Atomic Absorption and Atomic Fluorescence Spectrometry Section A

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Atomic Absorption and Atomic Fluorescence
Spectrometry Section A

• By Matt Boyd, James Joseph, Jon Blizzard, Jackie
  Freebery, Hunter Bodle

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Atomization Techniques

• AAS and AFS
• Two techniques
• Flame Atomization
• Electrothermal Atomization

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Flame Atomization

• Analyte is nebulized by flow of gaseous oxidants
• Desolvations
• Dissociation
• Volitalized

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Types Flames

• Figure 9-1

Fuel oxidant Temperature (Celsius) Maximum Burning (cm s-1)
Natural Gas Air 1700-1900 39-43
Natural Gas Oxygen 2700-2800 370-390
Hydrogen Air 2000-2100 300-440
Hydrogen Oxygen 2550-2700 900-1400
Acetylene Air 2100-2400 158-266
Acetylene Oxygen 3050-3150 1100-2480
Acetylene Nitrous oxide 2600-2800 285
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Flame Structure

• Primary Combustion zone
• Blue region, rarely used for spectroscopy
• Interzonal region
• Most widely used part
• Secondary combustion zone
• Products of inner core disperse

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Flame Atomizers

• Uses
• Atomic Absorption
• Fluorescence
• Emission Spectroscopy
• Laminar-flow Burners are Commonly Used
• Aerosol, oxidant, and fuel are burned in flame
• Performance Characteristics
• Most reproducible

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Electrothermal Atomizers

• Provides enhanced sensitivity
• Operates evaporating sample at low temps
• Ashing at higher temp
• Measures absorption and fluorescence
• Used in the ICP

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Electrothermal Atomization

• Occurs in open ended cylindrical graphite tube
• Held between two contacts in water cooled housing
  
• Two inert gas streams are provided

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Output Signal

• Transducer output rises to a maximum
• Rapid decay back to zero
• Quantitative determinations
• Peak height
• Peak area

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Performance Characteristics

• Advantages
• Sensitivity
• Relative precision
• Disadvantages
• Furnace methods
• Analytical range

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Analysis of Solids with Electrothermal Atomizers

• 1st- weigh grounded sample into a graphite boat
  and insert boat into furnace.
• 2nd- prepare slurry of powdered sample by
  ultrasonic agitation in an aqueous solution. The
  slurry is then pipetted into furnace atomization.

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Specialized Atomization Techniques

• Glow-Discharge Atomization
• Hydride Atomization
• Cold-Vapor Atomization

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Chapter NineAtomic Absorption and Atomic
Fluorescence Spectrometry

• Section 9A Sample Atomization Technique

By Rachel Conroy Katie Payne
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Flame Atomization

• Sample is nebulized by a flow of gaseous oxidant
  and fuel that carries it to a flame
• Process
• Desolvation
• Volatilization
• Dissociation
• Ionization
• Excitation to form spectra

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At each phase of atomization spectra can be
obtained.
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Types of Flames

• Oxidants
• Air 1700oC to 2400oC
• Oxygen
• Nitrous oxide
• Burning velocity states when flame is stable
• Too low causes flashback
• Too high flame will blow off

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Table 9-1 Properties of Flames
Fuel Oxidant Temperature oC Maximum burning velocity. cm s -1
Natural Gas Air 1700 1900 39 43
Natural Gas Oxygen 2700 2800 370 390
Hydrogen Air 2000 2100 300 440
Hydrogen Oxygen 2550 2700 900 1400
Acetylene Air 2100 2400 159 266
Acetylene Oxygen 3050 3150 1100 2480
Acetylene Nitrous oxide 2600 2800 285
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Flame Structure

• Primary Combustion Zone
• Interzonal Area
• Secondary Reaction Zone
• Flame Profile

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Flame Atomizers Variables

• Fuel and Oxidant Regulators
• Double-diaphragm pressure regulators
• Rotameter
• Performance
• Most reproducible
• Low sensitivity

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Schematic of a laminar-flow burner, the typical
atomizer used in AAS.
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Electrothermal Atomization

• Long residence time
• Measurements and vaporization
• Evaporated at a low temperature
• Ashed at a higher temperature

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Electrothermal Atomizers

• Graphite tube
• 2 inert gas streams provided
• Transverse configuration
• Pyrolytic carbon seal

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Shown is the cross-sectional view of a graphite
furnace atomizer. The Lvov platform and its
position in the graphite furnace.
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Other info

