Dear John,

Some comment on your posting. First of all the CO2 gas injected directly
into the open split in front of the ion source is NOT a reference gas! Using
such terms is very misleading. The gas is only used for monitoring purpose
of the IRMS performance (eg to detect drift during a run). It cannot and
should not be used for calibration or reference purpose at all. Because of
the single inlet set-up this is a must to have any certainty of the IRMS
behaviour - if using double inlet this certainty is included in the
continuous measuring against ref gas (here it is a ref gas in proper
sense!). This difference in procedure also is one of the factors why single
inlet IRMS is less precise than double inlet IRMS (although the contrary is
reported too many times - wonder how that can be?!?).

The real reference has to be a similar material as the sample to be measured
(carbonates in your case). A reference material has to pass the complete
analytical procedure and the gas produced from that material can be named
reference gas. If not following this procedure there is a risk the obtained
delta values do not represent the same scale (i.e. comparing apples with
pears).
Indeed gas under pressure released may (will) fractionate isotopically.

So, if you have shift in your monitoring gas caused by other factors than
drift in the IRMS, you have a problem. If such fractionation is used to
correct IRMS drift it will introduce quite some error.
Try a container with lower pressure - maybe that cures the isotopic drift of
the monitoring gas into acceptable proportions. Wonder to what extent this
is happening undetected in other CF-systems? Any opinions on this?

Best wishes,
Pier.

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> From: J T Hill <[log in to unmask]>
> Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
> Date: Tue, 11 Apr 2006 12:43:39 +0100
> To: <[log in to unmask]>
> Subject: [ISOGEOCHEM] CO2 Cylinder
> 
> Hello all
> 
> We have a Thermo Gasbench connected to a Delta XP for routine
> analysis of carbonates.  We are experiencing problems with the
> reference gas fractionating as it passes through the regulator.  We
> have to measure changes in the ref gas composition that occur over a
> diurnal cycle, by using very frequent standards in every run, and
> while we can get good analytical precision this way, it is
> inefficient.  We think the problem arises because the CO2 in the ref
> gas cylinder is pressurised at the liquid-gas critical pressure (~64
> bar), and therefore the CO2 is effectively changing phase as it
> passes through the regulator.  The changes in ref gas isotope
> composition seem to track changes in lab temperature.
> 
> If this diagnosis is correct, we should be able to solve the problem
> by putting ref gas in an intermediate reservoir at lower pressures,
> so that it is gaseous before it passes through the regulator.  We're
> having trouble obtaining high-purity gas at lower pressures
> commercially.  Another idea is to install a low pressure (up to 4
> bar) reservoir which can be connected directly to the GasBench refgas
> inlet, and topped up at quite long intervals from the main CO2
> cylinder.  This would require recalibration of the ref gas after each
> top up, but a large cylinder would last a couple of weeks or more.
> 
> Our question for IsoGeoChemists is:  how have other users of GasBench
> or similar equipment coped with this problem?  And what types of
> cylinder can anyone recommend?  We've made inquiries with Restek
> regarding low pressure reservoirs, and colleagues at Amsterdam say
> they use a diving tank.  Any advice out there, before we get the lab
> wallet out?
> 
> Best wishes,
> John Hill & Tim Atkinson
> University College, London, England
> 
> 
> John Hill
> Mass Spectrometry Facility
> University College London
> Chemistry Department
> 20 Gordon St.
> London
> WC1H 0AJ
> 
> Tel:  020 7679 4605
> Fax: 020 7679 7451
>