Demonstration Mass Spectrometer' TT'.
H. EBERHARDT
California Institrrte o f Technology, Pasadena, California model employing macroscopic balls for atoms is essentially impossible because of the very unfavorable ratio of charge to mass which may be obtained with such particles. On the other hand, the velocity selector type of mass spectrometer described by Smythe2 involves in an understanding of its operation only qualitative ideas of electrostatic attraction and repulsion which are more familiar and intuitively more easily grasped than electromagnetic concepts. In this technique, a beam of ions, which are characterized by various ratios of charge to mass but possess a uniform velocity, is produced by passing a beam of particles with nonuniform velocfties through a slit system in a transverse alternating electrostatic field. Only particles with certain velocities determined by the constants of the apparatus and the frequency of the applied field will pass through the slits. The beam leaving the velocity selector may be analyzed by electrostatic or magnetic fields into components of constant ratio of charge to mass, and i t is in this analysis that the application of the mass spectrometer to research problems becomes clear. Figure 1. The technique suggested here is designed to dernonstrate with macroscopic particles the analysis of a beam of particles of various masses but uniform velocity into groups of uniform mass. The apparatus is indicated diagrammatically in Figure 2. The particles are dropped starting a t rest from the point 0 and allowed to fall freely under the influence of gravity; thus, althouch the downward comDonent of the velocity of any particle is notconstant along its path, a t any point in the path this component of the velocity is independent of the mass of the particle--that is, the velocity of particles in the beam is uniform, but not constant over the path. After falling through the short distance a, the particles pass through an electrostatic field maintained between the plates P and P'. As they enter this field, they fall near the sharp tip of a bullet-shaped piece of metal, B, soldered to one of the deflecting plates, P, and pick up a charge from the corona discharge from this point. Because of this charge, which is of the same sign as that on the plate P, the particles receive a horizontal component of velocity as they pass through the field. After leaving the field, the particles fall freely in a parabolic orbit until they are caught by some collector a t the end of their paths. An analysis of the motion of particles in the apparatus is quite straightforward and results in the following expression for the displacement, s, in terms of the dimensions of the apparatus as indicated in the figure, the potential, V , applied to the
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N RECENT years the mass spectrometer has be-
come one of the most important tools in connection with the determination of atomic and molecular weights, and no discussion of this topic, even on an elementary level, may be considered complete unless attention is directed to the principles of mass spectroscopy. A discussion of the conventional types of mass spectrometers based upon the Aston, Dempster, or Bainbridge techniques is complicated by the uecessity of introducing concepts relating to the motion of a charged particle in a magnetic field; and, although these concepts are necessary for an understanding of the operation of spectrometers of the conventional type, they do not contribute greatly to an understanding of the value of the mass spectrometer in the problem of analyzing mixtures of components of differing masses. Moreover, a demonstration of these techniques with a
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Contribution Xumber 1030 from the Gates and Crellin Laboratories of Chemistry.
Saurns, W. R., (1932).
220
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J. MATTAUCH, Phyr. Rev.,40, 429-33
plates, and the charge, e, picked up by the balls falling through the corona discharge:
It will be seen that for a constant applied potential, V , the displacement is proportional to elm. If the potential remains constant, the charge picked up by the particles from the corona will be constant, providing the particles are of the same size and shape, and the, displacement will be inversely proportional to the mass of the ~articles: hence. a mixture of balls of diierent masses but the same size and shape may be separated into groups of the same mass by spacing collectors a t positions corresponding to various values of the displacement, s, from the position s = 0 where the balls will fall if no deflecting potential is applied. For atoms we have used wooden balls which are approximately spherical, about one-half inch in diameter, and which weigh about one gram. It is desirable to select from a large number of balls those which have nearly the same mass in order to get resolution as sharp as possible. For heavier atoms, holes are drilled through the center of these balls and filled with Wood's metal, the size of the hole determining the final mass of the ball. The deflecting plates are sheet metal about five X six inches in size, with the edges and corners rounded to reduce corona as far as possible. The plates are mounted on a sheet of lucite which serves as an insulating spacer, three and one-half inches wide, and which is fastened by a strut onto a backboard which supports the dropping mechanism and the rest of the apparatus. The "bullet" is a short length of onequarter-inch brass rod turned to a sharp point on one end and rounded on the other. It will be observed from the diagram that the plates are mounted asymmetrically with respect to the paths of balls falling freely in the absence of a deflecting field. Such an asymmetrical mounting allows a higher field strength to be maiutained for a given spacing of the plates