20
ANALYTICAL CHEMISTRY
(15) Engstrom, A., and Amprino, R., Esperientia, VI/7, 267 (1950). (16) Engstrom, A., and Click, A., Science, 111, 37%(1950). (17) Engstrom, A., and Wegstedt, L., Acta Chem. Scand., 3, 1442 (1949). (18) Friedlander, G., and Kennedy, J. W.,“Introduction to Radiochemistry,” Chap. IX, S e w York, John Wiley 8: Sons, 1949. (19) Glocker, R., and Frohnmayer, W.,Ann. Physik, 76, 369 (1925). (20) Heller, R. B., Sturcken, E. F., and Weber, A. H., Rev. Sci. Instruments, 21, No. 11, 898 (1950). (21) Hevesy, G. von, “Chemical Analysis by X-Rays and Its Applications,” Chap. VI and VIII, Kew York, McGraw-Hill Book Co., 1932. (22) Hughes, H. K., and Hochgesang, F. P., ANAL.CHEM., 22, 1248 (1950). (23) Hughes, H. K., and Wilcsewski, J. W., Proc. Mid-Year Meeting, Am. Petroleum Inst., 30M (III), 11 (1950). (24) Kehl, W’. L., and Hart, J. C., Proc. Am. PetroleumInst., 111, 28, 9 (1948). (25) Levine, S. W., and Okamoto, A. H., ANAL. CHEM.,23, 682 (1951). (26) Ibid., p. 699. (27) Ibid., p. 1293. (28) Liebhafsky, H. 9.,Ibid., 21, 17 (1949).
(29) Ibid., 22, 15 (1950). (30) Ibid.,23, 14 (1951). (31) Liebhafsky, H. A, Ann. N . Y . Acad. Sci., 53, 997 (1951). (32) Liebhafsky, H. A,, Pfeiffer, H. G., and Balis, E. W.,ANAL. CHEM.,23, 1531 (1951). (33) Liebhafsky, H. A., and Winslow, E. H., .48i”hf Bull., KO. 167, 67 (1950). (34) Liebhafsky, H. A., and Zemany, P. D., A s . 4 ~ . CHEM.,23, 970 (1951). (35) Michel, T. C., and Rich, T. A., Gen. Elec. Re$., 50, No. 2, 45 (1947). (36) Morrison, A., .Vucleonics, 5 , No. 6, 19 (1949). (37) Muller, R. H., and Wise, E. S . ,ANAL.CHEM.,23, 207 (1951). (38) Persico, E., and Goeffrion, C., Rev. Sci. Instruments, 21, 945 (1950). (39) Schwinn, TV. L., Welding Engr., 35, 24 (December 1950). (40) Vollmar, R. C., ASTM Symposium on Instrumental Methods of A4nalysis,San Francisco, Oct. 11, 1949. (41) Vollmar, R. C., Petterson, E. E., and Petrusselli, P. A , ANAL. CHEM.,21,1491 (1949). (42) Winkler, E. M., J . Applied Phys., 22, 201 (1951). (43) Zemany, P. D., Winslow, E. H., Poellmitz, G. S.,and Liebhafsky, H. A,, ANAL. CHEM., 21, 493 (1949). RECEIVED Nov. 17,1951.
X-RAY DIFFRACTION H. S. KAUFMAN, M . W . Kellogg Co., New York, N. Y . , AND ISIDOR FANKUCHEN, Polytechnic Institute of Brooklyn, Brooklyn, N. Y .
