摘要:

THE ANALYSTTHE ANALYTICAL JOURNAL OF THE ROYAL SOCIETY OF CHEMISTRYADVISORY BOARD*Chairman: J. M. Ottaway (Glasgow, U.K.)'L. S. Bark (Salford, U.K.)E. Bishop (Exeter, U.K.)W. L. Budde (U.S.A.)D. T. Burns (Belfast, U.K.)L. R. P. Butler (South Africa)H. J. Cluley (Wembley, U.K.)E. A. M. F. Dahmen (The Netherlands)L. de Galan (The Netherlands)A. C. Docherty (Billingham, U.K.)D. Dyrssen (Sweden)G. Ghersini (Italy)J. Hoste (Belgium)A. Hulanicki (Poland)' G . W. Kirby (Glasgow, U.K.)W. S. Lyon (U.S.A.)H. V. Malmstadt (U.S.A.)G. W. C. Milner (Harwell, U.K.)"A. C. Moffat (Aldermaston, U.K.)E. J. Newman (Poole, U.K.)H. W. Nurnberg (West Germany)'T. B. Pierce (Harwell, U.K.)E. Pungor (Hungary)P. H. Scholes (Middlesbrough, U.K.)D. Simpson ( Thorpe-le-Soken, U.K.)'J.M. Skinner (Billingham, U.K.)'J. D. R. Thomas (Cardiff, U.K.)K. C. Thompson (Sheffield, U.K.)*A. M. Ure (Aberdeen, U.K.)A. Walsh, K.B. (Australia)G. Werner (German Democratic Republic)T. S. West (Aberdeen, U.K.)"P. C. Weston (London, U.K.)' J. White head (Stockton- on- Tees, U. K.)J. D. Winefordner (U.S.A.)P. Zuman (U.S.A.)'G. J. Dickes (Bristol, U.K.)*Members of the Board serving on the Analytical Editorial BoafdEditor: P. C. WestonSenior Assistant Editor: R. A. YoungAssistant Editors: Mrs. J. Brew, Miss D. ChevinREGIONAL ADVISORY EDITORSDr. J. Aggett, Department of Chemistry, University of Auckland, Private Bag, Auckland, NEW ZEALAND.Professor L. Gierst, Universit6 Libre de Bruxelles, Facult6 des Sciences, Avenue F.-D.Roosevelt 50,Professor H. M. N. H. Irving, Department of Theoretical Chemistry, University of Cape Town, Ronde-Professor W. A. E. McBryde, Faculty of Science, University of Waterloo, Waterloo, Ontario, CANADA.Dr. 0. Osibanjo, Department of Chemistry, University of Ibadan, Ibadan, NIGERIA.Dr. G. Rossi, Chemistry Division, Spectroscopy Sector, CEC Joint Research Centre, EURATOM, lspraDr. I. Rubeska, Geological Survey of Czechoslovakia, Malostransk6 19, 118 21 Prague 1 , CZECHO-Professor J. R&icka, Chemistry Department A, Technical University of Denmark, 2800 Lyngby,Professor K. Saito, Department of Chemistry, Tohoku University, Sendai, JAPAN.Professor L. E. Smythe, Department of Chemistry, University of New South Wales, P.O. Box 1 ,Professor P.C. Uden, Department of Chemistry, University of Massachusetts, Amhesst, MA 01 003,Editorial: Editor, The Analyst, The Royal Society of Chemistry, Burlington House,Piccadilly, London, W1V OBN. Telephone 01 -734 9864. Telex No. 268001Advertisements: Advertisement Department, The Royal Society of Chemistry, Burlington House,Piccadilly, London, W1V OBN. Telephone 01 -734 9864. Telex No. 268001The Analyst (ISSN 0003-2654) is published monthly by The Royal Society of Chemistry, BurlingtonHouse, London W1V OBN, England. All orders accompanied with payment should be sent directly toThe Royal Society of Chemistry, The Distribution Centre, Blackhorse Road, Letchworth, Herts. SG6 1 HN,England. 1983 Annual subscription rate UK f93.50, Rest of World f 99.00, USA $201 .OO. Purchased withAnalytical Abstracts UK f 226.50, Rest of World f 238.50, USA $487.00. Purchased with AnalyticalAbstracts plus Analytical Proceedings UK €251 .OO, Rest of World €265.00, USA $539.00. Purchasedwith Analytical Proceedings UK fll7.50, Rest of World f 124.50, USA $253.00. Air freight and mailingin the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 1 1 003.USA Postmaster: Send address changes to: The Analyst, Publications Expediting Inc., 200 MeachamAvenue, Elmont, NY 11003. Second class postage paid at Jamaica, NY 11431. All otherdespatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe.PRINTED IN THE UK.Volume 108 No 1287 @ The Royal Society of Chemistry 1983 June 1983Bruxelles, B ELGl U M.bosch 7700, SOUTH AFRICA.Establishment, 21 020 lspra (Varese), ITALY.SLOVAKIA.DEN MARK.Kensington, N.S.W. 2033, AUSTRALIA.U.S.A

ISSN:0003-2654    

DOI:10.1039/AN98308FX021

出版商:RSC    

年代:1983

数据来源: RSC

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ANALAO 108 (1 287) 649-776 (1 983) June 198364968569170171 271 77227287337 38742748754757759763766769THE ANALYSTTHE ANALYTICAL JOURNAL OF THE ROYAL SOCIETY OF CHEMISTRYCONTENTSAuger Techniques in Analytical Chemistry.Scanning Potential Stopped-rotation Voltammetry-Joseph Wang and Bassam A. FreihaDifferential-pulse Polarographic Monitoring o f Permitted Synthetic Food ColouringMatters and Ascorbic Acid in Accelerated Light Degradation Studies and theSpectrophotometric Determination o f the Ammonia and Simpler Amines Formed-Arnold G. Fogg and Abdulhadi M. SummanPotentiometric Determination o f Sulphite by Use o f Mercury(1) Chloride - Mercury(l1)Sulphide Electrodes in Flow Injection Analysis and in Air-gap Electrodes-GeoffreyB. Marshall and Derek MidgleySynthetic Inorganic lon-exchange Materials. Part XXXII.Studies on an Araldite-based Membrane of Crystalline Antimonic(V) Acid as a Nitrate lon-selectiveElectrode-Sushma Agrawal and Mitsuo AbeUse o f an Argon - Nitrogen Inductively Coupled Plasma for the Analysis of AluminiumAlloys Subsequent t o Alkali Dissolution-JosB A. C. Broekaert, Franz Leis and GungorDinqlerAnalytical Errors Associated with Trace Element Determination in Freshwater Particu-late M atter by Atomic-absorption Spectroscopy-Renato Baudo, Gaetano Galantiand Pier Giorgio VariniEffects o f Temperature Variation on the Zero, Second and Fourth Derivative Ultra-violet Absorption Spectra of Benzenoid Drugs-Alexander G. DavidsonFluorimetric Determination o f Trace Amounts o f Gold as an lon-association Complexw i t h 2-Phenylbenzo[8,9]quinolizino[4,5,6,7-fe~]phenanthridinylium Perchlorate-Tomes PBrez-Ruiz, Concepci6n SBnchez-PedreAo, Joaquin A.OrtuAo and Pedro Molina-BuendiaPreparation of Fatty Acid Methyl Esters from Olive Oil and Other Vegetable Oils UsingAqueous Hydrochloric Acid - Methanol-Nikolaos B. Kyriakidis and George Dionyso-poulosModified Gas - Liquid Chromatographic Method for Determining Bromide/TotalBromine in Foodstuffs and Soils-John A. Roughan, Patricia A. Roughan and John P. G.WilkinsInter-laboratory Calibration f o r Pesticide Analysis in South Africa-Louis P. van Dyk,Laurraine Lotter, Pieter R. de Beer, Andre de Klerk, Awie J. Viljoen and Susan M. PrinslooA Review-J. C.RiviereSHORT PAPERSSpectrophotometric Determination o f Exchangeable Calcium in Soils by Chloro-Histochemical Demonstration o f Collagen in Comminuted Meat Products-F. OlgaEvaluation o f the Determination of High Levels of Total Cadmium in Foodstuffs UsingFurther Studies on the Recovery of Iodine as lodine-125 After Alkaline Ashing Prior tophosphonazo-mA-Qiu Xing-chu, Zhang Yu-sheng and Zhu Ying-quanFlint and Barry M. FirthFlame Atomic-absorption Spectrophotometric Measurement-Dorothy DellarAssay-G. Bryan BellingCOMMUNICATIONCortisol Antibody Electrode-M. Y. Keating and G. A. RechnitzBOOK REVIEWSSummaries o f Papers in this hue-Pages iv, v, vi, vii, viii, ix, x, xiiPrinted by Heffers Printers Ltd Cambridge EnglandEntered as Second Class at New York.USA, Post OfficSITUATIONS VACANTANALYSTWith at least several yearsexperience of analyticalprocedures in the pharmaceuticalindustry, required for Company inNorth London. The successfulapplicant will be expected tocontrol a small technical staff andreport to the Senior Analyst.Application Forms from theSecretary, Biorex LaboratoriesLimited, Biorex House,Canonbury Villas,London NI 2HB.A201 for further information. See page xiv“ A N A L O I D ”COMPRESSED ANALYTICALREAGENTSoffer a saving in the use of lab-oratory chemicals. A range of over50 chemicals includes Oxidizingand Reducing Agents, Reagents forPhotometric Analysis and Indicatorsfor Complexometric Titrations.For full particulars send for ListNO.513 to:-RIDSDALE b. CO. LTD.Newham Hall, Newby,Middlesbrough,Cleveland TS8 9EAor telephone Middlesbrough317216(Telex: 587765 BASRID)Annual Reports on AnalyticalAtomic Spectroscopy Vol. 11Edited by M. S. Cresser and 6. L. SharpThis volume reports on current developmentsin all branches of analytical atomic emission,absorption a nd f 1 uorescence spectroscopywith references to papers published andlectures presented during 1981. Much of theinformation is in tabular form for ease ofreference.Brief ContentsAtomization and ExcitationArcs, Sparks, Lasers and Low-pressure Dis-charges; Plasmas; Flames; ElectrothermalAtomization; Vapour Generation;InstrumentationLight Sources; Optics; Detector Systems;Instrument Automation; Complete Instru-ments; New Commercial Instruments;MethodologyNew Methods; Detection Limits, Precision andAccu racy ; Stand a rd s a n d Stand a rd i za ti o n ;ApplicationsChemicals; Metals; Refractories and MetalOxides, Ceramics, Slags, Cements; Minerals;Air Analysis; Water Analysis; Soils, Plants andFertilizers, Foods and Beverages; Body Tissuesand Fluids;Hardcover 388pp 0 85186 707 3f48.00 ($88.00) RSC Members f32.00RSC members should send their orders to: The Royal Society of Chemistry, The Membership Officer,30 Russell Square, London WClB 5DT. Non-RSC members should send their orders to: The RoyalSocietv of Chemistry, Distribution Centre, Blackhorse Road, Letchworth, Herts SG6 1 HN.The Royal Society of ChemistryBurlington HouseLondon WIV OB

ISSN:0003-2654    

DOI:10.1039/AN98308BX023

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

lzcne. 1983 THE ANALYST iiiV of Chemistry M . pyiKAnalytical Chemistry 82( I R K 82)Index of reviewsin Analytical Chemistry82(1RAC82) isthefirst issueof anew annuat periodical which contains information on review articlesdealing with analytical chemistry. IRAC 82 will contain approximately580 references to review articles which have been published in English,German and French during the period January to December 82 andwhich have been drawn from the chemical literature bya combination ofmanual and computerised search techniques. Future issues will containreviews published during the current year, or late in the previous year.Each entry includes the following information:-0 An index term0 An item number0 Title of the original document0 Bibliographic details0 A brief abstract if the scope ofthe review is not clear from the title0 An indication of the number ofreviews cited in the original reviewSoftcover 48pp 0 85186 576 3IRAC 82 may be purchased individually, or bysubscription whereby users will automaticallyreceive IRAC 82 and the next two issues.A substantial discount is available tosubscribers.PRICEIRAC 82lsingle issue only)€5.00($9.W) p+pextraIRAC 82 + next two issuesflO.W(S17.001 p + pextraIRAC 82 is a unique publication whichprovides the analytical chemist with rapidaccess to the review literature.Orders should be sent to:The Royal Society of Chemistry,The University,Nottingham NG7 2RD.The index is divided into two sections:Section 1 Section 2Covers individual compounds,classes of compounds, materialsand substance-related topics(e.g. clinical chemistry; forensicanalysis; inorganic analysis).Covers apparatus, techniquesand other items not easilyassignable to Section 1.All aspects of analytical chemistry are covered including inorganic,organic, biochemical, pharmaceutical, food, agricultural andenvironmental analysis; apparatus and techniques such aschromatography, spectroscopy, radiochemistry, electrochemistry,thermal analysis and particle size analysis.A list of books andconferences is also providediv SUMMARIES OF PAPERS IN THIS ISSUESummaries of Papers in this IssueAuger Techniques in Analytical Chemistry. A ReviewSummary of ContentsIntroductionHistoricalAuger effectChemical informationSurface specificity and vacuum requirementElectron energy analysisElectron sourcesData acquisition and handlingFirst principlesElemental standardsAuger peaksSatellite peaksIonisation loss peaksSecondary electron peakAnalysis in depthAngular dependence of Auger intensityErosion by ion bombardmentErosion by mechanical lappingChemical bonding informationApplicationsCorrosion and oxidationCatalysisReactions in the solid stateAnalyses using high spatial resolutionAdhesionAnalysis in depthConclusionsAdvantages and disadvantages of AESFuture developmentsComparison with other surface analytical techniquesTechnique and instrumentationQuantificationSpectral identificationReferencesKeywords : Review ; Auger electron spectroscopy ; surface analysisJ.C. RIVIRREMaterials Development Division, AERE Harwell, Oxfordshire, OX 1 1 ORA.June, 1983Analyst, 1983, 108, 649-684June, 1983 SUMMARIES OF PAPERS I N THIS ISSUEScanning Potential Stopped-rotation VoltammetryThe technique of scanning potential stopped-rotation voltammetry, whichis based on measuring the differences between currents with the electroderotation switched on and off while the applied potential is scanned linearly,is described. Asymmetric rotation pulses, without the achievement of therotation “off ” steady-state current, are employed. The resulting modulatedresponse is free of most background current components, directly proportionalto the analyte concentration and reproducible.Well defined current -potential graphs are obtained for ascorbic acid, dopamine, homovanillicacid and hexacyanoferrate(I1) ion at the micromolar concentration level.Extremely low background signals are achieved a t a glassy-carbon diskelectrode, allowing a detection limit of 7 x 10-8 M of dopamine. The tech-nique is simple and suitable for automation.Keywords : Scanning potential stofified-rotation voltammetry ; hydrodynamicmodulation ; solid electrode ; anodic oxidationJOSEPH WANG and BASSAM A. FREIHADepartment of Chemistry, New Mexico State University, Las Cruces, NM 88003,USA.Analyst, 1983, 108, 686490.Differential-pulse Polarographic Monitoring of Permitted SyntheticFood Colouring Matters and Ascorbic Acid in Accelerated LightDegradation Studies and the Spectrophotometric Determination ofthe Ammonia and Simpler Amines FormedPermitted food colouring matters and ascorbic acid were determined bydifferential-pulse polarography to monitor their interaction during lightdegradation studies at pH 5.5 in acetate buffer containing EDTA.Reduc-tive splitting of azo bonds in the food colours was apparent from the formationof amines such as aniline, sulphanilic acid and naphthionic acid, which weredetermined spectrophotometrically using diazotisation methods. Theseamines were shown to be degraded further in the light to yield ammonia,which was determined spectrophotometrically as indophenol.Keywords : Food colouring matters ; degradation ; ammonia ; amines ; digeren-tial-pulse polarographyARNOLD G.FOGG and ABDULHADI M. SUMMANChemistry Department, Loughborough University of Technology, Loughborough,Leicestershire, LE11 3TU.Analyst, 1983, 108, 691-700.vi SUMMARIES OF PAPERS I N THIS ISSUE June, 1983Potentiometric Determination of Sulphite by Use of Mercury(1)Chloride - Mercury(I1) Sulphide Electrodes in Flow InjectionAnalysis and in Air-gap ElectrodesFlow injection analysis, using as the detector a solid-state ion-selective electrodewith a mercury(I1) sulphide - mercury(1) chloride membrane, can be used fordetermining sulphite or dissolved sulphur dioxide in water. At concentra-tions in the range 1.5-10mg1-1 of sulphite, the method has a Nernstianresponse of 60 mV per decade, but at lower concentrations (down to 0.1 mg 1-l)the e.m.f.is linearly related to the sulphite concentration. Although the flowinjection method is less sensitive than direct use of the electrode, it avoidsthe problem of chloride interference and permits the determination ofsulphur dioxide in the commonly used tetrachloromercurate absorbent. Theonly serious interference found was from sulphide, although a small effectwas also obtained from thiosulphate. Measurements in the range 0.1-10 mg 1-1 of sulphite had relative standard deviations for single results of nomore than 2%. The method requires only two reagents (dilute nitric acidsolutions) and is simple to operate. Each analysis is complete in less than5 min.Air-gap electrodes, using the same sensor, had sub-Nernstian responses ofvery poor reproducibility and were not considered to be a practical means ofdetermining sulphite.Keywords : Flow injection analysis ; air-gap electrode ; ion-selective electrode ;sulphite and sulphur dioxide determination ; mercury(II)sulphide -mercury(I)chloride membrane electrodesGEOFFREY B.MARSHALL and DEREK MIDGLEYCentral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey,KT22 7SE.Analyst, 1983, 108, 701-711.Synthetic Inorganic Ion-exchange Materials. Part XXXII.Studies on an Araldite-based Membrane of Crystalline Antimonic(V)Acid as a Nitrate Ion-selective ElectrodeAn Araldite-based membrane of crystalline antimonic(V) acid, when acting as anitrate ion-selective electrode, shows a near-Nernstian response for concentra-tions of nitrate ions between and 10-1 M and can be used for determiningthe activity of the ions.Stable potentials are observed within 10-30 s andfor about 2 min. The useful pH range is 3.5-11 at a higher concentration(5 x M) and 4.5-9 at a lower concentration (5 x lo-* M) of nitrate ions.This membrane responds to nitrate ions in a solution containing 25% of non-aqueous solvent.Keywords : Crystalline antimonic( V ) acid; nitrate ion-selective electrode ;A raldite-based membrane ; potentiometrySUSHMA AGRAWAL and MITSUO ABEDepartment of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1,Ookayama, Meguro-ku, Tokyo 152, Japan.Analyst, 1983, 108, 7 12-7 16J N n e , 1983 SUMMARIES OF PAPERS I N THIS ISSUEUse of an Argon - Nitrogen Inductively Coupled Plasma for theAnalysis of Aluminium Alloys Subsequent to Alkali DissolutionThe determination of a series of elements (boron, copper, gallium, iron,magnesium, silicon, vanadium and zinc) in aluminium samples by inductivelycoupled plasma optical emission spectroscopy is reported.High-purityaluminium, as well as various types of aluminium alloys (Al- Cu, Al-Mg,A1 - Mg - Si, A1 - Si, etc.) were brought into solution to give an analyte con-centration of 0.125% m/V with an alkali dissolution procedure. Thedetection limits for the mentioned elements range from 5 to 150 pg g-l.Both trace elements and major constituents can be determined in the typesof aluminium alloys mentioned, by using the same calibration graphs.viiKeywords : Alkali dissolution ; aluminium analysis ; argon - nitrogen induc-tively coupled plasma ; optical emission spectroscopyJose A.C. BROEKAERT and FRANZ LEISInstitut fur Spektrochemie und angewandte Spektroskopie, Postfach 778, D-4600Dortmund 1, Federal Republic of Germany.and GONGOR DINGLERGrundenstrasse 65, CH-8247 Flurlingen, Switzerland.Analyst, 1983, 108, 717-721.Analytical Errors Associated with Trace ElementDetermination in Freshwater Particulate Matter byNine analytical errors associated with two procedures for determining thetrace element content of freshwater particulate matter by atomic-absorptionspectroscopy were investigated using both natural samples and suspensionsof candidate reference materials. The first method involves an ultrasoundtreatment of filters to remove collected particles, but in a second method thefilters are destroyed by ashing. The precision and accuracy of the twoprocedures have been determined.Atomic-absorption SpectroscopyKeywords ; Trace element determination ; particdate matter; precision andaccuracy ; reference materials ; atomic-absorption spectroscopyRENATO BAUDO, GAETANO GALANTI and PIER GIORGIO VARINIConsiglio Nazionale della Ricerche, Istituto Italian0 di Idrobiologia, Largo VittorioTonolli 50-52, 28048 Verbania Pallanza, Italy.Analyst, 1983, 108, 722-727Viii SUMMARIES OF PAPERS IN THIS ISSUEEffects of Temperature Variation on the Zero, Second and FourthDerivative Ultraviolet Absorption Spectra of Benzenoid DrugsJ w e , 1983The effect of temperature in the range 0-40 "C on the absorbance a t threewavelengths of maximum absorption and two wavelengths of minimumabsorption in the zero-order ultraviolet absorption spectra and on the second-and fourth-derivative amplitudes of eight benzenoid drugs, displaying finevibrational structure in the region 250-270nm, was studied.All of thedrugs show a linear increase in absorbance a t the Amin. between the bandsof the fine structure with an increase in temperature, with temperature co-efficients ranging from + 0.06 to + 0.325% per degree. A concomitant linearreduction occurs in the second- and fourth-derivative amplitudes withtemperature coefficients in the range from -0.60 to -1.12% and -0.7 to- 1.2 yo, respectively.The implications of these high temperature coefficientson the accuracy and precision of derivative spectrophotometric assay pro-cedures for benzenoid drugs are discussed.Keywords ; Benzenoid drugs ; ultraviolet absorbawe ; derivative spectrofihoto-metry ; tem#erature effctsALEXANDER G. DAVIDSONDepartment af Pharmacy, University of Strathclyde, Royal College Building, 204George Street, Glasgow, GI 1XW.Analyst, 1983, 108, 728-732.Fluorimetric Determination of Trace Amounts of Gold as anIon- Association Complex With 2- Phenyl benzo[8,9] Quinolizino-[4,5,6,7-fed]phenanthridinylium PerchlorateThe synthesis, characterisation and applications of 2-phenylbenzo[8,9]-quinolizino[4,5,6,7-fed]phenanthridinylium perchlorate (PQPP) as a reagentfor the formation of ion-association complexes is described. This reagentreacts with AuC1,- to produce a 1 : 1 complex in 0.5 M hydrochloric acid,which is slightly soluble in water and can be extracted into isoamyl acetatewith an extraction efficiency of 90.7% The PQPP shows an intensefluorescence and is used for the fluorimetric determination of trace amountsof gold in the range 0.2-1.75 pg per 5 ml of organic layer. The interferencesof many metallic ions have been examined and under appropriate workingconditions the method is applicable to the determination of gold in lead.Keywords : 2-PhenyLbenzo[8,9]quinolizino[4,5,6,7-fed~hemamthridinylium pev-chlorate ; spectyofluorirnetry ; gold ddermination ; leadTOMAS PaREZ-RUIZ, CONCEPCION SANCHEZ-PEDRERO andJOAQUIN A. ORTUNODepartment of Analytical Chemistry, University of Murcia, Murcia, Spain,and PEDRO MOLINA-BUENDfADepartment of Organic Chemistry, University of Murcia, Murcia, Spain.Analyst, 1983, 108, 733-737

