X-Ray Absorption and Emission William I . Campbell, U. College Park, Md.
S.
Department o f the Interior, Bureau of Mines, College Park Metallurgy Research Center,
lames D. Brown, University o f Western Ontario, Faculty o f Engineering Science, london, Ontario, Canada
A s
IK OUR 1964 (116) and 1966 (117)
reviews, we continue to use the format established by our predecessors Liebhafsky, Winslow, and Pfeiffer (413). This 1968 review consists of a critical evaluation of new developments and tabular summaries of X-ray spectrography and electron probe microanalysis. Stoddart and Dowden (614) recently compiled a comprehensive bibliography on fluorescent X-ray spectrography. Their bibliography is indexed by the element determined, matrix analyzed, author, equipment, method, and theory. At present the principal application of X-ray emission and absorption techniques is to determine the concentration of specific elements. I n the future X-ray emission techniques will find additional applications as analytical chemistry becomes more sophisticated in characterizing materials. The Committee on Characterization of Materials, Materials Advisory Board, National Research Council developed the following definition (441): “Characterization describes those features of the composition and structure (including defects) of a material that are significant for a particular preparation, study of properties, or use, and suffice for reproduction of the material.” Lowenergy X-ray spectroscopy supplemented by photoelectron and Auger spectroscopy can provide information regarding valence, coordination, bonding energy, and the composition of surface layers. We agree with Liebhafsky et al. (412) that the future of low energy X-rays lies in the interpretation of spectra rather than the determination of concentration. Low-energy X-ray and electron emission techniques are emphasized in this review. Energy dispersion techniques using radioactive isotopes as the source of excitation have grown rapidly during the past two years. This growth is due, in large part, to improvements in detector technology and to the commercial availability of a wide selection of isotopic sources. The lithium-drifted semiconductor detector with its high resolution capability opens new avenues of X-ray analysis. Energy dispersion and the semiconductor detector are extensively covered in this review. I n electron probe microanalysis, there has been significant progress in defining and evaluating the parameters that are used to relate measured X-ray intensi-
346 R
ANALYTICAL CHEMISTRY
ties to composition. The evaluation of these parameters and their effect on the calculated concentration have been greatly accelerated by the use of computers. Heinrich outlined a plan for cooperative action on the determination of X-ray mass absorption coefficients (300). Better knowledge of mass absorption coefficients is necessary for theoretically reliable methods for computing concentration. Cooperative efforts in determination of the various parameters, such as mass absorption and fluorescent yield, are to our mutual advantage. Your support is requested in these efforts. The critical tabulation of K, L, RI, N, and 0 spectral lines by Bearden was recently published in Review of JIodern Physics (47). This tabulation was available previously as an AEC report. Atomic energy levels were compiled by Bearden and Burr (48). I n their calculations, the wavelengths of all available emission lines of an element were used to form a n overdetermined set of equations that were solved by least squares to give a best set of relative energy levels. These values were then placed on an absolute scale by X-ray photoelectron measurements or by using wavelengths of the absorption edges. X-ray wavelength and absorption edge tabies were computed by Dewey for use in electron probe microanalyzer calculations (183). Brockman and Whittem (90) compiled a condensed 28 table for first order spectral lines. As stated in our 1966 review, there are excellent wavelength and 28 tables already available. We recommend the use of Bearden’s tables (47, 48) for any additional 28 tables. Recent textbooks and conference proceedings are listed in Table I. “Advances in X-Ray Analysis” (430, 474) provides excellent summaries of recent developments in X-ray emission and electron probe microanalysis. There are three new textbooks on X-ray emission techniques (5, 354, 468) and two books on electron microprobe analysis (496, 631). Hopefully some publisher will make Muller’s book available in English (468). There is occasionally a duplication of effort in the translation of textbooks. The book by Ulokhin (73), recently made available in English by Pergamon Press, was translated into English in 1962 by the Hindustan The Publishing Company (117).
