The figure to the right shows subtraction examples for the main CO2 band seen in the air with an uinpurged Digilab spectrometer. At the bottom are two spectra that were obtained at resolution 0.5 cm-1. These spectra were manipulated within GRAMS. Each was put into absorbance form using an artificial background spectrum created with ZAP. Subtraction of one spectrum from the other then gave the difference spectrum marked GOOD SUBTRACTION. Just above this is the difference spectrum obtained after the reference spectrum had its lines slightly broadened from 0.5 cm-1 to 0.6 cm-1. At the top of the figure is the difference spectrum obtained after the reference spectrum had been shifted by one data point (0.25 cm-1). The absorbance scales in the figure show the magnitude of the apparent “noise” resulting from the two types of spectral mis-match.
The figure to the left is a second illustration involving the water band of the two spectra. Again it is seen that slight broadening and frequency shift create highly damaged subtraction patterns.