MASS SPECTROMETRY JOHN 4.HIPPLE
4VD
I I I R T I K SHEPHERD, National Bt~reauof Standards, Washington 2 5 , D . C .
T
HE statisticallj inclined might dian inferences from the fact that Chernzcal Abstracts reported 11 references to mass spectronietry in 1943, 15 in 1944, 17 in 1945, 26 in 1946, and 40 in 1947. If the war jam could be properly discounted, an indication of increasing interest would probably remain. Actually, there is no doubt of the sharply increasing interest and importance of the mass spectrometer as an analytical tool. It is capable of resolving rather terrifying mixtures of gases, vapors, and even solids and liquids, within an astonishingly short time, and with reasonable and sometimes superior accuracy. Applications of the instrument are continually increasing, and its use in research and control has expanded beyond general anticipation, not only in the gas, petroleum] and chemical industries, but even in the biological as well as physical-chemical sciences. A census of the mass spectrometers in use today discloses a curious situation. There are a number of special-purpose instruments] designed for specific masses over narrow ranges. Aside from leak-detecting spectrometers, these are mostly assembled in the laboratories using them. There are a few metallurgical instruments designed for analysis of solids and an increasing number of isotope ratio instruments. But commercial presentation has so far centered on a general-purpose analytical instrument of wide mass range, about 0 to 200, with good resolution somewhat over 100. iZbout 70 of these are operating now, and they are performing a good share of the more accurate gas analysis being done in this country. Of these instruments] about 10% mere made by Restinghouse (Tvhich no longer manufactures this item) and almost all the others by the Consolidated Engineering Corporation, Pasadena, Calif. (General Electric and a fevi others are now entering the field.) Consolidated Engineering Corporation maintains research and engineering groups and exchanges technical information between users of the instrument through regular group meetings. Thus the corporation and its customers offer a contribution to this special analytical field that is unique at the momcnt. The reports of the Consolidated group meetings contain valuable material, all of tvhich has not been published. Occasional reference is made here to these reports, as this information may be made available by the authors.
reported, but the account is far from complete. Dibeler and blohler have discussed the analysis of a mixture containing Cs-C6 paraffins and olefins, IT-ith emphasis on sampling difficulties (58) The determination of hydrogen with the mass spectrometer has been thoroughly discussed by Honig of Socony-Vacuum Laboratories (68),and new techniques recommended correct what previously was a sometimes unsatisfactory determination. The analysis of hydrogen and hydrocarbons (79) and of Hg and DZ (9) has been reported. The determination of butene isomers in hydrocarbon mixtures has been discussed (6, 99, 126). The determination of oxygen (53),nitrogen (61), and of carbon (64) in organic compounds has been described. Analyses of specific mixtures include rare gases (39); and ethylene oxide with ethylene and carbon dioxide, silicanes, aliphatic derivatives, ethers, and other organic mixtures (163) Although the analytical mass spectrometer in general use is a gas analysis apparatus, it is, of courbe, capable of producing spectrograms representing mixtures of the vapors of various liquids. Heavier vapors, particulai 1)- those of polar substances, have caused trouble because of strong sorption; but recently this difficulty has been largely overcome by introducing the vapor sample almost directly into the ionization chamber, or heating the entire inlet system. As a result of these improved techniques, the analysis of water vapor and alcohols up through C1 has been made possible, and analyses of alcohols, aldehydes, mercaptans, and other liquid organic compounds have been reported (22, SO, 124, 158, 166). The analysis of liquid hydrocarbon mixtures in the C5 to CSrange is reported by Brown, Taylor, Melpolder, and Young (201, who found that paraffins, cycloparaffins, and aromatics may be individually determined, but olefins and cyclo-olefins only to a limited extent. The analysis of mixtures containing pentenes, pentanes, and isoprene (19) and of heptane mixtures (21) has been reported by Brown. Rock has discussed the analysis of octane mixtures (126). Thiophene mixtures have also been analyzed (145). While the mass spectrum of a complex mixture may be obtained in a few minutes, the analysis and computation of the spectrum may require several hours or even longer Sormally three computers per 8-hour shift are required for one spectrometer. Comparatively little has been published, nith the exception of the Consolidated Computing Manuals prepared by Rock, a similar manual prepared for the users of Westinghouse instruments, and a series of reports dealing with spectrometric computational