Teoria Básica ICP-OES

Liberty Series II Training Notes

Publication Date: 2/97 2-1

2 Series II course outline

General information

Basic ICP-ES

Inductively Coupled Plasma spectrometers are scientific instrumentsthat use emission spectroscopy to quantify or qualify elements in asample.

Samp le

Int roduc t i on

System

GasCon tro l

Op t i cs

P lasma

Generat ion

Data Acqu is t i on

Commun icat ions

Power Supp l i es

Wate r

To PC

Mains Supp l i es

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.

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.



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 atomin the 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.

Plasma

A plasma is a gas that contains a significant fraction of ions and freeelectrons.

So l id L iqu id Gas P lasma

Tempera tu re ( )

Mel tsBo i ls orVapor izes Ion izes

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 argongas in a radio frequency magnetic field.

An ion is an atom that carries a charge due to the loss or gain of anelectron.

Ionization in a plasma is triggered when argon is passed through arapidly changing magnetic field and is then seeded with electrons froma spark 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.

Ar , Ar+ , e-

Magne t i cF ie ld

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 atomno longer carries a charge and is no longer an ion.

Ionic emission

Ions also emit light through ionic emission.

An ion will absorb energy, usually by collision with other ions andatoms.

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 andtherefore the element.



Cont inuum

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.

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 flowin 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 10,000°K

In theory, the point of the greatest activity between ions, electrons andneutral atoms will be the point of the highest temperature.



As the magnetic field becomes less of an influence on the ions,electrons and neutral argon atoms, fewer recombinations and collisionsoccur.

As excitation decreases so does the temperature of the plasma.

This causes a formation of a temperature gradient over the area of theplasma.

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 asthe skin depth effect.

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.

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

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 willalso increase the amount of time that particles will preside in theplasma.

Reduction in the flow gas will tend to increase the intensity of hard lineswhile changes in the flow of gas will have little effect on the soft lines.



Sample introduction

Sample

In t roduc t ion

Sys tem

GasCont ro l

Opt ics

Plasma

Genera t ion

Data Acqu is t ion

Communica t ions

Power Supp l ies

Water

To PC

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 plasma’s central channel.

As the sample passes through the plasma it rapidly changes state.

The plasma as an excitation source offers two physical means foremission

• Atomization

• Ionization

Most elements when excited by a plasma source emit radiation in bothways.

Emission lines that result from atomic excitation are classified as a typeI lines .

Emission lines that from ionic excitation are classified as type II lines .

ICP-ES offers three commercially available solutions for sampling.

• Gas

• Solid

• Liquid

ICP-ES primarily is used to analyze liquids.



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 tubingfitted on 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.

The nebulizer forms an aerosol by pneumatic or ultrasonic means.

There are two basic types of pneumatic nebulizers.

• V-groove

• Concentric

V-groove

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

Sample in

Car r ie r gas

Sample is pumped through a 1 to 2 mm hole.

Carrier gas is fed through a second hole which is located close to thesample output hole.



The sample and carrier gas holes are positioned so that the output ofeach is aligned on the same axis in a V-shaped trough.

The sample flows along a V-shaped channel where it is captured by theventuri effect created by the carrier gas.

The carrier gas and sample combine to form an aerosol.

Concentric

A typical glass concentric nebulizer uses a venturi effect.

��

Sample solution is drawn through a central capillary that is surroundedby an outer channel that a carrier gas is fed through.

The carrier gas forced through the outer channel, passes by the end ofthe sample capillary, lowering the pressure surrounding the tip of thecapillary extracting the sample.

Ultrasonic

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

desolvatedaerosolto ICP

coolantin

condenser

coolantout

heated(140°C)U-tube

sample inlet

drain

drain

transducer

RF1.4 MHz

Argon in

aerosolchamber



The sample flows over a glass plate fixed to the transducer where theultrasonic vibrations cause the aerosol to form.

The carrier gas sweeps the sample aerosol into a heated tube that isconnected to a desolvator.

The dried sample is then introduced into the plasma.

Spraychamber

The aerosol must be injected into the plasma at a uniform rate withoutcausing plasma destabilisation.

In addition to this the aerosol that is injected into the plasma must alsocontain a sufficient number of small droplets that are reproducible andrepresentative of the sample.

A spraychamber is used to remove the larger droplets from the aerosolwhile 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 tube andtorch through an indirect route.

While passing through the chamber the larger droplets fall out of theaerosol and are removed through a drain, tubing and peristaltic pump towaste.

