180° Pulse Imperfection Effects on Fitting of Relaxation Curves Obtained by Low Field NMR Spectroscopy

Low field NMR spectroscopy is well adapted to the study of heterogeneous media, such as those found in the food industry or in life sciences, because it does not require preparation of samples. CPMG relaxation curves obtained with different pulse spacing times, on samples without exchange phenomenon, are not superimposable. We show that this phenomenon, due to pulse imperfections at 180°, is not a single drift; pulses reach a stationary regime towards the 80th pulse; values of T2 of the order of milliseconds, are then a bit perturbed. For the same reason, with a liquid sample, the total signal of a CPMG curve does not exactly correspond to the total signal obtained with a FID curve. With some fitting programs, by iterating on the value of the offset in order to minimise the standard deviation of the relaxation curve fitting, it is no longer necessary to know the value of the baseline of the CPMG relaxation curves; it enables one to better sample the first part of the curve.