> 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 Dear James, I'm afraid it just might be. Obvious that is. The answer that is. If you have a look at the EI mass spectrum of CS2 you will find fragment ions at m/z44 and m/z46 in the predicted ratio for natural abundant S isotopes. These fragments correspond to 12C32S and 12C34S, respectively. You either have contamination of CS2 present or it is formed in- situ. The EI mass spectrum of CS2 contains the following peaks in decreasing order of intensity: m/z76, mol peak and base peak (and its M+2), m/z32 (and 34) and m/z44 (and 46). Hope this helps. All the best, Wolfram ***************************************************** Dr. W. Meier-Augenstein, CChem MRSC Senior Research Fellow University of Dundee, Dept. of Anatomy & Physiology, OMS, DUNDEE DD1 4HN, United Kingdom Tel. (B): +44-(0)1382-34/5124 or /4574 Fax (B): +44-(0)1382-34/5514 e-mail (B): [log in to unmask] e-mail (H): [log in to unmask] URL: http://www.dundee.ac.uk/anatphys/wma/wolfram.htm *****************************************************