ISSN:0003-2654    

DOI:10.1039/AN98308FP057

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Jwae, 1983 SUMMARIES OF PAPERS IN THIS ISSUEPreparation of Fatty Acid Methyl Esters from Olive Oil andOther Vegetable Oils Using Aqueous Hydrochloric Acid - MethanolFatty acid methyl ester analysis is used for the examination of olive oiladulteration. Because of the toxicity of boron trifluoride - methanol reagent,which is currently used for the esterification of fatty acids, the use of aqueoushydrochloric acid - methanol as an esterification reagent has been studied.The method involves hydrolysis of lipids with a 2% sodium hydroxidein methanol solution. followed by esterification with aqueous hydrochloricacid in methanol (3 + 2) for 10 min on a steam-bath. A detailed study ofthe esterification of olive oil and of other vegetable oils with a high content ofunsaturated fatty acids has been undertaken.Comparison of this esterifica-tion with that using boron trifluoride and hydrochloric acid has given excellentagreement of results. Methyl ester hydrolysis has been found to take placeduring the esterification step. The hydrolysis does not affect the reliabilityof the method.ixKeywords: Fatty acids ; olive oil; methyl esters; gas chromatographyNIKOLAOS B. KYRIAKIDIS and GEORGE DIONYSOPOULOSState Chemical Laboratories Research Department, An. Tsocha 16, Ambelokipi,Athens, Greece.Analyst, 1983, 108, 738-741.Modified Gas - Liquid Chromatographic Method for DeterminingBromide/Total Bromine in Foodstuffs and SoilsThe widespread use of methyl bromide as a soil fumigant has necessitated thedevelopment of convenient and specific analytical methods for determiningbromide/total bromine in foodstuffs and soils subsequent to fumigation.The gas-chromatographic method described by Heuser and Scudamore wasinitially adopted by this laboratory.However, for the substrates underinvestigation, i.e., salad crops and soils, we found that the method lackedresolution and reproducibility and was hindered by tailing and long retention-time peaks. Themodified method, for dried ground substrates, is described. Mass spectro-metry was used on a non-routine basis to identify the chromatogram peaks.The mean recovery for dried vegetable substrates is 97% for a wide range ofbromide levels, equivalent to approximately 20-1 000 mg kg-1 on a freshmass basis.The method can be used to determine bromide down to0.1 mg kg-l of substrate fresh mass.The method proved suitable as a basis for development.Keywords : Gas chromatography - mass spectrometry ; bromide Jtotal bromineJOHN A. ROUGHAN, PATRICIA A. ROUGHAN and JOHN P. G. WILKINSMinistry of Agriculture, Fisheries and Food, Harpenden Laboratory, Hatching Green,Harpenden, Hertfordshire, AL5 2BD.Analyst, 1983, 108, 742-747.determination ; 2-bromoethanol ; f0odstu.s ; soilX SUMMARIES OF PAPERS I N THIS ISSUEInter-laboratory Calibration for Pesticide Analysis in South AfricaJzcne, 1983The organising and running of four inter-laboratory calibration exercises inSouth Africa in 1981 are described. Analyses of a solution containing pesti-cides, margarine and fruit pulp fortified with pesticides and a potato samplewith incurred residues were carried out.The results indicate that most labora-tories produced acceptable results. This type of exercise should be continued.Keywords : Inter-laboratory calibration ; pesticide analysisLOUIS P. VAN DYK, LAURRAINE LOTTER, PIETER R. DE BEER,ANDRE DE KLERK, AWIE J. VILJOEN and SUSAN M. PRINSLOOTask Group ICE, Working Group on Pesticide Analysis, Private Bag X134, Pretoria,South Africa, 0001.Analyst, 1983, 108, 748-753.Spectrophotometric Determination of Exchangeable Calcium inSoils by Chlorophosphonazo-mAShort PaperKeywords : Chlorophosphonazo-mA ; spectrophotometry ; calciMm determina-tion; soilsQIU XING-CHU and ZHANG YU-SHENGAgricultural Science Research Institute of Ganzhou Prefecture, Jiangxi, China.P.O.Box 82, Chengdu, China.and ZHU YINH-QUANAnalyst, 1983, 108, 754-757.Histochemical Demonstration of Collagen in ComminutedMeat ProductsShort PaperKeywords : S#ecifc collagen test; meat-product microscopy ; connective tissue ;rindF. OLGA FLINT and BARRY M. FIRTHF’rocter Department of Food Science, University of Leeds, Leeds, LS2 9 JT.Analyst, 1983, 108, 757-759Juute, 1983 THE ANALYST xiEnvironmentalists+The Royal Society of ChemistryPollution 9 Causes, Effectsand ControlEdited by Roy M. HarrisonPollution: Causes,Effects and ControlEdited by Roy M. HarrisonSpecial Publication No. 44Softcover 33Opp 0 85186 875 4Price f 12.00 ($21 .001Pollution isan inevitable consequence of Man’sexistenceon earth.As such, there will always be a requirement for the scientific studyof the pollutants themselves, their biological effects, and forengineering controls within a suitable legislative framework.Recent years have seen intense activity within all of these areaswhich has accompanied an increased public awareness ofpollution problems.Pollution is a truly inter-disciplinary subject area in which fewpractitioners have received formal training in more than one of themany disciplines contributing to the knowledge and understandingof pollution problems.For this reason there is much to be gainedfrom a broader perspective of the subject.The chapters in this book are in the main derived from the coursenotes provided by lecturers at an R.S.C.Residential School on thistopic held at Lancaster University in September, 1982. These havebeen supplemented by a few additional contributions aimed atimproving the overall coverage of this very broad subject area. TheResidential School had a teaching function and the chapters arepitched a t a level appropriate to this objective.This book will therefore be of value not only to teachers andstudents but also to scientists and technologists working in thefield of pollution.Contents00 Water Quality and Health000000000000 Epidemics of Non-infectious Disease00The Control of Industrial PollutionAspects of the Chemistry and Analysis of Substances of Concern in theWater CycleThe Role of Wastewater Treatment Processes in the Removal of ToxicPollutantsSewage and Sewage Sludge TreatmentThe Chemistry of Metal Pollutants in WaterEffects of Pollutants in the Aquatic EnvironmentImportant Air Pollutants and Their Chemical AnaylsisPollutant Pathways in the AtmosphereAtmospheric Dispersal of Pollutants and the Modelling of Air PollutionLegislation and the Control of Air PollutionCatalyst Systems for Emission Control from Motor VehiclesEvaluating Pollution Effects on Plant Productivity: A Cautionary TaleSystems Methods in the Evaluation of Environmental Pollution ProblemsOrganometallic Compounds in the EnvironmentORDERING:Orders should be sent to:The Royal Society of Chemistry,Distribution Centre, Blackhorse Road,, Letchworth, Herts SG6 1HN.Englandxii SUMMARIES OF PAPERS I N THIS ISSUEEvaluation of the Determination of High Levels of Total Cadmiumin Foodstuffs Using Flame Atomic-absorptionSpectrophotometric MeasurementJzcne, 1983Short PaperKeywords : Cadmium determination ; foodstuffs analysis ; flame atomic-absorption spectrophotometryDOROTHY DELLARDepartment of Industry, Laboratory of the Government Chemist, Cornwall House,Stamford Street, London, SE1 9NQ.Analyst, 1983, 108, 759-763.Further Studies on the Recovery of Iodine as Iodine-125After Alkaline Ashing Prior to AssayShort PaperKeywords : Iodine determination ; biological materials assay ; iodine-125G. BRYAN BELLINGCSIRO Division of Human Nutrition, Kintore Avenue, Adelaide, South Australia6000.Analyst, 1983, 108, 763-765.Cortisol Antibody ElectrodeCommunicationKeywords : Potentiometry ; membrane electrodes ; cortisol ; antibodiesM.Y. KEATING and G. A. RECHNITZDepartment of Chemistry, University of Delaware, Newark, DE 1971 1, USA.Artalyst, 1983, 108, 766-768... Jzcne, 1983 THE ANALYST X UAnalytical Sciences MonographsNo. 4 ElectrothermalAtomisation for AtomicAbsorption Spectrometryby C. W. FullerSince the introduction of atomic absorptionspectrometry as an analytical technique, by Walsh,in 1953, the use of alternative atomization sourcesto the flame has been explored. At the present timethe two most successful alternatives appear to bethe electrothermal atomiser and the inductively-coupled plasma.In this book an attempt has beenmade to provide the author's views on the historicaldevelopment, commercial design features, theory,practical considerations. analytical parameters of theelements, and areas of application of the first ofthese two techniques, electrothermal atomisation.Hardcover 135pp 0 85186 777 4E l 8.00 ($34.00) RSC Members f 13.50No. 5 Dithizoneby H. M. N. H. IrvingThe author of this monograph, who has beenclosely associated with the development ofanalytical techniques using this reagent for manyyears, and who has made extensive investigationsinto the properties of its complexes, has gatheredtogether a body of historical and technical data thatwill be of interest to many practising analyticalchemists.Hardcover 11 2pp 0 851 86 787 1f12.50 ($24.00) RSC Members f9.50No.6 lsoenzyme AnalysisEdited by D. W. MossThis monograph attempts to draw together the mostimportant experimental techniques which haveresulted from the modern recognition that enzymesfrequently exist in multiple molecular forms. Thismonograph also indicates the advantages andlimitations in isoenzyme studies of these modernexperiments.Brief Contents:Multiple Forms of Enzymes; Separation of MultipleForms of Enzymes; Selective Inactivation of MultipleForms of Enzymes; lmmunochemistry of MultipleForms of Enzymes; Catalytic Differences betweenMultiple Forms of Enzymes, Methods of ObtainingStructural Information, Selection of Methods ofAnalysis.Hardcover 171pp 0 85186 800 2f12.00 ($23.00) RSC Members f9.00No.7 Analysis of AirbornePollutants in WorkingAtmospheresThe Welding and SurfaceCoatings Industriesby J. Moreton and N. A. R. FallaThis Monograph covers the following:Part I The Welding Industry: Airborne Pollutantsin Welding; Sampling of Welding WorkshopAtmospheres; Analysis of Welding Fumes andPollutant Gases.Part II The Surface Coatings Industry: Origin ofAirborne Pollutants in the Surface CoatingsIndustry; Collection and Analysis of GaseousAtmospheric Pollutants in the Surface CoatingsIndustry; Collection and Analysis of ParticulateAtmospheric Pollutants in the Surface CoatingsIndustry Future Trends Relating to Sampling andAnalysis in the Welding and Surface CoatingsIndustries.Hardcover 192pp 0 85186 860 6f15.00 ($29.00) RSC Members f12.00No.8 The Sampling of BulkMaterialsby R. Smith and G. V. JamesThe literature of analytical chemistryexhaustively covers the many techniques nowavailable t o the analyst.feature common t o all analyses, is in contrastonly sparsely documented. Comparatively feworiginal papers on this subject have beenpublished in the last fifty years; there are veryfew reviews available, and perhaps as a resultsampling is badly neglected in most instructionalcourses i n analytical chemistry. ThisMonograph will go some way towards filling agap i n the literature and should stimulateinterest in the development of sampling as afield of study.Brief ContentsIntroduction; Glossary of Terms; Establishment of aSampling Scheme; Sampling Theories; Apparatusfor Sampling; Sampling Methods; Appendices 1-4.Hardcover 200pp 0 851 86 81 0 Xf16.50 ($32.00) RSC Members f10.75Orders:RSC Members should send their orders to:The Membership Officer, The Royal Society of Chemistry30 Russell Square, London WC1B 5DTAll other orders should be sent to:The Royal Society of Chemistry, Distribution Centre,Blackhorse Road, Letchworth, Herts.SG6 1 HNSampling, the oneThe Royal Society ofChemistrTUCK IN UNDER FLAP ATHE ANALYST June, 1983 iREADER ENQUIRY SERVICEFor further information about any of the products featured in the advertise-ments in this issue, please write the appropriate A number in one of the 3 ;Postage paid if posted in the British Isles but overseas readers must affix z;a stamp.boxes below. ; i(Please use BLOCK CAPITALS)NAME ......................................................................................................................................................................................... iOCCUPATION .................................................................................................................................................................ADDRESS ............................................................................................................................................................................. iSECOND FOLDPostagewill bePaid byLicenseeDo not affix Postage Stamps if posted inGt. Britain, Channei Islands or N. IrelandIBUSINESS REPLY SERVICELicence No. W.D. 106Reader Enquiry ServiceThe AnalystThe Royal Society of ChemistryBurlington HousePiccadilly London W1 E 6WFENGLANDFLAP

