Analytics & PAT
Capillary electrophoresis in biopharmaceuticals
Electrophoretic mobility, sieving matrices and isoelectric focusing — the physics behind CE-SDS and icIEF, and the artefacts they introduce into purity and charge-variant results.
An ion in an electric field accelerates until electrical force balances hydrodynamic drag, and then moves at a steady velocity set by the ratio of its charge to its friction coefficient. That ratio is the electrophoretic mobility, and on its own it separates less than intuition suggests. For a linear polyelectrolyte such as DNA, adding a monomer adds both charge and drag in nearly equal proportion, so mobility becomes independent of length. In free solution, a 100-base and a 1000-base fragment travel together. Almost everything in the field is a way around that.
Why a capillary
Resolution improves with applied voltage, but current through a conducting buffer dissipates as Joule heat, and a warm core with cool walls produces a radial viscosity gradient that broadens every band. A fused-silica capillary of 25–75 micrometres internal diameter has an extremely high surface-to-volume ratio, so heat generated in the lumen has almost no distance to travel before reaching the thermostatted wall. This is what permits field strengths of hundreds of volts per centimetre, and it is the single reason the geometry is a capillary rather than a slab.
The same silica wall introduces the method’s second characteristic behaviour. Above roughly pH 3 the surface silanols deprotonate, a cationic double layer forms, and the field drags it — and the bulk liquid with it — as electroosmotic flow. That flow is useful when it carries neutral and anionic species past the detector, but it drifts with buffer, temperature and wall history, so quantitative methods usually suppress it with a covalent or dynamic coating rather than try to control it.
The two workhorse modes
CE-SDS solves the charge-to-drag problem the way SDS-PAGE does: the detergent binds a protein at a roughly constant mass ratio, swamping its intrinsic charge, so mobility depends only on how the chain is retarded by a replaceable sieving polymer. It replaces the gel slab with an instrument that integrates peak areas directly, which is what makes it defensible for purity release testing under USP <1053>. Its weakness is optical: the detection path length is the capillary bore, so ultraviolet sensitivity is poor and low-level impurities often need laser-induced fluorescence after labelling. Sample preparation is also not neutral — heating an antibody in SDS can cleave susceptible hinge or asparagine sites, and a fragment peak can be created by the assay it is supposed to detect.
Imaged capillary isoelectric focusing exploits amphoteric carrier ampholytes, which distribute under the field into a stable pH gradient. A protein migrates until it reaches the pH where its net charge is zero and stops; diffusion out of that position restores a charge that pushes it back, so the band self-sharpens. Imaging the whole column with ultraviolet absorbance removes the mobilisation step that once dominated quantitative error.
Its limits are chemical. The measured value is an apparent isoelectric point, conditional on ampholyte composition, temperature and any urea added, and it is not transferable between methods as an absolute number. Worse, protein solubility is at its minimum at the isoelectric point, so the focusing step drives the analyte toward precipitation — for hydrophobic or aggregation-prone molecules, this, not resolution, is what determines whether the method is usable.