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X-ray techniques
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- 1X-ray techniquesElemental techniques:
X-ray fluorescence (XRF)
-Wavelength dispersive XRF (WD XRF)
-Energy dispersive XRF (ED XRF)
X-ray microprobe analysis
Crystallographic technique:
-X-ray diffraction - 2X-ray fluorescenceXRF is a widely used technique for the elemental analysis of solid samples for metals and some non-metals. It can also be used for liquids.
XRF offers the potential for high precision measurements of elements present at medium-high levels in samples (better than ICP-OES) and has similar quantitation limits to ICP-OES for solid samples (once dilution, as required for ICP analysis, has been taken into account).
XRF is unable to quantify the very light elements (H-Be), normally quantify Na and above. - 3Routine applicationsIndustrial raw materials analysis
- Steel, cement, high purity sands, scrap metal
-Geochemical analysis
-Contaminated land evaluation
-Oil analysis
-Metals in plastics and consumer products - 4Additional benefitXRF techniques can be used with solids or liquids and are inherently non-destructive.
Analysis can be undertaken of in situ materials without further prep.
-Archaeological analysis
-Art works etc
-Forensic - 5Electromagnetic Radiation
- 6X-raysX-rays: high frequency electromagnetic radiation with energy intermediate between the far-ultraviolet and gamma ray regions of the spectrum.
The wavelength of X-rays is measured in angstroms (Å), 1Å = 10-10m and their energy in terms of electron volts (eV)
There is a linear relationship between wavelength and energy, with wavelength inversely proportional to energy (the shorter the wavelength the higher the energy) - 7XRF fundamentalsSamples are presented to a beam of high energy X-rays
The X-rays penetrate the sample and cause excitation of electrons, especially inner orbital electrons, possibly completely displacing electrons from the atom.
Because the atom is unstable an electron falls from an outer orbital to fill the gap.
The fall prompts the release of energy in the form of an X-ray.
Because the electron structure of an atom is element specific, the energy / wavelength of the X-ray released is characteristic of the element.
Measurement of emitted X-rays enables quantification of elemental composition - 8TheoryA source X-ray strikes an inner shell electron. If at high enough energy (above absorption edge of element), it is ejected it from the atom.
Higher energy electrons cascade to fill vacancy, giving off characteristic fluorescent X-rays. - 9Basic instrument layout
- 10SourcesX-Ray Tube
-Side window
-End window
Radioisotopes
Other Sources
-Scanning Electron Microscopes
-Synchrotrons
-Positron and other particle beams - 11Side Window X-Ray Tube
- 12X-ray tubeElectrons are produced at a heated cathode.
The electrons are accelerated towards a metal anode (the target) by a high potential (10-100kV).
Upon collision, part of the beam is converted to X-rays, which are in turn directed towards the sample.
The composition of the anode is often dictated by the application - 13Emission from X-ray tubesThe emission of X-rays from the target takes two forms:
The tube continuum: X-rays with a range of energies from 0 to the operating potential of the tube. These X-rays are generated from when primary electrons from the filament are decelerated by collisions with outer electrons of the target material
A higher accelerating voltage gives more intensity and x-rays of a shorter wavelength (more energy)
- 14Emission from X-ray tubesLine spectra: X-rays derived from high energy collisions of primary electrons with inner orbital electrons of the target element. The are produced when the incoming electron strikes inner orbital electron with such force that it is displaced from the atom (producing an ion). This results in instability and an electron drops down to fill the gap, releasing energy in the form of an X-ray (at λs characteristic of the anode element)
- 15Emission from X-ray tubesLine spectra are only produced when the potential applied to the tube exceeds a threshold voltage. The line emissions have a much greater intensity than the emission produced by the tube continuum.
The wavelengths that the lines appear at are dictated by the element used for the anode
-Rh: Kα = 0.61Å, Kβ = 0.54Å, Lα = 4.6Å, Lβ = 4.2Å
-K series of lines derived from the removal and subsequent replacement of K series electrons, L series of lines from the removal and subsequent replacement of L series electrons. - 16Choice of anodeCommonly used tube anodes are composed of one of the following metals: Cr, Mo, Rh & W
Choice of anode is dictated by analytical requirements:
-The anode should not be made of an analyte
-The high intensity line spectra should be located at slightly shorter wavelengths than the emission lines of the analytes – this ensures maximum excitation of the analyte atoms.
