[ RadSafe ] Scintillation Counting Question

[jock thomson]

I have encountered this type of problem many times. The solution is relatively strightforward.
The problem is simply one of limited solubility. The count rate increases as the small volume sample slowly dissolves. Extended shaking or ultrasonic treatment will not work.
To resolve this problem simply add about 500 microlitres of either water or another low quenching solvent that the sample compound is known to be soluble in. Ideally you should add this 500 microlitres to the sample prior to adding the cocktail and that way you will not encounter variable count rates.
Be aware that certain electronegative solvents such as halogenated solvents are very quenching. I would normally suggest adding either water, ethanol, acetonitrile, ethyl acetate or hexane as these are all low quenching.
Hope this helps resolve the problem.
Regards
James Thomson
Meridian Biotechnologies Ltd.
Unit 12 Pottery Lane West
Chesterfield 
Derbyshire S41 9BN
England
Tel: (+44)-1246-559834
Cell: (+44)-7989-688143

 


----- Original Message ----
From: "Nielsen, Erik" <nielseec at nv.doe.gov>
To: radsafe at radlab.nl
Sent: Wednesday, August 27, 2008 11:18:57 AM
Subject: [ RadSafe ] Scintillation Counting Question


To expand on Cary's comment.

I would recommend vigorous shaking of your sample/cocktail mixture and
degassing it by setting it in an ultrasonic cleaning bath for a few
minutes.  The ultrasound will degas the sample as well as contribute to
the homogenization of the sample/cocktail mixture.

Erik C. Nielsen 
Senior Scientist 
Remote Sensing Laboratory 
P.O. Box 98521, M/S RSL-47 
Las Vegas, NV 89193-8521 

http://www.nv.doe.gov/nationalsecurity/homelandsecurity/frmac/default.ht
m 

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Message: 8
Date: Tue, 26 Aug 2008 17:41:38 -0700
From: "Cary Renquist" <cary.renquist at ezag.com>
Subject: RE: [ RadSafe ] 
To: "Marshall, Elaine" <emarshall at lrri.org>,    <radsafe at radlab.nl>

Just a thought...  
Perhaps it is a physical phenomenon.
Since the tritium compound is moderately soluble, perhaps it initially
forms 
small (microscopic) globules in the aqueous solution that eventually
"dissolve"
in the cocktail -- the globules might not be large enough to see, but
large enough 
to keep the tritium beta from interacting with the cocktail.

Cary

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Marshall, Elaine
Sent: Monday, 25 August, 2008 14:47
To: radsafe at radlab.nl
Subject: [ RadSafe ] Scintillation Counting Question

I have a researcher who posed the following question to me.  I have not
experienced this yet and was wondering if anyone else might have an
idea.

"For this study counting tritium (as estradiol), we are using Ultima
Gold cocktail, 10-12 ml for 40 microliter samples.  We are seeing
something that seems odd to me:  after holding for at least half an hour
in the dark, we get counts that seem low, and if re-counted a day or so
later, the counts go UP, by about 50%, and reach the expected levels.  I
know of a number of reasons for counts to go DOWN, but don't understand
how they can go up.  We do make a point of mixing well, without shaking
so much as to cause bubbles.  Any clue???"

The researcher wanted me to add that these are aqueous samples, although
the actual tritiated compound is only moderately aqueous-soluble.

Thanks!!

Elaine T. Marshall, CHP
Radiation Safety Officer
Lovelace Respiratory Research Institute
2425 Ridgecrest Drive SE
Albuquerque, NM  87108
(505) 348-9578
emarshall at lrri.org
------------------------------

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