Transcript Slide 1

INTRODUCTION TO OPTICAL ATOMIC SPECTROSCOPY (Chapter 8)
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Atomic spectroscopy techniques:
Optical spectrometry
Mass spectrometry
X-Ray spectrometry
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Optical spectrometry:
Elements in the sample are atomized
before analysis.
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Atomization:
Elements present in the sample are
converted to gaseous atoms or elementary
ions. It occurs in the atomizer (see Table 81).
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Optical spectroscopy techniques:
Atomic Absorption Spectroscopy (AAS)
Atomic Emission spectroscopy (AES)
Atomic Fluorescence Spectroscopy (AFS)
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Optical Atomic Spectra
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Figure 8-1a shows the energy level diagram for sodium.
A value of zero electron volts (eV) is arbitrarily assigned to
orbital 3s.
The scale extends up to 5.14eV, the energy required to
remove the single 3s electron to produce a sodium ion.
5.14eV is the ionization energy.
A horizontal line represents the energy of and atomic orbital.
“p” orbitals are split into two levels which differ slightly in
energy:
3s → 3p: l = 5896Å or 5890Å
3s → 4p: l = 3303Å or 3302Å
3s → 5p: l = 2853.0Å or 2852.8Å
There are similar differences in the d and f orbitals, but their
magnitudes are usually so small that are undetectable, thus
only a single level is shown for orbitals d.
Spin-orbit coupling
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Multiplicity: number of possible orientations of
the resultant spin angular momentum = 2S +1
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Atomic Line Widths
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Widths of atomic lines are quite important in
atomic spectroscopy.
Narrow lines in atomic and emission spectra
reduce the possibility of interference due to
overlapping lines.
Atomic absorption and emission lines
consists of a symmetric distribution of
wavelengths that centers on a mean
wavelength (l0) which is the wavelength of
maximum absorption or maximum intensity
for emitted radiation.
The energy associated with l0 is equal to
the exact energy difference between two
quantum states responsible for absorption or
emission.
A transition between two discrete, singlevalued energy states should be a line with
line-width equal to zero.
However, several phenomena cause line
broadening in such a way that all atomic
lines have finite widths.
Line width or effective line width (Dl1/2) of an
atomic absorption or emission line is defined
as its width in wavelength units when
measured at one half the maximum signal.
Sources of broadening:
(1) Uncertainty effect
(2) Doppler effect
(3) Pressure effects due to collisions
(4) Electric and magnetic field effects
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Uncertainty Effect
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It results from the uncertainty principle
postulated in 1927 by Werner Heisenberg.
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One of several ways of formulating the
Heisenberg uncertainty principle is shown in
the following equation:
Dt x DE = h
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The meaning in words of this equation is as
follows: if the energy E of a particle or system
of particles – photons, electrons, neutrons or
protons – is measured for an exactly known
period of time Dt, then this energy is
uncertain by at least h/ Dt.
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Therefore, the energy of a particle can be
known with zero uncertainty only if it is
observed for an infinite period of time.
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For finite periods, the energy measurement
can never be more precise then h/ Dt.
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The lifetime of a ground state is typically long,
but the lifetimes of excited states are
generally short, typically 10-7 to 10-8 seconds.
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Line widths due to uncertainty broadening are
called natural line widths and are generally
10-5nm or 10-4Å.
Note: Dl = Dl1/2
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Doppler Effect
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In a collection of atoms in a hot environment,
such as an atomizer, atomic motions occur in
every direction.
The magnitude of the Doppler shift increases
with the velocity at which the emitting or
absorbing species approaches or recedes the
detector.
For relatively low velocities, the relationship
between the Doppler shift (Dl) and the velocity
(v) of an approaching or receding atom is
given by:
Dl / l0 = v / c
Where l0 is the wavelength of an un-shifted
line of a sample of an element at rest relative
to the transducer, and c is the speed of light.
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Dl
Emitting atom moving: (a) towards a photon
detector, the detector sees wave crests more often
and detect radiation of higher frequency; (b) away
from the detector, the detector sees wave crests
less frequently and detects radiation at lower
frequency.
