X-ray absorption and emission - ACS Publications

(401) Whistance, G. ,R., Dillon, J. F., Threlfall, D. R., Bzochem. J., 111, 461 (1969). (402) Whittick, J. S., Muraca, R. F.,

Cavanagh, L. A., “Analytical Chemistry Instrumentation,” National Aeronautics and Space Administration, Washington, D. c., NASA sp-5083,

1967. (403) WileY, R. A., J* H*,J *Med. Chem., 12, 146 (1969). (404) Williams, D. R., Coffen, D. L.,

Garrett, P. E., Schwartz, R. N., J . Chem. SOC.,1968B,1132. (405) Wolken, J. J., Forsberg, R., Gallik, G., Florida, R., Rev. Sei. Instrum., 39, 1734 (1968). (406) W O O , W. s., J . Pharm. SCi.9 579 27 (1968). (407) Yamakawa, M., Kubota, T., Akazawa, H., Theor. Chim. Acta, 15, 244 (1969). (408) Yamakawa, M., Kubota, T., Akazawa, H., Tanaka, I., Bull. Chem. SOC. Jap., 41, 1046 (1968).

(409) Yamamoto, R. K., Cook, W. A,, Amer. Ind. Hyg. Assoc. J., 29, 238 (1968). (410) Yates, K., Klemenko, S. L., Csismadia, I. G., Spectrochim. Acta, 25A, 765 (1969). (411) Yudovich, E. E., .’ ’*, ~ ~ , ~ USSR ~ 1 $ ~transl.), ~ * (412) Zangieva, Z. G., Gurevich, N. A,, Sokolova, El. I., Russ. J. Inorg. Chem. (Engl. trawl.), 13, 274 (1968).

X-Ray Absorption and Emission William J. Campbell, U.S. Department of the Interior, Bureau of Mines, College Park Metallurgy Research Center, College Park,

Md. 20740 and John V . Gilfrich, U.S. Naval Research laboratory, Washington, D. C. 20390

A s

REQUESTED by the editors of ANALYTICAL CHEMISTRY, this 1970 review is more condensed than the 1968 review (74). The sections on electron probe microanalysis were authored by J. V. Gilfrich, while the other sections were prepared by W. J. Campbell. Please forward your questions and comments to the appropriate reviewer.

X*RAY SPECTROGRAPHY

I n a discussion on current trends in X-ray analysis, Birks listed the following subjects: Calculation methods, energy dispersion, inhomogeneous samples, crystals and gratings, effect of valence on spectral line shape and position, and electron spectroscopy (41). I a m in general agreement with Birks and I have included all of the above in this review with emphasis on calculation methods, energy dispersion using radioactive isotopic sources, and low energy electron spectroscopy. A bibliography on X-ray analysis prepared from the files of J. L. DeVries has been updated (24). This extensive selected compilation of worldwide literature covers theory, instrumentation, and applications. Russian accomplishments in X-ray analysis were summarized from the period of approximately 1930 through 1966 (367). The “Resource Letters on X-Rays” provides a n excellent educator’s guide to principles and instrumentation (361). Recent textbooks and conference proceedings are listed in Table I. “Advances in X-Ray Analysis” (17, 370) provides excellent papers on new developments in X-ray emission and electron probe microanalysis. There are three new textbooks on X-ray emission (32, 58,42) and an outstanding conference proceedings on electron microprobe theory and practice (235). Progress in low energy X-ray spectroscopy is summarized by Fabian (156) and low energy

electron spectroscopy by Siegbahn and his associates a t the University of Uppsala (464). N. V. Philips in Eindhoven made available printed proceedings of X-ray symposia held in Brussels 1964; Sheffield, 1964; Lausanne, 1965; and Swansea, 1966. One area in X-ray spectrography where there is obvious duplication of effort is the preparation of 28 tables. Separate tables were published for LiF, topaz, and ADP (171-173). In our 1968 review, we strongly recommended the use of Bearden’s wavelength tables for any future 28 tables. The new tables (29) by the Bureau of Mines lists 28 values for nine analyzing crystals and includes over 2300 X-ray lines from Bearden’s tables. Since these data are available in computer format, it is a very simple and inexpensive task to prepare tables for additional crystals. The precision and accuracy of the wavelength scale is a continuing study. Burr (69) discussed secondary wavelength standards-CrKa, CuLa, O K a and ALKa-for use in the long wavelength region. The value of CrKaz (2.293606 A) is considered to be precise to one ppm. The X-ray waveleggth scale based on WKa, 0.2090100 A, is summarized and compared to previous scales based on the kx unit derived from calcite (487). Radiation safety recommendations for X-ray spectrographic and diffraction equipment were prepared by the joint effort of the Pennsylvania Department of Health and the National Center for Radiological Health. These recommendations are intended for use by operators, administrators, manufacturers, and State radiological health personnel (356). From the numerous accident reports that I have examined, it is apparent that operator carelessness together with inadequate interlocks is the prime source of trouble. Because of the highly localized nature of most

248R * ANALYTICAL CHEMISTRY, VOL. 42, NO. 5 , APRIL 1970

X-ray beams, film badges are of limited value in evaluating new experimental setups. I strongly recommend careful monitoring with detectors of adequate sensitivity for the radiation being used. INSTRUMENTATION

X-Ray Spectrographs. Instrument manufacturers continue t o provide the analyst with improved models having a n increasing level of automation (536). Most of these models provide increased precision of analysis, convenience, and ease of data processing; however, they do not necessarily represent improved analytical accuracy. I n general, accuracy is still limited by the degree of sample preparation and the validity of the matrix correction rather than by any instrumental factor. Two X-ray spectrographs are commercially available that employ direct electron excitation. These instruments offer a means for a very rapid determination of low atomic number elements (230, 283). Instrumentation for the analysis of solutions under vacuum conditions was described by Chan (90). A general purpose spectrograph that employs large spherically-curved crystals was constructed. The 10 by 10-cm single crystals are bent to a 63.5-cm radius of curvature and ground to the 31.75-cm radius of curvature of the focal circle. A motor-driven remotely-controlled interchange mechanism has positions for four crystals. Resolution and sensitivity are claimed to be greatly superior to conventional flat-crystal optics (148). A simple two-crystal spectrometer was constructed by modification of a flatcrystal X-ray spectrograph. The detector, originally attached to the 28 arm, was replaced by a second crystal. This detector was placed a t an appropriate position relative to the 28 arm. Wavelength dispersion in this mode of opera-

tion is the sum of the dispersion of each crystal. Germanium (220) crystals gave the best resolution (3.3 eV FWHM for Cu Kal). Intensity with lithium fluoride was approximately one quarter that of a single-crystal spectrometer and the resolution was 9.5 eV FWHM for CU Kai (192). A total-reflection X-ray spectrograph was designed that consists of an efficient X-ray source, a totally reflecting mirror, and an open-window photomultiplier. Mea$urement of wavelength in the 15 to 80 A range is achieved by the wavelength dependence of the critical angle of reflection. As the incident angle is passed through the critical angle for a particular wavelength the reflected beam intensity is sharply reduced. Therefore, a periodic vibration of the incident beam through a small angular range about the critical angle furnishes a strong a x . signal characteristic of a narrow wavelength band (119, 243, 244). A source scanning X-ray grating spectrometer was proposed for scans of a very limited wavelength range. The scanning X-ray source is to be achieved by scanning the electron beam of the X-ray tube (436). Another approach for accurate measurement of fine structure is the moire fringe angular measuring system. Automatic iterative scanning gives digital output and facilitates summing of repeated spectra (523). A high-resolution, high-dispersion X-ray spectrometer developed for space applications may be applicable to laboratory-type problems. Using a cylindrically bent crystal, resolution for CrKal was 2.09 eV FWHM (443). Two data acquisition and processing systems for long wavelength X-ray specta were described by Baun (22). I n one system, the X-ray detector and crystal are advanced stepwise by a precision stepping motor that is interconnected with the step control, printing timer, printing scaler, print control, teletype, and paper tape punch. I n the other system, a multichannel analyzer is used in a multiscaler mode. I n both systems, the paper tape is converted to magnetic tape and fed to a computer. Line or point-by-point plots, linear in wavelength or energy, can be prepared for any selected spectral region. Sample holders were designed for use with various commercial instruments (79, 401, 467’). User of a thin plastic film over the surface of briquetted powders eliminated the problem of surface contamination by sulfur-containing vacuum pump oils (284). There was minor activity in development of improved X-ray milliprobes. Two new probes were described, both having curved-crystal X-ray optics. One has a microscope attached for examination of the irradiated area (461). The other probe has the unique feature

Table I,

Author Bermudez, P.

Summary of Recent Books

Title “Teoria y Practica de la EspectroscoDia de RayosX” “Principles and Practices of X-Ray Spectrometric Analysis” “X-Ray Spectrochemical Analysis-2nd

Bertin, E. P. Birks, L. S.

d i t,inn” ~_..

Dewey, R. D., Mapes, R. S., and Reynolds, T. W. Fabian, D. J., ed. Heinrich. K. F. J.. ed. Siegbahn, K. et al.

“Handbook of X-Ray and Microprobe Data” “Soft X-Ray Band Spectra and the Electronic Structure of Metals and Materials” “Quantitative Electron Probe Microanalysis” “Atomic, Molecular, and Solid State Structure Studied by Means of Electron Spectroscopy”

SECTIONS O F X-RAYSPECTROSCOPY Barrett, C. S., Newkirk, J. “Advances in X-Ray Analysis,” Vol. 12 B., and Mallett, G. R., eds. Buwalda, J., ed. “5th Conference on X-Ray Analytical Methods” __ .. Grove, E. L., and Perkins, “Developments in Applied Spectroscopy,” A. J., eds. Vol. 7A Herbstein, F. H. et al., eds. “Methods of Obtaining Monochromatic X-Rays and Neutrons” International Atomic Enerev “Nuclear Techniaues and Mineral ReAgency sources” Murt, E. M., and Guldner, “Physical Measurement’sand Analysis of W. G., eds. Thin Films” “Advances in X-Ray Analysis,” Vol. 11 Newkirk, J. B., Mallett, G. R., and Pfeiffer, H. G., eds. “Siemens Review-X-Ray and Electron Voment, G., ed. Microscopy News” ~~

I ”

of presenting concentration as a colored mosaic. The color presentation is achieved by an automatic raster scanning motion of the sample holder synchronized with the motion of a light beam across a colored film holder (73). Application of milliprobes and various instrumental factors affecting sensitivities were summarized (422). A compact X-ray spectrographic camera was constructed that employs curved-crystal optics and Polaroid film recording. Samples ranging from bulk materials to microgram quantities can be analyzed over the wavelength range 0.5 to 7 A. Using either spectrographic or diffraction-type tubes, the exposure times ranges from 1to 60 miutes depending on the sensitivity and concentration of the elements being determined (82, 296).

