Liquid chromatography under limiting conditions of enthalpic interactions: Enthalpic partition retention mechanism

摘要

Applicability of the enthalpic partition (absorption) retention mechanism was evaluated in liquid chromatography under limiting conditions of enthalpic interactions (LC LC). It was shown that the barrier principle of LC LC also held in the case of enthalpic partition retention mechanism. LC columns packed with porous silica C-18 bonded phase in combination with low-polarity PS and PnBMA were employed. The partition-promoting solvent was DMF and the partition-preventing solvent was THE Pore-permeating molecules of DMF moved slowly along column and acted as a barrier, which hindered fast progression of pore excluded macromolecules. The barrier was either the eluent itself, or a narrow zone of DMF injected immediately before polymer solution. In the former case, the eluent contained a high concentration of DMF and would not allow polymer elution. However, the sample was injected in pure THF and traveled within its zone. Being decelerated by the DMF barrier, both PS and PnBMA eluted independently of their molar mass in total volume of column liquid. They could be efficiently and independently of their molar masses separated from the medium- and high-polarity polymers, which did not exhibit enthalpic partition and eluted in the size exclusion mode. In contrast to recently evaluated adsorption based LC LC procedures, enthalpic partition produced broader, skewed, and often split peaks. This is assumed due to slow establishment of enthalpic partition equilibrium, possible presence of micropores in the C-18 bonded phase, and/or due to deformation of shape of barrier edge. Therefore, limiting conditions of enthalpic partition are to be applied preferably to characterization of polar SEC eluted polymers after their discrimination from the low-polarity macromolecules eluting in the LC LC mode.