Vrije Universiteit Brussel, Department of Chemical Engineering (CHIS-IR), Pleinlaan 2, 1050 Brussels, Belgium.
J Chromatogr A. 2011 Feb 25;1218(8):1153-69. doi: 10.1016/j.chroma.2010.12.086. Epub 2010 Dec 28.
We report on a general theoretical assessment of the potential kinetic advantages of running LC gradient elution separations in the constant-pressure mode instead of in the customarily used constant-flow rate mode. Analytical calculations as well as numerical simulation results are presented. It is shown that, provided both modes are run with the same volume-based gradient program, the constant-pressure mode can potentially offer an identical separation selectivity (except from some small differences induced by the difference in pressure and viscous heating trajectory), but in a significantly shorter time. For a gradient running between 5 and 95% of organic modifier, the decrease in analysis time can be expected to be of the order of some 20% for both water-methanol and water-acetonitrile gradients, and only weakly depending on the value of V(G)/V₀ (or equivalently t(G)/t₀). Obviously, the gain will be smaller when the start and end composition lie closer to the viscosity maximum of the considered water-organic modifier system. The assumptions underlying the obtained results (no effects of pressure and temperature on the viscosity or retention coefficient) are critically reviewed, and can be inferred to only have a small effect on the general conclusions. It is also shown that, under the adopted assumptions, the kinetic plot theory also holds for operations where the flow rate varies with the time, as is the case for constant-pressure operation. Comparing both operation modes in a kinetic plot representing the maximal peak capacity versus time, it is theoretically predicted here that both modes can be expected to perform equally well in the fully C-term dominated regime (where H varies linearly with the flow rate), while the constant pressure mode is advantageous for all lower flow rates. Near the optimal flow rate, and for linear gradients running from 5 to 95% organic modifier, time gains of the order of some 20% can be expected (or 25-30% when accounting for the fact that the constant pressure mode can be run without having to leave a pressure safety margin of 5-10% as is needed in the constant flow rate mode).
我们报告了一种普遍的理论评估，即在恒压模式下而不是在通常使用的恒流速模式下运行 LC 梯度洗脱分离的潜在动力学优势。给出了分析计算和数值模拟结果。结果表明，只要两种模式都使用相同的基于体积的梯度程序运行，恒压模式就有可能提供相同的分离选择性（除了由于压力和粘性加热轨迹的差异而引起的一些小差异），但时间明显更短。对于在 5%和 95%有机改性剂之间进行梯度运行的情况，水-甲醇和水-乙腈梯度的分析时间可预期缩短约 20%，且仅与 V(G)/V₀（或等效的 t(G)/t₀）的值弱相关。显然，当起始和结束组成更接近所考虑的水-有机改性剂体系的粘度最大值时，增益会更小。对获得的结果所依据的假设（压力和温度对粘度或保留系数没有影响）进行了严格审查，可以推断这些假设对一般结论的影响很小。还表明，在所采用的假设下，对于流速随时间变化的操作，动力学图理论也适用，这就是恒压操作的情况。在表示最大峰容量与时间的动力学图中比较两种操作模式，从理论上预测，在完全由 C 项主导的区域（其中 H 随流速线性变化），两种模式都可以预期表现同样良好，而恒压模式在所有较低流速下都具有优势。在接近最佳流速的情况下，对于从 5%到 95%有机改性剂的线性梯度，预计可以获得约 20%的时间增益（当考虑到恒压模式无需像在恒流速模式下那样留出 5-10%的压力安全裕度时，可以获得 25-30%的增益）。
