HPLC solvents and selection criteria MCQs With Answer

Introduction:

This quiz set on HPLC solvents and selection criteria is designed for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). It covers solvent properties, eluotropic strength, UV cutoffs, viscosity and backpressure effects, buffer compatibility, and detector considerations with a focus on practical method development and troubleshooting. Questions emphasize reasoning about solvent choice for reversed‑phase and normal‑phase separations, effects on retention and selectivity, and compatibility with LC‑MS and other detectors. Use these MCQs to strengthen conceptual understanding and to practice decision‑making relevant to real laboratory method development and validation.

Q1. Which solvent property is most directly responsible for increased column backpressure when used at high percentage in a reversed‑phase HPLC mobile phase?

• UV cutoff wavelength
• Dielectric constant
• Viscosity
• Polarity index

Correct Answer: Viscosity

Q2. For LC‑MS applications where low background and volatility are critical, which mobile phase additive is most appropriate?

• Phosphate buffer (10 mM)
• Trifluoroacetic acid (0.1%)
• Formic acid (0.1%)
• Nonylphenol ethoxylate surfactant

Correct Answer: Formic acid (0.1%)

Q3. Which organic modifier generally gives the lowest UV cutoff and is favored for UV detection at low wavelengths?

• Methanol (MeOH)
• Acetonitrile (ACN)
• Tetrahydrofuran (THF)
• Isopropanol (IPA)

Correct Answer: Acetonitrile (ACN)

Q4. When selecting a buffer for reversed‑phase HPLC of an acidic analyte (pKa 4.0) to improve peak shape, which mobile phase pH is most appropriate?

• pH 1.5
• pH 3.0
• pH 5.5
• pH 8.0

Correct Answer: pH 3.0

Q5. Snyder’s solvent selectivity parameter “s” is used to predict changes in selectivity. Which description best captures what this parameter quantifies?

• Solvent viscosity relative to water
• Solvent elution strength for normal‑phase only
• Solvent contribution to solute retention via specific interactions
• UV absorbance baseline noise

Correct Answer: Solvent contribution to solute retention via specific interactions

Q6. Which statement about methanol versus acetonitrile in reversed‑phase HPLC is correct?

• Methanol produces lower backpressure than acetonitrile at the same % organic
• Acetonitrile generally gives shorter retention times than methanol for nonpolar analytes
• Methanol has a lower UV cutoff than acetonitrile
• Acetonitrile is incompatible with UV detection

Correct Answer: Acetonitrile generally gives shorter retention times than methanol for nonpolar analytes

Q7. Which solvent is least recommended as a primary organic modifier for LC‑MS because of strong ion suppression and non‑volatile residues?

• Acetonitrile
• Methanol
• Tetrahydrofuran (THF)
• Isopropanol

Correct Answer: Tetrahydrofuran (THF)

Q8. When converting a method from acetonitrile to methanol while keeping retention similar, which change is most often required?

• Decrease column temperature to compensate
• Increase percentage of organic solvent
• Reduce flow rate by half
• Switch to a smaller particle size stationary phase

Correct Answer: Increase percentage of organic solvent

Q9. Which mobile phase consideration is most important to reduce tailing for basic analytes on silica‑based C18 columns?

• Use a buffer pH well below the analyte pKa
• Use a buffer pH above the analyte pKa so analyte is neutral
• Add a low concentration of an amine modifier to block silanol interactions
• Use THF as the organic modifier to mask silanols

Correct Answer: Add a low concentration of an amine modifier to block silanol interactions

Q10. For refractive index (RI) detection which mobile phase property must be strictly controlled to obtain stable baselines?

• Buffer ionic strength
• Solvent viscosity
• Temperature and mobile phase composition stability
• UV transparency

Correct Answer: Temperature and mobile phase composition stability

Q11. Which eluotropic strength concept best explains why increasing % organic reduces retention in reversed‑phase HPLC?