methods, probable errors, and reciprocal matrices, prepared by the Texas Company's Technical and Research Division (154). Other contributions are: computation of C1 to C, hydrocarbon mixtures containing carbon dioxide by Brown (18) and a new method of analysis by Johnsen (78). The catalog of mass spectral data issued by the American Petroleum Institute Research Project 44 and the Xational Bureau of Standards has been very helpful in the study of spectrograms, particularly so in the identification of substances for which no patterns are otherwise available (109). The accuracy and reproducibility of the mass spectrometric analysis and of the conventional chemical volumetric analysis have been compared by 50 laboratories throughout the country in the cooperative analysis of a standard sample of natural gas (155). The mass spectrometer in general gave the better account of this sample from the viewpoints of accuracy and completeness of composition. In many instances the mass spectral method could be a valuable supplement to the optical spectrograph in the analysis of solids, but there has been little attention to this field. Shaw and Rall
ANALYTICAL APPLICATIONS
The analytical mass spectrometer is primarily an apparatus for gas analysi.: and has offered the first actual systematic approach in this field. Within fairly wide limits it is possible to identify and determine the components of simple and complex mixtures of hydrocarbons, fuel gases, exhaust gases, rare gases, and special samples of many kinds. Instruments, procedures, and typical analyses of mixtures of various kinds have been reported by Washburn, Wiley, Rock, and their associates (169, 17O), Hipple (6S), Brewer and Dibeler (17), Brewer (15), Coggeshall (26), Farmer (44), and others. Somewhat less general applications have been discussed by Smyth ( 1 4 4 , Yier (115), Hipple (62), Rittenberg (1$3), Eltenton (4O), Watson, Buchanan, and Elder ( 1 7 l ) , Hipple and Condon (66), and others. These papers disclose spectrometric analyses of most of the common and rare gases, and of the hydrocarbons through the Cq group. More general treatment has been given in papers discussing industrial analv4s by the mass spectrometer and comparing results obtained \< ith those yielded by more conventional methods: Solomon and Rubin ( I & ) , Schaafsma (128), Milsom (104), Crone ( S I ) , Fulton and Heigl (50),Brewer (16), Webb (172), and Schlesman and Hochgcsang (132). Analyses of specific compounds and mixtures have been
32
V O L U M E 21, N O . 1, J A N U A R Y 1 9 4 9 (134) have redesigned an instrument of the Mattauch type Ivith this aim in mind. Hickam (60)has employed the mass spectrometer for the detection of small impurities in certain metals by completely evaporating the sample and comparing the integrated time-current curves for the various components. Grosse (5s) has made a limited application of the conventional instrument for gas analysis to this purpose. Extremely high sensitivity has been achieved, but reproducibility has not always been satisfactory. INSTRUMENTATION
There has been no major change in recent years in the type of instrument in general use for gas analysis-an ion source proviiling a monoenergetic beam t>hatis analyzed by a magnetic field. However, refinements have been made as the result of broadened experiencc gained in the vigorous extension to practical analytical problems. These refinements have permitted an increase in the accuracy and speed of individual analyses, resulted in somewhat simplified operation, or extended the rangr of problenis that may be handled by this general technique. I n most cases there is no published reference for these improvements which have rather naturally evolved since the time, approximately 5 years ago, when suitable instruments became more generally available. Descriptions of instruments that have been custom-built in various laboratories ( 5 , 51, 67, 85, 91, 107, 153) offer some individual variations of special interest to the particular designer. Nier (115) has continucd his role as instrument designer for those nonspecialists who dr,sirc to use instruments incorporating the latest developments by publishing very detailed design information. a styuel to his earlier paper on this subject. In the field of isotopic measurements the greatest siiigltx ad vance (118) has been the development of a null method for t,he comparison of two ion currents (116). The output of a feedback amplifier on the more abundant ion is used to balance out the simultaneously measured signal of the less abundant ion, so that the ratio of the two currents is continuously measured. In order to reduce the mass discrimination (a?)and widen the mass range that may conveniently be scanned without discontinuity, the trend has been toward magnetic rather than electric seannirig (67, 7 4 ) . The four-element oscillograph (169) has been the most widely usctl rcxcording method. The range limitation of the single element pen-and-ink recorder has been circumvented by the use of an autoniatic scale expander (55) with sonic’ modifications (37, 92).
