ISSN:0003-2654    

DOI:10.1039/AN97904FX045

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

ANALAO 104 (1 245) 1 105-1 21 6 (1 979)ISSN 0003-2654December 1979110511191124112911351138115111591165117111761181118511881191119511971201120412081213THE ANALYSTTHE ANALYTICAL JOURNAL OF THE CHEMICAL SOCIETYCONTENTSAbsorptiometric Determination o f Low Oxygen Concentrations in Power-stationWaters.Absorptiometric Determination of Low Oxygen Concentrations in Power-stationWaters. Part II. Continuous Automatic Method Using the Technicon Auto-Analyzer-G. I. Goodfellow, D. F. Libaert and H. M. WebberSingle Reagent Method f o r the Spectrophotometric Determination o f Phosphorus inSilicates-P. J. WatkinsModified Colorimetric Method f o r the Determination of Malathion-E. R. Clark andI. A. QaziHistochemical Identification o f Commercial Wheat Gluten-F.0. Flint and R. F. P.JohnsonInvestigation o f Atomiser Tube Design for Carbon Furnace Atomic-emission Spectro-metry-D. Littlejohn and J. M. OttawaySeparation o f Bismuth-21 0 and Polonium-21 0 from Aqueous Solutions by SpontaneousAdsorption on Copper Foils-A. B. MacKenzie and R. D. ScottDetermination of Mercury in Smelter Flue Dusts by Acid Digestion Methods-ChungH. Chiu and John C. HilbornH i g h -perform an ce Li q u i d Chroma t og ra phi c Deter m i n a t i o n o f Ac r y I i c Acid M on o me rin Natural and Polluted Aqueous Ehvironments and Polyacrylates-L. BrownDetermination o f Ochratoxin A in Pig'!; Kidney Using Enzymic Digestion, Dialysis andHigh-performance Liquid Chromatography with Post-column Derivatisation-D.C. Hunt, Lesley A. Philp and, N. T. CrosbyPreparation and Stability o f Dilute Insecticide Analytical Standards f o r Gas Chromato-graphy-D. L. Suett, G. A. Wheatley and C. E. PadburyAssay of Nicotinamide in Multivitamin Preparations Using an Ammonia Gas-sensingElectrode-D. P. Nikolelis, C. E. Efstathiou and T. P. HadjiioannouPart 1. Manual Method-G. I. Goodfellow and H. M. WebberSHORT PAPERSSpectrophotometric Method for the Determination of Residues of Carbaryl in Water-Selective Spectrophotometric Determination o f Zinc with 2,2'-Dipyridyl-2-quinolyl-Determination of Oxycarboxin Residues in Medicinal Plants-W. DFbska, B. GnusowskiDetermination o f Nitrate and Nitrite in River Waters-Mieko Okada, Haruo Miyata andExtraction o f Methylmercury from Fislh and I t s Determination by Atomic-absorptionDetermination of Antimony(ll1) by Titraition with Manganese(ll1) and Manganese( IV)-Determination of Conjugated Diolefins in Cracked Naphtha-S. S. Roy and B. K. GoelBook ReviewsInstructions t o Authorss. K. Handa and A. K. Dikshithydrazone-R. B. Singh, P. Jain, B. S. Garg and R. P. Singhand B. ZygmuntKyoji TBeiSpectroscopy-R. Capelli, C. Fezia, A. Franchi and G. ZanicchiMrs. N. Rukmini, V. S. N. P. Kavitha and K. Rama RaoSummaries of Papers in this Issue-Pages iv, vi, vii, x, xii, xivPrinted by Heffers Printer!; Ltd Cambridge EnglandEntered as Second Class at N e w York, USA, Post Offic

ISSN:0003-2654    

DOI:10.1039/AN97904BX047

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

i V SUMMARIES OF PAPERS IN THIS I S S U E December, 1979Summaries of Papers in this IssueAbsorptiometric Determination of Low Oxygen Concentrations inPower-station Waters. Part I. Manual MethodA simple, fast method for determining low concentrations of oxygen inpower-station waters has been developed, based on the reaction of dissolvedoxygen with the leuco-base of methylene blue to give a soluble blue oxidationproduct the absorbance of which is a function of the oxygen concentration.A special glass cell has been devised, which acts sequentially as a sample-collection vessel, a reaction vessel and a spectrophotometric cuvette. Thecell design permits the easy addition of the leuco-base and also the air-saturated water used for calibration. A novel technique of “zero-timeextrapolation” for the determination of the reagentlcuvette blank circumventsthe difficulty of making this measurement with oxygen-free water.The calibration graph is linear up to 50 pgl-l, but satisfactory measure-ments may be made up to 100 p g 1-l.The criterion of detection is approxi-mately 1.0 pg 1-1 with standard deviations ranging between 0.4 and 1.7 pg 1-1,depending on the concentration. The analysis time is 5-10 min for a singledetermination.Iron(I1) and copper(I1) ions are the only ions likely to be present in boilerwaters that cause serious interference and these must be removed beforeanalysis by passing the water sample through a cation-exchange column.Keywords ; Oxygen determination ; absovptiovlzetry ; water analysis ; manual!analysisG.I. GOODFELLOW and H. M. WEBBERCentral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey,KT22 7SE.Analyst, 1979, 104, 1105-1118.Absorptiometric Determination of Low Oxygen Concentrations inPower-station Waters. Part 11. Continuous Automatic MethodUsing the Technicon AutoAnalyzerA continuous automatic method for the absorptiometric determination oflow oxygen concentrations in power-station waters using the TechniconAutoAnalyzer is described.Keywords : Oxygen determinution ; absovptiometry ; water analysis ; continuousanalysisG. I. GOODFELLOW, D. F. LIBAERT and H. M. WEBBERCentral Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey,KT22 7SE.Analyst, 1979, 104, 1119-1123vi SUMMARIES OF PAl3ERS I N THIS ISSUESingle Reagent Method for the S:pectrophotometric Determinationof Phosphorus in SilicatesDecember, 1979A method has been developed for the spectrophotometric determination ofphosphorus in silicate rocks following fusion with a lithium borate flux anddissolution of the melt in dilute nitric acid.By using a single reagentincorporating antimony and relatively high concentrations of ascorbic acidand hydroxylammonium chloride, a stable colour has been achieved. Thecolour is fully developed within 30 min and is stable up to 4 h in solutionscontaining up to 0.8 pgml-1 of phosphorus(V) oxide with nitric acid con-centrations not greater than 0.04 N. Beer’s law is obeyed and E = 22 200a t 880 nm.Keywords : Phosphorus determination ; silicate rocks ; spectrophotonzetry ; singlereagent method; molybdonntimony1;t)hosphoric acidP.J. WATKINSGeology Department, Imperial College of Slcience and Technology, London, SW7 2BP.Analyst, 1979, 104, 1124-1 128.Modified Colorimetric Method for the Determination of MalathionA thorough investigation has been made of the recommended method for thedetermination of malathion, and the rnajor cause of two of the most seriousproblems in this method has been resolved. The method that uses acopper(I1) complex as the basis of colorimetric measurements suffers fromthe disadvantage that the colour fades quickly and that an increase of a fewseconds in the contact time of the copper(I1) solution and the hydrolysisproduct of malathion results in a reduction in the intensity of the yellowcolour.Attempts to overcome these drawbacks have been reported, someof which are tedious and others only partially successful, but it is suggestedthat if copper is replaccd with bismuth the problems may be more simplyresolved. Isomalathion does not react in this method.Keywords : Malathion alkaline hydrolysis ; malathion determination ; copperchelate colorimetry ; colour instabiliiy ; bismuth chelateE. R. CLARK and I. A. QAZIDepartment of Chemistry, University of Aston in Birmingham, Gosta Green,Birmingham, B4 7ET.Analyst, 1979, 104, 1129-1134.Histochemical Identification of Commercial Wheat GlutenThree methods for the microscopical identification of commercial wheatglutens are compared.A periodic acid - Schiff and an iodine - potassiumiodide method both indicate gluten by showing the wheat starch present.Toluidine blue contained in an aqueous mountant distinguishes gluten proteinfrom both soya and meat proteins. Each method identified commercialgluten present in a gluten - soya protein - meat mixture. Testing for bothstarch and protein is recommended for protein products that may containadded starch.Keywords : Commercial wheat gluten identification ; gluten - soya mixture ;gluten - soya - meat mixtureF. 0. FLINT and R. F. P. JOHNSONProcter Department of Food Science, University of Leeds, Leeds, LS2 9JTDecember, 1979 SUMMARIES OF PAPERS I N THIS ISSUEInvestigation of Atomiser Tube Design for Carbon FurnaceAtomic - emission SpectrometryviiModifications to a standard graphite furnace tube, designed for atomic-absorption measurements, are shown to give improved performance foratomic-emission measurements of a number of elements.Four differentmodified tubes are compared with a standard Perkin-Elmer HGA-72 graphitetube with respect to their thermal characteristics and the detection limits often selected elements of varying atom-appearance temperatures. Differentelements were found to give lowest detection limits in tubes of differentdesign. The results indicate the need for a new approach to tube design,specifically for atomic-emission measurements, in order that best detectionlimits can be achieved for all elements in a single graphite tube.Keywords : Modijied atomiser tubes ; atomic-emission spectrometvy ; carbonfurnace atornisationD.LITTLEJOHN and J. M. OTTAWAYDepartment of Pure and Applied Chemistry, Iinivcrsity of Strathclyde, CathcdralStreet, Glasgow, G1 1XL.Analyst, 1979, 104, 1138-1 150.Separation of Bismuth-210 and Polonium-210 from AqueousSolutions by Spontaneous Adsorption on Copper FoilsA description is given of factors controlling the adsorption of trace amountsof radioactive bismuth and polonium from aqueous solution on to thincopper foils. Optimum conditions are defined for the selective and quantita-tive adsorption of bismuth-210 and polonium-2 10 from aqueous solution andfor a subsequent desorption process to give separation of the extractedbismuth and polonium.The method is demonstrated to be suitable for thepreparation of thin sources for a-spectroscopy and for analysis of bismuth-210and polonium-210 in marine sediments. The kinetics of the adsorption anddesorption processes are briefly discussed.Key words : 210Pb, 210Bi and 210Po analysis ; spontaneous adsovption ; desovp-tion ; copper suvfacesA. B. MacKENZIE and R. D. SCOTTScottish Universities Research and Reactor Centre, East Kilbride, Glasgow,G75 OQU.Analyst, 1979, 104, 1151-1158.Determination of Mercury in Smelter Flue Dusts byAcid Digestion MethodsIn order to develop a mercury digestion method suitable for flue dust fromnon-ferrous and secondary lead smelters, seven acid digestions, two in aclosed system and five in an open system, were studied. Closed systemswere less convenient to use and had no real advantage over open systems.We conclude that the use of aqua regia in open systems is the most effectivedigestion method. A 96% recovery for spiked mercury(I1) sulphide andmercury(I1) oxide and a 98% recovery for mercury(I1) chloride were obtained.Keywords : Mevcuvy determination ; acid digestion ; ftue dust ; non-fervoussmelterCHUNG H. CHIU and JOHN C. HILBORNChemistry Division, Environment Canada, Air Pollution Technology Centre, RiverRoad, Ottawa, Canada, K1A 1C8.Analyst, 1979, 104, 1159-1 164

ISSN:0003-2654    

DOI:10.1039/AN97904FP109

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

x SUMMARIES OF PAPERS I N THIS ISSUEHigh-performance Liquid Chromatographic Determination ofAcrylic Acid Monomer in Natural and Polluted AqueousEnvironments and PolyacrylatesA procedure is described for the determination of acrylic acid monomer innatural and polluted waters and poly acrylates. Polyacrylates are extractedusing a mixture of methanol and water prior to analysis, but no preparationis required for water samples. Separation from interferences is achieved byhigh-performance liquid chromatography and quantification by ultravioletdetection. A detection limit of 0.05 mg 1-1 and a precision of 8% a t 1.17and 10 mg 1-1 of acrylic acid have been achieved using water samples. Samplestested included river, sea and estuarine waters, sewage and china clay workseffluents and potable waters.Acrylic acid can be detected a t levels in excessof 0.0005~0 of monomer in the polymer with a precision of 8% a t levels of0.05% and 0.003% of monomer.Keywords : A cvylic acid detevvvlination ; aqueous solutions ; polyacvylates ;December, 1979high-pevformance liquid clzromatogvaplay ; environmental pollutionL. BROWNSchool of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth,Devon, PL4 8AA.Analyst, 1979, 104, 1165-1170.Determination of Ochratoxin A in Pig's Kidney Using EnzymicDigestion, Dialysis and High-performance LiquidChromatography with Post-column DerivatisationA method for the determination of ochratoxin A in pig's kidneys is described.The detection limit is less than 1 p g k:g-l.By using enzymic digestion con-currently with dialysis and by controlling the pH conditions, the sample isextracted and interfering co-extracts removed without the use of columnchromatography. The final determination employs reversed-phase high-performance liquid chromatography wj t h either a phthalimidopropylsilane ora C,, (docosyl) bonded column, and the sensitivity of the fluorescence detectoris increased ten-fold by the formation of a post-column derivative withammonia solution.Keywords ; Ochvatoxin A ; high-pevfownance liquid chromatograplzy ; enzymes ;dialysisD. C. HUNT, LESLEY A. PHILP and N. T. CROSBYLaboratory of the Government Chemist, Cornwall House, Stamford Street, London,SE1 9NQ.Analyst, 1979, 104, 1171-1175.Preparation and Stability of Dilute Insecticide AnalyticalStandards for Gas; ChromatographyA convenient and economical procedure for preparing and storing accuratelymeasured amounts (1.00 mg) of insecticide analytical standards is described.The validity of the method was evaluated in a 2-year study of the storagestability of carbofuran, chlorfenvinphos, diazinon, dichlorvos, disulfoton anddisulfoton sulphone.Solutions (1 mg m-l) of each insecticide were preparedin hexane, toluene or acetone and aliquots sealed in glass ampoules were storeda t -20, 1 and 20 "C. After 0.25, 0.5, 1.0 and 2.0 years the mean concentra-tions of all compounds, except dichlorvos, were within 0.976 of fresh solutionsof the original standards; dichlovos was 3.6% greater.Neither the type ofsolvent nor the storage temperature had significant (P = 0.05) effects on theconcentrations but there were large differences in purity between differentbatches of primary standards of all the organophosphorus insecticides.Keywords : Insecticide analytical staizdards ; insecticide residues ; chromato-P P h YD. L. SUETT, G. A. WHEhTLEY and (C. E. PADBURYNational Vegetable Research Station, Wellesbourne, Warwick, CV35 9EF.Analyst, 1979, 104, 1176-1180xii SUMMARIES OF PAF'ERS I N THIS ISSUEAssay of Nicotinamide in Multivitamin Preparations Using anAmmonia Gas- sensing ElectrodeDecembev, 1979A simple potentiometric method for the determination of nicotinamide isdescribed. The sample is subjected to alkaline hydrolysis and the ammoniathus produced is determined with an ammonia gas-sensing electrode.Amounts of nicotinamide in the range 0.5-15 mg have been determined withan average error of about 1.7 % .The method has been applied to the analysisof multivitamin preparations. The analytical recovery of nicotinamide was95-107%. Comparison with an official method gave satisfactory results.Keywords : Ammonia gas-sensing electrode ; direct Potentiometry ; nico-tinanzide determination ; Ynultivitainin Preparations analysisD. P. NIKOLELIS, C. E. EFSTATHIOU and T. P. HADJIIOANNOULaboratory of Analytical Chemistry, University of Athens, Athens, Greece.Analyst, 1979, 104, 1181-1184.Spectrophotometric Method for the Determination of Residuesof Carbaryl in WaterShovt PaperKeywords ; Carbaryl residues ; water analysis ; spectvophotometryS.K. HANDA and A. K. DIKSHITDivision of Agricultural Chemicals, Indian Agricultural Research Institute, NewDelhi, India.Analyst, 1979, 104, 1185-1188.Selective Spectrophotometric Determination of Zinc with2,2'- Dipyridyl- 2 - cluinolylhydrazoneShort PapevKeywovds : 2,2'-Dipyridyl-2-quinolyllzydvazone reagent ; spectvophotometify ;zinc determination ; alloy analysisR. B. SINGH, P. JAIN, B. S. GARG and R. P. SINGHDepartment of Chemistry, University of Delhi, Delhi-110007, India.Analyst, 1979, 104, 1188-1191.Determination of Oxycarboxin Residues in Medicinal PlantsShort PaperKeywords : Oxycavboxin detevnzinatio tz ; fungicide residue analysis ; spectvo-photometry ; medicinal plantsW.DEBSKA, B. GNUSOWSKI and B. ZYGMUNTDepartment of Phytochemical Analysis, Institute of Medicinal Plants, Libelta 37,61-707 Poznari, Poland.Analyst, 1979, 104, 1191-1194... Decerutber, 1979 THE ANALYST XlllANALYTICAL SCIENCES MONOGRAPH No. 4Electrothermal Atomization forAtomic Absorption Spectrometryby C. W. FullerAt the present time the two most successful alternatives to the flame appear to bethe electrothermal atomizer and the inductively-coupled plasma. In this book anattempt has been made to provide the author‘s views on the historical develop-ment, commercial design features, theory, practical considerations, analyticalparameters of the elements, and areas of application of the first of these twotechniques, electrothermal atomization.The chapter headings are as follows: History; Theoretical Aspects of theAtomization Process; General Experimental Conditions; Analytical Conditionsfor the Determination of the Elements by Atomic Absorption Spectrometry;Applications (Oil and Oil Products; Metals; Rocks, Minerals and Soils; Waters;Plants; Food and Drugs; Biological Fluids; Biological Tissues; Air Particulates;Refractory Oxides and Related Materials; Other Analytical Applications;Theoretical).Clothbound 135pp 82” x 5” 0 851 86 777 4 f6.75 ($1 3.50)CS Members f5.50THE CHEMICAL SOCIETYDistribution Centre, Blackhorse Road, Letchworth,Herts., SG6 1 HN, EnglandAnnual Reports on AnalyticalAtomic SpectroscopyVOLUME 8, 1978This comprehensive and critical report of developments in analytical atomicspectroscopy has been compiled from about 1500 reports received from world-wide correspondents who are internationally recognised authorities in the fieldand who constitute the Editorial Board.In addition to surveying developmentsthroughout the world published in national or international journals, a particu-lar aim has been to include less widely accessible reports from local, nationaland international symposia and conferences concerned with atomic spectros-COPY.(Still available: Vols 3-7 covering 1973 to 1977)Clothbound 273pp 8 2 x 6” f 17.50 (CS Members f 13.00)Obtainable from : The Chemical Society, Distribution Centre,Blackhorse Road, Letchworth, Herts., SG6 1 H xiv SUMMARIES OF PAPERS I N THIS ISSUE December, 1979Determination of Nitrate and Nitrite in River WatersShort PaperKeywords : Nitrate determination ; nitrite determination ; water analysis ;spectrophotometryMIEKO OKADA, HARUO MIYATA and KYOJI TOE1Department of Chemistry, Faculty of Science, Okayama University, Tsusliima-naka3-1-1, Okayama-shi 700, Japan.Analyst, 1979, 104, 1195-1 197.Extraction of Methylmercury from Fish and Its Determinationby Atomic-absorption SpectroscopyShort! PaperKeywords : Methylmercury extraction ; metlzylmercury determination ; fisJzanalysis ; atomic-absorption spcctvoscopy ; cold-vapour methodR.CAPELLI, C. FEZIA, A. FRANCHI and G. ZANICCHIInstitute of General and Inorganic Chemistry, Oceanological Research Group,University of Genoa, Genoa, Italy.Analyst, 1979, 104, 1197-1200.Determination of Antimony(II1) by Titration with Manganese(II1)and Manganese(1V)Sh o ~2 P a p e rKeywords : Titrimetry ; antimony(II I ) determination ; manganese(III) ;manganese(I V ) ; iodine monochlorideMrs. N. RUKMINI, V. S . N. P. KAVITHA and K. RAMA RAODepartment of Chemistry, Andhra University, Waltair-530 003, India.Analyst, 1979, 104, 1201-1204.Determination of Conjugated Diolefins in Cracked NaphthaShort PaperKeywords : Conjugated diolejins ; cracked naphtha ; maleic anhydride - toluenereagent; naaleic anhydride value ; Diels - A ldev reactionS. S. ROY and B. K. GOELIndian Institute of Petroleum, Dehradun-248005, India.Analyst, 1979, 104, 1204-1207

