Crown ethers as buffer additives in CE MCQs With Answer

Crown ethers as buffer additives in CE MCQs With Answer

This short quiz collection is crafted for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). It focuses on the role of crown ethers when used as buffer additives in capillary electrophoresis (CE). The questions probe principles such as host–guest complexation, ring-size selectivity, effects on electrophoretic mobility and electroosmotic flow, chiral recognition, concentration and solvent effects, and practical considerations for detection and method development. Each multiple-choice item reinforces mechanistic understanding and analytical implications so you can apply crown-ether strategies effectively during CE method optimization and troubleshooting.

Q1. What is the primary mechanism by which crown ethers act as buffer additives in capillary electrophoresis?

• Formation of a stationary phase coating on the capillary wall
• Complexation with buffer cations and analytes, altering their effective electrophoretic mobility
• Acting as micelle-forming surfactants to solubilize hydrophobic analytes
• Direct covalent modification of analyte functional groups

Correct Answer: Complexation with buffer cations and analytes, altering their effective electrophoretic mobility

Q2. Which crown ether is most selective for potassium ions and therefore often used when K+ interactions are desired?

• 12‑crown‑4
• 15‑crown‑5
• 18‑crown‑6
• 21‑crown‑7

Correct Answer: 18‑crown‑6

Q3. How do chiral derivatized crown ethers enable enantioseparation of primary amines in CE?

• They serve as a chiral stationary phase by permanently coating the capillary wall
• They form transient diastereomeric host–guest complexes with enantiomers, giving different mobilities
• They racemize analytes so both enantiomers migrate together
• They selectively precipitate one enantiomer, leaving the other in solution

Correct Answer: They form transient diastereomeric host–guest complexes with enantiomers, giving different mobilities

Q4. What is the common effect of adding neutral crown ethers to a CE buffer on electroosmotic flow (EOF)?

• EOF typically increases due to enhanced double-layer charge
• EOF typically decreases because crown–cation complexation alters the counter‑ion distribution at the wall
• EOF becomes oscillatory and unpredictable
• EOF is completely abolished because crown ethers are surfactants

Correct Answer: EOF typically decreases because crown–cation complexation alters the counter‑ion distribution at the wall

Q5. Which concentration range of crown ether additives is typically explored first during CE method development to balance selectivity and current/joule heating?

• 0.001–0.01 mM
• 0.1–5 mM
• 10–50 mM
• 100–200 mM

Correct Answer: 0.1–5 mM

Q6. Regarding mass spectrometry (MS) compatibility, what is a major concern when using crown ethers as buffer additives?

• Crown ethers are highly fluorescent and interfere with MS detectors
• Crown ethers are volatile and increase baseline noise in MS
• Crown ethers are typically nonvolatile and can cause ion suppression or source contamination in MS
• Crown ethers catalyze analyte degradation in the ion source

Correct Answer: Crown ethers are typically nonvolatile and can cause ion suppression or source contamination in MS

Q7. What is the usual stoichiometry of complex formation between simple crown ethers (e.g., 18‑crown‑6) and monovalent metal cations or ammonium ions in solution?

• 1 crown : 2 cations
• 2 crowns : 1 cation
• 1 crown : 1 cation
• 3 crowns : 1 cation

Correct Answer: 1 crown : 1 cation

Q8. How does complexation of crown ethers with protonated amines typically affect the apparent pKa of those amines in the buffer?

• It increases the apparent pKa because the neutral form is stabilized
• It decreases the apparent pKa because the protonated form is stabilized by complexation
• It has no effect on pKa but changes UV absorbance
• It converts amines into permanently neutral species

Correct Answer: It decreases the apparent pKa because the protonated form is stabilized by complexation

Q9. In a buffer containing significant alkali metal contamination, what is the expected impact on crown‑ether‑mediated analyte separation?

• Alkali metals have no effect because crown ethers only bind organic bases
• Alkali metals can competitively bind crown ethers, reducing availability for analyte complexation and lowering selectivity
• Alkali metals enhance analyte binding by forming ternary complexes that improve resolution
• Alkali metals convert crown ethers into ionic surfactants, increasing EOF

Correct Answer: Alkali metals can competitively bind crown ethers, reducing availability for analyte complexation and lowering selectivity

Q10. Which of the following is a common practical modification of crown ethers used to improve chiral recognition in CE?

• Alkylation at the oxygen atoms to render them fully nonpolar
• Attachment of chiral side chains or aromatic appendages to the macrocycle
• Conversion to linear polyethers to increase flexibility
• Cross‑linking crown ethers into an insoluble polymer in the buffer

Correct Answer: Attachment of chiral side chains or aromatic appendages to the macrocycle

Q11. How can crown ethers increase the apparent mobility of a neutral analyte in CE?

