Introduction to X-ray fluorescence (XRF) analysis

Transcription

1 Introduction to X-ray fluorescence (XRF) analysis

2 Outline X-rays, what are they? What is XRF? History of discoveries in the field of X-rays Basics of XRF theory Evaluation of XRF spectra XRF instrumentation Analysis software

3 Discovery of X-rays November 11, 1895, Wilhelm Conrad Roentgen First Nobel prize in physics in December 22, 1895 photograph of Anna Röntgen s hand

4 Electromagnetic spectrum

5 X-rays, what are they? Bremsstrahlung Characteristic radiation

6 Spectrum of X ray tube Bremsstrahlung Short wavelength limit depends on tube voltage Characteristic radiation Depends of anode material

7 What is XRF? The XRF is the emission of characteristic (or fluorescent) X-rays from a material that has been excited with X-rays.

8 History of discoveries in the field of X-rays Max Theodor Felix von Laue Nobel Prize in Physics in 1914 for his discovery of the diffraction of X-rays by crystals.

9 History of discoveries in the field of X-rays Sir William Henry Bragg shared a Nobel Prize in Physics in 1915 with his son William Lawrence Bragg

10 History of discoveries in the field of X-rays Sir William Lawrence Bragg shared a Nobel Prize in Physics in 1915 with his father Sir William Henry Bragg The youngest Nobel Laureate having received the award at the age of 25.

11 History of discoveries in the field of X-rays Charles Glover Barkla Nobel Prize in Physics in 1917 For his discovery of the characteristic X-rays of elements.

12 History of discoveries in the field of X-rays Karl Manne Georg Siegbahn Nobel Prize in Physics in 1924 For his discoveries and research in the field of X-ray spectroscopy

13 History of discoveries in the field of X-rays Arthur Holly Compton Nobel Prize in Physics in 1927 For his discovery of the Compton effect

14 History of discoveries in the field of X-rays Henry Gwyn Jeffreys Moseley Moseley law in X-ray spectra (1924) Moseley was shot and killed during the Battle of Gallipoli, Turkey, on 10 August 1915, at the age of 27. Some prominent authors have speculated that Moseley could have been awarded the Nobel Prize in Physics in 1916, had he not died in the service of the British Army.

15 Characteristic (fluorescent) X-rays A source X-ray strikes an inner shell electron and removes it from the atom. Higher energy electrons cascade to fill vacancy, giving off characteristic X-rays.

16 ATOMIC STRUCTURE An atom consists of a nucleus (protons and neutrons) and electrons Electrons spin in shells at specific distances from the nucleus Electrons take on discrete (quantized) energy Inner shell electrons are bound more tightly and are harder to remove from the atom Adapted from Thermo Scientific Quant X EDXRF training manual

17 ELECTRON SHELLS Shells have specific names (i.e., K, L, M) and only hold a certain number of electrons n principal quantum number 2n2 number of electrons 2n-1 number of sublevels K shell, n=1, 2 electrons, 1 level L shell, n=2, 8 electrons, 3 sublevels M shell, n=3, 18 electrons, 5 sublevels N shell, n=4, 32 electrons, 7 sublevels X-rays typically affect only inner shell (K, L) electrons Adapted from Thermo Scientific Quant X EDXRF training manual

18 MOVING ELECTRONS TO/FROM SHELLS Binding Energy versus Potential Energy The K shell has the highest binding energy and hence it takes more energy to remove an electron from a K shell (i.e., high energy X-ray) compared to an L shell (i.e., lower energy X-ray) The N shell has the highest potential energy and hence an electron falling from the N shell to the K shell would release more energy (i.e., higher energy X-ray) compared to an L shell (i.e., lower energy X-ray) Adapted from Thermo Scientific Quant X EDXRF training manual

19 K, L, M Spectral Lines Ø Ø Ø Ø K - alpha lines: L shell etransition to fill a vacancy in K shell. Most frequent transition, hence most intense peak. K - beta lines: M shell etransitions to fill a vacancy in K shell. L - alpha lines: M shell etransition to fill a vacancy in L shell. L - beta lines: N shell etransition to fill a vacancy in L shell.

20 Moseley law in X-ray spectra

21 Evaluating Spectra In addition to elemental peaks, other peaks appear in the spectra: K & L Spectral Peaks Rayleigh Scatter Peaks Compton Scatter Peaks Escape Peaks Sum Peaks Bremsstrahlung

22 K & L Spectral Peaks K-Lines Ka kev Kb kev L-lines kev kev Rh X-ray Tube

23 Rayleigh Scatter X-rays from the X-ray tube or target strike atom without promoting fluorescence. Energy is not lost in collision. (EI = EO) They appear as a source peak in spectra. AKA - Elastic Scatter EO EI Rh X-ray Tube

24 Compton Scatter Energy is lost in collision. (EI > EO) Compton scatter appears as a source peak in spectra, slightly less in energy than Rayleigh Scatter. AKA - Inelastic Scatter EO EI Rh X-ray Tube Ka E0= kev kev Kb E0= kev X-rays from the X-ray tube or target strike atom without promoting fluorescence. E = kev shift= E = kev shift=

