How is the wavelength controlled in a FTIR spectrometer?
Not answering the control part but the desirable wavelength range is dictated by the characteristics of the gas species you’re trying to measure. Typically, they are in the range of 675 wave numbers to about 4,500 wave numbers for all the gases of interest. 675 wave numbers gets you benzene and 4,500 gets you HF. Then everything else is in between so you want to get those two ends.