Continuous Wave Free Precession Research Articles

Non-Invasive Method to Predict the Composition of Requeijão Cremoso Directly in Commercial Packages Using Time Domain NMR Relaxometry and Chemometrics

Low Field Time-Domain Nuclear Magnetic Resonance (TD-NMR) relaxometry was used to determine moisture, fat, and defatted dry matter contents in “requeijão cremoso” (RC) processed cheese directly in commercial packaged (plastic cups or tubes with approximately 200 g). Forty-five samples of commercial RC types (traditional, light, lactose-free, vegan, and fiber) were analyzed using longitudinal (T1) and transverse (T2) relaxation measurements in a wide bore Halbach magnet (0.23 T) with a 100 mm probe. The T1 and T2 analyses were performed using CWFP-T1 (Continuous Wave Free Precession) and CPMG (Carr-Purcell-Meiboom-Gill) single shot pulses. The scores of the principal component analysis (PCA) of CWFP-T1 and CPMG signals did not show clustering related to the RC types. Optimization by variable selection was carried out with ordered predictors selection (OPS), providing simpler and predictive partial least squares (PLS) calibration models. The best results were obtained with CWFP-T1 data, with root-mean-square errors of prediction (RMSEP) of 1.38, 4.71, 3.28, and 3.00% for defatted dry mass, fat in the dry and wet matter, and moisture, respectively. Therefore, CWFP-T1 data modeled with chemometrics can be a fast method to monitor the quality of RC directly in commercial packages.

Using TD-NMR relaxometry to assess the effects of diet type and stocking rate on the incidence and degree of severity of myopathies in broilers

This study aimed to evaluate different feeding strategies and stocking rates (SR) on the occurrence and severity of broiler myopathies. Additionally, a second aim was to test the potential use of Time-Domain Nuclear Magnetic Resonance (TD-NMR) relaxometry for assessing white striping (WS) and wooden breast (WB) incidence and the degree of severity in broiler chicken breasts. Treatments were arranged in a 2 × 2 factorial design and consisted of two nutritional levels (regular and superior diets) and two different SR (high 47 kg/m2 or regular 59 kg/m2). Infrared thermography (IRT) images were recorded on three different days to determine the average, minimum and maximum temperatures. After 49 days of rearing, birds were slaughtered, and 75 breasts were weighed and classified according to the absence (normal breast, without the occurrence of myopathy – NB) or occurrence and severity of WB and WS. The whole breasts were analysed by TD-NMR relaxometry using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and Continuous Wave-Free Precession (CWFP-T1) pulse sequence. There was no effect of diet and diet*SR interaction on the weight of breast meat; however, chickens raised at the higher density presented a lower breast weight (P < 0.0001; 490 vs. 539 g). The incidence and severity of myopathies was not influenced by the treatments (P = 0.9467); the frequencies of WB and WS were similar in the different treatments. For CPMG and CWFP-T1, three and two relaxation populations were noted, respectively. The sPLS-DA model optimisation showed a significant separation between WS and NB in terms of occurrence and severity. In conclusion, changes in diet and SR did not mitigate the occurrence or severity of WS and WB. Additionally, TD-NMR relaxometry showed to be a promising technology for evaluating and segregating breasts according to myopathy incidence and severity.

Recent 1D and 2D TD-NMR Pulse Sequences for Plant Science.

Time domain nuclear magnetic resonance (TD–NMR) has been widely applied in plant science in the last four decades. Several TD–NMR instruments and methods have been developed for laboratory, green-house, and field studies. This mini-review focuses on the recent TD–NMR pulse sequences applied in plant science. One of the sequences measures the transverse relaxation time (T2) with minimal sample heating, using a lower refocusing flip angle and consequently lower specific absorption rate than that of conventional CPMG. Other sequences are based on a continuous wave free precession (CWFP) regime used to enhance the signal-to-noise ratio, to measure longitudinal (T1) and transverse relaxation time in a single shot experiment, and as alternative 2D pulse sequences to obtain T1–T2 and diffusion-T1 correlation maps. This review also presents some applications of these sequences in plant science.

