( C S S R ) (English Transl.) 33, 3461 (1963). (390) Tucker, hI. A., Colvin, C. B., Martin, D. S. Jr., Znorg. Chem. 3, 1373 (1964). (391) Ueda, T., Fox, J. J., J . Am. Chem. SOC. 85? 4024 (1963). (392) Ukita, T., Yoshida, AI., Hamada, A., Kato, Y., Chem. Pharm. Bull. (Tokyo) 12, 459 (1964). (393) Unemoto, T., Zbid., 12, 65 (1964). (394) Ungnade, H. E., “Organic Electronic Spectral Ilata,” Vol. 11, 19531955, Interscience, New York, 1960. (395) Unicam Instruments Ltd., York St., Cambridge, England, Mfg. Bull. SP. 800. (396) “Us’ Atlas of Organic Compounds,” Buttenvorths, London, in press. (397) Van Poucke, L., Herman, l f . , Anal. Chim. Acta 30, 569 (1964). (398) Vinogradov, S. N., Gunning, H. E., J . Phys. Chem. 68, 1962 (1964). (399) Waack, R., Doran, hl. A., Zbid., 68, 1148 (1964). (400) Wallace, J., Biggs, J., Dahl, E. T., AXAL.CHEM.37, 410 (1965). (401) Wallenfels, K., Draber, W., Tetrahedron 20, 1889 (1964). (402) Wendling, E., Bull. SOC. Chim. France 1965, 427.
(403) Wexler, A. S., ANAL. CHEM.35, 1936 (1963). (404) Wexler, A. S.,Ibid., 36, 213 (1964). (405) White, Robert G., “Analytical Applications of Ultraviolet, Visible and Near - Infrared Absorption Spectrophotometry,” “Progress in Infrared Spectroscopy,” 5’01. 1, Herman A. Szymanski, ed., pp. 237-354, Plenum Press, New York, 1962. (406) White, R. G., “Handbook of Ultraviolet Methods,” Pleiium Press, New York, 1965. (407) White, R. G., “Irrelevant Absorption in Quantitative Ultraviolet Spectrometry,’] “Progress in Infrared Spectroscopy,” Vol. 2, Herman A. Szymanski, ed., pp. 275-298, Plenum Press, New York, 1964. (408) White, R. G., “Progress in Ultraviolet and Visible Spectrometry,” “Progress in Infrared Spectroscopy,” Vol. 2, Herman A . Szymanski, ed., pp. 253-274, Plenum Press, Xew York, 1964. (409) White, R. G., Seeber, R. E., A p p l . Spectry., 18, 158 (1964). (410) White, R. G., Ibid., p. 112. (411) Wierchowski, K. L., Shugar, D., Speclrochin. Acta 21, 931 (1965).
(412) Wilke, J. B., J . Assoc. OBc. Agr. Chemists 46. 920 11963). (413) Wilkinson, P.‘ G., ’Bryan, E. T., Appl. Opt. 4, 581 (1965). (414) Williams, E. J., Laskowski, AI., Jr., J . Biol. Chem. 240, 3580 (1965). (415) Williams, F. T., Jr., Flanagan, P. W. K.. Tavlor. W. J.. Schecter., H.., * J . Org. Chem.“30,’2674 (1965). (416) Williams, T., Krudener, J. McFarland, J., Anal. Chim. Acla 30, 155 (1964). (417) Wirth, T. H., Davidson, S., J . Am. Chem. SOC.86, 4314 (1964). (418) Woodhead, J. L., J . Znorg. Nucl. Chem. 27, 1111 (1965). (419) Yagi, Y., Popvv, A . I., J . Am. Chem. Soc. 87, 3577 (1965). (420) Yagil, G., Anbar, &I., J . Znorg. ”Vucl.Chem. 2 6 , 453 (1964). (421) Yamazaki, F., Fujiki, K., Murata, Y., Bull. Chem. SOC.Japan 38,8 (1965). (422) Young, J. P., ASAL. CHEM.36, 390 (1964). (423) Yoza, N. Ohaski, S.,Bull. Chem. Soc. Japan 36, 1485 (1963). (424) Zagorets, P. A., Bulgakova, G. P.,
Russ. J . Phys. Chem. (English Transl.) 39, 149 (1965). (425) Zatka, V.,Collection Czech. Chem. Commun. 29, 2607 (1964). (426) Zaugg, H. E., Schaefer, A. D., ANAL. CHEY.36, 2121 (1964).
X-Ray Absorption and Emission William J. Campbell, James D. Brown, and John W. Thatcher, Bureau of Mines, College Park Metallurgy Research Center, U. S. Department o f the Interior, College Park, Md.
A
IX THE 1964 REVIEW(loo), we continue to use the format set by our predecessors Liebhafsky, Pfeiffer, and Kinslow (368-$65)-that is, a critical review of fundamental developments and tabular summaries of applications of s-ray spectrography and electron probe microanalysis. An abbreviated review., with emphasis on papers by Russian researchers was prepared by Losev, Smagunova, and Stakheev (374). Continued expansion in the utilization of x-ray methods is the keynote of this review. One of the more promising advances has been the development and application of nondispersive systems using radioisotope sources. Progress has been made in gaining a better understanding of x-ray detector characteristics. bIathematica1 and graphical methods are being developed to convolute overlapping spectra characteristic of nondispersive techniques. Vacuum x-ray spectrographs with aindouless, selectable-target s-ray tubes and ultrathin nindow detectors are no\%available for escitation and detection, respectively, of spectra from elements, atomic numbers 4 to 11. +lvailability of improved long wavelength instrumentation has stimulated increased interest in s-ray spectral shifts as a function of valence and chemical bonding. Another noteworthy development is the use of computer-oriented
s
41 6 R
ANALYTICAL CHEMISTRY
mathematical methods for calculating composition from measured x-ray intensities. These high speed calculation procedures provide the means for comparing concentrations obtained by various mathematical correction procedures for absorption and secondary fluorescence in electron probe microanalysis. Recent textbooks and conference proceedings are listed in Table I. “Advances in X-Ray Analysis” (424, 426) continues to provide comprehensive coverage of s-ray spectrography and electron probe microanalysis. Applications of radioisotopes as x-ray sources are discussed in symposia held in *lustria (696) and in the United States (24). Developments in electron probe microanalysis are summarized in the proceedings of conferences held in Washington, D. C. (4O8), and Ispra, Italy (193). “Fifty Years of X-Ray Diffraction” (196) is highly recommended. The personal reminiscences of these outstanding scientists, both past and present, provide insight into their creative process as well as the history of x-ray analysis. A critical tabulation of K,L, M,N, and 0 spectral lines was prepared by Bearden and his associates a t Johns Hopkins University (35). The objective of their research program was to remeasure selected x-ray lines and reevaluate all published wavelengths on
a consistent and absolute scale. Extensive 28 tables for 11 crystals were prepared a t Pennsylvania State University under AST;\I sponsorship (610). Section 1 of these tables lists all lines for each element up to the 3rd order unless the multiple order exeeds 100A. Section 2 lists all lines arranged in order of increasing wavelength regardless of element or order of reflection. Many weak lines detectable only by high resolution techniques or soft x-ray spectroscopy are included in these tables. Twotheta tables for eight crystals were compiled at the Oak Ridge Kational Laboratory (13). Other recent tables are 28 values for germanium (4b) and for two crystals, K d P and Rb,lP (80d), that are used in the soft x-ray region. X handy personal size Periodic Chart with x-ray lines and absorption edges is also available (488). I n many instances the various 28 tables represent duplication of effort indicating the need for more coordination of efforts. The authors of this review are willing to serve as a source of information as to 20 tables that are presently available and those being compiled or considered for compilation. We suggest that Bearden’s \yavelength tables be used to prepare all additional 28 tables. With increasing utilization of x-ray apparatus, particularly by nonprofessional personnel, education in x-ray safety should be emphasized. Radiation
safety procedures are reviewed by Cook and Oosterkamp (129). Nonobservance of these procedures will lead to radiation accidents such as recently reported in Germany (244). A young laboratory assistant received an estimated dosage of 10,000 to 12,000 R to his right hand when holding an unshielded x-ray spectrographic tube. Electrical interlocking should be provided so that x-ray tubes are inoperable when shielding is removed. Instrument manufacturers continue to provide introductory education to those entering the field of x-ray analysis. While their ultimate purpose is to promote sales, they are performing a very worthwhile service. During 1965 Philips Electronics held its 50th x-ray school, the first session dating back to the 1940's. Other manufacturers provide training a t their plants or a t symposia held a t various locations in this country. Short intensive training programs in x-ray spectrography and x-ray microanalysis are held each summer a t Pomona College and Massachusetts Institute of Technology. Workships on x-ray analysis held as part of general conferences on spectroscopy are additional training aids.
Table 1.
Author Bearden, J. A. Blokhin, M. A. Engstrom, A. EURATOM Ewald, P. P. McKinley, T. D., Heinrich, K. F. J., Wittrv, D. B.. eds. Mallett, G. R., Fay, M. J., eds. Mueller, W. &I., Mallett, G. R., Fay, 31.J., eds. White. E. W.. Gibbk G. Johnson, G. G., Jr., Zechman, G. R.
v.,
Baker, P. S., Gerrard, ?*I., eds. Bovey, L. Davis, E. N., ed. Forrette, J. E., Lanterman, E., eds. International Atomic Energy Agency
Summary of Recent Books
Title X-Ray Wavelengths X-Ray Spectroscopy (English edition) X-Ray Microanalysis in Biology and Medicine On Electron Microprobe Analysis. Quantitative and Structural Analysis of Nuclear Materials Fifty Years of X-Ray Diffraction The Electron Microprobe Advances in X-Ray Analysis, Vol. 7 Advances in X-Ray Analysis, Vol. 8 X-Ray Emission Line Wavelength and Two Theta Tables
SECTIONSON X-RAYS Low Energy X- and Gamma-Sources and Applications Limitations of Detection in Spectrochemical Analysis Developments in Applied Spectroscopy, Vol. 4 Developments in Applied Spectroscopy, Vol. 3 Radiochemical Methods of Analysis, Vol. 2
ABSORPTION
Development and application of absorption techniques (11, 55, 185, 259, 291, 312-314, 333, 485, 507, 524, 587)
continue a t a low level compared to the interest in x-ray emission methods. Although the principles of absorption edge spectrometry have been known for several decades, the technique has not been widely exploited. This lack of attention is surprising when the advantages of this technique are considered. Absorption edge analysis eliminates the need for preparation and storage of standards; thus the method is very useful when the number of samples of a particular type are limited. Matrix effects are eliminated by using two spectral lines of similar wavelengths. Instrument instability is compensated for since ratios rather than absolute intensities are used in the calculations. If necessary, a monitored beam can be used to correct for fluctuating source intensity (454). Limitations of the absorption method are poor sensitivity for low atomic number elements and for concentrations less than 0.1%. Also there is a large uncertainty in many of the p / p values used in the calculations. -4 widely applicable routine procedure was described by Bertin, Longobucco, and Carver (53). Spectral line pairs and absorption edges for most elements are listed in their report. Optimization of absorption edge spectrography is the subject of a paper by Dodd and Kaup (166). Cullen (144) described an internal differential absorption edge tech-
nique whereby a n element in the sample emits spectral lines on both sides of the absorption edge of a second element present in the sample. For example, the concentration of nickel can be determined by the ratio of CuKa to CuKPl; these lines bracket the nickel absorption edge. Copper can be present in the original sample or added by the analyst. A general approach to the analysis of complex samples, proposed by Lefker (360) , requires measurement of Io/I for n wavelengths of a n n component system. The most accurate results are obtained by selecting wavelengths such that as many absorption edges are straddled as possible. Using as a n example a two-component system, the following equations are required:
where the A,, terms are the elements of the last row of the inverse matrix of the coefficients. Jacobson (998,300) and his associates a t the Karolinska Institute developed a multichannel x-ray spectrophotometer for analyzing in vivo. Special x-ray tubes were designed as sources of monochromatic x-rays. Although their instrumentation is intended primarily for medical research, the principles are applicable to other analytical problems. CHEMICAL EFFECTS
Recent improvements in low energy x-ray spectrographic instrumentation has stimulated increased activity in investigating the effects of chemical combination on the shape, intensity, and position of long wavelength x-ray spectral lines and absorption edges (154, 189, 253, 396, 398, 414, 573, 611).
