Capillary isoelectric focusing (cIEF) separates molecules by isoelectric point (pI) along a pH gradient inside a capillary. In research peptide analysis, cIEF is used in the laboratory to determine a peptide’s pI and to resolve charge variants that can arise from deamidation, terminal modifications, or other structural differences.

How the method works: The sample is mixed with carrier ampholytes and pI markers, focused under an electric field until each species reaches the pH where its net charge is zero, and then mobilized past a detector. A well-resolved cIEF profile gives researchers a fingerprint of charge heterogeneity to compare across lots, alongside identity and purity results from HPLC and mass spectrometry.

Applications and limits: cIEF is sensitive to small charge differences that reversed-phase methods may not separate, which makes it a useful orthogonal quality-control tool in research settings. Method performance depends on ampholyte range, focusing time, and sample salt content, so laboratories validate the method for each analyte and document lot numbers and conditions in their records.

Disclaimer: This article is for laboratory research purposes only. Not for human consumption. Not for medical, therapeutic, diagnostic, or veterinary use. No dosing, reconstitution-for-use, or efficacy information is provided or implied.

Citations: Standard cIEF method literature on isoelectric focusing and charge-variant analysis; HPLC/MS peptide characterization methods.