New insights on mass transfer kinetics in chromatography

Abstract

Wiley Online Library (wileyonlinelibrary.com).The mass transfer kinetics of thiourea, phenol, ethylbenzene, propylbenzene, butylbenzene, and amylbenzene were studied on a Gemini-C 18 (5 lm, 110 Š, 375 m 2 /g) column (150 mm  4.6 mm) eluted with methanol/water solutions (100, 90, and 20% v/v). Each of the successive steps of the mass transfer of these solutes (axial diffusion, eddy dispersion, film mass transfer resistance, and transparticle mass transfer resistance) was unambiguously measured, using a combination of the peak parking method, the total pore blocking method, and moment analysis, in a wide range of reduced linear velocities. The results obtained offer new insights on the mass transfer kinetics in chromatographic columns. They show first that the eddy dispersion A-term is strongly correlated with the particle porosity. The complex, anastomosed transcolumn flow pattern causes extra band broadening. This transcolumn effect was found to be markedly smaller with porous particles than with nonporous particles of the same size. Second, the film mass transfer coefficient of retained compounds is smaller for porous than for nonporous particles, a result consistent with concentration gradients being steeper at the wall of solid particles than across the entrance surface of pores. The external mass transfer coefficient decreases with increasing fraction of the surface area of the particles that is open to pores, e.g., with increasing particle porosity.

Theoretical investigation of diffusion along columns packed with fully and superficially porous particles

“…For strongly retained compounds,˝can be as large as 1.5 [39,36]. For˝= 1.5, the trends remain similar to those observed in Fig.…”

“…The shells of most particles of this type have a finite thickness and occupy typically 60-75% of the particle volume. From a physical point of view, the parameter varies typically between 0 (complete exclusion from the mesoporous network of the shell for large molecular weight compounds) to 1.5 in RPLC (maximum contribution of surface diffusion of small molecules to the diffusivity in the shell [36]). …”

“…It was empirically introduced because the B values should satisfy the constraint that for non-porous shells ( = 1), B = 2 e consistent with the obstruction to diffusion caused by fully impermeable spherical particles in a randomly packed bed [26,36,37] and e is usually taken as 0.6. This model is called the Garnett-Parallel model.…”

Shell particles, trials, tribulations and triumphs

“…This might be because packing nearly non-porous sub-3 m particles is uneasy. In fact, we showed that the trans-column velocity biases are more efficiently relaxed when the particles are porous than when they are not [98,112]. A compromise between a decrease of the B term and the increase of the A term has to be found and the best optimum particle size seem to correspond to values in the range of 0.6-0.7.…”

“…Pure water was used as the eluent and thiourea as the analyte. The advantage of using thiourea is that it does not partition between n-nonane trapped inside the mesopores and the interstitial eluent [111,112]. However, it was concluded that the eddy diffusion term measured by this method is systematically larger than the one that takes place in the same column when the mesopores are accessible to the eluent because diffusion through the mesopores of the particles contributes to efficiently relax the concentration gradients across the column [98,113].…”