“Handbook of X-Rays” was approximately five years in preparation. This comprehensive book covers all aspects of X-ray analysis, including X-ray emission and microanalyzer instrumentation and techniques (358). Because of the desire to achieve more rapid and detailed communications regarding their research, the Spectrochemical Section of the Kational Bureau of Standards is preparing detailed summaries of yearly progress (563, 664). These reports summarize facilities, publications, personnel, and more important, research in progress. Reports of this type from all of the major laboratories engaged in X-ray research would aid in reducing duplication of effort. Continuing education is becoming a way of life for most scientists, including those whose interest lies in X-ray methods of analysis. Excellent introductory courses are provided by several of the instrument manufacturers a t their plants or a t various locations throughout the United States. Summer workshops at various universities provide an excellent introduction to X-ray emission and electron probe microanalysis. Technical societies are offering short intensive courbes as part of professional development programs. For example, the Washington-Baltimore Section of the Society of Applied Spectroscopy is presenting a series of five 2-hour lectures on X-ray emission techniques. RIore emphasis on radiation safety is strongly recommended, particularly as there will be a significant increase in the use of isotopes for energy dispersion analysis and in applications involving X-rays in the 50- to 100-key range. INSTRUMENTATION
General. The manufacturers have continued the trend toward completely automated and programmed X-ray spectrometers. Instruments are now available t h a t can detect all elements from fluorine u p through t h e periodic table. T h e program ill automatically select the 28 angle, analyzing crystal, Soller slit, detector, and pulse height analyzer setting, and then print out both the counts collected and the concentration. Obviously the accuracy of the analysis is no better than the match between standard and un-
known, or the reliability of the mathematical correction procedures used. During the past two years more attention has been given to two possible souices of instrumental error. The crystal chamber must be kept constant to k0.5OC to avoid systematic errors resulting from small shifts in peak position due to changes in d-spacing of the crystal. This problem becomes more serious when using high resolution optics with aut omat ed spectrometers. Also, the temperature and pressure of the counting gas in flow proportional counters must be stabilized because the gas amplification factor is affected by both of these variables. Because of the difficulty and cost of obtaining an adequate supply of helium, the vacuum spectrometer is widely used outside the United States. This vacuum system results in slightly higher intendies owing to increased X-ray transmission of lowenergy Xrays; however, liquid samples are awkward t o analyze in a vacuum spectrograph. Squirrel1 (608) modified a P W 1540 spectrometer to use either a partial vacuum-helium atmosphere or helium only. Excellent results were obtained with volatile organic liquids using the helium path. Campbell and Hammond (114) automated a commercial X-ray spectrograph for continuous operation. The objective was to obtain the lowest possible limit of detection and to optimize analytical precision by taking 100 to 200 measurements over extended counting periods. A unique spectrograph was described by Zeitz (698) which uses a very thin transmission-scatter biological sample enclosed in a vacuum chamber. The amount of the element present is determined by intensity measurements using a curved-crystal spectrometer. The characteristic X-rays are transmitted through the sample to a slit on the focusing circle. The sample weight is obtained from scattered X-rays using a propo~tionalcounter readout. The Henke X-ray tube has been adapted to fit on a commercially available vacuum spectrometer (257). Samples are e x i t e d by low-energy X-rays, thus avoiding the problems of decomposition that occur when unstable compounds are subjected to electron bombardment. Although the grating spectrograph of Holliday (328) offers superior resolving power, the flat crystal spectrometer using an oriented soap film for dispersion is adequate for most applications (3f3). Probably the most versatile spectrograph for the lowenergy X-ray region is the instrument designed by Mattson and Ehlert (55, 449). By changing voltage on either the cathode or anode, samples can be excited by electrons or X-rays. I n another configuration wires suspended
Table I.
Author Adler, I. Blokhin, M. A. Brown, J. G. Castaing, R., Descamps, J., Philibert, J., Eds. Elion, H. A.
Summary of Recent Books
Title X-Ray Emission Spectrography in Geology Methods of X-Ray Spectroscopic Research X-Rays and Their Applications X-Ray Optics and X-Ray Microanalysis Instrument and Chemical Analysis Aspects of Electron Microanalysis and Macroanalysis Practical X-Ray Spectrometry Handbook of X-Rays Elektronenstrahl-Mikroanalyse
Advances in X-Ray Analysis, Vol. 9
Newkirk, J. B., Mallett', G. R., Eds. Theisen, R. h e r . SOC.Testing Materials Baker, P. S., Gerrard, N., Eds. International Atomic Energy Agency Parrish, W., Ed. Siegbahn, K.