P1+ P2 = 1 where p
= the mass absorption coefficient
P
=
d
=
fractional part, of nth component where 2 Pn = 1 thickness of sample, in cm.
The solution by matrix algebra is
P1 =
Al(,+l,
pz
-4z(n+i)
=
1 -d =
A(,+I)("+l,
Valence, coordination, electronegativity, conductivity, and crystal structure are important parameters that can be investigated by x-ray fine structure techniques. An international conference on the physics of x-ray spectra was held a t Cornel1 University, Ithaca, N. Y., June 1965. Russian research on soft x-ray spectrometry is summarized periodically in the Bulletin of the Academy of Sciences of the USSR. English translations of the Bulletin are available from Columbia Technical Translations, White Plains, N. Y. Baun and Fischer have collected extensive data for K spectra of Period I1 VOL. 38, NO. 5, APRIL 1966
417 R
Table II.
COMPOUND OR
Nondispersive Analysis
Principles (21, 192, 240, S43, 392, 393,
ALLOY
492,494, 520,523,562) Radiation safety (19’7, 392) Radioisotopes sources (92, 96, 108, 197, S44, 468, 469, 519, 681) X-ray sources (98, 303)
394 1
elements and the K and L spectra for Period I11 elements (32-34, 200, 2 0 f , 204, 205, 206,209). Their instrumentation, designed and constructed by Picker X-ray, consists of a horizontal flat crystal spectrometer mounted in a vacuum chamber (203, 208). X-rays are produced by primary excitation with electrons and detected by an ultrathin window flow-proportional counter. Layered stearate crystals are used to diffract x-rays longer than 25 A. Fischer and Baun state that the “soap film” crystals compare favorably with ruled gratings in regard to intensity and resolution. Because of the small depth of penetration of the electrons and escaping low energy x-rays, preparation of the sample surface is critical. Any change in the chemical state of the sample surface during analysis, e.g., oxidation, complicates data evaluation. Surface contamination probably explains many of the divergent results reported in the literature. In studying chemical effects in low atomic number elements, the analyst employs K satellite lines and Kp bands, whereas the K a lines are used for quantitative spectrochemical analysis. Satellite lines and Kp bands of low atomic number elements are easily measured with the soft x-ray spectrographs developed in the past few years. The intensity of the KaS and Kal satellite lines of magnesium are approximately 1 / 1 ~that of the Ka doublet and an order of magnitude more intense than the Kp band. Fischer and Baun presented extensive soft x-ray data relating changes in intensity, line shape, and peak position with various parameters. For example, the ratio of the Ka4/Kaa satellite lines of aluminum were correlated with electrical conductivity (see Figure 1). The ratio varied from approximately 0.5 for the metals (good conductors) to 1.0 for oxides (poor conductors). There was also a marked shift in the Kp band position with variation in conductivity; 41 8 R
ANALYTICAL CHEMISTRY
SEM I CONDUCTORS AN D I N S U L A T O R S
AND
I
AI
Applications Alloys (241, 288, 562) Cement (294, 296) Coal (392, 490) General (344) Ores (214, 391, 491, 492, 521, 622, 580) Periods I1 and I11 elements (494) Periods V I and VI1 elements (76, 216, Plant steam monitor (218) Portable field units ($14, 316-318, 491) Thickness gauge (96, 216, 369)
METAL
INT E R M ETA LL ICS
~
I
NbAI3 *Na3AIF, TIAIJ
Figure 1. Alumi~ num ( Y ~ / c Yintensity ratios for some aluminum compounds (206)
I NiAI,
I
I FeAl
I MnAl I AlSb
I
I
I
I
I I
I I I I
1
AIP A I As A12Se3 AI203
I
Kaolinite
3 AI203’ 2s102
* Decomposes
I
I
l
t
l
l
l
l
l
l
I
I
I
I
I
I
I
I
i n beom.
I using secondary e x c i t a t i o n .
Q ~ / Q ~ =
the maximum shift was approximately 0.75 28 going from aluminum to 3Xl2O3.22402. The Kp band was very unsymmetrical for magnesium and aluminum, whereas the band was symmetrical for their oxides; also the peak position was shifted toward longer wavelengths. Changes in symmetry of the Kp band of solid and liquid aluminum was observed (207). One of the more notable effects observed by Baun and Fischer was the difference between the L spectrum of sulfur in sulfides and sulfates. The 3s band, which was the strongest band for sulfides, was absent in sulfates. Dodd and Kaup (167) used x-ray absorption edge fine structure spectrometry to measure oxidation of iron in clay minerals. Christofferson, Hughes, and Klaver ( f l y ) developed an automated system for extended high resolution absorption edge measurements. Their instrument is similar to the step scanner used in x-ray diffraction and includes a paper tape printer, programmer, and facilities for processing the data on an IBM 7094 computer. The fine structure on the high energy side is characteristic of the valence of absorbing atoms and the identity and configuration of neighboring atoms. Theoretically the absorption edge measurements can provide information regarding the amount of each element present in a particular chemical form. Although studies of solid state
spectrometry are primarily of interest to physicists, the analyst should be aware of the many potential applications of this technique, in particular, determination of valence and coordination number. Also the analyst must be aware of the possible sources of error in quantitative analysis arising from changes in line shape, position, and intensity. Unless the element being determined is present in the unknown and standard in the same chemical form, a systematic error may result. Quantitative elemental analysis of Periods I1 and I11 elements by soft xray spectrographic methods should be evaluated with due regard for these additional problems. NONDISPERSIVE ANALYSIS A N D RADIOISOTOPE SOURCES
During the 1950’sradioactive isotopes were considered and rejected as a source of excitation to supplement the x-ray tube. This rejection of isotope sources was due to their low intensity which is 10-6 to 10-7 of that available from x-ray tubes. However, this low primary intensity can be partially compensated by using nondispersive systems which view 10-1 to 10-2 of the radiation emitted by a sample as contrasted to 10-6 for dispersive systems. Thus nondispersive systems with radioisotope sources are only 10-l to less sensitive than dispersive systems that use
w a y tubea as the source of excitation. Increased interest in radioisotope sources is evidenced by the number of excellent papers on this subject presented a t recent symposia (24, 296). Publications on the general subject of radioisotopes and nondispersive techniques are listed in Table 11. Radiosotope sources coupled with nondispersive techniques offer the following advantages over conventional dispersive systems : simplicity of design and low cost, compact lightweight instrumentation, elimination of variations in x-ray tube voltage and current as a source of error, and wide choice of excitation sources and excitation levels. Most of these advantages are demonstrated in the portable x-ray analyzer developed by the Argonne National Laboratory (316-318). Their instrument occupies a volume of approximately cubic foot and weighs about 10 pounds. The major components are a nickel-cadmium batterypowered transistorized power supply and recorder, a sealed proportional counter, and a small radioisotope excitation source. By u5e of a source, the range of elements that can be determined includes atomic numbers 19 to 92; greater sensitivity for lower atomic number elements can be achieved by a H3/Zr source. Other portable units mere developed by Florkowki and coworkers a t the Institute of Radioisotope Techniques, Poland ( Z l g ) , and Rhodes, in England (491, 492). I n conjunction with Rhodes, Hilger, and Watts have developed a commercially available unit suitable for mapping rock composition in situ (490). There are three types of radioisotope source+-alpha, beta, and gamma emitters. Direct bombardment of samples by either alpha or beta radiation has been suggested for the detection of Periods I1 and I11 elements. Beta emission has also been used for analysis of binary or pseudo-binary systems by relating variations in intensity of backscattered electrons as a function of the heavy element concentration. Typical alpha and beta emitters are Po21o and H3/Zr, respectively. Generally beta emitters are used to generate primary x-rays which, in turn, excite secondary x-rays in the sample. The photon spectrum resulting from the interaction of beta particles with matter consist of two principal componentsthe continuum and the characteristic K, L, and RI lines of the target elements. The Physics Department of the Edsel B. Ford Institute for RIedical Research has compiled the characteristics of betaexcited x-ray spectra from various sources (468, 469). Beta activities, target geometries, and other critical parameters were established. Various ratios of characteristic to continuous radiation were achieved by varying the
composition and thickness of the transmission foil. Preuss and coworkers found the highest ratio of K spectra to continuum when using a Pm147 source in conjunction with a 0.5-mil thick vanadium foil. Gamma sources may be either monoenergetic emitters, e.g. FeS5 (MnK), or high energy gamma-emitters used in conjunction with a secondary emitter, e.g., AmZ4l-Cs which has the CsK spectrum superimposed on the continuum. hIost of the monoenergetic sources are comparatively weak emitters and are used either for simple absorptiometry or for calibration of detectors. High energy gamma sources such as Gd'53, Am241,Sr'JO-Y90, and Cos5 have been utilized for excitation of K spectral lines of high atomic number elements. A SrgO-YgO source packed in silver foil and enclosed in a lead chamber was used by Gorski and Lubecki (241) for the determination of tungsten; lead K radiation was the principal component of excitation. llartinelli and Blanquet used an Ir192 source for excitation of lead K spectra (394). Effects of grain size, density, chemical form, takeoff angle, and depth of sample analyzed were discussed in their report. They concluded that the intensity of the high energy P b K a radiation was only slightly dependent on the above factors as contrasted to the strong dependence observed for lower energy PbLa lines. I n addition, surface preparation was not critical when lead K a was used because of the extended depth of penetration. In most quantitative applications, electronic stability is not a prime consideration since other parameters such as variable matrix and sample preparation predominate. However, there are specific problems, such as analysis of currency alloys, in which the accuracy is limited principally by instability in the x-ray source. Radioisotopes provide a constant source of excitation; the radiation flux changing very slowly with time a t a well known decay rate. For isotopes having half lives of greater than 1 year, the change in intensity is negligible over the time required fo. analysis. I n nondispersive analysis, energy discrimination is achieved by selective filtration, balanced filters, differential absorption in the detector, and electronic pulse amplitude discrimination; all of these methods may be used individually or in various combinations. Various investigators have achieved success in simple analytical problems using a single filter in front of a GeigerMueller detector. For survey type instruments, a rotatable filter head provides a means for efficient sequential detection of several elements. Balanced filters offer a reliable method for eliminating virtually all of the scattered background radiation except for a small
