Chromatography Forum

How do you prefer to run GC:
At constant pressure or constant flow?

It seems to me that there are a number of threads on this topic. And, while there are some who use constant pressure for one reason or another, my impression is that unless you have a compelling reason to go with constant pressure, constant flow is the more popular -- but I may be biased because that's my view.

It seems to me that there are a number of threads on this topic. And, while there are some who use constant pressure for one reason or another,

Old equipment or methods (pre-EPC) I would guess... That was my only choice on a 5890 for years...

- Karen

I seem to recall at least one person posting that even with instrumentation with an EPC, there was a need to match old methadology. I figure that if that was they case - and still is, they'll speak up. So, I won't bother go look to see if my memory is correct.

I use constant flow for two reasons: quicker separatiosn because the gas flow at the hot end of the temperature programme is not decreased by increased viscosity, and less baseline drift on flame detectors when using hydrogen as carrier gas.

Peter

Peter made good points.

Some other factors are if you are using programmed oven temperatures, especially over a wide range you can lower the flow at the low end to get better separations using a capillary column and not have the flow slow to unacceptable levels at the high temperature end of the program.

Even so using a non-polar phase column, try separating ethane and propane on a capillary versus a packed column! It is much easier to do with a packed column.

With packed columns this was not an issue insofar as the pressures and the flows and pressures were much larger than with capillary columns, and being of shorter length the change in flow over a temperature range was much less than with capillary columns.

Conversely, a capillary columns could reach higher temperatures and elute heavier molecules more easily than a packed column due to the reduction in bleed and better stability. Back in the day we were lucky to get good control over pressure, much less be able to keep a constant flow.

To sum it up, for NEW methods it is better to use constant flow if you are using capillary columns and if your GC allows you to do so. And if you can change OLD methods to constant flow it is better if you can make that change for both packed and capillary columns.

best wishes,

Rod