The result is an statistical distribution of frequencies
and thus a broadening of spectral lines.
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Pressure Effects Due to Collisions
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Pressure or collisional broadening is
caused by collisions of the emitting or
absorbing species with other atoms or
ions in the heated medium.
These collisions produce small changes
in energy levels and hence a range of
absorbed or emitted wavelengths.
These collisions produce broadening that
is two to three orders of magnitude grater
than the natural line widths.
E2
Energy (eV)
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lA
E2
lE
lA’
E1
l E’
E1
Atom 1
Atom 2
Example: Hollow-cathode lamps (HCL):
Pressure in these lamps is kept really low
to minimize collisional broadening.
Glass tube is filled with neon or argon at
a pressure of 1 to 5 torr.
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The Effect of Temperature on Atomic Spectra
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Temperature in the atomizer has a profound effect on the
ratio between the number of excited an unexcited atomic
particles.
The magnitude of this effect is calculated with the
Boltzmann distribution equation:
Nj / N0 = (gj / g0) x [exp(-Ej/kT)]
where:
- Nj and N0 are the number of atoms in the excited state
and ground state, respectively
- k is the Boltzmann’s constant
- T is absolute temperature (K)
- Ej is the energy difference between the excited and the
ground state.
- gi and g0 are statistical factors called statistical weights
determined by the number of states having equal energy
at each quantum level.
Example shows that a temperature fluctuation of only 10K
results in a 4% increase in the number of excited sodium
atoms.
A corresponding increase in emitted power by the two
lines would result.
An analytical method based on the measurement of
emission requires close control of atomization
temperature.
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Band and Continuum Spectra Associated with Atomic Spectra
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When atomic line spectra are generated, both
band and continuum radiation are usually
produced as well.
Molecular bands often appear as a result of
molecular species in the atomizer. Molecular
species can be associated or not to the element
of interest. For instance, the molecular bands
shown in the figure can be used to determine Ca.
Continuum radiation appears as a result of
thermal radiation from hot particulate matter in
the atomization medium.
Molecular bands and continuum
radiation are a potential
source of interference
that must be minimized
by proper choice of
wavelength, by background
correction, or by change in
atomization conditions.
Molecular flame emission and flame absorption spectra for CaOH
Background emission spectra from an ICP. The upper recording was taken favoring
continuum and band emission while the lower recording was taken under conditions
minimizing continuum and band emission.
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Achilles’ heel of Atomic Spectroscopy
because in many cases limits the accuracy,
the precision and the limits of detection of
analytical method.
Primary purpose is to transfer a reproducible
and representative portion of a sample into
one of the atomizers presented in Table 8-1.
Table 8-2 lists the common sample
introduction
methods
for
Atomic
Spectroscopy and the type of samples to
which each method is applicable.
Atomizers “fit” into two classes: continuous
and discrete atomizers.
Continuous atomizers: flames and plasmas.
Samples are introduced in a steady manner.
Discrete
Atomizers:
electro-thermal
atomizers.
Sample
introduction
is
discontinuous and made with a syringe or an
auto-sampler.
Continuous
Sample Introduction Methods
Discontinuous
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Nebulizers
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(a)
(b)
(c)
(d)
Direct nebulization is the most common method of
sample introduction with continuous atomizers. The
solution is converted into a spray by the nebulizer.
Types of nebulizers:
Concentric tube:
most common nebulizer. It
consists of a concentric-tube in which the liquid
sample is drawn through a capillary tube by a highpressure stream of gas flowing around the tip of the
tube.
Cross-flow: the high pressure gas flows across a
capillary tip at right angles. It provides independent
control of gas and sample flows.
Fritted disk: the sample solution is pumped onto a
fritted surface through which a carrier gas flows. It
provides a much finer aerosol than a and b.
Babington: it consists of a hollow sphere in which a
high pressure gas is pumped through a small orifice
in the sphere’s surface of the sphere. It is less
subject to clogging than a-c. It is useful for samples
that have a high salt content or for slurries with a
significant particulate content.
