Module0652383 - PowerPoint PPT Presentation

Title: Module0652383


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Module 06523/83 Intermediate Physical and
Analytical Chemistry 2
Atomic Spectrometry-9 lectures 1 exam question  
Books   D.C. Harris Quantitative Chemical
Analysis (5th Edition) Freeman Also any other
basic analytical text book i.e. Skoog, Kellner.
Leaning 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 and ICP mass spectrometry         
   Compare and contrast sample introduction
techniques            Understand and know how to
correct for sample matrix effects when making
measurement            Describe the difference
between energy and wavelength dispersive X ray
fluorescence spectrometry            Select the
optimal technique for a particular application
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AAtomic Spectra AAtomic spectra are the result of
the interaction of electromagnetic radiation with
matter. In this course we are studying the
interaction of high energy radiation (X-rays and
UV-visible radiation) with gaseous atoms.
XX-rays wavelength 1 x 10-11-1 x 0-8
m energy 1.2 x107-12000 kJ mol-1 Uultraviolet wave
length 1 x10-8 m -400nm energy 12000-310 kJ
mol-1 Vvisible wavelength 400-800 nm energy
310-150 kJ mol-   ? ? x ? c speed of light
2.998 x 108 m s-1 WWhere ? is the frequency in
Hz, ? is the wavelength in nm and c is the speed
of light which is 2.998 x 108 m s.-1  If the
frequency of electromagnetic radiation is 5 x1014
Hz what is the wavelength of this radiation and
in which spectral region does it occur?
3 x108 m s-1 / 5 x 1014 s-1 6 x10-7 m 600 nm
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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 ? or in relationship to
wavelength   Where h is Plancks constant 6.626
x10 34 Js   Calculate the energy in joules of
one photon of the radiation derived above.
E 6.626 x10-34 J s x 5 x 1014 s-1 3.31 x10-19J
In the first part of the course we will look at
interactions with UV-visible radiation.
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The Origin of Atomic Spectra in the UV-visible
region of the spectrum Atomic Spectrometry is
based on three fundamental processes occurring
in the atom.
These processes occur when valence electrons
transfer between electronic energy levels
resulting in photons of light being emitted or
absorbed. The energy of the light depends on the
energy levels in the particular atom. Therefore
a spectra is characteristic for a particular
element. DE hn

• These spectra are for gaseous atoms
• They are seen as well defined narrow lines
  (compare with
• molecular spectra) relating to the electronic
  transitions of
• valence electrons
• For metals transition energies are seen as line
  spectra in the UV, visible and NIR by a
  photodetector
•  

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Emission intensity
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An atom in its normal state is said to be in the
ground state. If electrons in the atom are
excited by thermal or light energy they move to
a higher energy level and the atom is said to be
excited. The energy is quantized as the electrons
must move to fixed energy levels. Transitions
involving the ground state are said to be
resonance transitions.
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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 orbitals but
  the
• differences in energy are too small to be easily
• detected.
•  

