Dynamic NMR microscopy of chromatographic columns

Pulsed magnetic field gradients encoded a discrete spatial and temporal motion of the molecules of a fluid percolating through the packed bed of a dynamic radial compression column for liquid chromatography. Two packing materials of porous, chemically bonded C18 silica were used: large irregularly shaped particles (55 – 105 μm) and small spherical particles (6 μm). By combining pulsed field gradient NMR and NMR imaging the distribution of the fluid dynamic displacement probability was determined quantitatively at each point of the static image space. The local axial dispersion coefficient, fluid velocity, and external porosity of the bed were also estimated. The resolution achieved was 250 μm × 250 μm × 2,500 μm per voxel, corresponding to a 250 μm × 250 μm pixel in a 2.5‐mm‐thick slice. Irregularly shaped particles showed slight external porosity heterogeneities over the column cross section, causing a dramatic loss of performance. Spherically shaped particles offered a nearly planar distribution of the transport characteristics across the packed bed, making radial compression of the bed enhance the performance.