P.F.A. versus Formaldehyde

[NasrallM]

Hello all. I am trying to perform ChIP seq for the first time and was wondering whether using paraformaldehyde (PFA) versus regular formaldehyde makes a huge difference wit respect to fixation efficiency ? I currently use P.F.A (37%) and crosslink for 8 minutes, after which, I quench with glycine for 5 minutes. When opening the P.F.A. bottle for the first time, I transfer the liquid to a 15ml conical tube, and use it on my cells. After usage I store at -20 Celsius, until further use. After the second time I use it, I dilute it down to 4% and store at 4 degree. Would using 37% P.F.A in this manner decrease crosslinking efficiency?

-M.N.

[cliffbeall]

Paraformaldehyde is the solid form, when its dissolved in aqueous solution it becomes formaldehyde. Formalin is 37% formaldehyde that is usually sold in solution and from what I read also contains a little methanol.

Many histologists tell you you should use para and make it fresh from the solid to preserve antigens, but not sure how good the evidence is for this.