ISSN:0003-2654    

DOI:10.1039/AN97904BP115

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

DECEMBER 1979 The Analyst Vol. 104 No. 1245 Absorptiometric Determination of Low Oxygen Concentrations in Power-station Waters Part I. Manual Method G. I. Goodfellow and H. M. Webber Central Electvicity Research Laboratories, Kelvin Avenue, Leatherhead, Suvrey, KT22 7SE A simple, fast method for determining low concentrations of oxygen in power-station waters has been developed, based on the reaction of dissolved oxygen with the leuco-base of methylene blue to give a soluble blue oxidation product the absorbance of which is a function of the oxygen concentration. A special glass cell has been devised, which acts sequentially as a sample- collection vessel, a reaction vessel and a spectrophotometric cuvette. The cell design permits the easy addition of the leuco-base and also the air- saturated water used for calibration.A novel technique of “zero-time extrapolation” for the determination of the reagent/cuvette blank circumvents the difficulty of making this measurement with oxygen-free water. The calibration graph is linear up to 50 pgl-l, but satisfactory measure- tnents may be made up to 100 pg 1-l. The criterion of detection is approxi- mately 1.0 pg 1-l with standard deviations ranging between 0.4 and 1 . 7 pg l-l, depending on the concentration. The analysis time is 5-10 min for a single determination. Iron(I1) and copper(I1) ions are the only ions likely to be present in boiler waters that cause serious interference and these must be removed before analysis by passing the water sample through a cation-e:.;changc column.Keywords ; Oxygen determination ; absovptiometvy ; watev analysis ; manual annly s is For many years the manual method recommended in the CEGB for determining oxygen in feed-water has been that described by Potter and Whitell the detailed procedure being given in BS 2690.2 This method is accurate if interfering ions are removed from the sample and is extremely precise, a standard deviation of 0.7 pg 1-1 being attainable. However, the technique is difficult and time consuming and even an experienced operator can analyse.no more than 10-12 samples per day. For these reasons the less sensitive and less precise indigo carmine spectrophotometric technique (BS 26902) or the North Hants Engineering Co. Ltd. portable] electrometric instrument (now supplied by Automatic Samplers Ltd.) has been preferred.A more convenient manual method was therefore desirable and the work of a number of Russian workers (Rotshtein and Shemyakin13 Sutotskii and Gramatchikov* and Devdariani et nZa5), who used a spectrophotometric procedure involving oxidation of the leuco-form of methylene blue in glycerol, seemed applicable. This reagent appeared to have a number of advantages over the indigo carmine reagent : (i) better stability; (ii) greater sensitivity; (iii) the formation of a single coloured species, the intensity of which is directly pro- portional to the concentration of oxygen. A detailed investigation of the experimental parameters was undertaken to develop a technique suitable for routine use in power stations, Basis of the Method Methylene blue is reduced in an alkaline glucose solution to the colourless leuco-form.The leuco-base is insoluble in water but is readily soluble in glycerol; this solvent has the 11051106 GOODFELLOW AND WEBBER: ABSORPTIOMETRIC DETERMINATION OF Analyst, vol. 104 additional advantage that because of its high viscosity oxygen diffusion is very slow and therefore the bulk of the reduced reagent soluition is relatively stable, even when no pre- cautions are taken to prevent contact with air. On adding the reagent to a sample containing oxygen a coloured oxidation product of the leuco-base is produced, the intensity of which is proportional to the concentration of oxygen as long as there is sufficient excess of the leuco-base. Method Apparatus Spectrophotometric cuvettes.The special cuvette shown in Fig. 1 can be obtained from either Chandos Intercontinental (New Mills, Nr. Stockport, Cheshire) or Hellma (England) Ltd. (Westcliff-on-Sea, Essex). The taps shou.ld be evenly but lightly greased with com- mercially available rubber grease. Silicone grease should not be used as this may be troublesome to remove should it get on to the internal optical faces. inlet Outlet Fig. 1 . Spectropho tometric cuvette. Sflectrophotometer. Most commercial spectrophotometers can be used for this deter- mination although minor modifications may be required to enable the cuvette to be con- tained in the sample compartment. Spectrophotometers with digital read-out are recommended as with null-point and analogue display instruments difficulties have been experienced in obtaining accurate readings during the initial stages of the reaction.However, many of these latter types of instrument have a recorder outlet to which a digital voltmeter can be connected. Although the voltage output may not be linear with respect to absorbance, a conversion graph may easily be obtained. A diagram of a suitable 5- or 10-m1 burette assembly with 0.1-ml graduations and a reservoir is given in Fig. 2. Alternatively, a self-filling micro-burette may be obtained from Baird and Tatlock Ltd., catalogue number 241/0420. The delivery tip of this burette must be modified to that shown in Fig. 2. Micro-syringes of 25- and 100-pl capacities fitted with 70-mm hypo- dermic needles. Make the column with approximately 20 ml of cation- exchange resin (Amberlite IR 120, analytical grade, has been found satisfactory) packed in a glass tube of approximately 180 x 15 mm.This volume of resin is sufficient to remove divalent cation impurities at concentrations of 100 pg 1-1 from approximately 5000 1 of water. (Ammonia present at the levels norma.lly in feedwater, i.e., 0.1-1.0 mg l-l, should not affect the efficiency of removal of divalent cations.) Burette. Micro-syringes. Cation-exchange resin column. Those supplied by Scientific Glass Engineering (U. K.) Ltd. are suitable. Reagents Water. in reagent preparations. Distilled water passed through a mixed-bed de-ionisation unit is suitable for useDecember, 1979 LOW OXYGEN CONCENTRATIONS I N POWER-STATION WATERS. PART I 1107 Leuco-methylene blue.Solution A: dissolve 0.123 g of methylene blue (technical dye grade) and 0.65g of glucose (analytical-reagent grade) in 35 ml of water and dilute the solution to 2000 ml with glycerol (analytical-reagent grade). This solution is stable for several weeks. Solution B : dissolve 100 g of potassium hydroxide (analytical-reagent grade) in water and dilute to 200 ml with water. To 100 ml of solution A add x ml* of solution B. Mix by shaking and transfer the solution into the burette. This reagent may be used for at least 3 weeks, although the sensitivity of the determination decreases slowly with the ageing of the reagent. The reagent in contact with air becomes oxidised (e.g., at the top of the burette) but this does not affect the bulk of the reagent although it does affect the ease with which the meniscus may be seen. The reagent in the burette should be stored out of sunlight and preferably in the dark, at 15-20 “C.Allow the solution to stand overnight for air bubbles to separate. 1 I Reagent reservoir 5-or 10-ml burette “Push-fit” 1 insert 35 mm PTFE tube (0.d. % 2.0 mm) Fig. 2. Reagent burette modified with PTFE tip. Air-saturated water. Pass a stream of air, from a sintered-glass frit held just below the surface, through a continuously stirred volume of water until equilibrium is reached; with volumes greater than 1 1 this may take several hours. The oxygen content of this water (the temperature of which must be known) may be obtained from Table I, together with the following correction for the barometric pressure : SP s = - 760 where P = the observed pressure in millimetres, S, = solubility at pressure P and S = solubility at 760 mm at the observed temperature, Volumes of “low-oxygen” water, sufficient for the preparation of the calibration graph, may be prepared conveniently by the following procedure.Fill a glass container (size 5-10 l), which can be either an aspirator or a bottle fitted with an inlet/ outlet syphon, with water from which much of the oxygen has been removed; this may be obtained either from a suitable power-station source taken in the same manner as a sample or by boiling de-ionised water vigorously for about 20 min and then cooling under an inert- gas atmosphere. The oxygen remaining in the water can be reduced to less than 1 pg 1-‘ by passing a stream of inert gas, e.g., white-spot nitrogen, through the water for 90-120 min at a flow-rate of 500-1000 ml min-I.“Lozel-oxygen,” water. * N.B. I t is not possible to state precisely the volume of solution B as it is dependent, among other factors, on the ambient temperature (see, for example, Table III), but it will be between 0.5 and 2.0 ml. The actual volume, x ml, must be determined by a preliminary trial as that volume, using a ca. 100 p g 1-1 oxygen solution, which gives a reaction time of 5-8 min to achieve maximum absorbance. For this test it is necessary only for the larger air bubbles to separate from the solution (by standing for ca. 30 min) as the oxygen present in any residual bubbles will be insufficient to affect the result of the trial.1108 GOODFELLOW AND WEBBER : ABSORPTIOME'TRIC DETERMINATION OF Analyst, I/'ol.104 TABLE I SOLUBILITY OF OXYGEN IN WATER^ Temperaturel'C Solubility* 0 14.63 1 14.23 2 13.84 3 13.46 4 13.11 5 12.77 6 12.45 7 12.13 8 11.84 9 11.55 10 11.28 11 11.02 12 10.77 13 10.53 14 10.29 15 10.07 Temperature/"C Solubility* 16 9.86 17 9.65 18 9.46 19 9.27 20 9.08 21 8.91 22 8.74 23 8.57 24 8.42 25 8.26 26 8.12 27 7.97 28 7.84 29 7.70 30 7.57 * Solubility of oxygen in water (mg 1-l) in equilibrium with air at 760 mm. Procedure Sample collection Attach the cation-exchange resin column to the sample point using thick-walled neoprene tubing and a butt joint. (Other tubing with low permeability to oxygen may be satis- factory.) Before initial use, flush the column with up to 100 1 of sample, at a flow-rate of approximately 100 ml min-l, ensuring that no air bubbles are trapped in the system.Leave the column attached to the sample point with the outlet closed. On subsequent samplings flush the column with 1-2 1 of sample immediately before collecting an aliquot for analysis. With both inlet and outlet taps of the cuvette open, connect the inlet port of the cuvette to the outlet of the resin column using a butt joint with tubing similar to that used above. (N.B. The temperature of the sample should be less than 30 "C and within &3 "C of the temperature at which the calibration graph was prepared.) At a flow-rate between 30 and 100 ml min-l all.ow at least 150 ml of sample to flow through the cuvette, ensuring that no air bubbles are trapped in the system. First close the inlet tap and then immediately close the outlet tap on the cuvette, disconnect it from the resin column and store it immersed in a suitable container filled with water low in oxygen (this may conveniently be collected from the overflow of the cuvette).(N.B. Failure to close the taps in the order given will cause breakage of the cuvette if the sample supply is under any significant pressure.) The sample should be analysed as soon as possible after collection. Analytical procedure The "reagent / cuvette blank" absorbance for each individual measurement is obtained by taking absorbance measurements over the initial stages of the reaction and from these calculating, by linear extrapolation, the "zero-time" absorbance. To obtain accurate values of this absorbance the mixing and insertion of the cuvette into the spectrophotometcr must be completed as quickly and efficiently as possible.I t has been found that efficient mixing is obt,ained in ca. 5 s by holding the cuvette with the first and second (or second and third) fingers on the inlet and outlet tubes and the thumb under the cuvette and vigorously oscillating the wrist through 180". The first absorbance reading should be made within 20-25 s of cornmencing shaking; a further two readings within the first minute should normally be adequate. Immediately before the reagent addition allow 0.1 ml of reagent to flow to waste from the tip of the burette. Open tap C on the cuvette and insert the PTFE tip of the burette so that it protrudes about 2 mm below the bore of the tap. Add 0.3 ml of reagent carefully so as to avoid mixing and reacting the sample with the reagent.This mixing is minimisedDecember, 1979 LOW OXYGEN CONCENTRATIONS IN POWER-STATION WATERS. PART I 1109 by tilting the cuvette slightly so that the leuco-base forms a shallow pool in one of the lower corners of the cuvette. Start the stop-watch and simultaneously begin to shake the cuvette for 5 s, ensuring complete mixing of the sample and reagent. Quickly dry the optical faces of the cuvette and place it in the spectrophotometer set at 647 nm. Take absorbance measurements at known intervals over the first minute from the start of the mixing procedure and then note the absorbance at longer intervals until maximum absorbance, A e , is attained.Remove the tip of the burette carefully and close tap C. Calculation of Results Determine the absorbance of the solution at zero time, A,, by extrapolating the readings made over the first minute of the reaction. The absorbance, As, due to oxygen in the sample is given by From A , and the calibration graph calculate the oxygen concentration in the sample. Preparation of the Calibration Graph The calibration graph is prepared from results obtained using a single batch of low-oxygen water. Known additions of air-saturated water using the micro-syringe are made via the inlet port of the cuvette to aliquots of the low-oxygen water contained in the cuvette and the resulting solutions treated as samples. This gives a range of added oxygen concentrations from 0 to approximately 70 pg 1-1 (depending on the temperature of the water, the atmospheric pressure and the volume of the cuvette). The determinations should be repeated enough times to define the calibration graph with the required precision.Subtract the average absorbance for the solution containing no added oxygen from the average absorbances for each of the other solutions and plot the corrected absorbances against concentrations of oxygen calculated from the following equation : Additions of 0, 20, 40, 60, 80 and 1OOpl of air-saturated water are recommended. Oxygen concentration (pg 1-11 = ES, v2 where V, pl = volume of air-saturated water added; V , ml = volume of cuvette; and S , mg 1-1 = concentration of oxygen dissolved in air-saturated water. The calibration graph is linear up to at least 50 pg 1-1.The sensitivity of the method depends upon the age of the leuco-methylene blue reagent and it is recommended that the calibration graph should be prepared afresh whenever tests give results outside the expected limits. Once the linearity of response of the method has been confirmed by the calibration procedure described above it is often more convenient to use an alternative calibration procedure. For this purpose samples of low-oxygen water should be obtained to which the reagent is added and the absorbance measured in the usual way. Immediately maximum absorbance has been attained a known volume of air- saturated water is introduced into the cuvette and the increased maximum absorbance measured. The increase between the two values of maximum absorbance is directly pro- portional to the concentration of oxygen added.Results Reagent Formulation Initial tests showed that the formulation of the leuco-methylene blue used by Rotshtein and Shemyakin3 was unsatisfactory owing to the formation of a precipitate in the glycerol solution, while that used by Devdariani et a1.5 gave a poor sensitivity under the conditions used owing to the reduction of the methylene blue by the excess of glucose present. The compositions of the Russian reagents, together with that finally used in the proposed method, are given in Table 11.11 10 GOODFEL1,OW AND WEBBER ABSORPTIOMETRIC DETERMINATION OF TABLE I1 REAGEKT COMPOSITIONS Rotshtein and Devdariani Sht:myakin3 et al.5 Methylene blue . . . . . . .. 0.246 g 0.3 g Water . . . . . . . . . . 70 ml 70 ml Reagent A (per litre) (in glycerol)- Glucose . . * . . . . . . . . 2.6 g 1.2 g Reagent B (per 100 ml) (in water)- Potassium hydroxide . . . . . . 50 g 40 g Leuco-base- Reagent B:reagent A . . . . . . ;1: 19 1 : 19 Analyst, VoZ. I04 CERL 0.062 Q 0.33 g 17.5 ml 50 g 1:50 to 1 : 200 I t will be seen that there is a variable ratio of reagent B to reagent A in the CERL prepara- The reason for this is discussed in detail below under Reagent comeosition a i d volzime tion. added to a sarYzple. Development of the Technique It was recognised at the outset that the technique would have to be simple, easy to cali- brate and precise if it were to be a satisfactory alternative to the Potter and White1 method. Four basic requirements were identified : the design of a leak-proof sampling vessel, capable of being used also as a spectro- photometric cuvette, which was simple to manufacture and easy to use ; a means of reagent addition without atmospheric contamination ; a technique for measuring a meaningful reagent/cuvette blank ; a suitable method of calibration.(i) (ii) (jii) (IV) The last requirement was necessary because the technique proposed by Rotshtein and Shemyakin3 of using standard solutions of fully oxidised methylene blue for calibration had been shown by Devdariani et to cause large errors because the absorption spectrum of such standards was not the same as that given by methylene blue formed in solutions by oxidation of the leuco-base. Spectrophotometific cztvette One of the primary requirements for a successful manual technique is that the sample should be manipulated as little as possible to minimise the risks of atmospheric contamina- tion.Ideally a vessel was needed which would serve for sample collection, for colour development and for spectrophotometric measurement. In addition, it not only had to be leak-proof to atmospheric oxygen, but had to allow the addition of reagent without intro- ducing extraneous oxygen. After trials with different designs of cell and different techniques for reagent addition the cuvette shown in Fig. 1 was adopted. I t was found that the reagent could be added without contamination by inserting through the bore of the stopcock a 2 mrn o.d. PTFE tube attached to the modified tip of a burette. The reagent inlet limb was designed so that ingress of air into the sample was minimised both during and after the time of reagent addition.Initially PTFE stopcocks were used as these required no lubrication. However, it was found that these often deformed easily and this resulted in ingress of air. Lightly greased borosilicate glass stopcocks were, therefore, used in later batches of cuvettes. Because of the necessary restriction in size of the cuvette for its accommodation in a spectrophotometer and also to allow access of the PTFE burette probe, specially manufactured glass micro- stopcocks of 2.5-mm bore were used. Teclznique of menswement The conventional method of spectrophotometric measurement is to measure the equilibrium absorbance of the sample to which reagents have been added, and to deduct from this reading the absorbance due to the reagent and any self-colour in the sample.This correctedDecember, 1979 LOW OXYGEK CONCENTRATIONS IN POWER-STATION WATERS. PART I 11 11 absorbance is then proportional to the concentration of determinand in the sample. The reagent blank is normally determined by measuring the absorbance of a solution containing no significant concentration of the determinand, to which reagents have been added. In the oxygen determination it is extremely difficult for routine analysis to ensure that this will be the case. An alternative technique for obtaining the absorbance of the reagent blank was therefore tested and found satisfactory. when the glycerol solution of reagent is initially added to the sample, because of the differences in viscosity an insignificant amount of mixing and reaction occurs, and by adjusting the reagent composition and concentration the time required for the reaction to give maximum absorbance can be controlled. By using a relatively slow reaction time to obtain maximum absorbance, the increase in absorbance of the solution may be measured accurately against time after the mixing of sample and reagent.The resultant line plot can then be extrapolated to “zero time” and the intercept with the absorbance axis gives the reagent blank absorbance plus the absorbance from any self-colour in the sample; this technique also eliminates the cuvette blank. The difference between the “zero-time” absorbance and maximum absorbance is then proportional to the oxygen content of the sample.This technique is based on two factors : (i) (ii) Reagent cona$osition and volume added to a sawfile The leuco-methylene blue reagent solution is prepared by mixing a solution of methylene blue and glucose in glycerol with a small volume of an aqueous potassium hydroxide solution and allowing the mixed solution to stand in normal daylight until colourless (15-30 min). The rate of reduction of methylene blue is dependent on the concentrations of both the glucose and potassium hydroxide. However, in alkaline solution the leuco-methylene blue is not excessively stable and decomposes slowly to an inactive form. The relative con- centrations of the three constituents have, therefore, to be carefully controlled to ensure : ( a ) reasonably fast reduction of the methylene blue; ( b ) adequate stability of the leuco-methylene blue reagent; (c) a colour development time commensurate with obtaining an accurate estimate of the reagent/cuvette blank ; ( d ) adequate stability of the final colour, when the reagent is added to aqueous solutions containing dissolved oxygen.Initially the leuco-base reagent used was a mixture of 1 ml of reagent B to 40 ml of reagent A (see Table 11) and this reagent was added in the ratio of 1 : 40 to water in the filled cuvette. Under these conditions maximum colour development occurred within 2-3 min of mixing the reagent with the sample, but the colour faded at >lyo min-l thereafter, and was there- fore not sufficiently stable for the precision required or for the mode of measurement used.The volume of reagent B added to reagent A was therefore reduced. Lsing the same ratio of the various leuco-methylene blue reagent solutions to water, it was found that as the concentration of alkali was reduced, the reaction time between the reagent and oxygen increased and also the final product became more stable. I t can be considered that there are two competing reactions: (i) (ii) oxidation of the leuco-methylene blue by dissolved oxygen, and reduction of the methylene blue formed in (i) by the glucose and hydroxide present. Decreasing the hydroxide concentration retarded the second reaction and the value of the maximum absorbance produced by a given concentration of oxygen was thereby increased. The most suitable ratio of reagent B to reagent A was found to lie between 1 : 50 and 1 : 200, the actual value depending upon conditions, such as ambient temperature.The most suitable ratio therefore needs to be experimentally optimised. The addition of 0.3 ml of the alkaline leuco-base to the cuvette (volume 12 ml) filled with sample gave maximum colour development times in the range 2-8 min. The time depended on the concentration of oxygen, the shorter time arising from lower concentration. The maximum absorbance with freshly prepared reagent remained constant for at 1 .ast 8 min. As the reagent aged over a number of days, when stored in the burette in the normal laboratory atmosphere, it was found that the reaction time decreased by about 50% and the absorbance sensitivity decreased by about 