Membrane filters – principle, construction, working, uses, merits, demerits MCQs With Answer

Introduction
Membrane filters are essential in pharmaceutical filtration — covering principle, construction, working, uses, merits and demerits — for B.Pharm students preparing for lab work and exams. These thin, porous polymeric or inorganic membranes (PVDF, nylon, PTFE, cellulose acetate, nitrocellulose) operate mainly by size-exclusion and adsorptive mechanisms; common pore sizes are 0.22 µm and 0.45 µm for sterilizing and clarifying. Understanding filter construction, flow dynamics, compatibility, validation (bubble-point, integrity), and limitations such as protein binding, extractables and clogging is critical for sterile processing, parenteral manufacturing and analytical sample prep. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the principal mechanism by which most membrane filters remove bacteria from aqueous solutions?

• Adsorption of bacteria onto the membrane surface
• Size-exclusion or sieving by membrane pores
• Chemical inactivation by membrane additives
• Electrochemical neutralization within the pores

Correct Answer: Size-exclusion or sieving by membrane pores

Q2. Which pore size is commonly accepted as sterilizing-grade for retention of bacteria in pharmaceutical liquids?

• 5.0 µm
• 1.2 µm
• 0.45 µm
• 0.22 µm

Correct Answer: 0.22 µm

Q3. Which membrane material is known for low protein-binding and is often used for biological samples?

• Nylon
• Cellulose acetate
• PVDF (polyvinylidene fluoride)
• Glass fiber

Correct Answer: PVDF (polyvinylidene fluoride)

Q4. What is a major disadvantage of high protein-binding membranes when filtering proteinaceous solutions?

• Increased flow rate
• Loss of analyte due to adsorption
• Improved microbial retention
• Complete chemical compatibility

Correct Answer: Loss of analyte due to adsorption

Q5. Which test is commonly used to check integrity of sterilizing-grade membrane filters after filtration?

• pH measurement
• Bubble point test
• UV-Vis spectroscopy
• Gravimetric analysis

Correct Answer: Bubble point test

Q6. What is the primary difference between depth filters and membrane (surface) filters?

• Depth filters use electrostatic removal, membranes use size-exclusion only
• Depth filters capture particles within a matrix; membranes retain on the surface
• Depth filters are always reusable, membranes always disposable
• Depth filters have uniform pore size, membranes have graded porosity

Correct Answer: Depth filters capture particles within a matrix; membranes retain on the surface

Q7. Which membrane is recommended for filtration of organic solvents due to its chemical inertness?

• Cellulose acetate
• PTFE (polytetrafluoroethylene)
• Nitrocellulose
• Polyethersulfone (PES)

Correct Answer: PTFE (polytetrafluoroethylene)

Q8. Which parameter primarily controls flow rate through a membrane under constant pressure?

• Pore size and porosity of membrane
• Color of the membrane
• Ambient humidity only
• Manufacturer’s logo

Correct Answer: Pore size and porosity of membrane

Q9. Syringe filters are typically used for which purpose in pharmaceutical lab work?

• Sterilizing large-volume intravenous solutions routinely
• Clarifying small sample volumes before HPLC or assay
• Replacing autoclave sterilization
• Measuring membrane tensile strength

Correct Answer: Clarifying small sample volumes before HPLC or assay

Q10. Which membrane material is most commonly used for western blot protein transfer detection (not filtration) and is sometimes used for filtration of proteins?

• Nylon
• PVDF
• Cellulose nitrate
• PTFE

Correct Answer: PVDF

Q11. What does the term “sterilizing-grade filter” imply?

• Filter that removes viruses by chemical reaction
• Filter validated to remove microorganisms to achieve sterility retention (e.g., 0.22 µm)
• Filter that sterilizes solutions by heat
• Filter that dissolves in the product

Correct Answer: Filter validated to remove microorganisms to achieve sterility retention (e.g., 0.22 µm)

Q12. Which property of membrane filters is critical to assess for compatibility with a given solvent or formulation?

• Colorfastness
• Chemical compatibility / resistance
• Brand popularity
• Filter packaging aesthetics

Correct Answer: Chemical compatibility / resistance

Q13. What is an extractable in the context of membrane filters?

