Flame Emission: it measures the radiation emitted by the excited atoms that is related to concentration.

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Part 2
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Relationship Between Atomic Absorption and Flame
Emission

• Flame Emission it measures the radiation
  emitted by the excited atoms that is related to
  concentration.
•  Atomic Absorption it measures the radiation
  absorbed by the unexcited atoms that are
  determined.
• Atomic absorption depends only upon the number of
  unexcited atoms, the absorption intensity is not
  directly affected by the temperature of the
  flame.
• The flame emission intensity in contrast, being
  dependent upon the number of excited atoms, is
  greatly influenced by temperature variations.

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Atomizers in emission techniques

• Type Method of Atomization
  Radiation Source
• Arc sample heated in an sample
• electric arc (4000-5000oC)
• Spark sample excited in a sample
• high voltage spark
• Flame sample solution sample
• aspirated into a flame
• (1700 3200 oC)
• Argon sample heated in an
  sample
• plasma argon plasma (4000-6000oC)

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Atomizers in absorption techniques

• Type Method of Atomization Radiation Source
• Atomic sample solution aspirated
  HCL
• (flame) into a flame
• atomic sample solution evaporated
  HCL
• (nonflame) ignited (2000 -3000 oC)
• (Electrothermal)
• Hydride Vapor hydride generated HCL
• generation
• Cold vapor Cold vapor generated (Hg)
  HCL

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Atomizers in fluorescence techniques

• Type Method of Atomization Radiation Source
• atomic sample aspirated sample
• (flame) into a flame
• atomic sample evaporated sample
• (nonflame) ignited
• x-ray not required sample
• fluorescence

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• Flame Atomization In a flame atomizer, a
  solution of the sample is nebulized by a flow of
  gaseous oxidant, mixed with a gaseous fuel, and
  carried into a flame where atomization occurs.
  The following processes then occur in the flame.
• Desolvation (produce a solid molecular aerosol)
• Dissociation (leads to an atomic gas)
• Ionization (to give cations and electrons)
• Excitation (giving atomic, ionic, and molecular
  emission)

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Processes that take place in flame or plasma
Sample Atomization For techniques samples need to
be atomized Techniques are useful for element
identification Molecular information destroyed by
atomization Flame Atomization Sample
nebulized Mixed with fuel Carried to flame for
atomization
T 1
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The Atomization Process
nebulization
vaporization
desolvation
M,X-aq
M,X-aq
MXsolid
MXgas
solution
mist
atomization
atomization
excitation or absorption
emission
X0gas
M0gas
Mgas
M0gas
(via heat or light)
ground state
excited state
Mgas
Xgas
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• Types of Flames
• Several common fuels and oxidants can be
  employed in flame spectroscopy depending on
  temperature needed. Temperatures of 1700oC to
  2400oC are obtained with the various fuels when
  air serves as the oxidant. At these temperature,
  only easily decomposed samples are atomized. For
  more refractory samples, oxygen or nitrous oxide
  must be employed as the oxidant. With the common
  fuels these oxidants produce temperatures of
  2500oC to 3100oC.

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• Burning Velocity
• The burning velocities are of considerable
  importance because flames are stable in certain
  ranges of gas flow rates only. If the gas flow
  rate does not exceed the burning velocity, the
  flame propagates itself back in to the burner,
  giving flashback. As the flow rate increases, the
  flame rises until it reaches a point above the
  burner where the flow velocity and the burning
  velocity are equal. This region is where the
  flame is stable. At higher flow rates, the flame
  rises and eventually reaches a point where it
  blows off of the burner.

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• Flame Structure
• Important regions of a flame include
• primary combustion zone
• interzonal region
• secondary combustion zone
• Primary combustion zone Thermal equilibrium is
  ordinarily not reached in this region, and it is,
  therefore, seldom used for flame spectroscopy.

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• 2. Interzonal region This area is relatively
  narrow in stoichiometric hydrocarbon flames, is
  often rich in free atoms and is the most widely
  used part of the flame for spectroscopy.
• 3. Secondary combustion zone In the secondary
  reaction zone, the products of the inner core are
  converted to stable molecular oxides that are
  then dispersed into the surroundings.

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• Temperature Profiles
• A temperature profile of a typical flame for
  atomic spectroscopy is shown in Fig. 9-3. The
  maximum temperature is located in the flame about
  1 cm above the primary combustion zone. It is
  important particularly for emission methods to
  focus the same part of the flame on the entrance
  slit for all calibrations and analytical
  measurements.

