Steve,

I'd suspect the same as Rod suggests, maybe some NO gas also present. Do the
results work out if the GC column is in line? Or can the O2 gas pulse be manipulated
to improve combustion and hence precision?

Tim

> 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
>


xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dr Tim Clough,
Research Scientist,
Soil, Plant and Ecological Sciences Division,
Soil and Physical Sciences Group,
P O Box 84,
Lincoln University,
Canterbury,
New Zealand.

Phone: (64) (3) 325 2811
Fax:  (64) (3) 325 3607