Dr.Syed Muzzammil Masaud

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Atomic Absorption Atomic Emission Spectroscopy

• Dr.Syed Muzzammil Masaud
• Mphill.Pharmaceutical Chemistry

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BASIC PRINCIPLE

• 
  ATOMIC ABSORPTION SPECTROSCOPY (AAS) is an
• analytical technique that measures the
  concentrations of
• elements. It makes use of the absorption
  of light
• by these elements in order to measure
  their
• concentration .
  

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• - Atomic-absorption spectroscopy quantifies the
  absorption of ground state atoms in the gaseous
  state .
• The atoms absorb ultraviolet or visible light and
  make transitions to higher electronic energy
  levels . The analyte concentration is determined
  from the amount of
• absorption.

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• Concentration measurements are usually determined
  from a working curve after calibrating the
  instrument with standards of known concentration.
  
• - Atomic absorption is a very common
  technique for detecting metals and
  metalloids in environmental samples.

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Elements detectable by atomic absorption are
highlighted in pink in this periodic table
                                                                                             
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The Atomic Absorption Spectrometer

• Atomic absorption spectrometers have 4 principal
  components
• 1 - A light source ( usually a hollow cathode
  lamp )
• 2 An atom cell ( atomizer )
• 3 - A monochromator
• 4 - A detector , and read out device .

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Schematic Diagram of an Atomic Absorption
Spectrometer


Detector and readout device
Light source (hollow cathode Lamp )
atomizer
monochromator
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Atomic Absorption Spectrophotometer
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1 Light Source

• The light source is usually a hollow cathode
• lamp of the element that is being measured .
  It contains a tungsten anode and a hollow
  cylindrical cathode made of the element to be
  determined. These are sealed in a glass tube
  filled with an inert gas (neon or argon ) . Each
  element has its own unique lamp which must be
  used for
• that analysis .

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Hollow Cathode Lamp

• cathode
• 
  Anode

Quartz window
Pyrex body
Anode
Cathode
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How it works

• Applying a potential difference between the
  anode and the cathode leads to the ionization of
  some gas atoms .
• These gaseous ions bombard the cathode and
  eject metal atoms from the cathode in a process
  called sputtering. Some sputtered atoms are in
  excited states and emit radiation characteristic
  of the metal as they fall back to the ground
  state .

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Scheme of a hollow cathode lamp
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• The shape of the cathode which is hollow
  cylindrical concentrates the emitted radiation
  into a beam which passes through a quartz window
• all the way to the vaporized sample.
• Since atoms of different elements absorb
• characteristic wavelengths of light.
  Analyzing
• a sample to see if it contains a particular
  element means using light from that element .

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• For example with lead, a lamp containing lead
  emits light from excited lead atoms that produce
  the right mix of wavelengths to be absorbed by
  any lead atoms from the sample .
• A beam of the electromagnetic radiation
  emitted from excited lead atoms is passed through
  the vaporized sample. Some of the radiation is
  absorbed by the lead atoms in the sample. The
  greater the number of atoms there is in the vapor
  , the more radiation is absorbed .

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2 Atomizer

• Elements to be analyzed needs to be in
  atomic sate
• Atomization is separation of particles into
• individual molecules and breaking molecules
  into atoms .This is done by exposing the
  analyte to high temperatures in a flame
  or graphite furnace .

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• The role of the atom cell is to primarily
  dissolvate a liquid sample and then the solid
  particles are vaporized into their free gaseous
  ground state form . In this form atoms will be
  available to absorb radiation emitted from the
  light source and thus generate a measurable
  signal proportional to concentration .
• There are two types of atomization Flame and
  Graphite furnace atomization .

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Flame

• Flame AA can only analyze solutions , where
• it uses a slot type burner to increase the
• path length, and therefore to increase the
  total
• absorbance .
• Sample solutions are usually
• introduced into a nebuliser by being sucked up
  a
• capillary tube .In the nebuliser the sample is
• dispersed into tiny droplets , which can be
• readily broken down in the flame.

