I have been performing redox sensing tests with bismuth oxychloride nanoflowers coated on a GCE for lead and zinc analytes in DI solution. The nanoflowers, mixed with Nafion, are applied to the GCE surface and paired with a Pt counter electrode and an Ag/AgCl reference electrode. A water baseline is taken, and then the heavy metal analyte is added, dropwise, 10 1 ppb, then 10 ppb, then 100 ppb. 3 CV scans are performed per concentration, for a total of 12 scans per test. The oxidation currents are fairly consistent between tests, with predictable increases with increasing concentration.

All that said, my question is about the seemingly unpredictable oxidation potentials between tests. The oxidation potentials go from positive (see attachment) to negative between tests with no apparent consistency. The oxidation current is not affected by the shift in potential (see attachment). Please help me understand how this phenomenon comes to exist, and help me understand why this isn't random, but must be due to an underlying and as yet unidentified (by me) reason.

Thank you.