ABSTRACT
Following rapid developments in capillary electrophoresis (CE), this technology has become an established analytical technique owing to its microscale characteristics, high speed, high efficiency, and versatility. However, the challenges of poor peak stability and/or reproducibility have consistently hindered its wider applications. CE has long been used as a measurement tool for plotting signal intensities versus the migration time; however, the migration time is not an independent variable in CE, but is affected by many direct and indirect parameters, including capillary (length, diameter, and inner surface properties), electric field (or voltage, current, and/or power), temperature, and running buffer (electrolytes, additives, solvents, and their concentration, buffering pH, etc.). These intricacies render the acquisition of reproducible electropherograms difficult. Various studies ranging from those on the early stages of CE development to those on the exploration of three important strategies have been conducted to address this issue. In the first strategy, the CE conditions, especially those parameters that can maintain a stable electro-osmotic flow, are strictly controlled and stabilized to significantly improve peak repeatability. In the second strategy, either the peak position is corrected using internal standards or the peak time is converted into other variables, such as electrophoretic mobility, to offset or eliminate some unstable factors, thereby improving the repeatability and even reproducibility of the peaks; this strategy is useful when plotting signals versus the migration time ratio, correlated migration time, effective mobility, or temperature-correlated mobility. In the third strategy, a new methodology called highly reproducible CE (HRCE) is established using theoretical studies to explore better principles for real-time CE with the aim of the complete removal of the challenge from the root. This strategy includes the development of novel methods that plot electropherograms based on weighted mobility, migrated charge, charge density, or partial differential molar charge density. Similar to ordinary CE approaches, this strategy can also draw electropherograms based on the ratios of these properties. As theoretically predicted, these novel methods can offset or resist changes in critical CE conditions (mainly electric field strength, capillary length and diameter, and/or some buffer parameters such as concentration). Our experimental results demonstrate that given certain prerequisites, a new set of methods can produce highly reproducible electropherograms. This review focuses on the theoretical basis and advancements of HRCE, and elucidates the link between electrophoretic migration/peak expression theories and their impact on reproducibility. Studies on the transformation of time-scale electropherograms in the CE literature are summarized and analyzed in general. However, this review does not directly discuss research on and progress in improving CE repeatability or reproducibility through instrument upgrades, parameter optimization, or practical method refinements.
