Measurement of the sensitivity and photometric accuracy of FT-IR spectrometers

A method has been developed to investigate the sensitivity and photometric accuracy of a Fourier transform infrared (FT-IR) spectrometer by statistica lly analyzing a set of spectra of a 75 mu m thick film of polyethylene tere phthalate (PET). These tests were applied to three commercial FT-IR spectro meters. Although the photometric accuracy of all three instruments was gene rally better than 0.1%, small artifacts that could not he removed by signal averaging were often observed where strong bands should have a transmittan ce of less than 0.01%. Binding of the moving mirror of one of the spectrome ters appeared to degrade its performance. When the standard deviation of th e noise on 100% lines was calculated in the conventional way, none of the i nstruments gave efficient signal averaging after 25 scans had been coadded, because of small slopes in the baseline. By calculation of the noise level by successive difference, signal averaging was improved, but even with thi s approach the noise level on one of the instruments did not decrease after 100 scans had been averaged.