Marine natural products: isolation and purification methods MCQs With Answer

Introduction

Marine natural products are a rich source of structurally diverse and pharmacologically active compounds, but their isolation and purification pose unique challenges due to high salt content, complex matrices, and often low abundance. This quiz collection focuses on practical and theoretical aspects of isolation and purification methods applicable to marine organisms — including extraction techniques, desalting strategies, chromatographic separations (normal/reversed-phase, counter-current, preparative HPLC), modern sample cleanup (SPE, ultrafiltration, lyophilization), and guidance for bioassay-directed fractionation and dereplication. Designed for M.Pharm students, these MCQs test understanding of method selection, sample handling, and instrumental choices critical for successful marine natural products research.

Q1. Which solvent is commonly used as a first-choice polar organic solvent for extracting a wide range of marine secondary metabolites from wet biomass?

• n-Hexane
• Dichloromethane
• Methanol
• Petroleum ether

Correct Answer: Methanol

Q2. Which sample cleanup technique is most effective for rapid desalting of marine extracts prior to chromatographic analysis?

• Direct injection into HPLC
• Solid-phase extraction using reversed-phase cartridges
• Soxhlet extraction
• Simple air-drying of extract

Correct Answer: Solid-phase extraction using reversed-phase cartridges

Q3. What is the fundamental separation principle of high-speed countercurrent chromatography (HSCCC)?

• Adsorption on silica gel
• Liquid–liquid partitioning between two immiscible phases
• Size exclusion based on molecular weight
• Ion exchange between charged groups

Correct Answer: Liquid–liquid partitioning between two immiscible phases

Q4. For efficient separation in countercurrent chromatography, the partition coefficient (K) of a target compound is ideally within which range?

• 0.5–2.0
• 5–10
• 0.01–0.1
• >10

Correct Answer: 0.5–2.0

Q5. Which extraction approach is safest for recovering heat-sensitive marine peptides and small proteins?

• Soxhlet extraction with boiling solvent
• Supercritical CO2 extraction at high temperature
• Microwave-assisted extraction with high power
• Cold maceration at low temperature

Correct Answer: Cold maceration at low temperature

Q6. What is the main objective of bioassay-guided fractionation in marine natural product research?

• To maximize total yield regardless of activity
• To identify and isolate bioactive compounds through iterative fractionation
• To remove salts and inorganic matter only
• To determine melting points of compounds

Correct Answer: To identify and isolate bioactive compounds through iterative fractionation

Q7. Why are antioxidants such as ascorbic acid or BHT sometimes added during extraction of marine samples?

• To increase solvent polarity
• To prevent oxidation and degradation of labile natural products
• To speed up solvent evaporation
• To promote bacterial growth for biotransformation

Correct Answer: To prevent oxidation and degradation of labile natural products

Q8. Which stationary phase is typically used for reversed-phase HPLC separations of moderately polar marine natural products?

• C18 bonded silica (octadecylsilane)
• Activated alumina
• Strong cation exchange resin
• Sephadex G-25 gel

Correct Answer: C18 bonded silica (octadecylsilane)

Q9. What is the preferred method to concentrate large aqueous marine extracts without exposing thermolabile compounds to heat?

• Rotary evaporation at high bath temperature
• Lyophilization (freeze-drying)
• Direct boiling to dryness
• Soxhlet concentration

Correct Answer: Lyophilization (freeze-drying)

Q10. In MS-guided dereplication of marine extracts, which piece of information is most valuable for rapid identification of known compounds?

• Color of the crude extract
• Exact mass and fragmentation pattern (MS/MS)
• Boiling point
• UV fluorescence only

Correct Answer: Exact mass and fragmentation pattern (MS/MS)

Q11. Which is a primary advantage of HSCCC compared with conventional column chromatography for marine natural products?

• No solid support, minimizing irreversible adsorption and sample loss
• Requires smaller solvent volumes always
• Uses silica gel to improve resolution
• Always faster for every separation

Correct Answer: No solid support, minimizing irreversible adsorption and sample loss

Q12. Supercritical fluid extraction commonly uses which supercritical solvent for extracting lipophilic marine metabolites?

• Carbon dioxide (CO2)
• Methanol
• n-Hexane
• Water

Correct Answer: Carbon dioxide (CO2)

Q13. Which approach is most effective for removing salts from polar marine extracts prior to NMR analysis?

• Direct concentration by rotary evaporation
• Reversed-phase solid-phase extraction with aqueous washes, then elution with organic solvent
• Adding more salt to precipitate impurities
• Heating the sample to drive off salts

Correct Answer: Reversed-phase solid-phase extraction with aqueous washes, then elution with organic solvent

Q14. If a marine metabolite lacks a strong UV chromophore, which HPLC detector provides a broadly applicable detection option?

• UV detector at 254 nm
• Evaporative light scattering detector (ELSD)
• Diode array detector only
• Colorimetric reaction detector only

Correct Answer: Evaporative light scattering detector (ELSD)

Q15. Partitioning an aqueous extract with ethyl acetate primarily transfers which class of compounds into the ethyl acetate layer?

• Highly polar inorganic salts
• Moderately lipophilic (medium polarity) secondary metabolites
• High-molecular-weight polysaccharides
• Strongly basic proteins

Correct Answer: Moderately lipophilic (medium polarity) secondary metabolites

Q16. Which chromatographic parameter most directly improves resolution when changed from 5 μm to sub-2 μm particles in a reversed-phase column?

• Particle size of the stationary phase (smaller particles increase efficiency)
• Color of the column housing
• Ambient humidity
• Sample vial volume

Correct Answer: Particle size of the stationary phase (smaller particles increase efficiency)

Q17. Desalting by gel filtration on Sephadex relies primarily on which separation mechanism?

• Size exclusion (molecular size differences)
• Ion exchange
• Hydrophobic interaction
• Covalent affinity binding

Correct Answer: Size exclusion (molecular size differences)

Q18. Which pretreatment step is most appropriate to remove high-molecular-weight proteins from a marine broth before small-molecule isolation?

• Ultrafiltration using an appropriate molecular-weight cutoff membrane
• Adding silica gel directly to crude broth
• Heating to 120 °C for 2 hours
• Freezing and thawing once

Correct Answer: Ultrafiltration using an appropriate molecular-weight cutoff membrane

Q19. In preparative HPLC, when is gradient elution generally preferred over isocratic elution?

• When the mixture contains compounds spanning a wide polarity range
• When the sample is already a single pure compound
• When only volatile solvents are used
• When sample complexity is extremely low and retention times are identical

Correct Answer: When the mixture contains compounds spanning a wide polarity range

Q20. In countercurrent chromatography, the partition coefficient (K) is defined as which ratio?

• Concentration in the mobile phase divided by concentration in the stationary phase
• Concentration in the stationary phase divided by concentration in the mobile phase
• Molecular weight divided by polarity index
• Retention time divided by flow rate

Correct Answer: Concentration in the stationary phase divided by concentration in the mobile phase

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