• Particulates that pass through the membrane
• Chemicals that leach out from membrane materials under specific conditions
• Air bubbles trapped in the filter housing
• Microbial colonies growing on filter surface

Correct Answer: Chemicals that leach out from membrane materials under specific conditions

Q14. Which membrane type is commonly used for microbiological membrane filtration for enumeration of bacteria on agar?

• Glass fiber depth filter
• Nitrocellulose or mixed cellulose ester (MCE) membrane
• PTFE hydrophobic membrane
• Metal mesh filter

Correct Answer: Nitrocellulose or mixed cellulose ester (MCE) membrane

Q15. Which filtration mode is preferred for sterile filtration of heat-sensitive parenteral solutions?

• Autoclave sterilization only
• Sterile membrane filtration using 0.22 µm filter under aseptic conditions
• Boiling for 5 minutes
• Filtration through paper towel

Correct Answer: Sterile membrane filtration using 0.22 µm filter under aseptic conditions

Q16. What is the main reason for pre-filtration (using a larger pore pre-filter) before final membrane filtration?

• To increase extractables
• To remove coarse particulates and reduce clogging of final filter
• To change the color of the solution
• To sterilize the solution completely

Correct Answer: To remove coarse particulates and reduce clogging of final filter

Q17. Which integrity test measures flow of gas through a wetted filter at a specified pressure differential?

• Visual inspection
• Dye-leak test
• Bubble point / diffusion test
• pH drift test

Correct Answer: Bubble point / diffusion test

Q18. For sterility assurance, what log reduction is typically expected from a validated sterilizing-grade membrane filter for bacteria?

• 0.1 log reduction
• 1 log reduction
• Sterile retention typically demonstrates high log reductions (e.g., >7 log for many challenge organisms) depending on validation
• No reduction

Correct Answer: Sterile retention typically demonstrates high log reductions (e.g., >7 log for many challenge organisms) depending on validation

Q19. Which membrane is hydrophilic and often chosen for aqueous biological samples but less compatible with aggressive organics?

• PTFE hydrophobic membrane
• Cellulose acetate or mixed cellulose esters (hydrophilic)
• Fluorinated ethylene propylene
• Stainless steel sintered membrane

Correct Answer: Cellulose acetate or mixed cellulose esters (hydrophilic)

Q20. What is a common consequence of filter clogging during sterile filtration?

• Increased flow rate
• Increased transmembrane pressure and reduced throughput
• Immediate sterilization of product
• Reduction of extractables

Correct Answer: Increased transmembrane pressure and reduced throughput

Q21. Which membrane is preferred when filtering solutions containing hydrophobic analytes to avoid adsorption?

• High protein-binding nylon
• Low-binding PVDF or PES membranes
• Nitrocellulose high-binding membrane
• Mixed cellulose ester with surfactants

Correct Answer: Low-binding PVDF or PES membranes

Q22. Which technique helps to minimize leachables and extractables from membrane filters before use?

• Rinsing or pre-wetting the filter with compatible solvent or buffer
• Storing filters in direct sunlight
• Heating filters to 200°C without guidance
• Using filters beyond expiry only

Correct Answer: Rinsing or pre-wetting the filter with compatible solvent or buffer

Q23. What is the role of a filter housing or holder in membrane filtration?

• To chemically modify the membrane
• To provide a secure, leak-free environment and support for the membrane during filtration
• To dissolve the membrane into the solution
• To sterilize the liquid passively

Correct Answer: To provide a secure, leak-free environment and support for the membrane during filtration

Q24. Which membrane is often recommended for low protein binding and good chemical compatibility with many aqueous buffers?

• Cellulose nitrate
• PES (polyethersulfone)
• High-binding nylon
• Paper

Correct Answer: PES (polyethersulfone)

Q25. What is meant by the term “retention rating” of a membrane filter?

• The tensile strength of the membrane
• Nominal definition of the pore size that determines particle retention efficiency
• The amount of extractables produced per hour
• The membrane color code

Correct Answer: Nominal definition of the pore size that determines particle retention efficiency

Q26. Why are 0.45 µm filters commonly used before 0.22 µm sterile filtration?

