Agilent 1100 drifting retention times

[SDCL]

Is there any experience with retention times drifting when using an Agilent 1100 binary pump with solvent switching valve? We are running a method that we have been using without change for 10 years. Mobile phase is PO4-TEA buffer/ ACN, column is Zorbax C18. The gradient runs from 10% ACN to 70% ACN over 17 minutes, post-column (re-equilibration) time 5 minutes. We have an Agilent vacuum degasser prior to the pump.

We have recently had problems that retention times will increase on subsequent injections (to about 4 minutes late). I have changed seals, piston, outlet ball valve. Measured flow rate is correct, and the flow appears to be consistent through the inlet tube, judged by pulling up a small air bubble into the line and observing that it does not pull backwards in the line with pump stroke. Changing B side to 90% ACN/10% water seemed to correct the problem, but changing parts did not correct.

Thanks in advance!

[thohry]

You can check the proportional valve. It the retention time shifts that much, possibly it is because of the incorrect gradient.

Carry out the OQ/PV and see.

[Ryklys]

I have very similar problem on Agilent as well. But my retention times go shorter, not longer. The assay is benzoapyrene on Agilent 1100. There is up to 2 minutes drift. But I have drift in the sample only. Last standart solution retention times goes almost back to initial times. All began after column reconditioning: retention times shortened for about 3 minutes (all cycle is 42 minutes long). Nothing have changed for years before. Our HPLC is not owerloaded with activity. Quite often it stays switched off for a month.

[bisnettrj2]

You can reverse the channels (PO4/TEA on B, ACN on A - flush everything with H2O first), and then create a method that mirrors your current one. If you have a reversal of your problem (RT decreases), then I would say your B proportioning valve is faulty, and you'll need to replace the whole proportioning valve system - or switch the B channel to the C or D channel, to see if those are working well.

Another problem may be a slight degasser failure, and you're getting small bubbles in the proportioning valve on the B channel, and the end result is that you get a higher proportion of A to B in the mobile phase that makes it to the column. However, that should be visible in the lines from the degasser to the pump. Happy hunting!