Pressure fluctuations

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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We are entering an Agilent 1100 HPLC into service. All new capillaries have been used through the system. The vacuum degasser had the vacuum tubes replaced. The pump was rebuilt with new seals in the pistons, inlet, outlet, and purge valves. The auto sampler analytical head assembly and injection valve assembly were also rebuilt. The system passes pressure tests. Despite these efforts, there are still pressure fluctuations to be seen. Most difficult is that these pressure fluctuations are not constant. There are periods where the pressure runs stable (~+/-1%) but then vary by 50%. The analysis is to be isocratic with 85% Acetonitrile and 15% Water so changes in composition are not responsible. The auto sampler was bypassed and the same fluctuations can be seen. This is a quaternary pump system and purging each leg for over 20 minutes each does not have a consistent impact.

The most significant impact to the stability of the system was replacing the vacuum tubes which makes this component an area of interest. According to the documentation the pump impedance is to be in the KOhm range which it is, 33 KOhm was measured. The operating voltage of the pump is 24V without any tolerance, but the voltage measured by the controller is only 22.4V. Not wanting to drop hundreds of dollars on a new controller, we are looking for some advice form this forum. Any help is appreciated.
Go to your local 'Radio Shack or equivalent' and buy a surge protector. It seems you have a 'poor quality' electrical supply. Where are you located?
Let's take a different approach after that if pressure still fluctuating.

Put the same solvent like IPA or a pre-mixed solvent in all four reservoirs. Purge all four lines at least 10 minutes at 5ml/min. If there are no pressure fluctuations doing this then the pump head itself is OK, then suspect the multi-channel mixing valve.

Obviously, check for leaks, even at the outlet of the purge valve and up underneath the pump head itself (use a mirror).
That's outside the box. I can check the line voltage under load.
Consumer Products Guy wrote:
Let's take a different approach after that if pressure still fluctuating.

Put the same solvent like IPA or a pre-mixed solvent in all four reservoirs. Purge all four lines at least 10 minutes at 5ml/min. If there are no pressure fluctuations doing this then the pump head itself is OK, then suspect the multi-channel mixing valve.

Obviously, check for leaks, even at the outlet of the purge valve and up underneath the pump head itself (use a mirror).


An added check on the mixing valve would be to put the lines into four 100ml graduated cylinders each containing 100ml IPA, then run 1ml/min of 25% of all four channels for an hour with the purge valve open. There should be 85ml remaining in all the cylinders, if not then one or more channels is not flowing as it should be.
The past is there to guide us into the future, not to dwell in.
This is a good idea to use IPA. In rare cases, I have seen piston decoupling issues (although, not on an agilent) but the issue was resolved by using high flow rate of 5ml/min IPA for 20 minutes on each line. remove column, just run pump out to waste at 5 ml/min.
Were the new seals worn-in with IPA at 350 bar minimum?
If your using buffers, be sure to rinse with water as seal wash.
Changing the autosampler parts should have no impact on pump pressure

Is your pressure fluctuation cyclical?
im not too concerned about the voltage, if you had a 24V power supply issue, it would be obvious.
normally I tackle pressure issues in this order:
check valves (OBV)
pump seals
AIV cartridge, AIV
usually mcgv is bad when new seals wont resolve retention time shift issues
rare cases: new pump head, deagsser


You can also remove the pump head and turn on pump and watch the pistons move and compare visually if there is an irregular pattern at different flow rates



Consumer Products Guy wrote:
Let's take a different approach after that if pressure still fluctuating.

Put the same solvent like IPA or a pre-mixed solvent in all four reservoirs. Purge all four lines at least 10 minutes at 5ml/min. If there are no pressure fluctuations doing this then the pump head itself is OK, then suspect the multi-channel mixing valve.

Obviously, check for leaks, even at the outlet of the purge valve and up underneath the pump head itself (use a mirror).
Try replacing the Pump's PTFE frit inside the Prime purge valve. This filter should be replaced EVERY month (very inexpensive) and protects the flow path from seal debris and large particulate matter. When it gets clogged up, the pressure will fluctuate. The solvent pickup filters (in the bottles) also clog over time. Replace them.

Also check that your HPLC degasser is operating perfectly. They require full service ~ 5-6 years as most of the parts inside wear out. When the membrane tubing leaks, it contaminates the mobile phase resulting in all kinds of problems.

Lastly, check the basics such as connections, leaks, pump compressibility settings (which will also cause fluctuations is set wrong).

Here are some links to help you:
"Diagnosing & Troubleshooting HPLC Pressure Fluctuation Problems (Unstable Baseline) "; https://hplctips.blogspot.com/2014/01/d ... -hplc.html

"HPLC PUMP SOLVENT COMPRESSIBILITY VALUES"; https://hplctips.blogspot.com/2011/10/h ... ility.html

"INLINE HPLC DEGASSING MODULES"; https://hplctips.blogspot.com/2011/02/i ... dules.html
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