INSTRUMENTAL ANALYSIS CHEM 4811

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 - Have shorter λ 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 ФK, ФL, ФM, …..

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

ELECTRONIC TRANSITIONS 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 Spectroscopy

ELECTRONIC TRANSITIONS - Some transitions are allowed and others are forbidden - Transitions are governed by quantum mechanical selection rules E = hν = hc/λ - For an X-ray photon released when an L electron drops from a specific sublevel to a K shell hc/λ = ΦL – ΦK ν = (ΦL – ΦK)/L

X-RAY LINES X-ray emission lines that terminate - in the K shell are called K lines - in the L shell are called L lines etc. - 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 8.2

X-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 Kα line - Two possible Kα lines for atoms with Z > 9 - These are Kα1 and Kα2 - Kαlines are usually not resolved so only one peak is seen

X-RAY LINES Siegbahn Notation - An electron that drops from the M shell sublevel to the K shell generates Kβ lines - Kβ1to Kβ3lines are usually not resolved so a single Kβ 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 lines

X-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 information

X-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 radiation

X-RAY LINES - Supposing electrons transfer all their energy in one collision - The λ of emitted photons is the shortest attainable - Is the minimum λ or the highest energy (λmin) - Energy of electrons (eV) = energy of radiation (hν = hc/λ) e = charge of electrons = 1.60 x 10-19 C V = applied voltage (Volts) h = 6.626 x 10-34 J·s c = 3.00 x 108 m/s

X-RAY LINES E = eV = hν = hc/λ - Implies when all energy of electrons is converted to X-ray - Substituting all values gives the Duane-Hunt Law - λmin depends on accelerating voltage but not metal (would be the same for different metals at the same V)

X-RAY LINES - From Duane-Hunt Law

X-RAY LINES Moseley’s Law - Developed by Henry Moseley - Relationship between characteristic X-ray lines and atomic number - Used to assign atomic number to newly discovered elements Z = atomic number of elements σ = screening constant (a function of repulsion of other electrons)

X-RAY METHODS X-Ray Absorption X-Ray Fluorescence (XRF) X-Ray Emission X-Ray Diffraction (XRD)

X-RAY ABSORPTION - 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 transmitted

X-RAY ABSORPTION From Beer’s Law I(λ) = transmitted intensity at wavelength λ Io(λ) = incident intensity at wavelength λ µm = mass absorption coefficient (cm2/g) ρ = density of sample (g/cm3) x = sample thickness (cm)

X-RAY ABSORPTION - For a sample containing different elements µtotal = w1µ1 + w2µ2 + w3µ3 + …… wi= weight fraction of element in sample µi= mass absorption coefficient of element - Longer λs are more readily absorbed than shorter λs (Longer λs are absorbed more) - Amount of absorbed light increases with increasing λ

X-RAY ABSORPTION EXAFS - 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 µm versus X-ray energy or λ

X-RAY FLUORESCENCE (XRF) - Results when atoms absorb incident X-radiation, become excited, and emit X-rays of characteristic λ - 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) - λmin of the primary beam must be shorter than the absorption edge of analyte element - λ 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 λ) 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) I R I΄ R΄ D θ θ d A C B

X-RAY DIFFRACTION (XRD) - θ = 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 λ 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 = nλ AB + BC = 2AB AB = BDsinθ = dsinθ nλ = 2dsinθ (called the Bragg Equation)

INSTRUMENTATION Components - 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 SOURCE Three 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 SOURCE Three common X-ray generation methods 3. Radioactive isotope which emits very high energy X-rays (gamma radiation) in its decay process A Fourth Method - Involves the use of massive high-energy particle accelerator (the synchrotron) - Particle induced X-ray emission (PIXE) - Uses alpha particles

THE X-RAY TUBE Consists of - An evacuated glass envelope - A tungsten wire filament cathode and a pure metal anode - A thin beryllium window sealed in the glass envelope - Window is transparent to X-rays and serves as the exit - Glass envelope is encased in a lead shielding and a heavy metal jacket

THE X-RAY TUBE - The cathode wire is a negatively charged electrode which gives off electrons when electrically heated - The process is called thermionic emission - Electrons are accelerated towards the positively charged anode (target) - High voltage (4 – 50 kV) between cathode and anode results in high acceleration of electrons - Electrons strike anode and release X-radiation of short λ

