****************************************************************************** From: E.A.Underwood@exeter.ac.uk Subject: CI BACKGROUND Date: Wed, 24 Jan 1996 16:31:00 +0000 (GMT) Can anyone help me to identify a background spectrum I get when running +ve CI using ammonia? I,ll try to draw it below. The spectrum is only there when the gas valve is open and is not seen in EI. When samples produce a CI spectrum the one shown is lost in the baseline. 193 207 224 267 281 297 371 388 l l l l l ll l ll l l ll ll ll l ll ll ll l ll lll ll ll lll lll lll ll I,m sure it has to come from the gas bottle or the line, but what is it? Thanks for your help, Eric Underwood. Chemistry Department University of Exeter Exeter England E.A.Underwood@exeter.ac.uk ****************************************************************************** From: marcu02@ibm.net Subject: Re: CI BACKGROUND Date: Thu, 25 Jan 96 20:09:14 Masses 207 and 281 ring a bell saying column-bleeding from a methyl-silicone/phenyl- silicone type of column. I'm not familiar with CI GC/MS, but I've seen plenty of peaks and background with 207 and 281 amu when running EI GC/MS using the DB-1- or DB-5-type columns from, say, J&W. Jorn Marcussen Flavour Analyst Danisco Ingredients, Denmark ****************************************************************************** From: m.vandeweert@chemail.chem.ruu.nl (Weird L. Marcowitz) Subject: Re: CI BACKGROUND Date: Fri, 26 Jan 1996 09:31:45 +0100 Organization: Academic Compter Centre Utrecht People here think the 207 and 281 are from siliconpolymers. Probably you have some silicon somewhere in the connection between the gas bottle and your MS, HTH, Marco ****************************************************************************** From: Heinrich Luftmann Subject: Re: CI BACKGROUND Date: 26 Jan 1996 10:36:06 GMT Organization: Org.Chem.Inst. Uni. Muenster For me it looks like "silicon oil" at least the peaks 207/281 (regard the isotope pattern). They are normally formed by an EI fragmentation. Where and how do you add your CI gas? If you put it through a piece of coated fused silica column you may mobilize (decompose) the stationary phase. But there are much more possibilities. Good luck. Heinrich L. ****************************************************************************** Date: Fri, 26 Jan 1996 11:14:10 -0700 From: Paul_Peterman@nbs.gov (Paul Peterman) Subject: Response to "CI Background" of EA Underwood Your abundant ions m/z 207 and 281 are very indicative of organosilicone, although usually recognizable in EI. Typical (M-73 or M-74)+ seem to be prevalent. Do you at least have a higher 73 ion in EI when the gas line is open? I suggest that you have some silicone oil or grease in the gas line, gas valve(s), connector, or other causing your background. Good Luck, Paul Peterman US Dept of the Interior Midwest Science Center Columbia, Missouri USA ****************************************************************************** From: Kirk Yeager Subject: background peaks Date: Sun, 28 Jan 96 14:22:23 MST While running a Finnigan TSQ, I ran across backgrounds similar to what you asked about. We attributed it to the slight decomposition of the siloxane stationary phase. This was sometimes accompanied by small peaks occurring at regular intervals in the GC trace we were analyzing. The analysis of the peaks gave us library matches indicative of siloxanes. The baseline had similar peaks, but not in as great of an abundance. Hope this helps. Kirk