Atomic Spectroscopy

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Atomic Spectroscopy

• Introduction

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Technique Flame Test
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An Introduction to Optical Atomic Spectroscopy

• In optical atomic spectrometry, compounds are
  first converted to gaseous molecules followed by
  conversion to gaseous atoms. This process is
  called atomization and is a prerequisite for
  performing atomic spectroscopy. Gaseous atoms
  then absorb energy from a beam of radiation or
  simply heat. Absorbance can be measured or
  emission from excited atoms is measured and is
  related to concentration of analyte.

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Atomic Energy Level Diagrams

• As a start, we should be aware that only valence
  electrons are responsible for atomic spectra
  observed in a process of absorption or emission
  of radiation in the UV-Vis region. Valence
  electrons in their ground states are assumed to
  have an energy equal to zero eV. As an electron
  is excited to a higher energy level, it will
  absorb energy exactly equal to the energy
  difference between the two states. Let us look at
  a portion of the sodium energy level diagram
  where sodium got one electron in the 3s orbital

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• The dark lines represent most probable
  transitions and in an atomic spectrum they would
  appear more intense than others. It should also
  be indicated that two transitions, of very
  comparable energies (589.0 and 589.6 nm), from
  the 3s ground state to 3p excited state do take
  place. This suggests splitting of the p orbital
  into two levels that slightly differ in energy.
  Explanation of this splitting may be presented as
  a result of electron spin where the electron spin
  is either in the direction of the orbital motion
  or opposed to it.

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• Both spin and orbital motion create magnetic
  fields that may interact in an attractive manner
  (if motion is in opposite direction, lower
  energy), or in a repulsive manner when both spin
  and orbital motion are in the same direction
  (higher energy). The same occurs for both d and f
  orbitals but the energy difference is so small to
  be observed. A Mg ion would show very similar
  atomic spectrum as Na since both have one
  electron in the 3s orbital.

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• In cases where atoms of large numbers of
  electrons are studied, atomic spectra become too
  complicated and difficult to interpret. This is
  mainly due to presence of a large numbers of
  closely spaced energy levels
• It should also be indicated that transition from
  ground state to excited state is not arbitrary
  and unlimited. Transitions follow certain
  selection rules that make a specific transition
  allowed or forbidden.

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Atomic Emission and Absorption Spectra

• At room temperature, essentially all atoms are in
  the ground state. Excitation of electrons in
  ground state atoms requires an input of
  sufficient energy to transfer the electron to one
  of the excited state through an allowed
  transition. Excited electrons will only spend a
  short time in the excited state (shorter than a
  ms) where upon relaxation an excited electron
  will emit a photon and return to the ground
  state.

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• Each type of atoms would have certain preferred
  or most probable transitions (sodium has the
  589.0 and the 589.6 nm). Relaxation would result
  in very intense lines for these preferred
  transitions where these lines are called
  resonance lines.
• Absorption of energy is most probable for the
  resonance lines of each element. Thus intense
  absorption lines for sodium will be observed at
  589.0 and 589.6 nm.

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Atomic Fluorescence Spectra

• When gaseous atoms at high temperatures are
  irradiated with a monochromatic beam of radiation
  of enough energy to cause electronic excitation,
  emission takes place in all directions. The
  emitted radiation from the first excited
  electronic level, collected at 90o to the
  incident beam, is called resonance fluorescence.
  Photons of the same wavelength as the incident
  beam are emitted in resonance fluorescence. This
  topic will not be further explained in this text
  as the merits of the technique are not very clear
  compared to instrumental complexity involved

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Atomic Line Width

• It is taken for granted that an atomic line
  should have infinitesimally small (or zero) line
  width since transition between two quantum states
  requires an exact amount of energy. However,
  careful examination of atomic lines reveals that
  they have finite width. For example, try to look
  at the situation where we expand the x-axis
  (wavelength axis) of the following line

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• The effective line width in terms of wavelength
  units is equal to Dl1/2 and is defined as the
  width of the line, in wavelength units, measured
  at one half maximum signal (P). The question
  which needs a definite answer is what causes the
  atomic line to become broad?

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Reasons for Atomic Line Broadening

• There are four reasons for broadening observed in
  atomic lines. These include
• 1. The Uncertainty Principle
• We have seen earlier that Heisenberg uncertainty
  principle suggests that nature places limits on
  the precision by which two interrelated physical
  quantities can be measured. It is not easy, will
  have some uncertainty, to calculate the energy
  required for a transition when the lifetime of
  the excited state is short.

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• The ground state lifetime is long but the
  lifetime of the excited state is very short which
  suggests that there is an uncertainty in the
  calculation of the transition time. We have seen
  earlier that when we are to estimate the energy
  of a transition and thus the wavelength (line
  width), it is required that the two states where
  a transition takes place should have infinite
  lifetimes for the uncertainty in energy (or
  wavelength) to be zero

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• DEgth/Dt
• Therefore, atomic lines should have some
  broadening due to uncertainty in the lifetime of
  the excited state. The broadening resulting from
  the uncertainty principle is referred to as
  natural line width and is unavoidable.

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2. Doppler Broadening

• The wavelength of radiation emitted by a fast
  moving atom toward a transducer will be different
  from that emitted by a fast atom moving away from
  a transducer. More wave crests and thus higher
  frequency will be measured for atoms moving
  towards the transducer. The same occurs for sound
  waves

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• Assume your ear is the transducer, when a car
  blows its horn toward your ear each successive
  wave crest is emitted from a closer distance to
  your ear since the car is moving towards you.
  Thus a high frequency will be detected. On the
  other hand, when the car passes you and blows its
  horn, each wave crest is emitted at a distance
  successively far away from you and your ear will
  definitely sense a lower frequency.