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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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Spectral Line width   The
lines seen in atomic spectra are theoretically
infinitesimally thin because DE h c/?  
I/A
Dl 1/2
l/nm
 The line width is defined as ??½ difference in
? between two sides of spectral line at half
peak height.  
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Natural line width This is determined by
lifetime of excited state which is formed when
photon is absorbed.   absorption process fast
(10-15s) life-time of excited state longer
(10-9s) These very short timescales are governed
by the Heisenberg Uncertainty Principle   cannot
know both lifetime energy with precision (of an
excited state)   this leads to uncertainty in the
natural line width ?N Dl N l2D n/c
Dn is the uncertainty in the frequency of the
emitted radiation, which is 1/t where t is the
lifetime of the excited state.
For a sodium emission line of wavelength 589nm
and an excited Life-time of 2.5 x 10-9 s, what is
the natural line-width?
Dl N (589 x10-9)2 x (1 / 2.5 x 10-9) / 3.00
x 108
Dl N 4.62 x 10-13 m Dl N 0.000 46 nm
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Actual line width x 500 broader   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  -This broadening can be described
by   ??D 2 ? / c (2RT/M)1/2  
PMT
observer
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For a sodium atom in a flame at a temperature of
2500 K calculate the Doppler line-width for the
589 nm spectral line. Note RAM sodium is 23g
mol-1 but must work in SI units here i.e. 23
x10-3 kg mol-1.  
? ?D (2 ? / c) x (2RT/M)1/2
?l D ( 2 x 589 x 10-9 m / 3 x 108 m s-1) x ( 2
x 8.314 J K-1 mol-1 x 2500 K /23 x10-3 kg
mol-1)1/2
Dl D (3.927 x 10-15 s) x (1807 391 J kg
-1)1/2 ( BUT 1J 1 kg m2 s-2) Dl D (3.927 x
10-15 s) x (1807 391 kg m2 s-2 kg -1)1/2
Taking the square root of the second bracket Dl D
(3.927 x 10-15 s) x (1344 m 2/2 s-2/2) Dl D
5.28 x 10 12 m or 0.0053 nm
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Pressure broadening (Lorentzian
profile)   -  Atoms are colliding - lose excess
energy - lifetime of excited atoms is very short
- line broadening affected by T P and nature of
gas.  
Gaseous Atoms The key difference between
molecular and atomic spectrometry is that 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. For atomic absorption the atoms
must be in their ground state and not excited to
a higher level so an atom source is used. In
atomic emission spectrometry the atoms needed to
be excited and therefore an excitation source is
used. Flames can be used as excitation and
atom sources, depending on how easily the
electrons in an atom are excited, so flame
emission spectrometry (photometry) can be used
for easily excited atoms such as sodium, lithium
and potassium (see experiment).
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The effect of temperature on atomic
spectra Boltzmanns Distribution
Temperature has a large effect on the ratio
between the number of excited Nj and unexcited
N0 atoms
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 is the Boltzmanns
Constant 1.38062 x 10-23 J K-1 T is the
temperature in K
At flame temperatures of 2500K there are 1 in
1000,000 atoms excited see tutorial 3   In
plasmas (temperatures of at least 5000 K) Nj/Ni
1.51 x 10-2 there are 1.5 of atoms in an
excited state see tutorial 3
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The intensity of emission is proportional to the
number of atoms in the excited state
let no. free atoms in a cm3 N area of
observation S cm2
?a volume l x S contains N x l x S atoms

• If A is the probability that an excited atom
  will emit then-
•  Nj A atoms will emit h? energy sec-1
• Therefore the light intensity emitted
• I l S Nj A h? subsititute from Boltzmanns eqn

I l S A Ni gj h? exp - ?Ej gi
kT
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Flame Chemistry Flames are used in atomic
spectrometry for excitation (emission
spectrometry) or as Atom Cells to produce
gaseous atoms for atomic absorption spectrometry
secondary combustion zone interzonal region
primary combustion (reaction) zone pre heating
zone
appearance depends on fuel/oxidant length of
zones depends on gas flow rate  gas flow too
high- lifts flame off burner gas flow too low-
flash back   pre heating zone gases heated
rapidly primary combustion region contains free
radicals but not in thermodynamic equilibrium
interzonal region used for spectrometry has
free radicals secondary combustion zone
combustion products formed i.e. CO and H2O
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Types of fuel/oxidant   air/acetylene 2300oC most
widely used. C2H2 2.502 10N2 ? 2CO2 H2O
10N2 stoichiometric reaction
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 C2H8 5N2O ? 2
CO2 H2O 5N2
Safe use ? correct burner ? fuel rich flame fuel
rich flame, smoking air/acetylene ? switch over
N2O/acetylene keeping high gas velocity ?
switch off turn up fuel the back to
air/acetylene ? turn fuel down first
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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 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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Lock and Key Effect Need a light source and
wavelength selector (monochromator) but this
can actually only isolate a range of
wavelengths, known as the BANDWIDTH (even good
ones - usually 0.02 nm for AAS).   If a continuum
source is used a range of wavelengths would be
detected but Beer Lamberts law only applies when
the bandwidth of the source is narrow with
respect to the width of the absorption peak.  
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Lock and Key Effect
Continum Source
Line Source
Ca AA profile 0.002nm wide
Emission intensity
Emission intensity
0.02nm
Wavelength/nm
Wavelength/nm
Io
Emission intensity
Emission intensity
I
Wavelength/nm
Wavelength/nm

• Absorption peak width 0.002 nm
• Problem non-linear relationship poor signal
• absorbed very small.
•  
• Need to use a line source with a very narrow
  emission
• wavelength range such as a hollow cathode lamp,
• the problems is however that you need a library
  of
• lamps for different elements.

