>The nature of ionisation would make it extremely unlikely for two ions to
>'meet' and form a molecule.  The ions are relatively few and far between,
>at a very low pressure and have the same charge.  I've never seen water
>dissociating to form mass 16.  This would involve both H atoms being
>dislodged, so you'd expect to have a huge 18, a smaller 17 (H20 + e -> HO+
>+ H) and a smaller still 16 (HO+ + -> O+ + H).

We see exactly this on our Finnigan 252. When water is present (mass 18) we
always have a smaller mass 17 and an even smaller mass 16. Of course, this
isn't a quadropole....







Paul Eby

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Biogeochemistry Lab
School of Earth and Ocean Science
University of Victoria

phone: 250-721-6183
fax:   250-472-4620