Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2THE SOLID-GAS INTERFACE239Detector- Preamplifier 4 AmplifierElRecorderHlghvoltageCount ratecomputerFig. 32. Block diagram of a pulse counter for radio-isotope measurements. From ref. 246.a Geiger-Muller counter or a scintillation counter, particularly the liquid scintillationdetector246. It is necessary to connect the detector to an electronic circuitwhich is able to count the radioactive pulses. Fig. 32 shows a block diagram oftypical components. The output from the detector must normally undergo oneor more stages of amplification before being counted and recorded. Isotopeeffects from 14C are known, i.e. the rate of reaction using 14C compounds is notthe same as that using "C compounds. This must be remembered when makingrate rnea~urements~~'~ z48.None of the other radioactive isotopes have been extensively used in catalyticreactions. However, sufficient data exists on the preparation of suitable compoundsto make more extensive use of other isotopes possible, where other methods arenot satisfactory. The problems and hazards associated with the handling of radio-active materials must be carefully considered and reference to the specialist publicationsdealing with radioisotopes is necessary246*249iz5 '.2.2.5 Mass spectrometryMass spectrometers have been widely used to study adsorption phenomena andcatalytic reactions. Several excellent reference books on mass spectrometryZS1 -253are available and advances in the subject are periodically reviewed254. The magneticdeflection spectrometer is the most popular type of instrument and manycommercial versions are available. Attention is concentrated in this section ontypes of mass spectrometer which have found particular application in the studyof heterogeneous reactions. Much of the relevant work can be found in the Transactionsof the American Vacuum SocietyzS5.One major difficulty is associated with the use of all mass spectrometers employinga hot filament. Interaction of gases with the filament by adsorption orchemical reaction tends to change its emission characteristics and thus alter thesensitivity. New chemical species may be produced which appear as part of theReferences pp. 270-278