For continuous visual observation of the niass spectrum the cathode ray oscilloscope has been used. Forrester and Khalley (46)have used a long persistence screen with a sweep of frequency cycle per second to obtain a sensitivity comparable as low as with instruments with conventional recorders. Siri (140) employed a sweep frequency of 200 cp. and his pattern indicated a resolutiori of mass unit in 65. These devices find application where the nierits of this type of observation permit the sacrifice of accuracy. One of the ever-present limitations on the extension of the range of the existing instruments is the first amplifying stage for the ion current. Consequently, new methods of detection or improvements i n old ones are of immediate interest. One development that has been announced since the end of the war is the capacitativc commutator or dynamic condenser electrometer (119, 120, 130, 131). This has extremely Ion drift, low background current, and high sensitivity. One of the chief objections to this system a t present is its high initial cost. The stability and ease of balancing the electrometer tube circuit have been improved with the advent of the split-beam electrometer tube (86). S e w circuits (25,111) have improved the performance with the tubes that have generally been used in the past. Millest (102)has concluded that the space charge detector is less sensitive than the electrometer for detecting small currents of positive ions. The use of Be-
33 Cu, which has been found to be very stable in electron multipliers ( Q ) , makes this very attractive for use where extremely high sensitivity is required or the specd of response vr-ould otherwise b e limited by the time-constant of tlic input circuit of the electronieter t ubcl. More careful attentioii t o tlic ion optics of the source has resulted in greater resolution in peaks varying widely in intensity. The “tails” a t the bottom of large peaks have been reduced by the use of an energy filter between the exit slit and the ion collector to reject those ions which have been scattered in the analyzer \vith an accompanying loss of energy. This has been done by the addition of a suppressor electrode to which a high retarding voltage is applied (118) or simply by operating with the ion source near ground and the analyzer a t a high negative potential ( 4 7 ) . A recording mass spectrometer for process analysis has rerently been described (114). Instruments for the continuous analysis of respiratory gases are under development, (35). Several rwiew articles have appeared in the past few years (1,34,97,ILX). The use of a mass spcctromrter of simplified design for leak dettbction was described in the Smyth report (144). There has been :til expanding interest in this application since the war hecause of its tvidcspread usefulness (76,118, 259, 160). S e n methods of mass analpsis have beon considered in the tlcssign of othw instruments. Several time-of-flight mass sprct ~ i m c ~ ehavc, r s 1 ) t w i proposed (11, 24, 147, 173) and two have reri~iitlybeen built (11, 24). I n the one type (@) a pulse of ions is sent down a drift tube and the arrival time depends on the mass with the spectrum displayed on a cathode ray screen. I n the other type (11) the ions are accelerated through a series of grids on which rf potentials are applied. For a particular geometry anti frequency, the phase will be appropriattt for ioris of one mass as th6.y pass through the grids to receive mol’(’r’nergy than others and henw thrse can overcome a stopping poteiitial at the end of the tube and reach the collec-tor. At the present stage it appears unlikelj- that these met,hods \vi11 ,supplant the convcntional ones in the near future for accurate isotopic ant1 gas analysis, as the requirements of the source, of’ the amplifier, or of both together are not eased (and in most respects t1it.y are made much more severe); the elimination of the magnet is of minor importance when precision of analysis is being stressed. Their possible use under less stringent circumstances warrants a continued exploration of their possibilities. Goudsmit (52) has described ail arrangement in which a pulse of ions describes a helical path in a uniform magnetic field. As the time for rotation through 360 O depmds only on &‘/e and the value of the magnetic field rather than the initial conditions, he proposes to make a precise coniparison of masses by timing the arrival of ions of different typm a t the collector after rotation through 2 X . Studies have continued on the focusing properties of static electric and magnetic fields. These have had for their purpose the increase of intensity and resolution hy the use of nonuniform ficlds of such form that higher than first-order focusing is obtained or to provide for axial focusing in the direction of the main component of the magnetic field (10, 13, 28, 29, 87, 13?-139, 150). The focusing of a uniform magnetic field superposed on a crossed electric field from a cylindrical condenser has rocently been investigated for relativistic energies (105). The effect of the fringing magnetic field and the angle of entry of the beam into the sectored magnetic field analyzer has been studied theoretically (80,88,96). Bainbriclge (8)has given formulas for the proper shapes of the edges of the pole faces, in order that second order focusing may be obtained. Koch (82, 83) has eliminated the necessity of a high degree of stability on the ion accelerating voltage by employing compensating electrostatic deflectors in the magnetic analyzer. Kier, Roberts, and Franklin (117 ) descriljc a double focusing mass spectrometer in which second-order focusing is obtained; stability of 1part in lo6is attained by controlling the ion beam by the signal from an auxiliary monitoring beam.
34
ANALYTICAL CHEMISTRY
The theoretical treatment customarily used in electron-optical devices has been applied to the mass spectrograph by Hutter (78).
such as the production of ions from hot filaments and the forrnation of negative ions at surfaces (77,129,141,l4S).
