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MS TIP No. 11 - Leak Checking Mass Spectrometers


Tip Mass Spec Tips is a collection of tips relating to the operation and service of mass spectrometers. Some of these Tips have been reported in our newsletter, "The Mass Spec Source." If you have a question relating to the operation or service of mass spectrometers or would like to add your comments to the below tip and would like to have it included in this forum, please contact us. Input to this database of MS Tips is needed from our readers to complete this section.

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.