20% over a period of 8 weeks. I t was also found that once the maxi-1112 GOODFELLOW AND WEBBER: ABSORPTIOMETRIC DETERMINATIOK OF AnaZyst, VOZ.104 mum absorbance for any one measurement had been obtained it remained constant for only 2-3 min before the colour started to fade. When leuco-methylene blue solution was freshly prepared by mixing reagent A with reagent B the sensitivity and stability of the reaction were essentially the same whether or not these two separate reagent solutions were them- selves freshly prepared or up to 2 months old. When a solution of potassium carbonate equivalent to the 50% potassium hydroxide solution was used to prepare the leuco-base solution (simulating the maximum absorption of carbon dioxide that could occur in the potassium hydroxide solution during storage) the sensitivity was not affected, although the reagent took 2-3 h to decolorise, and the subsequent reaction time to maximum colour was increased to such an extent (50 min) as to be impracticable.This was similar to the effects experienced when less potassium hydroxide was used, and indicates that both the reduction of methylene blue and the reaction between oxygen and the leuco-base are markedly pH dependent. Effect of Temperature Sample temperature This effect was tested by taking samples of low-oxygen water at known temperatures to which the leuco-methylene blue reagent was added. After the reaction had reached equilibrium, air-saturated water was introduced into the cuvette and the increase in absorbance recorded.At both 18.5 and 33 "C the increase in absorbance when 50 pl of air- saturated water was added was 0.194 (triplicate determinations, standard deviation of the means 0.006). However, at the higher temperature the maximum absorbance was obtained after only 2.5 rnin (90% at 1.35 min) compared. with 4.5 min (90% at 2.5 min) at 18.5 "C. At 33 "C, the absorbance started to decrease within 4 rnin of the air-saturated water being added. Once a water sample has been obtained and the taps have been closedl, temperature changes will cause differential expansion or contraction of the water and glass, and this may lead to air or air-saturated water being drawn into the body of the cuvette from the keyways of the stopcocks. To minimise this effect it is recommended that cuvettes filled with sample are stored under excess of sample-water collected in a suitable vessel during the cuvette flushing-out stage of the sampling procedure.Variations in temperature can affect the integrity of the cuvette. Reagent temperature It has been shown above that the leuco-methylene blue reagent is unstable and that the rate of deterioration increases with increasing temperature. This instability is related to the concentration of potassium hydroxide in the reagent, as the results in Table I11 show. Not only is there a decrease in sensitivity with increases in storage temperature, age of reagent and amount of potassium hydroxide, but there is a concomitant decrease in the time for equilibrium absorbance to be reached, TABLE I11 EFFECT OF TEMPERATURE ON REAGENT STABILITY Reagent (prepared on day 1) Sensitivity, absorbance per 100 pl of air-saturated A I 7 waterltime to equilibrium (min) Composition, Storage condition prior ,- A \ A/B* to test Day 2 Day 3 Day 4 18-21 "C 10.49 1 1 8 0.48817 0.51118 l O O / l .O Ambient temperature, lOO/l.6 28 "C for 24 h 0.46 81 5 1 001 1 .o 28 "C for 48 h 0.47017 1001 1.5 28 OC for 48 h 0.43014 100/0.8 28 "C for 72 h 0.47215 loo] 1 .o 28 "C for 72 h 0.46015 l00/1.6 28 "C for 72 h 0.41313 * A is the glycerol solution of methylene blue and glucose; B is 50% m l V potassium hydroxide solution.December, 1979 LOW OXYGEN CONCENTRATIONS IN POWER-STATION WATERS. PART I 1113 Tests with Different Batches of Methylene Blue Samples of methylene blue were obtained from three different sources: Aldrich Chemical Co., BDH and Eastman-Kodak.Dilute aqueous solutions (2 mg 1-l) of these were prepared, the visible absorption spectra of which indicated that all three samples were of essentially the same purity; this was confirmed when leuco-reagents prepared from each source gave similar absorbances when added to solutions containing excess of oxygen. Calibration graphs prepared from each of these reagents gave similar sensitivities and again indicated no significant differences in the composition of the three samples. The detailed results are given in Table IV. TABLE IV COMPARISON OF THREE BATCHES OF METHYLENE BLUE Absorbance a t peak wavelength (ca. 650 nm) t Aldrich BDH Eastman-Kodak (U.S.P.) (technical dye) (certified) I h 7 Aqueous solution (2 mg 1-I) .. . . 0.41 1 0.421 0.415 pg 0,1-1* 15.7 31.4 47.1 62.7 Excess 0,t 0.101 (0.002,) 0.094 (0.003,) 0.091 (0.002,) 0.189 (0.010) 0.187 (0.005,) 0.169 (0.002) 0.267 (0.014) 0.257 (0.001) 0.252 (0.002,) 0.355 (0.002,) 0.340 (0.004,) 0.330 (0.007) 0.899 (0.019) 0.918 (0.029) 0.913 (0.014) * Each figure for the absorbance is the mean of three results. t Each figure for the absorbance is the mean of five results. The figures in parentheses are the standard deviations of the mean results. Storage of Samples If the cuvette containing a sample is leak-tight it should be possible to store the sample indefinitely. However, the water remaining in the exterior limbs of the taps may become contaminated with oxygen and this may in turn contaminate the sample when the reagent is added.Eight cuvettes were filled with low-oxygen water and set aside for 2 h, four immersed in the overflow from the low-oxygen water supply and the remainder stored in air. In a second test the five cuvettes which showed little or no contamination during the first test were stored, filled with low oxygen water, for 2 h in air; the remaining three cuvettes were stored under water. The results of these tests are shown in Table V and indicate that two of the cuvettes (2 and 8) were definitely not leak-tight, two (1 and 4) gave slightly variable results while the remainder were unaffected by the mode of storage. As a safeguard it is recom- mended the cuvettes should be tested for leak tightness in air as above, but for routine practice samples should be stored under low-oxygen water until they are analysed.Addition- ally, samples should be analysed as soon as possible after collection. Wash- ou t of Cuvet tes When a new sample was collected this displaced the contents of the cuvette. The time and volume of water required to do this were determined by flushing a cuvette (volume 13.5 ml) con- taining a methylene blue solution, whose absorbance was 0.350, with water a t a flow-rate of 400 ml min-l and measuring the absorbances at 15-s intervals. After 15 s the absorbance was 0.005 and no further reduction occurred with two further periods of flushing. A more stringent test was carried out in which an empty cuvette was flushed with low- oxygen water for various time periods until a minimum absorbance value was obtained when the reagent was added.At a flow-rate of 200 ml min-1 the absorbance after 20 s was 0.020; after 40 s this was reduced to 0.008 and no further reduction occurred with flushing times up to 2 min. Further work showed that the flow-rate could be as low as 30 ml min-l provided that at least 150 ml of sample were flushed through the cell. The cuvettes were normally stored with the previous sample plus reagent in them.1114 GOODFELLOW AND WEBBER : ABSORPTIOMETERIC DETERMINATION O F Analyst, vol. 104 TABLE V STORAGE OF SAMPLES Storage for Absorbance, corrected for Equivalent oxygen Cuvettc No. 2 h original 0, concentration concentration/pg 1-1 1 Air 0.007 1.4 2 0.050 10.0 3 0.002 0.4 4 0.01 7 3.4 5 6 7 8 Water Air 2 Water 4 ,9 0.001 0.000 0.000 0.01 6 0.004 0.000 0.000 0.000 0.000 0.033 0.000 0.01 3 0.2 0 0 3.2 0.8 0 0 0 0 8.2 0 2.6 Performance of the Method range up to 30 ,ug 1-l) are summarised in Table VI.gave the equation The results from five determinations on each of five concentrations of oxygen (in the A regression analysis on these results y = 0.001, $. 0.004,x where y = absorbance units and x = pg 1-1 of oxygen, and the correlation coefficient was >0.99. TABLE VI PRECISION OF RESULTS added/ pg 1-' Mean absorbance* 1-1 Concentration of oxygen Standard deviation/ 0 7.28 14.55 21.83 29.10 0.003 0.0368 0.0'7 5 0.109 0.139, 0.40 1.02 1.33 1 .fi9 0.91 * Mean of five results corrected, where appropriate, for the absorbancc of the no-added- oxygen solution. To test the accuracy of the method a continuous supply of water with a known concentra- tion of oxygen was produced by passing a controlled mixture of nitrogen and oxygen through oxygen-free water.The water was supplied continuously to an EIL, Model 9430, oxygen monitor, and samples were taken periodically for analysis by both the Potter and White methodl and the methylene blue procedure. Because of the time taken to determine the oxygen concentration using the Potter and White method the results of the Potter and White determinations were used to show the accuracy of the results obtained with the EIL monitor. Results from the monitor were then compared with those obtained using the leuco-meihylene blue technique. The mean of five Potter and White determinations was 16.7 pg 1-1 (standard deviation 0.4 pg 1-l) compared with a mean of 16.8 (0.2) pg 1-1 obtained from readings taken simul- taneously on the EIL monitor.The results obtained using methylene blue are given in Table VII, together with the corresponding readings from the EIL monitor; they show no statistically significant difference.December, 1979 LOW OXYGEN CONCENTRATIOKS IN POWER-STATION WATERS. PART I 1115 TABLE VII DETERMINATION OF OXYGEN BY METHYLENE BLUE AND BY EIL 9430 MONITOR Methylene blue/ EIL monitor/ 17.5 16.8 18.6 17.3 17.7 17.0 17.9 17.0 16.7 17.0 17.0 16.8 16.5 16.6 16.8 16.8 Pf3 1-' Clg 1-' Difference/ P.g I-' 0.7 1.3 0.7 0.9 - 0.3 0.2 -0.1 0 Mean: 0.4 Interferences The effects of a number of substances likely to be present in feed-water samples were tested and the results are detailed in Table VIII.The test solutions were prepared by dissolving the substance in air-saturated de-ionised water and adjusting the concentrations so that suitable amounts of the substance and dissolved oxygen could be introduced into the cuvette, containing low oxygen de-ionised water, with a hypodermic syringe. TABLE VIII EFFECT OF OTHER SUBSTAXES Conccn tration/ Substance pu.g 1-l N,H, . . . . 90 T\;H,OH . . 1000 Fez+ . . .. 40 80 200 Fe3+ . . . . 200 cu2+ . . . . 100 Na+ . . . . 100 Ca2+ . . . . A l p + . . . . Zn2+ . . . . 100 c1- . . . . 150 NO,-- . . . . 1000 :::: ] Effect / p g 1-1 of oxygen I A > [O,] = 10 pg I-' [O,] = 30 pg1-I <0.5 t 0 . 5 - <0.5 <0.5 - [O,] = 60 - 13 - 26 - 58 - - - - - - - <0.5 - 25 32 - <0.5 <0.5 - <0.5 (0.5 <0.5 <0.5 To attempt to overcome the interference effects of copper(I1) and iron(I1) ions, complexing agents were introduced into the reagent.Both and 2 x lW4 M solutions of the di- sodium salt of EDTA increased the equilibrium response time to over 1 h, and although a 2 x 1 0 - 5 ~ solution gave a satisfactory response time its concentration was too low to eliminate the interference of either of the metal ions. Glycine, at a concentration of 0.25 g 1-1 of reagent, reduced the interference from copper(I1) ions to an insignificant amount but had no effect on the interference due to iron(I1) ions. As it appeared unlikely that normal complexing reagents would successfully eliminate the interference from copper(I1) and iron(I1) ions it was decided that a better alternative would be to remove them by ion exchange, as in the Potter and White technique.Removal of Cations by Ion Exchange Davies et aL7 showed that ions interfering with the Winkler determination could be removed successfully on fresh ion-exchange resin columns, but no work has been published on the effect of reducing cations [e.g., iron(I1) or hydrazinium ions] already absorbed on the resin on oxygen present in a solution passing through the resin. This situation would arise if a column were used for the removal of cations from many samples (or a continuously flowing sample) before regeneration.1116 GOODFELLOW AND WEBBER: ABSORPTIOMETRIC DETERMINATION O F AW&.St, Vd. 104 A cation-exchange column was saturated with iron(I1) ions, and was then placed in the flow-line from a supply of low-oxygen water.A second cation-exchange column in the hydrogen form was connected in series with the iron(I1) ion loaded column and sampling points were installed before and after the two columns. The second column ensured that any iron(I1) ions leaking from the first column would be removed prior to analysis. A small concentration of oxygen was chosen for these tests so that relatively small decreases in oxygen due to the effect of the reducing species would be analytically significant and this was achieved by passing low-oxygen water through a sufficient length of silicone-rubber tubing so that oxygen from the air diffused into the water to give a concentration of ca. 5pgl-I. The water was then passed through the ion-exchange columns, the flows from both sampling points being kept constant at about 50 ml min-l (i.e., a linear flow-rate of 30 cm min-l through the column).The experiment was repeated with the iron(I1) ion column replaced by a column saturated with hyclrazinium ions. The results given in Table IX show that no significant decrease in the oxygen content of the water occurred when it was passed through either the iron(I1) ion or hydrazinium ion column at the temperature of the experiment, 25, "C. TABLE I:X EFFECT OF REDUCING SPECIES ABSORBED ON CATION-EXCHANGE RESINS ON THE CONCENTRATION OF OXYGEN I N WATER PASSED THROUGH THE RESIN Oxygen concentration found*/pg 1-1 -7 -_ 7-- Reducing species Before ion exchanger After ion exchanger 17C2f . . .. 4.9 4.6 N,H,+ . . .. 5.2 5.2 * Each result is the mean of three determinations. The standard deviation in each instance was ca. 0.2 pg 1-'. Discussion Development of the Analytical Method The analytical method described is based on the work of Devdariani et aZ.,5 who showed that under controlled conditions the leuco-base of methylene blue is oxidised by oxygen dissolved in water and that the intensity of the blue colour produced is proportional to the oxygen concentration. The major obstacles to the development of a suitable manual technique were expected to be (i) the design of a suitable spectrophotometric cuvette, which could also be used as both the sampling and reaction vessel, (ii) the method of addition of the reagent and (iii) the calibration procedure. Once a technique had been developed for manufacturing a suitable cuvette it was found that the reagent could be added simply from a semi-micro burette that had been modified so that the tip of the delivery tube could be inserted directly into the cuvette.Two specialist spectrophotometric cuvette manufacturers can now make cuvettes to the CEGR specifications. The cuvettes fit into the cell compartments of Pye-Unicam SP600, SP6-500, SP1700 and SP1750 instrument:;; it is possible that with some other makes of spectrophotometer it may be necessary to make a simple modification to the cell cover to accommodate the cuvette. The development of a calibration technique involved two stages: (i) the preparation of solution containing known amounts of oxygen and (ii) the determination of the reagent / cuvette blank.Stage (i) involved the addition of air-saturated water to the cuvette con- taining low-oxygen water and it was found that this could be done conveniently and precisely by using a hypodermic syringe with the needle inserted through the bore of an open tap in the cuvette. I t was also found that air-saturated water could be prepared satisfactorily by passing a stream of compressed air through de-ionised water. Provided that sudden temperature changes of the water were avoided the possibility of super-saturation did not occur with this technique. To determine the reagent blank it is normally necessary to provide a sample containing zero concentration of the determinand, but to pi-epare a sample of water known to containDecember, 1979 LOW OXYGEN CONCENTRATIOSS I N POWER-STATION WATERS.PART I 1117 no dissolved oxygen on a routine basis is virtually impossible as the inert gases in the gas- stripping techniques used cannot be guaranteed to be 1 0 0 ~ o pure. The difficulty of measuring the reagent blank was overcome by a novel procedure. The reagent is an alkaline solution of leuco-methylene blue in !%yo glycerol and because of its high viscosity the reagent could be added to the sample in the cuvette as a narrow stream from the PTFE probe of the burette and collected at the bottom of the cuvette without any significant mixing occurring. Prior to the mixing of the two phases the integrated absorbance at the wave- length of measurement would be due to that of the reagent plus any self-colour of the sample.This is, of course, a hypothetical condition as it is not possible to distribute the leuco-reagent uniformly in the cuvette for spectrophotometric measurement, without initiating the reaction with dissolved oxygen in the water. With an appropriate choice of reagent composition the reaction rate between the leuco-metliylene blue and oxygen dissolved in the sample is made to be relatively slow and essentially linear for the initial reaction period. By moni- toring the increase in absorbance against time after mixing the sample and reagent, the reagent/cuvette blank can be estimated by extrapolating the absorbance curve to zero time, i.e., when the mixing commenced. In practice it was found that the linear part of the reaction curve extended for the first 40-60 s of the reaction so that two measurements within this period were sufficient to calculate by simple proportionation the absorbance at zero time resulting from the reagentlcuvette blank.This method eliminates the effect of any transmission losses of light passing through the optical faces of the cuvette. Range of the Method For power-station applications measurements of dissolved oxygen are normally required in the range up to 50 pg 1-l. I n this range the leuco-methylene blue method gives a linear calibration graph. At higher concentrations the graph becomes progressively non-linear and, with the reagent concentration used, tlie practical limit of the method is approximately 10Opgl-l. I t is possible that the range may be extended by increasing the amount of reagent added to the sample but this will affect the reaction rates.Maximum absorbance will be reached more rapidly and it is likely that this absorbance will not be stable so that accurate measurements will not be possible. However, as an indication of high oxygen concentrations such a modification to the method may prove useful for investigational purposes. Performance of the Method Tlie results given in Table VI show that at an oxygen concentration below 1 p g 1-1 the standard deviation was 0.40 pg 1-l. At higher concentrations, in the range 0-30 pg l-l, it varied between 0.91 and 1.69 pg 1-l and these higher values were thought to result from the increasing errors associated with the estimation of the zero-time absorbance when the absorbance increased rapidly over the initial stages of the reaction.However, when con- sidered as a proportion of the final concentration the errors were negligible. I3otli iron(I1) and copper(I1) ions caused serious interference with the determination and, as these ions are present in virtually all feed systems, it was considered necessary to remove them from samples prior to analysis. Cation exchange has been used successfully for many years in conjunction with the Potter and White method although doubts had been raised wlietlier concentrations of reducing agents, namely iron(I1) ions and hydrazine, might build up on the resin and reduce the oxygen dissolved in the sample passing through the resin column. The results obtained indicate that even when the resin is saturated with these reducing species no reduction of the oxygen in the sample occurs a t ambient temperature at the low concentrations normally found in power-station feed water and condensate.l'lic most probable cause of bias will be contamination of the sample by the atmosphere and tliis can occur a t any stage of tlie determination. l;or example, an air bubble of diameter 0.4 mm, which could easily escape visual detection, will increase the oxygen content of a samplc. in a cuvette containing 12 ml of water by 1 pg 1-l. However, the results obtained during the (levclopment of this method indicate that any bias was less than 0.5 pg 1-l. The results of comparative determinations made both with the EIL, Model 9430, oxygen monitor and by tlie Potter and Wliite method at a dissolved oxygen concentration of ca.17 pg I--' indicate that there was no significant bias between the methods.1118 GOODFELLOW AND WEBBER The leuco-methylene blue reagent deteriorates slowly with age and this causes a slow, but significant, decrease in sensitivity with a consequent change in the calibration graph. The sensitivity of the reagent should be checked during each batch of determinations by the analysis of a control standard. However, this deterioration in the reagent occurs over a number of weeks even when no special precautions, except the exclusion of direct sunlight, are taken with the storage of the reagent. Because the zero-time procedure is used to determine the reagent/cuvette blank absorbance, samples cannot be analysed in batches as with most types of conventional spectrophoto- metry. Each sample has to be treated individually and the rate of colour development measured until peak absorbance is obtained. The time required for this depends on the oxygen content of the sample and the concentration of reagent added. The concentration of reagent used in the recommended procedure was chosen to give a sufficiently fast over-all reaction rate to attain peak absorbance in a reasonable time whilst enabling the reagent/ cuvette blank to be determined with sufficient accuracy by extrapolation of the absorbance veysuus time graph to zero time. Using the given procedure the time required for the measurement of one sample is approxi- mately 10 min. This does not include the time taken to collect the sample or transfer it to the laboratory. Extension of the Technique Smaller experimental cuvettes (capacity 5 ml) have been made and used successfully for determining oxygen in water from corrosion rigs at CERL, where only limited volumes of sample are available. The technique has also been used for continuous measurement; this work is published separately in Part II.8 The authors record their appreciation to Mr. P. Madden, who advised on and made the experimental spectroplwtometric cuvettes, and to Mr. M. Owen of the CEGB South West Region, Scientific Services Dept., who provided the facilities for the comparative analyses and who made the Potter and White determinations. The work was carried out at the Central Electricity Research Laboratories and is published by permission of the CEGB. References 1 . 2. 3. 4. 5. 6. 7. 8 . NOTE-l-leference 8 is to Part 11 of this series. Potter, E. C., and White, J . F., J . A p p l . Chem., 1957, 7, 459. “Methods of Testing Water Used in Industry,” I’art 2, Methods 2 and 3, BS 2690 : 1965, British Standards Institution, London. liotshtein, V. P., and Shemyakin, V. N., Teplodnwgetika, 1962, No. 2, 54. Sutotskii, G. P., and Gramatchikov, M. V., Teploc’nergetika, 1966, 13(10), 86. Devdariani, 1. V., Partskhaladze, K. G., Petruzashvili, L. G., and Shmal’tsel’, G. N., Teplodner- Department of the Environment, “Analysis of Raw, Potable and Waste Waters,” HM Stationery Davics, l . , Redfearn, M. N., and Remer, I). E. Y . , Analyst, 1956, 81, 113. Goodfcllow, G. l., Libaert, L). F., and W’ebber, H. M., Analyst, 1979, 104, 1119. getzka, 1970, 17(10), 76. Office, Jdondon, 1972, p. 106. Received April 27th, 1979 Accepted M a y 30th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401105

