Diffusional pathlength

Basement Membrane (BM). The BM has an enormous surface area compared with the epithelium owing to connective tissue papillae, which may affect the effective diffusional pathlength. 

Figure 1.32 Schematic diagram showing a homogeneous barrier film on a microporous substructure. The actual average diffusional pathlength A of a component through a film of the thickness A0 is expressed as a function of the overall porosity and the pore radius.

Pellet, M.A., Watkinson, A.C., Hadgraft, J., and Brain, K.R., 1997, Comparison of permeability data from traditional diffusion cell and ATR-FTIR spectroscopy. Part II. Determination of diffusional pathlengths in synthetic membranes and human stratum corneum, Int. J. Pharm., 154, 217-227. 

Multiple minigrids were stacked between spacers to increase the optical pathlength of the cell while retaining thin-layer diffusional distances. This cell was used to demonstrate infrared thin-layer spectroelectrochemistry with ninhydrin. This was chosen for its three carboxyl groups, the reduction of which would be easily observable in the infrared. Another thin-layer cell was constructed with quartz plates in order to illustrate applicability in the UV (40). 

Equilibration of the redox protein is faster in the reflection cell, because the diffusional path length is only half the optical pathlength, which enables a rapid switching of the molecule between different redox states. Although direct electrochemistry is preferable, it turns out that redox cofactors are sometimes deeply buried in proteins and enzymes so that mediators are required to speed up the equilibration [4]. However, they can be kept at very low concentrations and thus do not interfere with the optical measurement. Under these conditions, ampero-metric, coulometric, or cyclovoltammetric experiments in combination with UVA IS/IR... 

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