Paul,

We run alot of N-15 here and have noticed the same thing, especially in the
summer when things are humid.  We have tried different manufacturers,
sometimes you get a bad bottle right "out of the box".  I know of no way to
"rejuvenate" the magnesium perchlorate, in fact that might be more
dangerous than just buying a new bottle.  We keep our stock in an evacuated
bell jar and only vent the jar and take out the bottles in a climate
controlled room that has a dehumidifier.  In our experience, if you leave
the top off a bottle of magnesium perchlorate off for too long in a humid
room while changing the water trap, the whole bottle picks up moisture and
is not of much use in the future.  By taking care during the water trap
change, you can keep you bottles dry until they are empty.

We also have found problems with the mass 30 background while running N-15
from a CE NC2500.  This was related to small leaks in the system, the
reference standard tank, and regulator.  We found minute leaks in the N2
regulator and connections by putting the regulator on a He tank and
charging the system with He (you do not need an adaptor for N2-He).  That
way you can detect small leaks with a He sniffer that you could not see
with N2 in the system.  After elimination of the leaks in the Conflow
hookup and buying a new regulator we still had problems with mass 30
backgrounds.  These backgrounds were reduced by going to the purest N2 we
could buy (6.0 Research Grade), but are still too high for our purposes
since we run CO as well.  However, mass 18 is low so we do not think this
is a water contamination problem, but something in our reference std gas at
mass 30.

Has anyone tried a N2 gas purifier trap in the reference gas line to the
Conflow?  Are there isotope effects as the trap "ages"?  We don't know what
to try next to reduce our mass 30 backgrounds with N2.  When we go over to
our pyrolysis system and run CO, we have to wait for a 24 hour period for
the mass 30 backgrounds to drop, and then we are OK.

If anyone has some advice, thanks in advance.

Bill

At 05:34 PM 05/04/2000 -0700, you wrote:
>When using an Elemental Anlayzer (Carlo Erba) to measure natural abundance
>15N I seem to find that the age of the magnesium perchlorate can affect the
>results.  If I get poor results, replacing the magnesium perchlorate seems
>to solve the problem, but eventually this doesn't seem to work.  Then I
>switch to a new previously unopened bottle of magnesium perchorate, and
>this again seem to solve the problem.  It is as though the magnesium
>perchlorate gradually deteriorates in the bottle after it has been opened a
>few times.
>
>Has anyone else experienced this?
>
>I recall someone telling me that they used a small amount of P2O5 after the
>Magnesium perchlorate in the water trap, and that this would take out
>additional water.  Has anyone tried this and does it affect 15N or 13C
>analysis?
>
>Thanks in advance,
>
>Paul Brooks.
>
>
>
>Paul D. Brooks.
>
>Dept. ESPM-Ecosystem Sci.,
>151 Hilgard Hall, MC 3110,
>UC Berkeley, Ca. 94720-3110.
>
>phone (510)642-3155
>FAX   (510)643-5098 (to Att. Paul Brooks)
>
----------------------------------------------------------
William J. Showers
Dept. of Marine, Earth, & Atm Sciences
1125 Jordan Hall
North Carolina State University
Raleigh, NC  27695

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