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Hollow Cathode Lamp
Ne or Ar at reduced pressure
On application of the voltage across the
electrodes the inert gas is ionised and a current
flows. When the voltage is high enough the
gaseous ions gain enough kinetic energy to
dislodge metal atoms from the cathode surface.
Some of the metal atoms will be in the excited
state and will emit characteristic monchromatic
radiation.
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Flame Atomisation Process Use an air/acetylene
or nitrous oxide acetylene flames( different
burners for different oxidiants because to
prevent flash back linear gas flow rate needs to
3 x speed of which flame can travel, burning
velocity). 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
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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 as pressure in capillary below
atmospheric pressure - sample solution forced up
capillary. high speed gas breaks solution through
turbulence as it emerges from capillary Also may
use impact bead (glass or alloy) to
encourage aerosol formation and remove large
droplets.
Factors effecting atomisation in flame
solution MX liquid aerosol droplets salt mist
of MX molecules of MX M ionisation
M excitation formation M
of compounds
process in flame
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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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Electrothermal Vaporisation Limitations with
flames High flow rate of sample through flame
(gas flow rate 200 cm/s) means that atoms spend
less than 0.001 s in flame. Only 10 of the
sample solution reaches the flame in the
nebulisation process. Solid sample must be
treated. Flame causes spectral interferences
especially near 200 nm (As 193.7 nm).  
Electro thermal Vaporisation There are several
designs but the most common is a graphite
furnace made from a hollow graphite tube (20-30
mm long, 5-10 mm i.d.) which is rapidly heated
with a high electric current (several hundred
amps) to a temperature high enough to atomise
the sample. Sample introduces to tube using a
micro pipette or auto-injector (1-100 µl). Use
inert gas as purge to minimise formation of
oxides i.e. argon or nitrogen.
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Furnace programming - get rid of as much light
absorbing material as possible prior to
atomisation Drying 100oC -remove solvent - must
not lose sample by spitting Ashing 400 -
500oC-pyrolyse organic matter most organic matter
breaks down to small volatile molecules which can
be flushed from the furnace Atomisation 2000 -
3000oC-to give rapid absorption choose
carefully, too high a temperature shortens
furnace life peak Cleaning-higher temperature
than atomisation to flush out furnace and
prevent carry over background correction must be
carried out simultaneously
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Temperature Programming
Advantages/Disadvantages All samples present in
atom cell therefore small samples can be
used. Lower limits of detection achieved
100-1000 x better. Solids can be atomised
directly. Background problems residence time of
matrix also increased so increase in molecular
absorbance too especially particulate matter from
solid samples. To avoid use background correction
and temperature programming. Loss of analyte
during ashing. Memory effects need to clean
between samples, use coated tubes. Temperature
gradients-platforms, cups. Precision poor as
injecting small quantities use auto
injector. Sample throughput lower.
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Hydride Generation For arsenic selenium antimony
to improve detection limits produce volatile
hydride of element. See diagram for set
up. Detection limits 0.01 ppb As sample HC1
(0.5-5.0 M) NaBH4 reaction time 10-100 sec
flush out with inert gas at low flow rate so
increased residence time. Cold Mercury Vapour
Method Instrumentation as for hydride but silica
tube needs no heating. Can use 184 nm resonance
line. Again lower limits of detection as longer
residence time and no flame. Hg 2 Sn 2 ?Hg0
Sn 4
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Wavelength Selection and Detection UV- visible
region and therefore same optics as for
molecular spectrometry. Only one element can be
detected at a time because of use of line
source. The monochromator comes after the atom
cell not needed before due to line source).
The monochromators only needs 0.02 nm
resolution due to use of lock and key effect.
The detector used is the photomultiplier tube for
its high sensitivity. (see Dr Aylotts lecture
notes)

• Modulation of signal
• Problems with basic layout
• i)Generates a constant voltage (DC signal) at the
  
• detector during sampling. DC signals are prone
  to
• electrical noise leading to poor sensitivity to
  (S/N)
• by comparison with that obtainable for an AC
  signal
• It is assumed that the only light reaching
• detector comes from lamp - but also light from
• flame - molecular or atomic emission.

