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.