Could it be that you are not achieving total oxidation, e.g., have some CO,
C2H4, etc. present?  Without GC, you would not be able to separate hence
would be unaware of the different constituents if they were present.
Rod

At 05:34 PM 04/11/2002 +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