Answer to Question #39034 in Physical Chemistry for EDWARD TONE

Question

dear sir
I HAVE A QUESTION ABOUT GAS CHROMATOGRAPHY.WHEN WE USE CONSTANT PRESSURE OR CONSTANT FLOW FOR COLUMN OR IN MAKE METHOD FOR ANALYSIS?
BEST REGARDS
TONE

Answer 1

If you're doing a relatively minor temperature program (say from 50C to 150C), then you won't notice much difference between constant flow and constant pressure modes. If you're doing a more "dramatic" temperature program (say from 35C to 350C), then the differences will be significant due to the relatively large reduction in column flow as you reach higher oven temperatures. Personally, I've just become used to using constant flow mode and that's what I normally default to when doing method development now.
With a more "dramatic" temperature program (larger temperature range), then you will see a reduction in retention time for late eluting compounds in constant flow mode. Additionally, you may see an improvement in resolution for late eluters in const flow mode as well, depending upon initial conditions. This is because there can be very significant changes in carrier gas linear velocity in constant pressure mode for temp programs with a large temperature range.