DIFFERENTIAL SPECTROPHOTOMETRY IN THE INFRARED

Abstract

The double-beam principle, basic to most effective differential spectrophotometry, permits direct comparison of the transmittance of two beams. Infrared double-beam instrumentation converts transmittance information directly to the ratio of the transmittance of the beams $(I_{sample}/I_{reference}$). This ratio (double-beam transmittance) is applied to quantitative analysis through the logarithm of its reciprocal [double-beam absorbance, log $I_{s}/I_{r} = A = (a_{r}-a_{s}$)bc]. The absorabance effects of all components in either beam are additive, consequently it is logical to handle interference relationships in terms of characteristic absorptivity differences $(a_{r}-a_{s})$ at each analytical frequency. The absorptivity-difference ratio treatment of multicomponent absorbance data is applied to practical systems. Calibration graphs of ratios are calculated from measured absorptivity differences of pure compounds. They indicate key concentrations directly from sample absorbance measurements. This data is provided directly by use of the major component as the reference. The inter-relationships of the most important operating variables in multi-component infrared differential spectrophotometry are examined.