32P-probe by PCR?

[Markus Piotrowski]

Dear netters,

although this should be a FAQ, I was not able to find the older 
discussions about this item. 
Okay, I want to prepare a 32P-probe by use of PCR. I did it in the past 
by simply avoiding the cold dATP and adding 2.5 ul of 32P-dATP (3000 
Ci/mmol, 10 uCi/ul) in an total volume of 50 ul. The probes I got had 
normally activities of about 20-30 million cpm. Now I want to do 
Southerns and people told me that this probes are not hot enough. I know 
I should add cold dATP, but I don't know how. Has anyone a PCR-protocoll 
that results in higher labelled probes?
I know that I can do ONP-labeling, but I think PCR should work as well.

Many thanks in advance,  Markus