• Output Signals
• Measures peak height
• Performance
• Slow because of cooling cycles
• Analytical range is narrow
• High sensitivity
• Analysis of Solids
• Finely ground samples, slurry

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Specialized Atomization Techniques

• Glow-discharge atomization
• Hydride atomization
• Cold-Vapor atomization

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Atomic Absorption Instrumentation9-B

• Brian May
• Mandi Kauffman
• Tyler MacPherson
• Carolyn Inga
• Ginny Harrison

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Atomic Absorption Instrumentation

• The AAS Consists of
• A radiation source
• Sample Holder
• Wavelength Selector
• Detector
• Signal Processor
• Read Out

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Radiation Sources

• Potentially highly specific because of narrow
  absorption lines.
• These narrow lines also cause problems because a
  linear relationship between absorption and
  concentration requires narrow source bandwidth
  relative to the width of an absorption line, but
  even good monochromators have bandwidths
  significantly larger than the absorption lines.

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Problems Created

• Non-linear calibration curves are inevitable when
  the AA is equipped with an ordinary
  spectrophotometer and continuum radiation source.
• Small calibration curves are obtained because
  only a small amount of the radiation from the
  monochromator slit is absorbed by the sample,
  this gives poor sensitivity

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Solutions

• The use of bandwidths narrower than the
  absorption lines. This is done by exciting the
  atoms with a lamp, filtering the light, and
  choosing appropriate operating conditions(source
  temperature and pressure).
• This disadvantage to this method is that it
  require an additional source lamps for each
  element, or group of elements.

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• Hollow Cathode Lamps (9B-1)Sample

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• -Most common source for atomic absorption
  measurements
• -Consists of a tungsten anode and a cylindrical
  cathode sealed in a glass tube which is filled
  with either argon or neon gas a pressure of
  1-5torr
• -Cathode is constructed from the metal whose
  spectrum is desired (or, if not constructed from
  the metal, it then serves to support a layer of
  that metal)

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• -When a potential of about 300V is applied across
  the electrodes, ionization occurs of the inert
  gas (argon or neon). The current is generated (of
  about 5-15 mA) as ions and electrons migrate to
  the electrodes.
• -if potential is large enough the gaseous cations
  gather enough kinetic energy to dislodge the
  metal atoms from the cathode surface and produce
  an atomic cloud in the process known as
  sputtering.
• -The excited metal atoms (a portion of those
  sputtered) emit their characteristic radiation as
  they return to ground state

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• -Efficiency of the cathode depends on its
  geometry and the operating potential
• High potentials (and thus high currents) ?
  greater intensities
• -A down-fall to high currents is that they
  produce an increased number of unexcited atoms in
  the cloud which have the potential of absorbing
  the radiation emitted from the excited atoms
  (Self-absorption)
• -This leads to lower intensities

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Electrodeless Discharge Lamps
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What are they made of?

• Sealed quartz tube
• Filled with an inert gas (Ar)
• Small amount of metal or its salt

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What does it use?

• Uses radio frequency
• Or microwave radiation to energize it

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What happens?

• The gas is ionized by the frequency
• Once enough energy is obtained it excites the
  atoms of metal
• The metal spectrum is the desired spectrum.

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What it provides

• Provides radiant intensities in greater supply
  than a Hollow-Cathode Lamp (HCL)
• Not as reliable as the (HCL)
• But better for elements such as
• Se, As, Cd, Sb

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Source modulation

• Emitted radiation is removed via the
  monochromator
• It is necessary to adjust the output the source
  so intensity will fluctuate at a constant
  frequency

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• Detector receives 2 signal
• An alternating from the source
• Continuous from the flame.
• These signals are then converted into electrical
  responses
• A high pass RC filter (section 2B-5) can be used
  to remove unadjusted signals

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• Adjusting the emission can be done by inserting a
  circular metal disc (chopper) into the system
  between the source and the flame
• Rotation of this disk at a constant rate will
  create a beam that is chopped to the desired
  frequency
• Tuning forks with vanes attatched to alternately
  allow the beam to pass and to not pass is another
  technique
• An alternative is the power supply being designed
  for intermittent or ac operation so the source
  can be switched oin and off at the desired
  frequency

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AA SpectrophotometerSee Figure 9-13 for block
diagrams

• Instrument must be capable of providing a
  sufficiently narrow bandwidth to isolate the line
  chosen for the measurement
• Glass filter alkali metals
• Only a few widely spaced resonance lines in the
  visible region
• Separate filter and light source for each element
• Most use photomultiplier tubes

