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Transcript of General2icp Aes
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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
u Basic ICP-ES
Inductively Coupled Plasma spectrometers are scientific instruments thatuse emission spectroscopy to quantify or qualify elements in a sample.
Atomic emission spectroscopy is the technique for detecting andmeasuring chemical elements in analytical samples. The techniquemeasures the intensity of light emitted by atoms or ions of the elementsof interest at a specific wavelength.
The sample to be analyzed must first be heated to a very hightemperature. This is done by introducing a sample into an excitationsource
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S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
General ICP-AES
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General ICP-AES
u Excitation
Atoms become excited by absorbing energy, usually by collision withother atoms (that is by heat).
The absorbed energy causes an electron in the outer shell to move to ahigher energy orbit.
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General ICP-AES
u Excitation
Such excited atoms are unstable, and the electron quickly returns to aless energetic orbit
The energy difference between the two orbits is ejected from the atom inthe form of light
The light is of a wavelength that is characteristic of the atom andtherefore the element
A spectrometer that is set to a wavelength of interest will then measurethe intensity of the light emitted at that wavelength
The intensity of the light is proportional to the number of atoms in theexcitation source of the element of interest
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S o l i d L i q u i d G a s P l a s m a
T e mp e ra t ur e ( )
M e l t sB o il s o r V a p o r i z e s I o n i z e s
General ICP-AES
u Plasma
A plasma is a gas that contains a significant fraction of ions and freeelectrons.
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General ICP-AES
u 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 aplasma. Argon gas in the ICP plasma normally has less than 1% ions
An inductively coupled plasma is achieved by the ionization of argon gasin a radio frequency magnetic field
An ion is an atom that carries a charge due to the loss or gain of anelectron
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General ICP-AES
u Plasma
Ionization in a plasma is triggered when argon is passed through arapidly changing magnetic field and is then seeded with electrons from aspark discharge
The electrons from of the spark discharge accelerate through the gasand the changing magnetic field
The accelerated electrons collide with argon atoms and knock electronsfrom them
The electron collisions with the argon atoms cause the release of moreelectrons 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 ionscontinue to collide forming more ions. The formation of ions allow aplasma state to form and become self sustained
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General ICP-AES
u Plasma
Ar , A r + , e -
cM agne ti
F i e ld
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General ICP-AES
u Plasma
When ions re-combine with free electrons, the approaching electronloses energy by emitting light over a wide range of wavelengths.
The emission over the wide range of wavelengths is known as acontinuum.
The plasma generates a baseline of continuum emission that comesmainly from the re-combination of ion pairs
Once the free electron is trapped by the ion it is constrained to exist inspecific orbits
Upon recombination of an electron with a singly-charged ion the atom nolonger carries a charge and is no longer an ion
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General ICP-AES
u 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 higherenergy 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 inthe form of light
The light is of a wavelength that is characteristic of the ion and thereforethe element
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General ICP-AES
u Ionic emission
As the atomic structure of an ion of a certain element is physicallydifferent from the atomic structure of an atom of the same element, anion of a certain element will emit light at different wavelengths than anatom of the same element.
The background emission of the plasma consists of the continuumemission of ion recombination, the ionic emission from the argon ionsand atomic emission of the excited argon atoms.
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General ICP-AES
u Atomization
Atomization is the physical process where gaseous molecules arebroken 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 oftemperatures of up to 10,000K
In theory, the point of the greatest activity between ions, electrons andneutral atoms will be the point of the highest temperature.
