26
ANALYTICAL CHEMISTRY
(154) Sewton, T. W., and Arcand, G. M.,J . Am. Chem. Sac., 75, 2449 (1953). (155) Nielsen, E., and Praetorius, E., Scand. J . Clin. & Lab. Intest., 4, 313 (1952). (156) Nisonoff, A., Henry, S. S.,and Barnes, W. W., Jr., J . Biol. Chem.. 199. 699 (1952). (157) Noda, L. H.,‘Kuby, ST’A., and Lardy, H. A., J . Am. Chem. Sac., 75, 913 (1953). (15X) S u r r i s , B.1. S.,and Coggeshall, S . D., .ANAL. CHEM.,25, 183 (1953). (159) O’C’onnor, R. T., Heinaelman, D. C., Pack, F. C., and Planck, R. W., J. Am. Oil Chemists’ Soc., 30, 182 (1953). (160) O’Connor, R. T., Pominski, C . H., Heinzelman, D. C., Howell, H., and Ham, P., von der, Ibid., 29, 220 (1952). (161) O’Connor, R. T., Stansbury, M . F., DamarQ,H. G., and Stark, S. M., Jr., Ibid., p. 461. (162) Olsson, G., and Brandberg, A . , Saensk Kem. Tiduhr., 63, 73 (19pl). (163) Paladini, A. C., and Leloir, L. F., - ~ N . A L .CHEM.,24, 1024 ( I 952). (164) Palilla, F. C., Adler, N., and Hiskey, C. F., Ibid., 25,926 (1953). (165) Pappenhagen, J. M., with Mellon, R.I. G., Ibid., 25, 341 (1953), (166) Parke, T. V., and Davis, W.W., Ibid., 24, 2019 (1952). (167) Passerini, R., and Ross, I. G., I . Sci. Instr., 30, 274 (1953). (168) Patterson, R. F., Keays, J. L., Hart, J. S., Strapp, R. K., and Luner, P., Pulp & Paper Mag. Can., 52, 105 (1951). (169) Peck, R. M., Miller, G. L., and Creech, H. J., J . $m. Chem. SOC.,75, 2364 (1953). (170) Potts, W. J., Jr., J. Chem. Phys., 20, 809 (1952). (171) Pratt, E. L., ANAL.CHEM.,25, 175 (1953). (172) Prokhovnik, S.J., Analyst, 77, 185 (1952). (173) Reiss, W., Hazel, J. F., and McNabb, W. M., ANAL.CHEM., 24, 1646 (1952). (174) Rieders, F., and Gruber, C. &I,, Jr., Proc. Sac. Ezptl. Biol. Med., 77, 684 (1951). (175) Rodrigues, L. D., and Rodrigues, R. D., AVotic.farm. (Port.), 18, 26 (1952). (176) Rose, H. E., iVature, 169, 287 (1952). ( l 7 i ) Rosenbaum, E. J., Ax.4~.CHEM.,24, 14 (1952). (178) Rulfs, C. L., and Meinke, W. W.,J. Am. Chem. Sac., 74, 235 (1952). (179) Scheibe, G., and Fauss, R., Kolloid-Z., 125, 139 (1952). 1180) Schnurmann. R.. Maddams. W. F.. and Barlow. h l . C.. ANAL. CHEM.,25, 1010 (1953). (181) . , Schuhert. W. M.,and Gardner, J. D , J . Am. Chem. Sac., 75, 1401 (1953). (18‘) Heagers, W. J., Neuss, J. D., and Mader, W. J., J . Am. Pharm. Assoc., Sci. Ed., 41, 640 (1952). (IS?,) Shanes, A. M., J . Pharmacol. Ezptl. Therap., 105, 216 (1952). (184) Shi,eve, 0. D., ANAL.CHEM.,24, 1692 (1952). ,(1S6) Shugar, D., Biochem. J. (London), 52, 142 (1952). (186) Shukis, A. J., Cristi, D., and Wachs, H., Soap Sanit. Chemicals, 27, KO.11, 124 (1951). (187) Simonin, Metais, and Weil, Ann. mid. legale criminal., police sci., mdd. aociale, et toxicol., 32, 372 (1952). (188) Sizcr, I. W., and Wagley, P. F., J . Biol.Chem., 192,213 (1951). \
I
Smith, K. C., and Allen, F. W., J . A m . Chem. Soc., 75, 2131 (1953). Sobel, A. E., Goldberg, AI., and Slater, S. R., ANAL.CHEM., 25, 629 (1953). Soloni, F. G., and Marquez. J. F , J . Am. Pharm. Assoc., Sci. Ed., 42, 20 (1953). Stearns, E. I., ANAL.CHEX.,25, 1004 (1953). Stenius, A. S., Ibid., 25, 1572 (1953). Strong, F. C., Ibid., 24, 338 (1952). Ibid., p. 2013. Strong, F. C., Appl. Spcctroucopy, 7, 12 (1953). Sutton, J., Nature, 169, 71 (1952). Swann, R. V., J . Pharm. and Pharmacal., 4, 886 (1952). Sweetser, P. H., and Brirker. C . E., .