desorption tubes decay

[christiana]

We own several thermal desorption tubes, like SKC 226-350, Carbotrap 349, Chromosorb 106, Tenax. A long time ago in some of them we injected very small quantities of solvents in liquid form (e.g. hexane in μL).
After desorption and conditioning the tubes for several times we still see in the chromatograms of 2-stage desorption these solvents (e.g. hexane, acetone, dcm).

Which your opinion? This is just a memory effect or there is a damage in adsorbent beds inside the tube.

For how long the desorption tubes could be effective for air sampling?

Thank you.

Christiana

[rb6banjo]

If I read what you're saying correctly, you are experiencing that you cannot get blank runs free from those analytes? I don't have much experience with these sorbents but 1 µL of neat material (hexane, acetone, etc.) is a lot. I'd bet that your traps are still usable. You just need to purge them at large temperatures and high flow of an inert gas (helium or nitrogen). I would see if I could devise a way to heat the traps to just below their maximum temperature and pump a lot of helium or nitrogen through them at the same time.

[AICMM]

christiana,

I am not sure where I read this but, as I recall, Tenax will decay with time and start giving you a toluene (or benzene?) peak. Since Tenax is a porous polymer, I would expect the same from the 106 but not necessarily the Carbotraps. On the other hand the carbotraps are very tenacious so it may take a bunch to get a heavier solvent off of them.

Can I suggest a call to Supelco tech support?

Best regards,

AICMM

[tdgcuser]

Are the solvents are compatible with the sorbent? Do you still see the lighter solvents on the weaker sorbents? For example, there shouldn't be much hexane remaining on the tenax tubes after a desorber plus conditioning cycle.

We routinely use hexane and IPA as solvents for spiking standards with pesticides on tenax TA tubes. We don't see the solvent because it elutes well before our target compounds.

I'm pretty happy with 99% of my analytes desorbing from the tube if I can get the othe 1% during conditioning. One percent of 1000000ng of solvent is a lot of solvent remaining (10000ng), if the conditioning step takes another 99%, you have 100ng solvent remaining assuming everything remains linear.

Give them a couple conditioning cycles (watch your temperatures, chromasorb 106 is fairly low) and make sure you don't have any solvent bottles in the lab where you store the tubes. The sorbent tubes also work as passive collectors, so keep them sealed up.

[christiana]

Thank you all for your helpful messages.

I will have some more experiments and I will inform you about.