New and rapid pulse sequences for two-dimensional D-T1 correlation measurements

Longitudinal relaxation time (T1), transverse relaxation time (T2) and diffusion coefficient (D) values have been widely used for the characterizations of materials using low field Time Domain Nuclear Magnetic Resonance (TD-NMR). Each parameter can be determined using one-dimensional techniques or their values and correlations by multi-dimensional experiments such as T1-T2, D-T2, and T1-D-T2. In this work, we studied four D-T1 sequences for TD-NMR combining Stejskal-Tanner Pulse Gradient Spin Echo (PGSE) diffusion measurement with Inversion-Recovery (IR), Saturation-Recovery (SR), Small-Angle Continuous Wave Free Precession (CWFP-T1) and Small-Angle Flip-Flop (SAFF) for T1 measurement. The results show that rapid D-T1 measurements can be obtained with single shot CWFP-T1 and SAFF sequences. The two sequences were two and eight time fast than sequences based on SR and IR, respectively. Although the two fast sequences yield low signal-to-noise ratio signal, they can be as fast as the traditional D-T2 experiment, or even faster, because it is not necessary to wait a recycle delay of 5 T1. Another advantage of the CWFP-T1 and SAFF methods, when compared to the one based on SR or CPMG (for D-T2) are the low specific absorption rate (SAR) of these sequences due the low flip angles in the sequences, that reduces the sample heating problem. These sequences were initially studied using phantom samples. They also were used to study plant tissues to observe the anisotropic diffusion in asparagus. Therefore, they can be useful methods for practical application in TD-NMR.

Using T1 as a direct detection dimension in two-dimensional time-domain NMR experiments using CWFP regime

The transverse relaxation time (T2), measured with Carr-Purcell-Meiboom-Gill (CPMG) sequence, has been widely used to obtain the direct dimension data in two-dimension time domain NMR (2D TD-NMR). In this paper we are demonstrating that Continuous Wave Free Precession sequence, with low flip angle (CWFP-T1), can be an alternative to CPMG as direct detection dimension. CWFP-T1 is a fast single shot sequence, like CPMG, and yields an exponential signal governed predominantly by the longitudinal (T1) relaxation time. To obtain the correlations between T1 and T2 (T1-T2 maps) we are proposing the use of CPMG-CWFP-T1 pulse sequence. In this sequence CPMG encodes T2 information (indirect dimension) that modulates the CWFP-T1 (direct dimension) signal amplitudes. CPMG-CWFP-T1 experiments were compared with classical 2D sequences such as Saturation-Recovery-CPMG (SR-CPMG) and Inversion-Recovery-CPMG (IR-CPMG) sequence and yields similar results in phantom sample. The experimental time for the 2D sequences, using single scan, shows that SR-CPMG ≤ CPMG-CWFP-T1 < IR-CPMG. Experimental and simulated results demonstrated that 2D-CPMG-CWFP-T1 maps have higher resolution in T1 dimension than the techniques that uses CPMG as direct dimension. CPMG-CWFP-T1 sequence was also applied to study beef samples, and 2D maps showed higher resolution in the two fat signals than the classical IR-CPMG method.

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

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 Tp 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, Tp should be as short as possible (limited by hardware). For Tp &lt; 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 |Mss|. 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.

Enhancing signal-to-noise ratio and resolution in low-field NMR relaxation measurements using post-acquisition digital filters.

The traditional way to enhance signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) signals is to increase the number of scans. However, this procedure increases the measuring time that can be prohibitive for some applications. Therefore, we have tested the use of several post-acquisition digital filters to enhance SNR up to one order of magnitude in time domain NMR (TD-NMR) relaxation measurements. The procedures were studied using continuous wave free precession (CWFP-T1 ) signals, acquired with very low flip angles that contain six times more noise than the Carr-Purcell-Meiboom-Gill (CPMG) signal of the same sample and experimental time. Linear (LI) and logarithmic (LO) data compression, low-pass infinity impulse response (LP), Savitzky-Golay (SG), and wavelet transform (WA) post-acquisition filters enhanced the SNR of the CWFP-T1 signals by at least six times. The best filters were LO, SG, and WA that have high enhancement in SNR without significant distortions in the ILT relaxation distribution data. Therefore, it was demonstrated that these post-acquisition digital filters could be a useful way to denoise CWFP-T1 , as well as CPMG noisy signals, and consequently reducing the experimental time. It was also demonstrated that filtered CWFP-T1 method has the potential to be a rapid and nondestructive method to measure fat content in beef and certainly in other meat samples.