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General ICP-AES
u Atomization
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General ICP-AES
u Atomization
As the magnetic field becomes less of an influence on the ions, electronsand 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 plasmashield the inner parts from the influence of the induction coil. Theinteraction between the plasma and the changing magnetic field of thecoil is concentrated in the outer parts of the plasma. This is known as theskin depth effect
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General ICP-AES
u Atomization
The gas flow pattern produced by the torch creates a region of lowerpressure in the center of the plasma
The skin effect and gas flow sustain a plasma that is more effective inthe outer regions of the plasma
The stream of gas from the nebulizer passes through the torch injectorand punches a channel through the center of the plasma
The central channel is cooler than the surrounding plasma(5000-7000 K)
Through the central channel particles in the form of an aerosol arecarried for excitation to atomic and ionic states
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General ICP-AES
u Hard and soft emission lines
The temperature gradient and shape of the plasma allows for theexcitation of both hard and soft emission lines
Hard lines react to power settings, gas flows and nebulizer pressuredifferently than soft lines
The energy difference emitted by electrons changing orbit levels in bothatoms and ions is characteristic of the wavelength of the light emitted
The shorter the wavelength the greater the amount of energy releasedas the electron returns to the less energetic orbit.
The greater the amount of energy released the larger the amount ofenergy required to achieve the excited state.
Hard lines are classified as wavelengths lower than 235 nm
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General ICP-AES
u 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, whilehigher 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
u Sample introduction
S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
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General ICP-AES
u Sample introduction
The function of the sample introduction system is to deliver uniformsample amounts to the plasma for excitation of atomic/ionic emission
The sample introduction system combines a sample together with acarrier 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
u Sample introduction
Most elements when excited by a plasma source emit radiation in bothways.
Emission lines that result from atomic excitation are classified as a type Ilines.
Emission lines that from ionic excitation are classified as type II lines.
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General ICP-AES
u 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
u Sample introduction
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General ICP-AES
u Sample introduction
The process of delivering a liquid sample into the plasma involves thebreaking 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 fittedon a peristaltic pump which rotates at user specified speeds
The speed of the pump and the physical size of the tubing regulates theamount of sample that enters the nebulizer
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General ICP-AES
u 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
u Sample introduction
V-groove
Most V-groove nebulizers are made from inert materials such asspecially selected plastic
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General ICP-AES
u 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 tothe sample output hole
The sample and carrier gas holes are positioned so that the outputof each is aligned on the same axis in a V-shaped trough
The sample flows along a V-shaped channel where it is captured bythe venturi effect created by the carrier gas
The carrier gas and sample combine to form an aerosol
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sample in
General ICP-AES
u Sample introduction
Concentric
A typical glass concentric nebulizer uses a venturi effect.
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General ICP-AES
u Sample introduction
Concentric
Sample solution is drawn through a central capillary that issurrounded 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 ofthe capillary extracting the sample
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desolvatedaerosolto ICP
coolantin
condenser
coolant
out
heated(140C)U-tube
sample inlet
drain
drain
transducer
RF1.4 MHz
Argon in
aerosolchamber
General ICP-AES
u Sample introduction
Ultrasonic
A typical ultrasonic nebulizer uses the vibration of a piezo-electrictransducer to form an aerosol.
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General ICP-AES
u Sample introduction
Ultrasonic
The sample flows over a glass plate fixed to the transducer wherethe 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
u Sample introduction
Spraychamber
The aerosol must be injected into the plasma at a uniform ratewithout 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 arereproducible and representative of the sample
A spraychamber is used to remove the larger droplets from theaerosol while providing a uniform flow of aerosol to the torch
The aerosol is sprayed directly into a spray chamber which removesthe larger droplets from the aerosol
The spraychamber allows the aerosol to travel to the transfer tubeand torch through an indirect route
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General ICP-AES
u Sample introduction
Spraychamber
While passing through the chamber the larger droplets fall out of theaerosol and are removed through a drain, tubing and peristalticpump to waste.
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General ICP-AES
u Sample introduction
Torch
The torch confines ionized argon gas in the RF field of the inductioncoil and introduces the fine sample aerosol from the spraychamberto 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
u 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 theplasma 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
u Sample introduction
Torch
The injector tube is the inner most tube and carries the sampleaerosol to the plasma
The flow of the sample aerosol is determined by the carrier gas flow
rateThe design of the torch produces low pressure at the center of theplasma
By design the sample is fed through this low pressure region
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General ICP-AES
u 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 argonthat forms the plasma
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General ICP-AES
u Sample introduction
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General ICP-AES
u Optics
S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
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General ICP-AES
u Optics
An ICP-ES optics system gathers the radiated emissions from theplasma.
