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- Mass Spec Tips1 - Freon for use in Mass Spectrometer Leak Checking 2 - Mass Spectrometer Probe Cooling 3 - Sample Vials for Direct Probes 4 - Selection of Vacuum Pump Oils for Lowest Mass Spec Background 5 - Determination Linkages in Biomolecules of Disulfide 6 - Transfer of H.P. ChemStation MS and GC Chromatograms from PC to MAC Computer 7 - Repairing Clogged Thermospray Probes 8 - Tuning a Finnigan 5100 to Meet BFB or DFTPP Criteria 9 - H.P. 5971 Transfer Line Tip for Direct Introduction of Capillary Column 10 - Troubleshooting Finnigan 5100 GC/MS Systems 11 - Leak Checking Mass Spectrometers 12 - Elimination of Memory Peaks and GC Background Noise 13 - SuperIncos Mapped Software Print Buffer Lockup 14 - Reduction of Peak Tailing 15 - Electron Multiplier Sensitivity 16 - INCOS Procedure for Calibrating on the Finnigan 4500 17 - Extending Electron Multiplier Life 18 - What techniques or methods do you use to determine if the electron multiplier 19- What techniques or methods do you use to detect vacuum leaks in your mass spectrometer 20 - Extending Lenear Range of the Mass Spec Article - Improving Sensitivity in the HP 5971 Mass Spectrometer - Part 1 and Part 2
- 11 - Leak Checking Mass Spectrometers (This Page)
Author: Richard A. Berger
Affiliation: Washington University
School of Medicine
The November, 1993 issue of THE MASS SPEC SOURCE contained a Mass Spec Tip concerning the use of Freon for leak checking Mass Spectrometers. We had used Freon-12 (Dichlorodifluoromethane) for this purpose until it was implicated in the ozone layer depletion phenomena. While Argon (MW 40) works well for leak detection in electron ionization (EI) mode, it does not work well in positive chemical ionization (PCI) mode using methane as the reagent gas since it is overshadowed by one of the methane complex ions (C3H5+, M/E 41). Argon does not show up at all in negative chemical ionization (NCI) mode.
We searched for some time for a gas which was:
1. Non-flammable, non-toxic, and non-reactive,
2. Inexpensive and readily available, and
3. could be used in EI, PCI, and NCI modes.
We found that sulfur hexafluoride (SF6 MW 146) satisfies most of these requirements. It is non-flammable and non-reactive; the TLV-TWA is 1000 ppm. It is available as a pure gas, usually shipped at its vapor pressure of 320 PSIG, or as an uncertified mixture with air, nitrogen, helium, or argon. Eighteen pounds of SF6 in a G size cylinder cost less than $200. This included the cost of a control valve and cart to move the cylinder (about 50 lbs gross weight) from machine to machine. In EI and methane PCI mode, the sulfur hexafluoride spectra has M/E 127 [M-F] + as its base peak. In NCI mode M/E 146 (M-) is the base peak.