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Concepts,Instrumentation andTechniques in AtomicAbsorption SpectrophotometryRichard D.BeatyandJack D.KerberSecond EditionTHE PERKIN-ELMER CORPORATIONCopyright 1993 by The Perkin-Elmer Corporation,Norwalk,CT,U.S.A.Allrights reserved.Printed in the United States of America.No part of this publicationmay be reproduced,stored in a retrieval system,or transmitted,in any form or byany means,electronic,mechanical,photocopying,recording,or otherwise,with-out the prior written permission of the publisher.iiABOUT THE AUTHORSRichard D.BeatySince receiving his Ph.D.degree in chemistry from the University of Missouri-Rolla,Richard Beaty has maintained an increasing involvement in the field oflaboratory instrumentation and computerization.In 1972,he joined Perkin-Elmer,where he held a variety of technical support and marketing positions in atomicspectroscopy.In 1986,he founded Telecation Associates,a consulting companywhose mission was to provide formalized training and problem solving for theanalytical laboratory.He later became President and Chief Executive Officer ofTelecation,Inc.,a company providing PC-based software for laboratory automat-ion and computerization.Jack D.KerberJack Kerber is a graduate of the Massachusetts Institute of Technology.He hasbeen actively involved with atomic spectrometry since 1963.In 1965 he becamePerkin-Elmers first field Product Specialist in atomic absorption,supporting ana-lysts in the western United States and Canada.Since relocating to Perkin-Elmerscorporate headquarters in 1969,he has held a variety of marketing support andsales and product management positions.He is currently Director of Atomic Ab-sorption Marketing for North and Latin America.iiiACKNOWLEDGEMENTThe authors gratefully acknowledge the contributions and assistance they have re-ceived from their colleagues in preparing this book.We are particularly indebtedto Glen Carnrick,Frank Fernandez,John McCaffrey,Susan McIntosh,CharlesSchneider and Jane Sebestyen of The Perkin-Elmer Corporation for the hours theyspent proofreading the several revisions and to Jorn Baasner,Horst Schulze,Ger-hard Schlemmer,Werner Schrader and Ian Shuttler of BodenseewerkPerkin-Elmer GmbH for their invaluable input on Zeeman-effect background cor-rection and graphite furnace atomic absorption techniques.ivTABLE OF CONTENTS1 Theoretical Concepts and Definitions The Atom and Atomic Spectroscopy .1-1Atomic Absorption Process .1-3 Quantitative Analysis by Atomic Absorption .1-4 Characteristic Concentration and Detection Limits.1-6Characteristic Concentration .1-7Detection Limits.1-72 Atomic Absorption InstrumentationThe Basic Components.2-1 AA Light Sources.2-2 The Hollow Cathode Lamp .2-3The Electrodeless Discharge Lamp.2-6Optical ConsiderationsPhotometers .2-7Single-beam Photometers.2-7Double-beam Photometers .2-8Alternative Photometer Designs .2-9Optics and the Monochromator System .2-10The Atomic Absorption AtomizerPre-mix Burner System .2-14Impact Devices .2-15Nebulizers,Burner Heads and Mounting Systems.2-16ElectronicsPrecision in Atomic Absorption Measurements .2-17Calibration of the Spectrometer .2-18Automation of Atomic AbsorptionAutomated Instruments and Sample Changers.2-19Automated Sample Preparation.2-20The Stand-alone Computer and Atomic Absorption.2-203 Control of Analytical InterferencesThe Flame Process .3-1Nonspectral Interferences .3-3Matrix Interference .3-3 Method of Standard Additions .3-4Chemical Interference.3-5v3 Control of Analytical Interferences(continued)Ionization Interference .3-6Spectral InterferencesBackground Absorption.3-7Continuum Source Background Correction .3-8Introduction to Zeeman Background Correction.3-11Other Spectral Interferences .3-14Interference Summary .3-144 High Sensitivity Sampling SystemsLimitations to Flame AA Sensitivity .4-1The Cold Vapor Mercury TechniquePrinciple .4-2Advantages of the Cold Vapor Technique .4-2Limitations of the Cold Vapor Technique .4-3The Hydride Generation TechniquePrinciple .4-3Advantages of the Hydride Technique .4-4Disadvantages of the Hydride Technique .4-4 Graphite Furnace Atomic AbsorptionPrinciple .4-5Advantages of the Graphite Furnace Technique .4-55 Introduction to Graphite Furnace Atomic AbsorptionConsiderations in Ultra Trace AnalysisPerformance Criteria .5-1Graphite Furnace Applications .5-2Components of the Graphite Furnace SystemThe Graphite Furnace Atomizer .5-2The Graphite Furnace Power Supply and Programmer .5-5Summary of a Graphite Furnace Analysis.5-5Sample Size .5-6 The Drying Step.5-7The Pyrolysis Step .5-8The Pre-atomization Cool Down Step .5-8The Atomization Step.5-8The Clean Out and Cool Down Step .5-9Fast Furnace Analysis .5-9vi5 Introduction to Graphite Furnace Atomic Absorption(continued)Measuring the Graphite Furnace AA SignalNature of the Graphite Furnace Signal .5-10Peak Height Measurement .5-10Peak Area Measurement .5-11Solid Sampling With the Graphite Furnace .5-126 Control of Graphite Furnace InterferencesInterferences