-A common compromise tube has a Rh anode.
X-ray generation is very inefficient and results in heating – all tubes with over 1 kW of power need water cooling - 17Choice of tube conditions
- 18RadioisotopesIsotopes have fallen out of favor, substituted by small X-ray tubes but small quantities are still used for some hand-held devices.
- 19Other SourcesSeveral other radiation sources are capable of exciting material to produce x-ray fluorescence suitable for material analysis.
Scanning Electron Microscopes (SEM) – Electron beams excite the sample and produce x-rays. Many SEM’s are equipped with an XRF detector for performing elemental analysis
Synchrotrons - These bright light sources are suitable for research and very sophisticated XRF analysis.
Positrons and other Particle Beams – All high energy particles beams ionize materials such that they give off x-rays. PIXE is the most common particle beam technique after SEM. - 20Excitation of the sampleBombardment of the sample by an X-ray beam results in the excitation of sample atoms and the release of secondary X-rays (fluorescence).
Providing the primary beam has enough energy it will cause line excitation of all elements within the sample, prompting the release of secondary X-rays at a range of wavelengths and intensities, reflecting the composition of the sample.
The most intense lines will be the K-series lines of each element in the sample as these are the most common transitions. - 21Measurement of the emitted x-raysEither
Dispersion of the x-rays from the sample into their individual wavelengths followed by quantification.
-Wavelength dispersive XRF (WD XRF)
Simultaneous measurement of all emitted x-rays and electronic deconvolution of the pattern into individual lines (elements) based on energy.
- Energy dispersive XRF (ED XRF) - 22Measurement of sample excitation – WD-XRFEach element within the sample will emit X-rays at specific, unique wavelengths / energies. To quantify the intensity of X-rays at each wavelength we need to disperse the spectrum
In WD-XRF a crystal with a regular structure is used to disperse the spectrum.
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 only occur when the wavelength strikes the surface at an angle q and the following relationship is satisfied:
- 23Bragg’s Lawn is an integer (1, 2, 3..); λ = the wavelength of interest; d = the lattice spacing between individual layers of atoms in the crystal; θ = the angle of incidence
- 24Bragg’s LawBragg’s law shows that at a given angle of incidence only one wavelength will be diffracted, all other wavelengths will suffer destructive interference.
By changing the angle of incidence a different wavelength can be diffracted.
Most WD-XRF are sequential measuring devices with a single detector, the crystal is rotated relative to the sample and the detector is rotated at twice the speed of the crystal to maintain the angular relationship. Accurate physical rotation is a key requirement of WD-XRF design - 25Wavelength Dispersive XRFWavelength Dispersive XRF relies on a diffractive device such as crystal or multilayer to isolate a peak, since the diffracted wavelength is much more intense than other wavelengths that scatter of the device
- 26Bragg’s LawExamination of Bragg’s equation also shows that crystals suitable for diffraction must have a lattice spacing (2d) of the same order of magnitude as the wavelength of the incident X-rays to be diffracted. For this reason a number of crystals are usually required:
- 27Source ModifiersSeveral devices are used to modify the shape or intensity of the source spectrum or the beam shape
WD-XRF:
Source Filters
Collimators
ED-XRF:
Collimators
Secondary Targets
Polarizing Targets
Focusing Optics - 28Source FiltersIn WD-XRD filters are usually used to reduce background
- 29Soller CollimatorsSoller and similar types of collimators are used to prevent beam divergence. The are used in WDXRF to restrict the angles that are allowed to strike the diffraction device, thus improving the effective resolution
- 30Detection of diffracted X-raysDetection is usually by one of two detectors
- Proportional flow counter: used to detect low energy / long wavelength X-rays – incoming X-rays ionize argon gas in the detector causing a current to flow between a plate and a wire held at high potential
- Scintillation counter: used to detect high energy / short wavelength X-rays. These strike a phosphor on the face of the detector prompting light emission which is measured and turned into an electrical signal by a photomultiplier.
Quantification is comparative, using standards to derive a calibration line.
The signal detected is not truly proportional to concentration. - 31Why ED-XRFWD-XRF requires diffracting crystals and goniometer ---> significant cost.
