Overview of capillary electrophoresis MCQs With Answer

Introduction: Capillary electrophoresis (CE) is a powerful, high-efficiency separation technique widely used in pharmaceutical analysis for small molecules, peptides, proteins, and enantiomeric separations. This blog provides an overview of core CE concepts relevant to M.Pharm students—covering separation mechanisms, major operational modes (CZE, MEKC, CIEF, ITP), instrumentation elements (capillaries, detectors, interfaces), sample injection and stacking methods, factors affecting resolution and sensitivity, and coupling with mass spectrometry. The multiple-choice questions that follow emphasize practical understanding, method development considerations, and troubleshooting strategies encountered in advanced instrumental analysis and pharmaceutical quality control. These MCQs are designed to deepen conceptual knowledge and prepare students for exams and laboratory applications.

Q1. What is the primary basis of separation in capillary zone electrophoresis (CZE)?

• Partitioning between stationary and mobile phases
• Difference in charge-to-size (charge/mass) ratio and electrophoretic mobility
• Affinity to micellar pseudo-stationary phase
• Hydrophobic interactions with capillary wall

Correct Answer: Difference in charge-to-size (charge/mass) ratio and electrophoretic mobility

Q2. Which factor primarily determines the direction and magnitude of electroosmotic flow (EOF) in an uncoated fused-silica capillary?

• Viscosity of the buffer only
• Surface charge (silanol groups) on the capillary wall and the zeta potential
• Capillary inner diameter exclusively
• Detector type at the capillary end

Correct Answer: Surface charge (silanol groups) on the capillary wall and the zeta potential

Q3. Why is UV absorbance detection in CE generally less sensitive compared with HPLC-UV?

• CE uses lower applied voltages
• In CE the optical path length through the capillary is very short
• CE detectors have inherently lower electronics quality
• CE mobile phases are not UV transparent

Correct Answer: In CE the optical path length through the capillary is very short

Q4. Which CE mode employs surfactant micelles as a pseudo-stationary phase to separate neutral compounds?

• Capillary zone electrophoresis (CZE)
• Capillary isoelectric focusing (CIEF)
• Micellar electrokinetic capillary chromatography (MEKC)
• Capillary gel electrophoresis (CGE)

Correct Answer: Micellar electrokinetic capillary chromatography (MEKC)

Q5. Field-amplified sample stacking (FASS) is used in CE for what purpose?

• To reverse EOF direction
• To increase sample solubility in the buffer
• To concentrate analytes on-line and improve detection sensitivity
• To separate enantiomers using chiral selectors

Correct Answer: To concentrate analytes on-line and improve detection sensitivity

Q6. Joule heating in CE increases with which of the following changes, and how is it commonly mitigated?

• Increases with lower current; mitigated by increasing voltage
• Decreases with higher ionic strength; mitigated by adding salts
• Increases with higher current (higher voltage and ionic strength); mitigated by using small ID capillaries and active temperature control
• Is independent of capillary dimensions; mitigated by changing detector type

Correct Answer: Increases with higher current (higher voltage and ionic strength); mitigated by using small ID capillaries and active temperature control

Q7. Which statement correctly distinguishes hydrodynamic injection from electrokinetic injection in CE?

• Hydrodynamic injection uses an electric field, electrokinetic uses pressure
• Hydrodynamic injection introduces sample by pressure or siphoning, electrokinetic injection depends on analyte electrophoretic mobility
• Both injection methods are identical in bias toward charged analytes
• Electrokinetic injection does not depend on sample conductivity

Correct Answer: Hydrodynamic injection introduces sample by pressure or siphoning, electrokinetic injection depends on analyte electrophoretic mobility

Q8. What is the main purpose of applying dynamic or permanent coatings to a CE capillary inner surface?

• To increase the detector path length
• To minimize analyte adsorption, reduce capillary wall interactions and control EOF
• To convert CE into HPLC
• To raise buffer ionic strength dramatically

Correct Answer: To minimize analyte adsorption, reduce capillary wall interactions and control EOF

Q9. Capillary isoelectric focusing (CIEF) separates amphoteric molecules based on which property?

• Molecular weight alone
• Hydrophobicity in the presence of micelles
• Isoelectric point (pI) using an immobilized pH gradient
• Charge-to-size ratio under high salt conditions

Correct Answer: Isoelectric point (pI) using an immobilized pH gradient

Q10. For successful coupling of CE to electrospray ionization mass spectrometry (ESI-MS), which background electrolyte property is most important?

