International Journal of Mass Spectrometry 349–350 (2013) 3–8 Contents lists available at ScienceDirect International Journal of Mass Spectrometry j ournal homepage: www.elsevier.com/locate/ijms Mass spectrometry—The early years ∗ K.S. Sharma Department of Physics and Astronomy, University of Manitoba, Winnipeg, Canada R3T 2N2 a r t i c l e i n f o a b s t r a c t Article history: In 1913 J.J. Thomson constructed his famous positive-ray parabola apparatus at Cambridge and discovered Received 21 March 2013 two isotopes of neon. He subsequently discovered other isotopes. His work confirmed the concept of Received in revised form 22 May 2013 isotopes and provided an explanation for deviations of atomic weights determined through chemical Accepted 23 May 2013 techniques from the “whole number rule”. This achievement marks the beginning of the field of mass Available online 5 June 2013 spectroscopy which is celebrating its 100th anniversary in 2013. His student Aston extended this work by constructing an instrument that we properly term a mass spectrometer and contributed significantly to Keywords: our first glimpses into the binding energy of the nucleus. Independently, Dempster constructed a similar Atomic masses instrument at the University of Chicago also provided contributions to our knowledge of nuclear masses. Nuclear physics The birth of this field of measurement has its roots in nuclear physics and chemistry. It has grown to be Mass spectrometry History the driver of a huge international industry and is utilized as a tool in almost every field of science. This Instrumental paper will recount the early days of the field. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Studies of these electrical discharges provided us with our first understanding of the electrical nature of gases and other Mass spectrometers as their name suggests are instruments substances. In the 1870s, Eugen Goldstein, working at the Berlin used to analyse beams of ions according to the masses of the ionic Observatory, conducted a series of systematic investigations of species that are present. It is therefore not surprising that the tech- the constituents of these electrical discharges in a Crookes dis- nique owes a considerable part of its utility to the techniques of charge tube. He confirmed earlier reports that negatively charged producing ion beams and the origin of the field is linked to studies rays (cathode rays) were emitted from the cathode and travelled of the electrical nature of matter. The study of electrical discharges towards the anode of the discharge tube. He surmised that these in gases provided us with the first glimpse into this aspect of nature. rays carried energy and could be detected by the fluorescence of One of the first studies of these phenomena was reported in 1675 the glass envelope behind the anode. He also observed that there by the French astronomer Jean-Felix Picard who observed flashes were also positively charged rays emitted from the anode that trav- of light in the empty space above the column of mercury in his elled towards the cathode. Holes in the cathode allowed these rays barometer when the column was agitated [1]. Many investigators to pass through the cathode and excite fluorescence in the glass tried to determine the cause of the phenomenon. In 1705 Francis envelope of the tube. He named these positive rays “kanalstrahlen” Hauksbee demonstrated that a small amount of mercury placed in a or canal rays [2]. There were competing explanations of whether partially evacuated bulb and charged by static electricity could pro- these rays were some form of wave or whether they were made duce enough light to read by. These events together with advances up of charged corpuscles. Wien deflected the canal rays in mag- in technology set the stage for the development of low-pressure netic and electric fields and discovered that the positively charged gas discharge tubes. Heinrich Geissler, a German glassblower, con- rays had a much smaller charge to mass ratio (by a factor of ∼2000) structed (in 1857) artistic cold-cathode gas discharge tubes that than the cathode rays. Similar studies were conducted by numerous glowed with many vibrant colours. The technology was further researchers but the results were unclear because of the effects of developed by French engineer Georges Claude in 1910 and became residual gas pressure inside the discharge tubes, the varying ener- the basis for the many “neon” signs that we see around us today. gies of the particles and the presence of ions from molecular species Between 1869 and 1875 William Crookes and others invented and and other contaminants in the gas. experimented with low pressure discharge tubes that made many of the scientific experiments on the nature of these phenomena 2. The first mass spectrometers possible. Joseph John Thomson (Fig. 1) was born in Manchester in 1856. He entered Trinity College, Cambridge as a minor scholar in 