Transcript Slide 1

INTRODUCTION
TO OPTICAL
METHODS
Many analytical methods are based on the interaction of radiant
energy with matter.
THE NATURE OF RADIANT ENERGY
Dual nature of electromagnetic energy – behaves as:
- waves or
- discrete packets of energy (photons)
Recall:
E  h
c


hc
E

h = Planck’s constant = 6.62610-34 J s
 = Frequency
 = Wavelength
c = velocity of radiation = 2.998108 m s-1 through a vacuum
Energy
All electromagnetic radiation travels at the same speed, c
The interactions of radiations with chemical systems
follow different mechanism and provide different kinds
on information.
Atomic/
molecular
transitions:
Valence
electrons
Molecular
vibrations
Molecular
rotations
INTERACTION OF RADIATION WITH MATTER
Electron configuration of Na: 1s2 2s2 2p6 3s1
wavelengths
Outer valence
electrons can absorb
photons and move to
higher energy level
Ground state
Partial energy level diagram for valence electrons in sodium atoms.
Irradiated with light containing wavelengths 589.00 and 589.59 nm
 outer valence electrons absorb photons and transfer to 3p levels
excited state absorb photons
h
Excited electrons have a strong tendency to return to ground state
 emit photons of definite amount of energy
h
Ground state
Na line (589 nm):
3p  3s transition
Analytical application of resonance absorption and radiation
= atomic absorption spectrometry
Other alkali metals also emit characteristic colours when placed in
a high temperature flame.
Li line:
2p  2s transition
K line:
4p  4s transition
EMISSION
With a highly energetic source, many electrons (not only outer
electrons) can be excited to varying degrees
 Resulting radiation contains many discrete and reproducible
wavelengths
 Mostly in UV-Vis regions
Analytical application = emission spectrometry
FLUORESCENCE
The energy gained by a molecule on the absorption of a photon
does not remain in that molecule, but is lost by several
mechanisms.
For example:
Part of the energy is converted to heat, lowering the net energy of
the molecule to the lowest vibrational and rotational level within the
same electronic level
The remainder of the energy is the radiated, returning the molecule
to the ground state