Much signal from the sample is lost during the diffraction process and hence to maintain sensitivity and maximise speed of analysis a powerful X-ray source is required (up to 4kW) ---> more cost and infrastructure (esp water cooling).
WD-XRF is normally a sequential technique – measurement times per sample can be 10-15 minutes ---> relatively slow
Simultaneous WD-XRF are available with multiple fixed crystals and detectors (and normally a variable goniometer as well) --> even more cost! - 32Why ED-XRFED-XRF uses a single energy dispersive detector capable of simultaneous measurement which does not require diffracting crystals & goniometer ----> significant cost saving.
Because there are no crystals or goniometer the detector can be placed very close to the sample giving a 100-fold increase in intensity entering the detector – enables the use of lower power X-ray tubes ---> more cost saving.
Combining the above with modern electronics also means that the instrument has a much smaller physical size ---> commonly benchtop, transportable in the boot of a car, + some low powered units can be hand held for in-situ measurements - 33ED-XRF developmentEarly XRF instrumentation was all WD-XRF – a mechanical challenge but electronically relatively simple.
ED-XRF came about because of:
-development of semi-conductor technology in the late 1960’s – mid 1970’s.
- Parallel developments in computing power to de-convolute the signal from the detector. - 34ED vs WD XRF
- 35Reasons for non-proportional detector responseThe auger effect
Rayleigh scatter
Compton scatter
The auger effect:
-The primary X-ray beam excites a volume of the sample. A secondary X-ray from a sample atom in the volume can, instead of escaping to the detector, be absorbed by an atom of a different element, prompting it in turn to release an X-ray.
-It is also possible for low energy X-rays from light elements to simply be absorbed with no subsequent excitation of another atom - 36Absorption-Enhancement EffectIncoming source X-ray fluoresces Fe.
Fe fluorescence is sufficient in energy to fluoresce Ca.
Ca is detected, Fe is not. Response is proportional to concentrations of each element. - 37Reasons for non-proportional response (2)Rayleigh scatter: primary X-rays are reflected by atoms and no fluorescence occurs. The amount of Rayleigh scatter due to a given atom is proportional to its atomic number – 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 - 38Rayleigh ScatterX-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 - 39Compton ScatterX-rays from the X-ray tube or target strike atom without promoting fluorescence.
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 - 40Reasons for non-proportional responseThe response of a sample to all three of these effects will be controlled by the elemental composition of the sample – changes in elemental composition will cause changes in these secondary phenomena and hence change the analyte signal at the detector.
In modern XRF systems numeric matrix correction routines are used to correct for these effects providing all elements are quantified as part of the analytical protocol. - 41Semi-quantitative analysisSLFP Standardless Fundamental Parameters
-The algorithm computes both the intensity to concentration relationship and the absorption affects
- Results are typically within 10 - 20 % of actual values.
FP (with Standards) NBS-GSC, NRLXRF, Uni-Quant, TurboQuant, etc…
-The concentration to intensity relationship is determined with standards, while the FP handles the absorption affects.
-Results are usually within 5 - 10 % of actual values - 42Reducing the influence of non-proportional responseCareful compositional matching of samples and standards is key:
-Use standards of very similar composition to the unknowns
-Ensure that the sample is very 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 the production of a fused glass disk
-Modern software also compensates for many influences that previously required operator involvement - 43Quantitative AnalysisXRF 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. - 44StandardsStandards (such as certified reference materials) are required for Quantitative Analysis.
Standard concentrations should be known to a better degree of precision and accuracy than is required for the analysis.
Standards should be of the same matrix as samples to be analyzed.
Standards should vary independently in concentration when empirical absorption corrections are used. - 45Quantitative analysisBecause there is no physical interaction between the sample and the instrument recalibration need only take place every 6 months to 1 year, a monitor is usually all that is required to check the calibration is still valid.
- 46Sample prep – pressed pellet
- 47Sample prep – fused disk
- 48Sample prep - liquid
- 49Sample prep – metallic solid
- 50XRF summaryXRF offers a relative simple route to high precision measurements.
Measurement times are typically of the order of 5-20 minutes but this is offset by the relatively simple prep requirements - 51X-Ray Diffraction A crystallographic technique OverviewX–ray Powder Diffraction (XRPD) is a standard technique used to determine structural information from crystalline substances
Non–destructive phase identification of bulk powders including multi–phase assemblages
Identify polymorphs
Assess the degree of crystalline/amorphous structure within active compounds
Qualitative and quantitative analysis - 52Crystalline materialsCrystalline materials consist of a regular three dimensional arrangement of atoms, ions or molecules.
