ANALYTICAL CHEMISTRY
724 vestigated and shown to be a real source of error with thick samples. The analysis of this multiple scattering has been extended to include another method of average particle size determination, by plotting the width of the central scattering versus the square root of the mass per area of the sample. This method was applied to carbon blacks (9). Multiple scattering may be eliminated as an error source by the employment of a sufficiently thin sample. Similarly, interparticle interference effects may be reduced or eliminated by sufficient dilution of the sample where feasible. Nonuniformity in particle size and shape are not amenable to simple analysis. It is best, of course, if the material under examination is also studied by other techniques which yield some information on the uniformity of particle size and shape. Inversion procedures for analysis to yield particle size distribution have been described (S),but again not demonstrated in application. The linearity of the gyration radius graph is the simplest indication of particle size uniformity. No serious attempts have been made to analyze particle shape distributions from low-angle Scattering data. APPLICATION
The most widely applied analysis is that which yields the gyration radius. This method of analysis has been applied to catalyst8 ( 1 7 ) ,biological materials, and all manner of colloids. Specifically studies have been made on alumina (18), carbon black (6, 17), coal and coke (21),oil-water emulsions @4), argon (18)and nitrogen (31) near their critical points, hemoglobin ( 8 ) , albumin and other proteins (13),soap and other micelle solutions ( e l ) , evaporated metallic films (6), clays (SO), latex (SC), silica ( d ) , bacteria ( 2 6 ) , gold sols (17, Sd), viruses (12, do), oxides (17, 19), and a few others. Great opportunities for application remain in soils and soil conditioners, industrial emulsions like foods and lubricating greases, catalytic activation and deactivation, paint and ink pigments, cements, and polymers such as latex, and in biology where the bulk of fundamental processes involve macromolecules in that misty land just beyond the reach of the microscope.
LITERATURE CITED
(1) Backus, R. C., and Williams, R. C., J . A p p l . Phys., 20, 174 (1949). (2) Barton, H. M., and Brill, R., Ibad., 21,783 (1950). (3) Bauer, S. H., J . Chem. Phys., 13, 450 (1945). (4) Brusset, H., Bull. SOC. chim., 16, 319 (1949). (5) Brusset, H., Compt. rend., 225, 1002 (1947). (6) Carroll, B., and Fankuchen, I., J . Chem. P h y s . , 16, 153 (1948). (7) Debye, P., Ann. P h y s i k , 46, 809 (1915). (8) Dervichian, D. G., Fournet, G., and Guinier, A,, “Haemoglobin,” pp. 131-4, New York, Interscience Publishers, 1949. (9) Dexter, D. L., and Beeman, W. W., P h y s . Rev.,76, 1782 (1949). (10) DuMond, J. W. M., Ibid., 72,83 (1947). (11) Fankuchen, I., and Jellinek, M. H., Ibid., 67, 201 (1945). (12) Firth, F. G., Rubber A g e , 57, 561 (1945). (13) Fournet, G., Compt. rend., 228, 1471 (1949). (14) Frank, R. M., and Yudowitch, K. L., P h y s . Rev., 88, 759 (1952) (15) Gingrich, N. S.,and Warren, B. E., Ibid., 46, 248 (1934). (16) Guinier, A,, Ann. Phys., 12, 161 (1939). (17) Guinier, A,, Discussions Faraday Soc., 8,344 (1950). (18) Jellinek, M. H., and Fankuchen, I., I n d . Eng. Chem., 41, 2259 (1949). (19) Kikindai, T., Compt. rend., 230, 1772 (1950). (20) Leonard, B. R., Anderegg, J. W., Kaesberg, P., Shulman, S., and Beeman, W.W., J . Chem. Phgs., 19,793 (1951). (21) Philippoff, W., J . Colloid Sci., 4, 169 (1950). (22) Riley, D. P., “The Ultrafine Structure of Coals and Cokes,’’pp. 232-9, London, British Coal Utilisation Research Association, 1944. (23) Ritland, H. N., Kaesberg, P., and Beeman, IT. W., J . P h y s . Chem., 18,1237 (1950). (24) Schulman, J. H., and Riley, D. P., J . Colloid Sci., 3, 383 (1948). (25) Shenfil, L., Danielson, ’iV. E., and DuMond, J. W. M.,J . A p p l . Phys., 23, 845 (1952). (26) Stone, L. L., and Yudowitch, K. L., Pittsburgh Conference on X-Ray and Electron Diffraction No. 11, Pittsburgh, 1950. (27) Turkevich, J., and Hubbell, H. H., J . Am. Chem. Soc., 73, 1 (1951). (28) Vineyard, G. H., P h y s . Rev., 74, 1209 (1948). (29) Warren, B. E., J . A p p l . Phys., 20, 96 (1949). (30) West, W. J., J . Colloid Scz., 7, 295 (1952). (31) Wild, R. L., J . Chem. Phys., 18, 1627 (1950). (32) Yudowitch, K. L., J. A p p l . P h y s . , 20, 174 (1949). (33) Ibid., p. 1232. (34) I b i d . , 22, 214 (1951). RECEIVED for review November 19. 1952. rlccepted January 15, 1953.
