# 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 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.
Last updated 2017-09-17