Which kind of ATP do you usually use in you experiment? - (Sep/04/2012 )

Hi, everyone!
I need to order ATP to test the activity of an ATPase. However, I cannot find pure ATP which is not complex with sodium or magnesium in the market. Most of ATP sold in the market is in their salt form. I don't know why. Maybe ATP salt is more stable than free ATP. I want to know which kind of ATP do you usually choose for enzyme activity assay----ATP disodium salt, ATP magnesium salt or free ATP complex with no metal ion? If I want to use free ATP, where can I order this kind of ATP?
Thank you very much!

Basically ATP will not crystallize without a salt component. I don't think you can buy free ATP. The sort you chose depends on whether you want Na or Mg in your solution...

OK! I think I need to order ATP disodium salt.