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Reduce by design of optics and modulation of
light beam -Optics arrange so light source is
focussed at centre of flame. Then at entrance
slit of monochromator. -Emission from flame which
is undesirable is minimised as it occurs over a
wide range of flame and will not be focussed on
the entrance slit When lamp the beam is
modulated (coded) the detector is tuned to
receive only coded signals. Modulate by applying
AC supply to lamp or interrupting lamp with
chopper.
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Produces a beam of regular varying intensity
which generates alternating signals at detector.
As signal fixed frequency by using an AC
amplifier which is tuned it only amplifies at the
same frequency as beam modulation. All noise
at other frequencies can be rejected improving
S/N - further improvement if a phase sensitive
detector is used. Only amplifies signals which
are both at the same phase and frequency as the
modulation of the light beam. Improved
sensitivity emission signals eliminated.
Interferences
Spectral interferences These cause enhanced
signals Vn 308.211 nm Al 308.215
nm Not too important in AAS (lock and key
effect) but do see molecular broad band
absorption due to scattering effects from
products of atomisation (see background
correction) Chemical interferences These cause
depressed signals because atoms are either
ionised or from compounds in the flame.
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Ionisation in flame bigger problem in hotter
flame - worse for easily ionisable elements such
as alkali, alkali earth Al.B. Ti
M ? M e-
The equilibrium process is affected by other
easily ionisable metals. The presence of e- from
other element will effect equilibrium and
suppress ionisation. Use large concentration of
suppresser i.e. 1000 ppm to ensure large excess
of electrons. IONISATION BUFFER for Na use K
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Formation of low volatility compounds oxides-
formation of refractory oxides i.e. Al3 03 etc.
Use hotter flame (also reducing) Other compound
formation i.e. Ca3 (PO4)2 (actual structure
uncertain) two possibilities PROTECTING
AGENT Complex Ca with EDTA to form a compound
that decomposes easily in flame. RELEASING
AGENT Strontium and Lanthanum form R complexes
with interfering with the interfering anion thus
releasing the Ca
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Background Correction
Signal absorbance
Absorbance due to matrix, difficult to remove
good sample preparation helps, otherwise need
background correction
A
wavelength
Blank absorbance can be subtracted
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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Problem

• Standard additions is an important calibration
  technique for overcoming matrix affects.
  Standard additions was used to determine the
  concentration of iron in a sea water sample. 10
  ml aliquots of the sea water were added to each
  of five 50 cm3 volumetric flasks. Then 0, 5.00,
  10.00, 15.00 and 20.00 ml of an Fe3 standard
  solution (10.00 mg 1-1) were added to the flasks,
  before they were made up to the mark with
  analytical grade water The following
  absorptions were obtained

