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

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 < 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.

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.

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.