fraction having the same wavelength as the spectral line of interest (96, 169, 492). In the proportional counter, gas discrimination can be accomplished by varying the composition and pressure of the counting gas. Robert (494) increased the intensity ratio of F K a / C K a by more than tilo orders of magnitude by reducing the pressure of the methane counting gas from 500 mm. to 70 mm. Variable pressure detector systems can be easily adapted to instrumentation designed for use in laboratories; portable field applications do not appear practical because of vacuum pumping requirements. I n nondispersive analysis the experimentally derived curves are the sums of peaks or distribution functions. I n an effort to mathematically separate overlapping peaks to improve the interpretation of these curves, analog and digital computers are being used (560). In the case of the digital computers, the theory of leaqt squares (63, 535) appears very promising. Here the best fit between the observed experimental curve and the component curves is calculated for each point on the experimental curve. An important feature of this approach is that counting statistics can be used in the weighing factor of each observational equation if the experimental curve is derived from the output of a counting device. This procedure increases the accuracy of the convolution. Also, an estimate of the precision of any parameter may be made while the curve fitting is being done. In the analog approach (1, 443) to the synthesis of the experimental curve, the computer generates Gaussian, Lorentzian, or modification of these functions and electrically combines these under the direction of an operator to match the experimental curve. This approach has been improved so that the experimental curve can be projected on an oscilloscope screen where the matching of curves takes place (172);instrumentation of this type is available commercially. This analog method has the traditional advantage of producing a resolved curve simultaneously as the measuring process is taking place. However, there is a certain amount of subjective fitting by the operator which limits the accuracy of this approach. Application of Fourier analysis has not become popular because interpretation is more involved. Furthermore, there appear to be complications arising from series termination errors which limit the accuracy of this method. Regardless of the method used for convolution of spectra, the user must be aware that significant changes in pulse shapes and amplitudes can result from changes in counting rate, detector voltage, and detector condition. Either some way must be found to eliminate these changes or some correction must be VOL. 38,
NO. 5 ,
APRIL 1966
419 R
built into the convolution procedure. At the present state of the art, only the latter is feasible. INSTRUMENTATION
General. There has been significant progress by the instrument manufacturers in the development of programmed x-ray spectrographs. Programmed variables include x-ray tube voltage and current, crystal, 20 setting, collimator, detector, pulse amplitude and amplifier setting, number of counts or counting time, and method of data presentation (19, 141-143, 152). Programs are prepared on individual pegboards so that these programs can be retained for future use. Many of the new models are available with automatic sample changers. The present trend is to couple the output of the x-ray spectrograph with either a modestly priced computer located in the laboratory or by teletype with a large computer center. Spielberg and Ladell (544) designed a scanning single crystal multichannel spectrometer that uses several reflecting planes of the crystal. Their instrument offers the possibility of examining several wavelength regions simultaneously; also the dispersion parameters can be optimized for each wavelength. They also received a patent for their Laue spectrograph that employs both transmission and reflection planes of the analyzing crystal (349). Several recent Russian contributions to instrumentation include a 130-kv. x-ray source for excitation of the K series of the rare earths, 2 dual channel systems to correct for instrument drift (67, 98), and a 10-channel spectrograph designed for the short wavelength region (549)*
A gas target x-ray tube was developed at Johns Hopkins University for the precise evaluation of the high frequency limit of the continuous x-ray spectrum (87, 545). The equivalent target thickness ranged up to 500 A. This tube was operated up to 30 kv.-50 ma. Of special interest is the photoelectron (beta ray) spectrometer developed by Hagstrom, Nordling, and Siegbahn (255). The lines of adjacent elements are widely separated, and relative intensities correspond to relative abundance. When CrKal was used as the primary radiation for exciting Sic, a Si to C ratio of 100 to 95 was obtained. This technique appears promising for surface analysis, chemical state determination, and detection of low atomic number elements. Recently several methods were described for coupling pulse amplitude discrimination with the 20 drive of the spectrometer. This combination provides improved line-to-background ratios and eliminates interfering multiordered radiation. Weber and Marchal
420 R
ANALYTICAL CHEMISTRY
(600) continuously adjust the bias voltage and the window width by means of a function generator using only linear potentiometers. Their circuit can be adapted to different orders of reflection as well as other crystals. Wytees and Augustus (681) utilize the inverse proportionality of pulse voltage to d sin 8. All pulses are amplified by a value proportional to sin 0. A potentiometer is used that has a sinusoidal resistance; the tap is moved proportionally to the angular velocity of the crystal. Salmon (510) drives the base line voltage as a function of 28; the channel width is maintained a t 21 volts when using a scintillation counter. The curve of base line voltage us. 28 was cut to scale from light opaque material and the outline mounted on a cylinder. An optical follower was electrically and mechanically linked to a servo mechanism which, in turn, positioned a 10-turn potentiometer controlling the base line voltage. Salmon's procedure is practical but does not achieve maximum line-to-background ratios. I n one of the new automated spectrographs, the means pulse amplitudes are made equal for all x-ray quanta that enter the detector. Then they are attenuated by a series of resistor networks that takes into account the order of the reflection, 20 and 2d spacing of the analyzing crystal. For those researchers whose automatic data readout consists of recording on chart paper, Pepper (460) has designed a simple switching circuit that allows the chart recorder to stay on scale automatically. The device senses the recorder position and initiates a correction to the data source (e.g., the ratemeter output) to keep the recorder on scale. Williams (612) investigated the effect of minor variations in vacuum path conditions on transmission of long wavelength x-rays. Pressures below 100 to 200 microns are required to minimize fluctuations in intensity with small changes in pressure. Excitation. I n the 1964 Fundamental Review (100) two contrasting philosophies for achieving increased sensitivity in the soft x-ray region were described. One philosophy argued for a continual modification of conventional instrumentation, while the other stated that only a completely new spectrograph design employing drastic changes in the excitation source would be adequate to excite and detect Period I1 elements. Developments in the last 2 years have borne out the second philosophy. New soft x-ray spectrographs have been completely redesigned to eliminate the x-ray tube window and include the excitation source as an integral part of a whole unit. Old ideas for excitation, such as electron, proton, or the gas x-ray tube, have been revived and adapted to modern instrumentation.
One of the most original and wellengineered soft x-ray spectrographs to appear in the literature in the last two years is the instrument designed by Mattson (396). His design keynotes convenience, sensitivity, and versatility. The entire spectrograph is enclosed in an 18-inch bell jar so that all operations are visible. Sensitivity is increased relative to a conventional spectrograph by the windowless construction of the primary source, the close coupling of anode to sample, and the 90" take-off angle from the sample to the detection system. The versatility is demonstrated by the following features: The sample can be excited by either electrons or x-rays; samples may be cleaned by ion bombardment while in the spectrograph; gases, liquids, or solids may be analyzed in the vacuum spectrograph; the anode may be rotated to present a clean surface to the electron beam; the anodes are replaceable and may be cleaned by ion bombardment while in the spectrograph : samples may be changed without disturbing the spectrograph high vacuum; all operations can be performed with controls located outside the bell jar; and locations are available on the baseplate to attach other feedthrough controls if necessary. Fischer and Baun (203) obtain good intensities for long wavelength spectra using electron excitation of samples in a soft x-ray spectrograph. Henke (276) described an accessory to his well known ultrasoft x-ray fluorescent spectrograph that allows electron excitation of the sample. Fearon (199) used a Van de Graaff generator to accelerate monoenergetic electrons for excitation of characteristic x-ray spectral lines. Telsec Instruments, Ltd., Oxford, England, recently announced the availability of a direct electron excitation x-ray spectrograph for elements, atomic 11 to 22. The instrument can be preset for any combination of six elements. A theoretical limit of 1 part in 105 is indicated from their calibration curve for aluminum in steel. Toussaint and Vos (576) investigated direct electron excitation using the demountable x-ray tube-spectrograph manufactured by Compagnie Generale De Radiologie, Paris. Using a modified diffraction tube, Spielberg (543) showed that for spectra in the wavelength range from 1 to 4 A., electron excitation gives less than an order of magnitude increase in sensitivity. For longer wavelengths, electron excitation gives roughly two orders of magnitude increase in smsitivity when compared to a conventional fluorescence unit. He obtained minimum detectable limits of 1 to 2 p.p.m. for elements in NBS low alloy steel having characteristic lines in the 1.5 to 7.13 A. wavelength range. He showed that the highest
sensitivity for the determination of low atomic number elements was obtained with a low excitation voltage-e.g., the excitation efficiency and peak-to-background ratio for silicon in steels reached a maximum a t about 12 kv. Wyckoff and Davidson (627-629) designed a windowless x-ray tube that can be used for either gas or hot filament operation. It is built around a casting for a standard FA-60 x-ray tube and fits as a direct replacement for such a tube. An inexpensive power supply is used; the authors state that because of the efficiency of the source none of their experiments required more than 100 watts of power. I n the gas mode the tube operates with about 20 microns of air pressure which is obtained very simply with a mechanical pump and a controlled air leak. They report that the tube is sufficiently stable for quantitative analysis. When the gas tube was combined with a vacuum spectrograph (628), excellent intensities were obtained for the light elemente.g., 1100 c.11.s. for carbon for 1 watt of excitation power. The “quasifluorescent” excitation, Le., primary photons plus backscattered electrons, explains the high intensities reported. -4significant feature of this tube is the reported lack of carbon contamination on the anode. This is unusual since mechanical pumps operated without cold traps or baffles usually backstream oil into the system. The lack of contamination on the sample is due to the presence of gases in the tubes and not because the electrons are backscattered before reaching the sample. Backscattered electrons are as capable of causing contamination as electrons coming directly from the filament. Thatcher and Campbell (568, 569) showed the interdependence of x-ray tube window absorption, x-ray tube voltage, and anode composition on the excitation of the aluminum K-series. They achieved a 20-fold intensity improvement over a commercial x-ray tube by using a selected target and no filtration of the primary beam. When backscattered electrons were added to the primary beam, a 100-fold increase in intensity was observed and the relative target efficiencies correlated well with the backscatter coefficients of the target materials. I n contrast, Wyckoff (6g9) found no difference in excitation efficiencies of targets when backscattered electrons were used. Caruso and Neupert (107) have designed a small soft x-ray source with a rotatable anode structure to allow selection of radiation from any of six different targets. The source is windowless and is attached to a vacuum chamber with an O-ring seal. There is no provision for water cooling, but none was needed for the targets used even