Spektrochemische Analysen mit Rontgenfluoreszenz Advances in X-Ray Analysis, Vol. 10 Quantitative Electron Microprobe Analysis Sections on X-Rays Fifty Years of Progress in Metallographic Techniques Second Symposium on Low Energy X- and Gamma Sources and Applications Radioisotope Instruments in Industry and Geophysics-Vol. 1 X-Ray and Electron Methods of Analysis Alpha, Beta, and Gamma Ray SpectrosCOPY
across two insulators are excited by electrons. The temperature of the wire can be varied by ohmic heating to remove any surface contamination. Gases can be introduced into the chamber containing the wire to investigate gas-solid reactions at selected temperatures. It is also possible to ionize gases at low pressures by electron excitation. The sample chamber and X-ray optical system are designed so that X-ray photons emitted by the chamber material do not enter the X-ray spectrometer optics. Their instrument has been used for surveying a wide variety of materials (2O4,443). Campbell et al. constructed a soft X-ray spectrometer for the study of valence bands of metals (112). Their instrument uses a blazed concave replica grating at grazing incidence, with a photomultiplier as the detector. A moderately low-energy Xray spectrometer was employed t o measure X-ray emission from the deuterium plasma of a theta pinch reaction (395)*
Direct electron excitation X-ray spectrometers are commercially available. The Telesec instrument can be preset for any combination of six elements (37). The manufacturers state that in the past there were three major objections to electron excitation-need for high vacuum around the electron gun, instability of system, and high background. According to Telesec, these objections have been eliminated in their instrument. Japan Electron Optics Laboratory (379) also developed a primary X-ray analyzer. The electron
beam diameter is approximately 5 millimeters; the incident electron power is 0.5 to 5 watts in normal operation. Specimen holders are insulated from ground by a Teflon bushing enabling the absorbed electrons to be measured by a micro-microampere meter. Signalto-noise ratios of 1700 to 1 and 1100 t o 1 were obtained for silicon K a and aluminum Ka,respectively, using pure element standards. Further information on electron excitation can be obtained from publications on electron probe microanalysis ($3). Excitation. There has been increased activity in t h e development of more efficient excitation on both t h e low- and high-energy X-ray regions. T h e stability of a commercially available X-ray generator was critically evaluated by Ashby and Proctor (28). Over a 23.5-hour test with line voltage fluctuations of *15% the X-ray tube current and voltage were constant to 1 0 . 0 2 and +0.05%, respectively. During these evaluations Ashby and Proctor observed drifts in X-ray intensity which could not be explained by electronic instability. These drifts in intensity were found to be a result of changes in the X-ray transmission of the air path between the sample and the detector. The density of the air was changing with variations in temperature and pressure, Because of the high electronic stability, this small temperature-pressure effect could be observed. Yee and Deslattes (691) found that a transistorized current stabilizer for X-ray tubes using directly heated cathodes was operational over VOL. 40, NO. 5, APRIL 1968
347 R
the 20- to 1000-mA range. The X-ray tube current was constant to =kO.l% for periods of a t least one-half hour. Linear polarization of X-rays was investigated by Huffman (558). The linear polarization of X-rays produced in a mercury vapor target tube was analyzed by measuring the scattered intensity from beryllium parallel and perpendicular to the emission plane. Because fluorescent X-rays are isotropic, line-to-background ratios can be optimized by viewing the sample so that scattering of primary X-rays is minimized. Machlett Laboratories (382) announced the development of an experimental thin-window platinum target X-ray tube. K i t h a 0.005-inch-thick beryllium window, this tube has characteristics equivalent to the conventional tungsten tube and the thin window chromium target tube. The dissipation of heat arising from backscattered electrons limits the size of their window to 0.5-inch diameter. Soviet scientists (594) designed a rhenium target X-ray tube that could operate up to a load of 3.5 kilowatts. The distance from the focal spot to the beryllium mindow is approximately 11 millimeters-about one half the distance of the Machlett OEG 60 tube. Operating at maximum power the beryllium window reached a temperature of 350°C. Electrostatic deflection of the