Carlos Jose Maestro
and Juan Carlos Maestro Antonio Del Viso 234 Cordoba, Argentine Republic ~
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Mass Spectrometer for Solid Samples
This article describes the construction and operation of a mass spectrometer for solid samples. The work was done by undergraduate students based on the original paper by Dewdney ( I ) . The mass spectrometer uses magnetic deflection. Though very simple, i t acquaints the student with techniques for high vacuum, glass working, electronic equipment, and, consequently, scientific and analytical techniques. Introduction
according to their qlm relation. Thus, through electrometric detection on a metallic electrode and oscilloscope recording, it is possible to determine the type of ions of a given sample. The theory of mass spectrometry is discussed clearly in several texts (2-5). Equipment
The equipment was divided into four parts: (a) working tube (Fig. 1); (h) high vacuum equipment; (c) electro-
Among different types of spectrometers available a t the present time, this model was found to he of the simplest construction and most useful for laboratory demonstration a t the undergraduate level. Ions are obtained by heating a filament and all the ions get the same kinetic energy through a difference potential, variable and oscillating in connection with the horizontal sweep of the oscilloscope, used as a recorder; the ions, going through an area of the magnetic field, will deflect
b Figure 1. The working tube
Figure 2. (A) Power supply: ( 8 )mass spectrometer control
Volume 50,Number 6, June 7973 1 439
magnet; and (d) electronic console (Fig. 2, A and B ) . Parts (a) and (b) were constructed entirely of Pyrex glass in our laboratory. The equipment was assembled in two separate sections mounted on wheels. This made the instrument more versatile for demonstrations since it could be moved easily from one laboratory to another. The Working Tube. The working tube has three zones: (1) the ionization chamber; (2) the magnetic deflection chamber and (3) the collection chamber. The sample is ionized over an ionization surface (6); in this case, a tungsten filament on which one drop of saline solution is placed. The filament is heated by means of approximately 100 MA, activating atoms which, if their potential of ionization is small relative to the work function of the tungsten, will be liberated as positive ions. The alkaline metals: Li, Na, K, Rb, and Cs have a sufficiently low potential of ionization, and therefore, they constitute a good source of ions. On both sides of the filament are parallel metallic electrodes whose voltage can be regulated to focus the ions in the principal slit whose opening is 0.025 mm. The ionization and focusing chambers described above are mounted on a standard 19/38 terminal, which permits rotation of the chamber without loss of vacuum t o place the slit in parallel with the magnetic field. The standard terminal is covered with an oil reservoir to avoid high vacuum losses. The tube of the magnetic field chamber has a diameter of 9 mm and a fixed curvature radius of 40 mm to give a dispersion of 0.04 for this equipment. The collection chamber is a Stainless Steel tube closed a t one end and located beyond the image slit whose gap is 0.025 mm; this is all mounted on a standard 19/38 terminal with the same care as in the ionization chamber. High Vacuum System. This system consists of a mechanical pump whose final vacuum is 10 3 mm Hg; a 3stage oil diffusion pump, metal glass with high vacuum Octoil S oil (from Consolidated Vacuum Co.); a 3-stage mercury diffusion pump connected through a dry trap; and between the pump and working tube there is a Dewar trap filled with liquid air to prevent mercury vapor from poisoning the working tube. Vacuum measurements are taken with a Leybold (Catalog 16315) penning type, vacuum gauge whose range is 1.J0-3 to 1.10-5 mm Hg. For lower pressures an ionizing Leybold vacuum gauge was used. Electromagnet. The electromagnet used was made according to the method of S. Boersma who recommends the use of mamets made with transformer cores. Therefore. a Phywe tr&former for teaching purposes was used. he mamet consists of two bobbins with 600 turns of 1 mm copper wire, with a total resistance of 5 ohms. The electromagnet is fed with batteries, and a calibration curve is made using a Hall effect fluximeter, obtaining fields of 3,000 Gauss. Electronic Console. The console consists of 5 main parts: (1) current supply-the electromagnet is fed by a battery through a rheostat. (2) High voltage source and control console (Fig. 2, A and B ) , stabilized variable power supply from 0 to 500 V with a 1%MA gain and a maximum intensitity of 100 mA. The accelerating plates are fed by the control console and an aggregate of alternate voltage maintains a continuous sweeping of the spec-
440 /Journal of Chemical Education
MAGNETIC FIELD : 3000 Gmurr
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