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Hi Anindya,

We have been facing similar issues with analyses of N-poor Precambrian
black shales from the Aravalli.  I think you are right that CO remains in
the line and that it interfers with your N2 peaks, given poor accuracy and
reproducibility of d15N.  We solved the problem by adding a strong
oxidizer to the powdered sample, like vanadium pentoxide, to oxidize CO. 
In my tests on organic-rich shales, I found that when the amount of V2O5
is more than about 30% the weight of the sample, the CO is completely
oxidized.  Also, you will want to change your reactor columns more
frequently with these kinds of very large samples.

I hope this helps,
Dominic



-------------------------------------------------
Dominic Papineau  Ph.D.
Geophysical Laboratory
Carnegie Institution of Washington
5251 Broad Branch rd NW
Washington DC 20015

Office: 202-478-8908
Cell: 303-514-8978
-------------------------------------------------


>
> Hi all
> We have been trying to measure d15N in sediment samples in EA 1112 Delta
> plus XP (furnace temp 1000oC, GC column 45oC, carrier He flow 100 ml/min,
> Oxygen 300 ml/min, oxygen injection for 5 sec). The samples have high C/N
> ratios from ~5 to >35 (some are black shale or lignites). Initially we
> were getting a a small peak  towards the end of the N2 peak (overlapping)
> following which a large peak (fragmented CO2?) appeared. Knowing that the
> large peak might come from very high CO2 we put an ascarite trap after the
> moisture trap. This removed the large peak, however the smaller one
> continued to be present (open http://www.sendspace.com/file/izyrap for the
> scan). The reproducibilty was extremely poor. We ran pure lab KNO3 just to
> check what happens in samples without any carbon and got excellent
> reproducibility; we therefore think that reduction or NOxs is not a
> problem. The small peak is possibly also CO  but coming from incomplete
> combustion (unlike fragmented CO2 in source). We suspect that due to high
> carbon content the combustion is incomplete and chromium oxide or silver
> cobaltous cant oxidise it completely.  We checked this by analysing
> synthetic compounds of various C/N ratios (like Nicotinammide and
> Atropina) and found that the small peak, interfering with nitrogen peak,
> increases with higher C/N ratio. Important to mention that to get N2
> signal of even 3000 mv we had to use about 20-40 mg sediment. Any idea
> about how to circumvent the problem particularly how to get best results
> on d15N on high C/N samples? If the interfering peak is CO then how to
> remove this? I was thinking to increase the length of silver cobaltous but
> being a single reactor in EA 1112 its a problem. Any help?
>
> ---------------------------------------------------------------------
> Anindya Sarkar
> Professor & Coordinator, National Stable Isotope Facility
> Department of Geology and Geophysics
> Indian Institute of Technology
> Kharagpur 721302
> West Bengal, INDIA
> Tel.: 0091-3222-283392 (O) 283393 (R) 220184 (R)
>         Cell: 09434043377
> Fax.: 0091-3222-282268
> http://anindya-sarkar.tripod.com
> ------------------------------------------------------------------