A water vapour proof film that is permeable to organics ?

[GOM]

Hi,

Picking up on GCGUYs suggestion of wax.

Would it help to counteract the surface depletion problem if the wax was a thin film? I am thinking of dispersing the component into a wax then coating onto a diatomaceous earth - in other words making an old fashioned column packing like Carbowax 20M on 60/80 mesh Chromosorb W but preloaded

Regards

Ralph

[Peter Apps]

Thanks Ralph, you have got me thinking. A dispersed thin film would not necessarily need a film matrix with a high diffusion co-efficient, because the film itself would be thinner than a mechanically stable membrane, and would have a larger area. On the other hand it would be possible to use a thick film of something with a high diffusion co-efficient - in other words a silicone coating on an inert support. The difference from a packed GC column is that the support does not need to be thermally stable (at least not above about 60C anyway).

There would need to be a flow of air through the particle bed, otherwise gas phase diffusion would limit the emission rate, or the particles would need to be spread out as a thin layer.

Definitely worth considering, thanks again.

Peter

[GOM]

Cheers - a silicone sounds good

My thinking about the diatomaceous earth is that its porous structure and thus large surface area (up to 4sqm/g), can take a large silicone loading(10%+).You could also use brick dust.

With a packed density of approx. 0.5g/ml and. say, a 10% silicone loading would give 0.05g silicone per ml

I agree with the thin layer spread - sprinkle over some adhesive tape that can easily be cut up into sections? - perhaps check the diffusion profiles with headspace GC, first for consistency and then with time?

Another option could be to pack a 5mm bed in a short 2cm length of glass/PP (a cut down 2ml syringe) tubing held in place at each end with a frit or glass wool. This would give 2 surfaces for diffusion and perhaps allow air to percolate through

Cheap and cheerful

Ralph

ralph.calvert@gmail.com