Separation of polyesters by gradient reversed-phase high-performance liquid chromatography on a 1.5 microm non-porous column

Efficient separation of polyesters composed of a large number of individual oligomers was achieved on a 1.5 microm "non-porous" octadecylsilyl (ODS) silica support by gradient high-performance reversed-phase liquid chromatography (gRP-HPLC) with a mobile phase of acetonitrile, aqueous trifluoroacetic acid (0.2%) and tetrahydrofuran at ambient temperature and signal monitoring by UV absorption at 280 nm. Substantial signal splitting of oligomers in the low molecular weight (Mr) region is indicative that separation not only occurs with respect to molecular weight distribution (MWD) but also to chemical composition distribution (CCD) and functionality type distribution (FTD). Although separation according to CCD and FTD decreases with increasing number of oligomers, co-elution of species with identical number of repeat units but differing in either structure of repeat units or end-groups can be assumed from the relatively broad signals succeeding the aforementioned peaks showing at least partial resolution. Despite the observation that high Mr oligomers elute as sharp signals, the preceding observations suggest that each of these peaks presumably composes of more than one individual component. The polyester oligomers are eluted in the range of increasing Mr and therefore, either separation according to MWD or CCD/FTD was at least achieved for the low Mr sample constituents. Some principal mechanistic aspects of separation are discussed and adsorption seems to play the dominant role. The detection limit, defined as that sample amount yielding an unequivocal recognition on the base of its characteristic chromatographic fingerprint pattern was about 5,000 ppm for the pair Alftalat 3258 - Alftalat 3352 and 10,000 ppm for the pair Crylcoat 430 - Crylcoat 801.