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• The line width (Dl) due to Doppler broadening can
  be calculated from the relation
• Dl/lo v/c
• Where lo is the wavelength at maximum power and
  is equal to (l1 l2)/2, v is the velocity of the
  moving atom and c is the speed of light. It is
  noteworthy to indicate that an atom moving
  perpendicular to the transducer will always have
  a lo, i.e. will keep its original frequency and
  will not add to line broadening by the Doppler
  effect.

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• In the case of absorption lines, you may
  visualize the line broadening due to Doppler
  effect since fast atoms moving towards the source
  will experience more wave crests and thus will
  absorb higher frequencies. On the other hand, an
  atom moving away from the source will experience
  less wave crests and will thus absorb a lower
  frequency. The maximum Doppler shifts are
  observed for atoms of highest velocities moving
  in either direction toward or away from a
  transducer (emission) or a source (absorption).

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3. Pressure Broadening

• Line broadening caused by collisions of emitting
  or absorbing atoms with other atoms, ions, or
  other species in the gaseous matrix is called
  pressure or collisional broadening. These
  collisions result in small changes in ground
  state energy levels and thus the energy required
  for transition to excited states will be
  different and dependent on the ground state
  energy level distribution.

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• This will definitely result in important line
  broadening. This phenomenon is most astonishing
  for xenon where a xenon arc lamp at a high
  pressure produces a continuum from 200 to 1100 nm
  instead of a line spectrum for atomic xenon. A
  high pressure mercury lamp also produces a
  continuum output. Both Doppler and pressure
  contribution to line broadening in atomic
  spectroscopy are far more important than
  broadening due to uncertainty principle.

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4. Magnetic Effects

• Splitting of the degenerate energy levels does
  take place for gaseous atoms in presence of a
  magnetic field. The complicated magnetic fields
  exerted by electrons in the matrix atoms and
  other species will affect the energy levels of
  analyte atoms. The simplest situation is one
  where an energy level will be split into three
  levels, one of the same quantum energy and one of
  higher quantum energy, while the third assumes a
  lower quantum energy state. A continuum of
  magnetic fields exists due to complex matrix
  components, and movement of species, thus exist.
  Electronic transitions from the thus split levels
  will result in line broadening

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The Effect of Temperature on Atomic Spectra

• Atomic spectroscopic methods require the
  conversion of atoms to the gaseous state. This
  requires the use of high temperatures (in the
  range from 2000-6000 oC). Thee high temperature
  can be provided through a flame, electrical
  heating, an arc or a plasma source. It is
  essential that the temperature be of enough value
  to convert atoms of the different elements to
  gaseous atoms and, in some cases, provide energy
  required for excitation. The temperature of a
  source should remain constant throughout the
  analysis especially in atomic emission
  spectroscopy.

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• Quantitative assessment of the effect of
  temperature on the number of atoms in the excited
  state can be derived from Boltzmann equation
• Where Nj is the number of atoms in excited state,
  No is the number of atoms in the ground state, Pj
  and Po are constants determined by the number of
  states having equal energy at each quantum level,
  Ej is the energy difference between excited and
  ground states, K is the Boltzmann constant, and T
  is the absolute temperature.

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Boltzmann distribution
Nj /N0 at 3000 K
Wavelength
Atom
852.1 nm
Cs
7.24 ? 10-3
589.0 nm
Na
5.88 ? 10-4
422.7 nm
Ca
3.69 ? 10-5
213.9 nm
Zn
5.58 ? 10-10
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• To understand the application of this equation
  let us consider the situation of sodium atoms in
  the 3s state (Po 2) when excited to the 3p
  excited state (Pj 6) at two different
  temperatures 2500 and 2510K. Now let us apply the
  equation to calculate the relative number of
  atoms in the ground and excited states
• Usually we use the average of the emission lines
  from the 3p to 3s where we have two lines at
  589.0 and 589.6 nm which is

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Therefore, at higher temperatures, the number of
atoms in the excited state increases. Let us
calculate the percent increase in the number of
atoms in the excited state as a result of this
increase in temperature of only 10 oC
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Effect of Temperature on Atomic Absorption and
Emission

• The question here is which technique would be
  affected more as a result of fluctuations in
  temperature? The answer to this important
  question is rather simple. Atomic emission is the
  technique that will be severely affected by
  fluctuations in temperature since signal is
  dependent on the number of atoms in the excited
  state. This number is significantly affected by
  fluctuations in temperature as seen from the
  example above. However, in the case of atomic
  absorption, the signal depends on the number of
  atoms in ground state that will absorb energy.

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• very high as related to the number of excited
  atoms
• Nj/No 1.72x10-4
• or
• 172 excited atoms for each 106 atoms in ground
  state
• This suggests a very high population of the
  ground state even at high temperatures.
  Therefore, atomic absorption will not be affected
  to any significant extent by fluctuations in
  temperature, if compared to atomic emission
  spectroscopy.

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• However, there are some indirect effects of
  temperature on atomic absorption spectroscopy.
  These effects can be summarized as
• Better sensitivities are obtained at higher
  temperatures since higher temperatures can
  increase the number of vaporized atoms at any
  time.
• Higher temperatures will increase the velocities
  of gaseous atoms, thus causing line broadening as
  a result of the Doppler and collisional effects.
• High temperatures increase the number of ionized
  analyte and thus decrease the number of atoms
  available for absorption.

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Band and Continuum Spectra Associated with Atomic
Spectra

• When the atomization temperature is insufficient
  to cause atomization of all species in the sample
  matrix, the existent molecular entities, at the
  temperature of the analysis, impose very
  important problems on the results of atomic
  absorption and emission spectroscopy. The
  background band spectrum should be removed for
  reasonable determination of analytes. Otherwise,
  the sensitivity of the instrument will be
  significantly decreased.

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• As the signal for the blank is considered zero
  and thus the instrument is made to read zero,
  when the analyte is to be determined, it got to
  have an absorbance greater than the highest point
  on the continuum and the instrument will assume
  that the absorbance related to analyte is just
  the value exceeding the background blank value.
  This will severely limit the sensitivity of the
  technique.