ISSN:0003-2654    

DOI:10.1039/AN98308BP063

出版商:RSC    

年代:1983

数据来源: RSC

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JUNE 1983 Vol. 108 No. 1287 The Analyst Auger Techniques in Analytical Chemistry A Review J. C. Rivisre Materials Develo9ment Division AERE Harwell Oxfordshire OX11 ORA Summary of Contents Introduction Historical Auger effect Chemical information Surface specificity and vacuum requirement Electron energy analysis Electron sources Data acquisition and handling First principles Elemental standards Auger peaks Satellite peaks Ionisation loss peaks Secondary electron peak Analysis in depth Angular dependence of Auger intensity Erosion by ion bombardment Erosion by mechanical lapping Technique and instrumentation Quantification Spectral identification Chemical bonding information Applications Corrosion and oxidation Catalysis Reactions in the solid state Analyses using high spatial resolution Adhesion Analysis in depth Conclusions Advantages and disadvantages of AES Future developments Comparison with other surface analytical techniques References Keywords ; Review ; Auger electron spectroscopy ; surface analysis Introduction Historical Although the Auger effect was discovered1 as long ago as 1925 during experiments in a cloud chamber in which X-radiation was used as the ionising source the first report of stimulation of Auger emission from solids by incident electrons did not appear until 1953.This was by Lander,2 who can justly be regarded as the founder of the modern technique of Auger electron spectroscopy (AES). At that time however both lack of sensitivity and inadequate vacuum capability limited the development of the technique and the possibilities lay dormant 64 650 R I V I ~ R E AUGER TECHNIQUES IN Analyst Pol.108 until 1967 although in 1958 an isolated application was published by Powell et aZ.,3 who used it to monitor carbon contamination on tungsten. By 1967 production of ultra-high vacua had become routine and in that year two groups of workers Scheibner and Tharp4 and Palmberg6 showed that the sensitivity problem could also be reduced significantly by using the spherical-grids-and-screen arrangement already in use for LEED as a retarding-field energy analyser. In the same year it was demonstrated by Harris6 and by Weber and Peria' that visual enhance-ment of small or badly resolved Auger features could be achieved simply by electronic differen-tiation of the energy distribution N ( E ) of secondary electrons to give dN(E)/dE.Most Auger spectra are still recorded in this form. It soon became obvious that retarding-field analysers (RFA) suffered from poor signal to noise characteristics representing a serious limitation on the development of the technique, and attention was turned to the possibilities of dispersive analysers. In 1969 Palmberg et aZ.* showed that the cylindrical mirror analyser (CMA) had properties that made it emi-nently suitable for AES (see Technique and Instrumentation) and in a short space of time the CMA had almost completely superseded the RFA. Nowadays the only situations in which an RFA is used for AES are those in which the experimentalist wishes to perform LEED also.The CMA is available commercially in a variety of designs and where AES alone is to be performed is likely to be the accepted analyser for the foreseeable future. On the other hand, where both AES and X-ray photoelectron spectroscopy (XPS) are combined in the same instrument the preference is for a concentric hemispherical analyser (CHA) in which better energy resolution is obtainable. The basic technique of AES has not changed since 1967 in as much as differentiation of the spectra is still almost universally employed but of course there have been continual refinements and improvements to improve the quality of the spectra. The greatest changes since the early days have been in the interpretation of the spectra in the understanding of the physical pro-cesses involved in the production and ejection of an Auger electron and in the ever widening field of application of the technique.This review will endeavour to describe these and other aspects of AES with particular emphasis on those features of relevance to analytical chemistry. Auger Effect Inter-action with an incident electron creates a hole in a core atomic binding level that is causes ionisation ; the level shown ionised in the example is the K or Is. Relaxation of the atom back towards its ground state occurs by the filling of the core level with an electron from an outer level. The excess of energy represented by the difference in the binding energies can then cause emission either of a photon of characteristic energy or of an Auger electron. The two processes The Auger effect taking place in an isolated atom is shown schematically in Fig.1 (a). t (a) VAC v.0. either Fig. 1. (a) Schematic diagram of competing Auger and X-ray emission processes in an isolated atom following ionisation of a core level and (b) schematic diagram of competing Auger and X-ray emission processes in a solid following ionisation of a core level Jwae 1983 ANALYTICAL CHEMISTRY. A REVIEW 65 1 compete but for core level binding energies of less than about 2000 eV the probability of Auger emission is close to unity. As can be seen in Fig. 1 (a) the Auger electron also comes from an outer level which may or may not be the same as the de-exciting outer level. In the specific example shown the de-exciting level is the L and the emitting level the L2,3 and the Auger transition depicted would thus be designated the KL,L, according to convention.Where spin - orbit splitting cannot be resolved in any sub-shell of any particular atom e.g. as be-tween L and L3 in Fig. l ( a ) then the convention combines the levels together as for instance, L,,,. The simple notation based on X-ray spectroscopy is widely used but becomes inade-quate in those instances in which the two holes in the final doubly ionised state cannot decay independently but remain together sufficiently long for coupling between them to occur. This leads to fine structure appearing in the Auger spectrum interpretable in terms of final state spectroscopic terms. When atoms combine to form a solid atomic energy levels shift and broaden to form energy bands and in particular the outermost levels containing the valence electrons go to form the valence band.Auger transitions can take place just as readily in atoms in the condensed phase as in the gaseous phase and those transitions which involve electrons in the valence band are often the most intense in the spectrum. Such a transition involving an electron from the valence band of a solid is shown diagrammatically in Fig. l ( b ) . The designation of that transition would be KL,V; if both the de-exciting and emitted electrons had originated in the valence band it would have been designated KVV. Once an Auger electron has been ejected either from an atom or from the surface of a solid, its energy can be measured. Reference to Fig. l(a) shows that the energy is equal to where ABC is the Auger transition being considered where Ei are the binding energies of the levels i in the atom in a singly ionised state and where El’ is the binding energy of level i in a doubly ionised state.For Ei binding energies measured by X-ray or photoelectron methods can be used but for Ei’ it is necessary to consider the effect on the electrons in outer orbitals of the atom due to the presence of the additional hole or positive charge. The effect is to give the outgoing Auger electron some additional kinetic energy arising from the relaxation of the outer orbitals towards the hole in order to provide additional screening. The contribution of this atomic relaxation energy to the Auger energy was first pointed out by S h i r l e ~ ~ who showed that it could be large of the order of 10-20 eV.When the ionised atom is not isolated but forms part of a solid an additional relaxation energy appears called the extra-atomic relaxation energy; this is gained from the shift of electrons in immediately adjacent atoms towards the positive charge. Relaxation energies and therefore Auger energies can now be calculated with great accu-racy; the reader is referred to the compilation of such energies by Larkinslo as an example and also as a manual. It will be clear from the above that each element in the Periodic Table (except H He and atomic Li) has a unique Auger spectrum as no two elements have the same set of binding energies and that analysis of the energies of the Auger electrons therefore pro-vides a means of elemental analysis.Even when an element is in solid form or in combination with other elements in a solid the fact that the binding energy of the initially ionised core level is the dominant term in equation (1) allows unambiguous elemental identification. Chemical Information The combination of one atom with another causes changes in the electron density surround-ing the atom that is changes in the electron binding energies. These changes are most pro-found in the outermost or valence electrons but inner levels can be affected too to a lesser degree. Direct observation of these “chemical shifts” forms the basis of the technique of X-ray photoelectron spectroscopy (XPS) but in AES it should be obvious from Fig. 1 that chemical effects will be much harder to interpret as there is an additional ionisation in the final state of the atom.In general the shifts observed in AES have been used in a qualitative “fingerprinting” way without much effort to understand their precise origin. Some of these shifts can be very large greater than any seen in XPS; e.g. from Si to SiO reported values range from 10 to 15 eV from A1 to Al,03 -13 eV and from Mg to MgO -11 eV. Amongst th 652 RIVIERE AUGER TECHNIQUES IN Analyst Vol. 108 transition metals of the first series the shifts are lower between 1 and 4 eV. For a comprehen-sive tabulation of the shifts published in the literature the reader is referred to the paper by Madden .ll The other source of information on chemical changes in the vicinity of an atom comes from study of the shape of an Auger peak based on a transition in which either one or both electrons originate in the valence band.Again the changes observed in Auger peaks can and have been used in a purely “fingerprinting” way and indeed for carbon have been so used from the earliest days of AES. Among the observable spectral changes are the presence or absence of fine structure components and changes in their widths and relative intensities. However al-though the use of line-shape changes in such a qualitative way provides some chemical informa-tion more recent developments seek to derive fundamental knowledge about the local density of electronic states around individual surface atoms. This is achieved by unravelling or self-deconvoluting spectra in which both electrons originate in the valence band because of course each electron carries the same information about conditions in the band.Because the Auger process is atomic in nature the self-deconvolution then produces a density of states appropriate to the immediate surroundings of the ionised atom. Examples of the ways in which chemical information has been extracted from Auger spectra will be given later. Technique and Instrumentation Surface Specificity and Vacuum Requirement If the characteristic energy of an Auger electron in a solid is to be measured then the electron must be able to diffuse to and escape from the surface of the solid without losing energy by inelastic scattering. Although the range of primary energies used in AES is 5-20 keV the energy range of the AHger electrons used in the technique is normally 20-2000 eV.Fig. 2 shows how the kinetic energy of low-energy electrons varies with inelastic mean free path Le., the average distance travelled before losing energy by an inelastic collision ; the compilation is by Seah and Dench.12 It can be quickly appreciated that for an Auger electron of energy in the above range to be able to escape from the surface with its original characteristic energy it must originate within the outermost few atomic monolayers ; hence the highly surface-specific nature of AES. 100 -10 -/ x 1 -Fig. 2. Variation of inelastic mean free path of electrons in a solid with kinetic energy. The inelastic mean free path is related to the average escape depth of an electron ejected from a solid into the vacuum.The dashed lines are theo-retical relationships.’ Jzcne 1983 ANALYTICAL CHEMISTRY A REVIEW 653 As AES is so sensitive to the state of the surface and as most applications of AES involve treatment of the surface at some stage e.g. cleaning by heating or ion erosion or both reaction with introduced gases and vapours fracture etc. it follows that interference from the ambient must be eliminated. In practice this means reducing the total pressure to the level at which the build-up of contaminant layers on the surface by adsorption of gas molecules is so slow that there is enough time to carry out the AES analyses without such interference. Consideration of gas kinetics shows that AES like the other surface-specific techniques must be performed in ultra-high vacuum at pressures of the order of lo-* Pa ( Torr).To achieve such vacua requires accelerated outgassing by bakeout at -200 "C and hence the materials of construction are limited to those which will withstand such treatment and which do not themselves contribute anything to the residual gas background. Electron Energy Analysis As stated in the Introduction the first electron energy analyser to be used extensively for AES was the retarding field analyser (RFA) simply because the RFA used the existing electron optical arrangement for LEED and there were at the time a large number of LEED systems in existence. For those engaged in LEED measurement for its own sake the RFA is still con-venient for checking the cleanliness of a surface or for monitoring surface reactions but for all other applications energy analysis in AES is now performed in either of two types of analyser, the cylindrical mirror (CMA) or the concentric hemispherical (CHA).The former is the more popular for AES alone the latter for combined AES and XPS. It consists of two co-axial cylinders with annular entrance and exit apertures cut in the inner cylinder. If a deflecting (Le. negative) potential I' is applied to the outer cylinder while the inner cylinder is earthed and if the radii of the inner and outer cylinders are rl and yZ respectively then electrons emitted at the source on the sample surface with a kinetic energy E are refocused according to the equation A diagraml3 of a modern CMA and its associated circuitry is shown in Fig. 3. * (2) E/eV = 1.31 ln(rl/r2) .. . . for a critical emission angle of 42" 18'. At that angle the first-order aberration terms vanish and the device becomes second-order focusing. Obviously r2 must be large enough to allow free passage of electrons and it is convenient to make it about double yl. Typical dimensions for a CMA would be outside diameter 15 cm and length about 50 cm. UHV chamber Cylindrical mirror analyser Fig. 3. A modern cylindrical mirror analyser (CMA) and its associated circuitry. The electron gun is co-axial with the CMA. Electrons ejected from the irradiated point on the target positioned at the source of the CMA are re-focused at the electron-multiplier detector by a deflecting potential applied to the outer cylinder.1 654 RIVIBRE AUGER TECHNIQUES IN Analyst Vol.108 As dispersive analysers such as the CMA allow refocusing of only those electrons whose energies are in a narrow range whose width is dependent on the energy resolution of the indi-vidual instrument simply ramping the deflecting potential V through the energy range of interest produces an energy distribution EN(E). If the differential distribution EdN(E)/dE is required a small modulation of a few volts is superimposed on V and the collecting circuitry is then tuned to the fundamental frequency of the a.c. component of the collected current. Typical energy resolution to be found in a commercially available CMA would be 0.3% (Le., resolving power 330). The high luminosity and low time constant of the CMA enable it to be operated at high scanning speeds if required so that an AES spectrum could be displayed on a TV screen.Alternatively the incident electron beam generated from an internal electron gun on axis can be scanned rapidly over the specimen surface while the CMA is set to the kinetic energy of a chosen Auger peak thus producing a distributional map of a particular element over the surface. This variation of the technique is called scanning Auger microscopy (SAM). Fig. 4. Schematic diagram of Concentric hemispherical analyser (CHA). The radius of the inner spherical surface is y1 and that of the outer Y,. If the potential applied between the hemispheres is V then electrons entering the analyser at S with energy E are re-focused at F according to equation (3). The other commonly used dispersive analyser the CHA is shown diagrammatically in Fig.4. As the name indicates it consists of two concentric hemispheres whose included angle can be 180" as shown in Fig. 4 or 150". If the radii of the inner and outer hemispheres are y1 and y2, respectively and if the deflecting potential applied across the hemispheres is 'V then electrons entering the analyser with energy E are refocused according to the equation The CHA is often operated under constant resolution conditions that is the deflecting poten-tial V is fixed at some chosen value according to the required energy resolution and electrons approaching the entrance slit are retarded to that potential. Modern versions have an input lens system to decelerate and focus electrons on to the slit and thus improve the over-all sensitivity by accepting a greater input solid angle of electrons from the specimen.A typical size of CHA in common use would be of average radius about 10 cm and have an energy resolution of about 0.1% (ie. a resolving power of 1000). Modulation to produce a differential distribution is normally applied to the appropriate input lens electrodes and the effect of that practice is that the distribution is unfortunately not necessarily EdN(E)/dE but some com-plicated function of it. The CHA is in general more applicable to XPS where differentiation is not employed J w e 1983 ANALYTICAL CHEMISTRY A REVIEW 655 Electron Sources As the only basic physical requirement of a primary source of electrons for AES is that of ionisation of an atomic core level the energy conditions required are not stringent.Because of the shape of the variation of ionisation cross-section with energy,14 it is desirable that the primary energy be greater than about five times the binding energy of the core level but there are no restrictions on energy spread in the beam. Only if characteristic loss peaks also observable in the spectrum are to be studied should the spread be restricted to about 5 0 . 5 eV. The principal direction of development in recent years has been towards ever smaller irradiated areas i.e. electron spot sizes in order to achieve high spatial resolution. Electron optical design parameters such as space charge and lens aberrations inevitably push the development towards lower beam currents and higher beam voltages so that the current state-of-the-art instrument would operate typically at 1-10 nA and 20-30 keV as primary beam conditions.Under those conditions spot sizes of -50 nm can be obtained although it should be realised that the Auger resolution will be worse than that 100-200 nm owing to back-scattering effects that broaden the region from which Auger emission occurs compared with the irradiated region. As electrons can be deflected easily by electrostatic or electromagnetic fields it is relatively simple to raster the primary beam across a surface and use either the low-energy secondary electrons to produce a topographical image as in the scanning electron microscope or the Auger electrons at a selected energy to produce an elemental map. Much of the drive in development is towards smaller spot areas and the scanning mode in this context is proving invaluable in quality control in such areas as semiconductor device manufacture.Data Acquisition and Handling A typical spectrum of the number of electrons N(E) at a particular kinetic energy E ejected from a surface as a function of E is shown in the lower part of Fig. 5 for a boron specimen. 1 @--==- Ep= 1000 eV 175 167 I B Ep= 1000 eV A 0 200 400 600 800 1000 E ne rg y/eV Fig. 5. (A) Secondary electron distribution N(E) from a boron specimen at a nominal primary energy of 1000 eV. To the right are the elastic peak and associated plasmon loss peaks. To the far left is the steep slope leading up to the true secondary peak. At 167 eV is the boron KVV Auger peak. (B) Portions of the differential distri-bution dN(E)/dE in the regions of the elastic peak and the Auger peak.In the differential distribution the position of a peak is taken conventionally as that of the high energy minimum e.g. that for the boron Auger peak is at 175 eV 656 RIVIBRE AUGER TECHNIQUES IN Awalyst Vol. 108 The N(E) distribution as it is called contains two prominent peaks one at the primary energy due to elastically scattered electrons and the other at very low energy due to so-called “true” secondary electrons. Elsewhere there are minor features and in particular there is a peak at -180 eV due to electrons ejected by the boron KVV Auger transition. As can be seen not only is the Auger feature superimposed on a high background but in parts of the spectrum the background is also changing rather rapidly.For those reasons and also for visual enhance-ment of Auger features as they are not always as clear as the boron peak in Fig. 5 it is still normal to differentiate the spectrum electronically with respect to E to provide the dN(E)/dE spectrum [it would in practice be EdN(E)/dE when using a dispersive analyser]. The differentiated boron peak is shown in the upper part of Fig. 5. The energy resolution of a differentiated Auger peak will be a function of the way in which the differentiation is performed, i.e. of the sinusoidal modulating voltage applied to the outer cylinder of a CMA and if too great an amplitude of modulation is used in an effort to achieve greater sensitivity then the peak will suffer serious distortion.To a first approximation the difference between the maximum positive and negative excursions of an Auger peak in the differential distribution is taken as a quantity proportional to the concentration of the element giving rise to that peak. This approximation has been shown to be valid and useful in a large number of instances but as more knowledge is gained about the changes in Auger peak shapes as a result of changes in chemical environment so it is being realised that alteration of a peak shape can also alter the proportionality between differentiated peak height and concentration. The more appropriate quantity to measure is the area under an Auger peak in the undifferentiated or N(E) spectrum. Unfortunately this is not as easy as it sounds because of the problems of removing the inelastic background in a physically correct way before integration and of removing the effects of instrumental broaden-ing.The first problem in particular has not yet been solved in a satisfactory way and for most quantification purposes it is still the differentiated peak-to-peak height that is used. Most modern Auger spectrometers are now controlled by dedicated minicomputer systems. These will acquire spectra according to pre-programmed acquisition parameters such as energy region modulation voltage and counting statistics and store the acquired spectra for subsequent manipulation. Some systems will also allow a set of spot analysis positions to be chosen and stored by placing a cursor on features of interest in a scanning electron display of the specimen surface.Once the positions have been recorded then the primary beam returns to them at each subsequent acquisition without requiring re-positioning. Such a facility is useful when following changes in individual features as a function of surface treatment e.g., erosion by ion bombardment. Quantification First Principles Suppose the surface of a multi-component specimen contains i atomic species and that the current of Auger electrons of kinetic energy Ei resulting from the transition ABC in the ith species is measured. Then the relationship between that current I i and the density N i of atoms of the ith species within the analysed volume is where 1 = primary electron current; EA = binding energy of level A ionised in atom i; Ep = primary electron energy; u(Ep,EA) = ionisation cross-section of E at E,; h(E,) = inelastic mean free path for electrons of energy E i ; T(E,) = transmission of analyser at energy E,; D(Ei) = efficiency of detector at energy E,; P(ABC) = transition probability of ABC including contributions to ionisations of A from both direct and Coster - Kronig transitions from other levels; G = geometric factor; R = roughness factor; and y(EP,EA) = back-scattering factor taking account of additional ionisation of A due to energetic secondary electrons produced by Ep Jane 1983 ANALYTICAL CHEMISTRY A REVIEW 657 Under normal experimental conditions the primary energy and current would be held constant the geometry of the system would be fixed the detector efficiency would be sensibly constant as in most analysers the electrons are accelerated through a few hundred electronvolts into the multiplier and across the surface of any one specimen the degree of roughness is also likely to be constant.Equation (4) can thus be reduced to Ii(ABC) = Ko(Ep,E,) X (Ei)T(Ei)P(ABC)[l + r (E,,E,)]N . . * (5) where K includes the experimentally constant factors. Even in this expression there are still quantities that are not well known. There are various theoretical expressions that fit the cross-section CJ reasonably well the inelastic mean free path X is known fairly accurately as a function of energy from the work of Seah and Dench,12 the transmission function T is becoming better known for most commercial analysers but the probability P and the back-scattering factor r are much less well known.Calculations exist for both the latter quantities but their accuracy is doubtful. Approximations have always to be made in using equation (5) and the first-principles approach is rarely accurate to better than 560y0. Elemental Standards In this approach to quantification use is made of compilations of elemental Auger spectra recorded under well characterised conditions of energy resolution and of amplification and the assumption is made that the ratio of the intensity of an Auger peak from a given element in any situation to that of the same Auger peak from the pure element can be taken as proportional to the atomic fraction of that element. With the help of the compilations the proportionality constants can be regarded as relative elemental sensitivity factors Si so that the atomic per-centage concentration Ci of the ith element in an analysed volume containing j elements can be written as Equation (6) is used frequently and leads to an accuracy in quantification of about &20y0, but reference to equation (5) shows that there is total neglect of matrix effects in equation (6).In certain simple situations e.g. binary alloys the matrix effects can be included so that the accuracy can be improved to &loyo but in the more commonly encountered multi-component systems data on the modifications of inelastic mean free paths and back-scattering factors are inadequate. In either method of quantification the assumption is implicitly made that all elements are uniformly distributed within the analysed volume.Non-uniform distribution with depth cannot yet be treated although in certain instances e.g. where the surface is sufficiently smooth some information about depth distribution can be obtained by variation of take-off angle. See also the section on depth distribution by ion profiling. Spectral Identification Auger Peaks Prediction of the expected Auger energies from any element can be made using equation (1) ; until recently a semi-empirical approximation was used that was accurate enough for most purposes particularly in the earlier days of the technique but now the theory for the calcula-tion of Auger energies has become very exact and tables of calculated energies are available. The most comprehensive are those of Larkins.lo Obviously the number of possible Auger transitions increases progressively with atomic number owing to the proliferation of atomic energy levels but fortunately for AES there is a large variation in the probabilities of the transitions so that in practice within the energy range normally encompassed in the technique, only a few transitions are observed.Each region of the Periodic Table tends because of the similar electronic structure of atoms in that region to have a spectrum of characteristic appearance. For example in the first series of transition metals the three major LMM Auger peaks act as the “fingerprint”; their relative intensities and separations alter from scandium to zinc but they are unmistakable 658 R I V I ~ R E AUGER TECHNIQUES IN Analyst Vol. 108 Some of them are shown in Fig.6 (from Weber15) for the metals chromium manganese and iron. In the same spectra it is also possible to see the low-energy M2,3M4,5M4,5 Auger peaks. Another set of spectra based on MNN transitions characteristic of another region of the Periodic Table is shown in Fig. 7 (also from Weberls) for the elements silver cadmium, indium and antimony. In both Figures there are low-intensity features at energies below those of the principal peaks due to minor Auger transitions. Commercially produced hand-books have been available for some time in which Auger spectra in the energy range 0-2000 eV are displayed for nearly all elements; most elements have been cleaned ilz sitzc for the recording, but obviously most of the non-metallic elements would have to have been used in the form of compounds.A Ic Ni Ni Ni 0 200 400 600 800 1000 0 200 400 600 800 1 Electron energylev Electron energylev DO Fig. 6. Auger spectra from three of the first-series transition metals (A) chromium (B) manganese and (C) iron. The LMM triplet appearing for those metals in the energy region 450-700 eV is a characteristic “finger-print” for the series. The low-energy peak is due to the Ma,8M4 IM4 transition. l6 Fig. 7. Auger spectra from (A) silver (B) cadmium (C) indium and (D) antimony. The sharp doublet due to MNN transitions is very characteristic of these metals.16 It should be realised when attempting to identify Auger spectra that chemical effects are common. They take the form of shifts in peak energy and changes in peak shape generally both together and are strongest when the Auger transition is based on one or more electrons originating in the valence band of the solid.The first example of this and still one of the most striking is that of carbon in various states of combination; Fig. 8 shows the carbon KVV Auger spectra from graphite and from various carbides recorded by several workers.lG-l* Examples of large chemical shifts in particular for Mg A1 and Si have already been given in the Intro-duction. Unless the possible presence of such shifts is taken into account misinterpretation could result Jzcne 1983 ANALYTICAL CHEMISTRY A REVIEW 659 272 eV 1 273 eV Sic* Sic N isC T 1 271 eV 272 eV 240 260 280 300 240 260 280 En erg y I eV Fig. 8. Differences in the carbon KVV Auger peak shape in various chemical situations.(a) In silicon carbide and in graphite; the asterisk refers to the ion-bombarded surface. (b) In nickel carbide. (c) In titanium vanadium and chromium carbides.l6-l8 Satellite Peaks Any electron of sufficient energy moving within a solid can interact with the Fermi sea of electrons setting up collective oscillations (“waves”) within the sea. These oscillations have characteristic frequencies and so the interaction involves the loss of a characteristic amount of energy from the electron termed the plasmon loss. When the energy spectrum of electrons ejected from the surface is measured the plasmon loss peaks can be seen on the low-energy side of the principal peaks in the spectrum. Often multiple losses can be observed owing to the excitation of second third etc.harmonics of the fundamental oscillation although of course they decrease progressively in intensity rather quickly. All such losses are called “bulk” plasmon losses. Sometimes observable particularly when a low primary excitation energy is used is a loss peak at a smaller energy separation from the parent peak. This is the so-called “surface” plasmon loss arising from interaction with collective electronic oscillations in the layers of the solid near the surface; the presence of the surface i.e. the termination of the solid changes the characteristic frequency so that the energy of interaction is 1 / 4 2 that of such interaction in the bulk. Bulk plasmon losses are typically in the range 15-25 eV depending on the material so that the corresponding surface plasmons would be in the energy range 10.5-17.5 eV.Neither peak in the energy spectrum is more than 10% of its parent often less but clearly if the parent peak is one of the major features then there is the possibility of confusion of a loss peak with the Auger peak of a minor constituent. Occasionally a satellite peak appears at higher energy than the parent but is invariably small. Such peaks have been attributed in the past to plasmon gains but are now believed to be due to double ionisation of the atom 660 R I V I ~ R E AUGER TECHNIQUES IN Analyst VoZ. 108 Ionisation Loss Peaks As stated in the Introduction the initial stage in the process leading to Auger emission (or photon emission) in AES is the ionisation of a core level by a primary electron.The ionisation occurs by excitation of an electron from the core level to the first unoccupied state above the Fermi level. If there is a narrow band of such unoccupied states and a high probability of transition to it then at the threshold for ionisation a substantial number of electrons are lost from the primary current and a significant step loosely called a peak appears in the spectrum separated from the primary energy by the ionisation energy. Two such steps after differentia-tion are shown in Fig. 9 in which can also be seen bulk plasmon loss peaks associated in this instance with the elastic peak at the primary energy; the material is carbon-contaminated boron. The binding energies of the K shells of boron and carbon are 188 and 284 eV respec-tively and the corresponding ionisation loss peaks appear at 807 and 71 1 eV below the elastic peak.952 I I loss plasmon I I I” I 600 700 800 goo 1000 1100 En e rg yleV Fig. 9. Differential spectrum from a piece of carbon-contaminated boron in the energy region near the elastic peak. Primary energy was nomi-nally 1000 eV. The peaks a t 71 1 and 807 eV arise from losses from the primary energy owing to ionisation of the carbon and boron K shells a t 284 and 188 eV respectively. Near the elastic peak can be seen the peaks due to plasmon losses from the primary energy. Secondary Electron Peak All electrons in the energy distribution ejected from a surface except those elastically scattered are strictly secondary electrons but conventionally the term “secondary electrons” is taken to mean the so called t m e secondary electrons produced in the solid by a cascade pro-cess.The current of true secondaries peaks at very low energies of the order of a few electron-volts but the peak is very broad as can be seen in Fig. 5 and extends to 200-300 eV; it is the most prominent feature in the energy distribution at high primary electron energies while at low primary energies (<lo00 eV) it is the next most prominent after the elastic peak. The presence of the secondary electron peak is inconvenient in that Auger transitions of low energy are thus situated on a steeply sloping background June 1983 ANALYTICAL CHEMISTRY A REVIEW 661 Analysis in Depth Although the average escape depth of Auger electrons in the energy range used by AES is of the order of only a few atomic layers as can be seen from Fig.2 in practice it is often necessary to obtain information from greater depths. Compositional variations through corrosion-formed films through multi-layer structures on semiconductor devices and through surface layers altered by implantation to name but a few as a function of depth are needed in order to understand the mechanisms involved in the formation of the films etc. The methods that have been used to study such variations differ according to the thickness of the layer or film involved and will be discussed here in order of increasing thickness. Angular Dependence of Auger Intensity For an ideally smooth surface it is easy to show that if h is the inelastic mean free path for electrons of a particular kinetic energy and if the angle to the surface at which the energy analyser is placed (called the take-off angle) is 8 then the average escape depth is hsin8.In other words by varying 8 from 90 to O” it is possible in principle to vary the average escape depth from h to zero. In practice of course no surface is ideally smooth and it is normally impossible to use take-off angles lower than about 15”. Nevertheless there are special cases, e.g. very thin films of SiO on Si and segregated layers where the angular dependence method has proved useful. Where the information on compositional variation is required from depths greater than the inelastic mean free path i.e. in the overwhelming majority of instances then it is obvious that the surface must be removed in order to allow analysis by AES at the depths required.The removal to achieve the depth profile has been carried out either by erosion by ion bombardment or by mechanical lapping to produce a taper section. Its major advantage is that it is non-destructive. Erosion by Ion Bombardment A beam of positive ions almost invariably those of argon of energy between 500 and 5000 eV, and of current density between 5 and 50 pA crn-, is directed at the surface from an ion gun for a chosen length of time corresponding to the depth of erosion required. As data acquisition in AES is relatively fast and as the pressure of argon in the spectrometer chamber is generally between 10-7 and 10-5 Torr during ion bombardment it is perfectly feasible to carry out simultaneous bombardment and analysis thus achieving a continuous compositional profile down to the chosen depth.In practice the energy analyser is programmed to record the intensities at several selected energies corresponding to major Auger peaks of the elements whose profile is required in continuous succession during bombardment ; this is known as “multiplexing”. The other method of recording a compositional depth profile is to stop the bombardment at selected intermediate intervals and record the Auger spectra from the newly exposed surfaces. The advantages of the latter procedure are that the surface condition is not changing further during analysis that a more complete analysis can be carried out and that the possibility of synergistic effects due to simultaneous ion and electron irradiation is removed.However it is of course much slower than the multiplexing procedure. Profiling by ion bombardment as described above is a very widely used technique but its range of usefulness in terms of interpretable information is limited by the effects of the bom-bardment itself on the solid surface. Argon ions of a few kiloelectronvolts kinetic energy can penetrate several atomic layers below the surface before finally stopping during which they can transfer varying amounts of energy to many atoms; the more energetic of the atoms excited in this way can transfer energy to other atoms and so on in a cascade process. The result is that for each incident ion there might be several atoms in the solid displaced from their normal positions.This can result in the effects of atomic mixing and of “knock-on,” in which a certain species might not be removed completely from the surface but simply driven further into the solid. These effects cause a local smearing of the compositional depth distribution that is carried forward with the advancing bombardment front and is constant with depth once equilibrium has been reached. More serious is the effect of the statistical nature of the sputtering process. Atoms in the solid are not peeled off layer by layer but in a statistical sequence which means that if the average number of layers removed is N then statistically N i layers remain only partially sputtered so that the compositional information comes from man 662 RIVIBRE AUGER TECHNIQUES IN AutaZyst Vd.108 fractional layers rather than from an ideally flat surface. In this instance the smearing of the compositional depth information clearly becomes progressively worse with the amount of material removed and an effective limit is eventually set to the depth to which it is worth going before the information is lost. The problem has been considered in detail by Lea and Seah," and the very general conclusion can be drawn from calculations that for real situations in which the outer surface has a roughness of about 50 nm there is no point in attempting to use ion bombardment to depths greater than 2 pm where the constituent sputtering yields are similar or to much smaller depths if the sputtering yields differ substantially.Where a compositional profile is required to depths greater than about 2 pm the analysis will be less prone to errors due to artefacts if mechanical methods of surface removal are used, as described below. Erosion by Mechanical Lapping The principle involved in mechanical lapping is that of exposing the depth to be analysed by cutting across it at an acute angle to the surface. If the cut is made by lapping the surface at a constant angle then the result is the familiar taper section used with great success in electron probe microanalysis. Thus if the depth of interest is d and the taper angle is a the length of section to be analysed is dcosecct; a taper angle of 5" would therefore spread out the depth by a factor of about 11.5 and an angle of 1" by a factor of about 60.Of more general use in AES is the variation of mechanical lapping called ball-cratering. The principle is illustrated in Fig. 10 (from Walls et A highly polished stainless-steel ball is caused to rotate in contact with the specimen surface fine diamond paste being used as the lapping medium in the area of contact. If the specimen surface consists of a film or layer of a material A on a dissimiliar material B then lapping would continue until the interface had been crossed and B became visible. Alternatively if it is required simply to profile to a certain chosen depth then for a given ball diameter (usually 30 mm) it is easy to calculate from the geometry what the diameter of the resultant crater should be. Distance can of course be measured very accurately with a travelling microscope.The advantages of ball-cratering are that relatively small areas of the sample are eroded that it can be used on curved surfaces and that where there is an interface the taper angle can be made very small indeed. \ \ \ \ I / 4 R\ I I I I I \ i I 4 t d Coating b Fig. 10. Principle of depth profiling by ball-cratering. A highly polished ball of radius R is smeared with fine diamond paste and rotated against the surface to be cratered until an area of the substrate of diameter D is revealed. If D is the diameter of the crater then the thickness of the film or coating is given by C = (D2* - D1*)/SR).*O In all instances of erosion by mechanical lapping it is necessary to sputter briefly with argon ions to remove contamination after introduction into the vacuum system for analysis but such sputtering does not affect the depth resolution unduly.Not all materials are suitable for mechanical lapping e.g. brittle oxides may break away from an underlying metal substrate where the section is thinnest near the interface and it is also possible for soft materials to be smeared across the eroded section Jzcne 1983 ANALYTICAL CHEMISTRY A REVIEW 663 Chemical Bonding Information When discussing any technique in relation to analytical chemistry there are two questions to which it must be capable of providing a t least partial answers. These are (1) what is the elemental composition? and (2) what is the nature of the inter-elemental bonding? Up to now the answers obtained from AES have been overwhelmingly to the first question and any answers to the second have been obtained indirectly and incidentally e.g.by “matching” the atomic proportions of elements found on a surface to compounds suspected to be present from other evidence. In the various areas of application of AES discussed in the next section, therefore the analyses with a few exceptions could not be considered fully chemical on the above criteria as the only information produced has been compositional. However it is worth considering the exceptions because the ways in which some workers are starting to use Auger spectra to extract answers to the second question above are pointers to one of the directions of rapid expansion of the technique in the near future. The three properties of any peak in the secondary electron spectrum are intensity energetic position and shape.In AES just as in XPS the first two of these are used for the derivation of elemental composition the intensity for quantification and the energetic position for identi-fication. Again as in XPS it is not usually necessary to measure energetic position particu-larly accurately for mere elemental identification especially where an element has several Auger peaks in the energy range recorded but in both techniques additional chemical informa-tion can be obtained by more accurate measurement of the peak energy. An example of the changes observed in the silicon LMM and KLL Auger peaks in going from elemental silicon to SiO and Si,N is shown in Fig. 11 from the work of Holloway.21 The energy shift in the Si KLL LMM 9 eV Si T 3 I I 1618 I 191 I I I artifact 506 I I Fig.11. Variations in the silicon LMM and KLL Auger peak shapes between (A) elemental silicon (B) SiO and (C) Si,N,. The peaks a t 91 eV in the LMM spectrum and a t 161 1 eV in the KLL spectrum from SiO are due to reduc-tion effects by the incident electron beam.a 664 RIVIBRE AUGER TECHNIQUES IN Analyst Vol. 108 LMM peak is from 91 eV in silicon to 87 eV in Si,N4 and to 78 eV in SiO, with accompanying shifts in the same sense if not of the same magnitude in the KLL peak. (Note also in Fig. 11 one of the problems not infrequently encountered in AES namely the effect of the incident electron beam on the decomposition of compounds near their surfaces; the peak at 91 eV in the SiO LMM spectrum is due to elemental silicon produced by beam reduction.) As men-tioned in the Introduction there are by now many published figures for chemical shifts in Auger spectra and those observed during oxidation have been listed by Madden.1l Provided that the energy calibration of ones spectrum is always the same as that used in the listed observations then the figures can be used to determine the nature of the oxidation product at a surf ace.Although intelligent guesses can often be made about the chemistry of a surface from the derived elemental composition more direct chemical information should be available from the detailed analysis of Auger line shapes. Some of the most intense Auger transitions from solids are of the CVV type that is ionisation of a core level C followed by an Auger process in which both the de-exciting and the ejected electrons are from the valence band V.As either electron can originate from anywhere within the valence band the CVV Auger spectrum should in principle have the same shape as a self-convolution of the density of electronic states within the valence band. Thus a self-deconvolution of such an Auger spectrum might after the correct background has been subtracted and instrumental broadening removed be expected to pro-duce an energy distribution looking very like the valence density of states (DOS). The DOS so derived would not necessarily resemble that of the bulk material but would be related to the local DOS in the immediate neighbourhood of the ionised atom Where the atom is situated at or very near the surface as in AES then it would be the surface local DOS that would be observed with a distribution probably rather different from that in the bulk.Such informa-tion could be very valuable in studying the chemistry of surface reactions. Of course Auger transitions involving only one valence electron i.e. of the CCV type should have the DOS information directly reflected in their spectrum if the direct relationship held good but often their intensity is insufficient. The more or less direct relationship described above between the local DOS and the Auger line shape has been found to be true at the time of writing for only a few elements and some of their compounds but has been useful nevertheless. Those elements which include Li Be C, Al Si and some transition elements with valence bands less than half full are said to have “band-like” CVV Auger spectra because the spectral line shapes can be related to the valence band structure.The relationship is not necessarily one-to-one as Auger transition matrix elements vary across the valence band and indeed it is possible for different Auger transitions to give different apparent local DOS after line shape analysis. The differences can be used to derive information about the contributions of individual components of the valence band to 15 10 5 0 EnergyleV 15 10 5 0 Fig. 12. (a) Comparison of the local density of states derived from (1) analysis of the silicon L,L,,3V Auger spectrum with (2) the theoretical prediction.(b) Local density of states derived from the silicon L,,,VV Auger spectrum by (1) self-deconvolution and by (2) convolution square root.% Jane 1983 ANALYTICAL CHEMISTRY A REVIEW 665 the Auger process so that in fact not just the total local DOS can be derived but also a partial local DOS. This is well illustrated by the work of Brockman and on silicon to which most attention has been devoted for both theoretical and technological reasons. Fig. 12 compares the DOS extracted from analysis of the shapes of the L,L,,,V and L2,,VV Auger transitions in silicon; the full lines are the experimental results. The two derived DOS curves are different. That based on the L,L2,3V agrees well with the theoretical (dashed line) calculation of the DOS near a silicon surface in other words all the contributory terms s*s s*p, and p*p from the valence band are included but the L,,,VV-derived DOS is almost entirely p*p i.e.represents a partial local DOS. Clearly then analysis of the shapes of these two transitions during chemical reaction at a silicon surface can provide detailed information about the ways in which electrons in different regions of the local valence band are affected by what is happening at the surface. A good example of the application of Auger line shape analysis to the study of the differences in local or site-specific DOS has been given by Davis et aL2 for the compound semiconductor GeSe. In order to avoid the problems involved in self-deconvolution of a CVV Auger transi-tion they recorded the M,M,,,V transitions of Ge and Se in the compound in other words, CCV-type transitions even though they were admittedly much weaker than the CVV.Their results after integration and appropriate data processing are shown as the lower graphs (3) in Fig. 13 the solid line for Ge and the dashed line for Se with energies referred to the Fermi level. . . -20 -15 -10 -5 0 E - EV/eV Fig. 13. Differences in the local densities of states around germanium and selenium atoms in GeSe studied by line shape analysis of the X-ray excited MlM4,,V Auger transitions. (1) Valence band spectrum of GeSe obtained by un-monochromatised XPS dotted line before solid line after data processing. (2) Same using monochromatised X-rays. (3) Solid line, local DOS from germanium M,M4,,V spectrum and dashed line local DOS from selenium M1M4, V spectrum.3 666 R I V I ~ R E AUGER TECHNIQUES IN Analyst Vol.108 For comparison the valence band spectrum for GeSe obtained by XPS is shown in the upper part of Fig. 13 as graph (l) the solid line being the result after data processing. XPS of course provides a general picture of the DOS in the near-surface region without being able to distinguish variations in DOS at individual sites. The significant result is that the Auger line shapes at (1) in Fig. 13 reflecting the local DOS each show only some of the features in the XPS valence band spectrum. Hence the Ge shape shows only two of the peaks and similarly the Se shape only two but not the same two. In other words the CCV Auger transitions in GeSe involve electrons only in those states that overlap with the site of the core hole and the DOS derived from their line shapes relates to the immediate vicinity of the ionised atom.Additional information can be derived from the relative magnitudes in each instance of the low-energy bonding p peaks and the higher energy non-bonding s peaks in comparison with the XPS result. At the opposite extreme from those elements that exhibit “band-like” CVV Auger transitions is the group of elements in which the CVV peaks are said to be of “quasi-atomic” character. This term has been applied because the observed line structure is much too narrow to have any possible direct relationship with the band structure the peak widths being closer to those observed from free atoms than from atoms making up a solid.The narrowness arises from distortions imposed on the spectra by the interaction of the two holes left behind after the ejection of the Auger electron (see the section Auger Effect in the Introduction) ; in the group of elements in question the two holes can be localised near the ionised atom long enough for their interaction to be significant. Hence for these elements it seems unlikely that any chemical information can be obtained from analysis of the CVV line shapes although such analysis has given much information about the Auger process itself. Between the extremes of purely band-like and purely quasi-atomic character lies a largely unchartered region in which there may be many elements whose CVV Auger spectra contain components of both. Moreover it seems possible that surface reaction involving transfer of electronic charge in one direction or another may alter the local DOS sufficiently for a change in character towards either extreme to be observable and therefore usable for chemical informa-tion.As yet there is insufficient work in this area for confirmation or otherwise of this possi-bility. One area in which the use of AES as a chemical tool looks definitely promising is that of the Auger spectra of gas-phase molecules. There are many papers in which the Auger spectra of molecules adsorbed on solid surfaces have been reported but only recently has attention been turned to molecules in the gas phase. Obviously there are some experimental problems as a sufficiently high pressure of the gas must be maintained at the focus of the energy analyser, while at the same time the pressure in the analyser itself must be low but they seem to have been largely solved.The Auger data can and have been used in two ways firstly as a “fingerprint” of the molecule as the spectra can show large variations between neighbouring molecules in a series and secondly by comparison of the gas-phase and condensed (adsorbed) molecular Auger spectra to derive information about the local chemical environment at the surface via the spectral changes. The first of the ways is exemplified in Fig. 14 (from Rye et in which the carbon KVV Auger spectra from the three hydrocarbons methane, ethylene and acetylene in the gas phase are shown. The carbon hybridisation is of course changing markedly from one to another of these three and this is reflected in the spectra.From the same paper24 is taken Fig. 15 as an example of the second way of using Auger spectra of molecules. Here the gas-phase and condensed (on nickel) phase oxygen KVV spectra are superimposed for molecules in the series H,O CH,OH and (CH,),O with naturally energy shifts as appropriate to take account of transition to the solid state. It can be seen that the two spectra almost coincide for (CH,),O but differ increasingly from CH,OH to H20. This reflects directly the nature of the intermolecular forces in the condensed layer or in other words the local density of electronic charge around each atom in the molecule. In (CH,),O for instance the forces are mostly of the weak Van der Waals type Le. with little movement of charge whereas in the other two molecules hydrogen bonding between the condensed particles becomes increasingly important.The changes in the degree of hybridisation of the oxygen orbitals as a result of condensation are so significant that the condensed phase spectrum starts to look very much like a solid-state valence band rather than that of a molecule. Clearly there is much useful chemical information to be obtained from measurements such as these J m e 1983 ANALYTICAL CHEMISTRY A REVIEW 667 G z Gas phase C(KVV) 180 220 260 3 E I ect ro n ene rg yIeV I I 1 470 51 0 t Electron energylev i0 Fig. 14. Carbon KVV Auger spectra from (A) Fig. 15. Comparison of the oxygen KVV Auger methane (B) ethylene and (C) acetylene in the gas spectra from several molecules in (1) the gas phase phase.In these three molecules the carbon hybri- [(A) water (B) methanol and (C) dimethyl ether] disation is drastically different and the differences and (2) condensed multi-layer phase on nickel at are revealed in the carbon Auger spectra empha- 110 K. The departure of the condensed phase sising the sensitivity of the Auger process to the spectra from the gas phase spectra reflects the local electronic environment.