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.
Yttria coated filament at start
Yttria coated filament after 16,000 cycles