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 T-p shorter than the effective transverse relaxation time (T-2*), is applied to a sample. Unlike regular pulsed experiments, in the CWFP regime, the amplitude is not dependent on T-1. 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 T-1/T-2; 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 (). Therefore, T* has been used as a single shot experiment to measure T-1 using a low flip angle (5 degrees) or T-2, using = 180 degrees. For measuring T-1 and T-2 simultaneously in a single experiment, it is necessary to use = 90 degrees, the values of T* and M-0, 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 T-2. 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.