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, December, 1979, Vol. 104, $9. 1119-1123 A bso r pt iornet r i c Deter m i nation of Low Oxygen Concentrations in Power-station Waters 1119 Part 11.” AutoAnalyzer Continuous Automatic Method using the Technicon G. I. Goodfellow, D. F. Libaert and H. M. Webber Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, K T 2 2 7SE A continuous automatic method for the absorptiometric determination of low oxygen concentrations in power-station waters using the Technicon AutoAnalyzer is described. Keywords Oxygen determination ; absorfitiowtetry ; water analysis ; continuous analysis Within the Central Electricity Research Laboratories, a method was required for the continuous determination of low concentrations (<50 pg 1-l) of oxygen in water from high- pressure/high-temperature corrosion research rigs.Because of the limited capacity of these rigs it was possible to abstract sample streams at flow-rates of only a few millilitres per minute ; this restriction virtually eliminated the use of any commercial dissolved oxygen on-line instrument. Goodfellow and Webberl (Part I of this series) have developed a manual absorptiometric technique for determining oxygen using leuco-methylene blue. This paper describes the modifications to the technique necessary to enable it to be used on a continuous basis with a Technicon AutoAnalyzer. For a detailed discussion of the various aspects of the technique the reader is referred to Part I. Experimental and Results Apparatus AutoAnaZyxer components. The proportioning pump, recorder and colorimeter were standard AutoAnalyzer I system components.The recorder was fitted with a range- expansion unit so that the chart width could be made to correspond to transmission ranges of 0 to 100, 50 to 100, 75 to 100 or 90 to 100%. The colorimeter was fitted with a flow- through cell having an optical path length of 50 mm. All glass coils, tubing and glass connections were standard items; Tygon pump tubing was used throughout. The term “reaction system” denotes the assembly of tubing, glass coils, etc., used to achieve the formation of the desired coloured product. The manual method described in Part I1 was shown to be suitable for determining oxygen up to concentrations of 100 pg 1-1 in water and the method was adapted to the AutoAnalyzer.Because of the permeability of plastic tubing to atmospheric oxygen it is necessary to construct the system in glass or stainless steel with butt-joint connections made with thick-walled PVC tubing. I t is also necessary to provide the sample at a positive pressure and use the pump after the reaction and measuring systems for metering purposes only. To add the reagent, it is necessary for it to pass through pump tubing where some oxygen contamination occurs; however, by subsequently passing the reagent through a delay coil (ca. 20 min) auto-reduction of any oxidised reagent occurs before its addition to the sample stream. It was known from previous work1,2 that both the chemical reaction rates and the electrical components of the AutoAnalyzer are affected by temperature fluctuations. To stabilise the system the components were housed in a Perspex box as is shown in Fig.2. The recorder was housed separately. Additionally, that part of the system used for the chemical reaction was protected from light. Reaction system. A diagrammatic representation of the reaction system is shown in Fig. 1. * For Part I of this series, see p. 1105.1120 Analyst, Vol. 104 A perfusor pump is required to inject known amounts of air-saturated water into the sample stream for calibration purposes. A pump suitable for this purpose, with a number of fixed flow-rates, for generating oxygen concentrations of about 4, 8, 20, 40 and 80 pg 1-l (and capable of much greater flow-rates if required), is manufactured by B. Braun, Melsungen, West Germany; the British agents are F.T. Scientific Instruments Ltd., Tewkesbury, Gloucestershire. GOODFELLOW et al. : ABSORPTIOMETRIC DETERMINATION OF Perfusor pump. Hypodermic needle, 25 mm X 23G (stainless steel) Luer fitting 0.42 ml min-' 2.5 ml min-' Cation-exchange resin column 3.9 mI min-1 Fig. 1. Reaction system for the AutoAnalyzer. Reagents Water. Distilled water passed through a mixed-bed de-ionisation unit is suitable for use in reagent preparations. Leuco-methylene blue. Solution A : dissolve 0.154 g of methylene blue (technical dye grade) and 0.81 g of glucose (analytical-reagent grade) in 44 ml of water and dilute the solution to 5 1 with glycerol (analytical-reagent grade). This solution is stable for several weeks. Solution B : dissolve 100 g of potassium hydroxide (analytical-reagent grade) in water and dilute to 200 ml with water.To 700 ml of solution A add 7 ml of solution €3. Mix by shaking and allow the solution to decolorise (ca. 30 min). This is sufficient reagent for 28 h continuous running of the AutoAnalyzer, and should not be used later than 1 week after preparation. Low-oxygen and air-saturated water. Prepare these solutions as described in Part 1.l Preparation of the Calibration Graph Setting the base line The base line on the recorder is set using low-oxygen water in place of the sample. Determining the response characteristics Having set the base line, the sensitivity and linearity of response to changes in oxygen concentration are determined by injecting, with the perfusor pump, known amounts of air-saturated water into the stream of low-oxygen water prior to the addition of the reagent.December, 1979 LOW OXYGEN CONCENTRATIONS IN POWER-STATION WATERS.PART 11 1121 Fig. 2 . Layout of reaction-system components: A, sample inlet; B, reagent; C, peristaltic pump; D, mixing coils; E, colorimeter; and F, perfusor syringe-pump. The concentration of oxygen at each calibration point is determined from the equation flow-rate of air-saturated water( 1 h-l) total flow-rate of water (1 h-l) x 103s~ 0 2 (pFLg1-l) = where S,mgl-l is the concentration of oxygen in the air-saturated water at a known temperature and pressure. S, is calculated from the equation SP 760 s, = - where P mm is the observed pressure and S is the solubility at 760 mm and the observed temperature; this value may be obtained from the table of solubilities in Part I,l Table I.The recorder trace of a typical set of calibration points is shown in Fig. 3 and the corre- sponding absorbances are given in Table I. The results show that a linear response is obtained up to 88 pg 1-1 of oxygen; above this concentration the response becomes pro- gressively non-linear although meaningful results can be obtained up to at least 220 pg 1-1 (about 10% transmission). TABLE I RESPONSE TO KNOWN OXYGEN CONCENTRATIONS Concentration of oxygen added/pg 1-I 0 8.8 22 44 88 220 Absorbance corrected Absorbance for the blank 0.045* - 0.098 0.053 0.187 0.142 0.333 0.288 0.599 0.554 1.022 0.977 * Base line arbitrarily set a t 90% transmission. An estimate of the precision of the method was determined by analysing two solutions, of 0 and about 20 pg 1-1 of added oxygen, alternately for 30-min periods over 6 h.During this time the base line (blank) drifted by 0.33 pg l-l, and the short-term noise (maximum range over a 10-min period) at both concentrations was less than 0.25 pg 1-1. The precision data, together with that obtained by the manual method,l are given in Table 11.1122 GOODFELLOW ef al. : ABSORPTIOMETRIC DETERMINATION OF Analyst, “01. 104 1 oo L- 0 30 60 90 1 Time/min 10 Fig. 3. Recorder trace for standard solutions. The numbers on the peaks are the concentration of added oxygen in p g 1-l; the base line is att 0 pg 1-I. TABLE I1 PRECISIONS OBTAINED WITH THE MANUAL AND AUTOANALYZER METHODS Manual method* AutoAnalyzer method? Standard deviationlpg 1-1 (i) a t 0 pg 1-1 (ii) a t ca. 20 p g 1-l Criterion of detectionlpg 1-1 0.40 11.69 0.93 0.11 0.37 0.25 * Calculated from 5 determinations.7 Calculated from 6 determinations made by alternating the 0 and 20 p g 1-1 solutions for 30-min periods. Sampling To avoid the interference effects from iron(I1)i and copper(I1) ions the sample must pass through a cation-exchange column prior to anal.ysis (see Part I1). The size of the column required will depend on the degree of contamination but should be made as small as possible to reduce the delay in response to a minimum. I[f the sample is extracted from a pressurised system it is necessary to arrange for the sample to be taken from a stream flowing to waste at atmospheric pressure. In this instance care inust be taken to ensure that the sample is not contaminated by the back-diffusion of air.Sources of Error Errors in the base-line setting This water should ensure that any bias is less than 1 pg 1-1. However, this setting does not take into account any self-colour in the sample. The true base line for the sample can be obtained by replacing the reagent with 98% glycerol solution, but this is an extremely time-consuming procedure owing to the long rinse-out period of the viscous reagent and is not recommended for routine use. The base line is normally set using low oxygen water in place of the sample.h X m b e Y , 1979 LOW OXYGEN CONCENTRATIONS I N POWER-STATION WATERS. PART I1 1123 E f e c t of temperature Changes in temperature affect the chemical reaction rate and the stability of the Auto- Analyzer system, while the stability of the reagent is adversely affected by an increase in temperature. The first two effects are minimised by enclosing the system in a Perspex box; the amount of reagent prepared (700 ml) ensures that it is replenished daily so that little deterioration does, in fact, occur.Reagent composition hence on the observed response of the AutoAnalyzer system. system is re-calibrated with each new batch of reagent. Small changes in the reagent composition have a marked effect on the reaction rate and It is recommended that the E f e c t of other substances Details of the effects of other substances were given in Part 1.l Of the substances normally present in power-station feed waters only iron(I1) and copper(I1) ions interfere with the determination and these are removed by cation-exchange resin. I t may be found necessary to fit a pre-filter into the sample line to protect the ion-exchange column from premature fouling. Any particulate matter in the sample will affect the absorbance of the solution. This work was carried out at the Central Electricity Research Laboratories and is published by permission of the CEGB. References 1. 2. Goodfellow, G. I., and Webber, H. M., Analyst, 1979, 104, 1105. Dolbear, S. A,, Riley, M., and Tetlow, J. A., Central Electricity Research Laboratories Report No. RD/L/R 1623. NOTE-Reference 1 is to Part I of this series. Received April 27th, 1979 Accepted May 30th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401119

