Strong negative deflection [August 27, 2004]

[admin]

By nick andrews on Friday, August 27, 2004 - 12:47 am:

I am using a coulometric electrochemical cell (ESA 5011) at E1 -150mV, E2 +200mV. When I inject on a dopamine standard made in mobile phase or water I get a normal looking chromatogram with a standard looking void and then elution of peak at 4 minutes. However, when I inject a biological sample (dialysate) or standard made in artificial CSF (basically NaCl, KCl, CaCl2, glucose) I get a very strong negative deflection that only returns to baseline within 6 minutes if I autozero.

Is there a way i can modify the electrochemistry (change the potentials on E1 or E2?) to prevent the trace going off scale like this?

thanks for any help
nick

[mwoodman]

Have you considered injecting the individual components to see which (or to what degree if all) causes the problem. Depending on mobile phase pH and conditions you may be upsetting the equilibrium of the column and it will take a while for the detector to recover. Presumably you could bypass the column and inject directly to the detector. A similar delay in recovering the baseline would tell you that you are actually affecting the detector cell.
Sample prep may be the best answer. If this is RP, just elute the sample thru an SPE cartridge to waste polar salts and glucose (all unretained) and elute the analytes via backflush to a vial.
Tell me more after you know if it's the column or cell that is most upset.