Re: [ISOGEOCHEM] Isotope fractionation WITHIN a
mass spect
Tim
Apologies if I have missed other replies to your question - I am
just catching up on a mass of e-mail after absence at EGU and
subsequent travelling.
I think the delta 18O will be fractionated to almost the greatest
extent possible. Nevertheless, its introduction to the mass spec aims
to leave it unfractionated. Viscous flow through the capillary ensures
that there can be no preferential transport of the lighter isotope due
to the bulk flow pushing along behind. However, the source is pumped
to high vacuum and most of the sample is pumped away before it gets
ionized and in this case the lighter isotope which is more mobile
(longer mean free path or higher diffusion coefficient) is
preferentially removed. So for m/z 46 relative to 44 the increase in
measured delta 18O is about 22.5 per mil I think.
If anyone would check my arithmetic, I would be grateful.
Equally, if any of the manufacturers has better data on the extent of
fractionation in modern, more closed and higher efficiency sources, it
would be good to hear from them.
This fractionation is very constant, but it is also why use of a
reference gas is needed - sample and reference suffer the same effect
and so their difference remains the same.
Two further associated points from the history of mass spec.
Viscous flow through a capillary ensures that even relatively
small samples are not fractionated during measurement. However,
in contrast, I believe that when Al Nier was measuring the absolute
isotopic abundances of elements in the late 1930s and the 1940s he
took advantage of molecular flow to get the right answers. A small
opening in the gas reservoir and no capillary ensured that the lighter
isotope entered the source vacuum preferentially which would have
given a lighter ratio had it not been for the source pump
preferentially removing the light isotope to the same extent leaving
gas of the original composition to be measured. This approach
fractionates the gas in the sample reservoir but if the amount of gas
is big enough the effect is minimal.
A further point from history: many people refer to the
change-over valve used for dual inlet as the "McKinney valve"
since it is described in the famous McKinney et al (1950) paper.
I referred to the McKinney valve in a paper I wrote in the 1970s but
in his review of it, Al Nier asked me to correct the name. Even though
it was decades later, Al was still annoyed that the Chicago group
visited his lab in Minnesota copied the idea of the change-over valve
but did not acknowledge it. The paper that appeared two years
before McKinney et al. and described the change-over valve for the
first time was by Byron Murphey from Al's lab (Murphey, B.F.
(1947) Phys. Rev. 72, 834-837).
best wishes
Max
At 11:08 AM +0100 4/23/07, Heaton, Timothy HE wrote:
Does anyone
know the extent of oxygen isotope fractionation which occurs for
carbon dioxide WITHIN a mass spectrometer?
In other
words, by how much would the ratio of 12C18O16O to 12C16O16O arriving
at the collectors differ from the ratio of the gas entering the
source? Qualitatively?
Tim H.E.
Heaton
NERC Isotope
Geosciences Laboratory
British Geological Survey
Keyworth, Nottingham NG12 5GG,
England
(www.bgs.ac.uk/nigl/index.htm)
Tel.
+44(0)115 936 3401
Email: [log in to unmask]
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Dr. Max Coleman
Senior Research
Scientist
Science Division
Jet Propulsion Laboratory,
Caltech
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