I would like to oxidize flavins and if possible also quinones in bacterial supernatant. Is there an enzyme one could use for that? Or any other "oxidant"?
If you are growing your cells aerobically i would expect most if not all of your extracellular flavin to be oxidized already. If you have a specific reason to suspect significant accumulation of reduced species, perhaps adding a small amount of ferricyanide would be sufficient to ensure its oxidation, though i cannot say how it might affect your bacteria.
as Silas wrote, oygen should do it, but if there is no oxygen or you want to use it for something else (cell growth?) you should concider electrochemial oxidation. Cheap, easy and as it seems like you have many different substances it could be used to oxidise them all.
now my issue is that I have reducing species in my supernatant and am trying to find out their identity. Now the problem with FeCl3 and oxygen is that they are not specific to oxidation of flavins. Is there something more specific?
Electrochemical oxidation works but I get very low current and therefore no peaks on CV that could be used for compound identification.