Content area
Abstract
Diffuse reflection (DR) spectroscopy is a sampling technique for infrared spectrometry. In diffuse reflection, the incident radiation enters a powdered matrix and is transmitted through one or more particles before being remitted from the top surface of the powder. Even though DR spectrometry is now accepted as one of the standard sampling techniques for mid-infrared (MIR) and near-infrared (NIR) spectroscopy, its precise mechanism is still poorly understood. Previous studies to determine the path-length that a photon has traveled within the matrix have been reported. The goal of these studies was to increase the understanding of the mechanism of diffuse reflection. In all of these studies, the path-length was examined utilizing near-infrared radiation because of its potential for the analysis of tissue and agricultural products. Some of these studies involve time-of-flight measurements that require expensive, specialized equipment. The method that is described in this dissertation involves standard sampling techniques in the mid-infrared, such as transmission, attenuated total reflection (ATR) and diffuse reflection, in combination with the Beer-Lambert law and other well known equations to determine the average path-length that a photon travels within a matrix.
This method has revealed that as the amount of absorbing substance, whether analyte or matrix, increases, the average path-length decreases. In the case of strongly absorbing liquids such as poly-(phenyl methyl siloxane) or solids such as caffeine dispersed on powdered KCl, the average path-length is ∼1 mm. This is for spectral regions where the absorptivity of the analyte is low and contains moderately absorbing bands when the concentration of the analyte is ∼1%. This path-length decreases in value when the concentration is increased. When a strongly absorbing material is added to KCl, even at very low concentration (e.g., 0.01% carbon black), the average path-length decreases to below 0.1 mm. From the experimental evidence reported in this dissertation, the average path-length that a photon travels in a non-absorbing matrix is at least 50 mm.
