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.