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Re: What is the cause of this baseline step
Posted: Mon Sep 12, 2011 10:05 am
by unmgvar
beside the main topic, and it is interesting,
a leak is a pressure or a flow problem or actually a combination, since in this case one does not go without the other, like in the pump seals, or maybe at the connection of the column
a bubble would be maybe a pressure problem that causes a flow problem right?
Re: What is the cause of this baseline step
Posted: Mon Sep 12, 2011 10:14 am
by danko
a bubble would be maybe a pressure problem that causes a flow problem right?
Still the other way around.
Best Regards
Re: What is the cause of this baseline step
Posted: Mon Sep 12, 2011 6:05 pm
by unmgvar
Danko can you please elaborate,
let's say we are talking on a bubble that came from one of the solvent lines and survived the degasser and moved from the proportion valve to the check valve and then.....
can you tell why that is a flow problem first in your view?
Re: What is the cause of this baseline step
Posted: Mon Sep 12, 2011 8:02 pm
by danko
Very simple, I should say: The flow is disrupted.
Unmgvar, it's in your interest not to waste your time on trying to find something to argue about.
The flow it the primary entity and the pressure is an impact.
Best Regards
Re: What is the cause of this baseline step
Posted: Tue Sep 13, 2011 6:18 am
by unmgvar
I do think that in this case you are wrong that is why I stepped in
in physics things work by combination as well
in this specific case
the flow and pressure work together. you cannot say that one is dominant over the other for this case.
i can say as well, no the pressure is disrupted, because an air bubble allows for pressure compression and this effect the flow to a halt.
true that we care in the end of the flow but it does not come to say that it is the dominant factor. hence the name is HPLC.
Re: What is the cause of this baseline step
Posted: Tue Sep 13, 2011 9:05 am
by HW Mueller
Due to lack of support by superiors, I started HPLC in the neurosurgery lab in Giessen with N2 over mobile phase pressure containers (made by myself from appropriate pipes and needle valves). Any backpressure changes changed the flows, I had to keep everything super clean to get repeatable results. Later I built in a flow regulator, that took care of this problem. As I understand this, a piston pump regulates flow, so that pressure drops out as a factor, that is apparently what Dancho is saying. Now if a faulty pump is not compensating backpressure fluctuations, such that the flow changes, the baseline does not change as sharply as in the present chromatogram, I have seen many flow change-baseline changes at the time when the pressure vessels were used.
The above looks to me as if somene/something just changed the baseline setting as someone mentioned already in the form of an attenuation change.
Re: What is the cause of this baseline step
Posted: Tue Sep 13, 2011 1:59 pm
by Mike H.
What would cause a step change in the baseline like this?
If this happen frequently at different time points, I am also think the lamp is bad.
The baseline between 3 and 7 minutes is also interesting, is this corresponding to a pressure fluctuation or is this a mixing problem?
The UV lamp failed the monthly intensity PV at the lowest wavelength, so this might be the cause. The step is random.
I'm running samples with surfactant matricies like PEG600 and sodium lauryl sulfate. I'm guessing that the early baseline between 3-7 is because of this. The noise doesn't appear in MeOH blanks so it's definitely sample related.