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• Putting this conclusion in other words we may say
  that if the analyte signal is less than the
  background blank, the instrument will read it as
  zero. Therefore, it is very important to correct
  for the background or simply eliminate it through
  use of very high temperatures that will
  practically atomize all species in the matrix. We
  will come to background correction methods in the
  next chapter.

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Atomization Methods

• It is essential, as we have seen from previous
  discussion, that all sample components (including
  analytes, additives, etc.) should be atomized.
  The atoms in the gaseous state absorb or emit
  radiation and can thus be determined. Many
  ionization methods are available which will be
  detailed in the next two chapters. Generally,
  atomization methods can be summarized below

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Sample Introduction Methods

• The method of choice for a specific sample will
  mainly depend on whether the sample is in
  solution or solid form. The method for sample
  introduction in atomic spectroscopy affects the
  precision, accuracy and detection limit of the
  analytical procedure.

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Introduction of Solution Samples

• 1. Pneumatic Nebulizers
• Samples in solution are usually easily introduced
  into the atomizer by a simple nebulization,
  aspiration, process. Nebulization converts the
  solution into an aerosol of very fine droplets
  using a jet of compressed gas. The flow of gas
  carries the aerosol droplets to the atomization
  chamber or region. Several versions of nebulizers
  are available and few are shown in the figure
  below

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2. Ultrasonic Nebulizers

• In this case samples are pumped onto the surface
  of a piezoelectric crystal that vibrates in the
  kHz to MHz range. Such vibrations convert samples
  into homogeneous aerosols that can be driven into
  atomizers. Ultrasonic nebulization is preferred
  over pneumatic nebulization since finer droplets
  and more homogeneous aerosols are usually
  achieved. However, most instruments use pneumatic
  nebulization.

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3. Electrothermal Vaporization

• An accurately measured quantity of sample (few
  mL) is introduced into an electrically heated
  cylindrical chamber through which an inert gas
  flows. Usually, the cylinder is made of pyrolytic
  carbon but tungsten cylinders are now available.
  The signal produced by instruments which use
  electrothermal vaporization (ETV) is a discrete
  signal for each sample injection. Electrothemal
  vaporizers are called discrete atomizers to
  differentiate them from nebulizers which are
  called continuous atomizers

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4. Hydride Generation Techniques

• Samples that contain arsenic, antimony, tin,
  selenium, bismuth, and lead can be vaporized by
  converting them to volatile hydrides by addition
  of sodium borohydride. Volatile hydrides are then
  swept into the atomizer by a stream of an inert
  gas.

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Introduction of Solid Samples

• A variety of techniques were used to introduce
  solid samples into atomizers. These include
• 1. Direct Sample Insertion
• Samples are first powdered and placed in a
  boat-like holder (from graphite or tantalum)
  which is placed in a flame or an electrothermal
  atomizer.
• 2. If the sample is conductive and is of a shape
  that can be directly used as an electrode (like a
  piece of metal or coin), that would be the choice
  for sample introduction in arc and spark
  techniques. Otherwise, powdered solid samples are
  mixed with fine graphite and made into a paste.
  Upon drying, this solid composite can be used as
  an electrode. The discharge caused by arcs and
  sparks interacts with the surface of the solid
  sample creating a plume of very fine particulates
  and atoms that are swept into the atomizer by a
  flow of an inert gas. This process of sample
  introduction is called ablation

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• 3. Laser Ablation
• Sufficient energy from a focused intense laser
  will interact with the surface of samples (in a
  similar manner like arcs and sparks) resulting in
  ablation. The formed plume of vapor and fine
  particulates are swept into the atomizer by the
  flow of an inert gas. Laser ablation is becoming
  increasingly used since it is applicable to
  conductive and nonconductive samples

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4. The Glow Discharge Technique

• A low pressure envelope (1 to 10 torr argon) with
  two electrodes with the conductive solid sample
  is the cathode, as in the figure below. The
  technique is used for sample introduction and
  atomization as well. The electrodes are kept at a
  250 to 1000 V DC. This high potential is
  sufficient to cause ionization of argon which
  will be accelerated to the cathode where the
  sample is introduced. Collision of the fast
  moving energetic argon ions with the sample
  (cathode) causes atomization by a process called
  sputtering.

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Atomic Absorption Spectroscopy

• We will cover two main techniques of atomic
  absorption spectroscopy (AAS), depending of the
  type atomizer. Two atomization techniques are
  usually used in AAS

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1. Flame Atomization

• Flames are regarded as continuous atomizers since
  samples are continuously introduced and a
  constant or continuous signal is obtained.
  Samples in solution form are nebulized by one of
  the described nebulization techniques discussed
  previously. The most common nebulization
  technique is the pneumatic nebulization.
  Nebulized solutions are carried into a flame
  where atomization takes place.

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• Several processes occur during atomization
  including
• a. Nebulized samples are sprayed into a flame as
  a spray of very fine droplets
• b. Droplets will lose their solvent content due
  to very high flame temperatures in a process
  called desolvation and will thus be converted
  into a solid aerosol.
• c. The solid aerosol is volatilized to form
  gaseous molecules

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• d. Gaseous molecules will then be atomized and
  neutral atoms are obtained which can be excited
  by absorption of enough energy. If energy is not
  enough for atomization, gaseous molecules will
  not be atomized and we may see molecular
  absorption or emission
• e. Atoms in the gaseous state can absorb energy
  and are excited. If energy is too much, we may
  observe ionization.

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• The different processes occurring in flames are
  complicated and are not closely controlled and
  predicted. Therefore, it can be fairly stated
  that the atomization process in flames may be one
  of the important parameters limiting the
  precision of the method. It is therefore
  justified that we have a closer look at flames
  and their characteristics and the different
  variables contributing to their performance.

