topic/ in chemic~linjtrument~tion
edited by
FRANK A . SETTLE, JR. vir~nia~itarylnstitute Lexington, VA 24450
X-ray Fluorescence Spectrometric Analysis of Geologic Materials Part 1. Principles and Instrumentation John A. Anzeimo Applied Research Laboratories, 15300 Rotunda Drive, Suite 301, Dearborn, MI 48120
James R. Lindsay US. Geological Survey, Branch of Geochemistry, 345 MiWlefield Rd., MS.938, Menlo Park, CA 94025
X-ray fluorescence (XRF) methods of analysis fmd widespread application in the analysis of geologic materials, steels, cements, and other materials. Formany years, XRF instrumentation has been used in research, quality control, and process-control analyses throughout the steel and cement industries. In geologic studies, XRF has become the method of choice for the analysis of the major and minor rock-forming elements and, in many cases, for measurement of trace constituents as well. Because of the broad application of X-ray spectrometric analyses in geology, many universities and colleges introduce XRF methods in the undergraduate curriculum for geology majors. Most often, X-ray fluorescence spectrometers are housed in the geology department rather than in the chemistry department. For this reason, graduate chemists often enter industrial, government, or university laboratories with little or no exposure to X-ray fluorescence John A. Anrdmo received his BS degree in chemistry from Loyola University in 1969 and pursued graduate studies at the University of Wisconsin and University of Penmylvanie. He joined Applied Research Laboratories (ARL) at its Dearborn, Michigan, facility in 1976 as a research scientist where he worked on universal calibration pmpms forsteel and cement plants using X-rav fluorescence iXRFl soechomebv. ~ r o 197910 i 1984. Anrelmomanagsdthe AR. X-ray labormay and was responsible tor appl~caions,demonsbat ons, nahmg. andcalibrations. Today, he isX-ray product manager for ARL wilh responsibility fn simultanwus and sequential XRF instrumew tation in the Western Hemisphere and Souiheast Asia. Anrelmo is a member of the AmarlcanChemical Society,Society 01 Applied Spectroscopy. a d task group chairman f a ASTM E02.09 on X-ray analysis of stainless steels. F a lhs last four years, he has served as a faculty member at Um State University of New Yark at Albany's X-ray clinic.
instrumentation or methodology. This article seeks to provide some insight into the principles and applications of XRF methods, especially in the analysis of geologic materials. X-ray spectrometry has a more impressive list of advantages than any other spectrographic method of chemical analysis (la). Its advantages include: the relative simplicity of X-ray spectra compared to optical spectra minimizes spectral-line interference; absorption-enhancement (matrix) effects are predictable and readily evaluated; and its versatility, speed, accuracy, and economy of operation commend it highly as an analytical technique. Its Limitations include: difficulties in analyzing the light elements (below atomic number 11, sodium) in liquids, relatively shallow penetration of solids, the need for standards in the same physical form as that of the analyte, and high initial cost of instrumentation. James R. L l n h y received a BA degree in chemisay from Rutgers Univershy in 1961 and an MS degree in inaganic chem istry from the University of Maryland in 1965.He joined the US. OeologicalSurvey as a chemist In January 1966 and was assigned lo the X-ray spechoscopy project in Washington, oC.His early work involved X-ray fluorescencemethods farthe quantltative analyses M geologic materials and electron microprobe analyses. In 1982. Lindsay became project chief of the X-ray specnoscopy prqect at the National Center in Reston. Virginia. In August 1984, he moved to the U.S. Oeological Survey laboramry in Menlo Park. CalifMia,where he is currently project chief of X-ray specnor oopy in me Branch of Geochemistry. h Bddition to his Work in X-ray spectroscopy. he is invoivec h research on the surface chemistry of bullding stones wing surface analytical memads such as X-ray photoelectron spectroscopy, secondary ion -s spectrometry, and omerrelatedtechniques.
