Is there any upper limit for gradient time (40-50 hours)

[zf12345]

Hi,

I am just wondering if there is any upper limit for gradient time. Can I run gradient like 40-50 hours and still obtain decent separation? Thanks!

[bartjoosen]

Your peaks will get broader when the runtimes increases.
Normally you use a gradient to elute things faster as they would with an isocratic system.

If you use gradient times of 40hours, this is almost isocratic, your peaks will get minutes wide, and you maybe see a bump in the baseline, or even nothing.

Bart

[zokitano]

Ask yourself another question: is that run of 40-50 hours will be the best cost/effective solution for your problem (beside that what bartjoosen has already said) ?

Regards

[Jackus]

I cannot find any foundation for such a long run time (only if you have 2 598 impurities ... )

[shaun78]

I don't know that much about instruments, maybe someone more knowledgable could aid us, but I would be incluned to think that a 40-50 hour gradient would be very hard for the instrument to accuractly reproduce.

Say you are going from 100% aq. to 100% organic over 50 hours. This would mean a 2% change in pump composition per hour.... which seems a little slow and too fine tuned for an instrument to pull off consistently.

But I could be wrong.

Definately not economically feasible though. Assuming your flow is 0.5 mL/min, thats 1.5L per injection. How are you going to store and degas enough solvent to even run an analysis?

[Bryan Evans]

Is this a theoretical question or do you have a specific problem in mind?

For complicated mixtures, options to try are:
- 2D LC
- LC-Fl
- MS/MS or MS^n

Or any combination of the above