backscattered electrons reduced the window temperature to 12O-15O0C. Taylor designed a highintensity rotating-anode X-ray tube that could be operated at 250-275 milliamperes at 30 kilovolts or 150 milliamperes a t 50 kilovolts (625). The focal spot is 10 by 1 millimeters, and the rotational speed is 1750 revolutions per minute. H e anticipated construction of a 25-kilowatt X-ray tube for spectrographic analysis. Kathren (362) utilized a 200-kilovolt industrial X-ray unit to generate “monochromatic” X-rays. The “monochromatic” X-rays were generated by bombarding secondary sources Ivith a high-energy primary beam. A 150kilovolt industrial unit was used to excite uranium K lines for detection of uranium contamination (561). With the significant improvements in detector technology, we anticipate many new applications of 50- to 100-keV X-rays. Small X-ray tubes (30-mm diameter) were evaluated for drill hole logging of minerals. These tubes could be operated up to 100 kilovolts, giving a n X-ray output of 1Olo to lo1’ photons per second ( 2 ) . Gas-filled tubes operated intermittently and their output was only one half to one fourth that of a heated-filament X-ray tube. Miniaturized X-ray tubes were evaluated for excitation of low atomic number elements on the lunar surface (172, 198, 464). 348 R * ANALYTICAL CHEMISTRY
The Cristallobloc 31 X-ray tube has a cylindrical rotatable anode with six faces for fluorescent excitation (635). When the primary excitation mode is used, the sample is placed in slits machined in the copper block. The Raymax demountable tube (580) was used to evaluate excitation parameters in the lowenergy X-ray region. Maximum intensity was obtained with high takeoff angle, low angle of incidence of the electron beam, and optimum voltage setting. Losev et al. (417) made similar studies on the escitation of silicon K radiation. I n addition they evaluated the output from 10 anodes ranging from beryllium to gold. Walker (659) used a commercially available demountable X-ray tube in a windowless configuration. Results were compared with those obtained with 30- and 100-micron beryllium windows. When the window was absent deflection plates were used to eliminate scattered electrons. One relatively simple way to increase intensity of the low-energy X-rays is to eliminate the plastic film holding the sample. Beard and Proctor (45) designed a windowless solution holder that gave a reproducible height of solution. The uncovered holders gave a 3- and 6-fold increase for sulfur and aluminum K a , respectively, compared with holders with 1/4-mil mylar windows. Davidson and Wyckoff (176) reexamined the cold cathode gas tube for use in the low-energy X-ray region. Contamination of the target was not a problem, in contrast to the buildup of impurities using the heated cathode. Their air-cooled tube could be operated up to 200 watts. A demountable X-ray tube was operated at 10 to 20 kilovolts and 200 to 300 milliamperes (182). The power on the tube is limited by the rate of dissipation of heat; this power can be increased by using pressurized coolant. Dispersion. Activity reported on t h e development of improved analyzing crystals has decreased during t h e past two years. Resolution and intensity of spectral lines are directly related t o t h e analyzing crystal; therefore a greater research effort is warranted. Isomet Corporation recently announced the commercial availability of OHM (octadecyl hydrogen maleate) crystals for use in the long wavelength region. These crystals offer high r,eflectivity and a 2d spacing of 63.5 4. Sparks (598) found that oriented hoteressed pyrolytic graphite, 2d = 6.70 A, has excellent potential as an X-ray monochromator. The diffraction efficiency of the 002 reflection for copper K a is approximately 50% if the divergence of the X-ray beam does not exceed the mosaic spread of the graphite. The peak width is full-width half-maximum. Sparks found that graphite
gives a 5-fold increase in intensity as compared to lithium fluoride. Crystals and oriented soap films used for X-ray analyzers are summarized in Tables I1 and I11 taken from a paper by Baun and Fischer (41). Of the newer crystals, clinochlore (a naturally occurring cLystal) looks promising for the 10 to 20 A region. As there appears to be a useful upper limit to the d-spacing of soap films, Baun and Fischer suggest alternate deposition of a thin film of a strong scatterer and a precisely controlled sandwich layer of a low scatterer. Although their approach is restricted by the present art of vacuum deposition, the suggestion merits further study. Chan (142, 143) found that certain PET crystals grown in his laboratory gave 