this work, water absorption of untreated and heat treated Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) heartwood was investigated using various NMR methods. The heat treatment temperatures were varied between 160 °C and 220 °C. According to the spin-spin relaxation time (T₂), there were two components of water in the samples heat-treated at 160 °C and 180 °C as well as the untreated sample, while three components of water were found in the samples heat-treated at 200 °C and 220 °C, and the mass of each component was calculated by the integral peak areas of the T₂ curve. The amount of bound water and free water in heat-treated samples were less compared to the untreated ones, and the water absorption decreased correspondingly, due to the increasing heat-treated temperature. The results obtained by one dimensional frequency coding indicated that the heat treatment made wood difficult to be accessed by moisture. Besides, NMR images revealed that the free water absorption in latewood was faster than in earlywood, but earlywood could absorb more water than latewood. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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13 pages, 1976 KiB

Open AccessArticle

Low-Field NMR and MRI to Analyze the Effect of Edible Coating Incorporated with MAP on Qualities of Half-Smooth Tongue Sole (Cynoglossus Semilaevis Günther) Fillets during Refrigerated Storage

by Na Li, Yong Shen, Wenru Liu, Jun Mei and Jing Xie

Appl. Sci. 2018, 8(8), 1391; https://doi.org/10.3390/app8081391 - 17 Aug 2018

Cited by 30 | Viewed by 4731

Abstract

Nondestructive and fast measurement and characterization of fish is highly desired during various processing treatments. This research investigated the effectiveness of low field LF-NMR and MRI as fast monitoring techniques to estimate the qualities of half-smooth tongue sole fillets treated with edible coating [...] Read more.

Nondestructive and fast measurement and characterization of fish is highly desired during various processing treatments. This research investigated the effectiveness of low field LF-NMR and MRI as fast monitoring techniques to estimate the qualities of half-smooth tongue sole fillets treated with edible coating combined with modified atmosphere packaging during refrigeration. T₂ relaxation spectra showed three peaks representing bound water (T₂₁), immobile water (T₂₂), and free water (T₂₃), respectively. pT₂₂ accounted for the largest proportion of three types of water, followed by pT₂₃. The weighted MRI provided the internal structure information associated with different samples, indicting the combination of edible coating and MAP (70% CO₂ + 30% N₂) is the best performance in the maintenance of qualities and freshness of HTS fillets. All results demonstrated that the combination of LF-NMR and MRI as fast and nondestructive methods have great potential to monitor qualities deterioration and predict shelf life in of HTS fillets during refrigerated storage. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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18 pages, 2637 KiB

Open AccessArticle

Leaf Development Monitoring and Early Detection of Water Deficiency by Low Field Nuclear Magnetic Resonance Relaxation in Nicotiana tabacum Plants

by Clément Sorin, François Mariette, Maja Musse, Laurent Leport, Florence Cruz and Jean-Claude Yvin

Appl. Sci. 2018, 8(6), 943; https://doi.org/10.3390/app8060943 - 7 Jun 2018

Cited by 10 | Viewed by 3788

Abstract

Drought is the main abiotic stress worldwide affecting harvest quality and quantity of numerous crops. To enable better water management, low field NMR (nuclear magnetic resonance) relaxometry was assessed as a developmental marker and a new method for early detection of water deficiency. [...] Read more.

Drought is the main abiotic stress worldwide affecting harvest quality and quantity of numerous crops. To enable better water management, low field NMR (nuclear magnetic resonance) relaxometry was assessed as a developmental marker and a new method for early detection of water deficiency. The effect of a foliar biostimulant against water stress was also investigated. Two leaves of different ranks (four and eight) were studied. The leaves of different ranks were characterized by different NMR T₂ spectra which validated the ability of NMR to describe the developmental stage of tobacco. Results also showed that T₂ NMR relaxation spectra allow the detection of mild water stress (80% of the field capacity) through the precise characterization of the leaf water status while other water stress markers (relative water content, photosynthetic related parameters…) were not yet impacted. The agricultural impact of the mild water stress was determined through the nitrogen rate in shoots and amino acids assay six weeks after the beginning of the stress and results shows that foliar application of biostimulant limits the negative consequences of drought. Our results demonstrate the sensitivity of NMR to detect slight changes triggered in the leaf by water stress at the tissue level. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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Review

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33 pages, 4753 KiB

Open AccessReview

Time Domain NMR in Polymer Science: From the Laboratory to the Industry

by Denise Besghini, Michele Mauri and Roberto Simonutti

Appl. Sci. 2019, 9(9), 1801; https://doi.org/10.3390/app9091801 - 30 Apr 2019

Cited by 81 | Viewed by 11558

Abstract

Highly controlled polymers and nanostructures are increasingly translated from the lab to the industry. Together with the industrialization of complex systems from renewable sources, a paradigm change in the processing of plastics and rubbers is underway, requiring a new generation of analytical tools. [...] Read more.

