Measuring oxygen at the bubble–liquid interface: how can the oxygen concentration in the bubble boundary layer be determined in an aerated bioreactor?

Introduction / Context:
Gas–liquid mass transfer into cell-containing broths occurs across thin boundary layers surrounding bubbles. If we want the oxygen concentration very near the gas–liquid interface, bulk measurements are insufficient; specialized micro-probes resolve steep gradients at sub-millimeter scales.

Given Data / Assumptions:

• Reactor is aerated and well-mixed at the bulk scale.
• Boundary layer around bubbles is much thinner than bulk length scales.
• Dissolved oxygen can vary strongly from the interface to the bulk.

Concept / Approach:
Mass transfer is driven by C* − C at the interface versus bulk. To characterize C near the interface, micro-electrodes positioned in the boundary layer measure local dissolved oxygen directly, revealing gradients that determine kLa effectiveness. Bulk steady-state DO cannot resolve interfacial concentrations.

Step-by-Step Solution:
1) Recognize that bulk DO reflects reactor-averaged conditions, not boundary layers.2) Use micro-probes with high spatial resolution to measure DO very close to bubble surfaces.3) Interpret profiles to estimate interfacial driving forces and validate mass-transfer models.

Verification / Alternative check:
Micro-probe studies show steep DO gradients in viscous or cell-laden systems; these are invisible to standard bulk sensors placed in the tank wall or headplate.

Why Other Options Are Wrong:
Options A and B measure bulk steady state only; D and E are indirect operational metrics that do not yield boundary-layer concentration.

Common Pitfalls:
Assuming uniform DO throughout; in reality, bubble boundary layers dominate transfer resistance in many broths.

Final Answer:
By directly probing the boundary layer with a micro-electrode (micro-probe)