Excitation. A simple inexpensive cold-cathode X-ray source was developed t h a t operates a t a mechanical pump vacuum level. Stability is within =tl%for the first hour and 1.5% for longer time periods. X-ray characteristics are controlled by either a finemetering valve or an automatic pressure controller (46‘4). A miniature X-ray tube designed for X-ray astronomy may be useful for educational purposes. The tube uses a miniature lamp filament and 0.05-mm thick aluminum foil as the transmission target. An anode current of 1p A and a potential of 4 kV produce loa A1 K photons per second (200). Heating of the window and housing in

X-ray spectrographic tubes can be significantly reduced by grounding the cathode, thus resulting in more efficient collection of backscattered electrons (3), Very high intensity X-ray beams were achieved by operating an electron gun of the Pierce Muller type a t 1 to 6 kV and up to 500 mA (102). There were several studies on the use of primary filters (filters between the X-ray tube and sample) to reduce general background and interfering lines characteristic of the X-ray tube target (86, 187, 307). We investigated this technique for reduction of background around the AuLa position using a molybdenum target tube. Line-tobackground was significantly improved by the use of a filter but a t a substantial loss in signal so that the figure of merit was not improved. Although proton excitation in the low energy X-ray region has great promise, there was again little activity in this field. Very encouraging results were obtained for 100- to 1000-ppm levels of boron and carbon using a 200 kV proton beam and a flat-crystal spectrometer (232). Variation in X-ray intensity due to channeling of the proton beam was studied on single crystals of aluminum, copper, and tungsten (289). Dispersion. There was moderate activity in improving crystals and collimators. The 200 reflection of ADP (2d = 7.50 A) was found to be superior to E D D T (19). Clinochlore, a member of the chlorite group of min-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970

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erals, has perfect 001 cleavage, is mechanically strong and stable in air at elevated temperatyre, and has a 2d spacing of 28.392 A. Large flat sheets of analyzer-grade clinochlore are now commercially available (23). The diffraction properties of LiF were improved by abrading cleaved crystals with 15 to 20 p carborundum and by flexing and reflatting. It was concluded that approximately theoretical mosaic crystals can be achieved by either distorting near-perfect crystals, such as LiF, or by orienting powders, such as graphite (327, 610). A mosaic of hundreds of small LiF crystals were used in a n X-ray telescope to concentrate parallel X-rays onto a small detector (518). Monochromators composed of highly oriented graphite look very promising in the long wavelength region (198). The graphite compares favorably to E D D T and LiF with good reflectivity but broad lines. Optimum conditions for building u p lead stearate crystals were reported to be different from those found earlier by Henke. Reflection coefficients, halfwidth, and peak-to-background ratios were determined as a function of the number of layers and the lead content (95). An improved collimator for long wavelength X-rays was designed in which plates with regularly-spaced apertures were replaced with plates having a random array of openings. Fewer plates are required t o suppress off-axis radiation (600). A new concept in X-ray focusing was proposed based on the piezoelectric effect. Deformation of a crystal by a n electric field results in changes in interplanar spacings. This approach may be useful for investigating fine structure over a small angular range ($73). When it is necessary to measure radiation distribution in short-lived phenomena, normal scanning procedures are not applicable. A prismatic singlecrystal slab whose principal surfaces are parallel to a crystal plane of high reflectivity is held at both ends and twisted several degrees. A wavelength range of 1.25 to 1.75 A has been measured at one spectrometer setting by means of the twisted-crystal analyzer (168). Detection. There was moderate activity in development of improved proportional and scintillation detection systems. A quantitative theory for t h e space-charge effect (change in energy and width of peak as a function of counting rate) was developed and confirmed experimentally (239). These space-charge effects are one of the principal limitations in energy dispersion X-ray spectrometry (408). Dead-time corrections using the simple Geiger formula were found to be adequate up to 250R

0

65,000 counts per second (212). I n proportional counters, the rise time of X-ray-produced pulses are shorter than the rise time for background pulses. This characteristic was used to reduce detector background (469). A continuously variable amplifier gain was employed to achieve pulse-amplitude discrimination during scanning over a wide wavelength range (214). Various gas compositions were proposed to reduce escape peak interferences (276). A variable geometry end-window proportional counter can provide energy selectivity in the long wavelength region. For example, the relative response for CuL (930 eV) as compared t o SiK (1.7 keV) can be varied over one order of magnitude (72). A low cost do-it-yourself flow-proportional counter was designed out of hexagonalaluminum bar stock (21). Various suggestions were offered for improved thin windows for flow-proportional counters. Polyproplyene can be stretched down from 26 p to 1 p by allowing atmospheric pressure to work against a thin film under a moderate vacuum (83). Makrofol KG, the polycarbonate of 4,4'-dihydroxidiphenyl2,2' propane, is commercially available as 2-p thick films (61). Thin mylar films can be prepared by dissolving mylar in phenol, then evaporating to form a film less than 0.1 p thick (467). Quantum counting efficiencies for krypton, argon, and neon scintillation counters were measured for chromium, cobalt, and copper K radiation. Efficiency for copper K a radiation ranged from 14% for krypton to 3.5% for neon (290). The relative efficiencies for various photocathodes to be used in windowless electron multipliers were studied using aluminum K radiation (466). A new wide-band windowless photon detector was examinsd in the wavelength range 25 to 2500 A (206). The real breakthrough in detector technology is the commercial availability of reliable high-resolution lithiumdrifted silicon and germanium detectors (4, 129, 267, 381, 468). Pulse distributions of 200 eV FWHM are now possible for 7 to 8 keV photons. These detector systems, coupled with multichannel analyzers and radioisotopic sources, are referenced in the section on energy dispersion X-ray analysis. I n the high energy X-ray region, greater than 20 keV, the resolution of the semiconductor detector is equal to that of a crystal spectrometer. Previously the analyst could not efficiently use high energy K X-rays because of the lack of suitable crystals. This limitation can now be overcome by the use of the semiconductor detector. APPLICATIONS

Quantitative Analysis. X-ray spectrography continued t o find a n increasing number of applications, both


for research and for process and quality control. Methods for specific elements are summarized in Table 11. Analyses for various types of samples are listed in Table 111. An important source of methods is the Applications Review, published every two years by ANALYTICAL CHEMISTRY. The feasibility of employing the Xray spectrograph for general semiquantitative analyses was explored (8). Results for 27 elements were found to

Table II.

Specific X-Ray Spectrographic Analysis

Aluminum I n alloys and metals (246) In minerals and ores (SO, 99, 132, 272, 303,432, 436,499)

In miscellaneous materials (229,406) Antimony (158, 325) Argon (249) Arsenic (66, 158, 441) Barium 1297) Beryllium ( 3 b ) Boron (282, 283, 324) Bromine (117, 136) Cadmium (91) Calcium I n cement materials (48, 281, ,917) I n minerals and ores (30, 99,-isi, 146, 272,303,432, 493) In miscellaneous materials (194, 371, 406,520) Carbon (282, 283) Cerium (106) Chlorine (181, 229, 269, 333, 388, 541) Chromium I n alloys and metals (497) In miscellaneous materials (4O,359,371) Cobalt (329, 354, 359,371, 400) Copper In alloys and metals (76, 197, 405,646) I n miscellaneous materials (269, 354, 359,400,453, 475, 509) Fluorine (322) Gold (67, 188, 545) Hafnium (321, 374, 518) Iodine (253, 488) Iron I n alloys and metals (197, 497) In cement materials (48) I n minerals and ores (30, 158, 143, 146, 159,179,272,303,41 3,4SZ149S) In miscellaneous materials (194, 229, 329,371,400,406) Lead (189,297,325) Lithium (323) Magnesium (30, 132, 272, @2) Manganese (146, 303, 359, 371, 400,493) Molybdenum I n alloys and metals (497, 504) In miscellaneous materials (47, 179, 506, 47.6, 609) Nickel I n alloys and metals (197, 263, 332) In miscellaneous materials (354, 369, 371,400,406,541) Niobium (195, 329, 442) Nitrogen (283) Osmium (480) Oxygen (155, 283) Phosphorus (310, 323, 388) Plutonium (153, 180, 384) Potassium (303, 306, 406, 432, 493) Rare earths (13, 31, 50, 177, 218, 366, 406, L2L

R & & m (465) Rhodium (188) Rubidium (43, 297) Ruthenium (480) (Continued)

Table It. Specific X-Ray Spectrographic Ana lysis (Continued)

Scandium (217) Selenium (6, 194) Silicon I n alloys and metals (246, 497, 620) I n minerals and ores (SO, 99, 132, 1.46, 303,432,455)

I n miscellaneous materials (406)

Silver (64, 67, 68, 406,646) Sodium (324, 340) Strontium (93, 194, 197, 376) Sulfur (28, 30, 61, 284, 310, 321, 388, 641) Tantalum (216, 247, Sag, 39s) Thallium (154) Thorium (61, 162) Tin (168,182,189,269, 326,406,477,604) Titanium I n minerals and ores (169, 179, 272, 399, 493)

I n miscellaneous materials (194, 229, 329,406, 449)

Tungsten (80) Uranium I n minerals and ores (61, 186) I n miscellaneous materials (161, 162, 384,416)

Vanadium (304, 369, 406, 442) Yttrium ( 1 2 ) Zinc (117, 194, 369, 400, 405) Zirconium I n alloys and metals (68, 321, 604) I n minerals and ores (135,169,264,624) I n miscellaneous materials (364, 442) Table 111. Applications to Specific Types of Samples

Alloys Copper base (66, 274) Dental (406) Ferrous (49, 66, 106, 282, 376, 393, 450, 457, 498, 613)

General (183, 496) High temperature (197) Lead (168, 326, 646) Light metal (180, 246, 378) Precious metal (76) Titanium (157, 604) Zirconium ( 3 L l ) Archaeology (65)’ Biology (88, 112, 368, 488) Brines and salt water (136, 569) Carbides (329) Catalysts (406) Cement (48. 317) Clays (29?)’ Clinical chemistry (40, 194, 228, 368, 376, ’

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Co’al-and coal ash (28,30,261,479) Cotton (588) Criminalistics (446) Gases (546) Geochemistry (118,132, 303, 432, 478, 493, 61L) Glasses (425) Lake sediments (110, 111) Meteorites (371) Ores (146, 150,157, 179, 182,306, 423, 4741 dr7J 524) Paints (344). Pharmaceuticals (253) Photography (64) Plastics (184, 229) Plating and protective coatings 188, -,*\ 041 )

Petroleum (642) Radioactive and nuclear materials (1621 217, 415)

Rare earths (13,31,177,366,424) Rocket propellants ( 7 ) Silicates and slag (15, 99, 132, 158, 272, 413, 435)

Soils (61, 110, 306) Water (334, 354, 365, 423, 442) Wood (269)

be correct within a factor of 2 with most values within h 20%. It would be worthwhile to critically compare the X-ray spectrograph and optical spectrograph for this important analytical service. Factors involved in correlating measured intensity to concentration continue t o receive the high level of attention that these factors merit. The two principal factors that limit analytical accuracy are grain size and interelement (matrix) effects. Variations in scattered and fluorescent intensity was the subject of several reports with particular emphasis on mineral analysis (65, 146, 252, 316, 476). Particle-size effects can be eliminated by a n appropriate dissolution step, such as the borax fusion; however, when chemical treatment is not feasible, the particle-size effect may be a serious source of error. Matrix effects (362) may be corrected by a variety of techniques-fundamental parameter, external standard, dilution, addition, thin film, absorption, and scatter. We will discuss each of these very briefly. A source of error in the long wavelength region is the chemical effect, that is, a change in wavelength due to bonding or valence. Using spectrometers with preselected peak positions this wavelength (28) shift can result in a significant analytical error (402). I n the fundamental parameter method proposed by Criss and Birks (115), a single set of equations is valid for any sample and number of components. For this fundamental method, knowledge of the spectral distribution of the primary X-ray beam (589, 499) is required. Spectral distribution for tungsten, molybdenum, copper, and chromium target X-ray tubes were determined over the 15 to 50 kilovolt range (186). Matrix effects are corrected using published mass absorption coefficients (87, 345) and fluorescent yields (308). Uncertainties in these values impose the principal limitation on the fundamental parameter approach. Iteration methods are computerized to solve for concentration; the first step is to use measured relative intensities to obtain the first set of compositions; then the data is recycled until measured and calculated intensities agree to a preselected value. As in past reviews, I again stress the need for long range support of a program to obtain basic parameters, such as mass sorption coefficients and fluorescent yields. Obviously the need for reliable values is increasing whereas research in this area is practically nil. B a t t applied the fundamental parameter method to the analysis of sulfide and oxide samples (18). H e points out t h a t mineralogical effects are not corrected by the fundamental parameter approach; therefore, extra steps mag be required. Cooper and Vaughn

provided a new approach in computer automated X-ray analysis for the mining industry (108). They achieved improved reliability in their analysis by performing two independent calculations of composition from measured intensities. Their first calculation procedure is a multiple regression based on assay standards to give a linear best-fit equation. The second procedure is the fundamental parameter method. Composition calculations by the two procedures are compared and accepted if they agree to within a preselected limit (usually 1a). Most X-ray analyses are based on the external standard method in which the sample is either compared to a standard similar in composition or the matrix correction is determined by an empirical approach (33, 34). The breakthrough in the empirical correction method is the widespread availability of computers, including large in-house installations for batch processing, dedicated computers coupled to the X-ray spectrograph, or connections wia telephone lines to time-sharing computer services. I n the Washington, D. C., area we have over a dozen companies offering timesharing services a t a cost of 5 t o 15 dollars per terminal hour. The X-ray analyst is now faced with a new problem-how to select the optimum computer system (351). We will need some expert guidance to assist in selecting the most economical and practical computer approach. Margoshes and Rasberry presented an excellent discussion on the fitting of spectrochemical data with digital computers. One procedure is described for user interaction during the running of the program (conversational mode) and another procedure is based on the batch-loading approach where the decisions are incorporated into the program (335). A general correction procedure, similar to that proposed by Lucas-Tooth, was developed by LaChance and Trail1 (302, 495). I n their method “the relative intensity of a constituent is directly proportional to its weight fraction and inversely proportional to 1 plus the sum of the weight fraction of the remaining constituents times their respective alpha constants.” These alpha constants, determined empirically, correct for absorption and enhancement. The Lachance-Trail1 procedure has been modified to take into account the polychromatic nature of the exciting radiation (101). Determination of effective exciting radiation was also studied (144). Other computer programs are used for analysis of silicate rocks (99, 208, 210, 390) and rocket propellants ( 7 ) . Precision and accuracy of comparative analyses of silicates were studied in regards to sample collection,