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• FLAME ATOMIZERS
• Used in all Atomic Spectroscopic techniques
• Converts analyte into free atoms in the form of
  vapor phase free atoms
• Heat is required
• Routes for sample introduction

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Various flame atomization techniques
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Types of Flames Used in Atomic Spectroscopy
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Processes that take place in flame
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Effect of flame temperature on excited state
population
atoms in Excited state
Boltzmann constant
Temperature
atoms in Ground state
Energy difference
Statistical factor
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For Zn N/No 10-15
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• Thus 99.998 of Na atoms are in the ground state
• Atomic emission uses Excited atoms
  
• Atomic absorption uses Ground state atoms

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Effect of flame temperature on excited state
population

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• ATOMIZATION DEVICES
• ATOMIZATION
• A process of forming free atoms by heat
• Atomizers are devices that carry out atomization
• Continuous
• Non-continuous
• Continuous (Constant temperature with time)
• Flame
• Plasma
• Non-Continuous (temperature varies with time)
• Electrothermal
• Spark discharge

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• SAMPLE INTRODUCTION SYSTEMS
• In continuous atomizers sample is constantly
  introduced in form of droplets, dry aerosol,
  vapor
• Nebulizer A device for converting the solution
  into fine spray or droplets
• Continuous sample introduction is used with
  continuous nebulizers in which a steady state
  atomic population is produced. Sample is
  introduced in fixed or discrete amounts.
• Discontinuous samplers are used with continuous
  atomizers

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1- Discrete samples are introduced into atomizers
in many ways Electrothermal atomizers a
syringe is used a transient signal is produced
as temperature changes with time and
sample is consumed 2- Indirect insertion
(Probe) sample is introduced into a probe
(carbon rod) and
mechanically moved into the atomization region
vapor cloud is transient because sample
introduced is limited
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3- Flow Injection The analyte is introduced
into the carrier stream into a nebulizer as
mist 4- Hydride Generation the volatile
sample is stripped from the analyte solution and
carried out by a gas into the atomizer. This
strip is followed by chemically converting the
analyte to hydride vapor form.
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5- With Arc Spark Solids are employed 6- Laser
Microbe Technique A beam of laser is directed
onto a small solid sample, gets vaporized,
atomized by relative heating. Either sample is
probed by encoding system or vapor produced is
swept into a second absorption or fluorescence
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• Nebulization gas is always compressed, usually
  acts as the oxidant it is oxygen (O2) in flame
  and argon (Ar) in plasma
• Nebulization chambers produce smaller droplets
  and remove or drain larger droplets called
  aerosol modifiers
• Aspiration rate is proportional to compressed
  gas pressure. The pressure drops through
  capillary, here 1/4 capillary diameters are
  recommended. This is inversely proportional to
  viscocity of the solution
• Peristaltic and/or syringe pumps could be used

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• Oxidant and fuel are usually brought into the
  nebulization chamber through a separate port.
  They mix and pass the burner head called premixed
  burner system.
• Add organic solvents to reduce the size of the
  drop

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The Atomic Absorption Spectrometer Sample
Introduction System
Nebuliser
Capillary
Solution
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• The fine mist of droplets is mixed with fuel
  ( acetylene ) , and oxidant ( nitrous oxide)
  and burned.
• The flame temperature is important
  because it influences the distribution of
  atoms. It can be manipulated by
  oxidant and fuel ratio.

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Graphite Furnace

• The graphite furnace has several advantages over
  a flame. First it accept solutions, slurries, or
  solid samples.
• Second it is a much more efficient atomizer than
  a flame and it can directly accept very small
  absolute quantities of sample. It also provides a
  reducing environment for easily oxidized
  elements. Samples are placed directly in the
  graphite furnace and the furnace is electrically
  heated in several steps to dry the sample, ash
  organic matter, and vaporize the analyte atoms.
• It accommodates smaller samples but its a
  difficult operation, because the high energy that
  is provided to atomize the sample particles into
  ground state atoms might excite the atomized
  particles into a higher energy level and thus
  lowering the precision .

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3- Monochromators

• This is a very important part in an AA
  spectrometer. It is used to separate out all of
  the thousands of lines. Without a good
  monochromator, detection limits are severely
  compromised.
• A monochromator is used to select the specific
  wavelength of light which is absorbed by the
  sample, and to exclude other wavelengths. The
  selection of the specific light allows the
  determination of the selected element in the
  presence of others.

