Dear Dr. Keisuke,
Some time ago I need to collect soil air from different horizons
and  to analyzed it composition and d13C in laboratory. We used two
methods.
1) In 1 liter glass flask was added about 100 cub. cm of vacuum
oil (which use in rotary pump, it has very low partial pressure).
Then flask was germetically closed by rubber cork with two holes: in
first hole glass tube have to be up to the bottom of flask and in
second hole- short tube (no longer then cork). On the external ends
of glas tubes a plastic tubes (about 5 -7cm) was added (do not use
silicon tube) and closed  by spiral clamp. In the field in flask was
added water up to the top; flask turn upside-down and keep in
support. Longer glass tube is connected with object and throw other
tube water move. The rate of water we regulate by the clamp. During
sample taking the oil layer has to prevent contact between water and
sample gas.
2) One end of gas glas pipette (it has two conic stopcocks) is
connected with object, other- with lower placeed vessel with water.
In this case 10 volumes of pipette have to remove as water.
-Both methods can be used at atmosphere presser.
-We had 0,8-1,5 per cent of CO2.
-d13C was equal for both method and not changed from 3 hours to 4
days (more period does not tested).
I will be glad if my experience help you.

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Dr. Sergey A. Oleynik
Laboratory of soil geochemistry and mineralogy
Institute of basic problem of biology
Russian Academy of Sciences
Pushchino, 142292
Russia
e-mail: [log in to unmask]
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