SEPARATION OF POLYESTER OLIGOMERS BY GRADIENT HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

A highly efficient method for the separation of commercially available polyesters composed of a multitude of individual oligomers by gradien t reversed-phase high-performance liquid chromatography (gRP-HPLC) was developed. Oligomers up to a molecular mass of more than 10 000 could be sufficiently resolved on an octadecylsilyl silica (C-18) stationar y phase using a ternary gradient consisting of acetonitrile, tetrahydr ofuran (THF) and aqueous acetic acid. Detection was performed by measu rement of signal responses from either UV detection at 230 nm or evapo rative light scattering detection (ELSD). Addition of THF to the mobil e phase is essential and omission of the co-solvent provided elution o f only a few oligomers, the residual amount being trapped on the highl y hydrophobic stationary phase. Due to the substantial UV mismatch inv oked by THF exhibiting marked absorbance at the chosen wavelength, ELS D, which is not associated with baseline drift phenomena, is much more suited for identification of individual polyester samples on the basi s of the chromatographic fingerprint. In one case about sixty sufficie ntly resolved peaks each attributable to a single oligomer could be ob served. A concentration of about 10 mg/ml of sample was required for a n unequivocal distinction of the individual polyesters. The use of a C -18 stationary phase is an ultimate prerequisite for efficient oligome r resolution as impressively evidenced by an almost complete lack of s eparation into individual oligomers on C-8 or C-Phenyl matrices. In th e latter cases only few low-molecular-mass oligomers are preceding the broad and unresolved bulk peak envelope of sample constituents. (C) 1 997 Elsevier Science B.V.