Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

244 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSW A S2 S1 ZFa1miFig. 36. Schematic construction of the farvitron gauge head and general indication of the potentialdistribution in the longitudinal axis. The potential trough is obtained by the different negativevoltages applied at the accelerating grids S1, SI and Z.forced by the potential distribution in the instrument to oscillate along the tubeaxis between A and S. The oscillating frequency (Mc . sec-') is given bywhere C is an instrument constant and U is the maximum potential drop throughwhich the ions pass. Those ions whose oscillating frequency is in phase with theapplied field will be gradually accelerated towards the signal electrode J. Thissignal is amplified, demodulated and transmitted to the screen of an oscilloscope.If the applied r.f. voltage is oscillated in phase with the mains frequency the massrange can be scanned 50 times per sec, allowing rapidly changing phenomena to bestudied. The instrument operates over the range torr to 2 x torr althoughthe resolution is less than that of the omegatron or the topatron.(e) Other typesA cycloidal mass spectrometer was devised by Bleakney and Hip~le''~ whichoperates with crossed magnetic and electrostatic fields. Measurements by otherworker^''^ show that with a magnetic field of 5000 gauss, masses 1-500 can bedetected. Adjacent mass numbers are separable up to 100, but the resolution decreasesfor higher mass numbers. Partial pressure detection down to torris claimed.The quadruple mass filter was described by Paul and Raether'". It consistsof four parallel cylindrical rods to which is applied a sinusoidal voltage. Undergiven conditions of voltage and frequency, only ions of certain e/M ratio reach the