** degree of charge transfer between the condensed molecules very little for dimethyl ether and con-siderable for water.24 Applications Most of the more recent applications of AES have been those in which the high spatial resolution capability of the technique has been a necessary requirement i.e. applications in the metallurgical and semiconductor fields.Nevertheless the total number of applications of all types is now so great that there is no difficulty in selecting examples of a more chemical nature. These will be discussed under the headings of the various fields from which they have been chosen. Corrosion and Oxidation In general because information about the detailed chemical nature of corrosion films in terms of elemental oxidation states is always required the preferred technique for surface analysis in corrosion has been XPS rather than AES. Many workers believe too that the high spatial resolution obtainable in AES is wasted in corrosion studies but it is likely that more situations arise than are realised in which that capability could be useful. I t was found to be very useful for example by Lumsden et ~ 1 .~ 5 in their study of the susceptibility of iron to pitting in solutions containing both chloride and phosphate ions. Whereas aggressive ions such as chlorides accelerate the general dissolution of an iron surface that occurs in the presence of water inorganic inhibitors such as phosphates can passivate the iron if added in adequate concentration. When both types of additive are present together the form of attack of the iron is intermediate between general dissolution and passivation and pitting of the surface can result 668 RIVIBRE AUGER TECHNIQUES IN Analyst Vol. 108 The iron sample was held at a constant potential in a solution of disodium hydrogen orthophosphate and potassium chloride and then scratched.Immediately after scratching faceted pits of the type shown in Fig. 16(a) appeared where dissolution occurred along crystallographic planes. The pits soon became hemispherical as in Fig. 16(b) and were then surrounded increasingly by reaction products eventually becoming completely covered [Fig. 16(c) and (41. Using an incident electron beam of diameter -1 pm Auger spectra could be obtained from the inside walls of the pits and from the corrosion product covering a pit. The films covering the walls were found to be either of iron and oxygen only or of the composition corresponding to the Auger spectrum of Fig. 17(a) ; in the latter instance the film contained the ionic species from the solution. The Auger spectrum from the pit cover Fig. 17(b) showed two prominent phosphorus peaks whose energies were those of phosphorus in a phosphate.The only other peaks were those of oxygen and iron suggesting that the corrosion product was iron( 111) phosphate. Further confirma-tion comes from the fact that the Auger peak heights are in approximately the right relative magnitudes for FePO,. Fig. 16 (from Lumsden et al.25) shows a sequence during pit growth. - 3 Y 3 !2 W 0 500 1000 0 500 1000 EnergyleV Fig. 17. (a) AES spectrum from the inside wall of a faceted pit showing residues from the ionic (b) AES spectrum from the reaction The positions of the phosphorus peaks indicate a phosphate, species in solution (Fe; 0.1 N Na,HPO + 0.001 N KCl). product covering a pit as in Fig. 16(d). suggesting that the material is iron(II1) pho~phate.,~ From its inception AES has been used very extensively in studies of the earliest stages of reaction between gases and solids.Its high surface specificity and its ability to determine both the nature of elements present on a surface and their approximate concentrations have made it an obvious technique to use in such studies although it is now realised that care must be taken to avoid artefacts that may be introduced by the incident electron beam itself. Of all the gas - solid reactions studied those between oxygen and the first series of transition metals form probably the largest single section both for technological reasons and because they are intrinsically interesting. Typical of some of the careful measurements made of that type is the work of Benndorf et aL26 on the chemisorption of oxygen on and the initial oxidation of the nickel (1 10) surface.Starting with an initially atomically clean surface they followed the changes in the nickel low-energy MVV spectra and high-energy LVV spectra and in the oxygen KLL spectrum as a function of oxygen exposure. The work function of the surface was monitored at the same time. Fig. 18 shows some of the changes they observed in the nickel spectra after oxidation at room temperature; the M,,,VV Auger peak has shifted split and decreased and the L,VV spectra have also decreased in intensity substantially. There is also additional structure below the M,,,VV peak at -30 eV after oxidation. When the oxygen KLL peak intensity (measured as the peak-to-peak height) was plotted against exposure along with the variations in work function and energetic position of the oxy-gen peak the result shown in Fig.19 was obtained. Three distinct stages in the room-temperature reaction could be distinguished. The initial stage was that of dissociativ Fig. 16. Scanning electron micrographs showing different stages in the formation and growth of a pit In the earliest stages the pit is of crystallographic Corrosion products build up in a circle and its subsequent burial under reaction products. morphology with faceted sides but soon becomes hemispherical. around it and eventually cover the pit completely.2 Jzcne 1983 ANALYTICAL CHEMISTRY A REVIEW 669 Isr I 20 60 100 700 800 900 Kinetic energyIeV Fig. 18. Nickel MVV and LMM Auger spectra from a nickel (110) surface before and after oxidation a t room temperature in ca.Torr of oxygen. The M,,,VV peak shifts splits and decreases and the LMM spectra also decreases in intensity. There is also additional structure a t 30eV. below the M,,VV peak after oxidation.,6 chemisorption of oxygen molecules marked by rapid linear increases in the oxygen Auger intensity and in the work function and an equally rapid decrease in the oxygen peak position. In the second stage the rate of uptake of oxygen decreases and passes through an inflection, while both the work function and the oxygen kinetic energies reach a plateau; the interpreta-tion here is that a rearrangement of the chemisorbed layer is occurring with oxygen being incorporated in the nickel sub-surface layers.Finally with further oxygen exposure the third stage is reached in which the rate of oxygen uptake increases again and then saturates the work I I I I 10 30 50 70 90 Oxygen exposure11 Fig. 19. The variations in oxygen KLL peak intensity and energy and The unit of Torrs. Oxidation was in the work function as a nickel (110) surface is oxidised. exposure is the Langmuir which is equal to carried out at room temperature and 5 x lo-* Tom of oxygen.* 670 RIVIBRE AUGER TECHNIQUES IN Analyst VoL 108 function decreases to below that of the clean surface and the oxygen Auger kinetic energy also decreases sharply and then becomes constant. This stage is said to be due to the growth of islands of nickel oxide over the surface. From the oxygen KLL intensities and from the attenuation of the nickel L,VV and M,,,VV intensities the authors were able to calculate the limiting oxide thicknesses at temperatures from 300 to 670 K; these varied from -3.5 layers of NiO at 300 K to -10 layers at 670 K.The broadening of the M,,,VV peak on oxidation is due to alterations in the local electron densities of states at the surface as the d band of nickel is broadened and pushed to lower energies on oxidation and oxygen-induced 3p states appear. The additional structure below the M,,,VV peak is due to an interatomic or “cross,” transition involving the nickel M,, levels and the oxygen L and L,, levels. Catalysis There is not a large amount of published work to be found in which AES has been applied directly to the study of commercial catalysts.The reasons are probably three-fold firstly, that AES does not provide all the chemical information required secondly that on supported catalysts the effect of the incident electron beam might be catastrophic and thirdly that more commercial secrecy surrounds the specification and performance of catalysts than is found in any other field. Nevertheless many interesting basic studies of model catalyst systems have been carried out using AES usually in combination with other surface analytical techniques. Generally these studies have taken the form of experiments in which two or more pure gases are adsorbed either simultaneously or sequentially on the surfaces either of evaporated films or of single crystals of compositions corresponding to those of materials of known catalytic activity.Changes in surface composition during reaction and the nature of the reaction products are monitored. The relationship between such studies and the real situation tends to be regarded as tenuous, but there are instances where it is demonstrably not so. One of these has been studied by Cros et at?.,,’ and it originated in an earlier observation that silicon atoms that had diffused through a thick gold layer on silicon could be oxidised at surprisingly low temperatures below 400 “C. Their experiment consisted in evaporating small amounts of gold on to a clean silicon surface produced by cleavage in vacuum and then observing the effects of oxidation at various temperatures by AES inter alia. The basic result at room temperature is shown in Fig.20. The uppermost spectrum (A) is of the clean silicon surface typified by the intense LVV peak at 92 eV. The next spectrum (B) is of the same surface now covered by four monolayers of gold. The large complex peak at 69 eV is due to the gold NVV Auger transition while the presence of the silicon LVV peak and its splitting into two peaks at 90 and 94 eV show that silicon has diffused to the surface and alloyed with the gold. Exposure of the gold-covered silicon to oxygen at 0.2 Torr for 3 h at room temperature then produced spectrum (C) in which the gold NVV peak is reduced in intensity but otherwise unchanged but in which the elemental silicon LVV peak has been reduced considerably and a new silicon peak at 78 eV has appeared. Comparison of (C) with (D) which is the spectrum obtained from SiO grown at 900 “C shows that the 78 eV peak is indeed due to the oxide.However the comparison of (C) with (E) is the most interesting for the latter is the spectrum obtained from a clean gold-free surface of silicon (k spectrum A) exposed to exactly the same oxidising conditions as for (C). Apart from a reduction in the intensity of the elemental 92 eV peak a weak feature appears at 84 eV ascribable to the formation of SiO (where x ml) but there is no suggestion of the character-istic SiO peak at 78 eV. Thus the presence of the gold on the silicon surface enhances con-siderably the oxidation of the silicon in that SiO is formed where it is not formed in the absence of gold but the gold itself is unaffected. In other words the gold is acting as a cata-lyst for the oxidation.By extending their experiment to higher temperatures and with the help of depth profiling, the authors were able to show that the crucial factor was indeed the gold - silicon alloy formed at the surface. When present in a sufficient concentration of gold atoms silicon atoms adopt different hybridisation states which results in the disappearance of the normally strong covalent silicon bonds and stabilisation in a quasi-metallic state. In this condition the Si-0, tetrahedra can grow easily as their free energy of formation is greater than that of the gold -silicon alloy and thus an SiO layer is produced June 1983 ANALYTICAL CHEMISTRY A REVIEW 671 Reactions in the Solid State In addition to the study of gas - solid and liquid - solid reactions AES has been used to observe the effects of reactions between solids and the thermal decomposition of solids.Often such experiments are easier to perform than those involving gases or liquids and in general the potentially disturbing effects of the incident electron beam do not intrude to the same extent. In several technologically important fields such as device fabrication and the design of therm-ionic electron sources information about the composition and thickness of surface films pro-duced by solid - solid reactions as well as the nature of the interface between the film and the substrate is vital. One of the requirements in the fabrication of certain types of integrated circuits is the ability to produce thin films of metal on a semiconductor in which the metal - semiconductor interface has the correct electronic properties.This is known as Schottky barrier formation. It has been found that a relatively simple and attractive way of generating interfaces of the correct properties is to deposit thin films of those metals which will react easily with the clean semi-conductor substrate. One such system that has been the subject of much study is that of 84 v I I I I 60 70 80 90 Electron energytev AES spectra recorded before and after oxidation of clean and of gold-covered silicon surfaces. (A) Clean silicon surface with characteristic LVV Auger peak a t 92eV. (B) The same surface covered by four monolayers of gold. The new peak near 69eV is due to the gold NVV Auger transition while the splitting of the silicon LVV peak into two peaks a t 90 and 94 eV indicates alloying of silicon with the gold.(C) Exposure of the gold-covered surface to 0.2 Torr of oxygen for 3 h a t room temperature. A new peak a t 78eV has appeared. (D) SiO grown on silicon at 900 OC showing that the 78eV peak is due to the oxide. (E). Exposure of gold-free silicon surface as in (A) to the same oxidising conditions as in (C). The 92 eV peak is reduced slightly and a weak peak appears a t 84eV due to SiO, but no 78-eV peak appears.97 Fig. 20. - Electron energylev Fig. 21. Auger spectra from a silicon surface taken a t succes-sive stages of deposition of palladium a t room temperature : (A) clean (B) 20 A evaporated ; (C) 60A evaporated; and (D) 120A evaporated.After a deposition of only 20 A the silicon L,,3VV spectrum has changed from the single domi-nant peak at 92 eV to four peaks a t 81 86 91 and 98eV in the same positions as found for bulk Pd,Si. No changes are observed in the palladium MNN spectra.2 672 RIVIBRE AUGER TECHNIQUES IN Analyst Vol. 108 palladium on silicon. When Okada et aL28 deposited palladium on to silicon at room tempera-ture after cleaning the silicon surface by the well tried method of ion bombardment and annealing they observed a sequence of Auger spectra of which a selection is shown in Fig. 21. It can be seen that although there was no change in the shape of the palladium MNN peak at 330 eV as the thickness of the palladium increased that of the silicon LVV spectrum changed drastically.After only a few gngstroms of palladium the 92 eV peak characteristic of clean silicon had disappeared and was replaced by four peaks at 81 86,91 and 95 eV. With increase in the thickness of the palladium deposit to over 100 A the silicon peaks vanished altogether, and only the palladium spectrum was left. The complex four-peak silicon spectrum observed at the beginning of deposition was compared with the spectrum from the alloy Pd,Si and found to be identical. Thus even at room temperature palladium will alloy spontaneously with a clean silicon sur-face. However the authors were able to extract more information from the spectra by realis-ing that the silicon spectra in the very early stages of palladium deposition were in fact com-posite spectra being made up of a superposition of the elemental (it?.covalent) silicon spectrum and of the alloy (i.e. metallic) spectrum. Using the standard spectra from silicon and from Pd2Si in linear combination they were able to synthesise silicon spectra to match exactly the observed spectra as shown in Fig. 22. When therefore a film of palladium on silicon of thickness about 300 A was depth profiled by ion bombardment the authors could determine at each stage the proportions of covalent and of silicide silicon in the film with the result given in Fig. 23. The profile consists of three regions unreacted palladium palladium silicide and the silicon substrate. In consideration of the Schottky barrier formation it is thus necessary to take into account two interfaces that between palladium and the silicide and that between the silicide and silicon.Measu /J r/ d Pd Synthesised Pd2Si Fig. 22. Synthesis of silicon spectra at various stages of palla-dium deposition on silicon from standard L,,,VV spectra of silicon in elemental silicon and in Pd,Si. Comparison with the observed spectra on the left then allowed the proportions of covalent and of silicide silicon to be determined at each stage of deposition.28 100 s d 5 m 50 0 100 200 300 400 ! 10 Fig. 23. Profile across the various Depthh inteaaces in the silicon - palladium system (A) covalent silicon (B) silicide and (C) palladium. The proportions of covalent silicon and of silicide silicon were determined as illustrated in Fig. 22. Three regions can be distinguished un-reacted palladium palladium silicide and the silicon substrate so that the system thus contains two interfaces one between palladium and the silicide and the other between the silicide and In the manufacture of thermionic electron emitters for vacuum tubes which over the years has progressed from being a black art to having a sound scientific basis oxides of the alkaline earth metals are used in varying proportions and the efficient operation of the emitter depends on one or more of the oxides being reduced at the surface of the device to free metal.The point is that a monolayer of free alkaline earth metal has a low work function lower than that of most other materials except some of the alkali metals which of course means that it is a copious source of electrons.The operation to produce the thin film of free metal is calle Jzcne 1983 ANALYTICAL CHEMISTRY A REVIEW 673 activation. One of the methods used to maintain a supply of free atoms during the life of the emitter is to include in the recipe a reducing agent as the supply must balance the loss by evaporation if the device is to have an acceptable working life. The surface interactions between typically employed alkaline earth oxides and reducing agents have been studied by Verhoeven and Van Doveren,29 and Fig. 24 shows the results from one such interaction that between barium oxide and magnesium. The upper spectrum is that from pure barium oxide prepared by complete oxidation of a thick barium film and shows peaks at 52 and 65 eV characteristic of the oxide.The three lower spectra were recorded following increasing amounts of deposition of magnesium metal. As the thickness of magnesium increases the size of the magnesium LVV peak at 44 eV of course also increases but the interesting changes are those taking place in the barium spectrum for even after a few mono-layers of magnesium a substantial peak due to metallic barium at 71 eV appears. The v 65 52 '44 44 I I 50 1 oc Electron energylev Fig. 24. Surface reaction at room temperature between barium oxide and a magnesium film deposited on it. (A) Barium Auger peaks from a completely oxidised barium film, showing peak positions a t 52 and 66eV characteristic of the oxide. Lower spectra the result of deposi-ting increasing amounts of mag-nesium on the barium oxide.(B) BaO after 6 s Mg deposition. (C) BaO after 12 s Mg deposition. (D) BaO after 112 s Mg deposition. The peak at 71 eV is due to metallic barium and that a t 44eV to mag-nesiu m .a* 50 100 Y 0 Deposition time/s Fig. 25. Peak-to-peak heights of various Auger signals from a barium oxide film plotted as a function of time of deposition of magnesium a t a constant deposition rate (A) Mg; (B) 0; (C) Ba; and (D) BaO. An equilibrium situation develops in which there is always free barium a t the surface.2 674 RIVIBRE AUGER TECHNIQUES IN Analyst VoE. 108 intensities of the original barium oxide peaks quickly become too small to measure. The changes as a function of deposition time (it?. thickness) of the magnesium are summarised in Fig.25 where it can be seen that an equilibrium situation develops at room temperature in which there is always a substantial amount of free barium at the surface. Notice that Auger effects due to oxidised magnesium (at -34 eV) were very small or absent; MgO would be expected to be the other product of the reaction between Mg and BaO. The interpretation is either that the reduction is taking place below the outermost layers or that the barium is segregating to the surface at once. Analyses Using High Spatial Resolution As described in the section on Technique and Instrumentation one of the most useful variations of AES is scanning Auger microscopy (SAM) in which the incident electron beam is rastered over a chosen area of the sample allowing production either of topographical images using the low-energy secondary electrons or of elemental maps using selected Auger energies.The optimum spatial resolution obtainable in SAM is currently about 0.2 pm but in many applications such as those discussed below such high spatial resolution is not always necessary. Cast iron often contains nodules of diameter 10-30pm as shown in the SEM image of Fig. 26(a) (from Joshi30) which is that of a cast iron surface polished and then cleaned in vacuo by ion bombardment. Each nodule is surrounded by an annular region of contrast between those of the nodule and of the matrix. The two SAM images in Fig. 26(b) and (c) are those of carbon and of iron respectively. Clearly the nodule itself must be very rich in carbon and indeed a complete Auger spectrum from the centre of the nodule showed that apart from the carbon peak there was only a very small oxygen peak.The shape of the carbon peak was typical of that of graphite. Interestingly the annular region corresponds to a denudation of carbon from the cast iron matrix presumably caused by diffusion of carbon to the growing graphite nucleus. Over the rest of the surface iron is fairly uniformly distributed and the Auger spectrum from a point in the matrix between the nodules and their haloes showed that the carbon was in the form partly of graphite and partly of carbide indicating a mixture of iron carbide or pearlite and finely divided graphite. Solar cells for energy conversion should be of high efficiency and reliablity have a long life and be cheap to produce.Costs can be reduced by using polycrystalline rather than single crystal materials but then it is found that the presence and the properties of grain boundaries become the limiting factors in the performance and reliability of the cells. The electrical characteristics and thus the eventual efficiency will depend on the chemistry of the grain boundaries i.e. on the nature and amount of impurities that segregate there and on any separate phases that might form. Kazmerski31 has studied the grain boundary chemistry by a variety of techniques including SAM. In order to expose the grain boundary in such a way that the analysis would be unambiguous a piece of polycrystalline large-grain cast silicon was fractured inside the vacuum system in ultra-high vacuum conditions and the fracture surface analysed immediately by SAM.In that way interference from atmospheric and other con-tamination was avoided. Fig. 27 (from Kazmerski’s paper31) compares the features seen in the topographical image of the fracture surface at the top with elemental maps for Ni Al C and 0. Clearly the particle labelled 1 is mostly A1 and 0 probably alumina while particle number 2 is a mixture of Ni A1 and 0. On the other hand the unlabelled particle in the top right-hand corner of the topographical image is mostly carbon and indeed the Auger spectrum revealed that it was in the form of graphite. When the fracture path passed thyough a silicon grain rather than along a grain boundary Auger analysis revealed no elements present apart from silicon itself.It seems that the casting process had concentrated impurities present in the bulk at very low levels at the grain boundaries with subsequent mutual interaction and the precipitation of particles of separate phases. Analyses such as these when combined with electrical measurements allowed identification of those impurities whose action was the most detrimental to the performance of the cell. The ability of AES to provide elemental maps of high spatial resolution has been particularly valuable in the study of processes occurring during the activation of thermionic cathodes, which as already mentioned are complex multi-component devices. For these it has been found that so-called conventional AES in which spot analyses are performed with a relatively large beam size does not provide much useful information as the area analysed is too large an Fig 26.AES analysis of nodules in a cast iron surface. The nodule appears black in the secondary electron image in (a) and is surrounded by a grey annular halo. The SAM images in (b) and (c) are those of carbon and of iron respectively. The nodule is almost pure carbon, probably graphite while the surrounding halo corresponds to a carbon-denuded region. Between the nodules and their haloes is a matrix of iron carbide.30 Fig. 27. SAM analysis of the surface of a grain boundary in polycrystalline cast silicon. At the top a secondary electron image of the surface showing particles embedded in the boundary surface. The elemental maps for Ni Si C and 0 below show that particle 1 is probably alumina and particle 2 probably nickel aluminate.The unlabelled particle in the top right hand corner is mostly carbon probably graphite.31 [to face P. 67 Fig. 28. Elemental maps by SAM analysis at mom temperature of an impregnated thermionic cathode before activation. (a) Absorbed current image; (b) tungsten (1 736 eV); (c) barium (584 eV); (d) oxygen (604 ev); (e) calcium (292 eV) ; (f) sulphur (150 ev) ; (g) carbon (278 ev) ; and (h) osmium (1 850 eV).32 Fig. 29. Elemental maps pregnated thermionic cathode, while the cathode temperature Barium (584 eV) ; (b) oxygen (d) sulphur (149 eV) ; (e) osmium (274 eV) ; and (g) distributio J m e 1983 ANALYTICAL CHEMISTRY A REVIEW 675 a spatially averaged analysis results.For instance in their study of impregnated commercial cathodes Jones et found no correlation between the elemental surface concentrations measured with a beam size of 100 pm and the emission performance of eight different cathode surfaces. However when they turned to SAM with a beam size of less than 1 pm and com-pared the cathodes before and after activation significant correlations both between the distributions of various elements and between the electron emitting areas and the distributions of certain elements were found. Fig. 28 from their paper shows the distribution of seven ele-ments of interest and the adsorbed electron current image (k the negative of the emitted electron current) for an unactivated cathode. In this instance the cathode consisted of a sintered porous tungsten matrix impregnated with a mixture of BaO CaO and A1203 and then sputtercoated with about 0.5 pm of an 0 s - Ru alloy.There is clear correlation between the distributions of tungsten barium oxygen calcium and sulphur the rest of the surface being occupied by carbon. The osmium seems uniformly distributed. After activation and while the cathode was held at 1 145 "C the elemental maps in Fig. 29 were recorded; also shown is the electron emission distribution on the same scale. The correlations have now changed; tungsten and ruthenium are found associated together with sulphur and their areas correspond to the regions of low electron emission. The barium and oxygen distributions correlate and those elements occupy the regions not occupied by the sulphur.In addition the barium and oxygen regions correspond to the areas of high electron emission. The osmium is still more or less uniformly distributed while carbon has disappeared. Hence of the impregnating materi-als BaO is the only one found at the surface after activation and the efficiency of the cathode is directly related to the amount of BaO present. Also clear from Fig. 29 is that sulphur is an effective cathode poison so that the efficiency will also depend on the distribution and amount of sulphur still present after activation. As there is no basic limitation to the temperature at which analysis by AES may be carried out high-resolution elemental maps such as those in Figs. 28 and 29 can be recorded continuously if required at all stages of cathode activation and operat ion.Adhesion Clean metal surfaces when brought into contact adhere to each other very strongly because of the interatomic bonds formed between them. The presence of other elements at the surfaces before contact either as native oxides as deliberately introduced thin films or as airborne contamination is bound to have an effect on the nature and strength of the adhesion and such an effect will not necessarily be deleterious. For example Hartweck and Grabke33 measured the force of adhesion between two iron surfaces that were first cleaned and then covered with fractions of a monolayer of various pure gases and found that at certain coverages the ad-hesion was considerably enhanced. This result is in contrast to that observed when much N 'E 2000 ; 1200 E G 0 .-v) m w-W 2 400 LL 0.2 0.6 1.0 1.4 ( a ) Monolayers 0.2 0.6 1 .o 1.4 1.8 I I I I I ( b) 0.1 0.2 0.3 0.4 0.5 0.6 0.2 0.4 0.6 0.8 1 .o Nitrogen in the boundary AN/& Carbon in the boundary Ac/AF, Fig.30. Variation of the force of adhesion between two pure iron samples with the coverages of (a) nitrogen and of (b) carbon. The coverages are expressed as relative in terms of the ratio of the peak-to-peak height of the principal Auger peak of the adsorbent to the peak-to-peak height of a principal Auger peak of iron.3 676 RIVII~RE AUGER TECHNIQUES IN Analyst Vol. 108 thicker adsorption or reaction films are present when adhesion is definitely reduced. Their results for adsorbed films of nitrogen and of carbon are shown in Fig.30; the coverages of nitrogen and carbon were measured by AES before contact was made between the iron sur-faces in an adhesion testing apparatus built in a UHV chamber. In both instances a maximum in adhesion occurs at a coverage of about 1.0 mondayer the force of adhesion at that point being about three times that for the bare metal. Similar results were found for other adsorbed species at fractional coverage and the authors concluded that the iron - non-metal - iron interatomic bonds formed at the interfaces were obviously much stronger than the iron - iron bonds formed by clean surfaces. At much higher coverages where the formation of three-dimensional compounds is possible it is logical that the cohesion should be weak as the bonds between metals and compounds are generally weaker than between the metals themselves, Analysis in Depth The combination of AES with surface erosion by ion bombardment or occasionally by mechanical lapping as described previously has been used very extensively to obtain mainly qualitative information about the variation in composition with depth; qualitative because the relationship between ion dose and the depth of erosion is usually known only approxi-mately and because the ion beam itself can cause changes in the composition.Nevertheless, the combination has been able to provide information not obtainable in any other way. A few examples will give an indication of how the AES depth analysis has been used. I Sb I 0 10 20 30 40 50 t/m i n Fig. 31. (a) Auger spectrum of an AlSb surface in the “as received” state ie.with the native air-formed oxide film present and the contaminant species silicon chlorine and carbon. (b) Auger spectrum of the same surface after ion bombardment has removed the contaminants but not the oxide. (c) Depth profile through the oxidised AlSb surface showing the five zones identified by the Roman numerials. Region (A) corresponds to the oxide film (B) to the oxide - semiconductor interface and (C) to the bulk emi icon duct or.^ Juvte 1983 ANALYTICAL CHEMISTRY A REVIEW 677 The oxide film formed in dry air on the compound semiconductor AlSb has been studied by Guglielmacci et the material has potential use in optoelectronics particularly in the photo-voltaic conversion of solar energy and the properties of its native oxide film are important.Fig. 31 shows the results of ion bombardment of the surface of the AlSb. The contaminant species silicon chlorine and carbon seen in the as-received state disappeared after bombard-ment light enough to remove only a few atom layers as demonstrated by a comparison of spectra A and B. The Auger peaks corresponding to the remaining elements aluminium, antimony and oxygen were then monitored as a function of bombardment time until the oxy-gen had also disappeared. The authors identified five zones in the film of characteristic composition zone I in which the surface contamination was removed; zone 11 the bulk of the oxide film of constant composition; zone 111 an interface region in which the antimony signal decreased and the antimony to aluminium ratio increased significantly over that in either the oxide or the substrate; zone IV another interface region in which both antimony and alumin-ium signals increased as the oxygen signal fell; and zone V the substrate AlSb.The depth removed to the end of zone IV was estimated to be 750A The presence of an excess of antimony in the interface region of zone I11 would probably modify the electrical charac-teristics of the surface. who were interested in knowing what were the effects of different surface treatments on the compositions of films formed on a 50 + 50 Fe - Ni alloy. Their treatments included ultrasonic cleaning in methanol boiling in hydrogen peroxide and subjecting to an oxygen radiofrequency plasma. Depth profiling in each instance gave the results shown in Fig.32. The nickel-to-iron ratios Oxide films of a different sort were examined by Wittberg et (a) ( b) Ni Ni Fe Fe C C 0 0 I I I 1 I I 0 10 20 0 10 20 I I I I I 1 I 0 10 20 30 40 50 60 Sputtering tirne/rnin Fig. 32. Depth profiles of the surface films formed on a iron - nickel 50 + 50 alloy after being cleaned in (a) methanol (b) hydrogen peroxide and (G) oxygen plasma. The oxide films on the peroxide- and oxygen plasma-treated surfaces are much thicker than on the alloy simply cleaned in methanol. However, the levels of carbon contamination are much lower than after the latter treatment.3 678 RIVIBRE AUGER TECHNIQUES IN Analyst Vol. 108 could be measured and it was found that after a conventional ultrasonic cleaning treatment in methanol the ratio was the same as that of the bulk alloy but that after the other two treat-ments there was either a surface enrichment of iron or a surface depletion of nickel.The oxide films on the peroxide and plasma treated surfaces were thicker than on the ultrasonically cleaned surface being particularly thick after the oxygen plasma treatment as expected. On the other hand the levels of carbon both at the surface and in the bulk were significantly lower after the plasma treatment. Measurements such as these can provide a guide to the surface treatment that should be used to produce a surface condition of the desired properties. In the field of ceramic fabrication the stability of surface composition under severe environ-mental conditions can be a problem in some applications e.g.in the electrical distribution industry and AES with profiling has been used to study progressive changes of ceramic surfaces subjected to such conditions. exposed alumina con-taining a variety of impurity elements to saturated steam (266 "C) at a pressure of 5.3 x lo6 Pa, and carried out depth analyses at regular intervals on specimens removed after various times. The principal result they found was a progressive segregation of calcium (present as an impur-ity at 0.1% m/m in the bulk) to the surface of the alumina. As can be seen from the calcium profiles in Fig. 33 the calcium concentration at the surface increased more or less monotonic-ally with time up to an exposure of 12 d beyond which its concentration at the surface did not increase further.At that point the calcium level at the surface was about two orders of magnitude greater than that in the bulk. Fig. 33 also gives an indication of the observation made by other techniques that after the time at which the surface saturates in calcium the calcium-rich region starts to extend into the bulk and eventually was found some 10 pm from the surface. In parallel work the authors established a correlation between the enhancement of calcium concentration at the surface and the reduction in flexural strength of the alumina. Clearly then commercial-grade alumina used for fabrication of components for electrical and other applications must not be used in conditions where one or more of the impurities added for mechanical strength reasons is likely to segregate to the surface in significant amounts and be lost from the bulk.Another area in which depth profiling with AES has proved most useful is that of ion im-plantation. The latter technique is being applied more and more to the fabrication of certain For example Sinharoy et 14 I I I -12 I I I 0 20 40 60 Sputtering time/min Fig. 33. Depth profiles for calcium into the surfaces of commercial alumina specimens (Coors AD-99 alumina) treated in high-pressure saturated steam (750 lb in-2) for different periods of time (A) Unexposed; (B) I d ; (C) 2 d ; (D) 3 d ; (E) 6 d ; and (F) 12d. The amount of segregated calcium increased with time of exposure up to about 12 d. After that the calcium-rich region starts to extend further into the bulk of the material.36 1019 0 0.04 0.080 0.04 O.Ot Depth/pm Fig.34. Depth profiles into surfaces of GaAs that have been implanted (a) with tellurium (120 keV 10l6 cm-2) and (b) with cadmium (120 keV 10le cme8). The observed profile (1) in (a) corre-sponds well to (2) the theoretically expected profile although the maximum is at a lower concentration. The observed cadmium profile (3) however does not bear much resemblance to (4) the theo-retical profile. The reason is suggested to be diffusion of cadmium towards the surface owing to a temperature rise caused by the implantati~n.~ Jame 1983 ANALYTICAL CHEMISTRY A REVIEW 679 solid-state devices but its usefulness is severely limited without reliable information about the distribution of the implanted species with depth and also the depth a t which its concentration has reached a maximum; in semiconductor jargon the dopant profile.Such profiles of tellurium and cadmium implanted into GaAs were examined by Park et aZ.37 The implants were carried out at 120 keV and a t fluences from 1015 to l O l 6 cm-2 at toom temperature. Tellurium and cadmium are used to alter the local electronic characteristics of the semi-conductor and it is important therefore to know where they are in the surface after implanta-tion. Depth profiling for the two implants gave the results shown in Fig. 34 where the Auger profiles are compared with the dopant profiles predicted from theory. For tellurium the pro-files are similar in shape although the concentration at the maximum is lower than expected, but the cadmium Auger profile is different from the theoretical profile.The maximum is flattened and there is a tail of concentration towards the surface. Indeed some cadmium was detected at the external surface before Auger profiling started. The explanation for such a profile is that cadmium has probably diffused towards the surface during implantation as the current density was high and some local heating may have occurred. From observations of this type can be deduced the implantation conditions needed to produce a locally altered sur-face layer of the depth and dimensions required for the fabrication of the device. In the above example for instance it is clear that implantation of cadmium to achieve the correct profile should be carried out either a t a lower current density or with efficient cooling of the host material .Conclusions Advantages and Disadvantages of AES No one analytical technique whether its application is in the field of surface analysis or in any other field is ever perfect and AES is no exception. The content of this review so far has concentrated on a description of the mode of operation of the technique and of some of the ways in which it has been useful and little mention has yet been made of the drawbacks. It is only fair then to those who might be thinking of applying AES to their own problems to provide a more balanced picture by setting the advantages and disadvantages against each other and they will be mentioned again later when a brief comparison is made of AES with other surf ace analytical techniques.The advantages should have become apparent from the previous sections but for complete-ness are summarised as follows: 1. high spatial resolution ; 2. fast data acquisition; 3. sensitivity to light elements; 4. relatively uncomplicated spectra ; and 5. continuously variable primary energy. The first two of these allow elemental mapping to be performed of which examples have been given. The third represents an advantage over other electron-probe techniques such as electron-probe microanalysis (EPMA) where analysis of the light elements is always difficult. The fourth advantage is again a relative one particularly in comparison with the complemen-tary technique of X-ray photoelectron spectroscopy (XPS) in which the spectra contain both photoelectron and Auger peaks.The fifth is useful if either the sensitivity for a particular element is to be maximised by choosing a primary energy at the maximum of the ionisation cross-section or if a charging problem is energy dependent or if loss spectra are required to be highly surface-specific which can be realised by reducing the primary energy to a low value. Among the inherent disadvantages the two principal ones stem directly from the use of an incident electron beam as the ionising source and they are electron-beam damage and surface charging. Even in the early days of AES when the irradiated area was large of the order of 1 mm diameter and the current density was relatively low typically A cm-2 effects on the surface composition arising from prolonged electron irradiation were observed.With the development of high spatial resolution the electron spot size has been reduced progressively to less than 1000 A but the beam current although also reduced has not been so in proportion, and as a result the current density has leaped to -lo2 A cm-2 for “conventional” AES and to -lo8 A cm-2 for high-resolution AES. Under these conditions the number and variety o 680 RIVI~RE AUGER TECHNIQUES IN Analyst Vol. 108 observations of disturbance of the surface by the incident beam have proliferated. These disturbances can take the forms of electron-induced desorption electron-assisted adsorption, electron-induced or -enhanced surface diffusion and surface decomposition. Some of the most severe effects have been observed with glasses.For example in Fig. 35 (from the work of Ohuchi et aZ.)35 are plotted the decays of the sodium Auger signal for two glass films of different thicknesses and for bulk glass as a function of the time of irradiation at room temperature by an electron beam. A cm-2 must be regarded as mild by today’s standards. The authors were able to show that the rate of decay i.e. of surface decomposition increased with increasing current density but decreased at higher electron energies and at lower temperatures. The irradiation conditions 3 keV and a current density of 5 x 0 1 3 5 7 9 Beam impingement timehin 1 Fig. 35. Decay with time of electron irradiation of the sodium Auger signal from the surfaces of glass films of different thicknesses (A) 1000 A (B) 2000 A and of !C).bulk glass at room temperature.The incident current density was 6 x 10-3A cm-2.3B During ion profiling of glasses there may also be synergistic effects between the ion and electron beams. When Ahn et aL39 carried out simultaneous ion erosion and Auger analysis of a soda-lime glass for a time long enough to have produced a crater of depth about 0.3 pm and then mapped the topography of the crater with a profilometer they obtained the result shown in Fig. 36. The diameter of the ion-eroded area was 3.2 mm and the electron beam being of much smaller diameter (-0.25 mm) was located in the centre of the crater. The profilometer traces show that the bottom of the crater is reasonably flat except where the electron beam has been impinging where there is an additional small crater of depth 600 A.The irradiation conditions chosen were such that for the same length of time the electron beam by itself did not remove measurable amounts of material. Thus both beams are required to be impinging on the surface together for the effect to be produced. The authors attributed the effect to electron-stimulated desorption of surface oxygen leading to enhanced sputtering in the electron irradiated area. The accumulation of surface charge during electron irradiation of insulating or semi-insulating surfaces can also be a problem in AES. It arises of course from the inability of the material to provide an electron current to balance the loss of electrons from the surface by secondary emission so that the surface becomes positively charged.The effect occurs at primary electron energies above that energy at which the secondary emission yield becomes greater than one. For most insulators or semi-insulators this threshold energy is only a few hundred electronvolts much lower than the primary energies normally used in AES. By the same token if the charging surface is flooded with electrons from an auxiliary source at energies well below the threshold e.g. 10-20 eV the excess positive charge at the surface can to a greater or lesser extent be neutralised. This is known as charge neutralisation and the neutralising source as a “flood gun.” Severe charging in AES can produce shifts in the entire spectrum by as much as several hundred electronvolts e.g. for powdered insulating materials Jane 1983 ANALYTICAL CHEMISTRY A REVIEW 681 1 m m Horizontal scale I I Depth scale 1 prn I Fig.36. Profilometer traces across the circular crater of diameter 3.2 mm formed in the surface of a soda-lime glass by ion erosion. In the centre of the ion-eroded area is another crater of additional depth, formed where both ion and electron beams have impinged together. Without the ion beam present the electron beam under the same energy and current density conditions and for the same length of time did not cause any measurable removal of material from the glass.30 and the shift can vary in a random manner following treatment of the material by for instance, ion bombardment. Use of a flood gun for charge neutralisation will not necessarily return the spectrum to the right position but it will stabilise its shift so that spectra recorded after successive surface treatments are at least comparable.The other present disadvantages of AES are not inherent but are the subjects of considerable research and are therefore likely to be minimised progressively. One is the difficulty of quantification and the other is the relative (compared with XPS) absence of chemical informa-tion. Most Auger spectra in AES are still recorded in the differential energy mode in which the raw measure of an elemental concentration is taken from the peak-to-peak height. This is a reasonable approximation as long as the undifferentiated peak width is not changing from one chemical situation to another but unfortunately there is now plenty of evidence that the width does change as does the associated fine structure.Thus the correct measure to use is the area under a peak in the undifferentiated N(E) distribution but here again there are substantial unresolved problems particularly with regard to the treatment of the inelastic tail that follows each peak. The exact amount of the tail to be included in the peak area is still a matter of argument. Further no general quantification treatment has yet been formulated that takes account of concentration inhomogeneities near the surface ; most treatments assume a homogeneous region within the analysed volume. Chemical information is implicit in many Auger peaks in particular those in which one or more electrons arise in the valence band of the solid but because of the very nature of th 682 RIVIERE AUGER TECHNIQUES IN Analyst Vol.108 Auger process the information has been largely inaccessible. In a few instances the changes in the spectra with chemical state are sufficiently dramatic for the various forms of the spectrum to be usable in an empirical “fingerprinting” way but it is only recently that improved physical interpretation coupled with the colossal advances in computing capability has allowed the correct unravelling of the peak shapes. Compared with the technique of XPS in which chemical information is available directly and in general comprehensibly AES still has a long way to go. On the other hand it does seem that the type of chemical information that AES will provide will be rather different from and complementary to that provided by XPS.In the same way as for the advantages the disadvantages of AES may thus be summarised as follows : 1. disturbance of the surface by the incident electrons; 2. surface charging due to the incident electrons; 3. difficulties in quantification; and 4. little direct chemical information as yet. Future Developments To a large extent the foreseeable developments in AES will be those intended to reduce or eliminate the disadvantages listed above. It seems unlikely given the current knowledge of the physics of electron - solid interactions that the potentially damaging effects of the electron beam can ever actually be removed but at least they can be understood and allowed for if that knowledge is extended beyond its present limits which is certainly one direction of develop-ment.In the same direction that of better physical understanding will go the development of quantification of AES as many of the poorly known quantities that enter the quantifica-tion procedure are of physical origin such as the back-scattering factor as a function of energy and atomic number and the inelastic mean free path. Theoretical calculation of these quantities has benefited enormously from the rapidly increasing capacity of computers over the last few years and as a result the models set up to simulate electron paths and electron interactions with atoms and with other electrons are becoming more sophisticated and realistic. Similarly there is no doubt that the amount of effort devoted to extracting chemical informa-tion will expand rapidly in the near future now that several workers have shown the way forward both experimentally and theoretically.On the technical side there will undoubtedly be further moves towards ever higher spatial resolution although a more pressing need in this connection is reduction of the incident current density that goes into the highly focused beams and that can only be achieved use-fully if the sensitivity of detection is increased. Current density reduction without added sensitivity simply means that recording times become unacceptably long. With the use of computers to control data acquisition store the data and then carry out data processing it is not now necessary to record Auger spectra in the differential mode by the conventional modula-tion techniques because spectra acquired in the N(E) mode can be differentiated by the computer if required.There will therefore be a general trend towards initial acquisition in the undifferentiated mode and that in itself could lead to greater sensitivity. Not all the developments of a technique can be foreseen and it is always possible that some major step forward will appear from an unexpected quarter. Comparison with Other Surface Analytical Techniques In every analytical field one can find several techniques in use because invariably no one technique can provide all the information required and they are used therefore in a comple-mentary fashion. This is certainly true of surface analysis and it is becoming the exception rather than the rule to find surface analytical equipment devoted to a single technique.Probably the most common combination as they can both use the same electron energy analyser is that of AES and XPS. In several ways their advantages and disadvantages balance each other. Both provide elemental compositional information at about the same ultimate sensitivity but XPS provides direct chemical information as well. Against that AES can analyse with high spatial resolution whereas at the moment XPS is restricted to an average analysis of a relatively large area. Again AES is a fast technique while XPS is slow but there is always the worry about electron-beam damage in AES such effects being virtually absent in XPS. As it is possible to switch from one technique to the other quickl June 1983 ANALYTICAL CHEMISTRY A REVIEW 683 tion 1.2. 3. 4. 6. 6. 7. 8. 9. 10. 11. 12. 13. 14. 16. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. and without altering the sample position the combination is an obvious and very useful one. The other techniques frequently employed in the same system as AES or XPS or even with both are static secondary ion mass spectrometry (SSIMS) and ion scattering spectroscopy (ISS) . Both use ion probes SSIMS of argon and ISS of helium in the low-energy range (1-4 keV). As their names suggest SSIMS is concerned with the mass analysis of secondary ions positive or negative and ISS with the energy analysis of elastically scattered primary ions. For SSIMS therefore the detector is a mass spectrometer usually of the quadrupole type and for ISS an energy analyser.In fact it has been shown to be possible to use the same energy analyser for ISS as for AES and XPS by reversing the polarities of the deflecting potentials as the energies of positive ions rather than of electrons are analysed. Each of these ion probe techniques has its advantages and disadvantages compared with AES and XPS. SSIMS has much higher sensitivity to some elements by some orders of magnitude can analyse for hydro-gen and hydrogen-containing species and in principle is capable of establishing the nature of surface compounds. On the other hand it is inherently destructive in that the surface has to be removed to be analysed and still very difficult to quantify because of matrix-dependent effects.ISS is the most surface-specific of the surface analytical techniques and quantification is straightforward but there is no possibility of obtaining chemical information while indi-vidual identification and analysis of heavy elements when there are several present together, is difficult. Ideally a surface analysis system should contain all the techniques whose elemental and chemical information is complementary so that as complete a picture as possible is obtained. In practice it is found that such multi-technique systems tend to be counter-productive in that, when more than two or three separate techniques are associated at least one of them is significantly under-used. I t is necessary to decide in advance for any particular problem or range of problems that combination of just two or three techniques will maximise the informa-required and concentrate on making optimum use of those.References Auger P. J . Phys. Rad. 1926 6 206. Lander J. J . Phys. Rev. 1953 91 1382. Powell C. J . Robins J . L. and Swan J . B. Phys. Rev. 1958 110 657. Scheibner E. J . and Tharp L. N. Surface Sci. 1967 8 247. Palmberg P. W. J . Appl. Phys. 1967 38 3137. Harris L. A. GEC Res. Dev. Rep. No. 67C201 (1967); J . Appl. Phys. 1968 39 1419. Weber R. E. and Peria W. T. J . Appl. Phys. 1967 38 4355. Palmberg P. W. Bohn G. K. and Tracy J. C. Appl. Phys. Lett. 1969 15 254. Shirley D. A. Chem. Phys. Lett. 1972 17 312. Larkins F. P. A t . Data Nucl. Data Tables 1977 20 311. Madden H. H. J . Vac. Sci. Technol. 1981 18 677. Seah M. P. and Dench W. A. Surf. Interface Anal. 1979 1 2 . Landolt D. and Mathieu H. J Oberfluche 1980 21 8. Powell C. J . Rev. Mod. Phys. 1976 48 33. Weber R. E. J . Cryst. Growth 1972 17 352. Kny E. J . Vac. Sci. Technol. 1980 17 658. Kleefeld J. and Levenson L. L. Thin Solid Films 1979 64 389. Smith M. A. and Levenson L. L. Phys. Rev. B 1977 16 1365. Lea C. and Seah M. P. Thin Solid Films 1981 75 67. Walls J. M. Hall D. D. and Sykes D. E. Surf. Interface Anal. 1979 1 204. Holloway P. H. Surf. Sci. 1976 54 506. Brockman R. H. and Russell G. J. Phys. Rev. B 1980 22 6302. Davis G. D. Viljoen P. E. and Lagally M. G. J . Electron Spectrosc. 1980 21 135. Rye R. R. Madey T. E. Houston J . E. and Holloway P. H. J . Chem. Phys. 1978 69 1504. Lumsden J. B. Stocker P. J. and Tsai S. C. Appl. Surf. Sci. 1981 7 347. Benndorf C. Egert B. Nobl C. Seidel H. and Thieme F. Surf. Sci. 1980 92 636. Cros A. Derrien J. and Salvan F. Surf. Sci. 1981 110 471. Okada S. Oura K. Hanawa T. and Satoh K. Surf. Sci. 1980 97 88. Verhoeven J. and van Doveren H. Thin Solid Films 1981 77 367. Joshi A. in Strauss B. M. and Cullen W. H. Editors “Fractography in Fdilure Analysis,” ASTM Kazmerski L. L. Appl. Surf. Sci. 1981 7 55. Jones D. McNeely D. and Swanson L. W. Appl. Surf. Sci. 1979 2 232. Hartweck W. and Grabke H. J . Acta Metall. 1981 29 1237. Guglielmacci J . M. Charfi F. and Joullie A. Thin Solid Films 1981 76 69. STP 645 American Society for Testing arid Materials Philadelphia 1978 p. 275 684 RIVI~RE 35. 36. 37. 38. 39. Wittberg T. N. Hoenigman J. R. Moddeman W. E. and Salerno R. L. Appl. Surf. Sci. 1980, Sinharoy S. Levenson L. L. and Day D. E. J. Vac. Sci. Technol. 1979 16 603. Park Y. S. Theis W. M. and Grant J. T. Aflpl. Surf. Sci. 1980 4 445. Ohuchi F. Ogino M. Holloway P. H. and Pantano C. G. Surf. Interface Anal. 1980 2 86. Ahn J. Perleberg C. R. Wilcox D. L. Coburn J. W. and Winters H. F. J. Appl. Phys. 1976, Received December Sth 1982 Accepted December 29th 1982 4 531. 46 4581

