Infrared analyzer

Principle

• Gas molecules with 2 or more dissimilar atoms absorb infrared radiation
  --> argon, helium, xenon, oxygen, nitrogen do NOT absorb IR radiation
• Because of Beer-Lambert's law,
  --> by measuring the fraction of radiation absorbed by a gas mixture, the partial pressure of a particular gas can be determined.

Collision broadening effect

Presence of one gas affects measurement (by IR) of another gas.
--> Due to interactions of the molecules, the wavelength at which they absorb IR is affected (different from when the gas was measured by itself).

• e.g. N2O and CO2
  --> 10% N2O causes 0.1% increase in CO2 measured

Setup

• Infrared radiation emitted by a hot wire, and filtered to obtain a particular frequency.
• Infrared detector and a reference point
  * reference gas should not contain any of the gas that is being measured.

NB:

• When setup as capnographs, can be side stream or mainstream

Disadvantage

• Affected by water vapour
• Inaccuracy in measuring gas mixture with N2O and CO2
• Cannot measure oxygen

Other notes

Spectrum

• CO2 absorbs best at 4200-4400nm
• N2O absorbs best at 4400-4600nm (?????? plus 3900nm)
  --> The closeness of the spectrums also affect measurement accuracy of N2O/CO2 gas mixture
• Anaesthetic agents at 3300 and 9000-12000nm

NB:

• IR analyzer cannot distinguish volatile anaesthetic agents because they all peak at 3300.
• Wavelengths in 9000-11000nm range used to distinguish them.