I built a vacuum line to trap CO2 from atmospheric samples for isotopic
analysis a couple years ago, and initially tried a magnesium perchlorate
trap to remove water vapor from the samples. The line produced poor
results for stable carbon isotopes. While investigating potential
problems, we heard that magnesium perchlorate works fine near atmospheric
pressure, but does not perform well under vacuum.
We elected to replace the magnesium perchlorate trap with a dry ice/alcohol
slush trap, and eventually got excellent results. Unfortunately, we
changed several other aspects of our line at the same time, so I can't
conclusively say that this was the problem.
While the isogeochem group is on this subject, I'd be very interested to
know if others have experienced problems with magnesium perchlorate at high
vacuum... or if these problems can be eliminated by using an anhydrous
PS. Also a safety warning: dry magnesium perchlorate can explode
violently when shocked severely.
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