Advanced extraction: microwave-assisted extraction, SCFE, CCC, preparative HPLC, flash chromatography MCQs With Answer

Introduction

This quiz collection focuses on advanced extraction and preparative separation techniques—microwave-assisted extraction (MAE), supercritical fluid extraction (SCFE), counter-current chromatography (CCC), preparative HPLC, and flash chromatography—tailored for M.Pharm students studying Phytochemistry (MPG 103T). The set combines conceptual, operational and troubleshooting MCQs that probe principles, instrument parameters, solvent selection, scale-up considerations, and method optimization widely used for isolating phytoconstituents. Each question targets practical understanding and decision-making in method development, including solvent effects, partition coefficients, pressure/temperature control, detector-directed collection, and stationary phase choices. Answers are provided for self-assessment, helping students prepare for examinations and lab applications by reinforcing both theory and practical strategy.

Q1. Which fundamental mechanism primarily enables microwave-assisted extraction (MAE) to release phytoconstituents from plant tissue?

• Mechanical agitation of cell walls by microwave radiation
• Dielectric heating of polar molecules and solvents causing rapid internal heating
• Generation of ultrasound cavitation by microwaves
• Photochemical cleavage of bonds by microwave photons

Correct Answer: Dielectric heating of polar molecules and solvents causing rapid internal heating

Q2. In MAE, which solvent property most directly affects microwave absorption and extraction efficiency?

• Boiling point only
• Dielectric constant (polarity) and loss tangent
• Viscosity exclusively
• Molar mass of solvent

Correct Answer: Dielectric constant (polarity) and loss tangent

Q3. Compared to conventional Soxhlet or maceration, a principal advantage of MAE is:

• Complete elimination of solvent use
• Markedly longer extraction times to increase yield
• Reduced extraction time and lower solvent consumption with selective heating
• Requirement for cryogenic conditions to preserve thermolabile compounds

Correct Answer: Reduced extraction time and lower solvent consumption with selective heating

Q4. What are the critical temperature and pressure of carbon dioxide used in SCFE?

• 15.6 °C and 1.01 bar
• 31.1 °C and 73.8 bar
• 100 °C and 1013 bar
• 0 °C and 0.5 bar

Correct Answer: 31.1 °C and 73.8 bar

Q5. Why is a polar co-solvent (modifier) like ethanol often added to supercritical CO2 during SCFE of polar phytochemicals?

• To decrease the density of CO2 and reduce solvating power
• To increase polarity of supercritical phase and improve solubility of polar analytes
• To react chemically with analytes forming volatile derivatives
• To solidify CO2 for easier handling

Correct Answer: To increase polarity of supercritical phase and improve solubility of polar analytes

Q6. In SCFE, increasing system pressure at constant temperature typically affects solvating power by:

• Decreasing CO2 density and reducing solvating power
• Increasing CO2 density and enhancing solvating power
• Changing CO2 polarity to ionic behavior
• Having no effect on solubility of solutes

Correct Answer: Increasing CO2 density and enhancing solvating power

Q7. The core operating principle of counter-current chromatography (CCC) is:

• Solid-phase adsorption of analytes on silica followed by elution
• Electrostatic attraction between charged solutes and stationary phase
• Continuous partitioning between two immiscible liquid phases without solid support
• Gas–liquid partitioning under vacuum conditions

Correct Answer: Continuous partitioning between two immiscible liquid phases without solid support

Q8. For efficient separation in CCC, the partition coefficient (K) of target compounds is ideally within which range?

• 0.01 to 0.1
• 0.5 to 2.0
• 10 to 100
• Exactly 0

Correct Answer: 0.5 to 2.0

Q9. One key advantage of CCC over conventional column chromatography is:

• Higher irreversible adsorption due to strong solid support interactions
• Absence of a solid stationary phase, minimizing sample adsorption and denaturation
• Requirement for very high flow rates to maintain partitioning
• Inability to separate hydrophobic compounds

Correct Answer: Absence of a solid stationary phase, minimizing sample adsorption and denaturation

Q10. When scaling up from analytical HPLC to preparative HPLC, which parameter must be maintained to preserve chromatographic selectivity?

