Reece, I. H., Roper, R., Werner, R. L.,
(64) Mason. S. F., J. C h a . SOC.1958. 3619. (65) hfecke, R., Noack, K., Spectrochim. Acta 12, 391 (1958). (66) Milkey, R. G., ANAL. CHEW 30, 1931 (1958). (67) Miyazawa, T., Shimanouchi, T., Mizushims, S., J. Chem. Phys. 29, 611 1958). (68) hoccia, R., Thompson, H. W., Spectrochim. Acta 10, 240 (1957). (69) Mur hy, J. E., Schwemer, W. C., ANAL. &EM. 30, 116 (1958). (70) Naves, Y. R., Perfumery Essent. Oil Record 49, 290 (1958). (71) Novak, A., Whalley, E., Trans. Faraday SOC.55, 1484, 1490 (1959). (72) O’Connor, R. T., DuPre, E. F., Mitcham, D., Teztile Research J. 28, 382 (1958). (73) O’Dwyer, M. F., J . Mo!. Spectroscopy 2, 144 (1958). (74) Padgett, W. M., Hamner, W. F., J. Am. Chem. SOC.80.803 (1958). (75) Perry, J. A., ANAL.-CHEM. 31, 1054 (1959). (76) Pimentel, G. C., hlcclellan, A. L.,
Australian J . Chem. 11, 92 (1958). (60) Liang, C. Y., Krim. S., J. Polymer Sci. 27, 241 (1958). (61) Lutinski, C., ANAL. C ~ E N .30, 2071 (1958). (62) McDonald, R. S., J. Phys. Chem. 62, 1168 (1958). (63) Manning, J. J., Bull. Narcotics, U.N., Dept. Social Affairs 7 , 85 (1955).
263 (1958). (78) Powell, W. R., WADC Tech. Note 57413, (1958). (79) Rao, K. N., Ryan, L. R., Nielsen, H. H., J. Opt. SOC.Am. 49, 216, 221 ( 1959).
(48) Halman, M., Pinchaa, S., J. C h a . SOC.1958, 1703. (49) Harkins, T. R., Harris, J. T., Shreve, 0. D., ANAL.CHEM.31, 541 (1959). (50) Helme, J. P., Moiines, J., Peintures, pigments, vernis 33, 524 (1957). (51) Hershenson, H. M., “Infrared Ab-
sorption Spectra,” Academic Press, New York, 1959. (52) Hill. R. D.. Meakins. G. D.. J. Chem. Soc. 1958, 760. (53) Jenkins, J. W., Kellenbach, K. O., ANAL.CHEM.31, 1056 (1959). (54) Jones, R. N., Augdahl, E., Nickon, A., Roberts, G., Whittingham, D. J., Ann. N . Y . Acad. Sci. 69, 38 (19571. (55) Jones, R. N., Nadeau, A., Speclrochim. Acta 12, 183 (1958). (56) Kagarise, R. E., Mayfield, J. W., NRL Rept. 5088, (1958). (57) Kasatochkin, V. I., Konova, M. >I., Silberbrand, 0. I., Doklady Akad. Nauk S.S.S.R. 119, 785 (1958). (58) Kuzminskii, A. S., Borkova, L. V., Zhur. Priklad. Khim. 31, 648 (1958). (59) Le FBvre, R. J. W., Oh, W. T.,
Review
of
“The Hydrogen Bond,” Freeman, San Francisco, 1960. (77) Plyler, E. K., Tidwell, E. D., J. Research Natl. Bur. Standards til.
(80) Schweyer, E., ANAL. CHEM. 30, 205 (1958). (81) Sheppard, N., “Hydrogen Bonding,” Pergsmon, London, 1958. (82) Simon, A., Mucklich, M., Kunath, D., Heintz, G., J. Polymer Sci. 30, 201 (1958). (83) Smith, A. L., McHard, J. A,, ANAL. CHEM.31,1174 (1959). (84)Spurr, R. A., Byers, H. F., J. Phys. Chem. 62, 425 (1958). (85) Stephens, E. R., Appl. Spectroscopy 12, 80 (1958). (86) Stone, J. P., Thompson, H. W., Spectrochim. Acta 10, 17 (1957). (87) SUSl, H., ANAL. CHEM. 31, 910 f1959’1.
(8Sj Susi, H., Koenig, N. H., Parker, W. E., Swern, D., Zbid., 443 (1958). (89) Susi, H., Rector, H. E., lbid., 30, 1933 (1958). (90) Tanner, K. N., King, R. L., Nature 181, 963 (1958). (91) Ward, W. R., Philpotts, A. R., J . Appl. Chem. (London)8, 265 (1958). (92) White, J. U., Alpert, N. L., Weiner, s., Rev. Scz. Instr. 29, 511 (1958). (93) White, J. R., Jr., Eddy, C. R,. ANAL. CHEhf. 30, Petty, J., Hoban, s., 507 (19;s). (94) Willis, J. B., Revs. Pure A p p l . Chem. 8, 10: (1958). (95) Wilson, E. B., Jr., Spectrochim. Acta 12, l(1958). (96) Yamaguchi, A,, Miyazan-a, T., Shimanouchi, T., Mizushima, S., Spectrochim. Acta 10, 170 (1957).