2.5 times the reflectivity of E D d T and 9 times that of mica for silicon K a radiation. For a pure silicon sample, count rates in escess of 106 counts per second were achieved. Chan cautions that PET crystals should be kept in a desiccator when not in use. Advantages of soap films and gratings are compared by Henke (309). Previously, lowenergy X-ray spectroscopy was accomplished by diffraction gratings a t grazing incidence. Xew techniques for making low-angle blaze gratings have greatly improved intensities. More recently most of the research in lowenergy X-ray spectroscopy has been accomplished using Langmuir-Blodgett type of analyzers. Using the oriented soap films at a high B r a g angle, a significantly larger total solid angle of radiation is viewed as compared to the small angle grazing incidence geometry. Excellent discussions on the preparation and properties of soap films were presented by Ehlert and 3lattson (203, 205). Optimization of slit width for focusing spectrometers is discussed by Lavrent’ev and Vainshtein (405). Detectors. During the past two years there has been significant research on the pulse amplitude shift as a function of X-ray intensity (227). Burkhalter, Brown, and Nyklebust (104) compared a variety of sealed and flowproportional counters for peak shift as a function of counting rate. The two significant findings were that peak shifts were larger for flom-proportional counters than for sealed detectors and that the peak shift was a strong function of anode voltage. For esamole, a t 1500 volts the peak shift is 5 7 , coinpared with a 50% shift a t 1900 volts. I n contrast a sealed detector operated at 1750 volts gave no observable shift from 500 to 50,000 counts per second. Sanford and Cclhane (654) found that performance at high count rate could be improved by using a n inert gas of low atomic number, by increasing the detector capacitance (increasing the anode wire diameter), and by operating at a low gas multiplication factor. Spiel-
beig (601) concluded that the density 1011 space charge depends on the aiiode nile diameter, as most of thc charge multiplication in the detector takes place close to the anode wire. Therefole the peak shift should be reduced by incieasing the wire diameter. Veiy little shift was observed using a 0.012-cni-diameter wire. Higher ope1 atiiig voltage \\as required, but did not create any special problems. The type of anode material (602) was also found to have an effect on pulse shift. A detector using an anode wire of 50-,udiameter tungsten had a significantly loner shift thaii the same detector with a 50-p-diameter stainless steel wire. d gas pressuie control system for the flow counter was described by Spielberg (600). He concluded that, in an airconditioned room, additional temperature control on the flow counter gas was not required. Practical techniques for casting thin formvar films on water nere described by LIeriitt and Agazzi (.@2), A ruggedized thin window counter was designed for space applications (559). The variation of gain and resolution with time for a sealed thin window counter was investigated by Culhane et al. (166). The deterioration in gain and resolution mas restored by either placing the counter in a desiccator with phosphorus pentoside or by bubjecting the counter to dry ice temperature. dpparently the counter deterioration resulted from water vapor leaking through the windo\\. .ipplications of gas-flow and sealed proportional counters ale described in the following papers (111, 113, 15'9, 467, 654). An excellent source of information on scintillation counters is the recently revised book by I3irks (64). There have been major improvemeiits in the lithium-drifted silicon and germanium detectors during the past two years These detectors offer escellent resolution in the medium- and highenergy X-ray region. Excellent review paperq on the state of the art have been published recently (177, 270, 325327). If the reader is not familiar with the new detectors, the introductory papers by 13oanian et al. (83) aiid Heath (2Y6) are recommended. The active counting volume of a lithium-drifted detector is formed by drifting lithium into a p-type silicon or gernianiuni crystal under carefully controlled conditions (260). The function of the lithium i, to compensate for the effect of impurities The detector is coniprised of a region of compensated silicon or germanium sandwiched betneeii a very thin 1,-type and n-type region. 13y applying a bias voltage to the detector in the reverse direction, the detector becomes a high-impedance device. This electric field depletes the compensated region of free charge carot the po,itivc
Table
Name
II.
