Liquid–liquid extraction (LLE) MCQs With Answer

Liquid–liquid extraction (LLE) MCQs With Answer

Introduction: Liquid–liquid extraction (LLE) is a foundational sample-preparation technique in bioanalytical workflows, essential for isolating drugs, metabolites, and biomolecules from biological matrices. This quiz collection reinforces core theoretical and practical concepts of LLE that M.Pharm students must master — partitioning behavior, distribution ratios, pH control, solvent selection, salting-out, kinetics, multi-stage extraction, and modern miniaturized variants like DLLME. Questions emphasize how chemical properties (pKa, logP), phase ratios, complexation, emulsion control, and stability influence recovery and assay accuracy. Use these MCQs to test comprehension, prepare for exams, and sharpen decision-making for method development in therapeutic drug monitoring and pharmacokinetic studies.

Q1. What is the partition coefficient (P) in the context of liquid–liquid extraction?

• Ratio of solute concentration in organic phase to that in aqueous phase at equilibrium
• Ratio of ionized to unionized form of solute in aqueous phase
• Fraction of solute removed after one extraction
• Volume ratio of organic to aqueous phase used in extraction

Correct Answer: Ratio of solute concentration in organic phase to that in aqueous phase at equilibrium

Q2. How does the distribution ratio (D) differ from the partition coefficient (P) for ionizable drugs?

• D accounts for all forms of the analyte (ionized + unionized) in both phases and is pH-dependent
• P includes ionized and unionized species while D includes only the neutral species
• D is a constant for a compound independent of pH and ionic strength
• P and D are identical terms and interchangeable for ionizable drugs

Correct Answer: D accounts for all forms of the analyte (ionized + unionized) in both phases and is pH-dependent

Q3. For efficient extraction of a weak acid from plasma into an organic solvent, which pH adjustment is appropriate?

• Adjust pH to a value at least two units below the pKa to keep the acid ionized
• Adjust pH to a value at least two units above the pKa to keep the acid ionized
• Adjust pH to a value at least two units below the pKa to keep the acid unionized
• Adjust pH to a value at least two units above the pKa to keep the acid unionized

Correct Answer: Adjust pH to a value at least two units below the pKa to keep the acid unionized

Q4. What is the primary effect of adding a high concentration of inorganic salt (salting-out) to an aqueous sample during LLE?

• Decrease analyte solubility in aqueous phase and enhance transfer to the organic phase
• Increase solubility of nonpolar analytes in the aqueous phase
• Prevent coalescence of organic droplets and promote emulsions
• Neutralize ionizable analytes by changing their pKa

Correct Answer: Decrease analyte solubility in aqueous phase and enhance transfer to the organic phase

Q5. Which set of solvent properties is most desirable when choosing an organic solvent for LLE of small-molecule drugs?

• Immiscible with water, high selectivity for analyte, chemically inert, and easy to evaporate
• Completely miscible with water, high dielectric constant, and non-volatile
• Highly polar protic solvent, miscible with biological matrix, and reactive toward analyte
• Strongly basic, miscible with water, and forms complexes with proteins

Correct Answer: Immiscible with water, high selectivity for analyte, chemically inert, and easy to evaporate

Q6. Which practical approach best reduces stable emulsions during LLE of plasma samples?

• Vigorous vortexing for extended time without centrifugation
• Addition of salt or a small amount of alcohol followed by gentle mixing and centrifugation
• Heating the mixture to boiling to promote phase separation
• Increasing the organic solvent volume drastically without mixing

Correct Answer: Addition of salt or a small amount of alcohol followed by gentle mixing and centrifugation

Q7. Why is the distribution ratio (D) pH-dependent for weak bases?

• Because the fraction of unionized base (which partitions into organic phase) varies with pH relative to the pKa
• Because D reflects the solvent’s dielectric constant which changes with pH
• Because pH directly changes the organic solvent composition
• Because the partition coefficient P changes with pH for neutral species

Correct Answer: Because the fraction of unionized base (which partitions into organic phase) varies with pH relative to the pKa

Q8. What is the typical purpose of back-extraction (re-extraction) in LLE workflows for bioanalysis?

• To transfer the analyte from organic phase back into an aqueous phase for cleaner analysis or increased polarity for LC/MS
• To permanently immobilize analyte in the organic phase for storage
• To convert analyte into a more lipophilic derivative within the organic phase
• To evaporate the organic solvent directly without further cleanup

Correct Answer: To transfer the analyte from organic phase back into an aqueous phase for cleaner analysis or increased polarity for LC/MS

Q9. Why are multiple small-volume extractions often preferred over a single large-volume extraction?

• Multiple extractions with smaller organic volumes typically give higher overall recovery than one extraction with the same total volume
• A single large-volume extraction always gives higher recovery than multiple small ones
• Multiple extractions reduce the partition coefficient of the analyte
• Single extraction eliminates need for solvent evaporation and concentration

Correct Answer: Multiple extractions with smaller organic volumes typically give higher overall recovery than one extraction with the same total volume

Q10. Which factors primarily control the equilibration time required in an LLE step?

