INSTRUMENTAL ANALYSIS CHEM 4811

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INSTRUMENTAL ANALYSIS CHEM 4811. CHAPTER 8. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 8 X-RAY SPECTROSCOPY. X - RADIATION. - Consists of electromagnetic radiation with wavelength between 0.005 and 10 nm - PowerPoint PPT Presentation

Transcript of INSTRUMENTAL ANALYSIS CHEM 4811

  • INSTRUMENTAL ANALYSIS CHEM 4811

    CHAPTER 8DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciencesClayton state university

  • CHAPTER 8

    X-RAY SPECTROSCOPY

  • X - RADIATION- Consists of electromagnetic radiation with wavelength between 0.005 and 10 nm

    - Have shorter and higher energy than UV radiation

    - Generated by electronic transitions of core electrons

    - Can also be generated when a high-speed electron strikes a solid

  • ATOMIC ENERGY LEVELS- Atoms are made up of nucleus (p + n) and electrons

    - Electrons are arranged in shells

    - The different shells correspond to the different principal quantum numbers (n) with integral values 1, 2, 3, ..

    Shells are named as - K for n = 1, L for n = 2, M for n = 3, etc.

    - Energy of K, L, M, .. are denoted K, L, M, ..

  • ELECTRONIC TRANSITIONS- An X-ray or a fast moving electron (with sufficient energy) can knock off a core electron from an atom when they collide

    - The atom becomes ionized

    - An electron from a high-energy shell falls into the vacant position

    - An X-ray photon emits as the electron falls into the lower energy level

    - The of emitted X-ray is characteristic of the element being bombarded

  • Auger Electron

    - An alternative to X-ray radiation emission

    - The emitted energy knocks off an electron from a higher energy shell instead of emitting an X-ray

    - This is known as the Auger Electron

    - Applied in surface analysis (Auger Electron Spectroscopy) and X-ray Photoelectron SpectroscopyELECTRONIC TRANSITIONS

  • - Some transitions are allowed and others are forbidden

    - Transitions are governed by quantum mechanical selection rules

    E = h = hc/

    - For an X-ray photon released when an L electron drops from a specific sublevel to a K shell

    hc/ = L K

    = (L K)/LELECTRONIC TRANSITIONS

  • X-ray emission lines that terminate - in the K shell are called K lines- in the L shell are called L linesetc.

    - The K shell has only one energy level

    - The L shell has three L energy sublevels

    - The M shell has five M sublevels

    - Transitions and symbols are summarized in Table 8.2X-RAY LINES

  • Siegbahn Notation

    - Used to identify X-ray emission lines

    - An electron that drops from the L shell sublevel to the K shell emits transition that results in K line

    - Two possible K lines for atoms with Z > 9

    - These are K1 and K2

    - K lines are usually not resolved so only one peak is seenX-RAY LINES

  • Siegbahn Notation

    - An electron that drops from the M shell sublevel to the K shell generates K lines

    - K1 to K3 lines are usually not resolved so a single K peak is seen(high-resolution spectrometer is required to see all 3 lines)

    - Electrons may also drop from the M shell sublevels to the L shell to generate L linesX-RAY LINES

  • - Characteristic X-ray lines are seen as sharp peaks on a continuous background

    - Tables of characteristic X-ray emission lines of elements are available

    - X-ray lines are independent of bonding, oxidation state, and physical state

    - Due to the fact that core electrons do not take part in bonding

    - X-ray lines provide no molecular informationX-RAY LINES

  • - The broad continuous background is due to collisions of the electrons with atoms of the solid metal

    - Each electron undergoes a series of collisions

    - Each collision results in a photon of slightly different energy

    - The result is a continuum of X-radiation called bremsstrahlung or white radiationX-RAY LINES

  • - Supposing electrons transfer all their energy in one collision

    - The of emitted photons is the shortest attainable

    - Is the minimum or the highest energy (min)

    - Energy of electrons (eV) = energy of radiation (h = hc/)

    e = charge of electrons = 1.60 x 10-19 CV = applied voltage (Volts)h = 6.626 x 10-34 Jsc = 3.00 x 108 m/sX-RAY LINES

  • E = eV = h = hc/

    - Implies when all energy of electrons is converted to X-rayX-RAY LINES- Substituting all values gives the Duane-Hunt Law- min depends on accelerating voltage but not metal(would be the same for different metals at the same V)

  • - From Duane-Hunt LawX-RAY LINES

  • Moseleys Law

    - Developed by Henry Moseley

    - Relationship between characteristic X-ray lines and atomic number

    - Used to assign atomic number to newly discovered elementsX-RAY LINESZ = atomic number of elements = screening constant (a function of repulsion of other electrons)

  • X-Ray Absorption

    X-Ray Fluorescence (XRF)