251 R

Fluxes for Fusion of Materials for X-Ray Emission Analysis Flux Crucible Typical samples Flux A Graphite Silicate glasses, silica- or alumina-bearing refractories, geological samples, cements 90 wt % Li~B407 IO wt 7% LitCOa Vitreous carbon Samples containing components that wet raphite such as oxides of Pb, Bi, Sb, As, Ni, u and others Graphite Samples containing com onents that are reFlux B duced in graphite crucigles using Flux A 90 wt % LizB407 8 wt 90Lizcos 2 wt Yo CeOz Vitreous carbon Oxides of Pb, Bi, Sb, As, Nil Cu,Zn, and others that are reduced in graphite crucibles Flux C Graphite Oxides or carbonates of the alkaline earths, samples containing no glass-forming compo90 wt % of Li~B407 nents 8 wt % of L.izCOa 2 wt of SI0 ALO~,or Table IV.

i5

GA

sample preparation, and calibration technique (15 ) . Dilution of the sample and standard provides a very general procedure for minimizing matrix effects. If the diluent also dissolves the sample to form either a glass or a solution, then grain size and heterogeneity are eliminated. Techniques range from a small dilution to obtain a matrix correction factor to the addition of high levels of diluent so that sample and standard will have a common matrix (117, 209, 211, 346, 471, 472, 483, 485, 486). More details of the borax dilution are covered in the section on sample preparation and standards. The addition technique is another of the established methods of matrix correction. The basis of this method is that a small addition of the element being determined does not significantly alter the sample matrix. Hence, measurement of the line intensity before and after addition provides a reliable method of analysis. General procedures of sample preparation and statistical analysis of the addition method were given in recent papers (258, 331, 470).

It has been well known for many years that matrix correction can be reduced or eliminated by the use of very thin films. For this type of sample, the intensity is proportional to the amount of the element present without dependence on the other elements in the thin layer. This thin-sample approach was used to analyze mixed oxides in which standards were prepared by vacuum evaporation of metal films (201). Another approach is the analysis of major constituents by dilution, then spreading as a thin film (230, 407). Ion exchange resin-loaded papers for collection and subsequent determination provide a general approach to thin film analysis. Absorption measurements can be employed either as a means to calculate mineral compositions directly (355) or 252 R

0

to obtain factors to correct for matrix variations (193). Probably the chief drawback to the latter approach is the difficulty in preparing transmission samples for the absorption measure ment. This sample preparation step is particularly difficult in the medium and long wavelength X-ray region. Research continues on the application of Compton and coherent scatter intensities as a means to correct for matrix and grain size effects. Critical studies on scattering methods of correction are recommended by this reviewer as scatter intensities should provide the best practical matrix correction for field analysis and for on-line process control analyses (506). Conventional methods of treating samples are not applicable for real-time analyses. Isolation of coherent from incoherent scatter can be achieved by diffraction (524), diffraction plus filtration (369), or by high resolution semiconductor detectors. Two approaches to matrix correction are t o estimate mass absorption coefficients from scatter intensities (416) or to ratio Compton to coherent (76). Analysis of heterogeneous samples is a very important problem that has not received the attention that i t merits. Although most of our existing laboratory methods are based on preparation of homogeneous samples, this preparation step is not always practical in the field or plant (35). Examples where heterogeneous samples are encountered are in situ analysis of drill holes and drill cores, grab samples in the field, and on-line process monitoring of flotation cells. For these types of applications, the analyst must extract quantitative information from less than optimum samples. Claisse presented a qualitative discussion of the problem encountered in analyzing heterogeneous mineral samples (100). Gunn pointed out the analytical problem arising from iron occurring as suspended particles in hydrocarbons (213). The intensity


of iron particles 8 p in diameter yields only 65y0 of the intensity from an equivalent amount of iron in solution; for particles 70 p in diameter, there is a several-fold drop in intensity. For FeKa, their on particle must be less than 1 p in diameter for the intensity to be equal to that from iron in solution.

Sample Preparation and Standards, The best approach to achieving the required sample homogeneity is t o convert the sample into the form of a solution (37),thin film (407),or a borax glass (303). The last appears to be the most practical for the routine processing of a variety of powder materials, Devices for automatically preparing fused samples are now commercially available from at least one X-ray instrumentation manufacturer. Fusion eliminates particle size effects and drastically reduces interelement effects. Sensitivity is only slightly reduced as a result of dilution by borax (255, 471, 48.4). There were several critical studies made of the borax fusion procedure that included factors such as temperature, fusion time, sample-to-flux ratio, and particle size effects (330, 432, 468). Table IV is a listing of optimum fluxes and crucibles for various types of samples (468). The electron microprobe mas used to investigate the homogeneity of borax glasses. No variation in iron K a intensity was noted for samples prepared from one part of igneous rock and four parts lithium tetraborate (382). The need for analyzed rock standards is being met by several sources. The first computation of major, minor, and 57 trace elements in the new U. S. Geological Survey rock standards was recently published (164). Data on the U. S. Geological Survey rock standards G-1 and W-1 and the Canadian ilssociation of Applied Spectroscopy sulfide ore and syenite rock standards mere also summarized (165, 456). I n South Africa, six igneous rock samples were prepared as analytical standards. Six hundred pounds each of granite, syenite, lujavrite, norite, pyroxenite, and dunite were ground so that 98% was less than 200 mesh. Tests indicated an acceptable level of homogeneity was achieved (428). Analyses of ternary alloys of Sn-PbS b presents two problems-smearing during milling which arises from the variable hardness, and segregation that occurs during storage. Cooling the sample with liquid nitrogen immediately before milling eliminated the smearing. However, it is necessary to use fresh standards as after three days the segregation was such that the analyses fell out of the 3u confidence level (325). =iging experiments mere performed using 10/90 and 63/37 tin-lead alloys stored a t 0, 20, and 100 "C (189). The net tin intensity of the 63/37 samples

increased with time and temperature; for the 10/90 samples the increase was related only to time. After 240 days a t room temperature, the change was equivalent to an increase of 0.50% Sn for the 10/90 and 1.05y0Sn for the 63/37 alloy. They found that remelting and recasting restored the aged samples, the additional phase noted in the aged samples was not present in fresh samples, and the aging effect was not a surface phenomena since all aged samples mere freshly surfaced b e fore each analysis. Further research on the long time stability of alloy standards is recommended by this reviewer. A critical study was made of the effect of surface roughness on X-ray intensity measurements. Fourteen elements, ranging from lead to silicon, were added singly to trans-1 ,Ppolyisoprene. Reproducible rough surfaces were made by molding a fiquare wire-mesh pattern into the surface by the use of standard sieve screens, Roughness was controlled by the mesh of the screen (184). Problems associated with the preparation of metal sample surfaces for electron excitation were investigated in terms of the shallow penetration of low energy electrons (277). One of the problems with electron excitation is the buildup of surface charges on nonconductive samples. A fine metal grid placed on the sample surface eliminated the need for the time-consuming methods normally used to make the surface conductive (473). Trace Analysis. Trace elements are determined either in the original sample (class 1) or a n isolated microgram amounts chemically or physically separated from the host material (class 2). Recent papers on trace analyses are listed in Table V. The various instrumental factors that affect the sensitivity of X-ray spectrographs were summarized by Jenkins and DeVries (275). They conclude that the greatest improvement is likely to occur from an increase in total source power and improved source stability. In the short wavelength X-ray region, the principal limitation is the lack of crystals with small d-spacings. For the longer wavelength region, absorption of X-rays in the X-ray tube and the detector window still imposes limitations on sensitivity. As stated in the previous reviews, our approach to trace analysis is to employ chemical preconcentration prior to X-ray determination with particular emphasis on ion-exchange resin-loaded papers as the collecting media. Our most recent efforts have been on chelating resin-loaded papers that have a very high selectivity for gold and certain of the platinum metals. Similar work is now under way using chelating resin-loaded papers having a high selectivity for base metals such as mercury,

lead, and copper. Other preconcentration techniques are also suitable for geochemical (118) and metallurgical (120) samples. Luke (320) proposed a widely applicable method for trace analysis of inorganic and organic materials. The trace element (elements) are isolated from interfering elements, then coprecipitated with a suitable precipitant. The precipitate is collected on a filter paper disk for X-ray determination. Luke states that precipitation methods are available for determining 69 elements. I n my experience, the lower limit of detection for class 2 samples is set principally by chemistry rather than by X-ray sensitivity. As one pushes down to the microgram or picogram level, the purity of the reagents used for preconcentration becomes critical. I strongly recommend that reagent blanks be incorporated into any preconcentration X-ray method. Films and Coatings. Some of the earliest applications of X-ray emission techniques were the determination of coating thickness and composition. Theory and practice of these applications are covered in a recent extensive review (364). Recent applications include chlorine in Si02 films (333), nickel coating thickness (332), gold and rhodium plating on small electrical devices (188), film thickness and composition of CdS and other 11, VI compounds (89, 91, 92), and various coatings on steel (452). One of the more interesting applications is the determination of entrapped argon resulting from the use of argon in R F sputtering of insulators. Concentrations of argon from 0.05 to 7.4 weight percent were determined in deposits 0.5 to 5 p thick. Two techniques were used for calibration: weight loss of deposits heated a t 600°C in a helium atmosphere for several hours and the use of a KC1 film for estimating the argon intensity-mass relationship (311). X-ray emission is also finding increased application in the study of surface oxidation and diffusion. Excitation with 10-100 keV protons were used to measure the density of oxide films on aluminum. Films of several monolayers ( ~ 1 A) 0 were detectable by this approach because of the low penetration and high X-ray yield characteristic of proton excitation (438). Combination of a high energy electron diffraction unit with an X-ray spectrometer provided the means for simultaneously obtaining H E E D data and elemental composition of surfaces and their reaction products (447). Diffusion of uranium through fuelelement cladding alloys was also investigated. When the cladded element is heated in an oxidizing atmosphere, uranium that migrates to the surface can be determined by X-ray emission. Standards were prepared by vapor de-

Table V.

Trace Analysis-Samples a n d Techniques

Samples Biological and clinical (40, 110-112, 194, 228.368.376)

Inorganic 'solutions (37, 181, 322, 323, 442)

Metals (6, 25, 68, 66, 120, 217, 218, 321, 374, 480,618)

Minerals (31, 67, 93, 297, 466, 472, 614) Natural products (88, 269, 388) Nonmetals (320,406,441) Organics (213, 310, 320) Thin films (24249) Water and brines (136,334,364,359) Techniques Evaporated residues (40, 109-111, 216, 334 )

Extractions (334, 369,441) Ion exchange papers and membi:anes (217,564,480)

Micropore filters (68, 181) Mylar film (299) Precipitation (6, 66, 181, 320-324, 400, 44% 466) Thin layer chromatography (310)

position of uranium onto plastic tape then applying this tape to the clad alloy (439). By use of very thin oxide films, oxidation of stainless steels was investigated (169). Relative amounts of five elements could be determined by rapid examination of a large number of samples subjected to various treatments, Process and Quality Control. I n the 1964 review, substantial growth was predicted in the application of on-line process control analyzers. At that time the two major obstacles were the lack of highly reproducible sample presentation systems and problems associated with correlation of intensity to concentration for samples of varying composition and particle size. These obstacles have been overcome by new sample preparation-presentation systems that include continuous grinding and briquetting devices, equipment for automatically fusing and casting borax buttons, and improved systems for analyzing slurries that use transmission and scattered intensities to correct for interelement effects and particle size. At present, on-line analyzers are used 24 hours a day in ore processing plants throughout the world, and further growth is anticipated. One of the trade journals predicts that more than 50 process analyzers will be installed in Canada during the next decade. Cost of these computer-coupled analytical systems range up to one-quarter million dollars; thus, there is keen interest by the instrument companies. The following references provide a general review of the field and plus detailed descriptions of specific applications (78, 108, 150, 260, 300, 309, 459, 474, 4756).