• To sterilize the solution completely
• To remove larger particulates and protect the 0.22 µm filter from rapid fouling
• To change the pH of the solution
• To increase bacterial load intentionally

Correct Answer: To remove larger particulates and protect the 0.22 µm filter from rapid fouling

Q27. Which analytical method commonly requires pre-filtration through a low-extractable membrane to prevent instrument clogging?

• HPLC (High-Performance Liquid Chromatography)
• Infrared spectroscopy without liquid samples
• Thermogravimetric analysis of solids
• Mass measurement on balances

Correct Answer: HPLC (High-Performance Liquid Chromatography)

Q28. Which membrane property is most important when filtering samples for endotoxin testing (LAL assay)?

• Non-interference with LAL reagents and low extractables
• Bright coloration
• High protein-binding
• Magnetic properties

Correct Answer: Non-interference with LAL reagents and low extractables

Q29. What is a common method to sterilize membrane filters supplied as non-sterile before use?

• Microwaving for 10 seconds
• Gamma irradiation or ethylene oxide sterilization (if validated for the membrane type)
• Exposure to household bleach only
• Rapid freezing in dry ice

Correct Answer: Gamma irradiation or ethylene oxide sterilization (if validated for the membrane type)

Q30. Which membrane is hydrophobic and commonly used for air or solvent filtration, not for direct aqueous biological filtration?

• Cellulose acetate hydrophilic membrane
• PTFE hydrophobic membrane
• PES hydrophilic membrane
• Mixed cellulose ester hydrophilic membrane

Correct Answer: PTFE hydrophobic membrane

Q31. In membrane filtration, what is “flux”?

• The color change of the membrane over time
• Volume flow rate per unit membrane area (e.g., mL/min/cm²)
• The weight of membrane per square centimeter
• The electric current through the membrane

Correct Answer: Volume flow rate per unit membrane area (e.g., mL/min/cm²)

Q32. Which factor does NOT significantly affect membrane filtration performance?

• Solution viscosity
• Particle load and size distribution
• Operator’s shoe size
• Transmembrane pressure

Correct Answer: Operator’s shoe size

Q33. What is the typical reason for choosing a nylon membrane over a cellulose-based membrane?

• Nylon has better compatibility with many organic solvents and robustness
• Nylon dissolves in water to improve filtration
• Nylon inherently sterilizes solutions
• Nylon increases extractables intentionally

Correct Answer: Nylon has better compatibility with many organic solvents and robustness

Q34. Which test organism is commonly used in bacterial retention validation studies for sterilizing-grade filters?

• Staphylococcus epidermidis
• Pseudomonas diminutiva (often Brevundimonas diminuta)
• Saccharomyces cerevisiae
• Escherichia coli K-12 only

Correct Answer: Pseudomonas diminutiva (often Brevundimonas diminuta)

Q35. Which is a common merit of membrane filtration over heat sterilization for certain pharmaceutical products?

• It denatures all active ingredients
• It allows sterile processing of heat-sensitive and thermolabile solutions
• It is always faster than any other method
• It increases evaporative losses intentionally

Correct Answer: It allows sterile processing of heat-sensitive and thermolabile solutions

Q36. What demerit is associated with membrane filtration when applied to highly viscous solutions?

• Enhanced microbial growth during filtration
• Severe reduction in flow rate and frequent clogging
• Conversion of solutes into gases
• Spontaneous sterilization

Correct Answer: Severe reduction in flow rate and frequent clogging

Q37. Which practice helps reduce protein adsorption on membranes during filtration of protein solutions?

• Using high-binding nitrocellulose membranes
• Choosing low-protein-binding membranes and pre-wetting with buffer containing low concentration of compatible surfactant if allowed
• Heating the protein solution to 80°C prior to filtration
• Filtering at extremely high pressure without validation

Correct Answer: Choosing low-protein-binding membranes and pre-wetting with buffer containing low concentration of compatible surfactant if allowed

Q38. What is the consequence of using a membrane incompatible with the formulation solvent?

• Unchanged filtration performance
• Membrane swelling, dissolution or breakthrough leading to contamination
• Instant sterilization of formulation
• Improved analytical sensitivity automatically

Correct Answer: Membrane swelling, dissolution or breakthrough leading to contamination

Q39. In bacteriological membrane filtration for enumeration, what is the usual next step after capturing microbes on the membrane?

