X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

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SECTION I BASIC IN X-RAY FLUORESCENCE I. History of X-Ray Fluorescence The history of X-ray fluorescence dates back to the accidental discovery of X-rays in 1895 by the German physicist Wilhelm Conrad Roentgen. While studying cathode rays in a high-voltage, gaseous-discharge tube, Roentgen observed that even though the experimental tube was encased in a black cardboard box the barium-platinocyanide screen, which was lying adjacent to the experiment, emitted fluorescent light whenever the tube was in operation. Roentgen's discovery of X-rays and their possible use in analytical chemistry went unnoticed until 1913. In 1913, H.G.J. Mosley showed the relationship between atomic number (Z) and the reciprocal of the wavelength (1/λ) for each spectral series of emission lines for each element. Today this relationship is expressed as: 2 c / λ = a ( Z− s) ; (I.1) where: a is a proportionality constant, s is a constant dependent on a periodic series. Mosley was also responsible for the construction of the early X-ray spectrometer. His design centered around a cold cathode tube where the air within the tube provided the electrons and the analyte which served as the tube target. The major problem experienced laid in the inefficiency of using electrons to create x-rays; nearly 99% of the energy was lost as heat. In the same year, the Bragg brothers built their first X-ray analytical device. Their device was based around a pinhole and slit collimator. Like Mosley's instrument, the Braggs ran into difficulty in maintaining efficiency. Progress in XRF spectroscopy continued in 1922 when Hadding investigated using XRF spectrometry to analyse mineral samples. Three years later, Coster and Nishina put forward the idea of replacing electrons with X-ray photons to excite secondary X-ray radiation resulting in the generation of an X-ray spectrum. This technique was attempted by Glocker and Schrieber, who in 1928 published Quantitative Roentgen Spectrum Analysis by Means of Cold Excitation of the Spectrum in Ann. Physics. Progress appeared to be at a standstill until 1948, when Friedman and Birks built the first XRF spectrometer. Their device was built around a diffractometer, with a Geiger counter for a detection device and proved comparatively sensitive for much of the atomic number range. It might be noted that XRF spectrometers have progressed to the point where elements ranging from Beryllium to Uranium can be analysed. Although the earliest commercial XRF devices used simple air path conditions, machines were soon developed utilizing helium or vacuum paths, permitting the detection of lighter elements. In the 1960’s, XRF devices began to use lithium fluoride crystals for diffraction and chromium or rhodium target X-ray tubes to excite longer wavelengths. This development was quickly followed by that of multichannel spectrometers for the simultaneous measurement of many elements. By the mid 60’s computer controlled XRF devices were coming into use. In 1970, the lithium drifted silicon detector (Si(Li)) was created, providing
very high resolution and X-ray photon separation without the use of an analysing crystal. An XRF device was even included on the Apollo 15 and 16 missions. Meanwhile, Schwenke and co-workers have fine tuned a procedure known as total reflection X-ray fluorescence (TXRF), which is now used extensively for trace analysis. In TXRF, a Si(Li) detector is positioned almost on top of a thin film of sample, many times positioned on a quarts plate. The primary radiation enters the sample at an angle that is only slightly smaller than the critical angle for reflection. This significantly lowers the background scattering and fluorescence, permitting the detection of concentrations of only a few tenths of a ppb. II. Introduction X-ray Fluorescence (XRF) Spectroscopy involves measuring the intensity of X-rays emitted from a specimen as a function of energy or wavelength. The energies of large intensity lines are characteristic of atoms of the specimen. The intensities of observed lines for a given atom vary as the amount of that atom present in the specimen. Qualitative analysis involves identifying atoms present in a specimen by associating observed characteristic lines with their atoms. Quantitative analysis involves determining the amount of each atom present in the specimen from the intensity of measured characteristic X-ray lines. The emission of characteristic atomic X-ray photons occurs when a vacancy in an inner electron state is formed, and an outer orbit