Can someone explain the chemical phenomena behind this?
I spike my sorbent bed (tenax ta) with benzene, toluene, ethylbenzene, xylene, styrene, naphthalene, pyrene, and benzo[a]pyrene. The boiling point range is 70 C to 450 C. I heat my tube to 350 C which is the limit for tenax ta. However, my PAH compounds with boiling points higher than 350 C show up in my chromatogram. Even more weird, is that the second desorption of the same tube doe not show any compound retained on the tube so all my compounds have fully evaporated and transferred to the column. I do not understand the chemical basis of this. Is this due to pressure lowered in the tube because there is a 50 ml/min helium flow?
Just to be clear, I am not complaining. I am actually pleased that this is happening. But I need to understand why and how it's happening.
Thanks
Well, it's the same reason for which something that is wet dries in air despite your kitchen not being at 100°C. Every substance above 0K always has an associated vapour pressure, it's just negligibly small for some substances which makes their evaporation or sublimation rate negligible at low enough temperatures:
https://en.wikipedia.org/wiki/Vapor_pressure
When I did PAH+fullerene evaporation for my PhD work, I never heated up the effusion cell directly to the sublimation temperature - then it would have been empty very soon and my coverages certainly not in the submonolayer range.
- Badges
- Science topic