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• Types of fuel/oxidant
• air/acetylene
• 2300oC most widely used.
• nitrous oxide/acetylene
• 2750oC hot and reducing red feather zone - due
  to CN very reactive free radical scavenger for 02
  ? lowers partial pressure of 02 in zone reducing
  atmosphere

C2H2 2.502 10N2 ? 2CO2 H2O
10N2 stoichiometric reaction
C2H2 5N2O ? 2 CO2 H2O 5N2
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• Why do you need a different burner for different
  oxidants?
• because to prevent flash back linear gas flow
  rate
• needs to 3 x speed of which flame can travel,
• burning velocity).

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Role of Chemistry in the Flame
 sample atomised by thermal and chemical
dissociation H2 Q ? H? H? O2 Q ? O? O?
H? O2 ? OH? O? O? H2 ? OH? H?  
equilibrium achieved by 3rd body collision
(B)   i.e. N2, O2 H? H? B ? H2 B? Q H?
OH? B ? H2O B? Q Free reductions may react
with sample to produce atoms i.e. H? HO?
NaCl ? H2O Na? Cl? Na? Q ? Na?
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Flame Atomisation Process Sample must be in the
form of a fine mist so as not to put out
flame. Breaks down sample into very fine drops
to form liquid aerosol or mist. This assist
atomisation as sample only in flame
0.025s Sample drawn up capillary tube at high
velocity
Sample

oxidant
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• Suction caused by high flows of oxidant gas
  and Venturi effect.
• The high gas flow rate at the end of the
  capillary creates a pressure drop in the
  capillary the pressure in capillary is below
  atmospheric pressure and sample solution is
  pulled up.
• The high speed gas breaks the solution into a
  fine mist by turbulence as it emerges from
  capillary.
• How do we get a better aerosol?
• use impact bead (glass or alloy) to encourage
  aerosol formation and remove large droplets.

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• Flame absorbance Profiles
• Fig. 9-4 shows typical absorption profiles for
  three elements. Magnesium exhibits a maximum in
  absorbance at the middle of the flame. The
  behavior of silver, which is not readily
  oxidized, is quite different, a continuous
  increase in the number of atoms, and thus the
  absorbance, is observed from the base to the
  periphery of the flame. Chromium, which forms
  very stable oxides, shows a continuous decrease
  in absorbance beginning close to the burner tip.

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Flame absorbance profile for three elements
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• Flame Atomizers
• Figure 9-5 is a diagram of a typical commercial
  laminar flow burner that employs a concentric
  tube nebulizer. The aerosol is mixed with fuel.
  The aerosol, oxidant, and fuel are then burned in
  a slotted burner that provides a flame that is
  usually 5 or 10 cm in length.

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Laminar-Flow Burner
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• Advantages
• 1. Uniform dropsize
• 2. Homogeneous flame
• 3. Quiet flame and a long path length
• Disadvantages
• 1. Flash back if Vburning gt Vflow
• 2. 90 of sample is lost
• 3. Large mixing volume

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Sample introduction techniques
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Methods of Sample Introduction in Atomic
Spectroscopy
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Nebulization

• Nebulization is conversion of a sample to a fine
  mist of finely divided droplets using a jet of
  compressed gas.
• The flow carries the sample into the atomization
  region.
• Pneumatic Nebulizers (most common)
• Four types of pneumatic nebulizers
• Concentric tube - the liquid sample is sucked
  through a capillary tube by a high pressure jet
  of gas flowing around the tip of the capillary
  (Bennoulli effect).
• This is also referred to aspiration. The high
  velocity  breaks the sample into a mist and
  carries it to the atomization region.

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Types of pneumatic nebulizers
Concentric tube Cross flow
Concentric tube Cross flow
Fritted disk
Babington
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• Cross-flow
• The jet stream flows at right angles to the
  capillary tip. The sample is sometimes pumped
  through the capillary.
• Fritted disk
• The sample is pumped onto a fritted disk through
  which the gas jet is flowing. Gives a finer
  aerosol than the others.
• Babington
• Jet is pumped through a small orifice in a
  sphere on which a thin film of sample flows. This
  type is less prone to clogging and used for high
  salt content samples.
• Ultrasonic Nebulizer
• The sample is pumped onto the surface of a
  vibrating piezoelectric crystal.
• The resulting mist is denser and more homogeneous
  than pneumatic nebulizers.
• Electro-thermal Vaporizers (Etv)
• An electro thermal vaporizer contains an
  evaporator in a closed chamber through which an
  inert gas carries the vaporized sample into the
  atomizer.

