摘要:

THE ANALYSTTHE ANALYTICAL JOURNAL OF THE CHEMICAL SOCIETYEDITORIAL ADVISORY BOARD*Chairman: J. M. Ottaway (Glasgow)R. Belcher (Birmingham)L. J. Bellamy, C.B.E. (Waltham Abbey)L. S. Birks (U.S.A.)E. Bishop (Exeter)L. R. P. Butler (South Africa)E. A. M. F. Dahmen (The Netherlands)A. C. Docherty (Billingham)D. Dyrssen (Sweden)W. T. Elwell (Birmingham)J. Hoste (Belgium)H. M. N. H. Irving (Leeds)M. T. Kelley (U.S.A.)W. Kernula (Poland)'J. H. Knox (Edinburgh)G. W. C. Milner (Harwell)"H. J. Cluley (Wembley)"P. Gray (Leeds)G. H. Morrison (U.S.A.)H. W. Nurnberg (West Germany)E. Pungor (Hungary)D. 1. Rees (London)"R. Sawyer (London)P. H. Scholes (Sheffield)'W. H. C. Shaw (Greenford)S. Siggia (U.S.A.)"D. Simpson (Thorpe-le-Soken)A. A.Smales, O.B.E. (Harwell)*A. Townshend (Birmingham)A. Walsh (Australia)T. S. West (Aberdeen)"J. Whitehead (Stockton-on- Tees)A. L. Wilson (Medmenham)P. Zuman (U.S.A.)'G. E. Penketh (Billingham)"Members of the Board serving on The Analyst Publications CommitteeREGIONAL ADVISORY EDITORSDr. J. Aggett, Department of Chemistry, University of Auckland, Private Bag, Auckland, NEW ZEALAND.Professor G. Ghersini. Laboratori CISE, Casella Postale 3986,201 00 Milano, ITALY.Professor L. Gierst. Universit6 Libre de Bruxelles, Facult6 des Sciences, Avenue F.-D. Roosevelt 50,Professor R. Herrmann, Abteilung fur Med. Phy!;ik., 63 Giessen, Schlangenzahl 29, W. GERMANY.Professor W. A. E. McBryde. Faculty of Science. llniversity of Waterloo, Waterloo, Ontario, CANADA.Dr.W. Wayne Meinke. KMS Fusion Inc.. 3941 Research Park Drive, P.O. Box 1567, Ann Arbor,Dr. 1. Rubeika, Geological Survey of Czechoslovakia, Kostelni 26, Praha 7, CZECHOSLOVAKIA.Dr. J. RfiSiEka, Chemistry Department A, Technical University of Denmark, 2800 Lyngby, DENMARK.Professor K. Saito, Department of Chemistry, Tohoku University, Sendai, JAPAN.Dr. A. Strasheim. National Physical Research Laboratory, P.O. Box 395, Pretoria, SOUTH AFRICA.Bruxelles, BELGIUM.Mich. 48106, U.S.A.Published by The Chemical SocietyEditorial: The Director of Publications, The Chemical Society, Burlington House,London, WIV OBN. Telephone 01 -734 9864. Telex No. 268001Advertisements: Advertisement Department, The Chemical Society, Burlington House, Piccadilly,London, WIV OBN. Telephone 01 -734 9864Subscriptions (nonmembers): The Chemical Society, Distribution Centre, Blackhorse Road,Letchworth, Httrts., SG6 1 HNVolume 104 No 1234 January 19790 The Chemical Society 197

ISSN:0003-2654    

DOI:10.1039/AN97904FX001

出版商:RSC    

年代:1979

数据来源: RSC

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ANALAO 104 (I 234) 1-96 (1 979)ISSN 0003-2654January 1979THE ANALYSTTHE ANALYTICAL JOURNAL OF THE CHEMICAL SOCIETYCON'I'ENTS1 REVIEW. Extractions and Separatioms w i t h Foamed Plastics and Rubbers-G. J. Moody and J. D. R. Thomas16 Evaluation o f a Method f o r Determination o f Total Antimony, Arsenic and Tin inFoodstuffs Using Measurement by Atomic-absorption Spectrophotometrywith Atomisation in a Silica Tube Using the Hydride Generation Technique-W. H. Evans, F. J. Jackson and Dorothy Dellar35 Determination o f Germanium, Arsenic, Selenium, Tin and Antimony in ComplexSamples by Hydride Generation - M icrowave-induced Plasma Atomic-emission Spectrometry-Wayne El. Robbins, Joseph A. Caruso and Fred L. Fricke41 Determination o f Micro-amounts of Polythionates.Part XI. Spectrophoto-metric Determination o f Two Species o f Polythionates in Their Mixtures byCyanolysis and Solvent Extractialn-Tomozo Koh, Yuzi Aoki and lwaji lwasaki47 Determination o f Ammonia in Low Concentrations with Nessler's Reagent byFlow Injection Analysis-F. J. Krug, J. RGiCka and E. H. Hansen55 Determination o f Chloride in High-purity Waters in the Range 0-20 pg 1-1 ofChloride Using lon-selective Mernbrane Electrodes Incorporating Mercury( I )Chloride-G. B. Marshall and D. Mi,dgley63 Assessment of Glass Electrodes.for Determining pH in Boiler Feed Water-D. Midgley and K. Torrance73 Polarographic Study of Aflatoxins B,, B2, G, and G2: Application o f Differential-pulse Polarography' t o the Determination o f Aflatoxin B1 i n Various Food-stuffs-Malcolm R. Smyth, David W. Lawellin and Janet G. OsteryoungSHORT PAPERSDetermination o f S-Methylmethionirre in Plant Products by Use o f an Automatic 79Amino-acid Analyser-E. G. Kovatcheva82 Determination o f Bifunctional Compounds. Part IV. 4-lodobutaneboronicAcid as a Selective Reagent for the Trace Determination o f BifunctionalCompounds-C. F. Poole, S. Singhawangcha and A. Zlatkis87 Spectrophotometric Determination o f Silver with Ammonium 2-Cyano-3-iminodithiobutyrate-Motomu Muraoka, Tatsuo Yamamoto and Tatsuo Takeshima91 Book ReviewsSummaries o f Papers in this Issue-IPages iv, v, viii, ix.Printed by Heffers Printers Ltd Cambridge EnglandEntered as Second Class # 3 t New York. USA, Post Offic

ISSN:0003-2654    

DOI:10.1039/AN97904BX003

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

January, 1979 THE ANALYST ViiAnnual Reportson AnalyticalAtomicSpectroscopyVOLUME 7, 1977This comprehensive and critical reportof developments in analytical atomicspectroscopy has been compiled fromover 1700 reports received from world-wide correspondents who are inter-nationally recognised authorities in thefield and who constitute the EditorialBoard. In addition to surveying de-velopments throughout the worldpublished in national or internationaljournals, a particular aim has been toinclude less widely accessible reportsfrom local, national and internationalsymposia and conferences concernedwith atomic spectroscopy.Clothbound 300pp 8;" x 6" €1 7.50(CS Members f 13.00)(Still a vailable: Vo/s 3-6covering 7973 to 7976)Obtainable from: The ChemicalSociety, Distribution Centre,Blackhorse Road, Letchworth,Herts., SG6 I HNVERLAG CHEMIEI I II I+ q O l 5 n m A O,Ol%inrndWavelength ----Atomic AbsorptionSpectroscopyby Bernhard WelzThis volume is an English-language edition ofthe second edition of a highly successful Germanbook on the technique.After an introductory chapter on the physical-spectroscopical principles, a knowledge of whichis required for an understanding of the funda-mentals of atomic absorption, instrumentationand technique are treated in further chapters.Here, the analyst will find the necessary know-how for complete command of the method.Finally,short sections are devoted to the individualelements and to various specific applications.Comprehensive bibliographical references areprovided.The lucid, almost tabular. presentationnowhere allows the reader to become lost inminutiae, and the narrow interweaving of theoryand practice will be greatly appreciated.B r ief contentsIntroduction; Light Sources;Atomization; Optics;Electronics and Readout; Technique; RelatedAnalytical Methods; The Individual Elements;Specific Applications.Clothbound 277pp 98'' x 7" 3 527 25680 6 f20.00Orders to: THE CHEMICAL SOCIETY,Distribution Centre, Blackhorse Road,Letchworth, Herts. SG6 1 HN...V l l l SUMMARIES OF PAPERS I N THIS ISSUEDetermination of Ammonia in Low Concentrations withNessler's Reagent by Flow Injection AnalysisA turbidimetric procedure for the determination of ammonia in low concentra-tions with the use of Nessler's reagent is described.Both natural watersand soil extracts can be analysed a.t a rate of up to 120 samples per hourwith good precision and accuracy. The effects of reagent composition, flow-rate, temperature and protective c'olloids in the flow injection system arediscussed in detail.Keywords : Flow injection analysis ; ammonia determination ; Nessler'sJanuary, I979reagent ; turbidimetric determination ; continuous-$ow measurementF. J. KRUG, J. ReZIkKA and E. H. HANSENChemistry Department A, Technical University of Denmark, Building 207, DK-2800Lyngby, Denmark.Analyst, 1979, 104, 47-54.Determination of Chloride in High-purity Waters in the Range0-20 pg 1-1 of Chloride Using Ion-selective MembraneElectrodes Incorporating Mercury( I) ChlorideTwo types of solid-state mercury(1) chloride electrodes have been used todetermine chloride in the concentration range 0-20 pgl-l.At these lowconcentrations, more chloride will dissolve from the mercury (I) chloride inthe electrode than is present in the sample itself. The extent of the dissolu-tion is controlled, however, by the chloride in the sample. In these circum-stances, the electrode potential is linearly related t o the concentration ofchloride in the sample. With the electrode housed in a flow cell with athermostatically controlled water jacket, the correlation coefficient betweene.m.f. and concentration was alwa.ys greater than 0.99. The sensitivity(0.18 mV per pg 1-1 of chloride a t 25 "C and 0.4-0.5 mV per pg 1-1 of chlorideat 4 "C) was about ten times greater than that of the silver - silver chlorideelectrode.Total standard deviations a t 10, 5 and 2 pgl-l of chloride were0.4, 0.5 and 0.3 pg 1-1 of chloride, respectively.Keywords : Chloride determination ; ion-selective electrodes ; mercury(1)chloride electrodes ; high-purity zmtersG. B, MARSHALL and D. MIDGLEYCentral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey,KT22 7SE.Analyst, 1979, 104, 55-62.Assessment of Glass Electrodes for Determining pH inBoiler :Feed WaterSix types of commercial glass electrodes have been tested in the laboratoryfor their suitability for measuring p;H in ammonia-dosed boiler feed water ofmoderately low specific conductiv:ity (about 5 pS cm-l).The electrodeswere chosen to represent the range of pH-sensitive glasses available. All ofthe electrodes showed a near-theoretical sensitivity, had stable standardpotentials and responded sufficiently quickly. In the dilute ammoniasolutions, however, the electrodes indicated pH values that could differ byas much as 0.3 pH unit when the :solution was flowing slowly through themeasuring cell. When the solution was stirred the maximum bias was0.05 pH unit. For most industrial purposes, the differences in performancebetween the various types of electrode are unimportant and glass electrodesare less of a problem than reference electrodes for pH measurements in thistype of water.Keywords : pH determination ; glass electrodes ; boiler feed-waterD. MIDGLEY and K.TORRANCECentral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey,KT22 7SE.Analyst, 1979, 104, 63-73Januavy, 1.979 SUMMARIES OF PAPERS I N THIS ISSUEPolarographic Study of Aflatoxins B,, B,, G, and G,: Applicationof Differential-pulse Polarography to the Determination ofAflatoxin B, in Various FoodstuffsixThe polarographic behaviour of aflatoxins B,, B,, G, and G, has been investi-gated and found to parallel closely that of coumarin and its derivatives.Diff erential-pulse polarography has been applied to the determination ofaflatoxin B, in a variety of food products. Good agreement was obtainedbetween the diff erential-pulse polarographic method and visual comparisonof the fluorescence exhibited by the aflatoxin following thin-layer chromato-graphic separation.Electroactive interferences co-extracted from highlipid-containing foods were removed by separation on a Sephadex LH-20column.Keywovds : Aflatoxin determination ; differential-pulse polarography ; foodMALCOLM R. SMYTH, DAVID W. LAWELLIN and JANET G. OSTER-YOUNGDepartment of Microbiology, Colorado State University, Fort Collins, Colo. 80523,USA.Analyst, 1979, 104, 73-78.analysisDetermination of S- Methylmethionine in Plant Products byUse of an Automatic Amino - acid AnalyserShort PaperKeywords : S-Methylvnethionine determination ; amino-acid analyser ; plantproductsE. G. KOVATCHEVADepartment of Analytical Chemistry, Higher Institute of Food Technology, Lenin26, 4000 Plovdiv, Bulgaria.Analyst, 1979, 104, 79-81.Determination of Bifunctional Compounds4-Iodobutaneboronic Acid as a Selective Reagent forthe Trace Determination of Bifunctional CompoundsPart IV.Shovt PaperKeywords : Gas chromatography with electron-capture detection ; bifunctionalcompounds ; cyclic boronic estevs ; 4-iodobutaneboronic acidC.F. POOLE, S. SINGHAWANGCHA and A. ZLATKISDepartment of Chemistry, University of Houston, Houston, Texas 77004, USA.Analyst, 1979, 104, 82-86.Spectrophotometric Determination of Silver with Ammonium2- Cyano-3-iminodithiobutyrateShort PaperKeywords : Silver determination ; spectrofihotovnetry ; ammonium 2-cyano-3-iminodithiobutyrate ; silver complexMOTOMU MURAOKA and TATSUO YAMAMOTODepartment of Chemistry, Faculty of Science, Josai University, Keyaki-Dai,Sakado-Shi, 350-02, Japan.and TATSUO TAKESHIMADepartment of Chemistry, Faculty of Science, Chiba University, Yayoi-Cho,Chiba-Shi, 280, Japan.Analyst, 1979, 104, 87-90X THE ANALYST January, 1979AN ALYTl C A L S C I E N C ES M 0 N 0 G RAP H SHigh-Precision Titrimetr,yby C.Woodward and H. N. RedmanThis monograph was written in the hope that it will prove both helpful and interesting topractising analytical chemists.Brief contentsThe first section, on visual titrations, covers apparatus, preparation and assay of standardsubstances and preparation of standard solutions.The second section deals with instrumented titrations, including photometric and electro-metric techniques as well as miscellaneous instrumented methods.There are 83 key references to the literature on high-precision titrimetry.Paperbound 71 pp 83” x 6” 0 85990 501 2 f 2.50 ($5.50)CS Members f2.00The Chemical Analysis of WaterGeneral Principles a Techniquesby A.L. WilsonThe volume covers all stages of the complete analytical process including: deciding on theanalytical information required; sampling, including place, time and frequency, as well asdevices and techniques; the analysis proper and the reporting of results, their statisticaltreatment, and the factors involved in the choice of analytical methods (including on-lineand automatic methods) for particular purposes; and data handling.Clothbound 196pp f 7.50 ( $1 6.50)CS Members f5.7583” x 6;” 0 85990 502 0Pyrolysis-Gas Chromatographyby R.W. May, E. F. Pearson and D. ScothernMany papers have been published, particularly over the past decade, on aspects of pyrolysis-gas chromatography. A large number of different types of apparatus have been used, on awide range of samples. This monograph attempts to present the available knowledge in aform useful to the practising analyst, helping in the choice of an appropriate method and inthe avoidance of the more common pitfalls in this, perhaps deceptively, simple technique.Clothbound 11 7pp f7.20 ($1 5.75)CS Members f5.508%“ x 6“ 0 85186 767 7Electrothermal Atomization forAtomic Absorption Spectrometryby C. W. FullerSince the introduction of atomic absorption spectrometry as an analytical technique, byWaIsh, in 1953, the use of alternative atomization sources to the flame has been explored.At the present time the two most successful alternatives appear to be the electrothermalatomizer and the inductively-coupled plasma. In this book an attempt has been made toprovide the author’s views on the historical development, commercial design features, theory,practical considerations, analytical parameters of the elements, and areas of application ofelectrothermal atomization.Clothbound 135pp f 6.75( $1 4.75)CS Members f5.008:’ x 5.;’’ 0 851 86 777 4Dithizoneby H. M. N. H. IrvingThe author of this monograph, who has been closely associated with the development ofanalytical techniques using this reagent for many years, and who has made extensiveinvestigations into the properties of its compllexes, has gathered together a body of historicaland technical data that will be of interest to many practising analytical chemists.Clothbound I 1 2pp f7.25 ($1 6.00)CS Members f5.508%’’ x 59” 0 85186 787 1THE CHEMICAL SOCIETY,Distribution Centre, Blackhorse Road, Letchworth,Herts., SG6 IHN, England

