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By JA on Monday, August 23, 2004 - 02:30 pm:
Using a high-pressure binary pump and PDA system, we've recently noticed it consistently produces "wavey baselines" during all runs, complicating sample analysis to the point of causing system downtime
The mobile phase is a simple H2O/ACN mixture, with the sinusoidal (i'll call it that but it isn't a pure sine wave) baseline period decreasing with increasing ACN composition from Pump B.
Isocratically, there is no abnormal pressure fluctuation (delta < 20 PSI over 10 mins at 900 PSI backpressure) and the baseline is ideal when running 100% A or 100% B on their own.
We have a seperate low-pressure mixing system with an identical PDA connected to the same electrical supply trunk in the lab which is displaying no such problem.
Until I ran the two eluents separately I was thinking it was a check valve issue.. although there are no visible leaks, and no problem with the automated Leak Tests. I have yet to confirm the actual flow rate output from each Pumphead pair (it's a Waters 2525).
Since it's high-pressure mixing we don't degass the eluents. If it was outgassing at the PDA with 50:50 A/B could I confirm this by comparing with a 50:50 premixed MP (not degassed) run from a single Pump?
Ideas gratefully received..
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By A.Mouse on Monday, August 23, 2004 - 02:38 pm:
You should still degas the solvents, especially the water line. It can be done quickly (1 minute or so) with vacuum. If this does not make the problem go away, add a small percentage (5%) of acetonitrile to the water and water to the acetonitrile and degas.
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By Ann on Tuesday, August 24, 2004 - 04:03 am:
What detection wavelength are you using? If down in the far UV region, I bet you are seeing mixing noise. Is the wavy baseline still evident at a higher wavelength?
Ann
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By Consumer Products Guy on Tuesday, August 24, 2004 - 07:51 am:
Turn off pump, see if baseline stabilizes; if so, its a pump/solvent issue. If baseline doesn't stabilize, check out detector. Isolate the problem first, then solve. Like A. Mouse stated,I always de-gas solvents.
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By JA on Tuesday, August 24, 2004 - 10:35 am:
I took the two eluents and degassed them as A.Mouse suggested, although all I could manage was to do it by sonicating for 15-20 mins.
Do you apply vacuum directly to the mobile phase container Mr Mouse?.. we have a setup for filtering under vacuum but it uses it's own ground-glass fitted conical flask and filter adapter. I'm trying to think how would I degas using vacuum on a 5, 10 or 20 litre Duran bottle which we commonly use to hold eluents?
Anyhow, degassing solved the baseline problem, and it could be reproduced by putting the non-degassed eluents back on the system.
Also worth noting was that we had some orange (0.020") PEEK tubing from the PDA outlet to drain and when replacing it with yellow (0.007") we can run the non-degassed eluents without problems.
Thanks for the help
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By A on Tuesday, August 24, 2004 - 11:47 am:
Depending on the flow rate, the flow cell (standard, prep), and the length of that peice of 0.007" tubing, you may be putting that flow cell at risk due to backpressure caused by the outlet tubing. You do not want to blow that cell. Be cautious.
Regarding degassing, you might want to think about investing in an in-line degasser or using a He sparge for those big bottles.
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By A.Mouse on Tuesday, August 24, 2004 - 03:19 pm:
We have a rubber stopper that is larger than the neck of the solvent bottle. There is a short piece of metal (or glass) tubing going through it. We connect this to the vaccum line at the fosset, and we put the solvent bottle into the sonic bath. You turn on the sonic bath and the vacuum, then you put the rubber stopper on the solvent bottle. You get a lot of bubbels for maybe half a minute, and then much less bubbles. After about 1 minute, we declare that the solvent is degassed and use it. No problem!
Don't put it into the sonic bath for a long time, since it will heat up. Also, if you use solvent mixtures, this short degassing works, and you do not change the solvent composition.
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By JA on Tuesday, August 24, 2004 - 03:56 pm:
:thumbs up:
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By HW Mueller on Tuesday, August 24, 2004 - 11:23 pm:
Vacuum on a 20L bottle?? You may get an implosion unless it´s specifically manufactured for vacuum. You can use a desicator, put opened bottles with your mobile phase inside and evacuate.
