Stable Isotope Geochemistry


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"Gray Edward Bebout" <[log in to unmask]>
Tue, 12 Nov 96 10:32:09 EST
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To interested parties---

    Regarding Gert's assessment of the miserable "offline" extraction of N2
for stable isotope analyses:  We have been able to develop methods for
routine "offline" processing of N2 samples in the size range of ~1.0 to >50
micromoles. With care, excellent precision/accuracy can be demonstrated over
this N2 sample size range (see various papers by Macko, Fogel, Kendall,
Bebout, etc.).  The majority of our N2 samples are derived from silicate
rock and mineral samples and are produced by combusted in sealed quartz
tubes with varying amounts of Cu-Cu oxide reagents.  We are also
experimenting with an Ar-line-like, metal extraction line that is operated
on-line to the mass spectrometer (and using a microvolume with silica gel). 
Analyses of organic materials (plankton in our lab) are somewhat easier
because of the higher N concentrations and the likely more efficient release
of N with combustion.  Continuous flow techniques will likely result in a
loss of precision--we'll see.........ask Willi Brand.  At least for the
rock/mineral studies, we need all of the precision we can get.

    Gray E. Bebout (Lehigh University)


>The offline extraction of nitrogen is a misery, especially at natural
>abundances: you have to be careful to avoid fractionation, samples have to
>be relatively large (especially a problem for plant samples), etc. Don't do
>it: this is were continuous flow technology has really helped out.
>Regarding trapping of CO2 from a CN analyser: you can go high tech and use a
>CO2 collection system (like we do in our lab, we use a prep CN analyser to
>collect CO2 for measurement in our accelerator for dating Ref: Hedges et al
>1992, Radiocarbon 34:306-311), or just use a liquid N2 trap.
>Best regards, GJ
>Dr. Gert Jaap ("GJ") van Klinken,
>Radiocarbon Accelerator Unit,
>Research Laboratory for Archaeology and the History of Art,
>University of Oxford
>6 Keble Road, Oxford OX1 3QJ
>Tel (+44) (01865) 273939   Fax (01865) 273932

    Gray E. Bebout, Stable Isotope Chemistry Laboratory
    Department of Earth and Environmental Sciences
    31 Williams Drive
    Lehigh University
    Bethlehem, Pennsylvania  18015

Office phone: 610-758-5831
Laboratory phone: 610-758-6168
FAX: 610-758-3677