Torch

The torch confines ionized argon gas in the RF field of the induction coiland introduces the fine sample aerosol from the spraychamber to theplasma 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.



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 topof the auxiliary and injector tubes.

The injector tube is the inner most tube and carries the sample aerosolto the plasma.



The flow of the sample aerosol is determined by the carrier gas flowrate.

The design of the torch produces low pressure at the center of theplasma.

By design the sample is fed through this low pressure region.

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 thatforms the plasma.



Optics

S ampl e

In t ro duc t ion

S ys te m

Ga sCon t ro l

Op t i cs

P la sma

G ener a t io n

D a ta Acq u is t i on

Comm un i ca t i ons

P ower Sup p l i es

Wat e r

To PC

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.



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.

Sequential

A sequential ICP-ES uses a scanning monochromator that gathers theradiant 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.

PMT

Exit slit

Entryslit

Window

Plasmatorch

Grating

M5

M4

Simultaneous

There are two basic simultaneous configurations currently commerciallyavailable:

• Rowland circle

• Echelle

PMTs

Exit slits

Entryslit

Window

Plasmatorch

FixedGrating



Rowland

A Rowland simultaneous ICP-ES uses a stationary polychromator thatgathers the radiant emissions and focuses this incident light onto asingle spherical diffraction grating.

The grating is designed to direct the spectrum of light to a number ofPMT detectors which are arranged in a circle.

Each PMT is physically placed for each characteristic wavelength thatis to be analyzed.

Therefore for each wavelength of interest a detector in a specificlocation must be used.

Echelle

Detector

Entryslit

Lens

Plasmatorch

Grating

Prism Window

An Echelle simultaneous ICP-ES uses a polychromator that gathers theradiant emissions and focuses this incident light onto two stationarydispersive elements.

The first dispersive element is a grating. The grating is usually ruled todisperse the incident light into a spectrum across the vertical opticsplane.

The second dispersive element is generally a prism. The prism ismanufactured and mounted to project the vertical spectrum from thegrating into a two dimensional optical matrix.

The prism does this by further dispersing the vertically orientated fullspectrum 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 wherelowest wavelength is positioned in one extreme, (ie lower right handcorner) and the highest wavelength is positioned in the oppositeextreme, (ie upper left hand corner).

The two dimensional spectrum is then observed by a solid statedetector.



RF

Sa mpl e

In t rod uc t ion

Sy s tem

Ga sCon t ro l

Op t i cs

P las ma

Ge nera t io n

D a ta Acq u is t i on

Comm un ic a t i ons

Po wer Sup p l i es

Wate r

To PC

The function of the plasma generation system is to deliver high energyRF current through the induction coil. The alternating current throughthe induction coil provides the magnetic fields required to produce andsustain a plasma as an excitation source.

Plasma generation systems for commercially available ICP-ESinstruments are generally PC controlled. The software allows theoperator of the instrument to select the level of RF power required bythe type of analysis of interest.

Plasma generation systems consist of an RF system and controlcircuitry.

RF system

There are two frequencies currently commercially available:

• 27 MHz

• 40 MHz

40 MHz RF systems are seen to have reduced background and providegreater plasma stability, particularly for organic analysis.

ICP-ES RF systems are required to produce uniform power levelsunder the varying conditions of sample loading.



Two main requirements have to be met to reduce these effects:

• Impedance matching

• level control

Impedance matching

Impedance matching is required to maintain oscillations in a tunedcircuit.

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.

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 lessefficient at that given frequency

To improve the matching and maintain the oscillations in the circuit thefrequency or the coupling must be varied.

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 powerlevel.

The difference of the desired value to the known value then results inthe control circuitry increasing or decreasing the amount of energyapplied to the RF system.

Control circuitry

The control circuitry provides a computer interface for the level controlof the RF system.

It also provides an interlock monitoring system for operator safety andequipment protection.



Gas control

S am p le

In t r od uc t i on

S ys te m

Ga sCo n t ro l

Op t i cs

P l as ma

Ge ne ra t i on

D a t a Ac qu is t i o n

C om mu n ic a t io ns

P ow er S up p l i es

W at e r

To P C

Ma in s Su p p l ie s

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

• 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 arebeing analyzed.



Data Acquisition/communication

S am ple

In t ro du c t i on

S ys t em

Ga sC on t ro l

O p t i cs

P l asm a

Ge ne ra t io n

D a t a A cq u is t i o n

Com mun ic a t i on s

P owe r Sup p l ies

Wa te r

T o PC

M ain s Sup p l ies

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 andthe various instrument accessories.

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Teoria Básica ICP-OES

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