Study of transport phenomena in chromatographic columns by pulsed field gradient NMR

Pulsed field gradient NMR has been applied to study mass transfer, flow, and dispersion in packed chromatographic columns. A single measurement allows the determination of the full displacement probability distribution of all fluid particles located in the measurement volume. Depending on the orientation of the pulsed magnetic field gradient with respect to the net flow direction, these-called averaged propagator is obtained independently and quantitatively for either the axial or the transverse fluid particle displacements, over an experimentally adjustable observation time. Thus, this technique can act on a dynamic time scale ranging from a few to several hundred milliseconds. This enabled us to detect the stagnant mobile phase in packed chromatographic columns and to follow the mass transfer between the stagnant solvent and the stream of mobile phase percolating through the column bed. With field gradients in the direction of net flow velocity, mean fluid particle displacements ranging between 0.07 and 100 times the average diameter of the stationary phase particles could be analyzed in terms of the intimately associated averaged propagator, with observation times between 30 and 960 ms. Starting in the absence of flow, the development from the Gaussian propagator for molecular diffusion in the packing (centered at zero net displacement) toward the fully established Gaussian propagator (parallel to the onset and increase of externally driven flow) is recorded, characterizing convection-driven dispersion in the packing. For short observation times, the mass transfer between the deep pools of; stagnant mobile phase in the totally porous support particles and the interstitial space in the packed bed lis found to be incomplete and the transition toward complete exchange is followed. The results indicate that the stagnant mobile-phase mass transfer in the classical HPLC silica-based stationary phase is purely diffusion-controlled.