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What is x-ray diffraction






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  • A given phase may be listed in several places in the inorganic alphabetical index.
  • Note that XRF is elemental analysis. XRD is used to determine the phase content of materials

What is x-ray diffraction What is x-ray diffraction Presentation Transcript

  • EBS 325 – Analytical Chemistry Laboratory Introduction To X-Ray Analysis By Mr. Samayamutthirian Palaniandy School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia.
  • or
  • X-RAY analytical errors
    • Sampling
    • Sample preparation
    • Instrumental
    • Standards
    • Statistical
      • A means by which units are taken from a
      • population in such a way as to represent the
      • characteristics of interest in that population.
  • homogeneous. Well-mixed random accurate representative The equipment does what we want. Our sampling frequency is fine. FAQ about samples and sampling
  • Reasons for poor procedures, equipment, and practices of SAMPLING. Lack of knowledge of the consequences of poor sampling. Lack of knowledge of the sampling theory. Trying to save money.
  • Questions to be answer before sampling WHAT is being sampled? WHY is the sample being taken? WHO is taking the sample? WHERE is the sample taken? WHEN and with what frequency is the sample taken? HOW is the sample taken? HOW MUCH material is in the sample • ?
  • EXAMPLES OF SAMPLING METHODS Coning & quartering Riffle splitter Paper cone riffle splitter Grab sampling Fractional shoveling
  • Fractional Shoveling
  • Grab Sampling Consist of taking a sample using scoop or spatula by simply inserting the sampling device into the sample container and removing an aliquot
  • Sample Mixing Flowing Liquids or Gases A static mixer can reduce the Grouping and Segregation Error. A correct cross stream sample may be impossible to obtain.
  • Precision of Sub-sampling Methods Gerlach, Dobb, Raab, and Nocerino, 2002 Journal of Chemometrics “ Gy Sampling in experimental studies. 1. Assessing soil splitting protocols” 16 , 321-328
  • Your decisions are only as good as your samples. Your samples are only as good as your sampling systems. Your sampling systems are only as good as your audit and assessment. Summary
  • X-RAY analytical errors
    • Sampling
    • Sample preparation
    • Instrumental
    • Standards
    • Statistical
  • Analytical errors – sampling
    • Sample must be representative of the process
    • Sampling must be reproducible (i.e. should be able to take identical duplicate samples)
  • Sample preparation methods must Simple Low cost Rapid Reproducible
    • The quality of sample preparation is at least as important as the quality of the subsequent measurements.
    Quality of sample preparation
  • Quality of sample preparation
    • An ideal sample would be:
    • Representative of the material
    • Homogenous
    • Of infinite thickness
    • Without surface irregularities
    • With small enough particles for the wavelengths being measured
  • XRD Working Concept When a monochromatic x-ray beam with wavelength  is incident on the lattice planes in a crystal planes in a crystal at an angle  , diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by a complete number n of wavelengths. By varying the angle  , the Bragg’s Law conditions are satisfied by different d-spacing in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffraction peaks produces a pattern which is characterised of the sample. Where a mixture of different phases is present, the diffractogram is formed by addition of the individual patterns.
  • XRF Working Concept In X-ray fluorescence spectroscopy, the process begins by exposing the sample to a source of x-rays. As these high energy photons strike the sample, they tend to knock electrons out of their orbits around the nuclei of the atoms that make up the sample. When this occurs, an electron from an outer orbit, or “shell”, of the atom will fall into the shell of the missing electron. Since outer shell electrons are more energetic than inner shell electrons, the relocated electron has an excess of energy that is expended as an x-ray fluorescence photon.  This fluorescence is unique to the composition of the sample. The detector collects this spectrum and converts them to electrical impulses that are proportional to the energies of the various x-rays in the sample’s spectrum.
    • Solids
    • Pressed powders
    • Fused beads
    • Liquids
    Sample types
    • metal alloys, plastics & glass
    • relatively easy to prepare by cutting, machining, milling % fine polishing
    • Avoid smearing of soft metals (e.g. Pb)
    • Polishing may introduce contamination from the polishing material
    • do not have particle size problems
    • Surface needs to be flat
    • Surface needs to be homogeneous
    • Surface defects are more critical for light elements if good accuracy is required.
    • Typical samples types that are prepared as pressed powders include rocks, soil, slag, cements, alumina, fly ash, etc.
