PULSED-FIELD-GRADIENT NMR OF DIFFUSIVE TRANSPORT THROUGH A SPHERICAL INTERFACE INTO AN EXTERNAL MEDIUM CONTAINING A RELAXATION AGENT

The theoretical basis of the pulsed-held-gradient spin-echo experiment and its applicability to the study of diffusive transport through a s pherical interface into an external medium containing a relaxation age nt are examined in the long-time diffusion limit. The theory is compar ed with previous experimental findings involving bicarbonate diffusion in and across human red blood cell membranes, To a first approximatio n, the addition of a relaxation agent to the external medium results i n the interface becoming a ''partially absorbing wall.'' However, the results of this study show that it is experimentally difficult to achi eve the partially absorbing wall conditions, and, in any case, this on e-site model is not very sensitive to permeability differences. The tr eatment is then extended to include the case where the transported spi ns do not relax instantly but instead have a small but finite lifetime . This model is shown to be more sensitive to permeability differences as well as being experimentally realizable. (C) 1995 Academic Press, Inc.