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Types of Flames

• Flames can be classified into several types
  depending on fuel/oxidant used. For example, the
  following table summarizes the features of most
  familiar flames.
• Therefore, it can be clearly seen that
  significant variations in flame temperatures can
  be obtained by changing the composition of fuel
  and oxidant.

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• On the other hand, flames are only stable at
  certain flow rates and thus the flow rate of the
  gas is very important where at low flow rates
  (less than the maximum burning velocity) the
  flame propagates into the burner body causing
  flashback and, in some cases, an explosion. As
  the flow rate is increased, the flame starts to
  rise above the burner body. Best flames are
  obtained when the flow rate of the gas is equal
  to the maximum burning velocity. At this equity
  ratio the flame is most stable. At higher ratios,
  flames will reach a point where they will no
  longer form and blow off the burner.

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 Flame Structure

• Three well characterized regions can be
  identified in a conventional flame. A lower
  region, close to the burner tip, with blue
  luminescence. This region is called the primary
  combustion zone which is characterized by
  existence of some non atomized species and
  presence of fuel species (C2 and CH, etc.) that
  emit in the blue region of the electromagnetic
  spectrum. The second well defined region is
  called the interzonal region just above the
  primary combustion zone. The interzonal region is
  rich in free atoms and is the region of choice
  for performing atomic spectroscopy.

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• It also contains the regions of highest
  temperatures. The third region in the flame is
  the outer region which is called the secondary
  combustion region. It is characterized by
  reformation of molecules as the temperature at
  the edges is much lower than the core. These
  regions can be schematically represented by the
  following schematic

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Flame Absorbance profiles

• Since the temperature of a flame depends on the
  position from its tip, it is necessary to
  concentrate our work on one spot in a flame and
  preferably adjust the height of the flame to get
  best signal. In fact, not all elements require a
  specific height above burner tip but rather each
  element has its own requirements which largely
  reflect some of its properties. For example, one
  can use higher distances from the tip so that
  higher temperatures are achieved to analyze for
  silver. This is possible since silver will not be
  easily oxidized.

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Flame Absorption ProfilesWe have seen that there
are different temperature profiles in a flame and
temperature changes as the distance from the
burner tip is change
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• However, best results for the analysis of
  chromium occur at lower heights (fuel rich
  flames) since at higher heights oxygen from
  atmosphere will force chromium to convert to the
  oxide which will not be atomized at flame
  temperatures. A third situation can be observed
  for magnesium where increasing the height above
  tip will increase the signal due to increased
  atomization at higher temperatures. However, at
  higher distances the oxide starts to form leading
  to a decrease in signal.

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Flame Atomizers (Continuous Atomizers)

• Flame Atomizers (Continuous Atomizers)
• There are several types of flame atomizers
  available. The simplest is a turbulent flow
  burner that is very similar to conventional
  Bunsen burner. This type of burner suffers from
  fluctuations in temperature since there is no
  good mechanism for homogeneous mixing of fuel and
  oxidant. The drop size of nebulized sample is
  also inhomogeneous which adds to fluctuations in
  signal. The path length of radiation through the
  flame is small which suggests a lower sensitivity
  of the technique.

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• Turbulent flow burners are also susceptible to
  flashback. These drawbacks were overcome using
  the most widely used laminar flow burner where
  quite flames and long path length are obtained.
  Flashback is avoided and very homogeneous mixing
  between fuel, oxidant, and droplets take place.
  Larger droplets are excluded and directed to a
  waste container. A schematic representation of
  the burner is shown below

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Fuel and Oxidant Regulators

• The adjustment of the fuel to oxidant ratio and
  flow rate is undoubtedly very crucial. Although
  stoichiometric ratios are usually required,
  optimization is necessary in order to get highest
  signal. However, in the determination of metals
  that form stable oxides, a flame with excess fuel
  is preferred in order to decrease oxide formation.

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Performance Characteristics of Flame Atomizers

• Reproducibility of flame methods are usually
  superior to other atomization techniques.
  However, the residence time of an atom in a flame
  is in the order of 10-4 s which is very short.
  This is reflected in a lower sensitivity of flame
  methods as compared to other methods. Also,
  conventional flames with reasonable burning
  velocities can produce relatively low
  temperatures which make them susceptible to
  interference from molecular species.

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2. Electrothermal Atomization

• These have better sensitivities than flame
  methods. The increased sensitivity can be
  explained on the basis that a longer atom
  residence time is achieved (can be more than 1 s)
  as well as atomization of the whole sample in a
  very short time. As the name implies, a few mL of
  the sample are injected into the atomization
  chamber (a cylinder of graphite coated with a
  film of pyrolytic carbon) where the following
  processes take place

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• a. Evaporation the solvent associated with the
  sample is evaporated in a low temperature (120
  oC) slow process (seconds)
• b. Ashing sample is ashed to burn organics
  associated with the sample at moderate
  temperatures (600 oC, seconds)
• c. Atomization The current is rapidly increased
  after ashing so that a temperature in the range
  from 2000-3000 oC is obtained in less than1
  second.

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Electrothermal Atomizers (Discrete Atomizers)

• The heart of the atomizer, beside efficient
  heating elements and electronics, is a
  cylindrical graphite tube opened from both ends
  and has a central hole for sample introduction.
  It was found that porous graphite results in poor
  reproducibility since some of the analyzed
  materials will diffuse through porous graphite
  and will thus lead to a history effect.

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• Therefore, the cylindrical graphite is made from
  a special type of nonporous high quality graphite
  called pyrolytic graphite. The length of the
  cylinder is 2-5 cm and it has less than 1 cm
  diameter. When the tube is fixed in place
  electrical contacts are achieved which are water
  cooled. Two inert gas streams (argon) flow at the
  external surface and through the internal space
  of the tube to prevent oxidation and clean the
  tube after each measurement. Usually, samples are
  analyzed in triplicates where three consecutive
  reproducible signals are required for each
  sample..

