Transcript Interferences in AAS and AFS

Optical Atomic Spectroscopy
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Optical Spectrometry
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Absorption
Emission
Fluorescence
Mass Spectrometry
X-Ray Spectrometry
Optical Atomic Spectroscopy
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Atomic spectra: single external electron
Slightly
different
in energy
Atomic spectrum Mg
Spins are paired
No split
Singlet ground state
Singlet excited state
Spins are unpaired
Energy splitting
Triplet excited state
Atomic spectroscopy
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Emission
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Absorption
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Fluorescence
Line Broadening
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Uncertainty Effects
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Heisenberg uncertainty principle:
The nature of the matter places limits on the
precision with which certain pairs of physical
measurements can be made.
One of the important forms Heisenberg uncertainty
principle:
t ≥ 1
p156
To determine  with negligibly small uncertainty, a huge measurement time
is required.
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Natural line width
n Should be 
Superposition of tw sinusoidal wave of different frequencies but identical
amplitudes.
Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007
7
Line Broadening
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Doppler broadening
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Doppler shift:
The wavelength of radiation emitted or
absorbed by a rapidly moving atom decreases
if the motion is toward a transducer, and
increases if the motion is receding from the
transducer.
In flame, Doppler broadening is much
larger than natural line width
Line Broadening
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Doppler broadening
Line Broadening
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Pressure broadening
Caused by collisions of the emitting or
absorbing species with other ions or atoms
High pressure Hg and xenon lamps,
continuum spectra
Temperature Effects
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Bolzmann equation
Nj
N0
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
gj
g0
exp(
Nj
N0

gj
g0
E j
E j
kT
exp(
kT
)
)
Effects on AAS, AFS, and AES
Atomic spectroscopy
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Interaction of an atom in the gas phase
with EMR
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Samples are solids, liquids and gases but
usually not ATOMS!
Atomic Spectroscopy
Sample
Introduction
Flame
Furnace
ICP
Sources
for Atomic Absorption/Fluorescence
Hollow
Sources
Cathode Lams
for Atomic Emission
Flames
Plasmas
Wavelength
Separators + Slits +Detectors
How to get things to atomize?
How to get samples into the
instruments?
Sample Introduction
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Pneumatic Nebulizers
Break the sample solution into small droplets.
 Solvent evaporates from many of the droplets.
 Most (>99%) are collected as waste
 The small fraction that reach the plasma have
been de-solvated to a great extent.
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What is a nebulizer?
SAMPLE
AEROSOL
Concentric Tube
Cross-flow
Fritted-disk
Babington
What happens inside the flame?
FLAMES
Rich in
free atoms
FLAMES
TE
GOOD AND BAD THINGS
oxidation
Boltzmann Equation: Relates Excited State
Population/Ground State Population Ratios to
Energy, Temperature and Degeneracy
N* g*
 ( )  e -(E/RT)
No
go
Flame AAS/AES Spray Chamber/Burner
Configurations
Samples
are nebulized (broken into small droplets)
as they enter the spray chamber via a wire capillary
Only
about 5% reach the flame
Larger
Some
droplets are collected
of the solvent evaporates
–Flow spoilers
»Cheaper, somewhat more rugged
–Impact beads
»Generally greater sensitivity
ElectroThermal AAS (ETAAS or GFAAS)
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The sample is contained in a heated, graphite
furnace.
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The furnace is heated by passing an electrical
current through it (thus, it is electro thermal).
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To prevent oxidation of the furnace, it is sheathed in
gas (Ar usually)
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There is no nebulziation, etc. The sample is
introduced as a drop (usually 5-20 uL), slurry or
solid particle (rare)
ElectroThermal AAS (ETAAS or GFAAS)
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The furnace goes through several steps…
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Drying (usually just above 110 deg. C.)
Ashing (up to 1000 deg. C)
Atomization (Up to 2000-3000 C)
Cleanout (quick ramp up to 3500 C or so). Waste is blown
out with a blast of Ar.
The light from the source (HCL) passes through the
furnace and absorption during the atomization step
is recorded over several seconds. This makes
ETAAS more sensitive than FAAS for most
elements.