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4 - Detector and Read out Device

• The light selected by the monochromator is
  directed onto a detector that is typically a
  photomultiplier tube , whose function is to
  convert the light signal into an electrical
  signal proportional to the light intensity.
• The processing of electrical signal is
  fulfilled by a signal amplifier . The signal
  could be displayed for readout , or further fed
  into a data station for printout by the requested
  format.

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Calibration Curve

• A calibration curve is used to determine the
  unknown concentration of an element in a
  solution. The instrument is calibrated using
  several solutions of known concentrations. The
  absorbance of each known solution is measured and
  then a calibration curve of concentration vs
  absorbance is plotted.
• The sample solution is fed into the instrument,
  and the absorbance of the element in this
  solution is measured .The unknown concentration
  of the element is then calculated from the
  calibration curve

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Calibration Curve

• A 1.0 -
• b 0.9 -
• S 0.8 -
  .
• o 0.7 - .
• r 0.6 - .
• b 0.5 - . .
• a 0.4 - .
• n 0.3 - .
• c 0.2 -
• e 0.1 -
• 10 20 30 40 50 60
  70 80 90 100
  
  
• Concentration
  ( g/ml )
  

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Determining concentration fromCalibration Curve

• A 1.0 - absorbance measured
• b 0.9 -
• S 0.8 -
  .
• o 0.7 - .
• r 0.6 - .
• b 0.5 - . .
• a 0.4 - .
• n 0.3 - .
  concentration calculated
• c 0.2 -
  
• e 0.1 -
  
• 
  
  
• 10 20 30 40 50 60
  70 80 90 100
  
  
• Concentration
  ( mg/l )
  

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Interferences

• The concentration of the analyte element is
  considered to be proportional to the ground state
  atom population in the flame ,any factor that
  affects the ground state atom population can be
  classified as an interference .
• Factors that may affect the ability of the
  instrument to read this parameter can also be
  classified as an interference .

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• The different interferences that are
  encountered in atomic absorption spectroscopy
  are
• - Absorption of Source Radiation Element other
  than the one of interest may absorb the
  wavelength being used.
• - Ionization Interference the formation of ions
  rather than atoms causes lower
  absorption of radiation .This problem is
  overcome by adding ionization suppressors.
  
• - Self Absorption the atoms of the same kind
  that are absorbing radiation will absorb
  more at the center of the line than at the
  wings ,and thus resulting in the change of
  shape of the line as well as its intensity
  .

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• - Back ground Absorption of Source Radiation
  
• This is caused by the presence of a particle
  from incomplete atomization .This
  problem is overcome by increasing the flame
  temperature .
• - Transport Interference
• Rate of aspiration, nebulization, or transport
  of the sample ( e g viscosity,
  surface tension, vapor pressure ,
  and density ) .

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2Atomic Emission Spectroscopy

• Atomic emission spectroscopy is also an
  analytical technique that is used to measure the
  concentrations of elements in samples .
• It uses quantitative measurement of the emission
  from excited atoms to determine analyte
  concentration .

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• The analyte atoms are promoted to a higher
  energy level by the sufficient energy that is
  provided by the high temperature of the
  atomization sources .
• The excited atoms decay back to lower levels
  by emitting light . Emissions are passed through
  monochromators or filters prior to detection by
  photomultiplier tubes.

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• The instrumentation of atomic emission
  spectroscopy is the same as that of atomic
  absorption ,but without the presence of a
  radiation source .
• In atomic Emission the sample is atomized and
  the analyte atoms are excited to higher energy
  levels all in the atomizer .

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Schematic Diagram of an Atomic Emission
spectrometer
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Introduction to AES

• Atomization Emission Sources
• Flame still used for metal atoms
• Electric Spark and Arc
• Direct current Plasmas
• Microwave Induced Plasma
• Inductively Coupled Plasma the most important
  technique
• Advantages of plasma
• Simultaneous multi-element Analysis saves
  sample amount
• Some non-metal determination (Cl, Br, I, and S)
• Concentration range of several decades (105
  106)
• Disadvantages of plasma
• very complex Spectra - hundreds to thousands of
  lines
• High resolution and expensive optical components
• Expensive instruments, highly trained personnel
  required