ISSN:0003-2654    

DOI:10.1039/AN97904BP005

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

16 Analyst January 1979 VoL. 104 PP. 16-34 Evaluation of a Method for Determination of Total Antimony Arsenic and Tin in Foodstuffs Using Measurement by Atomic-absorption Spectrophotometry with Atomisation in a Silica Tube Using the Hydride Generation Technique W. H. Evans F. J. Jackson and Dorothy Dellar Department of Industry Laboratory of the Government Chemist Cornwall House Stamford Street Londo?z, SEl ONQ A method is described for the determination of antimony arsenic and tin in foodstuffs in which organic matter is destroyed using a wet-oxidation pro-cedure except for arsenic in samples of marine origin in which organic matter is destroyed by the dry-ashing technique. Each element is obtained in the highest valency state and converted into the respective normal hydride with sodium tetrahydroborate( 111) prior to atomisation in a flame-heated silica tube and atomic-absorption spectrophotometric measurement.The optimum conditions for this procedure are discussed and direct and indirect interference effects are described. The accuracy of the procedure is assessed for each element and where possible the accuracy of the method in application is considered Standard deviations of the results for levels normally found in foodstuffs have been calculated and derived limits of detection and confidence intervals are given. Keywords A ntimoizy determination ; avsenic determination ; tin determination ; foodstufls analysis; atomic-absorption spectrophotometry Antimony and arsenic are metalloid elements that occur naturally in foodstuffs and tin may be present at high levels originating from the canning of foodstuffs.None of the three elements has a known essential function in human physiology although it has been reported that tin is an essential element in animal nutriti0n.l Organic arsenicals have been used as additives in feeding stuffs to promote growth in farm animals. The toxic nature of arsenic in inorganic form and lower valency state is widely recognised. In the UK the level of total arsenic in foodstuffs is controlled under the Arsenic in Food Regulations 1959.2 Traditional recommended methods of analysis for these three elements in foodstuffs have used spectrophotometric end measurement^.^*^ Since the beginning of this decade attention has been directed to methods involving evolution of metalloid elements as their hydrides, followed by measurement with atomic-absorption spectroscopy.Various sources of nascent hydrogen have been proposed for the preparation of the hydrides. Of these sodium tetra-hydr~borate(III)~-~ is now generally accepted as the most suitable provided that for certain of the end measurements each element is first reduced to the lower valency state using for example iodide, Arsenic is normally measured at a wavelength of 193.7 nm and it was recognised at an early stage that the use of an argon - hydrogen entrained air flame was necessary in order to reduce flame absorption in this wavelength region.8 Subsequently the direct use of a flame was dispensed with and the arsine produced was entrained with argon in an electrically heated tube.g From combinations of these variables a number of systems have been evolved and applied to metalloid elements in foodstuffs for example by Fiorino et aZ.1° using a ternary acid wet oxidation followed by a pre-reductant stage with iodide evolution of the hydride with sodium tetrahydroborate(II1) in an automated system and measurement in a nitrogen -hydrogen entrained air flame.Crown Copyright EVANS JACKSON AND DELLAR 17 In this laboratory estimates are obtained for a number of elements for the total diet survey of the UK including the three elements considered here. The ability to use a common digest for the determination of ten or more elements reduces the cost of analysis. The extreme sensitivity of the heated silica tube method proposed by Thompson and Thomerson,ll together with the ability to measure when possible in either valency state and coupled to a digestion procedure slightly modified from that previously described,12 permits measure-ments to be made on small volumes of the common digest.An evaluation of this combina-tion is presented in this paper. Method Reagents be prepared with distilled water. Ultrar or equivalent grade acid be used. All reagents must be of the grade indicated or of analytical-reagent grade; solutions should Nitric acid sp. gr. 1.42. It is recommended that BDH Aristar Hopkin and Williams Sulphuric acid sp. gr. 1.84 (1 + 19) and (1 + 99). Recommended grade as for nitric acid. Hydrochloric acid (1 + 1). Hydrogen peroxide 30% m/V. Sodium hydroxide solution 5% m/V.Antimony( V ) chloride. Arsenic( V ) oxide. Sodium tetrahydroborate(III) solutions 1 .O% 1.5% and 3.0% mlV. Magnesium nitrate hexahydrate solution 50% m/V. Cz@ferron (ammonium N-nitrosophenylhydroxylamine) solution 5% nzj V . Chloroform . Antimony(V) standard solution. Freshly prepared. Dissolve 1.228 g of antimony(V) chloride in 1 1 of hydro-chloric acid (1 + 1) to give a solution containing 500 mg 1-1 of the element. Dilute 10 ml of this solution to 1 1 with sulphuric acid (1 + 19) to give a solution containing 5 mg 1-1 of the element. Immediately before use dilute 10 ml of the latter solution to 250 ml with sulphuric acid (1 + 19). Dissolve 1.534 g of arsenic(V) oxide in 100 ml of sodium hydroxide solution (5% m/V) and dilute to 600 ml with water.Add 30 ml of hydrochloric acid (1 + 1) and dilute to 1 1 with water to give a solution containing 1000 mg 1-1 of the element. To 10 ml of this solution add 10 ml of hydrochloric acid (1 + 1) and dilute to 1 1 with water to give a solution containing 10 mg 1-1 of the element ; immediately before use take 5 ml of the latter solution and dilute to 250 ml with sulphuric acid (1 + 19) giving a solution containing 0.2 mg 1-1 of arsenic. Tin ( I V ) standard solution. A primary standard solution specially prepared for atomic-absorption spectrophotometry and containing 1000 mg 1-1 of tin can be purchased from commercial sources. Dilute 5 ml of the primary standard solution to 100 ml with hydro-chloric acid (1 + 1) to give a solution containing 50 mg 1-1 of the element and immediately before use dilute 5 ml of the latter solution to 500 ml with sulphuric acid (1 + 99) to give a solution containing 0.5 mg 1-1 of tin.Prepare working standard solutions for arsenic and antimony separately taking 0 1 2 4 6 8 and 10 ml of the 0.2 mg 1-1 standard solutions and diluting to 100 ml with sulphuric acid (1 + 19) to give solution concentrations of 0 0.002 0.004, 0.008 0.012 0.016 and 0.020 mg l-l respectively of arsenic and antimony. For tin dilute 0- 1- 2- 4- 6- and 8-ml aliquots of the solution containing 0.5 rng 1-1 of tin to 100 ml with sulphuric acid (1 + 99) to give solutions containing 0 0.005 0.01 0.02 0.03 and 0.04 mg 1-1 of tin respectively. Store all solutions in calibrated flasks; in the absence of reducing atmospheres these solutions are stable for up to 1 month.This solution contains 0.2 mg 1-1 of antimony. Arsenic(V) standard solution. Working standard solutions. Apparatus All glass apparatus should be kept full of 1 N nitric acid when not in use; the glass apparatus used for preparing diluted sample solutions for measurement of tin should be kept full of 1 N hydrochloric acid 18 EVANS et al. DETERMINATION OF ANTIMONY ARSENIC AND TIN Analyst Val. 104 Silica T-piece tubes. Those used for the procedure described are 150mm long with internal diameter 5 mm and with a side-arm 70 mm long. A separate tube must be kept specifically for each element and must be pre-conditioned for that element by repeated applications of the most concentrated working standard solution until a constant response is obtained.In normal use for foodstuff digests such a tube has a limited life expectancy before the internal walls become poisoned or excessively coated with sodium sulphate. The useful life expectancy is about 400 injections. Nitrogen is supplied at a flow-rate of 0.5 1 min-l together with an air bleed of variable flow-rate of 5 10 and 50 ml min-l for antimony arsenic and tin respectively. The gas supply is connected to the hydride generator chamber. That used is 11 cm long with external diameter 2 cm with a septum entry point 3 cm from the base which is fitted with a drainage tap. The gas entry tube to the chamber reaches to within 5 cm of the base and the exit tube is connected by a 60 cm long silicone-rubber tube to the silica tube which is mounted on a 10-cm single-slot acetylene - air burner; the silica tubes should be readily interchangeable.Disposable (polypropylene syringes and needles. These should be of 1-ml capacity fitted with 21-gauge needles for standard and sample solutions and of 5-ml capacity for sodium tetrahydroborate(II1) solutions. Separate 1-ml syringes should be used for standards and samples and discarded after each series of measurements. This should be of the desired sensitivity preferably with an enclosed burner chamber. The instrument should be fitted with an integrator having a response time not exceeding 1 S. Electrodeless discharge emission sources are preferred for both arsenic and antimony. Gas supply. Hydride generator chamber. Atomic-absorption spectvoplzotometer. A hollow-cathode emission source can be used for tin.Procedure Preparation of sample digests Destroy the organic matter from 5-20 g of foodstuff depending upon the nature of the foodstuff according to Method (1)C of the Analytical Methods C0rnmittee.1~ Ensure that nitric acid (sp. gr. 1.42) is added prior to 5 ml of sulphuric acid (sp. gr. 1.84) at the com-mencement of the oxidation and take suitable precautions to avoid an excessively violent reaction. When oxidation is complete cool the mixture dilute with 10ml of water cool and add 1 ml of hydrogen peroxide (30% m/V); boil until white fumes of sulphur trioxide are evolved. Cool the mixture dilute with 10ml of water add a few drops of nitric acid (sp. gr. 1.42) (to remove any peroxides) and boil gently to fumes.Repeat the boiling to white fumes twice according to the method described previously. Cool the mixture and dilute to 100 ml with water to give a nominal 5% V/V sulphuric acid digest which is colour-less and contains no suspended solids. At the same time prepare two reagent blank solutions, from volumes of acid used in the sample oxidation and treat these in a manner identical with that for sample digests. Preparation of digests for arsenic in samples of marhe origin For any material of marine origin a method based on that of Leblanc and Jackson14 must be used. Weigh up to 1.00 & 0.01 g of wet sample (up to 0.25 g of dry sample) into cleaned silica dishes and add 8 ml of magnesium nitrate hexahydrate solution (50% m/V). Disperse the sample well and evaporate to dryness in an oven at 105 "C.Transfer the sample into a muffle furnace warmed to 100 "C and increase the temperature in steps of 50 "C allowing the heating to continue for 30 min at each higher temperature until a temperature of 500 "C is attained. Continue the ashing overnight for convenience. When the sample is cool dissolve it in sulphuric acid (1 + 19) and dilute the solution to 100 ml with sulphuric acid (1 + 19). Prepare simultaneously two reagent blank solutions. Additional stage in the preparation of sample digests in the presence of excessive amounts of t i n When measuring antimony or arsenic if prior indication is that the sample digest solution contains tin at concentrations that interfere i.e. greater than 0.5 mg 1-1 and 0.2 mg 1-1, respectively this tin must be removed by extraction with cupferron solution according to the following procedure January 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 19 Cool a diluted digest to 4 "C.Preferably digest a separate sample to prevent excessive loss of sulphuric acid from fuming. Add 2 ml of cupferron solution (5% m/V) and 10 ml of chloroform and shake vigorously for 2min. Discard the chloroform layer repeat the cupferron addition and extraction and extract with a further 10 ml of chloroform. Evaporate to white fumes cool and dilute to 100 ml for a separate digest or to the volume originally taken. Measurement If the elemental concentration in the digest exceeds the calibration range dilution must be made with sulphuric acid (1 + 19). For determination of tin 2 ml of the digest must be diluted to 10 ml with water and if the calibration concentration range is exceeded further dilution made with sulphuric acid (1 + 99).When dilution of the solution is required the actual concentration measured should always be greater than 0.008 mg 1-1 for antimony and arsenic and 0.01 mg 1-1 for tin. Adjust the emission sources in a suitable atomic-absorption spectrophotometer to give a maximum sensitivity to noise ratio according to the maker's instructions at wavelengths of 217.6nm for antimony 193.7nm for arsenic and 224.6nm for tin Insert the burner assembly fitted with the silica tube into the radiation beam and adjust the position of the tube vertically horizontally and on the horizontal axis to obtain maximum response.Ensure that the nitrogen and air flows into the hydride generator chamber are as described and heat the tube with a normal acetylene - air flame. After heating for 10 min ensure that the base line is free from drift on a millivolt recorder. By means of a syringe introduce a volume of sodium tetrahydroborate(II1) solution, appropriate to the element being measured into the hydride generator chamber. The volumes will be 2 ml of lyo 2 ml of 1.5% and 1 ml of 3% solution for antimony arsenic and tin respectively. Inject 1 ml of the standard solution of highest concentration and repeat renewing sodium tetrahydroborate( 111) and standard solutions until a uniform response is obtained washing the generator chamber with distilled water between measure-ments. Subsequently obtain a constant response for the zero standard solution.Arrange the sample standard and blank solutions in random order and record for each element in turn the response from each solution in at least duplicate. Measure the responses for each sample and standard calculate the average and subtract the mean of the relevant blanks. For each element calculate the response at each of the standard concentrations as a measure of the lowest concentration and calculate the average. If any individual standard concentration falls outside the range &20% of this average for arsenic or antimony and -+lo% for tin that response must be rejected and a new average calculated. This should be an infrequent occurrence except for the lowest standard in the series. If wz is the mass of sample taken then for a net sample response rl within the calibration range, that sample will contain 100 cr,/mr mg k g l for arsenic and antimony and 500 cr,/mr mg kg-l for tin.The digest can be used as prepared for measurement of antimony and arsenic. Let the average response be r for the lowest concentration c of one of these elements. Experimental Each of these elements is normally present in foodstuffs at low concentrations and it is these levels which dictate the range of measurement and the experimental conditions of the described method. Arsenic may be concentrated in animal offal and in samples of marine origin but a moderate dilution permits measurement within the calibration range. Tin may be present at elevated levels in canned foodstuffs but provided a limiting coefficient of variation is accepted such levels can be determined similarly following dilution.The requirements in the development of a complete analytical method have been described e1~ewhere.l~ The most important requirements for any method are that the variables in each stage of the method be considered in order to obtain the maximum response for the low levels present and the response and the variation in response for each stage of the method be investigated to obtain conditions for which no systematic bias exists. There will inevitably be exceptions to the application of the resulting procedure and these must be defined and if important alternative procedures recommended 20 Analyst vol. 104 Destruction of Organic Matter Gorsuch16 originally showed that no loss of inorganic antimony or arsenic would be expected during wet oxidation with nitric and sulphuric acids while such a wet oxidation has been recommended for the determination of tin in foodstuffs.* Subject to the stability of these elements in sulphuric acid solution this would appear to be a satisfactory procedure to adopt.In the preparation of these digests however several aspects require consideration. While the chemical states of these elements in foodstuffs may remain unknown the possibility must be considered that elements exist as volatile forms or in forms intractable to destruc-tion. There are no recorded instances for antimony and tin but arsenic in samples of marine origin may exist in different inorganic valency states as specific methylated arsenic compounds or as complex organic compounds of high relative molecular mass and of unknown composition.17J8 It is difficult to release arsenic from the organically bound states by normal wet oxidation and alternative methods of destruction have been recom-mended.l*@ This inability of wet oxidation to release all the arsenic can be illustrated by the results obtained during this investigation on a wet fish homogenate sample using several different analysts.Levels of 1.4 1.4 3.5 7.0 7.7 11.4 and 14.3 mg kg-l were returned to give an average and standard deviation of 6.7 and 5.0 mg k g l respectively. Clearly even without definition of the method these results reflect variation that is not under contr01.l~ Dry ashing in the presence of magnesium nitrate was chosen as the most suitable means of releasing the total arsenic from samples of marine origin.Because levels are generally high relatively small masses of sample are acceptable for the range of measurement adopted, provided that such samples are homogeneous. It is preferable not to overload even this system as is exemplified in Table I. There is a limiting mass of sample that may be taken, which could depend upon the nature of the organoarsenical or the sample itself. EVANS et al. DETERMINATION OF ANTIMONY ARSENIC AND TIN TABLE I ARSENIC CONTENT (mg k g l ) FOUND IN SAMPLES OF MARINE ORIGIN USING DRY ASHING OF DIFFERENT MASSES Mass of sample ashed/g I A \ Foodstuff 0.1 0.25 0.6 1 .o 2.0 NBS tuna . . . . 3.0 3.0 3.0 2.5 2.7 2.7 1.7 1.6 1.2 1.2 Fish meal .. - 11 12 6 4 -During the measurement of the total levels of these elements in the hydride formation stage it is necessary to start the sodium tetrahydroborate(II1) reduction with the element entirely in one valency state. Most workers have preferred the lower valency state attained by means of pre-reduction with for example iodide. Despite a loss of sensitivity (the responses obtained in the lower valency states are 2.0 and 1.4 times that given in the higher valency states for antimony and arsenic respectively) we prefer the higher valency states, which are more readily and simply obtained. Tin in dilute solutions can exist only as tin(IV), while application of wet oxidation or dry ashing to the destruction of organic matter yields arsenic in the pentavalent form.This was found not to be the case for antimony which invariably remains in the trivalent state during wet oxidation with nitric acid. The con-version of antimony into the pentavalent state however is accomplished satisfactorily by the hydrogen peroxide stage described in the procedure. Finally it has been reported that no loss of antimony or tin occurs in solution at a pH less than It would be expected that both antimony and arsenic would be stable in 3-5% sulphuric acid in the higher valency state in the absence of reductants. Tin can however, hydrolyse and precipitate in similar circumstances. No losses of tin were noted for solutions of concentration up to 10 mg 1-1 (equivalent to a determination of 100 mg k g l in foodstuffs) in 5% sulphuric acid for 1 month; losses were observed within days for tin solutions of con-centration greater than 1 mg 1-1 in 1% sulphuric acid.Conditions for Evolution of Hydrides I t is well known that the thermal breakdown of these hydrides (taken to be of compositions SbH, ASH and SnH,) is greatly affected by the heated surface with which they are i January 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 21 contact. The pre-conditioning of the silica tubes described in the method indicates the need for a catalytic film of the element on the silica surface before consistent response is achieved. The fragile nature of this catalytic film can be shown by the observation during this investigation that whenever the response from pure solutions was disturbed for some reason to give a higher or lower response the following injection compensated to give a lower or higher response thus maintaining the average response.It follows that poisoning of this catalytic film would be expected from evolved hydrides of other elements deposited on the silica tube surface or from carryover from the hydride chamber in particular of sodium sulphate. This would have the effect of reducing the sensitivity during repeated use. It has also been observed during this evaluation that variation increases as a tube ages with use. It was impossible to restore the silica tubes to their original performance by means of acid cleaning or scouring, and it was considered necessary to replace each tube after the use described in the procedure. To enable facile replacement therefore an electrically heated silica tube was not considered.Conditions that can vary in the hydride evolution stage for each element are as follows: silica tube dimensions; the volumes of sample digest solution used; the flow-rate of the carrier gas and any auxiliary oxidant; the volume and strength of sodium tetrahydro-borate(II1) solution; and the acidity of the sample digest solutions. The length of the silica tube is determined by the size of the enclosed atomic-absorption burner chamber and it was found that maximum response was obtainable with tubes of internal diameter 5 mm. The residence time for the evolved hydrides in such tubes is short and precludes the use of instrument integration times exceeding 1 s. As it has been reported that 1 ml of an acidified solution is adequate for hydride evolution,ll the adoption of such volumes maximised the use of the prepared foodstuff digests.When using such a system it was found that the flow-rate of the carrier gas nitrogen was most sensitive for the range 0.4-0.6 1 min-l for each element. It has been reported that the addition of air into the carrier gas stream improves the relative response,21 but for the present procedure no such improvement was observed. It was particularly noticeable, however that additions of air decreased the variation in response for a series of injections for each element e.g. at 0.04 pg of tin with 0,lO and 50 ml min-1 of air added to the nitrogen flow relative standard deviations of 0.10 0.07 and 0.04 respectively were obtained for replicated injections.The optimum additions of air described in the procedure were there-fore adopted. While it is desirable to maximise the amount of sodium tetrahydroborate(II1) used to overcome consumption of this reagent by other elemental species present the amount that can be used in the present procedure for antimony(V) and arsenic(V) is limited by the need to maintain acidic conditions for the establishment of an equilibrium between the pentavalent and the lower valency states in sit% prior to hydride evolution,22 and the need to avoid an over violent evolution of hydrogen and ignition of gases leaving the silica tube. Amounts of sodium tetrahydroborate(II1) between 20 and 30 mg for antimony(V) and 30 and 40 mg for arsenic(V) were found to give a relatively constant response and the lower amounts were subsequently used with the proviso that these amounts were contained in a 2 ml-volume of solution.For tin(1V) the acidity of the solution used for measurement is much reduced and constant response is attainable for the range 10-30 mg of sodium tetrahydroborate(II1) ; 1 ml of 3% m/V reagent could be used without danger of ignition. In all instances it was found advisable to add the sample solution to the sodium tetrahydroborate(II1) solution in order to ensure thorough mixing. It was also noted that these solutions of sodium tetra-hydroborate(II1) gave consistent responses for a period of at least 3 h. The prepared digest solutions are 3 4 % V/V in sulphuric acid and for the range 2.540% there is no change in response from hydride evolution of either antimony(V) or arsenic(V).The response for tin(1V) is constant for acid concentrations of 0.5-1.0% V/V sulphuric acid, and hence it is necessary to dilute the digest solution 5-fold with water to fall within this range and make any further dilution with 1% V/V sulphuric acid to ensure a constant acid concentration not exceeding 1%. It was also found that blank responses for each element were constant within the respective ranges of sulphuric acid concentrations. For atomic-absorption measurements stable base lines with minimum but detectable noise levels are advantageous and electrodeless discharge emission sources are preferable for both arsenic and antimony. Comparison of results measuring with and without background A separate tube was therefore retained for each element 22 EVANS et al.DETERMINATION OF ANTIMONY ARSENIC AND TIN Analyst VoZ. 104 correction on foodstuff digests displayed no significant difference and results described in this evaluation were obtained without background correction measurement being made on a Pye Unicam SP1900 spectrophotometer. It may be noted that the condition of the optical system of any spectrometer when measuring arsenic at 193.7 nm is important. The results contained in this evaluation were obtained over a period exceeding 12 months with the optical system in average condition and so reflecting normal practice. A new instrument would be expected to have lower noise levels that could be reflected in slightly lower relative standard deviations (R.S.D.) than those listed.Using the procedure described the average R.S.D.s of a single injection for a series of 10 replicates for each element after blank correction by one analyst are shown a t the top of Table 11. TABLE I1 RELATIVE STANDARD DEVIATIONS OF SINGLE INJECTIONS FOR DIFFERENT EXPERIMENTAL CONDITIONS Degrees of Parameter freedom Replicate injections. . 9 Triplicate injections in presence of non-interfering element . . 8 Triplicate standard injections in a series containing standards and non-interfering elements . . * . 8 All standard injections in the above series 11 Triplicate injections in presence of inter-Triplicate standard injections in a series containing standards and interfering elements . . 8 All standard injections in the above series 11 fering element .. 8 Element concentration/mg 1-1 r Antimony(V) Arsenic(V) Tin( IV) *- 0.004 0.020 0.004 0.020 0.01 0.04 0.13 0.06 0.15 0.06 0.11 0.04 A \ 0.16 0.03 0.14 0.08 0.09 0.05 0.10 0.05 0.18 0.06 0.11 0.04 0.12 0.03 0.27 0.07 0.14 0.06 0.08 0.06 0.15 0.11 0.10 0.05 0.10 0.04 0.11 0.09 0.04 0.05 0.17 0.04 0.12 0.08 0.10 0.05 Calibration These R.S.D.s are higher than those reported for automatic systems of measurement of these elements e.g. 0.018 for antimony and arsenic and 0.022 for tin,23 and this would be expected. The latter were obtained with unknown solution volumes at concentrations of 0.05 0.10 and 0.05 rngl-l respectively which may mean the determination if not the measurement at one time of larger amounts of element than the 440 ng tested in this evaluation (Table 11).The R.S.D. of single injections is only one contributor in practice to the variation of the hydride evolution and measurement stages. In routine use contributions to the variation may include variation between series of readings the variation inherent in the blank and also that from transient interferences from different sample digests. A better expression of the R.S.D. would be that obtained in actual routine use. This can conveniently be obtained by consideration of the calibration lines used in determining the results described in this paper. The series of standards were measured 20-28 times by three analysts for over 12 months using the defined procedure of duplicate injections. Expressing the average net response for each calibration line in terms of the lowest con-centration taking the over-all average as unity the range for these average responses together with the standard deviations are as shown in Table 111.This gives an indication of the likely range of response between series of measurements. The ratio of the average of two reagent blanks to the zero standard for each series of measurement is also displayed for each element. While the magnitude of this reagent blank for each element is normally about 2 ng it would appear that wet oxidation can reduce this response. Table I11 also illustrates the ratio of the sum of the responses at each standard concentra-tion compared with the sum of the average responses for each element together with the R.S.D.of the differences of each response with the average response for the individual calibration lines. The ratios obtained for antimony (V) are variable for the lower standard January 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 23 and exceed the 95% confidence limits (95 C.L.) imposed by the R.S.D.s but it is assumed that response within the range measured is linear and this is supported by the decreasing magnitude of the R.S.D. as the measured concentration increases. It is concluded from similar considerations that the response for arsenic(V) is linear each ratio obeying the statistical limits imposed by the R.S.D.s while for tin(1V) linearity diverges markedly above 0.04mg.1-1 from consideration of both the ratio and the elevated R.S.D. at 0.05 mg 1-l.TABLE I11 CALIBRATION DATA Parameter Range of average response per unit concentration. . Relative standard deviation (R.S.D.) of average response . . Ratio of reagent blank with standard blank . . Ratio of standard to average response f R.S.D. : 1st standard . . 2nd standard . . . . 3rd standard . . . . 4th standard . . . . 5th standard . . * . 6th standard . . Antimony (V) 0.75-1.20 h0.13 0.83 1.14 f 0.40 0.90 f 0.21 0.94 f 0.10 1.08 f 0.12 1.00 f 0.05 1.01 f 0.05 Arsenic (V) 0.82-1.24 h0.12 0.88 0.89 5 0.41 0.95 f 0.15 1.05 f 0.11 0.99 f 0.07 1.02 & 0.06 1.00 f 0.08 Tin(1V) 0.7 7-1.26 If 0.16 0.92 1.02 & 0.10 1.03 f 0.07 1.02 & 0.04 0.99 & 0.05 0.98 f.0.06 (0.93 & 0.14)* * This refers to a 0.05 mg 1-1 standard not used in the calculation of the mean. For comparison with succeeding tables the coefficient of variation is 100 x R.S.D. To obtain the variation for single injections of pure solutions during application of the procedure to food digests R.S.D. values listed in Table I11 must be multiplied by d2 to give in the region of the second and sixth standards (fifth for tin) values which are effectively 0.29 0.07 for antimony(V) 0.21 0.10 for arsenic(V) and 0.10 0.07 for tin(IV) respectively, for 20-28 degrees of freedom. These can be compared with values for replicate injections for pure solutions alone at the same concentrations defined in Table I1 for 9 degrees of freedom. The 95 C.L. for the ratios of standard deviations can be obtained from tables2* and for 24 against 9 degrees of freedom will be 0.53-1.64.If it is accepted that at the higher concentration of tin(1V) the slightly exceeded upper limit is further evidence for incipient curvature of the calibration range there is agreement between the two sets of results within these limits except for the lower concentration of antimony(V) .* This agreement suggests equivalence of analysts absence of undue between-series variation and little interference from the food-stuffs examined in this evaluation that could conceivably indirectly increase the variation of measurement. Each value is an estimate governed by the asymmetric x2 distribution. Interferences With a system of measurement such as atomic-absorption spectroscopy with atomisation in a silica tube interference may be of four distinct types (a) directly upon the response of a measured element in the presence of an interfering ion (b) that upon the variation of measurement of an element in the presence of interfering ions (c) that upon the variation of subsequent measurements during an extended series of measurements in routine use and (d) that upon the response of subsequent measure-ments in an extended series of measurements.The last statement is taken somewhat out of context. * It must be accepted that when measuring low concentrations of an element similar in magnitude to the standard blank e.g. the first standard in Table 111 the standard deviation derived for these standards could be affected by the variation inherent in the blank giving a value up to 2/2 of that obtained for single injections within a series.Similar but diminishing relative contributions could occur as the measured concentration increases 24 EVANS et aZ. DETERMINATION OF ANTIMONY ARSENIC AND TIN Analyst VoZ. I04 Direct interference is usually considered only and necessarily so to ascertain occasions when the procedure is invalidated through systematic bias. Significant direct interferences have been documented for measurement by the argon - hydrogen entrained air flame25 and with graphite furnace atomisation.26 No interference has been reported for cationic species for a fully automated system using a heated silica tube provided that concentrated hydro-chloric acid is added to the sample solutions of arsenic(II1) and selenium(1V) prior to the sodium tetrahydr~borate(III).~~ The present method is different to all three and hence these effects have been considered for two concentrations of each element for the ionic species listed in Table IV the latter reflecting amounts that could be encountered in foodstuff digests.The differences are those obtained for the net response for triplicate injections, with and without interfering ion each contained within the same series of readings. The 95 C.L. are those calculated from Table I I (first row) for the same concentrations of antimony(V) arsenic(V) and tin(1V) investigated and are reduced by a factor of 0.58 to account for the triplicate injections.