In all infrared spectrometers the output from the source (which varies greatly in intensity with wavelength) is focused through the entire optical system and eventually to the detector. Energy and hence detector output vs. wavelength is, as a consequence, a steep curve and shows evidence
of water vapor and carbon dioxide absorptions. In dispersion instruments it is normal to analyze the radiation in such a way that the transmission through the sample is compared with that which has avoided the sample completely, i.e., Itrans is compared with the background
source emission I0. In older instruments, this was achieved electromechanically with the use of the so-called "null balance" system; more recently, it has been achieved with the use of the digital electronic technique referred to normally as "ratio recording." With either
method the absorption spectrum of the sample appears as a series of bands on an otherwise flat background. If source emission attenuated by carbon dioxide or water absorption is high at specific wavelengths, and it frequently is, no useful energy reaches the detector, and hence the spectrum
displayed at these wavelengths is meaningless. Further, optical imbalance and/or instability in the level of water vapor and carbon dioxide can lead to uneven backgrounds.
Department of Chemistry, The University, Southampton, S09 5NH, U.K.
Publication date: November 1, 1985
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