Sensitive Detection of By-Products Formed in Electrically Discharged Sulfur Hexafluoride

A new method, based on fluoride ion exhcange reactions, for detecting many of the by-products of electrically discharged SF6 is described. The technique employs an ion-molecule reaction cell and a mass spectrometer capable of detecting negative ions from a high pressure gas (up to P=100 kPa). Because of the high rate constants (k> 10-10 cm3 s-1) for processes of the type SF6- + X ---> XF-~ + SF5, where X is a molecule having a high fluoride ion affinity such as SOF4, SiF4, SF4, WF6, etc., concentrations below 1 ppb (part per billion) of selected by-products are detectable. Results are shown for a sparked sample of SF6, analyzed by this technique, and they are compared with the standard positive ion-electron impact mass spectral analyses. Increased detection sensitivity via the addition of specific materials, to SF6 gas insulated high voltage equipment, to enhance the production of certain by-products is discussed also.