Atomic Emission Spectroscopy

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Atomic Emission Spectroscopy
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Atomic Emission Spectroscopy

• Atomic emission spectroscopy (AES), in contrast
  to AAS, uses the very high temperatures of
  atomization sources to excite atoms, thus
  excluding the need for lamp sources. Emission
  sources, which are routinely used in AES, include
  plasma, arcs and sparks, as well as flames. We
  will study the different types of emission
  sources, their operational principles, features,
  and operational characteristics. Finally,
  instrumental designs and applications of emission
  methods will be discussed.

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Plasma Sources

• The term plasma is defined as a homogeneous
  mixture of gaseous atoms, ions and electrons at
  very high temperatures. Two types of plasma
  atomic emission sources are frequently used
• Inductively coupled plasma
• Direct current plasma

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Inductively Coupled Plasma (ICP)

• A typical ICP consists of three concentric quartz
  tubes through which streams of argon gas flow at
  a rate in the range from 5-20 L/min. The outer
  tube is about 2.5 cm in diameter and the top of
  this tube is surrounded by a radiofrequency
  powered induction coil producing a power of about
  2 kW at a frequency in the range from 27-41 MHz.
  This coil produces a strong magnetic field as
  well.

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• Ionization of flowing argon is achieved by a
  spark where ionized argon interacts with the
  strong magnetic field and is thus forced to move
  within the vicinity of the induction coil at a
  very high speed. A very high temperature is
  obtained as a result of the very high resistance
  experienced by circulating argon (ohmic heating).
  The top of the quartz tube will experience very
  high temperatures and should, therefore, be
  isolated and cooled.

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• This can be accomplished by passing argon
  tangentially around the walls of the tube. A
  schematic of an ICP (usually called a torch
  plasma) is shown below

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• The torch is formed as a result of the argon
  emission at the very high temperature of the
  plasma. The temperature gradients in the ICP
  torch can be pictured in the following graphics

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• The viewing region used in elemental analysis is
  usually about 6000 oC, which is about 1.5-2.5 cm
  above the top of the tube. It should also be
  indicated that argon consumption is relatively
  high which makes the running cost of the ICP
  torch high as well. Argon is a unique inert gas
  for plasma torches since it has few emission
  lines. This decreases possibility of
  interferences with other analyte lines.

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Sample Introduction

• There are several methods for sample
  introduction the most widely used is, of course,
  the nebulization of an analyte solution into the
  plasma. However, other methods, as described
  earlier, are fine where vapors of analyte
  molecules or atom from electrothermal or ablation
  devices can be driven into the torch for complete
  atomization and excitation. For your convenience,
  sample introduction methods are summarized here
  again

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Samples in Solution

• 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.

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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 for convenience.

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• 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 vapors of molecules and atoms are swept into
  the plasma source for complete atomization and
  excitation.

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• 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 plasma 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. Conductive Samples
• 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 plasma by argon
  flow.

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• 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 vapors of molecules and atoms are
  swept into the plasma source for complete
  atomization and excitation. Laser ablation is
  becoming increasingly used since it is applicable
  to conductive and nonconductive samples.

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The Glow Discharge Technique

• 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. Samples should thus be conductive to
  use the technique of glow discharge. The vapors
  of molecules and atoms are swept into the plasma
  source for complete atomization and excitation by
  flowing argon. However, nonconductive samples
  were reported to be atomized by this technique
  where they were mixed with a conductor material
  like graphite or powdered copper.

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Plasma Appearance and Spectra

• A plasma torch looks very much like a flame but
  with a very intense nontransparent brilliant
  white color at the core (less than 1 cm above the
  top). In the region from 1-3 cm above the top of
  the tube, the plasma becomes transparent. The
  temperatures used are at least two to three
  orders of magnitude higher than that achieved by
  flames which may suggest efficient atomization
  and fewer chemical interferences.

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• Ionization in plasma may be thought to be a
  problem due to the very high temperatures, but
  fortunately the large electron flux from the
  ionization of argon will suppress ionization of
  all species.

