Mass Spectrometer- Electron Spectrometer:
Exchange Interaction A
Victor L. Talrose Institute of Energy Problems of Chemical Physics Leninsky Prospect 38 B-334, MOSCOW 117829 Russia
Gennadij V. Karachevtsev and lgor A. Kaltashov’ Moscow Institute of Physics and Technology lnstitutsky Pereulok 9 Dolgoprudny, Moscow Region 141700 Russia
Both mass spectrometry and electron spectrometry are analytical methods based on ionization processes and/or interactions of charged particles with matter. In both cases ionization is produced by electron impact, photon impact, or interactions of electrons or photons with excited atoms. The charged particles of interest may be produced either by interactions in the gas phase or with a surface. The fingerprint of a molecule studied in MS is the set of ions characteristic of the molecule. The masses and relative intensities of the ions produced during ionization give information on sample composition and molecular structure. J. J. Thompson, who invented charged- particle mass measurement, was the first to recognize the great importance of MS for analytical chemistry (1). 0003-2700/92/0364 -401A/$02.50/0 0 1992 American Chemical Society
The end of t Cold War h meant the opening of normal intellectual discourse a n d exchange between scientists in the former Soviet Union and those in the United States as well as other Western countries. Each group is interested in learning what the other has been doing for the past 40 years and who the players are in each speciality area. In this FOCUS article Victor Talrose, one of Russia’s leading physical mass spectrometrists, and his co-authors touch on the synergism of mass and electron spectrometers, the i i terrelation of physical and an lytical chemistry, and the ink play of Soviet a n d Westel-science. Gennadij Karachevtsev was Talrose’s Ph.D. student, and Igor Kaltashov was a student in
In addition to important applications in physics, MS is used widely in analytical chemistry, chemical kinet ics, and chemical thermodynamics. MS has played a significant role in investigations of the kinetics and
thermodynamics of processes involving charged particles (2, 3).
Analysis of ion energies in MS The analysis of ion energies by MS can provide information about structures and/or energy states of massselected ions. Techniques that have been used for this purpose include kinetic energy gain/loss spectroscopy (4-6), mass ion kinetic energy (MIKE) spectroscopy (7),ion collision spectroscopy (4), and translational energy spectroscopy (TES)(8). A powerful new method for the determination of ion structures is coulomb explosion imaging (CEI) (9). In this technique, a n accelerated molecular ion is passed through a thin solid film, which rapidly (in s) strips away the valence electrons of the ion, thereby initiating a fast dissociation process termed a “coulomb explosion.” By measuring the relative momenta of atomic fragment ions, their trajectories can be traced backward to deduce initial spatial positions before the explosion. Kinetic energy analysis of ions allows the possibility of selectively detecting only atomic ions. This method was used to selectively detect Yb ions that were obtained for the first time using high- resolution accelerator MS Current address: Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, MD 21228
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(AMs)in the presence of intense molecular interferences (10). Methods exist for ion energy measurements based on mass spectral peak shapes and on retardation or deflection of ions by an applied electric field. Applications include measurements of gas temperatures and translational energies of fragment ions, secondary ions produced in ionmolecule reactions (11, 121, and ions from superelastic collisions (13).The measurement of kinetic energies of the fragment ions produced in strong electric fields has been used to study the kinetics of ion decay (14-20). In analytical MS, ion kinetic energy filtering with retarding potentials in the detection system produces a much cleaner mass spectrum and increases resolution (21). Electron spectroscopy In electron spectroscopy molecules are characterized by the energy spectrum of electrons detached by irradiation or by the collision of particles with gaseous or solid targets. This energy spectrum provides information about the electron energies of atomic and molecular orbitals, the electronic and vibrational levels of ions, and the energy levels of electrons in solids. Recently the analytical applications of electron spectroscopy have been extended (22-24). In photoelectron spectroscopy a monoenergetic beam of photons of energy hv is collided with sample molecules. Electrons may be removed from any molecular orbital with electron energy Ii,which is less than the photon energy. The translational energy E of the resulting electron is expressed as