~NAL.CHEM.,24, 1107 (1952). Telep, G., and Boltz, I).F., Ibid., 24, 163 (1952). Ibid., p. 945. Ibid., 25, 971 (1953). Theorell, H., and Chance, B., Acta Chem. Scand., 5, 1127 (1951) Thomas, T. B., and Schneider, E. E., J . Opt. Soc. Am., 41, 1002 (1951). . Chemists, 34, 498 (1951). Tilden, D. H., J . Aasoc. O ~ c A4gr. Treiber, E., Kleinert, T., and W-incor, K., Holzforschung, 6, 101 (1952). Treiber, E , and Koren, € I . , Ostery. Chem.-Ztg., 52, 108 (1951). Ungar, G., Damgaard, E., and Wong, W.K., Proc. Soc. Erptl. Biol. Med., 80, 45 (1952). Ungnade, H. E., Kerr, V., and Youse, E., Science, 113, 601 (1951). Vallee, B. L., ANAL.CHEM.,25, 985 (1953). Vandenbelt, J. M., Henrich, C., and Bash, S.L., Science, 114, 576 (1951). Targas, B. ill., Anales fac. farm. y bioquim., Unit. nacl. mayor San Marcos (Lima, Peru), 1, 397 (1950). Vendt, V. P., Dvornikova, P. D., and Anina, I. -4..Doklady Akad. Nauk S.S.S.R., 86, 1167 (1952). Vespe, V. C., and h l t z . D. F., . ~ A LCHEM.,24, 664 (1952). Vignau, AI. and Dehodard, AI., Bull. soc. chim. biol.. 34, 831 (1952). Wadelin, C., and illellon, 11. G., - ~ N A CHEM.. L. 24, 894 (1952). Wadelin, C., and 1lellon, 11. G., Anolynt, 77, 708 (1952). Wadley, E. F., and .\nderson, J. A . , ,Jr., t-,8. Patent 2,577,640 (Dec. 4, 1951). Wanless, G. G., Eby, I,. T . , and Rehner, J., Jr., AXAI..CHEM., 23, 563 (1951). Warren, C . W., Appl. Spectroscopy, 7, 16 (1953). Watanabe, K., and Inn, E. C . Y., J . Opt. Sac. Atner.9 43, 32 (1952). Wedgewood, P., and Cooper, R. L., Analyst, 78, 170 (1953). Wedgewood, P., and (’ooper, R. L., Cheniistry &. Industry, 1951, 1066. Whetsel. K. €3.. ASAL. CHEM..25. 1334 (1953). Wiley, R. H., Cagle, A . W., and’TVilken, P. H., J . Am. Oil Chemists’ Sac., 28,459 (1951). Wolff, G., and Wolff, J. P., Bull. mens. inform. ITERG, 6, 379 (1952).
X-Ray Absorption and Emission HERMAN A. LIEBHAFSKY General Electric Co., Schenectady,
T
N. Y.
HE title of the present review has been expanded t o give
explicit recognition t o the increasing importance as a practical analytical method, of x-ray fluorescence-of x-ray emission spectrography, as most analytical chemists will no doubt prefer to call it. Thifi increaEing importance, and the continuing interest in the other methods reviewed here, are evident from Table I, where information is given about three recent symposia devoted exclusively t o x-ray methods. Almost all of the papers of t h e first symposium appeared in A v a ~ y n c . 4C~H E m s T R Y for M a y 1953. Only one of t h e other papers has so far been published (80), b u t those of the third symposium will soon be released as a special technical publication of the American Society for Testing Materials. Inasmuch as the recent literature shows that European activity in u-ray analytical methods is growing, it is somewhat surprising
to find only one reference (106) in the “Proceedings of the International Congress on .4nalytical Chemistry,” Oxford t o the methods under review. Willard, Merritt, and Dean (126) devote a chapter to x-ray methods in the second edition of their book on instrumental methods of analysis. Aireview by Shaw (113) also covers many of the methods c o n d e r e d here. X-RAY ABSORPTION
The field is reviewed here under three headings adopted in earlier reviews (87-91 ). I n conformity with the expanded title, x-ray emission (x-ray fluorescence) is discussed separately. X-Ray Absorption Spectrometry. I n this method, which was originated by Glocker and Frohnniayer (69),t h e absorption of x-rays is measured at wave lengths t h a t bracket an absorption
V O L U M E 2 6 , NO. Table I.
