What could be causing my cyclic voltammograms to have a sloped scan?

[![ol](https://i.stack.imgur.com/Pg5Bb.png)](https://i.stack.imgur.com/Pg5Bb.png)


I am running a CV (cyclic voltammogram) in a two compartment bulk electrolysis cell with pH 7 phosphate buffer (0.5 ion concentration) and a napheon membrane separating the two compartments. I'm using a platinum microelectrode with a Ag/AgCl psudo reference electrode and a large platinum flag counter electrode. Currently I am running the setup all in one side of the two compartment cell in order to isolate the distance issue. 
I took a first order derivative to show the slope of the line is rather constant.


[![enter image description here](https://i.stack.imgur.com/nsvll.png)](https://i.stack.imgur.com/nsvll.png)


This was a background taken, after degassing with both stirring and helium sparging, using a polished 25 micron platinum microelectrode. Any ideas what could be causing my background scan to be a literal diagonal line, rather than flat like a regular baseline? 