E = hv - Ii (1) The energy of the molecular orbital can be determined by energy analysis of the photoelectrons released. Another variant of photoelectron spectroscopy uses a n energy analyzer to transmit only electrons of a single energy. Usually the transmission energy is chosen to be at near-zero energy so that only “threshold” electrons are detected. The spectrum is then obtained by varying the energy of the ionization photons. Combining mass spectrometers and electron spectrometers The block diagrams of both electron and mass spectrometers are very similar. These devices operate at high vacuum and have a source of ionization and a n ionization chamber. Because t h e y have s i m i l a r charged-particle analyzers and data 402 A
acquisition and processing systems, it is possible that they can be combined. For example, it h a s been very fruitful to merge mass spectrometric and electron spectrometric techniques in one installation for the study of solid surfaces. The pumping system, vacuum chamber, samplehandling system, a n d computerbased data acquisition and processing systems are jointly used. The cost and dimensions of such an installation are comparable to those of each separate component. Such multitechnique systems-for example, combining X-ray photoelectron spectroscopy (XPS or ESCA), scanning Auger microprobe (SAM),secondary ion MS (SIMS), and ion scattering spectroscopy (1SS)-became available com mercially in the mid-1980s (25). At least one combined mass spectrometridelectron spectrometric instrument has been developed (26). Both electron transmission spectros copy (ETS) and dissociative electron attachment MS (DAMS) have been possible. The ETS technique takes advantage of the strong dependence of the electron-molecule scattering cross sections associated with a resonance process. On the other hand, DAMS measures negative ion current as a function of the incident electron beam energy. These methods provide important information about kinetic and thermochemical parameters as well as the electron structure of the negative ions. In some cases mass spectrometers have been used as electron spectrometers with simple changes in the operation of a magnetic mass spectrometer (27). In such cases, the magnetic field had to be reduced so that the mass analyzer could transmit electrons, and a Hall detector was used to regulate the magnetic field. A high-efficiency detector was required, and the deflection technique was used to measure the kinetic energy of the charged particles. Figure 1 presents the results from an SF,/air mixture studied at Torr under continuous ionization (28). The initial kinetic energies of the charged particles are proportional to the squared fwhm of the appropriate curves. The initial kinetic energies of F- and SF“,fragments are very close to each other, but they are many times smaller than those of electrons. The asymmetrical shape of the electron curve may include not only secondary electrons produced through the ionization of gas molecules but also electrons produced
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15,1992
Figure 1. Dependence of electron, F-, and SF; ion currents on deflecting voltage in a SF6/air mixture. through secondary electron emission off the surfaces of the ionization chamber. In addition, it is possible to detect not only the charged particles escaping the ionization chamber but also those produced outside the chamber as a result of ion decay. Figure 2 presents the electron current as a function of acceleration voltage in an exp e r i m e n t i n which t h e S F 6 / a i r mixture is continuously ionized at a pressure of 1.5 Torr (28). The spectrum contains two distinguishing features: a narrow peak that corresponds to electrons escaping the ionization chamber and a wider peak that corresponds to electrons produced outside the chamber by electron attachment from SF, ions
SF, + M + SF6 + e- + M (2) This important process is under study. Mass spectrometers have also been used to obtain spectra of threshold electrons formed as a result of electron interactions with molecules e-+ AB -+ AB* + e; (3) To detect low-energy electrons it has been necessary to add molecules for which the cross section of electron attachment has a narrow peak a t nearzero energy. For example, slow electrons are very effectively attached to SF, molecules (4)
The cross section of electron attachment to SF, molecules has a maximum at zero energy and sharply decreases with increasing electron energy (fwhm = 0.03 eV). When the energy of the electrons introduced into the ionization cham ber equals a n energy level in the molecule, the electron will transfer most of its energy to the molecule
cp”9cr-.
..