* When the direct effect of an ionic species was repeated, these limits should be reduced by a further factor of 0.71.Consideration of the interfering elements that exceed the 95 C.L. suggests that in the examination of foodstuffs by the described procedure only arsenic tin and copper could occur a t levels that directly interfere in the individual determination of the three elements of interest. I t would appear that each interference may be of a consistent level irrespective of the measured concentration. Copper may exceed 10mgkg-l in liver offal or tomato products while total arsenic in marine samples could frequently exceed 5 mg kg-1. The most common interfering species is however tin at levels exceeding 5 and 2 mg kg-1 for antimony(V) and arsenic(V) respectively. When canned produce is examined for these elements the tin must therefore be removed according to the procedure detailed in the method.I t is interesting to note that metaphosphate partly precipitates tin(IV) another example of systematic bias but in practice although metaphosphate must be obtained during the digestion procedure] dilution of the cooled acid digest with water ensures con-version into the orthophosphate. There remain possible indirect interferences of types (b) to (d). These can be assessed for the concentrations shown in Table I1 according to the following pattern separately for non-interfering species chosen at random and for species known to interfere directly. For the latter with antimony(V) and arsenic(V) 2 pg of copper(I1) and 1 pg of tin(1V) were used as direct interfering amounts and for tin(IV) 1 pg of copper(I1) and 0.2 pg of nickel(I1).Alternate triplicate injections of standard solutions (S) and these solutions plus interfering (non-interfering) species (Ill I,) were made in the sequence S I, S I, S I, S I, S each standard concentration constituting an individual series of injections. The R.S.D. of the net response for a single injection within triplicate injections for I, I may reflect indirect interference of type (b) for 8 degrees of freedom. The R.S.D. of the net response for a single injection within triplicate injections for the standard S excluding the first triplicate] may reflect indirect interference of type (c) for 8 degrees of freedom. The R.S.D. of the net response for a single injection for all injections for S excluding those contained in the last triplicate may reflect indirect interference of type (d) for 11 degrees of freedom.The R.S.D.s obtained are tabulated (Table 11) and can be compared with the R.S.D. of repli-cated single injections of standards (Table 11 first row) by consideration of the 95 C.L. of the ratios of standard deviations for 8 or 11 degrees of freedom against 9 degrees of freedom. These limits will be approximately 0.5-1.9 and none of the ratios exceed these limits. The evidence presented suggests therefore that the direct interference of single species is important only for the levels investigated and for these it is desirable in the application of the method to have a knowledge of the levels of the interfering species. Cumulative effects direct and indirect (b) of both statistically significant and non-significant single interferences can only be observed during the application of this tested method to foodstuffs.* A full statistical treatment has been reporteds8 for assessment of interferences upon both response and variation in the presence of interfering ions. This treatment takes account of both error of the first kind, a i.e. a hypothesis is rejected when in fact i t is true and of the second kind i.e. a hypothesis is accepted when i t is false in application of the t-test for differences in response and the F-test for differences in variation. Irrespective of the magnitude of the R.S.D. to obtain a clear indication of either type of interference equivalent to that of the more empirical study undertaken in this text would require 13 replicates.This is a number we feel unable to undertake because of the many possible interferences. These tests are applied one-sided Jawary 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY Results 25 The accuracy of the total method in the presence of foodstuffs was assessed by recovery experiments on selected total diet homogenates containing negligible or low amounts of the three elements evaluated and no level of species known to interfere according to a design TABLE IV DIRECT INTERFERENCE EFFECTS ON HYDRIDE EVOLUTION AND MEASUREMENT Antimony( V) Arsenic( V) Tin(1V) Parameter - 1 - 7 Concentration/mg 1-1 . . . . 0.004 0.020 0.004 0.020 0.01 0.04 95% confidence limits. . . . . . f0.17 j 0 . 0 8 f0.20 k0.08 k0.14 f0.05 ,Added ion Ag+ A l 3 + As5+ Bi3+ Ca2+ Cd2+ cu2+ co2+ Cra+ Fe3+ Hg2+ K+ Mg2+ Mn3+ Moa+ Na+ Ni2+ Pb2+ PO,-Po43-Sb5+ Se4+ Sn4+ Sr2+ Zn2+ Amount/ K* 0.1 100 1 0.5 0.2 0.1 0.2 5 000 5 2 1 0.5 0.2 0.1 5 1 5 100 20 5 000 6 000 100 20 5 5 000 1 0.5 0.2 0.1 0.05 5 2 000 1000 20 2 000 500 0.1 0.2 0.1 1 0.2 1 0.5 0.2 100 100 20 Relative difference in response with and without added element: 7 .A -0.20 -0.06 +O.Ol - 0.03 ---0.15 (- 0.05) (-0.08) (-0.15)t (- 0.08) ---- 0.29t -0.16 -0.03 -0.14 -- 0.14 - 0.08 -0.16 0.00 -0.03 -0.06 +0.10 0.00 --- - + 0.04 f0.04 -0.01 + 0.05 ----( - 0.12) __ ( - 0.05) (- 0.07) -0.06 -0.14t -( + 0.04) -0.03 - 0.06 + 0.07 + 0.02 +- 0.02 -0.06) - 0.08) -0.11) -0.05) ------0.17 -0.03 0.00 0.00 f0.03 f0.04 - 0.06 - 0.01 - 0.02 - 0.03 0.00 - 0.03 -----fO.01 -0.03 -0.06 --0.06 ---(0.00) -( - 0.14) (-0.07) +0.01 (- 0.04) --0.15 Over-all average non-significant differences .. . . . . . . -0.05 -0.02 - 0.08 -0.14 ----- 0.23 - 0.08 + 0.08 (- 0.11) t (- 0.07) ---- 0.51 t ( - 0.15) + 0.02 - 0.14 - 0.06 (-0.11) (- 0.03) -0.17 - 0.17 - 0.18 (+ 0.04) - 0.09 - 0.04t +0.11 -0.06 --- 0.08 +0.17 + 0.14 + 0.08 --- 0.04 + O . l l + 0.04 (-0.35)t (-0.10) ( - 0.03) 0.00 - 0.367 -(0.00) - 0.04 - 0.04 +o.o2 ---- + 0.01 - 0.02 - 0.07 (- 0.17) (- 0.03) ---- 0.24 (0.00) + 0.03 + 0.03 + 0.01 (- 0.05) (-0.02) - 0.02 - 0.09 - 0.03 (- 0.03) i o .0 1 - 0.14 0.00 - 0.06 -- + 0.01 - 0.05 - 0.Oi + 0.02 - 0.05 0.00 - 0.03 (- 0.23) ---(-0.11) (- 0.02) + 0.07 - 0.35 (- 0.06) - 0.02 -0.01 -0.04 +0.09 +0.06 - --0.30t -0.11 (-0.01) (-0.04) (-0.20)t (-0.11) 0.00 -0.06 S0.04 +0.02 +0.16 -0.03 (-0.29)t (-0.24) (-0.05)f (-0.10) (- 0.06) ( - 0.02) -0.52t +0.01 +0.05 -0.02 - -(-0.ll)f (-0.10) (-0.11) (-0.01) +0.01 +0.08 - 0.05) - 0.07 -+0.04 --0.03 + 0.10) - --0.01 0.00 ( + 0.02) +0.01 -+ 0.03 + 0.06 -(+0.01) ---0.19)t (-0.17) - 0.06) t (- 0.12) ,-0.06) (-0.04) -0.07 -0.04 - -(- 0.50) t (- 0.44) -0.46t -0.46 +0.05 -0.02 -0.06 +0.01 - -+O.ll +0.03 - --0.03 -0.06 - -- --0.03 +0.02 (+0.03) (-0.03) - -0.00 -0.01 * For 100 ml of digest prepared from 10 g of foodstuff the concentration of interfering species as t ~ 5 % significance.$ ( mg kg-l can be obtaimd from x 10 for antimony and arsenic and x 50 for tin. ) Implies a duplicated set of readings 26 EVANS et al. DETERMINATION OF ANTIMONY ARSENIC AND TIN AnaZyst VoZ. 104 previously de~cribed.1~ Amounts of all three elements were added in the inorganic form and in the lower valency state to 20 g of beverage infusion and milk 5 g of fat homogenate and 10 g of the remaining homogenates (cereals meat fish preserves root vegetables and green vegetables) and in amounts dictated by that noimally found in foodstuffs.Recovery was determined in duplicate each base level being determined simultaneously in duplicate and all values for a particular diet homogenate being contained within a series of determina-tions. For arsenic in fish a separate recovery exercise was made using dry combustion of 1 g (wet mass) of a fish homogenate sample for reasons already discussed. Separate recovery experiments were made for 40 and 100 pg of added tin (to meat preserves and green vegetable homogenates reflecting canning practice) o avoid interferences when measuring the other two elements under consideration. The results after subtraction of the mean base levels, are summarised in Tables V and VI.From these results standard deviations pertaining to the method in the presence of foodstuffs can also be calculated and where applicable confi-dence limits deduced (Table VI). The varying base levels in each food homogenate cannot be allowed for in these calculations and the standard deviations reflect variation only in the determination of added species with subsequent measurement as a different valency species. TABLE V AVERAGE RECOVERIES OF ADDED ANTIMONY ARSENIC AND TIN Each value is the average of six results a t three added levels. Recovery yo Food homogenate Cereal . . Meat . . . . Fish . . Fats . . . . Fruit . . . . Root vegetables . . Green vegetables . . Beverages . . . . Milk . . Average . . Antimony 0.021 pg* 108 89 103 103 102 102 103 106 103 102 Arsenic 0.095 pg* 99 88 92 99 102 92 97 84 94 -Tin 0.69 pg*t 95 97 104 93 95 99 100 98 91 97 I Average 101 91 104 96 99 101 98 100 93 98 * Average content in the homogenate.t The average content of homogenates used for recovery a t 40 and 100 pg was 5 pg of tin. For the method in application to foodstuffs a similar exercise on standard reference materials and retail foodstuffs was instituted without regard to possible interfering species. The effects of the latter on the results are discussed later. Duplicate total analyses were made by each of the analysts each duplicate result being obtained within the same series of measure-ments and these results are summarised in Tables VII and VIII.Values obtained by wet oxidation for arsenic in two fish samples are included for comparison as a matter of interest. For the sample of NBS tuna it has been reported that the first can examined gave inconsistent results for lead12 and similar inconsistency was observed for tin in the range 2-7 mg k g l . All results given in Tables VII and VIII for this material were obtained on a second can and consideration of the variation for tin results suggest this can to be normal. It has been mentioned that levels of these three elements in foodstuffs are normally low, but exceptions have been described that require dilution of digests to enable measurement to be made in the concentration range described. As there is the possibility that most or all of the variations inherent in the total method arise from the hydride evolution and measurement stages it is necessary to consider the standard deviations (and factors derived from them) in two ways.To maintain a constant practice the standard deviations in Tables VII and VIII do not take account of these dilutions and for comparison of concentra-tions measured necessary allowances must be made. The footnotes to Tables VII and VIII are therefore brought to the attention of readers TABLE IT1 Element Antimony Arsenic Tin . . Total No. of diet groups 9 9 9 8 8 8 119 1:s I f 7 6 6 9 37 37 RECOVERY OF ANTIMOWY(III) ARSENIC(II1) AND TIN ADDED TO TOTAL DIET Amount Mean Significance* Repeatability No.of No. of added/ recovery Range r-*-, results analysts p g Y O % Matrix Analyst -1 1s 3 0.2 106 70-152 NS NS 25 0.053 0.0053 18 3 0.4 103 56-154 NS NS 22 0.092 0.0092 18 3 1.0 96 56-109 NS NS 10 0.10 0.010 16 3 0.2 91 67-116 NS NS 18 0.032 0.0032 16 3 0.4 92 78-115 NS 1% 8.0 0.030 0.003 16 3 1.0 99 78-124 NS NS 8.9 0.088 0.0088 6 3 2.0 101 60-125 - NS 22 0.44 0.44 6 3 4.0 103 94-112 - NS 5.4 0.22 0.22 6 3 10 101 94-113 - NS 7.6 0.76 0.76 12 3 2.0 100 67-134 NS NS 18 0.36 0.036 12 3 4.0 95 76-113 NS NS 9.4 0.36 0.036 18 3 20 96 68-113 576 KS 7.2 1.4 0.14 6 2 40 96 91-104 NS NS 5.4 2.1 0.21 6 2 100 102 95-108 NS NS 4.3 4.4 0.44 * NS = not significant. t These intervals are for a single result the first value derived from repeatability and the second derived from $ For these samples the results represent recovery from 1 g of a fish homogenate using dry ashing for destruction Measurement was made on prepared solutions diluted by a factor of two.3 Measurement was made on prepared solutions diluted by a factor of five. on a 10-g sample mass. method the confidence intervals are calculated for a 1-g sample mass Element Antimony Tin . . TABLE VII REPLICATE ANALYSES FOR TOTAL ANTIMONY AND TIN ON SINGLE FOODSTUFFS AND Foodstuffs material* Dried milk . . Flour . . Apricot puree . . NBS tuna . . NBS liver . . Tuna . . . . Pig kidney . . Bowen’s kale . . Spinach . . NBS liver . , Flour . . Dried milk . . Pig kidney . . Bowen’s kale . . NBS tuna . . Apricot purees . . Spinach .. . . Tunas . . Mean Range of Repeatability Sample content/ content/ Amount/ (-Ap, mas& 2.5 10 10 5 5 10 10 10 2 5 10 2.5 10 10 2 5 10 10 mg kg-l 0.002 0.002 0.002 0.004 0.005 0.003 0.005 0.010 0.082 0.01 0.01 0.05 0.02 0.02 0.16 1.39 7.5 8.3 mg kg-i 0-0.007 0-0.005 0-0.005 0-0.01 1 0-0.012 0-0.011 0-0.016 0.002-0.03 1 0.047-0.12 0-0.03 0-0.05 0-0.21 0-0.04 0-0.04 0-0.30 1.15-1.56 6.7-8.4 7.4-9.1 Pg 0.005 0.017 0.017 0.020 0.027 0.027 0.052 0.10 0.16 0.05 0.10 0.13 0.15 0.18 0.33 7.0 75 83 % Pg - 0.008 - 0.017 - 0.015 - 0.026 - 0.021 - 0.042 - 0.054 - 0.11 33 0.053 0.06 - 0.20 - 0.17 - 0.16 - 0.12 67 0.22 6.1 0.43 4.8 3.6 5.8 4.8 -mg kg-l 0.003 0.002 0.002 0.005 0.004 0.004 0.005 0.011 0.027 0.012 0.020 0.067 0.016 0.012 0.11 0.085 0.36 0.48 * Each row represents six results obtained by three analysts.t These intervals are for a single result the first value derived from repeatability and the second derived from For repeatability and reproducibility concentrations are based on sample mass taken. Results are calculated on a dry-mass basis; recorded consensus means for Bowen’s kale are antimony 0.069 There was no analyst significance for any sample. on 10-g sample masses. Measurement was made on prepared solutions diluted by a further factor of five to that normally used TABLE VIII Method of destruction of organic matter Wet oxidation Dry ashing REPLICATE ANALYSES FOR TOTAL ARSENIC ON SINGLE FOODSTUFFS AND STANDARD Foodstuffs material* Dried milk Apricot puree Coffee Flour Bowen's kales NBS livers7 Spinach Tuna7 NBS tuna37 Pig kidney71 Sample mass/g 2.5 10 5 10 2 5 10 10 6 10 Mean content/ mg kg-l 0.007 0.002 0.01 1 0.011 0.091 0.041 0.024 0.47 2.55 1.50 Range of content/ mg kg-1 0-0.023 0-0.008 0-0.022 0.007-0.01 3 0.066-0.12 0.027-0.050 0.020-0.030 0.45-0.53 2.42-2.97 1.29-1.78 Amount/ Pg 0.017 0.023 0.055 0.11 0.18 0.20 0.24 4.7 12.7 15 Signifi-cance of analystst NS NS NS NS NS NS NS NS 5% 5 Yo Repeatability Pg mgkg-- 0.027 0.011 - 0.037 0.004 66 0.037 0.007 8.2 0.009 0.001 16 0.030 0.015 9.0 0.019 0.0037 15 0.035 0.0035 6.4 0.30 0.030 8.6 1.1 0.22 8.8 1.3 0.13 T---Bowen's kales 1 0.093 0.005-0.168 0.093 5% 24 0.022 0.022 Tuna 1 0.54 0.40-0.62 0.54 NS 12 0.063 0.063 NBS tuna 0.25 2.88 2.38-3.12 0.72 NS 10 0.072 0.29 Pig kidney 1 1.33 1.16-1.58 1.33 NS 6.7 0.089 0.089 Fish flourlr 0.20 11.8 10.6-12.4 2.35 NS 4.5 0.11 0.54 * Each row represents six results obtained by three analysts.t NS = not significant. These intervals are for a single result the first value derived from repeatability and the second derived For repeatability and reproducibility concentrations are based on the sample tj Results are calculated on a dry-mass basis; recorded consensus mean for Bowen's kale is 0.14 mg kg-1.2g 7 Measurement was made on a prepared solution diluted by a factor of five for tuna and ten for NBS tuna based on a 10-g sample mass.matter by dry ashing intervals are calculated for 1-g sample mass. the fish flour 30 EVANS et al. DETERMINATION OF ANTIMONY ARSENIC AND TIN Analyst VoZ. 104 Discussion Tin To avoid confusion information that can be derived from these results will be considered separately for each element and for convenience tin will be considered first. The accuracy of the total method in the presence of representative foodstuffs can be accepted by reference to Tables V and VI. To test whether the recovery from any total diet homogenate is abnormal each duplicate mean at each level must be tested for 95% confidence intervals (C.I.) of -!&s/l/2 where s is the reproducibility from the over-all mean at each particular level.A single low recovery of 68% for the fat homogenate at 20 pg of added tin is responsible and this is the source of the matrix significance listed in Table VI. (Discards are not allowed because of the low levels being monitored.) It can also be noted that the average recovery of 91% from milk at the three added levels is the lowest obtained. No reference foodstuff material exists with authenticated levels of tin and the accuracy of the method in application to foodstuffs must depend upon the consensus mean for Bowen’s kale and any certified future value of NBS tuna. (On the basis of the evidence in Table IV the copper content of NBS liver does not permit the determination of tin in that material by the method.) The variation of the results for the method and the method in application according to the design previously described,15 are each defined by two standard deviations at each estimated level of the element.The repeatability so is the standard deviation of analytical uncertainty obtained by known analysts on representative foodstuffs and is unencumbered by other sources of variation other than the homogeneity of the samples examined. The reproducibility s reflects that obtained by any analyst on any foodstuff to which the method is applicable. s will also contain the variation between series of determinations and the variation inherent in the blank and these sources of variation will be included within the matrix variance for recovery experiments and contribute to analyst variance for the indi-vidual foodstuffs examined.Each value for so and s is an estimate subject to variation; for 6 degrees of freedom the 95 C.L. for so will be 0.64-2.20 so. Within this context values listed for so expressed as the coefficient of variation in Tables VI and VII display a suitable gradation as the estimated amount increases. If allowance is made for the additional &fold dilution of digests to give a new sequence for the amount measured this gradation continues to apply and further, similar values are evident at similar measured levels whether the results originate from recovery experiments or from individual foodstuffs. Each individual result will be obtained from at least duplicate injections and the R.S.D.s of single injections of standards are defined in Table 11 first row.Consideration of the 95 C.L. of the ratio of the coefficients of variation obtained in the recovery and single foodstuff exercises with that for similar levels of dupli-cated injection of pure solutions at 0.01 and 0.04 mg 1-1 (equivalent to measurements of 5 and 20 pg of tin in 100 ml of original digest) of 8 and 3y0 respectively indicate only one ratio that falls outside these confidence limits namely for recovery a t 20 pg of tin and the reason has been isolated. Similar observations can be made for s and values compared with duplicate injections of pure solutions for between-series measurement in routine use (Table 111). In the region of 0.01 and 0.04 mg 1-1 the latter coefficients of variation are 7 and 5y0, respectively and the ratios at equivalent amounts measured do not exceed 95 C.L.with the exception of recovery for 20 pg of tin. The agreement of so from the recovery exercise with that for injection of standard solutions suggests that there is little contribution to variation from digestion of different foodstuffs. Similar agreement for the relevant individual foodstuffs indicates that tin in these samples is homogeneously distributed. The agreement for s with duplicate injections of standard solutions between series measurement suggests cumulative indirect interference type (b) is not a problem for the range of foodstuffs and foodstuff homogenates examined. The general agreement between recovery and foodstuffs exercises for so and s further signifies not sur-prisingly for tin the absence of variation from different chemical states in foodstuffs.Finally, the agreement for both exercises with injections of pure solutions suggests that the overriding variation in the determination of tin originates from the hydride evolution and measurement stages. Table VII indicates the difficulty of obtaining meaningful lower levels of tin in non-canned Only one out of 27 means exceeds this limit which is statistically acceptable January 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 31 foodstuffs. Even though the standard deviation expressed as an amount is constant the skew distribution caused by zero results makes it impractical to compare derived limits of detection of results defined by so or s for the relevant degrees of freedom from the two exercises.Those derived from so and s for the recovery exercise at 2 and 4 pg of added tin will be 1.3 and 1.2 pg respectively which for the described procedure implies detection limits of 0.13 and 0.12 mg kg-l. The so and s values for recovery at 2 4 and 40 pg of tin (the last amount measured as Spg) and for NBS tuna each expressed as amounts are effectively constant and this is reflected in the 95 C.I. for each. This suggests that for subsequent exercises undertaken single results for total tin in the range 0.2-0.8 mg k g l should be reported to a 95 C.I. of 0.2 mg kg-l and above 0.8 mg kg-1 to a 95 C.I. of 25%. Antimony Recovery of antimony( 111) added to representative foodstuffs with hydride evolution from digests containing antimony as antimony(V) is illustrated in Table VI.The figures are justification for the decision to avoid a pre-reducing stage before hydride evolution and also exemplify the efficiency of the additional hydrogen peroxide stage in completely oxidising the antimony present to the higher valency state. The added antimony(II1) originated from a commercial primary standard solution specially prepared for atomic-absorption spectroscopy while the antimony(V) calibration standards were prepared from a reagent of purity not less than 97%. The over-all mean (from 54 results) reflects this. No significance from either matrices or analysts is apparent for this exercise but closer inspection of the duplicate means reveals that 95 C.L.are exceeded for addition of 1 pg to the meat homogenate. This is caused by a single low recovery of 56% the remaining recoveries at this level being in the range 88-109%. No standard reference material exists containing low authenticated levels of antimony. The summary of results in Table VII, with the exception of Bowen’s kale are included only for interest reflecting the low levels found in practice. It is impossible therefore to consider the variation of results in a manner similar to tin. The significance of the ratio of the R.S.D.s within and between the series of measurements on standard solutions has already been noted (p. 23) and this significance could arise from insufficient care with blanks from excessive instrument variation between series of measure-ment or from transient indirect interferences singly or in total.Consideration of the ratio of standard deviations indicates that there is clear agreement for s at each recovered level and for Bowen’s kale expressed as the coefficient of variation with between-series measure-ments on standard solutions (Table 111) and clear disagreement at measured concentrations of 0.004mg1-1 (equivalent to the recovery of 0.4pg of antimony) for so for within-series duplicate measurements on standard solutions (Table 11). This increased variation for so to a level similar to s could not be reflected by blanks or between-series measurements. As none of the foodstuff homogenates or single foodstuffs (except NBS liver) contain levels exceeding those in Table IV that directly interfere and it has been shown that indirect inter-ference on variation from single ionic species does not occur (p.24) a tenuous conclusion is drawn that at these low determined levels transient indirect cumulative interferences cause this increase in variation for so. Even if this conclusion is accepted it cannot be said, on the basis of the evidence that there is little contribution to variation from digestion of different foodstuffs even though the single sample of Bowen’s kale returns values for so and s that are in a suitable gradation for the amounts determined in recovery experiments. It can be said however that cumulative direct interference is not a problem (from the accuracy) and that indirectly the principle source of variation in the determination of antimony is the hydride evolution and measurement stages.Values for so and s expressed as the amount for Bowen’s kale and recoveries at 0.2 and 0.4 pg of added antimony(II1) are relatively constant. The derived limits of detection from the recovery exercise will be 0.23 and 0.21 pg for so and s respectively. The 95 C.I. for these three examples suggest that single results for total antimony can be reported as 0.02 and 0.04 mg kg-l and above the latter to 95 C.I. of 50%. This is not effective for the levels of this element likely to be present in foodstuffs. Arsenic The evidence presented in Table VI confirms that following both wet oxidation and dr 32 EVANS et al. DETERMINATION OF ANTIMONY ARSENIC AND TIN Analyst Vol. 104 ashing of a fish homogenate as a procedure for the destruction of organic matter arsenic added as arsenic(II1) will be obtained in the resulting digest in the higher valency state The exercise reflecting the accuracy of the total method in the presence of representative food-stuffs does display analyst significance for a recovery of 0.4 pg of arsenic(II1) and this ori-ginates from single high recoveries for the cereal and root vegetable groups.The recovery a t 1.0 pg of arsenic(II1) added while displaying no significance also has a single low recovery, which causes the 95 C.L. for that duplicate mean to be exceeded. In general recovery from the meat homogenate appears to be low and the same remark applies to milk. For recovery experiments using dry ashing a single non-significant low result of 60% occurs at the 2.O-pg added level.While the accuracy of the method can be considered to be acceptable for the method in application to foodstuffs again few standard reference materials of known arsenic content exist. NBS liver possesses a certified arsenic content but unfortunately with the present method (and for similar methods) copper would be expected to interfere each 1-ml injection of acidified digest containing 10 pg of copper (see Table IV). The bias obtained for this material is 0.041-0.055 which is equal to -0.014 mg kg-l with a 95 C.I. for this mean value of &0.010 mg k g l based on six results which merely reflects the interference. NBS tuna, which does not contain levels of interfering species does not have an authenticated content, but several values are recorded in the literature e.g.3.3 mg kg-l with a standard deviation of 0.2 mg kg-1 based on three results.1° The mean value of NBS tuna in this evaluation, with destruction of organic matter by dry ashing is 2.88 mg k g l with a 95 C.I. of A0.31 Testing of the analyst means for the individual foodstuffs in Table VIII indicates 4 instances out of 45 that fall outside the relevant 95 C.I. and these are the sources of the analyst signifi-cance tabulated. The comparison of levels found in the two fish samples using different procedures for destruction of organic matter is of interest. In each instance the dry-ashing technique gives significantly higher mean levels confirming the previous literature informa-tion. The agreement for so and s for these samples within the meaning of variation is marked however which suggests that for NBS tuna itself the greater part of the arsenic exists in a form readily and consistently destroyed by the wet-oxidation technique.The variation defined by so and s separately follows an effective gradation expressed as the coefficient of variation for the total amount determined with the exception of the recovery of 2 pg of arsenic(II1) by dry ashing. This applies both for the recovery exercise and for the individual foodstuffs and whether so or s originate from foodstuffs treated by wet oxidation or by dry ashing of the sample. This gradation is undisturbed if allowance is made for the various digest dilutions described in the footnotes to Tables VI and VIII. Comparison of so and s for these latter measured levels with that for duplicate injections of standard solutions within and between series of measurements (Tables I1 and 111) by examination of 95 C.L.of the ratios indicates no single instance where these ratios are exceeded. The same conclusions obtained for tin therefore apply for the variation from digestion of different foodstuffs homogeneity cumulative indirect interferences and the principle source of variation in the determination of arsenic. Levels for so and s expressed as amount are relatively constant for low levels of measured arsenic and sufficient individual foodstuffs were examined for which zero results are absent. A comparison of the derived limits of detection from so and s can therefore be made for the two exercises. From the recovery of 0.2 and 0.4 pg of arsenic(III) values of 0.11 and 0.12 pg are obtained from so and s respectively and from the average of the two lowest standard deviations for the individual foodstuff's displaying no analyst significance values of 0.19 and 0.16 pg of arsenic are obtained; this implies detection limits for the procedure of 0.011-0.019 mg kg-l.From the 95 C.I. tabulated it does appear that single results for total arsenic could be reported as 0.02 and 0.04 mg kg-l and for higher concentrations to 95 C.I. of 50%; as for antimony this defines the effectiveness of the method. mg kK1. Conclusions A complete analytical method has been described for the determination of total antimony, arsenic and tin in foodstuffs. The digests obtained from the wet-oxidation procedure can also be used for the determination of a number of other elements (copper iron manganese Jcknaary 1979 IN FOODSTUFFS BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 33 zinc lead cadmium and nickel).In the application of this digestion procedure the deter-mination of arsenic in samples of marine origin is an exception and an alternative dry-ashing technique on a limited sample mass must be used. Each of the three elements will be in the highest valency state in these digests and as such can be measured using atomic-absorption spectroscopy with atomisation in a flame-heated silica tube after evolution as the normal hydrides. The optimum conditions for measurement are described and the effect of direct interferences from ionic species upon the measurement are assessed defining the occasions when the application to foodstuffs is invalidated.For these elements the most important invalidation is for antimony and arsenic by tin in canned foodstuffs. For such occasions it is advisable to include an additional procedure for the removal of tin. The effect of indirect interferences from ionic species to which atomic-absorption spectroscopy with atomisation in a silica tube is susceptible has also been considered and evidence is presented to show that such indirect interference from single ionic species does not occur for the levels investigated. The accuracy of the total method for the low levels of each element normally present in foodstuffs has been confirmed and the accuracy of application of the method to foodstuffs has been considered as far as it is possible at present.The standard deviations repeatability (so) and reproducibility (s) for hydride evolution and measurement have been obtained for standard solutions for both within- and between-series measurements. Similar standard deviations have been calculated from results for the method via recovery experiments and the method in application via individual foodstuffs. Testing the ratios of the series of repeatabilities and reproducibilities separately and against each other indicates no significant differences except for antimony at low concentrations. The absence of significance suggests that there is little contribution from the digestion of different foodstuffs; tin and arsenic distribution in individual foodstuffs tested are homogeneous cumulative indirect inter-ferences do not occur and the principle source of variation for each element originates from the hydride evolution and measurement stages.While the accuracy of low levels of antimony determined by the method is satisfactory the elevated variation at these levels may be caused by transient indirect cumulative interferences of ionic species. The standard deviations are relatively constant as amount for a very narrow range for the lowest amounts determined for each element but from these a practical limit of detection has been deduced from the results of each element. For results obtained above these levels, consideration of 95 C.I. suggests the effectiveness of the results for the three elements that can be obtained by this method and indeed by any similar manual method employing hydride evolution and atomic-absorption spectroscopy with atomisation in a silica tube.This paper is published with the permission of the Government Chemist. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. References Tech. Rep. Ser. Wld Hlth Org. No. 532 Geneva 1973. “Arsenic in Food Regulations 1959,” SI 1959 No. 1831 HM Stationery Office London. Hanson N. W. Editor “Official Standardised and Recommended Methods of Analysis,” Second “Official Methods of Analysis of the Association of Official Analytical Chemists,” Twelfth Edition, Schmidt F. J. and Royer J. L. Analyt. Lett. 1973 6 17. Fernandez F. J. Atom. Absorption Newsl. 1973 12 93. Pollock E. N. and West S. J. Atom. Absorption Newsl. 1973 12 6. Kahn H. L. and Shallis J. E. Atom. Absorption Newsl. 1968 7 5. Chu R. C. Barron G. P. and Baumgarner P. A. W. Analyt. Chem. 1972 44 1476. Fiorino J . A. Jones J . W. and Capar S. G. Analyt. Chem. 1976 48 120. Thompson K. C. and Thomerson D. R. Analyst 1974 99 595. Evans W. H. Read J . I. and Lucas B. E. Analyst 1978 103 580. Analytical Methods Committee Analyst 1960 85 643. Leblanc P. J. and Jackson A. L. J . A s s . OH. Analyt. Chem. 1973 56 383. Evans W. H. Analyst 1978 103 452. Gorsuch T. T. Analyst 1959 84 135. Lunde G. J . Sci. Fd Agric. 1973 24 1021. Braman R. S. and Foreback C. G. Science N.Y. 1973 182 1247. Uthe J. F. Freeman H. C. Johnston J. R. and Michalik P. J . Ass. Ofl. Analyt. Chem. 1974, Edition Society for Analytical Chemistry London 1973. Association of Official Analytical Chemists Washington D.C. 1975. 57 1363 34 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. EVANS JACKSON AND DELLAR Smith A. E. Analyst 1973 98 209. Goulden P. D. and Brooksbank P. Analyt. Chem. 1974 46 1431. Aggett J. and Aspell A. C. Analyst 1976 101 341. Schmidt F. J. Royer J . L. and Muir S. M. Analyt. Lett. 1975 8 123. Davies 0. L. and Goldsmith P. L. “Statistical Methods in Research and Production,” Oliver Boyd, Smith A. E. Analyst 1975 100 300. Pierce F. D. and Brown H. R. Analyt. Chem. 1977 49 1417. Pierce F. D. and Brown H. R. Analyt. Chem 1976 48 693. Maurice M. J. and Buiys K. 2. Analyt. Chem. 1969 244 18. Bowen H. J. M. J . Radioanalyt. Chem. 1974 19 215. Edinburgh 1972. Received June 21st 1978 Accepted July 25th 197

