cpm vs. dpm ?

Andrew Doherty A.Doherty at Bris.ac.uk
Thu Mar 6 04:17:06 EST 1997

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

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