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.