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

1124 Analyst, December, 1979, Vol. 104, p p . 1124-1128 Single Reagent Method for the Spectrophotometric Determination of Phosphorus in Silicates P. J. Watkins Geology Department, Imperial College of Science and Tecihology, London, S 6Y7 2BP A method has been developed for the spectrophotometric determination of phosphorus in silicate rocks following fusion with a lithium borate flux and dissolution of the melt in dilute nitric acid. By using a single reagent incorporating antimony and relatively high concentrations of ascorbic acid and hydroxylammonium chloride, a stable colour has been achieved. The colour is fully developed within 30 min and is stable for up to 4 h in solutions containing up to 0.8 p g ml-’ of phosphorus(V) oxide with nitric acid con- centrations not greater than 0.04 N.Beer’s law is obeyed and E = 22 200 at 880 nm. Keywords : Phosphorus determination ; silicate rock:; ; spectrophotometry ; single reagent method ; molybdoantimonylphosphoric aczd The determination of phosphorus is an importa,nt part of the chemical analysis of silicate rocks even though the element is seldom present in amounts exceeding 1%. Spectrophoto- metric methods of analysis1v2 are generally employed for phosphorus determination at the present time and these give reasonably accurate and precise results. The introduction of the versatile lithium borate - nitric acid solution technique for the decomposition of silicate samples has led to the rapid determination of .major and minor constituents in rocks and minerals by atomic-absorption spectrophotometry and spectrophotometric technique^.^^^ The need for a simple and yet dependable method for the rapid determination of phosphorus using the same solution prepared for the analysis of the major constituents has led to the development of the method described below.In previously reported methods of analysis of silicate rocks using a lithium borate - nitric acid solution technique, the determination of phosphorus has proved troublesome. The more sensitive methods depend on the reaction between orthophosphate ions and molybdate ions in an acidic medium, usually sulphuric acid, to form a molybdophosphate complex. This complex is then reduced in some way to folrm a blue colour, sometimes designated by the term heteropoly blue. Unfortunately, nitrate ions interfere with the stability of the heteropoly blue causing fading of the colour after its development.Ingamells5 heated an aliquot of the sample solution with acidified iron(II1) nitrate, added tin( 11) oxalate to reduce the molybdophosphate complex, and then determined the absorp- tion of the solution after standing for 5 min. Bodkin6 found that this method gave erratic results and he evaporated an aliquot of his sample solution first with hydrofluoric acid and then with fuming sulphuric acid. He then determined phosphorus, using ascorbic acid as the reductant, after boiling the solutions for 1 min. This method, while giving accurate results, is time consuming. Shapiro3 reduced the molybdophosphate complex with tin( 11) chloride and suggested that the solutions should only be allowed to stand for a fixed period of 10 min before analysis.Clemency and B(orden7 considered that Shapiro’s method produced unexplained difficulties when making phosphorus determinations. In this laboratory, excellent results have been obtained for the determination of phos- phorus using a single reagent incorporating antimony to develop the reduced blue colour as suggested by Murphy and Riley.8 These results were obtained using a solution prepared by the hydrofluoric acid - perchloric acid digestion method as described by Maxwell1 for the preparation of his solution B. I t was thought, however, that if this reagent could be adapted for use with the lithium borate -nitric acid solution technique it would be both convenient and time saving. Accordingly, experiments were performed to see if a suitable single solution reagent could be developed that would yield a stable colour for the deter- mination of phosphorus when added to an aliquot of a solution prepared using the lithium borate - nitric acid method.WATKINS 1125 Experimental Initial trials using the reagent concentrations recommended by Murphy and Riley* (0.4 N sulphuric acid, 42 mg of ascorbic acid, 48 mg of ammonium molybdate and 0.4 mg of antimony in 50 ml of final solution) produced a blue colour that faded in the 0.04 N nitric acid solutions used.When the ascorbic acid concentration was increased to 200mg per 50ml of final solution, the colour produced was much more stable. A further increase in colour stability occurred when hydroxylammonium chloride was added to the reagent at a level giving an amount of 1.0 g in the final solution of 50 ml. However, it was only when the antimony concentration was increased to 1.5 mg in 50 ml of final solution that a suffici- ently stable colour was achieved.When using the final mixed reagent, colours produced were fully developed within 30 min and were stable for up to 4 h for phosphorus concentra- tions of up to 0.8 pg ml-l of phosphorus(V) oxide in solutions containing 0.04 N nitric acid. Beer’s law is obeyed up to at least 0.8pgml-l of phosphorus(V) oxide and the molar absorptivity of the complex, E , is 22200 at 880 nm. The results are shown in Table I. TABLE I STABILITY OF THE COLOUR Net absorbance* after- A > 2 h 3 h 4 h P205I pg ml-l 0.20 0.125 0.125 0.124 0.125 0.40 0.249 0.249 0.249 0.250 0.60 0.375 0.377 0.375 0.376 0.80 0.500 0.501 0.500 0.498 * Absorbances were determined against 10 ml of blank solution.Employing the final mixed reagent and adding different amounts of nitric acid to the final solutions, it was found that, for phosphorus concentrations up to 0.8pgml-l of phosphorus(V) oxide, acidities up to 0.08 N nitric acid could be tolerated for periods of up to 2 h. If the nitric acid concentration was less than 0.04 N the stability of the colour was increased to periods of up to 4 h. In the recommended method the nitric acid concentration will seldom exceed 0.04 N, thus ensuring adequate stability of the solutions. It would seem that the stability of the reduced molybdoantimonyl- phosphoric acid in dilute nitric acid depends on both the nitric acid concentration and the level of phosphorus in the solution, with solutions of relatively concentrated nitric acid together with high phosphorus levels being the most unstable.It might be possible to increase the tolerance to nitric acid still further by increasing either the antimony or ascorbic acid concentration, or both. and [H+j in the final solution were 0.00566 and 0.4, respectively, giving a value of 70.7 for the ratio [H+]/[MoO,~-]. This compares with the recommended value of 70 5 10 given by Going and EisenreichQ in their detailed investigation into the formation conditions for the molybdoantimonylphosphoric acid complex. Their reported value for E was 22400 at 880 nm. The results are given in Table 11.The values of TABLE I1 TOLERANCE TO NITRIC ACID Net absorbance for Xet absorbance for 0.40 pg ml-l P,O, after- 0.80 pg ml-l P,O, after- Normality of r-*-, (-*-, nitric acid 2 h 4 h 2 h 4 h 0.00 0.250 0.251 0.502 0.501 0.02 0.252 0.251 0.500 0.502 0.04 0.250 0.251 0.501 0.498 0.06 0.250 0.249 0.501 0.494 0.08 0.250 0.250 0.499 0.4671126 WATKINS SINGLE REAGENT METHOD FOR THE Analyst, V o l . 104 The only reported interferences in the determination of phosphorus by the heteropoly blue method are from silicon, arsenic and germanium. In 0.4 N sulphuric acid there is a very small error due to silica, which is shown in Table 111. This effect, which is almost within experimental error, is less than the equivalent of 0.002% of phosphorus(V) oxide, and can usually be discounted for routine analyses.Arsenic and germanium are usually present in silicate rocks in only trace amounts and thus their interference is generally negligible. The high level of hydroxylammonium chloride ensures that all of the iron present in the sample will be reduced to the iron(I1) state, hence eliminating any possible interference due to iron(II1). TABLE I11 EFFECT OF SILICA Absorbance* r A 7 Added silica/ 0.00 p g ml-l 0.04 p g ml-l 0.20 p g ml-1 0.80 p g ml-l p g ml-I p20.5 p2c'6 '2O6 y 2 0 5 0 0.013 0.038 0.138 0.511 120 0.014 0.040 0.139 0.512 160 0.015 0.040 0.140 0.513 200 0.015 0.040 0.140 0.512 (= 0% SiO,) (= 60% 50,) (= 80% SO,) (= 100% SiO,) * Absorbances were determined in the preisence of 10 ml of blank solution after standing for 1 h.Instrumentation all determinations. A Pye Unicam SP600 spectrophotometer and 20-mm glass absorption cells were used for Reagents tetraborate. Mixed Jux. Nitric acid, 2 N. Sulphuric acid, 25% V/V. Stock phosphorus standard solution, 1000 pg ml-l of phosphorus( V ) oxide. Dilute phosphorus standard solution, 10.00 pg mI.-l of phosphorus( V ) oxide. of the stock phosphorus standard solution to 11. Antimony potassium tartrate, analytical-reagent grade. Ammonium molybdate, analytical-reagent grade. A scorbic acid, analytical-reagent grade. Hydroxylammonium chloride, analytical-reagent grade. One part of lithium metaborate is well mixed with two parts of lithium Dilute 10.00 ml Procedure Weigh 0.1000 g of sample, ground to pass a 200-mesh sieve, into a clean porcelain crucible.Mix thoroughly with 0.600 g of mixed flux using i l plastic stirring rod. Transfer the mixture into a pre-ignited graphite crucible and fuse in an electric muffle furnace at 1000 "C for 30 min. Remove the crucible and pour the red-hot melt into a 100-ml plastic beaker containing 60 ml of water and 10 ml of 2 N nitric acid. Dissolve the shattered melt using a magnetic stirrer without any heat for 30 min. Dilute to 100 ml in a calibrated flask, shake well to mix, and transfer into a well sealed plastic bottle. This is solution P, containing 1 000 pg ml-l of sample in 0.2 N nitric acid solution. Prepare also a mixed flux blank solution, omitting the sample.L>ecember, 1979 SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHORUS I N SILICATES 1127 Determination of Phosphorus Dissolve 1.25 g of ammonium molybdate in 56 ml of 25% V/V sulphuric acid in a 250-ml plastic beaker.Dissolve 5.0g of ascorbic acid in 30ml of water and add to the acidified molybdate solution, then dissolve 0.10 g of antimony potassium tartrate in 10 ml of water and add this to the mixture and stir well. Dissolve 25.0 g of hydroxylammonium chloride in 130 ml of water and add with thorough mixing. Finally, dilute the mixed reagent to 250 ml in a calibrated flask and shake well to ensure thorough mixing. Into a series of 50-ml calibrated flasks pipette 10.00-ml aliquots of sample solution P. Also prepare a blank using 10.00 ml of the mixed flux blank solution in place of the sample. Dilute each solution to about 30 ml. Using a safety pipette filler add 10.0 ml of the mixed reagent to each flask.Measure the absorbance of each solution at 880 nm using 20-mm glass cells with the blank solution as the reference. Mixed reagent. Prepare just before use in the following way. Dilute to volume, shake well to mix, and stand for 1 h. NOTE- with more than 0.8% of phosphorus(V) oxide a 3-ml aliquot should be taken. For samples with more than 0.4% of phosphorus(V) oxide a 5-ml aliquot should be taken. For samples Preparation of calibration graph phorus standard. phosphorus(V) oxide. blank solution to each of the five flasks and dilute to about 30ml with water. safety pipette filler, add 10.0ml of the mixed reagent to each flask. shake well to mix and stand for 1 h. using 20-mm glass cells with the blank solution as the reference. Calculation Into four 50-ml calibrated flasks pipette 1.00, 2.00, 3.00 and 4.00 ml of the dilute phos- These flasks contain, respectively, 0.20, 0.40, 0.60 and 0.80 pg ml-l of Leave a further flask empty to act as a blank.Add 10.00 ml of the Using a Dilute to volume, Measure the absorbance of each solution at 880 nm volume of standard solution taken (ml) absorbance of standard solution Factor, F = 0.10 x For a 10-ml aliquot of solution P, P205(yo) = F x absorbance of sample solution TABLE IV COMPARISON OF RESULTS FOR P,O, CONTENT OF STANDARD ROCKS BY THE PROPOSED METHOD WITH RECOMMENDED VALUES Sample NIM-D . . . . NIM-G . . . . NIM-L . . . . NIM-N . . . . NIM-P . . . . NIM-S . . . . ST- 1 A (2001) . . . . (2003) . . . . (2005) . . . . SY-2t . .. . SY-37 . . .. Bli* . . . . . . Dli-N . . . . G-1 . . . . . . G-2 . . . . .. AGV-17 . . . . BCR- 1 . . . . * 3-ml aliquot used. t 5-ml aliquot used. SGD- 1 A* SG- 1 A Net absorbance 0.015 0.020 0.070 0.044 0.032 0.148 0.250 0.401 0.018 0.264 0.330 0.395 0.296 0.097 0.168 0.301 0.448 P,O, content, Yo 0.012 0.016 0.056 0.035 0.026 0.118 0.200 1.07 0.014 0.422 0.528 1.05 0.237 0.078 0.134 0.482 0.358 Recommended value, yo 0.02 0.02 0.06 0.03 0.02 0.12 0.21 1.06 0.00 0.44 0.54 1.05 0.25 0.07 0.13 0.51 0.36 Ref. 10 10 10 10 10 10 11 11 11 10 10 10 10 6 12 12 121128 WATKINS Results The determination of the phosphorus content of a number of international standard rocks has been carried out using the proposed method and the results are listed in Table IV. It can be seen that for all of the samples the results obtained are very close to the recommended values.Precision tests on the proposed method. were performed using four standard rocks with widely different phosphorus contents. Each sample was assayed six times, and the mean values, standard deviations and coefficients of variation are given in Table V. The values obtained show that the procedure gives both accurate and precise results. TABLE V PRECISION DATA Mean P,O, Standar’d Number of Coefficient of Sample content, yo deviation, ”/b determinations variation, yo NIM-D . . . . 0.012 0.001 1 6 9.12 G-2 . . . . 0.134 0.001 0 6 0.75 BCR-1 . . . . 0.358 0.001 8 6 0.50 SGD-1A* (2003) . . 1.070 0.004 6 0.37 * 3-ml aliquot used. Conclusion The method proposed for the rapid determination of phosphorus in silicate rocks utilises the addition of a single mixed reagent to an aliquot of the same sample solution used to determine the major elements; this solution is prepared by the lithium borate - nitric acid solution technique.The evaporation of an aliquot of the sample solution with a hydro- fluoric acid - sulphuric acid mixture and the careful control of the time required for colour development and measurement, which are necessary in other methods, are avoided. The sensitivity of the method is high and no elements commonly present in silicate rocks interfere significantly. The colour is fully developed within 30 min and is stable for up to 2 h in solutions containing up to 0.8 pg ml-1 of phosphorus(V) oxide with nitric acid concentra- tions not greater than 0.08 N.For nitric acid concentrations not greater than 0.04 N, the stability of the colour is increased to periods of up to 4 h. At the 0.01% phosphorus(V) oxide level the coefficient of variation is 9.1%, while at the 0.36% phosphorus(V) oxide level the coefficient of variation is 0.5%. When using a 10-ml aliquot of the sample solution the limit of detection of the method as described would be about 0.002% of phosphorus(V) oxide. The author thanks Dr. G. D. Borley of the Geology Department, Imperial College, for critically reading this paper and offering a number of constructive comments and suggestions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Maxwell, J . A., “Rock and Mineral Analysis,” Interscience, New York, 1968, Riley, J . P., Analytica Chzm. Acta, 1958, 19, 413. Shapiro, L., Bull. U.S. Geol. Surv., 1975, No. 1401.. Medlin, J . H . , Suhr, N. H., and Bodkin, J . B., Atom. Absorption Newsl., 1969, 8, 25. Ingamells, C. O., Analyt. Chem., 1966, 38, 1228. Bodkin, J . B., AnaZyst, 1976, 101, 44. Clemency, Ch. V., and Borden, D. M., Geostandards Newsl., 1978, 2, 147. Murphy, J., and Riley, J. P., Analytzca Chim. Acta, 1962, 27, 31. Going, J . E., and Eisenreich, S. J., Analytica Chzm. Acta, 1974, 70, 95. Abbey, S., Bull. Geol. Surv. Can., 1977, paper 77-34. Abbey, S., and Govindaraju, K., Geostandards Nervsl., 1978, 2, 15. Abbey, S., Geostandards Newsl., 1978, 2, 141. Received April 5th, 1979 Accepted June Sth, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401124

