Can I ask what type of Pyrolyzer you use? Is it platinum filament (ie Pyroprobe), Curie-point induction, or the constant temperature furnace? I am assuming it is fixed (stationary) to GC inlet, or is it removable? What pyrolysis temperature are you using? I have a few ideas but need a bit more information. You may be able to do it without any GPC whatsoever.
I use a vertical pyrolyser unit with a sample cup oven. It is removable of the GC inlet. Pyrolysis temp. is 600 degrees Celsius.
The setup I use that is quite nice is where I no longer have the pyrolysis unit attached (stationary) to the GC inlet and instead it is now offline, where I cut a 20-30 cm length of a separate GC column and have that at the outlet of now stand-alone pyrolyzer to act as the chemical trap. Then, I use methanol in a syringe to wash the pyrolysis products out of cut column length into a GC vial and can now load in GC autosampler.
The benefits of this are you can run nitrogen (cheap) through pyrolyzer while you can use hydrogen as carrier gas (hydrogen & pyrolysis don't mix!
). Also, sample throughput is higher since all sample prep is performed at one time and vials with collected pyrolysates are loaded in autosampler with it running non-stop as opposed to having to pyrolyze a new sample every 30 minutes for only 8 hours in the day. Not to mention, the GC-MS is no longer designated to only pyrolysis but can be used for any other preparations.
You may have to play around with your setup to get it right (It took me a while to get it working right but was worth it). There may be some losses of the early eluters (low boiling point compounds eg. acetaldehyde), but for me these are lower information value. I suppose a cold trap could be set up on the cut length of GC column to possibly help in collecting these low boiling point compounds, if they are necessary.
Anyway, my thought was you may be able to control the pyrolysis products you get by the amount of methanol you rinse cut length of column with, so that only the compounds of interest (GC injectable) and not the crud that is causing carry-over are extracted into GC vial. You can then either dispose of that cut length again or further rinse with methanol and use again (you can test to see that only methanol is coming out -- and this will tell you if all compounds were extracted for first methanol rinse). I have only used methanol as solvent, since this is the known solvent for cleaning GC columns. If you need a stronger solvent, check with the column manufacturer to make sure you won't destroy the cut column length, but you could also do some experiments with solvents since it is only 20 cm of column you may be destroying).
So, if this works then perhaps you won't have to run the prep GPC prior to Py-GC since carry-over products (not fully fragmented polyethylene wax/oil) will not get injected into GC since not extracted by methanol.
As Marquis said above, another option then (if above idea is too much set-up/experimenting with conditions) is depending on the column max temperature, set GC program to a higher inlet temp (pulsed split if possible) and end GC oven temp and let hold for longer to ensure all carry over products are 'burned' out of column before next run, and run a blank run in between to ensure it is in fact blank.
ChemBuddy | 
Chem Reddit | 
Topic: components separation by means of Py-GC-MS and preparative GPC (Read 14232 times)