cpm vs. dpm ?

Judith Ann Eudaly JudyE at lilly.com
Thu Mar 6 04:00:03 EST 1997

Troels Wind wrote:

> a) What's the difference between cpm and dpm?

cpm = counts per minute.  This is the "hits" that the scint. counter 
gets per minute. 
dpm = disintegrations per minute.  1 Ci = 2.22 x 12E dpm

cpm/dpm = counting efficiency.  

Counting efficiency can depend on many things.  The instrument itself, 
the counting cocktail, the assay process, etc.  You have to use an 
internal standard with a known dpm (and you must account for decay). 

you count an unknown = cpm1, 
add your standard (known dpm) and recount = cpm2
cpm2-cpm1/std dpm =  counting efficiency  (CE) of the entire process

calculate the dpms of your unknown 
cpm1/CE = unknown dpm

> b) Which of these is in YOUR opinion the best in terms of correlating
>    the scintillation readout to the amount of bound GTP in the sample?

Using cpm assumes the CE of all samples is the same.
Using dpm takes into account all the variables of the counting process.
Unless you have told the scint. counter the CE of your process or  
calculated a quench curve and H#(see reference below), I don't see how 
the dpm value that it is giving you would be correct.  

Perhaps you should read Section 10 "Disintegrations Per Minute" of the 
"Operating Manual for the LS 1801, 3801, 5801 Series". It gives a 
detailed description of what you need to do.  If you do not have one, I 
am sure Beckman would be glad to send you one.


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