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The Direct Current Plasma (DCP)

• The DCP is composed of three electrodes arranged
  in an inverted Y configuration. A tungsten
  cathode resides at the top arm of the inverted Y
  while the lower two arms are occupied by two
  graphite anodes. Argon flows from the two anode
  blocks and plasma is obtained by momentarily
  bringing the cathode in contact with the anodes.
  Argon ionizes and a high current passes through
  the cathode and anodes.

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• It is this current which ionizes more argon and
  sustains the current indefinitely. Samples are
  aspirated into the vicinity of the electrodes (at
  the center of the inverted Y) where the
  temperature is about 5000 oC. DCP sources usually
  have fewer lines than ICP sources, require less
  argon/hour, and have lower sensitivities than ICP
  sources. In addition, the graphite electrodes
  tend to decay with continuous use and should thus
  be frequently exchanged. A schematic of a DCP
  source is shown below

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• A DCP has the advantage of less argon
  consumption, simpler instrumental requirements,
  and less spectral line interference. However, ICP
  sources are more convenient to work with, free
  from frequent consumables (like the anodes in
  DCPs which need to be frequently changed), and
  are more sensitive than DCP sources.

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Advantages of Plasma Sources

• No oxide formation as a result of two factors
  including
• Very high temperature
• Inert environment inside the plasma (no oxygen)
• 2. Minimum chemical interferences
• 3. Minimum spectral interferences except for
  higher possibility of spectral line interference
  due to exceedingly large number of emission lines
  (because of high temperature)

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• 4. Uniform temperature which results in precise
  determinations
• 5. No self-absorption is observed which extends
  the linear dynamic range to higher concentrations
• 6. No need for a separate lamp for each element
• 7. Easily adaptable to multichannel analysis

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Plasma Emission Instruments

• Three classes of plasma emission instruments can
  be presented including
• 1. Sequential instruments
• In this class of instruments a single channel
  detector is used where the signal for each
  element is read using the specific wavelength for
  each element sequentially. Two types of
  sequential instruments are available

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• Linear sequential scan instruments where the
  wavelength is linearly changed with time.
  Therefore, the grating is driven by a single
  speed during an analysis of interest
• b. Slew scan instruments where the monochromator
  is preset to provide specific wavelengths moving
  very fast in between wavelengths while moving
  slowly at the specific wavelengths. Therefore, a
  two-speed motor driving the grating is thus used.

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Radial vs. Axial Viewing
Radial traditional side view, better for
concentrated samples. Axial direct view into
plasma, lower sensitivity, shifts detection range
lower.
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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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Slew scan spectrometer

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

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2. Multichannel Instruments

• This class of instruments is also referred to as
  simultaneous instruments in which all signals are
  reported at the same time using two types of
  configurations

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a. Polychromators

• Multiple detectors, usually photomultiplier
  tubes are used. Beams of radiation emerging from
  the grating are guided to exit slits (each
  representing the wavelength of a specific
  element) are focused at several PMTs for
  detection. Detection, thus, takes place
  simultaneously

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b. Array-based systems

• This multichannel type instrument uses a
  multichannel detector like a charge injection
  device or a charge-coupled device. Diffracted
  beams from a grating pass through a prism where
  further resolution of diffracted beams takes
  place by a prism. The prism will disperse the
  orders of each diffracted beam. The multichannel
  detector can also be a linear photodiode array as
  in the figure below

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3. Fourier transform instruments (FT)

• Instruments in which the signal is coded will
  need a decoding mechanism in order to see the
  signal. FT is a very common technique for
  decoding time domain spectra. In such
  instruments, the detector records the change of
  signal with time, which is practically not
  useful. However, Fourier transformation of the
  time domain signal yield a frequency domain
  spectrum, which is the usual signal, obtained by
  conventional methods. Instruments that rely on
  decoding a coded signal is also said to have a
  multiplex design.

