Module 06520

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Module 06520 Structure and Synthesis
Atomic Spectrometry-4 lectures 1/2 exam question
(15/25)  
Books   D.C. Harris Quantitative Chemical
Analysis (5th Edition) Freeman Foundations Of
Analytical Chemistry, Skoog, West, Holler And
Crouch, Fundamentals of Analytical Chemistry,
Skoog, West, Holler And Crouch, 8th Edition,
Thompson, 2004.

• Learning Outcomes
• By the end of this course you should be able to-
•   Explain the origins of atomic spectra and the
  processes
• of absorption, emission and fluorescence
•   Identify the different instrumental
  requirements for flame atomic absorption,
  electrothermal vaporisation, flame atomic
  emission, inductively coupled plasma emission
  spectrometry.
•    Compare and contrast sample introduction
  techniques.
•   Understand and know how to correct for sample
  matrix effects when making measurement.     

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AAtomic Spectra AAtomic spectra are the result of
the interaction of electromagnetic radiation with
matter.
Ultraviolet wavelength 1 x10-8 m
-400nm energy 12000-310 kJ
mol-1 Visible wavelength 400-800 nm energy
310-150 kJ mol-1

• x ? c speed of light
• Where ? is the frequency in Hz,
• ? is the wavelength in m
• c is the speed of light (2.998 x 108 m s-1)

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• When investigating the energy of electromagnetic
  radiation it is convenient to think of it as
  discrete photons of light.
• The relationship between the energy and
  frequency of light is
• E h ?
• Where h is Plancks constant 6.626 x10 34 Js
• or in relationship to wavelength

E h c ?
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There are three fundamental processes that can
occur in the atom.
When UV-visible radiation interacts with an atom
it has sufficient energy to cause transitions in
the valence energy levels.
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• Where Eo is the ground state-
• The electronic energy level where the valence
  electrons normally reside. Transitions involving
  this energy level are called Resonance
  transitions.
• Where Ej and Ei are excited energy states-
• Higher energy levels to which the electrons move
  when the atom is excited by heat or light. The
  atom only stays for a very short time then drops
  back to a lower energy level and spontaneously
  emits a photon of light.

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• Write the equation for-
• Atomic emission
• Ej- Ei h ?
• Excited by heat energy, light emitted as electron
  spontaneously returns to lower energy level
• Atomic absorption
• Ej- Eo h ?
• A photon of light is absorbed by an electron in
  its ground state such that it is excited to a
  higher energy level

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What does this mean? An electron can only move
between s?p or p ?d NOT s?s
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• Note how the p energy levels have split to give
  doublets with slightly different wavelengths, at
  for example, 589.6 nm and 589.0 nm

• This occurs because the electrons spin about
  their own axis and that direction may be with or
  against the orbital motion. Both the spin and
  orbital motions create magnetic fields as a
  result of rotation of the charge carried by the
  electrons.

• If the motions are in opposite directions the
  fields attract and if the motion is parallel they
  repel, so the energy of an electron that spin
  opposes its orbital motion is slightly smaller
  than one where the motion is alike.

• These differences occur for d and f orbital but
  the
• differences in energy are too small to be easily
• detected.
•  

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Emission intensity
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Partial Energy Level Diagram for Magnesium
Triplet state
Singlet state
3s14p1
3s14p1
1503 nm
3s13d1
3s14s1
381 nm
3s14s1
1183 nm
517 nm
3s13p1
3s13p1
203nm
285 nm
457 nm
3s2
Time for electron to change spin 10-9 s, much
greater than time For photon to be observed.
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Example of an atom with 2 external
electrons   For Magnesium excited singlet and
triplet states with different energies
exist.   Magnesium excited singlet and triplet
states   3p ___________ 3p
___________   3s ___________ 3s ___________
3s ___________ singlet singlet excited state
triplet excited state Ground state paired
unpaired anti parallel spin
(lower energy)   The time for an electron
to change spin (10-9s) is much greater than for a
photon to be absorbed or transmitted.  
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Other elements As number of outer electrons
increase energy level diagrams become very
complex
hydrogen
sodium
iron
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• Line width
• The lines seen in atomic spectra are
  theoretically infinitesimally thin because
• ?E hc/?
• In actual fact the atomic line is wider and this
  is due to several factors.
• Heisenberg Uncertainty Principle
• Due to the very short life times of excited
  states (10-9s) the Heisenberg Uncertainty
  Principle applies and we

cannot know both lifetime energy with precision
(of an excited state)
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Doppler Broadening This occurs due to the rapid
speed of atoms in gas. (Gaussion profile). 1000 m
s-1 (2000 mph)   If a source (excited
atom emitting a photon) is moving towards a
stationary observer (PMT) the emitted wave will
appear bunched up to the observer and the wave
will appear to have higher frequency If the
excited atom emitting a photon is moving away
from the detector the emitted wave will appear
stretched out and will appear to have a lower
frequency  -
PMT
observer
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• Pressure broadening
• Atoms are colliding and therefore they loose
  energy.
• If the temperature or pressure increases the
  atoms are more likely to collide reducing the
  lifetimes of the excited atoms.
• Gaseous Atoms
• In atomic spectrometry the sample must be broken
  down to form free atoms. This is usually
  achieved either by flames, electrically heating
  or by plasmas.

