X-ray fluorescence 1

X-ray fluorescence

A Philips PW1606 X-ray fluorescencespectrometer with automated sample feed in a

cement plant quality control laboratory

X-ray fluorescence (XRF) is the emission of characteristic"secondary" (or fluorescent) X-rays from a material that has beenexcited by bombarding with high-energy X-rays or gamma rays. Thephenomenon is widely used for elemental analysis and chemicalanalysis, particularly in the investigation of metals, glass, ceramics andbuilding materials, and for research in geochemistry, forensic scienceand archaeology.

Underlying physics

Figure 1: Physics of X-ray fluorescence in aschematic representation.

When materials are exposed to short-wavelength X-rays or to gammarays, ionization of their component atoms may take place. Ionizationconsists of the ejection of one or more electrons from the atom, andmay occur if the atom is exposed to radiation with an energy greaterthan its ionization potential. X-rays and gamma rays can be energeticenough to expel tightly held electrons from the inner orbitals of theatom. The removal of an electron in this way makes the electronicstructure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind.In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the twoorbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The termfluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in there-emission of radiation of a different energy (generally lower).

X-ray fluorescence 2

Figure 2: Typical wavelength dispersive XRF spectrum

Figure 3: Spectrum of a rhodium target tube operated at 60 kV, showingcontinuous spectrum and K lines

Characteristic radiation

Each element has electronic orbitals ofcharacteristic energy. Following removal ofan inner electron by an energetic photonprovided by a primary radiation source, anelectron from an outer shell drops into itsplace. There are a limited number of ways inwhich this can happen, as shown in Figure1. The main transitions are given names: anL→K transition is traditionally called Kα, anM→K transition is called Kβ, an M→Ltransition is called Lα, and so on. Each ofthese transitions yields a fluorescent photonwith a characteristic energy equal to thedifference in energy of the initial and finalorbital. The wavelength of this fluorescentradiation can be calculated from Planck'sLaw:

The fluorescent radiation can be analysedeither by sorting the energies of the photons(energy-dispersive analysis) or by separatingthe wavelengths of the radiation(wavelength-dispersive analysis). Oncesorted, the intensity of each characteristicradiation is directly related to the amount ofeach element in the material. This is thebasis of a powerful technique in analyticalchemistry. Figure 2 shows the typical formof the sharp fluorescent spectral linesobtained in the wavelength-dispersive method (see Moseley's law).

Primary radiationIn order to excite the atoms, a source of radiation is required, with sufficient energy to expel tightly held innerelectrons. Conventional X-ray generators are most commonly used, because their output can readily be "tuned" forthe application, and because higher power can be deployed relative to other techniques. However, gamma raysources can be used without the need for an elaborate power supply, allowing an easier use in small portableinstruments. When the energy source is a synchrotron or the X-rays are focused by an optic like a polycapillary, theX-ray beam can be very small and very intense. As a result, atomic information on the sub-micrometre scale can beobtained. X-ray generators in the range 20–60 kV are used, which allow excitation of a broad range of atoms. Thecontinuous spectrum consists of "bremsstrahlung" radiation: radiation produced when high-energy electrons passingthrough the tube are progressively decelerated by the material of the tube anode (the "target"). A typical tube outputspectrum is shown in figure 3.

X-ray fluorescence 3

DispersionIn energy dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-statedetector which produces a "continuous" distribution of pulses, the voltages of which are proportional to the incomingphoton energies. This signal is processed by a multichannel analyser (MCA) which produces an accumulating digitalspectrum that can be processed to obtain analytical data. In wavelength dispersive analysis, the fluorescent X-raysemitted by the material sample are directed into a diffraction grating monochromator. The diffraction grating used isusually a single crystal. By varying the angle of incidence and take-off on the crystal, a single X-ray wavelength canbe selected. The wavelength obtained is given by the Bragg Equation:

where d is the spacing of atomic layers parallel to the crystal surface.

