Commercial Fourier Transform Infrared Research Articles

Effects of Detector Nonlinearity on Spectra Measured on Three Commercial FT-IR Spectrometers

The effect of the nonlinear response of mercury-cadmium-telluride (MCT) detectors has been evaluated on three commercial Fourier transform infrared (FT-IR) spectrometers. The greater the photon flux and the photon flux density, and the smaller the area of the detector on which the source image is focused, the greater are the effects of the nonlinearity. The signal-to-noise ratio (SNR) of spectra measured with an MCT detector under conditions of high photon flux and, especially, high photon flux density is significantly less than the SNR calculated by using the manufacturer's D* value. Detector nonlinearity usually leads to negative deviations in Beer's law plots. An empirical correction algorithm has been applied to Beer's law spectra acquired with the use of photoconductive MCT detectors and has been found to work well.

The effect of apodisation and finite resolution on Fourier transform infrared and Raman spectra

The interferograms measured on Fourier transform infrared (FTIR) and Raman (FT-Raman) spectrometers are routinely apodised before the Fourier transform is carried out. The effects of finite resolution and different apodisation functions on synthetic bands of known Lorentz-Gauss character have been investigated. (The apodisation functions chosen were those commonly available on commercial FTIR and FT-Raman spectrometers). The effects are dramatic, the ratio of resolution/(true bandwidth) required to obtain less than 10% distortion (as judged by the peak height and width) in the band varied from 1 for boxcar to less than 0.2 for Norton-Beer (strong). Increasing Gaussian character of the band results in initially (i.e. at high resolution) the band being less altered by resolution and apodisation than a pure Lorentzian band. As the resolution is degraded, the band is more strongly affected. These results are explained in terms of the effect of truncation and distortion of the Fourier transform of the bandshape. A comparison of calculated and experimental results gave good agreement for the 1216 cm −1 band of chloroform in deuterochloroform. The experimental results also showed an increasing degree of Gaussian character as the resolution became comparable with the full width at half height of the band.