Department of Chemistry, Ralph N. Adams Institute for Bioanalytical Chemistry, University of Kansas, Lawrence, Kansas, USA.
Electrophoresis. 2024 Oct;45(19-20):1764-1774. doi: 10.1002/elps.202400107. Epub 2024 Jul 25.
Electroosmotic flow (EOF) is the bulk flow of solution in a capillary or microchannel induced by an applied electric potential. For capillary and microchip electrophoresis, the EOF enables analysis of both cations and anions in one separation and can be varied to modify separation speed and resolution. The EOF arises from an electrical double layer at the capillary wall and is normally controlled through the pH and ionic strength of the background buffer or with the use of additives. Understanding and controlling the electrical double layer is therefore critical for maintaining acceptable repeatability during method development. Surprisingly, in fused silica capillaries at low pH, studies observe an EOF even though the capillary surface should be neutralized. Previous work has suggested the presence of an "induced electroosmotic flow" from radial electric fields generated across the capillary wall due to the separation voltage and grounded components external to the capillary. Using thin-wall (15 µm) fused silica separation capillaries to facilitate the study of radial fields, we show that the EOF mobility depends on both the separation voltage and the location of external grounds. This is consistent with the induced EOF model, in which radial electric fields embed positive charges at the capillary walls to create an electrical double layer. The magnitude of the effect is characterized and shown to have long-range influences that are difficult to completely null by moving grounded components away from the separation capillary. Instead, active EOF control using externally applied potentials or a passive approach using a negative separation voltage are discussed as two possible methods for controlling the induced EOF. Both methods can reverse the EOF and improve the resolution and peak efficiency in amino acid separations.
电渗流（EOF）是在施加电场时引起的溶液在毛细管或微通道中的整体流动。对于毛细管电泳和微芯片电泳，EOF 能够在一次分离中分析阳离子和阴离子，并且可以通过改变分离速度和分辨率来进行调整。EOF 源自毛细管壁的双电层，通常通过背景缓冲液的 pH 值和离子强度来控制，或者使用添加剂来控制。因此，了解和控制双电层对于在方法开发过程中保持可接受的重复性至关重要。令人惊讶的是，在低 pH 值的熔融石英毛细管中，即使毛细管表面应该被中和，研究人员也观察到了 EOF。先前的工作表明，由于分离电压和毛细管外部的接地组件，会在毛细管壁上产生径向电场，从而产生“感应电渗流”。我们使用薄壁（15 µm）熔融石英分离毛细管来促进对径向场的研究，结果表明 EOF 迁移率取决于分离电压和外部接地位置。这与感应 EOF 模型一致，其中径向电场在毛细管壁上嵌入正电荷以形成双电层。我们对该效应的大小进行了特征描述，并表明其具有远程影响，即使将接地组件从分离毛细管移开，也很难完全消除。相反，我们讨论了使用外部施加的电势进行主动 EOF 控制或使用负分离电压进行被动方法作为控制感应 EOF 的两种可能方法。这两种方法都可以反转 EOF，并改善氨基酸分离中的分辨率和峰效率。
