Spectrophotometer Design
• Based on
• Characteristics of the radiation source:
– Single beam
– Double beam
• Characteristics of the wavelength selector
– Non-dispersing
– Dispersing
• Characteristics of the detector
– Single channel
– Multi-channel
1
Single Beam and Double beam
Spectroscopic Methods
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Spectroscopic Methods
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Types of Optical Spectrometers
• Three types:
– Single channel
– Multi-channel
– Multiplexing spectrometers.
• Single Channel: Give information at a single wavelength or record a
spectrum sequentially. 4 types:
– Fixed wavelength spectrometers with no scanning mechanism
– Conventional scanning spectrometer – with a motor to turn a prism or
grating scanning.
– Rapid scanning spectrometer – with mechanism to oscillate the prism
or monochromator very rapidly  complete scan in second or milliseconds; requires use of v. Fast transducer, e.g., photomultiplier.
– Double-Beam spectrometer – compensates for source drift and
changes in monochromator efficiency with wavelength.
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Multi-channel Spectrometers
• Measure the spectral information at multiple
s simultaneously
• Consist of a dispersive system coupled to
either
– A Polychromator with multiple exit slit and
detectors, or
– A spectrograph with a multichannel detector, e.g.,
diode array.
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Multiplexing Spectrometers
• Note the majority of analytical techniques are based
on the wavelength as the basis for information
selection.
• Multiplexing spectrometers have additional criteria to
improve selectivity, e.g.,
– Time-division multiplexing – allows time sharing of a single
optical path or single detector, by multiple optical signals,
e.g., Double-beam-in-time spectrometer.
– Frequency-division multiplexing – Here all s strike a
single-channel detector simultaneously. The spectral
information at each  is encoded in such a way that it can
be resolved using mathematical transform techniques,
e.g., Fourier Transform Spectroscopy.
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Analytical Atomic Spectroscopy
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Techniques for Analytical atomic
spectroscopy
• Atomic Absorption Spectroscopy
– Flame
– Electrothermal
• Atomic Emission Spectroscopy
– Flame
– Plasma
• Atomic Fluorescence Spectroscopy
– Flame
– Laser
Spectroscopic Methods
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Techniques for Analytical Atomic
Spectroscopy (cont.)
• Atomic Ionization Spectroscopy
– Laser enhanced ionization spectroscopy
– Atomic Mass spectroscopy
• Atomic X-ray Spectroscopy
– X-ray Fluorescence
– X-ray absorption
– X-ray diffraction
– Electrone micro-probe
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Atomic Absorption Spectrometry
(AAS): Background
• Developed by Walsh in 1955
– Ref. A. Walsh, Spectrochimica Acta, 7, 108(1955);
353, 643(1980).
• Now the most widely used technique for
elemental analysis.
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AAS Principle
• E.g., for Na, electronic formula:
1s 2 2s 2 2 p 6 3s1 3 p o 4s o 3d o 4 p o 4d o
•
Ground stateHigher levels, empty
• Let the energy difference between the ground state and a higher energy
level be E, then from quantum mechanics
(1)
E  h
– Where h = Planck’s constant
–  = the frequency of the radiation
= c/
– C = velocity of electromagnetic radiation
–  = wavelength of the radiation.
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Implications of Eq. 1
• It is possible to excite an electron from the ground
state of an atom to a higher energy level by
irradiating the atom with radiation whose
wavelength is given by
hc

•
(2)
E
• Provided certain selection rules are satisfied.
• The process is represented thus
• Na + h  Na*
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AAS Experiment
hv
Io
Free
atoms
It
b
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AAS Experiment (cont.)
• Imagine some free atoms in a suitable
container which is irradiated with radiation of
  hc E
• Let the intensity of the incident radiation be Io,
and that of the transmitted radiation be It.
