Dear Jamie,

I would think your best bet is to separate, convert when appropriate,
and measure the gases you're interested in on-line on a
continuous-flow isotope ratio mass spectrometer, e.g. a GC-C-IRMS or
as lightly modified CF-IRMS as used for breath analysis.

In both cases there is no way getting around a tedious and time
consuming process of trial and error to find the optimal conditions
for separating all the gases you mentioned. Apart from that you have
to realise that measuring N2 and CO / CO2 in one run necessitates a
change of accelerating voltage or magnetic field during analysis. So
far, there is no system available the geometry of which would allow
you to measure 29/28 and 45/44 ratios without making those changes. I
have been discussing a potential solution with one of the IRMS
manufactures but even if we succeed we would still have to change
ions source parameters to accommodate the different characteristics
of N2 and CO/ CO2.

What you could try, however, is to measure 29/28 and 30/28 ratios for
N2 and CO, and use the same mass ratios for CO2 since CO+ is formed
from CO2 in the ion source. To do this in one analytical run you have
to separate the three gases using e.g. a CarboPLOT 7 or a PoraPLT Q
column (Chrompack). In addition, you require the means of introducing
aliquots of reference gases for isotopic calibration. Ideally, you
want to be able to introduce all three independent of each other.

I might be able to help if all this is supposed to happen under
"normal" laboratory conditions but since you are working for NASA I
wonder if the long term intention is to send a mini gas IRMS to Mars
or some other outlandish place.

Hope all this gives you a rough idea of what lies ahead.


Good luck,

                        Wolfram



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Dr. W. Meier-Augenstein
Lecturer

University of Dundee, Dept. of Anatomy & Physiology,
Small's Wynd, DUNDEE  DD1 4HN, United Kingdom

Tel.: +44-(0)1382-34/5124, /4968
      +44-(0)468 -314563
Fax:  +44-(0)1382-34/5514

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URL: http://www.dundee.ac.uk/AnatPhys/wolfram.htm
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