Dear James,
Have you perhaps generated CS+ ions?  Even if the  change in ion current at
mass 44 is minor, the contribution to mass 46 can be substantial since 34S
is 4 percent abundant as compared to 18O at 0.2 percent.  CS+ can arise from
ionizing COS or CS2 or some ion recombination  in the source.
Roy Krouse
-----Original Message-----
From: James Andrew Hall <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, September 28, 1999 3:51 AM
Subject: sulphur contamination


>Dear Group,
>                I have been investigating stable carbon isotope ratios of
>organic contaminants in the sub-surface of a disused manufactured gas
>plant. This involves injecting solvent extracts and getting the
>mass spectra and d13C ratio.  Whenever molecular sulphur is identified on
>the quadropole, the 46/44 ratio for the background subtraction on the IRMS
>(Isoprime, Micromass) goes off the scale, making automated or manual
>correction practically impossible. Has anyone any ideas what is causing
>the 46 ion? I can't figure it out. I hope the answer is not too obvious!
>                                Thank you
>
>                                James Hall.