Hi Shikha,
        It is a mass spec problem.  When the mass
spec does a fast jump to CO2 it is missing the
peak.  The first thing that I would do is a new
mass calibration and a new jump calibration.
Even if the mass calibration is way out of wack
you will get good d15N values as the Conflo
method does a peak center on masses 28 and 29 at
the beginning of each sample.  But it does not do
a center for CO2 because there is not time before
the CO2 comes off the EA.  The Gasbench on the
other hand does a center on CO2 with each sample
and does not do a jump.   Please let me know if
this helps.  Jason

>Hello,
>During the last few days I have been having a problem with my Costech EA which
>is fitted to a Delta plus XL via Conflo II. The mass spec is working fine with
>other peripherals ( i.e Gas bench and TC/EA)and has been tuned for EA work.
>Linearity for N2 and CO2 reference gases is about 0.05 permil. Both the
>reactors and water trap have been replaced recently and the GC has been
>conditioned. The He flow is 100 ml/min and the sample drop time is 10 sec as
>recommended. The problem is that when I measure the standards (both N and C in
>single run) the d15N results look fine i.e., the standard deviation is less
>than 0.1 permil. However, the d13C of the same samples give standard deviation
>ranging from 0.3-2 permil. This variation is not in any way linked to sample
>size, but appears to be random. There appears to be no leak, the chromatogram
>is neat and does not show any signs of incomplete combustion. For oxygen, I
>have tried all the three oxygen loops i.e micro, semimicro and macro mode, but
>the problem is still there. I donít think that mass-spec is the problem since
>it works fine for d13C determinations using the Gas Bench.I have run out of
>ideas and would greatly appreciate some suggestions.
>Thanks!
>Shikha
>
>-----------------------------------------------------------
>Dr. Shikha Sharma
>Research Associate
>Department of Geological & Atmospheric Sciences
>253 Science 1
>Ames, Iowa-50011-3212
>Phone:(515) 294-7584(Off)6090(lab)
>E-mail:[log in to unmask]
>http://www.ge-at.iastate.edu/people/staff/sharma/Webpage.htm


--
Jason Curtis, Ph.D.
Stable Isotope Mass Spec Lab manager
Associate-In Geochemistry
Department of Geological Sciences
241 Williamson Hall
University of Florida
Gainesville, FL 32611
[log in to unmask], office 352-392-2296
lab 352-392-3344, fax 352-392-9294