Principle of operation of infrared gas analyser for end-expired CO₂

1. Background

a) Gases are capable of absorbing electromagnetic radiation and use has been made of this fact to produce infrared meters for carbon dioxide & nitrous oxide

b) Infrared radiation in the range 1—15mm is absorbed by all gas molecules with two or more atoms providing these atoms are dissimilar (eg CO but not O₂)

c) Measuring energy absorbed from a narrow band of wavelengths of infrared light passing through a gas sample is called infrared spectroscopy

2. Unique absorption bands

a) Respiratory & anaesthetic gases exhibit absorption of infrared radiation at unique bands in the spectrum

b) Carbon dioxide absorbs strongly between 4.2 and 4.4 mm

c) Because of the proximity of the carbon dioxide & nitrous oxide absorption bands, some CO₂ analysers are affected by high concentrations of N₂O

3. Gas analyser

a) The infrared radiation is emitted by a hot wire and the particular wavelegth required is obtained by passing the radiation through a interference filter (that only passes the one chosen wavelength). This infrared source emits a beam that passes through an optical path. As glass absorbed infrared radiation, the sample chamber have windows made out of a material that is transparent to infrared (rg sapphire). After passing through the sample chamber, the radiation is focused on a photodetector. The greater the absorption of infrared radiation by the gas the less radiation is monitored by the detector. Consequently it is possible to process the detector output electronically to indicate the concentration of CO₂ present.

b) The respiratory gas from the patient is sampled and passes through the optical path.