Re: Sr-90 analysis QA

[Schoenhofer <schoenho@via.at>]

At 10:47 1996-04-11 -0500, you wrote:
>We're using a contractor laboratory to analyze for Sr-90 in potable water
>samples.  As part of our QA/QC oversight, we monitor analysis parameters
>like chemical yield, etc.  Under Standard Method SM-18 7500 or the EPA 900
>Series methodologies, I'm trying to establish data validation and acceptance
>criteria which cannot be found in the method documentation or any other
>reference that I have.
>
>What value or range of values would be considered acceptable for the following:
>
>1.  yttrium chemical yield
>2.  strontium chemical yield
>3.  Y-90 ingrowth time (fraction of 1.0), i.e., for low activity
>environmental samples , a full two weeks is recommended, but how much less
>than that would be considered acceptable?; and 
>4.  Y-90 decay time until count following chemical seperation.
>
>General feelings/guidelines/real references all welcomed.
>===============
>Gary L. Schroeder
>Brookhaven National Laboratory
>schroede@mail.sep.bnl.gov
>
>==============================================================
>Comments:

To check the quality of the analyses done by your contractor laboratory I
would rather suggest you send them spiked samples of uncontaminated water. I
wonder whether you can find any Sr-90 in potable water, unless it must be
some very special water. To be sure your spiked water is not contaminated
from the very beginning you could use deep well water or mineral water for
that purpose. 

The numbers you would like to know, depend of course heavily on the method
used, but as long as your contractor achieves an acceptable LLD (which you
have to define of course), these numbers are of minor interest. You should
probably ask the contractor for the LLD. 

In general I remember for the fuming nitric acid method a strontium chemical
yield of appr. 90%, which may vary and does not matter, if you determine it
correctly. In my opinion it  h a s   to be determined with every single
analysis, because from our experience the chemical yields which my very well
experienced technician achieved did vary considerably. The Yttrium yield
should be in the same range. If you have low level samples, then I would
recommend to wait for Y-90 for an ingrowth of at least 90%, which can be
calculated from the half life of Y-90. As to the decay of Y-90 after
separation I would recommend to measure it at once - you should not wait for
weeks to let the so gained Y-90 decay.

In my lab we have changed the procedures anyway, using an abbreviated
chemical separation method, which avoids the complete separation of calcium.
We use ultra low-level liquid scintillation spectrometers with a fantastic
low background ("Quantulus" from EG&G - Wallac). In LSC you have a very
simple sample preparation and very high counting yield (more than 80% for
each of Sr-90 and Y-90). There is no need for Y-90 separation, but of course
it is possible to separate it and measure it alone - both in mixture with a
gel forming cocktail and by Cherenkov counting. We reach LLD´s of at least
20 mBq per sample (500 min counting time and based on 3 sigma of the
background). For instance for Sr-90 in milk or in bone we usually start with
about 1g of ash because the use of our LSC instruments allows for
separations which can be done using centrifuge glasses only. Think of the
costs we save on fuming nitric acid, other chemicals, working time and labor
costs!

Best regards,

Franz Schoenhofer
Federal Institute for Food Control and Research
Head of Department of Radiochemistry and Radioactivity in Food
Kinderspitalg. 15
A-1095 Vienna
AUSTRIA 
Christian Schönhofer
Habichergasse 31/7
A-1160 WIEN
Tel.:		+43-664-3380333
Tel./Fax:	+43-1-4955308
e-mail:		schoenho@via.at