Probably the largest on-stream analyzer in North America is the 30-stream


253

R

Table VI. Energy DispersionInstrumentation a n d Applications

Instrumentation Absorption (137, 396, 491) Detectors (67, 68, 113, 129, 867, 878, 505,343,381,488)

Drill hole probes (103, 419) Filters (39, 142, 179,519) General (63, 77, 140,417) On-line (150, 261, 300) Portable systems (131, 286,418) Principles (103, 268,313,3l4, 378, 417) Semiconductor systems (55,56, 174,185, 279,488,540)

Sources (67, Y7, 178, 186, 898, 318, 337, 595-397,420, 444,445,489,516)

Applications Alloys (47, 418) Archaeology (56, 174) Biology (115, 488) Cement (48, $81, 317, 418) Coal and coal ash (261, 418, 479) Coating thickness and composition (91) Criminalistics and national security fLLh-1

DAil’cores (141) Education (421) General (35, 68, 113, 140, 305, 3Y8, 418) Grain size (35,252,315,316) High atomic number elements (67, 68,

151) I n situ (419) I n vivo (488) Low atomic number elements (420) Mineral processing (300) Ores (67,68,14l, 179,182, 254, 286,305, 399, 418) Pharmaceuticals (263) Silicates (138, 139, 143, 588) Slags (519) Slurries (150, 300) Solutions (286)

unit a t the Kidd Creek concentration of Texas Gulf Sulphur, Timmins, Ontario. This analyzer is interfaced to a process control computer. This computer system provides the control room operator with the amounts of each of five elements a t five points in each of three process streams. Quantitative analyses of the five elements a t a given point are achieved in less than one minute. With these real-time analyses, the mill operator can make the necessary adjustments to maximize the concentration process. Examples of other large installations are the Kristineberg mining complex in Bolidens Gruv, Sweden, Falconbridge nickel mines in Quebec, and the Outokumpu Oy Laboratory in Finland. Most of the present on-line analyzers are based on extension of laboratory instruments that use curved- or flatcrystal spectometers and a n X-ray tube as the source of excitation. Radioisotopes provide a stable, low cost, compact source of X-rays or gamma rays (78,103,260). When these sources are coupled with a high resolution semiconductor detector and a moderate size dedicated computer, rapid multielemental analyses can be achieved in real time. The intensity of the coherent and incoherent scatter measured with 254R

*

the semiconductor detector can be used for matrix and particle-size corrections. On simple analytical systems, scintillation or proportional counters provide adequate resolution. I predict that the major growth in on-line analyzers will be directed toward energy dispersion techniques using radioisotopic sources of excitation rather than the present modification of conventional X-ray spectrographs. ENERGY DISPERSION X-RAY ANALYSIS USING RADIOACTIVE ISOTOPIC SOURCES

During the past two years, the field of energy dispersion X-ray analysis using radioisotopic sources (EDXA) has matured. Instrumentation and applications are listed in Table VI. Presently there are various types of instruments commercially available that use proportional or scintillation counters with either single-channel pulse-amplitude discrimination or balanced filters. These instruments cost 3 to 5 thousand dollars and can be used either in the laboratory or as easily portable analyzers in the field. Semiconductor detector systems coupled with multichannel analyzers and automatic data processing equipment are also available commercially. A list of instrument suppliers is available upon request to me (WJC). Investment and operating costs for EDXA were compared to conventional X-ray spectrography for the determination of calcium in magnesite (681). One of the principal deterrents to more usage in the United States has been the difficulty, real or imaginary, in obtaining a license to use radioactive sources. The procedure for obtaining a license is explained in an AEC booklet, “HOWto Get a License to Use Radioisotopes,” available from the U.S. Government Printing Office for 30 cents. Preuss and coworkers compiled large number of spectra from three betaemitting sources: 9oSr-90Y 6aNi, and *04Tl(395). Cline and H e i t h collected spectra from a variety of neutron-deficient isotopes using lithium-drifted silicon and germanium detectors (106). They plan to publish a supplement to their NaI catalog and prepare a new catalog based on spectra measured with germanium detectors. Several excellent papers were published on selection of optimum excitation sources that take into consideration the resolution of the detection system (68, 77, 318). A high intensity 66Fe source was produced by electrodepositing a very smooth coating of the isotope, thus significantly reducing self absorption of the manganese K radiation (397). Aluminum K intensity was increased by approximately a factor of 50 by applying 6 kilovolts between the beta-


emitting source and the sample; also the air path was evacuated between the source-sample-detector (337). A novel approach to isotopic excitation is to blend the sample with a small amount of log. This mixture is irradiated with a neutron beam to form ’Li from the log. The lithium isotope has a half second and emits a monolife of energetic 477 keV gamma that will excite high atomic number elements in the sample (298). The use of nuclear techniques in prospecting and developing mineral resources was the subject of a major symposium held in Buenos Aires (868). I n the proceedings, Clayton reviewed present applications of EDXA in mining and mineral processing (103). Polish researchers described field methods for determining iron in drill cores using fluorescent X-rays and gamma scattering (241). Rhodes et al. (419) described a prototype of a drill-hole probe for logging uncased-dry holes for heavy metals. This probe consists of twin scintillation counters with balanced filters, one filter over each counter window. Using concrete models, the sensitivity for silver and uranium was 0.015 and 0.15 weight percent, respectively, using 10-second counting times. I n December 1969, we evaluated a probe of this design for delineation of lead in holes drilled in a lead mine at Bixby, Mo. Preliminary results of this logging study are encouraging. There is high interest in direct logging of minerals down a drill hole because of the cost and difficulty of obtaining drill core, particularly in deep holes. I n another paper a t Buenos Aires, Burkhalter discussed the use of semiconductor detectors and automatic data processing for the determination of gold and silver in ores. Under optimum conditions the limits for gold and silver are approximately 20 and 5 ppm, respectively (67). A semiconductor system was tested in Israel for field archaeology. The operating environment ranged from hot dry deserts to the seacost with the salty humidity. No difficulties were encountered with the instrumentation. Under field conditions, accuracies of 10 to 20% were obtained on metals, glasses, glazed ceramics, and mineral specimens (174). A program was initiated to determine the feasibility of using semiconductor systems for in vivo biological studies. This approach may provide an improvement in area resolution, decrease in the radiation absorbed by the body, and decrease in the time required to complete a scan as compared to the current use of radiotracers (488). A radioisotopic source-semiconductor system was optimized for determination of trace elements in low atomic number matrices. With very long counting times, detection limits could be ex-

tended to the nanogram range (5.40). Semiconductor detectors coupled with multichannel analyzer and automatic data processing provide the means for rapid multielement analysis. I predict substantial growth in this technique during the next several years. There is presently a wide variety of computer programs available for stripping of gamma spectra that can be applied to X-ray spectra with only minor modification (496). Portable hand-held analyzers are enjoying a wide range of application from the sorting of metals to analysis of slags. These devices are now being evaluated for various problems such as detection of lead in bullet holes and the identification of labeled coins placed in parking meters (446). The most compact of the portable instruments uses the avalanche detector and balanced filters (278). LOW ENERGY X-RAY

Low Energy X-Rays. There has been moderate progress in low energy X-ray spectroscopy and absorption. The higher level of sophistication in experimental techniques and interpretation of data limits widespread application as compared to elemental analysis using X-ray emission. The Institute of Materials Research of the National Bureau of Standards sponsored a symposium of “Electron Density of States” in November 1969. The symposium proceedings that include many X-ray papers, is expected to be available by the summer of 1970. The proceedings of the 1967 Conference held in Strathclyde, Scotland, “Soft X-Ray Band Spectra and the Electronic Structure of Metals and Materials” (156) covers experimental techniques and various methods for interpretation of spectra. Glen and Dodd (191) authored three papers that provide a good introduction t o interpretation of emission and absorption spectra (125, 126, 191). They point out that many of the structural problems in mineralogy involve questions of the assignment of cations to alternate crystallographic sites, the coordination number of specific cations with respect to oxygen, and the nature of the specific type of chemical bond. X-ray spectroscopy appears very promising in answering these types of questions. Glen and Dodd interpreted KP band spectra of magnesium, aluminum, and silicon on the basis of known crystal structure, using molecular orbital theory. Similar approaches were used to obtain chemical information from absorption spectra. Shifts in absorption peaks are a result of changes in orbital energy levels that are directly related to chemical bonding. One of the major problems in X-ray

spectroscopy is that true spectral details are greatly altered by various ininstrumental distortions. The complicated multiplet structure observed in some emission spectra is a result of selfabsorption rather than true spectra,. This problem of self-absorption has been studied by varying takeoff angle, using different electron energies for excitation, and comparing emission and absorption spectra (70, 160, 161). As these problems are better understood, it is possible to correct for or to minimize the effects due to self-absorption. Table VI1 lists references to representative types of problems investigated by X-ray spectroscopy. An example of a straight-forward analytical application is the determination of sulfur coordination in scapolite (78). Another problem that has received extensive study is bonding energies of carbides. Group IV and V transition metal carbides have narrow carbon bands characteristic of strong bonding. The weakly bonded carbides of Group VI and higher have broad bands and an intensity distribution close to the carbon band from graphite (261). Carbon spectra from a large number of organic compounds were measured in a gaseous state to eliminate nearest neighbor effects and sample contamination (147, 342) ’

Baun described a very practical application of low energy X-ray spectroscopy (20). The question was to determine if thin films of aluminumcopper alloys prepared by vapor deposition have the same stoichiometry and state of chemical combination as the original bulk material. He had previously noted that the aluminum K band (AlKp) exhibited strong splitting with aluminum-copper alloys and that the energy difference varied linearly with composition. Band spectra from 49A1-51C~were compared from bulk samples and films prepared by vaporization of the alloy a t a temperature just above the melting point. Results indicated that 49A1-51C~ is a constant evaporation alloy. With copperrich alloys, the copper-rich phase evaporated faster than the aluminum-rich phase. Low Energy Electron Spectroscopy. During the past two years there has been two major developments of interest to the analyst-availability of commercial instrumentation and publication of a series of reports directed toward analytical applications. M y objective is to direct your attention to appropriate sources of information regarding principles, applications, and instrumentation. First a few words regarding nomenclature used in electron spectroscopy (36). Electron impact spectroscopy is based on the transfer of kinetic energy from a free electron to the bound electrons of the target

Table VII. Applications of Low Energy X-Ray Spectroscopy

Problem Bonding in iron germanides Alloy composition and chemical state Sulfur coordination in scapolite Density of states in alloys L emission spectra of germanium Chemical bonding in silicates and oxides Carbon K spectra from gaseous compounds Vanadium L emission and absorption spectra Charge transfer in titanium compounds Measurement of X-ray absorption fine structure Bonding energies in carbides Chemical bonding of sulfur Bonding in fluorides and oxide8 Correlation of Si-0 bond lengths A1 coordination and A1-0 bond lengths

(499) (629)

(530)

molecule (scatterer). Penning electron spectroscopy is a new field in which excitation is achieved by a quantum of energy bound to a bombarding excited atom, A*, that is usually a metastable species. The energy transfer is represented as A* -1f + -4 -If+ e

+

+

+

There are two general terms used for excitation by X-ray or ultraviolet radiation. The technique is classified as photoelectron spectroscopy (PES) if the ionizing process is achieved by ultraviolet radiation that results in ejection of electrons from valence orbitals of molecules. When monoenergetlc Xray sources are used to eject inner shell and valence electrons, the process is labeled electron spectroscopy for chemical analysis (ESCA). Auger electrons are a product of ionization by either photons or electrons; thus Auger electrons are observed in the ESCA spectra. Approximately two decades of high quality research on electron spectroscopy at Uppsala, Sweden, is summarized in the book, “Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy” (464). Seigbahn and his associates describe numerous analytical applications and present an excellent introduction to theory and experimental techniques. This outstanding book is also available a t very small cost as a WrightPatterson Air Force publication (466). The journal ~ L ~ L Y T I C CHEMISTRY ~ ~ L published a feature article on Auger spectroscopy by Harris (220) and in the January 1970 issue featured papers on ESCA by Hercules and PES by Betteridge and Baker. Dr. Muller’s sec-


255 R

Table VIII.

