Marker compounds for the determination of retention factors in EKC

EKC and its sub‐techniques, such as MEKC and microemulsion EKC, have attracted wide interest in recent years. Investigations on this topic have covered several analytical applications, but attention has also been paid more and more to basic studies. This review provides an overview of the different approaches to calculating retention factors, which express the ratio of the amount of sample component in the pseudostationary and mobile phases. Special attention is given to the selection of markers for the determination of the electrophoretic mobility or migration time of a marker describing the behavior of the pseudostationary phase in EKC. Introduction of a hydrophobic marker is by far the most common approach, but the use of a homologous series of compounds is also quite popular. In addition, other possible approaches found in the literature will be described.