X-ray Fluorescence Spectroscopy, Applications

X-ray Fluorescence Spectroscopy, Applications

2478 X-RAY FLUORESCENCE SPECTROSCOPY, APPLICATIONS X-Ray Fluorescence Spectroscopy, Applications Christina Streli, P Wobrauschek and P Kregsamer, Ato...

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2478 X-RAY FLUORESCENCE SPECTROSCOPY, APPLICATIONS

X-Ray Fluorescence Spectroscopy, Applications Christina Streli, P Wobrauschek and P Kregsamer, Atominstitut of the Austrian Universities, Wien, Austria Copyright © 1999 Academic Press

Introduction X-ray fluorescence spectrometry (XRF) has been applied during the 1970s to 1990s as a versatile tool to many analytical problems. The analysis of major, minor and trace elements in various kinds of samples can be performed qualitatively as well as quantitatively. The working principle is based on the excitation of the sample atoms by high-energy X-rays, followed by the emission of characteristic photons with a certain energy, well correlated to the atomic number Z of each element (Moseley’s law). The determination of the energy (or wavelength) of the emitted photon allows qualitative analysis and the determination of the number of emitted characteristic photons allows quantitative analysis. The fundamental physical principle of X-ray fluorescence is described in the article about the theory of X-ray fluorescence spectroscopy. One of the features of XRF besides the accurate, rapid, multielement capacity, is that the analysis can be performed nondestructively. In fact, one has to consider that XRF is a surfacesensitive method, because of the energy of the excited and emitted radiation which is in the range 1–115 keV (Na to U K-radiation). The penetration depth of the primary radiation is some µm or so for low-Z elements and some 100 µm or so for heavy elements, depending also on the matrix or type of sample (solid, liquid and powder samples are common). In any case the surface of the object of investigation has to be representative of the entire volume, and thus requires a homogenous sample. Therefore, because of the sample preparation, the ‘nondestructiveness’ is lost. To perform XRF a spectrometer is required that consists of an excitation source, sample and a detection system, which can be either wavelengthdispersive or energy-dispersive. The excitation is mostly performed by X-rays produced in and emitted by an X-ray tube. The spectral distribution of the emitted radiation is partly the bremsstrahlung, with a maximum energy corresponding to the applied voltage, and is partly superimposed by the characteristic lines of the respective anode material. The intensity of the emitted radiation depends on the atomic number of the target and the applied voltage. The intensity of the measured

HIGH ENERGY SPECTROSCOPY Applications fluorescence signal depends on the intensity and energy of exciting photons hitting the sample atoms. Low-power X-ray tubes operating in the few W range and standard X-ray tubes dissipating up to 3 kW, as well as X-ray tubes with a rotating anode, up to 18 kW, are in use. Photons from radioisotopes are used for special applications, offering an excitation source independent of any power supply. The brightest excitation source is synchrotron radiation. It is emitted when bunches of electrons or positrons with energies in the GeV range are travelling along curved sections in a storage ring. An intensive continuous spectrum with a strong natural collimation in the forward direction is emitted. The radiation is continuous from the eV region to some hundreds of keV and linear polarized in the orbital plane. Due to the different working principles of wavelength-dispersive (WD) and energy-dispersive (ED) XRF the applications differ strongly and it is necessary to work out the special techniques and their advantages and disadvantages.

Instrumentation and methodology Wavelength-dispersive spectrometers

Two types of WD instruments are in use, the sequential spectrometer and the simultaneous spectrometer. The sequential spectrometer scans the radiation emitted by the sample by changing the angle sequentially. For different wavelength regions different analyser crystals must be used to fulfil Bragg’s equation. A set of 6 to 8 crystals with various lattice spacings d is properly mounted and can be changed automatically. The simultaneous spectrometers consist of various combinations of analyser crystals and detectors, arranged around the sample at fixed angle settings. So each ‘channel’ is optimized to detect an individual wavelength corresponding to an element. Mostly these channels use focusing optics at the detector to increase the signal. These spectrometers are called simultaneous multielement spectrometers, but the number of simultaneously detected elements depends on the number of channels of the spectrometer. LiF, topaz and other natural crystals are used for the

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medium-Z elements. The use of synthetic multilayer structures (consisting of alternating layers of a highZ and a low-Z material with a bilayer thickness of ∼ 1–10 nm) as dispersive elements and measurement in an evacuated environment allows the efficient determination of low-energy characteristic radiation down to even Be–K lines (O = 11 nm) if a flow counter with an ultrathin entrance window is used. The big advantage of the WD spectrometers is their excellent wavelength-to-energy resolution, especially in the low-energy region and the high count-rate range (106 cps) in operation. Energy-dispersive spectrometers

