Dipolar-Filtered Magic Sandwich Echo as an alternative method for NMR cryoporometry

Cryoporometry is a valuable approach for determining pore sizes and their distributions in mesoporous materials by monitoring the melting and freezing behavior of a probe fluid confined in nanopores. ¹H NMR is well suited for this purpose, as motional narrowing of the ¹H-¹H dipolar coupling enables selective detection of the melted fraction using spin-echo-based sequences, and the intrinsically low resolution requirements favor low-field instruments. Here, we present an alternative implementation based on the Dipolar-Filtered Magic Sandwich Echo (DFMSE) sequence. By selecting suitable filter times, DFMSE enables acquisition of signals corresponding either to the total sample (solid + liquid) or exclusively to the liquid fraction, allowing internal normalization at each temperature. This approach naturally compensates for Curie-law effects, susceptibility variations, and temperature-dependent changes in tuning and sensitivity, eliminating the need for external references. In addition, monitoring the normalized intensity as a function of filter time provides a simple and unambiguous route to suppress signals that compromise the cryoporometry analysis. Validation using controlled-pore samples saturated with cyclohexane and water yielded pore sizes and distributions in close agreement with conventional cryoporometry and N₂ adsorption measurements. Application to a cadmium imidazolate framework further demonstrates the ability of DFMSE cryoporometry to provide reliable characterization in more complex porous systems.