The regularity implies there is a basic repeating unit - the unit cell defined by six parameters: the length of the sides (in Å) and three angles (expressed in degrees) - 53Crystal systemsMathematical analysis shows that there are seven possible shapes for a unit cell
- 54Crystal planesCrystalline materials consist of repeated unit cells, with a repeat distance (d), called the interplanar or lattice spacing. We can identify more than one plane in a given crystalline structure but the repeat distances will be different
- 55Crystal planesAn infinite number of planes can be drawn through a structure but the only ones of crystallographic significance are those containing all or most of the atoms. These planes have the ability to diffract an X-ray beam.
We have used this property as the basis of an analytical technique to disperse X-rays derived from a sample ---> WD-XRF - 56Bragg’s Lawn is an integer (1, 2, 3..); λ = the wavelength of interest; d = the lattice spacing between individual layers of atoms in the crystal; θ = the angle of incidence
- 57Bragg’s Law
- 58Bragg’s LawBy taking a finely powdered sample we can, in theory, present to the beam of X-rays all possible crystal planes because the sample will be randomly orientated with respect to its crystal planes..
By changing the angle that the beam of X-rays strikes the surface of the powdered sample we can cause diffraction from all the important crystal planes. - 59XRD - theoryCrystal lattices have many diffracting planes
By rotating the sample all the d-spacings can be measured - 60Monochromatic X-rays?The key requirement of Bragg’s Law that must be adhered to for XRD to be a viable technique is the use of a monochromatic beam of X-rays.
This is achieved using a side-window X-ray tube, similar to that used in XRF, but of lower power.
Tubes usually have a copper or cobalt anode and a filter to block all but one wavelength. - 61X-Ray DiffractionX-ray diffraction is one of the few techniques available which can be used to identify and characterize crystalline materials.
This is because each crystalline material will have a unique set of crystal planes, defined by its atomic composition.
XRD can also distinguish polymorphs – materials capable of forming more than one crystal structure.
XRD can be used to observe the changes in structure after subjecting a material to a chemical or physical process
XRD can quantify % of amorphous material - 62XRD – in practice
- 63SamplesIdeally – powders ground to ~5µm
But be aware of altering crystal structure
- Use cryoginding if this likely to be a problem
Randomly orientated powders
Preferred orientation gives incorrect peak heights
Sample size 0.5 – 1g
For small samples <500mg low background holders can be used - 64ApplicationsQualitative
Identification of polymorphs
Multiphase identification
Detection of amorphous phases
Quantitative
Quantification of crystalline component with amorphous phase present
Quantification of mixtures of phases - 65Identification of polymorphsNo sample prep – No grinding - straight from bottle – hence poor relative peak intensities. BUT accurate peak positions enough to identify compound.
- 66Multiphase identificationPrevious examples were singles phases
XRPD can identify mixtures of components:
Natural materials (e.g. cement & construction industry or geoscience applications– up to 15 phases) - 67Amorphous materialsA demonstration using indomethacin
-Starting with crystalline indomethacin
-Heat to beyond melting point
-Quench rapidly in liquid nitrogen
-Grind the resulting glass
-Resulting product is amorphous indomethacin
-Both samples analysed by XRPD - 68Quantification - RietveldRietveld refinement
-Models an ideal diffraction pattern given a crystal structure
-For multiphase mixtures – input all structures
-Model is compared to raw data
-Parameters are adjusted until the model converges on the raw data by least-squares residuals
- The scale parameter gives weight % of components
-User must assess the quality of the result
-Goodness of fit measures
-Difference plot
-Visual assessment of the model
-Where doesn’t it fit and why?
-Selection of additional parameters to refine and improve the model - 69Quantification - Rietveld
- 70SummaryXRF – widely used high precision technique for elemental analysis of most elements across the periodic table. Limitations associated with non linear response and calibration.
XRD – a mineralogical technique (not elemental), exceedingly powerful for the measurement of various facets of mineral phases
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- Karteikarten am X-ray techniques, erstellt von megan trent am 22/03/2021.
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