ICPOES教程.pdf

Concepts,Instrumentation and Techniques in InductivelyCoupled Plasma Optical Emission SpectrometryCharles B.Boss and Kenneth J.FredeenSecond Edition Copyright 1997 by The Perkin-Elmer CorporationAll rights reserved.Reproduction in whole or in part withoutwritten permission from Perkin-Elmer is prohibited.PRINTED IN U.S.A.iiABOUT THE AUTHORSCharles B.Boss is an Associate Professor of Chemistry at North Carolina StateUniversity.He graduated from Wake Forest University with a B.S.degree inchemistry in 1968.After serving in the U.S.Navy,he entered Indiana University,where he received a Ph.D.in chemistry in 1977 under the direction of Prof.GaryHieftje.At N.C.State,Dr.Boss has worked on the development and characterizationof several types of flame and plasma sources for atomic spectroscopy.His presentresearch interests also include the use of computer automation and chemometricsfor enhancement of analytical techniques.Kenneth J.Fredeen is the Director of Environmental&Applied Inorganic Systemsat The Perkin-Elmer Corporation.He graduated summa cum laude from ThielCollege with a B.A.degree in chemistry in 1980,and from Texas A&M Universitywith a Ph.D.in chemistry in 1985.While at Texas A&M,his graduate studiescentered on the use of laser-excited atomic fluorescence spectroscopy for charac-terization of flames and plasmas used in analytical atomic spectrometry.Dr.Fredeenjoined Perkin-Elmer in 1985 and has since been involved in the development ofsoftware,instrumentation and applications for the ICP-OES and ICP-MS productlines.iiiivACKNOWLEDGEMENTSThe authors gratefully acknowledge the valuable contributions from all thosepersons who reviewed manuscripts for this book and provided many useful com-ments and suggestions.In particular,we would like to thank Dennis Yates,SabinaSlavin,Walter Slavin,Jack Kerber,Mark Werner,Dave Hilligoss,Deborah Hoult,Barbara Ruocco and DonnaJean Fredeen in this regard.Special thanks go toProfessor S.Roy Koirtyohann for some helpful insights and suggestions.We wouldalso like to acknowledge Charles Keil for his fine work on many of the illustrationsthroughout this book.vviTABLE OF CONTENTSPreface.ix1An Overview of Elemental Analysis via Atomic Spectroscopy TechniquesNature of Atomic and Ionic Spectra.1-2Analytical Techniques Based on Atomic Spectrometry.1-5Atomization and Excitation Sources.1-6A Short History of Optical Emission Spectroscopy.1-72General Characteristics of ICP-OESThe ICP Discharge.2-1Detection of Emission.2-7Extraction of Information.2-8Performance Characteristics.2-9Role of the ICP in an Analytical Laboratory.2-113ICP-OES InstrumentationSample IntroductionNebulizers.3-2Pumps.3-6Spray Chambers.3-8Drains.3-9Alternative Sample Introduction Techniques.3-9Production of EmissionTorches.3-13Radio Frequency Generators.3-16Collection and Detection of EmissionTransfer Optics.3-16Conventional ICP InstrumentationWavelength Dispersive Devices.3-17 Detectors.3-21Echelle Grating-Based ICP InstrumentationWavelength Dispersive Devices.3-23Advanced Array Detectors.3-24Axial or End-on Viewing ICP Instrumentation.3-28Signal Processing and Instrument ControlSignal Processing.3-32Computers and Processors.3-33Software.3-33vii3 ICP-OES Instrumentation(continued)Accessories for ICP-AES InstrumentationAutosamplers.3-34Sample Introduction Accessories.3-354 ICP-OES MethodologyAn Overview of ICP-OES Methodology.4-2ICP-OES Interferences-General Considerations.4-4Preparation of Samples and Standards.4-5Sample Introduction.4-7Operating Conditions.4-8Wavelength Selection.4-13Emission Measurement.4-14Instrument Calibration.4-15Samples Analysis.4-16Report Generation.4-16Correcting for Spectral Interferences in ICP-OES.4-16Simple Background Shift.4-16Sloping Background Shift.4-17Direct Spectral Overlap.4-17Complex Background Shift.4-21Other Spectral Interference Correction Techniques.4-225 ICP-OES ApplicationsAgriculture and Foods.5-3Biological and Clinical.5-3Geological.5-4Environmental and Waters.5-4Metals.5-5Organics.5-6Appendix A-Instrument Maintenance and Performance VerificationInstrument Care and MaintenanceSample Introduction System and ICP Torch.A-1RF Generator.A-2Spectrometer.A-2Computer.A-3Verification of Instrument Performance.A-3Appendix B-GlossaryAppendix C-Bibliography and ReferencesviiiPREFACEThe widely used analytical technique for the determination of trace elements,inductively coupled plasma-optical emission spectrometry(ICP-OES),marked itsthirty-third anniversary in 1997.In this book,the technique will be referred to asICP-OES though the reader may notice that many technical publications refer to itas inductively coupled plasma-atomic emission spectrometry(ICP-AES).Unfortu-nately,the latter designation is sometimes confused with Auger Electron Spectrome-try(AES).In the interest of clarity,therefore,the term iInductively coupled plasma-optical emission spectrometry(ICP-OES)has been rigidly adopted.Many advances have been made since the first commercial ICP-OES instrumentswere