Analysis of linear and cyclic oligomers in polyamide-6 without sample preparation by liquid chromatography using the sandwich injection method. III. Separation mechanism and gradient optimization

The first 6 linear and cyclic oligomers of polyamide-6 can be quant. detd. in the polymer using HPLC with the sandwich injection method and an aq. acetonitrile gradient. In this final part of the triptych concerning the detn. of the oligomers in polyamide-6, the irregular elution behavior of the cyclic monomer compared to the cyclic oligomers was investigated. The authors also optimized the sepn. of the involved polyamide oligomers, with respect to gradient steepness, stationary phase, column temp., and mobile phase pH. The irregular elution behavior of the cyclic monomer could be caused by its relatively large exposed/accessible hydrophobic surface, which permits relatively easy penetration into the hydrophobic stationary phase giving extra retention. The dipole moment of the different oligomers was used as a measure for this exposed/accessible hydrophobic area to correlate the retention factors using quant. structure-retention relationships. The authors also studied the retention behavior of the polyamide, which is injected each run directly onto the column and modifies the stationary phase. Using a 250-ml post gradient injection zone of formic acid on a 250*3 mm Zorbax SB-C18 column, the polyamide could be effectively removed from the stationary phase after each sepn. The linear solvent strength (LSS) model was used to optimize the sepn. of the first 6 linear and cyclic oligomers. As the LSS model assumes a linear correlation between the modifier concn. and the logarithm of the retention factor and the cyclic monomer and dimer show extreme curvation of this relation in the eluting region, the authors investigated different models to predict gradient elution from isocratic data. A direct translation of the isocratic data to gradient retention times did not yield adequate retention times using the LSS model. The LSS model worked acceptably if gradient retention times were used as input data. Even for fast non-linearly eluting components, an av. error of 0.4 resoln. units of 4s was obtained. Using the LSS model in combination with different column temps. and mobile phase pH values, a sepn. of the first 6 linear and cyclic oligomers was accomplished. [on SciFinder (R)