INSTRUMENTATION BY RALPH H. MÜLLER
Techniques of Electron Spectroscopy for Chemical Analysis Offer Challenge to Conventional Spectroscopy
WBedell of 1269 Druid Rd. E., ClearE ARE INDEBTED to Howard
M.
water, Fla. 33516, for further information on Electron Spectroscopy for Chemical Analysis (ESCA) and closely related techniques. Bedell, a physicist, has had a long and distinguished career in applied spectroscopy and as Regional Manager of McPherson Instrument Corp., at 530 Main St., Acton, Mass. 01720, has devoted much time to the exploitation of ESCA developments. McPherson is in production of two models, one limited to the study of gases and a larger, general purpose instrument. He informs us that the McPherson Corp. has been made the exclusive distributor of the book, "ESCA" by Kai Siegbahn et al., published by Almquist and Wiksells of Stockholm. Earlier studies on electron spectrometry, such as those of Lassettre, Berman, Silverman, and Krasnow, J. Chem. Phys., 40, 1232 (1964), afforded precise measurements of inelastic collision cross sections in helium, carbon monoxide, and oxygen. A Technical News Letter from the National Bureau of Standards of August 1968 reports briefly on an impact spectrometer found valuable in trace analysis. I t states that the instrument, called an electron impact spectrometer, developed four years earlier for research in atomic physics, has recently shown great potential in the fields of chemical trace analysis. By the use of the spectrometer, such hard-to-distinguish compounds as carbon monoxide can now be detected in concentrations as low as 15 parts per million. The instrument's high resolution and sensitivity should make it valuable for trace analysis and in the detection and control of air pollution. Its full capabilities are being investigated in a joint program of NBS and the National Aeronautics and Space Administration, Langley Research Center. As developed by J. A. Simpson, C. E. Kuzatt, and S. R. Mielczamk of the NBS Institute for Basic Standards, its operation depends on the collision of electrons with gas atoms or molecules.
Upon impact, the collision electrons transfer their energy to the electrons bound within the atom, which are thus raised from the stable state to an excited energy level. The spectrometer disperses the collision electrons according to their energies after impact and the resultant energy corresponds to the optical absorption spectrum of the gas. In a study of the atmosphere, the naturally occurring argon and helium in the sample (approximately 0.2 nanogram and 0.2 picogram, respectively) were detected. I n less favorable conditions where interference is a problem, as in detecting carbon monoxide in air, contaminants on the order of 10 parts per million have been detected. Simpson reports that, with a new design, 0.01 ppm may be detected. The electron impact spectrometer possesses a number of advantages over other analytical instruments. For example, in comparison with a mass spectrometer, it is smaller, lighter in weight, and, at least for atoms and simple molecules, it is less susceptible to interference between constituents of mixtures. For example, in the case of nitrogen and carbon monoxide, there are differences in the molecular weight occurring in the third decimal place which constitutes a real problem in the mass spectrometer b u t not in the impact spectrometer. Also, the problem of identifying the parent compounds from their fragments does not arise. The comparison with optical absorption methods is highly instructive. The electron impact spectrometer covers an enormous range because the energy losses which are measured correspond to optical wavelengths ranging from the X-ray to the visible region. Also, the response is linear rather than exponential with concentration, hence the sensitivity is greatly improved and the spectra, are easier t o interpret. References pertinent to this development are : NBS Tech. News Bull., 48, No. 4, 64-68 (April 1964) ; Absorption spectrum of SF e in the far ultraviolet by electron impact, J. Chem. Phys., 44, No. 12,
4403-4404 (June 1966) ; and Electron monochromator design, Rev. Sci. Instr., 38, No. 1, 103-111 (Jan. 1967). Dr. William J. Campbell has very kindly sent us a copy of his review paper with James R. Brown: X-ray Absorption and Emission, ANAL. C H E M . ,