ISOTOPES
LITERATURE CITED
Abundance. The unspectacular but very important measure ments of the natural abundance of the stable isotopes have continued ( 3 , 8 , 5 9 ,75,98,167, 175,176). Aldrich and Nier ( 2 )have been able to make the very difficult measurement of the He4/Hea ratio with the He3 peak completely resolved from the impurity H D and thus showed the large difference between atmospheric helium and well helium. This development is of particular importance in providing a means of testing the various methods for concentrating HeS. Urey (162) has recently extended the accuracy of comparing the isotopic abundances of different samples by using the balanced beam method (116)in conjunction with an arrangement for inserting a control sample every 2 minutes. A recent monograph describes the preparation and measurement of isotopic tracers (121). Mass. The accurate measurement of masses received practically no attention during the war, but it is a field in which much work remains to be done and attention is being directed to it (35, 48,43, 80, 122). A critical review of the existing data has recently been made by Bainbridge ( 7 ) . Although the photographic method has been used almost exclusively, the use of electrical detection is now being seriously attempted (110, l 17, 195). Concentration of Isotopes by Mass Spectrograph. The large has been described in the Smvth report scale concentration of U235 (144). A more detailed description of separation in quantity bv electromagnetic means has been given by Smith, Parkins, and Forrester (149). Because this is the only source of many of the separated isotopes, continued interest in this field is assured (25, 81,84, 108, 161, 164, 165).
Identification and Mass Assignment of Products of Nuclear Reactions. There have been many letters, abstracts, and papers on this subject principally by the Chicago group-A. J. Dempster, M. G. Inghram, R . J. Hayden, D. C. Lewis, Jr., A. E. Shaw, ITr. Rall, and others. Bainbridge has reviewed this work up to August 1947 ( 7 ) . I n the autoradiographic technique, the mass spectrum of an element is obtained in the usual manner and the radioactive isotopes are then identified by laying an unexposed photographic plate against the exposed one-only the radioactive isotopes will give lines on the second plate. I n some cases a counter is used instead of the second photographic plate (58, 90). The mass spectroscopic method in many cases offers the only clear-cut identification of the stable or very long-lived products of nuclear reactions (14,52, 75, 153, 156). STUDY OF IONIZATION PROCESSES
Applications to Chemistry. As the study of ionization processes with the mass spectrometer was the forerunner of the application to routine analysis, it is clear that these more fundamental studies will be of continuing interest to all workers in applied mass spectroscopy. Measurements have continued on the appearance potentials of the various fragment ions, the study of secondary processes of ion production, and the compilation of data on the mass spectra of additional substances (12, 36, 45, 56, 66, 69, 70, 89, 95-95, 100, 101, 105, 127, 163, 168, 174). The spontaneous dissociation of hydrocarbon ions has been reported and some additional data on these metastable ions have been obtained (18, 48, 64, 65, 101). The difference in the dissociation probabilities of C-H and C-D bonds and of H2 and D2 have been investigated (9, 41, 148). Positioned isotopic tracers have been used to study the nature of the dissociation by electron impact and the mechanism of catalytic cracking (57, 106, 149, 161). ,4 few of the many examples in n-hich the mass spectrometer has analyzed the gases produced in various processes may be noted (49, 71, 136, 167). Eltenton has studied reaction intermediates by means of a mass spectrometer (do), and others are pursuing this field. Other methods of producing ions have been investigated,
“Advances in Nuclear Chemistry and Theoretical Organic Chemistry,” New York, Interscience Publishers, 1945. Aldrich, L. T., and Nier, A. O., P h y s . Em., 70, 983 (1946). Ibid., 74, 876 (1948). Allen, J. S., Rev. Sci. Instruments, 18, 739 (1948). Anker, H. S.,Ibid., 19, 440 (1948).
Atlantic Refining Co., Consolidated Engineering Corp., Mass Spectrometer Group, Rept. 35 (1946). Bainbridge, K. T., Preliminary Report No. 1, Committee on Nuclear Srience, National Research Council, June 1948. Bainbridge, K. T., Seventh Solvay Congress in Chemistry, Sept. 21-28, 1947. Bauer, N., and Beach, J. Y., J . Chem. Phys., 15, 150 (1947). Beidilk, F. M., and Konopinski, E. J., Rev. Sci. Instruments, 19, 594 (1948).
Bennett, W.H., P h y s . Rev., 74, 12228 (1948). Bloom, E. G., Mohler, F. L., Lengel, J. H., and Wise, C. E., J . Research Natl. B u r . Standards, 40, 437 (1948). Bock, C. D., Rev. Sci. Instruments. 4, 575 (1933). Bradt, Paul, and Mohler, F. L., P h y s . Rev., 73, 925 (1948). Brewer, A. K., Am. Soc. Testing Materials, B u l l . 140, 38 (1946).
