General ICPAES

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General ICP-AES

• Basic ICP-ES
• Excitation
• Plasma
• Sample introduction
• Optics
• RF
• Gas control
• Data acquistion and communications

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General ICP-AES

• Basic ICP-ES
• Inductively Coupled Plasma spectrometers are
  scientific instruments that use emission
  spectroscopy to quantify or qualify elements in a
  sample.
• Atomic emission spectroscopy is the technique for
  detecting and measuring chemical elements in
  analytical samples. The technique measures the
  intensity of light emitted by atoms or ions of
  the elements of interest at a specific
  wavelength.
• The sample to be analyzed must first be heated to
  a very high temperature. This is done by
  introducing a sample into an excitation source

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General ICP-AES

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General ICP-AES

• Excitation
• Atoms become excited by absorbing energy, usually
  by collision with other atoms (that is by heat).
• The absorbed energy causes an electron in the
  outer shell to move to a higher energy orbit.

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General ICP-AES

• Excitation
• Such excited atoms are unstable, and the electron
  quickly returns to a less energetic orbit
• The energy difference between the two orbits is
  ejected from the atom in the form of light
• The light is of a wavelength that is
  characteristic of the atom and therefore the
  element
• A spectrometer that is set to a wavelength of
  interest will then measure the intensity of the
  light emitted at that wavelength
• The intensity of the light is proportional to the
  number of atoms in the excitation source of the
  element of interest

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General ICP-AES

• Plasma
• A plasma is a gas that contains a significant
  fraction of ions and free electrons.

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General ICP-AES

• Plasma
• A gas is an electrical insulator
• A plasma conducts electricity
• Only a small fraction of the atoms in a gas need
  to be ionized to form a plasma. Argon gas in the
  ICP plasma normally has less than 1 ions
• An inductively coupled plasma is achieved by the
  ionization of argon gas in a radio frequency
  magnetic field
• An ion is an atom that carries a charge due to
  the loss or gain of an electron

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General ICP-AES

• Plasma
• Ionization in a plasma is triggered when argon is
  passed through a rapidly changing magnetic field
  and is then seeded with electrons from a spark
  discharge
• The electrons from of the spark discharge
  accelerate through the gas and the changing
  magnetic field
• The accelerated electrons collide with argon
  atoms and knock electrons from them
• The electron collisions with the argon atoms
  cause the release of more electrons from other
  argon atoms, resulting in argon ions
• These collisions are sustained by the influence
  of the magnetic field
• Through the influence of the magnetic field the
  argon atoms and ions continue to collide forming
  more ions. The formation of ions allow a plasma
  state to form and become self sustained

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General ICP-AES

• Plasma

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General ICP-AES

• Plasma
• When ions re-combine with free electrons, the
  approaching electron loses energy by emitting
  light over a wide range of wavelengths.
• The emission over the wide range of wavelengths
  is known as a continuum.
• The plasma generates a baseline of continuum
  emission that comes mainly from the
  re-combination of ion pairs
• Once the free electron is trapped by the ion it
  is constrained to exist in specific orbits
• Upon recombination of an electron with a
  singly-charged ion the atom no longer carries a
  charge and is no longer an ion

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General ICP-AES

• Ionic emission
• Ions also emit light through ionic emission.
• An ion will absorb energy, usually by collision
  with other ions and atoms.
• The absorbed energy causes an electron of the ion
  to move to a higher energy orbit
• The electron in the ion quickly returns to a
  lower orbit level
• The energy difference between the two orbits is
  ejected from the ion in the form of light
• The light is of a wavelength that is
  characteristic of the ion and therefore the
  element

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General ICP-AES

• Ionic emission
• As the atomic structure of an ion of a certain
  element is physically different from the atomic
  structure of an atom of the same element, an ion
  of a certain element will emit light at different
  wavelengths than an atom of the same element.
• The background emission of the plasma consists of
  the continuum emission of ion recombination, the
  ionic emission from the argon ions and atomic
  emission of the excited argon atoms.

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General ICP-AES

• Atomization
• Atomization is the physical process where gaseous
  molecules are broken down into simple elements
• Molecules are atomized by heat
• Argon ions and electrons, under the influence of
  the magnetic field flow in the horizontal plane
  of the RF coil
• The ions and electrons collide with the neutral
  argon atoms.
• The collisions with the neutral argon atoms
  result in the generation of temperatures of up to
  10,000ºK
• In theory, the point of the greatest activity
  between ions, electrons and neutral atoms will be
  the point of the highest temperature.

