Variable spacing Fast-T_1ρ for the analysis of fast relaxing species at low-field

T_1ρ is a powerful analytical technique capable of probing motions on the timescale of molecular dynamics. Conventional T_1ρ approaches are performed on high-field NMR magnets (typically >2 T) and rely on step-by-step data acquisition, resulting in relatively long experiments and acquisitions that are affected by non-negligible internal gradient effects and strict Specific Absorption Rate limits. The work presented herein reports the development of novel one- and two-dimensional Fast-T_1ρ pulse sequences on a 2 MHz benchtop NMR spectrometer. The developed method enables the observation of fast-relaxing species down to the millisecond timescale through application of a variably-spaced spin-lock duration array with delay spacings as short as 100 μs. Fast-T_1ρ and Fast-T₁-T_1ρ pulse sequences have been developed which show significantly faster acquisition relative to conventional T_1ρ, illustrated with a 6-fold and 25-fold reduction in experiment time respectively. These techniques were demonstrated to be robust and accurate with measured T_1ρ values for bulk glycerol showing a maximum difference of 5.4% between Fast-T_1ρ and conventional T_1ρ experiments. The power of this method is exemplified through the simultaneous observation of multiple relaxation environments within a porous γ-Al₂O₃ sample imbibed with an ethanol:water mixture, where T_1ρ relaxation rates of two orders of magnitude difference (2.4 ms to 370 ms) were captured.