INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 8

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment 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

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

ELECTRONIC TRANSITIONS

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

- Kα lines 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

- Kβ1 to Kβ3 lines 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 CV = applied voltage (Volts)

h = 6.626 x 10-34 J·sc = 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

X-RAY LINES

eV

hcλmin

- Substituting all values gives the Duane-Hunt Law

V

12400λmin

- λmin depends on accelerating voltage but not metal(would be the same for different metals at the same V)

- From Duane-Hunt Law

X-RAY LINES

)A( λ

12.4(keV)Energy o

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

X-RAY LINES

2σ)a(Zλ

cν

Z = atomic number of elementsσ = 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 transmitted

X-RAY ABSORPTION

From Beer’s Law

X-RAY ABSORPTION

ρxμo

meλI λI

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 sampleµi = 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΄

A

θ

B

C

Dθ

d

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 andconsequently 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 detectorGe(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

MILLER INDICES IN XRD

- Three coordinates identify a plane in space

- Is the reciprocals of the intersection points of the plane with x-, y-, and z-axis

- If a plane intersects at 1/2, 1/2, 1, the reciprocals are 2,2,1 and the Miller indices for the plane is (221)

- A plane parallel to a given axis has infinity intercept and the reciprocal is 0

X-RAY DEFRACTOMETER IN XRD

- Made up of X-ray tube, a sample specimen, and a detector

- The detector rotates in an arc described by a Rowland circle

- Modern instruments are equipped with 2D imaging detectors such as CCDs

X-RAY DEFRACTOMETER IN XRD

Goniometer

- Is the analyzing crystal positioning system

- When the crystal rotates by θo the detector rotates through 2θo

- The detector is therefore always in the correct position (the Bragg angle) to detect dispersed and diffracted radiation

ELECTRON PROBE MICROANALYSIS

- The use of small diameter electron beam (0.1 – 1.0 µm) to excite a sample

- Employs X-ray emission spectrometer as microanalyzer

- Obtains elemental composition and distribution on a microscopic scale

- Samples must be solids (thin film, powder, small and bulk solids)

- Determines elements with Z between 4 and 92

ELECTRON PROBE MICROANALYSIS

- Two Instruments for Microanalysis

Electron Microscope Analyzer (EMA)- Operates at high electron beam currents- Detector is the gas proportions counter

- Provides high emission intensity but moderate resolution and magnification

Scanning Electron Microscope (SEM)- Operates at low electron beam currents

- Detector is the Si(Li)- Provides low emission intensity but high resolution

and magnification

APPLICATIONS

General

- Evaluation of internal cracks in metals, cavities in teeth, voids in metals, and broken bones in humans

- Qualitative and quantitative determination of elements in solid and liquid samples

- Security baggage screening at ports

- Crystal structure determination (X-ray crystallography)

APPLICATIONS

General

- Studies of the universe and rocks (Cosmic X-ray)

- 3D images of body tissuesTechniques involve Completed Topography (CT) and

Completed Axial Topography (CAT)

- Provides physical information rather than chemical information

APPLICATIONS

X-ray Absorption

- Useful for medical radiography (for the location of bones)

- Defines the shapes of arteries and capillaries

- For detection of cracks and holes in welds and joints of bridges, aircrafts, buildings

- For measuring the thickness of thin metal films

- For determining the liquid levels in sealed vessels or pipes

APPLICATIONS

X-ray Fluorescence

- For qualitative identification of elements present in a sample(Tables of X-ray lines are available)

- Permits multielement analysis of alloys

- Used for both liquid and solid samples

- Solvents should contain elements with low Z (H2O, HCs, HNO3)

APPLICATIONS

X-ray Fluorescence

- Intensity of fluorescence is used for quantitative analysis

- Flat surfaces are required for quantitative analysis

C = K(I)(M)(S)

C = concentration of elementK = operating conditions and is a function of the spectrometer

I = intensity of the measured signalM = interelement effects (absorption and enhancement effects

S = specimen homogeneity

APPLICATIONS

X-ray Fluorescence

- Both external and internal standards may be used

- Use of internal standards improves precision

- Particle size distribution of standards should be similar to that of unknown analyte

For analysis ofMinerals, metals, textiles, paper ceramics, wood, polymers,

cement, sulfur in fuels, metal content in engine oils

APPLICATIONS

XRD

- Nondestructive technique

- Provides detailed information on the structure of crystallinesamples (diffraction patterns are unique for each compound)

- Used for the determination of the degree of crystallinity of semicrystalline solids

- To determine the disorder in metals and alloys

- Single crystal XRD for the exact position of each atom in a crystal

APPLICATIONS

Limitations of XRD

- Cannot identify amorphous materials

- Cannot be used for trace analysis

- Difficult to identify mixtures due to overlapping peaks