Greetings,
     Two more cents for the group.  We have had accurate and precise
results for isotope
measurements of N2O by gas tight syringe injection through butyl rubber
stoppers (0.5
per mil for 15N, 1.0 per mil for 18O, 1.5 per mil for d31/30... for the
isotopomerists).  We
do use needles with side ports.  Having said this I do not like to use
syringe needles
if at all possible.  We have not analyzed O2 by syringe injection but have
excellent
results for O2 and N2 using an evacuated gas sampling loop locatec between
two Valco
valves that avoids having to use syringes (see Roberts et al., 2000 Env.
Sci. Tech. 34: 2337-2341).
Cheers,
Nathaniel


At 01:46 PM 6/6/2003 +0100, you wrote:
>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
>
>==============================================
>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
>==============================================

Nathaniel E. Ostrom, Ph.D.
Professor
Dept. of Geological Sciences
206 Natural Sciences Building
Michigan State University
East Lansing MI 48824-1115
517-355-4661