Atomic%20Emission%20Spectroscopy%20Lecture%2018

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Atomic Emission SpectroscopyLecture 18

• Arcs, Sparks And Plasma

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Atomic Emission Spectroscopy )AES)

• - (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
• plasma, 2) arcs and 3) sparks, 4) 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 (ICP).
• Direct current plasma (DCP).

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Inductively Coupled Plasma (ICP)

• A typical ICP consists
• 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 (1 inch) in
  diameter and the top of this tube is surrounded
  by a radiofrequency powered induction coil (RF)
  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.
• - The 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 (collisions of e- and cations
  with ambient gas) (ohmic heating).
• Why Ar used?? 1- inert 2- few emission lines

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• - The top of the quartz tube will experience very
  high temperatures and should, therefore, be
  isolated and cooled.
• Cooling
• 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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Approach spark or arc to ionize Ar and so Ar
ions circulate fatly and suffers from resistance
so temperature increases
Current passes in the coil and form MF ? to the
current in the coil
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Inductively Coupled Plasma Torch
Radio frequency induction coil 27-41 MHz 0.5-2KW
Plasma torch F 2.5 cm
Tangential flow isolates the torch from plasma
Plasma argon
Sample argon
Argon flow rate 5-20 L/min
Tangential argon plasma support flow
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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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• Problems
• Needs very large amount of Ar
• Needs large amount of Energy
• Main Advantage
• Pure Atomization

Cross flow nebulizer
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Viewing region for working
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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.
• High cost of the ICP torch
• Because argon consumption is relatively high
  which makes the running
• 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

• Several methods for sample introduction
• The most widely used is
• 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

• 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
• (Only for sample introd. Not for atomization)
• 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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Electrothermal Vaporization
To ICP
Sample
Graphite rod
Heater power
Water coolant
Argon inlet
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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 of the tube).
• 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 is not a problem
• It is 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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2) 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.
• 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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• DCP advantages
• Less argon consumption about 1/3 the ICP and
  less E of power supply.
• Simpler instrumental requirements.
• less spectral line interference (lower
  atomization temp. about 5000 0C).
• However
• ICP sources are more convenient to work with
• - ICP is free from frequent consumables (like
  the anodes in DCPs which need to be frequently
  changed)
• - 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 (no or few
  ionization) es from the ionized Ar suppress
  ioniztion.
• 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
  (simultaneous measurements of many elements).

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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
• The signal for each element is read using the
  specific wavelength for each element
  sequentially.
• each element is measured after the another.
• Two types of sequential instruments are
  available

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• Linear sequential scan instruments
• ???? ??????
• 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
• 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. and 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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Axial view

• Much more light available
• This gives you the opportunity to achieve Lower
  Detection Limits than Radial Plasma
• Disadvantages
• More Matrix Interferences
• Slightly Reduced Dynamic Range

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Radial view
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Filter wheel, to remove orders of radiation
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Sequential vs. multichannel

• Sequential instrument
• PMT moved behind aperture plate,(slits found for
  elements at their ? at the focal plane).
• 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 - FT Instruments
• charge-injection device/charge coupled device
• characteristics
• Expensive ( gt 80,000-150,000)
• 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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Slew Scan Spectrometer
Exit slit
Composite grating
Mirror
Filter wheel
Photomultiplier tubes
Motorized observation height
Plasma torch
Mirrors
Hg lamp
Entrance slit
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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 (do not confused with
monochromator)

• Multiple detectors each measure 1 ?
• 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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Many slits at each a detector present
simultaneous measring
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Grating
Many PMT for the elements to be analyzed
Measuring electronics
Dedicated computer
Instrument control electronics
Schematic of an ICP polychromator
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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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PDA
Problems Spectral line interferences should be
eliminated the detector need to be very small to
prevent more than one line to hit any detector
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To overcome the previous problem
Prism to further resolve unresolved lines and the
orders of any line
Echele grating (high Res, use the order)
Could be used for qualitative analysis
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Long distance for G and CM give large angle and
good dispersion
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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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Atomic Emission Spectroscopy

• Lecture 20

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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 only be analyzed under
  vacuum since air components absorb under 180 nm.
• Alkali metals are difficult to analyze since
  their best lines under plasma conditions occur in
  the visible or 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.
• Problem
• departure from linearity is observed at high
  concentrations
• due to self absorption as well as other
  instrumental reasons.
• Overcome
• 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 (sensitive to low con)
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background
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Typical Calibration Curves
Tl 334.94 nm
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Photocurrent ( na)
1
V 437.92 nm
Nb 371.30 nm
Ce 456.24 nm
0.001
0.0001
0.1
0.01
Impurity concentration in iron ( wt)
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INDUCTIVELY COUPLED PLASMA-MASS
SPECTROMETRY (ICP-MS)
The detector is the MS (used to measure AW for
different atoms) - 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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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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One electrode is graphite and the other is the
sample the two electrodes approach to each others
Arc
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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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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
• (in some old universities)
• photographic film detector
• Cheap
• Long integration times (20-30 s to obtain stable
  signal).
• Difficult to develop/analyze
• Non-linearity of line "darkness on the
  photograph film.

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Very long monochromator 2-3 m For good separation
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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
• Superior quantitative results.

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For Qual. and Quant. Analysis
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Characteristics of Arc Sources

• 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.
• Disadvantage
• Several elements have their most sensitive lines
  in this same region which limits the technique.
• Overcome
• 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 construction
• 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.
Two electrodes
Capacitor Charged and discharge periodically in
the electrodes rapidly giving spark (1000 A)