good morning

[Analia Alet]

Another reason for this is to improve southern quality.
at high temperatures, the electrons impact very fast in the x-ray film, making 
a very large spot, so you don't have "a band" rader it is a big spot without 
boundaries. This is the case for P32. 
But if you are using C (not in southern blot but in other techniques) you 
don´t need to place the [x-ray films+membrane] at -70ºC because de decay 
constant for this isotope is very low, so you just leave your films at room 
temperature for almost a week.

At least this is how they explained this to me
best regards
Analía



Mensaje citado por methods-request from oat.bio.indiana.edu:

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In article <mailman.156.1208280425.19248.methods from net.bio.net>, "kamalaker 
nasani" <agbiok4 from gmail.com> wrote:
>I have a small doubt regarding Southern blot.
>
>After blotting membrane and x ray film we will keep them in -70 degrees to
>develop image of the membrane. Can any one tell the logic behind this
>process.  

The way it was explained to me long ago made perfect sense 
so I never questioned it despite knowing next to nothing about 
chemistry of photography. Here it is: 

It takes more than one hit to the same atom to convert Ag+ into 
elementary silver with photons. The first hit/step is chemically 
reversible - excited Ag+* can spontaneously relax back to Ag+. 
In other words, it's a chemical reaction and therefore depends on 
temperature. The second is not - once Ag0 is formed, it's a done 
deal. That is why in old days it was recommended to "preflash" 
X-ray film for maximum sensitivity.  So in your case the lower 
temperature "freezes" the reverse reaction, thereby increasing 
sensitivity. 

DK