X-Ray Fluorescence Spectroscopy Notes PDF

X-RAY FLUORESCENCE SPECTROSCOPY: CHARACTERISTIC X-RAY EMISSION Incoming Ejected Electron Electron X-ray Emitted The emission Vacancy fi...

X-RAY FLUORESCENCE SPECTROSCOPY: CHARACTERISTIC X-RAY EMISSION Incoming Ejected Electron Electron X-ray Emitted The emission Vacancy filled by an of X-ray electron from a Takes place higher Electronic ≈10-4 sec sub shell Vacancy created in the inner electronic later sub shell The energy of the Characteristic X-ray emitted depends on the difference in the binding energies of the electronic sub-shells in an atom between which the electron transfer takes place: hγ = EK ˜ E L 1 CHARACTERISTIC X-RAY SPECTRUM Analytical Instrumentation 2023 2 X-RAY AND AUGER ELECTRON EMISSION Characteristic X-ray Emission IN201-Analytical Instrumentation:Aug-Dec 2024 3  X-RAY FLUORESCENCE Absorption of X-rays by matter results in electronically excited ions which return to ground state by electronic transition from higher energy shells to lower energy shells with the emission of Fluorescent X-rays (Characteristic X-rays) Analytical Instrumentation 2023 4  CONDITIONS FOR X-RAY FLUORESCENCE EMISSION ▪ Exciting primary X-radiation should have larger intensity ▪ λ0 of incident X-rays should be shorter than the corresponding absorption edge of the element being investigated ▪ λ of fluorescent X-rays produced are always slightly > than the corresponding absorption edges Reason : Absorption requires complete removal of electron (Ionization) from the atom Energy required = BE +P.E work function whereas emission involves transition of an electron from a higher level to lower level within the atom. eg. 0 K absorption Edge of Ag = 0.485 A 0 K Emission Line of Ag = 0.497 A 5  FLUORESCENCE YIELD 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓𝑒𝑚𝑖𝑡𝑡𝑒𝑑 𝑓𝑙𝑢𝑜𝑟𝑒𝑐𝑒𝑛𝑐𝑒 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝐹𝑙𝑢𝑜𝑟𝑒𝑠𝑐𝑒𝑛𝑐𝑒 𝑌𝑖𝑒𝑙𝑑 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 Depends on the rate constants of the of the excitation/ recombination states Is used to determine the relative effectiveness of X-ray emission. Fluorescence yield For Atomic number < 4, the fluorescent yield is Zero: Dominating Auger Processes Intrinsic disadvantage in the analysis of light (low Z) elements K-series yield is more effective with higher intensity Fluorescence yield for Ce (Z=58) atoms, K line: 0.91, L-line: 0.14 and M-Line: 0.001 Atomic Number 6 SCHEMATIC OF X-RAY FLUORESCENT (XRF) SPECTROSCOPY X-ray radiation Secondary/ Fluorescent radiation 7  FEATURES OF X-RAY FLUORESCENT (XRF) SPECTROSCOPY The Wavelength or Energy of the Fluorescent X-ray Identification of the Element The Intensity of Fluorescent X-ray Concentration / composition of the element Surface Technique (penetration depth of ~ 0.5mm) Non-destructive Analysis Minimal sample Preparation High energy Resolution High Sensitivity Multielement Analysis Both Qualitative and Quantitative Analysis Analytical Instrumentation 2023 8  X-RAY FLUORESCENCE METHODS 1) Wavelength Dispersive XRF (WDXRF) Spectroscopy. 2) Energy Dispersive XRF (EDXRF) Spectroscopy. Analytical Instrumentation 2023 9  WAVELENGTH DISPERSIVE XFS mono source SAMPLE λ1 , λ2 , λ3 chromator Detector Stainless steel sample Smaller d-spacing in the Analyzing crystal allows detection of fluorescence from heavier (high Z) elements 10  ANALYZING CRYSTAL It determines the range of detectable atomic numbers (elements) 𝟐𝒅 𝒔𝒊𝒏𝜽 Works on the principle of Bragg's law: 𝝀 = 𝒏 Maximum detectable angle () in the Wavelength dispersive system ~ 73 Therefore, the Maximum detectable wavelength of fluorescent characteristic X-rays : 𝝀 𝐦𝐚𝐱 = 2𝑑 sin 73° = 𝟏. 𝟗 𝒅 ; for n = 1 𝒅𝜽 𝒏 The detection resolution: = ; Smaller ‘d’ : Higher resolution 𝒅𝝀 𝟐𝒅𝒄𝒐𝒔(𝜽) Examples of the Analyzing Crystals: Lithium Fluoride (LiF), Sodium chloride (NaCl), Thallium acid Phthalate (TAP), Pentaerythritol (PE) and Layered synthetic microstructure (LSM) Analytical Instrumentation 2023 11  ANALYZING CRYSTAL Lower Z elements will have longer wavelength characteristic emission and therefore require large atomic spacing of analyzing crystal (but lower resolution of the intensity peaks) Narrow d-spacing crystals will restrict the window of detectable elements to larger Z values owing to λmax = 1.9 d limit, but provides high resolution data. Analytical Instrumentation 2023 12  WAVELENGTH