Analytical Instrumentation: X-Ray Spectroscopy

Nikhilbinoy.CAssistant ProfessorICE DepartmentAnalytical Instrumentation:X-Ray Spectroscopy

Powerful tools for analytical purposes.Several distinct ways in which X-radiation can be employed.Absorption of X-rays give information about the absorbing material just as in other regions of the spectrum.Fluorescence emission of X-rays enables to identify and measure heavy elements in the presence of each other and in any matrix.Diffraction of X-rays enables analysis of crystalline materials with a high degree of specificity and accuracy.

BasicsWhen an atom is excited by the removal of an electron from an inner energy level, it may return to its normal state by transferring an electron from some outer level to the vacant inner level.This energy either appears as X-rays or is used to eject a second electron from the outer shell.Characteristic wavelength of X-ray, or the energy level electron is used for the analysis.

Different Types X-Ray SpectroscopyX-Ray Emission Spectroscopy (XES)Auger Emission Spectroscopy (AES)X-Ray Fluorescence Spectroscopy (XFS)Electron Spectroscopy for Chemical Analysis (ESCA)X-Ray Absorption Spectroscopy (XAS)X-Ray Diffraction Spectroscopy (XDS)

X-Ray Emission SpectroscopyThe primary electron beam ejects electron from inner energy levels.Outer level electrons fall into vacant inner levels by emitting secondary X-rays.+

Auger Emission SpectroscopyThe primary X-rays eject electrons from inner energy levels.Just outer level electrons fall into vacant inner levels by non radiative processes.Excess energy ejects electrons from outer levels.+

X-Ray Fluorescence SpectroscopyThe primary X-ray ejects electron from inner energy levels.Outer level electrons fall into vacant inner levels by emitting secondary X-rays.+

Electron Spectroscopy for Chemical AnalysisThe primary X-ray ejects electron from inner energy levels.Energy of the emitted electrons are measured.+

X-Ray Absorption SpectroscopyThe intensity of X-ray is diminished as they pass through material.Discontinuities in absorptivity occur when X-ray have sufficient energy to eject electrons.

X-Ray Diffraction SpectroscopyX-rays are diffracted by the planes of a crystal.

InstrumentationCommon for all types of X-Ray Spectroscopy.

VoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

X-Ray Generating EquipmentVoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

X-Ray Generating EquipmentThe target becomes very hot due to the collision high-energy electrons.Cooled by water.Sometimes rotated when a very intense beam of x-ray beam is generated.The X-ray beam passes out of the tube through a thin window of Beryllium or a special glass.VoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

X-Ray Generating EquipmentElectrons emitted by cathode are accelerated through a high-voltage between the target and cathode.When the beam of electron hits the target (or anode), a variety of events occur.Rapid deceleration of electrons causes the emission of X-ray radiation, photoelectrons, Auger electrons, and a large amount of heat.Two types of X-rays are emitter.Continuous band of white radiation.Series of discrete lines that are the characteristics of the target material.VoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

X-Ray Generating EquipmentVoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

White Radiation

50KV40KV30KV20KV

Discontinuities in White RadiationIf sufficiently energetic electrons are available, the transfer of energy from the impinging electron beam may eject an electron from one of the inner levels of the target atoms.By absorbing electromagnetic radiation.Absorption lines.Within each atom, the place of the ejected electron is promptly filled by an electron from an outer level whose place in turn is taken by an electron coming from still further out.The ionized atom returns to its normal state in a series of steps, in each of which an X-ray photon of definite energy is emitted or the excess energy is released by ejection of second electron with characteristic energy.Emission lines.These lines are superimposed on the white radiation.

Discontinuities in White Radiation:Absorption Edge

KMK AbsorptionL Absorptions

KAbsorptionLAbsorptions

Discontinuities in White Radiation:Emission LinesKMK AbsorptionL Absorptions

CollimatorCollimators are inserted in the incident or diffracted beam path to shape the X-ray beam.Metal tubes are typically used in single crystal experiments.Incident beam collimators are usually manufactured with two narrow regions.The region closest to the X-ray source carries out the collimation function.The inside radius of the collimator is typically chosen to be somewhat larger than the size of the sample.The second narrow region has a slightly larger diameter than the first.Used to remove the parasitic radiation that takes a bent path due to interaction with edge of the first narrow region of the collimator.The diffracted collimator is used to remove the stray light.VoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

Filters

Crystal MonochromatorsVoltageStabilizerHigh VoltageGenerationX Ray Tube

DetectorCurrentStabilizerElectricPowerSampleCollimatorMonochromator

Lattice Spacing=dCrystal Plane

X-Ray Diffraction Spectroscopy

BasicsCrystalline materials are characterized by the orderly periodic arrangements of atoms.The unit cell is the basic repeating unit that defines a crystal.Parallel planes of atoms intersecting the unit cell are used to define directions and distances in the crystal.These crystallographic planes are identified by Miller indices.

The (200) planes of atoms in NaCl

The (220) planes of atoms in NaCl

Basics

Diffraction and Braggs Law

Lattice Spacing=dCrystal Plane

X-Ray Powder Diffractometers

qw

2qX-ray tubeDetector

X-Ray Powder Diffractometer:A Single Crystal Specimen

2qAt 20.6 2q, Braggs law fulfilled for the (100) planes, producing a diffraction peak.The (110) planes would diffract at 29.3 2q; however, they are not properly aligned to produce a diffraction peak (the perpendicular to those planes does not bisect the incident and diffracted beams). Only background is observed.The (200) planes are parallel to the (100) planes. Therefore, they also diffract for this crystal. Since d200 is d100, they appear at 42 2q.