Fundamental DeveloDments in Analvsis
X-Ray Absorption and Emission Herman A. liebhafsky, Earl
H. Winslow, and Heinz Pfeiffer
General Electric Co., Schenecfady,
B
N. Y ,
the literature that is important t o these reviews continues its accelerated growth, the following have been eliminated as topics for review: microradiography, x-ray microscopy, nondestructive testing, y-ray methods. Equipment, not covered as a separate topic in 1960, will probably be reinstated in the future. This article follows the plan that has evolved in the earlier review (122-126, 123, 150). hfajor sources of information in the recent general literature are: books wholly devoted to x-rays (67, 63, 70, 89) ; books partially devoted to x-rays (63, 101, 112, 145, 187, 205, 210, 212); tabulations of data (55, 82); and abstracts of x-ray papers presented at conferences and meetings (5-7, 11, 12, 58,70, 71, 110). I n addition, there are the following general articles, most of which deal with x-ray emission spectrography (18, 26, 23, 32, 40, 59, 78, 94, 127, 161, 208, 223). ECAUSE
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Two x-ray books that have been written for analytical chemists, one a t the S a v a l Research Laboratory and the other a t the General Electric Research Laboratory, should appear in 1960. One may hope that the appearance of these books will help to stabilize, with consideration for the analytical chemist, the confused and proliferating nomenclature in the field under review. SCATTERING
While x-ray methods were coming of age in analytical chemistry, it sufficed to know that atoms do scatter x-rays, and that the importance of this scattering increases relative to photoelectric absorption as the atomic number decreases. The time has now come to consider scattering in greater detail. After the scattering, the wave length of the x-rays either has remained unchanged (unmodified scattering) or has been increased (modified scattering)
by the Compton effect. The amount of the shift, 6X, to the center of the broad peak produced by Compton scattering is given by the equation (62): 6X = 0.0243 ( 1
- COS 6)
where 9 is the angle between the incident and the scattered beam. The shift 6x in wave length during modified (Compton) scattering is independent of wave length and of the nature of the atoms responsible for the scattering; it depends only upon the geometry of the u-ray optical system. However. the intensity of the scattered beam relative to that of the incident beam increases with decreasing atomic number in the scattering material and with decreasing wave length of the x-rays. On the long wave length side of any x-ray beam that interacts with matter, one may therefore expect to find a component due to Compton scattering, and this component needs to be considered as a
source of possible interference in analytical work. In a well-planned investigation, Johnson and Stout (107) have shown t h a t Compton scattering does occur as expected and that i t can strongly influence the x-ray emission results when, for example, plants and soils are analyzed. Because Compton scattering is predictable, the means of minimizing i t are clear. Dodd (74) reports objectionable Compton scattering in his work with the absorption-edge method. Ziegler, Bird, and Chleck (222) have thoroughly discussed the possibility of using unmodified scattering, which they choose to call Rayleigh scattering, as a means of detecting heavy elements in a light (low-Z) matrix. Simplicity of the x-ray optical system is an outstanding feature of the method. Though not specific, it may find uses in routine applications such as process control. T h e n it does, it will be reviewed more completely. ABS0 RPTION
Fine Structure. The fine structure of absorption edges is of growing importance in analytical chemistry. Absorptiometry of this kind differs from the absorptiometric methods of the earlier reviews mainly in the much higher resolution i t demands. Absorptiometry with polychromatic beams needs no spectral resolution a t all. Most monochromatic beams of the earlier reviews have wave length ranges measured in hundredths or tenths of angstroms. For the fine-structure determinations, the best resolution attainable is none too good, and t h a t of the best commercial instruments (beam width about 0.001 A.) is barely adequate. Traversing an absorption edge at high resolution requires a great many measurements; fortunately, x-ray spectrographs that are commercially available opcrate automatically so that they can present the investigator each morning with a chart containing many results for an experiment begun the night before. The fine structure of the absorption edge can give information about chemical bonding-about valence states, for example (PO, 21, 202, 203). This statement does not conflict with the broad gcneralization that x-ray absorption (measured a t low resolution) is independent of the chemical state. Van Nordstrand’s work (206), undertaken in connection with the catalysis prohlems of the petroleum industry, seems closest to practical application. Absorptiometry with Monochromatic Beams. Mackay (144) proposes an elaboration of conventional absorptiometry. I n his scheme, intended especially for the life sciences, a sample in which n constituents are sought would be traversed by a
series of monochromatic beams t h a t also travel through a series of wedges, each of which has the composition of one constituent in the sample. Each wedge moves independently of the others to maintain the intensity of each beam constant a t the detector during a scanning of the sample. A photographic record of the wedge motions leads to a composition map for the sample. A simpler scheme of the same kind has been used by Jacobson (106) to establish the distribution of iodine in a human thyroid gland. Lublin (138) has used absorptiometry with monochromatic beams to analyze nickel-aluminum diffusion couples, and Ogilvie (163) has applied the method to others. Sources that produce monochromatic x-rays by K-capture (129,130)continue to grow in popularity for absorptiometry in the laboratory (64, 154, 186, 218) and in the plant (23). Muller and coworkers (155-157) are continuing their interesting investigation of beta rays from radioactive isotopes as agents for exciting the characteristic lines of metallic transmission or reflection targets. Doughman, Sullivan, and Hirt (76) have compared x-ray emission spectrography with absorptiometry for the determination of sulfur in oils. The absorptiometric method, based upon the use of iron-55 as a K-capture source according to Hughes and Wilczewski ( l a g ) , proved superior in requiring a lower capital investment and less operator training and skill. The emission method proved more reliable on samples with unknown matrices. Precision and time (about 10 minutes) were comparable for both methods. Dodd (73) has strengthened the absorption-edge method (76, 130), by using pulse height selection (149) in connection with it. The problem here is to make intensity measurements on either side of an absorption edge so that the amount present of an element can be deduced from the magnitude of the jump in the sample a t the edge. Dodd has found that pulse height selection by means of a very narrow (0.1 volt wide) window gives differential curves narrow enough so that simple measurement of differences in the peak heights of these curves leads directly to the amount present of the element sought.
Absorptiometry with Polychromatic Beams. As the earlier reviews show, this method has found such a diversity of applications t h a t one can scarcely expect it t o yield many surprises in the future. Hanson, Flynt, and Dowdey (fl) have introduced two techniques into absorptiometry in order to simplify the taking of data and to eliminate the effect of intensity variations in the x-ray source. Instead of relying on a beam
chopper, they achieve the second objective by oscillating the sample in and out of the beam, a more sophisticated technique than an earlier primitive commutation between sample and standard ( 2 2 f ) . The new technique includes channeling the Io and I beams into two ratemeters, from the output of which a quantity proportional to the absorption coefficient is automatically calculated and plotted. The second technique has two detectors arranged in tandem with the sample between, and it yields Io and I simultaneously. Plant (1%) has a new feature in a composite wedge for a recording photometer designed for the determination of fuel content and distribution in 8 fuel element core of fixed thickness in which uranium oxide is dispersed in a matrix of stainless steel. The composite wedge is composed of two wedges arranged head to tail, one wedge of uranium and the other of stainless steel. The fuel content of a core of fixed thickness can be determined after suitable calibration by establishing in a split-beam photometer the location on the wedge that matches the core in x-ray absorbance Lambert (118) describes a new automatic photometer designed a t Hanford. Extensive experience in the examination of plutonium-aluminum fuel plates showed the x-ray photometer to be the best instrument for this purpose. All plates for the Materials Testing Reactor were scanned in a prototype photometer, only 2 minutes being required for a 2Pinch plate. A polychromatic beam of effective wave length 0.413 A. has been found satisfactory by Vose (207) for the rapid determination of the organic-inorganic ratio in bone tissue. Brown and Keir (41) describe a compact photometer for the determination of tetraethyllead in gasoline. Knight and \-enable (114) have improved, partly by adding a heavy rare gas as diluent, the method of Kistiakowsky (129, 130), in mhich the absorption of long wave length x-rays is used for the study of shock and detonation waves in gases. The use of polychromatic beams in thickness gaging continues to expand (10, 24, 148, 164, 169). The x-rays (Bremsstrahlung) made by bombarding a p e t a l with beta rays from a radioactive isotope (64) are finding new applications (80, 196). Control of U. S. coinage has been entrusted to x-rays a t the Denver Mint (79) and this has resulted in fewer rejects and increased production-no doubt an important step in this age of inflation. The mint’s problem is more interesting than most. A single mill must accommodate thicknesses ranging from 0.038 to 0.196 inch and the varieky of alloys (gold alloys apparently excepted) in our coins. The superiority of automatic over manual control is clear: VOL 32, NO. 5, APRIL 1960