ISSN:0003-2654    

DOI:10.1039/AN9790400016

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, Janztayy, 1979, Vol. 104, pp. 35-40 35 Determination of Germanium, Arsenic, Selenium, Tin and Antimony in Complex Samples by Hydride Generation - Microwave-induced Plasma Atomic-emission Spectrometry Wayne B. Robbins and Joseph A. Caruso and Fred L. Fricke Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1, USA Cincinnati District Food and Drug Administration, 1141 Central Parkway, Cincinnati, OJzio 45202, USA This paper describes the utilisation of a semi-automatic hydride generation device coupled to a microwave-induced argon - helium plasma that is used with a 0.5-m monochromator for the analysis of germanium, arsenic, selenium, tin and antimony in several complex samples. Incorporated in the system are a condensation tube and a Chromosorb 102 column that are used to separate the analyte species from hydrogen evolved during the course of the generation reaction, and to separate the analytes from condensed contaminants that cause spectral background interferences. Results reported are standard recoveries and relative precision for ger- manium, arsenic, selenium, tin and antimony in whole blood and in enriched flour. Also reported are arsenic and antimony values for NBS orchard leaves.Results obtained on complex samples are compared with the precision obtained for aqueous standards. Keywords : Hydride generation ; microwave-induced filasnza ; atomic-absorption spectrometry ; germanium, arsenic, selenium, tin and antimony determination The utilisation of hydride generation in atomic-spectrometric analysis is continually increasing in p~pularity.l-~ This type of analysis allows the separation and pre-concentration of the analyte from sample matrices.Elimination of the sample matrix serves to circumvent the problem of chemical interferences in various emission sources and atom reservoirs that are a frequent complication in techniques utilising solution nebulisation. The resulting advantages of hydride generation include lower detection limits and extended linear ranges. Fricke et aL6 described the coupling of a semi-automatic hydride generator to a microwave-induced argon - helium plasma via a condensation tube immersed in liquid nitrogen and chromatographic separation of background interferences on a Chromosorb 102 column. Advantages over the conventional hydride generation - atomic-absorption systems included extension of linear ranges, elimination of the use of hollow-cathode lamps, and adaptability to multi-element analyses.Linearity of up to three orders of magnitude, and detection limits in the sub parts per billion (parts per lo9) range were reported for germanium, arsenic, selenium, tin and ant imony.6 As complex samples were not investigated in that study, it was believed that the applica- bility of the system could be established more clearly by investigation of several complex samples. This was the purpose of the work described in this paper. Experimental Apparatus A 2450-MHz microwave generator and Eversen &wave cavity were supplied by Opthos Instrument Co. A silica plasma containment tube (110 x 1.7 mm id.) was supplied by Amersil Inc.The monochromator was a Jarrell-Ash 0.5-m instrument with a 1180 lines mm-l grating blazed for 300 nm, with a reciprocal linear dispersion of 1.6 nm mm-l in the first order. A Jarrell-Ash MVAA 82500 atomic-absorption spectrometer was used in the emission mode with a 100-pm straight-edge fixed slit for the entrance slit and a 1P 28 photomultiplier36 Analyst, Vol. 104 tube mounted at a 150-pm exit slit as the detector. The recorder was a Hewlett Packard, Model 7107 B. The chromatograph was a length of Polypenco Nylaflow pressure tubing, 3ft x 4.7 mm i.d., packed with Chromosorb 102, 60-80 mesh. The condensation tube was a siliconised glass tube of 1.5 mm i.d. with a 40-cm length packed with 1.0-mm siliconised glass helices.The hydride generator was as described by Fiorino et al.2 A schematic diagram of the instrumental arrangement is shown in Fig. 1 and a discussion of the development of the system has been published elsewhere.6 ROBBINS et al.: DETERMINATION OF GERMANIUM, ARSENIC, I - . - , Microwave Plasma generator - containment Monochromator PMT 1 Argon column tube - CaC12 .2H20 Vent to d-. Three-way valve, V2 Condensation - 4- Three-way valve, V1 a Helium CaC12 Q I Hydride I generator Electronics EI- 1 Recorder I I ~ Fig. 1. Block diagram of the instrumental arrangement. Reagents Stock solutions of Se4+, Sn4+ and Sb3+ were prepared from Fisher atomic-absorption standards while Ge4+ and As3+ solutions were prepared from Ventron [and E.M. Laboratories standard stock solutions, respectively.A 4% m/V solution of sodium tetrahydroborate(II1) stabilised with 1% m/V of sodium hydroxide was used to generate the hydrides from solutions in hydrochloric acid. Hydrochloric acid. Reagent grade. Arg0.n. Commercial grade, 99.998%. Helium. Calcium chloride. dihydrate. Acid mixture. acids supplied by G. Frederick Smith Chemical Co., Columbus, Ohio. Commercial grade, 99.999%, passed through powdered calcium chloride. Fisher ACS-grade %mesh granular and Baker ACS-grade powdered Nitric - sulphuric - perchloric acids (4 + 4 + l), prepared from re-distilled Instrumental Conditions power and 0 W reflected power. respectively. The microwave-induced argon - helium plasma (MIP) was operated at 110 W forward Argon and helium flow-rates were 400 and 300 ml min-1,January, 1979 TIN AND ANTIMONY I N COMPLEX SAMPLES 37 The hydride generator was operated as described by Fiorino et aL2 This device serves to meter a precise amount of sodium tetrahydroborate( 111) solution into the acidic mixture of analyte through solenoid valves controlled by a timer.Solutions are transferred by means of carefully controlled gas pressure. The chromatographic column was maintained at room temperature (approximately 23 "C) . Deviations of &3 "C did not significantly affect the chromatographic characteristics of the system. Analytical wavelengths used during this work were germanium 303.9, arsenic 193.7, selenium 196.0, tin 317.5 and antimony 259.8 nm. Preparation of Samples Whole blood A 5.0-ml volume of expired whole blood obtained from the Paul I.Hoxworth Blood Bank, Cincinnati, Ohio, was mixed with 30 ml of the acid mixture and was heated until fumes of sulphur trioxide were evolved. The final acid concentration was adjusted to that deter- mined as described under Optimisation of Signal with hydrochloric acid. Enriched $our Flour (2.00 g) that had been dried in an oven overnight at 85 "C was mixed with 30 ml of the acid mixture and heated until fumes of sulphur trioxide were evolved. The final acid concentration was adjusted to that determined as described under Optimisation of Signal with hydrochloric acid. NBS orchard leaves 1571 Portions of approximately 1.0 and 1.5 g for arsenic and antimony, respectively, that had been dried as specified by NBS, were accurately weighed, mixed with 30ml of the acid mixture and heated until fumes of sulphur trioxide were evolved.The final acid concentra- tion was adjusted to that determined as described under Optimisation of Signal with hydro- chloric acid. Owing to possible positive or inhibitive effects in the quantitative generation of the hydrides because of the presence of various salts in the digestion matrix, a standard additions graph was made to determine the concentration in each instance, rather than a determination from a calibration graph. Although no attempt was made to determine the final oxidation state of the elements after the digestion procedure, the recoveries from the spiked samples indicate clearly that the oxidation states present are converted into the hydrides with no apparent difficulty.Wet as opposed to dry oxidation was chosen as it presented fewer problems when all five elements were taken into consideration. Determination of Germanium, Arsenic, Selenium, Tin and Antimony With reference to Fig. 1, the condensation tube was isolated from the reaction chamber by adjusting the three-way valve V1 and then isolated from the vent leading to the hood and the chromatographic column by adjusting the second three-way valve V2. The con- densation tube was cooled in liquid nitrogen for 2.5 min, then V1 and V2 were adjusted so that the condensation tube was open on one end to the reaction chamber and on the other end to the hood vent. A 20-ml aliquot of the solution containing the element of interest was placed in the reaction tube, the tube attached to the head of the hydride generator and the timer activated. The resulting reaction swept the hydrides through granular calcium chloride desiccant into the condensation tube where they were condensed, together with any volatile contaminants, while hydrogen was vented to the hood.Valve V1 was then adjusted to close the condensation tube to the reaction chamber and allow helium to pass through to the hood for 0.5 min; this operation removed any residual hydrogen not vented during the course of the reaction. Valve V2 was opened to the chromatographic column and the con- densation tube was immediately immersed in a water-bath at 80 "C. The helium flow then caused the vaporis2d mixture of hydrides and other condensed contaminants to pass through powdered calcium chloride desiccant into the chromatographic column, where the hydrides and contaminants were separated, and finally into the MIP where the elemental excitation took place.Areas under the peaks obtained with the strip-chart recorder were used to represent the signal, and were measured with a planimeter.38 ROBBINS et at?. : DETERMINATION OF GERMANIUM, ARSENIC, Analyst, T/d. 104 Optimisation of Signal Owing to the nature of the contaminants in the sodium tetrahydroborate(II1) and its varying quality, depending upon the manufacturer and the batch, it is necessary to optimise the reaction conditions needed to produce the elemental hydrides each time a different batch or a different manufacturer's material is used. To do this, the concentrations of the acid solutions from which the hydrides were generated were varied until a maximum signal was achieved.The hydrides of germanium, arsenic, tin and antimony are optimally generated from solutions of the same acid concentration (1.2-2.4 N), while the selenium hydride is optimally generated from more acidic solutions than the other four hydrides. The wavelength for each analytical line was optimised with the analyte signal. Standard Recoveries of the desired element to the sample and acid mixture prior to digestion. calculated by comparing peak areas with those obtained from spiked samples. Recoveries were measured by adding several Inicrolitres of a concentrated aqueous solution Recoveries were Spiked Samples Owing to possible positive or inhibitive effects in the generation of the hydrides, because of the presence of various salts in the digestion matrix, standard solutions were added to a sample after the digestion process was complete, in order to produce the spiked sample.Results and Discussion Standard recoveries for germanium, arsenic, selenium, tin and antimony aresh own in Tables I and 11. As was mentioned previously, these recoveries were determined by using spiked samples as the standard of comparison. Hydride generation interference effects have been noted with the addition of a variety of different cations to solutions of the elements being determined. The average deviations of four determinations indicate the relative precision as determined with this technique. Such precisions are satisfactory, especially when the TABLE I STANDARD RECOVERIES OF ELEMENTS FROM WHOLE BLOOD Average deviation* Amount added/ Amount recovered, from mean, Element Pg % Mean, yo % Ge 0.4 91, 91, 84, 82 87 4 As 1 .o 100, 108, 94, 95 97 3 Se 2.0 99, 105, 100, 101 101 2 Sn 1 .o 113, 108, 119, 107 112 5 * Average deviation from the mean value of four replicate determinations. TABLE I1 STANDARD RECOVERIES OF ELEMENTS FROM ENRICHED FLOUR Average deviation* Amount added/ Amount recovered, from mean, % '% Mean, yo Element Pg As 1.0 105, 95,.91, 105 99 6 Se 2.0 77, 114, 97, 77 91 14 Sb 0.5 87, 84, 77, 84 83 3 * Average deviation from the mean value of four replicate determinations.January, 1979 TIN AND ANTIMONY I N COMPLEX SAMPLES 39 low detection levels and numerous manipulation steps are considered.When compared with the relative precision data obtained from aqueous solutions listed in Table 111, there is no noticeable increase in the scatter of results that could be attributed to the introduction of the complex samples. It is also important to note that the absolute masses listed do not reflect the final solution concentrations that are dealt with, or that could be dealt with. To illustrate this, a com- parison of detection limits in micrograms is listed in Table I11 along with the solution concentration, assuming that the analyte is generated from the customary 20 ml of solution. TABLE I11 DETECTION LIMITS FOR AQUEOUS STANDARDS Data taken from reference 6. Wavelength/ Detection Detection Relative standard Element nm limit/pg* limit, p.p.b.* deviation? Ge 303.9 0.003 0.15 3.2% at 1 p g As 193.7 0.007 0.35 6.5% at 0.25 pg Se 196.0 0.025 1.25 5.5% at 1 p g Sn 317.5 0.040 2.00 2.9% at 1 pg Sb 259.8 0.010 0.50 8.8% at 0.1 p g * Using a 20-ml sample volume and defined as 2 x the standard deviation of t Standard deviation of 10 determinations as emission peak areas divided by base-line noise. the mean emission peak area and converted to percentage.Values for arsenic and antimony in NBS orchard leaves are shown in Table IV, as deter- mined by the standard additions technique. It should be noted that the arsenic deter- mination is well within the expected range. The antimony determination gives lower results than the NBS values, but the deviations meet. The difference in the values could be attributed to the fact that the NBS value was obtained by neutron-activation techniques.Although a value for selenium was determined by the NBS, also using neutron activation, this element was not determined by these laboratories as the amount of leaves needed for each analysis became unreasonably large for digestion and subsequent determination with the standard additions technique. TABLE IV DETERMINATION OF ARSENIC AND ANTIMONY IN NBS ORCHARD LEAVES 1571 Arsenic content/pg g-l Certified ’ Sample Found value 1 9.3 2 9.4 3 8.3 4 8.9 Mean 9.0 & 0.4* 10.0 f 27 Antimony contentlpg g-l Found value A r 1 Certified 2.8 2.3 2.1 2.0 2.3 & 0.3* 2.9 f 3t * Average deviation from mean. t Two standard deviations “of entire range of observed results.” In summary, the results indicate that hydride generation coupled with microwave-induced plasma atomic-emission spectroscopy is a useful technique for analysis of complex samples. In fact, the low detection limits and the degree of reproducibility that the technique offers make it not only superior to techniques involving solution nebulisation, but the charac- teristic long dynamic linear ranges and potential for multi-element analysis, which the microwave-induced plasma offers, make it potentially more useful than the more widely used atomic-absorption - hydride generation techniques of analysis.40 ROBBINS, CARUSO AND FRICKE References 1. 2. 3. 4. 5. 6. 7. Rliyazaki, A., Kimura, A . , and Umezaki, Y., Analytica Clzim. Acta, 1977, 90, 119. E'iorino, J . A., Jones, J . W., and Capar, S. G., Analyt. Chem., 1976, 48, 120. Drinkwater, J . E., Analyst, 1976, 101, 672. Seimer, D. D., and Koteel, P., Analyt. Chenz., 1977, 49, 1096. Shaikh, A. U., and Tallman, D. E.. Analyt. Chern., 1972, 49, 1093. Fricke, F. L., Robbins, W. B., and Caruso, J. A., J . Ass. Off. Analyt. Chem., 1978, 61, 11s. Smith, A. E., Analyst, 1975, 100, 300. Received March 14tli, 1978 Accepted July loth, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400035