Point-by-Point Chemical Analysis by X-Ray Spectroscopy Electronic Microanalyrer RAYMOND CASTAING, University of Toulouse, Toulouse, France, AKD ANDRE GUINIER, University of Paris, Paris, France
T
HE control of the quality of alloys and the development of new ones are obviously based on the knowledge, as complete as possible, of the ultimate structure of the solid. The metallographic microscope shows that most of the usual metals are not homogeneous but are formed by the aggregation of constituents of varying nature: They are differentiated from each other by their aspect in photomicrographs in such a way that it is possible to determine the size, the shape, and the distribution of the grains of the various phases; but the microscope does not give any direct information about a fundamental point, the chemical composition of the metals a t each point. The usual methods of analytical chemistry give only the average composition of domains which contain various phases, even if the attempt is made to cut very small specimens, because the individual grains are often SO minute that there are no practicable means of isolating them from the bulk of the metal. To solve, a t least partially, this major problem in metallography-point-by-point chemical analysis-the electronic microanalyzer has been very recently built in France. The complete description of this apparatus has been now published ( 1 ) and this
paper gives only a brief account of the possibilities offered by the microanalyzer to the chemical analyst. The electronic microanalyzer is able to perform a quantitative chemical analysis of a volume of about 1 cubic micron-Le., a mass of 10-11 gram. The analysis, which can be made in a few minutes, does not alter the sample, and there are no special difficulties involved in measurements a t elevated temperatures. The microanalyzer is a combination of three different pieces of apparatus: a metallographic optical microscope, an electronic microscope, and an x-ray spectrograph. The electron microscope is not a conventional one; its purpose is to give an extremely narrow electron beam n-hich is focused on the surface of the specimen within an area of only 1 or 2 square microns. The specimen is a small piece of metal of any form which must have a surface plane and is polished by ordinary metallographic techniques. This surface is vierred by means of the optical microscope. The specimen can be moved from the outside of the apparatus very accurately in such a way that any point of its surface can be reached by the electrons; these movements are con-
V O L U M E 25, N O . 5, M A Y 1 9 5 3
725
An electronic microanalyzer is described which eonsists of three parts: a metallographic optical mioroseope, an electron microscope, and an x-ray spectrograph. The first is for the purpose of delineating surface structure such as the grains in metals and alloys; the second is for collimating an extremely fine beam of eleotrons to impinge upon areas as small as 1 micron square; the third is to analyze the characteristic x-rays emitted in these small dimensions. The result is a point-by-point chemical analysis, both qualitative and quantitative, for a heterogeneous fine-grained specimen. Several examples especially for metallic systems are cited.