An ED detector consists of a semiconductor crystal (Si, Ge) prepared as p-i-n diode, mounted under vacuum, generally operated at 77 K and cooled with liquid nitrogen (LN) and the necessary connected electronics. Therefore a large, heavy dewar (7–30 L of LN) is required for an ED detector. LN consumption is ~1 L a day. The crystal environment has to be a vacuum, and as an entrance window in front of the crystal generally Be is used. Be is chosen as it is available with 8–25 µm thickness, vacuum and light-tight and its absorption of low-energy photons is tolerable. If very low energy photons (E < 1 keV) are to be measured, an ultrathin (< 1 µm) entrance window is required. Generally, the energy resolution is much larger with ED detectors than with WD systems, so ED is more susceptible to line overlaps. Especially in the low-energy regions this leads to difficulties in the interpretation of the measured spectrum and mathematical procedures for spectrum deconvolution are required. To overcome the problem of LN cooling, new Peltier cooled detectors are available offering smaller size and lighter weight, but worse resolution. As common practice the value for the energy resolution is given at 5.9 keV and is in the range 130– 180 eV for LN-cooled detectors, and 175–200 eV for Peltier cooled detectors. ED spectrometers measure all photons coming from the sample simultaneously. On one hand this is an advantage, because many elements can be detected within a short time, but on the other hand it is a disadvantage, because the maximum count-rate is limited to 50–80 kcps. The processing of the measured signal from each photon requires a certain length of time and during that interval the system is not ready to process the signal from the next arriving photon. The result is a deadtime, which has to be corrected. The reason for the saturation is not the number of fluorescence photons from the sample mainly, but the exciting radiation being scattered by the sample and the sample carrier. Therefore special

techniques have been developed to reduce the scattered radiation. Use of monochromatic radiation

The scattering can be drastically reduced, if monoenergetic radiation is used for excitation. Then only monoenergetic photons are arriving at the sample and can be scattered and so contribute to spectral background. Various methods of producing monoenergetic radiation are in use. Filtered radiation The easiest way to reduce the number of photons not used for sample excitation is the insertion of a filter. It mainly absorbs the lowenergy bremsstrahlung, but also a filter with Z as Zanode−1 can be used to effectively reduce the characteristic Kβ radiation from the anode. Secondary target excitation The radiation from an X-ray tube is used to excite a suitable target and the fluorescence radiation from the secondary target is used to excite the sample. This method of achieving quasi-monochromatic radiation suffers from a tremendous loss of photon flux, but this can be compensated for by using higher current from highpower tubes. Radioisotope excitation (RIXRF) A few radioisotope sources (with acceptable half-life) emit radiation in the X-ray region, such as Am-241 (59.5 keV), Cd-109 (Ag–K lines, 22 and 25 keV) and Fe-55 (Mn lines, 5.9 keV), and can be used for excitation. The advantage of radioisotope sources is their independence of a generator and power supply, which makes their use interesting for portable instruments, in field, on-line and even extraterrestrial applications. The disadvantage is the low photon flux in comparison to tube excitation. Crystal monochromators and multilayer structures Crystal monochromators in the beam path of an X-ray tube allow the selection of either the characteristic line from the anode or the selection of an energy from the continuous spectrum. Mono-chromators with a high reflectivity as well as a large energy band width are usually preferred, because the product of these two parameters determines the photon flux on the sample. In comparison to crystal monochromators, the multilayer offers high reflectivity as well as larger band width (dE/E = 10−2). Higher photon fluxes could be obtained. They are used with either X-ray tubes or synchrotron radiation. XRF using a linear polarized beam

If linear polarized radiation is scattered, in the ideal case no scattering radiation is emitted in the

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direction of the polarization vector. This effect can be used to reduce the scattering from the sample itself. Barkla polarizers or Bragg polarizers can be used. The Barkla polarizer scatters the entire spectrum of the exciting radiation; the Bragg polarizer acts additionally as a monochromator. Both polarizers use the scattering of the unpolarized radiation through an angle of 90°. Using polarized radiation the spectral background is drastically reduced in comparison to nonpolarized radiation, but again losses in intensity occur. Total reflection X-ray fluorescence analysis (TXRF)

