On the chromatographic efficiency of analytical scale column format porous polymer monoliths: Interplay of morphology and nanoscale gel porosity

Porous monolithic poly(styrene-co-divinylbenzene) stationary phases in 4.6 mm I.D. analytical-scale column format with varying porosity, globule scale polymer morphology and flow-through pore structure have been investigated with respect to their transport properties toward small retained solutes in isocratic elution, reversed-phase liquid chromatography. The current study was performed under kinetically and thermodynamically relevant conditions comprising retention factors from close to zero up to the order of 50–100 under most extreme conditions, while a linear chromatographic flow velocity up to 4 mm/s, in some instances up to 7 mm/s, was realized. Carefully designed experiments aimed at resolving issues associated with the monoliths performance, while a particular focus is given on gel porosity, chromatographic retention and band dispersion. Elucidation of three important metric properties gave orthogonal insight. These are: (i) the columns dry-state morphology and surface area, (ii) the gel porosity with tetrahydrofuran as solvent determined by size exclusion chromatography using a range of small subnanometer-sized molecules and polystyrene standards, as well as (iii) the isocratic reversed-phase performance of small molecules at varying binary acetonitrile/water mobile phase solvent compositions, modulating gel porosity. Consistently throughout the study, the adjustable and general retention-factor-dependence of the performance of these monolithic materials is shown. It can also be correlated to the analytes molecular weight and consequently size. Isocratic performance strongly depends on the amount of gel porosity of the scaffold, which can be changed by varying the percentage of organic modifier in the mobile phase and indicates the adjustable chromatographic nature of porous polymer monoliths. This gel porosity which is absent in the dry-state of the polymer monoliths and is characterized by sub-nanometer to nanometer-sized pore space induces, additionally to permanent porosity, stagnant mass transfer zones. The displayed major reason for mass transfer resistance implied by the use of polymeric monolithic columns determines dispersion behavior of small molecules and its varying importance with respect to morphology and size of the globular features containing stagnant mass transfer zones is addressed. This leads to the conclusion, that a reduction in polymer feature size and increase in number of flow-through pores per unit cross-section of the monolith with an improved homogeneity may be an interesting option of tailoring column performance. It is further concluded that dry-state methods (such as nitrogen adsorption analysis and scanning electron microscopy) or solvated-state methods (such as size-exclusion chromatography in tetrahydrofuran) by itself are insufficient measures to explain the adjustable chromatographic performance of porous polymer monoliths.