I have not found a reference that works out the relevant equations
specificially for fractionation in IRMS inlets. However, there is a large
literature on general vacuum technology and engineering, which is used to
design pumping systems, and applies to this general issue. A convenient and
condensed discussion with references can be found in the product guide from
Leybold Vacuum Products, Inc. "Product and Vacuum Technology Reference
Book"; the issue before me is dated 1992. There is a discussion of laminar,
molecular, and knudsen flow regimes, the latter of which cause isotopic
fractionation. Isotope flow through crimped aperatures is not specifically
covered since the vacuum folks care less about isotopes. However, the
equations are there that can be used to show that, for instance, heavy
species are selected against when moving through a molecular leak (defined
by mean free path). Check with the local physicists for this reference if
you don't have it.
I'm also curious to know if anyone knows of an available reference
that treats isotopic flow specifically.
At 01:58 PM 7/19/96 GMT0BST, you wrote:
>since I was not able to locate any information about it in the
>literature, allow me to post the question here:
>I am interested in the fractionation (13C/12C) that occurs
>when CO2 diffuses (or streams) through small openings or tubes. Is
>there any fractionation other than the mostly quoted +4.4 permille?
>If so, how is it related to the geometry (?) and size of the opening?
>Since this question must be of some importance for the design of mass
>spectrometers, I suspect quite some data will have been gathered
>Any hints or comments are VERY welcome !
>Stable Isotope Laboratory
>Department of Geography
>University of Wales, Swansea
>Swansea SA2 8PP
>Wales / GB
>Tel. +44 1792 295149
>Fax. +44 1792 205556