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Chemical Instrumentation feature Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers O ~ T H I SJOURNAL by calling attention to new developments in the h o q , design, or availability of chemical laboratory instrumentation, or by presenting useful insights a n d ezplanations of lopics that are of practical imporhnce to those who use, m leach (he use of, modem instrumentation a n d instrumenla1 lechnipues. The editor invites correspondence from prospective contributors.
LXVIII. X-Rays and Electrons in Analytical Chemistry with Emphasis on Instrumentation (Concluded) H. A. Liebhafsky, Texas A & M University, College Station, Texas a n d
H. G. Pfeiffer, General Electric Co.. S c h e n e c t a d y , N. Y .
X-RAY DIFFRACTION. AUTOMATION AND COMPUTERIZATION Camera Method. The Dow Chemical Company, in what is now its Chemical
Physics Research Laboratory, has done much to make x-ray diffraction of permanent importance in analytical chemistry. Two recent developments there are the ZRD Search-Match Program and the automation of a camera method for powder diffraction.
The use of x-ray diffraction as an analytical method becomes a classic example of information storage and retrieval as the pattern of an unknown is matched against the stored patterns of thousands of knowns that serve as standards. The commter is thus ideal instrumentation for analytical chemistry of this type. This fact is widely recognized, hut the traditional procedure has been reversed by Frevel a t Dow in that information about composition is placed ahead of the diffraction pattern in the finding of the appropriate diffraction standards. A camera method for powder diffraction is inherently more difficult to automate than is the diffractometric, in which exposure times are shorter and no film needs to be developed. Frevel has however succeeded in automating the measurement of powder patterns on film to produce digital d and I computer output readings that could then be used, if desired, as input for the computerized identification of the crystalline phases present. Three pieces of equipment were used: the 24-3404 JarrellAsh mieraphotometer, which measures the displacements on the film that give d values far the sample; the Bristol Dynamaster potentiometer recorder, which measures the transmittances that fix line intensities: and the IBM 526 card punch that (after the necessary encoding) presents the output data. The camera used bv Frevel is an AEG Guinirr douhle-cylinder camera (114.: mrn m diurnrter.. nnd the Cu Kr.1 linr velrrted hy a hent-crystal rnonorhrornator t t h e .lohansson arrangement) was used to obtain d-spacings and intensity data more precisely than is possible in the Hull-DebyeSchemer method. Diffmetometer Method. Samples for the diffractometer are adaoted to handline" bv . marhinr; there is no need for length, r x POSII~PL. and nu fdm to drvrlop; the d ~ t e c tor give* dlgitsl tcpsi \,aIues oi the i n t ~ n s ~ . ties-these are among the reasons why automation and computerization of a diffractometer system are natural developments. A successful system of this kind has resulted from the joint efforts of the Chevron Research Company, La Hahra, Calif., and the Datex Corporation, Monrovia, Calif. The system is shown in Figure 15. It is controlled by one computer and produces data for processing by another. The system is controlled by input information on command cards. When a stack of these has been put into the card reader, and the corresponding samples have been placed in the sample changer, the system is put into operation, which is fully automatic. Cards and samples may be added ~~~~~
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.---Manual transfer -Electrical OT automabc machine transfer Figure 15. Automated and computerized x-ray diffractometer system of the Chevron Research Company. Among the components: General Electric XRD-5 power supply: transistorized scaler and rate meter: high-intensity copper-target x-ray tube (40 mA; 50 kV): Tempres tube-mount: Datex central control computer. Encoderdyne drive motor. 28 display system: Precision instrument Go. incremental tape recorder: Leeds and Narthrup analog recorder; D. and 0.Machine. inc.. automatic sample changer; Tektranix oscilloscope for monitoring shape of pulses from proportional detector: modified IBM 026 card reader/punch, which transmits operating instruction for storage in the computer's memory and can be used during the 28 scan as a key punch far preparing transfer and command cards. See R. W. Rex. Advances in X-Ray Analysis, V o l 10. Plenum Press. New York. 1967. p. 366.