• By forming charged complexes with the neutral analyte, imparting electrophoretic mobility
• By increasing solution viscosity so neutrals migrate faster
• By causing neutrals to precipitate and migrate with electroosmotic flow
• By converting neutral analytes into ampholytes that focus at pI

Correct Answer: By forming charged complexes with the neutral analyte, imparting electrophoretic mobility

Q12. Are crown ethers useful additives in micellar electrokinetic chromatography (MEKC), and if so, how?

• No, crown ethers destroy micelles
• Yes, they can selectively complex ionic analytes or counterions, modifying micelle–analyte interactions and selectivity
• No, crown ethers precipitate with surfactants and clog the capillary
• Yes, because they act as strong anionic surfactants themselves

Correct Answer: Yes, they can selectively complex ionic analytes or counterions, modifying micelle–analyte interactions and selectivity

Q13. What is the typical effect of increasing temperature on the binding constant between crown ethers and their guests?

• Binding constant usually increases with temperature due to stronger solvation
• Binding constant usually decreases with temperature because complexation is often exothermic
• Binding constant is independent of temperature
• Binding constant first increases then suddenly drops at boiling point

Correct Answer: Binding constant usually decreases with temperature because complexation is often exothermic

Q14. How does the presence of organic modifiers (e.g., methanol, acetonitrile) in the buffer generally affect crown‑ether complexation and enantioselectivity?

• Organic modifiers typically strengthen crown–guest binding and improve enantioselectivity
• Organic modifiers generally weaken crown–guest binding and can reduce enantioselectivity
• Organic modifiers convert crown ethers into ionic species that increase EOF
• Organic modifiers have no measurable effect on crown‑ether interactions

Correct Answer: Organic modifiers generally weaken crown–guest binding and can reduce enantioselectivity

Q15. Which class of analytes is most commonly targeted by crown ethers as buffer additives in CE?

• Neutral aromatic hydrocarbons
• Primary and secondary amines, ammonium ions, and alkylammoniums
• Large proteins above 100 kDa
• Highly hydrophobic steroids only

Correct Answer: Primary and secondary amines, ammonium ions, and alkylammoniums

Q16. What is the likely effect on electrical current and Joule heating when crown ether concentration in the buffer is increased substantially?

• Current and Joule heating decrease because crown ethers reduce conductivity
• Current and Joule heating increase if crown–cation complexes increase ionic mobility or ionic strength
• No change in current because crown ethers are neutral
• Current oscillates but average Joule heating remains the same

Correct Answer: Current and Joule heating increase if crown–cation complexes increase ionic mobility or ionic strength

Q17. How does crown‑ether addition typically influence the zeta potential at the silica capillary wall?

• It increases zeta potential by introducing positive charges at the wall
• It lowers zeta potential by altering the counter‑ion distribution and reducing effective surface charge
• It converts the wall into a permanent cationic polymer
• It has no effect on zeta potential because crown ethers do not adsorb

Correct Answer: It lowers zeta potential by altering the counter‑ion distribution and reducing effective surface charge

Q18. When selecting a crown ether for separating a mixture containing Na+ and K+ complexed analytes, which choice reflects typical size selectivity?

• Use 12‑crown‑4 for K+ and 18‑crown‑6 for Na+
• Use 15‑crown‑5 for Na+ and 18‑crown‑6 for K+
• Use 21‑crown‑7 for Na+ and 12‑crown‑4 for K+
• Ring size has no bearing on Na+/K+ selectivity

Correct Answer: Use 15‑crown‑5 for Na+ and 18‑crown‑6 for K+

Q19. Which detector modality is generally least affected by nonvolatile crown ether additives in the buffer?

• Electrospray ionization mass spectrometry (ESI‑MS)
• Inductively coupled plasma mass spectrometry (ICP‑MS)
• Ultraviolet (UV) absorbance detection
• Atmospheric pressure chemical ionization (APCI) mass spectrometry

Correct Answer: Ultraviolet (UV) absorbance detection

Q20. During method development for crown‑ether‑mediated separation in CE, which of the following is the most comprehensive approach to optimize resolution?

• Only vary the crown‑ether concentration while keeping pH and ionic strength constant
• Screen crown‑ether ring sizes/derivatives, buffer pH, crown concentration, ionic strength and organic modifiers while monitoring temperature and detection compatibility
• Use the largest crown ether available at very high concentration to maximize binding
• Avoid pH control because crown ethers negate pH effects

Correct Answer: Screen crown‑ether ring sizes/derivatives, buffer pH, crown concentration, ionic strength and organic modifiers while monitoring temperature and detection compatibility

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