Mengerink Y, Peters R, van der Wal Sj, Claessens H A, Cramers C A
MI&Q, DSM Research, Geleen, The Netherlands.
J Chromatogr A. 2002 Mar 8;949(1-2):307-26. doi: 10.1016/s0021-9673(01)01440-6.
The first six linear and cyclic oligomers of polyamide-6 can be quantitatively determined in the polymer using HPLC with the sandwich injection method and an aqueous acetonitrile gradient. In this final part of the triptych concerning the determination of the oligomers in polyamide-6, the irregular elution behavior of the cyclic monomer compared to the cyclic oligomers was investigated. We also optimized the separation of the involved polyamide oligomers, with respect to gradient steepness, stationary phase, column temperature 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 quantitative structure-retention relationships. We 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-microl post gradient injection zone of formic acid on a 250x3 mm Zorbax SB-C18 column, the polyamide could be effectively removed from the stationary phase after each separation. The linear solvent strength (LSS) model was used to optimize the separation of the first six linear and cyclic oligomers. As the LSS model assumes a linear correlation between the modifier concentration and the logarithm of the retention factor and the cyclic monomer and dimer show extreme curvation of this relation in the eluting region, we 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. It was found that the LSS model worked acceptably if gradient retention times were used as input data. Even for fast non-linearly eluting components, an average error of 0.4 resolution units of 4sigma was obtained. Using the LSS model in combination with different column temperatures and mobile phase pH values, a separation of the first six linear and cyclic oligomers was accomplished.
采用夹心进样法和乙腈水溶液梯度洗脱，通过高效液相色谱法（HPLC）可对聚酰胺-6中的前六种线性和环状低聚物进行定量测定。在关于聚酰胺-6中低聚物测定的三联文章的最后一部分，研究了环状单体与环状低聚物相比不规则的洗脱行为。我们还针对梯度陡度、固定相、柱温和流动相pH值，对相关聚酰胺低聚物的分离进行了优化。环状单体不规则的洗脱行为可能是由于其相对较大的暴露/可及疏水表面，这使得它相对容易渗透到疏水固定相中，从而产生额外保留。不同低聚物的偶极矩被用作该暴露/可及疏水区域的量度，以利用定量结构-保留关系关联保留因子。我们还研究了每次运行时直接进样到柱上并改变固定相的聚酰胺的保留行为。在250×3 mm的Zorbax SB-C18柱上使用250微升甲酸后梯度进样区，每次分离后聚酰胺可从固定相中有效去除。线性溶剂强度（LSS）模型用于优化前六种线性和环状低聚物的分离。由于LSS模型假定改性剂浓度与保留因子的对数之间存在线性关系，而环状单体和二聚体在洗脱区域显示出这种关系的极端弯曲，我们研究了不同模型以从等度数据预测梯度洗脱。使用LSS模型将等度数据直接转换为梯度保留时间，未得到合适的保留时间。结果发现，如果将梯度保留时间用作输入数据，LSS模型的效果尚可。即使对于快速非线性洗脱的组分，平均误差也仅为4σ的0.4个分离度单位。结合不同的柱温和流动相pH值使用LSS模型，实现了前六种线性和环状低聚物的分离。
