Capillary electrophoresis terms

This chapter will first cover the nature of electrophoretic separations, especially those concerning capillary electrophoresis. Comprehensive multidimensional separations will then be defined, specifically in terms of orthogonality and resolution. The history of planar and non-comprehensive electrodriven separations will then be discussed. True comprehensive multidimensional separations involving chromatography and capillary electrophoresis will be described next. Finally, the future directions of these multidimensional techniques will be outlined. 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

Wong, K. S., Kenseth, J., Strasburg, R. Validation and long-term assessment of an approach for the high throughput determination of lipophilidty (log Pow) values using multiplexed, absorbance-based capillary electrophoresis. J. Pharm. Sci. 2004, 93, 916-931. 

The basic components of an LC-NMR system are some form of chromatographic instrument and an NMR spectrometer equipped with a flow-probe, as shown in Fig. 19.17. In terms of the chromatography of choice, there are many examples in the literature of a wide array of separation instruments employed, from SFC to capillary electrophoresis (CE) [87,88]. By far the most common method (not necessarily the best choice from a separation point of view) of achieving the desired separation is through HPLC. There are many commercial... 

In practice isotachophoresis is usually performed in narrow tubes with electrodes at either end and is one form of capillary electrophoresis. For the separation of a particular type of ion, e.g. an anion, two buffered electrolyte solutions are selected that have different anions but a common cation with a buffering capacity. One of the anions (termed the leading electrolyte) should show a greater mobility than the other anion and occupies the anodic end of... 

Reversed-phase HPLC can be compared in terms of its utility with other modes of chromatography and with other separation techniques such as gel electrophoresis, capillary electrophoresis (CE), and capillary electrochromatography. 

Methods development starts with a relatively high number of techniques to characterize and test samples. The number of protocols is often reduced once the critical parameters and the methods that identify them have been defined. The analyst must evaluate the initial techniques with respect to their purposes. If the goal is to generate research data, the practicality of the method and its limitations are not of primary concern if the goal is to use the technique as part of a test procedure, it has to be evaluated in terms of its potential to meet full validation. Critical procedures (e.g., release testing) that cannot be validated will bring a project to an expensive halt. For these reasons, this chapter provides basic principles as well as limitations of capillary electrophoresis (CE) as applied to the analysis of real biopharmaceutical molecules. 

Mayer, B. X., and Muller, M. (2001). Long-term analyses with capillary electrophoresis. LC-GC... 

Ohnesorge, J., Saenger-van de Griend, C., and Waetzig, H. (2005). Quantification in capillary electrophoresis-mass spectrometry long- and short-term variance components and their compensation using internal standards. Electrophoresis 26, 2360—2375. 

Complementary to IC, the capillary electrophoresis (CE) technique is useful for both anions and cations. The method is significantly faster than IC for screening and is relatively easy to automate which is advantageous when large numbers of samples require analysis. Although CE is currendy similar in terms of sensitivity to IC it is a relatively new technique and significant improvements in both selectivity and sensitivity continue to be made [26]. 

Tea is known to be a good sonrce of phenolic substances and its earliest HPLC characterization was pnblished in 1976 [359] The introdnction of DAD as a detection technique for HPLC greatly improved the possibility of fully characterizing it. In 2004, Yao et al. [360] published a study on tea cnltivated in Anstralia six phenolic acids were identified, including p-coumarylquinic, p-coumaric, and 3-(p-hydroxyphenyl) propionic acid. Previously, Bonoli et al. [361] carried out a comparison of HPLC with capillary electrophoresys and concluded that the latter shows advantages in terms of the time of analysis with respect to HPLC, even if in 2004, Pelillo et al. [362] were able to develop a fast analysis by means of HPLC. 

In its simplest form capillary electrophoresis is termed capillary zone electrophoresis . The conditions used in this type of analysis are relatively simple and the mobile phase used consists of a buffer with various additives. Many applications focus on critical separations which are difficult to achieve by HPLC. In many cases it is difficult to explain completely the types of effects produced by buffer additives. 

In capillary electrophoresis, components of a mixture are separated according to two main factors electrophoretic mobility and electro-osmotic flow. These terms apply to ions, molecules or micelles. 

In packed columns, all three terms contribute to band broadening. For open tubular columns, the multiple path term, A, is 0, so bandwidth decreases and resolution increases. In capillary electrophoresis (Chapter 26), both A and C go to 0, thereby reducing plate height to submicron values and providing extraordinary separation powers. 

Capillary electrophoresis provides unprecedented resolution. When we conduct chromatography in a packed column, peaks are broadened by three mechanisms in the van Deemter equation (23-33) multiple flow paths, longitudinal diffusion, and finite rate of mass transfer. An open tubular column eliminates multiple paths and thereby reduces plate height and improves resolution. Capillary electrophoresis reduces plate height further by knocking out the mass transfer term that comes from the finite time needed for solute to equilibrate... 

The electrophoretic separation technique is based on the principle that, under the influence of an applied potential field, different species in solution will migrate at different velocities from one another. When an external electric field is applied to a solution of charged species, each ion moves toward the electrode of opposite charge. The velocities of the migrating species depend not only on the electric field, but also on the shapes of the species and their environmment. Historically, electrophoresis has been performed on a support medium such as a semisolid slab gel or in nongel support media such as paper or cellulose acetate. The support media provide the physical support and mechanical stability for the fluidic buffer system. Capillary electrophoresis (CE) has emerged as an alternative form of electrophoresis, where the capillary wall provides the mechanical stability for the carrier electrolyte. Capillary electrophoresis is the collective term which incorporates all of the electrophoretic modes that are performed within a capillary. 

A powerful high-efficiency, high-resolution analytical technique is described for the separation, characterization and quantitation of minute amounts of analytes. This technique, termed capillary electrophoresis, offers the capability of on-line detection, the use of multiple detectors, micropreparative operation and automation. 

In general terms, capillary electrophoresis is the electrophoretic separation of a substance from (usually) a complex mixture within a narrow tube filled with an electrolyte solution which is normally an aqueous buffer solution. Although one example of separation performed in a totally non-aqueous solution has been reported (50), neutral and slightly basic buffer solutions are generally used. Small tubes dissipate heat efficiently and prevent disruption of separations by thermally driven convection currents. Therefore, capillary electrophoresis can use... 

---