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Along the same lines- have you tried combusting a organic compound with
no N, say, a sugar? This would allow you to check for interfering
masses (28, 28, 30) showing up. Without seeing everything it sounds
like you are producing some CO-it doesn't take much of this to throw
off your N isotope values.
Good Luck;
        Jay Brandes
On Monday, November 4, 2002, at 01:50 PM, Rod Savidge wrote:

> 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
>