and the Graphite Furnace .6-1Spectral InterferencesEmission Interference.6-2Background Absorption.6-3 Background Reduction Techniques.6-3Automated Instrumental Background Correction .6-6Interpolated Background Correction .6-6Nonspectral InterferencesDefinition .6-8Method of Standard Additions .6-8The Graphite Tube Surface .6-9The Lvov Platform .6-10Matrix Modification.6-11Maximum Power Atomization .6-12Peak Area Measurement .6-13Fast Electronics and Baseline Offset Correction.6-14Stabilized Temperature Platform Furnace The Goals .6-15The STPF System .6-157 Alternate Analytical TechniquesDirect Current Plasma(DCP)Emission .7-1Inductively Coupled Plasma(ICP)Emission .7-2Inductively Coupled Plasma-Mass Spectrometry(ICP-MS).7-3Summary .7-4vii1 THEORETICAL CONCEPTS AND DEFINITIONSTHE ATOM AND ATOMIC SPECTROSCOPYThe science of atomic spectroscopy has yielded three techniques for analyticaluse:atomic emission,atomic absorption,and atomic fluorescence.In order to un-derstand the relationship of these techniques to each other,it is necessary to havean understanding of the atom itself and of the atomic process involved in eachtechnique.The atom is made up of a nucleus surrounded by electrons.Every element has aspecific number of electrons which are associated with the atomic nucleus in anorbital structure which is unique to each element.The electrons occupy orbital po-sitions in an orderly and predictable way.The lowest energy,most stable electronicconfiguration of an atom,known as the ground state,is the normal orbital con-figuration for an atom.If energy of the right magnitude is applied to an atom,theenergy will be absorbed by the atom,and an outer electron will be promoted to aless stable configuration or excited state.As this state is unstable,the atom willimmediately and spontaneously return to its ground state configuration.The elec-tron will return to its initial,stable orbital position,and radiant energy equivalentto the amount of energy initially absorbed in the excitation process will be emitted.The process is illustrated in Figure 1-1.Note that in Step 1 of the process,the ex-citation is forced by supplying energy.The decay process in Step 2,involving theemission of light,occurs spontaneously.Figure 1-1.Excitation and decay processes.The wavelength of the emitted radiant energy is directly related to the electronictransition which has occurred.Since every element has a unique electronic struc-ture,the wavelength of light emitted is a unique property of each individual ele-ment.As the orbital configuration of a large atom may be complex,there are manyelectronic transitions which can occur,each transition resulting in the emission ofa characteristic wavelength of light,as illustrated in Figure 1-2.The process of excitation and decay to the ground state is involved in all threefields of atomic spectroscopy.Either the energy absorbed in the excitation processor the energy emitted in the decay process is measured and used for analytical pur-poses.In atomic emission,a sample is subjected to a high energy,thermal envi-ronment in order to produce excited state atoms,capable of emitting light.Theenergy source can be an electrical arc,a flame,or more recently,a plasma.Theemission spectrum of an element exposed to such an energy source consists of acollection of the allowable emission wavelengths,commonly called emissionlines,because of the discrete nature of the emitted wavelengths.This emissionspectrum can be used as a unique characteristic for qualitative identification of theelement.Atomic emission using electrical arcs has been widely used in qualitativeanalysis.Emission techniques can also be used to determine how much of an element is pre-sent in a sample.For a quantitative analysis,the intensity of light emitted at thewavelength of the element to be determined is measured.The emission intensityat this wavelength will be greater as the number of atoms of the analyte elementincreases.The technique of flame photometry is an application of atomic emissionfor quantitative analysis.If light of just the right wavelength impinges on a free,ground state atom,the atommay absorb the light as it enters an excited state in a process known as atomic ab-Figure 1-2.Energy transitions.1-2Concepts,Instrumentation and Techniquessorption.This process is illustrated in Figure 1-3.Note the similarity between thisillustration and the one in Step 1 of Figure 1-1.The light which is the source ofatom excitation in Figure 1-3 is simply a specific form of energy.The capabilityof an atom to absorb very specific wavelengths of light is utilized in atomic ab-sorption spectrophotometry.ATOMIC ABSORPTION PROCESSThe quantity of interest in atomic absorption measurements is the amount of lightat the resonant wavelength which is absorbed as the light passes through a cloudof atoms.As the number of atoms in the light path increases,the amount of lightabsorbed increases in a predictable way.By measuring the amount of light ab-sorbed,a quantitative