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

The separation of polypropylene glycol 1200 (PPG 1200) and polybutylen e glycol 1000 (PBG 1000) was investigated by reversed-phase high-perfo rmance liquid chromatography on octadecylsilyl silica gel (C-18) with aprotic (acetonitrile) and protic (methanol, ethanol, 2-propanol) orga nic modifiers. Detector responses were monitored by means of evaporati ve 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 hyd rophobic 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. Participa tion of substantial solvophobic solute-solvent influences was hypothes ized but the different mobile phase effects of the protic modifiers ma y also need to be taken into account. The former effect may be explain ed by interactions between the alkyl chains of ethanol and 2-propanol with the hydrophobic tetramethylene backbone of PBG 1000, which furthe r enhances the solubility increase elicited by hydrogen bond formation between the hydroxyl groups of the organic solvent and the ether oxyg ens of the analyte. The latter effect may particularly be assumed in t he case of methanol, where the methyl group seems to be too small to u ndergo efficient hydrophobic interactions with non-polar sites of the analyte.