Instrumentation
November ISSO
Several new mass spectrometers have been developed for continuous monitoring of process streams bw Rd&hH. M third of this series of columns published in January 1945 [IND. ENG.CHEM.,37, p. 75 ad. sec. (1945) 1, discussed the status of mass spectrometry at that time. Then the mass spectrometer was just beginning to be accepted as an instrument capable of performing routine analyses of complex hydrocarbon mixtures. Mass spectrometry has advanced greatly since then, and techniques have been developed for working with compounds of higher molecular weight. The instrument is no longer limited to hydrocarbon samples; mixtures to be analyzed may now consist of inorganic or organic compounds of virtually any class so long as the components have a vapor pressure greater than 50 microns a t room temperature.
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Perhaps the most remarkable development in the field of mass spectrometry is the adaptation of the mass spectrometer for routine monitoring of continuous industrial processes. C. F. Robinson, H. W. Washburn, 0. E. Berry, and G. D. Perkins of Consolidated Engineering Corporation, Pasadena, Calif ., have described the instrument which they developed for this purpose in
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make it safe to use in areas where fire and explosion hazards exist. Finally, the method of presenting the data is simplified so that average plant operating personnel can interpret it. The standard sample is selected to have the optimum composition for the process being controlled, and the instrument is set so that it records each of the peaks being measured with a sensitivity such that all have the same height on the record. Then, as long as all the peaks have the standard height, the composition of the sample stream is equal to that of the standard and therefore optimum. Unfortunately, instruments of this type are very expensive, too expensive to use except in very large scale continuous operations where there is no other means of controlling the process. Because of the complexity of these instruments, it does not seem very likely that their cost can be reduced materially unless a completely different approach to the problem can be found. Fortunately, two fundamentally different types of mass spec-
instrumentation Positive ions passing the second grid at the time when the third grid is negative are accelerated toward the third grid. Those which pass the third grid as its radio-frequency potential with respect to the second and fourth grids reverses are further accelerated toward the fourth grid. The direct current potential of the collector electrode is adjusted so that only those ions which have received maximum energy from the radio-frequency fields can reach it. If the interelectrode distances and potentials are held constant, the mass of the ions receiving maximum energy from the radio-frequency fields is a function of the frequency, so that by varying frequency, the mass spectrum can be scanned. To secure greater resolving power additional stages are used. Figure 1 shows the construction of a three-stage tube, using three sets of radiodrequency grids.
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Figure 2.
Schematic Diagram of Ion-Reaonant M ~ M Spectrometer Tube
The second type radio-frequency mass spectrometer is the ion resonant type or "omegatron." Figure 2 is a schematic diagram of this type. In this device electrons from a flament form ions in a region where crossed radio-frequency and magnetic fields are maintained causing the ions to follow spiral paths. Those ions with the proper mass resonate with the radiotfrequency field, spiraling outward until they reach the collector elec%rodeand are measured. As in the case of the Bennetttype tube, the mass spectrum can be scanned by varying the frequency. Both Bennett and ion resonant type mass spectrometer tubes can be secured from General Electric Company, Schenectady, N. Y. Both are so new that their full capabilities have not been explored. However, it seems likely they will prove to be valuable for continuous monitoring of plant process streams even though they do not have the high resolving power which can be obtaioed with the magnetic deflection type instruments in their present stage of development. 94 A