DetectionIn energy dispersive analysis, dispersion and detection are a single operation, as already mentioned above.Proportional counters or various types of solid-state detectors (PIN diode, Si(Li), Ge(Li), Silicon Drift DetectorSDD) are used. They all share the same detection principle: An incoming X-ray photon ionises a large number ofdetector atoms with the amount of charge produced being proportional to the energy of the incoming photon. Thecharge is then collected and the process repeats itself for the next photon. Detector speed is obviously critical, as allcharge carriers measured have to come from the same photon to measure the photon energy correctly (peak lengthdiscrimination is used to eliminate events that seem to have been produced by two X-ray photons arriving almostsimultaneously). The spectrum is then built up by dividing the energy spectrum into discrete bins and counting thenumber of pulses registered within each energy bin. EDXRF detector types vary in resolution, speed and the meansof cooling (a low number of free charge carriers is critical in the solid state detectors): proportional counters withresolutions of several hundred eV cover the low end of the performance spectrum, followed by PIN diode detectors,while the Si(Li), Ge(Li) and Silicon Drift Detectors (SDD) occupy the high end of the performance scale.In wavelength dispersive analysis, the single-wavelength radiation produced by the monochromator is passed into aphotomultiplier, a detector similar to a Geiger counter, which counts individual photons as they pass through. Thecounter is a chamber containing a gas that is ionised by X-ray photons. A central electrode is charged at (typically)+1700 V with respect to the conducting chamber walls, and each photon triggers a pulse-like cascade of currentacross this field. The signal is amplified and transformed into an accumulating digital count. These counts are thenprocessed to obtain analytical data.

X-ray intensityThe fluorescence process is inefficient, and the secondary radiation is much weaker than the primary beam.Furthermore, the secondary radiation from lighter elements is of relatively low energy (long wavelength) and haslow penetrating power, and is severely attenuated if the beam passes through air for any distance. Because of this, forhigh-performance analysis, the path from tube to sample to detector is maintained under vacuum (around 10 Paresidual pressure). This means in practice that most of the working parts of the instrument have to be located in alarge vacuum chamber. The problems of maintaining moving parts in vacuum, and of rapidly introducing andwithdrawing the sample without losing vacuum, pose major challenges for the design of the instrument. For lessdemanding applications, or when the sample is damaged by a vacuum (e.g. a volatile sample), a helium-swept X-raychamber can be substituted, with some loss of low-Z (Z = atomic number) intensities.

X-ray fluorescence 4

Chemical analysisThe use of a primary X-ray beam to excite fluorescent radiation from the sample was first proposed by Glocker andSchreiber in 1928.[1] Today, the method is used as a non-destructive analytical technique, and as a process controltool in many extractive and processing industries. In principle, the lightest element that can be analysed is beryllium(Z = 4), but due to instrumental limitations and low X-ray yields for the light elements, it is often difficult to quantifyelements lighter than sodium (Z = 11), unless background corrections and very comprehensive inter-elementcorrections are made.

Figure 4: Schematic arrangement of EDXspectrometer

Energy dispersive spectrometry

In energy dispersive spectrometers (EDX or EDS), the detector allowsthe determination of the energy of the photon when it is detected.Detectors historically have been based on silicon semiconductors, inthe form of lithium-drifted silicon crystals, or high-purity siliconwafers.

Figure 5: Schematic form of a Si(Li) detector

Si(Li) detectors

These consist essentially of a 3–5 mm thick silicon junction type p-i-ndiode (same as PIN diode) with a bias of −1000 V across it. Thelithium-drifted centre part forms the non-conducting i-layer, where Licompensates the residual acceptors which would otherwise make thelayer p-type. When an X-ray photon passes through, it causes a swarmof electron-hole pairs to form, and this causes a voltage pulse. Toobtain sufficiently low conductivity, the detector must be maintained atlow temperature, and liquid-nitrogen cooling must be used for the bestresolution. With some loss of resolution, the much more convenientPeltier cooling can be employed.

Wafer detectors

More recently, high-purity silicon wafers with low conductivity have become routinely available. Cooled by thePeltier effect, this provides a cheap and convenient detector, although the liquid nitrogen cooled Si(Li) detector stillhas the best resolution (i.e. ability to distinguish different photon energies).