though electron beam currents reached
as high as 1.0 ma. The production of characteristic x-rays by proton bombardment has been a subject of investigation for many years by nuclear physicists. Jopson and coworkers (308) determined the x-ray yields and ionization cross sections for 26 elements from atomic number 22 to 92, using proton bombarding energies of 441 m.e.v. Khan et al. (3%’) measured x-ray production and ionization cross sections for nine elements bombarded with protons of 15 to 1900 k.e.v. energy. A study of carbon contamination buildup is included as an appendix. Ogier and coworkers (445) measured the thick target yields of x-rays with wavelengths from 1.5 to 44 A. as a function of bombarding proton energy in the range of 1.1 to 1.6 m.e.v. A 23inch cyclotron was used as the proton source, and a flow-proportional counter with a 0.25-mil aluminized Mylar window was used as the photon counter. As predicted by theory, the x-ray yields in photons per proton per steradian increased with increasing proton energy and increasing x-ray wavelength. Sterk (547, 548)‘ designed a highintensity long wavelength x-ray generator using proton excitation of the target. Hydrogen gas is ionized in a plasma generator with an RF field, and the resulting protons are accelerated through an electrostatic lens to collide with the target. Proton currents range from 10 to 100 pa., and the voltage is adjustable from 0 to 150 kv. The soft x-ray source is presently being used for the alignment and calibration of x-ray spectrometers, including the testing of monochromators, crystals gratings, and detectors. Other applications in progress are development work in x-ray optics, investigation of the photoelectric effect, and simulation of the solar spectrum. Of most interest to the spectrographer, is the possible use of the generator as an analytical tool where the target itself becomes the object being investigated-Le., the characteristics of the radiation emitted are the means of performing the analysis, Sterk points out that the efficiency of x-ray production by protons increases rapidly with the wavelength of the excited line and, for wavelengths 7 to 10 A., is the same order of magnitude as that caused by electron excitation. In contrast to electron excitation, no continuum results with proton excitation; therefore higher peak-to-background ratios are possible. Included in his report are proton excitation efficiencies for the Ka-lines of carbon, oxygen, magnesium, aluminum, and copper. Korsunskii and Lukashenko (341) have provided a mathematical treatment of optimum excitation conditions for x-ray spectra in the 5- to 10-A. region. They used Kramers’ formula for the intensity of the continuum as a
function of wavelength. Widdington’s formula was used to determine the electron energy loss in the target. It was assumed that the motion of electrons in the target was rectilinear, and absorption discontinuities in the target were considered. The results of the calculations are plotted so that the reader can select the optimum anode and voltage to excite x-ray spectra in the 5- to 10-A. region. A procedure is also outlined to determine the optimum voltage when the anode characteristic line is the major excitation factor. In discussing the interaction of the electron beam with the anode, the authors conclude that greater intensity is obtained with a small angle between the direction of the electron beam and the surface of the anode. Pavlinskii and Losev (458) derived a semi-empirical relationship to calculate the intensity of fluorescence spectra excited by the primary radiation of an x-ray tube. Included in their discussion is a method to determine the intensity ratio for the characteristic and continuous parts of the spectrum. Greening (947) discussed the derivation of approximate x-ray spectral distribution with a mathematical treatment of absorption data. Pivovarov (463) derived an expression for the intensity of a fluorescent x-ray line excited by the continuum from a tungsten anode as a function of x-ray tube voltage, sample composition, angle of incidence of the primary radiation, and angle of emergence of the secondary radiation. The expression holds only when there are no elements in the sample with absorption edges between the absorption edge of the element of interest and the short wavelength limit of the continuum. Bernstein (47) measured the efficiency of chromium, tungsten, silver, titanium, and platinum target x-ray tubes a t 50 kv. and 50 ma. for exciting the K spectra of elements from tin to aluminum. He found that using a 10mil Be window, a chromium target tube gave a 2/1 gain in intensity over a tungsten target tube for elements with atomic number 22 and below. Dryer (170) discussed the relative merits of a Cr-target, 10-mil Be window x-ray tube and a W-target, 60-mil Be window x-ray tube for the excitation of low atomic number elements. He surmised, incorrectly, that a W-target, 10mil window x-ray tube would be an ideal choice for the excitation of both short- and long-wavelength spectra. Thatcher and Campbell (568, 569) have shown experimentally that with a 10-mil Be window either silver or chromium are the best target choices to excite long wavelength spectra such as -41Ka a t 8.34 A. Excitation studies in the medium and short wavelength regions have dealt mainly with the choice of target material VOL. 38, NO. 5, APRIL 1966
421 R
and operating voltage to give the highest analytical sensitivity. Pavlinskii and Losev (459) studied the effect of target material and voltage on the excitation of the K a lines of elements from chromium to erbium. They used a demountable x-ray tube with anodes of copper, molybdenum, silver, tungsten, and gold. They concluded that the intensity excited by the radiation from a specially selected anode may be 1.5 times as high as that excited by a tungsten anode. High intensity monochromatic x-rays are produced by an x-ray tube developed by Jacobson and Bordberg (699). Birks and coworkers (60) compared the x-ray yields for proton, electron, and primary x-ray excitation of titanium, chromium, iron, copper, germanium, zirconium, and gold. They plotted x-ray yields in photonsjsteradianlquantum for each element as a function of quantum energy and showed that the proton energies required are about 2 orders of magnitude greater than the electron energies for the same excitation. They found that the peak-to-background ratio for proton excitation was improved by one or two orders of magnitude as compared to electron excitation; in fact, the background using protons was so low as to be difficult to measure. They concluded that if a low cost proton source of 0.5 to 5 m.e.v. were available, it would have considerable advantage over electron excitation. Since analytical sensitivity is ultimately a function of the peak-to-background ratio, the background contributed by the excitation source must be considered. As stated earlier, the background with proton excitation is negligible; the background with x-ray excitation is composed mainly of scattered primary x-rays whereas the background with electron excitation is composed of continuous white radiation. According to Zemany (635) the peak-tobackground ratio for secondary excitation may be over 10,000 to 1, whereas the ratio for primary excitation is in the order of 200 to 1. Birks (61) has obtained peak-to-background ratios for electron excitation between 100/1 and 26,000/1 depending on the element and experimental conditions. These figures represent normalization to the natural x-ray line breadths. He obtained higher peak-to-background ratios a t low takeoff angles and concluded that the continuum was generated deeper in the sample than the characteristic radiation. I n contrast, Salem and Watts (509) determined that the average depth of formation of line radiation is greater than that of the continuum. Dispersion. The construction of soap film crystals by the LangmuirBlodgett technique for use in the long wavelength region received continued attention from researchers
422 R
ANALYTICAL CHEMISTRY
during the last two years. Mabis and Knapp (388) discuss the preparation of oriented soap film crystals, the choice of a cation, the choice of a fatty acid chain length, and the number of layers needed. Their work indicates that as the wavelength of the analyzed line increases, the strontium soaps become superior t o the lead soaps in diffracting efficiency. Ehlert (188, 189) studied the diffraction of x-rays by stearate soap film crystals as a function of the number of diffracting layers and the bivalent metal present. He stated that for detecting elements such as boron, nitrogen, and oxygen it is probably not necessary to build soap film crystals of more than 100 layers because of the high absorption by the carbon atoms. He found good agreement between the measured diffraction efficiency and the calculated efficiency for AlKa. For sodium radiation he found the lead stearate crystal gave a threefold increase in intensity with respect to a KAP crystal but with a lower peak-to-background ratio. An approximate packing model and unit cell were postulated for a multilayer soap film crystal by Henke (274). He applied the mosaic crystal theory t o the packing model and derived an expression for the integrated reflection efficiency of a multilayer crystal as a function of wavelength and crystal properties. The reflection efficiency plotted us. wavelength showed an abrupt decrease for wavelengths shorter than the carbon K edge. Henke states, “In this wavelength region, the scattering power of the material between the metal-atom reflecting planes, which is due mostly to carbon, is greatly enhanced so that the scattering power ‘contrast’ between the reflecting planes and the rest of the crystal material is greatly diminished.” He found that by mixing a relatively large amount of short-chained acid, such as decanoic, with the stearic acid, the reflection efficiency below 44 A. was increased by a factor of two or three and the d-spacing was unchanged. Detailed instructions for preparing multilayered crystals as well as a description of the equipment are provided in the paper. Improvements on the prototype to reduce dust contamination and vibration are described in a later paper by Henke (275). In addition to myristate (2d 79 A.) and stearate-decanoate layered crystals, Henke (276) has successfully made lead 130 A.) and lead lignocerate (2d 160 A). melissate (2d Kapp and Wainfan (311) outline a method to obtain the total multilayer film thickness and the double-layer spacing of layered soap-film crystals using the interference phenomena of both specularly and nonspecularly scattered x-rays. Jarvis (301) has described a method to measure the surface viscosities of monomolecular organic films.
