Infrared Intensities of Liquids III: The Photometric Accuracy of FT-IR Transmission Spectra of C5H10, CH3NO2, CH2Cl2, C6H6, C6H5·CH3, and C6H5Cl in the Liquid State at 25°C in 11- to 500-μm Cells

Abstract

This paper completes the report of our FT-IR measurements of transmission spectra of organic liquids. These measurements were undertaken in order to estimate the photometric accuracy of our FT-IR spectrometer for transmission spectra by comparing the results with those measured previously [Applied Spectroscopy 34, 657 (1980)] by a calibrated dispersive spectrometer. We have studied 97 bands of chlorobenzene, toluene, and dichloromethane, in 500-μm cells and 54 bands of benzene, chlorobenzene, toluene, cyclopentane, and dichloromethane, in cells 11 to 53 μm thick. The average value of the imaginary refractive index at the band peak, kmax, is reported for each band and compared with the calibrated value. When averaged over the 184 bands that are reported in this work and our earlier paper [Applied Spectroscopy 39, 401 (1985)], our measurements agree with the calibrated values to about 3% of kmax, and have a 90% confidence limit of about 0.7 to 1% of kmax, The calibrated values themselves have an estimated error of about 6%, so to obtain better knowledge of the accuracy of our measurements we need better standards and we need measurements to be made on other FT-IR instruments. We plan to publish simplified procedures that will encourage others to make such measurements. Our 90% confidence limit includes effects due to the use of different cells, daily realignment of the fixed mirror of the interferometer, realignment of the optics of the detector and sample compartments, change of beamsplitter, and a day-to-day variation of uncertain origin, in addition to the error sources that always contribute to typical analytical precision. Our data do not show these additional error sources to be systematic, so it is appropriate to include their effect in the evaluation of the confidence limit. We believe the main source of error to be the effect of the infrared cell on the path of the light beam through the instrument. The day-to-day variation of uncertain origin is thought to be due to very small changes in the beam path through the instrument, which in turn cause the effect of the infrared cell on the beam to vary.