Questions: Capillary Electrophoresis: Fundamentals and Applications

Question 1

In a CZE experiment at pH 7.5, a mixture of cationic, neutral, and anionic analytes is injected. In what order will the analytes reach the UV detector at the cathode end of the capillary?

• A. Anions first, then neutrals, then cations — because anions are attracted to the cathode
• B. Cations first, then neutrals, then anions — because EOF carries all analytes toward the cathode, but cation migration adds to EOF while anion migration opposes it
• C. Neutrals first, then cations, then anions — because neutrals are not retarded by the electric field
• D. All analytes arrive simultaneously because EOF sweeps them all at the same velocity

Answer: (B)

Electroosmotic flow moves the entire bulk solution toward the cathode (the detector end). Cations have electrophoretic mobility that is also directed toward the cathode, so their migration adds to EOF — they arrive first. Neutral molecules have no electrophoretic mobility and simply ride with the EOF bulk flow — they arrive as an unresolved band after the cations. Anions have electrophoretic mobility directed toward the anode (opposing EOF), but EOF is typically strong enough to overcome this opposition and still carry them to the detector — they arrive last. This is a fundamental principle of CZE: EOF enables the simultaneous detection of all three charge classes in a single run.

Question 2

Why does capillary electrophoresis typically achieve far higher theoretical plate counts (hundreds of thousands) compared to HPLC (typically tens of thousands)?

• A. CE uses much longer separation columns than HPLC, giving more time for separation
• B. CE uses higher pressures than HPLC, pushing analytes through the column faster and generating more theoretical plates
• C. The flat (plug) flow profile of EOF-driven flow in CE eliminates the zone broadening caused by the parabolic flow profile in pressure-driven HPLC
• D. CE uses smaller particles in the stationary phase, reducing resistance to mass transfer

Answer: (C)

In pressure-driven chromatographic flow (HPLC), the flow velocity is fastest at the center of the tube and zero at the wall — a parabolic (laminar) profile. This means analyte molecules at different radial positions travel at different velocities, which broadens analyte zones and reduces plate counts. In CE, the driving force is EOF: the bulk liquid is pulled uniformly by the layer of cations near the wall, producing a nearly flat (plug) flow profile where velocity is approximately equal across the entire capillary diameter. This eliminates the primary zone-broadening mechanism, allowing extraordinarily high efficiency. Note that CE has no stationary phase at all (option D is not applicable to CZE).

Question 3

Increasing the buffer pH in CZE generally increases the magnitude of electroosmotic flow.

• T. True
• F. False

Answer: True

EOF is generated by the negative charge on the capillary inner wall, which arises from deprotonated silanol groups (Si-OH → Si-O⁻). At low pH, most silanols are protonated and the wall is nearly neutral, producing little EOF. As pH increases, more silanols deprotonate, increasing the surface charge density, which increases the thickness and charge of the cation layer (electrical double layer) that drives EOF. Above pH ~4, EOF increases substantially with pH and is near maximum around pH 9. This is why buffer pH is the primary tool for controlling EOF magnitude in CE method development.

Question 4

Capillary electrophoresis can seldom detect anionic analytes because their electrophoretic migration is directed away from the cathode-end detector.

• T. True
• F. False

Answer: False

This is a common misconception. While anions do migrate electrophoretically toward the anode (opposite the detector direction), electroosmotic flow typically exceeds the electrophoretic mobility of most anions at the pH values used in CZE. The net velocity of an anion toward the detector equals the EOF velocity minus the anion's own electrophoretic migration velocity. As long as EOF > electrophoretic mobility of the anion, the anion still reaches the detector — just later than cations or neutrals. Only very highly mobile anions can outrun EOF; in practice, most analytes of interest (amino acids, small organic acids, peptides) are detected this way. EOF enabling detection of all charge classes in one run is one of CZE's practical advantages.

Question 5

What is electroosmotic flow, and why is it essential for making capillary zone electrophoresis practical for analyzing a wide range of analytes?

Model answer: Electroosmotic flow is the bulk movement of the liquid in the capillary toward the cathode, driven by the interaction between the electric field and the charged layer of counterions near the capillary wall. Fused-silica capillaries have negatively charged silanol groups at working pH; positive buffer cations accumulate near the wall, and when the electric field is applied, these cations drag the bulk solution toward the cathode. EOF is critical for three reasons: (1) it enables detection of all analyte types — cations, neutrals, and anions — in a single run, because EOF carries even anions (which would otherwise migrate away from the detector) toward the detector; (2) it produces a flat flow profile that eliminates zone-broadening, enabling very high separation efficiency; and (3) it provides a controllable internal force that can be tuned by adjusting buffer pH and ionic strength to optimize separation selectivity.

Without EOF, CZE would require separate runs for cations and anions, and neutral molecules could not be separated at all. EOF transforms what would be a limited technique for charged species into a practical tool for a diverse range of analytes. This is analogous to how the mobile phase in chromatography provides the driving force for all analytes regardless of their stationary phase interaction — but in CE, that driving force arises from electrostatics rather than pressure.