-
-
N
Frans and Davidson (223) measured aluminum Ka intensity and line breadth a t half-height for a series of multilayered lead stearate crystals of increasing number of deposited layers. They found that 110 layers were sufficient for maximum intensity and minimal line breadth in the long wavelength region. A 110-layer soap film crystal was four times as efficient in reflecting sodium K a radiation as a KAP crystal and gave similar resolution. Efforts to grow large d-spacing single crystals have lagged behind the development of multilayered soap film crystals. Ruderman et al. ( 6 0 4 , however, report success in growing small single crystals of fatty alcohol esters with 2 d-spacings from 58 to 97 A. The compounds and their 2 d-spacings are listed in Table 111. They stated that the ieflectivity of OXO was exceptionally high compared to the other crystals, but included no quantitative data. The Orbiting Solar Observatory will contain a spectrometer using the OHM and OAO single crystals to measure the spectral distribution of solar x-rays. I n the short and medium wavelength range of w a y spectrography, new instruments for handling crystals were constructed and more accurate measurements of crystal properties, such as reflectivity and lattice dimensions, were made. Volpe and Paschali (594) describe an apparatus for cleaving very thin wafers from crystals with great precision and reproducibility. Wafers of MgO and KaCl were successfully cleaved to a thickness of 0.025 cm. Deslattes, Simson, and Horton (160) describe an apparatus and techniques for x-ray alignment and solution polishing of oriented single crystals. Strain free surfaces are produced which are closely parallel to the atomic planes and are reasonably flat. Precision measurements of the cleavage pIane grating spacing of KAP were made by Bearden (36). He arrived a t a value of 13289.51 f 0.05 XU a t 26’ C. Henins and Bearden (273) used 17 silicon crystals of high quality to obtain a more accurate value of the conversion constant for 121: to Angstrom units. Domashevskaya (168) also calls attention to the potential of silicon crystals as analyzers in high resolution spectrographs. Highly perfect single crystals of ADP have been grown and evaluated by Deslattes and coworkers (161) for use in a double crystal spectrometer. The pyrolytic deposition of a hydrocarbon forms a polycrystalline graphite crystal which can be used as an x-ray analyzer according to Canon (101). When using the 303 reflection of topaz, Spielberg suggests that upper level reflections can be eliminated by rotating the crystal so that the b axis makes an angle of 83’ with the rotation axis of
the spectrograph (541). Lee and Campbell (359) have shown that a change in ambient temperature can cause a change in crystal d-spacing and become a source of error in analysis. The selection of the correct crystal to use for spectrographic analysis is aided by Duwez (I@), who measured the relative intensities from 10 crystal monochromators, and Wagner and Bryan (597), who considered the choice of crystals for the detection of light elements. Collimation systems have remained virtually unchanged in the last several years and are mentioned in the literature only in relation to operating conditions. For example, Baird and coworkers (23) show that fine collimation is not needed for the determination of low atomic number element30 seconds, is compensated. Also the instrument manufacturers are now quoting voltage and current stability to 10.03%. Variation in surface preparation can be minimized by using the highest energy x-rays available. Signal to noise Iatios can be increased by placing a filter between the x-ray tube window and the sample (111). This filter is particularly effective for reducing intensities of spectral lines originating from components of the x-ray tube. Most of the published applications of x-ray tube window filters indicate the authors' approach was empirical, whereas the optimum filter can be calculated readily from existing knowledge of x-ray absorption. Small increases in signal, up to 50%, can be accomplished for very thin samples by using a substrate whose characteristic lines are efficient sources of excitation of the element being determined (284). For example, the FeKa: line from thin films of iron on a zinc substrate is enhanced by the zinc K spectra. Another way to increase line to background is to rotate the x-ray tube and crystal by 90". The scattered radiation which is plane polarized is a minimum a t 90' whereas the distribution of fluorescent radiation is isotopic (112). Present limits of detectability for Class 2 samples are within practical limits imposed by impurities found in reagents used for concentration, in the sample support media, and introduced by atmospheric contamination during the chemical procedures. For example, in the analysis of high purity tungsten the
Table VI. Trace Analysis-Applications and Techniques
Class 1 Biological tissue (8) Deposits (471) Inorganic solutioris (330, 427, 589, 590) hletals (91, 326, 552, 598) Mineral (520) Organics ( 30,' 145, 183, 249, 575, 376, '
595
Thin films Oxides (496) (403) Class 2
Ashed samples (8, 183) Dried residues (17, S87, 455) Electrodeposition (415, 436) Impregnated papers (306, 435, 608) Ion exchange papers and membranes (99, 568, 382, 412, 539)
Precipitation (113, 14Y-149, 280, 505, 506, 536)
reagent blanks for several elements were an order of magnitude greater than statistical limits of detection (539). The most generally applicable procedure for Class 2 determinations is to use ion exchange techniques to isolate and collect the elements and to provide a media for presentation to the x-ray beam. Anion and cation resin-loaded papers and membranes provide a means for quantitative determination of most elements in the periodic table. Luke (382) evaluated cation and anion resinloaded membranes for determination of 28 elements. The Bureau of Mines (99, 539) has utilized ion exchange resin loaded papers for a wide variety of metallurgical problems. To a first approximation the spectral intensity for an element collected on a membrane or paper is independent of the number and kinds of other ions collected. In addition, the collection procedure is either quantitative, or only a simple correction for incomplete exchange is applied. Extensive use of ion exchange resin loaded papers and membranes is predicted, together with new approaches such as the use of reagent-loaded papers that are specific for a given element or class of elements. Quality Control and On-Stream Analysis. I n a recent advertisement the Wisconsin Centrifugal Foundry stated t h a t it is using x-ray instrumentation to provide 25,000 elemental determinations per month. This is a 24-hour-a-day, 7-days-a-week type of operation. This application typifies the potential of x-ray emission for production control of steels (230, 529) and nonmetallics (339). Close control of the copper content, &0.15'% absolute, in a smelting process was achieved by using a ratio system that compared standard and unknown simultaneously (41). Only 5 minutes were required for analysis including sample preparation. Automatic sample changers provide the VOL 38,
NO. 5 ,
APRIL 1966
427 R
means for unattended operation of the x-ray spectrograph (152). The precision is equal to that achieved by manual operation. Up to 11 elements on a total of 90 samples can be analyzed without need for operator assistance. X-ray methods provide the means for continuous analysis and control of dynamic processes. These on-stream applications will increase because the economic gains from improved quality and increased yield greatly exceed the cost of instrumentation. Munch (430) summarized desirable characteristics and advantages of process control. I n addition, he listed the cost of these systems, including many items which may be overlooked, such as interest on investment, depreciation, and research in connection with start-up. Ovcharenko and Shelkov reviewed 60 papers pertaining to x-ray process control instrumentation (453); Rotter (502) prepared a general discussion on process control. On-stream analysis of solutions does not present any special problems either in instrumentation or data interpretation (552). The only special accessories required are the means to sample the stream and a chamber for presenting the sample to the x-ray beam. In contrast, analysis of slurries and pulp creates major problems in sample preparation and correlation of concentration for measured intensities (389,417, 552,558). Lead and zinc in tailings of a mineral dressing plant were determined by taking a 6- to 12-gram sample of the slurry, vacuum filtering and drying to remove moisture, then analyzing this dried cake. Mylar windows were changed every 8 hours as a preventive measure. The lead content of 0.22 to 0.42y0 was measured t o +0.02%, while the zinc, ranging from 0.01 to 0.07’%, was measured to +O.Olyo. Intervals between samples were approximately 5 minutes. Matrix composition and particle size were essentially constant, so that a single intensity-concentration relationship could be used. Parameters to consider for the operation of an on-stream analyzer in a remote geographical location, such as the Rhodesian Copperbelt, are discussed by Moffat and Carson (417). These parameters include selection of electronic compounds, programming of operations, mechanical movements, operator safety, and correlation of intensity to concentration. I n their process, x-ray transmission measurements were used to establish pulp density; line intensity, after correction for pulp density, was used to calculate concentration. The pulp is conveyed by flexible pipes from steady head tanks to a series of sample chambers mounted on a wheel. A programmer controls the movement of the wheel so that the various sample chambers are presented to the x-ray optics. 428 R
ANALYTICAL CHEMISTRY
This analyzer has been successfully employed for the control of large flotation concentrators. X-Ray Probes. Several x-ray probe attachments are available as accessory items for fluorescent x-ray spectrographs. These macroprobes are an excellent supplement to an electron microanalyzer facility. Samples can be analyzed in any physical form and the same surface does not have to be electrically conductive. Surface preparation is not as critical as for electron microanalysis because of the much greater depth of penetration of x-rays as compared to electrons. Using either pinhole or slit optics, the cross-sectional area analyzed can be as small as 25 microns. A 25-micron slit gives 4 to 5 times the intensity of a 100-micron pinhole plus superior resolution. Reduction in spot size is accomplished by stopping down either the incident or secondary x-rays, and thus intensity is proportional to l / r z where r is the radius of the area being analyzed. Curved crystal optics and nondispersive techniques provide two ways to increase intensity. In macroprobe analysis the intensity to concentration relationship is the same as for conventional x-ray spectrographic analysis. Since the sample cannot be altered, matrix correction must be achieved by techniques such as comparing the ratio of line intensity to scattered intensity. For problems such as the major constituents in an inclusion or localized contamination, qualitative results are sufficient. Recent publications on macroprobes are of two general types : instrumentation modification and evaluation (48,49, 176, 307, 614, 615) and applications (52, 114, 307). ELECTRON PROBE MICROANALYSIS
General. The number of papers wTritten on electron probe microanalysis, particularly applications, reflects the large increase during the past 2 years in the number of operating instruments throughout the world. As the sign of the maturing of the field, many papers in which electron probe microanalysis is used as the major analytical tool no longer refer to the instrument in the title of the paper. -4 new edition of the biblioggraphy of Heinrich has been published in “The Electron Microprobe” (4Oa),an extensive volume containing the proceedings of the 1964 Microprobe Symposium held by the Electrochemical Society in Washington, D. C. In Europe, the Fourth International Congress on X-Ray Optics and Microanalysis, held in September 1965 in Paris, was the outstanding meeting in this field in the past two years. An earlier regional European meeting a t Delft was summarized by Mulvey (429).
For several years, in England and Japan, probe users groups have been meeting to discuss problems in quantitative analysis and to evaluate the performance of electron probe microanalyzers. The Birmingham England group reported some of their results at the Paris meeting. I n this country a Washington area group has been meeting monthly since December 1964, and a New York group has been meeting since the summer of 1965. Other groups are forming in Detroit and on the West Coast. The main purpose of such meetings has been to compare results of analyses on standard samples of known composition by many laboratories using many different types of instruments. This work has pointed out the necessity of obtaining homogeneous samples of known composition which can be used as standards to test the various calculation procedures. Adler (4,5) has emphasized this same need for mineralogical studies. The National Bureau of Standards will assist in the preparation and characterization of such samples and has studied the suitability of NBS low alloy steels as probe standards (410). A questionnaire which was circulated to approximately 70 active electron probe microanalysts showed an overwhelming desire to form some sort of national society. Such a society could take an active part in organizing electron probe symposia on an annual basis, thus reducing the necessity of attending meetings in all parts of the country to keep up with the field. The first Sational Conference on Electron Probe hhroanalysis will be held at the University of Maryland in May 1966. At that time the formation of a society may take place. QUANTITATIVE ANALYSIS
,4n excellent review of the subject of quantitative electron probe microanalysis has been published by Philibert (461) with discussions of excitation, absorption, fluorescence due to characteristic lines and continuum, and atomic number effects. Careful study of this article yields a good understanding of the importance of the various effects, as well as the physical basis for the corrections. Wittry (616, 617), in less complete reviews, discusses several sources of error in electron probe microanalysis and stresses the importance of choosing analytical conditions to minimize these errors. I n their review Duncumb and Shields (174) emphasized understanding the physics of interactions of electrons and x-rays with the sample. They feel that because of the complexity of the problem, the use of computers will assume increasing importance. Birks (58) also has recognized the importance of computer techniques.