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, January, 1979, Vol. 104, pp. 41-46 Determination of Micro-amounts of Polythionates 41 Part XI." Polythionates in Their Mixtures by Cyanolysis and Solvent Extraction Spectrophotometric Determination of Two Species of Tomozo Koh and Yuzi Aoki and lwaji lwasaki Department of Cltenaistry, Faculty of Science, Tokai Unive+i, Hiratsuka, Kanagama 259-12, Japan Depaytment of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan Two species of polythionates, when present in their mixtures, have been determined by solvent extraction and spectrophotometry of the thio- cyanate formed by cyanolysis of polythionates in both the absence and presence of copper(I1) ions. When 1 mol of polythionate, Sz0,2- (where x = 4, 5 or 6) undergoes cyanolysis, x - 2 mol of thiocyanate are formed in the presence of copper(I1) ions; in the absence of Cu(II), x - 3 mol of thio- cyanate are formed.Therefore, the value of x in Sz0,2- can be evaluated. By using these values, the composition of solutions containing two species of polythionates was calculated. The present method can be applied to the determination of two species of polythionates when mixed in various ratios at a concentration level of micromoles per litre. h'eywords : Spectrophotometry ; polytlzionate determination ; cyanolysis ; methy- kene blue ; thiocyanate There is a growing interest in sulphur chemistry, and accurate and sensitive methods for the determination of polythionates in their mixtures have become increasingly desirable. For example, when 1 mol of polythionate (tri-, tetra-, penta- or hexathionate) reacts with cyanide, sulphite, sulphide or hydroxide, the number of moles of thiosulphate formed is different in each instance.Therefore, these reactions of polythionates must be studied in detail in order to determine a specific polythionate in the presence of other polythionates. Many investigations have been made on the cyanolysis of polythionates and methods for the determination of trithionate,l-4 tetrathionate,3-7 pentathionate* and he~athionate,~ based on the photometric measurement of the thiocyanate formed, have been proposed. As a result of these investigations, we have developed a methodlo for the determination of polythionates in mixtures with each other. The sensitivity of the determination of poly- thionates could be increased about 60-fold by extracting the thiocyanate, formed by the following reaction, as an ion pair with methylene blue into an organic solventll: SSOG2- + (X - 1)CN- + H2O = S20,2- + SO,2- + ZHCN + (X - 3)SCN- - - (1) where x = 4, 5 or 6.One of the present authors has reported a similar method for the determination of thiosulphate,12 based on the copper(I1)-catalysed cyanolysis of thiosulphate. If copper(I1) is added to the reaction mixture in which equation (1) is stoicheiometrically complete, then the thiosulphate formed will be converted into thiocyanate. The over-all reaction of polythionates with cyanide is as follows : S,0G2- 4- xCN- + H20 = SO,2- + + ZHCN + (x - 2)SCN- .. * * (2) where x = 4, 5 or 6. Therefore, two species of polythionates in their mixtures can be determined by spectrophotometric measurement of the different amounts of the thiocyanate formed by cyanolysis of polythionates in the presence and absence of copper( 11) ions.In this work, we have increased the sensitivity for the determination of two species of polythionates in their mixtures about 50-fold, by solvent extraction of the thiocyanate * For Part X of this series, see Bull. Chew. SOC. Japan, 1978, 51, 164.42 KOH et al.: DETERMINATION OF MICRO-AMOUNTS Analyst, Vol. 104 formed by cyanolysis of polythionates as an ion pair with methylene blue. This method is suitable for the determination of 10-7-10-6~ polythionates and is more sensitive than any previous method. Experimental Apparatus photometer with 10-mm glass cells.Iwaki, Model KM, shaker. M-5, pH meter. Temperatures were regulated by a Taiyo, Model M-1, thermostat. Spectrophotometric measurements were carried out with a Hitachi, Model 139, spectro- Extractions were carried out by shaking with an pH measurements were made with a Hitachi-Horiba, Model Reagents without further purification. All chemicals, except the polythionates, were of analytical-reagent grade and were used Polythionates Potassium tetrathionate was prepared as described by Stamm et al.,13 and potassium pentathionate and potassium hexathionate as described by Goehring and Feldmann.14 The raw polythionates obtained were recrystallised and then dried at room temperature before storage at -10 & 2 "C. The water physically adsorbed on the polythionates and the water of crystallisation of pentathionate were determined by the Karl Fischer method.The polythionates were confirmed to be sufficiently pure for the present purpose; their purity was calculated from the total potassium and sulphur content^.^-^ Standard poly- thionate solutions were stored at 5 & 2 "C. Standard solutions Standard tetrathionate solution, 1 x M. Dissolve 151.3 mg of the potassium tetra- thionate (water content 0.07% m/m) in re-distilled water and dilute to 500 ml with water. This stock solution is stable for 6 month^,^ but a 1 x M solution is stable for 2 weeks, when stored at 5 & 2 "C. Standard pentathionate solution, 1 x M. Dissolve 186.8 mg of the potassium penta- thionate (water content including water of crystallisation 10.48% m/m) in re-distilled water and dilute to 500 ml with water.This stock solution is stable for 4 months,s but a working standard solution (1 x Dissolve 183.8mg of the potassium hexa- thionate (water content 0.29% m/m) in re-distilled water and dilute to 500ml with water. This stock solution is stable for 2 month^,^ but a 1 x 1 0 - 5 ~ solution is stable for 1 week, when stored at 5 &- 2 "C. Prepare a stock solution by dissolving potassium thio- cyanate in re-distilled water and standardise it by Volhard's method.l5 Working standards were used to confirm the stoicheiometry and completion of cyanolysis of the polythionates. B u f e r solutions. Prepare the buffer solution of pH 5.5 used in procedure A by mixing 150 ml of 0.2 M sodium dihydrogen orthophosphate solution with 11.2 ml of 0.2 M sodium hydroxide solution. Prepare the buffer solutions of pH 5.8 and 9.8 used in procedure B by mixing 150 rnl of 0.2 M sodium dihydrogen orthophosphate solution with 17.1 ml of 0.2 M sodium hydroxide solution and by mixing 100 ml of 0.3 M sodium hydrogen carbonate solution with 100 ml of 0.3 M sodium carbonate solution, respectively. CoP@er(II) solution, 0.6 M.Dissolve 37.64 6; of copper(I1) sulphate (CuS04.5H20) in re-distilled water and dilute to 250 ml with wa.ter. Dilute this stock solution with water to obtain a 0.06 M copper(I1) solution. Dissolve 48.22 g of ammonium iron(II1) sulphate [(NH,)Fe(S0,),.12H20] in re-distilled water containing 50 ml of 2 M sulphuric acid, and dilute to 500 ml with water to give a 0.2 M solution of iron(II1) in 0.2 M sulphuric acid.Methylene blue solution, 8 x 1 0 - 3 ~ . Dissolve 1.518 g of methylene blue (98.5%) in re-distilled water and dilute to 500 ml with water. Prepare a working solution by suitable dilution. M) is stable for 2 weeks, when stored at 5 5 2 "C. Standard hexathionate solution, 1 x 1 0 - 3 ~ . Standard thiocyanate solution. Solution of iron(II1) in sulphuuric acid.January, 1979 OF POLYTHIONATES. PART XI 43 Methods Procedure A (molar concentration of SCN- formed by this method = P) Pipette 1 ml of 0.06 M cyanide solution, 0.5 ml of phosphate buffer solution (pH 5.5) and a 10-ml aliquot of the sample solution containing two species of polythionates into a 50-ml calibrated flask. The pH of the solution is thereby brought to 7.4, which is in the optimum range of 7.2-7.6.Place the flask in a thermostat at 40 "C and allow it to stand for 5 h,ll in order to convert the polythionates completely into thiocyanate. Add 1 ml of 0.12 M formaldehyde solution for the removal of excess of cyanide, transfer the mixture into a 50-ml separating funnel, then add 1.5 ml of 2 x M methylene blue and 10 ml of 1,2- dichloroethane. Shake the funnel for about 2 min, in order to extract the ion pair between the methylene blue cation and the thiocyanate anion formed by cyanolysis of the poly- thionates. When the layers have separated, transfer the organic layer into a 15-ml glass- stoppered tube and add some anhydrous sodium sulphate. Shake the mixture vigorously by hand until transparent and measure the absorbance at 657 nm16 against dichloroethane.Procedure B (,molar concentration of SCN- formed by this method = Q) Add 1 ml of 0.1 M cyanide solution, 1 ml of phosphate buffer solution (pH 5.8) and a 10-ml aliquot of the sample solution containing two species of polythionates into a 50-ml calibrated flask, the pH of the solution thereby being brought to 7.3. Allow the flask to stand in a thermostat at 40 "C for 5 h, then add, by pipette, 1 ml of carbonate buffer solution (pH 9.8) and 1 ml of 0.06 M copper(I1) solution; the pH of the solution is brought to 7.9, which is in the optimum range of 7.4-8.4.12 Shake the mixture vigorously by hand immediately after the addition of copper(II), and allow it to stand at room temperature for 20min in order to convert completely the thiosulphate formed into thiocyanate.To this mixture, add 1.5 ml of 0.2 M iron(II1) in 0.2 M sulphuric acid in order to decompose the copper - cyanide complexes formed, which would interfere seriously with the determination. Transfer the mixture into a 50-ml separating funnel and add 1.5 ml of 2 x 10-3 M methylene blue solution and 10 ml of 1,2-dichloroethane. Shake the funnel for 2 min and allow the layers to separate. Transfer the organic layer into a 15-ml glass-stoppered tube, and add some anhydrous sodium sulphate. Shake the mixture vigorously by hand until transparent and measure the absorbance against dichloroethane at 657 nm. Results and Discussion Calibration Graphs The calibration graphs obtained by procedures A and B were found to show a good linear relationship. If the polythionate, S,0,2- (where x = 4, 5 or 6), is completely and stoicheio- metrically converted into thiocyanate, the calibration graphs for tetra-, penta- or hexa- thionate, prepared by procedure A, should coincide with that for thiocyanate when the molar concentration scales for thiocyanate and tetrathionate, and pentathionate, are drawn to one third and two thirds, respectively, of the scale for hexathionate concentration.The calibration graphs proved that the reaction according t o equation (1) had proceeded to completion. Similarly, the calibration graphs for tetra-, penta- or hexathionate, prepared by procedure B, were in full agreement with that for thiocyanate when plotted in terms of equivalent concentrations, demonstrating that cyanolysis of the three polythionates attained stoicheiometric completion according to equation (2).Evaluation of x in Sx062- in Solutions Containing Only One Species of Polythionate Ion Suppose that P and Q denote the molar concentrations of thiocyanate, which were deter- mined from the respective calibration graphs prepared by procedures A and B. Then, P/Q = (x - 3)/(x - 2), where x is the number of sulphur atoms in the formula for S,062-. When arranged with respect to x, the equation x = (3Q - 2P)/(Q - P ) can be obtained and, after rearrangement, x = [3(Q - P) + P]/(Q - P ) or x = P/(Q - P) + 3 is obtained. By using the second equation, values of x were determined for aqueous solutions of poly- thionates. The results in Table I show that tetra-, penta- and hexathionate, respectively, are present as pure polythionates even at a concentration level of micromoles per litre.44 KOH et al.: DETERMINATION OF MICRO-AMOUNTS TAE~LE I A?zalyst, Vol. 104 EVALUATION OF X I N s,062- I N SOLUTIONS CONTAINING ONLY ONE SPECIES OF POLYTHIONATE ION Concentration of polythionate/ Polythionate pmol 1--1 s,op 1 .oo s50,2- 1.00 s,o,2- 1 .oo 3.00 5.50 2.00 4.00 2.00 3.00 Determined concentration of thiocyanate/ pmoll--1 r - y Procedure A Procedure B X 0.97 1.91 4.03 3.05 6.02 4.03 5.42 10.93 3.98 1.90 2.89 4.92 3.99 6.08 4.91 7.94 12.00 4.96 3.09 4.10 6.06 6.05 8.00 6.10 8.88 11.91 5.93 The present method enabled a value for x in the formula S,OC2- to be determined with sufficient accuracy for the polythionates above trithionate. In previous work Urban6 determined these values to be 4.084.14 for tetrathionate, 4.214.34 for pentathionate and 4.87-5.35 for hexathionate.From these results, he deduced that polythionates, with the exception of tetrathionate, are present as mixtures containing the other polythionates in aqueous solutions. We disagree with his conclusion because the polythionates undergo partial alkaline decomposition caused by hydrolysis of the cyanide under the conditions of his method. Determination of the Composition of Solutions Containing Two Species of Poly- thionate Ions When two species of polythionate are present. simultaneously, the composition can easily be determined by using the values of x calculated in terms of x = P/(Q - P ) + 3. As can be seen in Table 11, there is close agreement between the calculated and the experimentally determined compositions.TABLE I1 DETERMINATION OF THE COMPOSITION OF SOLUTIONS CONTAINING TWO SPECIES OF POLYTHIONATE IONS Concentration of poly- thionate/pmol 1-1 7- s4OG2- S50,2- 3.0 1.0 2.0 2.0 1.0 3.0 S40,2- s,0,2- 3.0 1.0 2.0 2.0 1.0 2.0 s50,2- s,o,2- 2.0 1.0 1 .o 1.0 1 .o 2.0 Composition calculated, x a - 7 s40,2- s,o,2- 75 25 4.24 50 50 4.45 25 75 4.74 S40,2- s,o,2- 75 25 4.49 50 50 4.88 33.3 66.7 5.22 s,oG2- s,0e2-- 66.7 33.3 5.32 50 50 5.55 33.3 66.7 5.62 Coinposition determined, 7; p a - s,o,e- 5 4 0 6 76 24 55 45 26 74 s,0,2- s,o,2- 75.5 24.5 56 44 39 61 s50,2- s,o,2- 68 32 45 55 38 62 Analysis of the Tetrathionate - Pentathionate, Tetrathionate - Hexathionate and Pentathionate - Hexathionate Mixtures The method consists in the determination of different amounts of the thiocyanate formed according to equations (1) and (2) on two 10-ml aliquots of sample solution containing twoJanuary, 1979 OF POLYTHIONATES.PART XI 45 species of polythionates; one is treated by procedure A and the other by procedure B. the tetrathionate - pentathionate mixture For and all units being in molar concentration. The molar concentration of thiocyanate (P) obtained by procedure A corresponds to the sum of the tetrathionate and twice the pentathionate in the mixture, and that by procedure B (Q) to the sum of twice the tetrathionate and three times the pentathionate in the mixture, respectively. Hence, from simultaneous equations, (S4OS2-) = 2Q - 3P and (S,062-) = 2P - Q.Similarly, for the tetrathionate - hexathionate mixture (S40e2-) = (3Q - 4P)/2 and (s606%) = (2P - Q)/2, and for the pentathionate - hexathionate mixture (S5O6.2-) = 3Q - 4P and (s6062-) = 3P - 2Q. By using these equations, tetrathionate - pentathionate, tetrathionate - hexathionate and pentathionate - hexathionate mixtures were analysed. The results given in Table I11 show that the above technique can be applied successfully to the determination of two species of polythionates mixed in various ratios. The highest recovery was l08y0 and the lowest 92%, with a mean of 99.6%. TABLE I11 ANALYSIS OF TETRATHIONATE - PENTATHIONATE, TETRATHIONATE - HEXATHIONATE AND PENTATHIONATE - HEXATHIONATE MIXTURES Amount of polythionate/pmoll-l I -. Taken Found Recovery, yo r-----------l -7 7-Y S40,2- s50,2- s40,2- s50,2- S4O,2- S50,2- 1.0 3.0 1.05 2.96 105 97 2.0 1 .o 2.04 0.93 102 93 3.0 1.0 3.06 0.97 102 97 3.0 2.0 2.77 2.06 92 103 4.0 1 .o 3.96 1.02 99 102 s40,2- S,O,2- S40,2- s,0,2- s*o,2- s,0,2- 0.5 2.0 0.50 1.99 100 99 1.0 1.0 1.05 0.98 105 98 1.5 1.0 1.60 0.93 107 93 2.0 0.5 1.93 0.53 97 106 3.0 1.0 3.05 0.98 102 98 s,0,2- s,0,2- s50,2- s,o,2- s,op S,O,2- 0.5 1.5 0.54 1.44 108 96 1 .o 1 .o 0.92 1.06 92 106 1.5 0.5 1.55 0.47 103 94 2.0 0.5 1.99 0.48 99 96 2.0 1.0 2.05 0.97 103 97 A statistical study of 11 independent determinations by both procedures A and B was carried out on 10-ml volumes of solution containing various amounts of two species of polythionates but equivalent to a concentration of 7 x M thiocyanate. Procedure A gave a mean absorbance value of 0.613, with a standard deviation of 0.0045 absorbance unit and a relative standard deviation of 0.73%, and procedure B a mean absorbance of 0.437 with a standard deviation of 0.0050 absorbance unit and a relative standard deviation of 1.1%.Conclusion The sensitivity of the determination of two species of polythionates in their mixtures can be increased about 50-fold by extracting the thiocyanate formed by cyanolysis of poly- thionates as an ion pair with methylene blue into 1,2-dichloroethane. The value of x in46 KOH, AOKI AND IWASAKI SzOG2- in solutions containing only one species of polythionate ion and the composition of solutions containing two species of polythioriate ions were evaluated. Tetrathionate - pentathionate, tetrathionate - hexathionate and pentathionate - hexathionate mixtures can be analysed with good sensitivity and reproducibility. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. References Urban, P. J., Z. Analyt. Chem., 1961, 179, 422. Koh, T., Wagai, .4., and Miura, Y., Analytica Claim. A d a , 1974, 71, 367. Kelly, D. P., Chambers, L. A., and Trudinger, F. A., Analyt. Chem., 1969, 41, 898. Mizoguchi, T., and Okabe, T., Bull. Chem. SOC. Japan, 1975, 48, 1799. Nietzel, 0. A., and DeSesa, M. A., Analyt. Chem., 1955, 27, 1839. Urban, P. J., 2. Analyt. Chem., 1961, 180, 110. Koh, T., and Iwasaki, I., Bull. Chem. SOC. Japan, 1966, 39, 352. Koh, T., Bull. Chem. SOC. Japan, 1965, 38, 1510. Koh, T., and Iwasaki, I., Bull. Chem. SOC. Japan. 1965, 38, 2135. Koh, T., and Iwasaki, I., Bull. Chem. SOC. Japan, 1966, 39, 703. Koh, T., Saito, N., and Iwasaki, I., Analytica Chim. Acta, 1972, 61, 449. Koh, T., Bunseki Kagaku (Japan Analyst), 1973, 22, 322. Stamm, H., Goehring, M., and Feldmann, U., 2. Anorg. Allg. Chem., 1942, 250, 226. Goehring, M., and Feldmann, U., 2. Anorg. Chevn., 1948, 257, 223. Kolthoff, I. M., Sandell, E. B., Meehan, E. J.. and Bruckenstein, S., “Quantitative Chemical Analysis,” Fourth Edition, Macmillan, New York, 1969, p. 798. Koh, T., and Iwasaki, I., Bull. Chem. SOC. Japan, 1967, 40, 569. Received December 19th, 1977 Accepted J u l y 26th, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400041

