Several sources of errors leading to distortions in time-resolved step-scan Fourier transform infrared (FT-IR) measurements are described, and their effect on difference spectra and time traces is discussed. Fluctuations of the movable interferometer mirror as well as interference of the power line frequency are shown to be able to cause oscillations in the time traces of absorbance changes obtained by step-scan measurements. It is demonstrated that absorbance changes of the sample and thermal IR signals due to heating of the sample by laser excitation both contribute to the IR intensity changes at a fixed sampling position. Therefore, intensity variations of the laser pulses used to drive the sample reaction can cause considerable fluctuations of the time-dependent IR signals and, thereby, introduce multiplicative noise into the difference spectra. Means to correct these errors are proposed for some cases.

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