I bought the product as 'Sprint 330' at a local garden store, here in Jackson, Mississippi. They sold it in 5 lb. lots, $46.00. Needless to say, it is more than I will ever use in my lifetime. I think Ciba has changed the name and is now selling it as 'librel Fe-DP'. I dissolve 1 level teaspoon in 391 cc of water to get a stock solution. Adding 1 ml of the stock solution per gallon of tank water gives you 0.3 mg of iron per liter. If you don't have a graduated cylinder for measuring water, add one level teaspoon to two cups of water. That is pretty close. Some kind of fluffy stuff likes to grow in the stock, and so I either store it in the refrigerator or heat it to boiling to sterilize it.
TMG has it also.
I'm not sure one is better than the other(Gluconate ETDA etc), the DTPA stays in solution longer and is harder for the plant to remove this stronger chelator, but I do not think it makes much difference in terms of plant growth concerning iron.
There's the arguement that gluiconate is easier for the plants to use, but it'll not last as long in solution.
I think that a high level dose rather than something that stays in soltuion longer is much more the key.
Basically pulsues of high concentration is more important than contact time in solution in driving plant growth.
There are subtle difference when you get down to the traces.
I'd not sweat it personally. Get everything else right first, then keep adding more till hit a point where adding more will not improve plant growth.
Doesn't particularly matter which supplement/chelator you use...
Some have even more subtle effects.
In other words you will get more out of maintaining a good CO2 level vs using say DTPA vs Gluconate.
You might be able to dose less frequently with DTPA vs Gluconate but both will supply the iron to the plants.
A good porous substrate with iron added will also help with certain plants.
Most good all purpose trace mixes(Plantex/Flourish, etc.) will give you a decent amount of Fe. You only have to tweak by dosing small extra amounts of Fe and checking the plant's response to learn what your particular plants will need.
I have had good success recently by dosing small amounts of traces on a daily basis and increasing my mls. as needed. So far the growth has gotten better and better. When it levels off I'll know I'm there.