Stable Isotope Geochemistry


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[log in to unmask] (Peter Stow)
Tue, 19 Nov 1996 20:10:32 -0500
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>Dear Isotopers,
>recently I discovered that the zero enrichment tests (same gas on sample and 
>standard sides - by definition should give delta values of 0) I run routinely 
>on my 10 year old Finnigan MAT 251, do not yield the desired results. Delta 
>45/44 is all right, giving 0.0xx per mil. Delta 46/44, however, runs at 
>0.2xx (mostly positive). All this began after a series of power failures in 
>our lab, which flooded the instrument with air (up to three times a week).
>As I could figure out as yet, the most likely culprits are the inlet 
>capillaries; when I heat them up, they start outgassing, visible mostly on 
>mass 44 with different intensities on sample and standard side.
>My question: Can these capillaries build up some sort of a "memory effect" 
>which would explain the situation and what could be the solution? - Exchange 
>the capillaries? Bake them out?
>I would appreciate any advice, suggestion etc. which may help to sort out 
>this problem.
>Kind regards, 
>Dr. Uwe E. Horstmann
>Council for Geoscience - Geological Survey
>P/Bag X112
>0001 Pretoria
>South Africa
>Tel.: +27/12 841 1361
>Fax.: +27/12 841 1278
>e-mail: [log in to unmask]

If the zero enrichment is good for one of the 2 ratios but significantly off
for the other then the most likely cause is contamination.  Fractionation or
mis-balanced capillarioes will usually give a delta 46 with twice the value
of delta 45.

If we assume that this is contamination then baking the capillaries may
help.  I would run a B-SCAN (background mass spectrum scan) over the whole
mass range on both the standard and sample side before and after heating the

It can be shown mathmatically that a high backgroud in a collector cup will
give a delta for the ratio associated with that cup and will not cancel out
in the delta equation.

Peter Stow
                                 ISO-MASS Scientific Inc.
                                319-5940 Macloed Trail S.W.
                          Tel. (403)255-6631 Fax. (403)255-6958
                    Toll Free 1-800-363-7823  EMail [log in to unmask]