Volume 84
Principles X-rays are electromagnetic radiation in the wavelength range from about lo-& to about 10 nm (1 nm = 1 0 F m or 10 A; the latter is a nonstandard unit still widely used in X-ray spectrometry). They are produced when high-energy electrons decelerate or when electron transitions occur in the inner orbits of atoms. In conventional X-ray spectrometry, the spectral region of interest is -0.01 nm (UK,) to -2 nm (F KJ. In ultrasoft X-ray spectrometry, the region of interest is -1 nm to -10 nm (Be K.). In practice, X-ray spectrometry is eoncemed with two parameters for X-ray measurement: X-ray photon energy (or wavelength) and the intensity of emitted X-rays. The photon energy and wavelength are related as follows:
where h is Planck's constant (6.6 X 10-27erg s), c is the velocity of light (3 X l o 3 cmls), and A is the wavelength. Substituting and converting to kiloelectron volts (keV) and nanometers gives
E,, = 123.96lA (nm)
(2)
In spectrochemistry, the intensity I of an X-ray beam is proportional to the number of X-ray photons entering the X-ray detector per unit time, that is, the number of photons counted per unit time.
Production ol X-rays When an atom is irradiated by a sufficiently high energy source, an inner shell (usually a K or L shell) electron (called a photoelectron) is ejected leaving the atom in an excited energy state. That state is not stable, and the atom returns to its stahle ground state via one or more de-excitation * processes. Moat frequently, the inner-shell vacancy created by electron emission is filled by an electron from a higher energy level. The ex(Continued on page A182)
Number 8
August 1987
off at longer wavelengths. Figure 1 illustrates the continuous and characteristic line spectra of a heavy element. cess energy from the transition is often dissipated as electromagnetic radiation of sufficiently high energy to he called an X-ray photon. These X-ray photons have a narrow enerw bandwidth. are snecific for the narticular electron transitions that occur, and arc characteristic of the element from which they are emitted. Thus. they arecalled characteristic X-ray lines. Another mechanism for the dissipation of the excess energy is through another outer shell electron. When this happens, the second electron is emitted from the outer shell of the atom. leavine it in a douhlv ionized exriced state. l'he~ecmdemitledelectron is called an Auger eleetnm. Additional interactions can occur hetween inner and outer shell electrons to produce photons in the visible and ultraviolet regions of the spectrum or to dissipate their energy in the form of heat. ~
~
~~~
".
~~
When electrons strike matter, they decelerate giving up energy in numerous, unequal increments as X-rays. When large numbers of electrons strike the target, a continuous band of X-ray wavelengths, known as the continuum, the white radiation, or Bremsstrahlung (literally, "braking radiation") is emitted. I t is characterized by having a continuous range of energies, l i i i t e d bythe ene rw,of the strikine electrons. Its maximum intensity orrurs at appn,aimately 1.5 times the short wavelength limit. gradually falling ~
~
A182
c, "
I(-,
lam,
-
28 !Z
= !I - L.P
~
Continuum
~
I(=,
~
Journal of Chemical Educatlon
!@
Mr WAVELENGTH h
-
outer shell electrons. Thus, eaeh atom has several combinations of possible transitions that can occur, each one producing X-rays of a unique energy. If the transition is to a K shell, the X-ray is called a K X-ray; if to an L shell, an L X-ray. Each X-ray is further identified by a subscript a (alpha), 13(beta), -,(gamma), etc., that identifies the X-ray as originating from an electron transition between a specific energy level within the initial shell and a specific energy level within the final state. The electron transitions responsible for X-ray emission are limited by a set of rules based on the four quantum numbers that define the path and energy of eaeh electron in the atom. Moseley (2) observed that the X-ray photons within any series (that is, the K or L X-ray) produced by these transitions increase in energy with increasing atomic number of the element. The law he derived can be expressed as
Ma
-
Flaure 1. X-rav. soemum of a heaw element. . showing lhe K. L, and M haracteristic line spee trum and me continuous spectrum.