    • Particle size of powder needs to be controlled for light element analysis
    • If necessary, powders are ground to achieve a particle size of < 50 µm
    • Grinding can be introduce contamination (e.g. Fe from a chrome steel mill)
    • Binding agents (e.g. wax or cellulose) can be used to increase sample strength to avoid breakage in the spectrometer
    • Ground powders are pressed into a solid tablet under pressure using a hydraulic press & 40 mm die
    • Relatively slow method (≈5 minutes per sample) but relatively low cost
    • Pressed powders suffer from particle size problems for light elements
    • Preparation equipment needed includes :
    • Grinding mill and vessel (chrome steel, zirconia, tungsten carbide, etc.)
    • Hydraulic press and die (usually 40 mm)
    • Binding agents
    Pressed powders
    • Typical samples that are prepared as fused beads include rocks, cements, iron ores, etc. when higher accuracy is required.
    • Weighed sample is mixed with flux
    • Sample and flux are melted at ≈ 1000 o C
    • Melt is poured into a 40 mm mold
    • Bead surface needs to be homogenous (constant color without cracks)
    • Slow (10-15 minutes/sample)
    • High cost
    • Important benefit is that particle size problems disappear (fusion process results in a homogeneous glass)
    • An additional benefit is that the melting flux (usually Na or Li borate) dilutes the sample, reducing matrix variations, resulting in higher accuracy
    • Disadvantage –reduced sensitivity for trace elements
    • Preparation equipment includes :
    • Fusion device (manual or automatic)
    • Pt/Au crucible(s) & mould(s)
    • Fusion (melting) flux
    • A non wetting agent (e.g. KI or LiBr) is sometimes used to help produce a better quality bead and to assist with cleaning the Pt/Au crucible & mould between samples
    Fused beads
    • Typical samples include environmental (waters, mud) & oils
    • Easiest to prepare
    • Should have a constant volume that exceeds maximum penetration depth
    • Sample is poured into a liquid cell fitted with a thin plastic window
    • Range of window materials to suit different liquids
    • Fill to a constant height (e.g. 20 mm) to avoid errors from variable depth
    • Choose the correct thickness and material to suit the chemistry of the sample being measured
    • Na is lightest element that can be detected in liquids.
  • Influence of sample preparation
  • Factor of errors in Sample Preparation Grain size and surface roughness Uniformity of sample Contamination through the sample preparation
  • Grain size and surface roughness
  • Uniformity of sample Metallic Sample Casting condition of the sample in the molding. Sand molding Metal molding X-ray intensities differ according to the molding method which comes In the measurement of light elements. Quenching casting which makes the metallic composition fine produces good results Sample polishing 0.993 0.709 Mirror polishing 0.989 0.704 240# emery paper 0.983 0.699 100# emery paper 0.974 0.686 50# emery paper CrK  intensity NiK  intensity
  • Uniformity of sample Contamination during polishing Contamination effect when carbon steel and Ni-Cr alloy polish after polishing stainless steel. As the contamination form the polishing belt to the sample, the re contamination from The material of the polishing belt and from the remaining trace elements of polished Sample. 0.38 0.03 0.05 % Contamination 2.10 0.21 0.55 % Conc Fe Cr Ni
  • Powder Sample Grinding Condition Different grinding condition cause variation in particle size distribution which leads to variation in X-Ray intensity.
  • Powder Sample Brequetting Usual forming pressure – 20 tons with 40mm diameter. X-Ray intensities varies with variation of forming pressure (especially when pressure is low). Contamination Contamination from the grinding mill and media
  • CaO,CaCO 3 ,CaMg(CO 3 ) 2 Ca(OH) 2 etc. If you are given with four bottles of white powder. What will you do to identify them? Identification
  • What is X-ray diffraction?
    • non-destructive analytical technique for identification and quantitative determination of the various crystalline forms, known as ‘phases’.
    • Identification is achieved by comparing the X-ray diffraction pattern
  • Diffractograms and ICDD Card
  • What is X-ray diffraction?
    • XRD able to determine :
    • Which phases are present?
    • At what concentration levels?
    • What are the amorphous content of the sample?
  • How does XRD Works???
    • Every crystalline substance produce its own XRD pattern, which because it is dependent on the internal structure, is characteristic of that substance.
    • The XRD pattern is often spoken as the “ FINGERPRINT ” of a mineral or a crystalline substance, because it differs from pattern of every other mineral or crystalline substances.
    • A crystal lattice is a regular three-dimension distribution (cubic, tetragonal, etc.) of atoms in space. These are arrange so that they form a series of parallel planes separated from one another by a distance d, which varies according to the nature of the material. For any crystal planes exist in a number of different orientations- each with its own specific d-spacing
    Crystal lattice
  • Fourteen (14) Bravais Lattice
  • How does it work?