• It can be shown that
 abNo

I t  I o 10
•
(3)
• Or log I t I o  abNo  abC
(4)
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Symbols in Eqs 3 & 4
• a = a constant characteristic of the atom being irradiated
• b = the path traversed by the radiation through the container
of free atoms
• No = The number of free atoms per unit volume, i.e.,
the number density
 C, the concentration of free atoms in the
container,
• Note Eqs 3 & 4 are statements of the Beer-Lambert Law
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Beer-lambert (or Beer’s) Law
• For monochromatic radiation, absorbance, A,
is directly proportional to the path length, b,
through the medium, and the concentration,
c, of the absorbing species, i.e.,
•
A  abC
(5)
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Basis for quantitative atomic
absorption spectrometry
• From Eq. 4, a plot of log Io/It vs No (or C) is a st.
line
A
(Log Io/It)
C
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Thermal excitation of Atoms
• Note excitation of atoms can also be
accomplished through application of heat
• Excited states produced thermally can also
relax to the ground state radiatively
• When this happens, the atom is said to EMIT
radiation
• The energy emitted is referred to as EMISSION
•  Atomic Emission Spectrometry (AES).
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Atomic Emission Spectrometry
• In AES, the emission intensity, IE , is directly
proportional to the population of the excited
state, i.e.,
• IE  Nu
• This is the basis of quantitative atomic
emission spectrometry.
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AES (cont.)
• Note in thermal excitation, Nu is a function of temperature,
and is related to No by the Boltzmann population distribution
law for electronic bound states:
g
N u  N o  u e E / kT
 go 
• gu and go = statistical weights of excited and ground states
respectively
• K = Boltzmann constant = 1.380662x10-23 JK-1
• T = absolute temperature.
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Atomization Methods
• Note before atomic spectrometric measurements can be made, FREE
ATOMS of the element have to be generated.
• Numerous methods have been developed for generating atomic vapours:
–
–
–
–
–
–
–
Flame methods
Electrothermal methods
Direct current arcs
AC arcs
Microwave plasmas
Radio frequency plasmas
Laser based methods
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Instruments for Atomic Absorption
Spectrometry (AAS).
Power
Supply
Radiation
Source
Atomizer
Wavelength
selector
Transducer
Signal
Processor
& readout
Sample
introduction
system
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Instruments for Flame Atomic
Absorption Spectrometry (FAAS).
Power Supply
(modulated)
Hollow
Cathode
Lamp
Atomizer
Monochromator
Detector
Signal
Processor
Readout
A = abC
Sample
introduction
system
Source: Christian & O’Reiley.
Atomic Spectroscopy.
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Performance of the AAS Spectrometer
- Depends on:
•
•
•
•
•
•
Stability of power source.
Type of radiation source
Efficiency of atomization process
Efficiency of the monochromator
Sensitivity of the detector
Design of the spectrometer.
Atomic Spectroscopy.
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Common radiation source for AAS: The
Hollow Cathode Lamp
Anode
Fill gas
Ne or Ar (1 – 5 torr)
300 – 400 V
3 – 25 A
Quartz
window
Socket
Glass
Shield
Hollow
Cathode
Glass
envelope
Source: Christian & O’Reiley.
Atomic Spectroscopy.
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• It looks like any other lamp but its uniqueness lies in
the fact that its cathode is made of the same
element which you have to determine in your
sample
This means that you have to use a different lamp for
every element whose analysis is required.
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Efficiency of the HCL – depends on:
• Its geometry;
– cylindrical configuration of the cathode concentrates the
radiation in a limited region of the metal tube/cup.
– Enhances the probability of redeposition of sputtered
metal in the metal cup, rather than the glass walls.
• Operating potential
– High potentials  high currents  greater intensity of
emitted radiation.
– Note high currents  greater vaporization  increased
number of ground state atoms in atomic cloud, capable of
absorbing some of the radiation emitted by excited atoms.
– This self-absorption leads to reduced intensities
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HCL: Advantages & disadvantages
• Advantages:
– Available for most elements
– Produce very narrow emission lines,  high specificity
for each element.
