Here's my two penny's worth, a colleague here in Austria has the rather neat
idea of put a few granules of dry silica gel in each vial to ensure a dry
sample for N20 analysis. Would the isogeochemists throw their hands up in
horror at this approach? It also ensures that she can store the samples in
water bags thus preventing any leaks and if there are leaks the silica gel
turns pink....

Rebecca Hood

> ----------
> From:         bruce vaughn[SMTP:[log in to unmask]]
> Sent:         Monday, November 30, 1998 6:04 PM
> To:   [log in to unmask]
> Subject:      Re: Reliable stopcocks for isotopic measurement of CO2
>
> Dear Keisuke,
>
> Here is our two cents on Reliable stopcocks for isotopic measurement of
> CO2:
>
> The Isotope Lab at INSTAAR- University of Colorado shares the use of some
> 6,000 glass flasks to measure isotopes of carbon and oxygen in atmospheric
> CO2.  This is done in conjunction with the NOAA Carbon Cycle Group, who
> performs ongoing measurements of trace gases in the atmosphere at 53
> global sites on a weekly basis.  For more info on the flask network see
> www.cmdl.noaa.gov/ccg/.
>
> Currently our 2.5 liter flasks use stopcocks made by Glass Expansion of
> Australia (www.glassex.com.au ) with Teflon seals.  Our typical analytical
> reproducibility for isotopes is +/- 0.015 per mil for C13, and +/- 0.05
> per mil for O18.  Before first use, flasks are rinsed with deionized water
> and baked dry.  The flasks are covered with athletic tape for safety in
> handling.  The average time air samples are held in flasks before analysis
> ranges from 10 days to 5 months, depending on  shipping logistics (i.e.
> South Pole).  See Trolier et.al below for more details on flasks and
> methods.  Wet air samples are more problematic than dry ones- especially
> for oxygen isotopes. Over the years, sampling methods have been improved
> to dry the air sample as the flask is filled .  See Gemery et al. below
> for more details.
>
> I would caution users on the use of silicon septa where prolonged storage
> is a possibility.  From our experiments on CO2/H2O  headspace
> equilibration, we found evidence of exchange between the septa and the
> sample on times scales greater than 24 hours. I suspect this may have
> something to do with the filler in the silicon.  Using a septa sandwich of
> silicon and Kelef may help this.
>
> Viton seals are another possibility.  However, there are dozens of
> formulas for Viton, and the Carbon Cycle Group at NOAA has been testing
> some of them for use as seals on flasks and found as much as 6 ppm
> differences in CO2 mixing ratios on flasks stored for 30 days.  Isotopic
> tests have not yet been performed.  Doug Guenther at NOAA can be contacted
> for more info at: [log in to unmask]
> ----------------------
> Trolier et al. "Monitoring the isotopic composition of atmospheric CO2:
> Measurements from the NOAA Global Air Sampling Network". J. Geophsy. Res.
> 101 no.D20, p 25,897-25916, 1996.
>
> Gemery, P.A., Trolier, M., and White, J.W.C. "Oxygen isotope exchange
> between carbon dioxide and water following atmospheric sampling using
> glass flasks".  J. Geophsy. Res. 101, 14,415-14,420, 1996.
>
> Hope this helps,
>
> Bruce
>
> _________________________________________________________
>         Bruce H. Vaughn, Stable Isotope Lab
>         Institute of Arctic and Alpine Research
>         University of Colorado  CB 450
>         Boulder, Colorado  80309
>
>         Office: 303-492-7985                 [log in to unmask]
>
>         Lab:  303-492-5495                    FAX: 303-492-6388
> _________________________________________________________
>
> >>Dear all:
> >>We need containers for sampling and storage of CO2 gas for isotopic
> >>measurement.
> >>It will take a couple of days to bring the gas samplers from our site
> and
> >>undertake the isotopic measurement.
> >>Does anyone have comments about the materials of the gas samplers, and
> about
> >>the procedure for gas sampling?
> >>Previously, topic on materials of stopcocks is discussed on this mailing
>
> >>list.  Then we would like to know if this effect
> >>is detectable in carbon isotopic measurements of CO2 gas (several
> hundred
> >>ppm) stored for a week.
> >>Any comments are welcomed.
> >>
> >>Thanks in advance.
> >>Keisuke
> >>*********************************** In Japanese *****************
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> >>*****************************************************************
> >>Keisuke Koba (Assistant professor)
> >>Environmental informatics, Biosphere Informatics
> >>Department of Social Informatics, Graduate School of Informatics,
> >>Kyoto University  606-8501, Japan.
> >>Tel +81-75-753-3132    Fax +81-75-753-3133
> >>[log in to unmask] (temporal)
> >>http://rfecol.kais.kyoto-u.ac.jp/KOBA/kobaeng.html
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>