Martin M, Thevenon G, Tchapla A
Ecole Polytechnique, Laboratoire de Chimie Analytique Physique, Palaiseau, France.
J Chromatogr. 1988 Oct 28;452:157-73. doi: 10.1016/s0021-9673(01)81445-x.
The retention behaviour of saturated homogeneous triglycerides was compared to that of single chain homologous compounds in reversed-phase liquid chromatography on silica-based octadecyl bonded phases with non-aqueous binary eluents containing chloroform. For both series, in all eluents and at all investigated temperatures, a discontinuity (1-6% slope change) in log k' vs. the carbon atom number, nc, of each solute hydrocarbonaceous chain was observed for a given critical number of carbon atoms, nc,crit approximately 12-13. Similar discontinuities were observed in delta H degrees and delta S degrees vs. nc for the same nc,crit value. These and other phenomena (existence of two convergence points for log k' vs. nc plotted at different eluent compositions; existence of two convergence temperatures in log k' vs. 1/T plotted for several members of a given solute series) reflect the mechanism of penetration of the solute aliphatic chains into the bonded layer. The eluotropic strength of acetonitrile-chloroform mobile phases was determined and seen to have a larger rate of variation with the solvent composition than that of some other non-aqueous binary eluents. The ratio of the slope of plots of log k' vs. nc curves for triglycerides and single chain homologous series is not a whole number but has a value lying between 2 and 3. This explains why triglyceride retentions cannot be predicted from retention data for the fatty acid methyl esters. More importantly, this indicates that the triglycerides may interact with the stationary phase using various conformations, one, two or three chains penetrating into the bonded layer. These conformations, in dynamic equilibrium with each other, contribute differently to the retention. This offers, in principle, the possibility to separate unsaturated triglycerides having the same number of carbon atoms and of double bonds but differing in the distribution of the unsaturations along the chains, if the double bonds located in a non-penetrating chain can selectively interact with a mobile phase component.
在以硅胶为基质的十八烷基键合相上，使用含氯仿的非水二元洗脱剂进行反相液相色谱分析时，对饱和均相甘油三酯和单链同系物的保留行为进行了比较。对于这两个系列，在所有洗脱剂中以及在所有研究温度下，对于给定的临界碳原子数nc,crit（约为12 - 13），观察到log k'与每个溶质烃链的碳原子数nc之间存在不连续性（斜率变化1 - 6%）。在相同的nc,crit值下，还观察到ΔH°和ΔS°与nc之间存在类似的不连续性。这些以及其他现象（在不同洗脱剂组成下绘制的log k'与nc曲线存在两个收敛点；对于给定溶质系列的几个成员，绘制的log k'与1/T曲线存在两个收敛温度）反映了溶质脂肪链渗透到键合层中的机制。测定了乙腈 - 氯仿流动相的洗脱强度，发现其随溶剂组成的变化速率比其他一些非水二元洗脱剂的变化速率更大。甘油三酯和单链同系物系列的log k'与nc曲线斜率之比不是整数，而是介于2和3之间的值。这解释了为什么不能从脂肪酸甲酯的保留数据预测甘油三酯的保留情况。更重要的是，这表明甘油三酯可能以各种构象与固定相相互作用，一条、两条或三条链渗透到键合层中。这些构象相互处于动态平衡，对保留的贡献不同。原则上，如果位于非渗透链中的双键能够与流动相组分选择性相互作用，那么就有可能分离具有相同碳原子数和双键数但双键沿链分布不同的不饱和甘油三酯。