TXRF is an EDXRF technique, utilizing the total external reflection of X-rays on the smooth plane surface of a reflector material, e.g. polished quartz. If a low divergent beam impinges on the reflector surface at an angle smaller than the critical angle for total reflection, most of the beam is reflected from the surface; only a small part penetrates into the reflector, causing scattered radiation. This leads to a reduced spectral background. The fluorescence signal is enhanced because the primary and also the reflected beam excite the sample, which is deposited on the reflector. Due to the small incidence angle the detector can be brought very close to the sample, so the detection efficiency is high. All these features lead to detection limits in the range of pg. Generally the samples have to be in liquid form. A droplet of 2–100 µL is pipetted onto the sample reflector and the liquid matrix is evaporated. Also, thin films or thin layers, as well as atoms implanted in a reflecting material such as Si-wafers, can be measured and thickness and depths determined.

tube. In combination with a monochromator the exciting radiation can be tuned to the energy with the optimum value of the photoelectric cross section for the investigated sample. It is also possible to tune the energy below the absorption edge of a main element in the sample to excite an element at trace levels with Z < Zmain element. This method is called selective excitation and offers several advantages in trace element analysis. To perform microanalysis, focusing elements are inserted to produce high-intensity microbeams. Also TXRF can be done using SR as exciting radiation. Trace element analysis

To detect elements at trace levels – µg g−1 (ppm), ng g−1(ppb) or even pg g−1 (ppt) – with XRF, generally special techniques as well as special sample preparation methods have to be used. The relevant quantity for trace element analysis is the limit of detection (DL), which is given by the formula

IB is the background intensity, t the measuring time and S the sensitivity (cps ppm−1 or cps ng−1). Either increasing the sensitivity or reducing the background leads to a reduction of detection limits. Therefore, special techniques of XRF, mainly EDXRF techniques, are applied, like TXRF, MXRF or SRXRF, as methods for trace element analysis. Detection limits range from ng to fg or µg g−1 to pg g−1.

Microfluorescence analysis (MXRF)

Microfluorescence analysis indicates the analysed area to be very small, leading to spatially resolved information of the sample composition. There are several methods of obtaining a beam with a small diameter, from a simple pinhole to highly sophisticated X-ray optical elements with focusing characteristics. One very effective method is the use of capillaries using the principle of total reflection of X-rays on the inner walls of a glass capillary. Small diameters down to 1 µm can be obtained with satisfactory intensity. Synchrotron radiation induced XRF (SRXRF)

Synchrotron radiation, as described above, offers several advantages for use as an excitation source for XRF, especially the higher intensity, orders of magnitude greater than that offered by an X-ray

Applications The applicability of XRF is almost unlimited with respect to type of sample and concentration range, but depends very much on the chosen technique. It can be used for on-line analysis in production processes or in-field measurements of geological samples with portable instruments, or quality control, such as impurity determination on Si-wafer surfaces at ultratrace levels, and environmental investigations. Also sample preparation is an important factor influencing the applicability (see Figure 1). For fine art object investigation or precious objects from museums, absolute nondestructiveness is required, contrary to environmental sampling where homogenizing and pressing to a pellet is no problem. To describe the various fields of application in detail they are divided into categories.

X-RAY FLUORESCENCE SPECTROSCOPY, APPLICATIONS 2481

and brass. Typically, simultaneous WD spectrometers with an automatic sample changer are used. Mining and ore processing

In mining and ore processing XRF is used for quality and process control. The spectrometers used differ very much depending on their application. Laboratory spectrometers for quality control may be WD or ED systems whith tube excitation. The on-stream spectrometers are located at in-plant locations, can be WD and ED systems, with radioisotope excitation or X-ray tube excitation and equipped with flow cells. In-stream instruments can be installed in slurry streams, mainly equipped with radioisotope excitation and scintillation counters for single-element determination. Field instruments must be portable and battery operated. The big advantage of XRF over other analytical tools for that application is its simplicity and speed. The usefulness of X-ray instruments to selective mining is well established, the information being used for ore–waste sorting. Cement analysis

Figure 1 for XRF.

