Dear fellow Listmembers,

Has anyone else experienced non-reproducible (and 'light' by as much as 1
per mil) isotope values as a result of using a gas-tight syringe and septum
arrangement to remove gas aliquots from a sample bottle (with contents at
approx. 1 bar pressure)? My specific application is O2, but any information
gained using e.g. N2 or CO2 would be equally valid. Perhaps it is because of
the size of the aliquots (500 microlitres, from a 750mL vessel) in the case
in question. The 'large' size is necessary to provide sufficient gas for
dual inlet IRMS.

Gas transfer into a syringe, through a septum, seems to be a fairly routine
procedure for many isotope measurement applications; perhaps there is a
relatively small (<<500 microlitres) upper limit on the size of gas aliquot
that can be removed without significant fractionation - the latter
presumably being caused by diffusion effects into the syringe. One possible
contributory cause could be the occasional 'coring' of the septum during
injection of the syringe, thereby narrowing even further the bore of the
needle. But I'd be glad of any advice that others may have to offer...

Thanks,
Martin

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Martin Miller
Planetary & Space Sciences Research Institute,
The Open University,
Walton Hall,
Milton Keynes MK7 6AA
United Kingdom

E-mail: [log in to unmask]
Web (departmental): http://pssri.open.ac.uk
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