Capillary Electrophoresis: Principles, Modes, Instrumentation, and Method Development

Heat generated by electric current in the capillary causes temperature gradients, viscosity changes, and zone broadening; minimized by small capillary id and temperature control.

Sample injection length should be less than diffusion-controlled zone width to avoid loss of resolution; typically less than 1-2% of capillary length.

Adsorption of solutes, especially proteins, to the negatively charged silica wall causes peak tailing and efficiency loss; minimized by coatings or buffer additives.

Electrodispersion is peak broadening caused by conductivity differences between sample and buffer, leading to concentration-dependent local electric fields and velocity variations.

Resolution depends on differences in solute mobility and efficiency; increasing voltage improves resolution but is limited by Joule heating.

CZE separates ions based on charge-to-size ratio in a buffer-filled capillary, with EOF moving all solutes towards the detector; neutrals co-migrate without separation.

MEKC uses surfactant micelles as a pseudo-stationary phase to separate neutral and charged solutes by partitioning between micelles and buffer.

CEC combines liquid chromatography and CE by applying EOF through a packed capillary column, enabling high-efficiency separations of neutral and weakly charged solutes.

CGE separates macromolecules like DNA and proteins by size using a polymer sieving matrix inside the capillary, analogous to slab gel electrophoresis.

CIEF separates proteins by their isoelectric point (pI) in a pH gradient formed by ampholytes inside the capillary, focusing proteins at zones where net charge is zero.

CITP separates ions by sandwiching them between leading and terminating electrolytes, causing zones to migrate at the same velocity with sharp boundaries.

Sample is introduced into the capillary by pressure, vacuum, or siphoning; injection volume depends on pressure, time, capillary dimensions, and buffer viscosity.

Sample ions are injected by applying voltage, with loading dependent on ion mobility and concentration; can cause bias and lower reproducibility than hydrodynamic injection.

Techniques like field amplified sample stacking (FASS), isotachophoretic stacking, and high-salt stacking improve sensitivity by concentrating analytes during or after injection.

Common detection methods include UV-visible absorption, laser-induced fluorescence (LIF), contactless conductivity detection (CCD), and mass spectrometry (MS), each with specific advantages.

Permanent (covalent) and dynamic (buffer additives) coatings modify capillary walls to control EOF and reduce solute adsorption, improving separation performance.

Frequent buffer replacement and proper capillary conditioning maintain stable pH, EOF, and reproducible migration times, essential for robust CE methods.

CE is coupled to mass spectrometry using sheath flow or sheathless interfaces, balancing electrical contact and flow requirements for efficient ionization and detection.

Method development involves selecting CE mode based on analyte properties, optimizing buffer pH, ionic strength, additives, injection, voltage, and temperature for resolution and reproducibility.

PeakMaster software predicts CE separation behavior, including buffer properties, analyte mobility, peak shapes, and system eigenpeaks, aiding method optimization.

Chiral selectors like cyclodextrins are added to the buffer to separate enantiomers by forming diastereomeric complexes with different mobilities, optimized by selector type and concentration.

CGE is widely used for size-based separation of nucleic acids and proteins, with commercial kits and replaceable polymer matrices enabling high resolution and automation.

CIEF is used for high-resolution separation of protein isoforms based on pI, important in biopharmaceutical development and clinical diagnostics.