  The idea behind the research presented is based upon apparently contradictory
experimental results obtained here by means of photoacoustics modalities for
the same drug donor/acceptor membrane system, serving as a surrogate for a
transdermal delivery system. The first modality allowed for the monitoring of
the total amount of mass uptake (m(t)-type data), while the second technique
allowed for the quantification of time-dependent concentration distribution
within the acceptor membrane (c(x,t)-type data). Despite of a very good
agreement between the m(t) data and the 1st-order uptake fitting model
(standard Fickian diffusion with constant source boundary condition), the
standard approach failed during the c(x,t) data analysis. The results led to
the analysis of the interfacial transfer contribution to the overall mass
transfer efficiency, which eventually allowed to question reliability of the
m(t) data analysis for the determination and quantification of the mass
transport parameters. A more detailed analysis of the c(x,t) by means of the
newly introduced transport rate number parameter revealed, that the mass uptake
by the acceptor is almost equally influenced by interfacial and bulk transport
processes. The analyses performed were translated into a model-free
characteristic times, i.e. parameters common for any of the model scheme used.