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Applications of Plasma Sources

• 1. Since plasma sources result in a very large
  number of emission lines, these sources can be
  used for both qualitative and quantitative
  analysis.
• 2. The signal obtained from plasma sources is
  stable, has a low noise and background, as well
  as freedom from interferences.
• 3. Requires sample preparation similar to AAS

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• 4. Plasma sources are usually best suited for
  operation in the ultraviolet region, therefore,
  elements having emission lines below 180 nm (like
  B, P, S, N, and C) can be only analyzed under
  vacuum since air components absorb under 180 nm.
  Also, alkali metals are difficult to analyze
  since their best lines under plasma conditions
  occur in the near infrared.
• 5. An analytical emission line can easily be
  located but will depend on the other elements
  present since spectral line interferences are
  encountered in plasma sources due to the very
  high temperatures used.

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• 6. Linear calibration plots are usually obtained
  but departure from linearity is observed at high
  concentrations due to self absorption as well as
  other instrumental reasons. An internal standard
  is often used in emission methods to correct for
  fluctuations in temperature as well as other
  factors. The calibration plot in this case is a
  plot between the concentration of analyte and the
  ratio of the analyte to internal standard signal.
  The internal standard is a substance that is
  added in a constant amount to all samples,
  blanks, and standards therefore it must be
  absent from initial sample matrix. The internal
  standard should have very close characteristics
  (both chemically and physically) to analyte.

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Elements by ICP-AES
Different elements have different emission
intensities. Alkalis (Na, K, Rb, Cs) are weakly
emitting. Alkaline Earths (Be, Mg, Ca, Sr, Ba )
are strongly emitting.
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Concepts, Instrumentation, and Techniques in
Inductively Coupled Plasma Optical Emission
Spectrometry, Boss and Freeden, Perkin Elmer
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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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INDUCTIVELY COUPLED PLASMA-MASS
SPECTROMETRY (ICP-MS)
- Very sensitive and good for trace analysis -
Plasma produces analyte ions - Ions are directed
to a mass spectrometer - Ions are separated on
the basis of their mass-to-charge ratio - A very
sensitive detector measures ions - Very low
detection limits
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SUMMARY
Inductively Coupled Plasma Emission - High cost -
No lamp required - Low background signals - Low
interference - Moderate sensitivity Inductively
Coupled Plasma-Mass Spectrometry - Very high
cost - No lamp required - Least background
signals - Least interference - Very high
sensitivity
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Techniques for elemental analysis
ICP-MS ICP-AES FAAS GFAAS
Detection Limits Excellent Good Good
Excellent Productivity Excellent Very
good Good Low LDR 10 5 10 6 /10 10
HDD 10 3 10 2 Precision 1-3 0.3-2
0.1-1 1-5 Spectral
interference Few Common Almost none Very
few Chemical interference Moderate Few Many
Many Ionization Minimal Minimal Some
Minimal Mass efffects High on low none none
none Isotopes Yes none none
none Dissolved solids 0.1-0.4 up to 30
0.5-3 up to 30 No. of
elements 75 73 68 50 Sample
usage low medium high very
low Semi-quantitative yes yes no
no Isotope analysis yes no no
no routine operation Skill required easy easy
skill required Method
development skill required skill required easy
skill required Running
costs high high low
medium Capital costs very high high low
medium
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Emission Spectroscopy Based on Arcs and Sparks

• Samples are excited in the gap between a pair of
  electrodes connected to a high potential power
  supply (200 VDC or 2200-4400 VAC). The high
  potential applied forces a discharge between the
  two electrodes to occur where current passes
  between the two separated electrodes (temperature
  rises due to very high resistance).

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• The very high temperature (4000-5000 oC) realized
  in the vicinity between the two electrodes
  provide enough energy for atomization and
  excitation of the samples in this region or when
  the sample is, or a part of, one of the
  electrodes.
• Arc and spark methods are mainly used as
  qualitative techniques and can also be used as
  semiquantitative techniques.