.
troduced into the ionization chamber of a photoionization mass spectrometer (31).The dependence of SF; ion current on the ionization photon energy was measured while the processes i n Equations 5 and 6 took place in the ionization chamber
hv + M -+ M’ + ee-
+ SF,
-+ SF;
(5)
(6)
Each ionization threshold corresponds to an appropriate peak in the SF; ion current. This technique for measuring threshold photoelectron spectroscopy by electron attachment (TPSA) was first applied to the study of acetone (31) and subsequently extended to Ar, Xe, N,, CO, and C,H, (32-34). Under low pressure conditions one may increase the efficiency of slow electron a t t a c h m e n t by using a charged-particle magnetic trap (35). At low magnetic fields only slow electrons are retained in the trap while SF; ions escape the ionization chamber. In TPSA there is a possibility of producing spurious SF; peaks by processes involving excited atoms Xe* + SF,
Figure 2. Dependence of electron current on acceleration voltage (V,,,) obtained in a SF,/air mixture. AVaw = V,, - 2.5 kV
and be readily attached. When SF, is the target, attachment will produce SF;. Information on the molecular energy level is obtained by detecting the SF; ion current as a function of electron beam energy (29). An analogous method has been used to study energy degradation of fast electrons in gases at high pressures (30). The relative fraction of thermalized electrons was deter mined by monitoring the negative ion current of the acceptor of lowenergy electrons.
Studies with photoionization Photoionization methods provide the highest energy resolution. In a typi-. cal experiment, a mixture of molecules of interest M and SF, was in-
-+ SF; + Xe’
(7)
Spurious peaks may also be obtained by means of slow electrons produced by interactions of photons or electrons with the walls of the ionization chamber. On the other hand, the detection of such ions may produce important information about excited particles or about these surfaces. For example, TPSA was applied to the detection of slow photoelectrons escaping platinum and silver surfaces (36)to obtain information on the distribution of electron energy levels in the metals (37). In addition to the “chemical” detection method described above, there are other methods for threshold electron detection, such as cylindrical and semispherical electrostatic analyzers (38, 39).A simple and effective method for slow electron detection included line - of- sight steradiancy analyzers (40,41).Photoelectrons are accelerated by a uniform electric field through collimating slits. The electrons with zero initial energy re ceive a velocity perpendicular to the plane of the collimating slits and are therefore transmitted. The electrons with higher initial translational energy usually have a sizable velocity component parallel to the plane of the collimating slits and therefore do not pass through the collimating slits. The use of steradiancy analyzers in MS permits discrimination against
fragment ions that have greater than thermal translational energy. The r e s u l t is a m a s s spectrum with smaller fragment peaks and more abundant parent ions. By taking advantage of such discrimination effects, Taranenko and Kobelyansky (42) were able to produce a mass spectrum from n-eicosane in which the parent ion had the highest relative abundance. This instrumentation can simplify the interpretation of the mass spectra of complex mixtures. Some years ago Karachevtsev and Marutkin (43, 44) described another method for obtaining mass spectra with reduced fragmentation intensi ties. The method was based on the ion current dependence of fragment ions (usually formed with greater translational energy), which differs from that of thermal molecular ions as a function of deflecting voltage. Ions that have small initial translational energies are easily deflected, whereas fast fragment ions are not so strongly deflected. The result is that groups of fast and slow ions of the same masses may be separated. For example, Figure 3 shows the dependence of the ion current for mass 32 as a function of deflecting voltage in a n SF,/air mixture (28).It is possible to separate the peak corresponding to the thermal molecular 0; ions (narrow peak) and the peak representing fragment S’ ions that have acquired extra energy (broad peak). This method of mass spectral analysis may be useful for measuring the neutral composition of a gas discharge plasma that contains both atoms and molecules. Indirect analyzers equipped with non-line-of- sight trajectories have also been used (45,46). Weak electric fields are used to select only elec-
Experimenta A-4-
Figure 3. Dependence of ion current (M = 32) on deflecting voltage obtained in an SF,/air mixture. The solid line indicates experimentaldata; dashed lines indicate the contributions from the separated ions.
