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Consider removing sulfate from the mother liquor after crystals are formed.  I recall having success replacing ammonium sulfate with sodium malonate (it's a nice cryoprotectant as well).  This is assuming that ammonium sulfate is your major precipitant.  If it's just an additive (say, your conditions are some PEG + 0.2M ammonium sulfate), try transferring crystal to whatever PEG/glycerol combo you need for cryoprotection minus ammonium sulfate.

Strictly speaking, this is not answering your original question, as it suggests to recollect data instead of advice on how to distinguish sulfate from phosphate in your density.  

On Sat, Feb 16, 2019 at 4:07 AM 张士军 <[log in to unmask]> wrote:

Dear all

I have got a crystal grown at the condition both have ion of SO4 and PO4, and the diffraction resolution is very well, but the problem is coming: how to tell which is which just from electron density? I think they are exactly same. Thanks a lot !!!

Beat Regards

Shijun


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