241 R
Studies show that the new automatic controls make 10 times as many major and minor corrections during rolling as the mill operator can. The electronic circuits analyze the changes of thickness, predict the approach of signifkant change, and actuate the controls to forestall it, thus permitting compensation for variations before they are apparent to the operator. X-RAY EMISSION SPECTROGRAPHY
General. Sherman (189-191) is continuing his mathematical treatment of the intensities of characteristic lines emitted by mixtures with progress toward simpler expressions. Burnham, Hower, and Jones (42) have attempted to base a general scheme of analysis upon Sherman's earlier work (188) and used IBhI equipment to calculate the interaction coefficients required in setting up the methods. hnother novel feature is the introduction of a geometrical factor that makes it possible to handle samples of varying shapes (plate, wire, chips, turnings, and filings) with equal ease. The method \v&s tested successfully on a sample of National Bureau of Standards chips. The state of the sample has been the concern also of Friedlander and Goldblatt (88), who carried out a comparison of precision for solid, liquid, and powder sampling techniques as applied to high-temperature alloys. Mitchell (151) has worked out a general method of arithmetical corrections for the absorption and enhancement effects encountered in the combined oxides of tantalum, niobium, iron, and titanium prepared by cupferron precipitation from solution and subsequent ignition. The correction factors are established by measurements on standards similarly prepared. The correction scheme, though necessarily complex, gives good results and is applicable to a surpisingly wide range of compositions. The scheme would seem to qualify for extension to other routine cases-routine, in that many determinations must be carried out and that these are immune to unpleasant surprises caused by the presence of unsuspected elements. Andermann and Kemp (15) have made the important suggestion that scattered x-rays be used as internal standards in x-ray emission spectrography. Provided a suitable scattered wave length is selected, and this must be done empirically, this use of the scattered x-rays has much in its favor. It is certain to be better than using no internal standard. As their first two figures show, analytical-line intensity variations caused by variations in tube voltage, tube current, sample position, and particle size virtually disappear when each such intensity is divided by that of x-ray scattering in the region 242 R
0
ANALYTICAL CHEMISTRY
of 1.4 A. Whether scattered x-rays are in general as good standards as the best internal standards that can be added to the sample is another question. In any case, the greater convenience of the scattered x-rays and the considerable latitude permissible in their empirical selection as standards make them certain to prove useful in many applications. The ASTM Task Group (8, 95) has reported as follows: Of the four factors examined the following contributions to precision were found in terms of standard deviation: resetting operating conditions of the x-ray tube, 0.5%; repositioning the x-ray spectrometer a t the x-ray line peak, 0.5% or less; estimating composition under the most adverse conditions, 10% of amount present (an error of about 2% composition was obeerved to be rather constant over a wide range of composition); precision of mixing powders to a nominal composition, 7%. I n addition, the statistical nature of the x-ray process was checked, and experiment found to be in agreement with theory. Felten, Fankuchen, and Steigman
(83) have extended the method of PfeiEer and Zemany (168) to obtain a way of eliminating absorption and enhancement effects. By making identical spots of the sample on a series of filter papers, and then measuring analytical-line intensities of stacks containing different numbers of spotted papers, data for extrapolation to zero' thickness of spotted paper are obtained. I n the extrapolated value, line intensity or ratios of line intensities are proportional to composition. The general considerations pertaining to this method are those governing the determination of film thickness by x-ray emission spectrography (130). I n an extension of earlier work (128), it has been shown that considerstions stemming from probability theory and from the theory of errors govern the best precision attainable in x-ray emission spectrography even when a significant background is present (219). I n support of this statement, it was demonstrated that 90 results for a spot containing less than 0.2 y of zinc lay upon a theoretically predicted Gaussian curve, and that 216 results for another spot containing 4 y of strontium lay upon another such curve. Applications. Work on elements in solution had the follovhg points of particular interest. The x-ray emission method for plutonium (88) is to be preferred over chemical methods because these are complicated by the existence of various valence states of the element in solution; i t is also preferred over radiometric methods because these are atTected by variations in isotopic content. In the determination
of manganese in gasoline, Jones (105') used an iron rod as a convenient conipensative reference to serve in place of an internal standard. In the course of a year's experience, x-ray emission spectrography as a routine method was found to give results as reliable as the ASTM methods for sulfur and chlorine in petroleum liquids (216). The much shorter time (5 minutes for each element) contributed to a marked reduction of analytical costs and to improved refinery control. Bromine is a satisfactory internal standard for the determination of tungsten in alkaline solution (81), and tungsten serves as a built-in standard for the determination of molybdenum in ores of known tungsten content. The precision of this determination is considerably improved if the two elements are determined in solution. Dilution with silica (87, 106) of an ore in order to reduce absorption and enhancement effects in minerals is reminiscent of the work of Claisse (130). The application of x-ray emission spectrography to the analysis of product streams is described by Goodwin (go), and the feasibility of the method for continuous process control is discussed by McCune, Mueller, and Dunton (143). Two specific industrial applications will be mentioned. The first, by Eastmsn Kodsk (65), is the use of a specially designed automatic spectrograph that measures the mass concentration of silver in photographic emulsion-very like a determination of the film thickness of silver (130). The second is the impressive use of an automatic spectrograph for production control of alloys by Haynes Stellite, as described by Wittig (213). The following quotations sketGh the s t o r y : Within 4 hours after installation, standards were fed in to prove the analysis cycle. These standards were the same as had been used on the standard Norelco spectrometer. One half hour later, the performance of the Autrometer having been checked, the first unknowns from the Haynes Stellite rolling mill were surface-ground and analyzed. Currently, 47 different alloy grades are being analyzed by the x-ray spectrograph for 11 different elements whose concentrations vary from 0 . 5 to 30 weight %. On an average, 700 determinations per month are accomplished by the x-ray method. The highest monthly output has been 2200 element determinations. Our determinations show each analysis by x-ray to cost approximately $0.90,and completion time to be less than 20 minutes. A wet lab analysis averages $1.96 and takes 2 t o 4 hours to complete. The x-ray method as applied a p pears more accurate in almost all cases than the results obtained by wet chemistry.
The Autrometer is not the ultimate answer to a production laboratory, but it does supply analyses for more chemical problems than any other equipment on the market.