APPLICATIONS
Although the microanalyzer itself is somewhat complicated, its use is simple, so that i t can he regarded as an instrument for routine work. The sample, after polishing, is put in the apparatus and the analysis can begin after a few minutes, just as when an object is examined in an ordinary electron microscope. The measurement itself requires only a few minutes. The sample is not a t all deteriorated by the impact of the electrons, because the power of tho beam is extremely weak. This means that the
Figure 1. Diagram of Microanalyzer
trolled visually with the optical microscope until the point of par. ticular interest is submitted to the action of the electrons. Figure 2. Microphotograph of Copper-TinWhen electrons bombard a metallic surface, x-rays are emitted. Antimony Alloy Showing Three Phases In fact, the apparatus works simply as an x-ray tube, but with a (X250) focus 1,000,000th that of an ordinary tube used for crystallopraphic aork. As the electrons are very rapidly stopped in the metal, the active part of the specimen, which emits x-rays, has a volume of the order of only 1 cubic micron. The wave lengths of the x-rays depend on the nature of the elements bombarded by the electrons; to each element correspond a small number of characteristic wave lengths. So the analysis of the spectrum of the x-ray beam with a spectrograph (the third part of the microanalyEer) permits the rlot.ppmination of the elements present volume of matter touched by the trons. In this manner, point-by chemical analysis of the specirr effected. The analysis can be quantitative if the intensity of one acteristic wave length of a given ell is measured and if this intensity is pared with the intensity emitted re 3. Variation of Composition of Copper-Zinc-Alloy in Region of the same electronic conditions by a Diffusion ple of the pure element. Zn content ~~
ANALYTICAL CHEMISTRY
726
Another of the first applications of the apparatus was the study of the intermetallic diffusion. -4sample formed by a plating of copper and zinc was heated at 400’ C. for 1 hour. The whole diffusion area, as revealed by the microscope, was 0.1 nim. wide. The electronic probe was moved across the diffusion area and 0.411 the copper-zinc diffusion curve was ob0.12 tained in two different xvays. First, with I the spectrometer adjusted to register the 0.22 I radiation of zinc (ZnKa), the measured t 0.135 I intensity represents the variation of the / 003&4 zinc concentration in the sample betn-een 100 x I O 0 JW. 4W. S O P 100% (pure zinc) and 0% (pure copper). The abrupt variations correspond to the ? li d change from one intermediate phase t o the nest, but there are also steady variaFigure 4. Variation of Composition of Copper-Zinc-Alloy i n Region of tions inside a given phase. The data of Diffusion the equilibrium diagram of the two Cu content metals are shown also in Figures 3 and 4. Phase does not appear in the analysis; sample after being analyzed can be used for other measurements in fact, further measurements showed that this phase ip present, or that new analyses can be done a t the same place after various but its thickness is less than 1 micron. The agreement between treatments. the esperimental measurements and the predictions drna n from While the first apparatus, which is now in operation, is able t o the diagram is generally good. detect a limited number of elements only (zinc t o titanium), it is The analysis can also be performed in measuiing the conreneasy to increase this number by a modification of the x-ray spectration of copper across the diffusion area; this is done by changtrograph. But it is difficult to deal with the elements of low ing the adjustment of the x-ray spectrograph to receive the cop:itomic number. The wave lengths of the characteristic radiaper radiation instead of the zinc radiation. Figure 4 gives the tions increase and these long wave lengths are still difficult to deresults of this second measurement. It is easy to check that the tect. New researches are now in progress and it is hoped that the sum of the ordinates of the two independent curves is nt any point microanalyzer will be able to deal M ith all elements follolying aluequal to loo%, within maximum variations of 1%. This is a good test of the basic relations used for the calculation of the minum in the periodic table. TJp t o now the case of metals has been chiefly considered, but it concentration from the observed intensities of the characteristic radiations. seems possible to analyze nonmetallic solids as well. Evamples of application of the microanalyzer include the determination of IoLITERATURE CITED cal variations of the composition of an alloy, and the analysis of precipitates or inclusions of an unknown or incompletely known (1) Castaing, R., “Application des sondes electroniques h une mbthode nature. The possibility cannot be dismissed that for specimens d’analyse ponctuelle,” Office Sational d’Etudes e t Recherche5 involving nonconducting precipitate particles or inclusions in a -4eronautiques, Paris, France, 1952. conducting metallic matrix, the electron beam will be deflected RECEITED for review November 19. 1952. Accepted .January 9. 1953. Work from nonconducting to conducting phases, thus resulting in errodone in the laboratories of the OFfice Sational d’EtudeS et Recherchea neous spectral line intensities. This can be avoided, as proved in Aeronautiques, Paris, France. experiments just completed, by coating the specimen with an exceedingly thin transparent layer of aluminum (of the order of 40 -4.thickness). Figure 2 represents a micrograph of a copper-tin-antimony alloy which, in addition to blocks of antimony about 0.1 mm. wide, contains needles with a dark central area and a brighter rim. The problem was to determine the composition of these needles. A qualitative analysis shows immediately that they contain some copper; quantitatively the conclusion is reached that the central part has the composition CurSn, vhereas the bright rim corresponds to the formula Cu6Snd. The resolving power of the instrument is of the order of 2 microns. This was checked with an aluminum-copper alloy containing grains of the phase A12Cu,the dimensions of which ranged from 1 to 15 microns. If the probe is moved on particles of decreasing sizes, the intensity emitted in the characteristic radiation of c’opper remains constant as long as the diameter of the particle is larger than 2 or 3 microns. For smaller sizes, there is a loss of intensity Tvhich is due to the diffusion of electrons outside the grain of the precipitate. The analysis of the various phases of complicated ternary or quaternary alloy is a difficult problem with the ordinary methods of metallography, and it is very likely that the electronic microanalyzer will be a useful tool in this field.
....
T
’%
