2.9. Nuclear magnetic resonance (NMR)
Low-field NMR relaxation measurements were performed using
a Niumag Benchtop Pulsed NMR Analyzer PQ001 (Niumag Electric
Corporation, Shanghai, China) operating at a resonance frequency
for protons of 22.6 MHz as previously described (Han, Zhang, Fei,
Xu, & Zhou, 2009). One gramme of sample (1 1 1 cm) was
placed in an NMR tube. Spin–spin relaxation time (T2) was
measured using the Carr–Purcell–Meiboom–Gill sequence. The T2
measurements were made with a s-value (time between 90 and
180 pulse) of 150 ls. Data from 4000 echoes was acquired as 12
scan repetitions. The repetition time between subsequent scans
was 4000 ms. Each measurements was performed in triplicate.
Post-processing of NMR T2 data distributed exponential fitting of
CPMG (Carr–Purcell–Meiboom–Gill) decay curves was performed
using Multi-Exp Inv Analysis software (Niumag Electric Corp.,
Shanghai, China).