good morning

[Nick Theodorakis]

DK wrote:
> 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

You are essentially correct, but I want to add that implicit in your 
description is the assumption that one is using a light-based exposure 
method such as using an intensifying screen. If you are only using betas 
to expose, you don't need to cool the film.

The phenomenon is called "reciprocity failure," in case anyone wants  to 
google to learn more about it. It also used to be a problem for other 
long light exposures such as astrophotography.

Nick

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Nick Theodorakis
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