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General ICP-AES

• Atomization

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General ICP-AES

• Atomization
• As the magnetic field becomes less of an
  influence on the ions, electrons and neutral
  argon atoms, fewer recombinations and collisions
  occur
• As excitation decreases so does the temperature
  of the plasma
• This causes a formation of a temperature gradient
  over the area of the plasma
• An inductively coupled plasma tends to become
  hollow in the middle
• The plasma is an electrical conductor. The outer
  parts of the plasma shield the inner parts from
  the influence of the induction coil. The
  interaction between the plasma and the changing
  magnetic field of the coil is concentrated in the
  outer parts of the plasma. This is known as the
  skin depth effect

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General ICP-AES

• Atomization
• The gas flow pattern produced by the torch
  creates a region of lower pressure in the center
  of the plasma
• The skin effect and gas flow sustain a plasma
  that is more effective in the outer regions of
  the plasma
• The stream of gas from the nebulizer passes
  through the torch injector and punches a channel
  through the center of the plasma
• The central channel is cooler than the
  surrounding plasma (5000º-7000º K)
• Through the central channel particles in the form
  of an aerosol are carried for excitation to
  atomic and ionic states

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General ICP-AES

• Hard and soft emission lines
• The temperature gradient and shape of the plasma
  allows for the excitation of both hard and soft
  emission lines
• Hard lines react to power settings, gas flows and
  nebulizer pressure differently than soft lines
• The energy difference emitted by electrons
  changing orbit levels in both atoms and ions is
  characteristic of the wavelength of the light
  emitted
• The shorter the wavelength the greater the amount
  of energy released as the electron returns to the
  less energetic orbit.
• The greater the amount of energy released the
  larger the amount of energy required to achieve
  the excited state.
• Hard lines are classified as wavelengths lower
  than 235 nm

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General ICP-AES

• Hard and soft emission lines
• Soft lines are classified as wavelengths above
  235 nm
• Higher power levels will tend to increase the
  intensity of hard lines, while higher power
  levels tend to have little effect on soft lines
• Reducing the flow of the stream of gas through
  the central channel will also increase the amount
  of time that particles will preside in the plasma
• Reduction in the flow gas will tend to increase
  the intensity of hard lines while changes in the
  flow of gas will have little effect on the soft
  lines

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General ICP-AES

• Sample introduction

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General ICP-AES

• Sample introduction
• The function of the sample introduction system is
  to deliver uniform sample amounts to the plasma
  for excitation of atomic/ionic emission
• The sample introduction system combines a sample
  together with a carrier gas and transports it to
  the plasmas central channel
• As the sample passes through the plasma it
  rapidly changes state.
• The plasma as an excitation source offers two
  physical means for emission
• Atomization
• Ionization

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General ICP-AES

• Sample introduction
• Most elements when excited by a plasma source
  emit radiation in both ways.
• Emission lines that result from atomic excitation
  are classified as a type I lines.
• Emission lines that from ionic excitation are
  classified as type II lines.

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General ICP-AES

• Sample introduction
• ICP-ES offers three commercially available
  solutions for sampling.
• Gas
• Solid
• Liquid
• ICP-ES primarily is used to analyze liquids.

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General ICP-AES

• Sample introduction

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General ICP-AES

• Sample introduction
• The process of delivering a liquid sample into
  the plasma involves the breaking up of a stream
  of liquid with a carrier gas
• The liquid droplets and carrier gas combine to
  produce an aerosol
• This process is carried out by a device known as
  a nebulizer
• The flow of the liquid sample into a nebulizer is
  controlled by tubing fitted on a peristaltic pump
  which rotates at user specified speeds
• The speed of the pump and the physical size of
  the tubing regulates the amount of sample that
  enters the nebulizer

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General ICP-AES

• Sample introduction
• The nebulizer forms an aerosol by pneumatic or
  ultrasonic means.
• There are two basic types of pneumatic
  nebulizers.
• V-groove
• Concentric

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General ICP-AES

• Sample introduction
• V-groove
• Most V-groove nebulizers are made from inert
  materials such as specially selected plastic

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General ICP-AES

• Sample introduction
• V-groove
• Sample is pumped through a 1 to 2 mm hole
• Carrier gas is fed through a second hole which is
  located close to the sample output hole
• The sample and carrier gas holes are positioned
  so that the output of each is aligned on the same
  axis in a V-shaped trough
• The sample flows along a V-shaped channel where
  it is captured by the venturi effect created by
  the carrier gas
• The carrier gas and sample combine to form an
  aerosol

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General ICP-AES

• Sample introduction
• Concentric
• A typical glass concentric nebulizer uses a
  venturi effect.

