What precautions should I take when measuring in the reductive mode?
Reduction of oxygen causes an unacceptable high background current in reductive amperometric detection. This high current is caused by presence of oxygen in the mobile phase.
Therefore, the mobile phase and the sample must be virtually oxygen free. This is done by:
- Degassing of mobile phase using helium of the highest purity
- Degassing of sample using helium or argon of the highest purity
- Applying stainless steel tubing throughout the system. Only a small piece of PEEK tubing is used between column and flow cell to avoid grounded tubing to come close to the working electrode.
- The lowest oxygen levels are attained if the mobile phase is circulated. In that case PEEK tubing must be used instead of the oxygen-transparent Teflon. Stainless steel instead of PEEK may induce ground loops, leading to spikes in the chromatogram.
An example is the detection of artemether at -1 Volt vs. Ag/AgCl using a VT-03 flow cell with 2.8 mm glassy carbon working electrode. After thorough degassing the cell current must be lower than 60 nA. NB: A vacuum degasser is not effective enough for such applications!