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trons of low energies. A very high transmission efficiency was found for these analyzers, which appear to be useful for distinguishing slow and fast ions. The mass spectral detection of Hfragment ions with near-zero translational energy formed in the dissociative attachment process e-
+ H, + H- + H
(8)
can be used to measure concentrations of H, molecules in various vibrational states ( 4 7 ) . The sensitivity of this method is 10l1 cm-3 for H, in the ground vibrational state ( u = 0) and increases to 5 x lo5 cm-3 for high vibrational states ( v > 5). The special configuration of the electrostatic potential in the ion source permits efficient collection of slow negative ions only (47). Such electrode configurations in the ion source may be useful for producing spectra with smaller fragment ion peaks by discrimination against fragment ions that have higher translational energies. Thus some components of electron spectrometers may be used in mass spectrometers and vice versa.
Photoionization MS/ photoelectron spectroscopy To study unimolecular ion dissociation processes and ion-molecule reactions a t specific internal energies, coincidence MS can be combined with photoelectron spectroscopy. The mass spectrometer and the electron spectrometer share a n ionization chamber that permits detection of both the photoelectron and a corresponding photoion. The charged particles a r e detected by photoionphotoelectron coincidence (PIPECO). When threshold photoelectrons are detected, the internal energy of the reactant molecular ion is determined by the difference between the energy of the ionization photon and the molecular ionization potential. One may obtain reactant ions of selected electronic and vibrational states by varying the energy of the ionization photons (48-56). In this case the combination of mass spectrometer and electron spectrometer in one installation produces a quantitatively new device that is more powerful than its separate components. The PIPECO method h a s been shown to decrease the bias of thermodynamic characteristics (52-54). Used as a pulsed method, PIPECO provides good sensitivity and resolution and is promising for the study of radicals coexisting with other molecular species (54). 404 A
A combination of resonance enhanced multiphoton ionization photoelectron spectroscopy/MS can be used to study selectively excited vibrational modes of polyatomic cat ions, and these modes play different roles in controlling the collision dynamics (51). This combined instrument is indeed more powerful than the sum of its parts. The view that seemingly separate physical chemical techniques can be combined when their instrumental requirements are compatible should continue to lead to new discoveries about atomic and molecular energet ics and will provide the basis for powerful new analytical tools. We gratefully acknowledge Jeffrey P. Honovich for his assistance in the preparation of the manuscript.
References (1)Thomson, J. J. Rays ofpositive Electricity