Real‐time monitoring by proton relaxometry of radical polymerization reactions of acrylamide in aqueous solution

AbstractThe potential of time‐domain nuclear magnetic resonance (TD‐NMR) for the real‐time monitoring of solution radical polymerizations is demonstrated. A model system composed of a redox‐pair initiator system, acrylamide as monomer and water as solvent was investigated. A second‐generation continuous wave free precession technique was employed to measure the longitudinal relaxation time constant (T1) of the samples throughout the polymerization reactions. This parameter was shown to be sensitive to the reactant feed free‐radical enhancement of the water molecule relaxation time, making it a good probe to monitor monomer conversion in real time in an automated, non‐destructive fashion. It was found that the T1 value was better than the transverse relaxation time constant (T2) for describing the evolution of the polymerization reactions, due to its greater sensitivity to paramagnetic effects. The TD‐NMR signal variation observed was linked to the formation, propagation and termination steps of the radical polymerization kinetics scheme. These first results may contribute to the application of real‐time monitoring of radical polymerization reactions employing low‐cost and robust TD‐NMR spectrometers. © 2018 Society of Chemical Industry

Rapid and simple determination of [formula omitted] relaxation times in time-domain NMR by Continuous Wave Free Precession sequence

Longitudinal (T1) and transverse (T2) relaxation times have been widely used in time-domain NMR (TD-NMR) to determine several physicochemical properties of petroleum, polymers, and food products. The measurement of T2 through the CPMG pulse sequence has been used in most of these applications because it denotes a rapid, robust method. On the other hand, T1 has been occasionally used in TD-NMR due to the long measurement time required to collect multiple points along the T1 relaxation curve. Recently, several rapid methods to measure T1 have been proposed. Those methods based upon single shot, known as Continuous Wave Free Precession (CWFP) pulse sequences, have been employed in the simultaneous measurement of T1 and T2 in a rapid fashion. However, these sequences can be used exclusively in instrument featuring short dead time because the magnitude of the signal at thermal equilibrium is required. In this paper, we demonstrate that a special CWFP sequence with a low flip angle can be a simple and rapid method to measure T1 regardless of instruments dead time. Experimental results confirmed that the method called CWFP-T1 may be used to measure both single T1 value and T1 distribution in heterogeneous samples. Therefore, CWFP-T1 sequence can be a feasible alternative to CPMG in the determination of physicochemical properties, particularly in processes where fast protocols are requested such as industrial applications.

On resonance phase alternated CWFP sequences for rapid and simultaneous measurement of relaxation times

T1 and T2 relaxation times have been frequently used as probes for physical–chemical properties in several time-domain NMR applications (TD-NMR) such as food, polymers and petroleum industries. T2 measurements are usually achieved using the traditional Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence because it is a fast and robust method. On the other hand, the traditional methods for determining T1, i.e., inversion and saturation recovery, are time-consuming, driving several authors to develop rapid 1D and 2D methods to obtain T1 and T2 or T1/T2 ratio. However, these methods usually require sophisticated processing and/or high signal to noise ratio (SNR). This led us to develop simple methods for rapid and simultaneous determination of T1 and T2 using Continuous Wave Free Precession (CWFP) and Carr–Purcell Continuous Wave Free Precession (CP-CWFP) pulse sequences. Nevertheless, a drawback of these sequences is that they require specific adjustment of the frequency offset or the time interval between pulses (Tp). In this paper we present an alternative form of these sequences, named CWFPx−x, CP-CWFPx−x, where a train of π/2 pulses with phases alternated by π enable performing the experiments on-resonance and independently of Tp, when Tp<T2∗. Moreover, a CPMG type sequence with π/2 refocusing pulses shows similar results to CP-CWFP when the pulses are alternated between y and −y axis, CPMG90y–y. In these approaches, the relaxation times are determined using the magnitude of the signals after the first pulse |M0| and in the steady-state |Mss|, as well as the exponential time constant T∗ to reach the steady-state regime, as in conventional CWFP. CP-CWFPx−x shows the highest dynamic range to measure T∗ among CWFP sequences and, therefore, is the best technique to measure T1 and T2 since it is less susceptible to SNR and can be performed for any T1/T2 ratio.

Rapid and simultaneous relaxometric methods to study paramagnetic ion complexes in solution: An alternative to spectrophotometry

T1 and T2 relaxation times, measured with rapid and simultaneous continuous wave free precession (CWFP) and Carr–Purcell-continuous wave free precession (CP-CWFP) pulse sequences were used to study complexes of EDTA with paramagnetic ions in solution. The relaxation times of Cu(II)-EDTA and [CuEDTA.NH3]−2 complexes, from pH1 to 13, measured in a 0.47T time-domain nuclear magnetic resonance (TD-NMR) spectrometer show good agreement with the absorbance at approximately 730nm. These methods were also used to study colorless complexes containing paramagnetic ions, not accessible by electronic spectroscopy, such as Mn(II)-EDTA. Therefore, these sequences can be used in any bench top TD-NMR spectrometers as faster alternative to spectrophotometry in analytical routine analyses.

