Ion-pair chromatography MCQs With Answer

Ion-pair chromatography MCQs With Answer is a focused practice resource tailored for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). This blog presents twenty carefully crafted multiple-choice questions that probe theoretical concepts and practical aspects of ion-pair chromatography (IPC). Questions cover mechanisms of ion-pair formation, selection and concentration of ion-pair reagents, effects of pH, retention behaviour, method development, troubleshooting, detector compatibility (including LC–MS), and column chemistry. Each item is followed by four options and a clearly indicated correct answer to support self-assessment and exam preparation. Use this set to deepen understanding of IPC fundamentals and to refine analytical method design skills.

Q1. What is the primary mechanism by which ion-pair chromatography increases retention of ionic analytes on a reversed-phase column?

• Formation of neutral covalent adducts between analyte and stationary phase
• Ion-exchange between analyte and the silanol groups on silica
• Association of analyte with hydrophobic counter-ions to form ion pairs that partition into the stationary phase
• Formation of micelles in the mobile phase that solubilize analytes

Correct Answer: Association of analyte with hydrophobic counter-ions to form ion pairs that partition into the stationary phase

Q2. Which type of ion-pair reagent (IPR) is most commonly used to retain anionic drug molecules on a C18 reversed-phase column?

• Long-chain alkylammonium salts (e.g., tetrabutylammonium)
• Short-chain carboxylic acids (e.g., acetic acid)
• Alkyl sulfonates (e.g., sodium dodecyl sulfate)
• Neutral polymers (e.g., polyethylene glycol)

Correct Answer: Alkyl sulfonates (e.g., sodium dodecyl sulfate)

Q3. How does increasing the concentration of an ion-pair reagent in the mobile phase typically affect retention of the countercharged analyte?

• Retention decreases because the mobile phase becomes more polar
• Retention increases because more ion pairs form and partition into the stationary phase
• Retention is independent of ion-pair reagent concentration
• Retention becomes unpredictable due to micelle formation only

Correct Answer: Retention increases because more ion pairs form and partition into the stationary phase

Q4. Which mobile-phase parameter most critically affects the degree of ionization of acidic or basic analytes and thereby influences ion-pair formation?

• Flow rate
• Temperature
• pH
• Column length

Correct Answer: pH

Q5. What is a major drawback of using conventional long-chain ion-pair reagents when coupling IPC to mass spectrometry?

• They enhance ionization efficiency and cause signal saturation
• They are volatile and increase baseline noise
• They cause ion suppression and contaminate MS interfaces due to nonvolatile residues
• They improve MS sensitivity selectively for all analytes

Correct Answer: They cause ion suppression and contaminate MS interfaces due to nonvolatile residues

Q6. Which of the following statements about the selectivity of ion-pair chromatography is true?

• Selectivity is unaffected by the chain length of the ion-pair reagent
• Selectivity can be tuned by changing the identity and concentration of the ion-pair reagent
• Selectivity can only be altered by changing the stationary phase chemistry
• Selectivity is solely determined by temperature

Correct Answer: Selectivity can be tuned by changing the identity and concentration of the ion-pair reagent

Q7. In ion-pair chromatography, what role does the hydrophobic alkyl chain of an ion-pair reagent play?

• It increases the aqueous solubility of the analyte
• It provides hydrophobic driving force for the ion pair to partition into the reversed-phase stationary phase
• It decreases the overall ionic strength of the mobile phase
• It serves as a buffer to control pH

Correct Answer: It provides hydrophobic driving force for the ion pair to partition into the reversed-phase stationary phase

Q8. Which parameter is most important to monitor when switching from isocratic to gradient elution in an IPC method?

• Change in column internal diameter
• Gradient profile impact on ion-pair reagent concentration and therefore on retention and baseline stability
• Detector lamp age
• Viscosity of the solvent only

Correct Answer: Gradient profile impact on ion-pair reagent concentration and therefore on retention and baseline stability

Q9. For a basic analyte, which ion-pair reagent type would typically increase retention on a reversed-phase column?

• Alkyl sulfonates (anionic IPRs)
• Tetrabutylammonium salts (cationic IPRs)
• Neutral organic modifiers like acetonitrile
• Strong acids like hydrochloric acid

Correct Answer: Alkyl sulfonates (anionic IPRs)

Q10. Which of the following is an advantage of using volatile ion-pair reagents (e.g., perfluorinated acids) for LC–MS compatibility?

