Capillary electrophoresis separates substances as they move through a narrow capillary under an electric field. In capillary zone electrophoresis, peptide separation depends on differences in effective electrophoretic mobility. This supplies a different analytical perspective from reversed-phase chromatography, but a migration time still needs context before it supports an identity claim.
Start with mobility instead of a mass ranking
Electrophoretic mobility describes movement under an electric field. Charge and resistance to movement through the medium both matter. Consequently, two peptides cannot reliably be ranked by mass alone, and the phrase charge-to-mass separation is an incomplete explanation of their behaviour.
Grossman and colleagues measured 40 peptides while developing a semiempirical mobility model. Their experiments varied peptide size and charge within a particular solvent system. The resulting model included system-dependent constants, illustrating why a useful empirical relationship is not a universal molecular identifier.Grossman, Colburn and Lauer — A semiempirical peptide mobility model (opens in a new tab)
Imagine two equal-mass peptides with different ionisable groups. Their effective charges can differ under the same separation conditions. Conversely, two differently sized peptides can have similar effective mobilities. A sharp peak does not identify which of those possibilities applies.
Account for the movement of the liquid itself
Agilent describes electroosmotic flow as bulk movement of solution associated with the capillary surface and applied field. Observed analyte migration combines this transport with the analyte's own electrophoretic motion. Surface treatment and electrolyte conditions can alter that combination.Agilent — High Performance Capillary Electrophoresis: A Primer (opens in a new tab)
For an original signed-velocity illustration, let bulk flow towards a detector be +2.0 mm/s and an analyte's electrophoretic contribution be −0.5 mm/s. Its net motion is +1.5 mm/s, so it still travels towards the detector even though its own electrophoretic contribution points the other way.
| Bulk contribution | Analyte contribution | Net velocity |
|---|---|---|
| +2.0 mm/s | −0.5 mm/s | +1.5 mm/s |
| +2.0 mm/s | +0.5 mm/s | +2.5 mm/s |
These invented velocities explain the addition of contributions. They do not prescribe an instrument setting. A change in bulk flow could move many peaks without changing the molecular identity of every component.
Treat pH and surface behaviour as analytical variables
Lee and Desiderio examined synthetic opioid and tachykinin peptides at three pH values. Their comparison of observed and predicted mobilities showed the importance of the charge model and separation conditions. It supports examining peptide behaviour under a defined method rather than assigning one fixed mobility to each sequence.Lee and Desiderio — Capillary zone electrophoresis of synthetic peptides (opens in a new tab)
A migration shift between two runs therefore needs a controlled comparison. If electrolyte composition or capillary condition changed, the shift cannot be attributed solely to a new peptide impurity. Run suitability and a relevant reference help separate these explanations.
Agilent's primer also discusses solute–wall interactions, which can affect peak shape and migration. A distorted or poorly recovered component should not simply be interpreted as a lower abundance without assessing the separation.Agilent — High Performance Capillary Electrophoresis: A Primer (opens in a new tab)
The method record should make these variables visible enough to explain why two electropherograms are comparable. Merely reporting that both used CE leaves important differences unresolved.
Connect the separation to the detector evidence
An electropherogram plots detector response over migration time. It reveals resolved components under that method. Whether a peak is assigned by comparison with a reference, optical information or coupled mass spectrometry determines the strength and type of its identification.
A single unresolved zone can contain more than one species. Agreement between a migration time and a reference is useful, but additional selective evidence may be needed to distinguish closely related candidates.
The resulting conclusion should describe the actual separation and identification evidence. CE can add an independent analytical dimension without being treated as a complete structure determination.
Sources and further detail
- Agilent — High Performance Capillary Electrophoresis: A Primer (opens in a new tab)
Official primer 5990-3777EN; sections on mobility, electroosmotic flow and solute–wall interactions.
- Grossman, Colburn and Lauer — A semiempirical peptide mobility model (opens in a new tab)
Anal Biochem 179, 28–33 (1989), DOI 10.1016/0003-2697(89)90195-4. Primary study of 40 peptides; model constants are not generalised.
- Lee and Desiderio — Capillary zone electrophoresis of synthetic peptides (opens in a new tab)
J Chromatogr A 667, 271–283 (1994), DOI 10.1016/0021-9673(94)89076-5. Primary pH-dependent mobility study.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.