27
1, J A N U A R Y 1 9 5 4
Symposia on X-Ray lMethods
Symposium Date X-Rays as a n Analytical Chemical Tool, Segt. 16-17, 1952 122nd National ACS Meeting, Atlantic City X-Ray Fluorescence Spectroscopy March 4 , 1953 Pittsburgh Conference on Anal$tical Chemistry and Applied Spectroscopy Fluorescent X-Ray Spectrographic J u n e 2 9 , 1953 Analysis, 56th Annual Meeting, American Society for Testing hiaterials, Atlantic City
References ( 7 , 30, 3 1 ) (6, 6, 3%.33)
( 8 ,3)
edge. Qualitative and quantitative results are obtained concomi tantly. A significant step to advance the method may have been made by Drahokoupil (SS), whose work is available here only in abstract form. H e describes a recording instrument in which two x-ray b e a m differing slightly in wave length bracket an absorption edge, so that differential absorbance across the edge can be measured directly. It will be recalled (87) that the Dow x-ray absorption spectrometer has a single beam. The monochromatie (or nearly monochromatic) x-ray beams required in this method are ordinarily obtained either by filtering or by diffracting polychromatic beams. Engstrom ( 4 6 ) in 1947 reported using suitable characteristic lines excited by poll chromatic beams for this purpose; thus, the Kci line of nickel (1.66 .4.) and t h a t of cobalt (1.79 -1).were used to determine iron ( K absorption edge, 1.74 A , ) . Rogers (110, 111) has demonstrated that newly developed beryllium-window x-ray tubes provide beams of such high intensity as to increase the attractiveness of the foregoing scheme for producing monochromatic x-rays. Splettstosser and Seemann (11 7 , 118) independently accomplished the same result, as their published microradiographs show. Engstrom (28) has summarized his important work on the use of long-wave-length x-rays in the assay of biological materials. Though this fascinating activity differs a good deal from analytical chemistry as usually practiced, an examination of references t o Engstrom’s recent work (43,44,46, 51-89, 100) is sure to prove rewarding. The improved historadiographic apparatus described by Clemmons and dprison (34) is a welcome indication that this field of research is receiving increasing attention in the United States. The crystalline form and valence state of an element can be determined in some cases by considering the fine structure on the low-wavelength side of the absorption edge (66, 82 127). Ordinarily, of course, the effect of chemical state on x-ray spectra can be neglected. Absorptiometry with Monochromatic Beams. Some of Engstrom’s work falls in this category. Hughes and Wilczewski (68) use a monochromatic beam of wave length 0.71 A. for the determination of sulfur in hydrocarbons. Coppens ( 3 5 ) discusses the application of this method to analytical problems in mineralogy. Zircon ( Z r a . SiOa) absorbs t h e K a line of molybdenum (0.71 A . ) much less strongly than xenotime (YPOl), and this provides an easy and certain means of distinguishing between two minerals of otherwise nearly identical physical and mineralogical properties. By making measurements a t one or more additional wave lengths, it is possible to distinguish minerals such as the foregoing from others--e.g., sphene, CaTiSi06-that contain no heavy elements. Absorptiometry with Polychromatic Beams. The petroleum industry continues to use the method extensively for the determination of tetraethyllead in gasoline, sulfur in hydrocarbons, and additives to lubricating oils (86, 86, 91, 96, 106, 108). bfottlau and Driesens ( 9 6 ) have estimated t h a t the time saved by making sulfur and tetraethyllead determinations on a General Electric x-ray photometer paid for the instrument in less than 8 months.
