[theairisrunningout]

Hi all,

I am interested in learning more about the actual theory of coldtrapping as a means of preconcentration of analytes, specifically small hydrocarbons (let's say from methane to propane). I have found various papers on the actual instrumentation involved, which is fairly consistent across the board of sample types (whether it be atmospheric or freshwater). What I'm having the most difficulty finding is background theory on how cryotrapping actually works. I've read papers and books about cryopumping, but I believe this to be very different.

If I had a simple standard air sample (let's say it contains N₂, O₂, CO₂, H₂O, and CH₄, that I injected into my GC, which, before being pushed onto the column, was forced through a cryotrap (say, stainless steel tubing filled with a substrate submerged in LN₂), how exactly would each gas behave within the trap? Since CO₂, H₂O, and CH₄ have freezing points above 77 K, would these all freeze on the substrate, whereas O₂ would liquefy and N₂ be in vapor-liquid equilibrium, or am I looking at it with too much of a simplistic view? Are the only things that matter freezing points and boiling points of the gases relative to 77 K, or am I missing something? Then, if I were to warm the trap up to, say, 120 K, CH₄, O₂, and N₂ would all (re)-vaporize, and, if I further cryofocus the CH₄, I should have enhanced levels, no?

I apologize for the lengthy post, but if anyone could attempt to explain some of the theory, or provide relevant papers or even simple cartoons/animations, I would appreciate it greatly! Thanks!

[marquis]

I can't tell you about the theory.  Sometimes there is a quick and inexpensive way to try it.