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Chemical Instrumentation
Figure 17. Top-view schematic diagram of ARLPXQ with maximum number (22) of channels indicated. The channels come in pairs, one (broken line) being below the other. The standard (monitor) channel occupies the space allocated above to one pair. This figure and the next by courtesy of Davidson. Gilkerson. and Kemp. Pittsburgh Conference an Analytical Chemistry and Applied Spectroscopy. March 1958.
the charge, hence the voltage, on the fapacitor. This number of photons is equal to the intensity multiplied by the counting interval chosen, which is set for the standard channel. A short caunting interval makes for speed, a great advantage of multichannel systems. To attain speed, this counting interval must he minimized, and the increased intensity resulting from the use of Bragg crystals curved according to Johansson is therefore welcome. Spectrographs thus equipped are being described as "fully focusing." Among the multichannel x-ray spectrographs offered by ARL are models MXQ, VXQ, VPXQ, and PCXQ. The letters have the following meanings: X, x-ray; Q, Quantometer; M, modular; V, vacuum; P, production (i.e., rapid, large, for routine determinations); PC, process control. As the number and diversity of the features of these i n s t ~ m e n t are s reminiscent of Table 3, we cannot do more than give the most
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Figure 18. Schematic diagram related to Figure 17. Note that an end-window x-ray tube is used to maximize the number ol possible channels. The slits are adjustable to give the best balance between intensity and resolution for the determinations being mads.
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8. Equipment available far reading counts accumulated (NT). 9. Solids, slurries, and liquids can all be handled. 10. Precision seems comparable with best achievable under given conditions. 11. Provision can be made for incorporating channels (up to 3) for sequential determinations of elements suited to the channel thus incorporated. Each such channel replaces 2 fixed channels.
Figure 19. Schematic diagram, not to scale, iliustrating Operation of an electron microprobe. Only one of four spectrometer channels is shown. Sample and Bragg crystal (Johansson arrangement) lie an focusing circle. Detector is near enough this circle to intercept focused analytical line at full intensity. This figure and the next by courtesy ol Consolidated Electradynamics Corporation.
The Electron Microprobe. We turn now from "workhorse" spectrographs t o what was long a prima donna-the x-ray emission electron microprobe, which came into being when Castaing in 1951 modified an electron microscope to make a spectrograph ideal for localized determinations. It uses an electron beam to generate characteristic x-ray lines in, for example, a 1micron cube. See Figures 19 and 20.
Later developments bid fair to change the prima donna into a workhorse. The first of these is the incorporation by Cosslett and Duneumh of scanning capability, which makes it possible to select, one might say by television, areas of interest for subsequent localized determinations. The second development is the "interfacing" of microprohe and computer. When interfacing is done with the McCrone Micropmbe Data Converter, these things are possible: (1) all elements from boron upward can he determined in a l-cubie-micron volume element of interest; (2) data from the probe, corrected by computer, appear as analytical results in about 12 minutes; (3) overnight a square centimeter of sample can be scanned automatically far 3 different elements, about 10-14 g of each being detectable in each 1p3 volume element (not merely in the sample!). It is (Continued on pageA80)
important characteristics, not always with specific attribution. Figures 17 and 18 show the geometry of the Quantometers. Salient features not yet mentioned fallow: 1. End-window x-ray tubes. Be windows as thin as 0.127 mil. Targets: W, Rh, Cr, P t. 2. Wavelength range: 0.3G24.9 A (down to and including oxygen). 3. Pulse-height selection when needed. 4. Attenuation by filters used t o increase accessible composition range. 5. All determinations on a sample simultaneously completed in seconds to a few minutes. 6. Computer can be used to maximum capability (control, reading of capacitors, calculations, readout). 7. Reading of individual channels (voltages or ratio*): (a) Sequentially, strip chart recorder, 1 channel in 2 see. (b) Digital voltmeter-line printer, 1 channel/sec. (c) Analog output to computer, up to 50 ehannels/see.
Figure 20. Diagram of modern electron microprobe (Consolidated Electrodynamics Type 27101) with parts labeled. Each of the large drums houses two spectrometer channels. Compare this with Figure 19.
