General2icp Aes

7/27/2019 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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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

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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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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

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General ICP-AES


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


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


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


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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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


V-groove

Most V-groove nebulizers are made from inert materials such asspecially selected plastic


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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


Concentric

A typical glass concentric nebulizer uses a venturi effect.


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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


Ultrasonic

A typical ultrasonic nebulizer uses the vibration of a piezo-electrictransducer to form an aerosol.


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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


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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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


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


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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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


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



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u Optics

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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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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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General ICP-AES

u RF


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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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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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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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u Gas control

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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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u Data Acquisition / communication

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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.

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