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10A Plasam Source AES

• Plasma
• an electrically conducting gaseous mixture
  containing significant concentrations of cations
  and electrons.
• Three main types
• Inductively Coupled Plasma (ICP)
• Direct Current Plasma (DCP)
• Microwave Induced Plasma (MIP)

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ICP

• Inductively Coupled Plasma (ICP)
• Plasma generated in a device called a Torch
• Torch up to 1" diameter
• Ar cools outer tube, defines plasma shape
• Rapid tangential flow of argon cools outer quartz
  and centers plasma
• Rate of Argon Consumption 5 - 20 L/Min
• Radio frequency (RF) generator 27 or 41 MHz up to
  2 kW
• Telsa coil produces initiation spark
• Ions and e- interact with magnetic field and
  begin to flow in a circular motion.
• Resistance to movement (collisions of e- and
  cations with ambient gas) leads to ohmic heating.
• Sample introduction is analogous to atomic
  absorption.

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Sample introduction

• Nebulizer
• Electrothermal vaporizer
• Table 8-2 methods of sample introducton

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Nebulizer

• convert solution to fine spray or aerosol
• Ultrasonic nebulizer
• uses ultrasound waves to "boil" solution flowing
  across disc
• Pneumatic nebulizer
• uses high pressure gas to entrain solution

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Electro-thermal vaporizer ETV

• Electrothermal vaporizer (ETV)
• electric current rapidly heats crucible
  containing sample
• sample carried to atomizer by gas (Ar, He)
• only for introduction, not atomization

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Plasma structure

• Brilliant white core
• Ar continuum and lines
• Flame-like tail
• up to 2 cm
• Transparent region
• where measurements are made (no continuum)

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Plasma characteristics

• Hotter than flame (10,000 K) - more complete
  atomization/ excitation
• Atomized in "inert" atmosphere
• Ionization interference small due to high density
  of e-
• Sample atoms reside in plasma for 2 msec and
• Plasma chemically inert, little oxide formation
• Temperature profile quite stable and uniform.

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DC plasma

• First reported in 1920s
• DC current (10-15 A) flows between C anodes and W
  cathode
• Plasma core at 10,000 K, viewing region at 5,000
  K
• Simpler, less Ar than ICP - less expensive
• Less sensitive than ICP
• Should replace the carbon anodes in several hours

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Atomic Emission Spectrometer

• May be gt1,000 visible lines (lt1 Å) on continuum
• Need
• higher resolution (lt0.1 Å)
• higher throughput
• low stray light
• wide dynamic range (gt1,000,000)
• precise and accurate wavelength
  calibration/intensities
• stability
• computer controlled
• Three instrument types
• sequential (scanning and slew-scanning)
• Multichannel - Measure intensities of a large
  number of elements (50-60) simultaneously
• Fourier transform FT-AES

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Desirable properties of an AE spectrometer
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Sequential vs. multichannel

• Sequential instrument
• PMT moved behind aperture plate,
• or grating prism moved to focus new l on exit
  slit
• Pre-configured exit slits to detect up to 20
  lines, slew scan
• characteristics
• Cheaper
• Slower
• Multichannel instrument
• Polychromators (not monochromator) - multiple
  PMT's
• Array-based system
• charge-injection device/charge coupled device
• characteristics
• Expensive ( gt 80,000)
• Faster

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Sequential vs. multichannel
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Sequential monochromator

• Slew-scan spectrometers
• even with many lines, much spectrum contains no
  information
• rapidly scanned (slewed) across blank regions
  (between atomic emission lines)
• From 165 nm to 800 nm in 20 msec
• slowly scanned across lines
• 0.01 to 0.001 nm increment
• computer control/pre-selected lines to scan

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Slew scan spectrometer

• Two slew-scan gratings
• Two PMTs for VIS and UV
• Most use holographic grating

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Scanning echelle spectrometer

• PMT is moved to monitor signal from slotted
  aperture.
• About 300 photo-etched slits
• 1 second for moving one slit
• Can be used as multi channel spectrometer
• Mostly with DC plasma source

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AES instrument types

• Three instrument types
• sequential (scanning and slew-scanning)
• Multichannel - Measure intensities of a large
  number of elements (50-60) simultaneously
• Fourier transform FT-AES

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Multichannel polychromator AES

