Flowrate and head pressure relation

[thohry]

Hi all,

Say I've got a 30 x 0.25 column installed in a GC. If I set the head pressure (He) to be 11.5 psi, I will have the flowrate of 1 ml/min.

Now consider the same column in a GC/MS, the head pressure is 7psi to maintain a 1 ml/min flow. So the flowrate of the same system (column, gas and temp) is not dependent only on the pressure drop.
What is the point here?

Thanks for any input.

[Peter Apps]

The viscosity of a gas rises with the pressure, so at higher pressure a given pressure drop gnerates less flow.

Peter

[AICMM]

thorhy,

It is pressure dependent. Your GC/MS system has the column outlet into a vacuum whereas your GC outlet is at 1 bar outlet (typically.) Someone once told me that about the last 1/3 of your column is under the influence of the vacuum system.

Best regards.

[HW Mueller]

If the head pressure needs adjustment then the vacuum influences the whole column + flow control. The only physical exception, that I can think of, to this series flow behavior (law) are the ultrasonic flow regulators which Siemens used to have.

[thohry]

thorhy,

It is pressure dependent. Your GC/MS system has the column outlet into a vacuum whereas your GC outlet is at 1 bar outlet (typically.) Someone once told me that about the last 1/3 of your column is under the influence of the vacuum system.

Best regards.

Thanks to all.

If the last 1/3 column is under vacuum of some how, so we don't have the same velocity from inlet to the outlet? The more thin the gas in the column (toward the exit) the higher velocity (cm/s). Is that right?

[Peter Apps]

Yes, it's right. In any column (whether with an MS or an atmospheric pressure detector such as an FID), no matter what the outlet pressure the linear flow velocity at the high pressure end is lower than at the low pressure end. That is why when you inject a non-retained compound and measure its retention time to determine flow velocty you are measuring average flow velocity, where the averaging is done over the length of the column.

In the forum archives there is a lot of discussion of the relation between volume and linear flows, pressure and pressure drop, gas viscosoty, and what to set as optimum flows, and how to set and verify them. Please do a search; lmb is a good user to look for.

Peter