How much CO2 is created during water electrolysis with a graphite anode?

I'm experimenting with water electrolysis and wanted to use non-toxic materials, so I settled on pure graphite rods for both electrodes and Epsom salt (magnesium sulfate) as the electrolyte. The system works reasonably well, but upon further reading I found that some of the $\ce {O2}$ is likely reacting with the graphite and being converted into $\ce {CO}$ and $\ce {CO2}$ at the anode.

How much $\ce {O2}$ am I probably losing to $\ce {CO}$ and $\ce {CO2}$ ? Just a small percentage, or a majority of the potential $\ce {O2}$ production? How can I tell? I'm capturing the resulting mixed gas product in soap bubbles and igniting it but it seems to burn somewhat less vigorously than I was expecting, which makes me think there is rather more $\ce {H2}$ and less $\ce {O2}$ than optimal.

• Just out of curiosity, I would love to see your experimental setup. Could you take a picture? – Martin - マーチン Dec 11 '14 at 10:23