It is interesting t o compare this account in detail with earlier work (97) both as regards precision attained and the rapid progress being made. Trace Determinations. This seems a more precise description of x-ray emission spectrography on atoms relatively fen- in number than does microanalysis. Trace determinations divide themselves logically into those where the traces are major constituents (samples small in mass) and those n here the traces are minor constituents (qaniples possibly large). The first class is governed by the considerations involved in determining film thickness (130) and is exemplified by n-ork n i t h spots (85,168, 219); the second differs from the first in that deviations from proportionality on in6 to absorption and enhancement effects (130)are much more likely. Of the papers presented a t the important Second International Symposium, Stockholm (110), 15 belong in this section, but they will not be reviewed until the symposium proceedings have Iwtn published. Campbell (45) has recently discussed trace determinations. Natelson in his Report for Analytical Chemists (160), which deals with microanalysis in clinical chemistry, calls x-ray emission spectrography one of the most promising developments of recent years for determining elements in biological samples. Satelson and Bender (161) have refined the spot method mentioned above to the point where total calcium, chlorine, sulfur, and potassium could be determined directly on 0.02 ml. of serum. Lazar and Beeson (119) and Brandt and Lazar (35) have devised methods for various elements in plant materials. Scattered x-rays proved useful as an internal standard. Ion exchange membranes continue to he useful for collecting traces prior to their determination (92). Zemany, Kplbon, and Gaines (220) in this way collected and determined from 5 to 150 y of potassium from 7 5 ml. of aqueous solutions contained in polyethylene, the ultimate purpose of the work being to measure the amounts of potassium derived from various mica samples. Thorium from Zircalloy was collected and determined by Horton and Moak (45, l o g ) , and the Bureau of Nines has used the technique for copper ( 4 ) . Routine trace determinations have been carried out for some years a t Oxford in the Research Laboratory for Archaeology and the History of Art. Hall (94) describes a particularly interesting case in the following quotations:
One of the first problems presented was the analysis of the Piltdown skull fragments and flints. The question was whether the bones and flints had been artificially stained by an inorganic pigment in an attempt t o imitate the ferruginous colour imparted t o buried objects in the locality in which they were reported t o have been found. The conclusion finally reached was that the forger simulated true buried objects by boiling modern ape bones in potassium dichromate solution. The author has tried this and has obtained very similar results.
For reasons given in the 1958 review, this section concludes with a literature index, Table I, that is intended to supplement the 1958 Table VI (130). COMMERCIAL X-RAY EMISSION SPECTROGRAPHY
Because x-ray emission spectrography appears to excellent advantage over other methods in the analysis of minerals, a brief description of procedures a t the Fluo-X-Spec Analytical Laboratory of Denver will be given (178-182) to illustrate the operation of a commercial laboratory. Extensive experience (about 30,000 samples-standards included-since 1956) is proof that the procedures are rapid and successful. The work of the laboratory is qualitative, semiquantitative, and quantitative (least in demand). The relative advantages of x-ray emission spectrography are greatest in the first two categories. At present, a IOO-kv., constant potential, inverted spectrograph (2001) is used for most analyses. The high potential makes possible the excitation of the R spectra (more simple than the L ) of the heavier elements such as the rare earths. After being reduced in a crusher, samples are ground to about 200 mesh in a mortar. Silica sand is run between samples to avoid contamination. A portion of the ground sample is placed in the depression of a plastic bottle cap and covered with thin (l/Cmil) Mylar, which is clamped in place by forcing a close-fitting curtain ring over the outer rim of the plastic cap. The sample thus contained fits drectly into the sample holder of the spectrograph and is ready to be filed for reference (&year retention) after being run. Being a loose powder, the sample lies flat on the Mylar when the inverted cap is placed in the spectrograph. For qualitative analysis, scanning in the spectrograph is begun just above the 28 angle of titanium K , at 4 degrees per minute. When the scan is completed (automatic s h u h f f a t 28 = 3"), the completed chart is placed on a centrally illuminated translucent plastic drum on which are inscribed not only t h e import a n t analytical lines, but also important
interfering lines, such as those of higher order. The inscriptions are spaced to correspond with standard chart speed. Alignment of a n unknown chart by means of known lines (tungsten spectrum) then permits easy identification of the elements (heavier than titanium) in the sample by use of their characteristic lines. I n semiquantitative work (about f 10% of the amount present), scanning is restricted to the region of the analytical line. The estimate of amount present is based upon the intensity
Table 1. References to Recent X-pay Emission Spectrography Work on Various Elements
Aluminum (13, 14, 56, 64, 113, 116, 117, 198)
Antimonv (L5.120i k e n i c (57,'lkO) ' Barium ( 3 , 6 4 ) Bromine ( S , 46, 143) Cadmium (91) Calcium (3, 13, 14, 46, 4Y,56, 68, 160, 161) Chlorine (46, 113, 160, 161, 216) Chromium ( 3 , is, 16, 22, 42, 57, 64, 68, 91, 100, 103, 109, 21.9)
Cobalt (3,S5, 46, j Y , 83, 100, 213) Copper (3, 15, 45, 5Y, 68, 83, 91, 95>119, 150)
Germanium (3, 45, 166) Gold (91) Hafnium (45, 1.40, 159, 166) Iodine (46, 161, 165) Iridium (22, 93) Iron ( S , 9, l S , 14, 16, 22, 48, 46, 56, 57, 60, 68, 85, 91, 96, 100, 103, 134, 151, 166. 175. 189. 19L. 198. 213) Leach( 3 , 15, 91,'96, 143, 159, 166) Magnesium (IS,l 4 , 5 6 ) Manganese (3, 13, 35, 46, 56, 68, 100, 108, 213) Mercury ( i 7 7 ) Molybdenum (35, 60, 64, 68, 81, 85, 109, 119.615) 1 19,llS) Nickel (S, 15, 16, 22, 42, 45: 46, 57'3 68, Y 7 , 91, 100, 103, 109, 134,135, 813) Niobium (46. (46, 68. 68, 87, 109, 140, 151, 159 166. SlSi 81s) ' Palladium '( 95) Phomhorus f l S . 1L. 113. 161) I
- - - I
I
.