Overview of sample preparation techniques typical

Metals and alloys

Process control in today’s highly automated production facilities is strongly dependent upon fast, precise and accurate chemical analysis, and XRF has been found to be widely applicable in the metal industries. XRF of metallic samples includes several solvable problems, especially in the areas of sample preparation and modelling calculations to convert intensities into concentration data. In general, metallic samples do not need complicated sample preparation, but the analytical information is derived from a volume close to the surface which must be polished. XRF is applied to various kinds of alloys, such as Na–Mg alloys, and Al, Ti, ferrous, Ni, Cu, Zr, W and Au alloys, bronzes

Finishing cements and raw mixes typically contain Ca, Si, Al and O at high concentrations, plus Fe, K, Mg and Na at low concentrations. One of the major problems in accurate cement analysis is homogeneity, particularly in the case of raw mixes, where the source of raw material may be variable. Most of the elements to be determined are of low atomic number, hence the penetration of their characteristic lines will be of the order of a few micrometres only. Careful grinding and pelletizing will suffice, but the fusion bead technique with Li3BO4 is strongly recommended. Simultaneous multichannel WD systems are ideally suited for this kind of application. Lubricating oil analysis

Raw or used oils are usually analysed for additiveelement content including Ba, Zn, Mn, Ca, P and Cl, plus naturally occurring elements including S, N, Ni and Na. In blended stocks the concentration of these elements would typically lie in the range 0.01–2.5%. In a standard case the analysis will be performed in the liquid phase and under a He atmosphere. Large matrix effects are likely because of the variable concentration levels of relatively heavy elements in a very low average atomic number matrix. Geology

XRF offers a rapid, accurate, low-cost method of analysis. Fully automated XRF spectrometers, sequential as well as simultaneous WD instruments,

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and ED instruments are in use. The analysis of geochemical samples often involves the analysis of samples having concentrations ranging from 0.0001 to 80%. Elements from Na to U are routinely determined. Detection limits depend very much on sample preparation and are in the range 20–1000 µg g−1 for low-Z elements, 5–10 µg g−1 for medium-Z elements and 1–20 µg g−1 for high-Z elements. One spectacular application of EDXRF was the Mars Pathfinder which was designed to inspect the rocks on the surface of Mars. Radioisotope excitation was used and one of the new electrically cooled ED detectors mounted on a small vehicle. Surface analysis of rocks could be performed quickly and the data could be transferred to Earth. Fine art and archaeological objects

In principle, with EDXRF, nondestructive analysis can be performed, which opens up the wide field of art and museum objects. Coins, bronzes, paintings, pottery, ceramics and ancient glasses can all be analysed. Paintings can be analysed pixel by pixel. With a spectrometer mounted on an x–y stage, a selected area can be analysed and the whole painting can be scanned. In particular, MXRF techniques are very well adapted to analysing specific points on figures or vessels, as well as lines on ancient documents. Environmental analysis

This is probably the most versatile and important application as all kinds of biological material such as plants, roots, needles, food-stuff, algae and lichens as biomonitors can be analysed with XRF. Lichens offer the advantage of a natural sampler collecting aerosols without exchange with the substrate. See Figure 2 as example of a sample lichen measured with an EDXRF spectrometer. Soil, river sediments, sewage sludge, dust, coal fly ash, car exhaust and fog condensate or the aerosols, sampled in impactor stages, are well suited for XRF. All kinds of liquids can be analysed: river water, sea water, snow, ice. However, special techniques for sample preparation should be applied especially when trace element analysis is required. Microfluorescence analysis

MXRF allows spatial resolution of analysis. To perform it with sufficient intensity in the laboratory with X-ray tubes, special optics should be applied such as capillaries. Using capillaries, X-ray tube excitation with SR allows much higher intensities, and so lower detection limits can be obtained. Applications range from microelectronics and plating thickness,

Figure 2 Spectrum of lichen sample. Pressed pellet, measured with a standard EDXRF spectrometer, Rh tube 30 kV, 0.2 mA, 500 s, Pd filter, Si(Li) detector; concentration values of respective elements given in µg g−1.

maps of bone cross-sections, superconductor films, human hair, pig heart muscles, metal alloys, leaves, chinaware, environmental particles, tree rings and glass fragments. Medicine

In vivo measurements as well as in situ measurements and analysis of malignant cells, as well as tissue sampling have been performed. In vivo measurements started with the determination of iodine in the thyroid and range from Cd in liver and kidney and Pt in kidneys and tumours, to Hg in the wrists and skulls of dentists, Pb in various near-surface bones, Cu in the eye and Fe in the skin. For the in vivo measurements, the use of polarized radiation offers big advantages. Hg and Pb can be analysed by their K-radiation thus giving information from even deeper tissues. Also, the analysis of biopsy samples, whole blood and blood serum (Se) can be performed using EDXRF. Trace element content in malignant and benign tissues was investigated, as well as lung tissue from different factory workers, showing different elements at higher concentrations corresponding to their profession. Various body fluids were analysed, and correlation between trace element content and diseases found. EDXRF, TXRF and SRXRF were used, depending on the task. TXRF applications