    X-Ray Emission

    X-Ray Diffraction (XRD)X-RAY METHODS

  • - Absorption varies with atomic weight

    - When a beam of X-ray passes through a thin sample of pure metal

    - Some of the incident beam is absorbed and the remainder is transmittedX-RAY ABSORPTION

  • From Beers LawX-RAY ABSORPTIONI() = transmitted intensity at wavelength

    Io() = incident intensity at wavelength

    m = mass absorption coefficient (cm2/g)

    = density of sample (g/cm3)

    x = sample thickness (cm)

  • X-RAY ABSORPTION- For a sample containing different elements

    total = w11 + w22 + w33 +

    wi = weight fraction of element in samplei = mass absorption coefficient of element

    - Longer s are more readily absorbed than shorter s(Longer s are absorbed more) - Amount of absorbed light increases with increasing

  • X-RAY ABSORPTIONEXAFS

    - Extended X-ray absorption fine structure spectroscopy

    - A new absorption technique

  • ABSORPTION SPECTRUM- Characterized by absorption edges

    - Absorption edge is an abrupt change at wavelength of electron ejection

    - Only one K absorption edge is seen

    - Three L absorption edges are seen

    - Five M absorption edges are seen

    - Absorption edge wavelengths of elements are available

  • ABSORPTION SPECTRUM- Absorption spectrum is unique for each element

    - The mass attenuation coefficient is used in place of the mass absorption coefficient

    - The mass attenuation coefficient takes into account both absorption and scattering of X-ray by sample

    - An alternative way is to plot m versus X-ray energy or

  • X-RAY FLUORESCENCE (XRF)- Results when atoms absorb incident X-radiation, become excited, and emit X-rays of characteristic

    - Is a characteristic of the elements present and their concentrations

    - The process is X-ray emission when the excitation source is electrons

    - The process is X-ray fluorescence when the excitation source is a beam of X-rays

    - The X-ray excitation source is the primary beam and the X-ray emitted from sample is the secondary beam

  • X-RAY FLUORESCENCE (XRF)- min of the primary beam must be shorter than the absorption edge of analyte element

    - of fluorescence is characteristic of the element being excited

    - Is an elemental analysis method

    - Is a surface sensitive technique

    - Elements with Z between 12 and 92 can be analyzed in air

    - Air absorbs fluorescence of elements with Z between 3 and 11

  • X-RAY FLUORESCENCE (XRF)WDXRF- XRF in the wavelength-dispersive mode- Dispersive device separates X-rays of varying wavelength(diffracted at different angles proportional to their )

    EDXRF- XRF in the energy-dispersive mode- No dispersive device- All wavelengths arrive at the detector simultaneously- Detector measures and records the energies of individual detected X-ray photon- A filter is often used to improve S/N ratio

  • X-RAY DIFFRACTION (XRD)- Basis is diffraction of X-rays by crystals

    - Depends on the crystal properties of solids

    - Just like how light is diffracted by diffraction grating

    - Diffraction pattern can be used to determine atomic spacing in crystals

    - Used for the determination of the arrangement of atoms in crystals(that is the crystal structure; X-ray crystallography)

    - Useful for solid crystalline materials (alloys, polymers, metals)

  • X-RAY DIFFRACTION (XRD)IRIRABCDd

  • X-RAY DIFFRACTION (XRD)- = angle of incidence

    - d = distance between lattice planes (interplanar distance)

    - Incident waves (I and I) are in phase with each other

    - Reflected waves (R and R) should also be in phase with each other

    - The waves interfere destructively if they are out of phase

    - A beam of X-ray is reflected at each layer in a crystal if the spacing between the planes (d) equals the of radiation

  • X-RAY DIFFRACTION (XRD)- The wave I travels an extra distance AB + BC

    - AB + BC must be a whole number (n) of wavelengths for the waves R and R to be in phase (for reinforcement to take place)

    - Distance AB + BC = n

    AB + BC = 2AB

    AB = BDsin = dsin

    n = 2dsin (called the Bragg Equation)

  • INSTRUMENTATIONComponents

    - Excitation Source

    - Wavelength Selector

    - Collimators

    - Filters

    - Detector

  • INSTRUMENTATION- X-ray system operates under vacuum or helium atmosphere

    - Low energy X-rays by elements with Z < 11 are easily absorbed by air

    - Liquid samples cannot be analyzed in a vacuum

  • X-RAY SOURCEThree common X-ray generation methods

    1. Beam of high-energy electrons to produce a broad band continuum X-ray- Employs the X-ray tube- Used for XRF and XRD

    2. X-ray beam (primary beam) of sufficient energy to eject inner core electrons from a sample to produce secondary X-ray beam- Used for XRF

  • X-RAY SOURCEThree common X-ray generation methods

    3.