Introduction:
Measurement and control of carbon dioxide (CO2) is a critical aspect of bioprocess engineering, particularly for M.Pharm students focusing on cell culture, fermentation and bioreactor operation. Precise CO2 monitoring ensures optimal cellular metabolism, maintains pH stability through bicarbonate buffering, and prevents CO2 toxicity that can impair product yield and quality. This quiz covers sensor technologies (NDIR, Severinghaus, optical probes), sampling and calibration, gas–liquid mass transfer (kLa), off‑gas analysis, and control strategies such as PID, cascade and gas blending. The questions are designed to deepen understanding of practical measurement challenges, sensor limitations, and effective control actions used in pharmaceutical bioprocesses.
Q1. What is the primary principle by which non-dispersive infrared (NDIR) sensors detect CO2 in bioprocess off-gas?
- Measurement of electrical conductivity change due to dissolved CO2
- Absorption of infrared radiation at characteristic CO2 wavelengths
- Electrochemical oxidation of CO2 at an electrode
- Fluorescence quenching of an indicator dye by CO2
Correct Answer: Absorption of infrared radiation at characteristic CO2 wavelengths
Q2. Which sensor type is most commonly used for direct measurement of dissolved CO2 (pCO2) inside a cell culture medium?
- NDIR gas-phase analyzer placed in the headspace
- Severinghaus-style pCO2 electrode
- Electrochemical CO sensor for carbon monoxide
- Mass spectrometer sampling off-gas only
Correct Answer: Severinghaus-style pCO2 electrode
Q3. In mammalian cell culture, maintaining CO2 around 5% in the incubator atmosphere primarily controls which parameter of the medium?
- Osmolality through gas exchange
- Temperature via gas heat capacity
- pH via the bicarbonate/CO2 buffering system
- Dissolved oxygen concentration directly
Correct Answer: pH via the bicarbonate/CO2 buffering system
Q4. Which of the following is a common interference that can bias CO2 readings of NDIR sensors if not corrected?
- Low dissolved solids in the medium
- High humidity or water vapor in the sampled gas
- Magnetic fields from nearby motors
- Minor changes in liquid viscosity
Correct Answer: High humidity or water vapor in the sampled gas
Q5. The respiration quotient (RQ) is defined as which ratio relevant to CO2 monitoring in bioreactors?
- O2 produced / CO2 produced
- CO2 produced / O2 consumed
- CO2 consumed / biomass produced
- CO2 dissolved / CO2 in headspace
Correct Answer: CO2 produced / O2 consumed
Q6. Which control action is most directly used to reduce elevated dissolved CO2 levels in a stirred tank bioreactor?
- Decrease agitation rate to reduce shear
- Increase gas flow or sparging with air/N2 to enhance stripping
- Add a CO2-generating substrate
- Reduce temperature to increase gas solubility
Correct Answer: Increase gas flow or sparging with air/N2 to enhance stripping
Q7. For online off-gas CO2 measurement in fermentation, why is sample conditioning (drying and particulate removal) often required before the analyzer?
- To increase CO2 concentration by removing moisture
- To prevent condensation and contamination that damage sensors and bias readings
- To sterilize the gas for safety reasons
- To convert CO2 into a more easily measured species
Correct Answer: To prevent condensation and contamination that damage sensors and bias readings
Q8. Which statement best describes Henry’s law as applied to CO2 in a bioreactor?
- Dissolved CO2 concentration is independent of gas partial pressure
- Dissolved CO2 concentration is proportional to the partial pressure of CO2 in the gas phase
- Dissolved CO2 concentration equals the bicarbonate concentration at equilibrium
- Gas transfer coefficient kLa is identical to Henry’s constant
Correct Answer: Dissolved CO2 concentration is proportional to the partial pressure of CO2 in the gas phase
Q9. What is the effect of increasing kLa on CO2 accumulation in aerobic cultures, assuming metabolic CO2 production remains constant?
- Increased kLa will reduce CO2 accumulation by enhancing CO2 stripping
- Increased kLa will increase CO2 accumulation by producing CO2
- kLa has no effect on gas-phase CO2 concentrations
- kLa only affects oxygen and not CO2 transfer
Correct Answer: Increased kLa will reduce CO2 accumulation by enhancing CO2 stripping
Q10. Which calibration practice is recommended for CO2 gas analyzers used in pharmaceutical bioprocess monitoring?