• Higher organic reduces mobile phase viscosity so analytes diffuse faster
• Organic solvents increase the polarity of stationary phase
• Organic modifier weakens analyte–stationary phase hydrophobic interactions
• Organic solvents form micelles that carry analytes through the column

Correct Answer: Organic modifier weakens analyte–stationary phase hydrophobic interactions

Q12. Which solvent mixture consideration is most critical when preparing a gradient to avoid baseline disturbances at the detector?

• Using solvents with widely different UV cutoffs
• Matching degassing and eluent preparation procedures for all solvents
• Adding surfactants to minimize surface tension
• Adding salts to both solvents to equalize ionic strength

Correct Answer: Matching degassing and eluent preparation procedures for all solvents

Q13. For ionizable analytes, why is buffer concentration an important selection criterion?

• It controls mobile phase viscosity more than organic fraction
• It influences ionic strength, reducing secondary electrostatic interactions and improving peak shape
• Higher buffer concentration always increases retention
• Buffer concentration determines the column pore size

Correct Answer: It influences ionic strength, reducing secondary electrostatic interactions and improving peak shape

Q14. When using a volatile buffer such as ammonium acetate for LC‑MS, which parameter should be monitored to avoid salt buildup in the source?

• UV cutoff of the buffer
• Buffer concentration and total dissolved solids in waste
• Buffer pH above 9 only
• Buffer color and turbidity

Correct Answer: Buffer concentration and total dissolved solids in waste

Q15. Which statement correctly relates solvent polarity and retention in normal‑phase HPLC?

• More polar mobile phase increases retention of polar analytes on polar stationary phases
• Less polar mobile phase reduces retention of polar analytes on polar stationary phases
• Less polar mobile phase increases retention of polar analytes on polar stationary phases
• Solvent polarity has no effect in normal‑phase HPLC

Correct Answer: Less polar mobile phase increases retention of polar analytes on polar stationary phases

Q16. Which solvent property is most relevant when selecting an organic modifier for high‑temperature HPLC to control solvent compressibility and pump performance?

• Boiling point and vapor pressure
• UV cutoff wavelength
• Color and odor
• Solvent density at 0°C

Correct Answer: Boiling point and vapor pressure

Q17. Why is degassing of HPLC solvents recommended prior to use?

• To lower the solvent viscosity for better flow
• To remove dissolved gases that cause bubble formation, baseline noise, and pump irregularities
• To increase solvent polarity for improved retention
• To sterilize the solvent for biological assays

Correct Answer: To remove dissolved gases that cause bubble formation, baseline noise, and pump irregularities

Q18. When developing a reversed‑phase method for a mixture of neutral and ionic compounds, which approach to mobile phase selection often yields the best compromise in retention and peak shape?

• Use pure water as the mobile phase
• Use a high percentage of a single organic solvent without buffer
• Use a buffered aqueous phase at appropriate pH with a volatile organic modifier for LC‑MS if required
• Use THF only because it solvates all compounds equally

Correct Answer: Use a buffered aqueous phase at appropriate pH with a volatile organic modifier for LC‑MS if required

Q19. Which practical factor is often most limiting when choosing between acetonitrile and methanol during times of supply shortage?

• Difference in UV cutoff
• Solvent cost and availability, requiring method revalidation when switching
• Difference in chemical structure
• Methanol’s incompatibility with buffers

Correct Answer: Solvent cost and availability, requiring method revalidation when switching

Q20. Which criterion is essential when selecting solvents for gradient HPLC methods to ensure reproducible retention times and smooth baseline transitions?

• Using solvents with identical vapor pressures only
• Choosing solvents with similar miscibility, matched degassing, and compatible UV cutoffs to avoid abrupt detector artifacts
• Always using non‑volatile salts to stabilize pH
• Ensuring solvents have the same density at 25°C

Correct Answer: Choosing solvents with similar miscibility, matched degassing, and compatible UV cutoffs to avoid abrupt detector artifacts

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