Hi Bettina,
my questions regarding your problems:

1)What kind of mass spec do you run?

2)Did your turbo pump break down for a short time while the forevakuum pump
was still running? If yes, oil contamination of the ion source, flying tube
and inlet system could explain your mass independent fractionations.

3)Do you run a differential pumping system? This is a prerequisite for
oxygen isotope analysis under continuous flow conditions, especially for a
mass spec having small dimensions (systems with an accelerating voltage of
3 kV). Otherwise mass 32 is spread by He impact towards cup 32 and even
towards cup 34 (tailing effect). The manufacturing firms do not know about
this fact as those small mass specs have only been used for C and
N-determinations so far. For those isotopes tailing effects are neglegtibly
small because the rare isotopes 13 and 15 are much more abundant compared
to 17O. So, C and N are working without differential pumping, but not O and
systems are often sold without that knowledge.

4)If your source is new you have to precondition it with oxygen first. For
a 3kV system you also have to look for the best source conditions each day.
Especially at the beginning linearity conditions are changing from day to
day and could explain your daily drift. This is most probably caused by a
reaction of oxygen in the ion source, e.g. with the filament. This reaction
has to attain equilibrium first.


Jens


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