40, No. 5, 364 R (1968). These and several related topics are discussed in detail. The report on the general subject contains 695 references. He discusses low energy X-rays, excitation by heavy particles, Auger and photoelectrons, and ESCA. Dr. Campbell, who is a supervisory research chemist a t the Bureau of Mines, College Park, Md. 20740, states that he expects delivery of a combined LEED-Auger spectrometer in December. I t is being designed for them by Varian Associates and when combined with a low voltage, high current X-ray tube for excitation of photoelectrons will provide the capability of LEED, Auger, and photoelectron spectroscopy on a sample surface that can be heated up to 1000 ° C and exposed to a variety of atmospheres. ESCA and its closely related techniques are being studied and examined in all quarters of the scientific world, following the monumental contribution from Sweden. We hope Mr. Bedell will find it possible to report on centers of interest and activity—he seems to know just about everyone who is interested or working on the subject. We have heard enthusiastic comment on the talks given by Dr. David Hercules of M.I.T. and are looking forward to his formal lecture at the twenty-second Annual Louisiana State University Symposium on Analytical Chemistry next January. The essential antiquity of the subject has been pointed out by Siegbahn and others and its delayed development is easily understood in view of the great experimental difficulties in making electrical measurements of electron energies with a precision comparable t o the optical equivalent so easily obtained by spectroscopy. The professional spectroscopist knows that the first resonance line of mercury has a wavelength of VOL. 40, NO. 14, DECEMBER 1968 • 8 3 A
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ANALYTICAL CHEMISTRY
INSTRUMENTATION
Werner Centennial
2536.519 A but has to be wary of V, Fe, Rli, Bi, P, Ta. Indeed, a P t line only 1/20 the intensity of the Hg line has a wavelength of 2536.49 Â. The other neighbors differ by only 1 or 2 tenths of an Â. Anyone who wishes to plug these values into Ve — hv can note the minute potential differences in V associated with these transitions. Also, the Auger electrons representing a nonradiating transition are unique because the resources of optical spectroscopy are fruitless. Our interest and present excitement over the recent developments goes back to more than 40 years. Our postdoctoral studies at the Institute of the late James Franck at Gottingen were at a time when he was concerned with other important problems and hardly liked to be reminded of the simple but epochmaking experiment which he and Gustav Hertz had made in 1914. I n bombarding mercury vapor with electrons of controlled velocity, they had noted the inelastic impact at 4.9 volts and properly identified as the excitation energy leading to the emission of the first resonance line of Hg at 2537 À. This was the first experimental demonstration of the validity of quantized energy levels in the atom as postulated by Bohr a year earlier. Although Franck and Hertz jointly and singly determined many other critical impact values, the subject soon became of great interest and, in the hands of Bergen Davis, Foote, Mohler, and K. T. Compton, was soon established as a firm relationship between electrical excitation and optically established energy values, leaving no apparent extensions of further profit. It soon became accepted that real precision was to be gained from the spectroscopic data and not from electrical measurements. The superb achievements of modern spectroscopy from the X-ray region out to microwaves can well be regarded as a fait accompli. What more could be asked for certainty and precision? We have continued to ask, and bore our friends, with the question, "What more can be learned electrically about flames, arcs, sparks, and luminous gases?" The unquestioned advantage of optical methods may be put bluntly, "Seeing is believing." To do these things electrically is obviously doing it the hard way, but now genius has prevailed and we have a potent challenge to conventional spectroscopy. I t should convince the most doubtful that the phenomenological studies are still useful and all progress does not depend upon computers. Once more, it confirms the dictum of the inventor and designer, "Never let well enough alone."
ADVANCES
IN
CHEMISTRY SERIES
62
WERNER CENTENNIAL ADVANCES IN CHEMISTRY SERIES NO. 62 F o r t y - t w o papers survey the w o r k of A l f r e d W e r n e r , founder and systematizer of c o o r d i n a t i o n chemistry, evaluate progress since his time, and report current research in this active
field. •
Personal Werner
recollections
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The W e r n e r - J ^ r g e n s e n versy
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W e r n e r and cobalt complexes
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Nomenclature compounds
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C o o r d i n a t i o n in the second sphere
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Masking of ligand reactivity
of
of
Alfred contro-
coordination
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Linkage isomerism
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Chemistry of cyclobutadiene-iron tricarbonyl and many other topics.
661 pages with index Cloth bound (1967) $15.00 postpaid in U.S. and Canada; plus 20 cents foreign and PUAS. Sel of L. C. cards free with library orders.
Order from: Dept. Β
SPECIAL ISSUES SALES A M E R I C A N CHEMICAL SOCIETY 1155 SIXTEENTH ST., N.W. W A S H I N G T O N , D. C. 20036