Commercial Low Energy Spectrometers

(1) Associated Electrical Industries, Ltd.

Instrumentation Division Trafford Park, Manchester 17, England (2) McPherson Instrument Corporation 5301 Main Street Acton, Mass. 01720 (3) Perkin-Elmer Ultek Division P. 0. Box 10920 Palo Alto, Calif 94303

(4) Physical Electronics Industries, Inc. 7267 South Washington Avenue Edina, Minn.. 554351 ( 5 ) Varian Associates

Analytical Instruments Division 611 Hansen Way Palo Alto, Calif. 94303 (6) Veeco Instruments Inc. Terminal Drive Plainview, N. Y. 11803

tion on Instrumentation was directed to electron spectroscopy in the October and December 1968 issues of ANALYTICAL CHEMISTRY.The following references are also recommended for general reading (22221, 240,544). -4 selected bibliography on huger spectroscopy is available from Varian Associates. Energy distributions of back-scattered electrons were investigated along with L E E D and optical, and other data to ensure well-characterized surfaces (440). A bibliography on low energy electron diffraction and related techniques was compiled (270), and it will be updated annually. Commercially available Auger spectrometers are L E E D systems modified for electron spectroscopy. Because of the close relation to L E E D to low energy electron spectroscopy, I urge cooperation with them by forwarding copies of your publications on Auger and photoelectron spectroscopy to Dr. T. W. Hass, Bldg. 450, Wright-Patterson Air Force Base, Ohio 45423. Auger spectroscopy provides a means for detecting minute amounts of elements present on the surface. The depth of the surface region that contributes to Auger peaks was investigated by deposition of a metal onto a clean surface of a second metal. For Auger electrons in silver, the mean escape depth varies between 4 and 8 A for energies of 76 and 362 eV respectively (380). One tenth of a monolayer of cesium or potassium was detected on single crystals of germanium or silicon (525). Use of Auger electrons for contamination monitoring was considered by Taylor. With vacuum systems operating in the 10-10 to 1011 torr range, it requires from several hours to over one day to form a monolayer of oxygen (482). Analysis time can be greatly reduced by use of a high transmission coaxial-cylindrical analyzer. Scanning rates of 20,000 V per second are possible as compared to the usual 2 to 10 V per 256R

Electrostatic ESCA spectrometer

Electron impact spectrometer 3-grid Auger spectrometer 4-grid Auger spectrometer Electrostatic ESCA spectrometer 4grid Auger spectrometer 3-grid Auger spectrometer

second rate with sector analyzers (379). Applications of the Auger spectrometer for surface segregation and diffusion were described by Harris (222). When nickel or steel samples were heated in vacuum, sulfur diffused to the surface. B y varying the angle for collection of Auger electrons, it is possible to distinguish surface atoms from those more deeply distributed in the sample (223). The relationship between resolution and sensitivity was critically examined for the retarding grid spectrometer. The larger the modulating voltage, the greater the output signal but a t a decrease in resolution (481). Optimum energy for production of auger electrons was found t o be 3 to 3.5 times the critical ionization potential. Using retarding grid optics, a monolayer of oxygen on copper resulted in a detector current 2 X lodi1 ampere (44). A list of commercially available electron spectrometers is given in Table VIII. I am aware of two other companies that will consider construction of custom-made Auger spectrometers based on the 127' analyzer used by Harris (221). The electron impact spectrometer is based on instrumentation developed at the National Bureau of Standards. Descriptive literature on electron impact instrumentation and applications to the analyses of gases and vaporized materials are available from the McPherson Instrument Corporation. The Auger spectrometers are 3 or &grid LEED systems with a lock-in-amplifier and voltage sweep circuits for measurement of back-scattered electron energies. The second derivative of the retarding field plot is obtained electronically to give a practical signal-to-noise. I n our instrument at College Park, there is provision for bombarding the sample at 90" with a modified L E E D gun or for working a t selected angles using a special gun designed for Auger


spectroscopy. Also there is a quadrupole mass analyzer available in the vacuum chamber to measure composition of gases desorbed from the sample during heating or by electron bombardment. Circuitry is also included to measure the work function of samples subjected to various treatments. Characterization of surfaces will require a wide variety of new analytical techniques. Both of the commercial ESCA systems employ electrostatic analyzers. Magnetic analyzers such as those used in Sweden may not be practical in many locations because of difficulties in magnetic shielding. The ESCA analyzer developed at Varian's has a digital computer as an integral part of the system. They use a time-averaging technique to improve the signal-to-noise ratio and the computer memory provides for accumulation of signals from repeated spectrometer scans plus routines for data acquisition, start voltage, and data reporting. ELECTRON PROBE MICROANALYSIS

I n the 1968 review (74), it was said that the previous two years had shown significant progress in defining and characterizing the parameters (e.g., mass absorption coefficients) that are important in relating measured X-ray intensities quantitatively to composition. Unfortunately, it is difficult to justify such a statement for the two years since then. Progress has not ceased, but i t seems that the technique has reached a point in time when advances must come more slowly. For instance, the statement in the last review: "For X-ray wavelengths of approximately 3 A and shorter, the methods of quantitative analysis are reasonably wTell defined. I n this region analyses to within 2 percent relative are possible using pure elements as standards, provided . . . parameters are accurately known," is certainly still true today. However, no extension can be made to that statement as a result of the effort since 1968. There have been some improvements in the value of the parameters which are available, but even these have some uncertainties associated with them. There is considerable interest in improving the situation both by attempts to make better measurements and to develop better theories. One further step seems within reach a t the present time. The Transport Equation Program (60) makes it possible to convert relative X-ray intensity to composition without using many of the approximations which are necessary in the variety of techniques used until now. This is only possible because of the almost universal availability of high speed computers since the calculations are much too complex to be done by hand. The situation for wavelengths longer

than 3 A is also not significantly different than two years ago. Many more measurements are being made in this region, because instrumentation is available; but for true quantitative analysis, it is still necessary to use accurately characterized comparison standards in order to sidestep the problems associated with wavelength shifts, self-absorption, uncertain parameters, and band shapes. The application of various bonding theories (such as that of Molecular Orbitals) to the soft X-ray problem holds considerable promise in interpreting soft X-ray spectra for the future, but at this time these considerations are just beginning, and it is difficult to project what the outcome will be or when. Auger and photoelectron spectroscopy is being investigated as an analytical tool at an ever increasing rate, as is discussed in the earlier section of this review. Admittedly, the application of electron spectroscopy to the electron microprobe is perhaps not a simple adaptation, but it will be interesting to see what will develop in the immediate future. Publications and Meetings. The one new book (235), published in the last two years which is important to electron probe microanalysis is the proceedings of the seminar on quantitative analysis held a t the National Bureau of Standards in June 1967. I n the twelve papers published in this book, edited by Heinrich, the state of the art of converting X-ray intensities to composition is very well defined. nlot a great deal has changed since that time. The Proceedings of the Fifth International Conference on X-ray Optics and Microanalysis, held in Tubingen, Germany, in September 1968 have just become available as this is written and contain many good papers. The annual Denver X-ray Conference Books (1 7,370)have several papers on electron microprobe analysis of significant interest in the two volumes published since the previous review. To the best of this reviewer’s knowledge, no general book on the technique has been published recently, but the just published second edition of Birks’ book on X-ray spectrochemical analysis (42) does contain an updated chapter on microanalysis and his six-year old probe book is in the process of revision. Dewey et al. (124) have published a book in the Progress in Nuclear Energy series which contains mostly tables of data for microprobe analysis. Unfortunately, these tables are essentially duplications of data readily available elsewhere and do nothing to decrease the uncertainty in the values for the parameters reported (e.g., mass absorption coefficients). The authors admit that their values for wavelengths of X-ray lines and absorption edges and for critical excitation potentials, except

5

Table IX. Availability of EPASA Transactions Source of Transactions Meeting A. J. Tousimis 1st (1966)” Biodvnamics Res. Coro. College Park, Md. 6010“Executive Blvd.-= Rockville, Md. 20852 R. E. Ogilvie 2nd (1967) Metallurgy Dept. Boston, Mass. Mass. Inst. of Tech. Rm. 13-4009 Cambridge, Mass. 02139 D. R. Beaman 3rd (1968) Dow hIetal Products Co. Chicago, Ill. Metallurgical Lab. 241 Building Midland, hfich. 48640 A. A. Chodos 4th (1969) Calif. Inst. of Tech. Pasadena, Calif. Div. of Geol. Sciences 1201 E. California St. Pasadena, Calif. 91109 Only a very few copies still available.

for typographical errors, do not differ significantly from those previously reported by Bearden and Burr. Some of the other tables seem to apply only to the authors’ own technique for correction procedures, the value of which is not obvious. The book also includes a chapter on a “practical” approach to electron probe microanalysis. Again, the system used in the authors’ own, and no data are given to substantiate it. Generally the book seems to introduce further confusion into the technique where it is hoped that the last few years have otherwise produced some order and agreement among the majority of workers. Therefore, it cannot be recommended. There have been, however, three good review papers on the subject. Heinrich (234) discusses the last ten years in terms of instrumentation, quantitative analysis, and what will develop in the future. Duncumb (133) has done a magnificent job of examining the instrumental developments since his previous review in 1965 (publ. 1967). Poole and Martin (392) have covered a large time span and examine instrumental and experimental aspects going all the way back to the early 1950’s. There have also been two meeting reports of some interest. Il’in ($64) has reported on the Conference on Local Spectral Analysis, held RiIarch 23-25, 1967, in Moscow a t the A. A . Baikow Institute of Metallurgy of the Academy of Sciences of U.S.S.R., and Heinrich (233) has reported on the Third Kational Conference on Electron Microprobe Analysis and First National Meeting of the Electron Probe Analysis Society of America, held in Chicago, July 31 to Aug. 2, 1968. The Electron Probe Analysis Society of America, which was established in 1967, is now a functioning organization of over four hundred members. L. S.

Birks of the Xaval Research Laboratory was its first president (1968) followed by K. F. J. Heinrich of the National Bureau of Standards (1969). R. E . Ogilvie of Massachusetts Institute of Technology is the president for 1970, and T . 0. Ziebold, also of 1I.I.T. will be president in 1971. Since the Third National Meeting was held in Chicago in 1968, the Fourth was in Pasadena in 1969, with the Fifth scheduled for Kew York City in 1970, and the Sixth to be in Pittsburgh in 1971. The Transactions of the meetings held so far, containing extended abstracts of all the contributed papers, are available for a modest fee from the General Chairman for each meeting as listed in Table IX. This service wil1,be centralized in the near future. Many of the Probe Users Groups in the United States have affiliated with the national society as local sections and encourage attendance at their local meetings of all people involved in the use of the technique. The Probe Users Groups known to this reviewer are listed in Table X along with the name of the person to contact for information. Probe groups also esist in foreign countries. The International Conference on X-ray Optics and Microanalysis which was held in Tubingen, Germany in 1968, is scheduled for Japan in 1971. It is planned to hold the 1974 international meeting in the eastern part of the United States, in conjunction with the Ninth Annual Meeting of the Electron Probe Analysis Society of America. QUANTITATIVE ANALYSIS

This section deals with quantitative analysis as a whole, while the following sections will take up the individual corrections in order. Several general discussions of quantitative analysis have been published in the past two


257R

Table X.

Probe Users Groups

Canada

Metropolitan (N.Y.)