Volume 50, Number2. February 1973
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Chemical Instrumentation no wonder that computers with small memories are a t a disadvantage. The third development (mentioned above) is the introduction of energy resolution by use of a solid ionization detector and pulse-height discrimination by a multichannel analyz-
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Electron Excitation of Large Sample Areas. The success of the electron micro-
probe has stimulated a reappraisal of electron excitation over large (-1 cm2) sample areas and provided guide lines for the development of the needed equipment. This return to electron excitation could well be the most significant recent change in x-ray emission spectrography. We shall describe only the Telsec Betaprobe, as one spectrograph of this type is called. As usual, the names tend to confuse. Enough will be said about the Telsec Probe to make possible a meaningful comparison with the electron microprobe (see above). The Telsec Probe has two contiguous vacuum systems, each with its own backing and diffusion pump, joined by an aperture for the electron beam, the aperture being so small that the electron-gun chamber is maintained a t the necessary 10-s torr, even though the pressure in the sample (flat-crystal) spectrometer chamber is a hundred times as great. Results: pump-down time from atmospheric pressure is only 30 see for a metal samplelonger (up to 3 min) for clays not previously outgassed to reduce pump-down time. Contamination of the electron gun is thus prevented-even so, the filament therein has a limited operating life (say, 24 br; hut many determinations!) and planned replacements had best be made. A simple grid focuses the electron beam through the aperture. This beam would normally form an the sample an enlarged, but unstable. image of the tip of the filament. A simple electromagnetic lens is consequently added to focus the electrons t o a spot about 1 mm in diameter on the sample. Scanning coils cause the beam usually to traverse a sample area of 10 X 10 mm; the side of this square can be changed from 1 to 16 mm t o meet special requirements. Beam currents of 0.1, 0.2, 0.5, and 1.0 mA are available a t up to 15 kV. Beam stability is 0.1%. Nonconducting powdered samples must be made conducting as by admixing graphite; they are subsequently put in a lead ring or recessed lead disk and pressed. Lower beam currents are often desirable to reduce heating. A determination usually takes about 10 sec, once excitation has begun. The element coverage by the Telsec Probe is given in Table 4. It is an understatement to say this coverage is impressive. Note particularly that the light elements seem to give no trouble. Of the range of Bragg reflectors needed to aecommodate the range of wavelengths, the less well known are: PbSt, lead stearate; OHM, octadecyl hydrogen maleate; KAP, potassium acid (hydrogen) phosphate; ADP, ammonium dih-ydrogen phosphate; and EDDT, ethylene diamine ditartrate. Note also the use of M lines as analytical A80
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lines, necessary to keep electron beam energy law enough to ensure stability. Summary of Developments During Last Decade. Because many interesting developments in x-ray emission spectrography during the last decade or so could not be described above, we give below a brief summary of what occurred during that time: 1. Great success of the electron micropmhe. 2. Changes wrought by use of computers: for example, in control of equipment and in calculation and storage of results. 3. Introduction of solid-state circuitry. 4. Return to electron excitation. 5. Increasing numher of useful Bragg reflectors. 6. Increasing attention to light elements, hence to vacuum optical paths and detector windows. 7. Enormous emphasis on speed and convenience, as through provisions for easy interchangeability of Bragg reflectors, callirnaton, and deteethrs; multiple sample loading; and modular construction that permits adding components t o a simple system. 8. Growth of photoelectron spectrograph~. 9. Development of portable spectrographs, some with radioactive x-ray sources. 10. Progress in process control and largescale applications of x-ray emission spectrograph~. Examples: determination of plating thickness (e.g., Sn, Zn, or A1 on steel); the control of flowing mineral slurry streams, as by determinations of Cu, Zn, and Fe in concentrates from ore; and the completely automated manufacture of eement, in which Ca, Al, Si, and Fe are determined in a closed control loop.