The following quotation ( 2 7 ) summarizes the usefulness of t h e General Electric x-ray photometer in the atomic energy program. R i t h o u t disclosing security information we can state t h a t a t present we are making 10,000 heavy metal ion determiriations annually by means of the x-ray photometer and t h a t we expect this number to increase as we find new applications among essential materials, process reagents, and others. We estimate t h a t use of the instrument saves approximately 3000 man-hours annually. This figure is estimated on the basis t h a t analysis of a single solution by x-ray requires 20 minutes, whereas other methods require twice this time. The determination of uranium (8,9, 84,109) is probably typical of those referred to above. Korton (99) used point-to-point exploration with a polychromatic x-ray beam as a means of selecting uniform thin-walled bulbs for experiments on the diffusion of helium through various glasses. T h e use of polychromatic x-rays for thickness gaging and nondestructive testing ( 4 , 54, 70, 71, 92) is expanding rapidly, and only a ferv new examples can be mentioned. The coming 011 t h e market of cadmium sulfide detectors has contributed to this expansion (50, 51, 63, 67, 72-76); one interesting application of such a detector, the measurement of wall thickness on an airvraft propeller, is illustrated in Figure 2 of (91). Ettinger ( 4 7 ) describes a differential thickness gage employing two x-ray beams from a common source, and two scintillation counters as detectorsI n checking the filling of artillery shells, the gage can deteeh and record voids as small as lo-‘ cubic inch in the path of one Ream, if the path of the other is void-free. A third application is described as “a fully automatic x-ray inspection system which checks the internal condition of each orange and routes it into one of six classifications, depending on internal condition, at the rate of 10 a second (@).” But even this is not all-the equipment compensates for outsized oranges too! X-ray absorption by gases is potentialIy important in two kinds of measurements outside the realm of analytical chemistrynamely, t h a t of density in supersonic flow (I%), and t h a t of elevated gas temperatures (125). The latter application, being of more general interest, deserves more attention than it has so fnr received. Finally, Brewster’s compilation of absorption coefficients (gf) of special interest for ceramics, and several articles on coal and minerals (14, 16, 58. 6 6 ) indicate a growing appreciation of methods based on x-ray absorption in two fields where they should prove useful. A recent book ( 6 1 ) dealing with the nondestructive testing of metals places methods involving y-ray and x-ray absorption in correct perspective reIative t o the older ways of accomp!ishing such testing. X-RAY EMISSION
General. At t h e present time, no instrumental nietIiod of analysis is growing in popularity so fast as is x-ray emission spectrography. As with other x-ray methods, the principal factor responsible for this growth is the improvenient in methods for measuring beam intensity, but the availability of stahle x-ray sources powerful enough to yield characteristic (“fluoresc~ent”) x-rays of increased intensity has also contributed. The analytical chemist unfamilar with the method will do well to regard it as analogous to ordinary emission spectrography carried out on a direct-reading instrument. The earlier history is given by von Hevesy (66). Recent reviews by members of t h e Naval Research Laboratory (19, 53), where the method has been under investigation for more than 5 years, are excellent summaries of recent progress, which makes it possible to concentrate on general observations here. The fundamental facts of atomic structure ensure that x-ray emission spectrography will appear to greatest advantage in t h e determination of the metallic elements, either free or combined. In the periodic table, the proportion and industrial importance of
28
ANALYTICAL CHEMISTRY
lines of lighter elements occasionally o v e lap ~ fiist-older L-lines of these elements increase with the atomic number; as everyone some t h a t are heavier. Attainment of high resolution usually knows, the most important nonmetallic elements are found in the means loss of intensity, so that a workable conipromise must be region of low atomic numbers. This region is poorly suited to the sought in equipment designed for analytical applications It is practical application of x-ray emission spectrography because it usuallj advisable t o operate any given equipment a t the harbors serious problems arising from the long wave lengths of maximum intensity and the lowest resolving power satisfactory t h e characteristic x-ray lines. for the job in hand. The general position of x-ray emission spectrographyis strengthened further because certain of the metals to which it is readily Instruments. T h e t u o x-ray emission spectrographs in common use are diffraction instruments modified as necessary (12, IS, applicable-e.g., the rare earths, the platinum metals, hafnium a n d zirconium, niobium and tantalum, tungsten, molybdenum, 93, 94). A helium path is now available to extend the range of uranium-have grown greatly in importance, and remained difthe Philips instrument into the region of low atomic numbers ficult or cumbersome t o separate or determine by wet methods. ( I S ) . Birks ( 1 7 ) has described a vacuum spectrograph built a t the Naval Research Laboratoiy n i t h the same end in view. An Mode of Excitation. Provided the energy requirements are abstract ( 2 0 ) of an article hy Blokhin gives evidence of Russian satisfied, the characteristic lines can be excited by exposing activity in this field. the sample either to electrons, or to x-rays, or to both. Owing t o the great convenience of using sealed x-ray tubes, electron excitation-though first in the field (6Q-i~ virtually forgotten today. It is well to remember, however, that the most precise results recorded in x-ray emission spectrog675 raphy appear to be those obtained on a copperzinc alloy by Eddy and Laby (41) by use of electron excitation. I n precise work, deviations caused by absorption become important. These deviations decrease with the depth of penetration of the exciting beam. Electrons penetrate to, say, 10-6 cm. in the case of aluminum, and less deeply for heavier elements ( 1 2 2 ) . Koh and Caugherty have shown that the measurable penetration by x-rays is more than 200-fold 1 1 1 1 I I I I I 1 1 I ! , j I I , g r a t e r in a typical case ( 7 7 ) . Similar values !OOo 95' 90' 85' 80' 75' 70' 65' 60' 15' SOo 41' 40' 35' 30' 21' 20' 2 8 GONIOMETER R E A D I N G were obtained by Schaal (112). Deviations Caused by Absorption. I n presF i g u r e 1. C h a r t R e c o r d i n g from an X-Ray Emission S p e c t r o g r a p h for ent practice, the intensity of a characteristic a Typical H i g h - T e m p e r a t u r e Allojs-ray line is usually measured by counting. I
