Chromatography Forum

I get a drift in both curcumstances.. with a column and with a capillary instead of the column...

I use a aminophase column BioRad Aminex HP-87H with a guard column..

- Hubert

So it´s not the column, is the drift the same when you stop flow? What I am saying is that you have to isolate one possible source after the other. Degassing, flow problems, detector electronics, funny stuff like the leak mentioned above still come to mind. One can use H2O and a capillary for these checks.

I wouldn't guess that it is a temp. problem as the heat exchanger on the actual detector cell is very efficient for this Agilent model. (It'll take a good 8 hours or more for the detector flow cell to get back to ambient conditions if turned off from 50°C!).

Have you throughly flushed both your detector and reference cells with mobile phase? Consult your handbook to do this if your not sure.

One last test: what does the baseline look like under "no-flow" conditions? And how does that compare with the manufacturer's spec?

The idea here is that if you have the drift without flowing solvent, that eliminates all the column oven, column bleed, solvent, etc. possibilities and points toward a problem with the detector itself. If no drift under static conditions, then we at least know the detector's OK and can go back to looking at the upstream part of the system. (yes, I'm getting desperate here!)

Hi Folks,

Thanks for the info... I have flushed the detector many times.. and everytime it is flushing, the baseline is nice and level.. as soon as the purge is over.. drift again...

I ran the machine with no flow through it, pump off, column temp off.. And here are the results.. as you can see.. the drift is still there.... what does this mean?? Could it be detector related??



Thanks for any info you may have...

Thanks
- Hubert

Well, if you're still getting drift with no flow, that certainly suggests that the detector is the culprit. I can't be more specific than that, but at this point I think it would be worth getting a factory service person out to check out the detector.

I get the same type of drift from time to time with my Agilent RI detector. Our system runs a volitile solvent so it seems to be caused by eveaporation and/or temp. flucuation.

If your RI chamber is set at a high temp, that could cause it. Plus the usual Tech service accusations (is it near a vent, etc.)

You could also have a micro-leak in the ref. cell. I would have tech service check it out.

Hi everyone...

The saga continues...

The Agilent rep replaced the RI optics.. I calibated it and so far it looks like it is much better.. there is still a small negative drift, but much less than before.. the Agilent QC paper showed a drift of -90 nRIU' / hour and it is showing about that...

The temps are set to max.. the column compartment is 65 C and the RI optics are at 50 C (I get the best seperatons this way).. it's not near a vent and I don't see temp fluctuations on a digital temp I put in the RI case..

I have a hope that this new RI will clear things up and I'll be good to go...

One more thing... what kind of reproducibility should I be expecting from the RI? I'm looking at the results from run-to-run.. what is the accepted % change that's OK?

- Hubert

My experience with RI has been that reproduction can be problematic. We are using it for GPC so its not as much an issue. But I have read that using an RI for quantification can be tricky.

I am running the same setup in my lab, but my mobile phase is 6mM sulfuric acid - I believe you said yours was 50mM?

I've been happy with my quantitation with RiD - so long as I give the system 2-3 days to thermally equilibrate before injecting samples (ie, let it warm up over the weekend, then it's ready by Monday morning).

Hi again,

I am running a 10 mM sulfuric acid mobile phase (0.5 mL H2SO4 in 1 L dH2O).

I have been letting the system stabilize for many hours and the problem always persisted... I am trying to get the best reproducibility I can as small changes in some compound concentrations can mean large changes in my processes, and I need to be aware of them... what is the best way to ensure my results are reproducible?

Anyone???

Thanks
- Hubert

Hi Everyone,

It looks like the issue has been finally solved.. Agilent replaced the RI optics, and now I am getting much better results.. the drift is still there ~ -90 nRIU/hr, but it doesn't seem to be interfearing with the analysis runs..

Woohooo.. I am so happy...

Thanks to everyone for their hel and ideas..

- Hubert