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 attaction 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:
  • At the anode:
  • 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.
Last updated 2017-09-17

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