Amplifiers

The pulses generated by the detector are processed by pulse-shaping amplifiers. It takes time for the amplifier toshape the pulse for optimum resolution, and there is therefore a trade-off between resolution and count-rate: longprocessing time for good resolution results in "pulse pile-up" in which the pulses from successive photons overlap.Multi-photon events are, however, typically more drawn out in time (photons did not arrive exactly at the same time)than single photon events and pulse-length discrimination can thus be used to filter most of these out. Even so, asmall number of pile-up peaks will remain and pile-up correction should be built into the software in applicationsthat require trace analysis. To make the most efficient use of the detector, the tube current should be reduced to keepmulti-photon events (before discrimination) at a reasonable level, e.g. 5–20%.

X-ray fluorescence 5

Processing

Considerable computer power is dedicated to correcting for pulse-pile up and for extraction of data from poorlyresolved spectra. These elaborate correction processes tend to be based on empirical relationships that may changewith time, so that continuous vigilance is required in order to obtain chemical data of adequate precision.

Usage

EDX spectrometers are different from WDX spectrometers in that they are smaller, simpler in design and have fewerengineered parts, however they are not as accurate. WDX has greater resolution power than EDX. They can also useminiature X-ray tubes or gamma sources. This makes them cheaper and allows miniaturization and portability. Thistype of instrument is commonly used for portable quality control screening applications, such as testing toys for lead(Pb) content, sorting scrap metals, and measuring the lead content of residential paint. On the other hand, the lowresolution and problems with low count rate and long dead-time makes them inferior for high-precision analysis.They are, however, very effective for high-speed, multi-elemental analysis. Field Portable XRF analysers currentlyon the market weigh less than 2 kg, and have limits of detection on the order of 2 parts per million of lead (Pb) inpure sand.

Figure 6: Schematic arrangement of wavelengthdispersive spectrometer

Chemist operates a goniometer used for X-rayfluorescence analysis of individual grains ofmineral specimens, U.S. Geological Survey,

1958.

Wavelength dispersive spectrometry

In wavelength dispersive spectrometers (WDX or WDS), the photonsare separated by diffraction on a single crystal before being detected.Although wavelength dispersive spectrometers are occasionally used toscan a wide range of wavelengths, producing a spectrum plot as inEDS, they are usually set up to make measurements only at thewavelength of the emission lines of the elements of interest. This isachieved in two different ways:

• "Simultaneous" spectrometers have a number of "channels"dedicated to analysis of a single element, each consisting of afixed-geometry crystal monochromator, a detector, and processingelectronics. This allows a number of elements to be measuredsimultaneously, and in the case of high-powered instruments,complete high-precision analyses can be obtained in under 30 s.Another advantage of this arrangement is that the fixed-geometrymonochromators have no continuously moving parts, and so arevery reliable. Reliability is important in production environmentswhere instruments are expected to work without interruption formonths at a time. Disadvantages of simultaneous spectrometersinclude relatively high cost for complex analyses, since eachchannel used is expensive. The number of elements that can be measured is limited to 15–20, because of spacelimitations on the number of monochromators that can be crowded around the fluorescing sample. The need toaccommodate multiple monochromators means that a rather open arrangement around the sample is required,leading to relatively long tube-sample-crystal distances, which leads to lower detected intensities and morescattering. The instrument is inflexible, because if a new element is to be measured, a new measurement channelhas to be bought and installed.

• "Sequential" spectrometers have a single variable-geometry monochromator (but usually with an arrangement for selecting from a choice of crystals), a single detector assembly (but usually with more than one detector arranged in tandem), and a single electronic pack. The instrument is programmed to move through a sequence of wavelengths, in each case selecting the appropriate X-ray tube power, the appropriate crystal, and the appropriate

X-ray fluorescence 6

detector arrangement. The length of the measurement program is essentially unlimited, so this arrangement isvery flexible. Because there is only one monochromator, the tube-sample-crystal distances can be kept very short,resulting in minimal loss of detected intensity. The obvious disadvantage is relatively long analysis time,particularly when many elements are being analysed, not only because the elements are measured in sequence, butalso because a certain amount of time is taken in readjusting the monochromator geometry betweenmeasurements. Furthermore, the frenzied activity of the monochromator during an analysis program is a challengefor mechanical reliability. However, modern sequential instruments can achieve reliability almost as good as thatof simultaneous instruments, even in continuous-usage applications.