and applied potential and is desirable since the balls are deflected only in one direction. Care must be taken in the design of the device to see that adequate spacing of the deflecting plates is allowed so that balls exhibiting the maximum displacement desired will not hit the deflecting plates. The dimensions of the apparatus we have found convenient may be enumerated with reference to Figure 2 as a = one and one-quarter inches, b = six inches, c = four feet, and d = three and one-half inches. The maximum displacement observed is about one foot. None of these dimensions is critical, and it is clear that an apparatus may be designed to fit just about any desired circumstances. The requirements as to potential are difficult to estimate precisely but are easily filled by a Wimshurst Machine. The potential developed by such a machine is very much more than adequate to operate the spectrometer providing leakage is minimized. Since both deflecting plates are insulated by the lucite separator,
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O F ESSENTIAL DETAILS OB MASSSPECTROMFIGURE2. SKBTCH SHOWING PATHOF A DEFLECTED BALL. THESYMBOLS ARE EXPLAINED I N THE TEXT
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both from each other and from the ground, and since the charging may be accomplished by allowing the balls to fall through the corona, no difficulty should be experienced from leakage. It wlll be found that the potential developed by the Wimshurst Machine increases rapidly a t first as the rate of revolution of the discs increases, but when the corona discharge commences, the potential increases very much more slowly with increase in the rate of revolution; hence, when the corona appears and is sufficient to charge the balls, it is quite easy to maintain the voltage applied to the system constant by maintaining the rate of revolution of the discs of the generator approximately constant. The major difficultly in the use of the device consists in dropping the balls individually with no velocity in any direction a t the time they are dropped. A simple solution to the problem may be obtained if no attempt is made to make the feeding automatic. A hole large enough to accommodate a ball is drilled in a sheet of wood or lucite about one-quarter inch thick, and the sheet is fastened a t the top of the backboard so that the hole corresponds to the path of unddected particles. A lever is arranged so that the ball is kept from falling when the lever is under the hole, but is allowed to fall when the lever is swung aside. The balls must be
drilled in the disc come consecutively over the hole in the base sheet through which the balls drop. A feeding device, consisting of a vertical tube containing the balls one on top of the other, is located over another point of the disc so that the balls are dropped into the holes in the rotating disc as these holes come into position under the tube. The disc is about one-quarter inch thick, so that only one ball at a time may fall into a hole and is so spaced with respect to the base sheet that the ball rolls on the base sheet while it is being carried to the hole through which it is dropped. The disc is driven by a belt-and-pulley arrangement from a motor located on the ground. If this arrangement alone is attempted, it will be found that the balls still have a small component of velocity in the direction tangential to the disc a t the time the ball is dropped. This component may be nullified if a short tube, about one inch long, made by winding paper about a cylindrical form, is fixed to project downward from the hole through which the balls drop. If the disc is rotated reasonably slowly so that the balls drop a t a rate of about one per second, good results may be obtained with this device. A collector which will catch the balls a t the end of their paths and keep them segregated according to the displacement that they have experienced offers little difficulty. We have used a glass-fronted box equipped with suitable partitions to divide the box into secti'ons about two inches wide. The partitions are sharpened on their top edges to minimize as far as possible scattering of the balls by collisions with these regions. The floor of the box is slanted so that the balls roll toward the glass front to aid in observing the results of an exFrGunE 3. DETAIL OF DROPPING MECHANISM AND D E R L E ~ I N O PLATES periment, but it is hinged a t the front and provided with a lever so that the direction of slant may be reloaded and released individually and may be conven- versed. Under this circumstance the balls roll toward iently guided to the hole by a stemless funnel placed on the back of the box and into a channel which empties top of the plate. Although this system is simple and out of the side of the box so that they may be recovered adequate, it is considered desirable that many balls may easily. In order to prevent the balls from bouncing out be loaded a t one time and then automatically dropped of the box, the floor of the box is covered with an easily individually. See Figure 3. compressed, nonelastic material, such as is used for One device for accomplishing this purpose consists packing fragile articles; but, to allow the balls to roll of a rotating disc which picks up a ball a t one point of easily and freely, this material is covered with a layer of its revolution and drops it a t another through a hole moderately s t 8 paper. appropriately located with respect to the deflection The author wishes to express his thanks to Professor plates. A sheet of wood (the "base sheet") containing this hole is mounted on top of the backboard in a plane Linus Pauling and Mr. David Hagelbarger for their parallel to the ground. The disc rotates on this base interest and valuable suggestions in the design and sheet about a point such that four holes symmetrically construction of this demonstration mass spectrometer.