    • Diffraction
    • Bragg’s Law
    • n  =2dsin 
    • When a monochromatic x-ray beam with wavelength  is incident on the lattice planes in a crystal planes in a crystal at an angle  , diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by a complete number n of wavelengths.
  • How does it work?
    • In powder XRD method, a sample is ground to a powder ( ±10µm) in order to expose all possible orientations to the X-ray beam of the crystal values of  , d and  for diffraction are achieved as follows:
    •  is kept constant by using filtered X- radiation that is approximately monochromatic. (See Table 1).
    • d may have value consistent with the crystal structure (See Figure 5).
    •  is the variable parameters, in terms of which the diffraction peaks are measured.
  • Table 1: Monochromatic X-ray filters
  • Basic Component Of XRD Machine
    • Therefore any XRD machine will consist of three basic component.
    • Monochromatic X-ray source (  )
    • Sample-finely powdered or polished surface-may be rotated against the center – (goniometer).
    • Data collector- such as film, strip chart or magnetic medium/storage.
    By varying the angle  , the Bragg’s Law conditions are satisfied by different d-spacing in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffraction peaks produces a pattern which is characterised of the sample
  • Table 1: Typical experimental XRD data
  • Design and Use of the Indexes for Manual Searching of the PDF
    • Three search methods are used in the indexes – i.e.
      • The alphabetical index;
      • The Hanawalt index
      • The Fink index.
  • The Alphabetical Index
  • The Alphabetical Index Figure 3: Schematic search procedure when chemical information is known
  • Hanawalt Method
  • The Fink Method
  • XRF X-Ray Fluorescence is used to identify and measure the concentration of elements in a sample
  • XRF instrumental parameters
    • x-ray tube kv
    • x-ray tube mA
    • primary beam filters
    • collimator masks
    • collimator
    • crystal
    • detector
    • path
  • user benefits of wavelength dispersive XRF
    • versatile
    • accurate
    • reproducible
    • fast
    • non destructive
  • XRF is versatile
    • element range is Be to U
    • atomic numbers (Z) of 4 to 92
    • concentration range covers 0.1 ppm to 100 %
    • samples can be in the form of solids, liquids, powders or fragments
  • XRF is accurate
    • generally better than 1 % relative (i.e. 10% ± 0.1%)
    • accuracy is limited by calibration standards, sample preparation, sample matrix, sampling, instrumental errors & statistics
  • XRF is reproducible
    • generally within  0.1% relative
    • good reproducibility requires high quality mechanics, stable electronics and careful construction techniques
  • XRF is fast
    • counting times generally between 1 & 50 seconds for each element
    • semi-quant analysis of all matrix elements in 10 to 20 minutes
    • overnight un-attended operation
  • XRF is non-destructive
    • standards are permanent
    • measured samples can be stored and re-analysed at a later date
    • precious samples are not damaged
  • properties of x-rays
    • the following four slides list some of the more important properties of x-rays that contribute to the nature of XRF analysis
  • XRF analytical envelope
    • the following section describes the five major areas that define the analytical possibilities available with wavelength dispersive XRF spectrometers
  • XRF analytical envelope
    • elemental range
    • detection limits
    • analysis times
    • accuracy
    • reproducibility
  • elemental range
    • beryllium (4) to uranium (92) in solids
    • fluorine (9) to uranium (92) in liquids
  • range of elements in solid samples are shown in green (Be to U)
  • range of elements in liquid samples are shown in green (Na to U)
  • detection limits (LLD)
    • function of atomic number (Z) & the mix of elements within the sample (sample matrix)
    • < 1 ppm for high Z in a light matrix (e.g. Pb in petrol)
    • or > 10 ppm for low Z in a heavy matrix (Na in slag)
  • XRF applications summary
    • Na to U in all sample types
    • Be to U in solid samples
    • accuracy generally 0.1 to 1 % relative
    • reproducibility typically < 0.5% relative
    • typical LLD is normally 1 - 10 ppm (depends on element being measured and the sample matrix)
  • XRF errors
    • the following section describes major source of errors in XRF analysis, and investigates how these errors can be minimized to achieve maximize accuracy
  • overview of XRF methodology
    • good accuracy requires
    • careful sample preparation
    • fused beads for light elements
    • accurate standards
    • selection of optimum instrument parameters
    • collection of enough counts to avoid statistical errors
