KBr disks of caffeine were prepared quantitatively so that the absorptivity of caffeine in this environment could be measured. The disks were then ground up finely and their diffuse reflection spectrum was measured. The average path length traveled by remitted photons was then calculated by the simple application of the Beer–Lambert law. The path length that an average photon travels within a finely powdered matrix of KBr is approximately 4 mm when the concentration of a uniformly dispersed, strongly absorbing analyte such as caffeine is 0.01% or less. This path length then decreases as the concentration of the analyte increases as more photons are absorbed by the analyte. When carbon black is added to the mixture of caffeine and KBr and the measurements are repeated, the effective path length drops even further. The average photon that is remitted from a fine infrared-transparent powder containing a very low concentration of an analyte has been shown to encounter at least 400 particles, indicating a highly random path. The more strongly the matrix absorbs, the shorter the path length. When 0.1% of carbon black is added to the disk, the path length drops to about 100 μm.
Vol. 3, Iss. 5 Virtual Journal for Biomedical Optics
Lacey A. Averett and Peter R. Griffiths, "Mid-Infrared Diffuse Reflection of a Strongly Absorbing Analyte on Non-Absorbing and Absorbing Matrices. Part I: Homogeneous Powders," Appl. Spectrosc. 62, 377-382 (2008)