Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
J Chromatogr A. 2014 Jul 25;1352:20-8. doi: 10.1016/j.chroma.2014.04.089. Epub 2014 May 14.
The effect of the pressure on the dispersion of a low molecular weight compound along 0.508 and 1.016 mm i.d. × 50 cm long open circular tubes was investigated theoretically and experimentally. The theoretical predictions were based on the early models of dispersion derived by Aris and Taylor (1953) and on the approximate model of Alizadeh for the time moments (1980). Experimentally, the system pressure was increased at constant flow rate (0.15 mL/min) from less than 20 to nearly 1,000 bar by using a series of capillary tubes (25 μm i.d. PEEKSIL tubes) of increasing flow resistances placed upstream the detection cell of a commercial very high pressure liquid chromatograph (vHPLC) but downstream the 50 cm long tube. Theoretical and experimental results agree that the peak variance increases linearly with increasing pressure in the tube volume. The relative increase of the peak variance is 7% above that measured at low pressure (<20 bar) for each 100 bar increment in the tube volume. This result confirms that accurate measurements of the column efficiency corrected for extra-column contribution cannot be made by replacing the column with a zero dead volume union connector, because the pressures applied in the pre-column volume are significantly different in these two cases. This work shows also that increasing the pressure in the pre-column volume by increasing the flow rate affects the apparent column efficiency that is measured, independently of the direct effect of the flow rate. For a 2.1 × 50mm column packed with 1.3 μm core-shell particles run with a classic Acquity system, the associated relative decreases of the column efficiency are expected to be -30%, -20%, and -5% for retention factors of 1, 3, and 10, respectively. The column HETP is no longer independent of its length.
理论和实验研究了在 0.508 和 1.016mm i.d.×50cm 长的开环圆形管中，低压分子化合物在压力作用下的分散效应。理论预测基于 Aris 和 Taylor（1953）早期的分散模型和 Alizadeh 近似时间模型（1980 年）。实验中，在恒定流速（0.15mL/min）下，通过在商业超高压液相色谱仪（vHPLC）检测池的上游放置一系列流阻逐渐增大的毛细管（25μm i.d. PEEKSIL 管），将系统压力从低于 20 增加到近 1000bar。理论和实验结果均表明，峰方差随管内压力线性增加。与在低压（<20bar）下测量的结果相比，管内压力每增加 100bar，峰方差的相对增加量增加 7%。这一结果证实，通过用零死体积管接头代替色谱柱，无法对柱外贡献进行校正的柱效进行准确测量，因为这两种情况下，预柱体积内的压力有显著差异。本工作还表明，通过增加流速来增加预柱体积内的压力，会对测量的表观柱效产生影响，而与流速的直接影响无关。对于用经典 Acquity 系统运行的 2.1×50mm 柱，填充 1.3μm 核壳颗粒，对于保留因子为 1、3 和 10 的情况，预计柱效的相对下降分别为-30%、-20%和-5%。柱 HETP 不再与其长度独立。