FLUORESCENCE
heat
h
Lowering of energy
to the lowest
vibrational and
rotational level
within the same
electronic level
The remainder of
the energy is the
radiated, returning
to the ground state
Radiation
source
QUANTITATIVE ANALYSIS
The intensity of the response for each analyte must be calibrated
with standard solutions of known concentration of each analyte.
A calibration curve of signal vs concentration of analyte is then
drawn for each analyte.
Use concentration range where:
- calibration is linear
i.e. concentrations must not be too high to prevent curvature
- concentrations are high enough to give good signal-to-noise ratios
Match standards to samples as far as possible.
Intensity will depend on instrument parameters, therefore need to
calibrate each time instrument is turned on or the setting are
changed.
If a large batch of samples are being analysed at once, check signal
of standard periodically to ensure the is no drift in the signal.
Analyse reference materials to check accuracy.
NOMENCLATURE
Sensitivity:
Related to
- signal-to-noise ratio
- detection limit
Resolution:
Related to
- peak overlap
- selectivity
ATOMIC ABSORPTION
SPECTROMETRY
At sufficiently high temperatures most compounds decompose into
atoms in the gas phase.
Electronic transitions can then occur when energy is absorbed or
emitted.
In atomic spectroscopy:
Samples vapourised at 2000-6000 K
Signal measured - atomic absorption or emission at characteristic
wavelengths
High sensitivity – ppm levels
1 ppm = 1mg/kg
 1 mg/L for aq solutions
High resolution
– ability to distinguish one element from another in
complex samples
Ability for simultaneous multi-element analysis
1-2% precision – not as good as some wet chemical methods
FLAME AAS
Flame temperature = 2000-3000 K
Solution is aspirated into a flame
 causes solvent to evaporate
 remaining solid is atomised in flame
Some of these atoms can absorb radiant energy of a characteristic
wavelength and become excited to a higher electronic state.
In atomic absorption, energy from a light source is absorbed
 the radiant power decreases as it is transmitted through the
flame
The higher the concentration of a solution
 the more atoms there are
 the more radiation is absorbed.
Atomic absorption
FLAME
SOURCE
k = absorption coefficient
P 
ln o   k b
P
b = path length
Po = intensity of source
P = intensity of radiation measured
or
 Po 
A  log   k b loge
P
A = absorbance
Therefore:
A  k  concentration
Recall:
The higher the concentration of a solution
 the more atoms there are
 the more radiation is absorbed.
INSTRUMENTATION
~10 cm
HOLLOW CATHODE LAMP
Energetic Ne+ or Ar+ ions accelerated
towards and bombards cathode where atoms
vapourise and emit radiation
Apply potential
such that currents
of 1-50 mA flow
Contains inert
gas (Ne or Ar)
Inert gas ionises
at anode.
To create frequencies of radiation that are absorbed by the analyte,
the cathode must be of the same element as the analyte.
MONOCHROMATOR
 tuned to a specific wavelength and slit width
 separates the selected absorption line from other lines emitted
from the source
DETECTOR
 measures the amount of light that passes through the flame
(the rest is absorbed)
NEBULISER AND BURNER
Sample must be in the form of small droplets when it passes into
the flame – done by the nebuliser.
(Larger drops)
Droplets are
mixed with
combustion gas
Sample drawn up in
capillary by
decreased pressure
of expanding gas –
Venturi effect
Large droplets condense
(support gas)
Maximum flame temperatures:
!!!
Air-acetylene flame – most common, BUT…
- Some elements need hotter flame to atomise fully
- Some elements form refractory oxides in the flame which are not
atomised at the lower temperatures
Acetylene-nitrous oxide flame:
- reducing flame prevents oxide formation
- high temperatures remove many chemical interferences
BUT increased ionisation of many elements occurs at higher
temperatures:
e.g. Na  Na+ + e Results in loss of sensitivity (fewer neutral atoms)
NB: Careful when lighting and turning off the burners
– the order is important!
For example:
First light and air-acetylene flame, then convert to nitrous
oxide-acetylene flame. Reverse order for turning off.
Use the correct burner for the type of flame used
 hotter flame, narrower and shorter slot.
OPTIMISATION OF SIGNAL
 Choice of wavelength
 Ratio of gases in mix
 Aspiration rate of solution
 Height of burner  position of measurement in flame
Monitor absorption while aspirating solution of test element
and adjusting conditions.
FURNACE ATOMISERS
Instead of using a flame to atomise the sample, a furnace can be
used.
Heating occurs in an inert atmosphere to prevent oxide formation
Produces significantly lower detection limits than flame AAS.
Much smaller sample size is required.
BUT:
Interferences are great
Precision is poorer
Graphite
furnace
QUANTITATIVE ANALYSIS
See section under Optical Methods!
Interferences:
There are a range of interferences which can affect the absorption
signal which could lead to erroneous results.
A few of these are mentioned here.
Chemical interferences:
 Analyte element combines with other elements and production of
neutral atom in flame is decreased.
e.g.
Ca2+ combines with PO43- to produce calcium
pyrophospate in the flame
 Add releasing agent
e.g. EDTA complexes with Ca2+
 Matrix match standards
 Acids frequently cause depression in signal
 Matrix match standards
Ionization interferences:
 Some elements ionise easily in the flame e.g. alkali metals
 cause decrease in no. of atoms in flame
 decrease in sensitivity
 Add ionisation suppressant
high concentration (~200-1000 ppm) of other easily
ionisable elements e.g. Na, K to
(suppresses ionisation of analyte element)
 Matrix match standards
Physical interferences:
 Altering physical properties of sample solution
e.g. viscosity affects aspiration, nebulisation etc.
INDUCTIVELY COUPLED
PLASMA OPTICAL EMISSION
SPECTROMETRY (ICP-OES)
EMISSION SPECTROMETRY
Excitation sources powered by electrical energy
(we will consider the ICP source)
• Excitation source transforms the sample to a plasma of atoms,
ions etc. that can be electronically excited.
• Deactivation of these excited states produces radiation which are
sorted by wavelength.
Recall: Every element has characteristic spectra.
Simultaneous multi-element determinations!!
Atomic
emission
ICP DISCHARGE
ICP discharge is caused by the
effect of a radio frequency field
on a flowing gas.
Coil is energised by radio
frequency generator (5-75 MHz).
Ar(g) flows upward and
transports sample through a
quartz tube inside a copper coil
or solenoid.
The radio frequency signal
causes a changing magnetic
field inside the coil in the
flowing Ar(g).
The changing magnetic
field induces a circulating
(eddy) current in the Ar(g)
which in turn heats the
Ar(g).
Coolant gas to
protects quartz tube
from hot plasma
Forms a stable plasma that is extremely hot.
Quartz tube
Radio frequency load coil
Solution droplets
formed in the spray
chamber.
The solvent is
evaporated from the
solution droplets.
Only dried particles
flow with the argon to
the plasma.
NOTE:
There are other sources of radiation other than ICP that are used in
emission instruments, e.g.:
- AC or DC arc
- Spark
- Microwave plasma dicharge
- Laser microprobe
QUANTITATIVE ANALYSIS
See section under Optical Methods!
Internal standards used to minimise effect of variation in
instrument response
- useful for multi-element techniques
Interferences:
Some chemical interferences are reduced due to high temperatures
of the plasma
Spectral interferences:
 Spectral overlap as light is emitted by many different elements in
the sample (at the same wavelength)
DETECTION LIMITS OF SOME SPECTROMETRY
TECHNIQUES
NOTE:
GFAAS is more sensitive than FAAS
ICP-MS has extremely low detection limits