plst&Um(93, iii j ' Plutonium (86, 117 , 804) Potassium (56, 115, 160, 161, 220) Rare earth elements (19,58, 141,142, 150, 166,182)
Rhodium 193) Ruthenium ( 8 5 ) Scandium (99) Selenium ( 2 2 , 91, 166) Silicon ( I S , l 4 , 5 6 , 68,113, 198) Silver (55,91) Strontium (3, 55, &, 159, 176, 219) Sulfur (SO, 61, 75, 113, 160, 161, 216) Tantalum (45,100,1401151,159,213) Tellurium (166) Thorium (22, 45, 46, 100, 115, 141, 159, 195) Tin (16, 64, 68, 91, 166, 211) Titanium (56, 64, 87, 100, 105, 151, 97, 818)
Tungsten (68,81, 170, 21s) Uranium (46, 46, 60, 104, 115-117, 143, 169, 159, 198, 194) Vanadium (SO, 4.6, 67, 100, 135, 213) Yttrium (16, 98,141-143,159,184,193) Zinc (3,36, 67, 68,91, 96, 160, 166, 189, 819)
Zirconium (&%?,&, 100,1S6,l4O, 169,19S)
VOL 32, NO. 5, APRIL 1960
243 R
NEGATIVE D.C. X-RAY
CONVENTIONAL
4 Figure 1.
Schematic diagram of electron microprobe with auxiliaries omitted
TUBE
n
STAL 0CURVEMRY SPECTROMETER
\
SAMPLE
APERTURE
LENS X-RAY
i p + - . . - -
PROPORTIONAL COUNTER
SPECTROMETER
CRYSTAL
0
FINE-FOCUS X-RAY TUUBE
CURVED-CRYSTAL SPECTROMETER
/
/
SAMPLE
OPTICAL ^^ >COPE
...-/’ / X - R A Y
of the peak above background for the analytical line relative to the intensity of the adjacent background. As the background normally is due to scattered x-rays, this amounts to using scattering as a n internal standard (16). The relative intensity for an unknown is compared with that of an appropriate synthetic standard in order to give the required estimate of amount present. In many of the quantitatiye analyses, absorption and enhancement effects are present. Absorption effects can be allowed for after the absorption coefficient for the analytical line has been measured on a thin.section of the sample (183). MICROPROBES
During the last 2 years, devices permitting increased spatial resolution seem to have received more attention than any other instruments in which x-rays are used. I n conformity with the earlier reviews, these devices are called microprobes for the point-to-point exploration of surfaces. It seems unwise to call them microanalyzers: they seldom give composition, nor is it usually certain what is the sample they analyze. The second uncertainty arises not only from a lack of definition in the surface being explored, but also from an uncertainty in the effective depth of the sample. Especially when an electron beam is used, this depth can be so small as to give the surface of the sample an unusually important influence on the intensity of the emitted x-ray spectrum. Exploration is important in its on-n right; it should not be confused with analysis even when it yields quantitative results. The more common microprobes are shown schematically in Figure 1 (x-ray emission electron microprobe, or electron microprobe for short) and in Figure 2 (x-ray microprobes). An obvious difference between the two kinds of probes is in the method of 0
APERTURE
Figure 2. Schematic diagram of two x-ray microprobes
OETECTOR
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X-RAY SPOT
ANALYTICAL CHEMISTRY
exciting the characteristic line: electrons (Figure 1) us. x-rays (Figure 2). The electron microprobes are receiving somewhat greater attention a t present. Because all microprobes act as highly divergent (point) sources of the characteristic hnes, the x-ray spectrometer usually has a curved crystal to focus the line on the detector. X-Ray Microprobes. Several microprobes have been reported (4, 13.9, 199) which use a small diameter x-ray beam to excite a characteristic spectrum in the sample. These probes have the advantages of permitting work on samples outside of vacuum, and of avoiding electrical charge build-up on poorly conducting samples. The resolution, limited by the size of the x-ray spot or by the aperture (Figure 2), is considerably poorer than the best obtained in electron microprobes. The simpler type of x-ray probe (Figure 2) (4, 199) uses a standard OEG-50 x-ray tube and a pinhole system to generate a narrow x-ray beam. The resolution, though limited by the size of the pinhole, was adequate to prove that a small amount of metallic inclusion in a silver-containing mineral was arsenic-copper, not arsenic-silvercopper (4). Some other interesting applications (200) of the x-ray microprobe have been the identification of iron as an inclusion in high purity vanadium, identification of the inhomogeneities in iron-aluminum alloys, and measurement of surface enrichment of arsenic in arsenic-lead samples. Long and Cosslett (133) and Zeitz and Baez (217) used fine-focus x-ray tubes (36)to obtain high x-ray output over a small area of a very thin target. The x-rays leave the target by transmission (Figure 2) and excite the sample. With a thin sample as shown, it is possible to measure both the x-rays excited in the sample, and x-ray absorption by the sample (66, 132, 155). It is also possible to scan the sample surface with the x-ray beam by deflecting
(See Figure 1 for additional detail on spectrometers.) The fine-focus tube can be used for emission measurements without spectrometer (see dotted line), for absorption measurements with spectrometer, or for emission measurements with spectrometer displaced to avoid transmitted beam
the electron beam in the finefocus x-ray tube. Electron Microprobes (Figure 1). The electron microprobe first described by Castaing (61) has proved increasingly useful and has stimulated the development of similar laboratory (27, 31, 43, 66, 72, 84, 168,186,215) and commercial instruments (Table 11). All of the microprobes contain three standard components: an electron-optical system (usually that of an electron microscope) , an optical microscope, and an x-ray spectrometer. The more advanced instruments have elaborate positioning devices, provide for multichannel x-ray emission measurements, for surface scanning by the electron beam, and for measurement of the electron back-scattering by the sample. The electron-optical system demagnifies the electron source to a spot on the sample. The optical microscope is used to select and identify the area to be probed by the electron beam. The x-ray spectrometer resolves and measures the characteristic lines from the sample. Because the electron beam must operate in vacuum, the instrument has to be provided with a system of ports and vacuum pumps similar to that of the electron microscope. For best results, the electron-optical system must meet the following requirements. The electron beam current and voltage must be well-stabilized, the lenses must have low spherical aberration, and the objective (final) lens must be so arranged as to minimize the effect of magnetic samples on the electron beam. Su5cient clearance must also be provided a t the final lens to provide a path for the x-rays to the spectrometer, and to allow optical viewing of the sample.
~~
~~
Table It.