I
S THE course of the past two years, since the appearance of the last review on x-ray diffraction in this journal, there have been continued developments of apparatus and techniques which have further increased the utility of this tool for analytical purposes. These new developments, together with some of the most recent applications, are reviewed in this paper. APPARATUS AND TECHNIQUES
The redesigned x-ray diffraction units, manufactured by the General Electric Co. and North ilmerican Philips, are now in use in many laboratories. Both of these units are characterized by their highly stabilized x-ray outputs and the sensitive, high resolution Geiger counter spectrometers nith which they are equipped. Ohio X-ray, Bedford, Ohio, has also developed a recording x-ray Geiger counter spectrometer. Units such as these are responsible for the increased use of Geiger counter methods compared to the‘ previously favored photographic method for the detection and measurement of x-ray diffraction phenomena. Hilger and Watts (U. S.distributor, Jarrell-Ash Co., Boston, Mass.) have redesigned their demountable tube x-ray unit and are equipping it with a Geiger counter spectrometer with monitored x-ray beam to compensate for fluctuations. They have also announced a microfocus x-ray unit in Thich the loading per unit area is about 100 times that of ordinary tubes. This unit promises to be very useful for the analysis of micro specimens. For those who desire to build their own x-ray units, Garrod (38) has discussed the considerations in the design of demountable x-raj tubes. Lees and Armitage (60) have presented a circuit for automatic stabilization of continuously pumped tubes. A method has been described for increasing the safe power input into x-ray tubes by improving the water cooling efficiency (18). A great deal of attention has been given to camera modifications and design. Parsons (78)has described a method of adapting powder cameras for reflection patterns. An x-ray scanning camera (96) has been developed which eliminates diffraction line graininess due t o large crystallites. The realization that many liquid and gaseous compounds may be characterized by low temperature x-ray diffraction has provided a stimulating, rapidly growing interest in this field. -4
number of new or improved low temperature cameras and techniques have been described (1, 28, 7 4 ) . High temperature cameras have also received their share of attention. Goldschmidt and Cunningham (39) and Rilliams (105) have described such cameras. Gordon (41) has developed a precision camera and technique suitable for the high temperature measurement of the coefficient of expansion of beryllium. A novel high temperature camera utilizing a thin nickel foil as a combination x-ray filter and furnace has been described by Steward (86). The centering and mounting of high temperature camera specimens (87) and the calibration and manipulation (19) of such cameras have been discussed. Special cameras have been described by Lawson and coworkers (58, 69) for the examination of powder specimens a t high pressures. Alexander (3)has presented an analysis of the intensities from slits and pinholes in powder techniques. Considerations in the design of slits have been discussed by Garrod (38). Slits are effectively employed as a means of increasing the intensity, particularly where monochromatic radiation is employed. Wrightson and Fankuchen (111)have described a simple monochromator designed for powder work. A bent crystal monochromator has been described by Warren (103). Dumond (34) has developed point focus x-ray monochromators particularly adaptable to small-angle scattering. A collimator producing a beam of small divergence and high intensity (61) and an x-ray micro beam for examination of plastically deformed metals have been described (56). I n the field of small-angle scatter, Bolduan and Bear (81) have discussed the effective use of collimating apertures, while Yudowitch (21s) has evaluated the collimation errors encountered, Double crystal and slit methods for use in small-angle scattering have been compared (77). An improved double crystal spectrometer has been described by Broussard (86). Crystals suitable for this application have been discussed (85). A new x-ray method involving the use of a double crystal spectrometer for the study of imperfections has been described (76). Schulz (80, 81) has developed a method for determining preferred orientation in flat transmission and reflection samples using Geiger counter spectrometers.
