Chromatography Forum

HWM,

Wayne seems to have followed everything you suggested but the problem still seems to be there. Furthermore, in my mind, the likelihood of a gas/slow outgassing problems decreased from the moment he used 100% water as mobile phase and at the same time degassed his mobile phase (100% water have much less degassing problems than a water/ACN mobile phase...).

Einar seems to indicate that you can have sudden drops as well in addition to the drifting if you have lump or electrical problems with your UV. A change of the UV would be a straightforward way to know if the detector is to blame or not. Also I asked about the UV length as UV lamps at the end of their life are more trouble to use at low wavelengths (i.e. 190 nm) than higher wavelengths.

What Chris suggested is also a possibility and the quick fix is to use a longer or thinner diameter tube at the outlet of the UV cell (that goes to the waste). Another way is to buy commercial calibrated backpressure columns (from upchurch for example) of 50 psi or equivalent. However, among all the cases that I have seen this problem, the side effects were spiked peaks in the UV chromatogram... but again I do not eliminate completly the possibility...

Kostas,
well, I just thought it was appropriate to point out that Einar´s suggestion to see whether the pattern continues after stop flow was also straightforward. Also, leaks are not always easy to handle.
Now this water/air stuff is strange. Nonpolar gases (most of air) are less soluble in H2O than in organics. I have a part in my system which is particularily prone to holding and slowly releasing gases. These gases are flushed out in seconds with MeOH or MeOH/water as compared to tens of minutes with H2O. I use MeOH to remove gases faster from columns that have been "gassed".

Thanks a lot for all of your suggestions. I learnt a lot from the disscussion.

Finally, my problem was fixed.

After I flushed it with 100% methanol and then 100% Acetonitrile , a huge and broad peak came out. Then the baseline was stable. I believe the problem is due to a supper dirty system ( it was vacant there for a long time).

Thanks again.

Strange, Wayne, didn´t you say that you had the "elongated sawtooth" (that updrift, sudden downfall..) without a column also? Maybe it was an air problem after all, now you got it completely out of your entire system with the MeOH and ACN???