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Performance Characteristics of Electrothermal
Atomizers

• Electrothermal atomization is the technique of
  choice in case of small sample size. Also, higher
  sensitivities than flames are ordinarily
  obtained. Unfortunately, the analysis time is in
  the few minutes range and the relative precision
  is in the range of 5-10 as compared to 1 in
  flame methods. In addition, the linear dynamic
  range is usually small ( two orders of
  magnitude) which requires extra sample
  manipulation. It may be also mentioned that
  better experienced personnel can achieve the
  merits of the technique.

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Atomic Absorption Instrumentation

• Atomic absorption instruments consist of a source
  of radiation, a monochromator, a flame or
  electrothermal atomizer in which sample is
  introduced, and a transducer.

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Radiation Sources

• Although radiation in the UV-Vis region is
  required, we can not use broad band sources. This
  is because even the best monochromators can not
  provide a bandwidth that is narrower than the
  atomic absorption line. If the bandwidth of the
  incident radiation is wider than the line width,
  measurement will fail as absorption will be only
  a tiny fraction of a large signal which is
  difficult to measure and will result in very low
  sensitivities (figure a). Therefore, line sources
  with bandwidths narrower than that of the
  absorption lines must be used

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• This can be achieved by using a lamp producing
  the emission line of the element of interest
  where analyte atoms can absorb that line.
  Conditions are established to get a narrower
  emission line than the absorption line. This can
  in fact be achieved by getting an emission line
  of interest at the following conditions

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• 1. Low temperatures to decrease Doppler
  broadening (which is easily achievable since the
  temperature of the source is always much less
  than the temperature in flames).
• 2. Lower pressures this will decrease pressure
  broadening and will thus produce a very narrow
  emission line.

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• This may suggest the need for a separate lamp for
  each element which is troublesome and
  inconvenient. However, recent developments lead
  to introduction of multielement lamps. In this
  case, the lines from all elements should not
  interfere and must be easily resolved by the
  monochromator so that, at a specific time, a
  single line of one element is leaving the exit
  slit

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Hollow Cathode Lamp (HCL)

• This is the most common source in atomic
  absorption spectroscopy. It is formed from a
  tungsten anode and a cylindrical cathode the
  interior surface of which is coated by the metal
  of interest. The two electrodes are usually
  sealed in a glass tube with a quartz window and
  filled with argon at low pressure (1-5 torr).
  Ionization of the argon is forced by application
  of about 300 V DC where positively charged Ar
  heads rapidly towards the negatively charged
  cathode causing sputtering. A portion of
  sputtered atoms is excited and thus emit photons
  as atoms relax to ground state. The cylindrical
  shape of the cathode serves to concentrate the
  beam in a limited region and enhances
  redeposition of sputtered atoms at the hollow
  surface.

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• High potentials usually result in high currents
  which, in turn, produce more intense radiation.
  However, Doppler broadening increases as a
  result. In addition, the higher currents will
  produce high proportion of unexcited atoms that
  will absorb some of the emission beam which is
  referred to as self absorption (a lower intensity
  at the center of the line is observed in this
  case).

101
Electrodeless Discharge Lamps (EDL)

• An EDL is a sealed quartz tube containing a few
  torr of an inert gas and a small quantity of the
  metal of interest. Excitation of the metal is
  achieved by a radiofrequency or a microwave
  powered coil through ionization of argon, due to
  high energetic radiofrequency. Ionized argon will
  hit the metal causing excitation of the atoms of
  the metal of interest. The output power of the
  EDL lamp is higher than the HCL lamp. However,
  compared to HCL lamps, EDL lamps are rarely used.

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Emission in Flames

• There can be significant amounts of emission
  produced in flames due to presence of flame
  constituents (molecular combustible products) and
  sometimes impurities in the burner head. This
  emitted radiation must be removed for successful
  sensitive determinations by AAS, otherwise a
  negative error will always be observed. We can
  visualize this effect by considering the
  schematic below

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• The detector will see the overall signal which is
  the power of the transmitted beam (P) in addition
  to the power of the emitted radiation from flame
  (Pe). Therefore if we are measuring absorbance,
  this will result in a negative error as the
  detector will measure what it appears as a high
  transmittance signal (actually it is P Pe). In
  case of emission measurements, there will always
  be a positive error since emission from flame is
  an additive value to the actual sample emission.
  It is therefore obvious that we should get rid of
  this interference from emission in flames.

107
Source Modulation

• It turned out that excluding the emission signal
  from flames can easily be done by an addition of
  a chopper to the instrumental design. The chopper
  is a motor driven device that has open and solid
  (mirrors in some cases) alternating regions as in
  the schematic

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• The function of the chopper is to chop the light
  leaving the source so that when the incident beam
  hits the chopper at the solid surface, the beam
  will be blocked and detector will only read the
  emitted signal from the flame. As the chopper
  rotates and the beam emerges to the detector, the
  detector signal will be the sum of the
  transmitted signal plus that emitted from the
  flame. The signal processor will be able to
  subtract the first signal from the second one,
  thus excluding the signal from emission in
  flames.

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• This can be represented by the following
  equations
• Signal 1 (Blocked Beam)  Pe
• Signal 2 (Transmitted Beam) P Pe
• Overall Difference Signal (P Pe) - Pe P
  (Corrected Signal)
•  
• This correction method for background emission in
  flames is called source modulation.

111
The schematic of the AAS instrument with source
modulation correction can be represented by the
following schematic
112

• It should be recognized that addition of extra
  components to an instrument will decrease the
  signal to noise ratio and addition of a moving
  component is usually regarded as a disadvantage
  due to higher need for maintenance.
• Another procedure which can overcome the emission
  from flames is to use a modulated power supply
  that will give fluctuating intensities at some
  frequency (say for example pulsed radiation at a
  specific frequency).

