On the occurrence of very low intra-particle diffusion rates in zwitterionic hydrophilic interaction liquid...

Intra-particle diffusion is investigated in particle-packed and monolithic zwitterionic hydrophilic interaction liquid chromatography (HILIC) polymer columns. For this purpose, plate heights are first measured over a broad range of velocities for polar compounds with zone retention factors between k^″ =  2 and 10 on all column types, multiplied with the corresponding velocity, and extrapolated to zero velocity. The thus obtained B-term coefficients are additionally verified via peak parking experiments and indicate a very low degree of longitudinal diffusion in all columns, that moreover seems to be independent of the zone retention factor. This is in contrast to earlier observations on reversed-phase liquid chromatography columns and bare silica HILIC columns. To obtain more insight into the reasons for these low levels of longitudinal diffusion, the experimentally obtained effective diffusion coefficients are modeled to equations derived from the effective medium theory, such as the models developed by Torquato and Jefrey & Hashin. It is, however, demonstrated that these models fail to adequately describe effective diffusion in the studied zwitterionic HILIC columns, due to the low degree of intraparticle diffusion in these stationary phases. Fitting the experimental effective diffusion coefficients to the two limiting cases of the effective medium theory, i.e., the pure parallel-connection and pure serial-connection case, it is observed that to stay within these limits, intraparticle diffusion must be extremely low, i.e., D_pz/D_m ≤ 0.01, indicating that the diffusion inside the pores is at least 100 times smaller than the bulk diffusion coefficient. There are several possible explanations for these low intraparticle diffusion coefficients, such as a strongly hindered diffusion in the polymer matrix, extremely low surface diffusion rates in the aqueous layer adsorbed onto the stationary phase and/or the occurrence of slow localized adsorption events.