• Disadvantages
– The need for a different HCL for each element,  HCL
based AAS essentially a single element technique.
– Self-absorption at high currents
– Doppler broadening of emitted lines at high currents.
– Optimum lamp current different for different
elements.
Atomic Spectroscopy.
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The Flame Atomiser: Types.
• Direct injection or total consumption burner
– Fuel and oxidant mixed externally to the burner
– Sample solution sprayed directly into flame centre.
– Produces turbulent flames
• Premixed or Laminar-flow burner
– Sample, oxidant, fuel mixed before introduction into
flame.
– Provides relatively quiet flame, enhances sensitivity.
Atomic Spectroscopy.
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The Flame Atomiser: Laminar Flow
Burner.
Flame
Burner
Auxiliary
oxidant
Fuel
Nebulizer
Impact
bead
Blowout
plug
Capillary
tube
Spray Chamber
Sample
solution
Nebulizer
oxidant
To Drain
Flow spoilers
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Laminar Flow Flame Atomizer.
• Consists of
– Sample aspiration tube
– A concentric tube nebulizer
– A spray chamber
– A slot-type burner
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Sample introduction in FAAS
• Sample must be solution
• Can be aqueous solution, usually acidified to
keep the metal in solution.
• Organic solution, e.g., after complexation and
extraction of the organic complex
• Sample introduced via the nebulizer – most
commonly the pneumatic concentric tube
nebulizer.
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FAAS Spectrometer Design
• On basis of optics
– Single beam
– Double beam
• Single beam: 3 steps involved in taking absorbance or
emission readings
– Zero reading with shutter in front of transducer
– 100% T (or Io) taken while aspirating blank solution into flame
– Unknown T (or Io) while aspirating sample solution into flame
• Disadvantages: Flame conditions, e.g., temperature, can
change in between the 3 steps, leading to incorrect
readings.
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Double beam FAAS
• Beam from the HCL is split by a half-mirrored
chopper
– ½ passes through to flame (Sample beam)
– ½ directed around the flame (Reference beam)
• The 2 beams are recombined after the flame by a
½-silvered mirror, before passing through the
monochromator to the detector.
• Detector alternately senses Ref. Beam and
sample beam
– Ref. Beam  100%T (or Io )
– Sample beam  T (or IT ).
Atomic Spectroscopy.
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Double beam FAAS (cont.)
• Advantages:
– Io and IT measured simultaneously
• Disadvantages:
– Ref. Beam does not pass through the flame,
therefore does not correct for absorption or
scattering of radiant power by the flame itself.
– Methods for correcting for these loses have to be
incorporated into the instrument itself or into the
method.
Atomic Spectroscopy.
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Background correction methods
• Built into Instrument:
– Continuum source correction method
– Zeeman background correction method
– Pulsed Hollow Cathode Lamp method
• Method based:
– Two-line method
– Radiation buffer method
– Standard addition method.
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Continuum Source Background
Correction method
• Employs a Deuterium lamp – provides continuous
radiation throughout the UV region.
• Radiation from the D2 lamp and HCL are
alternately passed through the flame.
• The absorbance of the radiation from D2 lamp 
Abg broad band absorption or scattering by
sample matrix
• Absorbance of beam from HCL  AT = Analyte
absorbance + background absorbance.
• Analyte absorbance, Aan = AT - Abg
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Two-line Method
• Uses a second line from the source  Reference line
–
–
–
–
–
Ref. line should be as close as possible to analyte line
Should not be absorbed by the analyte
Can be another analyte line from HCL
Can be impurity line in HCL
Can be line from HCL filler gas, He or Ar
• If these conditions are met, then any decrease in
intensity of Ref. Line 
– Abg = Absorption &/or scattering by sample matrix
– Aan = AT - Abg
– Assumption: Background absorbance is the same at both
wavelengths.
Atomic Spectroscopy.
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X – RAY FLUORESCENCE
SPECTROSCOPY
3/12/2013
CH 208 Spectroscopic Methods
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