TXRF extends EDXRF to a method for trace and ultratrace element analysis. A special feature is the small sample amount required, which is in the range µg–ng of a solid material and less than 100–10 µL of a liquid. Therefore, TXRF is a microanalysis method

X-RAY FLUORESCENCE SPECTROSCOPY, APPLICATIONS 2483

and samples can seldom be analysed as-received. Pretreatment is generally required to prepare the samples as solutions, suspensions, fine powders or thin sections. For a determination of ultratraces, the matrix of the sample should be separated and removed. All preparation techniques can be applied that have been tested with other methods of atomic spectrometry, e.g. AAS, inductively coupled plasma mass spectrometry. The quantification is generally very simple, because the sample forms a thin layer, so the thin-film approximation is valid. One element of known concentration has to be added as internal standard. Quantification can be performed after the determination of the sensitivity factors of all elements relative to the internal standard. Also, surface and thin-layer analysis, as well as analysis of atoms below a reflecting surface, can be performed by varying the angle of incidence in the region of total reflection. This angledependent intensity profile allows a qualitative differentiation between contaminations on the surface, in the layers, in implantations or in so-called residues after evaporation of liquid samples. Quantitative determinations can be made by applying an algorithm deduced from theory in combination with an external standard. Table 1

It is obvious that the sample preparation technique used influences the detection limits. Table 1 shows this influence on various samples from different fields of application. Table 2 gives an overview of applications of TXRF already analysed. Figure 3 shows a spectrum of a water standard reference sample (NIST 1643c) obtained with a TXRF vacuum chamber, constructed at Atominstitut, Vienna. Generally, an excellent field of application of TXRF in trace element analysis can be seen in liquid samples. All kinds of liquids, ranging from different kinds of water to acids and oils, as well as body fluids, can be analysed. Environmental samples, like airborne particles, plant material or medical and biological samples such as tissue can be analysed directly on a reflector. The main industrial application of TXRF is the surface quality control of Si-wafer material. Wafers offer the required flatness and are polished, so that they can be directly analysed by TXRF. Several commercial instruments have been developed as wafer analysers and some 100 instruments are utilized in the semiconductor industry. TXRF is capable of checking the contaminations brought in by different steps during the production process. The required sensitivity is now 109 atoms cm−2 for transition

Influence of sample preparation on detection limits in TXRF (after Klockenkämper R (1997))

Sample Rain, river water

Drying

Freezedrying

Chemical matrix Open separation digestion

0.1–3 ng 20–100 pg 3–20 pg mL1 mL1 mL1

Blood, serum

Digest: 2–30 ng mL1

Ashing

Suspension

Solution

Pressure digestion

1–3 ng mL1 20–80 40–220 ng mL1 ng mL1

Air dust, ash, aerosols Air dust on filter

5–200 µg g1

Suspended matter

3–25 µg g1

10–100 µg g1 0.1–3 µg g1 0.6–20 ng cm

0.2–6 ng cm 10–100 µg g1

Sediment

10–100 µg g1

15–300 µg g1

Powdered biomaterial

1–10 µg g1

0.2–2 µg g1

Fine roots High-purity acids

1–10 µg g1

Digest: 0.1–1 µg g1

5–50 pg mL1

Tissue, foodstuff, biomaterial Mineral oil

0.5–5 µg g1 1–15 µg g−1

Mussel, fish High purity water

Freeze cutting

0.1–1 µg g−1 1 pg mL1

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Table 2

Applications of TXRF

Environment Water

Rain, river, sea, drinking water, waste water. Air Aerosols, airborne particles, dust, fly ash. Soil Sediments, sewage sludge. Plant material Algae, hay, leaves, lichen, moss, needles, roots, wood. Foodstuffs Fish, flour, fruits, crab, mussel, mushrooms, nuts, vegetables, wine, tea. Various Coal, peat. Medicine/biology/pharmacology Body fluids Blood, serum, urine, amniotic fluid. Tissue Hair, kidney, liver, lung, nails, stomach, colon. Various Enzymes, polysaccharides, glucose, proteins, cosmetics, biofilms. Industrial/technical applications Surface analysis Si-wafer surfaces, GaAs-wafer surfaces. Implanted ions Depth and profile variations. Thin films Single layers, multilayers. Oil Crude oil, fuel oil, grease. Chemicals Acids, bases, salts, solvents. Fusion/fission research Transmutational elements in Al Cu, iodine in water. Mineralogy Ores, rocks, minerals, rare earth elements. Fine arts/archaeological/forensic Pigments, paintings, varnish. Bronzes, pottery, jewellery. Textile fibres, glass, cognac, dollar bills, gunshot residue, drugs, tapes, sperm, fingerprints.