Midwest

Northern California

Pittsburgh

Southern California

Ramon Coy-Ill Ecole Polytechnique 2500 Marie Guyard Ave. Montreal, Quebec, Canada W. T. Kane Corning Glass Works Sullivan Research Park Painted Post, N. Y. 14870 D. R. Beaman Dow Metal Products Company Metallurgical Laboratory 241 Building Midland, hlich. 48640 R. J. Ruscica Materials Analysis Corn any 1060 East Meadow Circg Palo Alto, Calif. 94303 J. S. Makosey Electron & X-Ray Optics Dept. Research Laboratory Latrobe Steel Co. Latrobe, Pa. 15650 A. A. Chodos California Institute of Tech. Div. of Geological Sciences 1201 E. California St. Pasadena, Calif. 91109

Southwest

Upstate New York

Washington

years. The papers in the proceedings of the seminar a t the National Bureau of Standards cover all phases of the corrections as they existed in June of 1967 (936). Many of these papers were reviewed two years ago, (114) but now that they are in print, they areworth considering again. As has been said, there has not been a great deal of advancement since then. At least half of the discussions deal largely with the Atomic Number effect, this presumably being the least well understood of all the corrections. However, absorption, fluorescence by characteristic lines, and fluorescence by the continuum are also treated. There is, moreover, one paper which treats the problem differently. D. B. Brown (59) in his application of the Transport Equation to the conversion of X-ray intensities to composition, treats the problem as a whole, rather than as several independent phases. T h a t is, the calculation proceeds directly from an integration over the distribution in depth of primary radiation. The various corrections (for absorption, fluorescence, etc.), as we have come t o know them, do not enter explicitly. This reviewer found the discussion by Borovskii and Rydnik (54, 430) difficult to follow and therefore impossible to 258 R

J. L. Solomon IBM /FSD Dept: 581 /OOl-1 Owego, N. Y . 13827 J. V. Gilfrich Code 7681 Naval Research Laboratory Washington, D. C. 20390

evaluate. I n this paper, they describe the derivation of a general expression for the intensity of characteristic X-radiation produced by the impact of electrons on a thick target, based largely on the energy distribution of electrons in the anode as expressed in the form proposed by Makhov in 1960. However, the expression derived, cannot be presented in an analytic form for the general case. Therefore, these authors propose an approximation which allows a numerical calculation, and they test it by comparison with the experimental data of Poole and Thomas (1961) and of Ziebold and Ogilvie (1963). This comparison shows agreement t o within a few percent relative. I n addition to D. B. Brown’s treatment of the Transport Equation, there are two other papers in this volume which deals with the interaction of electrons with matter. Mulvey (362) discusses the choice of models for electron scattering and deceleration, while Shinoda, Murata, and Shimizu (449) performed Monte Carlo calculations to clarify the behavior of electrons, to obtain depth distribution functions, and to determine the shape of the diffused X-ray source. As Birks says in the introduction to this volume, “One of


the original objectives (of this seminar) was not reached or even approached. T h a t is, there was no agreement on any kind of priority list of problems to be attacked. Nevertheless, the exchange of views was valuable and informative and an excellent way to catch up on all phases on quantitative electron probe analysis.” Computer programs continue to be written (107, 202, 248, %?8),to ease the task of data reduction. As in the past, many of the programs are combinations of a few, or many, of the standard absorption, fluorescence, and atomic number corrections available from the literature. One program that falls into this general category is worthy of special note, if only for its popularity. Colby (107‘) has written this one, called by the acronym MAGIC, specifically for the analysis of thin films. However, it seems to be more or less universally applicable and is being used by a number of workers. A minimum of input is required, only the chemical symbols, X-ray lines employed, and the accelerating voltage being necessary in addition to the X-ray intensities. -411 constants are stored or calculated internally which requires a computer with a large memory. According to Colby, the special application of this program requires to films thinner than 2500 a new model which allows the X-ray spectra from the film and substrate to be unfolded to give the composition of the film only. The model proposed makes use of a mean electron energy concept. Some workers in the field question this model. This reviewer has also been told that there are errors in the way Colby handles the atomic number correction. As a tribute to the popularity of this program, Wise (634) has written a modification to it for use in computers with limited memory. Anderson (9) has examined in detail the difficulty of making quantitative any analysis in the ultrasoft region of the X-ray spectrum. H e defines the ultrasoft region as that where the critical excitation potential is 1.0 kV or less. -4s might be expected, the difficulties are many: poor parameters, line position and shape changes, and generally lack of complete understanding of the correction theory to be used in this wavelength range. His general recommendations to sidestep a t least some of the problems include operating at as low an accelerating potential as is practical and using standards that are similar in chemical composition to the sample. Dodd and Glen (126) have shown that it is possible to make calculations which tell something about how chemical combination affects the X-ray spectra of light elements. These are cases where the valence electrons are involved in the transitions resulting in X-ray emission. They have applied

elementary molecular orbital theory to studies of the Kp spectra of the metal itself and two simple crystalline oxides for magnesium, aluminum, and silicon. In each case, one of the oxides had oxygen in sixfold octahedral coordination with respect to the metal, and the other had oxygen in fourfold, tetrahedral coordination. Numerous correlations became apparent to the authors for the oxide cases. They say that band theory is appropriate for the metals. For the oxides, molecular orbital theory, together with the use of models of known crystal structures including orbital models, can be use to make tentative assignments of observed peaks to specific transitions. This opens the door for improved theoretical analysis of chemical bonding and electronic energy-level studies in solids, but not necessarily improved chemical analysis. Heinrich (232) has examined the sources of error in microprobe analysis and has concluded that care in the preparation and measurement of samples and standards is the first prerequisite. I n this paper, he presents a histogram of results on 98 of the 150 binary specimens previously reported by Poole and Thomas (1964). A more favorable error distribution in his histogram than that of Poole and Thomas is ascribed to some poorly characterized or measured specimens, or those with fx less than 0.6 (which he omitted), or to the previous omission of a necessary characteristic fluorescence correction (which he made). Salter (433) has statistically studied the analyses of over 900 specimens from both metallic and oxide systems. Nost of these were multicomponent systems and necessitated the use of a “matrix method of manual calculation.” This amounts to considering the niulticomponent system as a combination of pseudobinaries and iterating to a sum of approximately 1007,. The correction procedures used were the Duncumb-Shields modified Philibert absorption correction, with Heinrich’s mass absorption coefficients, and the Castaing fluorescence correction modified after Green and Cosslett such that the frequency ratio was replaced by an overvoltage ratio term. Friskney and Haworth ( I 75) have compared several correction methods for the analysis of metal oxides. They found that for their particular problems, the Duncumb and DaCasa correction for atomic number and absorption was the best. Where fluorescence corrections were necessary, they used the method of Reed and Long. Bence and Xlbee (26) have applied the Ziebold-Ogilvie technique to silicate and oxide minerals in the determination of ten major elements. Their results show almost remarkable agreement with chemical values and assumed stoichiometry. Judd and .insell (280) present argu-

ments concerning microsegregation in alloy thin-films as occurs in extraction replicas. Where the segregation is the same as the beam size (-2 pm) or smaller, the X-ray intensity changes. This is perhaps not surprising, but they have attempted to consider the effect quantitatively by simulating the problem. Silver prowder particles (-1 pm diameter) were supported on an evaporated carbon film. The silver particle serves as an analog to a highly concentrated segregation area, while the carbon film represents the surrounding matrix. The simplicity of this system enabled the X-ray intensity to be studied as a function of the position of the particle within the beam. A computer program was developed which predicts the relative intensity as a function of position of the particle within the beam. These calculated intensities compared favorably with experimental measurements. The authors concluded that by combining “quantitative melallography” with probe analysis results on areas of high segregation can be reasonably accurate. Grachev et al. (199) have presented a problem which until now has been largely ignored in any consideration of electron-solid interactions used to define a model for correction procedures. They have demonstrated that there is an angular preference for scattering of electrons (of tens of keV) in single crystals caused by channeling along the closest-packed crystal directions. This has been briefly discussed previously (Duncumb 1962, Bramman and Yates 1967). The magnitude of this effect has not been evaluated but it must be remembered that on the scale of the beam size and excited volume in a microprobe, most samples can be considered as single crystals. I n addition to his preliminary presentation a t the NBS Seminar (59), Brown et al. have published more detail on the prediction of X-ray production (60) using the Transport Equation. The predicted intensities are compared with experimental microprobe data for two alloy systems and three pairs of semiconductor compounds. The results compare quite favorably and are further improved when allowance is made for fluorescence by the continuum. Also Viegele (505) discusses the effect of the momentum of an electron on X-ray transport. The Office of Standard Reference Materials of the National Bureau of Standards now has available the first three standards intended primarily for electron probe microanalysis. The first of these is a 80% tungsten-20% molybdenum alloy wire embedded in pure molybdenum rod, onto which has been electroplated a layer of pure tungsten. Thus, the specimen and its end-member standards are included in one piece of

material. The preparation and microprobe characterization of this material (Standard Reference RIaterial 480) is very well documented in the literature (538, 559). The other two standards (SRM 481, gold-silver and SRbI 482, gold-copper) each consists of four alloy wires of 20% composition increments across the binary, plus the two end members. The only information concerning the preparation and characterization of these two materials is contained in the certificate which accompanies the material. The certificates are available on request from NBS. Absorption Correction. Anderson and Wittry (10) have performed an evaluation of absorption correction functions. Experimental data from a number of sources (Poole and Thomas, Colby, Colby and Conley, Heinrich, and Beaman) have been reviewed to determine their dependence on primary electron accelerating potential E,, excitation potential ratio U and atomic number 2. The data are compared by assuming that the mean depth of X-ray production (2) characterizes the distribution with accurately enough to show depth 6 (2) the influence of E,, L-, and 2 on the absorption correction function. However, has not the voltage dependence of (2) been well established. The authors therefore propose to define an “effec* and tive” mean depth of ionization (2) to determine the voltage dependence of this function from the available experimental data on f(x). They express the voltage dependence of (G)*by a change in the constants ill the modified .kchard and llulvey approximation to the Bethe range. Thus, 1 E,’.8 (pz)* = - Zl’a (1.8 In (174 E,/& -

E,’J 1.8 In (174 E,&?) It is then possible, they say, to show

f(x) as a function of the product of x and

(%)*. With the product of x and (2)” plotted on a logarithmic scale, a special scale is chosen for f(x) which causes the data to fall very close to a straight line. The basis for this special scale is given in an appendix to the paper. This special scale is a considerable aid in plotting the data and does not affect the dependence of f(x) on x and The results of this special plot led the authors to suggest a new absorption correction function which when compared with the Duncumb-Shields modified Philibert technique is a t least as good in the region of large f(x) values and considerably better for low f(x) values. However, when compared with the Heinrich refinement of the Duncumb-Shields modification, the improvement is more difficult to observe. Yakowitz and Heinrich (557) have examined all of the causes of uncer-


(z)*.

259R

tainty in the absorption correction. Their conclusion indicates that if an analysis is to be considered quantitative, f(x) must be 0.8 or higher. The effect of this is to minimize the effects of discrepancies among the various models for f(x). Because available experimental f(x) curves are very limited in number and in scope, Ranzetta and Scott (403) have calculated a series for C, Al, Ti, Cu, and Mo K radiation, L radiation of Mol Ba, Au, and U, and M radiation of Au and U,using the computer programs of Archard and hlulvey. Some experimental measurements of the distribution in depth of the primary X-ray emission [4(pZ)] have been made by Vignes and Dez (511 ) for titanium and lead a t various incident electron energies. They then calculated f(x) for these two elements which compared rather well with the curve3 predicted by the “Philibert correction.” Nagel (365) has reminded us that the mass absorption coefficient of an element has fine structure near the absorption edge. Tabulated values can be in error by 5 50% near the edge and by as much as =!=lo% up to 400 eV above the edge. This is of course only important where the analytical X-ray line is close to the absorption edge of another element in the specimen. Bishop (43)has used the Monte Carlo model for electron scattering to calculate the absorption and backscattering corrections needed. Agreement with experimental data is on the whole good, but, according to the author, the calculated values for the correction factors are not sufficiently accurate for general use. Nevertheless, in the case of light element analysis where very high absorption corrections are needed, those calculated from Nonte Carlo data are the best available a t present. Il’in and Loseva (265) have extended their use of the concept of an effective depth for Xray production, which was reported in the last review, by comparing their calculations with experimental data of their own and of others. The authors claim that the major cause of difficulty is the uncertainty in mass absorption coefficients and illustrate this fact by a statement that for a 20% Ag-Au alloy, the use of different tabulated values of ( p / p ) cause a 30y0 difference in the absorption correction. Mass absorption coefficients seem to be in a constant state of flux. The experimental work of Hughes et aZ. (262) has been republished, but there are few new measurements to report. However, there are several compilations of interest mainly because they make the uncertainties so obvious. The extensive tabulation by Mcllaster et al. which was reported two years ago has been revised (345) to produce a set of p / p tables somewhat different than the original, Veigele (507) has generated a 260 R