• Rowland circle
• Quantitative det.
• 20 more elements
• Within 5 minutes

In 10 minutes
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Applications of AES

• AES relatively insensitive
• small excited state population at moderate
  temperature
• AAS still used more than AES
• less expensive/less complex instrumentation
• lower operating costs
• greater precision
• In practice 60 elements detectable
• 10 ppb range most metals
• Li, K, Rb, Cs strongest lines in IR
• Large of lines, increase chance of overlap

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Detection power of ICP-AES
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ICP/OES INTERFERENCES

• Spectral interferences
• caused by background emission from continuous or
  recombination phenomena,
• stray light from the line emission of high
  concentration elements,
• overlap of a spectral line from another element,
• or unresolved overlap of molecular band spectra.
• Corrections
• Background emission and stray light compensated
  for by subtracting background emission determined
  by measurements adjacent to the analyte
  wavelength peak.
• Correction factors can be applied if interference
  is well characterized
• Inter-element corrections will vary for the same
  emission line among instruments because of
  differences in resolution, as determined by the
  grating, the entrance and exit slit widths, and
  by the order of dispersion.

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Physical interferences of ICP

• cause
• effects associated with the sample nebulization
  and transport processes.
• Changes in viscosity and surface tension can
  cause significant inaccuracies,
• especially in samples containing high dissolved
  solids
• or high acid concentrations.
• Salt buildup at the tip of the nebulizer,
  affecting aerosol flow rate and nebulization.
• Reduction
• by diluting the sample
• or by using a peristaltic pump,
• by using an internal standard
• or by using a high solids nebulizer.

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Interferences of ICP

• Chemical interferences
• include molecular compound formation, ionization
  effects, and solute vaporization effects.
• Normally, these effects are not significant with
  the ICP technique.
• Chemical interferences are highly dependent on
  matrix type and the specific analyte element.

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Memory interferences

• When analytes in a previous sample contribute to
  the signals measured in a new sample.
• Memory effects can result
• from sample deposition on the uptake tubing to
  the nebulizer
• from the build up of sample material in the
  plasma torch and spray chamber.
• The site where these effects occur is dependent
  on the element and can be minimized
• by flushing the system with a rinse blank between
  samples.
• High salt concentrations can cause analyte signal
  suppressions and confuse interference tests.

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Typical Calibration ICP curves
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Calibration curves of ICP-AES
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10B. Arc and Spark AES

• Arc and Spark Excitation Sources
• Limited to semi-quantitative/qualitative analysis
  (arc flicker)
• Usually performed on solids
• Largely displaced by plasma-AES
• Electric current flowing between two C electrodes

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Carbon electrodes

• Sample pressed into electrode or mixed with Cu
  powder and pressed - Briquetting (pelleting)
• Cyanogen bands (CN) 350-420 nm occur with C
  electrodes in air -He, Ar atmosphere
• Arc/spark unstable
• each line measured gt20 s
• needs multichannel detection

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Arc and Spark spectrograph
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spectrograph

• Beginning 1930s
• photographic film
• Cheap
• Long integration times
• Difficult to develop/analyze
• Non-linearity of line "darkness
• Gamma function
• Plate calibration

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Multichannel photoelectric spectrometer

• multichannel PMT instruments
• for rapid determinations (lt20 lines) but not
  versatile
• For routine analysis of solids
• metals, alloys, ores, rocks, soils
• portable instruments
• Multichannel charge transfer devices
• Recently on the market
• Orignally developed for plasma sources

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Comparison Between Atomic Absorption and
Emission Spectroscopy

• Absorption
• - Measure trace metal concentrations in
  complex matrices .
• - Atomic absorption depends upon
  the number of ground state
• atoms .

• Emission
• - Measure trace metal concentrations
  in complex matrices .
• - Atomic emission depends upon the number of
  excited atoms .

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• - It measures the radiation
  absorbed by the ground state atoms.
• - Presence of a light source ( HCL )
  .
• - The temperature in the atomizer is
  adjusted to atomize the analyte atoms in
  the ground state only.

• - It measures the radiation
  emitted by the excited atoms .
• - Absence of the light source .
• - The temperature in the atomizer is big
  enough to atomize the analyte atoms and
  excite them to a higher energy level.