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• The emission from flames is a continuous signal
  but that from the source is modulated. Now if we
  use a high pass RC filter, only the fluctuating
  signal will be measured as signal while the DC
  signal will be considered zero as it can not pass
  through the electronic filter. The high pass RC
  filter is a device which uses a resistor and a
  capacitor the impedance of which is inversely
  proportional to the frequency of the modulated
  signal. Therefore, only high frequencies will
  have low impedance and can pass through the
  capacitor while signals of low frequencies will
  suffer very high resistance and will not be able
  to go through the capacitor.

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AAS Instruments

•  Instruments in AAS can be regarded as single or
  double beam instruments.
•  
• Single Beam Atomic Absorption Spectrophotometers
• A single beam instruments is the same as the one
  described above (source modulation section) or
  generally

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The term spectrophotometer implies that the
instrument uses a dispersive monochromator
(containing a prism or a grating). Also, the
detector is a photomultiplier tube in most cases.
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Double Beam Atomic Absorption Spectrophotometers

• In this type of instruments, the incident beam is
  split into two beams of equal intensity by a
  chopper with the solid surface being a mirror.
  One of the beams will traverse the sample in the
  atomizer while the other is considered as a
  reference. Detector signals will be consecutive
  readings of both the reference and sample beams.
  The ratio of the reference to the sample beams is
  recorded to give the final signal.

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A schematic representation of a double beam
instrument is shown below
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• It should be emphasized here that in the absence
  of sample, Pr is not equal to P since the
  reference beam traverses through air while the
  other beam traverses through the flame. In
  flames, particulates and molecular species
  scatter and absorb a portion of incident
  radiation, which results in a lower intensity of
  the beam. To act as a real double beam, The AA
  spectrophotometer reference beam should pass
  through a reference flame.

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• But even if we do that, there are no guarantees
  that both beams will be of equal intensities
  because it is almost impossible to obtain exactly
  equivalent flames. It is therefore important to
  understand that the excellent features of a
  double beam configuration are not achievable in
  AAS instrumentation.

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Interferences in Atomic Absorption Spectroscopy

• There are two major classes of interferences
  which can be identified in atomic absorption
  spectroscopy. The first class is related to
  spectral properties of components other than
  atomized analyte and is referred to as spectral
  interferences. The other class of interferences
  is related to the chemical processes occurring in
  flames and electrothermal atomizers and their
  effects on signal. These are referred to as
  chemical interferences and are usually more
  important than spectral interferences.

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Spectral Interferences

• 1. Spectral line Interference
• Usually, interferences due to overlapping lines
  is rare since atomic lines are very narrow.
  However, even in cases of line interference, it
  can be simply overcome by choosing to perform the
  analysis using another line that has no
  interference with other lines. Therefore, line
  interference is seldom a problem in atomic
  spectroscopy.

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2. Scattering

• Particulates from combustion products and sample
  materials scatter radiation that will result in
  positive analytical error. The error from
  scattering can be corrected for by making a blank
  measurement. Scattering phenomenon is most
  important when concentrated solutions containing
  elements that form refractory oxides (like Ti,
  Zr, and W) are present in sample matrix.

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• Metal oxide particles with diameters larger than
  the incident wavelength will make scattering a
  real problem. In addition, samples containing
  organic materials or organic solvents can form
  carbonaceous (especially in cases of incomplete
  combustion) particles that scatter radiation.

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 3. Broad Band Absorption

• In cases where molecular species from combustion
  products or sample matrix are formed in flames or
  electrothermal atomizers, a broad band spectrum
  will result which will limit the sensitivity of
  the technique. It should be indicated here that
  spectral interferences by matrix products are not
  widely encountered in flame methods. Even if
  matrix effects are present in flames, they can be
  largely overcome by adjusting various
  experimental conditions like fuel/oxidant ratio
  or temperature.

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• Another method for overcoming matrix
  interferences is to use a much higher
  concentration of interferent than that initially
  present in sample material, in both sample and
  standards (this material is called a radiation
  buffer). The contribution from sample matrix will
  thus be insignificant.
• Spectral interferences due to matrix are severe
  in electrothermal methods and must thus be
  corrected for.

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Background Correction Methods

• a.      The Two Line Correction Method
• In this method, a reference line from the source
  (from an impurity in cathode or any emission
  line) is selected where this line should have the
  following properties
• 1.      Very close to analyte line
• 2.      Not absorbed by analyte
• If such a line exists, since the reference line
  is not absorbed by the analyte, its intensity
  should remain constant throughout analysis.

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• However, if its intensity decreases, this will be
  an indication of absorbance or scattering by
  matrix species. The decrease in signal of the
  reference line is used to correct for the analyte
  line intensity (by subtraction of the absorbance
  of the reference from that of the analyte). This
  method is very simple but unfortunately it is not
  always possible to locate a suitable reference
  line.

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b.      The Continuum Source Method

• This background correction method is the most
  common method although, for reasons to be
  discussed shortly, it has major drawbacks and
  fails a lot. In this technique, radiation from a
  deuterium lamp and a HCL lamp alternately pass
  through the graphite tube analyzer. It is
  essential to keep the slit width of the
  monochromator sufficiently wide in order to pass
  a wide bandwidth of the deuterium lamp radiation.

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• In this case, the absorbance by analyte atoms is
  negligible and absorbance can be attributed to
  molecular species in matrix. The absorbance of
  the beam from the deuterium lamp is then
  subtracted from the analyte beam (HCL) and thus a
  background correction is obtained.

