Factors affecting IR Spectrography

Effect of Nitrous Oxide

Nitrous oxide absorbs IR light.

Absorption is minimal at IR 4.3 µ m

Collision Broadening Phenomenon

Correction factors for the presence of nitrous oxide.

Because nitrous oxide absorbs IR (IR absorption spectra of N20 = 4.5 µm whereas C0₂ = 4.3 µm), the presence of N₂0 therefore can give falsely high C0₂ readings. This problem can be eliminated by using a narrow band IR filter that only transmits the  the wavelength most strongly absorbed by C0₂ (about 4.3 µm). Another problem relates to  N₂O concerns the  interaction between  N₂0 molecules and C0₂ molecules.  This  produces a "collision broadening effect" that affects the sensitivity of the IR analyzer and causes an apparent increase in C0₂ reading. "Collision broadening" is a phenomenon in which the spectral absorption peaks of a gas (C0₂) are broadened owing to the collision or proximity of molecules of another gas (N₂0).¹ The correction factors for the presence of various concentrations of N₂0 have been studied and range from 0.90 at 70% N₂O (corrected PC02 = observed PC02 x 0.90) to 0.94 at 50% N₂O.² Most monitors provide some system of electronic compensation to reduce this effect. Alternatively, the simplest method of eliminating this error is to calibrate the instrument with a gas mixture which contains the same background gas concentration as that to be analyzed.^3,4

References:

1.    Raemer DB, Calalang I. Accuracy of end-tidal carbon dioxide tension analyzer. J Clin Monit 1991;7:19-208.

2.    Kennell EM, Andrews RW, Wollman H.  Correction factors for nitrous oxide in the infrared analysis of carbon dioxide. Anesthesiology 1973;39:441-3.

3.    Carbon dioxide monitors.  Health Devices 1986;15:255-85.

4.    Paloheimo M, Valli M, Ahjopalo H. A guide to CO2 monitoring.  Finland:Datex Instrumentaitrium, 1988.