The techniques of quantitative analy4s are still crippled by a lack of accurate mass absorption coefficients, fluorescent yields, and absorption jump ratios, particularly for the wavelengths longer than 3 A . which are becoming increasingly important. The problem of uncertainties in mass absorption coefficients was discussed by Heinrich (271). Using the best available data, he calculated mass absorption coefficients for KCY,KO, LCY, and LO lines of all elements (271). These absorption c,oefficients are calculated from the expression p / p = CXn which holds with constant C and n for a single element in wavelength regions between absorption edges. C and n vary from element to element and from one wavelength region to another. The C’s and n’s were obtained by curve fitting to experimental data where available and by interpolation and extrapolation in other regions. One bf the assumptions on which Heinrich bases his calculations is that no curvature occurs in a logarithmic plot of p / p against wavelength. His measurements on 3 elements tend to support this assumption. Other recent measurements (408) in the 1- to 10-A. region show a slightly downward curvature for long wavelengths. Coefficients measured by Cooke and Stewardson (130) tend to be slightly higher than those found in Heinrich’s tables. The need for accurate measurements is still as acute as ever. Absorption Correction. Philibert’s equation (461) for f(x),Le., l+h
f(x) =
[1+:][1+
h (1
+
91
has become the most widely used expression in the absorption correction, partly because of the agreement with Castaing’s curves and partly because of its simplicity. Several authors (121,174, 476) have noted the decreasing accuracy of the simple Philibert expression a t low electron accelerating voltage E, when E , approaches E,, the excitation potential for the characteristic line measured. Duncumb and Shields (175) suggest t h a t by changing the value of u in Philibert’s equation to u =
2.39 X E,1.5
1Oj
- E,1.6
where E, and E, are in kilovolts, excellent agreement is obtained with the f(x) curves of Green (246) for all values of E,. Colby and Kiedermeyer (125) have tabulated values for f(x) for a 52.5’ take-off angle for elements of atomic number 4 to 94. By calculation of a single factor, these tables can be used for theDuncumb and Shields modification for any electron probe a t any voltage (123). Quataert and Theisen (470) developed a more complicated expression for f(x)
which contains an atomic number correction as well as an absorption correction. Their expression gave good agreement with the f(x) curves for Castaing a t relatively high accelerating voltage, and poor agreement forf(x) curves measured a t less than 15 k.e.v. With the aid of a computer Criss and Birks (140) approximated Castaing’s + ( p z ) curves by an exponential power series. Since an exponential series is used, exact integration is possible and the absorption and fluorescence corrections can be calculated with as great an accuracy as the + ( p z ) curves are known. The chief barrier to more general use of this method is the measurement or calculation of + ( p z ) curves for all elements a t several electron accelerating voltages. Atomic Number Correction. Considerable effort has been expended toward development of corrections for the atomic number effect. The atomic number effect is generally defined as the effects which make the primary x-ray intensity in the sample a nonlinear function of concentration. The effect becomes large as the difference in atomic number of the components in the sample increases. Because of the atomic number effect, the intensities of the lower and higher atomic number elements are increased and decreased, respectively. Increasing the electron accelerating voltage tends to reduce any atomic number effect. I n view of these facts, any correction must depend on electron accelerating voltage, excitation potential, composition, and average atomic number in the sample. The most prominent atomic number correction was developed by Thomas (571, 572). The correction parameter CY is defined in terms of the stopping power for electrons, and an effective current factor which takes into account the possible ionization energy lost because of electrons that are backscattered from the sample. The major justification for use of this correction procedure has been the agreement obtained between calculated and known composition for a large volume of data (465). However, her modification to the absorption equation makes this technique for atomic number correction less satisfying. Calais et al. (95) derived a slightly different expression for the atomic number correction in terms of stopping power for electrons and the backscatter coefficient. Their correction has the satisfying property of being applied separately from the absorption correction, but has yet to be proved for a large number of systems. Duncumb and Shields (174) feel that a t the present time “general use of CY values should be treated with caution.” The most successful applications of atomic number corrections have been where the CY values are determined for a given set of
instrumental parameters from a sample of known composition containing the same elements as the unknowns (121, 626).
Fluorescence Correction. After a n initial flurry in which about half a dozen corrections were developed for fluorescence due to characteristic lines, the field has settled down with Castaing’s and Wittry’s G methods being the most widely used. Colby (124) prepared tables for instruments with a 52.5’ take-off angle to simplify the use of the Wittry G correction. A change of parameters makes the table applicable to other take-off angles. In this same paper fluorescent yield and absorption jump ratios are given for the K shells of all elements. Reed (481)recommends the modification of Castaing’s equation using the expression
where C‘ is the ratio of the electron voltage to the absorption edge energy, to relate the characteristic line intensities from different elements. He also gives factors which can be used with Castaing’s expression to calculate fluorescence of K lines by L lines and L lines by K lines. Measurements on L line intensities indicate that the same expression as given above can be used to relate L line intensities from different elements. Some measurements have been made on total x-ray intensity generated by a beam of electrons as a function of atomic number and electron voltage. I n using these data it is important to consider the take-off angle a t which the intensities are measured outside the sample and if a correction has been applied for absorption in the sample. Some of the fluorescence correction equations require the use of intensities directly excited by the electrons within the sample, others the intensities as measured a t some specific x-ray take-off angle. Birks et al. (60) give efficiencies of x-ray production for several elements as a function of electron voltage measured outside the sample with a 45’ electron incidence and a 45’ x-ray take-off angle. L intensities are about one half the K intensities in the same wavelength region. I n another paper (61) x-ray yields for 10 elements were measured for electron energies from 11 to 38 k.e.v. and for x-ray take-off angles of 6’ to 45’. The angle between the incident electrons and the direction of the analyzed x-rays was held constant a t 90”. The expression relating intensity to the overvoltage (E, - E k ) ,
IcY(E,- E,)n was found to be invaid for these intensities which were measured outside the sample and not corrected for absorpVOL. 38, NO. 5 , APRIL 1966
429 R
tion. This is in disagreement with the observations of Reed (481) and Green (246). Metchnik and Tomlin (409) have also measured x-ray intensities outside the sample a t various take-off angles. Their measurements agreed with the theoretical expression of Worthington and Tomlin when modified for electron scattering in the target. Suoninen (559) calculated the intensity of x-rays generated by electrons in a thick target and obtained good agreement with measured values of intensity versus electron voltage. Using a Monte Carlo technique, Birks et al. (59) have calculated the distribution of fluorescence intensity due to absorption of characteristic lines. Five hundred thousand photons were used to approximate the distribution of primary intensity as measured by Castaing for copper a t 29 k.e.v. By generating random numbers, the absorption of the photon and generation of secondaries was calculated as a function of depth. The depth distribution of the secondary radiation for a given primary distribution was found to depend only on the mass absorption coefficient for the primary radiation. It was found that the secondary e x i t ation was significant to a depth approximately six times greater than the primary. The most neglected correction in quantitative analysis is fluorescence by the continuum. The argument has been that this fluorescence varies only slowly with composition and so can be neglected. Henoc, Maurice, and Kirianenko (277) have developed an expression to calculate the intensity due to fluorescence by the continuum relative to the intensity directly excited by electrons. The expression has not been widely applied because of its complexity. Xore universal application awaits computer programming of the equations in a form which relates x-ray intensity to composition. One of the problems and criticisms of the atomic number corrections is that they are not separated from continuum fluorescence. In most of the data on which the atomic number corrections are based or justified, the contribution due to continuum fluorescence has not been removed, although the systems in which a large atomic number effect occurs are the same ones for which a continuum correction is important. Empirical Methods. Because of the difficulties and inaccuracies of calculating composition from x-ray intensities, a number of empirical methods have been developed based on simple observations of the dependence of x-ray intensity on composition. The most prominent of these has been the Ziebold-Ogilvie method (636, 637) which is based on the observation that the ratio of the concentration CA of an element to the 430 R
ANALYTICAL CHEMISTRY
relative intensity K Aof its characteristic lines is a linear function of concentration
where ubB is a constant for a binary system. An accuracy of 2% is claimed for this equation. h method of calculating the constants of one system from the constants of other systems having elements in common is given. Moll (418) related the constant uABto the parameters used in the theoretical calculations of composition. However, his use of a defocused beam to determine intensities from inhomogeneous samples is not valid. Colby (122) suggests that the Ziebold-Ogilvie method does not adequately compensate for atomic number effects as a function of accelerating voltage. Dewey’s empirical method ( 163) based on graphical procedures used in emission spectrography should be accurate for the absorption correction to the degree that the univerial absorption curves suggested by Green (246) apply. The fluorescence correction suggested in the paper of Dewey is subject to error because the secondary radiation has a quite different distribution in depth than the primary. Belk (39, 40) applied several simplifying assumptions to the theoretical equations relating x-ray intensity to concentration. His equations for the absorption, atomic number, and fluorescence corrections can be applied in a few minutes and are valid where the corrections are small. The same data that Poole and Thomas (464) used to justify their atomic number correction is used by Belk to support the use of his equations. Moreau and Calais (4%)) have determined average atomic number by measuring the intensity of the continuum which was found to be a function of mass concentration. Brown (80) used the Philibert equation to estimate the inaccuracy which would be introduced by assuming that the observed x-ray intensity is a linear function of concentration. A method of calculating the maximum error introduced by this assumption for any system is given. COMPUTER TECHNIQUES
The models used to describe the interaction of electrons with matter to generate x-rays have become sufficiently complicated that computers are necessary to carry out meaningful calculations. Archard and Mulvey (18) have proposed a model for x-ray generation which is based on the idea of the electrons traveling into the sample in a straight line to some point P which is related to the average range for the