~haracterlsticX-ray Llnes The characteristic X-rays emitted by an atom arise from electron transitions from high energy states to Lower energy states. The difference in energy between the two states determines the energy of the emitted X-ray photon. Electrons ejected from inner K, L, and M shells of an atom create vacancies that are then filled by any of several
where h (lambda) is the wavelength of the X-ray photon in nanometers and Z is the atomic number of the element emitting the X-ray photon.
lnteracllons wlth Matter X-rays impinging on matter can undergo a number of interactions: (1) transmission-the X-rays may pass through unchanged, (2) scattering, with no change in energy (Rayleigh scattering), (3) scattering, with a loss of energy (Compton scattering), (4) absorption, causing an electron tran-
-
sition to occw with the subsequent emission of a characteristic X-ray photon (fluorescence) and a photoelectron. Both types of scatter and photoelectric absorption of X-rays make up the total process for the attenuation of the ineident X-rav beam In general, the intensity i,f an X-ray beam that P ~ F R P S through matter obey9 Lambert's Law:
I = &.-"."
(4)
where I is the attenuated intensity, l o is the incident intensity, x is the thickness, and p, is the linear absorption coefficient. The ahsorption coefficient, a measure of the stopping power of a material, decreases with decreasing wavelength (increasing energy of the incident X-ray beam). A plot of the absorption coefficient of a particular element versus wavelength reveals abrupt discontinuities or drops in intensity, called ahsorption edges, in the m e . The wavelengths of the discontinuities correspond to the binding energy of the electron of each of the inner shells. Thus, there is one absorption edge for K X-rays, three for L X-rays, five for M X-rays, and so on. Another absorption coefficient will be of more importance when matrix effects are discussed. The mass absorption coefficient, (in cm2/g),is related to the linear absorption eoefficient by 6 = PJP
(5)
where p is the density of the element (in gIcm3) absorbing the X-ray. X-ray Fluorescence Analysis X-ray fluorescence is the term used to indicate the phenomenon (not the method) of X-ray excitation of X-ray emission spectra. X-ray emission spectrometry can be further classified on the basis of dispersion made as wavelength-dispersive and energydispersiue, depending on the method used to disperse or separate the X-rays of differing energies. X-ray spectrometers consist of three major components: (1) an X-ray source,
(2) a dispersion system to separate the X-rays fluorescing from the sample, and (3) a detector and measuring system to record the X-ray intensity
X-rav Source
. ..
.
The most wid& aoolied source of X-rawis the X-ray tube. It is composed of a solid target called the anodeanda filament called the cathode (usually made of tungsten) heated to incandescence by an electric current to supply electrons (themionic emission), all enclosed in an evacuated glass envelope. A high-voltage power supply maintains a relative high potential between the filament and the anode to accelerate the electrons to a high kinetic energy before they strike the target or anode. The target consists of a thin disk or plating of the target metal plated on a heavy, hallow, watercooled copper block, which conducts the beat away from the electron-bombarded target area. X-raw are wnerated a t the targetwhen it is str"ck by {he electrons and are (Continued on page A184) ~~~
~~~
~~~~~~~~
~~
~
Volume 64
Number 8
August 1987
A183
inftrurnentation transmitted through a thin window (usually beryllium) sealed into the tube housing. can be made ofmanvdifferent X-rav targets " metals including chromium, copper, molybdenum, rhodium,silver,tungsten,platioum. and gold,
.
