I'm not PHd chemist, but... DMSA's chemical formula is:

HO2CCH(SH)CH(SH)CO2H

Doesn't seem to be any "Hg" atoms in there. :-) I don't think you need to worry about Hg being in DMSA. While I'd like to take credit for this discovery, I have to thank wikipedia. http://en.wikipedia.org/wiki/Dimercaptosuccinic_acid