Bumpy baseline for first 15 mins of gradient run- causes?

[EmpowersBane]

I run a gradient starting at 13% Acetonitrile and 87% Buffered Aqueous over a 42 minute run time. After the dead time the baseline usually settles out but last week I got a run which had a whole series of "potholes" in every injection (Blank, Standard and Sample) from 0.5 to 15 mins that is being integrated as a whole series of "peaks" even though I can see its in the blank and hence is a system problem. After 15 mins the issue settled out.

Whats the most likely cause of this? I have a very rigorous priming procedure which would have purged any air, could it be mobile phase or column? I should add none of the active peak shapes or areas etc were affected by this anomaly.

[HPLC chemist]

Was your mobile phase ultrafiltered? Sounds like dirt was temporarily trapped.

[tkubowicz]

Hello

First of all it'd be easier if you paste chromatogram.
Also try to plot pressure over signal to see if you have stable pressure. And check your pump, run diagnostic tests (leak/valve tests) to see if pump is working ok. Also check if degasser is working without any problems.

Regards

Tomasz Kubowicz

[Rndirk]

Also, which wavelength(s) do you measure at?

[EmpowersBane]

Thanks for the replies so far guys, Yes I had a pressure trace on the run and when overlaid it did match the bumps. I have 2 JPEG pics one of the bumpy baseline the other of a typical baseline, ill try attach them. I'm working at 2 wavelengths- 225nm for one active and 238nm for the second active.

Having trouble pasting in the pics, any ideas how to do this??

[tkubowicz]

Having trouble pasting in the pics, any ideas how to do this??

viewtopic.php?f=1&t=2617

[hossam kamal]

i think it may be mobile phase contamination ,
leave your column to equilibrate for 30 min with your aquoes solvent , then inject blank and run your gradient range , then leave to re-equilibrate for 5 min then inject againe blank , if this noise gets reduced in the second run it may be the aqueous solvent is the source of impurities , if it still the same it may be the organic solvent is the source of noise