High Resolution Infrared Spectroscopy in the Analysis of Liquids and Solutions

Abstract

Careful examination of organic liquids and solutions at very narrow spectral slit widths indicates that many compounds exhibit several absorptions of half band width narrower than 5 cm−1, particularly at the medium and long wavelengths. This suggests that sodium chloride prism spectrophotometers are currently being operated under conditions which produce rather significant errors in intensity and band width on some narrow bands. In order that this “finite slit error ‘be 3% or less in measuring peak absorbance, a spectral slit width about 1/5 of the natural half band width must be employed, e.g., a resolution of 1 cm−1 for a band of 5 cm−1 half band width The high resolution afforded by the grating instrument allows accurate intensity measurements on the typically narrow bands of such materials as indene at 1018 cm−1, cyclohexane at 861 and 903 cm−1 and acetonitrile at 2255 cm−1. Other advantages of the high resolution grating spectrophotometer for solution spectroscopy are discussed