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• CASE STUDY ONE
• Flame Atomic EmissionSpectrometry (Flame
  Photometry)

filter
flame
Application Widely used in hospitals for the
clinical analysis of sodium, potassium and
lithium in blood serum Used in the range 1 - 10
mg dm-3
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• Sample preparation
• Take about 5 mL of serum and dilute with
  deionsed water, making up to the mark in a
  volumetric flask.
• Advantages of the technique
• Easy to use
• Cheap (6000)
• Selective for Na K Li
• In this technique a natural gas/air flame is
  used to excite the atoms.
• This has a low temperature and can only excite
  a limited number of elements best for with those
  with only one valence electron.

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Why? The temperature is related to the number of
excited atoms via the Boltzmann Equation
Where gj, gi are statistical weightings
determined by the number of states having equal
energy of each quantum level (degeneracy). ?E is
the difference in energy between the excited and
lower or ground state k the Boltzmanns Constant
1.38062 x 10-23J K-1 T is the temperature in K
The more atoms in the excited state the higher
the intensity of the emission.
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• Example
• Work out the ratio of the number of atoms in
  the excited state to the number of atoms in the
  ground state for the sodium transition of 589 nm
  at a typical flame temperature of 2500 K.
• Look at the sodium energy level diagram.
• The lower energy levels i is which level? How
  many levels is this electronic energy level split
  into?

Lower level is the 3s level Not split
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• The higher energy levels j is which level? How
  many levels is this electronic energy level split
  into?

Higher level is 3p two levels Therefore gj/gi
2/1
Need to find ?E but ?E hc/ ? So ?E (6.626
x 10-34 x 3.00 x108) / 589 x10-9 E 3.37 x10-19 J
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• Substituting in to Boltzmanns distribution
• Ni/No
• 2 exp-((3.37 x10-19/
• (1.38x10-23 x 2500))
• 2 exp(-9.76)
• 1.15 x10-4

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• Fuel, oxidant (air) and the sample all pass
  through separate channels to the opening
• at the top of the burner where the flame rests.
  
• The sample is drawn up through capillary by gas
  flow around capillary tip
• With a flow rate of 1-3 ml min-1
• This leads to a very noisy signal, you need to
  signal average.

Take 3-10 measurements and find the mean
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• Other problems
• The flame temperature fluctuates which will
  effect the emission signal and therefore you need
  to use internal standard calibration (see example
  later).
• The calibration is a curve due to
  self-absorption-
• When the atoms in the outer part of the flame
  absorb the emission from the atoms at the
  centre.
• Ionisation see discussion of this in next
  section.

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FLAME
gas
air
sample solution
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• Summary
• Atomic spectrometry involves the interaction of
  UV-visible radiation with valence electrons.
• Atomic emission involves the excitation of the
  atoms by heat to a higher energy level, followed
  by spontaneous emission of a characteristic
  wavelength of light Ej-Ei hc/?.
• Atomic absorption involves the excitation of the
  ground state atoms to a higher energy level by
  absorption of a characteristic wavelength of
  light Ei-Eo hc/?.
• Flame photometry is a simple form of atomic
  emission spectrometry.

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• Test yourself
• Write an equation to describe the process of
  atomic absorption.
• Ej- Eo h ?
• Name two processes that cause the atomic lines to
  be broadened.
• Doppler broadening, pressure broadening
• Sketch a block diagram for a flame photometer.

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CASE STUDY TWO Flame Atomic Absorption
Spectrometry
sample

• Applications
• The analysis of calcium and magnesium in tap
  water at mgl-1 levels.
• Monitoring magnesium in tomato leaves to optimise
  addition of fertiliser in greenhouses.
• Also widely used in forensic science to measure
  bullet composition.

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• Sample preparation
• The water samples can be analysed directly but
  they must be acidified as soon as the sample is
  collected, why?
• Glass acts as an ion exchanger and the metals
  would stick to the surface.
• Acidification makes sure they stay in solution
  and do not precipitate out of solution.

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• Magnesium in tomato leaves
• The leaves need to be dried, ground and sieved
  first, why?
• You need to know the exact weight of leaves,
  moisture content can be be very variable.