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, December, 1979, Vol. 104, pp. 1129-1134 1129 Modified Colorimetric Method for the Determination of Ma I at hion E. R. Clark and I. A. Qazi Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham, B 4 7ET A thorough investigation has been made of the recommended method for the determination of malathion, and the major cause of two of the most serious problems in this method has been resolved. The method that uses a copper(I1) complex as the basis of colorimetric measurements suffers from the disadvantage that the colour fades quickly and that an increase of a few seconds in the contact time of the copper(I1) solution and the hydrolysis product of malathion results in a reduction in the intensity of the yellow colour. Attempts to overcome these drawbacks have been reported, some of which are tedious and others only partially successful, but it is suggested that if copper is replaced with bismuth the problems may be more simply resolved.Isomalathion does not react in this method. Keywords : Malathion alkaline hydrolysis ; malathion determination ; copper chelate colorimetry ; colour instability ; bismuth chelate The first colorimetric method for the determination of malathion was devised by Norris et aZ.,1 who applied it to technical product analyses and malathion residue analyses. In this method, malathion is decomposed by alkali to dimethyl dithiophosphate, sodium fumarate and ethanol. The dithiophosphate is then converted into the copper( 11) complex, which is soluble in organic solvents such as carbon tetrachloride and hexane with the forma- tion of an intense yellow colour. The colour intensity is proportional to the concentration of dimethyl dithiophosphate and is measured colorimetrically a t 420 nm, the absorption peak. Iron(II1) reagent is added in order to oxidise materials that would reduce copper(I1) ions to copper(1) ions.With dithiophosphate, copper( I) ions form a colourless complex, which is reported to be more stable than the yellow copper(I1) complex. This method has been recommended by the Malathion Panel, set up jointly by the Scientific Sub-committee of the Inter-Departmental Advisory Committee on Poisonous Substances Used in Agri- culture and Food Storage, the Analytical Methods Committee of the Society for Analytical Chemistry and the Association of British Manufacturers of Agricultural Chemicals2 as suitable for malathion residue analysis in fruits, vegetables and agricultural crops.In addition, it is the recommended method of the US Association of Official Analytical Chemists3 and has been listed in the WHO’S “Specification for Pesticides.”* In spite of such widespread use, the method suffers from the disadvantages that the yellow colour produced, which forms the basis of the colorimetric measurements, fades very quickly, and a slight increase in the contact time of the copper(I1) solution and the hydroylsis product of malathion results in a large decrease in the absolute ab~orbance.~ Many attempts have been made to overcome these drawbacks. Hill6 presented evidence to show that the copper(I1) complex of dimethyldithiophosphate exists in reversible equi- librium with its two dissociation products, copper( I) dimethyldithiophosphate and bis- (dimethoxyphosphorothiono) disulphide and that this dissociation accounts in part for the instability of the yellow complex observed by many workers.It was shown by Hill6 that incorporation of the disulphide would limit the dissociation of the coloured complex. Roussow7 found that the colour is stable for 1545 min if the temperature of the reagents is maintained between 16 and 20 “C. Wayne et aL8 developed a non-aqueous copper colorimetric method based on the reaction of Norris et aZ.l for the analysis of technical products containing malathion, but it has not been favoured by some workers because it is tedious and requires a large amount of glass- ware.Also, the availability of anhydrous solvents can be a problem in some in~tances.~ It has also been pointed out that strict cleanliness of glassware must be maintained in order to obtain reproducible results and that the method is time consuming in comparison with the origmal method. Further, the non- Sometimes there is difficulty with colour stability.1°1130 CLARK AND QAZI MODIFIED COLORIMETRIC Analyst, Vol. 104 aqueous method suffers from the disadvantage that the absorbance has to be measured exactly 2 min after the addition of the copper reagent and there is also some doubt whether the method could be conveniently extended to residue analysis. Visweswriah and Jayaramll suggested that the reaction of palladium(I1) chloride with the acid hydrolysis product of malathion would give a yellow complex suitable for the deter- mination of malathion.Their observations are conflicting, however, because they state that the acid hydrolysis gives primarily dimethylthionophosphoric acid but that the yellow colour is due to the reaction of palladium(I1) with dimethyldithiophosphoric acid, a product not formed under their recommended acid hydrolysis conditions. In this paper, the most probable cause of the problems encountered in the copper method for the determination of malathion are discussed and the development of a modified colori- metric method that does not suffer from any of these drawbacks is described. Experimental Reagents material in 200 ml of distilled water. 3 ml of concentrated nitric acid and dilute to 100 ml with distilled water.Dimethyldithiophosphate (DMDTP) solution, 5 x Bismuth solution. M. Dissolve 0.175 2 g of purified Dissolve 0.1 g of bismuth oxide (Bi,O,) (BDH laboratory reagent) in 1.0 ml of solution = 10.45 mg of bismuth. Copper solution. Dissolve 2 g of copper(I1) sulphate (CuS0,.5H20) (AnalaR grade, Hopkin and Williams Limited) in 100 ml of distilled wa.ter. 1.0 ml of solution E- 5 mg of copper. Carbon tetrachloride. Ethanol. Medical grade (99.5%). Malathion anazytical standard. Malathion emulsijable concentrate. Re-distil commercial-grade material and store in a glass bottle. Supplied by the American Cyanamid Company. Supplied by Murphy Chemical Limited. Apparatus SP6-100) spectrophotometers with 1 .O-cm quartz cells were used.Spectrophotometers. Double-beam recording (Unicam SP8-100) and single-beam (Unicam Procedure Puri$cation of ammonium dimethyldithiophosphate A 10-g amount of the commercial product (Aldrich Chemical Co., 95% purity) was dissolved in 50 ml of ethanol. The solution was filtered and to the filtrate were added 30 ml of carbon tetrachloride. The solution was kept overnight at room temperature, after which time the purified compound had crystallised in well defined crystals. The crystals were washed several times with diethyl ether, dried and the purity of the compound was confirmed by determining the melting-point (143 "C). Absorption spectra of the copper and bismuth coznplexes of DMDTP (Fig. 1) To a 50-ml separating funnel containing 9 ml of water, 0.3 ml of the DMDTP solution were added, followed by 10 ml of carbon tetrachloride and 1 ml of the copper (or bismuth) solution. The funnel was stoppered and immediately shaken vigorously for exactly 1 min.The organic layer was allowed to separate and then transferred into the 1-cm quartz cell, via a cotton-wool plug placed in the stem of the funnel. The absorption spectra of the com- plexes were immediately recorded on the Unicam SP8-100 spectrophotometer using carbon tetrachloride as reference solution.December, 1979 METHOD FOR THE DETERMINATION OF MALATHION 1131 Wavelength/nm Fig. 1. Absorption spectra (1-cm quartz cell) of bismuth and copper complexes of dimethyl- A, 0.3 ml of 5 x 10-3 M DMDTP M DMDTP solution with excess of dithiophosphate extracted into 10 ml of carbon tetrachloride.solution with excess of bismuth; and B, 0.3 ml of 5 x copper. Persistence of colour (Fig. 2) The change in the absorbance of the copper and bismuth complexes (produced as described above) with time was studied on the single-beam spectrophotometer connected to a chart recorder. 0.6 I I I 1 I 0 10 20 30 40 0.3 Timelmin Fig. 2. Variation of absorbance (1-cm quartz cell) with time of bismuth and copper complexes of dimethyldithiophosphate extracted into 10 ml of carbon tetrachloride. A, 0.2 ml of 5 X 1 0 - 3 ~ DMDTP solution with excess of bismuth, X = 325nm; and B, 0.2ml of 5 x 10-3M DMDTP solution with excess of copper, X = 418 nm. Beer’s law studies (Fig. 3) The absorbances of the copper and the bismuth complexes of DMDTP, using different amounts of DMDTP solution, were measured as described above.For the copper complex the wavelength used was 418 nm, whereas for the bismuth complex both the 325- and 390-nm absorption peaks were used. The resulting absorbances were plotted against DMDTP concentration. E f e c t of reducing agents (Table I> In the original method of Norris et al.,l the presence of any reducing agents with the hydro- lysis product of malathion is a major source of error. It was therefore decided to investigate the effect of a typical reducing agent (ascorbic acid) on the absorbance characteristics of the1132 CLARK AND QAZI : MODIFIED COLORIMETRIC Analyst, VoZ. 104 1.4 1.2 1 .o 8 0.8 m + 2 2 0.6 0.4 0 2 0 .o Volume of 5 x ~ O - ~ M DMDTP solution/ml Fig. 3.Beer's law gra.ph for DMDTP com- plexes of copper and bismuth: 5 x 1 0 - 3 ~ DMDTP solution extracted with excess of metal into 10ml of carbon tetrachloride. A, Copper complex, X = 418 nm; B, bismuth complex, X = 325nm; and C, bi!;muth complex, X = 390 nm. copper and bismuth complexes of DMDTP. For this purpose the absorbances of these complexes, for a fixed amount of DMDTP solu1:ion (0.3 ml), were measured after extraction from aqueous layers containing varying amounts of ascorbic acid. TABL~E I EFFECT OF ASCORBIC ACID ON THE ABSORBANCE OF COPPER AND BISMUTH COMPLEXES OF DIMETHYLDITHIOPHOSPHATE* Volume of M ascorbic acid/ml . . 0 3 4 6 8 10 Absorbance, Cu - DMDTP . . . . 0.82 0.62 0.51 0.44 0.38 0.37 Absorbance, Bi - DMDTP . . . . 0.74 0.'76 0.75 0.74 0.74 0.73 * 0.3 ml of DMDPT extracted with excess of copper and bismuth in the presence of various amounts of ascorbic acid.Lapse of time betweeiz the addition of metal reagent and subsequent extraction into organic solvent In the standard method for the determination of malathion it has been recommended that there should be a minimum lapse of time between the addition of the copper(I1) reagent and the subsequent shaking with the organic solvents. A delay of only a few seconds causes a decrease in the absolute absorbance and thus introduces errors. To confirm these findings and to investigate the effect of this lapse of time on the bismuth complex of DMDTP, a fixed amount of the DMDTP solution (0.7 ml) was used. It was found that if the complexes were extratcted immediately after addition of the metal ion, the absorbances obtained for the copper and bismuth complexes were 1.92 and 1.79, respectively.On the other hand, when the extraction was started 1 min after the addition of the metal reagents the absorbance for the bismuth complex remained virtually the same, whereas the average absorbance for the copper complex fell to 1.31 (average of ten readings).December, 1979 METHOD FOR THE DETERMINATION OF MALATHION 1133 Analysis of emulsiJiable concentrate For the analysis of 607' emulsifiable concentrate, the method in reference 5 was employed. When bismuth was used in place of copper, addition of iron(II1) solution was omitted. The results obtained for analyses of five samples by each method were 59.8 3 1.2% for the copper method and 60.2 & 0.8% for the bismuth method.Results and Discussion From the absorption spectra and Beer's law graphs (Figs. 1 and 3), it is clear that if bismuth is used for the determination of malathion in the visible range (390 nm) it is 4-5 times less sensitive than in the standard method. On the other hand, in the ultraviolet range (325 nm) the sensitivity is only slightly less than that of the standard method. From Fig. 2 it can be seen that the yellow colour due to the bismuth complex of DMDTP remains unchanged even after 40 min, whereas the absorbance of the copper complex falls appreciably during this time. In fact, it was found that the absorbance of the bismuth complex does not alter even 24 h after extraction. This makes the bismuth method for the determination of malathion much more attractive than the copper method because one of the major drawbacks in the standard method is removed without taking the extra trouble of developing the colour at a lower temperature,' using a precious metal such as palladiumll or incorporating the use of disulphide.6 Fig.3 shows that the bismuth complex obeys Beer's law at least within the concentration range for which the standard copper method is applicable. This is found to be true for absorption at both wavelengths of interest, i.e., 325 and 390 nm. Results of experiments with ascorbic acid show that whereas even a slight amount of reducing agent has a drastic effect on the results obtained by the copper method, no such detrimental effect is observed in the bismuth method.This is a distinct advantage in that the analyst has no longer to consider the impurities that have to be oxidised by the addition of iron(II1) reagent in the standard method. In fact, the addition of iron(II1) is no longer required. A further advantage of the bismuth method over the copper method is shown by the lapse of time between the addition of metal and subsequent shaking studies. Whereas a delay of 1 min has a noticeable effect on the absorbance of the copper complex of DMDTP, no such problem is encounFered with bismuth. This makes the bismuth method much simpler and reduces the probability of errors. Whereas the original method of Norris et a1.l requires strict adherence to time factors, the proposed method does not, and it is suggested that it gives equally good results.The application of the bismuth method for the determination of malathion to a technical product (60% malathion emulsifiable concentrate) confirmed its usefulness. As observed, within experimental error both the copper and bismuth methods yield similar results, proving that the standard method could well be replaced with the modified bismuth method with distinct advantages. The results not only prove the superiority of using bismuth in place of copper in the deter- mination of malathion but also give a good indication of the cause of all of the problems inherent in the standard method. All of the evidence supports the idea that the basic cause of all such problems is the ability of copper(I1) to be reduced to copper(1) not only by the impurities present in the hydrolysis product of malathion, but also by the major hydrolysis product DMDTP itself.It is known that aqueous solutions of diethyldithiophosphate reduce copper(I1) and iron(II1) to copper(1) and iron(II), respectively,12 and it can be assumed that DMDTP would behave similarly. This would account for the reduced absorbance of the copper - DMDTP complex when there is a delay between the addition of the copper reagent and subsequent extraction, i.e., with longer time more and more copper(I1) is reduced to copper(I), forming a colourless complex with DMDTP and resulting in a lower absorbance. Also, as suggested by this could account for the instability of the yellow complex in the organic solvent. Mention may also be made that, owing to the possibility of the oxidation of DMDTP, the addition of large amounts of iron(II1) solution used in the recommended method5 may be a source of error in itself.The dangers are two-fold in residue analysis, where some workers recommend repeated shaking of the aqueous solution of DMDTP and iron(IT.1) with the1134 CLARK AND QAZI organic solvent and discarding the extract. In this instance it is likely that even AnalaR- grade iron(II1) chloride (FeCl,.GH,O) may contribute sufficient copper(I1) to make serious losses of DMDTP possible. In conclusion, the advantages of using bismuth in place of copper for the determination of malathion can be summarised as follows: (1) less reagents are needed [there is no need to use iron(II1) solution or carbon disulphide]; (2) there is no need to use cooled solutions; (3) there is no interference from reducing agents; (4) there is no need for strict adherence to time factors; (5) the yellow colour is stable for extended periods of time; and (6) isomalathion has been found not to interfere.The authors thank the American Cyanamid Company and Murphy Chemical Limited for supplying malathion standard samples. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Norris, M. V., Vail, W. A., and Averell, P. R., J . Agric. Fd Chem., 1954, 2, 570. Malathion Panel, Analyst, 1960, 85, 915. “Official Methods of Analysis of the Association of Official Analytical Chemists,” Eleventh Edition, “Specification for Pesticides,” Fourth Edition, World Health Organization, Geneva, 1973. Zweig, G., Editor, “Analytical Methods for Pesticides, Plant Growth Regulators and Food Addi- Hill, A. C., J . Sci. Fd Agric., 1969, 20, 4. Roussow, S. D., S. Afr. J . Agric. Sci., 1961, 4, 435. Wayne, R. S., Groth, W. C., Miles, J . W., and Guerrant, G. O., J . Ass. Off. Analyt. Chem., 1972, 55,926. Wayne, R. S., J . Ass. Off. Analyt. Chem., 1973, 56, 579. Stiles, A. R., Miles, J . W., Wayne, R. S., and Newton, W. H., J . Ass. 08. Analyt. Chem., 1977, 60, Visweswriah, K., and Jayaram, M., Agric. Biol. Chem., 1974, 38, 2031. Handley, T. H., Analyt. Chem., 1962, 34, 1312. Association of Official Analytical Chemists, Washington, D.C., 1970. tives,” Volume 11, Academic Press, New York, 1973. 1148. Received March 29th. 1979 Accepted July 9th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401129

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, December, 1979, Vol. 104, pp. 1135-1137 H istochemical ldentif ication of Commercial Wheat Gluten 1135 F. 0. Flint and R. F. P. Johnson Procter Department of Food Science, University of Leeds, Leeds, LS2 9JT Three methods for the microscopical identification of commercial wheat glutens are compared. A periodic acid - Schiff and an iodine - potassium iodide method both indicate gluten by showing the wheat starch present. Toluidine blue contained in an aqueous mountant distinguishes gluten protein from both soya and meat proteins. Each method identified commercial gluten present in a gluten - soya protein - meat mixture. Testing for both starch and protein is recommended for protein products that may contain added starch. Keywords : Commercial wheat gluten identification ; gluten - soya mixture ; gluten - soya - meat mixture In addition to being a significant source of protein for many millions of people, wheat is an important source of industrial starch.l The separated undenatured wheat protein can be dried and forms the so-called “vital gluten’’ of commerce.According to Knight,l the dry commercial product contains 75430% of protein and 5-15% of carbohydrate, which consists largely of wheat starch. The most important use for commercial gluten is in flour protein enrichment for the baking of high-protein or starch-reduced breads, but it is also used in breakfast cereals, pastas and pet foods. Simmonds2 estimates that 10% of Australian gluten is used in processed meats and there are also techniques for the production of texturised vegetable proteins based on g l ~ t e n .~ Methods that distinguish commercial wheat gluten from soya and meat proteins are now needed in order to implement the present meat product regulations. Soya flours and texturised soya products can be identified by the cellular appearance of the carbohydrates that they contain, as shown by the periodic acid - Schiff (PAS) t e c h n i q ~ e . ~ Soya protein can be distinguished from meat proteins by the differential staining given by toluidine blue.5 The various colours (blue, purple and pink) shown by meat tissues and soya material reflect differences in the density of the bound stain and, as wheat proteins are chemically distinct from soya and meat proteins, they could be expected to stain differently. These two techniques and the classical iodine - potassium iodide method for the detection of starch have been compared for their success in identifying “vital gluten” powder and canned wheat glutens and in demonstrating commercial gluten in the presence of soya products and meat tissues.A more recent use for wheat protein is as a meat substitute. Experimental Materials Examined The following materials were examined: (1) laboratory-prepared vital gluten made by mixing bread flour to a stiff dough with distilled water, allowing it to stand for 20 min and then removing starch by washing in running water until the wash water appeared clear; (2) commercial canned gluten sold as a vegetarian food; (3) commercial canned spun soya protein and gluten binder sold as a simulated mince meat; (4) vital gluten powder from RHM Ingredient Supplies Ltd.; (5) vital gluten powder mixed with ground texturised soya protein (TSP); Protena P.M.l from RHM Ingredient Supplies Ltd.was used; and (6) vital gluten powder mixed with TSP [as in (5)] added to finely minced stewing steak. Sample Preparation The laboratory gluten and the com- mercial canned product [(l) and (2)] were sampled by extracting a “core” of material using a 5 mm diameter cork borer. The remaining materials [(3)-(6)] were stirred into a commercial Specimens for sectioning were prepared as follows.1136 FLINT AND JOHNSON HISTOCHEMICAL Analyst, Vol. 104 embedding medium (Tissue Tek OCT Compound from Miles Ames). After rapid freezing, 10-p" sections were cut from each material using a cryostat with a cabinet temperature of -20 "C.Staining Methods Gram's iodine solution (iodine - potassium iodide - water, 1 + 2 + 300) was prepared as a stock solution. Before use, 2 ml of stock solution were diluted to 10 ml with distilled water. The diluted iodine solution was applied directly to the sections, which were then covered with a cover-slip. The PAS method of Coomaraswamy and Flint4 for carbohydrate, with Light Green counterstain for protein, was used exactly as described. Toluidine blue contained in an aqueous mountant, as previously de~cribed,~ was used as a differential staining medium for proteins and other constituents with basic dye binding properties. Results and Discussion The results given by the three staining methods are summarised in Table I and illustrated They show the value of demonstrating both the carbohydrate and the protein in Figs.1-9. components in the identification of commercial wheat protein. TABLE I No. Material STAINING REACTIONS OF GLUTEN AND GLUTEN IN THE PRESENCE OF SOYA AND MEAT PROTEIN Iodine - potassium iodide PA3 and Light Green Toluidine blue A 7- . 7-- A 1 Laboratory vital 2 Canned gluten gluten 3 Cannedspun soya and gluten mince 4 Gluten powder 6 Gluten powder - TSP 6 Gluten powder - TSP - meat , I , Proteins Carbohydrate Proteins Yellow Starch granules, Green blue - black Yellow Gelled starch, Green dull purple Yellow Gelled starch in Green soya fibres and in gluten binder, similar dull purple shade Yellow Starch granules, Green blue - black Gluten, Starch granules in Green yellow; TSP, paler yellow gluten, ,blue - black Gluten, Starch granules in Green yellow; TSP, paler yellow; muscle fibres, paler yellow gluten, blue - black The distinctive size and shape of gluten and in the commercial gluten and the PAS method (see Figs.1, 2, I , Carbohydrate Proteins Starch granules, Pale blue magenta Gelled starch, Pale blue magenta; bran fragments (sparse), magenta Gelled starch in Fibres (soya), dark fibres (soya) and blue; binder binder (gluten), (gluten), pale blue similar magenta Starch granules, Pale blue magenta Starch granules in Gluten, pale blue; gluten, magenta; soya protein, dark intact and damaged purple - blue cell walls in TSP, niagen t a Starch granules in Gluten, pale blue; gluten, magenta; soya protein, dark soya cell walls in purple - blue; TSP, magenta muscle fibres, pale blue (shade distinct from gluten colour) ; muscle cell nuclei, red - violet Carbohydrate' Unstained Unstained Unstained Unstained Starch in gluten, unstained; cell walls in TSP, deep pink Starch.unstaikd; cell walls in TSP deep pink the wheat starch granules present in the laboratory powder are demonstrated clearly by both the iodine 7 , 8 and 9). Any commercial plant protein derived from a cereal source would hevitably contain starch granules, the morphblogy of which would be diagnostic for that cereal,6 e.g., a commercial plant protein derived from maize would contain corn starch,6 which has a characteristic polyhedral shape. Because wheat starch is widely used as an ingredient, the identification of wheat starch granules in a protein matrix is only a guide to the origin of that protein and there is a need for a confirmatory test that characterises the protein.The spun soya mince examined serves as an example: the fibres contain an amorphous PAS-positive material, which gave a purplish reaction toFig. 1 . Section of laboratory vital gluten stained Section of laboratory vital glutcn stained with iodine reagent. Magnification 90 x . Char- with PAS and light green. Magnification B O X . acteristic wheat starch granules appear deep blue - Characteristic wheat starch granules are magenta, black, wheat protein is yellow. wheat protein is green. Note the similarity of con- trast to Fig. 1. Fig. 2. Fig. 3. Section of canned gluten stained with Section of canned gluten stained with iodine reagent.Magnification 90 x . Gelatinised PAS and light green. Magnification 90 x . Darkcr starch appears dull purple, wheat protein is yellow. areas are of magenta-coloured gelatiniscd starch. Lighter areas are of protein stained green. Note thc similar contrast to iodine staining in Fig. 3. Fig. 4. Fig. 5. Section of canned spun soya and gluten Fig. 6. Section of canncd spun soya and glutcn inince stained with iodine reagent. Magnification mince stained with toluidine blue. Magnification 9 0 ~ . Darker areas are of dull purple staining 90 x . Soya fibres, in transverse and longitudinal gelatiniscd starch; both soya and glutcn protein are section, are de-p blue in contrast to pale blue-stained stained yellow. gluten. Starch fraction is unstained.Note thc differential staining of the two proteins comparctl with uniform staining in Fig. 5. [To fnrc p . 1136Fig. 7. Section of gluten powder stained with Fig. 8. Section of gluten powder stained with iodine reagent. Magnification 90 x . Starch PAS and light green. Magnification 9Ox. granules stained deep blue - black, wheat protein Starch granules stained magenta, wheat protein stained yellow. counterstained green. Note the similar contrast to iodine staining in Fig. 7 . Fig. 9. Section of gluten powder niixed with tcxturised soya protein and meat. Magnification 90 x . Top: particles of green - blue stained gluten (some coalesced), starch granules unstained. Bottom left: muscle fibres stained pale blue with red - violet nuclei. Bottom right: soya protein stained dark purple - blue, associated carbohydrate deep pink. Stain : toluidine blue.December, 1979 IDENTIFICATION O F COMMERCIAL WHEAT GLUTEN 1137 iodine, suggesting that it is gelatinised starch and the less well defined binder contains a similar material (see Fig.5). The toluidine blue serves to distinguish the two proteins present (see Fig. 6, which shows the deep staining of the fibrous soya set against the much paler colour of the declared gluten binder, the staining of which matches that of the other glutens examined). Spun soya protein (Fig. 6) gives a dark blue colour with toluidine blue, whereas extruded soya gives a more purple colour. In both instances the colour is much deeper than the pale shades given by commercial gluten and meat tissues.Both gluten and muscle fibres stain pale blue, but the gluten colour is a distinctly more greenish shade of blue (see Fig. 9). In addition to muscle fibres, meat contains connective tissue, which stains characteristically with toluidine blue. Collagenous connective tissue is coloured pale pink and elastinous connective tissue blue - green.5 The green - blue of the toluidine blue-stained glutens examined is similar to that shown by stained elastic tissue fibres, but there is little possibility of confusion of these proteins because of the characteristic fibrous form in which elastic tissue occurs and the amorphous “granular” appearance shown by raw and heat-processed gluten (see Figs. 9 and 6). The only other differential staining of protein that was observed was that given by iodine with the gluten powder-TSP-meat mixture [(S)].Here the gluten powder stained a full yellow and the TSP and raw meat a paler yellow, but the difference was not sufficiently well marked for this to form a reliable distinguishing test. The advantage of the iodine method is that by demonstrating the presence of wheat starch (even when gelatinised), an important fraction of present commercial glutens may be demonstrated, and this makes it a useful rapid first test although one which should be followed by subsequent protein staining. The PAS technique also demonstrates intact and gelati- nised starch but additionally it reveals plant cell walls and their debris so that TSP can be positively identified. An advantage of the PAS method is that the slide preparation can form a permanent record whereas the iodine and toluidine blue methods yield only temporary slides.Conclusions Each of the three micro methods can aid in the identification of fresh and processed com- mercial gluten. In the detection of commercial glutens, the toluidine blue technique is the preferred method but either the iodine or the PAS technique should be used to confirm that there is a starch fraction present and, with uncooked products, that it is wheat starch. Because it is a one-stage method of staining that clearly distinguishes commercial wheat gluten from meat tissues and soya protein, the toluidine blue method is the most suitable for the quantitative microscopy of processed meats containing added commercial gluten. The method previously described for the quantitative determination of texturised soya protein in comminuted meat5 could be used for determining added commercial gluten without further modification. References 1. Knight, J . W., “The Chemistry of Wheat Starch and Gluten and Their Conversion Products,” 2. 3. 4. 5. 6. Leonard Hill, London, 1965. Simmonds, D. H., Food Technol., Aust., 1976, 28, 84 and 116. Gutcho, M., “Artificial Meat: Textured Foods and Allied Products,” Noyes Data Corp., New Jersey Coomaraswamy, M., and Flint, F. O., A n a l ~ ~ s t , 1973, 98, 542. Flint, F. O., and Meech, M. V., Analyst, 1978, 103, 252. Kent- Jones, D. W., and Amos, A. J . , “Modern Cereal Chemistry,” Sixth Edition, Food Trade Press, and London, 1973. London, 1967. Received February 12th, 1979 Accepted July 9fh, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401135