electrons. The electrons then diffuse out from this point, losing energy according to Bethe’s law. From this simple model f(x) curves are obtained that agree with experimentally measured curves. The discrepancies which Clayton (119) observed in analyzing aluminum bearing binary compounds were shown by Archard and Alulvey to be due to the improper adjustment of the f(x) curves as the average atomic number changed from sample to sample. D. AI. Brown and Ogilvie (79),in criticizing this model, point out that agreement of backscatter coefficients 9 as a function of atomic number is not good, the calculated f(x) curve of copper in aluminum places photon production too deep in the sample, and the distribution in depth of x-ray production is completely erroneous for the .irehard model. D. AI. Brown in his thesis (78) used a computer-evaluated transport model to obtain accurate backscatter coefficients and distributions in depth which are in good agreement with Castaing’s measurements. Bishop (62),using Monte Carlo techniques to follow the paths of electrons in copper, has also obtained good agreement with the backscatter coefficient and the measuredf(x) curves for copper. Agreement with the energy distribution of the backscattered electrons and the + ( p z ) curve measured by Castaing was less satisfying. His work indicates that the expression (I - 6 cos26’) with 6 = 0.5 should be used to allow for nonnormal electron incidence. Suoninen (559) used the average properties of electrons measured by Russians in the early 1960’s with Worthington and Tomlin’s ionization cross-section to calculate the distribution in depth and intensity of directly excited x-rays. A computer was used for the numerical integration of his resultant equation. Again the calculated distribution in depth was not too satisfactory. The limitation on the accuracy of these techniques has been a lack of data on the number and distribution of energies of electrons a t each depth in the sample. I n a series of papers, Cosslett and Thomas (151--136) have described work on measurement of energies and angular distribution of electrons transmitted and backscattered from thin targets. Shimizu and Shinoda, using wedge shaped specimens, have attempted to determine the spacial distribution of electrons in the sample (525, 526). The spacial distribution was also studied by Ehrenberg and King (190) by using electrons incident on fluorescent materials. The only unanswered question in this study in regard to x-ray production is whether the spacial distribution for x-rays is the same as for visible light. Computers are also being used for calculating composition from x-ray
intensities. The method of Criss and 13irks (140) for fluorescence and absorption corrections in which ~ ( p z )is approximated by an exponential power series has already been mentioned. Brown (81, 82) has programmed Philibert’s absorption equation and Castaing’s fluorescence equation so that measured x-ray intensities from an electron probe can be converted directly into composition. These programs, along with other unpublished computer programs which various groups have developed, reduce the manual labor involved in the complicated procedure of reducing x-ray intensities to quantitative compositions. As time goes on, we can expect the evolution of more complicated programs for comparing and evaluating all of the correction prodedures. The goal of quantitative microanalysis-accuracy to within 1% of the amount present, using pure elements as standards-will be achieved only by very complex equations whose solution will require the use of computer\. ANALYSIS OF LOW ATOMIC NUMBER ELEMENTS
The analysis of low atomic number elements has received much impetus in the past two years as so-called “light element” kits have become available for most of the commercial electron probe microanalyzers (406, 449, 450). The preferred system a t the moment is a dispersive system consisting of a soap film crystal with a thin window proportional counter. The only exception is a nondispersive system designed as a plug-in to a Cambridge Instruments microprobe described by Ranzetta and Scott (474, 4‘75). Wardell and Cosslett (599) studied the stability necessary for effective use of nondispersive analysis of the low atomic number elements and achieved a stability of better than 1%. Franks (220-222) worked on the development of gratings for long wavelength x-ray analysis and showed that differences in OKa intensities could be detected in the oxides of copper. Applied Research Laboratories has developed a grating spectrometer for their electron probe microanalyzer. They report greater intensities with gratings than with soap film crystals. The availability of spectrometers should provide impetus to measurement of x-rays in the 10- to 100-A. range and stimulate the investigation of approaches to quantitative analysis for low atomic number elements. A number of unsolved problems remain before quantitative microanalysis in the wavelength region of 10 to 100 A. is possible using pure elements or compounds of quite different composition than the unknown as standards. Mass absorption coefficients are not accurately known to this region. The
tables of Heinrich (271), which are the most complete tables of mass absorption coefficients available, do not include wavelengths greater than 12 A, Some recent measurements by Cooke and Stewardson (130) for 5 elements cover the range from 7 to 17 A,, but hardly fill the need for reliable mass absorption coefficients for wavelengths a t least to the C Ka line (44.6 A). Because of the large absorption of the long wavelength x-rays, the measured x-rays come from extremely small depths of sample. Thus sample preparation is even more critical and sample contamination, which has negligible effect on intensities of shorter wavelengths, must be considered. Ranzetta and Scott (475) found that with the electron probe stationary, CKa intensities remained essentially constant since a ring of carbon contamination was formed around the point of impact with little contamination a t the center. During beam scanning, however, intensities were markedly affected because of a gradual buildup of contamination over the entire scanned area. They also point out the critical problems of sample preparation in the analysis of uranium carbides (474). Campbell and Gibbons (97) found that the most effective method of controlling the rate of contamination was by using a cold finger in the vicinity of the sample. Heating the sample to 300’ C. reduced contamination by an order of magnitude. Taeffner and Theisen (561) were able to eliminate contamination completely by using a controlled air leak directed toward the surface of the sample. Besides practical problems of obtaining stable and representative intensity measurements and accurate parameters for use in corrections for absorption and fluorescence, more serious theoretical problems remain to be solved. The shifts in wavelength reported by Fischer and Baun (201, 203-209) certainly indicate the necessity for some technique of scanning over a wavelength range to determine the peak area or maximum peak height. Even then it is no longer certain t h a t x-ray intensity is directly related to concentration. One of the most disturbing observations has been the differences in intensity reported from various forms of pure carbon (449, 476). The intensity from pyrolytic graphite was 72y0of that from diamond. This difference may be due to porosity, but independence of physical state has been one of the important conditions for quantitative analysis. Some work has been done toward evaluating correction procedures in the analysis of carbides by Ranzetta and Scott (476). Using Philibert’s method for the absorption correction, they found that the measured carbon content
was too low but increased as the electron voltage decreased in the range from 20 to 1 k.e.v. , thus minimizing the absorption correction. Using Archard and Mulvey’s correction proved more successful. =It this time, it is not certain if more accurate mass absorption coefficients or the modification of Duncumb and Shield’s would lead to better results using the Philibert equation. Manzione and Fornwalt (390) measured C Ka intensities from a large number of carbides. Their results are given in Table VII. Their intensity values appear to be almost a random function of composition, even after considering an absorption correction. For example, the intensity from UC is about a factor of 10 greater than the intensity from B4C, even though the concentration is only 1/4 as large. The difficulties can be resolved, however, if the work of Burk (87) is considered. From pure elements and for relatively long wavelengths, such as AlKa, a plot of intensity us. electron voltage is observed t o pass through a maximum. Beyond a certain voltage, the x-ray intensity from a pure element actually decreases with increasing voltage even though the mass absorption coefficient of an element for its own radiation is small. This is because x-rays are generated at a depth a t which large absorption takes place on leaving the sample. The same effect can be seen in the data of intensities from the carbides. For low atomic number carbides, intensities are relatively low since electrons penetrate deeply into the sample and the x-rays are then absorbed. For carbides of the heavier elements, the x-rays are generated closer to the surface and higher intensities result. At the present time quantitative analysis of low atomic number elements is possible only if standards close in composition t o the unknown can be obtained. Using carefully prepared standards, Andersen, Keil, and Mason (15) describe the characterization of a silicon oxynitride (Si2N20) phase found in a meteorite by quantitative analysis for both nitrogen and oxygen, They estimate an accuracy of their analysis of 2% for silicon and 15% for oxygen and nitrogen. ELECTRON CURRENT MEASUREMENTS
Measurement of sample current provides an alternate method of determining composition of microvolumes of samples containing only two elements. Such determinations are important since analysis by sample currents may provide reliable analyses which can then be compared to compositions determined from x-ray intensities, thus providing a means of establishing the accuracy of the theoretical corrections VOL. 38, NO. 5, APRIL 1966
431 R
for x-ray intensities. Considerable interest has been shown in the past two years toward measuring electron backscatter coefficients and relating sample current to composition. Weinryb (605) and Weinryb and Philibert (604) describe techniques for measuring backscatter electron coefficients in a Cameca electron probe. Using a biased grid between the sample and pole pieces to suppress the secondary electron currents they chim good agreement with measured values of backscatter coefficients. Their values, however, are about 10% lower than the generally accepted values for high atomic number elements. These lower values may be due to neglecting the backscattering from the grid and pole pieces. Burkhalter (89), also using a biased grid between the sample and pole piece but in an electron probe in which the sample is tilted relative to the electron beam, obtained accurate backscatter coefficients by careful analysis of the contributions of secondaries and currents from the grid and pole piece. A complete understanding and evaluation of the various contributions to sample current is a prerequisite to the general application of sample current to the analysis of binary compounds in probes of any geometry, since it is the backscatter coefficient which is fundamentally related to composition. Heinrich (268) studied the dependence of sample current on composition for binary compounds by using three proposed equations on data from six samples of known composition. He successfully showed that the sample current is a linear function of composition. At the same time, by biasing the sample, he demonstrated that sufficient electrical conductivity to prevent charging of the sample is important in current measurements. Colby (121, 122) using the same linear expression obtained quantitative results on six uranium samples with 30 k.e.v. electrons. Below Table VII.
~~
100 21.7 30.0 20.0 19.1 13.3 11.6 11.4 5.9 6.3 6.2 6.1 -~
uc1.5
4.8 7.0 9.2
uc2 a
Carbon content, wt. 70
WC
uc
SAMPLE PREPARATION
Sample preparation is one of the most important steps in quantitative microanalysis. The use of the most elegant calculation procedures are to no avail if the material analyzed a t the surface of the sample is not representative of the phases, inclusions, or precipitates present in the bulk of the sample for which analyses are required. Since analysis is performed on a layer only a few microns thick, the analysis is sensi-
Intensities Measured from Carbide Samples in the Electron Probe os a Function of Electron Accelerating Voltage”
Specimen Diamond BaC Sic Tic VC CrG ZrC NbC Mo~C HfC TaC ~
20 k.e.v. the results were less reliable, probably because of the increasing importance of secondary electron currents. Both Colby’s and Heinrich’s measurements were made using ARL microprobes. As a result of their work on the changing character of sample current beam scanning pictures with changing sample and grid bias, Weinryb and Philibert (604, 605) concluded that quantitative analysis is not feasible using sample currents. Ogilvie, in the discussion section following Heinrich’s paper (268),expressed the same opinion. Summing up, quantitative analysis using sample currents has been shown to be practical, a t least in the case of one experimental geometry. The studies of Burkhalter indicated the conditions necessary for analysis in any geometry; however his work has not been proved on the analysis of samples. Sample currents are quite sensitive to topographic features of the samples. Electron probe users have observed the large increase in current when the electron beam is incident on a crack or hole in the sample. Komoto and Hashimoto (558) have used the difference in output of electron detectors placed at the same take-off angle on opposite sides of the sample to gain a measure of the surface relief of the specimen. Conty (128) has obtained scanning pictures of “true” secondary current by combining backscatter, beam, and sample currents.
Electron accelerating voltage 20 k.e.v. count 30 k.e.v. count rate, c.p.s.* rate, c.p.s.b rate, c.P.s.~
10 k.e.v. count 488.4 5.3 11.1 43.9 30.1 12.6 7.6 10.5 4.1 9.7 7.8 5.7 50.7 64.1 74.9
Data of Manzione and Fornwalt (590). Corrected for background taken on the pure metal.