very similar set of tables which shows significant difference from McMaster, particularly in the low energy region. Frazer (170) has applied her coefficients to produce still another set of computer-fit mass absorption coefficient values. Also, Hubbell (259), who was a co-author with McMaster, has published a separate report of values for the higher energies. Gray (202, 203) has also attempted to analyze the data available in the literature in an attempt to produce values of mass absorption coefficients of a large number of elements from carbon to bismuth for C, N, and 0 characteristic lines. He claims 5y0 accuracy for most of his values with a few somewhat poorer (15%). These values were tested by analyzing defect T i c . When the probe data were corrected by a “full” absorption correction (from which Philibert obtained his form), the Reed fluorescence and Duncumb-Reed atomic number corrections, the carbon content of the T i c agreed to within 1% absolute (at the 18% level) with composition determined by other methods. Alexandropoulos (6) has described a grooved crystal spectrometer attachment for X-ray absorption measurements which, it is claimed, improves measurements, particulary near absorption edges. Generally, it can be said that the uncertainty in mass absorption coefficientshas not beenimproveda great deal, particularly for the soft radiation, even though there are a considerable number of new tabulations available, all of which claim to be quite accurate. Fluorescence Correction. The intensity of X-rays contributed by fluorescence due to the absorption of characteristic lines has been recognized as significant from the very beginnings of electron probe microanalysis. Until modified recently by Reed, as reported two years ago, the method proposed by Castaing in his thesis was probably the most used procedure. The past two years have not changed the suggestion that the procedure due to Reed is the most suitable. Heinrich and Yakowitz (238) have stated that the most significant source of error in the characteristic fluorescence correction is due to uncertainties in the fluorescent yields. Reference to the compilation of Fink et al. illustrates the uncertainties present in the values of fluorescent yield for some elements. Little experimental work has been performed to provide new data, but Price et al. (398) have measured the L2- and La- subshell fluorescence yields for elkments 2 = 71-83, 90 and 92. The precision estimated by the authors ( 2 0 ) was between 8 and 15’%. The values for the L2- subshell lie above most previous results while the values for the Lg subshell substantiate other recent experi-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5 , APRIL 1970

mental work but exceed earlier estimates. It is often said that we do not need to have any information about the absolute X-ray intensities in electron probe microanalysis, because relative intensities are the quantities of interest. However, when a characteristic fluorescence correction is required, this is no longer true (409). I n order to calculate the magnitude of the fluorescencecorrection, we must know the absolute intensity of the fluorescing radiation. Van Ark and van de Rotte (501) have measured the X-ray emission of various targets, bombarded by electrons. A Mo photocollector was used as an X-ray detector. The X-ray yield appears to be a periodic function of the atomic number. Green and Cosslett (204) have measured X-ray production efficiencies for the K a lines of C, Al, Ti, Fe, Nil Cu, Gel Mol Ag, and Sn, for the LaLlines of Nd, Ta, and Au, and for the M a line of Au. They used a crystal spectrometer calibrated for absolute intensity by a proportional counter. Discrepancies between theory and experiment for the L shell production efficiencies have been traced, according to the authors, to an error of a factor of -2 in Burhop’s theoretical L shell ionization cross sections (experimental results > theory). For the K shell measurements, the results agreed within *IO% with both Burhop’s theory and previous experimental values. In addition, the experimental technique used in this work has given new values of the absolute intensity of the continuous X-ray spectrum for twelve elements. The importance of fluorescence due to absorption of continum radiation remains a question. As has been mentioned, Brown et d.(60) have shown improvement between X-ray intensities measured and those predicted by the transport equation program when a correction for fluorescence by the continuum was made. Henoc (241) has described a continuum fluorescence correction based on the assumptions that the continuum radiation is generated a t the surface and that it is isotropic. Although his equation was developed in 1962, it was not used much, mainly because of its complicated form. The use of a computer, however, makes it practical for routine analysis. It seems a t this time that the question concerning the importance of the continuum fluorescence correction has to be evaluated for each individual situation, and this can be done using Henoc’s program. Atomic Number Correction. The need for a n atomic number correction is now universally accepted (404). The physical basis for this effect is the fact that the number of characteristic X-rays generated per electron incident on the specimen is not constant in matrices of varying atomic number because

of difference in back-scattering and electron stopping power as a function of atomic number. Philibert and Tixier (386, 387) claim that the validity of the expressions used for the various physical laws involved in the atomic number correction is not very good. Furthermore they suggest that the Bethe retardation law is invalid because i t neglects energy straggling. The recommendation is made that since Monte Carlo calculations and the Boltzmann transport equation give numerical solutions, these might be used t o create tables from which the corrections could be made, rather than attempting to synthesize analytical expressions. These authors agree with virtually all workers in the field of electron probe microanalysis in asserting that more reliable data concerning the basic parameters are needed. Weinryb (526) has studied backscattering between 5 and 35 keV, and Verdier and Arnal (508) have measured the backscatter coefficients of 26 elements for electrons above 50 keV. Poole (391) has presented a discussion of several different techniques for making the atomic number correction. The relative merits of these techniques were assessed by application to experimental data obtained in various laboratories. The results are presented in histogram form for 150 or 229 cases. This data includes those which were reevaluated by Heinrich (232). Duncumb and Reed (134) discuss the calculation of stopping po\Ter and backscatter effects: They have used the results of 48 measurements on a number of known binary compounds to make an empirical determination of the mean ionization potential J, which is necessary in the calculation of the stopping power of any element. The argument for doing this is that available information about J is inadequate to supply exact numerical values, but that J / Z is accepted to lie between 10 and 15 eV. Their empirical values show that J / Z is approximately constant a t 13.5 eV from Z = 92 down to Z % 40. J / Z then decreases gradually to about 11 eV a t Z = 13. At lower atomic numbers, it rises very rapidly to become about 24 eV a t 2 = 6. No physical reason could be advanced for this sudden change, but the authors claim it had been observed before. They then present a histogram which shows considerable improvement in the analytical result especially for A1 and Si when this empirical J factor is used. INSTRUMENTATION AND TECHNIQUES

The modern commercial electron probe microanalyzer is a well engineered instrument which, in the hands of a capable operator, can provide a great deal of information about the microcomposition of a sample. It should not

be necessary to dwell on the details of the instrument itself. However, some interesting developments have been reported which have special applications of interest. The combined electron microscope-electron probe microanalyzer is continuing to receive considerable attention (506). Rouberol et al. (426) have described the modification of a Cameca Microprobe to make i t a combination instrument. Philibert (385) has also discussed the same instrument. AEI in England has announced the availability of EMMA-4, a commercial model of the combined instrument developed by Duncumb. Murphy and bletzger (363)havedescribed a technique for performing conventional transmission electron microscopy with a commercial microprobe requiring essentially oiily a modified sample holder. Scanning Electron Microscopy (SEM) has developed tremendously in the last few years. While discussion of this technique is not germane to this review on X-ray emission and absorption, the fact remains that the samples being observed in scanning electron microscope are emitting X-rays and these X-rays can give information regarding the distribution of elementary composition in the specimen. This can be accomplished by placing an X-ray spectrometer in a position to view the irradiated area on the sample, although Xray counting rates are quite low a t the beam currents normally used in a SEN. Energy dispersive analysis, using either a proportional counter or a high resolution Si(Li) solid state X-ray detector, can achieve usable count rates a t conventional SERI: beam current. X very large number of commercial SEWS have provision for the installation of one or both of these capabilities. Of course, the installation of a secondary electron detector on a scanning microprobe makes it usable for scanning microscopy although spatial resolution is frequently not as good as with a regular S E X Tousimis (494) has described the use of a combined scanning electron microscope-electron probe to the study of biological soft tissures. Fitzgerald et al. (163))have shown the application of a Si(Li) detector for energy dispersive analysis on a conventional electron probe. Preparation and handling of the sample is an important part of performing analyses with meaningful results. Lachance and Plant (301) have described a method of mounting specimens and standards which is particularly applicable to minerals where standards are known composition minerals which may be in short supply. The mounting device is a small threaded cylinder with the standard mounted in a cavity on one end. After polishing, this device is held in a similarly threaded hole in a conventional probe sample

mount. Yates and Bramman (643) have developed a technique for mounting and polishing very small samples (down to 10 pm in diameter) so t h a t quantitative analysis could be performed on them. This was done by mounting the small specimen in resin in a block of steel. Abrasion and polishing rates of steel are slow enough to permit controlled removal of material. Gulson and Lovering (207) have analyzed rock samples by fusing the ground specimen with lithium tetraborate and lanthanum oxide and casting into a glass disk in a fashion identical to that used for X-ray fluorescence analysis. Thus the same preparation can be used for both EPMA and XRF. Chamberlain and Hodkin (86) described the preparation of plutonium containing samples in a fashion which avoids contaminating the electron probe by removing all loose surface contamination with successive applications of a cellulose acetate film. The problem of charging the surface of a nonconductor has conventionally been overcome by vacuum depositing a thin layer of a conductor such as carbon or a metal onto the specimen. Strasheim et al. (473) have used a metal grid to prevent the charging when exciting nonconductors with an electron beam. Smith and Pedigo (460) have examined the effects of specimen repositioning on the statistics of X-ray intensity. Modifications to instruments are continually being made, sometimes for the convenience of the operator, other times to improve its operation. Sander and Hiemstra (434) have described a safety device which protects the instrument in the event of a power failure by closing a gate valve between the diffusion pump and the vacuum tank. Tomura et al. (490) have used a ten-step concentration mapping system, while Kniseley et al. (293) have improved the beam scanning system of their instrument so that they can observe sample current images a t 45 frames/sec and a t sample currents as low as 1 nA. They accomplished this by placing an emitter follower circuit a t the probe console followed by a limited band-pass video amplifier. Ingersoll and Derouin (266) have taken color photographs as multiple exposures of the C R T on color film. This technique seeems less desirable than the other techniques available because of the necessity for changing the C R T to one having a different phosphor than normal, and the difficulty of achieving correct exposure on each of the multiples. Reid and Smith (412) have designed a cathodoluminescence attachment for their electron probe and Rosenbaum et al. (425) have made plans for a shielded electron probe facility for handling radioactive materials. Some of the spectrometers in use on electron probes are calibrated in wavelength for a (200) LiF crystal, rather


261 R

APPLICATIONS

Table XI. Some Applications of Electron Probe Microanalysis (11, 81, 219, 287, 494, 527)

Biological Cathodoluminescence Ceramics Coal Coatings Corrosion Diffusion Failure analysis Glass Inclusions Impurities Meteorites Mineralogy Phase diagrams Phase identification Refractories Segregation Soil Sputtering Thin films

(57;84, 97, 348, 478)

(45, 46, 62, 53, 121, 136, iL9,154,225,243,267, 295,360,383,414) (295) (154, 503) (271, 368, 512, 636) (14, 231, 521, 528) (167, 463) (96, 130, 190, 196, 224, 245, 250, 288, 294, 312, 339, 411, 517, 521, 532) (96, 226, 285, 295, 357, 377, 354) (16, 49, 167, 350) (2, 256, 353) (123)

(16, 273) (127,469) (27, 98, 104, 107, 251, SS6,4S1, 461 , 531 )

than in Bragg angle. When a crystal other than (200) LiF is used, a conversion is necessary to be able to set the spectrometer on the X-ray line of interest. Heinrich and Giles (237) have published a set of tables which facilitates making this conversion. Sawatzky and Jones (437) have devised a technique for determining the dead time of detector circuits particularly applicable to single detector probes with specimen and detector in a common vacuum chamber where most other techniques cannot be used. Friskney and Haworth (176) have recommended experimental procedures to be used for analyzing nonconducting specimens using metal standards. Reed (410) has discussed the stability of the electron beam and how to optimize gun parameters. Dorfler (127,128)deals with quantitative phase analysis of heterogeneous specimensie., what has been called “quantitative metallography” in metallurgy and “modal analysis” in mineralogy. Although Kossel techniques are not X-ray emission, they deserve mention herebecause frequently an electron probe is used as the X-ray generator to produce the patterns. Only a couple of papers are reviewed as an indication of what is being done in the area. Lutts (326) discussed the use of Kossel and divergent X-ray beam techniques in the determination of precision lattice parameters. Shinoda et al. (448) have studied the precipitation of alpha from beta brass in copper-zinc alloys with some discussion of microanalysis of the specimens. 262 R

There are some general comments which can be made concerning the applications of electron probe microanalysis which have appeared in the literature in the past two years. First, there are many published papers, and for any reviewer to claim, or even think, that he has managed to consider all of the publications in his field is very optimistic. Second, some of the publications are not easy to find or readily available. As with applications in any field, authors tend to publish in those journals which might be available to other workers in the same area of endeavor. Microprobe papers thus seem to appear in virtually all journals of the scientific community. It should be obvious that these first two generalizations are intended to be a t least in part an apology to those authors whose works are overlooked. A third general comment has to do with the reporting of results. Many of the references define the microanalytical composition determination as quantitative without giving any details about what correction procedures were applied to the relative X-ray intensities, or in some cases whether the raw data were corrected or not. This is particularly disconcerting when the results are reported to four significant figures. We must not allow ourselves to be overwhelmed by the number of digits which can be disgorged by a computer. Further, it seems very desirable that any paper containing the results of electron probe microanalysis should give details not only of the correction procedures which were applied but also what experimental conditions were used and where the necessary parameters were obtained. With this as a background, let us consider a few of the applications which are listed in Table XI. Kobren (295) has published a very good illustration of the kinds of work in which the probe is often applied. Methods of using the probe and some results are given by examples which include bonding of thermoelectric materials, alloy development, and analysis of materials failures. This paper illustrates very explicitly the value of a probe in materials research and development, when used in the scanning mode particularly, for qualitative and perhaps semiquantitative analysis to solve a myriad of problems. Bais (16) has given examples of industrial applicacations consisting of both scanning pictures to describe elemental distribution and quantitative analysis done during line scanning. The examples consist of the analysis of welding electrodes, identification of phases in an aluminum base alloy, and the analysis of sands. This publication is one in which the experimental conditions are listed but no details are given about the correction procedures which might have been made to the data.