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Liquid samples introduced to atomizer through a
nebulizer
Pneumatic nebulizer
Ultrasonic-Shear Nebulizer
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Atomization Atomizers Flame Electrothermal Spec
ial Glow Discharge Hydride Generation Cold-Vapo
r
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• Flame Chemistry
• Flames are used in atomic emission spectrometry
  for excitation (emission spectrometry) but in
  atomic absorption flames are used as Atom Cells
  to produce gaseous atoms.
• Why must the atoms not be excited for atomic
  absorption spectrometry?

If the atom is already in the excited state it
cannot absorb the light.
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Different Atomization Sources for Atomic
Spectroscopy
Typical Source Temperature Source Type
1700 - 3150 C. Combustion Flame
1200 - 3000 C Electrothermal Vaporization (ETV) on graphite platform
5000 -8000 C Inductively coupled plasma (ICP)
6000-1000 C Direct-current plasma (DCP)
2000-3000 C Microwave induced plasma (MI))
non-thermal Glow Discharge plasma (GDP)
40,000 C(?) Spark Sources (dc or ac Arc)
 
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• Flame Atomizers
• Superior method for reproducible liquid sample
• introduction for atomic absorption and
  fluorescence
• spectroscopy.
• Other methods better in terms of sampling
  efficiency
• and sensitivity.

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Laminar-Flow Burner
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Atomizers
Flame Atomic Absorption Spectrometry

• Atomization occurs in a flame created by mixing a
  fuel with an oxidant
• Analyte and background ions are atomized
  simultaneously
• Only a small percentage of the aqueous sample is
  atomized much of the sample goes to waste

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Laminar flame atomizer
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• Performance Characteristics
• Of Flame Atomizers
• In terms of reproducible behavior, flame
  atomization appears to be superior to all other
  methods for liquid sample introduction. In terms
  of sampling efficiency and thus sensitivity,
  however, other atomization methods are markedly
  better. A large portion of the sample flows down
  the drain and the residence time of individual
  atoms in the optical path in the flame is brief
  (10-4s).

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Electrothermal Atomization

• Atomization of entire sample in short period
• Average sample time in optical path is seconds
• Evaporation of sample
• Microliter volume
• Low temperature
• Sample ashed at higher temperature
• Increase current
• Sample temperature goes to 2000-3000 C
• Sample measured above heated surface
• High sensitivity for small samples

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• Electrothermal Atomization
• It provides enhanced sensitivity because the
  entire sample is atomized in a short period, and
  the average residence time of the atoms in the
  optical path is a second or more. A few
  microliters of sample are first evaporated at a
  low temperature and then ashed at a somewhat
  higher temperature in an electrically heated
  graphite tube or in a graphite cup. Then the
  current is rapidly increased to several hundred
  amperes, which caused the temperature to soar to
  perhaps 2000oC to 3000oC atomization of the
  sample occurs in a period of a few milliseconds
  to seconds.

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Atomizers
Electrothermal or Graphite Furnace Atomizer

• Atomization occurs in an electrically heated
  graphite tube
• The graphite tube is flushed with an inert gas
  (Ar) to prevent the formation of (non-absorbing)
  metal oxides

graphite tube
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• Performance Characteristics
• Electrothermal atomizers offer the advantage of
  unusually high sensitivity for small volumes of
  sample. Typically, sample volumes between 0.5 and
  10 ?L are used absolute detection limits lie in
  the range of 10-10 to 10-13 g of analyte. Furnace
  methods are slow-typically requiring several
  minutes per element. A final disadvantage is that
  the analytical range is low, being usually less
  than two orders of magnitude.

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Electrothermal atomizer
Sample concentration
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Atomization
From Skoog et al. (2004) Table 28-1, p.840
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Atomization and Excitation

• Atomic Emission Spectroscopy
• The heat from a flame or an electrical discharge
  promotes an electron to a higher energy level
• As the electron falls back to ground state, it
  emits a wavelength characteristic of the excited
  atom or ion

From Skoog et al. (2004) Figure 28-1, p.840
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Atomic Line Spectra
Each spectral line is characteristic of an
individual energy transition
E hn
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Flame atomic absorption spectrometry Beer
Lamberts Law  

A log (Po/P) A ? b c   where ?
is the molar absorptivity coefficient in units of
mol-1 dm3 cm-1 b is the pathlength in cm and c
is the concentration in mol dm-3   In limits
(below 0.8 Absorbance) A vs. concentration    
P
Po
sample
b