Highly controlled polymers and nanostructures are increasingly translated from the lab to the industry. Together with the industrialization of complex systems from renewable sources, a paradigm change in the processing of plastics and rubbers is underway, requiring a new generation of analytical tools. Here, we present the recent developments in time domain NMR (TD-NMR), starting with an introduction of the methods. Several examples illustrate the new take on traditional issues like the measurement of crosslink density in vulcanized rubber or the monitoring of crystallization kinetics, as well as the unique information that can be extracted from multiphase, nanophase and composite materials. Generally, TD-NMR is capable of determining structural parameters that are in agreement with other techniques and with the final macroscopic properties of industrial interest, as well as reveal details on the local homogeneity that are difficult to obtain otherwise. Considering its moderate technical and space requirements of performing, TD-NMR is a good candidate for assisting product and process development in several applications throughout the rubber, plastics, composites and adhesives industry. Full article

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14 pages, 3528 KiB

Open AccessReview

Applications of Continuous Wave Free Precession Sequences in Low-Field, Time-Domain NMR

by Tiago Bueno Moraes, Tatiana Monaretto and Luiz Alberto Colnago

Appl. Sci. 2019, 9(7), 1312; https://doi.org/10.3390/app9071312 - 29 Mar 2019

Cited by 15 | Viewed by 3342

Abstract

This review discusses the theory and applications of the Continuous Wave Free Precession (CWFP) sequence in low-field, time-domain nuclear magnetic resonance (TD-NMR). CWFP is a special case of the Steady State Free Precession (SSFP) regime that is obtained when a train of radiofrequency [...] Read more.

This review discusses the theory and applications of the Continuous Wave Free Precession (CWFP) sequence in low-field, time-domain nuclear magnetic resonance (TD-NMR). CWFP is a special case of the Steady State Free Precession (SSFP) regime that is obtained when a train of radiofrequency pulses, separated by a time interval T_p shorter than the effective transverse relaxation time (T₂*), is applied to a sample. Unlike regular pulsed experiments, in the CWFP regime, the amplitude is not dependent on T₁. Therefore, T_p should be as short as possible (limited by hardware). For T_p < 0.5 ms, thousands of scans can be performed per second, and the signal to noise ratio can be enhanced by more than one order of magnitude. The amplitude of the CWFP signal is dependent on T₁/T₂; therefore, it can be used in quantitative analyses for samples with a similar relaxation ratio. The time constant to reach the CWFP regime (T*) is also dependent on relaxation times and flip angle (θ). Therefore, T* has been used as a single shot experiment to measure T₁ using a low flip angle (5°) or T₂, using θ = 180°. For measuring T₁ and T₂ simultaneously in a single experiment, it is necessary to use θ = 90°, the values of T* and M₀, and the magnitude of CWFP signal |M_ss|. Therefore, CWFP is an important sequence for TD-NMR, being an alternative to the Carr-Purcell-Meiboom-Gill sequence, which depends only on T₂. The use of CWFP for the improvement of the signal to noise ratio in quantitative and qualitative analyses and in relaxation measurements are presented and discussed. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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9 pages, 1209 KiB

Open AccessReview

Monitoring Electrochemical Reactions in Situ with Low Field NMR: A Mini-Review

by Bruna Ferreira Gomes, Carlos Manuel Silva Lobo and Luiz Alberto Colnago

Appl. Sci. 2019, 9(3), 498; https://doi.org/10.3390/app9030498 - 1 Feb 2019

Cited by 11 | Viewed by 3710

Abstract

The number of applications of time domain NMR using low-field spectrometers in research and development has been steadily increasing in recent years with applications ranging from quality control of industrial products to the study of physical and chemical properties of a wide array [...] Read more.

The number of applications of time domain NMR using low-field spectrometers in research and development has been steadily increasing in recent years with applications ranging from quality control of industrial products to the study of physical and chemical properties of a wide array of solid and liquid samples to, most recently, electrochemical studies. In this mini-review we summarize the progress that has been achieved in the coupling between time domain NMR (using low-field spectrometers) and electrochemistry and how the challenges that this coupling poses have been overcome over the years. We also highlight the effect that the static magnetic field of the NMR spectrometer has on the electrochemical systems. Full article

(This article belongs to the Special Issue Applications of Low Field Magnetic Resonance)

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