electron makes a transition to that vacant state. The energy of the emitted photon is equal to the difference in electron energy levels of the transition. As the electron energy levels are characteristic of the atom, the energy of the emitted photon is characteristic of the atom. Molecular bonds generally occur between outer electrons of a molecule leaving inner electron states unperturbed. As X-ray fluorescence involves transitions to inner electron states, the energy of characteristic X-ray radiation is usually unaffected by molecular chemistry. This makes XRF a powerful tool of chemical analysis in all kinds of materials. In a liquid, fluoresced X-rays are usually little affected by other atoms in the liquid and line intensities are usually directly proportional to the amount of that atom present in the liquid. In a solid, atoms of the specimen both absorb and enhance characteristic X-ray radiation. These interactions are termed 'matrix effects' and much of quantitative analysis with XRF spectroscopy is concerned with correcting for these effects. While the principles are the same, a variety of instrumentation is used for performing Xray fluorescence spectroscopy. There are two basic classes of instruments: Wavelength Dispersive and Energy Dispersive. Wavelength Dispersive spectrometers measure X-ray intensity as a function of Wavelength while Energy Dispersive spectrometers measure X-ray intensity as a function of energy. An extremely important aspect of X-ray fluorescence spectroscopy is the method by which the inner orbital vacancy is created. Bombarding the sample with high energy X-rays is one method. Bombarding with high-energy electrons and protons are other approaches. An incident photon beam experiences a photon absorption interaction with the specimen while electron and proton beams primarily experience a Coulomb interaction with the specimen. X-ray tubes accelerate high-energy electrons at a target within the tube that is then caused to fluoresce X-rays. The resulting X-ray beam includes a continuum and characteristic lines of the tube target. Radioactive sources can also be used to generate X-ray, electron (beta
Page 1 and 2: X-Ray Fluorescence Analytical Techn
Page 3 and 4: II.4 Energy Resolution III. Spectru
Page 5: Module: Title: X-Ray Fluorescence A
Page 9 and 10: where: c = speed of light (2.99782
Page 11 and 12: Figure I.2: Bohr’s atomic model,
Page 13 and 14: decelerated as they penetrate the m
Page 15 and 16: The mass attenuation coefficient ha
Page 17 and 18: e neglected provided that momentum
Page 19 and 20: In higher atomic number materials,
Page 21 and 22: Figure I.14: A Bremsstrahlung (Cont
Page 23 and 24: filters but more often with pulse h
Page 25 and 26: SECTION II ENERGY DISPERSIVE X-RAY
Page 27 and 28: just above the absorption edges of
Page 29 and 30: energy. Electronic noise is minimiz
Page 31 and 32: E is the photon energy, ε is the a
Page 33 and 34: IV.2 Compton Edge Figure II.8: Esca
Page 35 and 36: The relative error resulting from a
Page 37 and 38: Figure II.11: Schematic diagram of
Page 39 and 40: factor of 2, because also the refle
Page 41 and 42: IV. Instrumentation The major compo
Page 43 and 44: One of the inherent advantages in T
Page 45 and 46: Figure III.6: Sample preparation me
Page 47 and 48: III.1 gives an overview of various
Page 49 and 50: crystal mounted on a 2θ goniometer
Page 51 and 52: Figure IV.4: Collimators with diffe
Page 53 and 54: II.3.2 Reflections of Higher Orders
Page 55 and 56: Ge InSb PET AdP TIAP OVO-55 OVO-N O
Page 57 and 58:
II.4.2 Scintillation Counters The s
Page 59 and 60:
When using other counting gases (Ne
Page 61 and 62:
SECTION V SAMPLE PREPARATION XRF an
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eground and pressed or cast as a di
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SECTION VI QUANTITATIVE ANALYSIS Th
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Cr 1.2 16 Sn 4.8 (Lα) 8 Mn 2.6 12
Page 69 and 70:
The mechanism of the enhancement ef
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Mineralogical effect were discussed
Page 73 and 74:
In general this equation is referre
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from sugar, but then this involved
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I. (The Photon Concept) EXERCICES A
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I. Solution SOLUTIONS An FM radio s
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charge = 15× 10 Cs second −3 -1
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h ∆λ = ( 1 − cosθ ) = 2λ c =
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