Multiple path length mid-infrared spectra of liquids obtained by using a modified button sample holder

Abstract

A modified mesh button sample holder is characterized by examining the effects of multiple absorption path lengths on infrared spectra. Unlike transmission cells, which are designed to provide a single radiation penetrating distance, reflections by the button stainless-steel mesh result in a wide distribution of absorption path lengths. Consequently, detector signals represent radiation traversing different distances through the sample. Higher absorptivity bands are measured with shorter effective path lengths than lower absorptivity bands. Therefore, band intensity ratios in measured infrared spectra differ from their relative absorptivities. In addition, overlapping band shapes are retained in spectral regions that would be opaque in transmission cell measurements. Wavenumber dependent effective path lengths can be systematically and reproducibly varied by changing the volume of liquid added to the button reservoir. Although Beer's law is not applicable, absorbance is proportional to concentration when sample volumes are constant, facilitating quantitative analyses.