Brewer, A. K., M i n i n g and Met., 27, 207 (1946). Brewer, A. K., and Dibeler, V. H., J.Research Nat2. B u r . Standards, 35, 125 (1945); R P 1664. Brown, R. A., Consolidated Engineering Corp., Mass Spectrometer Group, R e p t . 8 (1944). Ibid., Rept. 11 (1944). Brown, R. A., Taylor, R. C.; Melpolder, F. W., and Young, W. S., ABAL.CHEM., 20, 5 (1948). Brown, R. A., Taylor, R. C., and Young, W. S.,Consolidated Engineering Corp., Mass Spectrometer Group, Rept. 16 (1946). Brown, R. A , , and Young, TV. S.,“Rearrangement Peaks in Mass Spectra of Butyl Alcohols and Mercaptans,” Consolidated Engineering Corp., Mass Spectrometer Group, Rept. 52 (1948). Caldwell, P. A , , Rev. Sci. Instruments, 19,85 (1948). Cameron, A. E., and Eggers, D. F., Jr., Ibid., 19, 605 (1948). Chelius, L. G., and Keim, C. P., P h y s . Rev., 73,813 (1948). Coggeshall, N. D., Chem. Inds., 58, 420 (1946). Coggeshall, N. D., J . Chem. Phys., 12, 19 (1944). Coggeshall, N. D., P h y s . Rev., 70, 270 (1946). Coggeshall, N. D., and Muskat, M., Ibid., 66, 187 (1944). Consolidated Engineering Corp., Mass Spectrometer Group, Rept. 52, “Analysis of Alcohols by Mass Spectrometer,” 1948. Crone, H. G., B u l l . B r i t . Coal Utilisation Research Assoc., 10, 198 (1946). Dempster, A. J., Phys. Rev., 71, 8291. (1947). Ibid., 74, 1225A (1948). Dempster, A. J., S c i . Monthly, 67, 147 (1948). Dempster, A. J., Inghram, M. G., and Hess, D.C., Jr., Atomic Energy Commission Rept. D.2027. Dibeler, V. H., J . Research Nut!. B U T .Standards, 38, 329 (1947). Dibeler, V. H., Bernstein, R. B., and Taylor, T. I., Rev. Sd. Instruments, 19, 719 (1948). Dibeler, V. H., and Mohler, F. L., J . Research 12’atZ. B u r . Standards, 39, 149 (1947) ; RP 1818. Dibeler, V. H., Mohler, F. L., and Reese, R. M.,Ibid., 38, 617 (1947): RP 1799. Eltenton, G. C., J . Chem. Phya., 15, 455 (1947). Evans, M. W., Bauer, N., and Beach, J. Y., J . Chem. Phys., 14, 701 (1946). Ewald, H., 2. il’aturforsch, 1, 131 (1946). Ibid., 2a, 384 (1947). Farmer, I?. C., P h y s . Rev., 68, 235 (1945). Foner, S. H., Kossiakoff, A . , and McClure, F. T.. Ibid., 74, 1222 A (1948). Forrester, A . T., and Whalley, W. B., Rev. Sei. Instruments, 17, 549 (1946). Fox, R. E., and Hipple, J. A , , Ibid., 19, 462 (1918). Fox, R. E., Langer. A , . and Hipple, J. S., Phga. Rev., 74, 12228 (1948). Friedland, S. S., Ibid., 71, 377L (1947). Fulton, S. C., and Heigl, 6.J., Instruments, 20, 35 (1947). Graham, R. L.. Harkness, A. L., and Thode, H. G., J . Sci. Instruments, 24, 119 (1947). Goudsmit, S. A,, Ibid., 74, 622L (1948). Gro,se, 4.V., Hindin, S.G., and Kirshenbaum, A. D., J. Am. Chem. Soc.. 6 8 , 2118 (1946).
V O L U M E 21, NO. 1, J A N U A R Y 1 9 4 9 (54) Grosse, A . V ~Kirsheiibiliirii. , .\. E , , and flindin, 5. G., Scieuce. 105,100 (1947). (55) Grove, D. J., and IIipyle, J. .1.,Rev. Sei. Instruments, 18,837 (1947). (36) Hagstruni, H. D., P h y s . Rev., 72,947 (1947). 1,57) Hansford, 11. C . , Division of Petroleurn Cherni Meeting, AM. CHEM.SOC., 1946. ~ s 5 Q )Hayden, R. J., P h y s . Rev., 74,650 (1918). 39) IIess, D. C., J r . . I b i d . , 74, 773 (1948). ((io) FIickam, TV. >I., Ibid., 74,1222A (1948). t i l ) Hindin, S. G., and Grosse, A. V., Consolidated I