Solvent Suppression in High-Resolution 1H NMR Spectroscopy Using Conventional and Phase Alternated Continuous Wave Free Precession

Solvent suppression is frequently mandatory in 1H high-resolution nuclear magnetic resonance (NMR), especially for those experiments designed for non-deuterated solvent, normally used in protein and in vivo analysis, and also in liquid chromatography-NMR. Here, simple pulse sequences, which are based on continuous wave free precession (CWFP), consisting of a train of pulses separated by a time interval \( T_{\text{p}} \ll T_{2}^{*} \), is applied to suppress one or more solvent signals in 1H high-resolution NMR experiments, because of its dependency on the offset frequency. The conventional CWFP pulse sequence, that uses pulses with the same phase and duration, introduces some phase anomaly in the Fourier-transformed spectrum. This problem is minimized when the pulses are applied with phase alternation by π/2 in relation to the preceding pulse. Some problems with signal intensity can also be minimized using a shorter pulse width. Both CWFP and phase alternated CWFP can be easily used to suppress two solvent signals simultaneously, just using the correct T p value, that must be equal to the inverse of frequency difference (∆ν) between both signals to be suppressed. After modifications, we could introduce the CWFP train into 2D routine pulse sequences.

Fast determination of beef quality parameters with time-domain nuclear magnetic resonance spectroscopy and chemometrics

The noteworthy of this study is to predict seven quality parameters for beef samples using time-domain nuclear magnetic resonance (TD-NMR) relaxometry data and multivariate models. Samples from 61 Bonsmara heifers were separated into five groups based on genetic (breeding composition) and feed system (grain and grass feed). Seven sample parameters were analyzed by reference methods; among them, three sensorial parameters, flavor, juiciness and tenderness and four physicochemical parameters, cooking loss, fat and moisture content and instrumental tenderness using Warner Bratzler shear force (WBSF). The raw beef samples of the same animals were analyzed by TD-NMR relaxometry using Carr-Purcell-Meiboom-Gill (CPMG) and Continuous Wave-Free Precession (CWFP) sequences. Regression models computed by partial least squares (PLS) chemometric technique using CPMG and CWFP data and the results of the classical analysis were constructed. The results allowed for the prediction of aforementioned seven properties. The predictive ability of the method was evaluated using the root mean square error (RMSE) for the calibration (RMSEC) and validation (RMSEP) data sets. The reference and predicted values showed no significant differences at a 95% confidence level.

Simultaneous measurements of T1 and T2 during fast polymerization reaction using continuous wave‐free precession NMR method

Continuous wave-free precession (CWFP) pulse sequence employing time domain nuclear magnetic resonance spectroscopy (TD-NMR) was used to measure longitudinal (T(1)) and transverse relaxation times (T(2)), during the cure of a commercial epoxy resin (Araldite(TM)) with a 10-min solidification time. The intensity of the NMR signal after the first pulse and in the CWFP regime were used to monitor the concentration of the monomers, and the relaxation times were used to monitor the chain mobility. The main advantage of CWFP over the standard methods to measure relaxation times, inversion recovery (inv-rec) for T(1) and Carr-Purcell-Meiboom-Gill (CPMG) for T(2), is that the measurement of both relaxation times can be performed in a fast and single NMR experiment and, therefore, using a single reaction batch. CWFP is also as fast as the CPMG measurement but at least fivefold faster than the method to obtain T(1) using null point approximation in the inv-rec method. Therefore, the CWFP sequence can be used as a fast and general method to measure relaxation times in polymerization reactions, even with fast solidification time. As a TD-NMR technique, CWFP can be employed in any low-cost bench top TD-NMR equipment commonly used in an academic or industrial laboratory.

Rapid analyses of oil and fat content in agri‐food products using continuous wave free precession time domain NMR

Time-domain nuclear magnetic resonance (TD-NMR) is one of the most popular solutions for quality control in the food industry. Despite the recognized success of TD-NMR in quality control and quality assurance, the speed by which samples can be characterized by TD-NMR techniques is still a concern, primarily when considering online or high-throughput applications. Therefore, to enhance the speed of TD-NMR analysis, we developed rapid methods based on steady-state free precession of nuclear spins, which we denoted continuous wave free precession (CWFP). CWFP substantially increases the sensitivity of TD-NMR compared with free induction decay or spin-echo detection, which are traditionally used. The objective of this paper was to present the physical background of CWFP and review its recent developments and applications in fat and oil quantifications in agri-food products.

