Here is my tuppence worth of wisdom in the matter of N2 / air
injection for isotopic calibration purposes.

> One further point - this works fine for air when only a few tens of
> microlitres of gas are injected but attempts to inject larger volumes
> of gas (eg N2 diluted in Helium) have failed. The addition of larger
> volumes of gas seems to create a 'pressure wave' effect in the carrier
> gas and the production of a false peak.  Perhaps this could be
> overcome by changing from an SGE splitter valve to an open split at
> the EA/Mass spec interface but I have not had the chance to
> experiment. I would be interested in hearing comments on this.
>

In addition to the pressure pulse problem, Darren also mentioned
the problems associated with a manual injection port, i.e. timing
and reproducibility of injection.

As some of the list member might know, I developed a reference
gas inlet module that can be used in any CF-IRMS application. I
exemplified the technique on a GC-C-IRMS instrument, placing the
module in front of the combustion interface and introducing
reference gas (CO2 and / or N2) into a He make-up that
subsequently was fed into the carrier gas stream.

The principal design of the module is very robust and it can readily
adapted to be built into simple gas (= breath) - IRMS machines as
well as into EA-IRMS machines. Make-up and reference gas
pressure are balanced thus against the carrier gas stream/flow to
prevent pressure pulses, ghost peaks and mass discrimination.
The reference gas is equilibrated against ambient at all times
through a bleed line. Peak area is linear dependent on the "pulse",
i.e. for how long the solenoid controlling the reference gas flow into
the make-up is "open". Isotope abundance of the reference gas
peak is linear for peak size ranging from 7 nA s to 100 nA s (on a
GC-C-IRMS system). Due to the position of the module at a
relatively early stage of the gas train, the reference gas injections
are peak shaped rather than rectangular (direct ion source
injection). The actuating solenoid can be software controlled and
thus the peak size of the reference gas "pulse", making tasks such
as a linearity tests or introducing reference peaks of the same size
as sample peaks into a gas chromatogram a doddle.

The thingy has been described in Rapid Communications in Mass
Spectrometry, Vol 11, 1775-1780 (1997). A nitrogen application
(actually a dual isotope application, N2 and CO2) has been
described in J. Chromatogr. A, Vol 842, 351-371 (1999) with yours
truly as author.

For a dual isotope application, one simply connects two modules
in series, one hooked up to a N2 cylinder, the other hooked up to a
CO2 cylinder, both of 99.9995% (5.5) purity.


Best regards,

                     Wolfram



*****************************************************
Dr. W. Meier-Augenstein, CChem MRSC
Senior Research Fellow

University of Dundee, Dept. of Anatomy & Physiology,
OMS, DUNDEE  DD1 4HN, United Kingdom

Tel. (B): +44-(0)1382-34/5124 or /4574
Fax  (B): +44-(0)1382-34/5514

e-mail (B): [log in to unmask]

URL: http://www.dundee.ac.uk/anatphys/wma/meieraug.htm
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