Sample presentation

In order to keep the geometry of the tube-sample-detector assembly constant, the sample is normally prepared as aflat disc, typically of diameter 20–50 mm. This is located at a standardized, small distance from the tube window.Because the X-ray intensity follows an inverse-square law, the tolerances for this placement and for the flatness ofthe surface must be very tight in order to maintain a repeatable X-ray flux. Ways of obtaining sample discs vary:metals may be machined to shape, minerals may be finely ground and pressed into a tablet, and glasses may be castto the required shape. A further reason for obtaining a flat and representative sample surface is that the secondaryX-rays from lighter elements often only emit from the top few micrometres of the sample. In order to further reducethe effect of surface irregularities, the sample is usually spun at 5–20 rpm. It is necessary to ensure that the sample issufficiently thick to absorb the entire primary beam. For higher-Z materials, a few millimetres thickness is adequate,but for a light-element matrix such as coal, a thickness of 30–40 mm is needed.

Figure 7: Bragg diffraction condition

Monochromators

The common feature of monochromators is the maintenance of asymmetrical geometry between the sample, the crystal and the detector.In this geometry the Bragg diffraction condition is obtained.The X-ray emission lines are very narrow (see figure 2), so the anglesmust be defined with considerable precision. This is achieved in twoways:

•• Flat crystal with Soller collimators

The Soller collimator is a stack of parallel metal plates, spaced a few tenths of a millimetre apart. To improve angleresolution, one must lengthen the collimator, and/or reduce the plate spacing. This arrangement has the advantage ofsimplicity and relatively low cost, but the collimators reduce intensity and increase scattering, and reduce the area ofsample and crystal that can be "seen". The simplicity of the geometry is especially useful for variable-geometrymonochromators.

Figure 8: Flat crystal with Soller collimators

•• Curved crystal with slits

The Rowland circle geometry ensures that the slits are both in focus,but in order for the Bragg condition to be met at all points, the crystalmust first be bent to a radius of 2R (where R is the radius of theRowland circle), then ground to a radius of R. This arrangement allowshigher intensities (typically 8-fold) with higher resolution (typically4-fold) and lower background. However, the mechanics of keepingRowland circle geometry in a variable-angle monochromator is extremely difficult. In the case of fixed-anglemonochromators (for use

X-ray fluorescence 7

Figure 9: Curved crystal with slits

in simultaneous spectrometers), crystals bent to a logarithmic spiralshape give the best focusing performance. The manufacture of curvedcrystals to acceptable tolerances increases their price considerably.

Analysis Lines

The spectral lines used for chemical analysis are selected on the basisof intensity, accessibility by the instrument, and lack of line overlaps.Typical lines used, and their wavelengths, are as follows:

element line wavelength (nm) element line wavelength (nm) element line wavelength (nm) element line wavelength (nm)