Continuous
Atomizer
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Electro-thermal Vaporizers
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Discrete
Atomizer
Sample introduction in discrete atomizers is
typically made manually with the aid of a
syringe.
The steps that convert the liquid solution into a
vapor of free atoms are the same as those in
continuous atomizers.
The most common type of discrete atomizer is
the electro-thermal atomizer.
An electro-thermal atomizer is a small furnace
tube heated by passing a current through it
from a programmable power supply.
The furnace is heated in stages. The dry and
ash step removes water and organic or volatile
inorganic matter, respectively.
The atomization step produces a pulse of
atomic vapor that is probed by the radiation
beam from the hollow-cathode lamp (HCL).
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Continuous
Atomizer
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Free-Atom
Formation in
Atomizers
Discrete
Atomizer
Desolvation and Volatilization : desolvation leaves a dry aerosol of molten or solid particles. The solid
or molten particle remaining after desolvation is volatilized (vaporized) to obtain free atoms. The efficiency
of desolvation and volatilization depends on a number of factors: atomizer temperature, composition of
analytical sample (nature and concentration of analyte, solvent and concomitants) and size distribution. In
the case of nebulizers, it also depends on the nebulizer design, aerosol trajectories and resident times of
the particles.
Dissociation and Ionization: in the vapor phase, the analyte can exist as free atoms, molecules or ions.
In localized regions of the atomizer, molecules, free atoms and ions co-exist in equilibrium.
Dissociation of molecular species: molecular formation reduces the concentration of free atoms and
thus degrades the detection limits. The dissociation constant for a molecular species (MX) into its
components (MX <=> M + X) can be written as: Kd = nM.nX / nMX, where n is the number density (number
of species per cm3). For a diatomic molecule:
logKd = 20.274 + 3/2log MMMX/MMX + log ZMZX/ZMX + 3/2(logT) – 5040Ed/T
where Mi is the molecular or atomic weight of species i, Zi is the partition function of species i, Ed is the
dissociation energy in eV, and T is the temperature in K. Note: The final term in this equation describes
most of the temperature dependence: small values of Ed and high temperatures lead to large values of Kd
and thus high degrees of dissociation.
Ionization: it can also be consider an equilibrium process: M <=> M+ + e-. The ionization constant can be
written as: Ki = n M+ .ne / nM, where ne is the number density of free electrons. The ionization constant can
be obtained from:
logKi = 15.684 + logZM+ /ZM + 3/2(logT) – 5040Eion/T
where Eion is the ionization energy in eV. Note: The final term in this equation describes most of the
temperature dependence: small values of Eion and high temperatures favor the formation of ions.
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Atomic Absorption (AAS) and Atomic Fluorescence (AFS) Spectrometry
(Chapter 9)
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The two most common methods of sample
atomization encountered in AAS and AFS are flame
and electro-thermal atomization.
Flame atomization: A solution of the sample is
nebulized by a flow of gaseous oxidant, mixed with a
gaseous fuel and carried into the flame where
atomization occurs.
Oxidant (g)
Carrier (g)
Fuel (g)
Note:
• If the gas flow rate does not exceed the burning
velocity, the flame propagates back into the
burner, giving flashback.
• The flame is stable where the flow velocity and
the burning velocity are equal.
• Higher flow rates than the maximum burning
velocity cause the flame to blow off.
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Flame Structure
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Primary combustion zone: Thermal equilibrium is not achieved in
this zone and thus it is rarely used for flame spectroscopy.
Inter-zonal area: Free atoms are prevalent in this area. It is the most
widely used part of the flame for spectroscopy.
Secondary combustion zone: products of the inner core are
converted to stable molecular oxides that are then dispersed to the
surroundings.
Maximum temperature: it is located in the flame about 2.5cm above
the primary combustion zone.
Note: It is important to focus the same part of the flame on the
optical beam for all calibrations and analytical measurements.
Optimization: of optical beam position within the flame prior to
analysis provides the best signal – to – noise ratio (S/N). It depends
on element and it is critical for limits of detection.
Increased number of
Mg atoms produced
by the longer
exposure to the heat
of the flame.