DISPERSIVE XRF SPECTRA LiF Analyzing Crystal: smaller d-spacing TAP Analyzing Crystal: longer d-spacing Short wavelength range Short wavelength range Elemental analysis of Chromium, Cobalt, Vanadium, Aluminium, Tantalum, Tungsten and Rhenium alloy using WDXRF spectroscopy method The relative intensities of the spectrum lines of individual elements give an estimate of their relative concentration in an alloy 13  Advantages / Disadvantages of WDXRF Advantages ▪ Higher resolution → good spatial separation of X-ray lines. Resolution : WDXF : 10eV for 5.9keV peak. EDXF : 150eV for 5.9keV peak. ▪ The O/P count is proportional to elemental concentration → Compositional Analysis. ▪ Good peak to background (Signal to noise) ratio. Disadvantages of WDXRF ▪Mechanical system introduces errors during measurement. ▪Peak / background to be measured separately. ▪Higher currents needed for WDXF lead to a large beam size leading to specimen damage. Analytical Instrumentation 2023 15 ENERGY DISPERSIVE X-RAY FLUORESCENCE (EDXRF) ANALYSIS Cooled to 77K X-rays Semiconductor Characteristic SAMPLE Detector Amplifier X-rays Multichannel Analyzer The no of electron /hole pair produced by an incident X-ray Photon = n hν/ε ε = Energy required to create one e- / e+ pair. The K.E of the e- produced = h ν – ε. Semiconductor p-type Si(Li) detector Analytical Instrumentation 2023 16  DETECTION AND PROCESSING IN EDXRF In a semiconductor detector, the number of electron-hole pairs generated is proportional to the energy of the incident photon. Generally, the output is taken as voltage pulses which are amplified. The amplitude of the voltage pulse from the amplifier is proportional to the amount of energy deposited in the detector by the radiation. By examining the amplitudes of amplifier output pulses, it is possible to determine the energies of detected radiation. Selective counting of only those pulses within a certain amplitude range (pertaining to electrons of a given energy range) makes it possible to restrict counting to a selected energy range and to discriminate against background, scattered radiation, and so forth outside the desired energy range 19  Multichannel analyzer (MCA) for Counting the desired pulses above a signal (pulse amplitude) threshold A device that is capable of analyzing simultaneously within many different intervals or channels is called a multichannel analyzer (MCA) The MCA registers the no. of electrons produced in the energy range: (E+E) 20  ENERGY DISPERSIVE X-RAY SPECTRUM ANALYSIS Characteristic X-ray line intensity over the range of Emission energies Detected spectra is in the energy window of 0.1 to 20keV Shows (L or M lines of) Heavy and (K-lines of) Light elements EDS spectrum of glass containing Silicon, Oxygen, Calcium, Aluminium, Iron and Barium 21  Advantages / Disadvantages of EDXFA Advantages ▪Simple mechanical design, no moving parts in excitation as well as detection. ▪Absence of collimators / monochromators resulting in almost 100% increase in quantum of radiation reading the detector. ▪The detector can be positioned closer to the sample →Larger solid angle for X-ray Collection. ▪Even use of weaker [Radioactive] sources possible →Portable /field application. ▪Given beam current results in a higher count rate. ▪Smaller volumes of material are only needed. ▪Almost 100% detector efficiency for a wide range of X-rays from 3 to 20keV. ▪All the X-ray lines are measured simultaneously → Complete elemental spectrum →Rapid analysis. 22  Disadvantages of EDXFA ▪ Lower resolution for X-ray having  > 1Å, better resolution at lower . ▪ EDXF equipment requires detector to be operated at 77K ▪ Poor sensitivity to lower Z elements (Z < 11) Due to inherent properties of the semiconductor detector (dead layer effects) can not be used for X-rays with E < 1keV (cannot be used for analysis of elements with Z < 11. ▪ Stray signal / radiations affect the measurement → Higher background. Analytical Instrumentation 2023 23 Resolution obtained in the Wavelength (WDX) and Energy dispersion (EDX) techniques- the peaks of titanium (Ti-Kα) and barium (Ba-Lα) Red: WDX Blue: EDX Analytical Instrumentation 2023 24 COMPARISION OF WDXRF AND EDXRF SPECTROSCOPY 25 SUMMARY : WDXRF AND EDXRF Energy-dispersive TXRF Wavelength-dispersive TXRF Si(Li) Scintillation Detector Detector Analyzing Crystal (Johansson) X-ray X-ray Sample Sample Substrate Substrate Advantages Advantages Large solid angle (High detection efficiency). High energy-resolution. Collecting whole XRF spectra simultaneously. Good signal to background ratio. Scanning Electron Microscopy (SEM) Disadvantages Disadvantages Low detection-efficiency. Low energy-resolution. Limitation of counting-rate. Scattering background. 