Li Kα 22.8 Ni Kα1 0.1658 I Lα1 0.3149 Pt Lα1 0.1313

Be Kα 11.4 Cu Kα1 0.1541 Xe Lα1 0.3016 Au Lα1 0.1276

B Kα 6.76 Zn Kα1 0.1435 Cs Lα1 0.2892 Hg Lα1 0.1241

C Kα 4.47 Ga Kα1 0.1340 Ba Lα1 0.2776 Tl Lα1 0.1207

N Kα 3.16 Ge Kα1 0.1254 La Lα1 0.2666 Pb Lα1 0.1175

O Kα 2.362 As Kα1 0.1176 Ce Lα1 0.2562 Bi Lα1 0.1144

F Kα1,2 1.832 Se Kα1 0.1105 Pr Lα1 0.2463 Po Lα1 0.1114

Ne Kα1,2 1.461 Br Kα1 0.1040 Nd Lα1 0.2370 At Lα1 0.1085

Na Kα1,2 1.191 Kr Kα1 0.09801 Pm Lα1 0.2282 Rn Lα1 0.1057

Mg Kα1,2 0.989 Rb Kα1 0.09256 Sm Lα1 0.2200 Fr Lα1 0.1031

Al Kα1,2 0.834 Sr Kα1 0.08753 Eu Lα1 0.2121 Ra Lα1 0.1005

Si Kα1,2 0.7126 Y Kα1 0.08288 Gd Lα1 0.2047 Ac Lα1 0.0980

P Kα1,2 0.6158 Zr Kα1 0.07859 Tb Lα1 0.1977 Th Lα1 0.0956

S Kα1,2 0.5373 Nb Kα1 0.07462 Dy Lα1 0.1909 Pa Lα1 0.0933

Cl Kα1,2 0.4729 Mo Kα1 0.07094 Ho Lα1 0.1845 U Lα1 0.0911

Ar Kα1,2 0.4193 Tc Kα1 0.06751 Er Lα1 0.1784 Np Lα1 0.0888

K Kα1,2 0.3742 Ru Kα1 0.06433 Tm Lα1 0.1727 Pu Lα1 0.0868

Ca Kα1,2 0.3359 Rh Kα1 0.06136 Yb Lα1 0.1672 Am Lα1 0.0847

Sc Kα1,2 0.3032 Pd Kα1 0.05859 Lu Lα1 0.1620 Cm Lα1 0.0828

Ti Kα1,2 0.2749 Ag Kα1 0.05599 Hf Lα1 0.1570 Bk Lα1 0.0809

V Kα1 0.2504 Cd Kα1 0.05357 Ta Lα1 0.1522 Cf Lα1 0.0791

Cr Kα1 0.2290 In Lα1 0.3772 W Lα1 0.1476 Es Lα1 0.0773

Mn Kα1 0.2102 Sn Lα1 0.3600 Re Lα1 0.1433 Fm Lα1 0.0756

Fe Kα1 0.1936 Sb Lα1 0.3439 Os Lα1 0.1391 Md Lα1 0.0740

Co Kα1 0.1789 Te Lα1 0.3289 Ir Lα1 0.1351 No Lα1 0.0724

Other lines are often used, depending on the type of sample and equipment available.

X-ray fluorescence 8

Crystals

The desirable characteristics of a diffraction crystal are:•• High diffraction intensity•• High dispersion•• Narrow diffracted peak width•• High peak-to-background•• Absence of interfering elements•• Low thermal coefficient of expansion•• Stability in air and on exposure to X-rays•• Ready availability•• Low costCrystals with simple structure tend to give the best diffraction performance. Crystals containing heavy atoms candiffract well, but also fluoresce themselves, causing interference. Crystals that are water-soluble, volatile or organictend to give poor stability.Commonly used crystal materials include LiF (lithium fluoride), ADP (ammonium dihydrogen phosphate), Ge(germanium), graphite, InSb (indium antimonide), PE (tetrakis-(hydroxymethyl)-methane: penta-erythritol), KAP(potassium hydrogen phthalate), RbAP (rubidium hydrogen phthalate) and TlAP (thallium(I) hydrogen phthalate). Inaddition, there is an increasing use of "layered synthetic microstructures", which are "sandwich" structured materialscomprising successive thick layers of low atomic number matrix, and monatomic layers of a heavy element. Thesecan in principle be custom-manufactured to diffract any desired long wavelength, and are used extensively forelements in the range Li to Mg.Properties of commonly used crystals

material plane d (nm) min λ (nm) max λ (nm) intensity thermal expansion durability

LiF 200 0.2014 0.053 0.379 +++++ +++ +++

LiF 220 0.1424 0.037 0.268 +++ ++ +++

LiF 420 0.0901 0.024 0.169 ++ ++ +++

ADP 101 0.5320 0.139 1.000 + ++ ++

Ge 111 0.3266 0.085 0.614 +++ + +++

graphite 001 0.3354 0.088 0.630 ++++ + +++

InSb 111 0.3740 0.098 0.703 ++++ + +++

PE 002 0.4371 0.114 0.821 +++ +++++ +

KAP 1010 1.325 0.346 2.490 ++ ++ ++

RbAP 1010 1.305 0.341 2.453 ++ ++ ++

Si 111 0.3135 0.082 0.589 ++ + +++

TlAP 1010 1.295 0.338 2.434 +++ ++ ++

YB66 400 0.586

6 nm LSM - 6.00 1.566 11.276 +++ + ++

X-ray fluorescence 9

Detectors

Detectors used for wavelength dispersive spectrometry need to have high pulse processing speeds in order to copewith the very high photon count rates that can be obtained. In addition, they need sufficient energy resolution toallow filtering-out of background noise and spurious photons from the primary beam or from crystal fluorescence.There are four common types of detector:•• gas flow proportional counters•• sealed gas detectors•• scintillation counters•• semiconductor detectors