V O L U M E 2 4 , NO. 1, J A N U A R Y 1 9 5 2 The preparation of specimens is an important aspect of x-ray diffraction analysis. There have been continued developments in this field. Methods of preparing extruded specimens have been described (42, 50). Two methods (24, 36) have been developed for obtaining x-ray patterns from samples of about 0.001 mg. Beu ( 2 0 ) has improved a method for the preparation of powder sample capillary tubes from plastic. Ubbelohde ( 9 8 )has presented a technique involving the use of a low pressure head of water as an aid in the preparation of thin-walled glass capillary tubes. Capillary tubes of varying diameter and uniform 0.01mm. wall thickness are again available commercially from a source in Germany (Paul Raebiger, Franzstrasse 43, BerlinSpandau, Germany). A design has been presented for a sample holder adapted for deliquescent materials (62). Singer and Ellinger (83) have deecribed techniques for dealing with alloys requiring protective atmospheres. Perrine ( 7 3 ) has developed an optical system as an aid for centering x-ray diffraction specimens in powder cameras. A special rotating mount for use with a spectrometer has been prepared for the measurement of crystallite orientation in textile fibers (82). The calculation of mass absorption coefficients (101) and the dbsorption correction for encased diffractors for powder work ( 7 8 )have been discussed. There have been a number of papers relating to the measurement of x-ray intensities. The measurement of intensity is extremely important for the quantitative evaluation of x-ray patterns. Several densitometers have been described (17,26,71,9S). Considerable work is being done on the use of sensitive crystals such as zinc sulfide and cadmium sulfide ( 3 5 ) x-ray detectors. Cochran (25) has described a Geiger counter technique for the measurement of integrated reflection intensities. The intensities of single crystal reflections may be measured R ith a combination two-circle goniometer and x-ray spectrometer (12). The Eastman Kodak Co. has available an intensification formula for x-ray films which i t is claimed can yield a satisfactory pattern after 15 minutes of normal exposure. Considerable attention has been given to the measurement of diffraction line shapes with the Geiger counter spectrometer. This method provides a simple and effective manner of measuring crystallite size ( 4 , 110). Wilson (106) has treated the influence of size and absorption coefficient on the shape and position of powder diffraction lines. -4lexander ( 2 ) has discussed the geometrical factors that cause line broadening. The effects of crystal and particle size on the x-ray diffraction lines have been considered by Wilchinsky (104). Brentano ( 2 2 ) has given a quantitative theoretical evaluation of patterns from mixed powders. The revised ASTAT index of x-ray diffraction powder data has been made available. This is an invaluable aid in the identification of unknown compounds. Swanson and Tatge (92) have discussed some additions and corrections to the index. Frevel and North ( 3 7 ) have advocated the use of tungsten radiation of single wave length for obtaining high resolution powder diagrams. DeWijs (33)has presented a graphical method for indexing cubic materials and for obtaining the unit cell from powder data. The determination of precision lattice constants of noncubic crystals has been discussed by Taylor and Floyd (94). The Straumanis method of film measurement, which compensates for film shrinkage and does not require use of the camera radius, has been modified for use without high angle lines (107). APPLICATIONS
