October 02, 2022, 01:45:02 AM
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Topic: Method(like additive) that can avoid or reduce thermal decomposition in GC  (Read 819 times)

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Offline AOD120

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I have a memory that it's able to add antioxidants when analyzing easily oxidized substances, is there are similar additives for easily thermal decomposed substances´╝č

Offline rjb

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Re: Method(like additive) that can avoid or reduce thermal decomposition in GC
« Reply #1 on: February 28, 2022, 12:07:32 PM »
Not in the way that you're thinking I suspect!

If you are dead set on using GC, then you could potentially derivatize your compound of interest which could help address issues with its thermal stability. However this would (for me) be a last resort.

Personally I think that it might be wiser to consider derivatization only after you have satisfied yourself that changing run parameters cannot address the problem. Firstly, where do you think the decomposition is happening?

If it's the injector (which is common) then you could consider either trying to lower the injector dwell time or the temperature, or perhaps both. If you're running splitless, consider using a split flow which should lower injector dwell time, but obviously has consequences on sensitivity as you're dumping some of your sample overboard. Why not drop the injector temperature... Yes it can affect HMW components and lead to band broadening if you go too far, but at least give it a try. Is your liner dirty, or is it the type stuffed with glass wool? If so get a new liner (perhaps without glass wool), this can help minimise thermal degradation.

If its the column, it's mostly a matter of temperature programme (possibly slower ramps) and possibly thinking about whether a different column might help or not. From what I gather from people with a lot more experience than me, a thinner film thickness can help, but obviously as with all things, what you gain on the swings, you lose on the roundabouts.

Something to think about.

Offline AOD120

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I think the decomposition happened in both injector and column, so the first thing I came up with was dropping the injector temperature. Someone told me that there is some kind of "additive" that can reduce thermal degradation as I said before(really not sure about that).
I will do something with the liner and temperature programme since they are the most feasible way to try out. If they don't work, I will try the rest of them.
Thank you so much for the information. I appreciate it.

Offline rjb

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I think the decomposition happened in both injector and column, so the first thing I came up with was dropping the injector temperature. Someone told me that there is some kind of "additive" that can reduce thermal degradation as I said before(really not sure about that).
I will do something with the liner and temperature programme since they are the most feasible way to try out. If they don't work, I will try the rest of them.
Thank you so much for the information. I appreciate it.

Not a problem :)
Sounds sensible. Out of curiosity, what GC system are you using and what kinds of compound? It may be that you have a GC equipped with a multi mode (or PTV) injector rather than the standard split/splitless injector and this could solve some of the issues that you're having - it might be worth finding out if you can. Multi mode injectors can be really useful for compounds which are prone to thermal degradation and allow hot or cold split and splitless injections as well as injections with single or even multiple temperature ramps at rates of up to ~900C/min which can be really useful. They even allow (in certain configurations) insanely high samples volumes which increase LOD considerably. 

In terms of additives, there is nothing that I know (but I'm not the oracle) of and even if there were (and it was somehow able to address your injector degradation issues - which is unlikely) it would not solve any issues you're having with on column degradation and might even co-elute with one of your compounds which would not be ideal! To be honest, in a standard injector, the environment is about a friendly as a high temperature environment is ever likely to get. It's stuffed full of high purity helium, so the degradation processes are purely thermal or thermal and inlet activity related. By reducing inlet activity (hence me recommending a new deactivated sample appropriate liner without glass wool), you've minimised inlet activity and all that's left to worry about is temperature and other GC run parameters.

Beyond that, it's either a job for derivatization or HPLC!   

Offline AOD120

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I am sorry but I can't really tell the actual substances were analyzed because It may break the NDA...
I felt adding an additive was kinda sus too because it may pollute the graph eventually as you said. Anyway, by decreasing the injector temperature I got a fair result and I am happy with it:).
Thank you for your sharing.

Offline Babcock_Hall

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If it were possible to derivative the analyze (as rjb mentioned), this might help, especially if it increased volatility.  However, I imagine that doing so is not an option for you.

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