Py-GC/MS and split ratio

[pooch]

Hi,

I work with a Pyrolysis-GC/MS system. I use polystyrene as standard.
I can observe on the pyrogram styrene monomer(major peak), dimer and trimer.
Pyrolysis temperature has an influence on percentage of dimer and trimer. At 750°c I have less trimer than dimer.
Currently my GC method use a split ratio 30:1; but when I use a split ratio 50:1, trimer peak is higher than dimer.

I don't understand how split ratio can have an effect on high boiling compounds? Is it specific to the Py-GC/MS system?
Anyone have an explanation?

Thanks!
Damien

[tlahren]

Just a thought:

Could the cooling effect of the larger gas flow (50:1) cause a reduction in breakdown from trimer to dimer/monomer products? I.E., if thermal degradation is the process to take POLYstyrene into mono, di and tri-mer versions the added cooling effect of the increased split might be causing your problem.

[pooch]

Thanks for your answer.

So an increase split create a cooling effect in inlet? I don't known about that.
But it can explain the phenomen. The breakdown of polystyrene is made in the pyrolysor
chamber, but a temperature decrease in inlet could form more trimer (reaction between monomer and dimer)...

Thanks again!
Damien

[tlahren]

Sorry, I had the symptoms backwards in my head. I was thinking the trimer was decreasing with increasing split. Now it makes even more sense. I read about the cooling effect in an article about ultra fast GC methods once. The split rates were between 100 and 400 mL/min so I'm not sure if you would see such a noticeable effect going from 30 to 50. It's just the only thing I can think of.

[pooch]

Okey,
Thanks for your help!

[Peter Apps]

Total flow presumably has some effect on residence time in the pyrolyser, and the longer the residence the more the fragmentation.

Peter