I am running an agilent series 1100 pump and have been having trouble with varying retention times. I was troubleshooting expecting an internally leaking check valve, etc. I finally monitored the flow directly and found that it varies by about 10% although it seems to stay consistent during a run. The flow is set at 1.0 ml/min for this method but often is GREATER than 1 ml/min. It is always correct at the first injection after the pump is turned on. Subsequent runs have have a flow rate that is often above the set rate. Any suggestion?

Time to have that puppy serviced!

Seriously, I don't have any hands-on experience with the 1100, but a number of pumps monitor the back pressure and adjust the flow rate to compensate for solvent compressibility. A malfunction in that part of the system would be consistent with what you're seeing.

If you also have trouble with your baseline this could point towards a problem with compressibility or pressure ripple calibration, check valves or pulse dampener.You should also check pump seals.

Did you check your degasser unit? The low pressure gradient system is very sensitive towards gas in the system.

I`m not sure about the Agilent pump, but another possibility could be trouble with the flow sensor. We had a problem with a Dionex pump where an oil drop smeared over the optical sensor.

I have found the source of the problem. I should start by adding some information. The agilent pump is part of an LC-NMR system and is being controlled by Bruker HyStar software. It turns out that the HyStar application software is not always keeping time correctly. The HyStar 'clock' runs slow by as much as 10%. Because of the way the HyStar controls the pump this results in the gradient being run on a normal timescale but the response from the detector being plotted on an incorrect (intermittently) time axis as recorded by the HyStar software.