Diffusion Coefficients via IGC-ZLC

The Zero Length Column (ZLC) technique has been well-established by Brandani and Ruthven. The idea is both simple and powerful. A small sample (typically 1mg) of a "porous" material (i.e. anything that can absorb and release a probe gas of interest) of radius R is saturated with the probe gas and then the desorption is followed over a relevant timescale (t_max). The rate of desorption depends, of course, on the Diffusion Coefficient, D, and on a combination of R, the flow rate, F, the volume of the sample V, and the Henry constant K. In all the analyses, a parameter L is used to accommodate F, R, V, D and K:

L = FR²/(3KVD)

This is convenient for the app as we only need D, R and L plus t_max. Obviously in real life you cannot change D and R independent of L, but the point of the app is to illustrate the technique, not to try to analyse real data.

The desorption curve is rather complex at the start, depending on an infinite series of β values that in turn depend on L. However, rather rapidly the desorption becomes a simple exponential so by plotting on a log scale the detector signal c over its starting concentration c0 versus time, a straight line is obtained from which the key parameters can be extracted.

The ZLC technique does not require an IGC machine, but exactly the setup required for IGC desorption isotherms (IGC-FC) works wonderfully for ZLC because in both cases it requires a precise, careful injection of excess probe to gain the right degree of saturation, though for ZLC this is far lower.

The IGC apps are based on the inputs kindly provided by Dr Eric Brendlé of Adscientis who are specialists in IGC measurements.