Ag is not easily oxidized
so a continuous increase
in absorbance is
observed.
Secondary combustion
zone: oxidation of Mg
occurs. Oxide particles do
not absorb at the
observation wavelength.
Cr forms very stable
oxides that do not absorb
at this observation
wavelength.
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Commercial Flame Atomizers
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Typical commercial laminar-flow burner.
The aerosol formed by the flow of oxidant is
mixed with fuel and passes a series of
baffles that remove all but the finest solution
droplets.
The baffles cause most of the sample to
collect in the bottom of the mixing where it
drains to waste container.
The aerosol, oxidant, and fuel are then
burned in a slotted burner to provide a 5- to
10-cm high flame.
The quiet flame and relatively long-path
length minimizes noise and maximizes
absorption. These features result in
reproducibility and sensitivity improvements
for AAS.
AA
Spectrometer
Excitation
source
Flame
Monochromator
Detector
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Commercial Electro-thermal Atomizers
Cylindrical graphite
electrical contacts. These
contacts are held in a
water-cooled metal
housing.
Cylindrical graphite tube
where atomization
occurs. Dimensions:
about 5cm long and 1cm
internal diameter.
This tube is
interchangeable.
L’vov platform. Made of
graphite, sample is evaporated
and ashed on this platform.
Temperature on the platform
does not change as fast as it
changes in the walls of the
furnace. Atomization occurs in
an environment where
temperature does not change
so fast, which improves
reproducibility of
measurements.
Facilitates furnace cleaning,
which reduces memory effects.
Output signal
Longitudinal (b) and transversal (c) furnace heating.
Transversal mode is preferred because it provides a uniform
temperature profile along the entire length of the tube and
optical path.
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Flame Atomizers versus Electro-thermal Atomizers
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Advantages of Flame Atomizers
Better reproducibility of measurements
RSD:
Flame ≈ 1%
Electro-thermal ≈ 5% - 10%
Much faster analysis times than electrothermal atomizers
Wider linear dynamic ranges, up to 2 orders
of magnitude wider than electro-thermal
atomizers.
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Advantages of Electro-thermal Atomizers
Smaller sample volumes (0.5mL to 10mL of
sample) than flame atomizers.
Better absolute limits of detection (ALOD ≈
10-11 to 10-13 g of analyte) than flame
atomizers
Note: ALOD = [Sample Volume] x LOD
Electro-thermal atomization is the method of choice when flame atomization provides
inadequate limits of detection or sample availability is limited.
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Specialized Atomization Techniques
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Glow-Discharge Atomization
Cold-Vapor Atomization
Hydride Atomization
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Hydride Atomization
It provides a method for introducing samples
containing arsenic (As), antimony (Sb), tin (Sn),
bismuth (Bi) and lead (Pb) into the atomizer as a
gas.
This procedure improves limits of detection 10 to
100x.
Their determination at low levels is very important
because of their high toxicity.
Volatile hydrides are generated by adding an acidified
aqueous solution of the sample to a small volume of a
1% aqueous solution of sodium borohydride contained
in a glass vessel. A typical reaction is:
3BH4-(aq) + 3H+(aq) + 4H3AsO4(aq) →
3H3BO3(aq) + 4AsH3(g) + 3H2O (l)
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The volatile hydride is swept into the atomization
chamber by an inert gas.
The chamber is usually a silica tube heated to several
hundred degrees in a furnace or in a flame where
atomization takes place.
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Atomic Absorption Instrumentation
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Main components:
(1) Radiation source
(2) Sample holder = atomizer
(3) Wavelength selector
(4) Detector
(5) Signal processor and readout
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Radiation source:
Why not using a broadband source with a
monochromator for excitation?
The emission profile of the source should have a
narrower effective bandwidth than the absorption
line of the element of interest.
HCL and electrodeless discharge lamps (EDL)
satisfy this condition.
Disadvantage
over
broadband
source/monochromator: One source per element.
HCL
EDL
EDL provide intensities one to two orders of magnitude
better than HCL. EDL are only available for ≈ fifteen
elements. Particularly useful for Se, As, Cd and Sb
because it provides better LOD.