The emissions are then separated into their characteristic wavelengths.
The characteristic wavelengths of interest are then analyzed.
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General ICP-AES
u Optics
There are basically two different types of ICP-ES spectrometers on themarket:
sequential
simultaneous
These terms relate to the way optics separate the characteristicwavelengths for analysis
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General ICP-AES
u Optics
Sequential
A sequential ICP-ES uses a scanning monochromator that gathersthe radiant emissions and focuses this incident light onto a diffractiongrating.
The grating is rotated into a position to direct only the characteristicwavelength of interest onto a detector for analysis.
Most commercially available sequential ICP-ES instruments use aCzerny-Turner configuration.
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PMT
Exit slit
Entryslit
Window
Plasmatorch
Grating
M5
M4
General ICP-AES
u Optics
Sequential
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General ICP-AES
u Optics
Simultaneous
There are two basic simultaneous configurations currentlycommercially available:
Rowland circle
Echelle
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PMTs
Exit slits
Entryslit
Window
Plasmatorch
FixedGrating
General ICP-AES
u Optics
Simultaneous
Rowland circle
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General ICP-AES
u 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 gratingThe 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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e ec or
Entryslit
Lens
Plasmatorch
Grating
Prism Window
General ICP-AES
u Optics
Simultaneous
Echelle
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General ICP-AES
u 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 elementsThe 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
u 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 nowrepresents 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 statedetector
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S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
General ICP-AES
u RF
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General ICP-AES
u RF
The function of the plasma generation system is to deliver high energyRF current through the induction coil. The alternating current through theinduction coil provides the magnetic fields required to produce andsustain a plasma as an excitation source
Plasma generation systems for commercially available ICP-ES
instruments are generally PC controlled. The software allows theoperator of the instrument to select the level of RF power required by thetype of analysis of interest
Plasma generation systems consist of an RF system and control circuitry
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General ICP-AES
u 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
u Impedance matching
Impedance matching is required to maintain oscillations in a tuned circuit
Impedance is the measure of resistance in a given circuit to analternating current at a particular frequency
The free electrons in the plasma acquire energy from the inductivecoupling 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
u Impedance matching
The inductive coupling varies particularly at ignition
The coupling also varies during operation according to what type ofsample is being analyzed
As the inductance changes the impedance match becomes less efficientat that given frequency
To improve the matching and maintain the oscillations in the circuit, thefrequency or the coupling must be varied.
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General ICP-AES
u Level control
In order to control the amount of RF power supplied to the plasma asample of the RF energy must be made
The level of alternating current passing through the induction coil or theamplitude of the RF signal being transferred to the coil provide anindication 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 thecontrol circuitry increasing or decreasing the amount of energy applied tothe RF system
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General ICP-AES
u RF Control circuitry
The control circuitry provides a computer interface for the level control ofthe RF system
It also provides an interlock monitoring system for operator safety andequipment protection
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General ICP-AES
u Gas control
S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
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General ICP-AES
u Gas control
The purpose of a gas control assembly is to regulate and control thesupply of required gas flows throughout the ICP-ES
Most gas control assemblies supply the gas required for
torch/plasma
nebulizeroptics
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 arebeing analyzed
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General ICP-AES
u Data Acquisition / communication
S a m p l e
I n t r o d u c t i o n
S y s t e m
G a s
C o n t r o l
O p t i c s
P l a s m a
G e n e r a t i o n
D a t a A c q u i s t i o n
C o m m u n i c a t i o n s
P o w e r S u p p l i e s
W a t e r
T o P C
M a i n s S u p p l i e s
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General ICP-AES
u Data Acquisition
The purpose of the data acquisition assembly is to convert theproportional electrical current from the optical detector into suitabledigital information for data processing by the controlling PC software
Communications
The purpose of the communications system is provide a means forcommand and control of all the internal assemblies while providing aninterface for the instrument to communicate with the PC software and thevarious instrument accessories.