EBB 245. Materials Characterisation Lecture 3 X-ray Fluorescence Spectrometry (XRF) Dr Zainovia Lockman zainovia@eng.usm.my PPKBSM, USM (lecture presentations/notes 2011)

Contents of the Lecture Introduction to XRF XRF Instrument 2.1. WDS 2.2. EDS Analysis by XRF 3.1 Qualitative 3.2. Quantitative Variation of XRF Applications of XRF Revision & Active learning

1. Introduction to XRF: X-ray tube Sample Charactersitic X-ray Detector XRF analyses the chemical elements of specimens by detected the characteristic x-rays emitted from the specimen after being radiated by high energy x-rays. The characteristic x-rays can be analysed from either their : Wavelength: wavelength dispersive spectroscopy (WDS) or 2. Energies: energy dispersive spectroscopy (EDS)

Background theory When X-rays hit a sample the x-rays are absorbed. The absorbing atoms from the sample will be ionized (due to the x-ray beam ejects the electron in the inner shell). An electron from higher energy shell (like the L shell) then fall into the position vacated by dislodged inner electron and emit x-rays or characteristic wavelength. This process is called x-ray fluorescence. The wavelength emitted is characteristic of the element being excited. Measurement of this wavelength enable us to identify the element. The intensity of the fluorescence depends on how much of that element have absorbed the x-ray beam. Hence measurement of the fluorescence intensity makes possible the quantitative determination of an element.

The X-Ray Fluorescence Process Example: Titanium Atom (Ti = 22) X-ray hits titanium Titanium absorbed the X-ray The atom will ionise In the figure, electron from the K-shell is removed from the shell creating a vacancy at K level. What would happen to this vacant space?

The K-Lines: Kα x-ray, K x-ray As there is a vacancy at the K shell, electron from either the L or the M shell can jump into this vacancy. Electron from L or M shell has higher energy compared to electron in the K shell. When electron from L falls from its higher energy L level to a lower K level, energy in a form of characteristic secondary x-ray will be released. This characteristic x-ray is Kα x-ray unique to this element (titanium). When electron from M level falls to K level, the characteristic X-ray is called the K ray

The L Lines: Lα x-ray, L x-ray When electron from L shell falls into the K level, a vacant space will be created at the L level. Electron from M level will see a vacant space in the lower L level and will jump into the vacant space. As the M electron has higher energy, its transition from the high level to a lower level will be accompanied by a release of energy in a from of x-ray The characteristic x-ray from this transition will give out L X-ray Similarly electron from N can jump into the L level and the transition is from N level to L level then L X-ray will be emitted out

Example of XRF spectrum of metal Lead. How to ‘read’ the spectrum?

Characteristic X-rays Fluorescence Recall that XRF spectrum is gathered when the characteristic x-rays emitted from an element is detected by a detector in the XRF instrument. The characteristic x-rays are labeled as K, L, M or N to denote the shells they originated from. Another designation alpha (α), beta () or gamma (γ) is made to mark the x-rays that originated from the transitions of electrons from higher shells. Therefore K-lines will be produced consisting of: Kα x-ray is produced from a transition of an electron from the L to the K shell K x-ray is produced from a transition of an electron from the M to a K shell K x-ray s produced from a transition of an electron from the N to a K shell Similarly, transition from M and N to vacant L level will produce L-lines Since within the shells there are multiple orbits of higher and lower binding energy electrons, a further designation is made as α1, α2 or 1, 2, etc. to denote transitions of electrons from these orbits into the same lower shell.

2. XRF Instrument An XRF instrument is consisted of three parts: X-ray source Detection system Data collection and processing system X-ray source: generates primary X-rays to excite atoms of specimens (x-ray tube similar to in XRD) Detection: depending on WDS or EDS WDS: single crystal diffraction to detect characteristic wavelength EDS: photon detector, Si(Li) diode to separate the characteristic of X-ray photons according their energy

2.1. WDS WDS provides better resolution and wider range of elemental analysis compared to EDS WDS detect atom starting with Z~4 WDS apparatus is shown below.

Components of XRF-WDS X-ray tube To emit X-ray Similar tube concept as XRD Specimen camber To place the sample to be analysed Collimators To align the characteristic x-rays from the specimen and beam diffracted from the analysing crystal X-ray detector system comprising of: An analysing crystal Rotating X-ray photon counter scan a range of 2 to detect specific wavelength of characteristic X-ray from the specimen.

Components of XRF 1. X-ray generator part 2. Spectrometer part X-ray tube Specimen chamber Collimator To spectrometer part 2. Spectrometer part Analysing crystal Photon counter Collimator The fluorescent beam is collimated to a parallel beam. The collimated fluorescent beam will impinge on a face of a flat single crystal (analysing crystal) and will be diffracted by the single crystal. The diffracted x-rays are detected by the counter and through the electronic circuit, the intensities are recorded automatically on the charge recorder. The intensity will be plotted as a function of wavelength.

The analyzing crystal The diffraction phenomenon of x-ray through the single crystal is utilized for the dispersion of x-rays. This crystal is called the analyzing crystal. Must have an idea on the wavelength region of the fluorescence x-rays hence an the optimum analyzing crystal can be employed Analyzing crystal will determine the range of detectable atomic numbers. The wavelength that can be detected can be defined by Bragg’s law Normally, the maximum achievable  angle in a WDS system is about 73o Therefore the maximum characteristic wavelength that can be detected is about 1.9d of the analyzing crystal When the analysing crystal diffract the x-ray from the specimen, these diffracting angles (θ) are measured and λ of each element is determined using Bragg’s law. By determining the elemental spectra recorded on a chart, we can learn the name of elements containing in the specimen.

