gradient with different flow rates at different retention ti

[sriharshabio]

Hi,
I have come across a paper where for mobile phase gradient parameters, they have used different flow rates at different retention time points. for example 0 min (0.5 ml/min), 1 min (0.5), 2 min (0.6), 3 min (0.7) and so on....Can somebody please explain what is the usefulness of doing so other than the traditional method of keeping the flow rate constant. thanks

Regards,
Harsha

[Consumer Products Guy]

Maybe to help speed clean-up of the column once the analytes of interest had eluted?

In my career, that's the only time I actually used flow programming like that; I was using pre-mixed mobile phase in mode with bypass of multichannel gradient valve (essentially an isocratic system then) and I had a late-eluting peak of no interest (maybe a system peak) and low pressure, so I could bump up the flow rate after my peak of interest to increase throughput/save time.

[carlo.annaratone]

Same goes for me, used higher flows for washing and re-equilibration stages

[James_Ball]

If there is an analyte that normally gives a broad peak, it could possibly be used to increase the linear velocity to improve peak shapes. The down side is the increased flow rate often causes lower sensitivity, but if you sharpen the peak enough it would compensate for it. I believe one of the newer EPA HPLC/MS methods used this during the UCMR.

[sriharshabio]

Hi Guys,
Thanks for the replies. I found the gradient in this paper http://www.ncbi.nlm.nih.gov/pubmed/22305897.

Regards,
Harsha

[lmh]

Looking at that paper, it was probably just to keep the run as short as possible. Assuming that, like most LC runs, the flow rate wasn't so high that resolution was suffering, the analyst was probably trying to run the method as quickly as possible consistent with not getting excessive back-pressure. As the gradient became more organic, probably back-pressure dropped, so they took advantage of this by increasing the flow rate.
Increased flow-rate means they can steepen the gradient in the relevant sections, and still have the same percentage-change per mL pumped solvent (in effect, it's the same gradient, but everything is moving faster).
Not many people bother with this degree of time-optimisation (I never have) but I've met a few who find they can shave a minute or two off a method, and it all helps if you have a lot of samples.