Analyzing Crystals ( 4 7 )
2d,
A
Topaz Lithium fluoride Sodium chloride Calcite Silicon
2.71 4.02 5.64 6.06 6.28
Fluorite Germanium
6.30 6.53 6.58 7.50 8.74 8.80 8.80 10.64 14.15 14.92 15.12 15.18 15.96 18.50 19.91 26.63 28.42 32.84
Potassium bromide Ammonium phosphate Pentaerythritol Ammonium tartrate Ethylene diamine d-tartrate Ammonium phosphate Ammonium tartrate Ammonium citrate Sucrose Gypsum Beryl Itaconic acid Mica Potassium acid phthalate Clinochlore Bismuth titanate Octadecyl hydrogen maleate Octadecyl hydrogen succinate
Table 111.
Lead Lead Lead Lead Lead Lead Lead
Name laurate myristate palmitate stearate arachidate lignocerate mellissate
Soap Film-Multilayer Analyzers ( 4 7 )
Number of carbon atoms 12 14 16 18 20
24 30
Notes Natural mineral Durable, strong reflection Cleavage, easily bent Durable natural mineral, cleavage Semiconductor slices may be used, suppressed second order Cleavage Semiconductor slices may be used, suppressed second order N o cleavage, easily bent Large natural growth face Soft, difficult to grow, intense reflections Cleavage, easy to grow from HzO solution No cleavage Easy to grow from H20solution Large natural growth face Natural growth face Difficult to grow, spontaneous nucleation Dehydrates in high vacuum, easily bent Natural mineral Cleavage Natural mineral, cleavage, easily bent Growth from H20 solution, cleavage Natural mineral, cleavage, easily bent Platelets, may be bent Both difficult to grow, long growth cycles, spontaneous nucleation
2d Spacing,
70 80.5 90 100.79 110 131.45 165
riers (electrons and holes). This depleted region is the radiation-sensitive volume of the detector. Typical detector areas and depths are 30 to 100 square millimeters and 2 to 5 millimeters, respectively. A large number of ion pairs are formed per incident photon, but these semiconductor detectors do not have the high multiplication typical of the pulse formed in the gas proportional type counter. Therefore, a very low-noise, high-gain preamplification is required (207, 208). This preamplification is achieved b y operating a field effect transistor a t liquid nitrogen temperature (661). The detection efficiency is the function of the depletion depth and the atomic number of the semiconductor. A 3-millimeter-thick silicon diode will absorb approximately 50% of 30-keV X-ray photons; however the photoelectric absorption of silicon decreases rapidly with increasing X-ray energy. For higher energy X-rays, germanium is the detector of choice because of its higher photoelectric absorption coefficient.
-1
Notes Possible degradation in vacuum Easy to prepare Easy to prepare Easy t o prepare More difficult to prepare More difficult to prepare and usually poor quality
The principal advantage of the lithium-drifted detector is the significant improvement in resolving power as compared to proportional and scintillation counters. The theoretical width of the measured pulse is related to the statistical spread in the number of electron-hole pairs formed by photoelectric absorption. Resolution is conventionally expressed by the following relationship FTVHM (keV) = 2 . 3 5 5 a (1) where E = energy of incident electron or X-ray photon in keV e = average energy to create electron-hole pair = 3.6 eV for silicon, F = the Fano factor For small-diameter detectors, the Fano factors for germanium and silicon are 0.15 and 0.20, respectively (296). The observed line width also includes contributions from various sources of noise in the detector and in the electronics, Resolutions of 0.5 to 0.7 keV ( F W H M ) have been reported for silicon detectors in the 5- to 30-keV region. The exVOL. 40,
NO. 5 ,
APRIL 1968
* 349 R
celIent resolution of the germanium detector for high-energy K X-rays is shown in Figure 1 (83). Of particular significance is the separation of the Kala2doublets of thorium and uranium. By using amplifiers with pole zero cancellation, the problem of peak broadening at high count rate has been virtually eliminated. Pulse widths did not show any apparent broadening for count rates up to 15,000 counts per second (296). The lithium-drifted detectors have great potential for energy dispersion analysis. Information can be obtained in a few minutes regarding the overall sample composition using a lithium-drifted detector coupled with a multichannel analyzer. We predict these detectors will find extensive appli-
b
a z
:w:
Figure 1 . K X-ray spectra of high atomic number elements using a lithium-drifted germanium detector (83)
In
a W a In I-
z
3
0 V
2
Lr,
I
K82
LL
0
m a
z
a
-
In 3
I
I-
,
,
0
60
70
Table IV.