The study of metallurgical diffusion continues as one of the most prolific areas of application. Most of the references to diffusion in the table are concerned with metal systems, When coupled with those efforts in corrosion, phase diagrams, phase identifications, and the studies of inclusions, it can be seen that metallurgy uses the technique of electron probe microanalysis to a greater extent than any other field. Hekenkamp (231) claims that the use of the electron probe measurements are able to compete favorably with radioactive tracer techniques in impurity diffusion experiments and are superior to all other known methods for transport of dilute impurities in metals under the influence of external forces-e.g., electric fields or temperature. Eifert et al. (149) have used the technique to determine interface compositions, motion, and lattice transformations in multiphase diffusion couples. Solubility and phase diagram studies cover almost the entire periodic chart from atomic number 13 upward such as the solubility of Pu in F e (357) and the Pu-Th syst’em (394) and including some ternary systems, such as -4u-SnPb (285)and Sb-T1-Te (96). The field of corrosion studies is receiving considerable attention by workers having access to electron probes. Brewer and Valters (57) have examined t’he surfaces of a-brass which had been eroded by high voltage oscillatory spark discharges. Their choice of brass was dictated by the fact that the Cu-Zn system requires virtually no correction procedures. Khile this is not’ rigidly true-mass absorption coefficients are not exactly equal, and ZnKp does fluoresce CuKa-it is certainly close enough to make the use of relative intensities as a measure of concentration pract’ical. Chodos and Meites (97) have investigated the formation of oxide films on platinum-iridium electrodes. Stern and Light (469) have shown how the oxygen content affects the preferred orientation of Ni-Cr thin films produced by sputtering. Thin films can be examined by the electron probe either to determine their thickness, or their composition, and in some cases both. The MAGIC computer program (for), as has been mentioned, was designed to make the calculations necessary for the analysis of thin films. Bennett et al. (27) have determined the composition of oxide films formed during the oxidation of austenitic stainless steels a t elevated temperatures, including the highly irradiated corrosion products of in-reactor oxidation. Their technique was to chemically strip the oxide from the substrate and then analyze it. By using fairly high electron beam energy for thin films, the electrons lost very little energy in passing through. For

the thicker films, a low energy beam was used and the films were infinitely thick. This enabled them t o study films from 0.1 t o 4 pm thick. Cline (104) discussed the techniques for determination of film thickness by the electron probe. She estimates the ultimate sensitivity to be &6 angstroms of aluminum. Knausenberger et al. (891) have obtained compositional and morphological information about SiOz films on silicon by a combination of ellipsometry and electron probe microanalysis. Marshall and Hall (336) have developed a technique for analyzing films in the thickness range of 0 to 0.5 mg/cm2 by using the intensity of the continuous X-ray spectrum as a monitor of the film thickness. They claim that the ratio of the numbers of characteristic quanta to continuum quanta gives a measure of the concentration of an element in the film which is independent of film thickness. The applications of electron probe microanalysis to the field of mineralogy is steadily increasing. This is not to say that mineralogists have until now neglected the technique. Some of the finest work during the years of development has been done in this field. The difficulties are many. A large proportion of the elements of particular interest in minerals are of low atomic number, and the phases are not simple but many times very complex. It has become a matter of routine for the mineralogist or geologist doing electron probe microanalysis to use comparison standards as much like the unknown as he can obtain, rather than use the pure element standards which are more often used in metallurgy. Thus, he can avoid many of the difficulties associated with changes in the characteristic lines due to chemical combination, and, if the standard has the same elements as the unknown, the corrections for absorption, fluorescence, and atomic number are minimized. The lunar samples returned by d p o l l ~11 are being examined by several workers in the field, and although no official publications concerning their findings are available as this is written, the year 1970 should see these results circulated and possibly also those of specimens from dpollo 12. Some of the applications in Table TI1 have to do with uranium minerals and ores (246, 517), gold ore ($49, 350), garnets (130, 294), and soils and sands (16, 875). Some of the names of minerals being investigated are familiar to the laymen, some not so familiar: anandite ( $ l a ) , troctolite (339), kyanites (552),silicates (190),and perryite (411). Snetsinger et al. (463) have studied chondritic meteorites containing both vanadium and titanium. They discuss the problem of preventing the TiKp overlap with VK,. Biologists have many problems in

common with mineralogists: light elements, complex materials, etc.; but they are further hampered by the difficulty of obtaining or preparing adequate standards. T. Hall (219) has discussed in considerable detail the problems involved in quantitative analysis of biological specimens and suggested possible solutions for some of them. The work by Ansell and Judd (11) to attain quantitative results on particles smaller than the electron beam size is a worthwhile first step toward solving another of the problems in biology. Weinryb (527) has shown some typical biological problems, such as the determination of the distribution of fluorine in teeth, determination of foreign particles in lung tissue, and analysis of dried solutions. Carroll and Tullis (81) have made some observations on the presence of titanium and zinc in human leucocytes. They reported abnormally high concentrations of titanium and zinc (106 to 108 atoms/cell) in cells of patients suffering from lymphoblasticleukemia or Hodgkin’s disease. Tousimis (494) has shown how combining scanning electron microscopy and electron probe microanalysis can benefit the study of biological soft tissue because both morphological and chemical information can be obtained simultaneously. It was suggested here two years ago that significant contributions might be made in biological quantitative electron probe microanalysis if experts in the field of X-ray generation would apply their talents to these unique problems. While no great strides have been made, and this is not in general being done, it does still seem desirable. Certainly the work being done by increasing numbers of scientists is advancing the field. Let us here voice the hope that in the not-too-distant future almost any sample will be analyzed with reasonably accurate results.

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ACKNOWLEDGMENT

Practica de la Espectroscopia de Rayos X,” Editorial Alhambra, S.A., Madrid

We appreciate the cooperation of the numerous authors who forwarded copies of their publications. I n particular we thank R. Jenkins of N. V. Philips in Eindhoven for furnishing proceedings of important X-ray conferences held in Europe. T o assist future reviewers, a brief summary in English would be appreciated for those papers published in other languages. LITERATURE CITED

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Thermal Analysis C. 6. Murphy, Xerox Corporation, Rochester,

T

HIS REVIEW covers the major trends in thermal analysis from the period covered by the last review (203) to October 1969. During this period, thermal analysis has made remarkable strides. A number of conferences have been devoted to the subject, including the Second International Conference on Thermal Analysis (2&), the ilmerican Chemical Society Symposium on Analytical Calorimetry (ZSq), and the Third Toronto Symposium on Thermal Analysis (179). National groups devoted to thermal analysis have been established in Japan and Italy. The Yorth American Thermal Analysis Society has been formed; i t held its first meeting a t Battelle Memorial Institute in Noveniber 1969. Among the new books, there are “Applications of Differential Thermal Analysis in Cement Chemistry” (230), “Differentialthermoanalyse” (243), and the pocket book, “La Thermo-Analyse” (110). The last has sections devoted to thermogravimetry (TGA), differential thermal analysis (DTA), evolved gas analysis (EGX), dilatometry, equilibrium diagrams, and thermometric titrations. A recent volume of the “Treatise on Analytical Chemistry” contains sections entitled “Elements of Chemical Thermodynamics: Introduction to Thermal Methods” (282), “Principles of Thermometry” (58), “Cryoscopy” (96),“Calorimetry” ( I C s ) , “Thermometric Enthalpy Titrations” (125) , and “Differential Thermal Analysis,’ (205). Reviews have appeared on DTh (80, 105, 200, 204) and derivatography (219, 271). The development of thermography in the USSR has been reviewed (23). The application of DTA and TGA to high polymers also has been the subject of a recent review (198). The Elsevier Publishing Co. has announced the publication of a new journal, Thermochimica Acta, with W. W.Wendlandt editor-in-chief. Kultura

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N. Y.,

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Press, Budapest, Hungary, has inaugurated the new journal, Journal of Thermal Analysis. Both journals will publish articles, notes, reviews, etc., in the broad field of thermal analysis. Efforts have been made, particularly by the ICTA, to establish standards for the field of thermal analysis. An initial attempt has been made (168) to establish standard nomenclature in the field. I n addition to the previously recommended method for reporting thermal analysis data (178), extensive work has been applied to and reported on the selection of standards for thermal analysis (90, 180). However, recognition for standardization of reporting procedures (269) and the need for calibration standards for DTA have been voiced by others (75, 290). DIFFERENTIAL THERMAL ANALYSIS

Dynamic scanning calorimetry (DSC) also is considered in this section. A number of equipments have been developed during the period of the review. They include apparatus for operation at high pressures (5100 kbars) and high temperatures ( S 1000’) (19), for use in vacuum and controlled atmospheres to ca. 600’ (241), for operation under hydrogen over the range of 1 to 500 bars (33),and a sensitive micro equipment for operation in vacuum or air, X2, or He, employing a disk-type differential thermocouple, with a fixed distance between the junctions, as the detector (296, 297). A simplified apparatus for examination of solid fats has been described (220). h DSC has been reported (115) in which heating and control are by focused radiation from projection lamps. The accuracy of the last equipment was determined to be 1 to 270 from runs with In, Sn, and C6H&OOH. Equipment employing vacuum-deposited Ni and Xu thermocouples (136, 137) has been de-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5 , APRIL 1970

veloped, and it has been reported that changes in heat transfer coefficient between sample and sensor have been practically eliminated, resulting in flat base lines. High precision measurements were obtained with n-C32H66and In. Equipment for single-crystal DTA has been reported (89, 235) and it has been shown (89) that such equipment can show nonreproducible effects associated with distortions, etc., which contribute to an ill-defined effect when a multitude of particles are employed. Solid-state circuitry for temperature programming, incorporating siliconcontrolled rectifiers, has been described (256) and was incorporated in hot stage microscopic equipment previously described (19 7 ) . An electrical circuit for accurate measurement of small temperature differences, incorporating thermistors, has been described (273),and DTA equipment incorporating thermistors has been used with water-salt systems (225). The significance of sample holder construction materials on thermograms has been indicated when thermal decomposition of [ (NH4)J107024.4Hz0] was studied in P t , glass, corundum, quartz, and Ag crucibles (22). Abnormal effects were observed with P t due t o catalytic oxidation of “3. Equipment for simultaneous high temperature DTA and x-ray diffraction is available commercially (91), and such equipment with controlled atmosphere application has been described (12). Equipment for simultaneous DTA and TG.1 for application under vacuum or atmospheric pressure has been described and applied to CuS04.5H20 and cellulose (215). A simple, generalized theory for the analysis of dynamic thermal measurements has been presented (98). The influence of atmosphere on the decomposition of some metal oxalates (169) and the influence of water vapor pressure on the dehydration of CaC204.H20 (92)

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