pressure cycling?

[pdchemist15]

Having issues with instrument pressure cycling. The pressure increases and decreases approximately 20-30 psi every 10-12 minutes. This is using degassed HPLC methanol. The pressure has been monitored with 2 different columns attached. Check valves and in-line filters have been replaced. Delivered volumes are precise. Column is inside heater chamber (ambient and set to 22.0 C).
Any ideas? Any thoughts on pressure transducer? THis is a waters alliance 2695 instrument.

Thanks

[juddc]

What's your baseline backpressure? 1000 psi? 2000psi?

20-30 psi isn't really that big a deal, but I'd guess that you're actually seeing small variations in column temperature because the column oven isn't really good at maintaining temeratures precisely within 5 deg or so of ambient. Try repeating the experiment at 30-35 deg C, let the unit equilibrate for a good while (at least 45 min), and let us know what's going on then.

[pdchemist15]

BP= ~1650

It doesn't seem like much but it wreaks havoc on my analysis. I am quantitating three analytes. 2 large peaks and 1 very small peak(~140,000 response). The small quantity RSD's are affected by the change in the pressure which affects my absorbance baseline.
Prior to pressure issues the trace would have minor fluctuations..yes I would have the delta psi differences but the overall pressure would remain constant within 5 psi range through out a 20 minute run...This was run outside the heater chamber in the open lab..but maybe the weather has changed the dynamics of this analysis.
I'll do what you suggested and hopefully it will confirm what you are suggesting...

[juddc]

Just curious - what's your detector? 2487, 996, 2996, 2998? The reason I ask is that I've found the 2998 to be more sensitive to variations in ambient humidity than the 996 or 2996 and I'm wondering if you might be seeing that.

140,000 is small but not that small...unless it's broad peak. I wouldn't expect hat kind of pressure variation to bother the analysis much, but I've seen stranger things...

[pdchemist15]

Well..the column heater did the trick....noticed the LC is under a duct vent...maybe due to the weather change the cycling of the room heating and cooling system through the day is the cause for the change in dynamics...Now to set the system up with my method and MP and adjust my column heater settings...

Thanks for your help

Also its a 2996 and it is a broad peak

[kamel]

I am using shimatzu LC, I tried sucssefully to solve problem like yours (pressure fluctuation)

baffer salts can causes mechanism jam.
This problem appears because of buffer salts..Check one of two head pump, you can have pressure cycling if one pump does not run perfectly ( I mean pump with piston and small ball..Put out pump head and flush the inlet valve with solvent..or replace inlet seal pump.
I think it can be the solution.