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Flame atomic absorption spectrometry Beer
Lamberts Law  

A log (Po/P) A ? b c   where ?
is the molar absorptivity coefficient in units of
mol-1 dm3 cm-1 b is the pathlength in cm and c
is the concentration in mol dm-3   In limits
(below 0.8 Absorbance) A vs. concentration    
P
Po
sample
b
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• Flame Chemistry
• Flames are used in atomic emission spectrometry
  for excitation (emission spectrometry) but in
  atomic absorption flames are used as Atom Cells
  to produce gaseous atoms.
• Why must the atoms not be excited for atomic
  absorption spectrometry?

If the atom is already in the excited state it
cannot absorb the light.
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• Types of fuel/oxidant
• air/acetylene
• 2300oC most widely used.
• nitrous oxide/acetylene
• 2750oC hot and reducing red feather zone - due
  to CN very reactive free radical scavenger for 02
  ? lowers partial pressure of 02 in zone reducing
  atmosphere

C2H2 2.502 10N2 ? 2CO2 H2O
10N2 stoichiometric reaction
C2H2 5N2O ? 2 CO2 H2O 5N2
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• Why do you need a different burner for different
  oxidants?
• because to prevent flash back linear gas flow
  rate
• needs to 3 x speed of which flame can travel,
• burning velocity).

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Role of Chemistry in the Flame
 sample atomised by thermal and chemical
dissociation H2 Q ? H? H? O2 Q ? O? O?
H? O2 ? OH? O? O? H2 ? OH? H?  
equilibrium achieved by 3rd body collision
(B)   i.e. N2, O2 H? H? B ? H2 B? Q H?
OH? B ? H2O B? Q Free reductions may react
with sample to produce atoms i.e. H? HO?
NaCl ? H2O Na? Cl? Na? Q ? Na?
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Flame Atomisation Process Sample must be in the
form of a fine mist so as not to put out
flame. Breaks down sample into very fine drops
to form liquid aerosol or mist. This assist
atomisation as sample only in flame
0.025s Sample drawn up capillary tube at high
velocity
Sample

oxidant
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• Suction caused by high flows of oxidant gas
  and Venturi effect.
• The high gas flow rate at the end of the
  capillary creates a pressure drop in the
  capillary the pressure in capillary is below
  atmospheric pressure and sample solution is
  pulled up.
• The high speed gas breaks the solution into a
  fine mist by turbulence as it emerges from
  capillary.
• How do we get a better aerosol?
• use impact bead (glass or alloy) to encourage
  aerosol formation and remove large droplets.

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Premixed Burner
To burner stem
Nebuliser and spray chamber
fuel
Enlarged concentric nebuliser
Flexible tube to sample
mixing baffles
oxidant
Expansion chamber
Drain with U bend
Danger point if drain not full gas can escape
backwards resulting in EXPLOSION
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Solution MX
nebulisation
Liquid aerosol droplets
solvent evaporation
Salt mist of MX
salt vapourised

Processes In Flame
Molecules of MX
Dissociation Thermal and chemical
M
ATOMISED
MX compound formed
M excited
M ionised
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Photomultiplier Tube
Photosensitive cathode
LIGHT
dynodes
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Resolution- Is the ability to separate closely
spaced peaks R ? n x N ?? Where - ? is
the wavelength N is the number of grooves n is
the diffraction order (??) the difference between
two wavelengths
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• Spectral Resolution
• Vn 308.211 nm
• Al 308.215 nm
• Difference 0.004 nm
• The resolution of the spectrometer will not be
  sufficient to separate out the two wavelengths.

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• Ionisation in the Flame
• M ? M e-

• If you add a large excess of easily ionisable
  element
• The equilibrium is shifted and ionisation
  prevented
• M ? M e-

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• Other compound formation
• i.e. Ca3 (PO4)2 (actual structure uncertain)
• two possibilities
• 1 USE A PROTECTING AGENT
• Complex Ca with EDTA
• to form a compound that
• decomposes easily in flame.
• 2 USE A RELEASING AGENT
• Complex the phosphates with Strontium and
  Lanthanum
• thus releasing the Ca

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• What are the limitations of flame atomic
  absorption spectrometry?
• You can only analyse one element at a time, why?
• The sensitivity of the method is limited to ppm
  levels (mg l-1), why?

Need a different hollow cathode lamp for each
element-lock and key effect
-Only 10 of the sample reaches the flame by
the nebulisation process. -The sample only
spends 0.001 s in the flame due to the high gas
velocities 200 cm/s
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• CASE STUDY 3
• Electrothermal Vaporisation Atomic Absorption
  Spectrometry

AMPLIFIER AND READOUT
MONOCHROMATOR
PHOTO DETECTOR
LIGHT SOURCE
ATOM CELL
Applications Sample Preparation For small amounts
of samples and low concentrations in organic
matter the best approach is ashing the sample
The sample is weighed into a porcelain crucible,
cover with a lid and placed in a muffle furnace
at 500oC
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• Advantages
• All the sample is present in the atom cell
  and this has two advantages
• Small samples can be analysed
• Much higher sensitivity is achieved

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The atom cell
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• After heating the tube must be rapidly cooled,
  how could this be achieved?