elements like Cr, Fe, Co, Ni, Cu and Zn. TXRF has an ‘up-time’ of 90% and is nondestructive. Surface mapping can be performed and differentiation between film type or particle type is possible. The detection limits can be improved by more than two orders of magnitude, if the impurities of the entire surface of the wafer are collected and preconcentrated prior to TXRF analysis. The native oxide layer is dissolved by HF vapour and the impurities remaining on the surface are collected by scanning the wafer with a drop of a suitable liquid. This method has the advantage of higher sensitivity, but nondestructiveness is lost. It is also possible to measure the thickness of nearsurface layers in the range 1–500 nm on reflecting substrates with TXRF. Single layers as well as multilayer samples can be analysed. Also, atoms implanted in the reflecting surface can be detected.

The implantation depth as well as the depth profile can be determined. TXRF can also be applied to fine art and museum objects. The sampling technique – a dry cotton bud can be used to rub off a small amount of paint – can only be applied during restoration, because the varnish has to be removed. For analysis the bud is dipped onto a sample carrier by a single tip. An amount of less than 100 ng is transmitted and can be analysed. Application of SRXRF

The rapid development of the SR X-ray sources since about 1975 is starting to have an impact on X-ray analysis. Due to the features of SR, especially the small source size and therefore the high brilliance, the use of microprobes is obvious. There are several approaches to producing a microbeam; the simplest is to use a pinhole collimator, but more sophisticated systems use focusing optics. Monochromatic as well as continuous radiation is used. Because SR is linearly polarized in the orbital plane, scattering from the sample is reduced, leading to low detection limits. SRXRF is a trace element analytical method as well as an MXRF method. SRXRF is performed at several SR facilities, most prominently NSLS Brookhaven, HASYLAB Hamburg, SSRL Stanford, SLS Daresbury, Photon Factory Tsukuba, DCI Lure and ESRF Grenoble. The available spot sizes are in the region of 10 µm and the detection limits in the low pg to fg range. Interesting applications were found in the fields of geology (mineral inclusions can be analysed), as well as in biology and medicine (distribution of trace elements in bone, tooth, brain, hair or algae strands, tree rings and aerosol particles), giving interesting information. Also, application in archaeology is found, letters in different ancient papers being analysed to allow the differentiation of the ink used to help identify the workshop. Even extraterrestrial minerals and rocks have been analysed. TXRF can also be done using SR as the excitation source (SR-TXRF) offering the advantage of higher photon flux and improved detection limits. Experiments are performed at SSRL, HASYLAB, Photon Factory and ESRF. The main application is the surface quality control of Si wafers. With SR-TXRF, detection limits on wafer surfaces of 107 atoms cm−2 have been obtained. At SSRL there is a beamline dedicated for routine wafer analysis. SR is the ideal source for the excitation of low-Z elements. It offers high intensity also in the low-energy region, in comparison to standard X-ray tubes. They do not emit enough intensity in the low-energy region; therefore the analysis of low-Z elements always lacks intensity

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Table 3 Overview of various applications (from Török S and Van Grieken R (1994) X-ray spectrometry. Analytical Chemistry 66: 186R–206R; Török S, Labar J, Injuk J and Van Grieken R (1996) Analytical Chemistry 68: 467R–485R)

Field Archaeology General Paintings Obsidian Medals Pottery Pigments Biomedical General

