Hi Richard.

I agree, I should have said "the isotopic composition of the CO2 taken off will not change in a way that is immediately noticeable" (as long as there is any liquid CO2 left).

Any minute and slowly occurring changes over time are taken care off by always analysing 2 scale end-members as scale anchors with one's samples and by scale normalizing measured delta values on this basis.

I trust whenever you are reviewing manuscripts for a scientific journal you also will take as much issue with any results reported solely on the basis of comparison to cylinder gas (inappropriately referred to as "reference gas" for the reason you mention) or on the basis of comparison to a single standard (in conjunction with a cylinder gas).

Wolfram

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Richard Becker
Sent: 26 May 2016 10:25
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] weird standard values

Without going back through the whole message thread to see if I've missed something, I think I've got to take issue with what you say in the paragraph below. Yes it is true that as long as liquid remains, the equilibrium pressure of the gas phase will be maintained, and a difference in isotopic composition between liquid and gas, if one exists, will be maintained.
However, if the gas is isotopically different than the liquid, material balance requires that removing gas has to change the composition of the total remaining liquid plus gas in the tank. The contents of the tank will drift over time, although much more slowly than what happens in a Rayleigh process because of the buffering effect of  the liquid.

Richard


>Remember, compressed CO2 is a liquid.  As long as liquid CO2 remains in the tank, there is an equilibrium between liquid CO2 and gaseous CO2 sitting at an equilibrium pressure of about 55 bar at room temperature.  As long as there is liquid neither the cylinder pressure reading nor the isotopic composition of the CO2 taken off will change.  The moment the liquid reservoir is exhausted, the pressure will start to drop and the isotopic composition of the CO2 taken will continuously change.  Withdrawing amounts from a limited reservoir is a Rayleigh process (in the same way d2H and d18O values of meteoric water change over subsequent precipitation events from the same but perpetually decreasing cloud).
>
>
>Regards,
>
>Wolfram

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