In cement materials (178) In minerals and ores (184, 199, 384, 477, 653, 677) In miscellaneous materials (59, 376, 464, 616, 626, 642) In organics (54, 210, 263) In soils (360) Lead In alloys and metals (171, 432, 466, 697) In minerals and ores (666) In miscellaneous materials (433, 616, 630, 667) In organics (608) Magnesium In metals (372, 697) In minerals and ores (565) In miscellaneous materials (360, 394) In slags (178) Manganese (52, 77, 202, 372, 411, 466, 697, 677) Mercury (433)
350 R
ANALYTICAL CHEMISTRY
100
,
110
120
ENE'RGY.'keV
Specific X-Ray Spectrographic Analysis
Aluminum In alloys and metals (146, 162, 227, 234, 236, 400, 466) I n minerals and ores (199,360,384, 663, 677) In miscellaneous materials ( I 78, 394, 526) Antimony (400,457,479,681) Arsenic (431, 433, 457, 681 ) Barium (126, 372, 613) Beryllium (204) Bismuth (433) Boron (204, 230, 233) Bromine (542, 671, 608, 657) Calcium In cement materials (178. 677) In minerals and ores (199, 384, 477, 653, 677) In miscellaneous materials (394, 516, 626, 642) In organics (12, 54, 1.61, 499) Carbqn (230, 238, 370) Chlorine (12, 181, 384, 642, 668, 657) Chromium In alloys and metals (77, 202, 280, 372, 411, 457) In miscellaneous materials (64) Cobalt In metals and alloys (163, 411, 457, 697) In miscellaneous materials (464, 606, 630, 686) In organics (64) Copper In alloys (32, 171, 306, 229, 380, /ill,469) In metals (146, 372, 642, 697) In miscellaneous materials (184, 464) Fluorine (56, 443) Germanium (681) Gold (89, 144, 171, 206) Hafnium (3, 240, 259) Iron In alloys and metals (77, 1.45, 163, 234, 280, 372, 411, 466, 628, 597)
90
80 X-RAY
,
Molybdenum In alloys (457) I n miscellaneous materials (52, 184, 421, 668) I n steels (202) Nickel In alloys and metals (145, 153, 236, 280, 372, 466, 697) In miscellaneous materials (52, 63, 59, 163, 199, 464, 626, 642, 658)
Niobium I n alloys (400, 457) In miscellaneous materials (506. ~, 685) Nitrogen (230, 238) Oxvnen (LLc?) ar r Palladium (206) Phosphorus (12, 241, 400, 626, 677) Platinum (206) Potassium (12, 199, 384, 477, 499, 626) Rare earths In metals (146, 381, 628) In miscellaneous mixtures (11, 616, 683) Rhenium (457) Rubidium (621) Selenium (431, 433, 681 ) Silicon In alloys (202, 372, 400, 636) In cement materials (178) In minerals and ores (161, 199, 384, 653, 677) In miscellaneous materials (142, 143, 360, 394, 626) Silver (171, 206, 535) Sodium (59L 655) Strontium (740, 457, 621, 639, 682) Sulfur (12, 230, 262, 384, 451, 472, 658, 674, 608, 672, 673) Tantalum (457, 506) Technetium (421) Tellurium (103) Thallium (ASS. L57) Thorium (69, $0, 386, 683) Thulium (340) Tin (126, 139,400, 467, 465, 479, 630, 692, 681) Titanium In metals (400, 411) I n minerals and ores (384, 477, 563, 677) In miscellaneous materials (54, 506, 626, 683) Tungsten (31) Uranium (69, 293, 372, 386, 683) Vanadium (53, 180, 400, 457) Yttrium (27, 586, 578, 683) Zinc In metals (32, 77, 206, ?29, 372, 380, 434, 466, 697) In miscellaneous materials (69, 626, 666) In organics (263, $17,698) Zirconium In metals (269, 400, 624) In minerals and ores (3) In miscellaneous materials (464, 630) I
Y
~
\
I
cations in several phases of X-ray emission analysis. Grodski (272) and Uaun and Fischer (39)recently summarized the state of the art for photoelectric-type detectors. There are four basic types of these detectors : multidynode, secondary electron multipliers; resistance strip magnetic multipliers; continuous channel multipiers; and detectors in which photoelectrons are accelerated and subsequently counted. The detectors differ in the method by which the photoelectric signal is collected and amplified. All of the detectors are windowless-the incident X-ray photons are the direct cause of the photoelectric emission from a cathode. The photocathode material has a very pronounced effect on the detector efficiency. These detectors have a poorer signal-to-noise ratio than proportional counters, but their fast pulse rise time permits very high count rates, Presently application of the photoelectric detector is limited to the very low energy X-ray region. Characteristics of avalanche type semiconductor detectors are summarized by Huth and Locker (342). Possible advantages of the avalanche-type detectors are small size, ruggedness, and speed of response. At the present stage of development resolution is poorer than for the gas-filled proportional counters. QUANTITATIVE ANALYSIS
