Signal Acquisition and Data processing of T1(spin-lattice), T2 (spin-spin) and 2D (T2 - T1) spectra:
The 1H LF-NMR measurements was carried out on a Maran bench-top pulsed NMR analyzer (Resonance Instruments, Witney, United Kingdom) with a permanent magnet and a18-mm probe head operating at 23.4 MHz. Prior to the measurements, the samples were stabilized at 40 °C for 40 min and then allowed to equilibrate inside the instrument for 5 min. The receiver gain (RG) and magnetic field were calibrated before each measurement. The 1D spectrum of spin-lattice relaxation time (T1) was generated using an Inversion Recovery Sequence (INVRC). This sequence consists of a 180 degree radiofrequency pulse (inversion pulse) to the sample followed by a 90 degree radiofrequency pulse. The 1D spectrum of spin–spin relaxation time (T2) was generated using a Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence. The CPMG sequence consists in applying a 90 degree radiofrequency pulse to the sample, followed by many 180 degree pulses. Each time a 180 degree pulse was applied, the signal decay of the magnetic field was removed and a single data point was acquired (Carr and Purcell, 1954; Meiboom and Gill, 1958). The 2D cross-correlation experiments were performed by an Inversion Recovery- CPMG sequence, where the inversion recovery step [180°–t1] is inserted prior to the Carr-Purcell-Meiboom-Gill (CPMG) sequence (Resende et al., 2019ab; Weisman et al., 2018; Song et al. 2002).
The data processing methodology used to reconstruct the signal acquired, into a 2D (T1 vs. T2) spectrum used in this study was developed by our laboratory, as described in previous studies (Berman et al., 2013; Campisi-Pinto et al., 2018; 2019; Weisman et al., 2019; Resende et al., 2019a,b). NMR with low field magnets requires a special data processing methodology in order to maximize the amount of information extracted from the acquired signals. Our methodology uses the primal dual interior method for convex objectives optimization (PDCO) solver and provides an excellent spectrum reconstruction accuracy.