introduced to the analytical community.Advancements in the understandingof the ICP source and the measurement of emission signals have led to manyimprovements in the design of the components that comprise the instruments usedfor ICP-OES.Other advances have resulted through the use of computers with theseinstruments and the increasing levels of automation and sophistication that theyhelp to realize.Despite the advances in ICP-OES instrumentation and software,ICP-OES is not afoolproof technique.Because ICP-OES is a sensitive trace analysis technique,care must be taken in the preparation of standards,blanks and samples introducedinto the instrument.The instrumentation must be set up properly and parameters,such as wavelengths and background correction points,must be selected to fit theapplication.Attention to details seemingly as minor as changing the peristaltic pumptubing when it is worn can make the difference between acceptable and unaccept-able analysis results.There are several sources of specific,detailed information about the ICP-OEStechnique available.It was felt,however,that there was still a need for a generalintroductory guide for new and potential users of ICP-OES.Such a guide shouldprovide basic,practical information to help new users better understand the funda-mentals behind,and requirements for,performing good analyses using the ICP-OEStechnique.This book is intended as that introduction to the ICP-OES technique.It was writtennot only for those persons who have some familiarity with other analytical techniquessuch as atomic absorption spectrometry but also for novices in the field of analyticalchemistry.The book begins with some simple,yet fundamental,concepts regardingatomic spectroscopy and the analytical techniques based on this field of study.Asone progresses through the book,more detail regarding the ICP-OES technique isixpresented including information about ICP-OES performance,instrumentation andmethodology.The final chapter of the book briefly describes some representative ICP-OESapplications.The reader should not be alarmed if his or her particular application isnot mentioned here.So many different ICP-OES applications have been developedin the last thirty-three years that it would be difficult to describe them all in a singlevolume.We have also included in an appendix some information about instrumentmaintenance and performance verification.While this kind of practical informationcan be vital to obtaining good analytical results,it is sometimes difficult to find.Finally,in the back of this book there is a glossary of terms as they are commonlyused in atomic spectroscopy.The reader may find it helpful to refer occasionally tothis section for further details regarding terms that are used in the main body of thebook.We hope that this introduction to the ICP-OES technique will provide usefulinformation to those persons who are about to get involved with ICP-OES as wellas present ICP users and those with simply a curiosity about the technique.x1 AN OVERVIEW OF ELEMENTAL ANALYSIS VIA ATOMIC SPECTROSCOPY TECHNIQUESOne of the simplest questions that an analyst can ask about the chemical compo-sition of a sample is which elements are present and at what concentrations?Sincethere are only 92 naturally occurring elements and millions of different molecules,differentiating among the elements is a much easier task than differentiating amongthe molecules.Nonetheless,the elemental composition of a sample is often animportant part of the information needed to assess its properties.For example,consider a water sample which is determined to contain 88.2%oxygenand 11.0%hydrogen by mass,meaning that only 99.2%of the sample could bemade up of water molecules.Whether the water from which this sample was takenis useful for a particular purpose may well depend on the remaining 0.8%.If thiswater sample contained as much as a microgram of boron per gram of sample(0.0001%of the mass),the water would be perfectly useful for most purposes.If,however,you wanted to use that water in the fabrication of ceramic turbine bladesfor jet engines,purification would be required.When water containing as much asone part per million boron is used in the manufacture of the ceramics for theseturbine blades,their failure rate rises dramatically.Research has shown that boroncollects on the grain boundaries of the ceramic turbine blades,causing fracturesthat have been implicated in catastrophic failures of jet engines.There are many other examples of the need for determining the trace levelconcentrations of elements within samples.For example,the United States Envi-ronmental Protection Agency has strict rules concerning trace levels of dangerousmetals allowed in wastewaters.Some of these limits are in the parts per billion range.Determination of elemental concentrations at these trace levels requires the use ofsensitive scientific instrumentation.The most commonly used techniques for the determination of trace concentrationsof elements in sample are based on atomic spectrometry.As the name atomicspectrometry implies,these techniques involve electromagnetic radiation(light)thatis absorbed by and/or emitted from atoms of a sample.