X-Ray Flourescence Spectroscopy

• 1. Ray Techniques


  • Elemental Techniques
  • Crystallographic technique
• Elemental Techniques


  • X-ray fluorescence (XRF)
  • X-ray microprobe analysis
• 1. ray fluorescence (XRF)


  • wavelength dispersive XRF (WD XRF)
  • energy dispersive XRF (ED XRF)
• 1. ray fluorescence (XRF)


  A widely used technique for elemental analysis of metals and some non-metals
• 1. ray fluorescence (XRF)


  • Uses high frequency electromagnetic radiation (between far UV and gamma ray regions of the spectrum)
  • Mainly used with solid samples, but can be used for liquids
  • Potential for high precision measurements present at medium-high levels in samples
  • Unable to quantify very light elements (H-Be), normally elements above Na
  • Can get handheld devices – can measure out in the field
  • Inherently non-destructive, without preparations of sample
  • Can automate the process (more efficient)
• Wavelength of x-rays


  Measured in angstroms (Å). 1Å = 10-10m
• Energy of X-rays


  Measured in electron volts (eV)
• Wavelength and energy of X-rays


  Wavelength is inversely proportional to energy (shorter the wavelength, the higher the energy) - linear relationship
• How XFR works


  1. Samples are presented to a beam of high energy x-rays

  2. The x-rays penetrate the sample and excite the electrons (especially inner orbital electrons)

  3. If the x-ray beam's energy is above absorption edge of the element (needs a certain amount of energy), it ejects the electron

  4. Atom becomes unstable so an electron falls from an outer orbital and fills the gap

  5. The fall releases energy in form of x-ray at a discrete wavelength

  6. Measurement of emitted x-ray enables quantification of the elemental composition (concentration)

  7. The electron structure of an atom is element specific, so the energy/wavelength of the x-ray released is specific to that element

  8. The secondary x-rays reflect the composition of the sample
• Basic Instrument Layout


  • X-ray source
  • Sample
  • Source Modifiers
  • Detector
• Primary X-ray beam sources


  • X-ray tube (can switch on and off)
  • Radioisotopes (cannot control, must dispose appropriately)
  • Scanning electron microscopes – electron beam excites sample to produce x-rays, often paired with XRF analysis
  • Synchrotrons – bright light sources are used for research and very sensitive XRF analysis
  • Positron and other particle beams – All high energy particle beans ionise materials so that they give off x-rays. SEM and PIXE are most common beams used.
• 1. ray tube


  • Electrons are produced at a heated cathode
  • The electrons are accelerated towards a metal anode by high potential voltage (10-100kV)
  • Upon collision, part of the beam is converted into x-rays which are directed towards the sample
  • Window that sample sits on is often made of Beryllium (as its a light metal that the x-ray can penetrate)
  • The composition of the anode is dictated by the application (can change it)
  • Tube operating conditions can be varied depending on the analytical requirements (modern systems alter these on an element by element basis)
• Side window x-ray tube

  Filaments typically needs changing every 2 years
• End window x-ray tube


  • Same basic principle applied but electron beam is different (on either side of the anode)
  • Can put anode closer to sample, more interacts so has a higher sensitivity
  • Typically, end window x-ray tubes last 5-10 years (longer lasting)
• Emission from x-ray tubes


  • Get two forms of x-rays emitted:
  • The tube continuum – as you increase the accelerating voltage, the number of x-rays emitted increased, x-rays appear at lower wavelengths (higher intensity)
  • The line spectra – sharp intense beam at particular wavelengths that come off of the contimuum. It's made from the metals in the anion of the tube. When the incoming electron strikes the inner orbital electron with high force to displace it (produces an ion). Just at a shorter wavelength to elements interested in, which enhances them. Only produced when accelerating voltage exceeds the threshold voltage.
  • Most intense lines will be the K-series lines of each element in the sample as these are the most common transitions.
• Choice of anode


  • Commonly used anodes are made from Cr, Mo, Rh, and W
  • The anode should not be made of an analyte
  • High intensity line spectra should be located at slightly shorter wavelengths than the emission lines of the analytes
  • All tubes over 1kW of power need water cooling due to how much energy (heat) is used.
• Wavelength dispersion XRF – WD XRF


  1. Dispersion of the x-rays from the sample into their individual wavelengths using a crystal followed by quantification

  2. Each element emits x-rays at specific, unique wavelengths. To quantify the x-ray intensity, we need to disperse the spectrum

  3. A crystal with a regular structure is used to disperse the system

  4. Bragg's law states that when an x-ray beam is incident on a regular crystal substance, very strong diffraction of a given wavelength will occur when the wavelength strikes at an angle θ and the following relationship is followed: nλ = 2d x sin θ

  5. Many crystals are usually required. Bragg's equation shows that crystals suitable for diffraction must have a lattice spacing (2d) of the same order of magnitude as the wavelength of incident x-rays to be diffracted.