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Problems Associated with Background Correction
Using D2 Lamp

• 1.      The very hot medium inside the graphite
  tube is inhomogeneous and thus signal is
  dependent on the exact path a beam would follow
  inside the tube. Therefore, exact alignment of
  the D2 and HCL lamps should be made.
• 2.      The radiant power of the D2 lamp in the
  visible is insignificant which precludes the use
  of the technique for analysis of analytes in the
  visible region.
• 3.      Addition of an extra lamp and chopper
  will decrease the signal to noise ratio.

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c.       Background Correction Based on Zeeman
Effect

• Zeeman has observed that when gaseous atoms (but
  not molecules) are placed in a strong magnetic
  field ( 1 tesla), splitting of electronic energy
  levels takes place. The simplest splitting of one
  energy level results in three energy levels, one
  at a higher energy, another at a lower energy
  (two s satellite lines) and the third remains at
  the same energy as the level in absence of the
  magnetic field (central p line). Furthermore, the
  p line has twice the absorbance of a s line and
  absorbs polarized light parallel to direction of
  the magnetic field while the two s lines absorb
  light perpendicular to magnetic field.

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• Light from a HCL lamp will pass through a
  rotating polarizer that passes polarized light
  parallel to external magnetic field at one cycle
  and passes light perpendicular to field in the
  other cycle. The idea of background correction
  using this method is to allow light to traverse
  the sample in the graphite furnace atomizer and
  record the signal for both polarizer cycles using
  the wavelength at the p line.

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• a.       First cycle light parallel to field
  the p line of the analyte absorbs in addition to
  absorbance by matrix (molecular matrix absorb
  both polarized light parallel or perpendicular to
  field)
• Signal a Ap AMatrix
• b.      Second cycle light perpendicular to
  field the p line of analyte will not absorb
  light perpendicular to field and s lines will
  also not affect absorbance at the p line
  wavelength. Only matrix will absorb.
• Signal b AMatrix

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• The overall signal is the difference of the two
  signals Ap
• Therefore, excellent background correction is
  achieved using the Zeeman effect. This background
  correction method results in good correction and
  is usually one of the best methods available.

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Chemical Interferences

• These are interferences resulting from chemical
  processes occurring in flames and electrothermal
  atomizers and affect the absorption signal. To
  quantitatively assess the effects of the
  different chemical processes occurring in flames,
  one should regard the burnt gases as behaving
  like a solvent. This is necessary since our
  knowledge of gaseous state reaction equilibria is
  rather limited. Chemical interferences include
  three major processes

138
Formation of Compounds of Low Volatility

• Anionic species forming compounds of low
  volatility are the most important. The formation
  of low volatility species will result in a
  negative error or at least will decrease the
  sensitivity. For example, the absorption signal
  of calcium will be decreased as higher
  concentrations of sulfate or phosphate are
  introduced. Cations forming combined products
  with the analyte will also decrease the signal
  obtained for the analyte. For example aluminum
  forms a heat stable compound with magnesium.

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Elimination of Low Volatility Compounds

• Addition of a releasing agent cations that can
  replace the analyte (preferentially react with
  the anion) are called releasing agents. In this
  case the analyte is released from the compound of
  low volatility and replaced by the releaseing
  agent. Lanthanum or strontium are good releasing
  agents in the determination of calcium in
  presence of phosphate or sulfate. Also, lanthanum
  or strontium are good releasing agents in the
  determination of magnesium in presence of
  aluminum since both can replace magnesium.

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• Addition of a protective agent organic ligands
  that form stable volatile species with analytes
  are called protective agents. An example is EDTA
  and 8-hydroxyquinoline which will form complexes
  with calcium even in presence of sulfate and
  phosphate or aluminum.
• Use  of higher temperature is the simplest
  procedure to try if it is possible

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Dissociation Equilibria

• Dissociation reactions occur in flames where the
  outcome of the process is desired to produce the
  atoms of analyte. For example, metal oxides and
  hydroxides will dissociate in flames to produce
  the atoms as in the equations
• MO M O
• M(OH)2 M 2 OH

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• Remember that we are not working in solution to
  dissociate the compounds into ionic species. In
  fact, not much is known about equilibrium
  reactions in flames. It should also be remembered
  that alkaline earth oxides and hydroxides are
  relatively stable and will definitely show
  characteristic broad band spectra (more intense
  than line spectra), except at very high
  temperatures. The opposite behavior is observed
  fro alkali metals oxides and hydroxides which are
  instable even at lower flame temperatures and
  thus produce line spectra.

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• An equilibrium can be established for the
  dissociation of compounds containing atoms other
  than oxygen, like NaCl where
• NaCl Na Cl
• Now, if the signal from a solution of NaCl was
  studied in presence of variable amounts of Cl
  (from HCl, as an example), the signal will be
  observed to decrease as the concentration of Cl
  is increased a behavior predicted by the Le
  Chatelier principle in solutions.

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• The same phenomenon is observed when a metal
  oxide is analyzed using a fuel rich flame or a
  lean flame. Signal will be increased in fuel rich
  flames since the dissociation of metal oxides is
  easier due to less oxygen while the opposite
  takes place in lean flames (oxygen rich).

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• A good example on dissociation equilibria can be
  presented for the analysis of vanadium in
  presence of aluminum and titanium, fuel rich
  flames result in higher absorbance signal for
  vanadium since the little oxygen present in
  flames will be mainly captured by Al and Ti, thus
  more V atoms are available. However, in lean
  flames, excess oxygen is present and thus
  vanadium will form the oxide and addition of
  extra Ti and Al will not affect the signal. 

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Ionization Equilibria

•  Ionization in fuel/air flames is very limited
  due to relatively low temperatures. However, in
  fuel/nitrous oxide or fuel/oxygen mixtures,
  ionization is significant. Therefore, at higher
  temperatures an important portion of atoms can be
  converted to ions
• M M e
• K Me/M

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• Ionization in flames may explain the decrease in
  absorption signal for alkali metals at very high
  temperatures where as the temperature is
  increased signal will increase till an extent at
  some temperature where it starts to decrease as
  temperature is further increased a consequence
  of ionization. Therefore, usually lower flame
  temperatures are used for determination of alkali
  metals. A material that is added to samples in
  order to produce large number of electrons is
  referred to as an ionization suppressor, the
  addition of which results in higher
  sensitivities.