Radiation Sources for AAS
Hollow Cathode Lamp
Conventional HCL
Ne or Ar at
1-5 Torr
Hollow Cathode Lamp (Cont’d)
Ionize the inert gas at a potential of ~ 300 V
Generate a current of ~ 5 to 15 mA as ions
and electrons migrate to the electrodes.
a tungsten anode and a
cylindrical cathode
neon or argon at a pressure of 1
to 5 torr
The cathode is constructed of
the metal whose spectrum is
desired or served to support a
layer of that metal
The gaseous cations acquire enough kinetic energy to dislodge some of the
metal atoms from the cathode surface and produce an atomic cloud.
A portion of sputtered metal atoms is in excited states and thus emits their
characteristic radiation as they return to the ground sate
Eventually, the metal atoms diffuse back to the cathode surface or to the glass
walls of the tube and are re-deposited
Hollow Cathode Lamp (Cont’d)
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High potential, and thus high currents lead to
greater intensities
Doppler broadening of the emission lines from the
lamp
Self-absorption: the greater currents produce an
increased number of unexcited atoms in the cloud.
The unexcited atoms, in turn, are capable of
absorbing the radiation emitted by the excited
ones. This self-absorption leads to lowered
intensities, particular at the center of the emission
band
Doppler broadening
?
Improvement…….
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Most direct method of obtaining improved lamps
for the emission of more intense atomic resonance
lines is to separate the two functions involving the
production and excitation of atomic vapor
Boosted discharge hollow-cathode lamp (BDHCL)
is introduced as an AFS excitation source by
Sullivan and Walsh.
It has received a great deal of attention and a
number of modifications to this type of source have
been conducted.
Boosted discharge hollow-cathode lamp (BDHCL)
Operation principle of BDHCL
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A secondary discharge (boost) is struck between
an efficient electron emitter and the anode,
passing through the primary atom cloud.
The second discharge does not produce too
much extra atom vapor by sputtering the walls
of the hollow cathode, but does increase
significantly the efficiency in the excitation of
sputtered atom vapor.
This greatly reduces the self-absorption
resulting from simply increasing the operating
potential (increase Doppler broadening and
self-absorption) to the primary anode and
cylindrical cathode.
Electrodeless Discharge Lamps (EDL)
Electrodeless discharge lamps (EDL)
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Constructed from a sealed quartz tube containing a few torr
of an inert gas such as argon and a small quantity of the
metal of interest (or its salt).
The lamp does not contain an electrode but instead is
energized by an intense field of radio-frequency or microwave
radiation.
Radiant intensities usually one or two orders of magnitude
greater than the normal HCLs.
The main drawbacks: their performance does not appear to
be as reliable as that of the HCL lamps (signal instability
with time) and they are only commercially available for some
elements.
Single-beam design
DOUBLE BEAM FAA
SPECTROMETER
Note: the Ref bean does not pass
through the flame thus does not correct for the
interferences from the flame!
synchronized
Interferences in AAS and AFS
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Spectral Interferences
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Overlapping
Broadening absorption for air/fuel mixture
Scattering or absorption by sample matrix
Background Correction
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Two-line Correction (like Internal Standard)
Continuum-Source Correction
Zeeman Effect
Source Self-Reversal (Smith –Hieftje)
Continuum-Source Correction
Continuum-Source Correction
(The draw is not to scale)
A
B
0.04 nm
The light from the HCL is absorbed by both the sample and the background, but
the light from the D2 lamp is absorbed almost entirely by the background
A: HCL lamp, the shaded portion shows the light absorbed from the HCL. The emission
has a much narrower line width than the absorption line.
B: D2 lamp, the shaded portion shows the light absorbed by D2 lamp. The lamp
emission is much broader than the sample absorption, and an averaged absorbance taken
over the whole band pass of the monochromator.
Zeeman Effect Background
Correction:
Source Self-Reversal (Smith –Hieftje)
Self-absorption
Line broadening
A relative new technique
Source Self-Reversal (Smith –Hieftje)
Absorbed by sample reduced, not complete eliminate!
But the background absorbs the same portion of light.
Absorbed by sample and background
Vandecasteele and Block, 1997, p126
Interferences in AAS and AFS
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Chemical Interferences
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Formation of compounds of low volatility
Calcium analysis in the presence of Sulfate or phosphate
Solutions
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Higher temperature
Releasing agents: cations that react preferntially with the
interference ions.
Protection agents: form stable but volatile species with
the analytes (i.e. EDTA,APDC….)
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Chemical Interferences
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Atom ionization
M ↔ M+ + e
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Atomic Fluorescence Spectrometry
Commercial AFS instruments are
on the market!
Learn more in CHM 6157