Question for paraformaldehyde in PBS buffer

[Andrew Huang]

Dear Sir/Madam,
	Do you ever prepared for 4% paraformaldehyde in PBS buffer?
In immunohistochemical staining procedure, I found when I dissolved 4% 
paraformaldehyde in 0.1M PBS buffer, there will be lots of 
unsoluable, white materials in bottom. I'm so confused whether it is 
natural results or my procedure is wrong. Could anybody tell me how and 
why it is, please? Thank you!

Sincerely 

Andrew