STUDY OF TRANSPORT PHENOMENA IN CHROMATOGRAPHIC COLUMNS BY PULSED-FIELD GRADIENT NMR

Pulsed field gradient NMR has been applied to study mass transfer, flo w, and dispersion in packed chromatographic columns. A single measurem ent allows the determination of the full displacement probability dist ribution of all fluid particles located in the measurement volume. Dep ending on the orientation of the pulsed magnetic field gradient with r espect 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 adjust able 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 colu mns and to follow the mass transfer between the stagnant solvent and t he stream of mobile phase percolating through the column bed. With fie ld gradients in the direction of net flow velocity, mean fluid particl e displacements ranging between 0.07 and 100 times the average diamete r of the stationary phase particles could be analyzed in terms of the intimately associated averaged propagator, with observation times betw een 30 and 960 ms. Starting in the absence of flow, the development fr om the Gaussian propagator for molecular diffusion in the packing (cen tered at zero net displacement) toward the fully established Gaussian propagator (parallel to the onset and increase of externally driven fl ow) is recorded, characterizing convection-driven dispersion in the pa cking. For short observation times, the mass transfer between the deep pools of; stagnant mobile phase in the totally porous support particl es and the interstitial space in the packed bed lis found to be incomp lete and the transition toward complete exchange is followed. The resu lts indicate that the stagnant mobile-phase mass transfer in the class ical HPLC silica-based stationary phase is purely diffusion-controlled .