Pat,

We have seen that with our acetanilide, but after we put it in a drying
oven at 65 degrees C overnite, the N15 composition was back to +1.3 per
mil.  We found after a period of time Nh4, NO3 and Acet stds pick up water
and for some reason then run isotopically off.  Putting them in the drying
over improves things, why?.

As far as the discussion about carrier gases, we use UHP grade He and have
never found the need to use gas traps for the NC2500 EA or the pyrolysis
TCEA.  The big problem with gas contamination in the carrier gases
generally is not in the tank, it is using less expensive regulators that
may work fine when new, but start to leak over time.  The leaks then pull
in atm with water vapor that degrade your CF-IRMS performance and peak
shapes.  Putting gas purification traps on the He carrier line may help
initially, but then things really get bad when the traps fill up and water
comes from the regulator leaks and the gas trap into the system.  To see if
this is a problem with your system, look at the mass 30 background while in
the N2 cup configuration, if it is 3x to 5x the 28 & 29 levels, you might
have leaky regulators or other leaks in the system which causes the
formation of NO from the N2 and water and elevates the mass 30 backgound.

Hope this helps,

Bill

At 02:11 PM 11/9/00 -0600, you wrote:
>In my carrier gas line I use a moisture trap( indicating
>Drierite-Molecular Sieve) and a Helium Purifier with these I then use
>high purity helium. I don't have any traps on my reference gases.
>
>I have another problem. I recently changed my combustion column on the
>NA 1500, I have the Conflo II and a Delta-S. The delta-15N of my
>reference Acetanilide was running -1.1 to -1.3 after the change it is
>-1.4 to -1.8, I also have an integrator on the thermal conductivity
>detector of the EA. The nitrogen response factor increased after the
>column change meaning I am getting less N-area per unit weight N. I am
>about to pack another combustion column but thought I would seek
>opinions first.
>
>Thanks,
>Pat
>
>--
>
>Patrick D. Anderson
>Researcher
>Great Lakes WATER Institute
>Univ. of Wisconsin-Milwaukee
>600 E. Greenfield Ave.
>Milwaukee, WI 53204
>Phone: 414-382-1731
>
----------------------------------------------------------
William J. Showers
Dept. of Marine, Earth, & Atm Sciences
1125 Jordan Hall
North Carolina State University
Raleigh, NC  27695

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