The Introduction to Mass Spectrometry (Excerpts)
Robert W. Kiser
Kansas State University
Selected excerpts from chapter 2 of "The Introduction to Mass Spectrometry"
Two very significant discoveries are due to mass spectroscopic studies. First, J.J. Thomson discovered that neon consisted of a mixture of two different isotopes (masses 20 and 22) rather than only a single isotope. This observation of the existence of stable isotopes is perhaps the greatest achievement that can be claimed by mass spectroscopy. Later studies showed that there are three stable isotopes of neon. Since Thomson's early work many other isotopes have been discovered and studied by mass spectroscopy. The consequence of this discovery was the realization that the chemical properties of an element are determined by the atomic number rather than by the atomic weight of the element, as was beautifully shown also by H.G. J. Moseley's classic X-ray line spectra investigations. The second significant discovery due to mass spectrographic studies was made by F.W. Aston. He observed that the masses of all isotopes are not simple multiples of a fundamental unit, but rather they are characterized by a mass defect; i.e., isotopes do not have integral masses. (It is interesting to note that in this paper, Aston reports two new isotopes of xenon-- xenon-124 and xenon-126-- present to about 0.1% each in natural xenon, and shows that chlorine-39 is not present in natural chlorine.)
The discovery of positively charged electrical entities was made in 1886 by E. Goldstein, when he observed that an electrical discharge at low pressures caused a slightly divergent discharge to stream through the openings in a perforated cathode. Later, W. Wien, Nobel Laureate of 1911, showed that rays of Goldstein were deflected in a magnetic field and then established that these rays carried a positive electrical charge.
Upon introducing such polyatomic molecules as COCl2 and hydrocarbons into the discharge tube of the parabola mass spectrograph, Thomson noted the formation of many parabolas, revealing the formation of a variety of positively charged fragments (e.g., those corresponding to COCl2+ , CO+,C+, O+, etc., from the COCl2). But, since the photographic plate has different sensitivities for the various ions, quantitative measurements of the relative intensities of the ions were not possible. Therefore Thomson replaced his photographic plate with a Wilson tilted electroscope and Faraday cylinder behind a parabolic slit. By changing the magnetic field and taking 10-sec measurements, Thomson was then able to measure the current corresponding to the various postitive ions as they were brought over the slit, and thereby obtained the first mass spectrum: a plot of the ion current as a function of the mass-to-charge (m/e) ration. However it was not until 1920 that F.W. Aston first introduced the term "mass spectrum."
Thomson pointed out that for mixtures of hydrogen and oxygen, varying in composition over wide ranges, there was little difference in the intensities of the H2+ and O2+ ions on the photographic plate. By means of the electrical detection system, he was able to achieve a much more quantitative measure of the relative proportions of hydrogen and oxygen present in a given mixture. Thus, as well as being credited with the invention of the first mass spectrograph, Thomson must also be credited with the invention of the mass spectrometer.
One can readily see from these few paragraphs that the making of the history of mass spectroscopy has involved many scientists. However, a very important point remains: one cannot be unimpressed by the tremendous contributions made by Thomson, Nobel Laureate of 1906 and the father of mass spectroscopy.
From about 1915-1920 on, mass spectrometry developed along two main lines: one concerned with the precise determination of masses, and the other concerned with measuring the relative abundances of ionic species. The mass spectrograph which Aston used in many of his studies of stable isotopes was readily adapted to measurements of isotopic mass to a precision of 0.1%, but it was not suited to accurate determinations of the relative abundances of these isotopes, because of the photographic recording.
In 1918, A.J. Dempster reported the construction of an electron bombardment ion source mass spectrometer of simpler design that Aston's mass spectrograph. Dempster's mass spectrometer could not be used for precise mass measurements, but it was better suited than Aston's mass spectrograph for measuring the relative abundances of the ionic species and was suitable for studying electron impact processes in gases.
Thus, by 1920, the early instruments were capable of the three types of measurements which can be made in mass spectroscopy: (a) precise mass determinations, (b) measurement of relative abundances of ions, and (c) electron impact studies. However, the full potentialities of the methods of mass spectroscopy were not realized with the early instruments, and not until 1942 was the first commercial instrument built by Consolidated Engineering Corporation and delivered to the Atlantic Refining Corporation. Today there are a number of industrial organizations producing a wide variety of instruments.
...Thomson set aside then pages (pp. 106-116) in his Rays of Positive Electricity... for a discussion of the applications to chemical analyses. In fact, he even included this in the full title of his book: Rays of Positive Electricity and Their Applications to Chemical Analyses. He suggested the uses of mass spectrometry for determining atomic and molecular weights, and for obtaining both qualitative and quantitative information in analyses superior to emission spectrography, while pointing out the necessity of using only very small samples (about 0.1 cc at STP). By means of examples, Thomson even showed the possible use of mass spectrometry in the identification of the components of air. Yet, the first purely chemical application of mass spectroscopy appears to have been made by Conrad. Conrad published many very beautiful plates of groups of parbolas he obtained in studies of organic compounds and drew interesting conclusions from the appearances of the parabolas.
J.A. Hipple and D.P. Stevenson made the first satisfactory direct determinations of the ionization potentials of free radicals, and G.C. Eltenton was the first to successfully study free radicals with a mass spectrometer, although Aston had commented upon the value of such studies in 1933...
One other use of the mass spectrometer or the mass spectrograph, which is of more than just historical interest, is that of isotope separation. Ast achieved by A.O. Nier, E.T. Booth, J.R. Dunning, and A.V. Grosse. It was then established that uranium-235 was the uranium isotope responsible for the process of fission by neutrons, a belief which Dunning had held since 1939. L.W. Alvarez and R. Cornog had already used the 100-in. Berkeley cyclotron as a mass spectrograh to find helium-3 present to about 10-7% in natural helium. In late November of 1941, E.O. Lawrence began to convert the 37-in. Berkeley cyclotron to a large mass spectrograph. On December 2, 1941 an ion beam current of 5 µ amp was received at the collector, ten times as large as that by Nier, et al. By the end of 1941, the first 37-in. mass spectrograph had attained a separated sample enriched to 3% uranium-235. On January 14, 1942 a 9-hr run at 50 µ amp produced 18 µg of material enriched to 25% uranium-235. By February, a total of over 0.2 mg enriched to 30% uranium-235 had been obtained. With some further design changes, ion source modifications, and different collectors, a new unit was installed in the 37-in. magnet in February 1942; this was the birth of the "calutron."