AW: Standard addition of tritium; speciation problems?

["Franz Schoenhofer" <franz.schoenhofer@CHELLO.AT>]

-----Ursprüngliche Nachricht-----

Von: liste de distribution pour les RADIOCHIMISTEs

[mailto:RADCH-L@in2p3.fr]Im Auftrag von Arvic Harms

Gesendet: Dienstag, 01. Oktober 2002 14:50

An: RADCH-L@in2p3.fr

Betreff: Standard addition of tritium; speciation problems?





Dear all,



I have a question concerning liquid scintillation counting (LSC). Is the

efficiency for every chemical form of tritium under identical

experimental conditions the same (i.e., is it good idea to add a

standardised HTO source to a source containing an organic tritium form

(e.g., thymidine) in order to determine the activity of this organic

tritium form)? Anybody any ideas?



------------------------------------------------------------------



Arvic,



I had to think of such questions many years ago, when



a) measuring tritium in urine and



b) measuring low levels of C-14 in alcohol.



First of all I want to emphasize, that I believe that the old fashioned

internal standard method is still the best and most simple method and I have

never used standard quench curves in my more than 20 years of work with LSC.

But my problem was always low level measurements of environmental and also

biological samples for special radionuclides.



Second, if you use the internal standard method the easierst way is, to add

the internal standard in a chemical and physical form, which is as close as

possible to the form your sample is. This will hardly be possible to have an

identic standard solution. Therefore your internal standard should be

contained in a very small volume, because addition of a small volume of a

slightly different solution will not change the properties of your sample.



In detail I used for the tritium in urine sample 1 ml of urine, 9 ml of a

gelforming cocktail (Quickszint 400, Zinsser) in a PTFE lined PE vial (SLD,

Zinsser). Parallel I prepared the same sample and added a few tens of microL

of an aqueous solution of tritium. If I remember correctly it was HTO, but

it might have also been tritium-labelled sucrose. The instrument I used was

the Quantulus (Wallac Oy, Finland), which is equipped with an external

standard. In order to control, whether quench occurred I used the SQP

(Standard Quench Parameter) determined by the external standard of the

Quantulus. In all cases the SQP were identically for both sorts of measured

samples, which ensured that there was no change in efficiency. Actually I

chose this method to determine the efficiency, because the samples were

heavily coloured as you can imagine. Working with quench curves will in my

opinion not give reliable results.



For the measurement of low-levels of C-14 in alcohol I used also the

Quantulus and as an internal standard I used labelled n-hexadecane,

dissolved in absolute alcohol. Since again the amount added was very small,

the SQP confirmed that there was no change in quenching, though the alcohol

samples were not 100% free of water.



I recommend that you adopt the approach (similar composition of your

internal standard, high enough activity, so that a very small volume is

sufficient and check the samples with your counter (which one?) for any

differences in quench behaviour.



Any further questions are welcome!



Franz Schoenhofer



************************************************************************

You are currently subscribed to the Radsafe mailing list. To unsubscribe,

send an e-mail to Majordomo@list.vanderbilt.edu  Put the text "unsubscribe

radsafe" (no quote marks) in the body of the e-mail, with no subject line.

You can view the Radsafe archives at http://www.vanderbilt.edu/radsafe/