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Re: Fwd: Re: Rn-222 in water analysis
Barry,
The method you describe is what the EPA used years ago
for their inter labratory comparisons.
I wonder if the problem is related to oxygen quenching?
I have seen that happen before with minimal shift of the
spectrum.
Bill Field
Community of Science
http://myprofile.cos.com/Fieldrw
> Information from radch-l which may be of interest to some on this list ..
>
> >X-Mailer: Microsoft Outlook Express 6.00.2600.0000
> >Date: Tue, 14 May 2002 16:06:58 -0600
> >Reply-To: liste de distribution pour les RADIOCHIMISTEs <RADCH-L@in2p3.fr>,
> > Barry Stewart <bstewart@cemrc.org>
> >Sender: liste de distribution pour les RADIOCHIMISTEs <RADCH-L@in2p3.fr>
> >From: Barry Stewart <bstewart@cemrc.org>
> >Subject: Re: Rn-222 in water analysis
> >Comments: To: "Ewing, Judy" <jewing@SLD.STATE.NM.US>
> >To: RADCH-L@in2p3.fr
> >
> >Hi, Judy,
> >
> >Allow me to offer a suggestion which served us well when I was at ASU and we
> >were determining radon in water by LSC. Most would agree that there are
> >instability problems associated with long-term storage of LSC standards, so
> >we found a way to avoid having to do that. Briefly, what we did was sorb
> >the Ra-226 onto cation resin which was contained in a known volume and
> >sealed with no headspace. This releases the radon into the water, providing
> >(after secular equilibrium) water standards of radon. There is no
> >deteoriation of such samples. Details are given below if you are
> >interested.
> >
> >First, place one gram of Dowex-50 cation resin (50-100 mesh) into a glass
> >VOA-type vial using a teflon-lined cap instead of a septum cap. Weigh to
> >obtain the tare weight of vial, plus resin, plus cap. Then fill the vial
> >completely full (no headspace) with DI water, cap, dry any water on the
> >outside, and weigh. The net weight corrected for density will give the free
> >volume of the vial. Now carefully decant about half of the water.
> >
> >Next, add a weighed aliquot of NIST Ra-226 standard and mix for a few
> >minutes to ensure that the radium is sorbed onto the resin. Then fill the
> >vial with DI water until it overflows, and cap it. Make sure there is no
> >headspace. Set the standard aside for 30 days to allow secular equilibrium
> >between Ra-226 and Rn-222. Ra-226 is bound by the resin, but the radon is
> >released into the water. From the volume of the vial and the amount of
> >Ra-226 standard added, calculate the radon concentration (in whatever units
> >you choose). When equilibrated, you will have a standard solution of radon
> >in water that will exactly match your samples.
> >
> >After an aliquot is removed from the vial, top off again with DI water and
> >set aside for 30 days to re-establish equilibrium. Prepare enough of these
> >standards so that you have enough for initial calibration, plus the number
> >of QC samples you would use daily. But you must keep good records so that
> >you always know for each vial the exact condition with respect to secular
> >equilibrium.
> >
> >As long as you do not lose any of the resin, the standard will be stable
> >indefinitely.
> >
> >Barry Stewart
> >Associate Scientist
> >Carlsbad Environmental Monitoring and Research Center
> >1400 University Drive
> >Carlsbad, NM 88220
> >505-234-5543
> >505-887-3051 (fax)
> >
> >----- Original Message -----
> >From: "Ewing, Judy" <jewing@SLD.STATE.NM.US>
> >To: <RADCH-L@in2p3.fr>
> >Sent: Tuesday, May 14, 2002 2:40 PM
> >Subject: Rn-222 in water analysis
> >
> >
> >> Hello All,
> >>
> >> Greetings from sunny New Mexico, USA.
> >>
> >> We have been analyzing drinking water for Rn-222 by LS counting, using a
> >> modified version of the Hahn and Pia method. We are having problems with
> >> our efficiency standards. Over the timespan of a few months, these Rn-226
> >> supported standards turn yellow in the organic Optifluor-O phase, and
> >count
> >> at a reduced rate. The spectrum downshifts in energy, and the individual
> >> peaks become less resolved. We are constantly forced to make new
> >standards
> >> and/or widen the analysis window, both of which are best avoided.
> >>
> >> We speculated that "color quench" due to cocktail deterioration
> >(atmospheric
> >> or acid-induced oxidation, perhaps?) might be the cause. With this in
> >mind,
> >> our current set of efficiency standards were made with freshly purchased
> >and
> >> opened Optifluor-O cocktail, a fresh DI water dilution of a new NIST
> >Ra-226
> >> SRM, and no added acid. These standards were put into service in October
> >of
> >> 2001, and our efficiency factor has steadily decreased from 6.8 CPM/pCi to
> >> 6.2 CPM/pCi (1 sigma uncertainty at 0.06 CPM/pCi). There has been no
> >> spectrum downshift, and no visible yellowing of the organic layer,
> >however.
> >> Still, the CPM/pCi ratio continues to decline.
> >>
> >> Has anyone else experienced this problem? Does anyone have a possible
> >> explanation? Any ideas or suggestions as to what we can do about it?
> >>
> >> Thank you in advance.
> >> JL Ewing
> >> Analyst
> >> Radiochem Section
> >> Scientific Lab Division
> >> NM State Dept. of Health
> >>
> >
> +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
> Jean-Charles ABBE
> Délégation CNRS
> IRCCyN
> 1, Rue de la Noë
> BP 92101
> 44321 Nantes cedex 3
>
> Tel : 02 51 12 45 16
> Fax : 02 51 81 05 77
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