X-ray diffraction has continued to be an extremely useful tool for analytical purposes. Its application for the identification and characterization of crystalline compounds has become more widespread. The papers referred to in this section do not constitute all of those published which have involved the application of x-ray diffraction as an anal>-tical tool. Those chosen for revien repre-
21 sent a rather broad sampling of the various applications that have been made. There have been numerous applications of this tool in the field of inorganic chemistry. Beamer and Maxwell ( 1 5 ) have applied x-ray diffraction to the characterization of radioactive materials, including polonium. A great deal of work has been done on the characterization of metal oxide systems. including manganese dioxide (Si?), rare earth oxides ( 6 3 ) ,iron oxides and manganese oxide (64), and hydrous oxides and hydroxides (70). Effects of solid solution in calcite (6) have been studied, as have solid solutions of bromine and iodine mixtures (45). Hoffmann (51)has investigated the effect of electrolytes on the x-ray spacings of colloidal samples. rl structural investigation of platinum, niobium, and tantalum complexes in solution has been carried out by Vaughan, Sturdivant, and Pauling (100). I n the special field of catalysis there have been a number of x-ray studies. The various aluminum oxides and their hydrates have been characterized (50). Cobalt-thoria-kieselguhr ( 4 9 ) catalysts and Fischer-Tropsch iron ( 5 ) catalysts have been studied. Particular attention has been given to the study of iron carbides and their role in the Fischer-Tropsch process. The ironiron carbide equilibrium has been studied (27),as has the transition of the Hagg iron carbide to cementite (30). Hofer and Cohn have described the analysis of the iron carbide system by x-ray diffraction methods (47'). Jack ( 5 8 ) has presented a structural investigation of the iron-carbon and iron-nitrogen systems. In the mineralogical field there have been many applications, including x-ray examination of asbestos (16) and radioactive minerals ( 9 ) . Barshad ( 1 3 ) has studied the effect of interlayer rations on the expansion of the mica-type crystal lattice. Thermal transformations in magnesium chlorite minerals have been investigated ( 2 3 ) . Van der Marel ( 9 9 )and Jeffries and Jackson ( 5 3 ) have discussed the analyses of soils by x-ray diffraction. A rapid x-ray method for determination of silica in industrial dusts has been described (75). High temperature phase changes in metals have been studied by Heal and l l y k u r a (44). Tesche ( 9 5 )has investigated the formation of scale on iron between 400' and 700' C. The lattice parameters of martensite and austenite have been given by llazur (67). The measurement of retained austenite in carbon steels (10) and steel weld metal ( 6 9 )has been described. A number of alloy systems have been studied, including copper-titanium ( 5 4 ) and iron-nickel-chromium (7'5). X-ray line broadening in metals has been discussed by Hall (43). The particle size of colloidal gold and carbon black has been measured (57). There has been a very large increase in the number of applications of x-ray diffraction analysis to organic materials. Among the many compounds that have been characterized by x-ray methods are solid aromatic hydrocarbons (48), amides of saturated aliphatic acids (112),amides and silver salts of fatty acids ( 6 6 ) ,linoleic acids (108), saturated 1,3-diglycerides ( f 4 ) ,amylose complexes ( 5 5 ) ,and derivatives of epoxystearic acids and expoxyoctadecanols (109). Methods have been described for the x-ray diffraction identification of alkyl halides as sulfide and sulfone derivatives ( 6 8 ) and of n-aliphatic aldehydes as 2,Pdinitrophenylhydrazones (65). The identification of organic dyestuffs by x-rays has been discussed by Susich (51). Solid soaps (88, 85) and thermal transitions in grease (102)have been investigated. There have been numerous applications in the field of high polymers. Synthetic polypeptides have been examined (11). The identification of natural and synthetic rubbers has been described (40). Arlman (7') has compared the x-ray crystallinity of rubber a i t h values obtained by density measurements. The cr,mtallinity of polythene samples has been evaluated (67). The molecular weight and size of dried tobacco necrosis protein has been obtained b> x-ray methods and compared with electron microscope data (31). Yudowitch (114) has determined latex particle sizes from x-ray diffraction peaks. Small-angle scattering from polymers ( 8 )and cellulose fibers ( 4 6 ) has been investigated.