  6. By changing the angle of incidence, a different wavelength is diffracted

  7. Most WD CRF are sequential measuring devices with a single detector, the crystal is rotated relative to the sample and the detector is rotated twice the speed of the crystal to maintain the angular relationship.
• Detectors of diffracted x-rays


  • Proportional flow counter – used to detect low energy/long wavelength x-rays
  • Scintillation counter – used to detect high energy/short wavelength x-rays
• Quantification in WD XRF


  • Comparative, using standards to derive a calibration line
  • The signal detected is not truly proportional to concentration
• Disadvantages of WD XRF


  • Requires crystals and goniometer which are expensive
  • Much of the signal from the sample is lost during the diffraction process, so a powerful x-ray source is needed which is expensive
  • Normally WD XRF is a sequential technique which means is relatively slow
• Energy dispersive XRF - ED XRF


  Simultaneous measurement of all emitted x-rays and electronic deconvolution of the pattern into individual lines (elements) by the detector based on energy
• Benefits of ED XRF


  • Uses a single energy dispersive detector capable of simultaneous measurements which makes it cheaper
  • As there are no crystals or goniometer the detector can be placed close to the sample meaning a higher intensity enters the detector which allows a lower power x-ray tubes to be used which is cheaper (not as much heat produced)
  • Combining it with modern electronics means it's a smaller instrument which is transferable
• Source Modifiers


  • Devices that are used to modify the shape or intensity of the source spectrum or beam shape
  • For WD-XRF – source filters, collimators
  • For ED-XRF – collimators, secondary targets, polarising targets, focusing optics
• Source Filters


  Between the tube and the sample to reduce the background signals
• Collimators


  In the way of the beam after its hit the sample, which focus the beam and stop the beam from dispersing – improves signal quality
• Primary x-ray beam energy

  The higher the energy, the more it penetrated the sample
• Reasons for non-proportional detector response


  • Auger effect - The primary x-ray beam excited a volume of the sample, a secondary x-ray from the sample is absorbed by an atom of a different element (and releases an x-ray), instead of escaping to the detector
  • Rayleigh scatter - primary x-rays are reflected by atoms and no fluorescence occurs. The amount of rayleigh scatter is proportional to the atomic number of the sample (high atomic number elements cause more scatter)
  • Compton scatter - primary x-rays strike the atom without causing fluorescence but with transfer of energy. The intensity of compton scatter bears a strong relationship to the mean atomic number of the sample
• Modern XRF systems


  Numeric matric correction routine are used to correct the effect providing all elements are quantified
• Reducing the influence of non-proportional response


  • Use standards of very similar composition to the unknowns
  • Ensure that the sample is finely ground, and the surface presented to the x-ray beam is truly representative
  • Or dilute both samples and standards into a common matrix, normally by production of a fused glass disc
  • Modern software also compensates for many influences
• Semi-quantitative analysis


  • SLFP Standardless Fundamental Parameter - The algorithm computes both the intensity to concentration relationship and the absorption effects. Results typically within 10-20% of actual values
  • FP (with standards) NBS-GSC, NRLXRF, Uni-Quant ect.. - The concentration to intensity relationship is determined with standards, while the FP handles the absorption effects. Results typically within 5-10% of actual values.
• True quantitative analysis


  XRF is a reference method, standards are required for quantitative results. Calibration plot is needed to compare spectral intensities of unknown samples to those of known standards.
• There is no physical interaction between the sample and the instrument, so recalibration is only needed every 6 months to a year. Only a monitor is usually required to check calibration is still valid.
• Sample Preparation


  • Pressed pellet (for heavier elements)
  • Fused disk (for lighter elements)
  • Liquid
  • Metallic Solid
• Pressed pellet


  • Weigh the sample
  • Add binder
  • Into a mill sample to grind the sample
  • The milled powder is put into a press (ensure no cracking)
• Fused disk


  • Put weighed flux into mortar
  • Weigh the sample, add it to mortar and mix
  • Transfer to Pt crucible
  • Heat the crucible to melt the flux-sample mix
  • Tip when molten into a casting dish
• Liquid


  • Liquid holder
  • Cover bottom of the holder with transparent film
  • Check for holes (leave on tissue to see if any leakages)
  • Add a pre-defined sample weight
• Metallic Solid


  • Insert into mill
  • Check height
  • Mill the sample
  • Check the surface is smooth
• The preparation route dictates the quality of the results
• Applications


  • Industrial raw material analysis (steel, cement, scrap metal ect.)
  • Geochemical analysis
  • Contaminated land evaluation
  • Oil analysis (particularly sulphur)
  • Metals in plastic and consumer products