NO. 1 2 3 4
5
ManufacturerMetropolitan-Vickers (AEI Research Laboratory) Philips Electronics Cambridge Instrument Co. (Cosslett and Duncumb) Intercontinental Electronics Corp. (Castaing) Applied Research (Wittry)
Laboratories
NO. 6 1
~
Commercial Electron Microprobes
Samples Max. size '/pinch cubes 2
&v;
x
1/4
40 samples, or will take a sample up to X X inch 8 1-inch diameter, $/IS inch thick Comments
Beam Diameter,b Voltage, hlicrons Kv. 10-50 2
10-50
1
4-60
1
8-35
0.5
1-50
X-Ray Optics 1 curved-crystal spectrometer, LiF
crystal Maximum of six preset curved-crystal spectrometers; LiF, ADP, EDDT Curved-crystal spectrometer; LiF, gypsum 2 curved-crystal spectrometers; quartz, mica
3 curved-crystal preset spectrometers;
LiF, quartz, ADP
Sample viewed out of electron beam. Helium or vacuum path in one spectrometer; a scanning spectrometer may be used. Scanning and visual presentation of results. Scanning; diffraction pattern recorded photographically. Large (52.5') take-off angle for x-rays; vacuum spectrometers. Scientists reputedly associated with manufacturers are named in parentheses. * Some manufacturers seem to give the effective beam diameter, others the actual. The best resolution is 1 micron at voltages that nil1 excite the x-ray spectra of most elements. 1 2 3 4 5
The optical microscope must not interfere with the electron beam and must be good enough to resolve detail of the order of the electron beam diameter. Interference with the beam is usually avoided either by using mirrors or by rotating the sample mounting out of the electron beam for optical viewing. With the latter arrangement, viewing is not possible while the electron beam is on. Owing to the relatively low total x-ray intensities, the analyzing crystal is put as close to the sample as possible. Conventional detectors (gas-flow proportional counters. scintillation counters,and Geiger counters) are used. Most of the microprobes have provision for a t least two x-ray spectrometers, so that optics can be left in place for both light and heavy elements. Cosslett and coworkers (66, 147) have incorporated two nen- features in the latest versions of their microprobe: scanning of the surface with the electron beam and measurement of electrons backscattered from the sample (181). By synchronizing the sweep of one cathoderay tube with the x-ray output, and that of another with the electron output, a visual presentation of the composition and the topography of the scanned area is achieved. Scanning has also been incorporated into the latest model of the Castaing microprobe. The commercial instruments with their main characteristics as given by the manufacturers are listed in Table 11. The prices range from about $25,000 (No. 1) to $125,000 (No. 4). I n favorable cases, the electron microprobe can give highly reliable quantitative results. Often, however, the results are qualitative or semiquantitative
for one or more of the following reasons: inhomogeneities smaller than the effective beam area (see below), surface contamination or character, changes in excitation levels, and absorption and enhancement effects. No matter how good the focusing, the diameter of the electron beam cannot be reduced much below 1 micron because electrons fan out from the point of impact when they strike the sample a t the voltages required t o excite most analytical lines. Therefore, if the sample composition a t the surface changes within an area of about 1 square micron, an electron beam probing that area cannot give a quantitative result representative of the whole sample. Deviations from proportionality between weight fraction and analyticalline intensity (130) can usually be dealt with in x-ray emission spectrography by comparing the unknown with a standard of nearly identical composition. Such comparison is ruled out in most work with the electron microprobe because the beam is too small and does not penetrate deeply enough. In addition, the excitation process is more complex with the microprobe than in an ordinary x-ray spectrograph. I n the microprobe, the analytical line is excited mainly by the electrons, to a small extent (about 3%) by the continuous x-ray spectrum generated in the sample by the beam, and possibly by a characteristic line (other than the analytical) excited by the electrons. Each of these components is sensitive in a different way to the composition of the sample. Castaing and his coworkers (60, 62) have succeeded in making the electron microprobe nearly enough absolute in certain cases so that pure elements can
there serve as standards. Correction methods for deviations from proportionality have also been developed (52, 174, 214 ) . The back-scattering of electrons is especially important in this connection, especially when the sample contains as major constituents elements of greatly different atomic numbers. Most of the applications of the x-ray emission microprobes so far reported are qualitative, and they have often served only to determine its capabilities. In certain meetings (110, l y f ) , complete sessions have been devoted to the microprobe, and the American Society for Testing Materials has formed a task force to investigate it. Two interesting investigations of cosmological material have been reported. The ratio of iron to nickel to cobalt in spherules weighing less than 10- gram and believed to have been drawn from meteors by atmospheric fi-iction has been determined (53). Porous phases of meteoric iron have been examined for iron-nickel distribution (146). Diffusion studies have received considerable attention. Adda and Philibert (2) studied the diffusion of uranium in transition metals. They found that a concentration gradient present in the uranium-copper system disappeared when hydrostatic pressure was applied during diffusion. A study of grain boundary diffusion (17) of nickel into copper bicrystals showed a diffusion several orders of magnitude higher along the grain boundaries than in the grains. It is of interest that these results could be obtained even though the grain boundary was much narrower than the electron beam. Measurements on the uranium-niobium diffusion V O t 32, NO. 5 , APRIL 1960
245 R
couple (167) led to the discovery of a previously unreported phase. The x-ray emission microprobe has proved valuable also in the study of segregation. A copper-to-steel weld \vas found to contain a metastable phase (12y’ copper) a t the boundary (33). The same workers also studied segregation in a nickel-rhenium alloy containing tungsten and molybdenum. Segregation of niobium and titanium accompanied by a chromium-rich grainboundary precipitate was found in Inconel (39). I n stainless steel (68), the chromium to molybdenum ratio n’as found larger in the sigma than in the chi phase. The variation of chromium concentration with depth in 430 stainless steel was measured by Cartoll and Ohh ($8). Application to the study of oxidation, corrosion, phase identification. and diffusion has also been made (34,49,54,162). I n these microprobes, the analytical chemist has powerful new tools that will have to pay for themselves in solving important problems that cannot otherwise be solved. The microprobes will seldom be the most reliable means of establishing the bulk composition of a sample large enough for analysis in other ways. FILM THICKNESS
Absorption and emission are each useful in establishing film thickness (1SO). Emission has been successfully used by Loomis (134) on iron-nickel films deposited on glass slides with development of information storage devices in view. Achey and Serfass (1) have introduced balanced filters to remove interference by characteristic lines of the film when attenuation of characteristic lines of the substrate is used for measuring the thickness of the film. For cases in which previously reported procedures fail because the film contains a n element of the substrate or because of the manner in which the substrate reflects x-rays, Keating and Kammerer (111) have suggested the use of a difiractometer in reflection either for two orders of a reflected line or for two characteristic lines. (The use of two as described is intended to eliminate differences due to differences in reflection by different substrates.) To illustrate: The thickness of a zirconium nitride film on zirconium was measured by observing the attenuation of copper K , and of chromium K , as each passed through the film twice in its path from source t o substrate surface t o detector. Zemany (218) has found iron-55, the R-capture isotope, a highly convenient source for measuring the thickness of titanium on Kovar. I n such samples, the emitted manganese K lines excite titanium K , but no other characteristic lines not absorbed by air.
