Chromatography Forum

Hey guys, I've been having a problem with my HP1100 and can't seem to find the solution. We just replaced the arm on the autosampler and it seems that most of the major problems started after that. We had a major shift in retention time and found that we had a problem with the flow rate. We used the grad cylinder and stopwatch to find this solution. We also had fluctuating pressure on the column. We took the pump out and check all the valves and seals and didn’t find a problem with them. We did fix the retention time shift in this process; we were getting air in the line which was causing the change in flow rate. This however did not fix the fluctuating pressure problem. We checked and there are no leaks or clogs in the system and also have noticed an unstable signal from the detector. Not sure where else to look to find the problem. We have found that the pressure fluctuating stops for a short period of time after we take the pump out and put it back online. Anyone have any ideas for what the problem might be.
Thanks!

Some possibilities:

- bad check valve (I know you checked, but I would replace them just to be sure)
- a problem with the pressure transducer or the compressibility compensation settings (I'm not 100% sure, but I think the 1100 adjusts the actual flow rate in response to changes in pressure).
- a small leak in the inlet line or degasser allowing a bit of air to be aspirated

If the problems arose after the instrument was being repaired, as well as the above, look for partially blocked solvent or pump inlet filters.

Bruce Hamilton

I think that 1) pump issues are responsible for pressure fluctuations & wandering flow rates 2) air can be introduced by the autosampler if the little piece of tubing is not clamped in the correct position (plastic overflow tube that can also leak a drop or two during inject cycles). That tube is supposed to allow pressure equalization between the vial and other end of the sample loop (if memory serves...) but it can misbehave and siphon air into or MP out of the loop's side of the solvent path. Excessive dripping and poor RSDs prompt me to check the position of the end of that tube. Move up a bit if leaking, down if sucking in air...

"We did fix the retention time shit in this process"

Unfortunate spelling mistake, but it made me smile

If you are using a gradient pump, a partially blocked gradient valve could also be the cause. The valve could get blocked due to fungi or polymers from acetonitrile. You may change the solvent to each of the 4 channels and observe the pump pressure to verify the problem.