(Not implicit in the termatomic spectrometry,however,is that we generally include emission and absorp-tion of electromagnetic radiation by charged atoms,or ions,also under the headingof atomic spectrometry.)By using atomic spectrometry techniques,meaningfulquantitative and qualitative information about a sample can be obtained.In general,quantitative information(concentration)is related to the amount of electromagneticradiation that is emitted or absorbed while qualitative information(what elementsare present)is related to the wavelengths at which the radiation is absorbed oremitted.An affiliated technique to atomic emission or absorption spectrometry is atomicmass spectrometry.In mass spectrometry,instead of obtaining analytical informa-tion from the radiation of atoms or ions,ions introduced into a mass spectrometerare separated according to their mass to charge ratio and are either qualitatively orquantitatively detected.Nature of Atomic or Ionic Spectra The measurement of absorption and emission of electromagnetic radiation can bemore easily described once the nature of atomic and ionic spectra is understood.Consider the Bohr model of an atom shown in Figure 1-1.The atom is depicted asa nucleus surrounded by electrons which travel around the nucleus in discreteorbitals.Every atom has a number of orbitals in which it is possible for electrons totravel.Each of these electron orbitals has an energy level associated with it.Ingeneral,the further away from the nucleus an orbital,the higher its energy level.When the electrons of an atom are in the orbitals closest to the nucleus and lowestin energy,the atom is in its most preferred and stable state,known as its groundstate.When energy is added to the atom as the result of absorption of electromag-netic radiation or a collision with another particle(electron,atom,ion,or molecule),one or more of several possible phenomena take place.The two most probableevents are for the energy to be used to increase the kinetic energy of the atom(i.e.,increase the velocity of the atom)or for the atom to absorb the energy and becomeexcited.This latter process is known as excitation.Figure 1-1.Bohr model of an atom.As energy is absorbed by an atom,an elec-tron jumps to an orbital with a higher energy level.The atom may decay to alower energy state by emitting a photon,h.1-2Concepts,Instrumentation,and TechniquesWhen an atom becomes excited,an electron from that atom is promoted from itsground state orbital into an orbital further from the nucleus and with a higher energylevel.Such an atom is said to be in an excited state.An atom is less stable in itsexcited state and will thus decay back to a less excited state by losing energythrough a collision with another particle or by emission of a particle of electromag-netic radiation,known as a photon.As a result of this energy loss,the electronreturns to an orbital closer to the nucleus.If the energy absorbed by an atom is high enough,an electron may be completelydissociated from the atom,leaving an ion with a net positive charge.The energyrequired for this process,known as ionization,is called the ionization potential andis different for each element.Ions also have ground and excited states through whichthey can absorb and emit energy by the same excitation and decay processes asan atom.Figure 1-2 shows the excitation,ionization and emission processes schematically.The horizontal lines of this simplified diagram represent the energy levels of an atom.The vertical arrows represent energy transitions,or changes in the amount of energyof an electron.The energy transitions in an atom or ion can be either radiational(involving absorption or emission of electromagnetic radiation)or thermal(involvingenergy transfer through collisions with other particles).The difference in energy between the upper and lower energy levels of a radiativetransition defines the wavelength of the radiation that is involved in that transition.Figure 1-2.Energy level diagram depicting energy transitions where a and b rep-resent excitation,c is ionization,d is ionization/excitation,e is ion emission,andf,g and h are atom emission.An Overview of Elemental Analysis via Atomic Spectroscopy1-3The relationship between this energy difference and wavelength can be derivedthrough Plancks equationE=hwhere E is the energy difference between two levels,h is Plancks constant,and is the frequency of the radiation.Substituting c/for n,where c is the speed of lightand is wavelength,we getE=hc/This equation shows that energy and wavelength are inversely related,i.e.,as theenergy increases,the wavelength decreases,and vice versa.Using Figure 1-2 asan example,the wavelength for emission transition f is longer than the wavelengthfor emission transition g since the energy difference for f is less th