Method

Field Se in soil

Method SR-TXRF

MXRF

Soil, marine sediments

WDXRF

TXRF

Impurities in ice

TXRF

EDXRF

V, Ni, in oil, asphaltene

EDXRF

WDXRF

Bitumen solutions

EDXRF

WDXRF

Geology

TXRF

Rocks

MXRF

Mineral grains

SRXRF

SR-TXRF

Soils, sediments

MXRF

WDXRF

Single-cell analysis

EDXRF, TXRF

Fossilized bone

WDXRF

Biopsy samples

EDXRF, RIXRF

Crysolite

EDXRF

Blood

EDXRF

Geological samples

WDXRF

Skin in vivo

RIXRF

Phosphate in rocks

EDXRF

Bone

EDXRF

Oxides, silicates, carbonates

WDXRF

Bone in vivo

RIXRF

Liquid petroleum products

EDXRF

Leaves

TXRF, WDXRF

Microlayer of Fe–Mn nodules

SRXRF

Hair

EDXRF

Au in micas

SRXRF

Vegetables

TXRF

Materials science

Plants

WDXRF

Thin-film characterization

XRF, EDXRF

Lichens

WDXRF

Impurities on Si-wafers

TXRF, SRXRF

Moss

RIXRF

Multilayers

EDXRF

Cd in kidney

RIXRF

Superconductors

WDXRF

Mussel shells

WDXRF

Zirconium oxide

WDXRF

Cu, Se, Zn in kidney

SR-TXRF

Alumina

WDXRF

Marine bivalve shells

SRCXRF

Cu corrosions

EDXRF

Pt in tumour tissues

EDXRF

Ultrapure reagents

TXRF

Cu in human serum

EDXRF

Glasses

WDXRF, EDXRF

Pb in bones, serum, blood

RIXRF

Ferroalloys

EDXRF

Fe in vivo

RIXRF

High-purity Cu

EDXRF

Hg in vivo

RIXRF

GaAs-wafers

TXRF

Amniotic fluids

TXRF

Electrolytic solutions

RIXRF WDXRF

Plankton in polluted lakes

EDXRF

Al2O3 thin films

Meadow moths

SRXRF

Plastic materials

EDXRF, WDXRF

Ceramic materials

WDXRF EDXRF

Environmental Aerosols

WDXRF, EDXRF,

Hf in Zr matrix

TXRF, SRXRF

Ga in polyurethane foam

WDXRF

Fly ash

SRXRF

P in PbO films

EDXRF

Rain water

TXRF

Cu, Sr, Bi film on MgO

RIXRF

River water

TXRF

Molybdate crystals

EDXRF

Sea water

TXRF

Ferrous alloy

WDXRF

Sediments, suspensions

EDXRF, TXRF

Ta in Ti–Ta alloys

WDXRF

Waste

EDXRF

Pb in houseware

RIXRF

Coal

EDXRF, WDXRF

Textile fibres

TXRF

Dust

EDXRF

W analysis

TXRF

Pb in dust

WDXRF

HTSC films

SRXRF

HTSC = High-temperature semiconductor.

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Figure 3 Spectrum of water sample, NIST 1643c standard reference material, 10 µL, dried on a quartz reflector, measured with the TXRF vacuum chamber of Atominstitut, Mo monochromatic excitation, 40 kV, 50 mA, 1000 s; concentration values of respective elements given in µg L−1.

of the fluorescence lines. TXRF is also applied to the determination of low-Z elements using a special detector. Detection limits of 60 fg for Mg have been achieved using SR and have to be compared to 7 pg with windowless Si-anode tube (prototype) excitation. SR-TXRF generally is a very fast growing field and the problem of reduced access to SR sources for routine analytical applications is becoming less severe due to the large number of dedicated facilities. Table 3 gives an overview of applications and techniques published from 1994 to 1998.

Conclusions Applications range from on-line analysis and in-field inspections to ultratrace analysis of semiconductor surfaces. There is almost no sample that cannot be analysed by XRF as long as elemental analysis is required. The achievable detection limits depend on the method used and range from µg g−1 (ppm) to pg g−1 (ppt). Nondestructive analysis can be performed but sometimes sophisticated sample preparation techniques are required. The elemental range (Be to U) depends on the excitation source as well as the detection system. Generally XRF can be seen as a work-horse for elemental analysis and is easy to use.

List of symbols d = lattice spacing; DL = detection limit; IB = background intensity; S = sensitivity; t = measuring time; Z = atomic number; λ = wavelength. See also: Environmental and Agricultural Applications of Atomic Spectroscopy; Environmental Applications of Electronic Spectroscopy; Geology and Mineralogy, Applications of Atomic Spectroscopy; Inorganic Compounds and Minerals Studied Using XRay Diffraction; IR and Raman Spectroscopy Studies of Works of Art; IR Spectroscopy Sample Preparation Methods; MRI of Oil/Water in Rocks; X-Ray Fluorescence Spectrometers.