DiaperslngSystems The X-rays emitted by the sample must be separated intoaspectrum ofwavelengths so that the characteristic X-rays of each element can be measured separately. In wavelength-dispersive X-ray fluorescence (WDXRF). soectrometrv. . .. the several X-rav lines emitted by the specimen are dispersed spatially by crystal diffraction, prior to detection, on the basis of their wavelengths. In energy-dispersive X-ray fluorescence (EDXRF) spectrometers, the detector receives all excited lines of all the specimen elements a t once and dispersion-is done electranidv. For each incident X-rav nhoton, thedet&r generatea a pulse of eieetrir current havinganamplitude proportional to its photon energy. The output is amplified. analyzed, and separated according to pulse amplitude and thereby on the basisof the photon energy characteristic of the element detected. Figure 2 from Bertin ( I ) illustrates the wavelength- and energy-dispersive modes simply. Wavelength-dispersive techniques use the diffraction property of crystalline materials with interatomic distances of 0.14 to 1.26 nm to disperse the X-rays over a suffi-
ciently wide range so that the detector can be placed to receive anly the X-rays of the anslyte. The most common analyzing crystals used (and their abbreviations in brackets) are lithium fluoride (both the 200 and 220 crystal lattices), [LiF(200) a n d LiF(220)l; germanium ( I l l lattice), [Getlll)]; pentaerythritol tetrakis(hydroxymethyl) methane, [PET]; ammonium dihydrogen phosphate, [ADP]; and thallium hvdrwen nhthalate. ITIHPI. Newlv devel" oped synthetic pse&cryskls (3)"with dsparing of 2.2 nm up to8.0 nm are especially efficient for dispersing X-rays from light elements (boron to magnesium). For a beam of X-rays of a specific wavelength to diffract, it must exit the crystal with X-ravs phase. a condition that exists . in . only when the path length of the X-ray in the ervstal is exactlv eoual to an inteeral multidle of the wav;len&. By selecting a crystal with a suitable d-spacing and adjusting the angle of incidence of the X-rays on the crystal, a characteristic X-ray line can be diffracted into a suitable detector while most interfering X-ravs are eliminated. The Brsgg equationdescribes this relationship:
-
where n is an integer defining the order of the reflection, A is the wavelength in nanometers of the X-rav diffracted. d ia the distame in nanometers between Lttice nlanes in the crystal, and 0 is the angle of incidence of X-ray beam to the crystal lattice. In X-ray spectrochemical analysis, three types of detectors are in common use: gas-
filled proportional counters, scintillation counters, and solid-state semiconductor detectors. In the gas-proportional counter, the Xray photon enters through a thin window and ionizes the gas forming electron-pasitive ion pairs. Sealed gas proportional counters are used to detect intermediate energy X-rays (for example, chlorine K. through nickelK.) and usually have a beryllium window. Gas-flow proportional detectors are used to detect low-energy X-rays (for example, boron K. through sulfur K.) with a window of polypropylene. Because the windows are necessarily thin to permit transmission and avoid absorption of the low-energyphotons, some slight diffusion of the detecting gas occurs through the window. Although the diffused gas must be continually replaced, the amount diffusing is too small to affect the level of vacuum in the chamher. . . . . ..
In scintillation detectors, the X-ray photon striking a disk of single-crystallinethallium-activated sodium iodide, Nal(Tl), Drod u e s a small amount of light proportional to the enerw of the ohoton. Electrons oroduced by t i i t light akking a pbotocatl;ode are then amplified by a multiplier phototube. Scintillation detectorr are used for detecting high-energy X-rays (above Ni K,). Figure 3 shows the useful ranges of gas proportional and scintillation detectors. Thesolid-statedetectorconsistsofalithium-drifted silicon (Si(Li)) detector mounted in a liquid-nitrogen vacuum eryostar. Each X-ray photon absorbed in the lithiumdrifted layer transfers its energy to a photoelectron, which, in turn, expends its energy producing a proportionate number of electron-bole pairs that are subsequently amplified. The solid-state detector is used in energy-dispersive X-ray spectrometers.
-
Figwe 2 Rinciples of wavelen~ispaaweand ensrgldispersive spsctmmstry. The polychomatic primary beam smmed by the X-ray tube is rspresemed by a single shml wavelength The primary beem lnadlates the specme4 and excitesthree spectral linea having short. Intermediate,and long wavelengths. respectively. The separation of these three lines by wavelength and energy dkiperslon is represented to the right and I& of tha specimen, respsctively. In tha wavsien~isperaiveSpectmmetBT(A), the three wavelengths fall upon a aysial. The crystal and detector are made to rotate svnchronwslvthroum successive males 8 and 28. reswctivek. As me . cmlrotates. n oapse.thamrre*anole 8 I& its in&olanarsacina diodlffmcteachwavelen& A. The ,~ ~~. detector receiver the dithacted X-rays. mveriing each incident photon into a pulse of eleCtriC Cunem having heimt propational to the photon enagy. If dittraction orders an,disrsgarded, at any 8 sening,the detectw output consists of pulses having the same average hetght. In an energydkipersivespecbomster (E),all three wavelengths enter the detector at once so that all three pulse heights are present in the detector output at once. These pulses are separated electronically by height. (Reprimed with permission ham senin. E. P. lnimductlon to X-ray SpecirmMc Analysis; Plenum: New Ynk, 1978; p 86.) ~
~
~
~
F~~~~~
~~~
A184
~
Journal of Chemical Education
.
