Counting method for particle size MCQs With Answer

Counting method for particle size MCQs With Answer

The counting method for particle size involves directly counting individual particles to determine size distribution, number-based metrics, and particle morphology using microscopy, hemocytometer grids, image analysis, or automated particle counters. This topic is essential for B. Pharm students studying pharmaceutical powders, suspensions, and aerosols where particle size affects dissolution, stability, bioavailability, and quality control. Key concepts include number, volume and surface distributions, D10/D50/D90, Coulter principle (electrical sensing zone), sampling, dispersion techniques, calibration, resolution limits, and sources of error. Practical understanding aids method selection and regulatory compliance. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which instrument is traditionally used for manual counting of particles and determining number-based particle size distribution?

• Laser diffraction analyzer
• Hemocytometer with optical microscope
• Sieving shaker
• Dynamic light scattering instrument

Correct Answer: Hemocytometer with optical microscope

Q2. The Coulter principle measures particle size by detecting changes in which parameter as particles pass through an orifice?

• Light scattering intensity
• Electrical resistance (impedance)
• Acoustic attenuation
• Optical transmission

Correct Answer: Electrical resistance (impedance)

Q3. Which size descriptor represents the median particle diameter where 50% of the sample by number is smaller?

• D10
• D50
• D90
• Mean diameter

Correct Answer: D50

Q4. For number-based counting methods, which distribution is most directly obtained?

• Volume distribution
• Surface area distribution
• Number distribution
• Mass distribution

Correct Answer: Number distribution

Q5. Which limitation is most associated with manual microscopy counting for particle sizing?

• Overestimation of submicron particles due to Brownian motion
• Operator bias and low throughput
• Inability to detect particles larger than 1 mm
• Automatic conversion to volume distribution

Correct Answer: Operator bias and low throughput

Q6. Image analysis for particle counting provides which additional information beyond particle size?

• Chemical composition
• Particle shape and morphology
• Zeta potential
• Bulk density

Correct Answer: Particle shape and morphology

Q7. Which factor is critical in sample preparation for accurate counting method results?

• Using excessively high particle concentration to speed counting
• Ensuring representative sampling and proper dispersion
• Drying samples at very high temperature
• Adding dyes that dissolve particles

Correct Answer: Ensuring representative sampling and proper dispersion

Q8. Which counting method is best suited for measuring particles in the submicron range (e.g., 10–1000 nm) in colloidal suspensions?

• Sieve analysis
• Optical microscopy with hemocytometer
• Electron microscopy or nanoparticle tracking analysis
• Standard Coulter counter with large aperture

Correct Answer: Electron microscopy or nanoparticle tracking analysis

Q9. When converting number distribution to volume distribution, which mathematical operation is commonly applied to particle diameters?

• Square (d^2)
• Cube (d^3)
• Square root (d^0.5)
• Logarithm (log d)

Correct Answer: Cube (d^3)

Q10. The D90 value indicates what in a particle size distribution?

• 90% of particles by number are larger than this diameter
• 90% of particles by volume are smaller than this diameter
• 90% of particles by number are smaller than this diameter
• Mean diameter plus 90% confidence interval

Correct Answer: 90% of particles by number are smaller than this diameter

Q11. In Coulter counting, what is a common cause of coincidence error?

• Particles passing singly through the aperture
• Multiple particles passing through the aperture simultaneously
• Excessive dilution leading to too few pulses
• Use of a conductive electrolyte

Correct Answer: Multiple particles passing through the aperture simultaneously

Q12. Which of the following is NOT typically a counting method for particle size?

• Manual microscopy counting
• Coulter counter
• Laser diffraction
• Image analysis with automated segmentation

Correct Answer: Laser diffraction

Q13. When using a hemocytometer, what is the main unit for reporting counts?

• Particles per milliliter or per unit area
• Mass fraction (%)
• Volume percentage
• Optical density units

Correct Answer: Particles per milliliter or per unit area

Q14. Which principle governs sedimentation methods used for particle sizing in counting contexts?

• Stokes’ law describing settling velocity
• Beer-Lambert law for light absorption
• Ohm’s law for electrical flow
• Planck’s radiation law

Correct Answer: Stokes’ law describing settling velocity

Q15. Polydispersity index (PDI) is most relevant to which counting or sizing technique?

