XRF Flashcards
What electrons does AAS and AES utilise?
Valence electrons
How can the excitation of the valence electrons occur?
The excitation can occur at different degrees:
Low E tends to excite the outermost electrons first
When excited with a high E (photon of high v) an electron can jump more than on level
Even higher E can tear inner electrons away from the nuclei
What happens to excited electrons?
An electrons at its excited state is not stable and tends to return its ground state, if an electrons jumped more than one energy level because of absorption of high E the process of the electrons returning to its ground state may take several steps ie to the nearest low energy level first then down to the next
How to reach deeper energy levels?
Its impossible to work out the temperature needed to do this so need bombardment with x rays or electrons directly
XRF mechanism?
x ray or electron excitation –> sample –> x ray emission
XPS mechanism?
x ray excitation –> sample –> electron emission
What is the method of XPS/XRF?
Non destructive analysis of materials from visible absorption/ emission to x ray absorption/ emission (XRF), from x ray emission to electron emission (surface analysis XPS)
What is XRF?
An acronym for x ray fluorescence, considered one of the most widespread and useful methods for the determination if presence and quantity of the elements in a given substance
What is XRF a consequence of?
XRF is a consequence of the ionisation of atoms for which the excited state returns to the ground state emitting x ray photons of well defined energies, the characteristic x ray emission can be induced wither by interaction with charged particles or with x ray photons generated by a Coolidge tune or synchrotron radiation sources
How does XRF work?
Based on the primary excitation with an x ray beam, followed by the secondary x ray fluorescence emission. The x ray emission is then detected and analysed by a photon detector usually a solid state device characterised by a sufficient energy resolution
Difference between atomic spectroscopy and XRF?
Digestion of samples introduces errors
AAS, AES, AFS require sample preparation due to flame.plasma activation
Inner electron processes are independent of the chemical matrix and therefore there is no need to digest or to use flame/plasma
What is XRF based on?
XRF analysis is base don direct x ray fluorescence of solid/liquid samples
Activation is possible by electron beam or by x ray excitation
However calibration and quantitative analysis more difficult for XRF
How is XRF different for different materials?
Each material has different x ray cross sections so each material depending on how much there is will absorb x rays differently
How can x rays be generated?
A typical tube used as an x ray generator known as a Coolidge tube, bright, monochromatic
Rotating anodes
Radioisotopes (Fe55, Cd109, Am241) storage rings or synchrotron etc
Direct in situ generation during scanning electron microscopy (SEM)
How does a Coolidge tube work?
It works with a good quality vacuum (about 10-4 Pa or 10-9 Atm) in the Coolidge tube the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tub, the high voltage potential is between the cathode and the anode, the electrons are thus accelerated and then hit the anode
Whats important about the anode of the Coolidge tube?
In the sidewinder there is an electrostatic lens to focus the beam onto a very small spot on the anode, the anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons: mechanistically spun to increase the area heated by the beam. Cooled by circulating coolant, the anode is angled at 120 degrees off perpendicular to the electron current so as to allow escape of some of the x ray photons which are emitted essentially perpendicular to the direction of the electron current. The anode is usually made out of W, Mo or Cu, the tube has a Be window designed for escape of the generated x ray photons
What does the x ray emission spectrum at the anode consists of?
Bremsstrahlung (deceleration radiation)
X ray fluorescence pattern
What is Bremsstrahlung?
The first is a continuous spectrum extending up the lifting electrons energy (for example 60KeV or lambda = 20pm if the potential difference between the anode and the cathode is 60kV)
What is the X ray fluorescence pattern?
the x ray fluorescent pattern is instead composed by well defined peaks located at energy (or wavelengths) typical of the atomic constituent of the anode. These energies reflect the inner electronic structure of the atoms
What is the origin of the Bremsstrahlung?
The origin of the Bremsstrahlung is due to the deceleration imposed to the electrons arriving inside the anode from the surface
What does XRF work for?
XRF works for solids, liquids or gases/plasmas, not no thermal excitation
How is the sample affected by XRF?
XRF is a non invasive technique leaving the sample unchanged after application, in fact the electrons involved in the x ray emission processes are those related to the core levels and for this reason chemical bonds, structure and morphology are not affected
Excitations in XRF?
The number of excitation processes is low as compared to the available electrons, detectors for x rays are powerful
Sample preparation for XRF?
The sample do not usually need any specific preparation because the energy of the exciting photons is usually high enough (10-30 keV) allowing us to analyse depths in the order of 10 microns or more
How does the XRF detector work?
Good solid state detectors consist essentially of a 300-500 microns thick silicon junction type p-i-n diode (PIN) with a bias up to 1000V across it, when an x ray photon passes through it it causes a swarm of electron hole pairs to form and this causes a voltage pulse. The pulse magnitude is linked to wave length/energy
Inner atomic levels involved?
Inner atomic levels involved in the emission of x ray emission according to the accepted terminology (line Ka1 corresponds to a transition from the L3 (2p3/3) to the K (1s) level) note delta l = ±1
What does examination of XRF spectra allow?
The examination of the XRF spectra allows us to identify the atomic species present in a given sample, the energy of the x ray emission lines is a well defined function of the atomic number Z
For its powerful information content and simple application methodology, the XRF technique is applied in many different contexts and fields, from basic physics to chemistry, material science and mineralogy
What is an important feature of XRF?
An important feature of the XRF is the possibility of performing a chemical analysis in a rapid and non destructive way
How does XRF allows qualitative analysis?
Qualitative analysis for the identification of the elements through the position of the typical x ray emission lines in the XRF spectra
How does XRF allows quantitative analysis?
Quantitative analysis that needs an accurate data analysis accounting for the intensity of the different x ray emission lines. The measured intensity depends on a number of factors including the characteristics of the source, of the detector, of the geometry, emission cross sections and sample morphology. The use of standard with known concentration of the elements under investigation is particularly important for precise determinations
What does the Boltzmann distribution allow for thermal excitation?
For thermal excitation (equilibrium) the Boltzmann distribution allows emission intensities to be estimated from wavelength, for non equilibrium excitation (electrons, ions, x rays) also inner atomic levels can be excited to give x ray fluorescence
What does XRF require?
X ray fluorescence requires excitation with electrons or with x rays to give a finger print x ray spectrum reflecting the inner shell energy levels
What does quantitative XRF require?
Quantitative XRF required knowledge of the emission cross section and it varies for different types of sample and sample thickness. Standards are most likely to give reliable quantification
What is the origin of the x ray fluorescence pattern?
The x ray fluorescence pattern is due to the emission of photons as a result of a de excitation of the electrons of the atomic core levels (originally excited as a result of the collisions with the electrons of the incoming beam)