ANALYTICAL CHEMISTRY
22
Many organic compounds have been subjected to single crystal x-ray structural analysis. These are not considered here. One example is of particular interest for analytical purposes-namely, the structure of urea-hydrocarbon adducts (84). McCrone has continued to present useful crystallographic data on organic compounds as a monthly feature in this journal. LITERATURE CITED
(1) Abrahams, S. C., Collin, R. L., Lipscomb, U'. N., and Reed, T. B., Rev. Sci. Instruments, 21, 396 (1950). (2) Alexander, L . , J . Applied Phys., 21, 126 (1950). (3) Ibid., p. 779. (4) Alexander, L., and Mug, H. P., Ibid., 21, 137 (1950). (5) .4ndersen, R. B., Hofer, L. J. E., Cohn, E. M., and Seligman. B., J . Am. Chem. SOC.,73, 944 (1951). (6) Andrews, K. W., M i n i n g Mag., 29, 85 (1950). (7) Adman, J. J., Rubber Chem. and Technol., 23, 306 (1950). (8) Arnett, L. M., Meibohm, E. P. H., and Smith, A. F., J . Polymer Sci., 5, 737 (1950). (9) Arnott, R. J., Am. Mineral., 35, 386 (1950). (10) Averbach, B. L., Castleman, L. S., and Cohen, M., Trans. Am. SOC.Metals. 42. 112 (1950). (11) Bamford, G. ".,'Handy, ~. E., and Happey, F., Proc. Roy. SOC.(London),205A, 30 (1951). (12) Baron, RI. L., and DeBretteville, A., Jr., Rea. Sci. Instruments, 21, 458 (1950). (13) Barshad, I., Am. Mineral., 35, 225 (1950). (14) Baur, F. J., Jackson, F. L., Kolp, D. G. and Lutton, E. S., J . Am. Chem. Soc., 71,3363 (1949). (15) Beamer, W.H., and Maxwell, C. R., J . Chem. Phys., 17, 1293 (1949). (16) Beatty, S. V. D., Am. Mineral., 35, 579 (1950). (17) Bennett, J. A., Rev. Sci. Instruments, 20, 908 (1949). (18) Berman, A. J., Ibid., 21, 275 (1950). (19) Berry, R. L., Henry, W. G., and Raynor, G. V., J . Inst. Metals, 78, 643 (1951). (20) Beu, K. E., Rev. Sci. Instruments, 22, 62 (1951). (21) Bolduan, 0. E. A., and Bear, R. S., J . Applied Phys., 20, 983 (1949). (22) Brentano, J. C. M., Ibid.. 20, 1215 (1950). (23) Brindley, G. K., and .41i, S. Z., Acta Cryst., 3, 25 (1950). (24) Brooks, R., and .4lcock, T. C., J . Sci. Instruments, 28, 28 (1951). (25) Broussard, L., Rev. Sci. Instruments, 21, 396 (1950). (26) Brown, W.N., Jr., and Birtley, W. B., Ibid., 22, 67 (1951). (27) Browning, L. C., Dewitt, T. W., and Emmett, P., J . Am. Chem. Soc., 72,4211 (1950). (28) Clifton, D. F., Rev. Sci. Instruments, 21b339 (1950). (29) Cochran, R'., Acta Cryst., 3, 268 (1950). (30) Cohn, E. hl., and Hofer, L. J. E., J . Am. Chem. SOC., 72, 4662 (1950). (31) Cowan, P., and Hodgkin, D. C., Acta Cryst., 4, 160 (1951). (32) Delano, P. H., Ind. Eng. Chem., 42, 523 (1950). (33) DeWijs, J. C., Acta Cryst., 3, 394 (1950). (34) Dumond, J. W. M., Rev. Sci. Instruments, 21, 188 (1950). (35) Frericks, R., J . AppEied Phys., 21, 312 (1950). (36) Frevel, L. K., and Anderson, H. C., Acta Cryst., 4, 186 (1951). (37) Frevel, L. K., and North, P. P., J . Applied Phys., 21, 1038 (1950). (38) Garrod, R. I., J . Sci. I n s f m m e n t s , 27, 89 (1950). (39) Goldschmidt, H. J., and Cunningham, J., Ibid., 27, 177 (1950). (40) Goldspiel, S., and Bernstein, F., A S T M Bull., 171, 71 (1951). (41) Gordon, P., J . Applied Phys., 20, 908 (1949). (42) Grotenhuis, M., Durst, G. F., and Barkow, A. G., J . Iron Steel Inst., 161, 388 (1949). (43) Hall, W. H., Proc. Phys. Soc., 62A, 741 (1949). (44) Heal, H. T., and Mykura, H., Metal Treatment, 17, 129 (1950). (45) Heavens, 0. S., and Cheesman, G. H., Acta Cryst., 3, 197 (1950). (46) Heyn, A. N. J., J . Am. Chem. Soc., 72,5768 (1950). (47) Hofer, L. J. E., and C o b , E. M., ANAL.CHEM.,22, 907 (1950). (48) Hofer, L. J. E., and Peebles, W. C., Ibid., 23, 690 (1951). (49) Hofer, L. J. E., Peebles, W. C., and Bean, E. H., J . A m . Chem. Soc.. 72. 2698 (1950). (50) Hofer,'L. J. E., Peebles, W. C., and Guest, P. G., ANAL.CHEW, 22., 1218 ---- (1950). - -, (51) Hiffmann, 0. A., J . Phys. and Colloid Chem., 54, 421 (1950). (52) Jack, K. H., Acta Cryst., 3, 392 (1950). (53) Jeffries, C. D., and Jackson, M. L., Soil Sci., 68, 57 (1949). (54) Karlsson, N., J . Inat. Metals, 79, 391 (1951). (55) Katzbeck, W. J., and Keir, R. W., J . Am. Chem. Soc., 72,3208 (1950). \ - -
