Capillary electrophoresis - Applications

CE capillary electrophoresis is an analysis technique based on the use of a capillary with an internal diameter of a few tens of micrometers, filled with a separating electrolyte and subjected to an electric field. There are several separation modes, depending on the composition of the electrolyte and the presence of a gel, pH gradient or stationary phase in the capillary. Each mode separates compounds according to one or more criteria such as their charge-to-size ratio, hydrophobicity, chirality, size or isoelectric point, for example. There are also several detection modes that can be coupled to CE separation, such as UV absorbance spectrophotometry, fluorimetry, conductimetry or mass spectrometry, for example.

This diversity of separation mechanisms and detection modes makes CE suitable for analyzing a wide variety of compounds, such as small inorganic or organic ions, neutral and/or chiral molecules, amino acids, peptides, proteins, sugars and polysaccharides. This opens up a wide range of application fields, such as pharmaceuticals, biology, the environment, food or forensics, where its various qualities - miniaturization, high efficiency, speed, automation and low cost - are highly appreciated.

However, depending on the nature of the compounds targeted, some separation and detection methods are more appropriate than others. For CE analysis of ions and small ionized or ionizable molecules, capillary zone electrophoresis (CZE) and electrokinetic chromatography (EKC) are the preferred methods, most often combined with detection by UV absorbance spectrophotometry or conductimetry, and sometimes mass spectrometry. Not only the nature of the ions involved, but also the mode of detection, impose constraints on the composition of the separation electrolyte and any functionalization of the capillary surface. In the case of neutral compounds with zero electrophoretic mobility, the EKC mode is typically used: an additive is added to the separation electrolyte, such as a surfactant above its critical micellar concentration CMC, in order to separate the compounds according to their interactions with this so-called pseudo-stationary phase. If chiral separation is required, a chiral additive such as a CD cyclodextrin, for example, is added to the electrolyte to act as a chiral selector and separate the enantiomers in EKC mode. As regards the analysis of AA amino acids, their lack of chromophoric properties requires a prior derivatization step to make them detectable by UV absorbance spectrophotometry, if this detection mode is chosen for example. This is also the case for most saccharides, as this derivatization step facilitates their detection as well as their separation in CZE by giving them charges whatever the pH of the separation electrolyte. In the case of CE analysis...