Beverly,

Usually when I see a variance between the value of reference peaks at the 
start and finish of a run, it turns out to be due to water. Have you been 
keeping track of mass 18?

Paul Eby

University of Victoria


>===== Original Message From Stable Isotope Geochemistry 
<[log in to unmask]> =====
>Greetings All,
>
>We have a GC (Trace/PAL)- irms (Delta Plus Advantage) with a GC III
>combustion interface and have been experiencing three problems with d13C
>measurements of specific compounds over the last few months.
>
>1.  Selective loss of the low molecular weight compounds (n-alkanes <
>C24)  relative to the higher molecular weight compounds (n-alks> C26).
>This issue has become increasingly problematic over the last few weeks,
>and does NOT appear to be related to any dramatic changes in running
>conditions or instrument mainetenance.  Samples are injected in
>splitless mode.  Septa are changed every 20 injections, the inlet has
>been recently re-plumbed, and flow rates are as expected through the
>backflush valve (1.5 mL/minute in backflush mode) and the column (1
>mL/min).  The backflush vent capillary has been replaced and is the
>recommended dimension.  The sample itself is fine (all peaks are as
>expected and approximately equal in height) as evidenced by analysis on
>a different system (GC-FID).  Furthermore, there do not appear to be any
>leaks in the system as checked by  monitoring mass 40 under a stream of Ar.
>
>2. Isotopically enriched d13C values of the sample peaks (2-3 permil too
>enriched, in fact).
>
>2.  A non-systematic fractionation of +/- 1-2 permil between the first
>set of CO2 ref peaks, and the last set of CO2 ref peaks on each GC run.
>
>Any ideas would be greatly appreciated.
>
>Thanks,
>
>Bev
>
>--
>~~~~~~~~~~~~~~~~~~
>Beverly Johnson
>Assistant Professor
>44 Campus Avenue
>Department of Geology
>Bates College
>Lewiston, ME  04240
>ph: 207-786-6062
>fax: 207-786-8334