On dielectric surfaces (e.g. glasses, semiconductors etc.) the ideal angle of incidence may be different and in some cases also transmission spectroscopy is a suitable method. For the absorbance of ultrathin layers the typical order of magnitude is 10-3-10-4: detection of such weak signals calls for an FT-IR providing high signal-to-noise ratios in a short acquisition time.
Since atmospheric water vapor and CO2 absorb in the mid and far infrared as well, these absorptions may interfere with weak sample signatures, situated in the same spectral range. To achieve best results, the FT-IR instrument must be capable of keeping the water vapor interference to a minimum. Other important requirements in FT-IR spectroscopy for surface science include high dynamic range of the ADC (Analog-to-Digital Converter, 24bit for all current Bruker spectrometers) and low baseline drift.