Sorry, I don't have any info as to testing for SO2. Ascorbic acid is also vitamin C, and not the same thing as tannins.
So why are you testing for sulfur dioxide? If you're testing for sulfites because of an allergy, people that are allergic to sulfites are also allergic to many other foods as well. Red Wine Headache happens because of something else in the wine. The jury is out on what causes it though. Here
is an interesting article on Red Wine Headaches, which I get if I have several glasses of red wine.