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Single-Beam

• Several hollow- cathode sources
• Chopper or pulsed power supply
• Atomizer
• Simple grating spectrometer with a
  photomultiplier transducer
• 100 transmittance is set with a blank
• The blank is replaced with samples to determine
  absorbance and transmittance

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Double Beam

• Beam from hollow-cathode source is split by a
  mirrored chopper
• One half passes through the flame and the other
  half goes around it
• 2 beams recombine by a half silvered mirror and
  passed into a Czerny-Turner grating monochromator
• Photomultiplier transducer
• Output input to a lock-in amplifier
• Ratio between reference and sample is amplified
  and fed to the readout
• Since reference beam is not passed through the
  flame it cannot correct for loss of radiant power
  due to absorption or scattering by the flame

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Chapter 9 Section C

• Megan Seeger, Andrea Lando, Joe Bailey, and Sarah
  Duncan

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Spectral Interferences

• Can be caused by overlapping lines but is very
  rare due to the emission lines of the
  hollow-cathode sources being so narrow
• Can also result from the presence of combustion
  products that exhibit broadband absorption or
  particle products that scatter radiation
• Both reduce the power of the transmitted beam and
  lead to positive analytical errors

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Continued

• A more troublesome problem occurs when the source
  of absorption or scattering originates in the
  sample matrix
• Interferences because of scattering by products
  of atomization is most often encountered when
  concentrated solutions containing elements such
  as Ti, Zr, and W are aspirated in the flame

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Continued

• Interferences caused by scattering may also be a
  problem when the sample contains organic species
  or if organic solvents are used to dissolve the
  sample
• Flame atomization spectral interferences by
  matrix products are not widely seen and can be
  avoided by variations in the analytical variables

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Radiation Buffer

• When an excess of the interfering substance is
  added to the sample and standards
• If the concentration added is large compared to
  the concentration in the sample matrix then the
  contribution from the sample matrix is
  insignificant

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Two-Line Correction Method

• Uses a line from the source as a reference it
  should be as close as possible to the analyte
  line
• This makes any decrease in power of the reference
  line from that observed during the calibration
  arises from absorption or scattering and is then
  used to correct the absorbance of the analyte line

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The Continuum-Source Correction Method

• Deuterium lamp provides a source of continuum
  radiation throughout the ultraviolet region
• The radiation from the continuum source and the
  hollow cathode lamp are passed alternately
  through the electrothermal atomizer, the
  absorbance from the deuterium radiation is then
  subtracted from the analyte beam

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Chemical Interferences

• More common than spectral interferences
• Can be minimized by suitable operating conditions

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Most Common Interferences

• Occurs when anion form low-volatility compounds
  with the analyte only a fraction of analyte is
  atomized and the outcome is low results
• Ex. Decrease in calcium absorbance with
  increasing concentrations of sulfate or phosphate

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Common Interferences Cont

• Cation Interferences
• Outcome low results
• Ex Aluminum causes low results when determining
  magnesium (forms a heat stable compound)

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Solutions to Interferences

• When caused by formation of species of low
  volatility, interference can be eliminated by use
  of higher temps
• Releasing Agents cations that react preferably
  with the interferent and prevent analyte
  interaction
• Protective Agents prevent interferences by
  forming a stable, volatile species with the
  analyte

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Background Correction Based on the Zeeman Effect

• p.242-243

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Zeeman Effect

• When an atomic vapor is exposed to a strong
  magnetic field, a splitting of electronic energy
  levels of the atoms takes place that leads to the
  formation of several absorption lines for each
  electronic transition. The sum of the
  absorbencies of the lines is equal to exactly the
  value of the original line from which they were
  formed.
• A,B,C ? A
• --------------
  ---------
• B
• ---------
• C
• ---------

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Splitting Pattern

• Most common type of splitting
• Central line (p) and two equally spaced satellite
  lines (s). This is observed with a singlet but
  for more complex transitions these lines will be
  split further.
• The p line absorbs only plane polarized light in
  a parallel direction to the magnetic field
• The s lines absorb only polarized radiation at a
  90 degree angle to the magnetic field

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How it works

• Turn to page 243 in textbooks
• Radiation from a cathode tube
• Rotating polarizer
• Separates the beam into two parts that are
  polarized at 90 degrees to each other
• These go into a graphite furnace that splits the
  energy levels into three peaks (D)
• This information then goes to a monochromator,
  photomultiplier tube, and into a data analysis
  system.
• This system subtracts the perpendicular cycle
  from the parallel half cycle giving a background
  correction.

