Stable Isotope Geochemistry


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J T Hill <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Tue, 11 Apr 2006 12:43:39 +0100
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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.

Tel:  020 7679 4605
Fax: 020 7679 7451