determination of the amount of analyte element present canbe made.The use of special light sources and careful selection of wavelength al-low the specific quantitative determination of individual elements in the presenceof others.The atom cloud required for atomic absorption measurements is produced by sup-plying enough thermal energy to the sample to dissociate the chemical compoundsinto free atoms.Aspirating a solution of the sample into a flame aligned in the lightbeam serves this purpose.Under the proper flame conditions,most of the atomswill remain in the ground state form and are capable of absorbing light at the ana-lytical wavelength from a source lamp.The ease and speed at which precise andaccurate determinations can be made with this technique have made atomic ab-sorption one of the most popular methods for the determination of metals.A third field in atomic spectroscopy is atomic fluorescence.This technique incor-porates aspects of both atomic absorption and atomic emission.Like atomic ab-sorption,ground state atoms created in a flame are excited by focusing a beam oflight into the atomic vapor.Instead of looking at the amount of light absorbed inthe process,however,the emission resulting from the decay of the atoms excitedby the source light is measured.The intensity of this fluorescence increaseswith increasing atom concentration,providing the basis for quantitative determi-nation.Figure 1-3.The atomic absorption process.Theoretical Concepts and Definitions1-3The source lamp for atomic fluorescence is mounted at an angle to the rest of theoptical system,so that the light detector sees only the fluorescence in the flameand not the light from the lamp itself.It is advantageous to maximize lamp inten-sity with atomic fluorescence since sensitivity is directly related to the number ofexcited atoms which is a function of the intensity of the exciting radiation.Figure 1-4 illustrates how the three techniques just described are implemented.While atomic absorption is the most widely applied of the three techniques andusually offers several advantages over the other two,particular benefits may begained with either emission or fluorescence in special analytical situations.Thisis especially true of emission,which will be discussed in more detail in a laterchapter.QUANTITATIVE ANALYSIS BY ATOMIC ABSORPTIONThe atomic absorption process is illustrated in Figure 1-5.Light at the resonancewavelength of initial intensity,Io,is focused on the flame cell containing groundstate atoms.The initial light intensity is decreased by an amount determined bythe atom concentration in the flame cell.The light is then directed onto the detectorwhere the reduced intensity,I,is measured.The amount of light absorbed is de-termined by comparing I to Io.Figure 1-4.Atomic spectroscopy systems.1-4Concepts,Instrumentation and TechniquesSeveral related terms are used to define the amount of light absorption which hastaken place.The transmittance is defined as the ratio of the final intensity tothe initial intensity.T=I/IoTransmittance is an indication of the fraction of initial light which passes throughthe flame cell to fall on the detector.The percent transmission is simply thetransmittance expressed in percentage terms.%T=100 x I/IoThe percent absorption is the complement of percent transmission defining thepercentage of the initial light intensity which is absorbed in the flame.%A=100-%TThese terms are easy to visualize on a physical basis.The fourth term,absor-bance,is purely a mathematical quantity.A=log(Io/I)Absorbance is the most convenient term for characterizing light absorption in ab-sorption spectrophotometry,as this quantity follows a linear relationship with con-centration.Beers Law defines this relationship:A=abcFigure 1-5.The atomic absorption process.Theoretical Concepts and Definitions1-5where A is the absorbance;a is the absorption coefficient,a constant whichis characteristic of the absorbing species at a specific wavelength;b is the lengthof the light path intercepted by the absorption species in the absorption cell;andc is the concentration of the absorbing species.This equation simply states thatthe absorbance is directly proportional to the concentration of the absorbing spe-cies for a given set of instrumental conditions.This directly proportional behavior be-tween absorbance and concentration is ob-served in atomic absorption.When the ab-sorbances of standard solutions containingknown concentrations of analyte are meas-ured and the absorbance data are plottedagainst concentration,a calibration rela-tionship similar to that in Figure 1-6 is es-tablished.Over the region where theBeers Law relationship is observed,thecalibration yields a straight line.As theconcentration and absorbance increase,nonideal behavior in the absorption proc-ess can cause a deviation from linearity,asshown.After such a calibration is established,the absorbance of solutions of unknownconcentrations may be measured and the concentration determined from the