1876 ∗ and became a Fellow of Trinity College in 1880 where he remained Tel.: +1 204 474 6181; fax: +1 204 474 7622. E-mail address: [email protected] a member of the College for the rest of his life, becoming Lecturer 1387-3806/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2013.05.028 4 K.S. Sharma / International Journal of Mass Spectrometry 349–350 (2013) 3–8 Fig. 2. Cathode ray tube used by Thomson in a measurement of e/m for the electron. From Science Museum. The electrical discharge was struck in the bulb on the left. The electrical deflection was effected by the two plates seen in the middle. The coils used to create the magnetic field are displayed below. Reproduced with permission from http://www.scienceandsociety.co.uk/results. asp?image=10324719&itemw=4&itemf=0002&itemstep=1&itemx=4. conducted by Ernest Rutherford and find expression in the atomic theory constructed by Bohr. Early work in Chemistry had, through the laws of simple and multiple proportions in stoichiometry lead Dalton in 1808 to conclude that chemical reactions appeared to involve the com- bination of basic elemental entities termed “atoms”. In 1815 the English chemist, William Prout, noted that atomic weights of the elements appeared to be nearly integer multiples of the atomic Fig. 1. J.J. Thomson at work. Reproduced from http://commons.wikimedia.org/wiki/File:JJ Thomson %28Nobel weight of hydrogen [4]. These developments motivated Thomson %29.jpg; by the Nobel Foundation, 1906. to search for a basic building block from which atoms could be constructed. The existence of some elements that notably deviated from this whole number rule (like neon and chlorine) was known. in 1883 and Master in 1918. He was Cavendish Professor of Exper- In 1912 another British radiochemist, Frederick Soddy deduced imental Physics at Cambridge from 1884 to 1918 and Honorary the existence of isotopes for some radioactive elements that he Professor of Physics, Cambridge and Royal Institution, London. was studying. He was awarded the Nobel Prize in Chemistry, in Throughout his career Thomson attempted to discover the funda- 1921, for this discovery. From 1905 to 1914 Thomson turned his mental structure of matter and its basic building blocks. He was attention to the positively charged canal rays. He built a device, convinced that the study of cathode and anode rays would reveal shown schematically in Fig. 3, [5] that would deflect the posi- these fundamental constituents of positive and negative matter. His tive rays with magnetic and electric fields. Thomson was trying work on “vortex rings” as these fundamental particles inspired his to find a fundamental corpuscle of positive electricity as he had investigations of the products of gas discharges. He was noted for found with negative electricity. An experimental proof of the exist- his skill at finding interpretations for complicated scientific obser- ence of these isotopes and an accurate measure of their relative vations and suggesting new avenues of investigation but did not abundances could be a convincing proof of these theories. His pre- love the task of carefully constructing scientific apparatus and the conceptions about the structure of matter and technical challenges travails of coaxing results from it. His visionary approach to sci- with the instrument initially made the results hard to interpret and ence combined with the superb experimental skills of men who progress difficult (for a more definitive account see [6]). He was worked with him, like Aston and Rutherford, resulted in some very joined by F.W. Aston in 1910 as a research assistant. Aston’s early important discoveries about the nature of matter. researches on electrical conduction in gas discharges made him an In 1897, working with a discharge tube with a much improved expert on the construction and operation of discharge tubes and the vacuum, J.J. Thomson used electric and magnetic fields to deflect phenomena associated with them. Aston’s superb skills as an exper- cathode rays (see Fig. 2). Through his results he concluded that imental physicist played a major part in the successes that Thomson the cathode rays were composed of “charges of negative electricity enjoyed with his positive-ray parabola apparatus. In this apparatus carried by particles of matter” [3]. He was convinced that these Thomson deflected the positive rays, simultaneously, in two direc- particles formed a fundamental constituent of matter. Thomson tions perpendicular to their initial direction of travel using parallel, was awarded the Nobel Prize in 1906 “in recognition of the great coterminous electric and magnetic fields. The deflected ions were merits of his theoretical and experimental investigations on the detected on a photographic plate. conduction of electricity by gases”. He is credited for the discovery Let us assume a perfectly collimated ion beam that is initially of the electron in his studies of cathode rays in 1897 [3].