cpm vs. dpm ?

Guenter Jost Guenter.Jost at io-warnemuende.de
Thu Mar 6 05:51:00 EST 1997

In article <331E8B92.188E at Bris.ac.uk>, A.Doherty at Bris.ac.uk says...
>Troels Wind wrote:
>> Hi Netters,
>> I'm currently doing some GTP-binding assays with 3H-labelled GTP and
>> have stumbled into a small problem. The scintillation-counter, a
>> Beckman LS1801, gives me two different values for each measurement,
>> one in 'cpm' and one in 'dpm'.
>> I cant see any correlation between these figures, i.e. they are not
>> proportional, so I would like to know:
>> a) What's the difference between cpm and 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?
>> I thank you in advance!
>> Cheers,
>> Troels Wind
>> Aarhus University
>> Denmark
>cpm is the 'counts per minute' which the scintillation counter actually
>reads, while the dpm is the 'distintegrations per minute', which is the
>actual rate of decay of the tritium nuclide. The problem with tritium is
>that it's decay releases a low energy particle, hence the need for
>scintillation counting in the first place. Many of those particles
>however never cause the scintillant to flash, mainly due to quenching by
>the solvent in the scinillation fluid. So the counter has an internal
>readioactive standard (usually something like Cobalt 60) which has a
>known, long half life, with which the amount of quenching in each
>individual sample can be assessed against a standard quench curve so
>that the counts which the counter detects can be converted from cpm to
>dpm. The counter comes with factory set quench curves, but you can
>generate your own if you want.
>As far as which measure to use - it depends on the nuclide. For 3H or
>14C, use the dpm's, as uneven quenching across the experimental samples
>is then corrected for. But if you're using 35S or 32P, then the cpms are
>fine as these release much higher energy particles which are quenched
>much less efficiently.
>Hope this helps
>Andy D
>Dr Andrew Doherty               email -  a.doherty at bris.ac.uk
>Dept. Anatomy                   Tel (0117)9287421
>School of Medical Sciences      Fax (0117)9287402
>University of Bristol
>University Walk
>Bristol UK
>BS8 1TD
just some additional remarks:
- the efficiancy for 3H should be about 40 to 60 % (cpm/dpm).
I am wondering a little bit about "no proportionality" between cpm and 
dpm. Are the samples in its composition so different? Than I would prefer 
dpm. But You have to be sure that the quench correction by the internal 
standard is O.K..Otherwise I would suggest to run some external standards 
- thatt means to add a standard sample to the sample after counting and 
count it again.
We don't have a Beckman but there should also some measurements available 
to see some differences between the samples like internal ratio, 
homogenity etc.
Hope this helps a little bit.
Guenter Jost
guenter.jost at io-warnemuende.de 

More information about the Methods mailing list