Positive baseline drift issue

[sgmOPUS]

Hello everyone! I'm having some issues with my Shimadzu Prominence reverse phase HPLC. When it is set up in the analytic mode it has NO problems, but when the preparative cell is inserted in the PDA detector and the signal is plotted the baseline never stabilizes with a POSITIVE baseline drift of about 4 mAu/min. This behavior is independent of the solvent used (methanol/water and acetonitrile/water mixtures were used with NO gradients), the flow or even the nature or presence of the column (the issue persists with C8, C18, 150 and 250 mm long columns, and EVEN with NO column). The data aquisition system has been recalibrated several times using th software provided and the cell has been cleaned with pure methanol and isopropanol several times. Solvents used were all HPLC grade. I'd like to know if there's anything I'm missing in order to solve this problem. I really apreciate your answer!

Thank you!

[silmartras]

Are you using two channels for the mobile phase? You can try to use only one channel with the mix and view if the problem also occurs.

[sgmOPUS]

Are you using two channels for the mobile phase? You can try to use only one channel with the mix and view if the problem also occurs.

I'll try that! Thank you so much!

[sgmOPUS]

Are you using two channels for the mobile phase? You can try to use only one channel with the mix and view if the problem also occurs.

I've used pure methanol at pump's channel B and the problem persists. Any other suggestion?

[silmartras]

Really not. You can check if the problem persists using water and the channel B, and ckeck again channel A with methanol and water, in order to investigated if you have an organic impurity that is dissolved with organic solvents. But perhaps is a problem in the detector. It is possible have a problem with the sensitivity of the detector? To find more possibilities you can read the trobleshooting guide of Thermo that you can find in internet in pdf format (it's free).

Good luck

[sgmOPUS]

Really not. You can check if the problem persists using water and the channel B, and ckeck again channel A with methanol and water, in order to investigated if you have an organic impurity that is dissolved with organic solvents. But perhaps is a problem in the detector. It is possible have a problem with the sensitivity of the detector? To find more possibilities you can read the trobleshooting guide of Thermo that you can find in internet in pdf format (it's free).

Good luck

I'm pretty sure it's a problem in the preparative cell now, but I really appreciate your advice. Thank you for your time.

Santiago

[danko]

Set the flow rate to zero (no gradient either) and plot the signal over la onger time - say an hour.
The come back and tell us what you saw.

Best Regards

[sgmOPUS]

Set the flow rate to zero (no gradient either) and plot the signal over la onger time - say an hour.
The come back and tell us what you saw.

Best Regards

With no flow the problem persists but the slope decreases to 2 mAU/min aprox.

[danko]

OK, I’m not completely sure, but I think you are dealing with tiny air bubbles entering the flow-cell from the outlet, which is most probably of a larger diameter, compared to that of the analytical.
To solve the problem (if it is the problem, so you’ll have to do the test) try and create some backpressure in the flow-cell by mounting a flow-restrictor or something like that in the flow-path/on the outlet. Make sure the pressure does not exceed the pressure tolerance of flow-cell (to be found in the documentation it came with or the manufacturer’s website).

Best Regards

[danko]

Actally it is even more probable that the tiny air bubbles are released from the mobile phase when it enters the flow-cell. The cure is the same as described in my previous post.
In addition it might help significantly if you degass your mobile phase - with Helium perhaps.

best Regards

[sgmOPUS]

Actally it is even more probable that the tiny air bubbles are released from the mobile phase when it enters the flow-cell. The cure is the same as described in my previous post.
In addition it might help significantly if you degass your mobile phase - with Helium perhaps.

best Regards

I already tried pumping several kind of solvents through the cell with a syringe forward and backwards the flow direction. I think it would have taken out bubbles. In other hand, bubbles tend to make some kind of "waving" (ups and downs)on signal but not positive slopes, don't they?. And yes, we are truly considering to add a degasser to the line.

[danko]

I'm not thinking of air pockets (as opposed to microscopic air bubbles released when the pressure changes from high (in the column) to low in the flow-cell) This kind of air bubbles cause absorbance raise – due to light scattering in different directions and thus less light reaches the PMT or PDA.

This microscopic air bubbles will probably end up forming bigger air pockets if they're not moving along the flow which they usually do in larger flow-cells.

All of the above is almost eliminated when pressure is applied on the flow-cell. And a degassing the mobile phase (effectively) is always a good idea.

Best Regards

[sgmOPUS]

I'm not thinking of air pockets (as opposed to microscopic air bubbles released when the pressure changes from high (in the column) to low in the flow-cell) This kind of air bubbles cause absorbance raise – due to light scattering in different directions and thus less light reaches the PMT or PDA.

This microscopic air bubbles will probably end up forming bigger air pockets if they're not moving along the flow which they usually do in larger flow-cells.

All of the above is almost eliminated when pressure is applied on the flow-cell. And a degassing the mobile phase (effectively) is always a good idea.

Best Regards

Thank you very much Danko!