ISSN:0003-2654    

DOI:10.1039/AN9830800649

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst June 1983 Vol. 108 pp. 685-690 685 Scanning Potential Stopped-rotation Voltammetry Joseph Wang” and Bassam A. Freiha Deflartment of Chemistry New Mexico State University Las Cruces NM 88003 USA The technique of scanning potential stopped-rotation voltammetry which is based on measuring the differences between currents with the electrode rotation switched on and off while the applied potential is scanned linearly, is described. Asymmetric rotation pulses without the achievement of the rotation “off” steady-state current are employed. The resulting modulated response is free of most background current components directly proportional to the analyte concentration and reproducible. Well defined current -potential graphs are obtained for ascorbic acid dopamine homovanillic acid and hexacyanoferrate(I1) ion at the micromolar concentration level.Extremely low background signals are achieved at a glassy-carbon disk electrode allowing a detection limit of 7 x 10-8 M of dopamine. The tech-nique is simple and suitable for automation. Keywords Scanning potential stopped-rotation voltammetry ; hydrodynamic modulation ; solid electrode ; anodic oxidation Increasing effort is being directed towards the development of sensitive and reliable voltam-metric techniques employing solid electrodes. Many important compounds that oxidise at potentials too positive for them to be determined at mercury electrodes are analysed at solid electrodes. Pulse voltammetry which effectively corrects for the charging current at mercury electrodes is not effective for trace analysis at solid electrodes because of the additional (un-corrected) background current component due to redox reactions of the surface.lS2 Hydro-dynamic modulation at forced-convective solid electrodes has been shown to be a feasible technique for obtaining voltammograms for low concentrations of electroactive compound^.^ The convection pulsing produces a pulsation only in the convection-dependent current and thus it is free from most background interferences.In this way the analytical information for trace compounds can be extracted directly from the modulated response. Most modulation approaches for batch electroanalysis have utilised the rotating disk electrode (RDE) because of its effective mass transport and rigorous theoretical treatment.A sinusoidally modulated RDE about a centre rotation speed value w, has been incorporated with continuous potential scans to obtain sensitive volt ammo gram^.^ The peak to peak amplitude of the change in rotation speed (Am) is usually a small fraction (up to 10%) of w,. The current transients produced by a square-wave step of the rotation speed of a disk electrode have been e ~ a l u a t e d . ~ ~ Similar steppings in the rotation speed have been incorporated (in pulsed-rotation voltammetrp) with pointwise changes in the applied potential. In previous work in this laboratory we advocated the utilisation of stopped-rotation modula-tion in which the rotation is switched on and off while maintaining a constant applied poten-tial.’ By measuring the resulting current differences at a number of discrete potentials, sensitive voltammograms were developed and plotted pointwise.The main advantages of this approach over sinusoidal or square-wave modulation are its inherent sensitivity i.e. lo0 yo modulation and simple operation (on - off switching vs. level changes). However it suffers from the relatively long time (around 20 min) required to record the complete voltammogram. This paper presents a technique termed scanning potential stopped-rotation voltammetry, which involves the imposition of a linear potential scan on a disk electrode the rotation of which is sequentially switched on and off. The current difference between the “on” and “off” positions is measured at fixed time intervals after switching occurs and plotted as a function of the potential.By incorporating stopped-rot at ion volt ammetry with continuous potential scans sensitive voltammograms are recorded within 3 min while maintaining the high sensitiv-ity of discrete potential stopped-rotation voltammetry. A relatively rapid non-steady-state modulation (frequency 2-4 s) is incorporated with scan rates of 2-5 mV s-l. Similar method-ology has been described recently for obtaining sensitive voltammograms in flowing streams, * To whom correspondence should be addressed 686 WANG AND FREIHA SCANNING POTENTIAL Analyst Vol. 108 utilising scanning potential stopped-flow voltammetry.8 As a stringent test of the method a glassy-carbon electrode is employed because of its relatively large background current .9J* The characteristics and advantages of scanning potential stopped-rotation voltammetry were elucidated in this study.Experimental Apparatus The 200-ml capacity electrochemical cell the rotating disk assembly and the measuring system (i.e. polarograph and recorder) have been described in detail earlier.? A 0.75-cm diameter glassy-carbon disk served as the working electrode in conjunction with a silver -silver chloride reference electrode and a graphite rod counter electrode. Reagents The chemicals and reagents used have been described in detail previously,ll except that millimolar stock solutions of homovanillic acid (Sigma Chemical Co.) were prepared fresh every day. The supporting electrolytes were 0.1 M phosphate buffer (pH 7.4) and 0.1 M potassium chloride solution. Procedure A 200-ml volume of the phosphate buffer was introduced into the cell and de-aerated for 8 min.The nitrogen delivery tube was then raised above the solution and pre-treatment of the working electrode was begun. This consisted of scanning the applied potential between + l . O and -1.0 V at 50 mV s-l for three cycles. Following this measurements were made on blank and analyte solutions. The electrode was held at a potential for the start of the scan (usually -0.2 V) and after 20-30 s an anodic linear potential scan was initiated. During the scan period stopped-rotation modulation was provided by manual on - off switching (every few seconds) of the rotation speed. Because of the slow current decay when the rotation is “off” (compared with the rapid achievement of the “on” current steady state) asymmetric rotation pulses [e.g.2 (on) and 4 (off) s] have been used for obtaining a large limited modu-lated response Ail while using short cycling periods. Results and Discussion The long response times of steady-state stopped-rotation voltammetry [about 3 s (rotation “on”) and 25 s (“off ”)?I preclude its incorporation with continuous potential scanning. Instead of waiting so long a non-steady-state technique can be applied to obtain large current amplitudes utilising short cycling times. It has been shown in our constant-potential stopped-rotation approach? that a non-steady-state modulation with 3 s “on” and 3 s “off” (i.e. 5-fold reduction in the period required for steady-state operation) results in a diminution in current of only 20%.The resulting non-steady-state current amplitude is linearly dependent on the analyte concentration. Similar compromise between sensitivity and speed has been utilised in stopped-flow8 and stopped-stirringl2 voltammetry. The feasibility of combining a non-steady-state stopped rotation modulation with linear potential scans is discussed below. Fig. 1 illustrates typical linear-scan stopped-rotation voltammograms (raw and filtered data) for the oxidation of 5 p~ ascorbic acid and 10 p~ dopamine. As the potential is scanned anodically increased current oscillations followed by plateau regions are observed. The filtered a.c. current - potential data show well defined waves with half-wave potentials of about +0.48 V (ascorbic acid) and +0.24 V (dopamine); the plateau regions start a t +0.68 and +0.44 V respectively.The ascorbic acid wave is spread over a wider potential region as expected from the irreversible nature of the oxidation. Despite its irreversible oxidation the ascorbic acid response is quantifiable in contrast to that obtained at the micromolar concentra-tion level with potential-pulse techniques. The corresponding background stopped-rotation voltammograms (not shown) show zero a.c. response as will be discussed later. The sensitivity of stopped-rotation voltammetry compares favourably with that obtained by other voltammetric techniques currently being used for trace analysis a t solid electrodes. In Fig. 2 stopped-rotation diff erential-pulse and linear-scan voltammograms for the oxidation of 7.5 x M hexacyanoferrate(I1) are compared.The hexacyanoferrate(I1) ion is often use June 1983 STOPPED-ROTATION VOLTAMMETRY 687 0.6 0.3 0 0.6 0.3 0 EN Fig. 1. Linear scan stopped-rotation voltammograms for (a) 10 p~ dopamine and (b) 6 p~ ascorbic acid. Phosphate buffer 0.1 M. Conditions were as follows : pH 7.4; scan rate 2 mV s-l; cycling times 2 (on) and 4 (off) s; and rotation speed (on) (a) 1600 and (b) 3600 rev min-l. The lower graphs are the responses as they come off the chart-paper, while the upper plots are the filtered responses (plotted point-wise by measuring the individual current oscillations). I I I I I I 0.8 0.4 0 ” 0.8 0.4 0 EN Fig. 2. Comparison of stopped-rotation voltammetry (A) diff erential-pulse voltammetry (B) and linear-scan voltammetry (C) with and (D) without electrode rotation.Conditions were as follows 75 PM hexacyanoferrate(I1) in 0.1 M potassium chloride; scan rate 5 mV s-l; rotation speeds (A C) 2500 and (B D) 0 rev min-1; cycling times (A), 2 (on) and 4 (off) s; and pulse amplitude (B) 50 mV. in the evaluation of new electroanalytical techniques. Among the techniques compared in Fig. 2 stopped-rotation voltammetry yields the most defined and quantifiable response. Owing to the very low ax. background current the analytical information can be extracted directly from the stopped-rotation voltammogram ; in diff erential-pulse or linear-scan voltam-metry background correction should be made. A response similar to B was obtained by rotating the electrode during the differential-pulse scan (not shown) The large differential-pulse background is due to changes in the redox state of surface functional groups that occur during the potential step.l,l3 The linear-scan background currents were composed of the double layer charging and the surface redox reactions.The stopped-rotation response is free of these background current components (which are non-convective). At the 1 x 1 0 - 5 ~ level only stopped-rotation voltammetry permits quantification (not shown) ; the differential-pulse and linear-scan analytical responses were obscured by the high background currents. The lower background current of differential-pulse voltammetry at carbon paste electrodes permits quantification of hexacyanoferrate(I1) a t the micromolar concentration 1e~el.l~ Similar comparison between the voltammetric techniques using 7.5 x M dopamine and a glassy-carbon electrode yielded graphs similar to those in Fig.2 (not shown). The scan rate used in the stopped-rotation measurements 5 mV s-l is similar to that commonly employed in differential-pulse voltammetry; thus a complete voltammogram is recorded within about 3 min. The current - time transient following the rotation stoppage results primarily from the relaxation of the diffusion layer thickne~s.~ The thickness of the diffusion layer when the rotation is on (assuming steady-state conditions) is given by Levich behaviour : = 1.61D”3~-1/2~1/6 . . . . - - (1) where D is the diffusion coefficient of the species w is the rotation speed (“on,’) and v is the kinematic viscosity.Theoretically under conditions of immediate rotation stoppage an 688 WANG AND FREIHA SCANNING POTENTIAL Analyst Vol. 108 absence of solution motion (i.e. conditions of linear diffusion) as the rotation is stopped the diffusion layer thickness increases with the square root of time. Thus a t any time t following rotation stoppage 8ofp is given by Combination of equations (1) and (2) with the general equation for the convective-diffusion limiting current (3) . nFADC 21 = - 8 where n F A D and C having their usual meanings leads to the following expressions for the difference in the currents at the “on” and “off” positions : In practice “Cottrell conditions” are not achieved because of the time lag associated with the deceleration of the motor4 and the resulting continuous solution motion.Quantitative evaluation is based on the linear correlation between the limiting modulated response and concentration expected from equation (4). Two separate experiments were per-formed to confirm this linearity. Stopped-rotation voltammograms obtained after successive concentration increments of dopamine are shown in Fig. 3. These four measurements are 0.7 0.4 0.1 EN Fig. 3. (B-E) stopped-rotation voltammograms ob-tained after increasing the dopamine concentration in 2.5-p~ steps along with (A) the corresponding background cur-rent. Conditions as in Fig. l ( ~ ) . X .,o I” k 0.2 pA I I I I 0.8 0.4 0 EN Fig. 4. Stopped-rotation voltammogram for (A) 1.0 p~ dopamine and (B) its supporting electrolyte (phosphate buffer pH 7.4) solution.Conditions as in Fig. 1 ( b ) June 1983 STOPPED-ROTATION VOLTAMMETRY 689 a part of six concentration increments from 2.5 to 15 p~ dopamine. The plot of limiting modulated response (measured a t +0.4 V) against concentration was linear with a slope of 0.316 pA p ~ - l (correlation coefficient 0.999 ; intercept -0.07 PA). The background voltam-mogram (A) is characterised by its very low modulated response indicating good correction for nonconvective currents and the absence of electroactive contaminants (the small and random current spikes are electrical noises due to rotation stepping). A similar calibration experiment for ascorbic acid [six increments from 5 to 30 p ~ . Conditions scan rate 5 mV s-1; frequency 3 s; rotation speed (on) and buffer as in Fig.l ( a ) ] yielded a linear plot (slope 0.265 pA p ~ - l ; correlation coefficient 0.997 ; intercept +0.14 PA). Fig. 4 illustrates the utility of stopped-rotation voltammetry for obtaining defined voltam-mograms for low concentrations of electroactive species. Examination of the data for 1.0 p~ dopamine (B) and its corresponding background current (A) shows that the limiting current amplitude is about 0.54 pA whereas the average noise level is 20 nA. A detection limit of about 70 nM of dopamine may be determined if it is based on a signal to noise ratio of 2. Other rotating disk assemblies may result with lower noise levels i.e. lower detection limits. A series of eight successive stopped-rotation voltammetric measurements of 10 p~ dopamine, carried out over a total time of about 1 h was used to evaluate the precision of the technique [conditions as in Fig.1 ( b ) ] . Reproducible voltammograms at the same potential regions, were obtained. The mean limiting current amplitude found was 5.65 PA with a range of 5.58-5.75 pA. The relative standard deviation calculated for this series 0.96% indicates the feasibility of obtaining reproducible current - potential data for low concentrations of electro-active species at solid electrodes. Fig. 5 demonstrates the potential of stopped-rotation voltammetry for trace analysis of mixtures ; stopped-rotation current - potential graphs (raw and filtered response) for a mixture of 7.5 p~ dopamine and homovanillic acid show two defined waves and plateau regions with 0.8 0.4 0 EN Fig.5. Stopped-rotation voltammo-gram for a mixture of 7.5 p~ dopamine and homovanillic acid in 0.1 M phosphate buffer (pH 7.4). Conditions as in Fig. l(b). The lower graph is the response as it comes off the chart-paper while the upper plot is the filtered response 690 WANG AND FREIHA apparent half-wave potentials of +0.22 V (dopamine) and +0.69 V (homovanillic acid). In general species with a 0.3-0.4 V difference in their half-wave potentials can be measured simultaneously (depending on the reversibility and number of electrons). Derivatised hydro-dynamic modulation approaches have been developed recently for improving the selectivity of multi-components ana1y~is.l~ The a.c. current is not affected by the non-convective-dependent d.c. background current a t relative extreme potentials ; a defined (combined) plateau is observed at potentials more positive than +1.0 V (where the oxidation of water occurs) indicating the feasibility of using the technique for obtaining voltammograms of depolarisers with high positive potentials.In conclusion scanning potential stopped-rotation voltammetry appears to be a valuable technique for trace electroanalysis of oxidisable compounds at solid electrodes. Similar data may be obtained in the cathodic region (provided that pure blank solutions are used) thus avoiding the inconvenience (and hazard) of the dropping-mercury electrode. Switching the rotation speed of the electrode on and off is experimentally the simplest way to modulate the RDE; therefore the technique can be easily placed under automated control and can be per-formed with any RDE assembly without the need for especially programming circuitry for superimposing sinusoidal or square-wave modulation.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. References Kissinger P. T. Anal. Chem 1976 48 17R. Sokol W. F. and Evans D. H. A n d . Chem. 1981 53 578. Wang J. Talanta 1981 28 369. Miller B. Bellavance M. I. and Bruckenstein S. Anal. Chem. 1972 44 1983. Bruckenstein S. Bellavance M. I. and Miller B. J. Electrochem. Soc. 1973 120 1351. Blaedel W. J. and Engstrom R. C Anal. Chem. 1978 50 476. Wang J. Anal. Chem. 1981 53 1528. Wang J. and Dewald H. D. Anal. Chim. Ada 1982 136 77. Galus Z. and Adams R. N. J. Phys. Chem. 1963 67 866. Brumleve T. R. Osteryoung R. A. and Osteryoung J. Anal. Chem. 1982 54 782. Wang J. Anal. Chem. 1981 53 2280. Wang J. Anal. Chim. Acta 1981 129 253. Wang J. and Freiha B. A. Talanta in the press. Wang J. Talanta 1982 29 805. Received October 26th 1982 Accepted December 16th 198