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, JanGary, 1979, Vol.104, $$. 47-54 Determination of Ammonia in Low Concentrations 47 with Nessler‘s Reagent by Flow Injection Analysis F. J. Krug,* J. RiliiEka and E. H. Hansen Chemistry Department A Technical University of Denmark Building 207, DK-2800 Lyngby, Denmark A turbidimetric procedure for the determination of ammonia in low concentra- tions with the use of Nessler’s reagent is described. Both natural waters and soil extracts can be analysed a t a rate of up to 120 samples per hour with good precision and accuracy. The effects of reagent composition, flow- rate, temperature and protective colloids in the flow injection system are discussed in detail. Keywords : Flow injection analysis ; ammonia determination ; Nessler’s reagent ; turbidimetric determination ; continuous-flow vneasuvement In 1856 Nessler introduced a reagent consisting of mercury(I1) iodide and potassium iodide in alkaline solution for the qualitative and quantitative determination of ammonia.Known since then as Nessler’s reagent, it has been used extensively as the most sensitive test for ammonia. However, little was known regarding what happens when the components are mixed in aqueous solution until Sarkar and Ghosh1s2 undertook a detailed study over 20 years ago. They concluded that the composition of the colloidal precipitate formed was NH,-,Hg,I,, the value of TZ depending on the concentration of Hg142- and OH- in the reagent mixture and on the amount of ammonia. Thus, at low concentrations of ammonia, Nessler’s reagent was believed to react according to the equation 2Hg142- + NH, + 30H- + O<:i>H21 + 71- + 2H20 forming a brown precipitate, while at increasing ammonia concentrations and a constant OH- concentration the composition tended towards NHHg,I,.H,O (deep brown) and finally NH2Hg,I , (chocolate).Several analytical applications of Nessler’s reagent have been described in the The main advantages of the method are good sensitivity and simplicity. However, the Nessler method is accurate only if a number of conditions are carefully controlled.7 Previous work on the determination of sulphate by continuous flow injection turbidimetry has demonstrated that flow injection analysis can be used with this type of detection with very good control of the experimental parameters, such as precise timing of reaction sequences and reproducible mode of mixing and rate of nucleation, which allow good accuracy to be obtained even at high sampling rates.1° Several papers describing the use of flow injection analysis for the determination of various chemical species by different detection procedures have already been p ~ b l i s h e d .l ~ - ~ ~ The determination of ammonia as total nitrogen in plant digests was previously studied by using both potentiometric (air-gap electrode) and colorimetric (indophenol blue) dete~ti0n.l~ Although the air-gap electrode is suitable for the determination of ammonia in low con- centrations (down to 0.1 p.p,m. of NH4+-N), the method is slow, allowing a sampling rate of only about 60 determinations per hour. The indophenol blue method is reproducible, accurate and fast (120 samples per hour), but not sufficiently sensitive (applicable down to 5 p.p.m.of NH,+-N), and therefore it cannot be applied to soil extracts and natural waters. The aim of this paper is to show the feasibility of continuous-flow injection for the coloured turbidimetric determination of ammonia in low concentrations (0.1 p.p.m. of NH,+-N). A number of variables have been studied, such as optimum reagent concentrations, alkalinity, flow-rates, mixing coil lengths, temperature and presence of protective agents. Further, a practical approach to the determination of optimum reagent composition in a flow injection system is described. * Present address: Centro de Energia Nuclear na Agricultura, Caixa Postal 96, 13400 Piracicaba, ’350 Paulo, Brazil.48 KRUG et al.: DETERMINATION OF AMMONIA IN LOW CONCENTRATIONS Analyst, Vol. 104 Experimental Reagents and Samples All chemicals were of analytical-reagent grade. Nessler’s reagent. A procedure similar to that described by VogeP was used: 14g of potassium iodide are dissolved in 40 ml of distilled water and a 4% solution of mercury(I1) chloride is added while stirring continuously with a magnetic stirrer until a slight red precipitate is formed. Then 100 ml of 10 N sodium hydroxide solution are added and the volume is made up to 500 ml with distilled water. Further small amounts of mercury(I1) chloride are added until there is a permanent turbidity. The mixture is allowed to stand for Id, then decanted and the solution is stored in an amber-glass bottle.If any precipitate is formed during the next 7d, the solution must again be decanted or filtered to avoid deposition of solid particles in the optical path of the flow-through cuvette. A stock solution containing 100 p.p.m. of NH,+-N is prepared by dissolving 0.381 8 g of ammonium chloride in ammonia-free water and diluting to 1000 ml with ammonia-free water. Standard solutions in the concentration range 0.5-8.0 p.p.m. of NH,+-N are prepared by suita’ble dilutions of the stock solution. A 0.02~o stock solution is prepared by suspending 0.2g of poly(viny1 alcohol) in about 100ml of water with continuous stirring, then adding 800 ml of boiling water. After cooling, the solution is made up to 1000 ml with water. Working solutions are prepared by suitable dilutions of the stock solution. Standard nmmonia solutions.PoZy(vinyl alcohol) solutions. Du Pont Elvanol 71-30 was used as a stabiliser. Sodium hydroxide solution, 2.0 M. Water samples. Soil sample extracts. Samples were collected in the Sdlerprd Lake, north of Copenhagen, where the nitrogen balance is currently being investigated in connection with environmental studies. These were obtained from Aarhus University, Denmark, where they were prepared by two different acid digestion procedures and analysed by titration. Apparatus and Procedures The five manifolds (Fig. 1) were made from polyethylene tubing (internal diameter 0.50, 0.75 and 0.86 mm) and plastic toy components (Lego, Billund, Denmark). The experi- ments were carried out using 30-p1 sample volumes, injected by means of a precisely machined rotary valve described previously.18 In all experiments samples and standards were injected at least in triplicate.The coloured turbidity was measured at 410 rim using a Corning, Model 254, colorimeter, equipped with a Hellma flow-through cuvette, Type 0s 178.12 (volume 18p1, light path 10 mm), connected to a Servograph REC recordeir furnished with an REA 112 high-sensitivity unit (Radiometer A/S, Denmark). The reagent streams were pumped by an Isniatec, Model MP 13 GJ-4, peristaltic pump (Ismatec S.A., Switzerland) operated at speed 8, 9 or 10 with suitable pump tubes to obtain the desired flow-rates. Occasionally a slight precipitate might form, which would be deposited on tube walls and in the flow cell.In such an event, usually after a few hundred analyses, the system must be cleaned by pumping through it distilled water for 3 min, 5 M hydrochloric acid for 30 s and again distilled water for 3 min. Results and Discussion Reagent Concentration in Carrier Stream There are numerous methods for the preparation of Nessler’s reagent.3-8 The final choice was a slightly modified version of Nessler’s reagent as described by Vogel.3 The reason for modification is that in manual methods a large volume of sample (about 50 ml) and only 1 or 2 ml of reagent are ~ s e d . ~ ~ ~ ~ ~ ~ ~ In flow injection analysis the ratio between sample and reagent volumes is not as large, and therefore the reagent was further diluted, viz., with 2 N sodium hydroxide solution, so as to ensure the same alkalinity in all sample solu- tions investigated. Therefore, the concentration of Nessler’s reagent in the carrier streams used was 5-500/, V/V of that of the original stock solution.January, 1979 WITH NESSLER’S REAGENT BY FLOW INJECTION ANALYSIS Nessler (’I or NH4+ 49 50 cm 0.75 mm 1.8 12) Nessler S 50 cm -- 1 @ A 0.50 mm - Nessler (3) PVA 0.86 0.86 r - - - - 1 Nessler 15% Water-bath 7 I j 50cm i 4 50cm I I 1.66 , L-!!!-J (5) 4 50cm N essl er 0.86 mm Waste Fig..1. Flow diagrams for the determination of ammonia as used for investigating the various experimental parameters ( 1-4) and for routine analysis ( 5 ) . 1, Effect of reagent composition; 2, effect of pumping rate and alkalinity; 3, effect of using poly(viny1 alcohol) as protective agent; 4, effect of temperature; and 5, confluence system used for analysis of acid sample solutions.S = point of sample injection (30 PI); = pumping rate (ml min-I); the coil lengths are given in centimetres and the internal coil diameters in millimetres. TO find the optimum concentration of Nessler’s reagent two series of experiments were performed : (a) pumping a known ammonia standard solution and injecting reagent solutions of different concentration; and (b) pumping solutions of Nessler’s reagent of different con- centrations and injecting various ammonia standards. In both series of experiments the same manifold was employed (Fig. 1, manifold 1). Using a fixed flow-rate (1.8 ml min-l) and by adjusting the length of the coil and internal diameter of the tube a reaction time of about 10 s was obtained together with a medium dispersion of the sample (or reagent) zone in the reagent (or sample) stream.The resulting sampling rate of about 120 samples per hour was considered to be satisfactory for all practical purposes. The reproducibility of measurement for each point, showed in the family of curves, obtained in both experiments [Fig. 2(a) and ( b ) ] was &l.5y0. Fig. 2(a) shows, for procedure (a), the influence of the concentration of Nessler’s reagent on the peak height for a series of pumped ammonia standard solutions containing 0.0, 1.0 and 2.0 p.p.m. of NH,+-N. The maximum peak height for the 1.0 and 2.0 p.p.m. N samples was obtained by injecting 15y0 V/Y Nessler’s reagent. The linearity of the conventional calibration graphs as obtained with various concentrations of Nessler’s reagent in the carrier stream (10, 15, 20 and 30% V / V ) is shown in Fig.2(b) [procedure (b)]. The reagents were otherwise the same as used in procedure (a). The final choice of adopting the 15% V/V reagent solution concentration was made using Fig. 2(a) and (b). The linearity and slope50 KRUG at?.: DETERMINATION OF AMMONIA I N LOW CONCENTRATIONS AnaZyst, Vd. 104 1.0 - 0.8 - 0.6 - 0.4 - 0.2 r A * 4 0 10 20 30 40 60 Concentration of Nessler reagent, % V/V r A A A I I I 0 2 4 6 8 N concentration, p.p.rn. Fig. 2. Influence of the composition of Nessler’s reagent on the coloured turbidity produced using configuration 1 (Fig. 1). (a), Pumping sample solutions containing (A) 0, (B) 1 and (C) 2 p.p.m.of NH + N and injecting the reagent; and (b), pumping reagent solutions containing 10, 15, 20 and 30% Vl;, i n relation to the originally prepared Nessler s8tock solution) and injecting the samples. of the calibration graphs show that procedure (a) is suitable for selecting the best concentra- tion of reagent. In general, this approach should always be used for developing other methods in which expensive reagents are used. In addition, this method saves time as it allows a number of parameters to be investigated rapidly. An interesting point to consider is the difference between the peak heights for 1 and 2 p.p.m. of NH,+-N, as found by comparing Fig. 2(a) and (b), which represents 0.68 and 0.09 absorbance unit, respectively, for the 20% V/V Nessler’s reagent.This indicates that only a small volume of reagent is required in order to produce a significant coloured turbidity. During the dispersion of the reagent zone [procedure (a)] there is more ammonia available, resulting in a higher peak compared with the original procedure (b), where the sample zone is dispersed in the reagent stream. There- fore, it is possible to consider yet another flow injection system for a high-sensitivity deter- mination. Thus, by using manifold 1 in Fig. 1 and procedure (a), ammonia can be determined down to 0.02 p.p.m. of NH,+-N (theoretical limit of detection for 1% absorbance), but only 30 samples can be analysed per hour. The reagent consumption would be only 3Opl of 15% V/V Nessler’s reagent per determination for a 2-ml sample volume, which, in comparison with the manual methods,3-5 represents a drastic reduction [about 400 times less potassium mercury(I1) iodide].Yet another, recently suggested, technique, the so-called zone merging26 is being investi- gated with the aim of further reducing the consumption of reagents and increasing the sampling rate. This approach involves the use of a new type of valve, which simultaneously injects sample and reagent into a carrier stream of 2 N sodium hydroxide solution, permitting a higher sampling rate of about 100 determinations per hour. Alkali Concentration in Carrier Stream The influence of the concentration of sodium hydroxide on the effectiveness of the Nessler’s reagent is shown in Fig, 3. The experiment was carried out with manifold 2 (Fig. l), by pumping 1.67 ml min-l of 15% VlV Nessler’s reagent with final sodium hydroxide con- centrations of 0.5, 1.0 and 2.0 N.Increasing the alkali concentration increases the sensi- tivity,’~~ but unfortunately also increases the base-line noise owing to the generation of “schlieren patterns.” Also, on increasing the concentration of alkali an increase was noticedJanuary, 1979 WITH NESSLER’S REAGENT BY FLOW INJECTION ANALYSIS 51 in the accumulation of precipitate on the walls of the tube and on the windows of the flow cell, causing a gradual drift of the base line (cj., Fig. 7). Although the high blank values obtained for the reagent containing 2 N sodium hydroxide solution obviously affect the accuracy of the determination of ammonia at concentrations lower than 0.5 p.p.m.of NH,+-N, this alkali concentration was, however, considered to be the optimum. Flow-rate The influence of the flow-rate of the reagent (15% V/V Nessler reagent in 2 N sodium hydroxide solution) on the absorbance signal is shown in Fig. 4. This experiment was carried out using manifold 2 (Fig. 1). By varying the flow-rate from 0.67 to 1.67 ml min-l, the isoconcentration curves obtained for 2.0, 4.0 and 6.0 p.p.m. of NH,+-N indicate that the reaction is very fast. This is surprising as manual methods have been reported to require a considerably greater reaction time697,9 (10-60 min for completion), while a t a flow- rate of 1.67 ml min-l the reaction time (sample residence time) is approximately 6 s. At lower flow-rates (0.67 ml min-l) a double peak is formed owing to the lower dispersion of the sample zone in the reagent stream, which can be explained from the theory of di~persion.~’ A /f N concentration, p.p.m.1.2 al C 42 2 0.8 Q 0.4 Samples per hour 0 0.67 1.00 1.33 1.67 Flow-rate /mi min- ’ Fig. 3. Influence of the concentration of Fig. 4. Influence of flow-rate on the coloured sodium hydroxide on the ammonia calibration turbidity and the sampling rate for 16% V/V graphs, using manifold 2 (Fig. 1) with a pumping Nessler’s reagent in 2 N sodium hydroxide rate of 1.66 ml min-1, the carrier stream being solution,using manifold 2 (Fig. 1) with different 15% V / V Nessler’s reagent containing: (A), pumping rates (4). Isoconcentration curves 2 N ; (B), 1 N ; and (C), 0.5 N sodium hydroxide for: (A), 0.0; (B), 2.0; (C), 4.0; and (D) 6.0 solution.p.p.m. of NH,+-N. Basically, the lower the flow-rate, the lower is the Reynolds number and the lower is the dispersion of the sample zone, which means that the sample in certain regions is not so well mixed in the reagent stream. The high blank values noticed for the isoconcentration curve for 0 p.p.m. of NH,+-N are due to differences in refractive index between the reagent and reagent plus sample, caused by the high content of alkali. The “schliering” decreases on increasing the flow-rate and again the sample dispersion theory explains the differences obtained. As a compromise for performing the ammonia determinations at a high sampling rate and with good accuracy (a high ratio between sample and blank), a flow-rate of 1.67 ml min-l was chosen.52 KRUG et id.: DETERMINATION OF AMMONIA I N LOW CONCENTRATIONS Analyst, Vd. 104 Influence of Temperature The influence of the temperature during the nesslerisation on the ammonia isoconcentration curves is shown in Fig, 5. This experiment was executed with manifold 4 (Fig. l), pumping 15% V/V Nessler's reagent in 2 N sodium hydroxide solution. From the results it can be seen that in the temperature range 2142 "C the coloured turbidity formed is virtually the same for samples containing less than 4 p.p.m. of NH,+-N. Above this level, the iso- concentration curves for 6 and 8 p.p.m. of NH4+-N indicate an anomaly. This might be explained by the fact that at higher concentrations of ammonia, turbidity predominates over colour and the size of the colloidal particles is affected by temperature variations. Again, this observation is in good agreement with results obtained by manual that is, that an increase in temperature leads to an increase in turbidity. This was further confirmed by the drift of the recorded base line above 31 "C, and visually by inspecting the walls of the reaction coil, which showed the deposition of a slight, orange - brown precipitate.Use of a Protective Agent To prevent precipitation on the walls of the tube we tried the use of poly(viny1 alcohol) (PVA), which acts as a protective agent for colloidal suspension^.^^^^ This experiment was carried out with manifold 3 (Fig. 1). Although its use has been recommended for the determination of ~ulphate,~ it was not successful in the determination of ammonia; thus Fig.6 shows that on increasing the PVA concentration the sensitivity decreases. An explanation for this phenomena is the effect of PVA in altering the colour of the complex formed. There was no influence on the slight precipitation on the walls of the tube, but the colour of the precipitate changed from orange - brown to grey - brown. -D 20 30 40 Temperatu re/"C Fig. 5. Influence of temperature on the coloured turbidity produced as recorded with manifold 4 (Fig. 1). Iso- concentration curves for: (A), 0.0; (B), 1.0; (C), 2.0; (D), 4.0; (E), 6.0; and (F), 8.0 p.p.m. of NH,+-N. 1.2 0 0 5 0.8 d 13 L I 0 2 4 6 N concentration, p.p.m. Fig. 6. Influence of the presence of poly(viny1 alcohol) in the carrier stream on the ammonia calibration graphs (manifold 3, Fig.1). Poly- (vinyl alcohol) concentration: (A), 0; (B), 0.005; ( C ) , 0.010; and (D), 0.020%. Practical Applications The accuracy of the proposed method can be judged from the results in Table I and the precision and sampling rate from the recorder output shown in Fig. 7. On comparing the flow injection system with the manual indophenol blue method, a good correlation between the two methods was obtained for water samples. On the other hand, for the soil extracts the results were less satisfactory (comparing flow injection analysis with titration). ForJanuary, 1979 WITH NESSLER’S REAGENT BY FLOW INJECTION ANALYSIS TABLE I COMPARISON OF PROCEDURES FOR THE DETERMINATION OF AMMONIA IN NATURAL WATERS AND SOIL EXTRACTS Values are given in parts per million of NH,+-N.0.4 0 - Sample No. 3 4 5 6 9 11 12 18 19 24 28 29 - Water samples A -l Manual Flow injection indophenol method blue method 3.90 3.90 3.38 3.38 3.10 3.19 2.72 2.70 2.25 2.62 1.95 2.24 1.40 1.45 1.45 1.20 0.60 0.60 2.75 2.35 5.70 5.65 5.80 5.70 7 Sample No. 1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.2 5.1 5.2 6.1 6.2 Soil extracts Flow injection method 4.32 4.24 3.22 3.08 1 .oo 1 .oo 3.56 3.56 2.68 3.28 3.04 3.32 1 Titration method 4.44 3.52 4.08 2.77 1.03 1.49 5.00 3.89 3.67 3.91 2.92 2.53 53 the analysis of soil digests or acidified water samples the confluence manifold (manifold 5, Fig. 1) was used. The advantage of this system is the possibility of injecting acid samples without affecting the coloured turbidity formed. Thus, the samples are introduced into a carrier stream of 2 N sodium hydroxide solution, and the subsequent addition of the Nessler’s reagent, pre-diluted to 25% V/V, ensures an optimum reagent concentration in the last mixing coil.1.2 6, C m 0.8 4 I) Q 10 min I Scan --+ Fig. 7. Routine analysis of digested soil samples recorded with manifold 5 (Fig. 1). From left to right are shown a series of ammonia standard solutions (0.5, 1.0, 2.0, 4.0 and 6.0 p.p.m. of NH,+-N), followed by eight sample solutions and a second set of standards, and finally a blank, all solutions being injected in triplicate.54 KRUG, R ~ Z I C K A AND HANSEN Finally, it should be emphasised that the optical characteristics of the spectrophotometer used have a marked influence on the sensitivity of the method and the blank value. Thus, generally better results were obtained with an instrument that had a more collimated beam (Corning 254) than with an instrument with a wide beam (Beckman DB GT).Conclusion Nessler’s reaction has been adapted successfully to flow injection analysis, making possible the determination of ammonia in water samples and soil extracts in the range 0.5-6.0 p.p.m. of NH,+-N at a rate of 100 samples per hour. In comparison with manual methods the consumption of reagent is significantly reduced and the time required for analysis is shortened. The method would not be applicable if a significant amount of colloidal material is present in the solutions to be analysed. It would be advantageous if a protective agent could be found that would prevent the gradual deposition of precipitate on the walls of the system.The periodic cleaning of the system with acid .would then not be necessary. Thanks are due to Mr. H. Skotte of Lyngby-Taarbaek Town Council Control Laboratory, Denmark, and Dr. Per Nmnberg of Aarhus University, Denmark, for providing water and soil samples, respectively. The authors also extend their appreciation to DANIDA (Danish International Development Agency) for providing a stipend for one of us (F.J.K.) at the Technical University of Denmark, and for partial material support (DANIDA Project No. 104 Dan. 8/241). 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 26. 26. 27. References Sarkar, P. B., and Ghosh, N. N., Analytica Chzm. Acta, 1955, 13, 195.Sarkar, P. B., and Ghosh, N. N., Analytica Chzm. Acta, 1956, 14, 209. Vogel, A. I., “A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumental Marczenko, K., “Spectrophotometric Determin,ation of Elements,” Ellis Horwood, Chichester, 1976, Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis Including some Turbidimetric and Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis Including Photometric Methods,” Thompson, J. F., and Morrison, G. H., Analyt. Chem., 1951, 23, 1153. Williams, P. C., Analyst, 1964, 89, 276. Massmann, W., 2. Analyt. Chem., 1963, 193, 332. Krug, F. J., Bergamin Filho, H., Zagatto, E. A. G., and Jmgensen, S. S., Analyst, 1977, 102, 503. RbZiCka, J., and Hansen, E. H., Analytica C h i m Acta, 1975, 78, 145. RbiiEka, J., and Stewart, J. W. B., Analytica Chim. Acta, 1975, 79, 79. Stewart, J. W. B., RbiICka, J., Bergamin Filho, H., and Zagatto, E. A. G., Analytica Chim. Acta, RbiiEka, J., Stewart, J. W. B., and Zagatto, E. A. G., Analytica Chim. Acta, 1976, 81, 371. Stewart, J . W. B., and RbiiEka, J., Analytica Chim. Acta, 1976, 82, 137. RbiiEka, J., and Hansen, E. H., Analytica Chiwz. Acta, 1976, 87, 353. RfiiiEka, J., Hansen, E. H., and Zagatto, E. A. G., Analytica Chim. Acta, 1977, 88, 1. Hansen, E. H., RbiiCka, J., and Rietz, B., Anulytica Chim. Acta, 1977, 89, 241. RbiiEka, J., Hansen, E. H., and Mosbaek, H., Analytica Chim. Acta, 1977, 92, 219. Jorgensen, S. S., Bergamin Filho, H., Zagatto, E. A. G., Krug, F. J., and Bringel, S. R. B., Bolm RbiiEka, J., Hansen, E. H., Mosbaek, H., and Krug, F. J., Analyt. Chem., 1977, 49, 1858. Hansen, E. H., Krug, F. J., Chose, A. K., and RbiiCka, J., Analyst, 1977, 102, 714. Hansen, E. H., Chose, A. K., and RbiiCka, J., Analyst, 1977, 102, 705. Betteridge, D., and RbiiCka, J., Talanta, 1976, 23, 409. Bergamin Filho, H., Reis, B. F., and Zagatto, :E. A. G., Analytica Chim. Acta, 1978, 97, 427. Bergamin Filho, H., Zagatto, E. A. G., Krug, F. J., and Reis, B. F., Analytica Chim. Acta, 1978, RbiiEka, J., and Hansen, E. H., Analytica Chim. Acta, 1978, 99, 37. Analysis,” Longmans, London, 1968, p. 847. pp. 18-19. Nephelometric Methods,” Volume 11, Van Nostrand, New York, 1957, pp. 814-818. Volume IIA, Van Nostrand, New York, 1959, p. 705. 1976, 81, 371. C E N A , Piracicaba, 1977, BC 047. 101, 17. Received July loth, 1978 Accepted August 15th, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400047

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, January, 1979, Vol. 104, pp. 55-62 Determination of Chloride in High-purity Waters in the Range 0-20 pg I-' of Chloride Using lon-selective Membrane Electrodes Incorporating Mercury(1) Chloride 55 G. B. Marshall and D. Midgley Ceqztral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, KT22 7SE Two types of solid-state mercury(1) chloride electrodes have been used to determine chloride in the concentration range 0-20 pgl-l. At these low concentrations, more chloride will dissolve from the mercury (I) chloride in the electrode than is present in the sample itself. The extent of the dissolu- tion is controlled, however, by the chloride in the sample. In these circum- stances, the electrode potential is linearly related to the concentration of chloride in the sample.With the electrode housed in a flow cell with a thermostatically controlled water jacket, the correlation coefficient between e.m.f. and concentration was always greater than 0.99. The sensitivity (0.18 mV per pg 1-1 of chloride a t 25 "C and 0.4-0.5 mV per pg 1-1 of chloride a t 4 "C) was about ten times greater than that of the silver - silver chloride electrode. Total standard deviations a t 10, 5 and 2 pgl-1 of chloride were 0.4, 0.5 and 0.3 p g 1-1 of chloride, respectively. Keywords : Chloride determination ; ion-selective electrodes ; mercury(I) chloride electvodes ; high-purity waters We earlier developed a solid-state mercury( I) chloride electrode for determining low levels (0.01-1 mg 1-l)" of chloride by a manual methodl and suggested that even lower levels could be determined by using the electrode in a flow cell at a carefully controlled temperature, as has been done with a silver - silver chloride ele~trode.~ The mercury(1) chloride electrode is about ten times more sensitive than a silver chloride electrode because of the lower solubility of the mercury(1) salt.Boiler waters from modern power stations can have chloride contents as low as 0.01 mg 1-1 and even lower levels can be expected in condensed steam. The deter- mination of very low levels of chloride is particularly important for stations with once- through boilers, where the condensate should be monitored before being returned to the boiler. This paper describes the performance of mercury(1) chloride electrodes in the range 0-20 pg 1;l of chloride in flowing solutions, at a controlled temperature during several weeks of operation.Theoretical The potential of an ion-selective electrode is described by a form of the Nernst equation. For a chloride electrode, this is E = E" - k ln[Cl-] where E" is the standard potential of the electrode, [Cl-] is the total concentration of chloride in solution and k = RT/F, R being the gas constant, T the absolute temperature and F Faraday's constant. We have used concentrations instead of activities because in the method the nitric acid added to the sample provides a constant ionic medium and activity coefficients are, therefore, constant. The total concentration consists of the chloride originally present in the sample solution (m) and the chloride dissolved from the electrode itself (s), which are related by the solubility product equilibrium. For mercury(1) chloride * It should be noted that there were misprints in the title and summary of our previous paper.' In each instance the range of chloride concentration should have read 0.01-1 pg ml-1.256 MARSHALL AND MIDGLEY: DETERMINATION OF CHLORIDE IN AnaZyst, VoZ.104 Ks = [Hgz2+fI [Cl-,l2 = 0.5s(m + s ) ~ Ion-selective electrodes are generally considered to be useful only if nz is greater than s, so that there is a linear relationship between E and lnm, but, when nz is less than s, an expression for the potential can be derived as .follows: E = E" - kln(m -t s) = E" - klns -- kln 1 + - ( 3 Expanding In 1 + - and neglecting second.- and higher-order terms, we obtain ( 3 E m E" - 2ilns - km/s At such low values of m, s is virtually constant and hence E , to a very close approximation, is linearly related to m: E m E' - k'.zvt where E' = E" - klns and k' = k/s.Experimental Apparatus Potentials were measured with a Corning 110 digital pH meter reading to 0.1 mV and displayed on a Servoscribe 2s chart recorder. Two types of ion-selective electrodes were used, those made from Radiometer F3012 Universal Selectrodes, as described previously,ll and the Ionel Model SL-01 (Ionel Electrodes, Mount Hope, Ontario, Canada). Both have rnembranes made of a mixture of mercury(I1) sulphide and mercury(1) chloride; in the electrode developed at CERL the mixture is used to impregnate a graphite - PTFE electrode, while in the Ionel electrode the mixture is hot-pressed into a The reference electrode was a mercury - mercury(1) sulphate electrode with a 0.5 mol 1-1 sodium sulphate filling solution (instead of the usual 1 mol 1-1 solution, which would have precipitated at the lowest operating temperature of 4 "C).The reservoir containing the filling solution and the reference element could be raised up to 0.5 m above the remote liquid junction, which was of the ground-glass sleeve type. The chloride electrode and the remote junction were housed in a Perspex flow cell fitted with a water jacket through which thermostatically controlled water was circulated by means of a Techne C-100 thermocirculator operated in conjunction with a Techne, Model 1000, chiller unit. The remote junction was in a separate compartment downstream of the chloride electrode compartment, which contained a magnetic stirrer bar.The flow cell was painted black to exclude light. A Technicon, Model I , proportioning pump delivered the solutions to the flow cell, pumping the sample or standard solutions at 3.9 ml min--l and the acid reagent solution at 0.8 ml min-l. Air was injected at 1 ml min-l to improve mixing. The air-segmented mixture of sample and reagent solutions passed through a glass mixing coil to a T-piece, from which the air and a portion of the solution were extracted at 1.6 ml min-l. The rate of delivery of solution to the flow cell was, therefore, 4.1 ml min-l. The pump tubes were made of Y V C , but the transmission lines were made of PTFE. Before entering the cell, the solution passed through a short length of stainless-steel tubing,, which was connected to the chassis ground terrninal on the pH meter.A stainless-steel wire was immersed in the solution downstream of the electrode pair and also connected to the chassis ground point. Without these connections the signal was noisy. Reagents Water. Town mains water was distilled in .a stainless-steel still (Manesty Machines Ltd., Liverpool), and the distillate passed through a twin-column mixed-bed de-ionisation unitJanuary, 1979 HIGH-PURITY WATERS USING ION-SELECTIVE ELECTRODES 57 (Elga Products Ltd., Model B106/2). This water had a specific conductivity o€ less than 0.1 pS cm-1 at 20 "C as it left the unit and it has been found5 to have a chloride content of about 0.7 pg 1-l.A stock solution (1 000 mg 1-1 of chloride) was prepared by dissolving 1.649 g of sodium chloride in water and making up to 1 1 in a calibrated flask. Intermediate stock solutions (100, 10 and 1 mg 1-l) were prepared by successive dilutions of the above solution. The working chloride solutions were prepared by pipetting appropriate volumes of 1 mg 1-1 stock solution into polyethylene bottles of known mass, of capacity approximately 5 1, and then adding water until the mass of solution in each bottle was 5 kg (using a Mettler P11 balance weighing to 0.1 g). Nitric acid, 0.6 nzol Z-l. The reagent was prepared in 10-1 batches by dilution of 38 ml of concentrated nitric acid (BDH, Aristar grade). At the pumping rates used, this solution gave the same final concentration of acid as that used in the manual rneth0d.l Staizdard chloride solutions.Procedure method,l i.e., solutions were stirred, light was excluded and nitric acid was added. As far as possible, the experimental conditions were kept the same as in the manual Results Both the electrode developed at CERL and the Ionel electrode were tested at 25 "C and at a temperature close to 4 "C. In each instance, five batches of five standard chloride solutions (1, 2 , 5 , 10 and 20 pg 1-l) were analysed in duplicate in random order. Each batch was analysed on a separate day and all five within 7 days. The response to de-ionised water was also recorded. Sensitivity temperatures. Linear responses over the range 0-2Opg1-1 were obtained for both electrodes at both The results of linear regression analysis of the data are shown in Table I.TABLE I CALIBRATION OF MERCURY(I) CHLORIDE ELECTRODES Standard deviation Calibration slope/ of slope/ Temperature/ mV per pg 1-1 mV per pgl-' Correlation Electrode "C of chloride of chloride coefficient CERL . . 25.0 0.1785 0.003 0 0.9995 CERL .. 4.3 0.430 0.001 6 0 9999 Ionel . . . . 25.0 0.175 0.002 2 0.999 7 Ionel . . .. 3.6 0.549 0.0044 0.999 9 The calibration slopes of the two electrodes agreed fairly well at 25 "C. The agreement at the lower temperatures was not so close, but this was caused, at least in part, by the difference in temperature. The linearity of the plots was excellent, as shown by the correlation coefficients. Precision At 25 "C the Ionel electrode gave slightly smaller total standard deviations, but at 4 "C the CERL electrode was slightly better. None of these differences were significant at the P = 0.05 level of the F-distribution. Between-batch standard deviations were zero or non-significant, with two exceptions; one of these was only "possibly significant'' (CERL electrode at 4.3 "C and 5 pg 1-l) and the other appeared because of a freak result for the within-batch standard deviation (Ionel electrode at 25 "C and 1 pg 1-1). The results of precision tests are shown in Tables I1 and 111.58 MARSHALL AND MIDGLEY: DETERMINATION OF CHLORIDE IN Analyst, VoZ.104 TABLE I1 PRECISION OF MEASUREMENTS; WITH THE CERL ELECTRODE Chloride Standard deviation,? Temperature/ concentration/ A e.m.f.*/ A > "C CCg 1-1 mV Within-batch Between-batch Total 26 20 0 0.083 - - (0.47) - - 0.106 (0.69) 0.131 (0.73) 0.167 (0.93) 0.144 0.212 10 1.81 0.077 NSS (0.43) (;;) (0.71) (NOS) (0.79) (;;) 6 2.61 0.126 2 3.21 0.167 1 3.43 0.141 NS O§ (0.93) (0) (0.81) (1.19) (1.14) (NS) 20 0 0.100 - (0.23) - - 10 4.33 0.166 0 0.166 (0.36) 0.224 (0.36) (0) 6 6.49 0.122 O.lSST[ (0.29) (0.44) (0.62) 2 7.74 0.106 NS 0.131 (0.31) 0.166 1 8.16 0.141 (0.33) (NS) (0.36) 3.74 0.204 - (0.24) w;) * Mean e.m.f.normalised with respect t o 20 p g 1-1 solution, e.g., A, = E , .- E,. t Standard deviations are in mV except for the figures in parentheses, which are in concentration 9 De-ionised water. 7 Significant at the P = 0.05 level but not att the P = 0.01 level. units (pg 1-1). NS = not significant at the P = 0.05 levell. 4.3 Criterion of Detection The criterion of detections is given by 1.65;42aB, where U, is the within-batch standard deviation of the blank.The criteria for the Ionel electrode were 1.0 pg 1-1 at 25 "C and 1.7 pg 1-1 at 3.6 "C and for the CERL electrode were 2.6 pg 1-1 at 25 "C and 0.8 pg 1-1 at 4.3 "C. As the blank was not included in the precision trial of the CERL electrode at 4.3 "C, uB was approximated by the within-batch standard deviation of the lowest (1 pg 1-1) standard solution; in view of the criteria of detection obtained at 25 "C and by comparison with those for the Ionel electrode, this is a fair approximation. Accuracy Recovery tests were carried out on a number of power station waters, using the Ionel electrode at 25 "C. The solutions were analysed and then spiked with 5 pg 1-1 of chloride solution.From the standard deviations in Table 111, the recovery was predicted with 95% confidence to be 5.0 & 1.1 pg 1-1 for a single result. The results in Table IV show no definite exceptions to this, but as the condensed steam and feed water samples had concentrations below the criterion of detection, the recovery could only be quantified for the boiler water sample (104%). Response Time The time to reach an equilibrium e.m.f. was dominated by the characteristics of the flow system. About 5 min were required for each fresh solution to reach the sensing electrode. Once the electrode had started to respond to tlhe new solution, 5-10 min were taken to reach equilibrium, depending on the difference in concentration between successive solutions. This time was affected little by temperature or by which electrode was being used (Fig.l), although the Ionel electrode responded faster when immersed in fresh solution in a beaker (about 2 min compared with 5 min for the CElRL electrode). The response time was also affected by stirring.January, 1979 HIGH-PURITY WATERS USING ION-SELECTIVE ELECTRODES TABLE I11 PRECISION OF MEASUREMENTS WITH THE IONEL ELECTRODE Chloride Temperature/ concentration/ 25 20 "C pg 1-1 3.6 10 5 2 1 O§ 20 10 5 2 1 A e.m.f.*/ mV 0 1.77 2.69 3.16 3.31 3.66 0 5.43 8.32 9.82 10.43 10.98 Standard deviation, t Within-batch Between-batch r A 0.095 - 0.063 NS$ 0.063 NS 0.055 NS (0.54) - (0.36) (NS) (0.36) (NS) (0.31) (NS) 0.000 0.1027 (0.44) (0.0) (0.35) - (0.43) (0) (0.37) (NS) (0.00) (0.58) 0.078 0.0 0.190 - 0.236 0 0.205 NS 0.127 NS 0.288 0.410 (0.23) (y) (0.52) '3 (0.75) (0) 59 Total - - 0.072 (0.41) 0.087 (0.49) (0.067) (0.38) 0.102 (0.68) 0.078 (0.44) - 0.235 (0.43) 0.258 (0.47) 0.141 (0.26) 0.288 (0.62) 0.410 (0.75) * Mean e.m.f.normalised with respect to 20 pgl-l solution, e.g., A2 = E, - Ezo. t Standard deviations are in mV except for the figures in parentheses, which are in concentration units ( pg 1-1). $ NS = not significant a t the P = 0.06 level. 5 De-ionised water. 7 Significant at the P = 0.001 level. Effect of Stirring Without stirring in the electrode compartment of the flow cell, both electrodes took more than twice as long to reach equilibrium compared with when the solution was stirred. The e.m.f. shifted by up to 2mV when the stirring was stopped, but the sensitivity of the electrodes was not affected.Effect of Temperature The increased sensitivity at low temperatures is shown by the AmV results in Tables I1 and 111. The precision, however, was not improved by working at low temperatures, possibly because of the greater difficulty of maintaining a steady temperature at sub-ambient levels. Although the flow cell had a thermostatically controlled water jacket and was kept inside an insulated cabinet, at such low concentrations as were tested more precise control of the air temperature would have been desirable. TABLE IV RECOVERY TESTS WITH POWER STATION WATERS Chloride content/pg 1-1 r I Station Unit Sampling point Sample Sample + 5 pgl-l spike A 2 De-aerator <1 6.4 A 2 Extraction pump <1 7.0 A 3 Economiser inlet <1 6.8 A 3 Boiler 9.2 14.4 B 1 Extraction pump <1 6.2 A 3 Extraction pump <1 4.760 MARSHALL AND MIDGLEY: DETERMINATION OF CHLORIDE IN Analyst, VoZ.104 X A 4 20 pg I-' 2 pg ,\ I-' j--y m t B 1 20 pg I--'- t X X X c 2opg I-' Time Fig. 1. Response curves for mercury( I) chloride electrodes : A, CERL electrode a.t 25 "C; B, Ionel electrode at 25 "C : a n d C, Ionel electrode at 4 "C. X = change of solution. The standard potentials of the electrodes increased as the temperature decreased. The e.m.f.s observed a t 20 pg 1-1 of chloride changed by 15 and 21 mV for the CERL and Ionel electrodes, respectively, when the temperature changed from 25 to 4 "C. The main cause of the shift and of the change in sensitivity was the greater concentration of chloride contributed by the dissolution of the mercury(1) chloride of the membrane at the higher temperature.Stability of the Cell Potential and Durability of the Electrodes The CERL electrode operated continuously for a t least 2 months without needing to be re-impregnated. The Ionel electrode has operated for 1 month without needing to be re-polished. When standard solutions were not being passed through the flow cell the apparatus was run on de-ionised water. The day-to-day stability of the e.m.f. can be judged from the results in Table V. TABLE v STABILITY OF E.M.F. OF THE CERL ELECTRODE IN 10 pg 1-1 CHLORIDE SOLC'TION AT 4.3 "c Day I \ 1 2 8 9 10 11 16 E . m. f . /mV -50.6 -51.3 -51.0 -51.3 -50.9 -60.7 -50.9 Reference Electrodes It was noted previously1 that the condition of the reference electrode was critical for obtaining good results. If the head of electrolyte solution above the ground-glass sleeve junction was only a few centimetres, the junction This was also true in flowing solutions.January, 1979 HIGH-PURITY WATERS USING ION-SELECTIVE ELECTRODES 61 needed to be flushed out daily with fresh solution.With the head of electrolyte solution at about 50 cm, the reference electrode required no attention other than re-filling. Discussion Comparison with Silver - Silver Chloride Electrodes Compared with the limiting linear response of the silver - silver chloride electrode a t low concentration^,^ that of the mercury( I) chloride electrode is about ten times more sensitive.The mercury(1) chloride electrode at 25 "C is still four times as sensitive as the silver - silver chloride electrode at 5 "C. The within-batch standard deviations reported by Tomlinson and Torrance3 were smaller, in millivolt terms, than we found, but because of the different sensitivities were larger in concentration units. Silver chloride electrodes respond faster than mercury(1) chloride electrodes, but for most purposes the difference is unimportant. Sekerka et aZ.7 used an Ionel electrode in 1-1000 pg 1-1 of chloride solutions at 25 "C; the within-batch standard deviations of their results at 10 and 20 pg 1-1 were slightly larger (0.64 and 0.68 pg l-l, respectively) than those we obtained. Apparatus The apparatus is similar to that already used with the silver - silver chloride ele~trode.~ Unless the maximum sensitivity is required, however, the apparatus can run at 25 "C or possibly even higher temperatures; in this instance the chiller unit could be dispensed with, giving considerable savings in space and expense.For continuous use, one that has a fairly high head of electrolyte solution or some other means of maintaining a free-flowing junction is recommended. The most troublesome part of the apparatus was the reference electrode. Absolute and Relative Chloride Concentrations Because the de-ionised water used to prepare the standard solutions will never be perfectly free of chloride, absolute measurements at these levels are scarcely possible by direct potentio- metry, whether with silver or mercury(1) chloride electrodes.We have shown, however, that standard solutions can be prepared in a consistent way at concentrations between 1 and 20 pg 1-l. The absolute chloride concentration in the solutions was 0.7-1.0 pg 1-1 higher than nominal, as concentrations of this magnitude were measured in the water we used by Dimmock and Webber.5 Very similar chloride concentrations in other sources of de-ionised water have been found by Florence8 and by Tomlinson and Torrance3 in their work with silver chloride electrodes. No change in signal was observed when we replaced our standard solutions with corresponding solutions prepared with de-ionised water that had been passed repeatedly through a mixed-bed de-ionisation column. If the electrode is used to detect ingress of chloride, e.g., from cooling water leaking into condensed steam, the linear e.m.f. - concentration relationship has the advantage that an increase in chloride concentration is indicated directly by the change in e.m.f.even though the absolute levels are uncertain. If the change in e.m.f. was interpreted by means of the conventional logarithmic relationship, the increase in concentration would also be biased. Conclusions The use of mercury( I) chloride electrodes for determining chloride concentrations can be extended from the levels attainable by manual analysis1 to very low levels (less than 20 pg 1-l) by housing the electrode in a flow cell at a controlled temperature. At a given temperature, the electrode is about ten times more sensitive than the silver chloride electrode used at present. One advantage of this greater sensitivity is that the mercury(1) chloride electrode does not need to be operated at sub-ambient temperatures in order to obtain adequate precision in the concentration range 1-20 pg 1-1. Two kinds of electrode can be used, that developed at CERLl and the Ionel SL-01. The two electrodes are almost equally sensitive and precise, but the Ionel has a faster response time and requires no prepara- tion.62 MARSHALL AND MIDGLEY This work was carried out at the Central Electricity Research Laboratories and is pub- lished by permission of the Central Electricity Generating Board. 1. 2. 3. 4. 6. 6. 7. 8. References Marshall, G. B., and Midgley, D., Analyst, 1978, 103, 438. Marshall, G. B., and Midgley, D., Analyst, 1978, 103, 784. Tomlinson, K., and Torrance, I<., Analyst, 1977, 102, 1. Lechner, J. F., and Sekerka, I., J . Electroanalyt. Chem. Interfacial Electrochem., 1974, 57, 317. Dimmock, N. A., and Webber, H. M., C.E.R.L. Laboratory Note RD/L/N 56/77, Central Electricity Roos, J. B., Analyst, 1962, 87, 832. Sekerka, I., Lechner, J. F., and Harrison, L., ,/. Ass. Off. Analyt. Chem., 1977, 60, 625. Florence, T. M., J . Electvoanalyt. Chem. Interfacial Electrochem., 1971, 31, 77. Research Laboratories, Leatherhead, 1977. Received July 21st, 1978 Accepted August 15th, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400055