Figure 10: Arrangement of gas flow proportionalcounter

Gas flow proportional counters are used mainly for detection oflonger wavelengths. Gas flows through it continuously. Where thereare multiple detectors, the gas is passed through them in series, thenled to waste. The gas is usually 90% argon, 10% methane ("P10"),although the argon may be replaced with neon or helium where verylong wavelengths (over 5 nm) are to be detected. The argon is ionisedby incoming X-ray photons, and the electric field multiplies this chargeinto a measurable pulse. The methane suppresses the formation offluorescent photons caused by recombination of the argon ions withstray electrons. The anode wire is typically tungsten or nichrome of20–60 μm diameter. Since the pulse strength obtained is essentiallyproportional to the ratio of the detector chamber diameter to the wirediameter, a fine wire is needed, but it must also be strong enough to bemaintained under tension so that it remains precisely straight andconcentric with the detector. The window needs to be conductive, thin enough to transmit the X-rays effectively, butthick and strong enough to minimize diffusion of the detector gas into the high vacuum of the monochromatorchamber. Materials often used are beryllium metal, aluminised PET film and aluminised polypropylene. Ultra-thinwindows (down to 1 μm) for use with low-penetration long wavelengths are very expensive. The pulses are sortedelectronically by "pulse height selection" in order to isolate those pulses deriving from the secondary X-ray photonsbeing counted.

Sealed gas detectors are similar to the gas flow proportional counter, except that the gas does not flow through it.The gas is usually krypton or xenon at a few atmospheres pressure. They are applied usually to wavelengths in the0.15–0.6 nm range. They are applicable in principle to longer wavelengths, but are limited by the problem ofmanufacturing a thin window capable of withstanding the high pressure difference.Scintillation counters consist of a scintillating crystal (typically of sodium iodide doped with thallium) attached to aphotomultiplier. The crystal produces a group of scintillations for each photon absorbed, the number beingproportional to the photon energy. This translates into a pulse from the photomultiplier of voltage proportional to thephoton energy. The crystal must be protected with a relatively thick aluminium/beryllium foil window, which limitsthe use of the detector to wavelengths below 0.25 nm. Scintillation counters are often connected in series with a gasflow proportional counter: the latter is provided with an outlet window opposite the inlet, to which the scintillationcounter is attached. This arrangement is particularly used in sequential spectrometers.Semiconductor detectors can be used in theory, and their applications are increasing as their technology improves,but historically their use for WDX has been restricted by their slow response (see EDX).

X-ray fluorescence 10

A glass "bead" specimen for XRFanalysis being cast at around 1100°C in a Herzog automated fusionmachine in a cement plant qualitycontrol laboratory. 1 (top): fusing,

2: preheating the mould, 3:pouring the melt, 4: cooling the

"bead"

Extracting analytical results

At first sight, the translation of X-ray photon count-rates into elementalconcentrations would appear to be straightforward: WDX separates the X-ray linesefficiently, and the rate of generation of secondary photons is proportional to theelement concentration. However, the number of photons leaving the sample is alsoaffected by the physical properties of the sample: so-called "matrix effects". Thesefall broadly into three categories:

•• X-ray absorption•• X-ray enhancement•• sample macroscopic effectsAll elements absorb X-rays to some extent. Each element has a characteristicabsorption spectrum which consists of a "saw-tooth" succession of fringes, eachstep-change of which has wavelength close to an emission line of the element.Absorption attenuates the secondary X-rays leaving the sample. For example, themass absorption coefficient of silicon at the wavelength of the aluminium Kα lineis 50 m²/kg, whereas that of iron is 377 m²/kg. This means that a givenconcentration of aluminium in a matrix of iron gives only one seventh of the countrate compared with the same concentration of aluminium in a silicon matrix.Fortunately, mass absorption coefficients are well known and can be calculated.However, to calculate the absorption for a multi-element sample, the compositionmust be known. For analysis of an unknown sample, an iterative procedure istherefore used. It will be noted that, to derive the mass absorption accurately, datafor the concentration of elements not measured by XRF may be needed, andvarious strategies are employed to estimate these. As an example, in cementanalysis, the concentration of oxygen (which is not measured) is calculated byassuming that all other elements are present as standard oxides.