and Their Application to Chemical Analysis; Longmans: London, 1913. (2) Talrose, V. L. Adv. Mass Spectrom. 1980, 8A, 147. (3) Futrell, J. H. Gaseous Ion Chemistry and Mass Spectrometry; Wiley: New York, 1986. (4) Collision Spectroscopy; Cooks, R. G., Ed.; Plenum Press: New York, 1978. (5) Aparina, E. V.; Balakay, A. A,; Markin, M. I.; Talrose, V. L. Khim Vys. Energ. (Sov.) 1982,16,195. (6) Jonathan, P.; Hamdan, M.; Brenton, A. G.; Willett, G. J. Chem. Phys. 1988,119, 159. (7) Reid, C. J . J. Phys. B:At. Mol. Phys. 1990, 23,2783. (8) Hamdan, M.; Brenton, A. G. In Physics and Ion Impact Phenomena; Springer Series in Chemical Physics; Springer: Berlin, 1991; Vol. 54, Chapter 6. (9) Vager, Z.; Naaman, R.; Kanter, E. P. Sczence 1989,244,426. 110) Litherland. A. E.: Kilius. L. R.: Garwan, M. A.; Nadean, M. J.;'Zhao,' X. L. J. Phys. B 1991,24,L233. (11) Talrose, V. L. Chemzcal Effects of N u clear Transformations; I n t e r n a t i o n a l Atomic Energy Agency: Vienna, Austria, 1961; p. 103. (12) Talrose, V. L.; Karachevtsev, G. V. In Reactions under Plasma Conditions; Venugopalan, M., Ed.; John Wiley & Sons: New York, 1971; Vol. 11, p. 35. (13) Talrose, V. L.; Vinogradov, P. S.; Larin, I. K. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 1,p. 305. (14) Becky, H. D. Z. Naturforschung 1961, 16A, 505. (15) Becky, H. D. Field Ionization Mass Spectrometry; Pergamon: Elmsford, NY, 1971. (16) Derric, P. J.; Falick, A. M.; Burlingame, A. L. /. Am. Chem. SOC.1972,94, 6794. (17) Karachevtsev, G. V.; Talrose, V. L. Kin. Katal. (Sov.) 1963,4,923. (18) Karachevtsev, G. V.; Talrose, V. L. Kin. Katal. (Sov.) 1967,8,5. (19) Ottinger, C. Z Naturforschung 1967, 22a,20. (20) Ottinger, C. Z. Naturforschung 1972, 27a, 293. (21) Feng, R.; Konishi, Y. Abstracts of Pa-
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pers, 12th International Mass Spectrometry Conference, Amsterdam, The Netherlands, 1991; p. 298. (22) Vilesov, F. I.; Kurbatov, B. L.; Terenin, A. N. Dokl. Akad. Nauk SSSR (Sov.) 1961,138,1329. (23) Baker, A. D.; Betteridge, D. Photoelectron Spectroscopy: Chemical and Analytical Aspects; Pergamon: Oxford, 1972. (24) Carlson, T. A. Photoelectron and Auger Spectroscopy; Plenum: New York, 1977. (25) Product Bulletin A314-0282, Kratos Analytical Instruments (Manchester, U.K.), 1982. (26) Modelli, A,; Foffani, A,; Scagnolari, F.; Jones, D. Chem. Phys. Lett. 1989,163, 269. ( 2 7 ) Babaev, A. P.; Kaltashov, I. A , ; Karachevtsev, G. V.; Marutkin, A. Z. Instrum. Exp. Tech. (Sov.) 1990,4,200. (28) Kaltashov, I. A,; Karachevtsev, G. V. unpublished results. (29) Compton, R. N.; Huebner, R. H. Ado. Radiat. Chem. 1970,2,281. (30) Gregor, I. K.; Guilhaus, M. Int. /. Mass Spectrom. Ion Processes 1984,56,167. (31) Karachevtsev, G. V.; Potapov, V. K.; Sorokin, V. V. Dokl. Akad. Nauk S S S R (Sov.) 1972,206,144. (32) Ajello, J . M.; Chutjian, J. M. J. Chem. Phys. 1976,65,5524. (33) Chutjian, J. M.; Ajello, J. M. J. Chem. Phys. 1977,66, 4544. (34) Ajello, J . M.; Chutjian, J. M. J. Chem. Phys. 1979,71, 1079. (35) Karachevtsev, G. V.; Lipey, M. M.; Potapov, V. K. Khim Vys. Energ. (Sov.) 1975,6,487. (36) Karachevtsev, G. V.; Potapov, V. K.; Sorokin, V. V. Opt. Spectrosc. (Sov.) 1974, 3.5. - - , 960
(37) Karachevtsev, G. V. Opt. Spectrosc. (Sov.) 1985,59,202. (38) Villarejo, D.; Herm, R. R.; Inghram, M. G. I. Chem. Phvs. 1967.46.4995. (39) Kihg, J . C. I n Electron-Molecule Scatter and Photoionization; Proceedings of the International Symposium; Burke, P. G.; West, J. B., Eds.; Plenum: New York, 1988; p. 99. (40) Spohr, R.; Guyon, P. M.; Chupka, W. A.: Berkowitz. R. Rev. Sci. Instrum. 1971.'42.1872. 141) Karachevtsev, G. V.; Lipey, M. M.; Potapov, V. K. Instrum. Exp. Tech. (Sov.) 1976,6,141. (42) Taranenko, L. A,; Kobelyansky, P. V. J. Tech. Phys. (Sov.) 1989,50(10),195. (43) Karachevtsev. G. V.: Marutkin. A. Z. Instrum. Exp. Tech. (Sov.) 1979,6,i19. (44) Karschevtsev, G. V.; Marutkin, A. Z. Instrum. Exp. Tech. (Sov.) 1981,2,173. (45) Peatman, W. B.; Kasting, G. B.; Wilson, D. J. J. Electron. Spectrosc. 