ISSN:0003-2654    

DOI:10.1039/AN9830800685

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst June 1983 Vol. 108 691-700 691 Differential-pulse Polarographic Monitoring of Permitted Synthetic Food Colouring Matters and Ascorbic Acid in Accelerated Light Degradation Studies and the Spectrophotometric Determination of the Ammonia and Simpler Amines Formed Arnold G. Fogg and Abdulhadi M. Summan Chemistry Defiartment Loughborough University of Technology Loughborough Leicestershire LE 11 3T U Permitted food colouring matters and ascorbic acid were determined by differential-pulse polarography to monitor their interaction during light degradation studies at pH 5.5 in acetate buffer containing EDTA. Reduc-tive splitting of azo bonds in the food colours was apparent from the formation of amines such as aniline sulphanilic acid and naphthionic acid which were determined spectrophotometrically using diazotisation methods.These amines were shown to be degraded further in the light to yield ammonia, which was determined spectrophotometrically as indophenol. Keywords ; Food colouring matters ; degradation ; ammonia ; amines ; difleeren-tial-pulse polarography Ascorbic acid is added to certain soft drinks as a vitamin and/or as an antioxidant. Most soft drink preparations are coloured artificially with permitted food colouring matters and problems can arise due to interaction of some or all of the food colouring matter with ascorbic acid. This is particularly troublesome should the bottled soft drink be displayed for example, in bright sunlight in a shop window or on a garage forecourt. The problem is exacerbated in countries that experience strong sunlight which has to be taken into account when exporting soft drinks.Studies of the determination of food colouring matters and their degradation products are being undertaken in this laboratory. Diff erential-pulse polarography has been used to determine mixtures of food colouring matters1-3 and to monitor the degradation of individual food colouring matters in accelerated light and heat degradation ~ t u d i e s . ~ ~ ~ The addition of tetraphenylphosphonium chloride which alters the peak potentials of some food colours is useful in effecting separation of overlapping polarographic peak~.l-~#~ The use of voltam-metry with a glassy carbon electrode has been assessed with static and flow injection systems.6J The formation of Red 10B from the permitted food colour Red 2G by heat degradation has been monitored p~larographically.~ The interaction of permitted food colouring matters with ascorbic acid during an accelerated light degradation experiment has been studied here.Ascorbic acid gives a polarographic oxidation wave a t a dropping-mercury electrode with a half-wave potential of +0.06 V (pH 5.5) and can be determined polarographically at the same time as the food colours are determined by means of their reduction waves at more negative potentials. As ascorbic acid is a reducing agent it was expected that reductive splitting of the azo group to form the corresponding amines might occur in the presence of intense light and this was found to be so. For this reason the feasibility of using spectrophotometric methods to detect and deter-mine these amines was investigated.The structures of the permitted synthetic food colouring matters containing azo bonds are given in Table I. Clearly if reductive splitting occurs Red 2G should give aniline Sunset Yellow FCF Yellow 2G tartrazine Black PN and Brown FK should give sulphanilic acid and amaranth carmoisine Ponceau 4R and Chocolate Brown HT should give naphthionic acid in addition to larger amines. Aniline and sulphanilic acid are known to diazotise and couple with N-1-naphthylethylenediamine in dilute hydrochloric acid solution but naph-thionic acid was found to require more concentrated acid conditions in order to react and a satisfactory procedure was developed for this. Certain other large amines were tested an 692 FOGG AND SUMMAN DIFFERENTIAL-PULSE POLAROGRAPHIC Analyst VoZ.108 TABLE I STRUCTURES OF PERMITTED FOOD COLOURING MATTERS WITH AZO GROUPS Food colour Red 2G . . Sunset Yellow FCF Yellow 2G . . Tartrazine . . Structure OH NHCOCH3 eN=NIYl NaS03 ' ' S03Na HO NaS03 S03Na CI QC' S O ~ N ~ S O ~ N ~ OH NHCOCH3 NaSO, Black PN S03Na S03Na NH2 Brown FK (mixture of about six compounds*) R = H -26% R = CH3 -17% S03Na *Additional azo e.g. N H 2 + N = N e groups occur here in R" other constituents H p S03Na Amaranth NaS03 B N = \ S03N Jzcrte 1983 MONITORING OF FOOD COLOURS AND ASCORBIC ACID TABLE I-continued Food colour Carmoisine . . . . Ponceau 4R . . 693 Structure OH S03Na OH were found not to react under the conditions developed for the spectrophotometric deter-mination of naphthionic acid and it is possible that there is little or no interference in these methods from the other amines formed during the degradation of the food colours shown in Fig.1. Ascorbic acid was found to interfere with the reactions used to produce the spectro-photometric derivatives and any ascorbic acid remaining in the degraded solution was destroyed carefully with potassium permanganate before determining the amine. When these methods were applied to the degradation studies the yields of the simpler amines such as sulphanilic acid were found to increase to almost complete formation with increased time of degradation but then to decrease markedly again.Subsequently aniline, sulphanilic acid and naphthionic acid were shown to degrade under the conditions of the light degradation. Ammonia was shown to be formed and this was determined spectro-photometrically using the indophenol method. In this study the ammonia was distilled before being determined as minor interferences were observed from other degradation products which underwent indophenol-type reactions when the indophenol method was applied directly to the degraded solutions. Experimental Apparatus Polarographic measurements were made with a PAR 174 polarographic analyser (Princeton Applied Research) and a Gould H-2000 X - Y recorder. A three-electrode operation was employed using a dropping-mercury electrode a platinum counter electrode and a saturated calomel reference electrode.Diff erential-pulse polarography was carried out with a forced drop time of 1 s a scan rate of 5 mV s-l and a pulse height of 50 mV. Solutions were deoxygenated by means of nitrogen that had been passed through a vanadium(I1) scrubber. Light degradation studies were carried out in a specially constructed light box in which the sample solutions were held at a uniform close distance from a 500-W lamp (Philips G/74). This apparatus is described in more detail el~ewhere.~ The solutions were deoxygenated with nitrogen gas and were contained in tightly sealed screw-capped autoclavable bottles. Spectrophotometric measurements were made with a Pye Unicam SP600 spectrophoto-meter 694 FOGG AND SUMMAN DIFFERENTIAL-PULSE POLAROGRAPHIC Analyst VOZ.108 Light Degradation Studies Food colour solutions 1000 p.p.m. Dissolve 0.1 g of an authentic food colour sample in water and dilute to 100 ml in a calibrated flask. Acetate bufer solution (PH 5.5). Dilute 62.5ml of 1 M acetic acid solution and 50ml of 1 M sodium acetate solution to 11 and adjust to pH 5.5 with 1 M sodium hydroxide solution. Ascorbic acid - EDTA solution 2% m/V ascorbic acid. Dissolve 2 g of abscorbic acid and 0.05 g of ethylenediaminetetraacetic acid (disodium salt) in water and dilute to 100 ml in a calibrated flask. Ascorbic acid - EDTA solution 0.2% mlV ascorbic acid. Prepare as above to contain the same amount of EDTA but only 0.2 g of ascorbic acid. Solutions for degradation. In the general study of the interaction between food colours and ascorbic acid concentrations of 5 and 100 p.p.m.respectively were used. In attempting to determine the stoicheiometry of the reaction between the food colours and ascorbic acid, solutions 5 p.p.m. in food colour and 1000 p.p.m. of ascorbic acid were degraded in light. In later studies in which the amounts of small amines and ammonia were monitored solutions 10 p.p.m. in food colour and 1000 p.p.m. in ascorbic acid were degraded. In all instances 20 ml of acetate buffer (pH 5.5) were included in the preparation of each 100 ml of solution for degradation. Polarographic Determination of Food Colour and Ascorbic Acid in Degraded Solutions The food colour concentration was determined by applying diff erential-pulse polarography directly to the degraded solutions after deoxygenation.Ascorbic acid was determined similarly but using a 10-fold dilution with water before polarographing for solutions at the higher ascorbic acid concentrations. Spectrophotometric Determination of Amines Sodiwa nitrite solution 0.1% m/V. Hydrochloric acid 1 + 1 V/V. Mix equal volumes of concentrated hydrochloric acid and Sulphamic acid solution 0.5% mlV. Potassium bromide solation 20% m1V. N-1-Naphthylethylenediamine dihydrochloride solution 0.1 yo m/ V . Standard sulphanilic acid solution 1 x 10-4 M. Dissolve 0.433 g of sulphanilic acid in water and dilute to 250 ml in a calibrated flask. Transfer by pipette 5 ml of this solution into a 500-ml calibrated flask and dilute to volume with water. M. Dissolve 0.618 g of naphthionic acid in a small volume of water and 3 ml of 1 M sodium hydroxide solution and dilute to 250 ml with water in a calibrated flask.Transfer by pipette 5 ml of this solution into a 500-ml calibrated flask and dilute to volume with water. Dissolve 0.465 g of aniline in a small volume of water and 2 ml of 1 + 1 V/V hydrochloric acid and dilute to 500 ml in a calibrated flask. Transfer by pipette 5 ml of this solution into a 500-ml calibrated flask and dilute to volume with water. Dissolve 1.58 g of potassium permanganate in 100ml of water in a 250-ml beaker cover with a clock-glass boil gently for 15-20min allow to cool to room temperature filter and collect the filtrate in a flask covered with aluminium foil. Store this solution in the dark. As required dilute 10 ml of this solution to 100 ml.water. Standard naphthionic acid solution 1 x Standard aniline soktion 1 x 10-4 M. Potassium permanganate solution approximately 0.01 M. Procedure for destruction of the excess of ascorbic acid Place an aliquot (10 ml) of degraded solution in a 50-ml conical flask add a few drops (0.4-0.5 ml) of concentrated sulphuric acid and warm slightly. Add 0.01 M potassium permanganate solution dropwise until a faint pink tinge remains 10 s after addition. Trans-fer the solution quantitatively into the calibrated flask to be used for determining amines June 1983 MONITORING OF FOOD COLOURS AND ASCORBIC ACID 695 Procedure for determination of sulfihanilic acid and aniline Transfer by pipette an aliquot of standard sulphanilic acid or aniline solution or degraded food colour solution (previously treated with permanganate if necessary) into a 100-ml calibrated flask add 2 ml of 1 + 1 V/V hydrochloric acid and 2 ml of 0.1% m/V sodium nitrite solution mix and allow to stand for 20 min.Add 2 ml of 0.5% m/V sulphamic acid solution mix and allow to stand for 3 min. Add 2 ml of 0.1% m/V N-l-naphthylethylene-diamine dihydrochloride solution dilute to volume mix and allow to stand for 30min.* Measure the absorbance of this solution at 545 nm. Procedure for determination of naphthionic acid Transfer an aliquot of standard naphthionic acid or sample solution into a 50-ml beaker, adjust the pH to 7.0 if necessary by addition of dilute hydrochloric acid or sodium hydroxide solution and add 2 ml of 0.1% m/V sodium nitrite solution and 2.5 ml of 20% potassium bromide solution.Mix well and add carefully over a period of 5 min to a cooled mixture of 3ml of concentrated sulphuric acid and 3ml of water. Allow the mixture to stand for 3 min add 2 ml of 0.5% m/V sulphamic acid solution mix and allow to stand for 5 min. Add 3 ml of ethanol (96%) and after 3 min transfer quantitatively into a 50-ml calibrated flask containing 4 ml of 0.1 yo m/V N-1-naphthylethylenediamine dihydrochloride solution, dilute to volume and allow to stand for 30 min. Measure the absorbance of the solution at 550 nm. Spectrophotometric Determination of Ammonia Sodium hypochlorite solution. (10-14y0 available chlorine) to 25 ml. Alkaline phenol solution. in water and dilute to 100 ml. Sodium nitrofirusside solution 0.1 yo m/V.Dilute 10 ml of commercial sodium hypochlorite solution Carefully dissolve 30 g of phenol and 20 g of sodium hydroxide Distillation of ammonia from degraded solations Transfer an aliquot (e.g. 25ml) of degraded food colour solution into a 50-ml conical flask add a few drops (0.4-0.5ml) of concentrated sulphuric acid warm slightly and add 0.01 M potassium permanganate solution dropwise until a faint pink tinge remains for 20 s. Transfer the solution quantitatively into an ammonia distillation flask and add 25 ml of 10% m/V sodium hydroxide solution. Distil the ammonia directly into 25ml of 0.1 M hydrochloric acid solution contained in a 50-ml conical flask rinsing the condenser with a small volume of water into the flask after completion.Transfer the solution into a 50-ml calibrated flask and dilute to volume with water. Take aliquots of this solution (e.g. 20 ml) for the indophenol reaction; the pH of the aliquot is adjusted to >7 before adding the reagents. Procedare for spectrophotometric determination of ammonia Transfer by pipette an aliquot of neutral or slightly alkaline standard ammonium chloride solution or sample solution into a 25-ml calibrated flask. Add by pipette in turn with swirling and at 1-2-min intervals 0.2 ml of sodium hypochlorite solution 0.5 ml of alkaline phenol solution and 0.2 ml of sodium nitroprusside solution. Heat the flask at 60-65 "C in a water-bath for 3-5 min to form the indophenol blue. Cool dilute to volume and measure the absorbance at 630 nm. Results Ascorbic acid under these conditions gives a diff erential-pulse polarographic peak at +0.06 V.The necessity of adding EDTA to inhibit oxidation of ascorbic acid by air during the light degradation is illustrated by the results given in Table 11. In the absence of EDTA only 1% of the ascorbic acid in a 100 p.p.m. solution remains after 7 h whereas in * Coupling is only about 50% complete after 30 min for aniline under these conditions,O although good reproducibility was obtained. Similar results were obtained after full coupling in 3-24 h 696 FOGG AND SUMMAN DIFFERENTIAL-PULSE POLAROGRAPHIC Analyst VoZ. 108 the presence of 0.0025~0 of EDTA 76% remains after 64 h. Levels of EDTA greater than 0.005~0 m/V were not used as this distorted the polarographic wave of ascorbic acid.All results reported below were obtained for degradation solutions containing 0.002 5% m/V of EDTA when ascorbic acid was included. When ascorbic acid was omitted so was EDTA. TABLE I1 EFFECT OF EDTA IN STABILISING ASCORBIC ACID (100 p.p.m.) FROM OXIDATION BY AIR DURING LIGHT DEGRADATION (a) Without EDTA-Time/h . . 0 2 10 Remnant of ascorbic acid yo of original . 100 50.0 1.0 (b) With 0.0025~0 of EDTA-Time/h . . . . 0 64 90 132 Remfiant of ascorbic acid % of original 100 76 56 0 The interaction of ascorbic acid and a food colour is illustrated by the example of amaranth in Fig. 1. Graphs are shown for the degradation of ascorbic acid and amaranth separately, and in the presence of each other. Clearly in admixture both compounds degrade signifi-cantly more rapidly which was so for all the food colours.Information for the interaction with ascorbic acid for all the food colours is given in Table 111. For solutions 100 p.p.m. in ascorbic acid the approximate time taken for all the ascorbic acid to be oxidised is given together with the time for half of the food colour to be degraded under these conditions and also in the absence of ascorbic acid. In the former instance the ascorbic acid is degraded before all the food colour has been lost. For solutions 1000 p.p.m. in ascorbic acid the time taken for all the food colour to degrade is also given. The food colours are listed in order of increasing stability under the latter conditions. Time/h Fig. 1. Light degradation in acetate buffer (pH 5.5) of amaranth and ascorbic acid separately and in admixture.Initial amaranth concentration = 10 p.p.m. Initial ascorbic acid concentration = 100 p.p.m. EDTA con-centration = 25 p.p.m. A Amaranth alone; B amaranth in the presence of ascorbic acid; C ascorbic acid alone; and D ascorbic acid in the presence of amaranth J m e 1983 MONITORING OF FOOD COLOURS AND ASCORBIC ACID 697 An attempt to determine the stoicheiometry of the reaction of food colour and ascorbic acid was made for those food colours that degraded rapidly in the presence of ascorbic acid. A typical degradation graph is shown in Fig. 2. The initial degradation of ascorbic acid ceased after the food colour had disappeared visually. This amount of ascorbic acid corresponds to a stoicheiometry of 1:4 for food colour to ascorbic acid for Chocolate Brown HT Black PN and Brown FK.Some time after the disappear-ance of the food colour degradation of ascorbic acid accelerated again and the rate of degradation became more rapid than it would have been in the absence of the food colour degradation products. It may be significant that the time at which the rate of ascorbic acid degradation increases again (10 h see Fig. 2) corresponds with that at which there is a marked increase in the production of ammonia. The yield of ammonia remains essentially constant during the period of ascorbic acid stability (2-10 h). The other food colours gave ratios of 1 2. TABLE I11 LIGHT DEGRADATION OF FOOD COLOURS AND ASCORBIC ACID IN ADMIXTURE Food colour Chocolate Brown HT Carmoisine .. Black PN . . BrownFK Ponceau 4R . . Red 2G Sunset Yellow FCF . . Amaranth . . Tartrazine . . Green S Brilliant Blue FCF . . Patent Blue V . . Yellow 2G Quinoline Yellow . . Approximate time for complete loss of 100 p.p.m. of ascorbic acid/h * . 38 66 80 22 42 90 a . 90 62 66 70 62 80 66 62 * >2 weeks. t Ascorbic acid degrades completely first. p loo 0 0 g 8 m Time for loss of half of food colour in presence of 100 p.p.m. of ascorbic acid/h 62 38 75 18 30 96 36 44 42 56 42 62 60 62 Time for loss of half of food colour in absence of ascorbic acid/ h 73 * * 92 80 * * * * * * * 130 70 Time for loss of all food colour in presence of 1000 p.p.m.of ascorbic acid/h 1 2 2 3 12 23 24 24 42 62 64 64 > 96t >96t 50 0 10 20 Time/h Fig. 2. Light degradation of ascorbic acid (initial concentration = 1000 p.p.m.) in the presence of Chocolate Brown HT (initial con-centration = 6 p.p.m.). EDTA concentration = 26 p.p.m. The solution became colourless after 2 h 698 FOGG AND SUMMAN DIFFERENTIAL-PULSE POLAROGRAPHIC Andyst VoZ. 108 Data on the degradation of sulphanilic acid naphthionic acid and aniline in the presence and absence of ascorbic acid are given in Table IV. The percentage of amine remaining and the percentage yield of ammonia (based on the formation of one ammonia molecule per amine molecule) are given. Naphthionic acid clearly degrades rapidly giving a full yield of ammonia within 16 h even in the absence of ascorbic acid; in its presence this time is halved.Sulphanilic acid is considerably more stable than is naphthionic acid 15 d being required for TABLE IV LIGHT DEGRADATION OF SULPHANILIC ACID NAPHTHIONIC ACID AND ANILINE WITH AND WITHOUT ASCORBIC ACID PRESENT With ascorbic acid Compound Time 1.5 h 5 h 7.5 h 12 h 16h Sulphanilic acid . . l h 3 h 5 h 15h 20h 2 d 4 d 8 d 10 d 12 d 16 d 21 d Aniline . . 7 d 12 d 20 d 25 d Naphthionic acid . . 0 Compound remaining yo 100 27.2 4.2 1.5 0 0 92.2 77.8 73.8 71.7 70.8 62.3 49.3 27.4 19.1 11.3 0 100 6.6 0 --Molar yield of ammonia % 10* 84.1 98.6 100.1 100.6 100.4 9.8 18.5 23.1 26.8 28.3 36.5 45.6 67.4 77.0 87.1 95.7 0 95.9 99.9 --Without ascorbic acid r Compound Molar yield of remaining yo ammonia % 100 10* A \ 62.2 48.4 24.7 85.4 24.7 86.1 4.9 96.6 0 100.2 100 66.6 66.0 61.5 85.8 39.1 11.4 100 0 24.1 24.1 29.3 0 14.8 62.4 91.7 * These results indicate that some ammonia is obtained on heating naphthionic acid with sodium hydroxide solution.The results in this column should be viewed accordingly. complete loss in the presence of ascorbic acid and much longer in its absence. Aniline is much stabler than the other two amines although even for aniline 50% degradation to ammonia was observed after 14 d in the presence of ascorbic acid.In Table V data are given concerning the formation of amine (aniline sulphanilic acid or naphthionic acid) and of ammonia during the degradation of the food colours in the presence of lo00 p.p.m. of ascorbic acid. All the data were obtained after the complete loss of food colour from the solution and after removal of any remnant of ascorbic acid with potassium permanganate. Thus in all instances maximum formation of amine is seen at the f h t measurement time usually after an additional 1 h of light treatment and even by this time ammonia has been formed indicating that some of the amine has already degraded. Clearly ammonia can be formed from both amines formed by cleavage of the azo double bond and this is apparent in the results. Red 2G Sunset Yellow FCF tartrazine carmoisine Ponceau 4R and amaranth all of which contain a single azo group give a 200% yield of ammonia, indicating that not only do the amines that have been determined (aniline sulphanilic acid and naphthionic acid) degrade but also those with more complicated structures formed from the part of the molecule on the other side of the azo bond.This is probably to be expected in view of the increasing ease of degradation from aniline to naphthionic acid. Black PN and Chocolate Brown HT which have two azo groups in their structures give a yield of 400% of ammonia. Brown FK which is a mixture of related compound9 having 1-3 a June 1983 MONITORING OF FOOD COLOURS AND ASCORBIC ACID 699 TABLE V FORMATION OF SIMPLE AMINES AND AMMONIA ON LIGHT DEGRADATION OF A20 FOOD COLOURS I N THE PRESENCE O F ASCORBIC ACID The amines and ammonia were not determined until all traces of the food colour had disappeared visibly.The time for this is given in parentheses with the name of the food colour. The additional times of light treatment before measurement are given. Food d o u r Red 2G (23 h) . . Sunset Yellow FCF (24 h) Tartrazine (42 h) Black PN (2 h). . Brown FK (3 h) Carmoisine (2 h) Ponceau 4R (12 h) Amaranth (24 h) Chocolate Brown HT (1 h) * . Additional time l h 5 h 10h 20h 2 d 6 d 10 d 15 d l h 5 h 10 h 20h 2 d 6 d 10 d 15 d 5 h 10h 20h 2 d 6 d 10 d 15 d l h 5 h 10 h 20h 2 d 8 d 10 d l h 5 h 10 h 20 h 3 d 8 d 16 d 1.5 h 5 h 7.5 h 24h 1.5 h 5 h 7.5 h 12 h 24 h 1.5 h 5 h 7.5 h 12 h 24 h 1.5 h 5 h 7.5 h 12 h 24 h Amine formed Aniline Sulphanilic acid Sulphanilic acid Sulphanilic acid Sulphanilic acid Naphthionic acid Naphthionic acid Naphthionic acid Naphthionic acid Molar yield of amine yo 84.7 59.6 52.1 45.7 40.1 34.7 18.6 0 120.4 66.0 58.4 52.2 44.9 34.9 18.9 0 67.3 58.3 51.4 44.9 34.8 18.6 0 62.1 37.9 34.8 29.3 17.6 9.7 0 86.7 59.8 51.2 47.6 43.4 25.5 0 26.3 3.7 1.4 0 25.9 3.6 1.2 0 0 24.8 3.3 1.4 0 0 25.6 3.8 1.0 0 0 Molar yield of ammonia % 145 148 155 160 168 182 196 90 137 141 147 153 166 181 196 138 148 149 163 158 181 196 140 158 162 176 24 1 363 402 110 141 145 151 157 173 198 183 199 199 201 184 200 205 206 207 186 199 203 203 204 182 199 201 22 1 399 700 FOGG AND SUMMAN groups gives a yield of 200%.No results have been given in Table V for Yellow 2G due to the good light stability of this food colour (see Table 111); the eventual yield of ammonia was confirmed as being 200%. Discussion This study has provided information on the enhanced degradation of food colouring matters and ascorbic acid in the presence of each other during light degradation. The formation of aniline sulphanilic acid and naphthionic acid during these degradations has been demonstrated and their concentrations have been determined spectrophotometrically.These simple amines have been shown to degrade further to give ammonia. Naphthionic acid in particular degrades rapidly in this way even in the absence of ascorbic acid although the degradation is even more rapid in its presence. Sulphanilic acid gives appreciable amounts of ammonia within a few hours in strong light in the presence of ascorbic acid, whereas ammonia is observed only after several days in the absence of ascorbic acid. Aniline is much stabler than is sulphanilic acid. Quantitative yields of ammonia are obtained in all instances indicating that both nitrogen atoms in the azo linkages are converted into ammonia eventually. With Red 2G and Black PN additional ammonia might have been expected from photolysis of the acetamido groups in these molecules but this was not observed.Brown FK gave a yield corresponding to the formation of two ammonia molecules per molecule; as Brown FK is reported to be a mixture of compounds containing 1-3 azo groups and additional amino groups a higher yield than this might have been expected. The light degradation technique might serve as a useful means of assessing samples of Brown FK. Details of a high-performance liquid chromatographic (HPLC) study of the light and heat degradation of food colours will be reported elsewhere.* Studies similar to those reported here are also being made of the heat degradation (130 “C in sealed vials) of food colours in the presence and absence of vitamin C.Preliminary results indicate that simple amines and ammonia are formed but in much lower yields than in the light degradation studies. Phenols are the likely products of the degradation of the amines in light but further HPLC studies will be required in order to confirm this. Degradation in this study was carried out in fairly intense light. The purpose of using these accelerated degradation conditions was to make the identification of products easier. When a complete list of products is obtained it will still be necessary to show that these are formed under less extreme and more normal food processing and storage conditions and to study the extent to which any of them are formed. It should be noted that the pH at which these studies were carried out (5.5) was chosen as being convenient for the polarographic determination of ascorbic acid. Addition of EDTA was made so that the reaction of ascorbic acid with dissolved molecular oxygen which is extremely rapid could be disregarded. Clearly EDTA is not normally present in foodstuffs. Further soft drinks normally have a pH of 2.5-3 and other additives such as preservatives, antioxidants and sugars may also have significant effects on the stability of the food colours. The authors thank Pointing Limited and Williams (Hounslow) Limited for providing samples of food colouring matters and the latter for useful comments Dr. N. T. Crosby (Laboratory of the Government Chemist) and Mr. B. A. Saturley (Beecham Products) for helpful advice and the Government of Saudi Arabia for financial support for A.M.S. References 1. 2. 3. 4. 5. 6. 7. 8. 9. Fogg A. G and Yoo K. S. Analyst 1979 104 723. Fogg A. G. and Yoo K. S. Analyst 1979 104 1087. Fogg A. G. and Whetstone M. R. Analyst 1982 107 455. Fogg A. G. and Whetstone M. R. to be published. Fogg A. G. and Bhanot D. Analyst 1980 105 234. Fogg A. G. and Bhanot D. Analyst 1980 105 868. Fogg A. G. and Bhanot D. Analyst 1981 106 883. “Report of the Food Additives and Contaminants Committee Interim Report on the Review of Norwitz G. and Kellher P. N. Anal. Chem. 1981 53 1238. Received July 27th 1982 Accepted August 23vd 1982 Colouring Matter in Food Regulations 1973,” H.M. Stationery Office London 1979

