Offscale peaks for DA following normal runs [June 24, 2004]
Posted: Tue Aug 24, 2004 8:21 pm
Bizarre offscale peaks for DA following normal runs
Chromatography Forum: LC Message Board: Bizarre offscale peaks for DA following normal runs
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By Maggie on Thursday, June 24, 2004 - 01:03 pm:
Hi --
I wonder if anyone has any tips on the following:
I'm analyzing dopamine using a recycled mobile phase, detector set at +650 mV, sensitivity 500 nA. I have a beautifully stable baseline, and getting a dopamine peak at approx. 6 minutes when injecting either standard (1 ng DA/ml mixed in mobile phase) or samples (taken by microdialysis from rat nucleus accumbens). Occasionally, after several injections of standards or samples in a row where the peaks are normal and consistent, suddenly I get a chromatogram that looks normal in every respect except that the peak for DA goes completely offscale for about 20 seconds (the preceding peaks might have been less than 1/5 the size). Every time this has happened the offscale peak is in exactly the same place as the DA peak -- but the last time, the offscale peak came out about 1.5 minutes earlier.
Any ideas what might be causing this?
Maggie
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By DR on Friday, June 25, 2004 - 06:10 am:
recycled mobile phase
That's the first thing I'd change. If that is of no help, that leaves:
Sample cleanup (there's always room for improvement in the cleaning of biological samples).
Switching to a gradient method (I assume that since you were recycling, you were running isocratically). Use of a gradient step at the end of the isocratic part of the run may push whatever it is off the column between injections.
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By Maggie on Monday, June 28, 2004 - 08:51 am:
Thanks, DR. I will try that. If anyone else has any suggestions that would be appreciated too!
Chromatography Forum: LC Message Board: Bizarre offscale peaks for DA following normal runs
-------------------------------------------------------------------------------------------------------
By Maggie on Thursday, June 24, 2004 - 01:03 pm:
Hi --
I wonder if anyone has any tips on the following:
I'm analyzing dopamine using a recycled mobile phase, detector set at +650 mV, sensitivity 500 nA. I have a beautifully stable baseline, and getting a dopamine peak at approx. 6 minutes when injecting either standard (1 ng DA/ml mixed in mobile phase) or samples (taken by microdialysis from rat nucleus accumbens). Occasionally, after several injections of standards or samples in a row where the peaks are normal and consistent, suddenly I get a chromatogram that looks normal in every respect except that the peak for DA goes completely offscale for about 20 seconds (the preceding peaks might have been less than 1/5 the size). Every time this has happened the offscale peak is in exactly the same place as the DA peak -- but the last time, the offscale peak came out about 1.5 minutes earlier.
Any ideas what might be causing this?
Maggie
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By DR on Friday, June 25, 2004 - 06:10 am:
recycled mobile phase
That's the first thing I'd change. If that is of no help, that leaves:
Sample cleanup (there's always room for improvement in the cleaning of biological samples).
Switching to a gradient method (I assume that since you were recycling, you were running isocratically). Use of a gradient step at the end of the isocratic part of the run may push whatever it is off the column between injections.
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By Maggie on Monday, June 28, 2004 - 08:51 am:
Thanks, DR. I will try that. If anyone else has any suggestions that would be appreciated too!