Chromatography Forum

Hi,

Is a gradient of flow rate suitable?

For example:

Time Flow Rate A% B%
0,00 1,00 10 90
5,00 1,50 10 90
10,0 1,50 50 50
15,0 1,00 10 90

Thanks!

in principal it is possible, but why you want to make your life that difficult?
Why you want to run a flow rate gradient???? If you want to get reproducibility - forget it.
Nice idea if you need a better baseline separation, but I would than use a different separation mode or minimum a different column.
Maybe some more details about your chromatographic conditions and let us know what you want to improve.

I've only used flow gradient myself in the clean-out part of a gradient run, to increase throughput a little.

I'd say if it passes your system suitability, then OK. But have to wonder if you really need that.

As Gerhard mentioned, reproducibility will most probably be a nightmare. If you use UV-detection, remember that peak areas depend on flow-rate. Imagine what a flow-rate gradient means in this context...

If the flow gradient is only from faster column cleaning and initial mobile phase reconditioning than I think it's ok. If you want to use flow gradient to improve separation somehow it's not a good idea. There is so many variety in achiving good separation but the flow gradient it is not one of them

it won't do much anyway; since the flow-ramp is done at constant solvent strength, basically it's isocratic with the time-axis gradually squished as time increases, which could improve the looks of things because it would make the broader peaks at later retention times a bit narrower, but since it will also make them closer to one another, it won't actually improve the separation. There is an ideal flow-rate for the column, and if you can run at that flow rate without excessive back-pressure, then you should do so - for the entire method, and not just part of it. Modern columns, however, have pretty flat van Deemter plots (I believe), so if you can't, it doesn't matter (much). 50% changes in flow rate won't make much difference.

Yes, I've also used higher flows for the cleaning and/or re-equilibration portions of a run, usually motivated by a desire to shorten run time, and sometimes in the knowledge that I was using a slower flow not for the column, but for the detector.