Absorption Anomalies in Ratio and Subtraction FT-IR Spectroscopy.

Subtraction and ratioing of strong absorption bands in Fourier transform infrared (FT-IR) spectroscopy produces anomalous absorption errors. One source of error is the instability in the wavenumber scale of the FT-IR spectra. The possible causes of this error are explored. The thermal expansion and contraction of the cavity of the HeNe reference laser from a typical commercial instrument was found to produce changes in the laser wavenumber of ±0.034 cm(-)(1). Changes of this size are shown to introduce errors into the wavenumber scales of FT-IR spectra which are sufficient to produce the observed anomalies. The dependence of the error on instrumental and spectroscopic parameters is explored. Solutions to the problem are proposed.