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

1138 Analyst, December, 1979, Vol. 104, pp. 1138-1150 Investigation of Atomiser Tube Design for Carbon Furnace Atomic-emission Spectrometry 0. Littlejohn" and J. M. Ottaway Department of Pure and Applied Chemistry, University #of Strathclyde, Cathedral Street, Glasgow, G1 1 X L Modifications to a standard graphite furnace tube, designed for atomic- absorption measurements, are shown to give iimproved performance for atomic-emission measurements of a number of elements. Four different modified tubes are compared with a standard Perkin-Elmer HGA-72 graphite tube with respect to their thermal characteristics and the detection limits of ten selected elements of varying atom-appearance temperatures. Different elements were found to give lowest detection liimits in tubes of different design.The results indicate the need for a new approach to tube design, specifically for atomic-emission measurements, in order that best detection limits can be achieved for all elements in a single graphite tube. Keywords ; Modified atomiser tubes ; atomic-emission spectrometry ; carbon furnace atomisation It has been suggested in previous publications1--6 that the application of commercial carbon furnace atomisers in atomic-emission spectrometry may, in certain analyses, be an attractive alternative to carbon furnace atomic-absorption spectrometry. The operation of the carbon furnace as an emission source facilitates the use of electrothermal atomisation in simul- taneous multi-element analysis.' However, a number of features of the design and operation of commercial atomisers are unsuitable for the measurement of optimum atomic-emission signals of many elements.With standard instrumentation, the maximum atomic-emission intensity occurs after the maximum atom concentration has been attained and when the tube temperature is still As atomisation and excitation occur in the same central section of the tube, the thermal energy available to populate energy levels depends greatly on the atom-appearance temperature. Maximum atom concentrations of volatile and medium-volatile elements therefore exist at temperatures much lower than the maximum available and it is often difficult to detect the atoinic emission of these elements in the presence of the rapidly increasing background signal. Carbon furnace atomic-emission spectrometry (CFAES) detection limits are dependent in part on the atomic emission to tube-background emission ratio.Both signals represent the summation of the photons emanating from each infinitely narrow cross-section of the tube vapour (analyte emission) and directly or indirectly, the tube surface (background emission). If a vapour phase temperature gradient exists along the tube, it is clear that the maximum atomic emission to background emission ratio and lowest detection limit will be achieved when the maximum possible concentration of atoms is formed in the hottest temperature zone, when that section of the tube has attained the maximum temperature available. This condition is generally not achieved with standard commercial furnaces but is fulfilled for L'vovlO and Woodriffll furnaces, and to an extent in T-shaped atomisers12 where the sample is introduced into a hot tube.To our knowledge, however, there are no reported analytical applications of these atomkers as emission sources, which is surprising considering the inherent advantages of the design of these furnaces. Although optimisation of the tube temperature enhances the detection of a number of elementsg the application of this procedure is limited to particular carbon furnace atomisers and is unsatisfactory for many volatile elements. To achieve universal improvements in the CFAES detection limits obtained with commercial atomisers, it appears necessary therefore to separate the processes of atomisation and excitation and to allow each to occur in separate sections of the tube assembly.I t would also be advantageous if the temperature and hence the background signal of the excitation zone were almost constant at the time of measure- * Present address : Imperial Chemical Industries Ltcl., Petrochemicals Division, P.O. Box 90, Wilton, Middlesbrough, Cleveland, TS6 8 JE.LITTLEJOHN AND OTTAWAY 1139 ment, as this would reduce errors introduced when subtracting the appropriate background intensity from the combined analyte plus background peak-height signal when the facility of automatic background correction is not available. Modifications to the design of a standard Perkin-Elmer HGA-72 atomiser tube, to allow more sensitive atomic-emission measurements, have been d e s ~ r i b e d . ~ ~ ~ ~ ~ ~ ~ Lower detection limits were obtained for a number of volatile elements by reducing the thickness of the carbon wall towards the ends of the tube, making the temperature of these end sections greater than the tube centre at any atomisation setting.13 Improvements were greatest for gallium, thallium and indium, but detection limits substantially lower than 1 pg ml-l for other elements such as lead, bismuth and cadmium were only achieved when a small sample cup was fitted to the centre of the modified tube a~semb1y.l~ An alternative modification that gave higher temperatures at the centre of the tube was observed to give sensitive emission signals only for elements of higher atom-appearance temperatures.8 The thermal and analytical parameters of these tube modifications were not considered in detail in the initial communications.A more rigorous investigation is therefore presented in this paper to establish the important aspects of tube design that influence the sensitivity of atomic-emission signals generated from a carbon furnace during sample volatilisation and atomisation. Five HGA-72 tube designs are considered and the analytical utility of each tube is assessed, where possible, by the measurement of tube wall and vapour temperatures and a consideration of the tube-temperature gradient. Detection limits for ten elements of varying volatility, namely lead, gallium, silver, tin, manganese, chromium, iron, titanium, molybdenum and gadolinium, are presented to indicate the useful range of application of each tube design. Experimental Instrument ation The instrument used for all measurements was a Perkin-Elmer HGA-72 carbon furnace atomiser mounted in a Perkin-Elmer 306 atomic-absorption/emission spectrophotometer and coupled to a Servoscribe RE541.20 potentiometric strip-chart recorder.The spectral band width of the spectrometer was set at 0.07, 0.14 or 0.2 nm as indicated in the text. The monochromator was adjusted to the required wavelength using the appropriate hollow- cathode lamp, which was then disconnected. If suitable lamps were not available, the monochromator was adjusted to the approximate wavelength and then peaked on the line using the atomic-emission signal obtained during the atomisation of a suitable standard solution. Sample solutions were transferred into the centre of the standard and modified HGA-72 tubes with 20- and 50-pl Oxford micropipettes.The solutions were dried a t 373 K for 40- 45 s and then at 400-900 K for a further 20 s, depending on the volatility of the element. The instrument settings that corresponded to these tube-wall temperatures varied with the design of the HGA-72 atomiser tube. All samples, however, were atomised a t maximum power (999 units) for 10 s unless otherwise stated, in an atmosphere of research-grade argon (99.996%). The interrupted gas flow facility (gas stop) was used to enhance atom-residence times and to allow a higher vapour temperature to be achieved during the lifetime of the atom population. The tube-wall temperatures achieved by the different tube designs during this atomisation sequence will be referred to in the text.Time-resolved atomic-emission signals were recorded on the strip-chart recorder set at a speed of 2 cm s-l, using a trigger mechanism that initiated the recorder motor a t the start of the HGA-72 atomisation stage. The Perkin-Elmer 306 spectrophotometer does not exhibit the facility of automatic wave- length modulation background correction. I t was necessary therefore to record first the combined analyte and background emission and then, on the same section of chart paper, the tube background signal only, for the same instrumental conditions. The background intensity at the time of the combined signal peak was then subtracted to give the net maxi- mum atomic-emission intensity. HGA-72 Atomiser Tube Design than most commercial atomisers.The Perkin-Elmer HGA-72 carbon furnace atomiser is more amenable to tube modification Different temperature gradients along the 53 mm long,1140 AnaZyst, VoZ. 104 8.6 mm i.d. tube can be created by reducing the 1.85 mm wall thickness a t a number of positions on the outside of the tube. In this work experiments were performed with the standard and four modified tubes. The “high-temperature’’ tube was prepared as described by Ottaway and Shaw,8 by removing 1 mm of carbon with a lathe from the outside of the standard tube up to 0.5 cm either side of the injection hole [Fig. 1 (A)]. The “tapered” tube was designed to reduce to a minimum the temperature gradient along the I-IGA-72 atomiser. Starting from a position 6mm either side of the central injection hole, the wall thickness was tapered towards the ends of the tube by removing up to 1 mm of ca.rbon over a 12-mm section on the external surface, as shown in Fig.1 (B). The initial “volatile-elements” tube, designed by Ottaway and Hutton,13 was prepared by removing 1.2 mrn (0.05 in) from a 12-mm (0.5-in) portion of the outside of the tube at each end, starting 6 nim (0.25 in) from the central injection hole. In this work however, only 0.75-1.0mm of carbon was removed from the end sections [Fig. 1 (C)]. The “cup” tube is an adaptation of the “volatile-elements” tube and has been described in detail in a recent Cornmuni~ation.~~ The bottom 5.0-mm section of a 9 mm long and 4.5 mm i.d. Varian CRA 90 atomiser cup was positioned 2.5 mm inside the volatile- elements tube through a 5.0 mm diameter hole cut in the centre of the tube beneath the injection hole.The cup was then fixed permanently in position in the modified tube by pyrolysis of the tube assembly at 2400 K for 10 min in a 5% methane - 95% argon gas atmosphere in the HGA-72 atomiser workhead [Fig. 1 (D)]. This pyrolysis procedure was also used to form a coating of pyrolytic graphite on the standard and modified tubes for the measurement of molybdenum, titanium and gadolinium emission signals. LITTLE JOHN AND OTTAWAY : INVESTIGATION OF ATOMISER TUBE Fig. 1. Modified HGA-72 carbon tubes used in this study: (A) the “high-temperature” tube; (B) the tapered tube; (C) the “volatile ele- ments” tube; and (D) the cup -tube. Temperature Measurements Tube-wall temperatures were measured with ;in Ircon, Series 1100, optical pyrometer as described previ0us1y.l~ The pyrometer was focused on the inside of the tube through the injection hole and the ends of the tube.The temperature during atomisation was recorded on a Servoscribe RE54130 chart recorder at a speed of 2 cm s-1. All vapour-phase tempera- tures were obtained from iron electronic excitation temperature measurements using the iron-atom lines at 370.56, 373.49, 373.71 and 392.29 nm using the procedure described in detail e1~ewhere.l~ Atomic-emission signals generated at these wavelengths during the atomisation of 20-4 aliquots of a 2.0 pg ml-l iron standard solution were recorded on the Servoscribe recorder at 2.0 cm s-l. Corrections for the variation in the spectral response of the spectrometer were applied to the emission intensities at various times during the atomisa- tion sequence and graphs of ln(lXij/giAij) against E , were constructed. From the slopes of -l/kT, the absolute temperature T can be obtained at each point in time.In this expression, I is the corrected emission intensity measured under conditions of negligible self-absorption at wavelengths Aij, g, is the statistical weight of the upper energy level i of energy E,, A i j is the transition probability and K is Boltzmann’s constant. Chemicals and Detection Limits Stock solutions of the required elements were prepared from reagents of the highest available purity by dissolving the appropriate atmount of a suitable salt in distilled water and nitric or sulphuric acid to give a final acid concentration of M.Working standards were prepared from these stock solutions when required.December, 1979 DESIGN FOR CARBON FURNACE ATOMIC-EMISSION SPECTROMETRY 1141 Detection limits were calculated from 7-10 injections of a standard solution giving an atomic emission to background emission ratio of between 1.0 and 2.0, and are defined as that concentration giving a signal above the background equal to twice the standard deviation of the maximum emission signals from the standard solution injections. Results and Discussion The modifications to the HGA-72 tube design described in this paper have been achieved by altering the thickness of the graphite wall at various positions along the tube. Before describing the effects that each of these alterations has on CFAES detection limits, it is of interest to note, for comparison, the thermal and analytical parameters of the standard tube design.Standard HGA-72 Tube The variation with time of the carbon wall temperature at the centre of a standard HGA-72 tube at a number of atomisation settings has been discussed in detail in a previous paper.g At each setting the vapour temperature is controlled by the temperature attained by the carbon wall and the gradient that exists along the tube surface. An indication of the temperature gradient between the centre of a standard tube and a point 10mm from the end is illustrated for atomisation settings of 600, 690 and 999 units in Fig. 2. The wall temperatures were obtained by focusing the Ircon, Series 1100, pyrometer on the inside of the tube wall through the injection hole and from the end of the furnace.There appears to be little variation in the temperature difference between the two positions at any time from 2 to 7 s during the heating cycle of each setting. The temperature of the carbon wall 10 mm from the end of the tube appears to be 200-280 K lower than the centre throughout each sequence and the effect that this gradient has on the vapour temperature at 620 units is shown in Fig. 3. As might be expected the apparent vapour temperature is averaged over the tube-wall temperature gradient but is only, at maximum, 120 K lower than the tempera- ture at the centre of the tube. A similar deviation for the HGA-72 has been reported previously,15 for atomisation at 999 units (maximum power).2800 - 2600 - 2400 - 2200 - C C 2000 L ' 1 1 1 I I I I I I I I 1 3 5 7 1 3 5 7 1 3 5 7 Time/s Fig. 2. Variation with time of the temperature at the centre (C) and 10 mm from the end (E) of a standard HGA-72 atomiser tube operated a t (a) 600, (b) 690 and (c) 999 units. Detection limits for the ten sample elements of varying volatility are given in Table I. All atomic-emission measurements were made a t an atomisation setting of 999 units, and this maximum power setting was chosen for each of the HGA-72 tube designs. The tempera- tures given in Table I are those of the carbon wall at the centre of the tube, measured with the Ircon optical pyrometer at the time of the maximum atomic-emission intensity. Discus- sion of the detection limit values will be considered in a later section, but it appears that the standard tube design, a t maximum power, is most suited to the determination of elements of medium to high atom-appearance temperatures. Although the maximum intensities of volatile elements occur, in general, before those of involatile elements, maximum atomic1142 LITTLEJOHN AND OTTAWAY: INVESTIGATION OF ATOMISER TUBE AnaZyst, VoL.104 I I 1 3 ‘5 7 9 Tirne/s Fig. 3. Comparison of the vapour temperature (0) with the wall temperature (0) at the centre (C) and 10 mm from the end (E) of a standard HGA-72 atomiser tube operated a t 620 units. emission for lead occurs after the maximum atomic emission for a number of less volatile elements, such as manganese, iron and tin. This may be related to the comparatively high concentration of lead standard solution required to obtain measurable signals.A shift in the time of the maximum emission intensity, further from the start of the atomisation sequence, has been observed for a number of elements, as the sample solution concentration is increased. This will be discussed in greater detail elsewhere.16 TABLE I CFAES DETECTION LIMITS FOR ELEMENTS OF VARYING VOLATILITY ATOMISED AT MAXIMUM POWER IN A STANDARD HGA-72 ATOMISER TUBE Central tube-wall Time of maximum temperature at time Detection limit using Wavelength/ atomic-emission of maximum atomic a 50-p1 aliquot/ Element nm* intensit y/s emission/K pg ml-l Lead . . . . . . Gallium . . . . Silver . . .. Tin . . . . .. Manganese . . . . Iron . . . . . .Chromium . . . . Molybdenum? . . Titanium? . . . . Gadolinium?. . . . 405.78 403.30 328.07 286.33 403.08 371.99 425.43 379.83 399.86 440.19 5.0 3.0 4.4 4.0 3.5 4.0 4.5 8.0 6.5 5.0 2 843 2 703 2 823 2 803 2 773 2 803 2 783 2 873 2 863 2 803 5.37 0.049 0.016 0.24 0.007 4 0.025 0.001 6 0.051 0.020 1.32 * Spectral band width 0.2 nm (except Gd, 0.14 nm; Mo and Ti, 0.07 nm). t Pyrolytically coated tube. High-temperature HGA-72 Tube The variation with time of the carbon wall temperature at the centre of the cut-down section of this tube is illustrated in Fig. 4 for a number of atomisation settings. The heating rate at the‘tube centre is much faster than for the standard tube design15 and exceeds 2500 K s-l at 550 units. Clearly the heating rate depends on the mass of carbon at the centre of the tube and will increase as the wall thickness is reduced.As might be expected, the atomisation settings give higher temperatures with the “high-temperature” tube than for the standard version. A maximum equilibrium temperature of 3000 K was obtained for this tube at 550 units. Increasing the control to 999 units did not give further increases in either the tube temperature or heating rate. At settings close to the maximum voltage, for example for 400-550 units, similar heating rates were observed, making the concept ofDecember, 1979 DESIGN FOR CARBON FURNACE ATOMIC-EMISSION SPECTROMETRY 1143 temperature optimisation applicable to the high-temperature tube, as discussed previo~sly.