Determination and use of secondary infrared intensity standards

The presentation of absorption intensities in infrared spectra is usually limited to relative intensities instead of absolute intensities. The measurement of absolute intensities can be facilitated by the use of secondary intensity standards. Such standards have been accepted by the Commission on Molecular Structure and Spectroscopy and the Physical Chemistry Division of the International Union of Pure and Applied Chemistry and were published recently. The secondary standards are based on the complex refractive index and molar absorption coefficient spectra of benzene, chlorobenzene, toluene, and dichloromethane. They have been used in this laboratory to calibrate the effective pathlength of a transmission cell and the effective number of reflections in a Circle(R) multiple attenuated total reflection cell. A computer program, IRYTRUE, has been developed to standardize the routine use of these intensity standards to calibrate the effective pathlength of a transmission cell. The program has been used to calibrate three transmission cells. The agreement between the calibrated values of the effective pathlength obtained from the use of different standard band groups was determined. The calibrated cell pathlength agrees with that calculated from the interference fringe pattern of the empty cell within 3% for very thin cells and within 1% for cells thicker than 100 mu m. We propose that the effective pathlength evaluated in this manner be called the cell constant, and that this cell constant be used in place of the pathlength in quantitative infrared analysis. The calibration of multiple attenuated total reflection measurements in the Circle cell has been achieved in two ways: by the use of peak heights and by the use of areas. Programs PCCALC and CIRCLCAL and its associated program RSCALC are described for this purpose. The intensity standards allow one to measure absolute infrared absorption intensities of liquids with confidence to an estimated accuracy of 2-3% by either transmission or calibrated ATR methods.