Response time effective diffusion length

Fig. 18 Influence of the effective diffusion length on the response time of a hydrogel component, (a) Bulk hydrogel component. The effective diffusion length is (7a- (b) Component with hydrogel particles. The component volume equals with that of the bulk component. The effective diffusion length is approximately rp...

There are four unknown parameters in the theoretical impulse response for porous particles, h(t) the pellet diffusion time, tdif (which contains the effective diffusion coefficient of the pair T-C, Dtc, td.fs R p/D.f( , R is the radius of the pellet equivalent sphere), the mean residence time of the carrier-gas in the interparticle space, tc (tc = v/L with the carrier gas linear interstitial velocity, v, and column length, L), Peclet number, Pe (Pe = L.v/E, with E the effective axial dispersion coefficient) and the adsorption parameter, 5q (see below). Because matching with four unknown parameters would give highly correlated parameters, it is better to determine some parameters independently,... 

Past efforts allow us to formulate three objectives for the present work. First we would like a technique that is roughly 2 to 4 (or more) orders of magnitude more sensitive than existing spectro-electrochemical methods. If this were achieved, the techniques could be applied to high-sensitivity analysis where one has a complex mixture and one makes use of the selectivity of spectroelectrochemis-try. Second, it would be valuable to lower the usable time scale of spectroelectrochemistry down into the microsecond region for a variety of chemical systems. With an optically transparent electrode and virtually all spectroelectrochemical methods, the response is limited by an effective path length which decreases with the time scale. Therefore, it is very difficult to monitor species on a microsecond time scale simply due to the low sensitivity of the techniques. The third objective is spatial.resolution of the diffusion layer. It would be very informative from both fundamental and practical standpoints to be able to accurately observe concentration vs. distance profiles. 

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