Dear Sue,

although I did not understand fully what your problem exactly is I recommend
the following solution:
1. Use standards for all desired compounds and analyse them by EA-IRMS
2. Do this again with one or two compounds which are not in your samples and
which you use as internal standards
3. Adjust your GC-C-IRMS system to the isotope ratio of one of these
internal standards measured by EA-IRMS
4. Measure standards with different amounts covering the whole amount range
of your samples
4. Calculate offsets between measured EA-IRMS values and calculated
GC-C-IRMS values via isotope mass balance calculation of desired fatty
acids. Offsets might be a function of signal intensity
5. Measure your samples and standards.
6. Calculation of "real" isotopic composition of your samples should be made
as follows:
6.1. Correct for reference gas drift during each run
6.2. Correct for amount dependence
6.3. Correct for derivatization C (H)
6.4. Correct for offset between EA-IRMS and GC-C-IRMS

For more details look at the following papers:
Glaser B. (2005) Potential and constraints of compound-specific stable
isotope (d13C) analysis in environmental research. Journal of Plant
Nutrition and Soil Science, 168, 633-648.
Glaser B., Amelung W. (2002) Determination of 13C natural abundance of amino
acid enantiomers in soil: methodological considerations and first results.
Rapid Communications in Mass Spectrometry, 16, 891-898.
Gross S., Glaser B. (2004) Minimization of foreign carbon addition during
derivatization of organic molecules for compound-specific d13C analysis of
soil organic matter. Rapid Communications in Mass Spectrometry, 18,
2753-2764. DOI: 10.1002/rcm.1684.
Schmitt J., Glaser B., Zech W. (2003) Amount-dependent isotopic
fractionation during compound-specific isotope analysis. Rapid
Communications in Mass Spectrometry, 17, 970-977.
Meier-Augenstein W. (1997) The chromatographic side of isotope ratio mass
spectrometry: Pitfalls and answers. LC GC International, 10, 17-25.
Meier-Augenstein W., Watt P.W., Langhans C.D. (1996) Influence of gas
chromatographic parameters on measurement of 13C/12C isotope ratios by
gas-liquid chromatography-combustion isotope ratio mass spectrometry.
Journal of Chromatography A, 752, 233-241.


-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
Behalf Of Susan Ziegler
Sent: Freitag, 28. April 2006 18:29
To: [log in to unmask]
Subject: [ISOGEOCHEM] GC-C-IRMS issues


Hello All:

I am seeking any possible hints to solve a problem that has developed
in our GC-IRMS system.

At the moment the suite of standards we run for d13C of FAMEs in
order to check for consistency no longer exhibits a constant
correction factor across all the compounds in each standard
chromatogram which range from 14:0 through 23:0 in size. In fact we
are seeing greater depletion in 13C for the longer (or later RT)
FAMEs by as much as a few per mil relative to the first compounds
(14:0).  I have been trying to isolate the variables and this is what
I've found so far:

1. Flow is constant throughout the entire GC run (2 mL/min).
2. CO2 injected throughout the run remains constant in it isotopic
composition so does not exhibit the depletion see in compounds that
must be combusted.
3. I figured it must be the combustion column so it has been replaced
and we are again seeing the same problem.
4. Peak shape looks pretty good with widths remaining within 15-20s
and fairly sharp ratio traces.  The ratio traces do become a little
broader with the last few more deplete compounds but not particularly
worse than runs where we've gotten good consistent values.

We are using an Agilent (HP) 6890 GC with a split/splitless injector
in splitless mode.  I have changed the septa (done frequently
anyway), liner, and the gold seal and washer.  The GC column is very
new but also has been cut (`0.5m) to see if reactive sites may have
been a problem.  We are using the metal Valco cross piece and have
been now for years without a problem, could this have developed
active sites that may yield such symptoms?

Thanks for any information anyone might have regarding this
problem...I've been at this for some time now and am in danger of
pulling out all my hair!

Best regards,
Sue