The use of water cooling
Furnace Programming What is the problem with
adding the whole sample to the tube?
You are also adding a matrix that will form a
burnt ashy material that could block the light
path and interfere with absorption measurements.
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• Disadvantages
• Memory effects-
• Contamination from previous analysis- happens
  when the tube is not cleaned properly between
  analysis

What is the problem with having to carry out
temperature programming?
Each analysis takes about 1 minute so sample
throughput is very low compared to flame AAS
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• Background Correction Techniques
• Used to eliminate interference from sample
  matrix, for example
• salts with high salt concentration cause
  light scattering.
•  Absorption due to sample will be over
  estimated, especially bad for graphite furnace
  AAS.
•  Three techniques used including Zeeman, and
  Smith Hieftje
• Background Correction but the most common is -
•  Deutrium Background Correction- Use a hollow
  cathode lamp
• and a deuterium lamp with arotating sector
  mirror.
• Get alternating voltage as the two different
  beams reach the detector.

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Temperature Programming
Tube cleaning
atomisation
ashing
drying

• Drying 100oC -remove solvent -must not lose
  sample by spitting

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• CASE STUDY 4
• Inductively Coupled Plasma Atomic Emission
  Spectrometry
• A plasma is sometime called the fourth state
  if matter.
• What is a plasma?

a hot partially ionisied gas-the sun is a giant
helium plasma
The hottest part of the inductively coupled
plasma is at temperatures of 7 000K - 10
000K
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Plasmas
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This means more atoms are excited and it can be
used for multi-element analysis (important see
tutorial on Boltzmanns Distribution). How do
interferences differ in the ICP compared to the
flame? chemical interferences spectral
interferences
Less because the hotter temperature breaks
everything down into atoms

More because the higher energy allows More
electron transitions
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• Advantages
• Multi-element analysis
• longer linear working range (need fewer standards
  for calibration)
• higher sensitivity than flame AAS (ppb levels)

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1.Production of an ICP
2.The ICP torch
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• What is the advantage of this design?
• The emission from the fireball of plasma is
  intense so analytical measurement are made in the
  cooler tall plume 10-30 mm above the core which
  is more optically transparent.

The sample stays in the central channel instead
of spreading throughout the plasma (as compared
to the flame). This means it is more
concentrated and you can get better sensitivity.
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• Sample Introduction
• The sample needs to be introduced into the plasma
  without affecting the temperature. How would you
  introduce a liquid sample into the plasma?

The sample needs to be in the form of an aerosol
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Sequential ICPMonochromator

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• Sequential Scanning Detection
• Advantages
• Inexpensive, sensitive
• Flexible, any wavelength can be detected
• Therefore good for semi-quatitaitve work.
• Disadvantages
• Slower
• Needs lots of sample to keep running through
  plasma as the monochromator is scanned.

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• Simultaneous Detection
• Polychromator
• Advantages
• Very rapid
• Sensitive (using PMT detector)
• Disadvantages
• Very expensive and large
• Inflexible, designed to work for a suite of
  pre-selected wavelengths
• Up to approx 20 maximum

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Echelle Spectrometers and Solid State Detectors
Advantages Nearly simultaneous Covers nearly the
whole spectrum New solid state detectors
sensitive Disadvantages Some problems with the
technology of the solid state detectors
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Radial versus Axial Detection
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• Spectral Interference
• The main problem with ICP-AES is that because it
  is so efficient at excitation complex spectra are
  obtained.
• If a sample has a complex matrix components of
  the matrix may also be excited and emit light at
  wavelengths close to the analyte wavelength. How
  can this problem be overcome?

Use wavelength tables to select the analyte
wavelengths to be monitored away from any
wavelength where there might be interferences.
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• Calibration
• Which is the best type of calibration technique
  for ICP AES and why?

Internal calibration as changes in the plasma
temperature will affect the intensity of the
emission signal
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• Conclusions-
• You should now be able
• Explain the origins of atomic spectra and the
  processes of absorption, emission and
  fluorescence.
• Identify the different instrumental requirements
  for flame atomic emission, flame atomic
  absorption, electrothermal vaporisation,
  inductively coupled plasma. emission spectrometry
  Compare and contrast sample introduction
  techniques.
• Understand and know how to correct for sample
  matrix effects when making measurement.
• You should also be able to apply your knowledge
  to select a suitable sample preparation and
  analytical method for a specific application.