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General ICP-AES

• Sample introduction
• Concentric
• Sample solution is drawn through a central
  capillary that is surrounded by an outer channel
  that a carrier gas is fed through
• The carrier gas forced through the outer channel,
  passes by the end of the sample capillary,
  lowering the pressure surrounding the tip of the
  capillary extracting the sample

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General ICP-AES

• Sample introduction
• Ultrasonic
• A typical ultrasonic nebulizer uses the vibration
  of a piezo-electric transducer to form an aerosol.

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General ICP-AES

• Sample introduction
• Ultrasonic
• The sample flows over a glass plate fixed to the
  transducer where the ultrasonic vibrations cause
  the aerosol to form.
• The carrier gas sweeps the sample aerosol into a
  heated tube that is connected to a desolvator.
• The dried sample is then introduced into the
  plasma.

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General ICP-AES

• Sample introduction
• Spraychamber
• The aerosol must be injected into the plasma at a
  uniform rate without causing plasma
  destabilisation
• In addition to this the aerosol that is injected
  into the plasma must also contain a sufficient
  number of small droplets that are reproducible
  and representative of the sample
• A spraychamber is used to remove the larger
  droplets from the aerosol while providing a
  uniform flow of aerosol to the torch
• The aerosol is sprayed directly into a spray
  chamber which removes the larger droplets from
  the aerosol
• The spraychamber allows the aerosol to travel to
  the transfer tube and torch through an indirect
  route

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General ICP-AES

• Sample introduction
• Spraychamber
• While passing through the chamber the larger
  droplets fall out of the aerosol and are removed
  through a drain, tubing and peristaltic pump to
  waste.

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General ICP-AES

• Sample introduction
• Torch
• The torch confines ionized argon gas in the RF
  field of the induction coil and introduces the
  fine sample aerosol from the spraychamber to the
  plasma preheating zone.
• A standard torch assembly consists of three
  concentric tubes.
• The outer wall forms the channel that carries the
  plasma gas flow
• The plasma flow keeps the plasma from overheating
  the torch

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General ICP-AES

• Sample introduction
• Torch
• The intermediate tube separates the auxiliary
  flow from the plasma.
• The auxiliary gas flow provides a positive
  pressure at the base of the plasma which lifts
  the plasma and keeps it from interacting with the
  top of the auxiliary and injector tubes.

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General ICP-AES

• Sample introduction
• Torch
• The injector tube is the inner most tube and
  carries the sample aerosol to the plasma
• The flow of the sample aerosol is determined by
  the carrier gas flow rate
• The design of the torch produces low pressure at
  the center of the plasma
• By design the sample is fed through this low
  pressure region

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General ICP-AES

• Sample introduction
• Torch
• The four processes a liquid sample undergoes are
• Desolvation
• Vaporization
• Molecular decomposition into elements
  (Atomization)
• Excitation and ionization
• The sample aerosol under goes the same
  transitions as the argon that forms the plasma

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General ICP-AES

• Sample introduction

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General ICP-AES

• Optics

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General ICP-AES

• Optics
• An ICP-ES optics system gathers the radiated
  emissions from the plasma.
• The emissions are then separated into their
  characteristic wavelengths.
• The characteristic wavelengths of interest are
  then analyzed.

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General ICP-AES

• Optics
• There are basically two different types of ICP-ES
  spectrometers on the market
• sequential
• simultaneous
• These terms relate to the way optics separate the
  characteristic wavelengths for analysis

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General ICP-AES

• Optics
• Sequential
• A sequential ICP-ES uses a scanning monochromator
  that gathers the radiant emissions and focuses
  this incident light onto a diffraction grating.
• The grating is rotated into a position to direct
  only the characteristic wavelength of interest
  onto a detector for analysis.
• Most commercially available sequential ICP-ES
  instruments use a Czerny-Turner configuration.

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General ICP-AES

• Optics
• Sequential

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General ICP-AES

• Optics
• Simultaneous
• There are two basic simultaneous configurations
  currently commercially available
• Rowland circle
• Echelle

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General ICP-AES

• Optics
• Simultaneous
• Rowland circle

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General ICP-AES

• Optics
• Simultaneous
• Rowland circle
• A Rowland simultaneous ICP-ES uses a stationary
  monochromator that gathers radiant emissions and
  focuses incident light onto a single spherical
  diffraction grating
• The grating is designed to direct the spectrum of
  light to a number of PMT detectors which are
  arranged in a circle.
• Each PMT is physically placed for each
  characteristic wavelength that is to be analyzed.
• Therefore for each wavelength of interest a
  detector in a specific location must be used.

