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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>>$B=u<j (J
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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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