THE X-RAY TUBE - Cathode is tungsten wire filament - The X-ray tube is named for the anode (A copper X-ray tube has a copper anode) - The wavelength of X-ray radiation emitted depends on the target metal - Intensity and energy of electrons depend on the voltage between cathode and anode

THE X-RAY TUBE - The target element should have a greater atomic number than the analyte elements in the sample - Energy of the X-ray emitted should be greater than that required to excite element in XRF - This is not required in XRD or X-ray absorption - Water cooling of the anode may be necessary due to excessive heating Read about secondary XRF sources

RADIOISOTOPE SOURCES - Gamma ray is an X-ray resulting from radioactive decay of certain isotopes - Alpha decay, beta decay, electron capture processes can result in the release of gamma rays Advantages - Small, rugged, portable, do not require power supply - Ideal for XRF spectra for bulky samples Disadvantages - Weak intensity compared to that of X-ray tube - Voltage cannot be changed to optimize source - Source cannot be turned off

COLLIMATORS - Provides parallel beam of radiation - Made up of two sets of closely packed metal plates separated by a small gap - All radiation but the narrow beam between the gap are absorbed - Stray radiation is decreased when gap width decreases or when gap length increases - This decreases background - Not needed for energy dispersive and curved crystal spectrometers

FILTERS - Used to absorb certain λs but permit desired λs - Is placed between the X-ray source and the sample - Both continuum and characteristic line radiations are seen at certain operating voltages - Only one type of radiation is however desired - Chosen filter should have its absorption edge between Kα and Kβ emission lines of the target element - Commonly made of metal foils of pure element, brass, cellulose

DETECTORS - Transform photon energy into electrical pulses - The pulses (photons) are recorded as counts per second (known as count rate) - Count rate is a measure of the intensity of the X-ray beam Three classes of X-ray detectors - Gas-filled detectors - Scintillation detectors - Semiconductor detectors

DETECTORS Gas-Filled Detectors - Filled with helium gas - He gas is ionized by X-ray photons to produce He+ ions and e- - The e- move to the positively charged center wire and are detected

DETECTORS Gas-Filled Detectors - Three types Ionization Chambers - Number of electrons reaching the anode is reasonably constant Proportional Counters - Number of electrons increases rapidly with applied voltage Geiger Muller Tubes - Enormous amplification of the electrical pulse

DETECTORS Gas-Filled Detectors - Two Types of Proportional Counters Flow Proportional Counter - Has thin polymer film window - Filler gas leaks out because of thin film - Coated on the inside with Al for electric field homogeneity Sealed Proportional Counter - Window is thicker which prevents leakage

DETECTORS Scintillation Detector - X-ray falls on a compound that absorbs X-rays and consequently emits visible light - The process is called scintillation - PMT detects the visible light scintillations - The scintillation compound could be organic or inorganic crystal, or organic compound dissolved in a solvent - Example is NaI(Tl): thalium-doped sodium iodide crystal

DETECTORS Semiconductor Detectors - Involves generation of ion pairs (electron e- and a hole e+) - Principle is similar to that of the gas ionization detector - The most common is the lithium-drifted silicon diode Si(Li) called ‘silly’ detector - Another example is the lithium-drifted germanium detector Ge(Li)

SAMPLE HOLDERS - Samples are analyzed face-up or face-down - Solid and liquid samples can be analyzed Two Classes - Cassettes for bulk solid samples and - Cells for loose powders, small drillings, and liquids

SAMPLE HOLDERS Cassette - Metal cylinder with a screw top and a circular opening - Maximum size of sample is 52 mm diameter and 30 mm thick - Sample is spun at ~ 30 rpm to homogenize the surface Cell - Plastic cylinder with one end covered with plastic film - A plastic disk is placed on top of sample that presses the sample against the film - A cap is then screwed on top of the disk - The top is vented to prevent pressure build-up

SAMPLE HOLDERS - Face-down configuration gives better quantitative results for liquid samples - Polymer film thickness is about 3 – 8 µm Examples of Polymer Film - Mylar (polyester) - Kapton (polyimide) - Teflon (fluoropolymer) - Polycarbonate - Polypropylene

INSTRUMENTAL ANALYSIS CHEM 4811