432 R
ANALYTICAL CHEMISTRY
847.5 7. . 6_ 16.7 66.0 45.6 28.2 14.6 17.2 9.5 16.3 14.3 11.7 113.0 139.0 164.8
1284.0 10.1 24.3 95.7 74.7 45.2 25.7 28.0 12.4 23.2 28.2 20.9 211.4 253.6 281.0
tive to changes in concentration caused by leaching or deposition of materials during the polishing operation. Extreme caution must be exercised in the use of etchants, particularly when analyzing minor phases or inclusions, since major compositional changes can take place. Deposition of polishing materials in soft phases or crevices left when hard inclusions are plucked from the sample can lead to an analysis of the polishing material rather than the sample. The smaller the inclusions, the harder it is to determine if the inclusions are really there. More attention is being given to sample polishing from the standpoint of electron probe microanalysis. For large mineral samples two techniques have been described for obtaining relief-free surfaces. Cadwell and Weiblen (94) describe polishing procedures for minerals using diamond abrasives and alumina on polyethylene film, which reduces relief compared to the more common types of polishing cloth. Taylor and Radtke (563) describe a five-step procedure using automatic polishing equipment, diamond for rough polishing, and alumina for final polishing, which is suitable for all types of mineral samples. Several procedures for mounting specimens were also described. One novel technique for preparation of metal sample surfaces was using a microtome having a diamond knife. Kirchberg (329) claimed that flat smooth surfaces with no smearing of soft phases could be obtained which were suitable for electron probe microanalysis without further treatment. Knives with carbide tipped edges were used for preliminary leveling of the sample to prevent excessive wear of the diamond knife. A number of mounting techniques are described for samples which are porous or are small grains or pieces. For porous samples Reichard and Coakley (482, 484) found that impregnation with epoxy in vacuum yielded a sample which would polish well. The same authors (484) also describe techniques for mounting powders in indium or soluble cement mixed with graphite. If embedded in a conducting medium, nonconducting particles up to 1 or 2 mm. cross-section did not require a coating. Landis, Merchant, and Zemany (3b5) used lucite for mounting wear debris. I n problems in which surface properties or contamination are being investigated, it may not be possible to polish the sample a t all. Schreiher (515) studied electrical contact surfaces by pressing specimens into lead powder. Mellors (404) analyzed bone sections by imbedding them in epoxy and coating them with a thin layer of carbon (-200 A.) for electrical conduction. He had no success in preparing thin sections of bone for electron
probe microanalysis. Tousimis (575) has outlined several methods of preparing biological specimens using a microtome on fixed tissues embedded in methacrylate or epoxy resins. Gall6 (227, 228) and Hall, Hale, and Switsur (264) also describe techniques for preparation of biological specimens. Degradation of the sample due to action of the electron beam is seldom a problem, even in the case of biological materials, provided proper techniques of coating the sample are used and beam currents are kept low. The only reported problems of beam interaction with mineral samples are the decomposition of carbonates, the loss of potassium in certain systems, and drilling of holes into silica samples observed by A4dler (5). Sulfur also decomposed under the action of the electron beam, even when coated. Gallium cannot normally be used as a standard be'cause of its low melting point. However, Hakkila et al. (263),by mounting the gallium in '/,-inch diameter copper tubing and cooling with dry ice prior to inserting the standard in the probe, were able to use gallium as a standard for 1 to 2 hours without melting under the electron beam. Almasi et al. (10) had observed some degradation of low thermal conductivity semiconductors due to localized heating by the electron beam. A thorough study of the problem showed t h a t a '/(-micron layer of aluminum reduced the temperature effects sufficiently SO that the material remained unaffected. Radioactive materials present special problems to keep the electron probe free of contamination. Hakkila et al. (263) used normal techniques of polishing in a glove box for preparation of galliumplutonium samples. Ultrasonic cleaning was used to remove loose particles which could contaminate the instrument. They do not recommend the use of formvar coatings since these char on interaction with the electron beam and small pieces could thereby flake off and contaminate the instrument. Dupuy, Moreau, and Calais (180),on the other hand, claim that ultrasonic cleaning deteriorated polished surfaces and describe a method of coating samples with formvar which they find is entirely satisfactory. INSTRUMENTAL
+illmosta half-dozen new instruments have been introduced during the past two years. Most of these are improved instruments developed by the established manufacturers of electron probe microanalyzers. Several of these have been described, a t least in part, in the literature (84, 151, 347, 451). There are two major trends which can be seen in the design of these instruments. First, they have tended to become modular in construction so t h a t a sim-
ple, low-cost basic instrument can be purchased and then as funds become available, be expanded to a full instrument with all accessories such as electron beam scanning, multiple x-ray spectrometers, and Kossel line cameras. The second trend is toward simplification of the electron and x-ray optics so that maintenance, servicing, and assembly are much more rapid. One hears reports of electron probe microanalyzers being uncrated, assembled, and aligned ready for operation in 3 or 4 hours-something unheard of just a few years ago. The increase in number of designs of electron probe microanalyzers has not simplified the choice of an instrument. Most of the manufacturers can offer all of the glamorous accessories, such as beam scanning, light element kits, and sample current display. However, because of differences in geometry, methods of viewing the sample, sample stage motions, number of spectrometers, or ease of operation, a particular instrument can have advantages for particular applications. No instrument is universally superior for all applications. The requirement3 for an electron optical system for an electron probe microanalyzer have been outlined by Ogilvie (447) and Fisher ( % I O ) , while Fitzgerald (211) has determined the factors affecting beam stability. The conditions necessary to maintain constant current and a beam size of + O . l micron are listed. The most exciting development in electron optics has been the miniature lens of LePoole (361). This lens consists of a thin-walled tube, 25-50 mm. in length having a bore of 1-3 mm. and only 6-10 layers of 0.3mm. diameter wire. Because of its small size, the problems of space in the vicinity of the sample necessary for large take-off angles should be greatly reduced. Development of these lenses would drastically alter the appearance of electron optical columns. The geometry of the commonly used types of curved crystal spectrometers was reviewed by Ogilvie (446). The relative merits of several types of crystals are also listed by Ogilvie. More information on crystals is given in the x-ray section of this 1966 review. Usually, the information from flat crystals can be carried over to curved crystals. However, in somecases, such as mica, which is a very satisfactory curved crystal but has never been a satisfactory flat crystal, the bending process can either improve or degrade the crystal. A Russian curved crystal x-ray spectrometer (165) in which the detector is a constant distance from the crystal, and not on the focusing circle, has been reported. Compactness is the major reason for moving the detector off the focusing circle. Peak-to-background ratios of the order of 200 to 1
were obtained with a LiF crystal. Duncumb (173) has attached both an x-ray spectrometer and a nondispersive detection system to EMMA and describes some of the results obtained from carbide particles down to approximately 0.1 micron in diameter. Heinrich (269) has continued to develop the techniques of x-ray readout, particularly in reference to beam scanning. The use of color filters on multiexposure beam scanning pictures, to obtain color photographs in which areas rich in different elements are different colors, is particularly intriguing. Others (331, 606) describe techniques of oscilloscope presentation of both linear scans and beam scanning pictures. h few devices have been described which are particularly useful for one type of instrument but with ingenuity may be applied to others. A specimen holder for the Xorelco probe for viewing mineral samples in transmitted light has been developed (353). Another useful device is a removable Mylar window for isolating the x-ray spectrometers in a Cambridge probe so they may be adjusted (483). Finally, a current amplifier for use in beam scanning pictures of sample currents for one of the British instruments has been described (150). APPLICATION
The number of papers in which electron probe microanalysis has been used is increasing rapidly as the number of instruments has increased. One striking aspect in reading the papers on applications has been the use of the electron probe as a sleuth to unearth clues regarding the failure of components or to trace down sources of contamination (118, 120, 304, 511,516). Applications are gathered under various headings in Table VIII. This table can no longer be a complete listing of applications of electron probe microanalyzers which have appeared in the literature. The references have been selected to illustrate effectively the use of the instrument and the information which can be obtained. The table has been broken into two sections; one for qualitative and semiquantitative and one for quantitative analysis. This division, while somewhat arbitrary, can serve as a guide to those authors who have attempted to obtain quantitative results. One of the important areas in which electron probe microanalyzers have found wide application has been in the analysis of integrated circuits; whether by x-ray emission or currents generated within the device due to interaction of the electron beam (194, 195, 379, 438, 479). Wittry and Kyser (349, 618, 61 9 ) have published several papers on the generation of infrared radiation in GaAs semiconductor material on interVOL. 38, NO. 5, APRIL 1966
* 433 R
Table VIII.
QUANTITATIVE Meteorites Corrosion products Solid state solubility Biological General metallurgical Metal phases Mineral phases Diffusion Surface and thin layers Ceramic and semiconductor materials Low atomic number elements QUALITATIVE A X D SEMIQUANTIT.4TIVE Meteorites Corrosion and oxidation
(31, ‘119,’ 121,’ 245, 263, 336, 457, 499, 626) (15, 322, 426, 537, 554, 555) ( 1 6 , 179, 239, 254, 332, 391, 527, 570, 586) (292, 580) (477) (16, 390, 476)
2r2, 500, 513,
Biological General metallurgical Inclusions and precipitates Metal phases Surface and thin layers Ceramic and semiconductor materials Beam scanning
action with the electron beam. The intensity varies by several orders of magnitude depending on impurity content. I n the visible region, cathodoluminescence continues to be an intriguing but illusive phenomenon. Long and Agrell (371) have made some correlations between minor impurities and color of cathodoluminescence in mineral samples, but these are not quantitative. Colby (1.26)has used the color of cathodoluminescence as preliminary identification of hydrides in uranium shot. Developments have also occurred in the Kossel line technique. These have centered around the interpretation of patterns. For the cubic case a computer program has been written which can be used to determine the orientation and wavelength necessary to obtain intersection of conics for accurate lattice parameter measurements (231). Evaluation of the precision which can be achieved has been carried out by Yakowitz (652). Continued development can be expected in the analysis of noncubic Kossel line patterns. Several electron probe microanalyzers have Kossel camera attachments available, but because of the time-consuming nature of this technique and the lack of free time in typical electron probe installations, Kossel line developments primarily will occur separate from electron probe microanalyzers. ACKNOWLEDGMENT
The reviewers wish to acknowledge the extensive clerical and technical assistance received in the preparation of this manuscript; in particular, preparation of the list of references cited was greatly facilitated by the computer program prepared by Robert L. Myklebust. We also acknowledge the cooperation of the many authors 434 R
(21) Badgett, C. O., ORNL llC-5 p. 72 (1964). (22) Baird, A. K., Henke, B. L., ANAL. CHEM:37, 727 (1965). (23) Baird, A. K., RIcIntyre, D. B., Welday, E. E., Develop. Appl. Spectry. 4, 3 (1965). (24) Baker, P. S., Gerrard, AI., Eds., 1964 Symp. on Low Energy X- and
Applications of Electron Probe Microanalysis
ANALYTICAL CHEMISTRY
Gamma Sources and Applications, Illinois Inst. Tech. Res., ORNL 11C 5. (25) Ball, D. F., Analyst 90, 258 (1965). (26) Ball, T. K., Filby, R. H., Geochim. Cosmochim. Acta 29, 737 (1965). (27) Banerjee, B. R., Bingle, W. D., Develop. Appl. Spectry. 3, 3 (1964). (28) Banerjee, B. R., Bingle, W. D., “The Electron Microprobe,” p. 653, Wiley, New York, 1966. (29) Baronin, V. N., Betin, Y. P., Verk-
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