(56) Kellar, J. K.,Hirsch, P. B., and Thorp, J. S., A'ature, 165, 554 (1950). (57) Krimm, S., and Tobolsky, A. V., J . Polymer Sci., 7, 57 (1951). (58) Lawson, 8. W., and Riley, N. A., Rev. Sci. Instruments, 20, 763 (1949). (59) Lawson, A. W., and Ting-Yuan, T., Ibid., 21, 815 (1950). (60) Lees, C. S., and Armitage, M. D., J . Sci. Instruments, 27, 300 (1950). (61) Lely, J. A., and Van Rijssel, T. W., d c t a Cryst., 2, 337 (1949). (62) Llewellyn, F. J., Ibid., 4, 185 (1951). (63) McCullough, J. D., J . Am. Chem. Soc., 72, 1386 (1950). (64) XIcSIurdie, H. F., Sullivan, B. M., and Marrer, F. A , , J . Research Natl. Bur. Standards, 45, 1 (1950). (65) Malkin, T., and Tranter, T. C., J . Chem. Soc., 1951, 1178. (66) Matthews, F. W., Warren, G. G., Miohell, J. H., ANAL. CHEM., 22, 514 (1950). (67) Marur, J., A'ature, 166, 828 (1950): (68) hferritt, L. L., Jr., Cutter, H. B., Golden, H. R., and Lanterman, E., ANAL.CHEM.,22, 519 (1950). (69) Miller, D. S., Welding J., 29, 456 (1950). (70) Milligan, IT. O., J . Phys. and Colloid Chem., 55, 497 (1951). (71) O'Connell, T. C., and Barkow, 1.G., Rev. Sci. Instruments, 21, 573 (1950). (72) Parsons, J., Ibid., 21, 185 (1950). (73) Perrine, Ibid., 21, 262 (1950). (74) Post, B., Schwartr, R. S., and Fankuchen, I., Ibid., 22, 218 (1951). (75) Rees, W.P., Burns, B. D., and Cook, A . J., J . Iron Steel Inst., 162, 325 (1950). (76) Reis, A. J., Slade, J. J., Jr., and Weissmann, S., J. Applied Phys., 22, 665 (1951). (77) Ritland, H. N., Kaesberg, P., and Beeman, IT. W., Ibid., 21,838 (1950). (78) Ritter, H. L., Harris, R. L., and Wood, R. E., Ibid., 22, 169 (1951). (79) Schmelser, L. L., I n d . Hyg. Occupational Med., 3, 121 (1951). (80) Schulr, L. G., J . Applied Phys., 20, 1030 (1950). (81) Ibid., p. 1033. (82) Sepal, L., Creely, J. J., and Conrad, C. &I., Rev. Sci. Instruments, 21,431 (1950). (83) Singer, J., and Ellinger, F. H., J . Applied Phys., 21,461 (1950). (84) Smith, A. E., J . Chem. Phys., 18, 150 (1950). (85) Stephenson, S. T., and Martin, D. L., Rev. Sci. Instruments, 21, 1023 (1950). (86) Steward, E. G., J . Sci. Instruments, 26, 371 (1949). (87) Ibid., 28,29 (1951). (88) Stosick, A . J., J . Chem. Phys., 18, 757 (1950). (89) Ibid., 18, p. 1035. (90) Stumpf, H. C., Russell, -4.S., Kewsome, J. W.,and Tucker, C. M., I n d . Eng. Chem., 42, 1398 (1950). (91) Susich. G., AFAL.CHEM.,22, 425 (1950). (92) Swanson, H. E., and Tatge, Eleanor, J . Research Natl. Bur. Standards, 46, 318 (1951). (93) Taylor, A., J . Sci. Instrument.9, 28, 200 (1951). (94) Taylor, A., and Floyd, R. W., Acta Cryst., 3, 285 (1950). (95) Tesche, 0. A., Trans. Am. SOC.Metals, 42, 641 (1950). (96) Thewlis. J., and Pollock, A. R., J . Sci. Instruments, 27, 72 (1950). (97) Turkevich, John, and Hubbell, H. H., J . Am. Chem. Soc., 731, (1951). (98) Ubbelohde, A. R., J . Sci. Instruments, 27, 208 (1950). (99) Van der Marel, H. W., Soil Sci., 70, 109 (1950). (100) Vaughan, P. A., Sturdivant, J. H., and Pauling, L., J . Am. Chem. Soc., 72, 5477 (1950). (101) Victoreen, J. A., J . Applied Phys., 20, 1141 (1950). (102) Vold, M. J., Hattiangdi, G. S., and Vold, R. D., Ind. Eng. Chem., 41, 2539 (1949). (103) Warren, B. E., Rev. Sci. Instruments, 21, 102 (1950). (104) Wilohinsky, 2. W., Acta Cryst., 4, 1 (1951). (105) Williams, E. C., J . Sci. Instruments, 27, 154 (1950). (106) Wilson, A. J. C., Ibid., 27, 321 (1950). (107) Wilson, A. J. C., Rm. Sci. Instruments, 20, 831 (1949). (108) Witnauer, L. P., Nichols, P. L., Jr., and Senti, F. R., J . Am. Oil Chemists' Soc., 26, 653 (1949). (109) Witnauer, L. P., and Swern, D., J . Am. Chem. Soc., 72, 3364 (1950). (110) Wood, W. A., and Raschinger, W. A., J . Inst. Metals, 75, 571 (1949). (111) Wrightson, F. M., and Fankuchen, I., Rev. Sci. Instruments, 22, 212 (1951). (112) Wurz, D. H., and Sharpless, N. E., ANAL.CHEM.,21, 1446 (1949). (113) Yudowitch, K. L., J . Applied Phys., 20, 1232 (1949). (114) Ibid., 22, 214 (1951).
RECEIVED November
2, 1951