  • Methods of Analysis the following presentation describes the requirements for quantitative and semi-quantitative analysis
  • overview of XRF methodology
    • the objective of XRF is to determine as accurately as possible the composition of unknown samples
    • measured x-ray line intensities are converted to concentrations using an appropriate algorithm
  • overview of XRF methodology
    • each specific application needs to be looked at in detail to determine which method will be the most appropriate
  • XRF analytical methods
    • the atomic number (Z) of each of the elements to be determined will have
    • an influence on the type of sample preparation to be used, and the quantitative or semi-quantitative method that will be the most suitable
  • XRF analytical methods
    • the quantitative method is the most accurate, but requires calibration standards
    • semi-quantitative method is less accurate, but does not require standards
  • overview of XRF methodology
    • first determine the following:
    • which elements are to be measured
    • what are their concentration ranges
    • what accuracy is required
    • how many samples are to be measured
    • are suitable standards available
  • overview of XRF methodology
    • elements to be measured
    • low Z will require careful preparation
    • low Z may have lower accuracy
    • low Z may require fusion of powders
    • semi-quant does not measure the very light elements (Be to N)
  • overview of XRF methodology
    • concentration ranges
    • as the concentration range for each element increased, accuracy generally decreases
    • large concentration ranges will require more standards
  • overview of XRF methodology
    • good accuracy requires
    • careful sample preparation
    • fusion of powder samples for Z  13
    • longer analysis time
    • accurate calibration standards
    • careful selection of each variable instrument parameter
  • overview of XRF methodology
    • calibration standards
    • require the same sample preparation as unknown samples
    • accurate chemical analysis
    • need to cover concentration ranges
    • mechanically stable
  • XRF applications summary
    • Na to U in all sample types
    • Be to U in solid samples
    • accuracy typically 0.1 to 1 % relative
    • typical LLD is between 1 - 10 ppm
  • semi-quant (standardless analysis)
    • accuracy is limited by
    • particle size
    • inhomogeneity
    • non-measured elements (H to N)
  • semi-quant (standardless analysis)
    • accuracy of the semi-quantitative method can be as good as 1% relative; typically accuracy is between 5% and 10%
  • quantitative analysis
    • calibration graph (x-ray intensity v/s % element) is established for each element that is to be measured
    • measure unknowns using the established calibrations
  • quantitative analysis - calibration
    • for a single element (a), the concentration C is a function f of the intensity I
    • C a = f a x I a
  • quantitative analysis - calibration
    • for multiple elements (a & b) in a sample matrix, the concentration is related to both a & b:
    • C a = f(I a , I b ) or C a = f(I a , C b )
  • quantitative analysis - calibration
    • the object is to obtain the best fit of experimental data to a given algorithm
    • e.g. method of least squares fitting
    • Σ (C chem – C calculated ) 2 = minimum
    • where Σ = sum from all standards
    • and C = concentration
  • quantitative analysis - calibration
    • XRF software typically includes several quantitative methods. The most simplistic method is a straight line calibration where matrix (or inter-element ) effects are absent
  • Soalan Pramakmal
    • Nyatakan 5 punca kesalahan analitikal analisis X-Ray.
    • Takrifkan sampel.
    • Apakah punca prosedur pensampelan yang lemah?
    • Nyatakan 5 perkara yang mempengaruhi kualiti penyediaan sampel yang ideal.
    • Terangkan prinsip kerja XRD.
    • Terangkan prinsip kerja XRF.
    • Berikan 5 contoh kaedah pensampelan.
    • Terangkan cara penyediaan “fuse beads”.
    • Nyatakan faktor kesilapan dalam penyediaan sampel yang mempegaruhi analisis X-Ray.
    • Apakah maklumat yang boleh diperolehi daripada keputusan XRD.
    • Tuliskan persamaan Bragg.
    • Nyatakan komponen asas dalam mesin XRD.
  • Soalan Pramakmal
    • Nyatakan 3 kaedah pencarian index unsur dengan manual PDF.
    • Apakah perbezaan kaedah Hanawalt dan Fink?
    • Lakarkan carta alir kaedah Fink.
    • Lakarkan carta alir kaedah Hanawalt.
    • Nyatakan julat no. atom yang boleh dikesan dengan kaedah XRF pada sampel pepejal dan cecair.
    • Apakah kaedah penyediaan sampel yang baik untuk unsur yang mempunyai no. atom yang rendah.
    • Kejituan keputusan XRF dipengaruhi oleh 3 faktor. Nyatakan fator-faktor itu.
    • Apakah itu LOI?