• They produce higher viscosity mobile phases
• They avoid nonvolatile salt deposition and reduce ion suppression in the MS source
• They always give better chromatographic selectivity than nonvolatile reagents
• They eliminate the need to optimize pH

Correct Answer: They avoid nonvolatile salt deposition and reduce ion suppression in the MS source

Q11. Which experimental observation suggests the stationary phase has been modified by adsorption of ion-pair reagent?

• Sudden decrease in retention with increasing IPR concentration
• Stabilized retention after repeated injections as an equilibrated ion-pair-coated stationary phase forms
• Complete loss of retention for neutral compounds only
• Instantaneous equilibration within one injection regardless of column history

Correct Answer: Stabilized retention after repeated injections as an equilibrated ion-pair-coated stationary phase forms

Q12. Which mathematical parameter is commonly used to quantify retention in IPC and other LC modes?

• pKa
• Capacity factor (k’)
• Partition coefficient of water
• Log P of the ion-pair reagent

Correct Answer: Capacity factor (k’)

Q13. When developing an IPC method for a zwitterionic drug with pKa values near the mobile phase pH, what strategy is most appropriate?

• Ignore pH and focus only on organic modifier percentage
• Adjust pH to favor formation of a single charged form and choose an appropriate IPR to pair with that charge
• Use only neutral ion-pair reagents
• Increase flow rate to minimize ion-pair formation

Correct Answer: Adjust pH to favor formation of a single charged form and choose an appropriate IPR to pair with that charge

Q14. Which troubleshooting step is most appropriate if peak tailing for basic analytes is observed in IPC on silica-based columns?

• Decrease temperature only
• Use a higher concentration of acidic IPR or reduce free silanol interactions by using endcapped or polymeric stationary phases
• Reduce the concentration of ion-pair reagent to zero
• Replace mobile phase with pure water

Correct Answer: Use a higher concentration of acidic IPR or reduce free silanol interactions by using endcapped or polymeric stationary phases

Q15. In IPC, what is the likely effect of increasing the alkyl-chain length of an alkylammonium ion-pair reagent from C4 to C18?

• Decrease in retention of anionic analytes
• Increase in retention and greater hydrophobic interaction of ion pairs with the stationary phase
• No change in retention but improved mass spectrometric response
• Complete loss of ion-pairing ability due to steric hindrance

Correct Answer: Increase in retention and greater hydrophobic interaction of ion pairs with the stationary phase

Q16. What is the effect of high ionic strength (e.g., addition of salts) in the mobile phase on ion-pair chromatography?

• It always increases retention by forming stronger ion pairs
• It can compete with analytes for ion-pair reagent and reduce ion-pair formation, often decreasing retention
• It makes the column permanently charged
• It has no effect on ion-pair formation

Correct Answer: It can compete with analytes for ion-pair reagent and reduce ion-pair formation, often decreasing retention

Q17. Which statement best describes the stoichiometry of ion-pair formation relevant to chromatographic retention?

• Ion-pair formation is always 2:1 analyte:IPR
• Stoichiometry depends on charge and molecular structure; often 1:1 but can vary, influencing retention and selectivity
• Stoichiometry is irrelevant to retention
• It is always 1:2 IPR:analyte

Correct Answer: Stoichiometry depends on charge and molecular structure; often 1:1 but can vary, influencing retention and selectivity

Q18. When performing IPC for a mixture of ionizable compounds with widely differing pKa values, which approach provides the best overall separation?

• Set pH at the midpoint of pKa values to partially ionize all compounds
• Optimize pH to control charge states and select appropriate ion-pair reagents, possibly using gradient elution to resolve strongly retained species
• Use zero ion-pair reagent and rely only on organic modifier changes
• Run at the highest possible temperature to collapse selectivity differences

Correct Answer: Optimize pH to control charge states and select appropriate ion-pair reagents, possibly using gradient elution to resolve strongly retained species

Q19. Which of the following is a recommended practice to minimize column contamination and memory effects when using strong ion-pair reagents?

• Use no wash step and increase sample load
• Flush the column with high organic solvent and/or a mobile phase without IPR, and include periodic column rinses with aqueous-organic mixtures
• Store the column dry after each run without flushing
• Always use the maximum concentration of IPR to saturate the column

Correct Answer: Flush the column with high organic solvent and/or a mobile phase without IPR, and include periodic column rinses with aqueous-organic mixtures

Q20. In method validation for IPC, which factor is particularly important to assess due to the dynamic nature of ion-pair equilibria?

• Repeatability of retention times and peak areas after column aging and different equilibration histories
• Color of the mobile phase only
• Manufacturer of the column packing
• Time of day when analyses are performed

Correct Answer: Repeatability of retention times and peak areas after column aging and different equilibration histories

Download