Pressure drop after bypass-to-mainpass switch/injection

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

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First time poster. Just trying to troubleshoot a pressure drop on an Agilent 1100.

With the quaternary pump on (1 mL/min) I can switch between mainpass and bypass just fine, with just a small pressure drop (15 bar with C18 column attached) when going from bypass to mainpass which recovers within about 10 secs. However, sporadically this pressure drop increases to >50 bar during injections (immediately after switch from bypass back to mainpass) which recovers within about 30 secs, with concomitant loss of sample peak size. I can't see any sign of leaks, and pressure test (in bypass) is fine.

Things I've tried:
1) Removing 900uL seat extension (the pressure drop coincided with dripping out of the waste port throughout the injection cycle)
2) Replacing rotor seal and stator face, and replaced analytical head (with a used one)

Everything was back to normal for a few days, but it's happening again (even without the seat extension) with increasing frequency. There must be a leak somewhere (the waste port is fine and only drips as analytical head positions itself to aspirate), but no signs of a leak at the 6-port valve or the analytical head.

Any suggestions appreciated.
Probably not a leak, but not enough information provided to be sure. It sounds like you are using the 900 ul A/I head with the 900 ul loop kit and software set to the 900 ul option in the A/I (if not, then we have another problem). Depending on the flow rate (1.00 mL/min), the mobile phase composition (?), the liquid composition inside the A/I at the time you switch (it it has not had time to clear when you make the switch, then the pressure could vary due to composition changes or lack of equilibration. If everything is equilibrated and you allow enough time for the two flow paths to equilibrate (then they are 100% the same), then the only pressure change which should be observed would be related to the pressure difference between the two flow paths (one with the giant 900 ul loop filled with liquid in-line, the other without it).

If the pressure drop under identical conditions varies widely (and the system IS equilbrated and flushed properly), then a partially plugged seat, bad injector rotor seal, and/or an air bubble (from poorly degassed MP) may be the cause.


Related article: "Backpressure Changes, Pressure Drop from HPLC Tubing Selection (0.007, 0.005, 0.010") "; https://hplctips.blogspot.com/2019/06/b ... -from.html
One more thing that it can be when using large injections like this, especially if you are drawing from the vial multiple times to reach the injection volume being used. The short piece of clear tubing that comes from the valve, it has to be attached to the post beside the valve, there is a thin slot in the post, which the line will slide into from the top.

The line is the waste line from the large loop, and the mobile phase that is displaced by the multiple injections into the loop exits there and drips down the post and into the waste line. If you do not put the short line into that groove and instead attach it to a longer line going to a waste container, it will create a siphon that will draw air into the loop between injections causing air bubbles to be injected thus causing the pressure drop on valve switch. (Don't ask me how I know that :roll: ).
The past is there to guide us into the future, not to dwell in.
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