IR Instruments - The Spectrometer

Dispersive Spectrometers

Dispersive spectrometers expose a sample to several narrow bands of infrared radiation frequencies at a time. This is done using a diffraction grating to disperse the radiation from the energy source and by rotating the grating the particular frequencies reuired can be selected. Typically the instrument scans through a large range of frequencies by rotating the grating. Frequencies that have not been absorbed by the sample reach the detector at 100% transmittance, while the remainder of frequencies that have been absorbed reach the detector at much lower percent transmittance. This data is used to construct a spectrum. Dispersive IR instruments are normally double beam, which means that the source beam is split into two beams. One beam passes through the sample cell, the other passes through the reference cell (usually air). The reference beam subtracts out any variation in signal over time.

Fourier-Transform Spectrometers

Fourier-Transform spectrometers simultaneously expose a sample to the entire range of IR frequencies. Frequencies not absorbed by the sample constitute the "sample beam," which reaches the detector at 100% transmittance. As this takes place, another beam of infrared radiation reaches the detector. This "reference beam" has not passed through the sample. The sample beam and the reference beam are allowed to interfere with each other as they reach the detector and the resulting data is used to construct an interferogram. This interferogram is converted to a normal spectrum by performing a Fourier Transformation (FT).