246 R
ANALYTICAL CHEMISTRY
Owing to the compactness of the source, it can be combined with a detector to serve as a useful probe that should find other applications. The practical aspects of the field have been surveyed by Goodwin and Winchester (91), and two practical gages have been described (173, 209). h recent important determination being carried out with x-rays is that of cladding thickness of nuclear fuel elements (117, 118, 137, 139). ACKNOWLEDGMENT
For help in the preparation of this review, the authors thank their colleagues, Helen Huff and P. D. Zemany. LITERATURE CITED
(I),Achey, F. A., Serfass, E. J., J. Elec-
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on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., hfarch
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(Corrosion-lnds,) 34, Nos. 407-408, 302 (1959). (30) Black, R. H., Forsyth, W. J., Norelco Reptr. 6, 53 (1959). (31) BorovskiI, I. B., “Miscellaneous
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Abstrack, 10th Annual Symposium on Spectromopy, Chicago, Ill., June 1959. (7) American Chemical Society, Abstracts, 133rd-136th Meetings, April 1958-September 1959. (8) Am. Soc. Testing Materials Task Group on X-Ray Fluorescence Spectroscopy, Appl. Spectroscopy 13, 3 (1959). (9) Amundsen, H. R., Hashim, A. H., Hai, A., Rizvi, S. H., Palcistan J. Sci. Znd. Research 1, 207 (1958). (10) ANAL. CHEM.31, No. 4, 90A (1959). (11) Analytical Chemistry Group, ACS
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43 (1954): (38) Brit. J . Appl. Phys. 10, 105 (1959). (39) - , Brooks. E. J.. Birks. L. S.. Am. Soc. \-
Testing MLterials, Spec. Tech. Publ. No. 245, 100 (1958). (40) Brown, F., Anulvst 84,344 (1959). (411 Brown, J. F., Weir, R. J., J . Sci
Iwtr. 33,’222 (1956). (42) Burnham, H. D., Hower, J.; Jones, L. C., ANAL.CHEM.29, 1827 (19571. (43) Buschmann, E. C., Proc. 6th Annual Conference on Industrial Applications ’
~
of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 207, 1957. (44) Campbell, J. T., Shalgosky, H. I., Nature 183, 1481 (1959). (45) Campbell, W. J., Eastern Analytical
Symposium and Instrument Exhibit, New Yark. November 1959. (46) -Campbell, W. J., Leon, M., Thatcher, J. W., U. S. Bur. Mines, Rept. Invest. 5497, (1959). (47) Campbell, W. J., Thatcher, J. W., U. S. Bur. Mines, Rept. Invest. 5416 1958); Proc. 7th Annual Conference on ndustrial Applications of X-Ray Anal-
i
ysis, Denver Research Institute, Denver, Colo., p. 313, 1958. (48) Carroll, K. G . , International Nickel Company Research Laboratory, Bayorme, N. J., private communication to H. A. Liebhafsky, November 1959. (49). Carroll, K. G., Ohh, s., 17th Annual Pittsburgh Dflraction Conference, Mellon Institute, Pittsburgh, Pa., November 1959.
(50) Castaing, R., Ph.D. thesis, University of Paris, 1952. (51) Castaing, R., Recherche a t o n a u t . 23, 41 (1951). (52) Castaing, R., Descamps, J., J . phys. radium 16, 304 (1955). (53) ,Castaing, R., Fredriksson, K., Geochzm. et Cosmochim. Acta 14, 114 (1958). (54) Castaing, R., Philibert, J., Crussard, C., J . Metals 9, 389 (Apnl 1957); Tram. Am. Inst. Mining, Met. Petrol. Engrs. 209, 389 (1957). (551 Chem. Em. News 35, 86 (September 9, 1957). ” (56) Chodos, A. A., Bracco, J. J. R., Engel, C. G., Proc. 6th Annual Conference on Industrial Applications of );-Ray Analysis, Denver Research Institute, Denver, Colo., p. 315, 1957. (57) Chodos, A. A., Nichiporuk, W., Proc. 7th Annual Conference on Industrial Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 247, 1958. (58) Chupp, E. L., DuMond, J. W. M., Gordon, F. J., Jopson, R. C., Mark, H., Phys. Rev. 112, 1183 (1958). (59) Clark, G. L., I S A Journal 5, 40 (February 1958). (60) Clark. G. L.. Hunt. R.. 9th Annual ‘ Symposikn on ’Spectroscopy, Chicago, Ill., June 1958; Spedrochim. Acta 13, 158 (1958) (Abstract). (61) Clark, G. R., Hunt, R. E., Davis, C. M., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959. (62) Compton, A. H., Allison, .S. K;! “X-Rays in Theory and Expement, 2nd ed., pp. 206-7, Van Nostrand, New York, 1943. (63) Condon, E. U., in “Handbook of Physics,” E . U.,,Condon, H. Odishaw, Pt. 7, Chap. 8, eds., “X-Rays, McGraw-Hill, New York, 1958. (64) Cook, G. B., Mellish, C. E., Pa e J. A., Proc. 2nd International ference on Peaceful Uses of -4tomic Energy, United Nations, Geneva, Vol. 19, p. 127, 1958. (65) Cosslett, V. E., Duncumb, P., Proc. 1956 Stockholm Conference on Electron Microsco- 6 p. 12, Academic Press, New Yor?, 1957. (66) Cosslett, V. E., Duncumb, P., Long, J. V. P., Nixon, W. C., Proc. 6th.Annual Conference on Industrial Apphcations of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 329, 1957. (67) Cosslett, V. E., Engstrom, A., Pattee, H. H., Jr., “X-Ray Microscopy and Microradiography,” Proc. 1956 Symposium, Cavendish Lab., Cambridge, Academic Press, New York, 1957. (68)Davis. C. M.. Clark. G. R.. Proc. ‘ 6th Annual Conference’ on Industrial Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 351, 1957. (69) Denver Research Institute, Ab~tracts, and Proc. 6th Annual Conference on Industrial Ap lications of X-Rav Analvsis. Denver flesearch Instit&, DenGer,‘ Colo., August 1957. (70) Ibid., 7th Annual Conference, August 1958. (71) Ibid., Abstracts, 8th Annual Conference, August 1959. (72) Descam s J Philibert, J., Comptes rendus du %.‘’Congr&s pour l’avancement des methodes spectrogra hiques (Paris 1957) ( & W s en 1958) du &roupment. (73) Dodd, C. G., 8th Annual Conference on Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., August 1959. (74) Dodd, C. G., University of Oklahoma, Norman, Okla., letter to H. A. Liebhafsky, November 23,1959.