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Practical Details in AAS

• 1.        Sample Preparation
• The most unfortunate requirement of AAS may be
  the need for introduction of samples in the
  solution form. This necessitates the dissolution
  of the sample where in many cases the procedure
  is lengthy and requires very good experience.
  Care should be particularly taken in order not to
  lose any portion of the analyte and to make sure
  that the reagents, acids, etc. used in the
  dissolution and pretreatment of the sample are
  free from analyte impurities.

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• I suggest that you follow exact procedures for
  preparation of specific samples for analysis by
  AAS. In some cases where the sample can be
  introduced directly to an electrothermal atomizer
  without pretreatment (like serum samples),
  definitely, electrothermal atomizers will have an
  obvious advantage over flame methods which
  require nebulization.

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Organic Solvents

• Increased nebulization rate due to lower surface
  tension of organic solvents which produces
  smaller droplets as well as faster evaporation of
  solvents in flames will result in better
  sensitivities.
• Immiscible organic solvents containing organic
  ligands are used to extract metal ions of
  interest and thus concentrate them in a small
  volume (thus increasing sensitivity) and
  excluding possible interferences due to matrix
  components.

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  Calibration Curves

• The absorbance of a solution is directly
  proportional to its concentration but due to the
  large number of variables in AAS, usually this
  direct relationship may slightly deviate from
  linearity. The standard procedure to do is to
  construct a relation between the absorbance and
  concentration for a series of solutions of
  different concentrations. The thus constructed
  graph is called a calibration curve.

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• The unknown analyte absorbance is found and the
  concentration is calculated or located on the
  curve. Neither interpolation nor extrapolation is
  permitted to the calibration curve. A sample can
  be diluted or the calibration curve may be
  extended but always the analyte absorbance should
  be within the standard absorbance range recorded.
  Usually, the concentration axis has the ppm or
  ppb units.

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Standard Addition method

• Chemical and spectral interferences can be
  partially or wholly overcome by the use of a
  special technique of calibration called the
  method of standard addition. In addition, the use
  of this method provides better correlations
  between standards and sample results due to
  constant nebulization rates. The method involves
  addition of the same sample volume to a set of
  tubes or containers.

154

• Variable volumes of a standard are added to the
  tube set followed by completion to a specific
  volume. Now, all tubes contain the same amount of
  sample but different concentrations of analyte. A
  plot is then made for the volume of standard and
  absorbance. This plot will have an intercept (b)
  with the y axis and a slope equals m.

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• The concentration of the analyte can be
  determined by the relation
• Cx bCs/mVx
• Where, Cx and Vx are concentration and volume of
  analyte and Cs is the concentration of standard.
• One can only use two points to get the analyte
  concentration using the relation
• Cx AxCsVs/(At Ax)Vx

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Detection Limits

•  Usually, atomic absorption based on
  electrothermal atomization has better
  sensitivities and detection limits than methods
  based on flames. In general, flame methods have
  detection limits in the range from 1-20 ppm while
  electrothermal methods have detection limits in
  the range from 1-20 ppb.

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• This range can significantly change for specific
  elements where not all elements have the same
  detection limits. For example, detection limits
  fro mercury and magnesium using electrothermal
  atomization are 100 and 0.02 ppb while the
  detection limits for the same elements using
  flame methods are 500 and 0.1 ppm, respectively.

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Accuracy

• Flame methods are superior to electrothermal
  methods in terms of accuracy. The relative error
  in flame method can be less than 1 while that
  for electrothermal method occurs in the range
  from 5-10. Also, electrothermal methods are more
  susceptible to molecular interferences from the
  matrix components. Therefore, unless a good
  background correction method is used, large
  errors can be encountered in electrothermal
  methods depending on the nature of sample
  analyzed.

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Flame Photometry

• The technique referred to as flame photometry is
  a flame emission technique. We introduce it here
  because we will not be back to flame methods in
  later chapters. The basics of the technique are
  extremely simple where a sample is nebulized into
  a flame. Atomization occurs due to high flame
  temperatures and also excitation of easily
  excitable atoms can occur.

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• Emission of excited atoms is proportional to
  concentration of analyte. Flame emission is good
  for such atoms that do not require high
  temperatures for atomization and excitation, like
  Na, K, Li, Ca, and Mg. The instrument is very
  simple and excludes the need for a source lamp.
  The filter is exchangeable in order to determine
  the analyte of interest and, in most cases, a
  photomultiplier tube is used as the detector.

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b. Charge Transfer and Charge Coupled Transducers

• The photosensitive elements are, in contrary to
  PDAs, arranged in two dimensions in both charge
  injection devices (CID) and charge-coupled
  devices (CCD). Therefore, these are very similar
  to photographic films. For example, a
  commercially available transducer is formed from
  244 rows with each row containing 388 detector
  elements. This will add up to a two-dimensional
  array holding 16672 detector elements (pixels) on
  silicon chip that is 6.5 mm by 8.7 mm.

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• The full description of the system and its
  mechanism will not be covered here as this is
  behind the scope of this course. However, we
  should qualitatively know that these important
  transducers function by first collecting the
  photogenerated charges in different pixels and
  then measuring the quantity of the charge
  accumulated in a brief period. Measurement is
  accomplished by transferring the charge from a
  collection area to a detection area.

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Optical Atomic Spectra

• We have briefly described in an introductory
  chapter that atomic spectra are usually line,
  rather than band, spectra due to absence of
  vibrational and rotational levels. The existence
  of quantized electronic energy levels explains
  the origin of the observed line spectra and exact
  locations of possible lines