MS TIP No. 1 - Freon for use in Mass Spectrometer Leak Checking

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Author: John J. Manura

Affiliation: Scientific Instrument Services

Many mass spectroscopists have routinely utilized Freon (Dichlorodifluor-methane) for locating mass spectrometer vacuum leaks around flanges, feedthroughs, probe inlet seals, diffusion pumps and other seals on the mass spectrometer vacuum system. The product was convenient to use since it was supplied in a small compressed air can with a nozzle (DustOff by Falcon Products). The gas could easily be sprayed around a suspected leaking fitting while the mass spectrometer was scanned for the predominant ion in the Dichlorodifluoromethane spectra. If a fitting was leaking, the Freon would produce a mass spec ion of 85 M/e in the EI mode. In the last two years, the availability of Dichlorodifluoromethane has been discontinued by all manufacturers in small compressed gas cans due to its ozone depleting properties. As a result other products will need to be utilized for mass spectrometer leak checking.

A similar product, Monochlorodifluo-romethane, sold under the same name of Dust-Off is available from Scientific Instrument Services (part # FGB). This product can be used in the same manner as the previous product, however the mass spectrometer should be scanned for the 51 M/e ion in the spectrum. Monochlorodifluoromethane (M.W. = 86) produces a major peak at 51 (100%) with minor peaks at 67 (15%) and 31 (17%) when the mass spectrometer is scanned in the EI mode. However it is expected that sometime soon this product will also be eliminated because it too is a fluorocarbon and may have ozone depleting properties much like its predecessor Dichlorodifluoromethane.

Perhaps the best product to use for leak checking the flanges on the mass spectrometer is Argon gas. Argon (M.W. = 40) produces a predominant molecular ion at M/e 40 when scanned in the EI mode. Argon can be purchased in larger gas cylinders from your local gas supplier. To this tank you will need a two stage regulator and a length of flexible 1/8Ó tubing for directing the gas flow to the area to be leak checked. This product will not have the harmful environmental effects that the previous fluorocarbons have on the environment.

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