ISSN:0003-2654    

DOI:10.1039/AN9830800691

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst June 1983 Vol. 108 pp. 701-711 701 Potentiometric Determination of Sulphite by Use of Mercury(1) Chloride - Mercury(l1) Sulphide Electrodes in Flow Injection Analysis and in Air-gap Electrodes Geoffrey B. Marshall and Derek Midgley" Central Electricity Research Laboratories Kelvin Avenue Leatherhead Surrey KT22 7SE Flow injection analysis using as the detector a solid-state ion-selective electrode with a mercury(I1) sulphide - mercury(1) chloride membrane can be used for determining sulphite or dissolved sulphur dioxide in water. At concentra-tions in the range 1.5-10mg1-1 of sulphite the method has a Nernstian response of 60 mV per decade but a t lower concentrations (down to 0.1 mg 1-l) the e.m.f. is linearly related to the sulphite concentration. Although the flow injection method is less sensitive than direct use of the electrode it avoids the problem of chloride interference and permits the determination of sulphur dioxide in the commonly used tetrachloromercurate absorbent.The only serious interference found was from sulphide although a small effect was also obtained from thiosulphate. Measurements in the range 0.1-10 mg 1-1 of sulphite had relative standard deviations for single results of no more than 2%. The method requires only two reagents (dilute nitric acid solutions) and is simple to operate. Each analysis is complete in less than 5 min. Air-gap electrodes using the same sensor had sub-Nernstian responses of very poor reproducibility and were not considered to be a practical means of determining sulphite.Keywords ; Flow injection analysis ; air-gap electrode ; ion-selective electrode ; sulphite and sulphur dioxide determination ; mercury(II)sulphide - mercury(I) chloride membrane electrodes Ion-selective electrodes with mercury( 11) sulphide - mercury( I) chloride membranes are known to respond to sulphite in solution1 according to the reaction Hg,CI,,, + 2S03,- -+ Hg(SO,),,- + Hgo + 2C1- . . - - (1) Reaction (1) occurs at the surface of the electrode which is sensitive to chloride ions; hence the electrode's e.m.f. is determined by the amount of chloride released and therefore by the original concentration of sulphite. The use of an electrode for determining sulphite directly in aqueous samples is limited be-cause the electrode responds primarily to chloride which is very commonly present in such samples.In order to avoid chloride interference we have separated the electrode from the sample by a diffusive barrier across which the sulphur dioxide but not chloride or other ions, can pass. We have used two techniques the air-gap electrode2 and flow injection analysis In the air-gap electrode acidification of the sample releases sulphur dioxide which diffuses across a small air space and is absorbed in a film of chloride-free collecting solution on the surface of the membrane. In flow injection analysis small volumes of sulphite solution are injected into a continuously flowing carrier stream of nitric acid which passes on one side of a gas-permeable PTFE membrane. Sulphur dioxide volatilised by the low pH of the carrier stream diffuses across the PTFE membrane into a continuously flowing absorbent stream of less concentrated nitric acid.This absorbent stream then flows over the surface of a mercury( I) chloride - mercury( 11) sulphide membrane electrode. (FIA) .3 * To whom correspondence should be addressed 702 MARSHALL AND MIDGLEY POTENTIOMETRIC DETERMINATION Analyst VoZ. 108 Theoretical Mercury( I) chloride is sparingly soluble and for the reversible reaction Hg2C12+Hg22+ + 2C1- - ’ (2) K = [Hga2+][C1-I2 = 10-17-88 The dissolved mercury(1) ions disproportionate to mercury(0) and mercury( 11) as follows : Hg22++Hg0 + Hg2+ - * (3) Kd = [Hg”] [Hg2+]/[Hg2”] = 10-Sulphite forms a strong complex with mercury(I1) ions: Hg2+ + 2S03”+Hg(S0,),” * (4) /3 = [Hg(SO3),2-]/[Hg2+] [S032-]2 = The introduction of sulphite into a solution in which the mercury(1) chloride electrode is immersed reduces the concentration of free mercury(I1) ions and therefore forces equilibrium (3) and then equilibrium (2) to the right.The result is that two chloride ions dissolve from the electrode for each two sulphite ions added according to the over-all reaction (1). As the electrode responds to chloride ions its e.m.f. is a measure of the sulphite ion concentration, provided that reaction (1) is the only source of chloride ions apart from the solubility of mercury(1) chloride itself. The e.m.f. follows the usual Nernstian response : E = E” -Kl0g[SO32-] . . (5) where E” is the standard potential and k is the slope factor theoretically equal to RTln(lO)/F where R is the gas constant T is the temperature (K) and F is Faraday’s constant.It may be noted that the slope factor has the value expected for the singly charged chloride ion (-60 mV per ten-fold change in concentration) and not the 30 mV per ten-fold change in concentration that would be obtained with an electrode responding directly to a doubly charged ion such as sulphite. For operation of the electrode at low chloride (and therefore sulphite) concentrations the solution should be below pH 5.4 At pH 34 as used in this work most of the sulphite would be present as the hydrogen sulphite ion and the conditional stability constants /3* would be lOl6 and 10l8 at pH 3 and 4 respectively: when TBoz is the total concentration of dissolved sulphur dioxide.These constants are large enough to ensure that reaction (1) goes to the right and that equation (5) can be rewritten as where E* represents the apparent standard potential in the conditions concerned. Experimental Appa ratus Sensing electrodes The mercury(1) chloride electrodes used were SL-01 (Ionel Electrodes Ontario) and PHI 91100 (Graphic Controls London). The reference electrodes for the air-gap electrodes were 1370-230 mercury - mercury(1) sulphate electrodes with 1 mol 1-1 sodium sulphate filling solutions (Electronic Instruments Ltd., Chertsey). Those used in flow injection analysis were Radiometer K.601 mercury - mercury(1) sulphate electrodes with saturated potassium sulphate filling solutions June 1983 OF SULPHITE BY FLOW INJECTION ANALYSIS 703 E m .f. measurements Air-gap electrode e.m.f.s were measured with a Beckman 4500 digital pH meter reading to 0.1 mV and the time course of the e.m.f. was followed on a chart recorder. For flow injection analysis the e.m.f.s were read as peak heights from the chart recorder. Hole for brass swivel Injection port (centred over sample cup) Stud location holes 5-mm thick Perspex lid Reference electrode holder /---- l-mm step 5 mm mm Location stud Electrode holder 5 mm diameter hole (for sample cup) Channel cut to fit ceramic junction 10 mm diameter hole x 20 mm dee Diameter appropriate to electrode (lone1 1.0 cm) graphic contro 0.95 cm l+- 70 mmd to reference electrode Fig. 1. Inverted air-gap electrode. A ir-gap electrodes Air-gap electrodes were constructed by mounting the sensing and reference electrodes in suitably machined PTFE blocks.The first apparatus was similar to that used by RBiiEka and Hansen2 and because of the upright position of the sensing electrode it was difficult to achieve a thin stable film on the surface unless a high concentration of non-ionic detergent was added to the solution. With this design the sample was acidified before the electrodes were in place, which might have led to a variable loss of sulphur dioxide. This is subsequently described as the upright air-gap electrode. A modified apparatus was devised (Fig. 1) that held the electrode in an inverted position. With this arrangement the solution on the surface did not have to contain detergent unless very small volumes (<25 p1) of solution were used.It also had the advantage that acidification of the sample took place only when the apparatus was closed so that losses of sulphur dioxide were minimised. This is subsequently referred to as the inverted air-gap electrode. membrane flow-through cell valve Lay-out of FIA system. Fig. 2 704 MARSHALL AND MIDGLEY POTENTIOMETRIC DETERMINATION Analyst VoZ. 108 FZow injection apparatus The flow system (Fig. 2) including the injection valve the gas transfer block and the elec-trode holder was part of the Bifok FIA-05 apparatus (EDT Research London). The flow streams were pumped by a Gilson Minipuls 2 peristaltic pump. Reagents A stock solution (1 000 mg 1-1 of S032-) was prepared by dissolving 1.575 g of anhydrous sodium sulphite (BDH Chemicals AnalaR grade) in 500 ml of de-ionised water containing about 1 ml of glycerol.The volume was made up to 1 1 with de-ionised water. A working solution (10 mg 1-1 of SOS2-) was prepared by dilution of 5 ml of stock solution to 500 ml. About 1 ml of glycerol was added before the volume was made up to the mark. Further solutions were prepared as required by dilution of the working solution. Some solutions also contained 0.1 mol 1-1 of sodium tetrachloromercurate preservative [11.7 g 1-1 of sodium chloride + 27.2 g 1-1 of mercury(II)chloride]. The stock solution (1 mol 1-1) was prepared by diluting 31.5 ml of concentrated nitric acid (BDH Chemicals Aristar grade) to 500 ml. The carrier solution (0.1 moll-l) and the absorbent solutions and 10-4 moll-l) for flow injection analysis were prepared by successive dilution of the stock solution.Interfered soZzdions. Solutions containing 10-3 moll-1 of each of the interferents were prepared from BDH Chemicals AnalaR materials sodium acetate formic acid sodium carbonate sodium sulphide and sodium thiosulphate. Sodium sulphide was also tested at the and mol 1-1 levels. mol 1-1 nitric acid from dilution of the 1 mol 1-1 stock. Other components were sometimes added a non-ionic detergent (ICI Lissapol NX or BDH Nonidet P40) and mol 1-1 mercury(1) nitrate in Procedure Flow injection analysis The carrier sample and absorbent streams were pumped continuously at rates of 1 1 and 0.23 ml min-l respectively. When a steady base line had been reached a 30-pl slug of sample was injected into the carrier stream.The output of the pH meter acting as a unity gain amplifier was registered on the 100-mV scale (0-10 mg 1-1 sulphite solutions) or the 20-mV scale (0-1.5 mg 1-1 sulphite) of the chart recorder. When the base line was reached again, another slug of sample could be injected. The minimum time between samples was about 4 min. It is important to adjust the electrode holder properly if noise is to be minimised. The inlet should be positioned at the top edge of the indicator electrode so that the sample flows evenly over the whole sensing surface. The outlet should be positioned so that the level of spent sample in the sump of the holder does not reach the sensing surface of the indicator electrode yet remains in continuous contact with the solution flowing over the surface.The formation of large drops at the bottom of the indicator electrode should also be avoided. A ir-gap electrodes A volume of film solution (5400 pl) was applied to the surface of the sensing electrode by means of a syringe pipette care being taken to see that the film also covered the end of the frit leading to the reference electrode. The electrode was allowed to stand until a reading of -30 to -50 mV was obtained i.e. similar to the reading in a bulk solution free of chloride and sulphi t e . A 1-ml portion of sulphite solution was added to the sample container containing a magnetic stirrer bar but with the stirrer off. The procedure then varied according to the apparatus used. Sulphite solutions. Nitric acid solutions.Film solutions. Film solutions for the air-gap electrode contained mol 1-l nitric acid. The amounts added are described in the text as they occur. U$wight air-gap electrode When the electrode was reading in the range -30 to -50 mV 0.3 ml of 1 mol 1-1 nitric acid was added to the sample container the electrode holder was quickly placed on top of the sample container and the stirrer was started. The subsequent change in e.m.f. was recorded Jme 1983 O F SULPHITE BY FLOW INJECTION ANALYSIS 705 Inverted air-gap electrode The lid was placed on the electrode holder and when the electrode was reading in the range -30 to -50 mV 0.2 ml of 1 mol 1-1 nitric acid was injected into the sample cup by a self-filling syringe pipette. The stirrer was started and the subsequent change in e.m.f.was recorded. Results Preliminary Experiments Before use in FIA or air-gap electrodes the mercury(1) chloride electrodes were tested for their response in bulk solutions of sulphite ion. A 1000 mg 1-1 sulphite solution was added from an Agla micrometer syringe to 50 ml of 10-3 mol 1-1 nitric acid; the steady e.m.f. was recorded after each addition. Fig. 3 shows that the calibrations obtained with electrodes from the two manufacturers were very similar having Nernstian responses of 55-56 mV per ten-fold increase in concentration above about 1 mg 1-1 of sulphite. Deviations above the line in the region 0.2-1.0 mg l-l are typical of a loss of sulphite by oxidation or volatilisa-tion. Deviations below the line (<0.2 mg 1-l) are caused by chloride dissolved from the mercury(1) chloride in the electrode itself.The chloride calibration (moll-1) of the Ionel electrode is also shown; as predicted it coincides with the sulphite calibration. The response to each increment in sulphite concentration was complete in 1-2 min and the calibrations were reproducible to &2 mV. 0 E ui -100 Su I p hite concent rat ion/mg I - ’ 0.05 0.1 0.5 1.0 5.0 - 1 I I 1 1 10-7 10-6 10-5 I O - ~ Sulphite concentration/mol I- ’ Fig. 3. Calibration in bulk solution. 0 Graphic controls. Ionel 0 SOae-; and x C1-. Flow Injection Analysis Carrier solution The 1.0 mol 1-1 nitric acid was tried first but at this high concentration nitric acid diffused through the membrane and lowered the pH of the absorbent (10-4 mol 1-1 nitric acid absor-bent emerged at pH 3.5).Even a slug of de-ionised water was sufficient to give a signal, because interrupting the flow of carrier caused an increase in the pH at the electrode’s surface and hence a negative shift in e.m.f.4 With 0.1 mol 1-1 nitric acid carrier the problem did not occur and this concentration was used for all subsequent tests. Absorbent solution The peak heights obtained for 10 mg 1-1 sulphite in 10-4 and 10-3 mol 1-1 nitric acid absorb-ents were 67.6 & 0.5 and 68.3 & 1.5 mV respectively. The difference was negligible any increase in the efficiency of absorption of sulphur dioxide and formation of the mercury(I1) -sulphite complex at the higher pH presumably being balanced by the effect of the small but significant increase in the formation of hydroxo - mercury complexes 706 MARSHALL AND MIDGLEY POTENTIOMETRIC DETERMINATION Analyst VoZ.108 \ + E ui H 10 min + Time Fig. 4. FIA responses to 1-10 mg 1-1 of sulphite. 1.5 mg I-' I mg I-' -4 I C- Time Fig. 5. of sulphite. FIA responses to 0-1.5 mg 1-' Sensitivity The responses for 1-10 mg 1-1 of sulphite are shown in Fig. 4 and for 0-1.5 mg 1-1 of sulphite (on a five-fold expanded scale) in Fig. 5. When the peak heights are plotted against the logarithm of the concentration (Fig. 6) it can be seen that a linear response is obtained in the region 10-1.5 mg 1-l. At 60.5 mV per decade change in concentration the sensitivity is almost Nernstian at 25 "C. At concentrations below 1.5 mg 1-1 the response increasingly deviates from Nernstian linearity but plotting peak heights directly against concentration in this region gives a linear calibration with a correlation coefficient of more than 0.99 (Fig.7). The limit of Nernstian response for an electrode directly immersed in sulphite solution was about 0.1 mg 1-1 (Fig. 3) implying that flow injection analysis is less than 10% efficient in its use of the sulphite available in the sample solution. 20 I 1 0.1 0.5 2 10 Sulphite concentration/mg I-' 0 0.5 1.0 1.5 Sulphite concentration/mg I- ' Fig. 6. FIA calibration at concentrations up to 10 mg 1-' of sulphite 0. results from normal scale (Fig. 4) ; x results from expanded scale (Fig. 5). Fig. 7. FIA calibration at concentrations up to 1.6 mg 1-l sulphite.Precision Within-batch standard deviations were determined from four successive injections of each standard solution. For measurements on the 100-mV scale of the recorder the standard deviations for a single result were 0.5 mV for 2 5 and 10 mg 1-1 solutions and 0.3 mV a June 1983 OF SULPHITE BY FLOW INJECTION ANALYSIS 707 1 mg 1-l; these correspond to relative standard deviations in concentrations of less than 2%. For measurements made on the 20-mV recorder scale the standard deviations for single results were 0.1-0.2 mV for solutions in the range 0.1-1.5 mg l-l corresponding to relative standard deviations in concentration of 1 4 % . Interferences In order to interfere a substance must be capable both of crossing the PTFE membrane in the FIA apparatus and of reacting at the electrode’s surface either by forming a less soluble precipitate than mercury( I) chloride or by forming complexes with mercury(1) or mercury(I1) ions.In order to cross the membrane the potential interferent must exist in a volatile non-ionic form in the acidified sample stream; halides therefore do not reach the electrode and so cannot interfere. A negative interference could occur if a substance hindered the diffusion of sulphur dioxide across the membrane because it formed complexes with sulphite that were either thermodynamically or kinetically stable in the acid carrier. Solutions of mol 1-1 sodium acetate formic acid and sodium carbonate produced peaks of less than 0.3 mV with the recorder set to 20 mV full scale i.e. less than the blank readings shown in Fig.6. Sodium thiosulphate moll-1) produced a peak of 0.6 mV equivalent to 3.5 x 10-7 mol 1-1 (0.04 mg 1-l) sulphite. The only serious interference was sulphide as this can diffuse across the PTFE membrane as hydrogen sulphide and displace chloride from mercury(1) chloride as follows : A concentration of mol 1-1 of sulphide produced a peak of 2.5 mV equivalent to about 1.8 x loA6 mol 1-1 (0.2 mg 1-l) sulphite and 10-4 moll-1 sulphide gave a peak of 52 mV which is equivalent to 5 x mol 1-1 of sulphide produced a very large off-scale peak and the electrode’s response was subsequently very noisy. On inspection the surface of the membrane seemed to be tarnished and after it was polished the performance was restored. In the determination of sulphur dioxide in air the sulphur dioxide is often collected in a preservative solution of sodium tetrachloromercurate.The possible interferences of this solution were investigated in case the complexes formed by the preservative HgSO,Clnn- with n = 1 2 or 3 could not be decomposed by the acid. Sulphite solutions of concentration 10 mg 1-1 with and without tetrachloromercurate gave peaks of 65.4 & 1.1 mV and 64.6 & 1.0 mV respectively and hence no significant interference could be detected. This also con-firms that chloride does not interfere. mol 1-1 (5.6 mg 1-l) sulphite. A concentration of Response time As shown in Figs. 4 and 5 the peak was reached in less than 1 min after breakthrough but the wash-out was slower corresponding to the usual differences in rate of response for increases and decreases in c~ncentration.~~~ Sharper but smaller peaks were obtained by increasing the rates of flow of the carrier and absorbent streams.Air-gap electrodes Sensitivity All the variations of physical configuration and film composition in the air-gap electrode gave a fairly large response to sulphite (Fig. 8) but in most instances the response was unstable. Even where stable responses were reached in an acceptably short time (< 10 min) the repra-ducibility was poor and the sensitivity was less than the theoretical 60 mV per decade. The response took the shape of the two curves on the left of Fig. 8 when the film of solution on the sensing electrode had contracted so that part of the electroactive surface was directly exposed to the atmosphere inside the air-gap electrode; in the worst instances no plateau was observed.A more stable film was obtained if the proportion of detergent was increased but this had the effect of increasing the blank reading. Fig. 9 shows calibrations obtained from two successive applications of the same film solution (1 drop of Lissapol in 500 ml) to the same Tone1 electrode. The sensitivities of 35 mV per decad 708 MARSHALL AND MIDGLEY POTENTIOMETRIC DETERMINATION Analyst VoZ. 108 (A) and 40 mV per decade (B) are typical of the range found for all the variations of the air-gap electrode tested but lower than found for the response in bulk aqueous solutions of sulphite, even though the e.m.f.s are in the same range (compare Figs. 3 and 9). Time ___+ f A / Contracting Stable films films Fig.8. Response curves of the air-gap electrode. Curves are labelled with the sulphite concentration (mg 1-l). F indi-cates the renewal of the film solution. -130 > y: E E ui -150 -170 -190 1 2 5 10 Su I p hi te concent rat ion/mg I- ’ Fig. 9. Calibration of two suc-cessive assemblies (A and B) of an air-gap electrode. T shows the theoretical slope arbitrarily placed on the e.m.f. axis. Reproducibility Fig. 9 shows how much successive assemblies of an air-gap electrode can differ in the e.m.f. produced in response to identical samples. The differences between assemblies were much larger than were obtained for repeated presentation of identical sulphite solutions to the same assembly. In later tests with a 50% m/m Nonidet film solution the standard deviations of the e.m.f.s for 10 and 5 mg 1-1 solutions were 7.1 and 8.6 mV respectively; with a sensitivity of 40 mV per decade these standard deviations are equivalent to about 50% in concentration.Resportse time With film volumes of 25 pl or less equilibrium was reached in less than 10 min (Table I), i.e. the time taken for sulphur dioxide to be distributed between the sample and the film was longer than the response time of 1-2 min for the electrode itself. The acidified sample had to be stirred; with quiescent solutions the response was very slow presumably being limited by the diffusion of sulphur dioxide in water. TABLE I EFFECT OF FILM VOLUME Volume of filmlpl 100 60 25 E.m.f. for 10 mg 1-l SOa2-/mV .. . . -152 -157 -167 Time to equilibriumlmin . . 40 20 10 Efect of fdm volume The smaller the volume of the film applied to the sensing electrode surface the quicker the response of the electrode and the more negative the e.m.f. taken up (indicating an apparently higher sulphite concentration) ; Table I shows results for the Ionel electrode with films of 1% Nonidet + 10-5moll-1 of Hg,(NO& The sensitivity over the range 1-10mg1-1 varied between 30 and 45 mV per decade but could not be correlated with the film volume. The changes observed would (qualitatively) be expected for equilibrium distributions between a fixed source volume and a variable receiver volume but the film volume was observed to affect the e.m.f. even when no gas transfer occurred (in tests with chloride solution).With applications of 100-p1 volumes of 0.1 mg 1-1 Cl- solution to the electrode surface a steady reading could not be obtained the e.m.f. reaching a maximum of about -70 mV in 2 min before becoming more negative. With 200- and 400-pl applications steady e.m.f.s of abou June 1983 OF SULPHITE BY FLOW INJECTION ANALYSIS 709 -63 mV and -52 mV were obtained. The sensitivity of the electrode to 200-4 films of chloride solution was less than that in bulk solution the difference in e.m.f. being 41-43 mV compared with a range of 50-54 mV over the range 0.1-1.0 mg 1-1 of C1-. The results with chloride solutions show that using small volumes of solution is in itself sufficient to reduce the sensitivity of the electrodes by as much as 50% which closely parallels the difference between the sulphite response in solution and with air-gap electrodes.Detergent content of Elm solzttion In order to be able to achieve a stable covering of the electrode surface with a small (5-25 pl) volume of film solution a high detergent content was required (up to 50% m/m). As the pro-portion of detergent in the film solution increased the “blank” electrode potential i e . before absorption of any sulphur dioxide became more negative as in Table 11. The main cause of this shift was inferred to be a chloride impurity in the detergent; an e.m.f. of -60 mV in bulk solution is normally indicative of 0.1-0.2 mg 1-1 of chloride. Adding mercury(1) nitrate to the detergent solution should precipitate the chloride so that when an exactly equivalent amount is added the resultant solution should contain the same amount of chloride as a pure aqueous solution.The addition of 67 pl of mol 1-1 of Hg,(NO,) per gram of Nonidet was required to produce the same e.m.f. (-40 mV) in solutions containing 0 and 50% m/m Nonidet indicat-ing that the Nonidet contained about 50 pg g-l of chloride. This result is subject to error because the Nonidet may influence the e.m.f. not only through chloride contamination but by changing the dielectric constant and viscosity of the solution which would affect the activity coefficients and liquid-junction potential respectively. TABLE I1 EFFECT OF NONIDET CONCENTRATION Nonidet % / V / V . . . . 0 0.1 1 10 20 50* 50**t Blank e.m.f./mV . . -40 -43 -50 -60 -75 -150 -40 Sensitivity/mV per decade .. . 30-40 30 - 28 30-40 - 45 * Nonidet % mlm. t Nonidet including Hg,(NOJ,. Design of the air-gap electrode The design with the inverted-sensing electrode was easier to use as regards acidification of the sulphite solution and application of the film solution. It also enabled tests to be carried out with relatively large (>50 p1) volumes of film solutions. The inverted arrangement gave no better sensitivity or reproducibility than the original design. The inverted design had the advantage that the surface could be inspected without dismantling the assembly so that a slow response was not confused with the drift associated with exposure of the sensing surface (Fig. 7). No significant difference was observed between the performance of Ionel and Graphic Controls sensors in the air-gap electrode.Discussion Flow Injection Analysis Sensitivity The loss of sensitivity in flow-injection analysis compared with the equilibrium performance of the detector is not unexpected as the sulphite is diluted during the gas transfer and absorp-tion stages and the actual concentration at the electrode’s surface will be smaller than in the original sample. Interference Except for hydrogen sulphide commonly occurring volatile acids do not interfere even though they can cross the FIA membrane to some extent. Because the absorbent stream is itself acidic (pH 3) most of the substances absorbed will be protonated and only those capable of reacting very strongly with the electrode material will interfere.Pre-treatment of the sample solution with lead nitrate6 or bismuth nitrate‘ should remove sulphide interference 710 MARSHALL AND MIDGLEY POTENTIOMETRIC DETERMINATION Analyst VoZ. 108 The thiosulphate ion interferes with the mercury(1) chloride electrode.8 In the 0.1 mol 1-1 nitric acid carrier about 4% of the thiosulphate should be present as the uncharged thio-sulphuric acid which may be capable of diffusing into the absorbent stream and so causing the small interference observed. Decomposition of thiosulphate to sulphite is also possible, although the solution used was freshly prepared. The strongest metal - sulphite complexes are those formed with the mercury(1) ion and as these had no effect on the size of the peaks sulphito complexes of other metal ions would not be expected to interfere either.Alkaline substances present in such concentration as to raise the pH of the carrier stream and so reduce the extent of sulphur dioxide formation would cause a negative bias but approxi-mate neutralisation of the sample before analysis would prevent this occurring. Comparison with other methods Compared with direct use of the mercury( I) chloride electrode flow injection analysis avoids the problem of the commonest interferent chloride although at the cost of raising the lower limit of Nernstian response. Any substance that interferes in flow injection analysis would also interfere in the direct method probably to a greater extent. Potentiometric gas-sensing membrane electrodes based on the principle of measuring the pH change produced on absorption of sulphur dioxide in sodium hydrogen sulphite solution, show9 similar precision and limits of Nernstian response as found for flow injection analysis with the mercury(1) chloride electrode but are more susceptible to interference by acidic gases, hydrogen sulphide excepted.The commonest method of determining sulphite is probably the colorimetric method based on the fuchsin - formaldehyde - sulphurous acid complex. The relative standard deviation of measurements by this method are 1-3% and the limit of detection is about 0.015 mg 1-1 of sulphite.1° This method has also been used in flow injection analysis with a stopped-flow stage,3 but not below 5 mg 1-l. The loss of sensitivity in the flow injection variant of the pro-cedure is not surprising in view of the 30-min development time allowed in the conventional method.Flow injection analysis is therefore much more sensitive when using a mercury(1) chloride electrode and although it is less sensitive than the conventional colorimetric procedure, it avoids the development time. Flow injection analysis with potentiometric detection is more selective than instrumental techniques commonly used for determining sulphur dioxide in air12 (e.g. coulometry conductiv-ity and flame-emission spectrometry). Mercury displacement detectionlsJ4 is very selective and much more sensitive than flow injection analysis but lacks the adaptability of a general-purpose technique. The proposed method is more sensitive than results so far reportedl5~ls for ion chromatography but these did not seem to utilise the full sensitivity available from the ion chromatograph and the comparison may be misleading.Air- gap Electrodes The air-gap electrodes examined in this work cannot make full use of the response of the mercury(1) chloride electrode to sulphite ion. The critical factor is that in order to achieve an acceptably short response time (<lo min) the volume of film solution on the electrode’s membrane surface must be small (<25 pl). The use of such small volumes necessitates the inclusion of detergent to stabilise the film on the membrane but the chloride content of the detergents tested was high enough to interfere with the sulphite measurements ; adding mercury( I) nitrate to precipitate the chloride removed this interference.The sensitivity of the mercury(1) chloride electrode with films of small volume (G400 p1) is markedly less than when immersed in much larger volumes of solution. This is so even for dilute aqueous chloride solutions i e . neither the sulphite reaction nor the detergent is the main factor in this loss of sensitivity. Marshall and Midgley4ps found that in unstirred bulk solutions mercury(1) chloride electrodes responded slowly but the equilibrium e.m.f .s differed little from those in stirred solution. In this work the response time was longer with small volumes (<400 p1) of chloride film solution than in stirred bulk solution (5-10 min compared with about 2 min) but not as long as in unstirred bulk solution (20-30 min). Although the sensing electrodes gave reproducible e.m.f.s (&-2 mV) when directly immersed in bulk sulphite solutions and when in small volumes of chloride solution e.m.f.s in the air-ga Jzcne 1983 OF SULPHITE BY FLOW INJECTION ANALYSIS 71 1 mode were very variable (&8 mV) with a standard deviation equivalent to about 50% in concentration terms.With such poor reproducibility air-gap electrodes cannot be considered a practical means of determining sulphite. Conclusion Flow injection analysis using a mercury(1) chloride membrane electrode as detector is a use-ful alternative to the established methods of determining sulphite at concentrations down to 0.1 mg 1-1 of sulphite and is more sensitive than previously reported flow injection methods for sulphite. The air-gap electrode is not a suitable means of utilising the sensitivity of the mercury(1) chloride membrane electrode to sulphite.The authors acknowledge the help of Mr. C. Gatford with the experimental work. This work was carried out at the Central Electricity Research Laboratories and is published by permis-sion of the Central Electricity Generating Board. 1. 2. 3. 4. 6. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. References Tseng P. K. C. and Gutknecht W. F. Anal. Chem. 1976 48 1996. RbfiEka J. and Hansen E. H. Anal. Chim. Acta 1974 69 129. RbfiCka J. and Hansen E. H. “Flow Injection Analysis,” John Wiley Chichester 1981. Marshall G. B. and Midgley D. Analyst 1978 103 438. Marshall G. B. and Midgley D. Analyst 1979 104 55. Frant M. S. Ross J . W. and Riseman J . H. Anal. Chem. 1972 44 2227. Sekerka I. and Lechner J. F. Water Res. 1976 10 479. Midgley D. unpublished work. Midgley D. and Torrance K. “Potentiometric Water Analysis,” John Wiley Chichester 1978, Dimmock N. A. and Goodfellow G. I. unpublished work. Scaringelli F. P. Saltzman B. E. and Frey S. A. Anal. Chem. 1967 39 1709. Forrest J. and Newman L. J. Air Pollut. Control Assoc. 1973 23 761. Marshall G. B. and Midgley D. Anal. Chem. 1981 53 1760. Marshall G. B. and Midgley D. Anal. Chem. 1982 54 1490. Steiber R. and Statnick R. M. in Sawicki E. Mulik J . D. and Wittgenstein E. Editors “Ion Chromatographic Analysis of Environmental Pollutants,” Ann Arbor Science Ann Arbor 1978, p. 141. Frazier C. D. in Mulik J. D. and Sawicki E. Editors “Ion Chromatographic Analysis of Environ-mental Pollutants,” Volume 2 Ann Arbor Science Ann Arbor 1979 p. 211. Received November 29th 1982 Accepted January 14th 1983 p. 271

