Characterization of polymer monolithic columns for small-molecule separations using total-pore-blocking conditions

• Peak-dispersion measurements were carried out with blocked stagnant pores.
• Diffusion in and out mesopores is not the main cause of band broadening.
• NMR measurements show that the surface polymer monolith is wetted when using ACN.
• Severe band broadening may take place due to mass-transfer via surface diffusion.

This study involves the investigation of the meso- and micropores in polymer monolithic materials and the performance characterization of polymer monoliths for the separation of small molecules. Pore-blocking experiments, that involve the blocking of the stagnant pores with a solvent which is immiscible with the mobile phase, were conducted to determine interstitial volumes of a commercially-available polymer monolithic column. After blocking the meso- and micropores a clear reduction in the column void time was observed. Using this approach, the internal porosity (defined as the sum of the meso- and micropores with respect to the volume of the monolithic material) was determined at 12.5%. Peak-dispersion measurements were conducted by applying reversed-phase (RP) conditions. The high plate-height values for small-molecule separations are mainly attributed to the large eddy-diffusion and mobile-phase mass-transfer contributions to band broadening, related to the inhomogeneous structure and presence of parabolic profiles in the macropores. The C-term contribution of early eluting (retained) compounds was higher than that of the late eluting compounds. This could be attributed to the low zone-retention factors of early-eluting compounds and consequently a large stationary-phase mass-transfer contribution. However, peak-dispersion measurements with blocked meso- and micropores carried out at RP conditions indicated that the Cs-contribution alone is likely not be the main cause of peak broadening. Finally, 1H spin–spin (T2) relaxometry NMR measurements were conducted with water and ACN in the monolithic material