[ RadSafe ] Uranium assay; relative sensitivities

[Bob Shannon]

Detection sensitivity depends on the method and instrumentation used. 

Alpha spectroscopy measures radioactivity directly and generally has MDAs in
the vicinity 10^-2 pCi/L range for each of the isotopes. 

KPA (Kinetic Phosphorimetry) is capable of total uranium measurements only.
Labs often convert mass concentration results assuming a specific activity
of 0.67 pCi total U alpha / ug total U for reporting. Any conversion to
activity, however, presupposes that the isotopic make-up of the sample is
known, so be sure to know what assumptions are made and only use this
technique when you know the isotopic makeup of your uranium. Also be careful
since 'natural' uranium does not always exist in 'natural' abundance (i.e.
secular equilibrium of U-238 and U-234) in nature. 

In terms of activity, ICP-MS is significantly more sensitive for U-238 or
U-235 than it is for U-234. Modern instrumentation can typically detect low
part-per-trillion levels and below depending on the type of instrument and
the type of sample. This would yield U-238 and U-235 detection levels in the
attocurie / L range, while sensitivity is probably comparable to alpha
spectroscopy values for U-234. 

Sensitivity is not the only consideration if you want to use ICP-MS. Many
labs have not set up adequately to provide quality assured isotopic
measurements by ICP-MS (as opposed to total Uranium - see KPA above for
restrictions). I would ask for documented performance data and check to see
what protocols are in place for doing isotopics. I would especially
recommend asking if the lab does intercomparison samples (PE samples) and
check these results before I trusted my samples to the lab.


Bob Shannon

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of james.g.barnes at att.net
Sent: Tuesday, June 14, 2005 5:15 PM
To: RadSafe Bulletin Board
Subject: [ RadSafe ] Uranium assay; relative sensitivities

Folks;

For measurement of mass concentration of uranium (i.e., micrograms of
U/Liter), is a radiometric analysis (i.e., pCi/Liter converted to ug/Liter)
or a chemical assay measurement (directly measuring for chemical forms of U)
more sensitive?

Jim Barnes
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