Enhancement occurs where the secondary X-rays emitted by a heavier element aresufficiently energetic to stimulate additional secondary emission from a lighterelement. This phenomenon can also be modelled, and corrections can be madeprovided that the full matrix composition can be deduced.

Sample macroscopic effects consist of effects of inhomogeneities of the sample, and unrepresentative conditions atits surface. Samples are ideally homogeneous and isotropic, but they often deviate from this ideal. Mixtures ofmultiple crystalline components in mineral powders can result in absorption effects that deviate from thosecalculable from theory. When a powder is pressed into a tablet, the finer minerals concentrate at the surface.Spherical grains tend to migrate to the surface more than do angular grains. In machined metals, the softercomponents of an alloy tend to smear across the surface. Considerable care and ingenuity are required to minimizethese effects. Because they are artifacts of the method of sample preparation, these effects can not be compensatedby theoretical corrections, and must be "calibrated in". This means that the calibration materials and the unknownsmust be compositionally and mechanically similar, and a given calibration is applicable only to a limited range ofmaterials. Glasses most closely approach the ideal of homogeneity and isotropy, and for accurate work, minerals areusually prepared by dissolving them in a borate glass, and casting them into a flat disc or "bead". Prepared in thisform, a virtually universal calibration is applicable.Further corrections that are often employed include background correction and line overlap correction. The background signal in an XRF spectrum derives primarily from scattering of primary beam photons by the sample

X-ray fluorescence 11

surface. Scattering varies with the sample mass absorption, being greatest when mean atomic number is low. Whenmeasuring trace amounts of an element, or when measuring on a variable light matrix, background correctionbecomes necessary. This is really only feasible on a sequential spectrometer. Line overlap is a common problem,bearing in mind that the spectrum of a complex mineral can contain several hundred measurable lines. Sometimes itcan be overcome by measuring a less-intense, but overlap-free line, but in certain instances a correction is inevitable.For instance, the Kα is the only usable line for measuring sodium, and it overlaps the zinc Lβ (L2-M4) line. Thuszinc, if present, must be analysed in order to properly correct the sodium value.

Other spectroscopic methods using the same principleIt is also possible to create a characteristic secondary X-ray emission using other incident radiation to excite thesample:• electron beam: electron microprobe;• ion beam: particle induced X-ray emission (PIXE).When radiated by an X-ray beam, the sample also emits other radiations that can be used for analysis:• electrons ejected by the photoelectric effect: X-ray photoelectron spectroscopy (XPS), also called electron

spectroscopy for chemical analysis (ESCA)The de-excitation also ejects Auger electrons, but Auger electron spectroscopy (AES) normally uses an electronbeam as the probe.Confocal microscopy X-ray fluorescence imaging is a newer technique that allows control over depth, in addition tohorizontal and vertical aiming, for example, when analysing buried layers in a painting.

Instrument qualificationA 2001 review, addresses the application of portable instrumentation from QA/QC perspectives. It provides a guideto the development of a set of SOPs if regulatory compliance guidelines are not available.Further information: Verification and Validation

Notes[1] Glocker, R., and Schreiber, H., Ann. Physik., 85, (1928), p. 1089

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X-Ray Fluorescence". In Banci, Lucia (Ed.). Metallomics and the Cell. Metal Ions in Life Sciences 12. Springer. doi: 10.1007/978-94-007-5561-1_2 (http:/ / dx. doi. org/ 10. 1007/ 978-94-007-5561-1_2).

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ISBN 978-94-007-5560-4.electronic-book ISBN 978-94-007-5561-1 ISSN  1559-0836 (http:/ / www. worldcat.org/ search?fq=x0:jrnl& q=n2:1559-0836)electronic-ISSN  1868-0402 (http:/ / www. worldcat. org/search?fq=x0:jrnl& q=n2:1868-0402)

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External links• Spectroscopy (http:/ / www. dmoz. org/ Science/ Instruments_and_Supplies/ Laboratory_Equipment/