• Column length only, regardless of diameter
• Linear velocity (or equivalent gradient slope scaled by column volume)
• Exact same injection volume in microliters
• Detector wavelength exclusively

Correct Answer: Linear velocity (or equivalent gradient slope scaled by column volume)

Q11. Which detector strategy is most useful for automated fraction collection in preparative HPLC for natural product isolation?

• Refractive index detection only
• Mass spectrometry-directed fraction collection (mass-directed purification)
• Turbidity detection without mass information
• Naked-eye collection based on color alone

Correct Answer: Mass spectrometry-directed fraction collection (mass-directed purification)

Q12. In preparative HPLC, reducing stationary phase particle size offers what trade-off?

• Lower resolution and lower backpressure
• Higher resolution but increased backpressure and pump power requirements
• No change in efficiency or pressure
• Complete elimination of solvent consumption

Correct Answer: Higher resolution but increased backpressure and pump power requirements

Q13. What distinguishes flash chromatography from traditional gravity column chromatography?

• Use of high vacuum to draw solvents through a stationary phase
• Application of positive pressure (or pumped flow) to speed solvent flow through sorbent
• Exclusive use of water as the mobile phase
• Operation only at cryogenic temperatures

Correct Answer: Application of positive pressure (or pumped flow) to speed solvent flow through sorbent

Q14. For a complex crude plant extract with multiple polarities, which flash chromatography strategy generally yields better separations?

• Strictly isocratic elution with a single solvent ratio
• Gradient elution from nonpolar to more polar solvent
• Using only supercritical CO2 without modifiers
• Avoiding silica and using pure water columns

Correct Answer: Gradient elution from nonpolar to more polar solvent

Q15. When packing a flash column for reproducible separations, which packing method is generally preferred?

• Dry packing by pouring dry powder without solvent
• Wet slurry packing to achieve homogeneous bed packing
• Packing with random large voids intentionally
• Freezing the sorbent and shattering it into the cartridge

Correct Answer: Wet slurry packing to achieve homogeneous bed packing

Q16. At constant pressure in SCFE, increasing the temperature typically causes what net effect on solvating power of supercritical CO2?

• Always increases solvating power due to higher analyte vapor pressure
• Generally decreases solvating power because CO2 density falls, though analyte vapor pressure may offset
• Has no effect because temperature is irrelevant above critical point
• Instantly converts CO2 into a solid matrix enhancing extraction

Correct Answer: Generally decreases solvating power because CO2 density falls, though analyte vapor pressure may offset

Q17. Which vessel material is suitable and safe for microwave-assisted extraction?

• Thin metal containers that reflect microwaves
• Glass or quartz vessels (microwave-transparent)
• Aluminum foil-wrapped tubes
• Pure copper-lined flasks

Correct Answer: Glass or quartz vessels (microwave-transparent)

Q18. In CCC, the maximum practical sample load is primarily determined by:

• Detector sensitivity only
• Stationary phase volume and the partition coefficient (K) of compounds
• Column color and labeling
• Injector needle gauge alone

Correct Answer: Stationary phase volume and the partition coefficient (K) of compounds

Q19. For preparative HPLC intended for MS-guided isolation and downstream evaporation/lyophilization, which buffer is most appropriate in the mobile phase?

• Non-volatile phosphate buffer at high concentration
• Volatile buffers like ammonium acetate or ammonium formate
• High-concentration sodium chloride solution
• Strong acids that are non-volatile (e.g., trifluoroacetic acid at non-volatile counterions)

Correct Answer: Volatile buffers like ammonium acetate or ammonium formate

Q20. How does decreasing silica particle size in a flash chromatography cartridge affect separation?

• Decreases separation efficiency and reduces resolution
• Increases separation efficiency and resolution but raises backpressure
• Makes separation independent of solvent polarity
• Eliminates need for gradient elution entirely

Correct Answer: Increases separation efficiency and resolution but raises backpressure

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