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General ICP-AES

• Optics
• Simultaneous
• Echelle

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General ICP-AES

• Optics
• Simultaneous
• Echelle
• An Echelle simultaneous ICP-ES uses a
  polychromator that gathers the radiant emissions
  and focuses this incident light onto two
  stationary dispersive elements
• The first dispersive element is a grating. The
  grating is usually ruled to disperse the incident
  light into a spectrum across the vertical optics
  plane
• The second dispersive element is generally a
  prism. The prism is manufactured and mounted to
  project the vertical spectrum from the grating
  into a two dimensional optical matrix

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General ICP-AES

• Optics
• Simultaneous
• Echelle
• The prism does this by further dispersing the
  vertically orientated full spectrum across the
  horizontal optics plane
• Having been dispersed in two planes the resulting
  image now represents a two dimensional optical
  matrix
• The matrix is composed of a composite of the
  entire spectrum where lowest wavelength is
  positioned in one extreme, (ie lower right hand
  corner) and the highest wavelength is positioned
  in the opposite extreme, (ie upper left hand
  corner)
• The two dimensional spectrum is then observed by
  a solid state detector

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General ICP-AES

• RF

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General ICP-AES

• RF
• The function of the plasma generation system is
  to deliver high energy RF current through the
  induction coil. The alternating current through
  the induction coil provides the magnetic fields
  required to produce and sustain a plasma as an
  excitation source
• Plasma generation systems for commercially
  available ICP-ES instruments are generally PC
  controlled. The software allows the operator of
  the instrument to select the level of RF power
  required by the type of analysis of interest
• Plasma generation systems consist of an RF system
  and control circuitry

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General ICP-AES

• RF
• There are two frequencies currently commercially
  available
• 27 MHz
• 40 MHz
• 40 MHz RF systems are seen to have reduced
  background and provide greater plasma stability,
  particularly for organic analysis
• ICP-ES RF systems are required to produce uniform
  power levels under the varying conditions of
  sample loading
• Two main requirements have to be met to reduce
  these effects
• Impedance matching
• level control

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General ICP-AES

• Impedance matching
• Impedance matching is required to maintain
  oscillations in a tuned circuit
• Impedance is the measure of resistance in a given
  circuit to an alternating current at a particular
  frequency
• The free electrons in the plasma acquire energy
  from the inductive coupling of the high energy RF
  magnetic field
• The amount of inductance between the electrons
  and the fields vary

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General ICP-AES

• Impedance matching
• The inductive coupling varies particularly at
  ignition
• The coupling also varies during operation
  according to what type of sample is being
  analyzed
• As the inductance changes the impedance match
  becomes less efficient at that given frequency
• To improve the matching and maintain the
  oscillations in the circuit, the frequency or the
  coupling must be varied.

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General ICP-AES

• Level control
• In order to control the amount of RF power
  supplied to the plasma a sample of the RF energy
  must be made
• The level of alternating current passing through
  the induction coil or the amplitude of the RF
  signal being transferred to the coil provide an
  indication of the amount of RF energy available
  to the plasma
• This level must then be compared with the
  operator selected power level
• The difference of the desired value to the known
  value then results in the control circuitry
  increasing or decreasing the amount of energy
  applied to the RF system

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General ICP-AES

• RF Control circuitry
• The control circuitry provides a computer
  interface for the level control of the RF system
• It also provides an interlock monitoring system
  for operator safety and equipment protection

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General ICP-AES

• Gas control

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General ICP-AES

• Gas control
• The purpose of a gas control assembly is to
  regulate and control the supply of required gas
  flows throughout the ICP-ES
• Most gas control assemblies supply the gas
  required for
• torch/plasma
• nebulizer
• optics
• Commercial ICP-ES instruments use argon for the
  plasma
• Nitrogen is used on some instruments as an optics
  purge
• Oxygen is often used as carrier gas additive when
  organic solvents are being analyzed

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General ICP-AES

• Data Acquisition / communication

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General ICP-AES

• Data Acquisition
• The purpose of the data acquisition assembly is
  to convert the proportional electrical current
  from the optical detector into suitable digital
  information for data processing by the
  controlling PC software
• Communications
• The purpose of the communications system is
  provide a means for command and control of all
  the internal assemblies while providing an
  interface for the instrument to communicate with
  the PC software and the various instrument
  accessories.