• High non-volatile buffer concentration for conductivity
• Use of highly viscous, non-volatile organic solvents
• Volatile buffers (e.g., ammonium acetate) and low non-volatile salt content
• Buffers that produce strong coloration for UV detection

Correct Answer: Volatile buffers (e.g., ammonium acetate) and low non-volatile salt content

Q11. Which additive is commonly used in CE to enable chiral separations of drug enantiomers?

• Sodium dodecyl sulfate (SDS) only
• Cyclodextrins (e.g., beta-cyclodextrin) as chiral selectors
• High concentrations of phosphate for buffering
• Non-ionic polymers to increase viscosity

Correct Answer: Cyclodextrins (e.g., beta-cyclodextrin) as chiral selectors

Q12. Which detection technique in CE is most suitable for detecting inorganic ions without chromophores?

• UV-Vis absorbance detection
• Laser-induced fluorescence (LIF) without derivatization
• Conductivity detection (contactless or contact conductivity)
• Refractive index detection with large path length

Correct Answer: Conductivity detection (contactless or contact conductivity)

Q13. In CE terminology, what is the effective length of a capillary?

• The total capillary length from inlet to outlet reservoir
• The distance from injection point to detector window
• The length of capillary buried inside the instrument only
• Half of the total length regardless of detector position

Correct Answer: The distance from injection point to detector window

Q14. How does increasing buffer pH (within the silica capillary stability range) generally affect EOF and why?

• EOF decreases because silanol groups become protonated
• EOF increases because silanol groups are more deprotonated, increasing negative surface charge
• EOF becomes zero because buffer ions stop moving
• EOF reverses direction to move toward the anode

Correct Answer: EOF increases because silanol groups are more deprotonated, increasing negative surface charge

Q15. In MEKC, what is the role of sodium dodecyl sulfate (SDS) when used above its critical micelle concentration?

• To act as a volatile buffer for MS coupling
• To form micelles that act as a pseudo-stationary phase enabling separation of neutral and hydrophobic compounds
• To coat the capillary permanently and eliminate EOF
• To increase capillary inner diameter through swelling

Correct Answer: To form micelles that act as a pseudo-stationary phase enabling separation of neutral and hydrophobic compounds

Q16. Which of the following will most likely reduce CE separation efficiency (broaden peaks) if not controlled?

• Excessive Joule heating leading to thermal gradients and increased diffusion
• Using very low applied voltages with excellent temperature control
• Applying dynamic coatings to reduce adsorption
• Optimizing sample stacking to concentrate analytes

Correct Answer: Excessive Joule heating leading to thermal gradients and increased diffusion

Q17. Which inner diameter (ID) of fused-silica capillary is most commonly used in routine CE to balance sensitivity and heat dissipation?

• 5–10 µm
• 25–75 µm (commonly ~50 µm)
• 200–500 µm
• 1–2 mm

Correct Answer: 25–75 µm (commonly ~50 µm)

Q18. Laser-induced fluorescence (LIF) detection offers very high sensitivity in CE. What is a common requirement for many analytes to be detected by LIF?

• No sample preparation is ever required
• Analytes often need fluorescent derivatization if they are not inherently fluorescent
• Non-volatile salts must be added to the buffer
• Use of large-bore capillaries (>200 µm ID)

Correct Answer: Analytes often need fluorescent derivatization if they are not inherently fluorescent

Q19. In an uncoated fused-silica capillary at pH 8 with EOF directed toward the cathode (detector at cathode), what happens to strongly anionic analytes whose electrophoretic mobility is toward the anode?

• They cannot be detected because they move only toward the anode
• EOF can overwhelm their electrophoretic mobility and carry them toward the cathode for detection
• They precipitate at the injection end due to pH
• They convert to cations spontaneously and migrate to the cathode

Correct Answer: EOF can overwhelm their electrophoretic mobility and carry them toward the cathode for detection

Q20. When validating a CE method for limit of detection (LOD), which common criterion is used to define the LOD?

• Signal-to-noise ratio of 3:1 (or equivalent statistical approach)
• Signal-to-noise ratio of 100:1
• Only the slope of the calibration curve
• Peak width at 50% height regardless of signal intensity

Correct Answer: Signal-to-noise ratio of 3:1 (or equivalent statistical approach)

Download