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Source Modulation
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The output of the source is modulated so its
intensity fluctuates at a constant frequency.
The detector receives two types of signal, an
alternating signal from the source and a continuous
signal from the flame.
A high-pass RC filter is then used to remove the
continuous signal and pass the alternating signal for
amplification.
Source modulation can be done with a chopper or
rotating disk or a power supply.
Emission from the sample + emission from the flame
Monochromator is able to eliminate flame interference based
on wavelength separation. However, when the wavelength of
interference is the same as the analyte wavelength the
monochromator is unable to eliminate interference.
High-pass filter
H2 - O2
Choppers
C2H2 – O2
C2H2-N2O
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Spectrophotometers for AAS
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Single beam and double beam instruments.
Double beam instruments provide the advantage
of correcting for source fluctuations. In this
arrangement, however, the reference beam does
not pass through the flame and, therefore, it does
not correct for loss of radiant power due to
absorption or scattering by the flame itself.
Loss of radiant power due to absorption or
scattering in the flame could have different
sources:
(a) fuel and oxidant mixture alone
(b) concomitants in sample matrix
(c) all of the above
When the source of loss of radiant power is only
due to the fuel and the oxidant of the flame the
solution is simple: make blank measurements
and correct analytical data. This type of
correction should be done with both types of
spectrophotometers, i.e. single and double beam.
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Loss of radiant power due to absorption or
scattering from concomitants in the sample matrix
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a)
b)
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(1)
(2)
(3)
Example of spectral matrix interference due
to absorption: presence of CaOH in the
analysis of Barium.
Solution: raise the temperature of the flame.
Higher temperatures will decompose CaOH
and remove its potential interference.
Example of spectral interference due to
scattering by products of atomization: most
often encountered when particles with
diameters greater than the absorption
wavelength are formed in the flame:
Concentrated solutions containing Ti, Zr, and
W. These elements form refractory oxides.
Organic species or when organic solvents are
used to dissolve the sample. Incomplete
combustion of the organic matter leaves
carbonaceous particles.
Methods for correcting spectral interference:
The two-line correction method
The continuum source correction method
Zeeman background correction method
Scattering
HCL
l0
F > l0
F
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It takes place when an atomic vapor is
exposed to a strong magnetic field
(≈10KG).
It consists of a splitting of electronic
energy levels, which leads to formation
of several absorption lines for each
electronic transition.
The simplest splitting pattern is
observed with singlet transitions, which
leads to a central (p) line and two
equally spaced (s) satellite lines (≈
0.01nm).
The central line corresponds to the
original wavelength. It has an
absorbance
that
is
twice
the
absorbance of the satellite lines. Both
s lines have the same intensity.
Energy
Zeeman Effect
No
Yes
A
No
l
A
p
s-
Yes
s+
l
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Background Correction Based on the Zeeman Effect
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The behavior of p and s lines is different with respect to plane polarized radiation:
p lines absorb plane polarized radiation parallel to the external magnetic field (II).
s lines absorb plane polarized radiation perpendicular (90°) to the external magnetic field.
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Ionization Equilibria
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Ionization of atoms and molecules in atomizers
can be represented by the equilibrium:
M <=> M+ + eThe equilibrium constant for this reaction is:
K = [M+] [e-] / [M]
The ionization of a metal will be strongly
influenced by the presence of other ionizable
metals in the flame:
B <=> B+ + eThe ionization of M will be decreased by the
mass-action effect of the electrons formed from
B.
B can then act as an ionization suppressor.
Ionization suppressors are often added to the
flame to improve sensitivity of analysis.
Ionization suppressors are commonly used with
higher temperature flames such as N2Oacetylene.
The concentration of ionization suppressor
needs to be controlled to avoid primary inner
filter effects, i.e. absorption of excitation
radiation.
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Atomic Fluorescence Spectroscopy (AFS)
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There are five basic types of fluorescence: resonance fluorescence, direct-line fluorescence,
stepwise-line fluorescence, sensitized fluorescence and multi-photon fluorescence. The figure
shows an energetic diagram level for resonance fluorescence.