• Interfacial area, diffusion rates, viscosity, and intensity of mixing/agitation
• Only the chemical reactivity of the solvent with the analyte
• The pH of organic solvent and its dielectric constant alone
• The molecular weight of the analyte exclusively

Correct Answer: Interfacial area, diffusion rates, viscosity, and intensity of mixing/agitation

Q11. What describes the working principle of dispersive liquid–liquid microextraction (DLLME)?

• A ternary solvent system where a disperser creates fine droplets of extractant, producing large interfacial area and high enrichment factors
• Use of a single large volume of organic solvent to slowly extract analyte over hours
• Solid-phase support is used to trap analyte which is eluted by organic solvent
• Analyte is extracted by evaporating aqueous phase and re-dissolving residue in organic phase

Correct Answer: A ternary solvent system where a disperser creates fine droplets of extractant, producing large interfacial area and high enrichment factors

Q12. Why is an internal standard used in LLE-based bioanalytical methods?

• To correct for variability in extraction recovery, matrix effects, and sample handling losses
• To act as a phase-transfer catalyst and accelerate equilibrium
• To increase the solubility of analyte in organic solvent
• To change the pKa of the analyte during extraction

Correct Answer: To correct for variability in extraction recovery, matrix effects, and sample handling losses

Q13. How does a high logP (octanol-water partition coefficient) value affect LLE of a drug from blood?

• Indicates lipophilicity and a greater propensity to partition into nonpolar organic solvents, facilitating extraction
• Indicates hydrophilicity and that drug will remain in aqueous phase
• Means the drug will be highly ionized at physiological pH
• Suggests that the drug forms stable emulsions and cannot be extracted

Correct Answer: Indicates lipophilicity and a greater propensity to partition into nonpolar organic solvents, facilitating extraction

Q14. What is the basis of ion-pair extraction for ionic analytes?

• Formation of a neutral ion pair between analyte and counter-ion to increase solubility in organic solvents
• Changing pH to permanently ionize analyte so it moves into organic layer
• Using chelating agents to increase analyte polarity for aqueous retention
• Increasing ionic strength to keep analyte in aqueous phase

Correct Answer: Formation of a neutral ion pair between analyte and counter-ion to increase solubility in organic solvents

Q15. For extraction of a weak base with pKa 8.5 from plasma, which pH of the aqueous phase favors maximal extraction into an organic solvent?

• pH 6.0 to keep the base predominantly ionized
• pH 8.5 to have equal ionized and unionized forms
• pH 10.5 to keep the base predominantly unionized
• pH 3.0 to form salts that partition better into organic phase

Correct Answer: pH 10.5 to keep the base predominantly unionized

Q16. How does the organic-to-aqueous phase ratio influence extraction efficiency in a single-stage LLE?

• Increasing the organic/aqueous ratio generally increases fraction of analyte extracted into the organic phase
• Increasing the organic/aqueous ratio always reduces extraction efficiency
• Phase ratio has no effect; only partition coefficient matters
• Higher organic volume prevents phase separation and lowers recovery

Correct Answer: Increasing the organic/aqueous ratio generally increases fraction of analyte extracted into the organic phase

Q17. After collecting the organic extract, which step is commonly used to remove residual water before evaporation or analysis?

• Add anhydrous drying agents such as sodium sulfate or magnesium sulfate and filter
• Boil the organic extract to drive off water without drying agents
• Mix extract with excess aqueous buffer to neutralize traces of water
• Freeze the extract to solidify water and decant solvent

Correct Answer: Add anhydrous drying agents such as sodium sulfate or magnesium sulfate and filter

Q18. Which stability consideration is most critical during LLE for labile drug analytes?

• Control pH, temperature, light exposure, and addition of stabilizers to prevent degradation or conversion during extraction
• Always perform extraction at elevated temperature to speed up kinetics
• Use strong oxidizing organic solvents to ensure complete extraction
• Avoid internal standards as they destabilize analytes

Correct Answer: Control pH, temperature, light exposure, and addition of stabilizers to prevent degradation or conversion during extraction

Q19. How does chelation-assisted liquid–liquid extraction improve recovery of metal-containing analytes?

• Chelating agents form neutral or less polar complexes with metals that partition into organic solvents
• Chelating agents increase ionic strength so metals remain in aqueous phase
• Chelation oxidizes metals and makes them non-extractable
• Chelating agents always precipitate metals out of solution preventing extraction

Correct Answer: Chelating agents form neutral or less polar complexes with metals that partition into organic solvents

Q20. What information does a liquid–liquid phase diagram with tie-lines provide for method development?

• Compositions of coexisting aqueous and organic phases at equilibrium and the direction of phase separation for given mixture ratios
• Exact extraction time required for equilibrium regardless of mixing
• The pKa of analytes in each solvent without experimental measurement
• Solvent boiling points and toxicity rankings only

Correct Answer: Compositions of coexisting aqueous and organic phases at equilibrium and the direction of phase separation for given mixture ratios

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