Use of Carr–Purcell pulse sequence with low refocusing flip angle to measure T1 and T2 in a single experiment

The Carr–Purcell pulse sequence, with low refocusing flip angle, produces echoes midway between refocusing pulses that decay to a minimum value dependent on T 2 ∗ . When the refocusing flip angle was π/2 (CP 90) and τ > T 2 ∗ , the signal after the minimum value, increased to reach a steady-state free precession regime (SSFP), composed of a free induction decay signal after each pulse and an echo, before the next pulse. When τ < T 2 ∗ , the signal increased from the minimum value to the steady-state regime with a time constant ( T ∗) = 2 T 1 T 2/( T 1 + T 2) , identical to the time constant observed in the SSFP sequence, known as the continuous wave free precession (CWFP). The steady-state amplitude obtained with M CP90 = M 0 T 2/( T 1 + T 2) was identical to CWFP. Therefore, this sequence was named CP-CWFP because it is a Carr–Purcell sequence that produces results similar to the CWFP. However, CP-CWFP is a better sequence for measuring the longitudinal and transverse relaxation times in single scan, when the sample exhibits T 1 ∼ T 2. Therefore, this sequence can be a useful method in time domain NMR and can be widely used in the agriculture, food and petrochemical industries because those samples tend to have similar relaxation times in low magnetic fields.

High-throughput non-destructive nuclear magnetic resonance method to measure intramuscular fat content in beef

High intake of saturated fat from meats has been associated with cardiovascular disease, cancer, diabetes, and others diseases. In this paper, we are introducing a simple, high-throughput, and non-destructive low-resolution nuclear magnetic resonance method that has the potential to analyze the intramuscular fat content (IMF) in more than 1,000 beef portions per hour. The results can be used in nutritional fact labels, replacing the currently used average value. The method is based on longitudinal (T(1)) and transverse (T(2)) relaxation time information obtained by a continuous wave-free precession (CWFP) sequence. CWFP yields a higher correlation coefficient (r = 0.9) than the conventional Carr-Purcell-Meiboom-Gill (CPMG) method (r = -0.25) for IMF in beef and is just as fast and a simpler pulse sequence than CPMG. The method can also be applied to other meat products.

High-Throughput, Non-Destructive Determination of Oil Content in Intact Seeds by Continuous Wave-Free Precession NMR

A new method, based on continuous wave-free precession nuclear magnetic resonance, is proposed as a high-throughput technique for measuring the oil content of intact seeds. The method has the potential to analyze more than 20 000 intact seeds per hour and is shown to be applicable even to mixtures of seeds of different species with similar fatty acid composition.

Thermal diffusivity and nuclear spin relaxation: A continuous wave free precession NMR study

Continuous wave free precession (CWFP) nuclear magnetic resonance is capable of yielding quantitative and easily obtainable information concerning the kinetics of processes that change the relaxation rates of the nuclear spins through the action of some external agent. In the present application, heat flow from a natural rubber sample to a liquid nitrogen thermal bath caused a large temperature gradient leading to a non-equilibrium temperature distribution. The ensuing local changes in the relaxation rates could be monitored by the decay of the CWFP signals and, from the decays, it was possible to ascertain the prevalence of a diffusive process and to obtain an average value for the thermal diffusivity.

Fast and simultaneous measurement of longitudinal and transverse NMR relaxation times in a single continuous wave free precession experiment

The purpose of this communication is to describe a method for rapid and simultaneous determination of longitudinal ( T 1) and transversel ( T 2) relaxation times, based on a single continuous wave free precession (CWFP) experiment which employs RF pulses with a π/2 flip angle. We analyze several examples, involving nuclei such as 1H, 31P, and 19F, where good agreement with T 1 and T 2 measurements obtained by traditional methods is apparent. We also compare with the more time-consuming steady-state free precession (SSFP) method of Kronenbitter and Schwenk where several experiments are needed to determine the optimum flip angle. The role of an inhomogeneous magnetic field on the observed decays and its effect upon the accuracy of relaxation times obtained by these methods is examined by comparing numerical simulations with experimental data. Possible sources of error and conditions to minimize its effects are described.