En:
(75) Doughman, W. R., Sullivan, A. P., Hirt, R. C., ANAL. CHEX. 30, 1924 (1958). (76) Dunn, H. W., 3rd Conference on Analytical Chemist in Nuclear Reactor Technology, Xatlinburg, Tenn., October 1959. (77) Dwiggins, C. W., Jr., Dunning, H. N., ANAL.CHEM.31, 1040 (1959). ( i 8 ) Ebert, F., Wagner, A., 2. Metallk. 48,646 (1957). (79) Elec. Eng. 77,768 (1958). (80) Electronics (Eng. ed.) 31, No. 9, 84 (1958). (81) Fagel, J. E., Jr., Liebhafsky, H. A,, Zemany, P. D., ANAL. CHEM. 30, 1918 (1958). (82) Fagel, J. E., Jr., Madden, R. B., Pikler, H., Stephenson, H. T., Appl. Spectroscopy 11, 131 (1957). (83) Felten, E. J., Fankuchen, I., Steigman, J., ANAL.CHEM.31, 1771 (1959). (84) Fisher, R. M., Swartz, J. C., 5th Bnnual Conference on Industrial Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., August 1956. (85) Flikkema, D. S., Schablaske, R. V., Ibid., Proc. 6th Annual Conference, p. 387, 1957. (86) Flikkema, D. S., Schablaske, R. V., U. S. Atomic Energy Comm. Rept. ANL-5804 (1957). (87) Fornwalt, D. E., Komisarek, J., 2nd Conference on Analytical Chemistry in Nuclear Reactor Technology, Gatlinburg, Tenn., September 1958. (88) Friedlander, S., Goldblatt, A., Pittsburgh Conference on Andflical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959. (89) Glocker, R., “Materialprfifung mit Rijntgenstrahlen,” 4th ed., SpringerVerlag, Berlin, 1958. (90) Goodwin, P. S., Control Eng. 5, 94 (August 1958). (91) Goodwin, P. S., Winchester, C. L., Plating 46,41 (1959). (92) Grubb, W. T., Zemany, P. D., Nature 176, 221 (1955). (93) Hakkila, E. A., Univ. Microfilms (Ann Arbor, Mich.), L. C. Card No. Mic 58-532; Dissertation Abstr. 18, 782 (1958). (94) Hall, E. T., Endeavour 18, 83 (April 1959). (95) Hall, T., Proc. 6th Annual Conference on Industrial Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 297, 1957. (96) Hansen, J., 10th Annual Symposium on Spectroscopy, Chicago, Ill., June 1959. (97) Hanson, J. P., Flynt, W. E., Dowdey, J. E., Reu. Sci. Zmtr. 29, 1107 (1958). (98) Heidel, R. H., Fassel, V. A,, ANAL. CHEM. 30, 176 (1958). (99) Heidel, R. H., Fassel, V. A., 8th Annual Conference on Apphcations of X-Ray Analysis, Denver Research Institute, Denver, Colo. , -4ugust 1959. (100) Heinrich, K. F., McKmley, T. D., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959. (101) Hinslep, J. F., Won-Destructive Testing.” Macdonald and Evans. London, 1g59. (102) Horton, W. S., Moak, W. D., U. S. Atomic Enerw Comm. ReDt. KAPLM-WSHG (lg59). (103) Houk, W. W., Silverman, L., ANAL.C ~ M31,. 1069 (1959). (104) H o d , W. W., Silverman, L., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., M a c h 1959. (105) Ivoilov~A. S., h v , N. .F., Zwt. Akad. Nauk S.S.S.R.. Ser. Fia. 21. 1465 (1957).
(106) Jacobson, B., Science 128, 1346 (1958). (107) Johnson, C. M., Stout, P. R., ANAL.CIiEM. 30, 1921 (1958). (108) Jones, R. A., Ibid., 31, 1341 (1959); Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959. (109) Jones, R. W., Ashley, R. W., ANAL.CHEM. 31, 1629 (1959). (110) Karolinska Institutet, Dept. for Medical Physics, Abstracts, 2nd International Symposium on X-Ray blicroscopy and X-Ray Microanalysis, Stockholm, June 1959. (111) Keating, D. T., Kammerer, 0. F., Rev. Sci. Zmtr. 29, 34 (1958). (112) Keinath, ,I=;., “The Measurement of Thickness, Nate. Bur. Standards (U. S.),Circ. 585 (1958). (113) Kiley, W. R., 9th Annual SJ-niposium on Spectroscopy, Chicago, Ill., June 1958: Saectrochim. Acta 13) 158 (1958) (Abstract). (114) Knight, H. T., Venable, D., Rei’. Sci. Instr. 29, 92 (1958). (115) Lambert, M. C., A.C.S. Xorthwest Regional Meeting, Seattle, Wash., June 1959. (116) Lambert, M. C., Noreclo Reptr. 6, 37 (1959). (117) Lambert, M. C., Proc. 7th Annual Conference on Industrial Applications of X-Ray Analysis, Denver Research Institute, Denver, Colo., p. 193, 1958. (118) Lambert, M. C., U. S.Atomic Energy Comm. Rept. HW-57941 (1958). (119) Lazar, V. A., Beeson, K. C., J . Assoc. Ofic. Agr. Chemists 41, 116 (1958). (120) Leon, M., Campbell, W. J., Pittsburgh Conference on Analytical Chemistry and A plied Spectroscopy, Pittsburgh, Pa., Larch 1958. (121) Lewis, R. K., Ogilvie, R. E., 17th Annual Pittsburgh Diffraction Conference, Mellon Institute, Pittsburgh, Pa., November 1959. (122) Liebhafsky, H. A., ANAL. CHEM. 21, 17 (1949). (123) Ibid., 22, 15 (1950). (1241 Zbid.. 23. 14 (1951). (izjzbid.: 24; i6 (i952j. (126) Ibid., 26, 26 (1954). (127) Liebhafsky, H. A., J . Metals 11, 273 (1959). 11281 Liebhafskv. H. A,. Pfeiffer. H. G., Zimany, P. D.’. A I 257 ~
i195.5).
‘ 31, 1317 (1959). ’ (132) Long, J. V. P., J. Sci. Instr. 35, 323 (1958). (133) Long, J. V. P., Cosslett, 5’. E., in “X-Ray Microscopy and Microradiography,” V. E. Cosslett, A. EngstrBm, H. H. Pattee, Jr., eds., p. 435, -4cademic Press, New York, 1957. (134) Loomis, T. C., 9th Annual Symeposium on Spectroscopy, Chicago, Ill., June 1958; Spedrochim. Acta 13, 158 (1958) (Abstract). (135) Lopp, V. R., Claypool, C. G., 8th Annual Conference on Apphcations of X-Ray Analysis, Denver Research Institute. Denver, Colo., August 1959. (136) Losev, N. F., Glotova, A. S . , Zauodskaya Lab. 24, 619 (1958). (137) %we, B. J., Sierer, P. D., Jr., Ogilvle, R. B., Proc. 7th Annual Conference on Industrial Applications of X-Ray Analysis Denver Research Institute, Denver, kolo., p. 275, 1958. (138) Lublin, P., 8th Annual Conference on Applications of X-Ray Analysis, Denver h r c h Institute, Denver, Colo., August 1959.
VOL. 32,
NO. 5, APRIL 1960
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