Looking inside the pores of a chromatographic column. I. Variation of the pore volume with mobile phase composition

The dependence of void volume and pore volume on mobile phase composition was studied on a reversed phase column based on poly(divinyl benzene) in mobile phases consisting of 100% tetrahydrofuran (THF) and 40-90% THF-water. Void volumes V0 are obtained from a series of polymers with different molar mass by extrapolation of elution volume to M = 0, the interstitial volume Vi from the elution volume of polymers with a sufficiently high molar mass. The pore volume Vp is obtained as the difference (V0 - Vi). While Vi does not depend on mobile phase composition, a dramatic decrease of the pore volume is observed, when the mobile phase is changed from THF to THF-water. This effect of the mobile phase composition on the pore volume is observed using PEG and PPG as test macromolecules, regardless the retention mechanism: at 40-50% THF both PEG (in exclusion mode) and PPG (in adsorption mode) experience the same reduced pore volume, which is considerably lower than that in 100% THF.