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Fwd: Re: Rn-222 in water analysis



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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