Separation of polypropylene glycol 1200 and polybutylene glycol 1000 by reversed-phase high-performance liquid chromatography on a C 18 stationary phase with different organic modifiers and detection by evaporative light scattering

Abstract

The separation of polypropylene glycol 1200 (PPG 1200) and polybutylene glycol 1000 (PBG 1000) was investigated by reversed-phase high-performance liquid chromatography on octadecylsilyl silica gel (C 18) with aprotic (acetonitrile) and protic (methanol, ethanol, 2-propanol) organic modifiers. Detector responses were monitored by means of evaporative light scattering. It was shown that the retentions of all oligomers of PPG 1200 decrease in the order methanol > acetonitrile > ethanol > 2-propanol. A biphasic elution pattern was observed with the more hydrophobic PBG 1000 and the retentions of low-molecular-mass homologues decreased in the order methanol > ethanol > acetonitrile > 2-propanol, whereas those of medium- and high-molecular-mass oligomers decreased in the order acetonitrile > methanol > ethanol > 2-propanol. Participation of substantial solvophobic solute-solvent influences was hypothesized but the different mobile phase effects of the protic modifiers may also need to be taken into account. The former effect may be explained by interactions between the alkyl chains of ethanol, and 2-propanol with the hydrophobic tetramethylene backbone of PBG 1000, which further enhances the solubility increase elicited by hydrogen bond formation between the hydroxyl groups of the organic solvent and the ether oxygens of the analyte. The latter effect may particularly be assumed in the case of methanol, where the methyl group seems to be too small to undergo efficient hydrophobic interactions with non-polar sites of the analyte.