Even if diffraction occurs, no peak would be observed if the plane normal is not parallel to the diffraction vector.A single crystal specimen in a Bragg-Brentano diffractometer would produce only one family of peaks in the diffraction pattern.

X-Ray Powder Diffractometer:Poly Crystalline SampleFor every set of planes, there will be a small percentage of crystallites that are properly oriented to diffract (the plane perpendicular bisects the incident and diffracted beams).Basic assumptions of powder diffraction are that for every set of planes there is an equal number of crystallites that will diffract and that there is a statistically relevant number of crystallites, not just one or two.

2q

2q

2q

X-Ray Fluorescence SpectrometeryAn extremely powerful tool for qualitative and quantitative determination of heavy elements in the presence of each other and in any matrix.Characteristic X-ray spectra are excited when the sample is irradiated with a beam of primary X-rays of greater energy than the characteristic X-radiation.XRF is based on the principle that the energy of the emitted X-rays depends on the atomic number of the atom (Z) and their intensity depends on the concentration of the atom in the sample.

Working PrincipleWhen a photon or charged particle of sufficient energy interacts with an atom, the atom may be excited releasing an electron out of an inner, K or L shell.An outer shell electron can fall into the vacated inner shell.Releases energy as an X-ray.By measuring the photon energy of this fluorescent X-rays, the atom can be identified.+

Working Principle+

X-Ray Fluorescent Spectrometer:Excitation SourcesNecessary to use monoenergetic source inorder to reduce unwanted background due to scatter occurring over a broad range of wavelengths obscuring the fluorescence of interest.Commonly used devices are X-ray tubes.Special devices are used to make the X-ray beam nearly monochromatic.Transmission anode X-ray Tube.Operates on the principle that any metal is good transmission filter for its own X-rays.Produces characteristic radiation of the desired energy by properly selecting the anode material.Characteristic X-ray radiation is maximized by using the accelerating potential.Lower lines are filtered out by using the thickness of anode.Secondary fluorescent target.X-ray from primary tube is used to excite the secondary fluorescence from the target, whose material is such that it produces the most appropriate exciting radiation.Filter.More nearly monochromatic excitation beam are isolated by using a thin metal foil.

X-Ray Fluorescent Spectrometer:SpectrometersTwo main types are there:Energy dispersive system.Fast time resolved experiments.Small beams.Transmission geometry (more concentrated samples).Time consuming to setup.Wavelength dispersive system.Dominates (almost 98% experiments).Flexible, various detection techniques, etc.Usually fairly straight forward to set-up.

X-Ray Fluorescent Spectrometer:Spectrometers: - Energy Dispersive System (EDS)The fluorescent radiation resulting from irradiation of the sample reaches a detector, which produces an electrical pulse proportional to the energy of the fluorescent X-rays.The energy level indicates the element involved.The number of pulses counted at each energy level over the entire counting time is related to the concentration of the element.

ExcitationSourceSpecimenCharacteristic andScattered radiationCollimatorDetector

X-Ray Fluorescent Spectrometer:Spectrometers: - Energy Dispersive System (EDS)The detector is usually a semi conductor detector.Si or Li DetectorThe Li-drifted centre part forms the non conducting i-layer.When an x-ray beam passes through the i-layer, a swarm of electron-hole pairs are formed.Causes a voltage pulse.To obtain sufficiently low conductivity, the detector must be maintained at low temperature, and liquid-nitrogen must be used for the resolution.With some loss of resolution, the much more convenient Peltier cooling can be used.

ExcitationSourceSpecimenCharacteristic andScattered radiationCollimatorDetector

X-Ray Fluorescent Spectrometer:Spectrometers: - Wavelength Dispersive System (WDS)The technique derives its name from the fact that the analysis of the X-ray beam by diffraction is similar to spectrum analysis carried out with a diffraction grating.A crystal is used as a diffracting element.The crystal possess regular three dimensional lattice array of atoms and acts as an X-ray grating from which photons can be coherently scattered, so that they are in phase at certain angles. They reinforce each other producing a diffraction pattern.Two commonly used arrangements are:Plane Crystal WDS.Curved Crystal WDS.

Wavelength Dispersive System (WDS): -Plane Crystal WDSThe specimen is usually held in an aluminium cylinder.Plastic cylinder is also used for the analysis of acid or alkaline.The primary and secondary slits and the analyser crystal are placed on the focal circle, so that Braggs law is always satisfied as the Goniometer is rotated.The detector is rotated by an angle twice the angular change in crystal setting.Goniometer is used to track and control the movement of crystal and detector.Due to absorption of long length X-rays by air and window materials, some intensity losses of X-rays take place.Reduced by evacuating the Goniometer.

Sample SpinnerSample HolderPower Source

Crystal

Detector

Goniometer

Wavelength Dispersive System (WDS): -Curved Crystal WDSCurved crystal is more suitable for the analysis of small specimens.In this technique, collimators are not required.Intensity is increased by focusing the fluorescence lines.The crystal is bend to the focusing circle.The angular velocity of the detector is twice that of the crystal, and the two of them move along the periphery of the circle.

Advantages of X-Ray SpectroscopyIt is element specific.It provides the information about the local environment of a particular element even in complex systems.Dilute or concentrated systems can be studied.It can be used on solids, liquids, solutions and even gases.There is no need for the sample to be crystalline.Wide temperature and pressure range.

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