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.