Constant flow v/s Constant pressure

[Rhodium]

What are the basic guidelines for using constant flow v/s constant pressure on an Agilent 6890?

[Don_Hilton]

My rule of thumb is easy. Unless I need to follow a method that specifies constant pressure, I use constant flow. I try to run columns close to the optimum linear velocity - or just a bit above that. If the low rate drops - as happens with temperture programs and contant head pressure - the linear velocity may drop below optimum. And the van Deemter curve is much less forgiving at low flow than high flow.

[willnatalie]

I agree, constant flow is better.

[aldehyde]

yeah

[larkl]

I worked in one lab that ran a LOT of methods in constant pressure. Problem was, they used a lot of 100 meter capillary columns and temperature program methods. With constant flow, the head pressure has to increase as the temperture increases. You run into a limit if you have a very restrictive column, like the long ones.

Either one will work. There have been many, many samples run in constant pressure for many years before electronic flow controllers were developed.