Calculate the concentration of Fe3 in the sea
water
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Deuterium Lamp Background Correction
.
hollow cathode lamps (emitting 0.002 nm
linewidth) deuterium arc lamp (emitting over a
wide range of wavelengths)
Continuum source
Line source
Background absorption
Atomic absorption
The signal from the deuterium lamp is the
absorbance of the background (analyte small
) The signal from the hollow cathode lamp is for
the analyte and background Therefore subtract
the one from the other and get signal due to
analyte.
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Atomic emission Spectrometry Flame Atomic
Emission Spectrometry In this technique a flame
is used to excite the atoms. Use either flame
photometer or atomic absorption spectrometer in
emission mode (AAS notes) air/natural gas
flame Low temperature ? only some elements can be
excited mainly alkali metals some alkaline earth
elements. Li 670 nm, Na 589 nm, K 766
nm. Wavelength selection ? required emission
lines well separated Use a filter to isolate
? Sample introduction/flame ? total consumption
burner laminar flow Fuel, combustion supporting
gas and aerosol all pass through separate
channels ? to opening where flame rests sample
drawn up through capillary by gas flow around
capillary tip (1-3 ml min-1) Use of lithium as
internal standard to correct for flame
temperature, fuel flow rates background
radiation
Ej
E h?
Ei
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Applications Mainly clinical analysis of sodium,
potassium and lithium in blood serum and other
biological samples. Can do all three together,
range 1 - 10 mg dm-3 Advantages 1 Cost - cheap
(6000) 2 freedom from spectral interferences ?
only a few elements excited 3 Less ionisation of
elements in low temperature flame.
Problems 1 limited applicability 2 poor
sensitivity 3 interferences i) chemical
interferences ii) ionisation interferences iii) sp
ectral interferences
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Inductively Coupled Plasma Source The
inductively coupled plasma is a hot partially
ionised gas with a very high temperature 7
000K-10 000K (plasmas). This means more atoms
are excited and it can be used for multi-element
analysis (important see tutorial and homework on
Boltzmanns Distribution). The high temperature
also means i)chemical interferences reduced ?
no problem ii)spectral interferences will be
worse - more transition
Other advantages i) longer linear working range
ii) potential for multielement analysis Sample
confined to channel in centre of plasma, self
absorption negligible. Use for minor and major
components.
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The inductively coupled plasma is induced by
radio frequencies (15 - 50 MHz) using high power
levels (0.5 - 3.0 kW).
The gas used for the plasma is usually argon RF
current is passed through a metal induction
coil and the current has an associated magnetic
field. The lines of force of the magnetic
field pass along the axis of a quartz tube
placed inside the coil. Electrons accelerated by
the field to travel in circular orbits inside
quartz tube. Energy from electrons is
transferred to the argon gas via collisions so
the gas heats up. The temperatures reached
produce high concentrations ions of both excited
atoms and ions. The plasma is doughnut shaped
with the sample introduced into the relatively
cool (6000/8000 K) hole in the centre.
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Plasmas
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The emission from 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. In the ICP the plasma
gas flows through three concentric tubes in the
torch with large amount of gas being needed (10
dm3 min-1). The sample is drawn up in an aerosol
of argon into the centre of plasma via the
central tube. 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. The second gas flow in the inner
tube maintains plasma and holds the plasmas
position on top of the torch. The third outer
stream (sometimes N2) maintains the plasma,
acts as a coolant to prevents the quartz melting
and gives the plasma it shape.
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Sample Introduction   This is usually as a
solution with a nebuliser, although solids can
be introduced by ablating the sample with a
laser.   The nebulisers used with the ICP work
on similar principles to the concentric nebuliser
used in flame atomic absorption
spectrometry.   The commonly used nebulisers have
transparent efficiencies of 0.5 - 1.5. As well
as the concentric nebuliser some specialised
nebuliser have been design for ICP-AES to improve
its performance. This is because the concentric
nebuliser is not very efficient and easily
blocks due to the narrow orifice. The cross
flow nebuliser is less likely to block (see
design).
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Ultrasonic nebuliser The sample solution is
pumped into a vibrating crystal transducer (at
approx 10 MHz). This makes the proportion of
small particles (less 10µm) in the aerosol
higher giving four times better sensitivity.
This nebuliser is often used with a desolvation
unit.
Babington Nebuliser This is good for high solids
suspended in solution (i.e.10 sodium or
slurries of powders). The sample is trickled
down a V groove and an argon jet issues from
capillary hole in middle of the groove causing
nebulisation of the sample.
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Multi-wavelength Detection To make full
advantage of atomic emission spectrometry
multi-wavelength detection is required. This
can be achieved in several ways. Sequential
Scanning Detection In this system a conventional
monochromator is used, but this must have good
resolution compared to those used for atomic
absorption spectrometry. The diffraction
grating in the monchromator is driven by a
stepper motor, each step being 0.007 nm. Often
two grating are used one for 175 -460 nm and the
other for 460-900 nm. Using this method about
twenty elements can be determined, 3-4 elements
per minute. A mercury lamp is used to
calibrate the wavelength. Can detect at any
wavelength and software is used with wavelength
tables that allows you to choose the best
wavelength of the particular application and
matrix, to avoid spectral interference.
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Sequential ICPMonochromator