25 Compton effect

26 SUM PEAKS Example from analysis of Fe sample Detector Fe Kα peak 6.40 kev Fe Kα πηοτον κες Fe Kα πηοτον κες Sum peak kev Sum Peak = Fe + Fe = Adapted from Thermo Scientific Quant X EDXRF training manual Fe sum peak kev Artifact peak due to the arrival of 2 photons at the detector at exactly the same time (i.e., Kα + Kα, Kα + Kβ ) More prominent in XRF spectra that have high concentrations of an element Can be reduced by keeping count rates low

27 ESCAPE PEAKS Example from analysis of Pb sample 700 Detector 600 Si Kα photon 1.74 kev 500 Pb Lα photon kev Escape Peak = Pb Si 8.81 = Adapted from Thermo Scientific Quant X EDXRF training manual Pb escape peak (from Lβ) 400 Intensity (cps) Escape peak 8.81 kev Pb Lα line Pb Lβ line kev kev 300 Pb escape peak (from Lα) Energy (kev) Artifact peak due to the absorption of some of the energy of a photon by Si atoms in the detector (Eobserved = Eincident ESi where ESi = 1.74 kev) More prominent in XRF spectra that have high concentrations of an element and for lower Z elements

28 X-ray absorption Io Ix

29 Properties of absorption coefficient

30 XRF Instrumentation The basic concept for all XRF spectrometers is a source, a sample, and a detection system. The source irradiates the sample, and a detector measures the fluorescence radiation emitted from the sample. Two types of instruments: Wavelength Dispersive XRF spectrometers Energy Dispersive XRF spectrometers

31 Wavelength Dispersive XRF spectrometer

32 Energy Dispersive XRF spectrometer

33 Detector for EDXRF Great efforts in the design of detector and signal processing system Very small magnitude of signals Example: Ca (Z=20) Ka =3.691 kev Energy required to produce 1 electron-hole pair =3.86eV Number of electron-hole pairs produced=3691/3.86=956 Electron charge= C Charge from Ca Ka interaction Q= = C With capacitance 0.1pF the output voltage Q/0.1pF=1.53mV Example: Na (Z=11) Ka =1040eV, Mg (Z=12) Ka =1253 ev E=213eV Energy resolution of 200 ev requires 80 V voltage resolution

34 Silicon drift detector(sdd) 200µ Ultralow capacitance detector: C= pF / cm2 The concept of the SDD was introduced in 1984 : E. Gatti, P. Rehak, Semiconductor drift chamber an application of a novel charge transport scheme, Nucl. Instrum. Meth. 225, pp (1984).

35 Energy Dispersive Electronics Fluorescence generates a current in the detector. In a detector intended for EDXRF, the height of the pulse produced is proportional to the energy of the respective incoming X-ray.

36 Multi-Channel Analyser Detector current pulses are translated into counts (counts per second, CPS ). Pulses are segregated into channels according to energy via the MCA (Multi-Channel Analyser).

37 DIFFERENT TYPES OF XRF INSTRUMENTS Bruker Tracer V Benchtop/Lab model/ Portable/ Handheld/ Innov-X X-50 Thermo/ARL Quant X EASY TO USE ( point and shoot ) COMPLEX SOFTWARE Used for SCREENING Used in LAB ANALYSIS Can give ACCURATE RESULTS when used by a knowledgeable operator Primary focus of these materials Designed to give ACCURATE RESULTS (autosampler, optimized excitation, report generation)

38 ARL QUANT X Energy Dispersive XRF spectrometer Rh anode X-ray tube Voltage 50kV Power 50W cooled Si Peltier (Li)detector (6 stages) Temperature : -90o C, Energy resolution 150ev pump 10-3 Vacuum bar time 2 hours Warm-up size:30x40x5 cm Sample max

39 Element range and chamber atmosphere Organic elements (i. e., H, C, N, O) do not give XRF peaks Air absorbs low energy X-rays from light elements particularly below Ca, (Z=20). For detection of light elements 2 kinds of chamber atmosphere can be used: Vacuum - For solids or pressed pellets. Helium - For liquids or powdered samples

40 Qualitative Analysis

41 Quantitative Analysis Concentrati on XRF is a reference method, standards are required for quantitative results. Standards are analysed, intensities obtained, and a calibration plot is generated (intensities vs. concentration). Intensity XRF instruments compare the spectral intensities of unknown

42 Analysis software ARL QUANT X has two types of software for quantitative analysis WinTrace Calibration standards are required. Composition of standards must be similar to that of the sample under study. UniQuant - Standardless analysis software Pre-calibrated at the factory no standards required.

43 Uniquant analysis software Automatically collects and processes spectra with 8 excitation conditions for each sample Each excitation condition (tube voltage and filter) is optimized for a range of elements of different atomic numbers Z (low Z, mid Z, high Z) Measurement time: 15 min per sample

44 Analytes and conditions

45 Sample specific information is provided by the user

46 Example of analysis report

47 Results of XRF analysis for 14 ceramics samples from Byurakan Typical error

48 XRF analysis result: composition diagram (average for 14 samples)

49 Summary XRF is based on detection of X-rays that are characteristic of the elements in the sample. These characteristic X-rays (fluorescence) are excited in the sample by an external radiation source (X-ray tube or isotope source) Energy Dispersive XRF systems detect elements between Sodium (Na, Z=11) and Uranium (U, Z=92). XRF is a fast, non-destructive, and usually requires only minimal sample preparation.