DYNAMIC NMR MICROSCOPY OF CHROMATOGRAPHIC COLUMNS

Pulsed magnetic field gradients encoded a discrete spatial and tempora l motion of the molecules of a fluid percolating through the packed be d of a dynamic radial compression column for liquid chromatography. Tw o packing materials of porous, chemically bonded C18 silica were used: large irregularly shaped particles (55-105 mu m) and small spherical particles (6 mu m). By combining pulsed field gradient NMR and NMR ima ging the distribution of the fluid dynamic displacement probability wa s determined quantitatively at each point of the static image space. T he local axial dispersion coefficient, fluid velocity, and external po rosity of the bed were also estimated. The resolution achieved was 250 mu m X 250 mu m X 2,500 mu m per voxel, corresponding to a 250 mu m X 250 mu m 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 pa rticles offered a nearly planar distribution of the transport characte ristics across the packed bed, making radial compression of the bed en hance the performance.