Baseline Problem

[MJ]

Hi all,

We have a Varian Prostar HPLC and when i run a blank injection I get a saw tooth looking baseline. The pattern repeats approximately every 12 mins. Initially I thought it was a bubble in the pump but after purging everything it still appears. We have changed the pump seals and cleaned the flow cell.

I have noticed salt (from the mobile phase) on the front of the inlet valve where the rubber seal is and where the piston goes in and out. We have just ordered an inlet valve in see if this is the cause.

Does anyone know what the problem could be or things i could try to correct the problem.

MJ

[HW Mueller]

It seems unlikely that sawteeth of 12 min periodicity are due to cycles of pump malfunction. I have seen sawteeth due to air bubbles forming in the cell and suddenly being swept out when big enough. The sawtooth shape is a slow rise with a sudden drop back down. I have seen peaks several minutes wide, but 12 min? If the sawteeth don´t have the described shape maybe some fluctuation in the power supply (I have never seen this, some members have reported it before) is causing it?
Note that I was talking about dissolved air of insufficiently degassed mobile phase that outgassed in the cell. Very seldom it happened that the system syphoned in air at a badly sealing fitting.

[Anthony_Ng]

Did the sawteeth started at 12 min. and last until the analysis finished?

Does the baseline very stable during the equilibration and first 0-12 min.?

I am thinking if there is some contamination in the "blank" and sawteeth actually are peaks.

What is the size of sawteeth if you inject, say, double or triple the injection volume?

[MJ]

Not a contamination in the blank because I wash with 100% methanol at the end of my run and I get the same picture here. I dont know what the baseline is before starting as there is no realtime trace with the varian software (that i can find) however my run is 70mins and I see at least 3-4 sawteeth profiles in a uniform pattern.

Yes they start at the baseline and then slowly rise up and then a sudden drop off. Then it starts again.

We filter out mobile phase prior to use but then dont have He or anything bubbling in the mobile phase to constantly degas. I know this wont help my problem but this problem has just appearred so it might not be related to my mobile phase storage although this would not be helping my problem.

The power supply is an interesting one. Never thought of this and guess if i get a UPS i can see if the problem is resolved.

I didnt think of injecting different volumes and see if the sawteeth profile chances. I might give this a try on my next run.

[HW Mueller]

Can´t imagine that this typical outgassing (in the cell) shape is due to power surges. Try to see if vacuum degassing helps.

[Peter Apps]

Hi MJ

Check your method and instrument settings to make sure that you do not have some strange autozero event happening.

Peter

[Friederike]

Hi MJ,

I saw a similar pattern whith a detector UV-lamp which was near exhaustion.

[MJ]

The UV lamp is about 1 month old so i dont think its this but thanks for the suggestion. We cleaned up the pump valves and the problem seemed to be in some chromatograms and not in others so I think the problem must be the inlet valve which we have ordered. When it arrives we will replace it and see if it fixes the problem.

mj

[MJ]

Done a bit more investigating and it seems when we run a method with no flow the same saw tooth pattern is seen. The problem also seemed to appear when we installed a new lamp into the detection. Therefore the problem seems to point to the detector as being the cause of the saw tooth baseline. Does anyone have any suggestions on what might be the case.

Power Fluctuations might be one i guess so would a UPS fix this problem?

regards

MJ

[danko]

Do you see the same “toothâ€

[MJ]

Yes I see the same "tooth" appearance with zero flow (0ml/min) compared to 1ml/min. I get the same "tooth" appearance with and without the column. So do you think this could be air trapped in the flow cell? how do i remove it?

MJ

[danko]

My feeling is that you would go to the â€

[HW Mueller]

You would have to have a dancing air bubble in the flow cell to get sawteeth during stop flow. On the other hand I wonder that you still have this problem after one month, are the sawteeth still the same?

[danko]

If the outlet capillary is short and - more importantly - somewhat large, then it is possible that air bubbles periodically find their way into the flow cell while some liquid leaks out of the cell.
That reminds of a couple of important question: How is the detector outlet configured? Is there some backpressure on the flow cell?

Best Regards

[Kazimierz]

I had similar symptoms with Varian Prostar pump when I was remove
back pressure column from mobile-phase autflow (special device always
attached with Varian HPLC).

After again connecting this device problem was fixed. Please unsure that
backpressure is active on outflow. Other HPLC (Shimadzu, Ultimate) not
require it.