Spectroscopy/ ) at DMOZ

Article Sources and Contributors 13

Article Sources and ContributorsX-ray fluorescence  Source: http://en.wikipedia.org/w/index.php?oldid=594202222  Contributors: A2tori, AbigailAbernathy, Accounting4Taste, Aeolodicon, Aleksa Lukic, Anthony,Austinfighting, Beetstra, Beland, Bobbya111, Bullet2010, CWenger, Cabbage1031, Cae rob, Calvero JP, Cdang, Cgingold, Chem-awb, Cmacey, Crayderfish, Dan Sarandon, DanMS, Dekisugi,Dougher, Ewlyahoocom, Excirial, Graeme Bartlett, Hakseng, HenrikMidtiby, Hmqcnoesy, Homer Landskirty, Hooperbloob, InstEng, Itub, Jag123, Jcwf, Jeffateastern, Jenpen, Jhenssky, Jshor05,Julesd, Ketiltrout, Kkmurray, Kumaravel17804, Laurascudder, Lectonar, Linas, LinguisticDemographer, Luna Santin, Lupo, Mange01, Marangkerapu, Marie Poise, Materialsandeng,Materialscientist, Mav, Mco seals, MelbourneStar, MikeW25, MogRuith, Motic, MrJones, MrOllie, Nanonani, Natelipkowitz, Nikevich, Nybergt, Oosoom, OwenBlacker, Particles en, Petergans,Phatom87, PunkyMcPunkersen, R'n'B, RadXman, Raomap, Richard.chx, Rickterp, Rjwilmsi, Rob Hooft, Rod57, Ryancwa, Salamurai, Sancho s rancho, Skier Dude, SkyrayXRF, SmoJoe,Spexuk, Statsone, Stephanie.kowalyk, SuperHamster, SwisterTwister, T.vanschaik, Tagishsimon, Talu42, TastyPoutine, TenOfAllTrades, Tillman, Tmacent1, TomMilner65, Tonyrex, Twisp,Viriditas, Vkj1987, Vsmith, WackyBoots, Wolbo, Zephyric, 168 ,علی ویکی anonymous edits

Image Sources, Licenses and ContributorsFile:LDAutoXRFPic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:LDAutoXRFPic.jpg  License: Public Domain  Contributors: Original uploader was LinguisticDemographer aten.wikipediaFile:X-ray fluorescence simple figure.svg  Source: http://en.wikipedia.org/w/index.php?title=File:X-ray_fluorescence_simple_figure.svg  License: Public Domain  Contributors: Calvero.File:XRFScan.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:XRFScan.jpg  License: Public Domain  Contributors: Original uploader was LinguisticDemographer at en.wikipediaFile:TubeSpectrum.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TubeSpectrum.jpg  License: Public Domain  Contributors: Original uploader was LinguisticDemographer aten.wikipediaFile:dmedxrfschematic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dmedxrfschematic.jpg  License: Public Domain  Contributors: Original uploader was LinguisticDemographerat en.wikipediaFile:DmedxrfSiLiDetector.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:DmedxrfSiLiDetector.jpg  License: Public Domain  Contributors: Original uploader wasLinguisticDemographer at en.wikipediaFile:dmwdxrfschematic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dmwdxrfschematic.jpg  License: Public Domain  Contributors: Original uploader wasLinguisticDemographer at en.wikipediaFile:XRFgoniometer manual.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:XRFgoniometer_manual.jpg  License: Public Domain  Contributors: Original uploader was Tillman aten.wikipediaFile:dmbraggreflection.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dmbraggreflection.jpg  License: Public Domain  Contributors: Original uploader was LinguisticDemographerat en.wikipediaFile:dmwdxrfFlatXtalMonochrom.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:DmwdxrfFlatXtalMonochrom.jpg  License: Public Domain  Contributors: Original uploader wasLinguisticDemographer at en.wikipediaFile:dmwdxrfCurvedXtalMonochrom.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:DmwdxrfCurvedXtalMonochrom.jpg  License: Public Domain  Contributors: Originaluploader was LinguisticDemographer at en.wikipediaFile:dmwdxrfFlowDetector.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:DmwdxrfFlowDetector.jpg  License: Public Domain  Contributors: Original uploader wasLinguisticDemographer at en.wikipediaFile:LDHerzogBeadMaking.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:LDHerzogBeadMaking.jpg  License: Public Domain  Contributors: Original uploader wasLinguisticDemographer at en.wikipedia

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