• Sieve analysis
• Dynamic light scattering (DLS)
• Manual counting with hemocytometer
• Optical microscopy for coarse powders

Correct Answer: Dynamic light scattering (DLS)

Q16. In image analysis, thresholding is used primarily to:

• Measure refractive index
• Differentiate particles from background for counting
• Determine particle mass
• Stabilize suspension zeta potential

Correct Answer: Differentiate particles from background for counting

Q17. Which regulatory guideline is most relevant to particle size analysis in pharmaceuticals?

• ICH Q6 for impurities
• USP General Chapters on particle size and testing
• ICH Q1A for stability
• FDA color additive regulations

Correct Answer: USP General Chapters on particle size and testing

Q18. A number-based distribution is skewed toward smaller particles compared to a volume-based distribution because:

• Smaller particles occupy more volume
• Larger particles dominate number counts
• Each large particle contributes disproportionate volume relative to its count
• Number-based distributions weight particles by surface area

Correct Answer: Each large particle contributes disproportionate volume relative to its count

Q19. Which sample dispersion technique helps prevent particle agglomeration before counting?

• Using ionic salts to increase aggregation
• Ultrasonic sonication and appropriate dispersants
• Evaporation of suspension solvent
• Heating to induce sintering

Correct Answer: Ultrasonic sonication and appropriate dispersants

Q20. Which metric represents the width of a particle size distribution commonly used in counting analyses?

• Span = (D90 – D10)/D50
• Mean absolute deviation only
• Optical density at 600 nm
• Zeta potential in mV

Correct Answer: Span = (D90 – D10)/D50

Q21. In flow imaging particle analysis, what advantage is gained over simple Coulter counting?

• Direct chemical identification of particles
• Simultaneous imaging allows morphology and size assessment
• Lower cost and simpler maintenance
• Exclusive measurement of ionic strength

Correct Answer: Simultaneous imaging allows morphology and size assessment

Q22. Which error affects the accuracy of number-based counts when particles overlap in images?

• Edge effect increasing particle size
• Under-counting due to particle overlap
• Overestimation of zeta potential
• Improved resolution of small particles

Correct Answer: Under-counting due to particle overlap

Q23. For aerosol particle counting, which instrument provides number concentration and size classification in real time?

• Optical particle counter (OPC)
• Sieving tower
• Scanning electron microscope
• Hemocytometer

Correct Answer: Optical particle counter (OPC)

Q24. Which parameter must be known to convert electrical pulse amplitude in a Coulter counter to particle volume?

• Refractive index of particles
• Calibration constant or cell constant
• Particle zeta potential
• Laser wavelength

Correct Answer: Calibration constant or cell constant

Q25. When comparing counting methods, which is TRUE about number vs. volume weighting?

• Number weighting emphasizes large particles
• Volume weighting emphasizes large particles
• Both weightings give identical distributions
• Number weighting converts directly to surface area without calculation

Correct Answer: Volume weighting emphasizes large particles

Q26. Which practice improves accuracy when using automated image analysis for particle counting?

• Using poor contrast images to test robustness
• Optimizing threshold and validating segmentation against manual counts
• Counting only edge-touching particles
• Mixing multiple samples without cleaning between runs

Correct Answer: Optimizing threshold and validating segmentation against manual counts

Q27. In pharmaceutical suspensions, particle counting can help predict which property most directly?

• Color stability under light
• Suspension settling rate and homogeneity
• Chemical degradation pathways
• Tablet hardness

Correct Answer: Suspension settling rate and homogeneity

Q28. Which statement about detection limits for counting methods is correct?

• Optical microscopy cannot detect particles larger than 100 μm
• Coulter counters have a lower detection limit determined by orifice size and electronics
• All counting methods can detect down to single nanometers without modification
• Sieve analysis is best for nanometer particles

Correct Answer: Coulter counters have a lower detection limit determined by orifice size and electronics

Q29. Which conversion is necessary to compare a number-based histogram to a mass-based specification?

• Multiply each particle count by particle density only
• Convert counts to individual particle volumes (using d^3), then multiply by density
• Divide counts by particle surface area
• No conversion is possible between number and mass

Correct Answer: Convert counts to individual particle volumes (using d^3), then multiply by density

Q30. Which best practice ensures representativeness in particle counting for a powder blend?

• Sampling only from one corner of the container
• Using statistically valid multiple subsamples and proper mixing before counting
• Measuring just the first aliquot collected
• Avoid homogenization to preserve original structure

Correct Answer: Using statistically valid multiple subsamples and proper mixing before counting

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