Whether a particular chelating agent is useful in protecting ascorbate is rather complicated. It depends on the stability constant for a particular species like iron III but also on the redox potential of the complex. Ryuji did a lot of research on the net and recommentded that EDTA not be used. One of the problems he encountered was in finding all the redox potentials for chelating agents. The data is rather sparse. He settled on salicylic acid because it is readily available and cheap. BTW, the TEA is to complex any copper II which also catalyses the Fenton reaction.
As a source of Fe III ions may I suggest ferric ammonium sulfate and for iron II ferrous ammonium sulfate. Another posibility would be ferric nitrate. These salts are quite stable and can be obtained in high purity. Other ferric salts tend to decompose to various hydrated oxides in water. For the amounts needed for a test the ammonium ion will not be a problem.