Oxygen Analysis

Describe the principles of measuring oxygen concentration

As oxygen is a molecule containing two similar atoms, its partial pressure cannot be determined using infrared techniques (unlike CO₂). Oxygen content of a gas is instead determined using:

• Paramagnetic analyses
• Fuel Cells

Paramagnetic Analysis

Principles of paramagnetic analysis:

• Oxygen is paramagnetic
  This means it is attracted by magnetic fields, but does not propagate the field.
  • This is because its two unpaired valent electrons have the same spin.

• Many other gases weakly repelled by magnetic fields (diamagnetic)

• The attraction of a gas mixture to a magnetic field is therefore proportional to its oxygen content

• Many different methods exist which use this property to determine oxygen content

Pressure Method

• Gas tested flows into a tube
• A reference gas flows into a parallel tube
• Both gases then pass through:
  • Flow restrictors
  • Magnetic field
    This is being turned on and off at ~100Hz.
• The gases combine in the magnetic field
• The greater the oxygen content of the gas, the more it will move into the magnetic field
  This movement creates a negative pressure behind the gas.
• The pressure difference between the tested gas and the reference gas is proportional to the oxygen content of the test gas.

Temperature Method

Used in many modern devices.

• Gas flows through a magnetic field, causing the particles to align
  This changes the thermal conductivity of the oxygen molecules.
• The change in thermal conductivity of the gas mixture is proportional to the oxygen content
• This is detected by measuring current passing through a heated wire

Pros

• Accurate
• Rapid response time
  Modern analysers can identify breath-to-breath variation in FiO₂.
• Don't require regular calibration

Cons

• Water vapour reduces accuracy
• Interference from other paramagnetic gases
  • Nitric oxide
    Effect is minimal as nitric is delivered in far smaller volumes than oxygen, and is only weakly paramagnetic.

Fuel Cells

Fuel cells rely on reduction of oxygen to measure oxygen partial pressure. They consist of:

• Oxygen permeable membrane
• KOH solution
  This contains:
  • Lead anode
    Lead is consumed as the fuel cell operates.
  • Gold cathode

Method

• Oxygen diffuses across the membrane into the potassium hydroxide solution
• At the cathode:
  $O_2 + 4e^- + 2H_2O \rightarrow 4OH^-$
• At the anode:
  $Pb + 2OH^- \rightarrow PbO + H_2O + 2e^-$
• The oxygen consumption is proportional to the current generated, which is measured with an ammeter

Pros

• No power required
• Small
• Accurate

Cons

• Will accumulate nitrogen in the presence of N₂O
  Results in an under-reading of PO₂.
• Must be replaced after 6-12 months
• Requires regular two-point calibration
  21% and 100% oxygen are used.
• Relatively slow response time compared to paramagnetic analysers
  ~20s.

References

1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.