>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,
>
>Uwe
>
>***************************************************
>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
capillaries.
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]
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