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Background Corrections with Source Self Reversal

• Also known as the Smith-Hieftje method
• Based on the self reversal or self absorption of
  radiation from a cathode lamp
• the absorbance is collected at periods where the
  lamp is running at a low current
• The background is collected when the lamp is at
  high voltage
• High currents high number of nonexcited
  electrons that will absorb the radiation of the
  excited species

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Dissociation Equilibria

• Dissociation reactions involving metal oxides and
  hydroxides play an important role in determining
  the emission and absorption spectra for an
  element.
• MO? M O
• The M is the analyte atom and the OH is the
  hydroxyl radical.

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Dissociation Equilibria

• Dissociation equilibria which involve anions
  other than oxygen may also influence flame
  emission and absorption.
• Line intensity for Na is decreased by presence of
  HCl
• NaCl ? Na Cl
• Adding HCL decreases Na concentration thereby
  lowering line intensity.

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Dissociation Equilibria

• V, Al, and Ti interact with such species as O and
  OH. These are represents as Ox. These are always
  present in flames.
• VOx ? V Ox
• AlOx ? Al Ox
• TiOX ? Ti Ox

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Ionization Equilibria

• Ionization of atoms is small in combustion
  mixtures that involve air as the oxidant, it is
  often neglected.
• Ionization is important in higher temp. flames
  where oxygen or nitrous oxide is the oxidant.
  There are free electrons produced by the
  equilibrium.
• M ? M e-

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Ionization Equilibria

• The equilibrium constant K for the reaction
• K Me- / M
• Degree of Ionization of metals at flame temps.
  Table 9-2 pg. 246

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Ch. 9 Atomic Absorption Spectrometry

• Section D
• Atomic Absorption Analytical Techniques

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Sample Preparation

• 1. Flame Spectroscopic Methods
• Sample materials
• Soils
• Animal tissues
• Plants
• Petroleum products
• Minerals
• Common problem most are insoluble in aqueous
  solutions so preliminary treatment to the sample
  is required

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Preliminary Treatments

• Decomposition of material
• Rigorous treatment of the sample at high
  temperatures
• Con risk losing the analyte by volatilization or
  as particulates in smoke
• Treatment with specific reagents
• Con can cause chemical and spectral
  interferences or can cause the analyte to appear
  as in impurity in the solution
• Common Decomp. Methods
• 1. Treatment with hot mineral acids
• 2. Oxidation with liquid reagents (sulfuric,
  nitric, or perchloric acids wet ashing)
• 3. Combustion in an oxygen bomb (or other
  closed container)
• 4. Ashing at high temperatures
• 5. High temperature fusion with reagents (
  boric oxide, sodium carbonate, sodium peroxide,
  and potassium pyrosulfate)

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Sample Preparation

• Electrothermal Atomization
• Sample Types
• 1. Liquid Samples blood, petroleum products,
  and organic solvents.
• liquid solvents can be pipetted directly into
  the furnace for ashing and atomization.
• 2. Solid Samples plant leaves, animal tissues,
  and inorganic substances.
• solids can be weighed directly into a
  cup-type atomizer or into specific containers for
  introduction into a tube type furnace.

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Sample Introduction by Flow Injection

• Introduce samples into a flame atomic absorption
  spectrometer
• Peristaltic pump and valve arrangements help
  insure efficiency while conserving the sample
• Carrier system Deionized water or diluted
  electrolyte are used to provide continuous
  flushing of the flame atomizer
• This reduces build up from samples containing
  high levels of salts or suspended solids

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Organic Solvents

• Low Molecular-weight organic solvents
• 1. Alcohols
• 2. Esters
• 3. Ketones
• Why Organic Solvents?
• Increased nebulizer efficiency- increases the
  amount of sample that reaches the flame
• Rapid evaporation of the solvent
• Solvent Ratios
• Leaner fuel-oxidant ratios must be used to offset
  the presence of any added organic material
• This produces lower flame temperatures, which can
  increase the potential for chemical interferences

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Organic Solvents (cont.)

• Immiscible Solvents
• ex Methyl isobutyl ketone
• These solvents extract chelates of metallic ions
• The resulting extract in then nebulized directly
  into a flame
• Enhance absorption lines
• Only small amounts are required to extract from
  relatively large volumes of aqueous solutions
• Enhance the sensitivity of the sample, which
  reduces interferences
• Common Chelating Agents-
• Ammonium pyrrolidinedithiocarbamate
• Diphenylthicarbazone
• 8-hydroxyquinoline
• Acetylacetone

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Calibration Curves

• Should Follow Beers Law
• A abc
• A absorption (L/ g?cm)
• a absorptivity
• b path length through medium
• c concentration

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Calibration Curves (cont.)

