Separation of acetylated polypropylene glycol di- and triamines by gradient reversed-phase high-performance liquid chromatography and evaporative light scattering detection

Abstract

Polypropylene glycol di- and triamines, the so-called Jeffamines, were reacted with a mixture of equal volumes of acetic anhydride and pyridine to give the corresponding acetamides prior to chromatographic separation by reversed-phase high-performance liquid chromatography using a linear binary solvent gradient and evaporative light scattering detection (ELSD). The procedure was applied to decrease silanophilic interactions of the solutes with the column matrix in order either to improve the peak resolution R s or to decrease peak tailing. Removal of excess of derivatizing agent prior to sample injection is not required owing to the volatility of pyridine, which does not yield any ELSD response. Therefore, the signals of oligomers with lower retention that are hidden by the broad and strongly tailing pyridine peak when measured by UV detection, are now clearly detectable. Separation was performed on C 18, C 8, C 6, C 4 and C 1 stationary phases with either acetonitrile or methanol as organic modifiers. With acetonitrile either complete elution or excellent separation of the low-molecular-mass samples ( M r ⋍ 400) is achieved on the C 18, C 8, C 6 and C 4 matrices. However, R s decreases with increasing M r but the recoveries of high- M r samples ( M r ⋍ 2000–5000) increase markedly with decreasing hydrophobicity of the stationary phase in the order C 18 < C 8 < C 6 < C 4. Complete elution of the whole family of investigated polyether amines was accomplished with either acetonitrile on a C 1 column or methanol on all sorbents used in the study. The optimum peak resolution was obtained on a C 4 column with either acetonitrile or methanol. Complete elution of all samples in particular on matrices with either high or intermediate hydrophobicity with methanol compared with acetonitrile is presumably attributable to a better solvation of the polypropylene glycol backbone by hydrogen bonding between the ether oxygens and the hydroxyl group of the protic solvent.