Question

Chapter 6, FC 6: If the resolution of an FTIR depends on the distance the mirror travels, why are relatively long moving distances such as 2 cm or 4 cm not used? What is the trade off?

Answer 1

If the resolution is increased, a smaller aperture is selected , and the intensity of the light entering the detector is reduced, increasing the relative amount of noise in the spectra. Therefore, it is undesirable for the resolution to be increased more than necessary. The apperture is source of the radiation. Ideally a point source should be used. But the point point source does not exist in reality and ideally tha source of a certain size is used in order to ensure a sufficient level of light intensity at the surface of the infrared detector.

Also the high distances will change the  degree of fineness of the data obtained by measurement (i.e., the minimum peak interval that can be distinguished) and will vastly perturb the resolution of the peaks.For example , if 4 cm-1is selected, spectra will be obtained at intervals of approximately 2 cm-1. To obtain sharper spectra (i.e., spectra with a higher resolution), a value such as 2 cm-1or 1 cm-1is set.