calcium chloride precipitation problem - (Feb/27/2007 )

I'm using annexin to stain my cells and that requires a higher concentration( 3mM) of CaCl2 in my buffer (PBS). Unless both are ice cold, sometimes even when they are cold, i'm seeing the CaCl2 ppt out. I've noticed if there is FBS in the solution , it seems to prevent precipitation.
It doesnt ppt out so much with HBSS... but HBSS does not work for my experiments, but why HBSS is better is also a mystery, since HBSS and DPBS are nearly identical in composition.

This entire thing is very wierd,has anybody oberserved something like this ?

HBSS contains 0.3 mM KH2PO4, 0.3 mM Na2HPO4.
DPBS contains 1.2 mM KH2PO4, 8.1 mM Na2HPO4.
Lower phosphate levels in HBSS reduce CaPO4 precipitation problem observed with Dulbecco's PBS.