Further reading Bertin EP (1978) Introduction to X-ray Spectrometric Analyis. New York: Plenum Press. Carpenter DA (ed) (1997) Special Issue on Micro X-ray Fluorescence Analysis. X-ray Spectrometry 26(6): Ellis A, Potts Ph, Holmes M, Oliver GL, Streli C and Wobrauschek P (1996) Atomic spectrometry update: X-ray fluorescence spectrometry. Journal of Analytical Atomic Spectrometry 11: 409R–442R. Ellis A, Potts Ph, Holmes M, Oliver GL, Streli C and Wobrauschek P (1997) Atomic spectrometry update: X-ray

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fluorescence spectrometry. Journal of Analytical Atomic Spectrometry 12: 461R–490R. Herglotz HK and Birks LS (eds) (1978) X-ray Spectrometry. New York: Marcel Dekker. Holynska B (1993) Sampling and sample preparation in EDXRS. X-ray Spectrometry 22: 192–198. Iida A and Gohshi Y (1991) Trace element analysis by Xray fluorescence. In: Ebashi S, Koch M and Rubenstein R (eds) Handbook on Synchrotron Radiation, Vol. 4, pp 307–349. Amsterdam: North Holland, Elsevier. Jenkins R, Gould RW and Gedcke D (1981) Quantitative X-ray Spectrometry. New York: Marcel Dekker. Klockenkämper R (1997) Total-Reflection X-ray Fluorescence Analysis. New York: Wiley.

Sparks CJ (1982) X-ray fluorescence microprobe for chemical analysis. In: Winick H and Doniach S (eds) Synchrotron Radiation Research, pp 459–509. New York: Plenum Press. Török S and Van Grieken R (1994) X-ray spectrometry. Analytical Chemistry 66: 186R–206R. Török S, Labar J, Injuk J and Van Grieken R (1996) X-ray spectrometry. Analytical Chemistry 68: 467R–485R. Van Grieken R and Markowicz A (eds) (1993) Handbook of X-ray Spectrometry. New York: Marcel Dekker. Wielopolski L and Ryon RW (eds) (1995) Workshop at the Denver X-ray conference on in vivo XRF measurement of heavy metals. Advances in X-ray Analysis 38: 641.

X-Ray Spectroscopy, Theory Prasad A Naik, Centre for Advanced Technology, Indore, India

HIGH ENERGY SPECTROSCOPY Theory

Copyright © 1999 Academic Press

X-ray is the region of the electromagnetic spectrum lying between gamma rays and extreme ultraviolet (XUV / EUV) corresponding to a wavelength range of about 0.1 to 100 Å. The radiation on the lower end of the XUV region, up to about 300 Å, is also sometimes referred to as X-ray. On the lower wavelength side, radiation of shorter wavelengths is termed X-ray if it is nonnuclear in origin. The wavelength of the radiation is related to the photon energy by the standard relation E (keV) = 12.4/λ(Å). In terms of energy, the X-ray region is roughly between 125 eV and 125 keV. Being electromagnetic radiation, X-rays can be reflected, refracted, scattered, absorbed, polarized etc. They also show interference and diffraction effects. There are several sources of X-rays such as a Coolidge tube, vacuum sparks, hot-dense fusion plasmas, synchrotron, pinch devices, muonic atoms, beam-foil interaction, stellar X-ray emitters, solar flares, etc. The X-rays originating from all these sources can be broadly categorized into main types: (1) atomic inner shell transitions, (2) emission by free electrons, (3) X-rays from few electron systems. The basic spectroscopic aspects of the various types of X-rays are discussed in this article.

X-rays from inner shell transitions in atoms X-rays are produced when an electron in an outer shell of an atom jumps to an inner shell to fill an electron vacancy. The difference in energy is emitted as an X-ray photon. The vacancy giving rise to such a transition can be produced by an energetic photon, bombardment of charged particles (e –, p, α ..), or by nuclear processes such as internal conversion, K-capture, etc. If a charged particle collision or a nuclear process produces the vacancy, the resulting X-ray emission is called primary. If the vacancy is produced by an X-ray photon, the subsequent emission is called secondary or fluorescence radiation. In all these cases the singly ionized atom lowers its energy by emission of a photon of definite wavelength which is characteristic of the emitting atom. Hence, these X-rays are also called characteristic Xrays. Characteristic X-rays

The most energetic X-ray emission comes when a vacancy in a K shell (n = 1) is filled by an outer electron. Removal of a 1s electron from a neutral atom raises it to the highest energy state represented by lsls–1 or 1 2S1/2 or KI. Removal of a 2s electron