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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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