Turbidity analysis MCQs With Answer

Turbidity analysis MCQs With Answer

This collection of MCQs on turbidity analysis is tailored for M.Pharm students studying Bioprocess Engineering and Technology. It covers fundamental principles, instrumentation, standards, and practical considerations used when measuring turbidity in bioprocess streams, cell cultures, and pharmaceutical waters. Questions explore light scattering theories, nephelometry versus turbidimetry, calibration with formazin/NTU, wavelength and angle effects, interference from coloured or dissolved materials, and the relationship between turbidity and biomass concentration. These items are designed to deepen conceptual understanding and prepare students for lab practice, instrumentation troubleshooting, and quality control applications in pharmaceutical bioprocess settings.

Q1. What physical phenomenon primarily causes turbidity in aqueous bioprocess samples?

• Absorption of light by dissolved chromophores
• Scattering of light by suspended particles
• Fluorescence from cellular metabolites
• Refraction due to temperature gradients

Correct Answer: Scattering of light by suspended particles

Q2. Which instrument measures turbidity by detecting light scattered at 90 degrees from the incident beam?

• Turbidimeter (turbidimetry)
• Nephelometer
• Spectrophotometer measuring absorbance at 600 nm
• Total organic carbon analyzer

Correct Answer: Nephelometer

Q3. Which standard material is most commonly used to calibrate turbidity instruments and define NTU?

• Polystyrene latex beads
• Formazin suspension
• Kaolin clay dispersion
• Silver nitrate solution

Correct Answer: Formazin suspension

Q4. In turbidity measurement, NTU stands for which of the following?

• Neutral Turbidity Unit
• Net Transmittance Unit
• Nephelometric Turbidity Unit
• Normalized Transmission Unit

Correct Answer: Nephelometric Turbidity Unit

Q5. Which light scattering regime applies when particle diameters are similar to the wavelength of incident light, commonly encountered in bacterial suspensions?

• Rayleigh scattering
• Mie scattering
• Geometric optics scattering
• Thomson scattering

Correct Answer: Mie scattering

Q6. Why is Beer-Lambert law often not strictly applicable to turbidity measurements of dense suspensions?

• Because suspended particles fluoresce and re-emit light
• Because multiple scattering and non-uniform attenuation violate linearity
• Because dissolved salts change absorbance non-linearly
• Because path length in cuvettes is variable

Correct Answer: Because multiple scattering and non-uniform attenuation violate linearity

Q7. Which wavelength is most frequently used for estimating microbial biomass via optical density correlated with turbidity?

• 280 nm
• 405 nm
• 600 nm
• 750 nm

Correct Answer: 600 nm

Q8. How does particle size influence the angular distribution of scattered light in turbidity measurements?

• Larger particles concentrate scattering in forward angles; small particles scatter uniformly
• Smaller particles increase forward scattering; larger particles scatter backward
• Particle size has no effect on scattering angle distribution
• Larger particles shift scattering to ultraviolet wavelengths

Correct Answer: Larger particles concentrate scattering in forward angles; small particles scatter uniformly

Q9. When measuring turbidity in coloured culture media, which practice reduces bias from sample colour?

• Measure at shorter wavelengths to avoid colour
• Use nephelometric 90° scattering rather than direct transmittance
• Add chemical clarifiers to remove colour before measurement
• Rely solely on OD260 for better correlation

Correct Answer: Use nephelometric 90° scattering rather than direct transmittance

Q10. What is the primary limitation of using OD600 (spectrophotometric absorbance) to estimate cell concentration in high-density cultures?

• Absorbance detectors lack sensitivity at low densities
• Multiple scattering leads to non-linear relation between OD and cell concentration
• OD600 selectively measures only viable cells
• OD600 cannot detect particles smaller than 10 nm

Correct Answer: Multiple scattering leads to non-linear relation between OD and cell concentration

Q11. Which sample handling step is essential when turbidity readings exceed the instrument’s linear range?

• Filter the sample through a 0.22 µm membrane
• Dilute with instrument-appropriate blank and apply dilution factor
• Increase path length to reduce apparent turbidity
• Heat the sample to dissolve particulates

Correct Answer: Dilute with instrument-appropriate blank and apply dilution factor

Q12. In-line turbidity sensors used in bioreactors often measure at multiple angles. What is the main advantage of multi-angle detection?

• It eliminates the need for calibration standards
• It provides information on particle size distribution and reduces angular bias
• It increases sensitivity only for dissolved compounds
• It prevents fouling of optical surfaces permanently

Correct Answer: It provides information on particle size distribution and reduces angular bias

Q13. Which of the following interferents most directly causes overestimation of turbidity in protein-rich fermentation samples?

• Dissolved salts
• Colloidal protein aggregates and foam bubbles
• Gas composition changes
• Sterile filtration aids

Correct Answer: Colloidal protein aggregates and foam bubbles

Q14. Regulatory methods for turbidity in pharmaceutical water frequently reference which organization’s standard methods?

• World Health Organization (WHO)
• American Public Health Association (APHA) / Standard Methods
• Pharmacopeia of India only
• International Electrotechnical Commission (IEC)

Correct Answer: American Public Health Association (APHA) / Standard Methods

Q15. Which approach best distinguishes between high turbidity caused by live cells versus inert particles?

• Measure turbidity at multiple wavelengths only
• Combine turbidity with viability assays (e.g., plate counts or flow cytometry)
• Assume turbidity is proportional to viable cell count always
• Filter the sample and measure filtrate turbidity

Correct Answer: Combine turbidity with viability assays (e.g., plate counts or flow cytometry)

Q16. What effect does an increased refractive index contrast between particles and medium have on measured turbidity?

• No effect; turbidity depends only on particle size
• Increases scattering intensity and measured turbidity
• Decreases absorption but not scattering
• Transforms scattering into fluorescence

Correct Answer: Increases scattering intensity and measured turbidity

Q17. Which mathematical or empirical model is commonly used to relate turbidity measurements to cell biomass for calibration?

• Henderson-Hasselbalch equation
• Linear or polynomial regression between NTU/OD and dry cell weight
• Michaelis-Menten kinetics
• Van’t Hoff equation

Correct Answer: Linear or polynomial regression between NTU/OD and dry cell weight

Q18. What is the recommended action to minimize sensor fouling on in-line turbidity probes in proteinaceous fermentations?

• Operate probes at higher temperatures only
• Implement automated wipers or periodic chemical/steam cleaning
• Reduce aeration to minimal values
• Use only single-use disposable probes

Correct Answer: Implement automated wipers or periodic chemical/steam cleaning

Q19. Which statement best describes the difference between nephelometry and turbidimetry?

• Nephelometry measures transmitted light; turbidimetry measures scattered light
• Nephelometry measures scattered light at an angle; turbidimetry measures decrease in transmitted light
• They are identical techniques with different names
• Turbidimetry requires fluorescence detectors while nephelometry does not

Correct Answer: Nephelometry measures scattered light at an angle; turbidimetry measures decrease in transmitted light

Q20. When validating a turbidity method for QC of purified water, which performance characteristics are essential to document?

• Only linearity and brand of instrument
• Linearity, precision, accuracy, limit of detection, and robustness
• Only the calibration curve slope
• Only visual clarity and operator training records

Correct Answer: Linearity, precision, accuracy, limit of detection, and robustness

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