In all cases, the basic instrumentation is the same. EDL are the best excitation sources for AFS.
The advantage of AFS over AAS is that it provides better limits of detection for several elements.
Energy
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Atomic Emission Spectrometry (Chapter 10)
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The figure shows the typical configuration of a flame or
plasma emission spectrometer.
There are three primary types of high temperature
plasmas:
ICP = inductively coupled plasma
DCP = direct current plasma
MIP = microwave induced plasma
ICP and DCP are commercially available.
Both types of plasmas sustain temperatures as high as
10,000K.
ICP:
• It consists of three concentric quartz tubes through which streams of argon
flows. The diameter of the largest tube is 2.5cm.
• Surrounding the top of the largest tube is a water-cooled induction coil that
is powered by a radio-frequency generator, which radiates 0.5 to 20KW of
power at 27.12MHz or 40.68MHz. This coil produces a fluctuating magnetic
field (H).
• Ionization of the flowing argon is initiated by a spark from a Tesla coil.
The interaction of the resulting ions, and their associated electrons, with H
makes the charges to flow in closed annular paths.
• The resistance of ions and electrons towards the flow of charges causes
ohmic heating of the plasma.
• The tangential flow of argon cools the internal walls of the ICP.
• Spectral observations are generally maed at a height of 15 to 20mm above
the induction coil, where the temperature is 6,000-6,500K and the region is
“optically transparent” (low background).
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DCP
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It consists of three electrodes configured in an inverted Y. A
graphite anode is located at each arm of the Y and a tungsten
cathode at the inverted base.
Argon flows from the two anode blocks toward the cathode.
The plasma is formed by bringing the cathode into momentary
contact with the anodes.
Ionization of the argon occurs an a current develops that
generates additional ions to sustain the current indefinitely.
The temperature at the arc core is between 5,000K and 8,000K.
ICP versus DCP:
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DCP present lower background than ICP.
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ICP is more sensitive than DCP; LOD in ICP are approximately one order of magnitude better.
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DCP and ICP have similar reproducibility of measurements.
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DCP requires less argon usage.
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ICP is easier to align because the optical window of a DCP is relatively small.
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Graphite electrodes must be replaced every few hours, whereas the ICP requires little maintenance.
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Instrumentation
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The main two types of instruments for AES
fit into two general categories: sequential or
multi-channel spectrometers.
Sequential instruments are designed to read
one line per element at the time.
Multi-channel instruments are designed to
measure simultaneously the intensities of
emission lines for a large number of
elements (50 or 60 elements).
Both instruments require wavelength
selectors with high spectral resolution.
Sequential instrumentation:
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It uses a slew-scanning monochromator.
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Hg lamp is used for calibration.
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Two PMT, one is used for the UV and
the other for the VIS.
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A flipping mirror selects the exit slit and
the PMT.
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Scanning Echelle instrumentation
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It can be used either as a single channel or as a
“simultaneous multi-channel spectrometer”.
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Scanning is accomplished by moving the PMT in both x and
y directions to scan an aperture plate located on the focal
plane of the monochromator.
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The plate contains as many as 300 slits. The time it takes to
move the PMT from one slit to another is approximately 1s.
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This arrangement can be converted to a multi-channel
system by placing small PMT behind several exit slits.
Polychromators
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The entrance slit, the exit slits, and the grating
surface are located along the circumference of a
Rowland circle.
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The curvature of a Rowland circle corresponds to
the focal plane of a monochromator.
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Each exit slit is factory configured to transmit lines
for selected elements.
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The entrance slit can be moved tangentially to the
Rowland circle to provide scanning.
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Charge-Coupled Device Instrumentation
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Typically incorporate two CCD, one for the
UV (165nm – 375nm) and one for the VIS
(375nm – 782nm).
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The Schmidt cross-disperser separates the
UV from the VIS radiation and the orders at
each emission wavelength.
Elements Determined
Tl and Nb curves are not linear probably because of incorrect background
subtraction. Self-absorption is another cause of non-linearity. It occurs at
high concentrations where the non-excited atoms absorb emitted radiation.
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