ISSN:0003-2654    

DOI:10.1039/AN9830800701

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

712 Analyst June 1983 "01. 108 pp. 712-716 Synthetic Inorganic lon-exchange Materials Part XXXII." Crystalline Antimonic(V) Acid as a Nitrate Ion-selective Electrode Studies on an Araldite-based Membrane of Sushma Agrawalf and Mitsuo Abe Department of Chemistry Faculty of Science Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku, Tokyo 152 Japan An Araldite-based membrane of crystalline antimonic(V) acid when acting as a nitrate ion-selective electrode shows a near-Nernstian response for concentra-tions of nitrate ions between 10-5 and 10-1 M and can be used for determining the activity of the ions. Stable potentials are observed within 10-30 s and for about 2min. The useful pH range is 3.5-11 at a higher concentration (5 x M) and 4.5-9 at a lower concentration (5 x 10-4 M) of nitrate ions.This membrane responds to nitrate ions in a solution containing 25% of non-aqueous solvent. Keywords Crystalline antimonic( V ) acid ; nitrate ion-selective electrode ; A raldite-based membrane ; potentiometry Inorganic ion exchangers can be used as ion sensors with an appropriate membrane and suit-able support matrix.l This is implied by the fact that the ion exchangers have fixed ionic groups in their structure ensuring the occurrence of a selective ion-exchange reaction. Liquid membrane nitrate ion-selective electrodes are now acceptable for making analytical and general chemical measurements and specifications of various nitrate ion-selective electrodes were reported by Davies et aZ.2 Various nitrate ion-selective electrodes have been based on an Orion liquid membrane (Model 92-07-2) together with wax-treated carbon powder,3 liquid nitron nitrate electrodes ,* quaternary ammonium compounds in non-porous polymer mem-branes5-7 and a solid-state membrane involving precipitated nitron nitrate.8 Crystalline antimonic(V) acid (C-SbA) behaves as a cation exchanger and shows specific selectivity for cations such as Na+ Cd2+ and Sr2+ with crystal ionic radii of about 0.1 nm.B Many chromatographic applications have been reported by using a relatively small column of this exchanger.1°-13 An attempt to extend the application of C-SbA in Araldite (a polyamide-type epoxy polymer) for use as an ion-selective electrode is described.Experimental Preparation of C- SbA The C-SbA was prepared as previously described.1° A 75-ml volume of antimony(V) chloride was preliminarily hydrolysed with 75 ml of cold water and then hydrolysed with 5 1 of de-mineralised water.The precipitate was kept in the mother solution at 40 "C for over 20 d and washed with cold de-mineralised water using a centrifuge (about 10000 rev. min-l) in order to free it from chloride ions. The precipitate was dried and ground into small grains of size under 325 mesh. Preparation of the Ion-selective Membrane The membrane was prepared by mixing homogeneously 0.7 g of the C-SbA in the hydrogen ion form with 0.3 g of Araldite spreading thinly on a piece of filter-paper and setting aside overnight. The hardened membrane was cut into a circular disc about 2.5 cm in diameter. The filter-paper was then removed with a razor blade and the membrane was equilibrated in a 0.1 M salt solution for about 15 d.The equilibration time can be shortened whenever a * For Part XXXI of this series see M. Abe M. Tsuji and M. Kimura Bull. Cham. SOC. Jpn. 1980 54, Present address Department of Chemistry State University of New York a t Buffalo Buffalo NY 130. 14214 USA AGRAWAL AND ABE 713 concentrated solution is used. The membrane was fixed with Araldite to one end of a Pyrex glass tube 8 cm in length and the tube was immersed in a 50-ml Corning beaker through a plastic lid. The electrode assembly consisted of the following cell: S.C.E. 1 test solution 11 membrane I 0.1 M NaNO 1 S.C.E. E.m.f. Determination Two sleeve saturated calomel electrodes (S.C.E.) were inserted into the solution and the e.m.f.was measured on a digital Toa Model HM-15A pH meter and on a Model R-02 recorder (Rikadenki Kogyo Ltd.). By using a magnetic stirrer all of the measurements could be taken at 25 & 0.1 "C. Between the measurements the electrodes were washed with water and dried with tissue-paper to prevent cross-contamination. The dried electrode was immersed in 0.1 M sodium nitrate solution for 2 h before each use. Supporting Material An epoxy resin first used by Coetzee and Basson14 proved to be the most suitable material. Polyamide Araldite (Araldite standard Ciba-Geigy Switzerland) was used as the supporting material for preparing the membrane. Reagents All of the reagents used were of analytical-reagent grade. Results and Discussion Prepared Membrane Homogeneous distribution of C-SbA in Araldite was confirmed by electron microscope observation.The prepared membrane was stable physically and chemically even in strong mineral acid and alkaline solution. The preparation and electrochemical properties of a number of inorganic ion-exchange membranes have been studied by Alberti and co-workers,l+l7 Coetzee,l Coetzee and Basson'* and Jain and co-workers.18-20 The membranes used were heteropolyacid salts,15J8 chromium hexacyanoferrate(II1) ,l9p2o zirconium phosphate and antimonate(V) ,15-17 with suitable supports. All of these showed a near-Nernstian response for cations. The C-SbA showed very high Kd values for sodium calcium strontium and cadmium ions and was very stable in an aqueous solution with a wide pH range.g For sodium and cadmium nitrate preliminary experiments showed a linear response to the concentration range of 10-5-10-1 M while the direction of potential change was opposite to that of the cations indicat-ing that the membrane was responding to the anion.The e.m.f. values of this membrane were independent of the presence of various cations such as Li+ Na+ K+ Rb+ Cs+ T1+ NH4+ Mg2+ Ca2+ Sr2+ Ba2+ Ni2+ Co2+ and Zn2+ at equivalent concentration. The response of the electrode was relatively fast and the potential reached a constant value within & 0.2 mV (Fig. 1). After 2 min the potential was increased very slowly at a rate of 0.04 mV s-l. When the membrane was prepared without C-SbA using Araldite only the potentials showed almost linear relationships with the concentrations of nitrate ions.Steady potentials were not obtained by immersing the membrane for a long time but were observed when the concentration of C-SbA was increased to more than 50% in the membrane (Fig. 1). Ion-selective electrodes based on ion exchangers involve ion-exchange processes at the electrode interface. The C-SbA is essentially a cation exchanger and has no adsorptive properties towards anions in the pH range ~tudied.~Jl Earlier reports indicate that the C-SbA has a pyrochlore structure and the lattice constant and diffraction intensities of the C-SbA in the H+ form change when H+ is replaced by various cations without any change in the crystal s y ~ t e r n . ~ l - ~ ~ Powdered X-ray analysis revealed that no change was observed on the membrane even when it was immersed in the solution of sodium nitrate or cadmium nitrate for a long time.This result indicates that no apparent cation-exchange reaction occurs on the membrane. It is known that fast conduction of protons occurs within the pyrochlore framework in C-SbA in the H+ form.23 On the other hand the commercial hard 7 14 60 50 40 > E % % . 0 a 50 40 30 AGRAWAL AND ABE SYNTHETIC INORGANIC Analyst Vol. 108 0 20 40 60 80 100 120 140 Response time/s 200 > E E 100 -. - m .-4-0 0 I I I I 6 4 2 0 -Log (activity of NO3) Fig. 1. Response time of the electrode with the membrane containing C-SbA at different concentra-tions A 0% ; B 30%; and C 50-70%. Solution, 0.01 M NaNO,. ener of the Araldite contains polyamide.The functional group of =N= may remain in the polymer after polymerisation and can behave as an anion exchanger. The response on this membrane may be due to the anion-exchange contribution of the polymer and the C-SbA acts as an electro-conductive material. It is known that a small amount of liquid ion exchanger leaks from the commercial liquid membrane and epoxy membrane containing liquid e~changer.~ The prepared membrane gives no leakage of the ion-exchange material. The potentials obtained were plotted against -log(activity of NO3-) (Fig. 2). The activity coefficient for a single nitrate ion was calculated by using the extended Debye-Huckel equation : where p is the ionic strength of the solution. For sodium nitrate the potential response was linear in the range 10-5-10-1 M with a near-Nernstian slope of 56 mV per decade of the activity.However the change in the potential is sufficiently large to permit the determination of the concentration of NO3-. The response of the membrane compares favourably with that of the commercial nitrate ion-selective electrode2 and is much better than any other fabricated nitrate ion-selective electrode. 2-* Fig. 2. Response of the electrode to nitrate ions contained in various solutions. 0 Activity in water; A 10 and 25% ethanol solution; and a 10 and 25% acetone solution. + Logf = -0.15 r-lf/(l + /A+) It deviated from the linear graph in the range lO-'-lO-5 M. Effect of pH The effect of pH on the determination of NO3- was investigated in the pH range 3-13 with a solution of acetic acid and sodium hydroxide.The useful pH ranges were 3.6-1 1 and 4.5-9 for higher (5 x M) and lower (5 x lo-* M) concentrations of nitrate ions respectively (Fig. 3). Selectivity Coefficients The mixed-solution method was employed for studying the response of the membrane to common anions with the solution containing between 10-2 and 10-3 B ions in sodium nitrate solution of different concentrations and the conventional separate-solution r n e t h ~ d ~ ~ ~ * ~ was used for a large variety of the cations and anions at M concentration. Sodium salts were used for anions and chloride salts for cations. The selectivity coefficients (Kg6J-,B) of the separate-solution method were calculated by the following equation June 1983 ION-EXCHANGE MATERIALS.PART XXXII 715 TABLE I SELECTIVITY COEFFICIENTS (kg$; B) FOR THE C-SBA MEMBRANE ELECTRODE f Interfering ion F- . . c1- . . Br- . . I- 10,- . . NO,- . . HSO,-HS0,-H,P04-CH,COO-ClO,- . . * . k d - B I A* B t Interfering ion 0. lob 0.18 SO,e- 0.31a 0.30 0.34a 0.57 C032- . . 0.55a 1.38 W0,2- . . 0.08 MOO,,- . . 0.66 B40,,- . . 0.26 %Fez- . . 0.42b 0.52 S20,2- . . 0.082b 0.15 HASO,&- . . . . 0.058b 0.09 OH- 0.75a kPOt NOj- B - A* Bt 0.051 0.033a 0.047 0.015b 0.012 0.012 0.035 0.017 0.068 0.11 0.042 0.62 * A Mixed-solution method t B Separate-solution method M B ions; b lo- M B ions. M B ions). where E is the potential a is activity z is the charge of ion B and S is the slope of the calibra-tion graph.The k 6 - values are summarised in Table I. The membrane is selective for nitrate ions with respedt to many anions except I- for which the response is more marked than for NO3-. Interferences such as those by I- are known for many nitrate ion-selective elec-trodes.2-8 The interference of I- on this membrane is less than for other electrodes. The value for C1- remains unchanged but that of I- decreased considerably. The selectivity co-efficients for B r and SO,"- decrease to lesser extents and the value for C10 was less than 1. These results indicate that this electrode can determine NO3- concentration in the presence of B r ClO and SO:- which is a great advantage over other electrodes (the &A3- values were higher than 1).Similar selectivity and response are found for ion-selective elecirodes for anions involving an epoxy coated wire matrix membrane.27 E.m.f. Titrations The membrane can be used as an end-point indicator electrode for the potentiometric titration of nitrate ions with nitron. The e.m.f. titration curve is demonstrated in Fig. 4. The curve has almost classical shape which shows that the electrode is more specific to nitrate than to any other anion. The end-point is not very sharp because of the high solubility of the nitron nitrate precipitate which is reported to be 1.3 x M of nitrate ions.28 120 > E % 100 .- ta a to a 80 2 4 6 8 1 0 1 2 PH Fig. 3. Effect of pH on the electrode potential with two differ-ent concentrations of NO3= (A) 5 x 1 0 - a ~ ~ ; and (B) 5 x 1 0 - 4 ~ .> 100 E . -(D al 0 .- to ta 0 80 60 Volume of nitron added/ml 1.0 2.0 3.0 4.0 Fig 4. Potentiometric titration graph of 0.02 M NaNO (20 ml) with 0.2 M nitron 716 AGRAWAL AND ABE Application to the Determination of Nitrate Ion Concentration in Non-aqueous Solution The membrane can be applied to the determination of the concentration of nitrate ions in a medium containing non-aqueous liquid. The potentials are plotted against log (concentration) in Fig. 1. The working curve for the non-aqueous medium was tested for solutions of ethanol and acetone at different concentrations. The stability of the response decreased in 25% non-aqueous solution which contained nitrate ions at concentrations lower than lov4 M.The working curve in the 10% non-aqueous solution is the same as that in the aqueous solution. The membrane responded well to nitrate ions at a concentration of up to low4 M although the drift increased in 50% non-aqueous solution. Life of the Membrane The membrane can be used for a period of 2 months without any significant change in the potentials while the linear range and the slope (-2 mV) decreased slightly after continuous use for about 4 months. The absolute value of the response varies from one preparation to another by a few millivolts. A similar conclusion was found by La1 and Christian,29 but a solid membrane electrode is preferable over a liquid electrode for many reasons as reported by Rechnitz3* Conclusion The Araldite-based membrane of C-SbA can be fruitfully used for the determination of the nitrate ion concentration in aqueous and non-aqueous solution and especially as an indicator electrode in potentiometric titration.The authors express their thanks to Mr. Nobuyuki Hayashi for his help with some of the experiments. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. References Coetzee C. J. Ion Sel. Electrode Rev. 1981 3 105. Davies J. E. W. Moody G. J. and Thomas J. D. R. Analyst 1972 97 87. Qureshi G. A. and Lindquist J. Anal. Chim. Acta 1973 67 243. Tateda A. and Murakami H. Bull. Chem. SOC. Jpn. 1974 47 2885. Nielsen H. J. and Hansen E.H. Anal. Chim. Acta 1976 85 1. Dobbelstein T. N. and Diehl H. Talanta 1969 16 1341. Hulanicki A. Maj-zurawska M. and Lewandowski R. Anal. Chim. Acta 1978 98 151. Lal U. S. Chattopadhyaya M. C. and Dey A. K. Mikrochim. Acta 1980 11 417. Abe M. Denki Kagaku 1980 48 344. Abe M. and Ito T. Bull. Chem. SOC. Jpn. 1968 41 333. Abe M. and Ito T. Bull. Chem. Soc. Jpn. 1967 40 1013. Abe M. and Uno K. Sep. Sci. Technol 1979 14 355. Abe M. and Kasai K. Sep. Sci. Technol. 1979 14 895. Coetzee C. J. and Basson A. J. Anal. Chim. Acta 1970 56 321. Alberti G. Cont A. and Torracca E. Atti Accad. Nazl. Lincei Rend. Classe Sci. Fis. Mat. Nut., Alberti G. Costantino U. and Zsinka L. J . Inorg. Nucl. Chem. 1972 34 3549. Alberti G. Costantino U. and Gupta J. P. J. Inorg. Nucl.Chem. 1974 36 2103. Srivastava S. K. Jain A. K. Agrawal S. and Singh R. P. J . Electroanal. Chem. 1978 90 291. Jain A. K. Srivastava S. K. Agrawal S. and Singh R. P. Talanta 1978 25 531. Jain A. K. Agrawal S. and Singh R. P. J . Indian Chem. Soc. 1980 57 343. Abe M. J . Inorg. Nucl. Chem. 1979 41 85. Abe M. and Sudoh K. J . Inorg. Nucl. Chem. 1980 42 1051. Abe M. in Clearfield A. Editor “Inorganic Ion Exchange Materials,” CRC Press Cleveland OH, England W. A. Cross M. G. Hamnett A. Wiseman P. J. and Goodennough J. B. Solid State Ammann D. Pretsch E. and Simon W. Anal. Lett. 1972 5 843. Levins J. Anal. Chem. 1972 44 1544. Suzuki K. Ishiwada H. Shirai T. and Yanagisawa S. Bunseki Kagaku 1981 30 751. Hulanicki A. and Maj M. Talanta 1975 22 767. Lal S. and Christian G. D. Anal. Chem. 1971 43 410. Rechnitz G. A. Chem. Eng. News June 12 1967 146. 1963 35 548. 1982 Chapter 6 p. 232. Ionics 1980 1 231. Received July 20th 1982 Accepted January 12th 198

ISSN:0003-2654    

DOI:10.1039/AN9830800712

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst June 1983 Vol. 108 $9. 717-721 Use of an Argon - Nitrogen Inductively Coupled 717 Plasma for the Analysis of Aluminium Alloys Subsequent to Alkali Dissolution* Jose A. C. Broekaert and Franz Leis Institut fur Spektrochemie und angewandte Spektroskopie Postfach 778,0-4600 Dortmund 1 Federal Republic of Germany and Gungor Dingler Grundenstrasse 65 CH-8247 Flurlingen Switzerland The determination of a series of elements (boron copper gallium iron, magnesium silicon vanadium and zinc) in aluminium samples by inductively coupled plasma optical emission spectroscopy is reported. High-purity aluminium as well as various types of aluminium alloys (A1 - Cu A1 - Mg, A1 - Mg - Si A1 - Si etc.) were brought into solution to give an analyte con-centration of 0.125% m / V with an alkali dissolution procedure.The detection limits for the mentioned elements range from 5 to 150 pg g-l. Both trace elements and major constituents can be determined in the types of aluminium alloys mentioned by using the same calibration graphs. Keywords Alkali dissolution ; aluminium analysis ; argon - nitrogen induc-tively coupled plaswaa ; optical emission spectroscopy The analysis of various types of aluminium samples in the solid state with the aid of X-ray spectroscopic methods or conventional spark optical emission spectrometry generally requires extensive calibration. For off-line analyses sample dissolution and subsequent determinations by flame atomic-absorption spectroscopy are appropriate ; however they may suffer from dynamic range limitations and insufficient power of detection for some important elements (e.g.boron silicon and vanadium). Inductively coupled plasma optical emission spectroscopy (ICP-OES) in our view is a worthy alternative that opens the possibility of simultaneous multi-element analyses. Firstly it enables high-purity aluminium as well as a wide variety of aluminium alloys to be brought into solution; and secondly for alloys with a high silicon concentration it provides a considerable gain in time compared with acid dissolution including the use of fluoric acid as described in ref. 2. According to Greenfield et aL3 (and as applied in this work) the use of a high-power argon -nitrogen ICP may be advantageous because large amounts of sodium are introduced into the solutions when using an alkali dissolution procedure.With this type of ICP a high pressure of aerosol gas (up to 7 bar) can be used which lowers nebulisation effects and the risk of nebuliser blocking in the analysis of solutions with high salt contents. The procedure for the alkali dissolution of aluminium samples described here introduces as little analyte dilution as possible covers a wide variety of alloys and keeps the salt concentration at a tolerable level. Sequential multi-element determinations demonstrate the capabilities of the argon - nitrogen ICP for the trace element analysis of aluminium. Aqalyses were performed over a wide concentration range for various types of aluminium alloy standard sample. Its use for the analysis of aluminium was investigated in this work.An alkali dissolution procedure1 was applied for two reasons. Experiment a1 Instrumentation An argon - nitrogen ICP powered by a free running radiofrequency generator was used. The working coil is an integral part of the oscillator circuit and the output power is stabilised (to within 0.2y0) by feed-back from the radiofrequency stray field which is measured in the * Part of a paper presented at Euroanalysis IV Helsinki August 23rd-28th 1981 718 BROEKAERT et al. ARGON - NITROGEN INDUCTIVELY COUPLED Analyst Vol. 108 vicinity of the coil.* The sample solutions #were freely aspirated using a concentric glass nebuliser (Meinhard ASS.),^ mounted in a glass nebulisation chamber according to Scott et aLs A 0.9-m microcomputer controlled Czerny - Turner monochromator with photoelectric measurement equipment was used for the sequential multi-element analyses.Instrumental details and operational parameters are listed in Table I. TABLE I INSTRUMENTATION ICP-Radiofrequency generator . . FS-10 (Linn supplied by Kontron GmbH); frequency 27.12 MHz (free running); and maximum output power 10 k W with power stabilisation Burner . . . . Three gas flow burner3; outer gas flow 251min-' of nitrogen; Nebuliser . . . . Meinhard Ass. Type B 11; with free aspiration of samples; argon Nebulisation chamber . . . . Dual-wall glass spray chamber Illumination . . . . Three-lens system; and observation zone 4 x 4 mm selected at the intermediate image Spectral apparatus . . . . 0.9-m Microcomputer-controlled Czerny - Turner monochromator; grating 90 x 90 mm a = 1/2400 mm; entrance slit width, 17 pm; exit slit width 25 pm; thermostatically controlled at 30 f 0.1 "C; photomultiplier 9789 QB; quartz refractor plate for background measurements in front of the exit slit4; and microprocessor Intel 80/20 and intermediate gas flow 8 1 min-l of argon flow 2 1 min-l; and argon pressure 4 bar Sample Dissolution Procedure An amount (0.5 g) of sample drillings is transferred into a platinum dish and 15 ml of sodium hydroxide solution (16.5% m/V) prepared from pellets (Merck No.6498) are added. For all types of aluminium alloys investigated (high-purity aluminium A1 - Cu A1 - Mg A1 - Mg - Si, A1 - Si) analyte masses of up to 0.5 g can be completely dissolved. Subsequently the mixture is evaporated to dryness 5 x 1 ml of concentrated nitric acid (Suprapur Merck) are added and after the vigorous reaction a further 10 ml are then added.Aluminium hydroxide is precipi-tated but is re-dissolved after adding 50 ml of distilled water and warming for a few minutes. The clear sample solution is transferred into 100-ml flasks made up to the mark and subse-quently diluted 1 + 3 with distilled water and stored in polyethylene bottles. Using this procedure pure aluminium and all investigated aluminium alloys are rapidly dissolved and their solutions remain stable for several weeks. Analyte solutions contain 0.125% m/V alloy and 0.35% m/V sodium and can be nebulised with a Meinhard nebuliser without the risk of clogging. At these analyte concentrations salt depositions at the tip of the ICP burner did not occur.Results and Discussion Analytical Lines and Detection Limits All measurements were made under working conditions giving optimum line to background intensity ratios. As investigated earlier* they are as follows operating power (defined as the product of plate voltage and current of the radiofrequency generator admitting an efficiency of 0.7) 3 kW; observation zone (4 x 4 mm) located at 4-8 mm above the coil; and aerosol gas pressure 3 4 bar (aerosol gas flow 1-2 1 min-l). The latter were determined according to Kaiser and Specker,lo and were calculated using the equat ionll The selected analytical lines and the obtained detection limits are given in Table 11 June 1983 PLASMA FOR THE ANALYSIS OF ALUMINIUM ALLOYS 719 TABLE I1 DETECTION LIMITS A 3-kW argon - nitrogen ICP was used and the analyte concentration was 0.125%.Aluminium solid samples, Element linelnm % mlm B I 249.7 2.4 x 10-4 Cr I1 286.3 . . 5.5 x 10-4 Cu I 324.8 1.7 x 10-3 Fe I1 259.9 . . 8.2 x 10-4 Mg I1 279.6 . . 2.8 x 10-4 Mn I1 257.6 . . 6.1 x 10-4 Si I 251.6 8.9 x 10-3 V I1 292.4 1.1 x 10-3 Zn I 213.8 4.3 x 10-3 Ga I 294.4 1.5 x Aluminium samples in solution/ng ml-l 3 7 21 51 200 3 8 110 14 53 Aqueous solutions9/ ng ml-l 2 17 37 5 1 2 ---40 where cL is the detection limit c is the background equivalent concentration IB is the blank signal Iu the background intensity and ar(IB + I,) is the relative standard deviation for a series of blank samples.In this work the true background intensity I is determined from the signals measured in the vicinity of the analytical line (as this is possible with the quartz refractor plate technique’). Blank contributions which arise from the reagents used could not always be measured owing to a lack of aluminium samples in which the elements to be determined are completely absent. In this instance they are calculated from the intensity signals for samples in which the concentrations of the respective elements are low. The detection limits referring to the solutions are higher than the values in pure aqueous solutions for iron manganese and magnesium. This may relate to ionisation interferences and electron density changes at high alkali concentrations.12 The lower value for copper may be due to the same reasons.The large difference for iron may arise from the blank contribu-tions. Analysis of Pure and Alloyed Aluminium Samples Determinations of iron and silicon in aluminium (Fig. 1) and of copper in aluminium (Fig. 2) show that widely varying concentrations of third elements (e.g. for the samples in Fig. 1, magnesium concentrations vary from 0.0002 to 2% m/m) do not cause matrix effects; and i+! r‘ 0.5 !!? 0 .- c CI al C u 0 0 2000 0 2 000 Intensity arbitrary units Fig. 1. Determination of (a) iron and (b) silicon in various aluminium alloys. Samples were as follows: <0.2%. mlm; and A1 - Mg (515 525) CMg >2% m/m) (Schweizerische Aluminium AG) . Alkali sample dissolution 0.125% m/ V . Argon - nitrogen ICP 3 kW and Fe(I1) 259.9 nm and Si(1) 251.6 nm lines.A1 (113 114 115 132 134 135 141 142 144) c ~ g 1000 2000 3000 Intensity arbitrary units Fig. 2. Determination of copper in alu-minium alloys. Concentrations (%) referr-ing to theholid samples. Samples were as follows A1 - Mg - Si (614 61G) ; A1 - Mg -Si - Ni (636); A1 - Si (414 416); A1 - Si -Cu - Ni (431 433) (Schweizerische Alu-minium AG). Alkali sample dissolution a 0.0250,b m/V and 0 0.125% m / V . -Argon - nitrogen ICP 3 kW; and a Cu(1) 324.8 nm line 720 BROEKAERT et al. ARGON - NITROGEN INDUCTIVELY COUPLED Analyst Vol. 108 calibration graphs are linear over at least two decades of concentrations. In this work, limitations arose from the dynamic range of the measurement system. It is also shown that a gain in the dynamic concentration range can be obtained by varying the amount of analyte.For example for copper (Fig. 2) samples with low copper contents were dissolved at a con-centration of 1.25 g 1-' and high concentration alloyed A1 - Cu samples at 0.25 g 1-1 and, despite the different concentrations of aluminium in the solutions the same calibration graphs could be used. The method was applied to the analysis of a series of aluminium samples including high-purity aluminium A1 - Mg A1 - Mg - Si A1 - Mg - Si - Mn A1 - Si - Cu - Ni and Unialloy types. For the calibration a series of aluminium standard samples (Table 111) was used. For these calibration samples and for a series of samples to be analysed three replicate measurements were made.The determined concentrations the standard deviations of the estimate and the certified values of the analysed samples are given in Table IV. TABLE I11 CALIBRATION SAMPLES USED FOR THE ANALYSIS OF ALUMINIUM BY ICP-OES All samples Schweizerische Aluminium AG; the results are in Boron . . 0.022 . . 0.007 . . 0.015 ------Chromium Copper 0.0005 -- 0.003 0.021 -- 0.003 - 0.02 0.02 -- -0.2 -- -Gallium Iron 0.013 0.16 - 0.24 - 0.40 - 0.54 - 0.72 - 0.005 5 0.035 -- -Mag- Man-nesium ganese - 0.003 5 - 0.0013 - 0.021 - 0.037 1.93 -- -- -0.1 -- -Sample A1 114 A1 132 A1 133 All34 A1 141 A1 142 A1 144 Unialloy iO7/4 Al-Si414 A1 - Si 416 A1 - Si - Cu - Ni 442 A1 - Mg - Si 614 .. A1 - Mg - Si 616 . . A1 - Mg - Si - Mn 636 Al-Mg515 Silicon 0.096 0.15 0.41 0.52 0.65 ---12.2 13.5 7.9 0.05 0.6 1.2 1.4 Vanadium -0.008 - - -0.045 0.036 -Zinc 0.018 0.014 0.079 0.055 ----- -0.006 -TABLE IV ANALYSIS OF ALUMINIUM BY ICP-OES SUBSEQUENT TO ALKALI DISSOLUTION Results in %' m/m. Certified concentration values (Sck-.veIzerische Aluminium AG) are given in parentheses. Mag- Man-Sample Boron Chromium Copper Gallium Iron nesium ganese Silicon Vanadium Zinc A1115 A1135 All42 Unialloy 207/2 . . Unialloy 207/3 . . A1 - Si - Cu - Ni 431 AI-Mg 511 Al-Mg 525 A1 - Mg - Si 611 . . A1 - Mg - Si - Mn 6:3l A1 - Mg - Si - Mn 633 0.0028 0.002 9 f 0.001 1 (0.0028) (0.004) 0.0047 0.021 f 0.010 f 0.001 (0.0045) (0.02) - 0.039 f 0.001 (0.04) f 0.001 4 - -0.003 9 f 0.0001 (0.0038) -0.002 4 -f 0.0008 (0.0025) 0.0060 -f 0.0008 (0.005 6) 0.038 0.496 f 0.001 * 0.051 (0.37) (0.52) - -0.001 8 -f 0.0005 (0.002 3) 0.0051 0.053 f 0.0005 f 0.003 (0.004) (0.05) - 0.078 f 0.004 (0.079) - -.. 0.011 f 0.002 (0.011) -0.306 f 0.015 (0.31) - 0.517 f 0.015 (0.54) - - 1.58 0.06 (1.61) 1.95 rt 0.05 (1.98) - 7.62 f 0.72 (7.9) - 0.031 - . - . ~ ~ f 0.003 (0.026) - 0.187 f 0.0005 10.181 - 0.062 f 0.001 (0.055) - -0.023 0.287 f 0.006 f 0.014 (0.026) (0.28) - -2.78 f 0.05 (2.87) -- 0.033 f 0.002 (0.033) - - - 0.309 f 0.030 (0.3) - 0.520 f 0.030 (0.53) - 0.890 f 0.030 (0.98 J m e 1983 PLASMA FOR THE ANALYSIS OF ALUMINIUM ALLOYS 721 A standard deviation of the estimate [s(cx)] is calculated from the measured intensities and the concentrations according to Nalimov12 : where cx is the mean of m replicates of the unknown sample as calculated from the regression equation cX = BIx which is obtained from the intensities I measured for n standards with concentrations cl.B is the slope of the calibration graph. The term Z1 is the mean of the c values for all standards and s ( 1 ) is the standard deviation over the regression that follows: Ii are the measured intensities and Ii the intensities for the standard samples calculated from equation (3). The experimental values agree reasonably with certified values.The error of the complete analytical procedure relating to a l-s confidence level is lower than 574 provided that the concentrations being determined are two orders of magnitude above the detection limit which is so for most copper iron magnesium and silicon and some manganese and zinc values. The analytical precision reported as shown by the concentrations of the calibration samples used, can be realised in a range of two decades of concentrations. For the analysis of aluminium alloys with silicon concentrations above 5% m/m an analyte concentration of 0.25 g 1-1 was taken. However the same calibration as for samples with lower silicon concentrations but analyte concentrations of 1.25 g l-l could be used. Conclusion The results show that the use of an argon - nitrogen ICP for alkali dissolution permits trace However the method is limited in some instances by A linear dynamic range of more than two orders of magnitude and low element analysis in aluminium samples.blank contributions. matrix effects make the method attractive for the analysis of random samples. The authors thank Mrs. W. Bartusch for contributing to the experimental work. The standard aluminium samples were kindly supplied by Schweizerische Aluminium AG. The work was supported by the Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen and by the Bundesministerium fur Forschung und Technologie. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Wilson L. Anal. Chim. Acta 1968 40 603. Bell G. F.At. Absorpt. Newsl. 1966 5 73. Greenfield S. Jones I. Ll. and Berry C. T. Analyst 1964 89 713. Dorn G. in Barnes R. M. Editor “Developments in Atomic Plasma Spectrochemical Analysis, Proceedings of the International Winter Conference 1980 San Juan Puerto Rico,” Heyden, Philadelphia 1981 p. 369. Meinhard J. E. ICP Inf. Newsl. 1976 2 163. Scott R. H. Fassel V. A, Kniseley R. N. and Nixon D. E. Anal. Chem. 1974. 46 75. Nordmeyer N. 2. Anal. Chem. 1967 225 247. Broekaert J. A. C. Leis F. and Laqua K. in Barnes R. M. Editor “Developments in Atomic Plasma Spectrochemical Analysis Proceedings of the International Winter Conference 1980 San Juan Puerto Rico,” Heyden Philadelphia 1981 p. 84. Aziz A. Broekaert J. A. C. and Leis F. Spectrochim. Ada Part B. 1982 37 369. Kaiser H. and Specker H. 2. Anal. Chem. 1956 149 46. Aziz A. Broekaert J. A. C. and Leis F. Spectrochim. Ada Part B 1981 36 261. Kalnicky D. J . Fassel V. A. and Kniseley R. N. Appl. Spectrosc. 1977 31 369. Nalimov V. V. “The Application of Mathematical Statistics to Chemical Analysis,” Pergamon Received August 18th 1982 Accepted January 14th 1982 Press Oxford 1963

ISSN:0003-2654    

DOI:10.1039/AN9830800717

出版商:RSC    

年代:1983

数据来源: RSC