INSTRUMENTATION FOR ELECTRONS IN ANALYTICAL CHEMISTRY Auger Electrons. It was mentianed earlier that Auger electrons are ejected when a radiationless transition follows x-ray absorption. They can be ejected also when excitation is produced by ather electrons. The production of Auger electrons thus competes with the generation of charaeteristic x-ray lines. At low atomic numbers, the production of Auger electrons easily wins this competition. The possibility that Auger electrons might be useful in chemical analysis was neglected until recently, when Harris in valuable investigations proved that they are well adapted to quolitatiue determinations on surfaces. His apparatus consisted of a n electron gun to excite the sample in its holder, an electron-energy analyzer, and an electron multiplier. The Augerelectron energies range up to about 2000 eV, and the experimental data are electron-multiplier output (response) recorded against Auger-electron energy (analogous to x-ray intensity as a function of wavelength). The characteristic Auger peaks are difficult to isolate from the large and complex background. Harris made an important contribution by using the deriuotiue of the response curve, automatically
Chemical Instrumentation istic line (in this ease, a K line) of the atom. The Einstein postulate about the energy relationships in elementary processes should apply. That it does apply was elegantly demonstrated by use of a magnetic photoelectron spectrograph, used in primitive form hy Robinson and Rawlinson in 1914. then by de Braglie, and in the form shown in Figure 21 by Robinson in 1923 and as late as 1940. The operation of the spectrograph is outlined in the caption of Figure 21; its function is to measure the energy of photoelectrons emitted by a target that absorbs monochromatic x-rays. Focusing is only approximate, for the electrons that pass the slit S do not all travel the same circle. Using a compnss, it is easy to show what also appears in the figure; namely, that circles approaching each other most closely a t S will approach each other most closely again a t L. As ultimately refined, the magnetic electron spectrograph became one of the most reliable instruments for the determination of physical constants. The early work established the energies of the various shells and subshells in the atom and gave an excellent value (for the time) of the Planck constant, 11. Since then, K. Siegbahn and his colleagues have made notable progress in the field and increased Robinson's great precision still further so that it is now possible to mea-
sure binding energies of electrons to a fraction of an electron volt. Even more recently, the magnetic field for controlling the electron path has been replaced by an electrostatic, with the advantages that size and cost of equipment are reduced. In eleetmstatic photoelectron spectrography, the necessary magnetic shielding is accomplished by using mu-metal, or something similar. Equipment of this kind is now affered by Varian and by others. As concerns us, photoelectron spectrography (or Electron Spectroscopy for Chemical Analysis. ESCA, acronym indicated) is the substitution of a characteristic (photo) electron line, related to binding energy of the electron, for a characteristic x-ray line emitted because this photoelectron was ejected. During 1971, the MePherson Instrument Corporation marketed a versatile, precise electrostatic photoelectron spectrograph named ESCA 36 because the hemispherical shells that maintain the electrostatic potential have a mean radius of 36 cm. In such instruments resolution increases with this diameter, being for this instrument 0.05% of the kinetic energy of the photoelectrons collected. Samples can be solid. liquid, or gaseous; cooled or heated. Targets of Mg, Al, or Cu supply K lines for excitation. Monoenergetic electron or UV sources are optional extras. The detector is an electron multiplier exposable to air. The spectrograph system has a n automated control and data acquisition system that includes a dedicated computer. A
sample wheel holds up to eight samples. the surfaces of which can be cleaned by argon-ion bombardment. Sample changing is easy. The ESCA 36 measures electron kinetic energies over the range 4 to 4WO eV. Auger electrons within this range will naturally be recorded along with the photoelectrons. The energy spectrum can be scanned a t variable rates: a rapid scan over this spectrum will reveal whether a n element is present, be it free, or combined, or bath; a slow scan over the narrow spectral region thus marked out for an element will give more detailed information about it. McPherson also markets an ESCA 2.5 Elecnan Impact Spectrometer for determinations on gases and vapors. This instrument identifies gaseous molecules via the energy loss that occurs when monoenergetic electrons strike them. It promises to be useful for the identification and determination of pollutants in sir, and to have research applications as well. To oversimplify: in x-ray emission spectrograph~,we use the characteristic line for the determination of a n element and ignore the photoelectron. ESCA does the reverse. In our opinion, the two methods are complementary rather than competitive. We do not believe that ESCA will replace x-ray emission spectrography in the quantitative determination of elements. We are certain that x-ray emission speetrography cannot compete with ESCA in determinations of the binding energies of electrons.
CONCLUSION Even this incomplete account of instrumentation shows the important contribution of physics, including nuclear and solid-state, of electronics, of the computer, and of systems engineering toward making x-ray and electron methods widely useful today. Of course, instrumentation is only part of the story. The relationship of xrays and electrons, the simplicity of x-ray spectra, the magnitude end the range of the energies involved-fundamental factors such as these made successful instrumentation possible. Actually, the advantages x-rays offer in analytical chemistry have been known far a long time: Hevesy demonstrated many of them in his work on thucolite and hafnium. Modern instrumentation has turned these advantages to widespread use in applications a t once numerous and diverse. The end is not in sight. As the chemistry goes out of analytical chemistry, physics, the computer, and engineering continue to move in.
ACKNOWLEDGMENT We thank the Robert A. Welch Foundation for partial support.
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