1975, 7, 233. (46) Tanaka, K.; Kato, T.; Koyano, I. J. Chem. Phys. 1986,84,750. (47) Popovic, D.; Cadez, I.; Landau, M.; Pichou, F.; Schermann, C.; Hall, R. I. Measurement Science and Technology 1990, 1, 1041. (48) Jochima, H. W.; Ruhl, E.; Baumgartel, M. Z. Naturforschung 1989,44B,13. (49) Koyano, I.; Tanaka, K.; Ibariki, K. Proceedings of the International Symposium on Advanced Nuclear Energy Research: Near-Future Chemistry in the Nuclear Energy Field, J a p a n Atomic Energy Research Institute: Tokyo, 1989; p. 341. (50) Golovin, A. V.; Akopian, M. E.; Reingang, L. M.; Chistyakov, A. B. Khim. Vys. Energ. (Sov.) 1989,23,387. (51) Yang, B.; Chui, Y.; Orlando, T. M.; Anserdon, S. L. Abstracts of Papers, SASP '90: Symp. Atomic and Surface Physics, Obertraun, 1990, p. 276.
(52) Norwood, K.; Ng, C. Y . J. Chem. Phys. 1990,93,6440. (53)Weitzel, K. M.; Boose, J. A.; Baer, T. J. Chem. Bhys. 1991, 150, 263. (54) Norwood, K.; Ng, C. Y. Chem. Phys. I A t . 1989, 156, 145. ( 5 5 ) Hanson, D. M.; Ma, C. I.; Lee, K.; Lapiano-Smith, D.; Kim, D. Y. J. Chem. Phys. 1990, 93, 9200. (56)Golovin, A. V.; Cheremnykh, P. G . Instrum. Exp. Tpch. (Sov.) 1991,2, 141. I
i r
Victor L. Talrose (left) is director of the Institute of Energy Problems of Chemical Physics and professor and former dean of the Moscow Institute of Physics and Technology. He holds an M.S. degree (1947) and a Ph. D. in physics (1952) from Moscow State University, as well as a D.Sc. degree in chemistry (1962) from the Institute of Chemical Physics of the Academy of Sciences. He was elected to the Russian Academy of Sciences in 1968 and has written more than 400 papers and four monographs. His primary fields of interest include chemical physics, radiation chemistry, MS, chemical lasers, atmospheric chemistry, and the ecological consequences of energy. He discovered ionmolecule reactions of organic substances with the CH.; ion. Gennadij V. Karachevtsev (right) is a professor in the department of molecular and chemical physics at the Moscow Institute of Physics and Technology. He received his M.S. degree (1961) and his Ph.D. (1966) from the institute. His research interests include kinetics and thermochemistry of gas-phase ion and electron processes, M S , a n d electron spectrometry.
Supercritical Fluid Extraction/ Chromatography Monday-Thursday, May 1 1 - 14, 1992 Virginia Tech Blacksburg, VA
s e v e n Key Ways You'll Benefit from Attending This Course:
1.
2. 3.
4.
Learn state-of-the-art techniques for performing SFE and SFC Learn to interpret SFC and SFE data Discover why SFE offers advantages over convent ionaI extraction K n o w which SFC and SFE strategies to use w h e n your analyte will not dissolve in
co2
5. 6.
7,
Obtain guidelines for column selection Gain hands-on experience working with chromatographs, column detectors, and extraction devices Understand the limitations o f SFE and SFC
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Title Organization
Igor A. Kaltashov received his M.S.degree (1989) from Moscow Institute of Physics and Technology,where he studied ion processes with MS. He is currently a graduate student in the department of chemistry and biochemistry at the University of Maryland Baltimore County.
Address
City, State, Zip
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