- Never calibrate; factory calibration is sufficient for life
- Calibrate regularly using certified calibration gases spanning the expected concentration range
- Calibrate only with pure nitrogen regardless of expected CO2 levels
- Calibrate with distilled water to simulate humid conditions
Correct Answer: Calibrate regularly using certified calibration gases spanning the expected concentration range
Q11. Optical dissolved CO2 sensors often rely on which mechanism to quantify CO2 concentration?
- Colorimetric change in pH-sensitive dye enclosed by a gas-permeable membrane
- Direct measurement of ionic strength of the medium
- Electrochemical oxidation at a silver electrode
- Gravimetric detection of CO2 mass in the headspace
Correct Answer: Colorimetric change in pH-sensitive dye enclosed by a gas-permeable membrane
Q12. In a cascade control scheme for CO2, which variable is typically the primary (master) setpoint that indirectly controls CO2?
- Dissolved oxygen (DO) concentration
- Gas flow rate as the only measured variable
- pH, using base addition that affects bicarbonate equilibrium
- Cell density measured optically
Correct Answer: pH, using base addition that affects bicarbonate equilibrium
Q13. Which is a likely biological consequence of chronic elevated CO2 (~10% pCO2) in mammalian bioreactors?
- Enhanced recombinant protein folding due to alkalinization
- Growth inhibition, altered metabolism and decreased productivity
- Complete sterilization of culture due to CO2 toxicity to microbes
- Improved oxygen uptake by cells
Correct Answer: Growth inhibition, altered metabolism and decreased productivity
Q14. When using off-gas mass balance to estimate CO2 production rate, which measurements are essential?
- Inlet gas composition, outlet gas composition, and total gas flow rates
- Only dissolved CO2 in broth is sufficient
- Only cell wet weight is needed
- Sterility test results and pH
Correct Answer: Inlet gas composition, outlet gas composition, and total gas flow rates
Q15. Why might a CO2 electrochemical sensor be less suitable for long-term continuous bioprocess monitoring compared with NDIR or optical sensors?
- Electrochemical sensors are insensitive to humidity
- They typically have shorter lifetimes and suffer from drift and electrolyte depletion
- They cannot detect CO2 at any concentration below 50%
- They are unaffected by temperature changes
Correct Answer: They typically have shorter lifetimes and suffer from drift and electrolyte depletion
Q16. In the design of a CO2 control loop adding base to offset pH changes, what unintended effect must be considered?
- Base addition will always increase dissolved oxygen
- Base addition alters bicarbonate equilibrium and can mask true pCO2 changes
- Base prevents any gas transfer across the membrane
- Base addition directly removes CO2 by chemical reaction with oxygen
Correct Answer: Base addition alters bicarbonate equilibrium and can mask true pCO2 changes
Q17. Which off-gas analysis method provides the highest specificity and sensitivity for simultaneous measurement of CO2 and other gases but is more complex and costly?
- Non-dispersive infrared (NDIR) analyzer
- Gas chromatography (GC) or mass spectrometry (MS)
- pH electrode in the broth
- Simple chemical CO2 indicator strips
Correct Answer: Gas chromatography (GC) or mass spectrometry (MS)
Q18. Which maintenance practice best reduces long-term drift and ensures reliable CO2 sensor performance?
- Leaving the sensor powered continuously without calibration
- Periodic calibration, cleaning of humid condensate traps, and replacement per manufacturer schedule
- Submerging the sensor in culture media for sterilization
- Applying random voltage spikes to the sensor electronics
Correct Answer: Periodic calibration, cleaning of humid condensate traps, and replacement per manufacturer schedule
Q19. In a fed-batch fermentation producing CO2, which strategy helps lower CO2 accumulation without changing metabolic pathways?
- Reduce aeration and agitation to decrease kLa
- Increase headspace pressure to raise dissolved gas partial pressures
- Increase sparging with N2 or air and/or increase agitation to enhance stripping
- Stop feeding substrate entirely to eliminate CO2 production immediately
Correct Answer: Increase sparging with N2 or air and/or increase agitation to enhance stripping
Q20. Which parameter must be compensated for in many CO2 sensor readouts because it alters gas solubility and sensor response?
- Medium color
- Temperature of the sampled gas or liquid
- Bioreactor material composition
- Operator shift schedule
Correct Answer: Temperature of the sampled gas or liquid
