We are currently running a C18 RP column attached to ECD (phosphate mobile phase, pH=3.5, MeOH=21%, OSA=1mM) for separation of DA and 5-HT. A negative deflection has appeared which interferes with the DA peak - does anyone know what causes this and how to avoid this in the future?

Nick: There are several possible causes for
negative peaks when using an electrochemical
detector. One of the most common causes is
from the passage of a non-conductor/poor electrolyte
through the cell. For example,
one usually sees large negative peaks at the
beginning of a run if one injects the sample
with rather large amounts of organic solvent.

Hi, Nick. What is your sample solvent? pH? as said by the 1st Anonymous sample solvent can often produce system peaks not only due to a non-conductor/poo electrolyte but also differences in ionic strength or pH. Once I changed the HClO4 I had been using for protein precipitation of plasma by TCA and it worked.

Hello to all. Since you have experience with ECD, perharps you can advise me. I am running a C18 RP column attached to an electrochemical cell to perform an oxidation(800mV) prior to fluorescence detection. Mobile phase is ammoniun phosphate pH=7.0, OSA=1.5mM (not organic modifier: The problem is that I cannot obtain a good calibration curve since the responses increase along the day (and night). Retention times are constant. I haven`t had this problem before when I used a very similar mobile phase but with a lower concentration of OSA (0.5 mM)and 4 % of acetonitrile. I think that the problem might be related to the mobile phase itself. Perhaps it affects the electrochemical cell? Any suggestions are welcomed.