Table: Analyzing Crystals used in WDS

The detector The collimated fluorescent beam will impinge on the face of the analyzing crystal The analysing crystal will diffract the beam A radiation detector (counter) will rotated simultaneously with the crystal, twice the angular speed of the crystal and will record any diffracted beam from the crystal The immediate instrument output is count rate (intensity) versus angular position of the counter Knowing the Bragg’s Law, the angular position can be converted into wavelength The XRF spectrum is intensity versus wavelength plot

2.2. EDS The EDS system does not have moving part such as the rotation detection employed in WDS EDS is relatively faster and much cheaper to purchase and to run A detector collects the characteristic x-ray from the specimen Can detect element starting at Z~ 8 (lightest element that can be detected is O)

EDS Detector The Si(Li) is the most commonly used as detector It is a p-type silicon with lithium in a form of Si(Li) diode. X-ray photon will be collected by the detector will excite electrons from the conduction band of the silicon forming electron-hole pairs The higher the photon energy the more electron-hole pairs can be generated Characteristic X-ray photons can be separated by their energy levels according to the number of electron-hole pairs that they manage to generate The electron hole pairs will be swept by the diode and will be converted into electrical pulse whose voltage amplitude is proportional to the X-ray photon energy.

3. Analysis by XRF Given an unknown solid specimen. The specimen can be analyzed by XRD. For this, XRF can be used for: Qualitative analysis Identification of element in the specimen Unknown elements are identified by comparing their fluorescent spectra with the known and tabulated atomic spectra So we know what element(s) present(s) in the specimen Quantitative analysis Fluorescence increases with concentration of element in the specimen The intensity of the fluorescence can be used to determine the concentration of the elements in the specimen We know the quantity of the element(s) present in the specimen

3.1. Qualitative analysis Example: XRF spectrum of a stainless steel

For this case the primary X-ray source of XRF is supplied by tungsten-target tube operated at 50 kV. Often uncommon element like rhodium (Rh) is used. The primary X-ray includes both the continuous and characteristic radiation. The continuous radiation will generate background and the characteristic X-ray generates additional peaks in the spectrum. The XRF spectrum shown is for stainless steel containing 18%Cr and 8% Ni. The K lines of all the major constituents (Fe, Cr and Ni) and of some of the minor constituents (Mn and Co) are apparent. In addition tungsten L lines can be seen. The tungsten lines can always be seen when a tungsten tube is used. The copper K lines are due to copper existing as an impurity in the tungsten target

3.2. Quantitative Analysis In XRF, the concentrations of elements must relate to their peak intensities of the spectrum. Chemical compositions should be calculated by comparing the ratios of integrated peak intensities among elements in the specimen. Given: C = M x K x IR C = weight fraction of an element relation to the relative intensities of its peaks (IR) K = instrument factor M = Matrix factor of the specimen K = conditions of primary source, the geometrical arrangement of specimen respect to radiation and detection and detector characteristic The matrix factor refers to the interactions among the elements in the specimen

Example of Quantitative Analysis Consider the quantitative analysis result of a dry limestone sample which is obtained from XRF analysis : Mg and Ca are found as carbonates, while Fe and Si as oxides. Convert the data to mol % of the actual substances (CaCO3, MgCO3, Fe2O3 and SiO2) present in the limestone. Mol weight : Ca = 40.1; Mg = 24.3; C = 12.0; O = 16.0 and Fe = 55.8, Si=28.1. Spectra of oxygen and carbon are not considered. ELEMENT SPECTRUM WT% Mg Mg Kα 1.2 Ca Ca Kα 90.0 Fe Fe Kα 2.3 Si Si Kα 6.5

Quantitative Analysis 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). XRF instruments compare the spectral intensities of unknown samples to those of known standards. Concentration Intensity

4. Variation of XRF XRF facilitates rapid, nondestructive analysis, and has a wide range of applications, including production processes and quality control. For example in semiconductor industry, XRF is used to identify (qualitative) and measure the concentration (quantitative) if impurities. XRF however must measure large area for decent spectrum formation (1cm2) The penetration depth of X-ray is typically several micron (dependent on the absorption coefficient of the material) therefore XRF cannot be used for thin film, coating and surface analysis

TXRF However, several variations of XRF have been developed especially the TXRF  Total reflection XRF: a surface sensitive technique which X-rays strike the sample at a very shallow angle and penetrate only a small distance into the sample. This technique can be used for determination of thin film, contamination on a surface of a material (for example contamination on silicon wafer) and level of impurities on a surface of a specimen

5. Examples of the applications of XRF The development in X-ray detectors has established the XRF method as a powerful technique in a number application fields, including: Ecology and environmental management: measurement of heavy metals in soils, sediments, water and aerosols Geology and mineralogy: qualitative and quantitative analysis of soils, minerals, rocks etc. Metallurgy and chemical industry: quality control of raw materials, production processes and final products Paint industry: analysis of lead-based paints Jewelry: measurement of precious metals concentrations Fuel industry: monitoring the amount of contaminants in fuels Food chemistry: determination of toxic metals in foodstuffs Agriculture: trace metals analysis in soils and agricultural products Archaeology and archaeometry Art Sciences: study of paintings, sculptures etc.

Revision & Active learning (AL): to be done with a partner Given a metal foil with dimensions of 2cm x 5 cm and thickness of 100mm. Write down step by step procedures to analyse the unknown metal foil by XRF.