Application Problem RE: High MW Hydrocarbons

[cawarhur]

Chemistry brief: We're injecting C5's-possibly C20's (heat-soaked DCPD has all sorts of rearrangement products from trimer, to tetramer, to possibly pentamer. We're not concerned with pentamer because it isn't likely to be in our samples.

The problem: tetramer won't elute.


Our GC column is a 105m Rtx-5 (equiv. to HP-5) x 250um x .25um.
Carrier gas is He.

I've tried increasing the run time but it is getting impractical. I've tried increasing the final ramp temp, but we see substantial column bleed (though the tetramer does elute). I've tried ramping flow but it wasn't exactly effective (the flow we run at is already moderately high and going above that makes me uncomfortable and the GC sort of rolls its digital eyes at me).

What other options do I have? Help me out!

[cawarhur]

Is it possible to have seen the tetramer peaks on a 30m column and then not on the 105m? The obvious answer is yes, i will just be a matter of retention time and method length...but the method is already an hour long and in comparison to the 30m runs, should things elute proportionally, the tetramer should be eluting before the 60 minute mark.

[chromatographer1]

two obvious options:

easiest to implement, hydrogen carrier, reduction of cycle time to about 2/3 to 1/2 that of helium

harder, multidimensional chromatography. two column back flush to detector, or 3 column reverse column step, or two column forward column step should work for your application.

best wishes,

Rod

using a thinner film or a more narrow column can introduce problems in your application

[cawarhur]

Did you suggest hydrogen as a carrier due to viscosity issues with helium at higher temperatures? If that is the case, am I correct in thinking that we would get roughly the same separation and retention times but we could run at lower temperatures and not see the column bleed?

Hrm...on second thought, we are running all sorts of things with double bonds, so hydrogenation might be an issue....thoughts on this, too?

[chromatographer1]

This is a possibility and I know that styrene can react with hydrogen even at 80C. Now what is the rate of conversion (1 part in 1000?) and will you separate the hydrogenated olefins.... and if you can see them can you merely add the areas for the analysis. These are good questions and I am afraid I can only offer you the possibility of problems, not the answers.

I would suggest a forward step multidimensional application (a single six port 0.010" ID valve) and using a 10 or 15 meter column A followed by a 105m or 60m column B. Lengths are only a suggestion for initial trials.

Another thought for the future: helium will become scarce and will be expensive. Is it worth the time to investigate hydrogen? Or does SFC with pentane carrier have a means of escape from GC?

Ain't science grand?

Rod

[Peter Apps]

Presuming that you really need the resolution of the 105 m column, a thinner film will allow all the components to elute more quickly but will be more suceptible to overloading. If you go to narrower bore you will suffer from very high back pressures. The very low phase ratios of a thin-film megabore column might get you what you need without pressure problems (finding a 105 m megabore might be a problem ?).

If you can, move to hydrogen as well.

Peter

[cawarhur]

Problem solved! Thanks guys!

[chromatographer1]

cawarhur

Please don't be unkind. Tell us the solution you found.

Thanks.

Rod