Using a high-pressure binary pump and PDA system, we've recently noticed it consistently produces "wavey baselines" during all runs, complicating sample analysis to the point of causing system downtime
The mobile phase is a simple H2O/ACN mixture, with the sinusoidal (i'll call it that but it isn't a pure sine wave) baseline period decreasing with increasing ACN composition from Pump B.
Isocratically, there is no abnormal pressure fluctuation (delta < 20 PSI over 10 mins at 900 PSI backpressure) and the baseline is ideal when running 100% A or 100% B on their own.
We have a seperate low-pressure mixing system with an identical PDA connected to the same electrical supply trunk in the lab which is displaying no such problem.
Until I ran the two eluents separately I was thinking it was a check valve issue.. although there are no visible leaks, and no problem with the automated Leak Tests. I have yet to confirm the actual flow rate output from each Pumphead pair (it's a Waters 2525).
Since it's high-pressure mixing we don't degass the eluents. If it was outgassing at the PDA with 50:50 A/B could I confirm this by comparing with a 50:50 premixed MP (not degassed) run from a single Pump?
Ideas gratefully received..
-------------------------------------------------------------------------------------------------------
By A.Mouse on Monday, August 23, 2004 - 02:38 pm:
You should still degas the solvents, especially the water line. It can be done quickly (1 minute or so) with vacuum. If this does not make the problem go away, add a small percentage (5%) of acetonitrile to the water and water to the acetonitrile and degas.
-------------------------------------------------------------------------------------------------------
By Ann on Tuesday, August 24, 2004 - 04:03 am:
What detection wavelength are you using? If down in the far UV region, I bet you are seeing mixing noise. Is the wavy baseline still evident at a higher wavelength?
Ann
-------------------------------------------------------------------------------------------------------
By Consumer Products Guy on Tuesday, August 24, 2004 - 07:51 am:
Turn off pump, see if baseline stabilizes; if so, its a pump/solvent issue. If baseline doesn't stabilize, check out detector. Isolate the problem first, then solve. Like A. Mouse stated,I always de-gas solvents.
-------------------------------------------------------------------------------------------------------
By JA on Tuesday, August 24, 2004 - 10:35 am:
I took the two eluents and degassed them as A.Mouse suggested, although all I could manage was to do it by sonicating for 15-20 mins.
Do you apply vacuum directly to the mobile phase container Mr Mouse?.. we have a setup for filtering under vacuum but it uses it's own ground-glass fitted conical flask and filter adapter. I'm trying to think how would I degas using vacuum on a 5, 10 or 20 litre Duran bottle which we commonly use to hold eluents?
Anyhow, degassing solved the baseline problem, and it could be reproduced by putting the non-degassed eluents back on the system.
Also worth noting was that we had some orange (0.020") PEEK tubing from the PDA outlet to drain and when replacing it with yellow (0.007") we can run the non-degassed eluents without problems.
Thanks for the help
-------------------------------------------------------------------------------------------------------
By A on Tuesday, August 24, 2004 - 11:47 am:
Depending on the flow rate, the flow cell (standard, prep), and the length of that peice of 0.007" tubing, you may be putting that flow cell at risk due to backpressure caused by the outlet tubing. You do not want to blow that cell. Be cautious.
Regarding degassing, you might want to think about investing in an in-line degasser or using a He sparge for those big bottles.
-------------------------------------------------------------------------------------------------------
By A.Mouse on Tuesday, August 24, 2004 - 03:19 pm:
We have a rubber stopper that is larger than the neck of the solvent bottle. There is a short piece of metal (or glass) tubing going through it. We connect this to the vaccum line at the fosset, and we put the solvent bottle into the sonic bath. You turn on the sonic bath and the vacuum, then you put the rubber stopper on the solvent bottle. You get a lot of bubbels for maybe half a minute, and then much less bubbles. After about 1 minute, we declare that the solvent is degassed and use it. No problem!
Don't put it into the sonic bath for a long time, since it will heat up. Also, if you use solvent mixtures, this short degassing works, and you do not change the solvent composition.
-------------------------------------------------------------------------------------------------------
By JA on Tuesday, August 24, 2004 - 03:56 pm:
:thumbs up:
-------------------------------------------------------------------------------------------------------
By HW Mueller on Tuesday, August 24, 2004 - 11:23 pm:
Vacuum on a 20L bottle?? You may get an implosion unless it´s specifically manufactured for vacuum. You can use a desicator, put opened bottles with your mobile phase inside and evacuate.