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, January, 1979, Vol. 104, pp. 63-72 63 Assessment of Glass Electrodes for Determining pH in Boiler Feed Water D. Midgley and K. Torrance Central Electricity Research Laboratories, Kelvirt A venue, Leatherhead, Surrey, K T22 7SE Six types of commercial glass electrodes have been tested in the laboratory for their suitability for measuring pH in ammonia-dosed boiler feedwater of moderately low specific conductivity (about 5 pS cm-l). The electrodes were chosen to represent the range of pH-sensitive glasses available. All of the electrodes showed a near-theoretical sensitivity, had stable standard potentials and responded sufficiently quickly. In the dilute ammonia solutions, however, the electrodes indicated pH values that could differ by as much as 0.3 pH unit when the solution was flowing slowly through the measuring cell.When the solution was stirred the maximum bias was 0.05 pH unit. For most industrial purposes, the differences in performance between the various types of electrode are unimportant and glass electrodes are less of a problem than reference electrodes for pH measurements in this type of water. Keywords : pH determination ; glass electrodes ; boiler feed water Experience of continuous pH measurements in power station waters shows that there is considerable dissatisfaction with some aspects of this long established technique. Much of the trouble can be attributed to reference electrodes, which have been the subject of a previous study.l This work constitutes a similar appraisal of glass electrodes. The conditions in which the electrodes were tested were intended to simulate those prevailing in boiler feed water.The pH of de-ionised water was adjusted to about 8.5 by adding ammonia to give a concentration of 0.5 mg 1-1. The resultant solution had a specific conductivity of about 5 pS cm-l. Four general-purpose, screened, industrial electrodes were chosen for testing, together with two types of low-resistance electrode suitable for meas- urements at low temperatures or in high-purity water. Of the latter type, one (type F) had a glass with a low specific resistance and limited working ranges of temperature and pH, while the other (type D) had the same type of glass as the general-purpose electrode (type C) from the same company. The electrodes and their properties are summarised in Table I.Experimental Apparatus Two electrodes of each of the types listed in Table I were tested. They were placed in a 126 mm diameter water-jacketed Perspex flow cell such that electrodes of the same type were opposite each other. This arrangement minimised the risk of local conditions being TABLE I ELECTRODES TESTED Resistance/ Electrode Mi2 EIL 1072-110t . . .. 100 Coming 476022t . . .. Schott 9201s . . .. 100 Schott 92025 . . .. 20 Metrohm EA-107T . . 600 Metrohm EA-lO7Tq . . 30 - Working temperature range* /"C -6 to 100 -6 to 100 0 to 80 10 to 100 -30 to 80 -15 to 40 Working pH range* 0-14 0-14 0-14 0-14 0-14 0-1 1 Code A B C D E F * Manufacturers' figures. t Electronic Instruments Ltd., Chertsey, Surrey. $ Corning - EEL, Evans Electroselenium Ltd., Hdstead, Essex.3 H. V. Skan Ltd., Shirley, Solihull.64 MIDGLEY AND TORRANCE : ASSESSMENT OF GLASS ELECTRODES Analyst, Vol. 104 favourable to particular types of electrodes. The solution in the cell could be stirred, as required, by means of a magnetic stirrer bar. The temperature in the cell was maintained at 30.0 j, 0.1 "C by means of a Churchill thsermocirculator. A Pye 305 calomel reference electrode with 3.0 mol 1-1 potassium chloride solution as the reference electrolyte was situated in the central position of the flow cell, 45 mnn from all the glass electrodes. The flow cell was kept in a cabinet maintained at 30 "C by a 300-W heater controlled by a mercury relay thermometer. The flow cell and the cabinet have been described in more detail e1sewhere.l Potentials were measured with a Corning 110 digital pH meter connected through its recorder output terminals to the Central Datat Acquisition and Processing System (CDAPS) at CERL, which automatically gave a paper-tape record of the e.m.f.s and the time of measurement.The recorder span control of the pH meter was adjusted so that the meter acted as a unity-gain amplifier. The electrodes were switched in turn through the pH meter by a modified signal multiplexer with reed relays. Each relay was latched shut for 4 s before the e.m.f. was recorded. Reagents Buffers. Standard NBS phosphate (pH 6.853 at 30 "C) and disodium tetraborate (borax) (pH 9.139 at 30 "C) buffers were prepared fresh each time the electrodes were standardised. Stock ammonia solution.A 20-ml volume of 35% ammonia solution, sp. gr. 0.88, (BDH, Aristar) was added to 2.47 kg of de-ionised water in the glass reservoir of an automatic pipette fitted with a soda-lime guard tube on the air inlet. This solution had an ammonia concentration of approximately 2.5 g 1-l. Each week a 96-1 batch of approximately 0.5 mg 1-1 ammonia solution was prepared in two interconnected 50-1 aspirators by dilution of the stock solution with de-ionised water that was fed directly into the aspirators from the outlet of a mixed-bed de-ionisation unit. Working ammonia solzttio?~. Procedure The working solution was pumped from the aspirators to a header tank above the cabinet. The solution flowed from the header tank, through a glass capillary into the flow cell, at a rate of about 9 ml min-l and was then run to waste.The solution was not stirred while passing through the flow cell, except as detailed below. The header tank and the aspirators were protected from atmospheric carbon dioxide by means of soda-lime guard tubes. The aspirators held enough solution for continuous running for 1 week. The electrodes were monitored by CDAPS every 2 h for changes in their potentials. This enabled us to obtain an estimate of the short-term standard deviations of the electrode potentials [step (i)], to test the effects of stirring [step (ii)], to measure the response times [steps (iii) and ( i v ) ] and to check the calibration of the electrodes [steps (v) and (vi)]. (i) The potentials were recorded once a minute for at least 10 min. (ii) The stirrer was started and the potentials were again recorded at intervals of 1 min for at least 10 min.(iii) With the stirrer in operation, the flow of solution was stopped. The potentials were recorded at 1-min intervals. (in) After at least 10min a 1-ml portion of stock ammonia solution was injected into the solution in the cell by means of a syringe. 'The potentials were recorded until they were steady. ( v ) The electrodes were removed from the flow cell, rinsed with de-ionised water and placed in phosphate buffer solution. The temperature of the rinse water and the buffer had previously been brought to 30 "C. The potentials were recorded every minute for a t least 6 min after a steady value had been reached. (vi) The electrodes were rinsed with de-ionised water and placed in borax buffer solution.The potentials were recorded every minute for at least 6 min after steady values had been attained. (vii) The electrodes were rinsed with de-ionised water and returned to the flow cell. (viii) A new batch of working solution was connected to the pump and CDlAPS set to record at intervals of 2 h again. Step (i) was repeated on two other occasions during each week. For the next 12 weeks, only step (viii) was carried out, except in weeks 16, 20 and 25, when the full procedure was followed. After a further 11 months, during which time the electrodes were stored in de-ionised water, exclept for a few tests in dilute ammonia solution and one in acidic solution, the calibration was checked as in steps (v) and (vi). Once a week for the first 13 weeks the procedure below was carried out.January, 1979 FOR DETERMINING PH I N BOILER FEED WATER 65 Results In most instances the results are shown for only one electrode from each pair as electrodes of the same type gave very similar results.Exceptions to this rule are noted. Stability of the Standard Potential The e.m.f.s measured when the electrodes were immersed in the phosphate buffer solution were used to check the variation of the standard potential with time. Electrodes of the same type behaved similarly, but there were considerable differences between the rates of change of standard potential for electrodes of different types (Table 11). The standard potentials changed steadily from week to week, except for electrode B1, the standard poten- tial of which was constant for the first 7 weeks.The potentials of the type A electrodes changed in the opposite direction to the others. TABLE I1 RATES OF CHANGE OF STANDARD POTENTIAL Electrode Rate of change*/mV week-1 Al, A2 -0.2, -0.3 B1. B2 0.6t, 0.2 c1, c 2 0.5, 0.3 D1, D2 0.2, 0.2 E l , E2 0.1, 0.1 F1, F2 0.2, 0.1, * Average over 12 weeks. t The potential was constant for 7 weeks before it started to increase; the figure given is for the next 5 weeks. Because reference electrodes may not be truly constant, the observed changes in standard potential cannot be assigned exclusively to the behaviour of the glass electrodes. The differences between the changes in standard potential are real, however, as all the potentials were measured against the same reference electrode.Stability of the Slope Factor The slope factor was calculated from the difference between the e.m.f.s when the electrodes were immersed in the two buffer solutions. A selection of the results, expressed as a per- centage of the theoretical value at the same temperature, is shown in Table 111. TABLE I11 SLOPE FACTOR AS A FRACTION (yo) OF THE THEORETICAL VALUE* Electrode Week 1 3 5 7 9 11 13 16 20 25 73 A1 99.421. 99.641 99.271. 99.34 98.911. 99.051. 97.821. 98.691. 98.621. 98.251. 97.70t B1 99.85 99.85 99.20: 99.64 99.131. 99.051. 99.131. 98.761. 98.47t. 98.401. 98.071. c1 99.71: 99.711 99.85 99.201. 99.56: 99.20t 98.761. 98.05t 99.49: 98.911. 98.91f D1 100.29 99.85 100.29 99.491 99.271. 99.277 99.277 99.131. 98.83t 98.16t 98.981. El 99.71; 99.85 99.71: 100.36 99.341.99.201. 99.277 98.691. 98.767 98.761. 98.501. F1 99.93 99.93 99.85 99.93 99.341. 99.341. 99.341. 98.917 99.05t 98.83t 98.531. * Not significantly different from the theoretical value at the 95% confidence level, unless t Significantly different from the theoretical value a t the 99% confidence level. : Significantly different from the theoretical value a t the 95% confidence level. otherwise noted.66 MIDGLEY AND TORRANCE: ASSESSMENT OF GLASS ELECTRODES Analyst, VoZ. 104 All the electrodes showed a tendency for the slope to decrease with time, but the variations from week to week were small. The slope factors of all the electrodes except those of type A were essentially theoretical at the start of the test, but from the eighth week onwards all the electrodes deviated significantly from the theoretical value ; none, however, would have been rejected for normal use.Precision The precision of the e.m.f. measurement was estimated by measuring the e.m.f .s at intervals of 1 min, assuming that there were no changes in pH or temperature during the period of the test (6-10 min). Results for tests in different solutions, all performed on the same morning, are shown in Table IV. The standard deviations varied erratically from week to week, but there was no over-all tendency for the standard deviations to increase or decrease during the 25-week test. Table V shows an extract of the results for the weekly variations for the electrodes in the borax buffer solutions. TABLE IV STANDARD DEVIATIONS OF THE ELECTRODE POTENTIALS IN DIFFERENT SOLUTIONS Standard deviation/mV for a single result' in- Phosphate Electrode buffer t A1 0.43 A2 0.26 B1 0.38 c1 0.29 c 2 0.31 D1 0.37 D2 0.13 El 0.28 E2 0.47 F1 0.15 F2 0.10 Borax buffer t 0.33 0.44 0.30 0.37 0.35 0.42 0.18 0.2 1 0.43 0.15 0.20 Unstirred ammonia solution: 0.58 0.19 0.19 0.27 0.20 0.29 0.15 0.27 0.44 0.14 0.14 Stirred ammonia solution: 0.56 0.32 0.37 0.27 0.59 0.41 0.36 0.50 0.59 0.43 0.42 * Results taken from the ninth week of testing.t Five degrees of freedom. Nine degrees of freedom. The standard deviations observed in the phosphate buffer, borax buffer and unstirred working solution were generally similar on any one occasion. The standard deviations observed in the stirred working solution were generally 2-3 times larger than in the other solutions, the biggest proportional increases being found with the electrodes with the smallest standard deviations.STANDARD DEVIATIONS* (mV) IN BORAX BUFFER SOLUTIONS Week 1 3 5 7 9 11 13 16 20 25 A1 0.21 0.43 0.50 0.67 0.33 0.11 1.01 0.96 1.13 0.36 B1 0.20 0.26 0.46 0.26 0.30 0.28 0.12 0.23 0.25 0.26 (: 1 0.20 0.21 0.34 0.28 0.37 0.32 0.44 0.29 0.32 0.28 D1 0.15 0.14 0.39 0.55 0.42 0.08 0.08 0.08 0.08 0.19 E l 0.33 0.26 0.33 0.34 0.21 0.39 0.61 0.25 0.37 0.42 F1 0.15 0.14 0.12 0.34 0.15 0.14 0.22 0.18 0.17 0.18 * Standard deviation for a single result with five degrees of freedom.JnnNary, 1979 FOR DETERMINING PH IN BOILER FEED WATER 67 Electrodes with a low electrical resistance, k., types D and F, had lower standard devi- ations than the others, type D electrodes being slightly better than type F.The electrodes with relatively high impedances had, on average, very similar standard deviations, but the type A electrodes were less consistent than the others. Response Time The equilibrium response times varied in the range 1-7 min for almost all the electrodes. I t was assumed that the mixing characteristics of the cell were the same in each test, as the temperature, the arrangement of the electrodes, the volume of solution in the cell and the stirring rate were kept constant. Typical response curves are shown in Fig. 1. y: E LLi 7th week - s' 20th week - 0 4 8 0 4 8 Time after injectiodmin Fig. 1. Time response curves for pH electrodes: OI A l ; Al B1; A, C1; a. D1; ., El, 0, F1.There was a tendency for theiresponse times to increase as the tests progressed over the 25-week period, but the changes were irregular. The increases in response times were most pronounced for the C and D electrodes and least pronounced for the A and F electrodes. Electrode B2 had consistently the best response time before a faulty relay made measure- ments unreliable. Electrode E2 was markedly slower than the others, although El was among the fastest. The electrodes can be grouped in terms of their equilibrium response times, which include a mixing time of less than 1 min, as follows. Initially the B electrodes were faster, but the F and E electrodes were more consistent over the full period of the tests. Typical response times were 1-3 min. Electrode A1 gave response times of 3 4 min through- out the trial, but the others were fairly rapid at first (2-3 min), becoming slower (5 min) over the second half of the trial.Group 3: the response of electrode E2 was consistently the slowest (5-7 min) and did not change significantly during the trial. Group 1: B1, B2, F1, F2, El. Group 2: A l , A2, D1, D2, C1, C2. Bias The electrodes indicated different pH values for the approximately 0.5 mg 1-1 ammonia solution, which flowed through the flow cell at 9 ml min-l but was otherwise unstirred. Types A, C and D indicated lower pH values than average and types B, E and F higher than average values. The same pattern of electrode behaviour was observed with all batches of68 MIDGLEY AND TORRANCE ASSESSMENT OF GLASS ELECTRODES Analyst, vd.104 solution, regardless of any differences in pH between the batches, but the absolute value of the mean bias between electrodes varied cclnsiderably from batch to batch. The mean biases between electrodes were calculated with respect to one electrode of each type for each weekly batch of solution. These mean biases were significantly different from zero at the 5% level in almost all instances. Typical examples, measured with respect to electrode D2, are shown in Table VI for 6 consecutive weeks. TABLE VI MEAN BIAS (PH UNITS) OF ELECTRODES AGAINST ELECTRODE D.2 Week Electrode A1 B1 c 1 D1 El F1 Number of readings 7 8 -0.006* 0.124 0.017 0.036 0.093 0.029 68 9 -0.076 0.096 0.020 0.036 0.073 0.019 66 10 - a m 6 01.122 0.043 0.062 0.128 0.075 81 11 o.ooo* 0.221 0.045 0.068 0.183 0.098 56 12 o.ooo* 0.132 0.017 0.040 0.127 0.030* 70 -----l 13 0.042 0.200 0.063 0.105 0.208 0.105 71 * Not significantly different from zero a t the 5% level.When the dilute ammonia solution was stirred, the bias between the pH values indicated by the different electrodes was reduced; high and low pH values in the unstirred solution always showed a decrease and an increase, respectively, when stirring started. Results from a typical batch are given in Table VII. Over all the batches, the standard deviations for the determination of the mean pH of the results from the 12 electrodes varied from 1.4 x 10-3 to 6.1 x to 7.2 x Because of the good agreement between electrodes in stirred solutions, the mean of the values they indicated was taken as the best estimate of the true pH of the dilute ammonia pH unit in the stirred solutions, compared with from 1.6 x pH unit in the unstirred solutions.TABLE VII pH VALUES GIVEN BY DIFFERENT ELElCTRODES I N DILUTE AMMONIA SOLUTION Electrode A1 A2 B1 B2 c 1 c 2 D1 D2 E l E2 F1 F2 Mean Standard deviation** I Instirred solution* 8.656 8.603 8.727 8.694 8.648 8.633 8.668 8.562 8.712 8.728 8.658 8.729 8.667 0.055 Stirred solution* 8.690 8.650 8.678 8.684 8.694 8.644 8.690 8.655 8.684 8.648 8.686 8.648 8.671 0.020 Average bias? between pH indicated in unstirred solution and true pH -0.076: - 0.0695 N.S.7 N.S.7 - 0.115: -0.129; -0.103; -0.1875 0,103; 0.052j 0.058j 0.07611 * Results for the ninth weekly run: each p€I is the mean of 10 readings. t Average of 10 different batches; the true pH for each batch is the mean of all pH values in Significantly different from zero at the 59; level.$ Significantly different from zero a t the lT, level. 7 Non-significant. (1 Significantly different from zero a t the 0.1% level. ** Standard deviation for a reading by a single electrode (11 degrees of freedom). the stirred solution.January, 1979 FOR DETERMINING PH IN BOILER FEED WATER 69 solution. The errors of the pH electrodes in unstirred solutions were determined by com- paring the true pH with the individual pH values indicated immediately before stirring started. The means (over ten batches) of such errors are also shown in Table VII. Except for the type B electrodes, the errors in unstirred solutions were significantly different from zero.Fig. 2 shows, for one electrode of each type, the error in the unstirred solution plotted against the true pH. For the types C and D these errors increase with the pH itself (linear correlation coefficients of 0.94), but in the other instances no relationship is evident, except possibly for the type I;. 8.1 8.3 8.5 a. 7 Mean pH in stirred solution Fig. 2. Deviation of pH in unstirred solution from the mean pH in stirred solution : solid lines, correlation coefficient > 0.94; broken lines, correlation coefficient G0.60. 0, A l ; x , B1; A, C1; 0, D2; 0, El; A, F2. The experiments with stirred and unstirred solutions were repeated with neutral and acidic solutions. In de-ionised water the same pattern of electrode behaviour was observed as in dilute ammonia solution, i.e., low-reading electrodes indicated an apparent increase in pH when stirring started and conversely for high-reading electrodes. The standard devi- ations of measurements with a single electrode (9 degrees of freedom) were 0.048 and 0.012 pH unit in unstirred and stirred solution , respectively. In unstirred 10-3 moll-1 hydrochloric acid solution the electrodes showed far less bias between one another than in de-ionised water or dilute ammonia solution. In contrast to the effects of stirring the neutral and alkaline solutions, all of the indicated pH values changed in the same direction (lower) when the solution was stirred (cj. , Tables VII and VIII) , although these changes were relatively small. TABLE VIII pH VALUES GIVEN BY DIFFERENT ELECTRODES IN mol 1-1 HYDROCHLORIC ACID Electrode Unstirred solution Stirred solution A2 3.087 3.077 B1 3.106 3.087 c1 3.128 3.113 D1 3.112 3.096 E l 3.122 3.108 F1 3.111 3.101 Mean* 3.118 3.102 Standard deviation? 0.015 0.013 * Mean of results for 10 electrodes (Al, B2 excluded).t Standard deviation for a reading by a single electrode (nine degrees of freedom).70 MIDGLEY AND TORRANCE : ASSESSMENT OF GLASS ELECTRODES AnaZyst, VoZ. 104 Effect of Temperature The temperature dependence of the e.m.f. of a glass electrode cannot usefully be discussed without simultaneously considering the nature of the reference electrode, the pH of the solution in which it is immersed and the teimperature dependence of that pH. When an electrode is used to give direct meter readings of pH, the temperature compensation circuit of the pH meter will also influence the apparent temperature dependence of the electrode.It was considered that a study of the effect OE temperature on different glass electrodes used with an arbitrarily chosen reference electrode in a small number of particular solutions would not be a useful guide to the practical aspects of pH measurement at different tempera- tures. Attention was concentrated therefore on the recovery of electrodes from a rapid change in temperature, such as may occur in a process stream. This is a matter of general applicability and is independent of the other factors mentioned above. The electrodes were immersed in phosphate or borax buffer at 30 "C and allowed to reach a steady e.m.f. They were then placed (in blatches of six) in a second solution of the same buffer at 20 or 40 "C for 1 h, before being returned to the original solution at 30 "C.The reference electrode remained in the solution at 30 "C throughout. The e.m.f.s were recorded on the CDAPS system as the electrodes returned to their original temperature. All the electrodes regained their original potentials (A0.5 mV) within 30-60 min and, although the initial displacement of the e.1n.f. from the value at 30 "C varied between electrodes in the order A > C w D > E m F m B, for most purposes the differences between the electrodes were of no importance. Disciussion Stability The stability of both the standard potential and the slope factor, as determined by measure- ments with buffer solutions, were very good for all types of electrodes over a period of 17 months.The changes observed would not have produced a significant error (0.05 unit) in pH over a period of at least 4 weeks between standardisations. Changes in performance in the dilute ammonia solutions as the electrodes aged were hard to assess because of the large variations in precision and bias between weekly batches. The response times of the electrodes increased during the period of the tests, but the changes occurred slowly and were unimportant for the first 8-12 weeks of operation. The precision of measurements in buffer and dilute ammonia solutions did not change with time over a 6-month period. Performance The slope factors of all the electrodes were initially very close to the theoretical value and declined to 99% of the theoretical value over' a period of about 12 weeks.The precision of e.m.f. measurements was approximately the same in buffer and dilute ammonia solutions; a typical electrode had a. standard deviation of 0.5 mV (0.008 pH unit) for a single measurement, which is adequate for almost all industrial purposes, and the electrodes with low-resistance membranes (types D and F) had standard deviations as low as 0.15 mV. The response times of all of the electrodes were adequate for most industrial purposes even after operation for 6 months. The recovery of the electrodes from large changes in temperature (10 "C) was fairly satisfactory (30-60 min), but was better for types B, E and F than the others. The recovery time of a pH electrode is only one aspect of the effect of temperature changes on pH measure- ment and should not be the only consideration for choosing between electrodes; the nature of the reference electrode and the temperaturle compensation circuitry in the pH meter must be considered simultaneously if errors caused by temperature variations are to be mini- mised.In unstirred dilute ammonia solutions, the bias between electrodes could be as large as 0.3 pH unit (Table VI), but in stirred solutions the bias between individual electrodes was rarely above 0.05 pH unit and usually there was no significant bias from the mean of the pH values indicated by all the electrodes. 'The bias developed differently in alkaline andJanuary, 1979 FOR DETERMINING PH I N BOILER FEED WATER 71 acidic solutions (cf, Tables VII and VIII) and only a small part of it could be attributed to changes in standard potential, as the largest change in Table I1 is equivalent to only 0.015 pH unit per week. Pairs of similar electrodes showed similar deviations from the true pH, independently of their positions in the flow cell.It was inferred, therefore, that any local variations in flow or temperature within the flow cell were of little importance. These results show the importance of adequate stirring for measurements in dilute ammonia solutions; the type B electrodes were the least affected by this problem and the worst affected were type C and D electrodes. The significance of solution agitation for on-line industrial pH measurements arises in relation to the rate and turbulence of the flow of sample.Bias is further discussed below in terms of the glass composition. Glass Composition All were predominantly lithium oxide - silica glasses modified by small amounts of other oxides, of which the most important were those of tantalum or niobium. The electrodes with a negative bias in dilute ammonia solutions had the higher tantalum contents, but the bias could not be correlated directly with the tantalum content, presumably because of the modifying influence of the other oxides present. Electrodes that gave a negative bias also had slower responses than the others, but the stability of the standard potential and the slope factor could not be correlated with other properties. The short-term standard deviation depended on the total resistance of an electrode rather than on the glass composition, at least under the conditions tested in this work.Two mechanisms may be postulated to explain the effect of stirring on the bias, the positive and negative deviations from the true pH in alkaline solution and the difference of behaviour in alkaline and acidic solutions. Firstly, water hydrates the glass and extracts alkali metal hydroxides from the structure. The hydroxide may accumulate in pores near the surface of the glass and cause the electrode to indicate a higher pH than that of the bulk of the solution. Stirring will displace the accumulated hydroxide and the indicated pH will decrease towards that of the bulk solution. Secondly, hydroxide ion in solution can attack the silicate lattice, as follows: At the end of the tests, the electrode glasses were analysed.This process consumes hydroxide in the surface layers of solution adjacent to the glass and causes the electrode to indicate a lower pH than exists in the bulk of the solution. Stirring will supply more hydroxide and the indicated pH will increase towards that of the bulk solution. In acidic solution the first reaction should predominate, because the rate of the second reaction depends on hydroxide concentration. As expected from this hypothesis, the indicated pH decreased when the acidic solution was stirred and this occurred with all of the electrodes. Even in the unstirred dilute ammonia solutions the type B, E and I-; electrodes gave high pH readings, showing that the first reaction was still dominant for these electrodes. The types A, C and D electrodes, however, gave low pH readings, indi- cating that the second had become more important. The bias between electrodes in slowly flowing solutions can, therefore, be explained qualitatively as being caused by the relative susceptibilities of the electrodes to both mechanisms under a particular set of solution conditions. Conclusions All the electrodes tested had nearly theoretical responses and for most industrial purposes any difference in performance between them would be of no importance. If, however, the sample was unstirred, poorly buffered and either static or flowing only very gently, errors could reach a level of 0.2 pH unit. Under the conditions of the tests, electrodes of types A, B, E and F were more satisfactory than the others in this respect. Low-resistance electrodes had no significant advantages over general-purpose electrodes for the application to power station waters, although they were capable of slightly higher precision.72 MIDGLEY AND TORRANCE The good performance obtained from a variety of glass electrodes, in contrast to the poor performance of some reference electrodes,l confirms power station experience that reference electrodes are the less satisfactory half of industrial pH cells. This work was carried out at the Central Electricity Research Laboratories and is pub- lished by permission of the Central Electricity Generating Board. Reference 1. Midgley, D., and Torrance, K., Analyst, 1976, 101, 833. Received July 25th, 1978 Accepted August 14th, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400063

