Baseline hump depending on standby time

[FSE]

Hi!

I am running a gradient from 100 % Water (C) to 100 % Methanol (A) in an Agieltn 1100 with online degasser and DAD detector. The program is:

5 min 100 % Water
35 min 100 % Methanol
60 min 100 % Methanol
70 min 100 % Water
80 min 100 % Water

at a flowrate of 0.2 ml/min.
Channel B is Dichlormethane and Channel D is Acetonitrile - they are not used during those runs. If Channel A and C were both used just before I get a nice rising baseline (approx. trapezoid) but if I do several programs (blank runs!) where there is some standbytime for channel A (C - water is still flowing) I see a "hump" in the middle of the rise of the baseline. The intensity of the hump increases with idle time of channel A whereas the width stays approx. constant. I am not using any column just a backpressure regulator! I have changed solvents and still the same. I have used No-Ox tubing for channel A but no effect. Does anybody have any experience with "dissolving" vacuum degassers or other contaminant leaching compounds in Agilent systems?
The UV spectrum of the hump is featureless and shows an absorption at low wavelength <240 nm.

[Mark Tracy]

First, confirm that the degasser is the suspect by routing the methanol around it. I have seen degassers with contamination problems, but not since the early days of the HP-1050. I have also seen where a volatile, UV-absorbing liquid in one channel of the degasser migrates to another channel by vapor diffusion, but that takes quite a while.

Have you eliminated water contamination? The symptom you are describing is more commonly caused by dirty water. Water treatment cartridges do go bad.