Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

240 EXPERIMENT A L MET H 0 D S F 0 R H ET E R 0 GENE 0 US RE ACT I0 NSmass spectrum and confuse or falsify the results of the experiments. There areways round this difficulty by careful choice of filament, which are discussed inSection 2.1.3b with reference to ion gauges.(a) Magnetic defection instrumentsThe conventional magnetic deflection mass spectrometer has a very highresolution, but suffers from the disadvantages of size and a relatively high backgroundpressure. In order to study reactions at low pressures, it is usually necessaryto operate the spectrometer in the high vacuum or ultra high vacuum region.This requirement demands a small spectrometer which can be baked out to reducebackground contaminants. Davis and VandersliceZs6 have developed a suitableinstrument for ultra high vacuum work and this design is now sold commerciallyby the General Electric Corporation, Schenectady.The instrument is a 90" sector, 5 cm radius analyser and can resolve adjacentmasses up to about 140. It can detect partial pressures of lo-'' torr and whenused with a suitable electron multiplier, the mass scan can be displayed on anoscilloscope. The spectrum can be scanned in a few milliseconds thus enablingrapid reactions to be followed.(b) The omegafronSommer et aZ.257 were the first to construct this type of mass analyser, whichwas later simplified by Alpert and BuritzZs8. The principle of operation is similarto that of the cyclotron (Fig. 33). Ions, formed by an electron beam passing throughCollector plate/VoltageFig. 33. Diagram of the simplified type of omegatron developed by Alpert and BuritF. Fromref. 19.