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Steve, I don't know if you have solved your problem yet, but I have some
additional suggestions.

Our old NA 1500 did not work well for N, and it now appears that it was the
autosampler.   The older autosamplers can leak and produce quite a variable
blank.  However, if the inorganic samples work then this is less likely to
be a problem but worth considering.

I would also make sure that the samples are in the same beam area size as
the standards, as many older combinations of mass spectrometer/EA are not
that linear and also change linearity with time.  Cleaning the source
usually improve the linearity again.  However, as samples get smaller at
some point every instrument starts to get more negative for 15N, so it is
very important to check at what mass of N in the tin capsule this occurs.

 From my experience,  linearity should be checked on every run with
different sized organic standards that cover the range of N in the unknown
samples.

You should also try running the samples with the GC column back in line,
since this is the standard method.  In principle is shouldn't make any
difference, but in practice might.

Also, what size O2 loop are you using and what pressure O2?  As others have
pointed out, putting in enough O2 for a good burn is very important.  We
use 22 mL of O2 at STP for 4 mg of plant material.  That would be a little
over 1 atm on the O2 pressure gauge with a 10 mL loop.

I have not used the Carlo Erba extensively for C/N analysis, but I have
been told that for best N2 results, the Helium and O2 pressure should
be  set to the same pressure.

Hope this helps,

Paul.



At 05:34 PM 11/4/02 +0000, you wrote:
>Dear Colleagues,
>
>We have recently attached an old Carlo-Erba EA to one of our mass specs
>with a view to measuring carbon and nitrogen isotope ratios in samples of
>organic matter using a continuous flow prep method. Unfortunately, we've
>experienced some problems in obtaining accurate nitrogen isotope data and
>have so far been unable to come up with a solution. I'll explain our
>problem and if anyone out there has an answer perhaps you'd be kind enough
>to tell us where we're going wrong.
>
>Our system runs along 'conventional' lines. The sample, weighed into a
>silver container, is combusted at a nominal temperature of 1050oC in a
>stream of pure oxygen and the product gases are flushed under He through a
>column containing silvered cobaltous oxide and chromium oxide also at
>1050oC. Oxidised gases are then passed through a copper reduction furnace
>at 650oC to reduce any nitrous oxides. We have dispensed with the GC column
>and instead remove CO2 and H2O using carbosorb. The carbosorb seems to be
>capable of removing all CO2 as evidenced by measurement of mass 44 which
>remains at background values. There is no evidence that water is
>transferred to the ion source either. Ion peaks are collected for mass 28,
>29 and 30 and a 29/28 ratio integrated from the peak areas. So far, so good
>(I hope). We tested the accuracy and precision of this procedure using a
>combination of IAEA and USGS inorganic nitrogen-bearing intercomparison
>standard and obtained values which were both accurate and precise. However,
>when we came to repeat the procedure with organic samples we obtained
>results which typically enriched in 15N, giving delta values which were
>between 1 and 2 per mil more positive than they should be.
>
>We have stripped down the EA, repeatedly changed the chemicals etc, but
>still continue to get good data for inorganic salts, but poor data for
>organic materials. We even re-checked isotopic values for both organic and
>inorganic materials using conventional sealed tube preparation techniques
>and obtained satisfactory results. Our problems seem to be associated with
>organic samples. Does anybody have any suggestions which might help us.
>
>Regards,
>
>Steve Crowley



Paul D. Brooks,
Dept. ESPM-ES,
147 Higard Hall MC 3110,
UC Berkeley, Ca. 94720-3110.

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