26 QUANTITATIVE ANALYSIS USING XRF Analytical Instrumentation 2023 27  SEMI-QUANTITATIVE INFORMATION FROM XFA → The peak heights corresponding to each line is measured. → Rough estimate of the concentration of the species emitting X-ray Fluorescence is obtained from: IX = IS WX. IX = Relative Line Intensity in terms of no. of counts / Fixed time. WX = Weight fraction of desired element in sample. IS = (Relative) Intensity that would be observer with pure (100%) element (WX = 1 , Standard Sample). Wx = Ix / IS 28  Wx vs Atomic concertation plot The above estimation assumes that : ◼ The emission from an element is unaffected by the presence of other elements. ◼ Error is estimation which is however smaller in comparison with optical emission methods. 29  FACTORS AFFECTING XRF INTENSITY Matrix Absorption Multiple Excitation: Secondary Absorption/Emissions The X-ray fluorescence emitted by an element in a matrix, has to pass through a certain depth of the matrix before reaching the surface. IX-ray d IX = IS WX The intensity of the XRF is decreased by the absorption of the matrix. Absorption effects by matrix results in Lower / Higher estimates of the concentration. If matrix absorbs incident radiation / emitted radiation, strongly compared to inclusion → WX will be Low as IS is estimated from a lower absorbing standard. If matrix absorbs incident / emitted radiation weakly compared to the inclusion →WX estimated will be higher, as IS will be estimated from a higher absorbing standard. Analytical Instrumentation 2023 30  Effect of Matrix Absorption Calculate the ratio of the intensity of Fe in the alloy (Fe-Al and Fe-Ag) to the Intensity of the Pure Fe 𝐼 element: 𝐹𝑒 𝐼𝑆 The absorption coefficient of Fe-Al is smaller than the absorption coefficient of Fe-Ag sample Analytical Instrumentation 2023 31  Effect of Multiple excitations 𝐼𝐹𝑒 𝐼𝐹𝑒 𝐼𝑆 for Fe-Ni  𝐼𝑆 for Fe-Al eventhough their absorption coefficients are very different Multiple excitations in Fe (Z = 26) are caused by the interaction of Ni (Z = 28) with X-rays : Producing secondary absorption and Secondary emission of X-rays 32  Applications Solid / Liquid samples (small, large, regular, irregular sample). Quality control in material preparation. Quantitative estimation of heavy elements. Analytical Instrumentation 2023 34  Film thickness determination using XRF Metal Plating Thickness Determination of oxide layers thickness Determination of paint layer thickness. Can be used for planar / irregular samples Analytical Instrumentation 2023 35 The contribution to the intensity of the primary fluorescence Ii (x) of element i is given by the product of the above factors. Fluorescence Dependence on the Thickness of Samples Analytical Instrumentation 2023 36  The Thick Sample For a thick sample, where h becomes sufficiently large, the exponential term of equation becomes negligible. then we are left with For a thin film, h is very small. For x 1 gram XRF is a reference method, standards are required for quantitative results. XRF analyses cannot distinguish variations among isotopes of an element. Analytical Instrumentation 2023 39  Applications of X-ray Fluorescence Chemical Analysis Soil surveys Mining (e.g., measuring the grade of ore) Cement production Ceramic and glass manufacturing Metallurgy (e.g., quality control) Environmental studies (e.g., analysis of particulate matter on air filters) Petroleum industry (e.g., sulfur content of crude oils and petroleum products) Field analysis in geological and environmental studies (using portable, hand- held XRF spectrometers) Analytical Instrumentation 2023 40  Advantages / Disadvantages of XFA - Summary Advantages ▪ Simple & non-destructive. ▪ Accuracy & precision comparable to other concentration analysis methods. Disadvantages ▪ Less sensitive – concentration limit – 0.01% ▪ Not very suitable for analysis of lower Z elements (Z < 11) , due to a Competing process called “Auger Emission” (re-absorption of X-rays emitted) Analytical Instrumentation 2024 42