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.