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Simultaneous Detection Polychromator (See
diagram) In this method a concave fixed grating
is used and the detectors (photomultiplier tubes)
are positioned at slits on a Rowlands circle.
This is a fixed system and lacks
flexibility, Because you have to decide which
wavelengths needed in advance, and it is very
costly to change the software. Very fast though
and only need a small amount of sample.
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Echelle Spectrometers and Solid State
Detectors Many new instruments have solid-state
light detectors such as charge-coupled devices
as are used in CCD cameras. In recent years the
sensitivity of these detectors has greatly
improved and rivals photomultipler detection.
If commercially available CCDS are used then an
Echelle spectrometer is needed. Instead of just
using first order diffraction for light
dispersion the Echelle spectrometer uses other
multiple orders (typically between 50-120th
orders) too cover the normal spectral range.
It is less than 0.4 m long and has a course
grating (120 groves/mm) and a prism as a cross
disperser. Instead of having a linear spectrum
with intensity against wavelength the Echelle
using a cross disperser to create a two
dimensional grating with wavelengths across and
in a series of lines going down (like text in a
book). This optical design is ideal to fit
the shape and size of solid-state detector and
simultaneous detection is possible.
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Radial versus Axial Detection ICP plasmas can be
viewed either radially or axially. The normal
mode has been to monitor the plasma in radial
mode, because you can observe the hottest part of
the plasma directly. In recent years new systems
have also been design with radial viewing in
which the plasma is viewed end on. This
increases the sensitivity of the instrument to
ppb levels for many elements but you have to view
through the tail flame of the plasma and sheath
gases have to be used to overcome this problem.
 
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Practical Considerations in ICP atomic emission
spectrometry 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. This problem can easily be overcome
if you have some knowledge of the matrix. The
instrument software has wavelength tables that
you consult to see if there will be an
interference effect. f the matrix is unknown
preliminary optimisation experiments will be
necessary to check for interferences.
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Chemical Interferences The ICP AES operates at a
much higher temperature than the flame AAS and
therefore nearly all compounds are broken down in
the plasma. Ionisation effects can however be
seen from easily ionisable compounds. Internal
standards The emission signal in a plasma depends
on the temperature of the plasma (see Boltzman
equation). This means that if the temperature
of the plasma fluctuates during a run the
results will be affected. The way to overcome
this problem is to use the internal standards
method of calibration.
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ICP-Mass Spectrometry (ICP/MS) ICP mass
spectrometry is elemental mass spectrometry so
it is also very different from the organic mass
spectrometry you use for identification of
molecules in organic chemistry. In organic
chemistry a soft ionisation source is used to
produce molecular fragments for
interpretation. In elemental mass spectrometry
the molecule is completely broken down into its
elements.
Instrumentation The ICP is the ion source in
ICP/MS. Different types of mass spectrometers can
be used with the ICP. The most usual and least
expensive is the quadrupole mass spectrometer
(100K). For more specialised work a double
sector instrument can be used that give better
mass resolution but this is very expensive.
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The ICP is at high temperature and operates at
atmospheric pressure and the mass spectrometer
perates at room temperature under a vacuum.
An interface is need between the two. The ions
from the ICP are sampled into the mass
spectrometer using two nickel cones, the sampler
and the skimmer cones,which are about 10mm
apart. The cones have a 1mm orifice in the
centre and the first one is located in the
central channel of the ICP. The pressure is
reduced between the cones by a rotary pump. A
molecular supersonic beam is formed between the
cones and this reduces the temperature because
of the expansion of the plasma. Ion optics are
used to line up the ion beam and to trap photons
(that would give a noisy signal if they reached
the detector). The detector in the mass
spectrometer is an electron multiplie and this is
more sensitive than a photomultiplier.
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Advantages/Disadvantages The ICPMS is very
sensitive and can detect very low levels of
elements (sub ppb in many cases). You can also
measure different isotopes of an element and use
isotopic dilution methods instead of normal
calibrations. Isotopic dilution techniques give
very high accuracy and precision. Problems
include the blocking up of the skimmer cones if
salts are present in samples and mass
interferences as opposed to spectral
interferences. For example the species 40Ar16O
can be formed which has the same mass units as
56Fe. Applications
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