RE: Standards for the Taking

[Franz Schoenhofer <schoenho@via.at>]

At 16:15 05.01.2000 -0600, you wrote:


>Chris and Group,
>
>At one time I tried to establish just why or what the basis of 10 year
>expiration for sealed standards including but not limited to LSC standards.
>I got some responses, but all can be grouped under identifiable things that
>did not need a time limit.  That is half life was given, but typically LSC
>standards are H-3 [12.25 y] and C-14 [5730 y] so by t-1/2 they can go for a
>while past 10 years.  
>
  But I am still interested in the rational or theoretical basis to
>retire a sealed standard, SOLELY ON AGE.
>
>Peter G. Vernig,


Peter,

There seem to be two answers for your question - a scientific one and a one
related to marketing. I refer only to liquid scintillation standards.

You know that all cocktails contain very complex aromatic compounds. These
compounds are sensitive to deterioration by the influence of light and of
course also radiation. They are not only "excited" by the radiation, but
double bonds and other weak chemical bonds can also  break easily. 500 000
beta rays per minute is quite a lot. So I can imagine that after some time
the efficiency of the cocktail will become lower. A standard with an
exchanged efficiency is useless.

The second reason has to be understood from the standpoint of the
manufacturers. Warranty is a big problem for the industry - especially in
the USA. In a country where you find warnings everywhere for hazards - not
only on cigarette packages - a company simply does not want to risk
probably tremendous payments to a user, because a standard has changed
after 15 years of use. So they rather put a limited time for use on it.
This is even more advisable, if an ANSI standard exists. 

I do not have personal experience with such sealed standards, because I
never used them - only the technician of the company of our four Quantulus
has used them to check the instrument after service and to adjust the
instrument after a few changes which were made in the electronic. We always
used in our work freshly prepared standards of the same composition
regarding matrix, cocktail and mixing ratio with the radionuclide in
question, for instance tritium, C-14, Sr-90, Ra-226 etc. In some cases with
severe quench we have very successfully used the internal standard method.
I have never really understood what these sealed tritium and C-14 standards
are good for except for checking the instrument. 

Franz


Franz Schoenhofer
Habicherg. 31/7
A-1160 Vienna
Austria
Tel.: +43-1-495 53 08
Fax.: same number
mobile phone: +43-664-338 0 333
e-mail: schoenho@via.at

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