• Should cover range of concentration found in the
  sample
• 1 standard solution should be measured after each
  time an analysis is performed. Using 2 standards
  that bracket the analyte concentration would be
  more efficient in identifying any uncontrolled
  variables that result from atomization and
  absorbance measurements

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Application of AAS

• Sensitive men for the quantitative determination
  of more than 60 metals or metalloid elements
• Table 9-3 shows Detection Limits
• Columns 2 3 present detection limits for a
  number of common elements by flame and
  electrothermal atomic absorption
• Detection Limits
• Flame Atomization 0.001 0.020 ppm
• Electrothermal Atomization 2 x 10-6 1 x 10-5
  ppm
• Accuracy
• Relative error
• Flame Analysis 1-2
• Electrothermal Analysis errors extend flame
  errors by a factor of 5-10

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9D ATOMIC ABSORPTION ANALYTICAL TECHNIQUES

• Roa Al-Qabbani
• and
• Ashley Appell

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9D-1 Sample Preparation

• Sample has to be introduced into the excitation
  in the form of a solution (disadvantage).
• Many materials are not soluble in common
  solvents extensive treatment is required.
• Treatment with hot minerals, oxidation with
  liquid reagent, ashing at high temperature, etc.
• Some minerals can be atomized directly. Solid
  samples are weighed into cup-type atomizers
  (advantage).

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9D-2 Sample Introduction by Flow Injection

• FIA Introduces samples into a flame atomic
  absorption spectrometer.
• Carrier stream of the FIA system provides
  continuous flushing of the flame atomizer
  (advantage).

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9D-3 Organic Solvents

• The effect of organic solvents is attributable to
  increased nebulizer efficiency. More rapid
  solvent evaporation also contribute to the
  effect.
• Use of immiscible solvents is the most important
  analytical application of organic solvents to
  flame spectroscopy.
• Resulting extract is nebulized into the flame
  (sensitivity increases).
• Part of the matrix components remain in the
  solvent (advantage).

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9D-4 Calibration Curves

• Theory is that calibration curves should follow
  Beers Law which does not happen very often
• Absorbance should be directly proportional to
  concentration
• Use two standards that bracket the concentration
  of the analyte.

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9D-5 Standard Addition Method

• Should use method found in Section 1D-3
• Need to compensate for chemical and spectral
  interferences of the sample

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9D-6 Application of AAS

• A sensitive way of determining 60 metals and
  metalloid elements
• Detection Limits
• Flame Atomization Atomic Absorption Spectroscopy
  are in the range of 1-20ng/mL,or .001-.020ppm
• Electrothermal Atomization are in the range of
  .002-.01ng/mL or 2x10-6 - 1x10-5ppm
• Accuracy
• Error in Flame Ionization Atomic Absorption
  Spectroscopy 1-2
• Electrothermal Atomization increase by a factor
  of 5-10

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Atomic Fluorescence Spectroscopy

• Keri Franz
• Kyle Howard
• Lauren Kaminsky

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Fluorescence
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Atomic Fluorescence Spectroscopy

• Useful and convenient for quantitative
  determination of many elements
• Not used as often as atomic emission and atomic
  absorption
• Useful for determining elements that form vapors
  and hydrides- Pb, Hg, Cd, Zn
• Fluorescence instruments are generally harder to
  maintain and thus more expensive

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Instrumentation Sources

• Sample container is usually a flame or
  electrothermal atomization cell, glow discharge,
  or an inductively coupled plasma
• Continuum source is ideal
• Hollow cathode lamps were used frequently but now
  EDLs (electrodeless discharge lamps) are more
  common
• EDLs have greater intensity than hollow cathode
  lamps
• Lasers are good sources despite increased costs
  and operational intricacies

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Dispersive Instrumentation

• Contains
• Modulated Source
• Atomizer (flame or nonflame)
• Monochromator or Interference Filter System
• Detector
• Signal Processor
• Readout

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Nondispersive Instrumentation

• Contains
• Source
• Atomizer
• Detector
• Advantages
• Low Cost Simplicity
• Multi-element Analysis Adaptability
• High Sensitivity
• Simultaneous Collection of Energy
• For accuracy, make sure source output lacks
  elemental contamination and background radiation
  should not be emitted

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Applications

• Determination of Metals
• Lubricating Oils
• Seawater
• Geological Samples
• Clinical Samples
• Environmental Samples
• Agricultural Samples