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Awalyst, January, 1979, Vol. 104, pp. 73-78 73 Polarographic Study of Aflatoxins B,, B,, GI and G, : Application of Differential-pulse Polarography to the Determination of Aflatoxin B, in Various Foodstuffs Malcolm R. Smyth, David W. Lawellin" and Janet G. Osteryoung Department of ilficvobiology, Colorado State University, Fovt Collins, Colo. 50523, USA The polarographic behaviour of aflatoxins B,, B,, G, and G, has been investi- gated and found to parallel closely that of coumarin and its derivatives. Differential-pulse polarography has been applied to the determination of aflatoxin B, in a variety of food products. Good agreement was obtained between the differential-pulse polarographic method and visual comparison of the fluorescence exhibited by the aflatoxin following thin-layer chromato- graphic separation.Electroactive interferences co-extracted from high lipid-containing foods were removed by separation on a Sepliadex LH-20 column. Keywords : _4flatoxin determination ; diffeerential-pulse polarogvaphy ; food a w l y s i s Aflatoxins are a group of toxic metabolites produced by certain strains of the common mould Aspergillus jlavus (e.g., Aspergillus parasiticus) and by various other mould genera. They have been shown to be highly carcinogenic in various animal species and hence pose a potential health risk to man. Accurate and sensitive methods of analysis are therefore required for the determination of these compounds in foodstuffs that have sustained mould growth. Aflatoxins B,, B,, G, and G, are difuranocoumarin derivatives that give rise to intense fluorescence in the blue (B, and B,) and green (G, and G2) regions of the spectrum.As a result of this characteristic fluorescence there are many published methods for the determina- tion of these compounds based on thin-layer chromatography with either visual or fluoro- densitometric measurement.l-3 Although these methods can detect concentrations of individual aflatoxins down to 1 ng per spot, the error associated with visual comparison of fluorescence intensity is usually of the order of &20-28%, whereas that associated with fluorodensitometric measurement is about & 643% .l The use of polarography for the identification of aflatoxins was first reported by Gajan ei aL4 but no attempt was made either to explain the complicated electrochemical behaviour of these compounds or to produce a working method for their determination in foodstuffs.It was decided, therefore, to study the polarographic behaviour of these compounds in greater detail and to investigate whether differential-pulse polarography could provide an accurate and sensitive method for their determination. This technique has recently found wide application for the determination of many other foreign organic compounds in biological materials. Experimental Apparatus Polarograyhic measurements were carried out using a PAR, Model 174, Polarographic Analyser in conjunction with a three-electrode cell system with a saturated calomel electrode (S.C.E.) as the reference electrode and platinum as the counter electrode. The dropping- mercury electrode (D.M.E.) used had a flow-rate of 0.82 mg s-l and a drop time of 7.9 s at a mercury head of 76 cm.* Present address : Department of Biochemistry, The Medical and Dental School, North Western Uni- versity, Chicago, Ill., USA.74 SMYTH et al. : APPLICATION OF DIFFERENTIAL-PULSE POLAROGRAPHY Analyst, VoZ. 104 Reagents Samples of aflatoxins B,, B,, G, and G, were obtained from Aldrich Chemical Co., Inc. Stock solutions of these compounds (100 pg ml-l) were prepared in AnalaR methanol and stored in the dark under refrigeration. A stock Britton - Robinson buffer solution, 0.04 M in glacial acetic acid, orthophosphoric acid and boric acid, was prepared from analytical- reagent grade compounds; buffer solutions of various pH (2-12) were prepared by the drop- wise addition of 0.2 M sodium hydroxide solution and measurement of the pH using a glass elect rode.Thin-layer chromatograms were run on silica gel G plates (0.25 mm thick) obtained from E. Merck. All other compounds and solvents used were of analytical-reagent grade. Yeast extract was obtained from Difco. Procedures Polarographic studies were carried out on solutions that had previously been de-gassed with oxygen-free nitrogen for 10 min. Current - potential curves were recorded in the direct-current, normal-pulse polarographic arid diff erential-pulse polarographic modes. The samples were blanketed with an atmosphere. of nitrogen during analysis and each solution was scanned between - 1 .O V and the potential of electrochemical reduction of the supporting electrolyte.Scan rates of 1-5 mV s-1, drop times of 1-2 s and a modulation amplitude of 100 mV (in the d.p.p. mode) were typically employed. For the thin-layer chromatographic investigations, 5 pl of various aflatoxin standard solutions (containing 1-20 pg ml-1 of the aflatoxin) and solutions containing unknown concentrations of aflatoxins were spotted on the plate and elution was achieved using a chloroform - acetone - water mixture (88 + 10 + 2). The spots were rendered visible under longwave ultraviolet light. The concentratiton of individual aflatoxins in the unknown was then determined by a visual comparison of t'he fluorescence intensities of the standards and the unknown. Aflatoxins were produced in vivo by inoculating Asfiergillus parasiticus (ATCC 1551 7) spores into yeast extract - sucrose medium (YES: 20% of sucrose, 2% of yeast extract and 88% of tap water) and whole milk and incubaked at 28 "C for 7 d.The surfaces of moistened rice, corn and pelletised rabbit feed (the last medium was chosen to represent silage) were also inoculated with Aspergillm parasiticus spores and incubated under the same conditions. In each instance, aflatoxin B, was found to be the predominant mycotoxin present following incubation. For the aflatoxin produced in YES medium, 5-ml aliquots of the medium were taken and extracted twice with chloroform -I methanol (9 + 1). The chloroform fractions were then pooled and reduced to dryness by flash evaporation. The residue was taken up in 0.5 ml of methanol and 5 pl of this extract were spotted on a thin-layer chromatographic plate.The remaining solution was diluted to 5 ml with Britton - Robinson buffer (pH 9) and the concentration of aflatoxin determined by diff erential-pulse polarography (operating conditions: scan rate, 1 mV s-1; drop time, 2 s; modulation amplitude, 100 mV). With the other growth media, 50 g were homogenisedl with 200 ml of water - methanol (1 + 1) for 15 min. Following centrifugation a t 5000 g for 15 min, the supernatant was extracted twice with butan-1-01 and three times with chloroform. The butan-1-01 and chloroform layers were then pooled and reduced to dryness by flash evaporation. The residue was taken up in 1 ml of Britton - Robinson buffer (pH 9) - methanol (9 + 1) and applied to a Sephadex LH-20 column (100 x 10 mm).Elution was carried out with the same solvent and the fraction (5ml) exhibiting fluorescence and eluting at the same retention time as aflatoxin B, was examined by differential-pulse polarography for aflatoxin content. Results and Discussion Polarographic Behaviour Aflatoxins B,, B,, GI and G, were found to exhibit similar polarographic behaviour over the pH range 4-11. The effect of pH on the E, and ilim values of the two waves exhibited by aflatoxin B, in direct-current polarography is shown in Fig. 1. At pH values below 8 both waves are dependent on pH with the first wave, i a , exhibiting a change of E+ with pH of 35 mV pH-1 and the second wave, ib, exhibiting a change of 70 mV pH-l. At pH values greater than 8 wave ib disappears and wave i a becomes independent of pH.A study onJanuary, 1979 TO THE DETERMINATION OF AFLATOXIN B, IN FOODSTUFFS 75 the variation of ilim with h and h* (where h = height of mercury column) for the two waves exhibited by aflatoxin B, in Britton - Robinson buffer (pH 7) indicated that both processes are diffusion controlled. The normal-pulse polarograms, however, exhibit a maximum, which indicates that reactant adsorption is involved in the over-all electrode process. Assuming that the aflatoxins have a diffusion coefficient (D) similar to that of coumarin, Le., 8 x cm2 s-l (ref. 6), and substituting into the Ilkovic equation, it was found that in the pH range 6-7 the total height of the two waves corresponded approximately to a two-electron process. As the heights of the individual waves are not equal, it is assumed that competing reactions are involved in the over-all electrode process.;~ t -1.2 -1.1 0.04 0.03 < 0.02 5 - .- 0.01 0 4 5 6 7 8 9 10 11 12- PH Fig. 1. Plots of E* (solid lines) and ilim (broken lines) against pH for waves i, (0) and ib (A) exhibited by aflatoxin B,. This behaviour is similar to that reported for coumarin by Polievktov and Lomadze,' who found that the first wave of coumarin in neutral aqueous alcoholic solutions corre- sponded to the formation of an anion-radical species, accompanied by dimer formation. They attributed the second wave to the reduction of the undimerised anion-radical and reported that in acidic media protonation of the molecule preceded reduction. The forma- tion of an anion-radical followed by dimerisation has also been reported by other workers in studies on the electrochemical behaviour of coumarin.6s8 As the electrochemical behaviour of the aflatoxins closely parallels that of coumarin, it is likely that the electroactivity of these compounds is associated with the coumarin moiety in their molecular structure.The main difference in behaviour lies in the fact that the reduction of the aflatoxins occurs at a potential about 300400 mV more positive than that for coumarin. This can be explained by the increased conjugation caused by the keto group in the neighbouring cyclopentanone (B, and B,) or &lactone (G, and G,) rings. Slight differences in the potential of reduction of the aflatoxins have also been observed owing to their slight differences in structure, e.g., in solutions of pH 8-10, the half-wave potentials of reduction of the various aflatoxins were found to be B, = -1.26, B, = -1.27, G, = -1.21 and G, = -1.23V (all versus S.C.E.).These potentials are in good agreement with the values quoted by Gajan et aZ.4 The mechanism of reduction of these compounds can therefore be postulated to be as shown on the next page.76 SMYTH et aE. : APPLICATION OF DIFFERENTIAL-PULSE POLAROGRAPHY Analyst, VoZ. 104 pH 8-11 2e- -+ 2H20 --dimer +20H- Choice of Wave for Analytical Purposes The best defined waves for analytical purposes were obtained in Britton - Robinson buffer of pH 9 where, although the peak current in differential-pulse polarography was slightly smaller than that obtained in solutions of pH 7-8, measurement of the peak height was made easier owing to the disappearance of the second wave.A graph of i p against con- centration for aflatoxin B, in the range 1 :K lO-'-l X 1 0 - 5 ~ was linear with a slope of 0.039 nA ng-1 ml. The limit of detection for the differential-pulse polarographic determina- tion of aflatoxin B, in pure solution was found to be about 8 x M (25 ng ml-1). The E , value (in differential-pulse polarography) was also found to be dependent on concentra- tion for each of the aflatoxins studied. This :is illustrated in Fig. 2 for the effect of concentra- tion on the E, value of aflatoxin B, in Britton - Robinson buffer (pH 9). Determination of Aflatoxin B1 in Foodstuffs Using Differential-pulse Polarography When aflatoxin B,ywas produced by Aspergillus parasiticus in YES medium, extracted I 0 200 400 6 Concentrationhg ml-' -1.27 Fig.2. Effect of concentration on E , value (in differential-pulse polarography) of aflatoxin B, in Britton - Robinson buffer (pH 9). Conditions: scan rate, 2 mVs-l; drop time, 2 s ; and modula- tion amplitude, 100 mV.January, 1979 TO THE DETERMINATION OF AFLATOXIN B, IN FOODSTUFFS 77 twice with chloroform-methanol (9 + 1) and determined by both thin-layer chromato- graphy and diff erential-pulse polarography, a good correlation was obtained between the two sets of results (Table I). The differential-pulse polarographic method did, however, permit the earlier detection of aflatoxin B, (because only 5 pl of the 0.5 ml of extract was applied to the thin-layer chromatographic plate) and gave rise to more precise results.TABLE I DETERMINATION OF AFLATOXIN B, IN YEAST EXTRACT - SUCROSE MEDIUM BY THIN-LAYER CHROMATOGRAPHY AND DIFFERENTIAL-PULSE POLAROGRAPHY Results are averages of three separate innoculations. Concentration of aflatoxin BJpg ml-1 Time of incubation/h Thin-layer chromatography Diff erential-pulse polarography 24 N.D.* 0.148 f 0.08 72 1.30 f 0.64 1.21 f 0.22 108 3.20 & 0.46 2.96 f 0.29 0 0 0 * N.D. = Not detected. When aflatoxin B, was grown on rice, corn, milk or pelletised rabbit feed, and extracted in a similar fashion, it was found that the concentration of aflatoxin B, in the extract could not be determined by diff erential-pulse polarography owing to the high level of electroactive interference (Table 11).In order to overcome this problem, aflatoxin B, grown on these foods was first extracted with butan-1-01 and then separated on a Sephadex LH-20 column. This method of purification has often been used in the analysis of aflatoxins, most recently by Josefsson and Moller.9 When the eluent was analysed by differential-pulse polarography , a good correlation was again obtained between the diff erential-pulse polarographic and thin-layer chromatographic assays (Table 11). The limit of detection for the differential- pulse polarographic method was found to be 0.15 pg ml-, for a 5-ml fraction of eluent. This permits the determination of 1-2 pg g-l of the aflatoxin in the original material when 50 g of material were taken for analysis. TABLE I1 DETERMINATION OF AFLATOXIN B, IN VARIOUS FOOD PRODUCTS BY THIN-LAYER CHROMATOGRAPHY AND DIFFERENTIAL-PULSE POLAROGRAPHY Concentration of aflatoxin BJpg nil-' A r > Diff erential-pulse polarograph y Food matrix Thin-layer chromatography A* Bt Rice .. .. .. 17.0 Pelletised rabbit feed . . 1.5 Milk .. .. .. .. 0.18 Corn .. .. .. .. 3.0 U.Q.$ 16.0 U.Q. 0. 2 U.Q. 1.3 U.Q. 2.6 * Following extraction with chloroform - methanol (9 + 1). t Following extraction with butan-1-01 and chloroform and separation on a Sephadex LH-20 $ U.Q. = Unable to quantitate. column. These results indicate, therefore, that diff erential-pulse polarography can provide an accurate and sensitive method for the determination of aflatoxin B, in various foodstuffs. However, when a mixture of aflatoxins is produced in vivo, a separation procedure, e.g., high-performance liquid chromatography, would have to be applied prior to analysis.It is suggested that diff erential-pulse polarography or a combination of high-performance liquid chromatography with electrochemical detection could provide information complimentary to that obtained with fluorescence methods for the screening of these carcinogenic myco- toxins in situations of environmental significance.78 SMYTH, LAWELLIIX AND OSTERYOUNG The authors thank Drs. Dale W. Grant and Jeffrey W. Whittaker for valuable discussions. This paper was presented in part at the 173rtl ACS National Meeting in New Orleans, March 20th-25th, 1977. Partial financial support was provided through NSF Grant No. MPS 75-00332. References 1 . 2. 3. 4. 5. 6. 7. 8. 9. Pons, W. A., Jr., Cucullu, A. F., Franz, A. O., Jr., and Goldblatt, L. A., J . Am. Oil Cliem. Soc., Romer, T. R., J . Ass. Off. AnaZyt. Chem., 19’75, 58, 500. Schuller, P. L., Horwitz, W., and Stoloff, L., J . Ass. OH. Analyt. Chem., 1976, 59, 1315. Gajan, R. J., Nesheim, S., and Campbell, A. D., J . Ass. Off. Agric. Chem., 1964, 47, 27. Smyth, M. R., and Franklin Smyth, W., Analyst, 1978, 103, 529. Patzak, R., and Neugebauer. L., Mh. Chem., 1961, 82, 662. Polievktov, M. K., and Lomadze, I., Zh. Obsch. Khim., 1977, 47, 1383. Brook, P. A., and Crossley, J . A., Electrochim. Acta, 1966, 11, 1189. Josefsson, B. G. E., and Moller, T. E., J . As:;. Off. Amlyt. Chem., 1977, 60, 1369. 1968, 45, 694. Received June 22nd, 1978 Accepted August 23rd, 1978

ISSN:0003-2654    

DOI:10.1039/AN9790400073

出版商:RSC    

年代:1979

数据来源: RSC