Chromatography Forum

I have trouble with my Agilent 1100. There are huge pressure fluctuations when running a wide gradient.
At pure water or organic the fluctuation is minimal but around 50:50 it is about 20 bar! Now I made an isocratic method
that wants to pump 45:55 Water/ACN but the fluctuations are killing the method and disturbing the UV baseline.
In addition to the large high-frequency fluctuation there is a longer term and pretty random slope to the pressure
vs time. Any thoughts or suggestions for further experiments?

A bit more. The screenshot is pumping at 0.5 mL min and the high frequency fluctuation
is about 15 bar and the frequency is 16 hz. If I halve the flowrate the frequency also
halves to 8 hz and the overall fluctuation remains unchanged at about 15 bar. ?Odd!

This is a quaternary pump but I tried running water on A and running organic on B and D
and observed the same thing. The example shown has water on 'C' and ACN on 'B'.

Are you using a degasser after the mobil phases, feeding the pump?
The water especially will need 15 minutes degassing , then prime the pump, fresh, and look athe baseline.

I have trouble with my Agilent 1100. There are huge pressure fluctuations when running a wide gradient.
At pure water or organic the fluctuation is minimal but around 50:50 it is about 20 bar! Now I made an isocratic method
that wants to pump 45:55 Water/ACN but the fluctuations are killing the method and disturbing the UV baseline.
In addition to the large high-frequency fluctuation there is a longer term and pretty random slope to the pressure
vs time. Any thoughts or suggestions for further experiments?

As the previous poster asked - are you using a degasser?

Whether you are or are not using a degasser, it looks like you have an air bubble stuck in the pump. A regular poster, HPLCCONSULT (I believe, but I may be mis-attributing) has often recommended priming all lines/channels with isopropanol, especially with the quaternary pump. Try to prime each degasser channel and each quaternary pump channel with isopropanol (you can attach a restriction capillary or long piece of capillary tubing after the pump with the purge valve closed to add a bit of back pressure during the purge process). After that, re-equilibrate just the water and acetonitrile channels and see if the problem is resolved. If it doesn't solve the problem, then run through the Diagnostic View in Chemstation and go through the Leak and Pressure Tests to see if you can identify a leak in your pump or system.


And if you aren't using a degasser, then either degas online with helium sparging or buy a degasser from Agilent.

hi
you can also check out the gold seal of purge-valve assembly. If it is creasy purge valve doesn't close tight enough and this causes pressure fluctuation

Or you could have a bad purge valve altogether - which would be evident when you ran the Leak or Pressure Tests (you would see isopropanol leaking from the waste port on the valve, even though it was closed). Purge the pump, re-equilibrate with your mobile phases, then go from there.

*Edit* - You could also have a bad Multi-Channel Gradient Valve. See this article from LC/GC on steps to diagnose a bad gradient proportioning valve.

http://chromatographyonline.findanalyti ... rticle.pdf

also, see John Dolan's HPLC Solutions article for more detail on the test...

http://www.sepscience.com/emails/HPLCSol74NA.pdf

And always purge ALL four channels, even if you are only using one or two.

Hmmm... I kind of shrugged mentally at some of your suggestions. Nothing explains
what I observed. I wasn't using a degasser before but set one up last night. I purged
with tons of IPA on all four channels. The problem has completely gone away.
Thanks all for your help.