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

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.

Thanks, DR. I will try that. If anyone else has any suggestions that would be appreciated too!