Emission. X-ray spectrography continues t o be used for a n increasing number of applications, both for research and for quality and process control (674). Methods for specific elements are listed in Table IV. Analysis of various classes of samples are summarized in Table V. In the comprehensive bibliography by Stoddart and Dowden (614), there is an index listing the element determined and the matrix being analyzed. Another excellent source of methods is the Applications Review, pnblished by Analytical Chemistry every two years. X-ray emission methods were compared to chemical (402) and optical (201) techniques with regard to accuracy, precision, range of application, difficulty, time, and cost. Although there has not been any major development, there has been steady progress in quantitative aspects of X-ray analysis. These applications range from incinerator slags (626) to logging minerals in situ (45.5) and the analysis of the Moon’s surface (172, 637). Backerud (32) presented a critical discussion of the determination of copper in complex brasses. The ratio of copper Kpl to zinc K a was used to eliminate dependence on surface preparation, sample size, and samp1e-to-Xray tube distance. Applications of various techniques were described in the following papers-
addition (425), dilution (167, 584), emission-transmission (410), external standard (507), fusion (184, 287, 690), line to scatter (74), and solutions (655). Smagunova et al. (585) compared the accuracy of seven standard methods for analyzing ores. Their studies included absorption correction, addition, dilution, internal standard, and line-tobackground. They concluded that the internal standard approach gave the most reliable values. Czamanske et al. (167) compared dilution techniques with simple briquetting for powdered samples. They reported that uncertainties associated with undiluted Samples, moderate dilution, and moderate dilution-fusion methods, make these techniques less generally applicable than suggested in the literature. They conclude that the analyst must either account for absorption and enhancement effects or create an essentially identical matrix for sample and standard. Formulas for the calculation of optimum amounts of diluent are covered by Duimakaev and Blokhin (193). Champion and Whittem (141) compared solution analysis on a weight basis rather than the conventional volume basis. Their paper merits widespread consideration because their approach reduces systematic errors. Matrix effects, the magnitude of which are related to the type and amount of acid or base added, are significantly reduced when samples are compared on a weight basis rather than by volume. A critical investigation of their approach is recommended. Applications of computer techniques to achieve quantitative results continue to increase in the production and control laboratories. The Lucas-Tooth computer program is the one that is widely used (206, 260, 390, 419, 553). Sanderson and Yeck list a program for use with a small computer (553). Their program is used for ferrotitanium ores and residues of widely varying composition. Alley and Myers (13) used multiple regression to analyze ingredients in a rocket propellant mix. Another regression technique (324) was used to obtain estimates of mass absorption coefficients as a function of wavelength. These coefficients were then employed in the calculation of concentration from measured X-r
