On the isotope analysis issue, do the porous cups cause isotopic fractionation?

I thought about this some more over the weekend and I am not so sure that you wouldn't see a fractionation if you are looking at D or 180 at natural abundances. I can imagine the cup matrix and tension acting very much like a mini (make that micro) chromatography column. I think a test of the effect would definitely be in order. It shouldn't be too hard. A sealed container with known delta-D and 18O would give you a good idea of any effects and their magnitude. I would love to hear how this turned out. The solutes I don't think would be affected as long as they don't interact with the column.