-
-
The construction of wavelength-dispereive spectrometers is done in two basic configurations called sequential and simultaneous. In the sequential spectrometer, a single-channel instrument is mounted on an angle-measuring device known as a goniometer. By varying the set of instrument parameters, the selection of optimal conditions for a large range of elements is possible. These oarameters include the selection ofsmg~eorhua~ target X-ray tubes,multiple colltmators or attenuators, programmable pulse he~ghtdlacriminatorsas well as multiple-position crystal changers, choice of multiple detectors, and one or more moveable goniometers covering the range from 0' to 152'. Anv element can then be analvzed bv selectine an aoorooriate set of oarameters. This versatility provides the sequential spectrometer with its major advantage; a wide range of elements may be analyzed as needs change. Because each element must he analyzed sequentially, however, multiple-element samples require lengthy measurement times, for anly one X-ray line may he detected and orocessed a t a time. With each fi&d channel having a single crystal and its own detector and preamplifier, the simultaneous spectrometer can speed the analysis of multiple-element samples although there must be a separate channel for each element to be analyzed. The simultaneous spectrometer may also ~~
~~~
-~~~ . ..
REGION FOR SEALEDKRYPTON DETErnR
WAVELENGTH
lh
Figure 3. Efficiency of detectors lor goniometer versus elemems and wavelengms. From reference 4.
have scanning channels similar to the sequential spectrometer. On the fixed channels, preset angular relationships between theentrance port, the crystal, and the detector permit measurement of a particular characteristic X-ray line from one element. Curved-crystal or focusing techniques are generally used on fixed-channel instruments (as contrasted with flat-crystal or nonfocusing techniques). Two types of curved-crystal arrangements are in common use ( I b ) . In the Johann geometry, used in semifocusing spectrometers, the crystal is curved to a radius equal to the diameter of the focusing circle. This arrangement is optimal a t the wavelength for which the distances from the center of the crystal to the source and to the detector.. all lvine . .. on a common f m s i n y circle, are equal. However, the focus rs less optin~alat higher or lower wavelengths. In the Johansson arrangement, although substantially the same as the Johann arrangement, the focusing defect is corrected by cylindrically curving the crystal to the radius eanal to the diameter of the focusing circle then mindine" its inner surface to onehalf that radius, brinp;ing the inner surface of the crystal into contact with the focal circle. This achieves a fully focusing configuration. For analyzing a restricted group of elements, the simultaneous spectrometer offers several advantages aver the sequential soectrometer. Analvsis time is reduced considerably because all of the elemenu are measured nimultsnrously and operating conditions remain more nearly constant. More samples can be processed making it more practical to analyze standards more frequently. A third type of instrument equipped with both fixed ehannels and scanning goniometers combines the advantages of simultaneous and sequential spectrometers in one instrument. Up to four sequential goniometers can be placed in one instrument or one goniometer and up to 24 fixed channels; intermediate combinations are possible with the space required for one goniometer ~~
equivalent to that far eight fixed channels. 0 to 155' Each goniometer can scan from ' and can be equipped with a six-position crystal changer, a flow proportional detector, a sealed proportion detector, and a scintillation detector.
and Aeeuraey": papar pnented at the Pittsburgh Conference on Analytical Cbemiatry aod Applied Speetraaeopy, AtlaotieCity. NJ.March 1W.
~~
~~~~~~
~~
~
~.~~~~
Volume 64
Number 8
August 1987
