I am coming across claims that only ultrapure (>99.5%) or enzyme-grade ammonium sulfate should be used. Is there any material difference between 99% and 99.5% for antibody precipitation purposes?

And what is the concern about the purity, is it residual metals? Or just excessive absorption of water? If it's the latter, I think I probably don't care, since I'm using the powder to make a supersaturated solution, and precision of weighing isn't critical.