• Direct injection into HPLC
• Placing the membrane on an appropriate growth medium and incubating
• Autoclaving the membrane instantly
• Discarding the membrane without analysis

Correct Answer: Placing the membrane on an appropriate growth medium and incubating

Q40. Which membrane characteristic is important to specify on the filter cartridge label for regulated sterile filtration?

• Pore size, membrane material, lot number and sterilization method
• Color of the package and barcode only
• Name of the lab technician who assembled it
• Ambient room scent

Correct Answer: Pore size, membrane material, lot number and sterilization method

Q41. Why is validation (e.g., filter integrity and microbial challenge) critical in pharmaceutical membrane filtration?

• To ensure filters are aesthetically pleasing
• To ensure consistent sterile retention, product safety and regulatory compliance
• To reduce the need for any documentation
• To increase the shelf-life of the membrane only

Correct Answer: To ensure consistent sterile retention, product safety and regulatory compliance

Q42. Which membrane is best suited for filtration of aggressive organic solvents used in sample preparation for HPLC?

• Cellulose acetate
• PTFE or solvent-compatible nylon
• Mixed cellulose ester
• Paper coffee filter

Correct Answer: PTFE or solvent-compatible nylon

Q43. What is a common method to determine whether a membrane filter has been compromised during filtration?

• Checking the color of the filtrate only
• Performing a post-filtration integrity test such as bubble point or diffusion test
• Smelling the filter for contamination
• Measuring the filter’s thickness by eye

Correct Answer: Performing a post-filtration integrity test such as bubble point or diffusion test

Q44. Which membrane type usually exhibits the lowest levels of extractables for aqueous pharmaceutical use?

• Unwashed mixed cellulose ester with adhesives
• Biocompatible, pre-washed PES or PVDF membranes supplied and validated for low extractables
• Untreated natural fiber membranes
• Colored plastic membranes without certification

Correct Answer: Biocompatible, pre-washed PES or PVDF membranes supplied and validated for low extractables

Q45. What is the effect of increasing temperature (within limits) on filtration viscosity and flux for aqueous solutions?

• Viscosity increases and flux decreases
• Viscosity decreases and flux generally increases
• No change occurs
• Solution becomes solid instantly

Correct Answer: Viscosity decreases and flux generally increases

Q46. Which membrane property is most critical for successful filtration of cell culture media without significant nutrient loss?

• High protein-binding capability
• Low adsorption of growth factors and nutrients (low binding)
• Hydrophobicity to bind media components
• Opaque color to block light

Correct Answer: Low adsorption of growth factors and nutrients (low binding)

Q47. What is “fouling” in membrane filtration?

• The intentional sterilization of the membrane
• Accumulation of particles, colloids or macromolecules on/in membrane reducing performance
• The membrane becoming more porous over time
• The membrane emitting foul odors

Correct Answer: Accumulation of particles, colloids or macromolecules on/in membrane reducing performance

Q48. When filtering a solution for HPLC analysis of a hydrophobic drug, which filter choice minimizes sample loss?

• High-binding nitrocellulose membrane
• Low-extractable, low-binding PVDF or PES membrane compatible with solvent
• Paper filter designed for coffee
• Hydrophobic PTFE with no solvent compatibility

Correct Answer: Low-extractable, low-binding PVDF or PES membrane compatible with solvent

Q49. In aseptic filling, what is a key benefit of using in-line membrane filters just prior to filling?

• They increase particulate load intentionally
• They provide a final microbial and particulate control step to protect fill operation
• They lengthen batch processing time without benefit
• They sterilize containers only

Correct Answer: They provide a final microbial and particulate control step to protect fill operation

Q50. Which demerit should be considered when selecting membrane filtration for large-scale production of parenterals?

• Membranes are always reusable without validation
• Potential for frequent replacement, scale-related pressure drops and validation burden for large volumes
• Filtration eliminates need for aseptic environment
• Membrane filtration guarantees zero extractables in all cases

Correct Answer: Potential for frequent replacement, scale-related pressure drops and validation burden for large volumes

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com