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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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Sample Handling and Preparation

• 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 excited and emission is
  collected. The figure below shows some common
  shapes of graphite electrodes used in arc and
  spark sources.

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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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Instruments for Arcs and Sparks

• In most cases, emission from atoms in an arc or
  spark is directed to a monochromator with a long
  focal length and the diffracted beams are allowed
  to hit a photographic film. This typical
  instrument is called a spectrograph since it uses
  a photographic film as the detector.

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spectrograph
Beginning 1930s photographic film Cheap Long
integration times Difficult to develop/analyze Non
-linearity of line "darkness
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• The blackness of the lines on the photographic
  film is an indication of the intensity of the
  atomic line and thus the concentration of the
  analyte. The location of emission lines as
  compared to standard lines on a film serves to
  identify the wavelengths of emission lines of
  analyte and thus its identity. The use of
  spectrographs is not very convenient since a lot
  of time and precautions must be spent on
  processing and calibrating the photographic film.

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• Qualitative analysis is accomplished by
  comparison of the wavelengths of some emission
  lines to standards while the line blackness
  serves as the tool for semiquantitative analysis.
• Polychromators are also available as multichannel
  arc and spark instruments. However, these have
  fixed slits at certain wavelengths in order to do
  certain elements and thus they are not versatile.

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• Recently, arc and spark instruments based on
  charge injection and charge coupled devices
  became available. These have extraordinarily high
  efficiency and performance in terms of easier
  calibration, short analysis time, as well as
  superior quantitative results.

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Characteristics of Arc Sources

• 1. Typical temperatures between 4000-5000 oC are
  high enough to cause atomization and excitation
  of sample and electrode materials.
• 2. Usually, cyanogens compounds are formed due to
  reaction of graphite electrodes with atmospheric
  nitrogen. Emission bands from cyanogens compounds
  occur in the region from 350-420 nm.
  Unfortunately, several elements have their most
  sensitive lines in this same region which limits
  the technique. However, use of controlled
  atmosphere around the arc (using CO2, Helium, or
  argon) very much decreases the effect of
  cyanogens emission.

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• 3. The emission signal should be integrated over
  a minute or so since volatilization and
  excitation of atoms of different species differ
  widely. While some species give maximum signal,
  others may still be in the molecular state.
• 4. Arc sources are very good for qualitative
  analysis of elements while only semiquantitative
  analysis is possible. It is mandatory to compare
  the emission spectrum of a sample with the
  emission spectrum of a standard. In some cases, a
  few milligrams of a standard is added to the
  sample in order to locate the emission lines of
  the standard and thus identify the emission
  wavelengths of the different elements in the
  sample. A comparator densitometer can be used to
  exactly locate the wavelengths of the standard
  and the sample components.

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The lines from the standard are projected on the
lines of the combined sample/standard emission
spectra in order to identify sample components.
Only few lines are shown in the figure.
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Why use Carbon in Atomic Spectroscopy?

• We have previously seen the use of graphite in
  electrothermal AAS as well as arc and spark AES,
  even though molecular spectra are real problems
  in both techniques due to cyanogens compounds
  absorption and emission. The reasons after
  graphite common use in atomic spectroscopy can be
  summarized below

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1. It is conductive.
2. It can be obtained in a very pure state.
3. Easily available and cheap.
4. Thermally stable and inert.
5. Carbon has few emission lines.
6. Easily shaped.

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Spark Sources

• Most of the instruments in this category are arc
  based instruments. Spark based instruments are of
  the same idea except for a spark source
  substituting an arc source. The spark source is
  constructed as in the figure below where an AC
  potential in the order of 10-50 KV is discharged
  through a capacitor which is charged and
  discharged through the graphite electrodes about
  120 times/s resulting in a discharge current of
  about 1000 A.

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This very high current will suffer a great deal
of resistance, which increase the temperature to
an estimated 40000 oC. Therefore, ionic spectra
are more pronounced.