~ At the settings normally selected for atomic-emission analysis, the maximum temperature is reached within 2-3 s.In this period, however, there is a peak in the wall temperature at the tube centre, for a number of settings. This is most pronounced at 400-550 units (Fig. 4) and causes an equivalent peak in the background signals measured at these atomisation settings . 3100 I 2 900 450 2700 3 50 2 300 I ' I I I 1 3 5 7 Time/s Fig. 4. Variation with time of the wall temperature at the centre of a high-temperature HGA-72 atomiser tube operated a t settings between 350 and 550 units. As expected, the temperature gradient from the centre of the tube to a point 10 mm from the end is greater than for the standard tube, but is not altered significantly by changing the temperature setting as illustrated in Fig. 5 for 400, 450 and 550 units.In the initial seconds of the atomisation sequence, the gradient is as much as 600-800 K between the measurement points. However, as the equilibrium temperature is reached, the gradient is reduced to 300400K. The effect of this temperature gradient on the apparent vapour temperature is shown in Fig. 6 for atomisation at 500 units. When iron solutions were injected it was found that, during the initial drying sequence at 373 K for 40 s, some of the vaporised water condensed in the cooler end sections of the tube. It was therefore necessary 3100 2 700 Y 3 2300 2 --. L K l 6 1900 I- 1500 I I E I I 0 2 4 6 0 2 4 6 0 2 4 6 Time/s Fig. 5. Variation with time of the temperature a t the centre (C) and 10mm from the end (E) of a high-temperature HGA-72 atomiser tube operated at (a) 400, (b) 450 and (c) 550 units.to introduce a second drying stage at 873 K for 20 s to dry the tube completely. As with the standard tube design, the vapour temperature is averaged over the carbon wall tempera- ture gradient. Because the gradient is greater than in the standard tube, the vapour-phase temperature deviates more from the tube-centre temperature, in this instance by between 200 and 400 K.1144 LITTLEJOHN AND OTTAWAY: INVESTIGATION OF ATOMISER TUBE Analyst, Vd. 104 3000 2 800 Y --. L + 3 F 2600 a, a E l- 2400 2 200 0 2 4 6 8 lime/s Fig. 6. Comparison of the vapour temperature (0) with the wall temperature (m) at the centre (C) and 10 mm from the end (E) of a high- temperature HGA-72 atomiser tube operated at 500 units.Detection limits for the ten sample elements atomised in the high-temperature tube at 999 units (maximum power) are given in Table 11. As the tube attains the equilibrium temperature much faster than the standard tube the maximum atomic emission is reached within 4.0 s for all elements, irrespective of volatility. As expected the detection limits of elements of high atom-appearance temperature are enhanced when the high-temperature tube is used (shown here for gadolinium and in reference 8 for several other elements), while those of more volatile elements are degraded. The analytical utility of the high-temperature tube is somewhat restricted by the short lifetime of the tubes. Standard HGA-72 tubes can be used, in general, for more than 100 analyses, even when the furnace gas flow is interrupted during atomisation.The lifetime is reduced to about 30 injections when the carbon wall thickness is reduced at the tube centre. This can. be extended by forming a pyrolytic graphite coating on the tube after every 20-25 analyses, vvhich increases the lifetime by up to a factor of three times. TABLE I1 CFAES DETECTION LIMITS FOR ELE:MENTS OF VARYING VOLATILITY ATOMISER TUBE ATOMISED AT MAXIMUM POWER IN A HIGH-TEMPERATURE HGA-72 Time of maximum atomic-emission Element* intensity I s Lead . . . . . . Gallium . . . . . . Silver . . . . . . Tin . . .. .. Iron . . . . . . Chromium . . . . Manganese . . . . Molybdenumt . . . . Titanium? . . . . Gadolinium? . . . . 3.8 4.0 3.5 3.5 3.1 3.5 3.0 4.0 3.5 4.0 Central tube-wall -temperature a t time of maximum atomic emission/K 3 063 3 063 3 063 3 063 3 063 3 063 2913 2 993 2 953 2 973 * Wavelengths and spectral band widths as in Table I.7 Pyrolytically coated tube. Detection limit using a 50-pl aliquot/ pg ml-l 16.32 1.30 0.033 0.67 0.080 0.16 0.005 0.052 0.017 0.86 Tapered HGA-72 Tube This tube was designed in an attempt to reduce to a minimum the temperature gradient along the HGA-72 atomiser. Although it was difficult to reproduce accurately the same taper on different tubes, the temperature gradient from the central position to 8mm from~ ~ C 6 V Z b e Y , 1979 DESIGN FOR CARBON FURNACE ATOMIC-EMISSION SPECTROMETRY 1145 the end of the tube was usually reduced to a maximum of 100-150 K during the atomisation sequence. Because the temperature gradient is reduced, the vapour-phase temperature is very similar to that of the carbon wall at the centre and ends of the tube.Detection limits for the ten sample elements atomised at 999 units (maximum power) in the tapered tube are given in Table 111. Tapering the ends of the HGA-72 tube tends to give a lower equilibrium tube temperature at each atomisation setting than for the standard or high-temperature tube designs. This is illustrated in Fig. 7 for atomisation at 999 units. 2 700 2 600 Y 1 2500 3 + e 2400 P 2 300 Fig. 7. Comparison of the vapour temperature (A) with the wall temperature (-) at the centre (C) and 8 mm from the end (Ej of a tapered HGA-72 atomiser tube operated at 999 units. Consequently, the detection limits of the more involatile elements are degraded but as the temperature gradient is decreased, the detection limits of volatile and medium volatile elements are imprdved slightly.Tapering the tube does not appear to affect the lifetime of the tube significantly in comparison with the standard design. TABLE I11 ATOMISED AT MAXIMUM POWER I N A TAPERED HGA-72 ATOMISER TUBE CFAES DETECTION LIMITS FOR ELEMENTS OF VARYING VOLATILITY Tube-wall temperature atomic-emission maximum atomic a 5O-pl aliquot/ Time of maximum at time of Detection limit using Element* intensity 1s emission/K pg m - l Lead . . . . . . 4.0 2 693 2.3 Gallium . . .. . . 2.9 2 593 0.036 Silver , . . . . . 3.9 2 693 0.014 Tin . . .. . . 3.6 2 663 0.195 Manganese * . . . 3.0 2613 0.001 5 0.009 6 Iron . . . . . . 3.6 2 673 Chromiuni ... . 3.5 2 783 0.000 7 Molybdenum? . . . . 7.0 2 673 0.084 Tit ani urn t .. . . 5.5 2 653 0.047 Gadoliniunit . . . . 4.5 2 653 9.5 * Wavelengths and spectral band widths as in Table I. Pyrolytically coated tube. Volatile-elements HGA-72 Tube The initial volatile-elements tube design13 allowed the ends of the tube to be heated faster and to a higher temperature than the central section where the sample is atomised. At maximum power (999 units), the central section reached a maximum temperature of 2053 K and the end sections, of reduced wall thickness, 2443 K.13 The temperature gradient was thus reversed, compared with the standard HGA-72 tube design. Atoms formed in the central section were thought to diffuse into the hotter end-sections giving greater atomic1146 Analyst, VO,?.104 emission to tube-background emission ratios folr volatile elements than for the standard tube. The end sections are also nearer their maximum temperature when the atom popula- tion is present, and the temperature and background signal are therefore changing less rapidly, giving improved analyte signal reproducibilities. Significant improvements in detection limits were observed for a number of volatile elements but the application of this tube to the CFAES determination of elements with higher atom-appearance temperatures is limited by the comparatively low temperature of the central atomisation section of the tube wall. The concept of atomising samples into an already hot tube seems to be a positive step in the development of a furnace tube design specific to the requirements of carbon furnace atomic-emission analysis.Therefore, to extend the range of application of the volatile-elements tube design, only 0.75-1 .O mm of carbon was removed from the end sections of the tubes used in this study. By removing slightly less carbon than the original design, higher temperatures are achieved at both the end and centre of the tube. This is illlustrated by the optical pyrometer tempera- tures given in Fig. 8 obtained at an atomisation setting of 999 units. The apparent vapour temperature, estimated as before from iron electronic excitation temperatures, is considerably greater than the central tube-wall temperature in the initial seconds of the atomisation sequence. Consequently, the atomic emission of all elements with appearance temperatures less than 2273 K is enhanced compared with atomisation to the same central wall tempera- ture in a standard, high-temperature or tapered 11GA-72 tube.LITTLEJOHN AND OTTAWAY: INVESTIGATION OF ATOMISER TUBE 2600 Y \ 2500 3 + 2 f 2400 c 2 300 . - t + + c :&/ 1 I I I 0 2 4 6 E Time/s Fig. 8. Comparison of the vapour temperature (+) with the wall temperature a t the central (C) and end sections (E) of a volatile-elements HGA-72 atomiser tube operated a t 999 units. Detection limits for nine of the sample elements of varying volatility atomised at maxi- mum power (999 units) in the modified volatile-elements tube are given in Table IV. When the volatile-elements tube was pyrolysed to enhance atomisation of molybdenum, titanium and gadolinium, the wall temperature at both the end and central sections was observed to increase slightly.However, as expected, detection limits for these elements are poor and owing to severe between-run memory effects, it was not possible to calculate a detection limit for molybdenum, although atomic-emission signals could be measured. I n contrast, the detection limits for gallium to chromium are adequate for many analytical purposes. However, the values for lead and gallium are higher than those reported for the initial volatile-elements tube design.13 This is undoubtably related to the observation that at the atom-appearance temperatures of these elements, the tube temperature a t the end sections is still increasing rapidly. Consequently, the analyte-emission signal tends to appear as a shoulder on the rising background signal and is difficult to detect at low atom concentrations.Volatile-elements HGA-72 Tube Fitted with a Sample Cup measurement of atomic-emission signals. further it is apparently necessary to fulfil two conditions. The principle of the volatile-elements tube design appears to be particularly suited to the To extend the range of application of this tube Firstly, the end sections must beDecember, I979 DESIGN FOR CARBON FURNACE ATOMIC-EMISSION SPECTROMETRY 1147 heated far faster than the central atomisation section and should reach an approximately constant temperature before atoms of even the most volatile elements are generated. This implies that the difference between the end and central sections should be as great as possible during the initial seconds of the atomisation sequence.Secondly, the temperature attained by both sections should be as high as possible. In an attempt to achieve these conditions, a new type of atomiser tube was prepared that is substantially different from the others described above in that it incorporates a sample cup placed in the centre of the graphite tube, as well as end sections of reduced wall thickness.14 TABLE I V CFAES DETECTION LIMITS FOR ELEMENTS OF VARYING VOLATILITY ATOMISED AT MAXIMUM POWER IN A VOLATILE-ELEMENTS HGA-72 ATOMISER TUBE Tube-wall temperature at time of maximum atomic Time of maximum emission/K Detection limit using atomic-emission -7 a 50-p1 aliquot/ Element* intensityls Centre End pg ml-I Lead .. . . .. . . Gallium . . .. . . Silver . . . . . . Tin . . . . . . . . Manganese . . . . . . Iron . . . . . . . . Chromium . . . . .. Titanium? . . .. .. Gadolini um t . . . . 4.2 2.3 2.4 3.0 3.5 6.0 7.0 --$ t -+ 2 563 2 373 2 353 2 393 2 323 2413 2 373 2 793 2 793 2 693 2 593 2 613 2 2 3 2 893 2 893 .F -+ t -+ 5.4 0.026 0.008 9 0.063 0.001 5 0.007 3 0.001 3 0.37 18.85 * Wavelengths and spectral band widths as in Table I. t Pyrolytically coated tube. $ Not measured. Optical pyrometer temperature measurements of the cup and reduced wall sections at maximum applied power were given in the original Communication.14 The incorporation of a sample cup as part of the tube wall at the centre enhances the temperature difference between the atomisation (cup) and excitation (tube vapour) sections compared with the volatile- elements tube described in the previous section. The cup is resistively heated with the rest of the tube but because of its greater mass, the temperature rises more slowly and to a lower final value than the sections of reduced wall thickness.Comparison of the temperature profiles in Fig. 8 with those in Fig. 2 of reference 14 indicates that the cup attains a similar equilibrium temperature to the central section of the original volatile-elements tube design. However, the end sections of the cup - tube assembly are almost 300 K higher than those of the tube operated without the sample cup attachment. Fig. 2 of reference 14 illustrates that during the initial few seconds of atomisation, differences of at least 1000 K exist between the cup and the end sections of reduced wall thickness.I t appears, however, that the cup - tube will be restricted in application to elements with atom-appearance temperatures below 2 200 K. Excluding molybdenum, titanium and gadolinium, detection limits for the remaining seven of the sample elements are given in Table V, along with the values obtained for a number of other volatile elements.14 Aliquots of 20 p1 were found to be the maximum suitable for the cup - tube, and comparison of concentration detection limits with the other tubes where 50 pl can be used is at a significant disadvantage. A larger cup size may allow larger aliquots to be used with this type of atomiser but this has not been attempted to date. The tempera- tures in Table V correspond to those of the cup and reduced wall sections at the time of the maximum atomic-emission signal, which was between 2.5 and 60 s after the start of atomisa- tion depending on the volatility of the element.By following the procedure described above and elsewhere14 it is possible to create reproducibly cup - tubes with th. described heating parameters and a tube lifetime similar to that of the standard HGA-72 design.1248 General Comparison The detection limits obtained for the ten sample elements chosen at the start of this study are collated in Table VI for the standard and modified HGA-72 tube designs. The optimum detection limit for each element is in italics. As expected, the high-temperature tube is most suited to the determination of carbide-forming or involatile elements, while the most sensitive atomic-emission signals of volatile elements are obtained with the cup - tube.The volatile-elements and tapered tubes give lowest detection limits for medium-volatile elements and it is interesting to note that of the ten elements investigated only molybdenum achieves optimum sensitivity in the standard tube when operated at maximum power. The detection limits of volatile elements are poor in the high-temperature, standard and tapered tubes as the maximum atom concentrations of these elements are present at low tube vapour tempera- tures and the weak emission signals cannot be easily detected from the rapidly increasing background signals. However, the detection limit for lead in the volatile-elements tube is higher than expected in comparison with the values obtained with the other tube designs.LITTLE JOHN AND OTTAWAY : INVESTIGATION OF ATOMISER TUBE Analyst, vol. 104 TABLE 'v' CFAES DETECTION LIMITS FOR ELEMENTS OF VARYING VOLATILITY ATOMISED AT MAXIMUM POWER IN A VOLATILE-ELEMENTS HGA-72 ATOMISER TUBE FITTED WITH A SAMPLE CUP Element Wavelength/nni* Cadmium . . . . . . 326.1 1 Lead . . . . . . . . 405.78 Zinc . . . . . . . . 307.59 Bismuth . . . . . . 306.77 Galliilm . . . . . . 403.30 Silver . . .. . . 328.07 Gold . . . . . . . . 267.60 Tin . . . . , . . . 286.33 Manganese . . . . . . 403.08 Copper . . . . . . 324.75 Iron . . . . . . . . 371.99 Chromium . . . . . . 425.43 Tube-wall temperature a t time of maximum atomic emission/I< r-----hp 7 c u p Tube wall <: 1573 2 443 1823 2 523 2 003 2 613 1698 2 773 1773 2813 1698 2613 2 253 2 693 2 223 2 683 2 223 3 683 2 093 2 733 - - - - Detection limit using a 20-pl aliquot! pg ml-l 0.06s 0.027 1.57 0.033 0.000 66 0.00044 0.16 0.030 0.003 1 0.002 0 0.10 0.006 3 * Spectral band width 0.2 nm (Pb and Bi, 0.07 nm).With the exception of molybdenum17 the optimum detection limits in Table VI represent the best achieved to date with a standard commeircial furnace - spectrometer system. The values also compare favourably with detection limits presented recently using wavelength modulation background correction to compensate automatically for the tube wall back- ground emission using a Perkin-Elmer HGA-2 100 a t ~ m i s e r . ~ Conclusions The design and temperature gradient of a graphite tube greatly influence the intensity of atomic emission and, consequently, the CFAES detection limits that are obtainable with a Perkin-Elmer HGA-72 atomiser.When the carbon wall temperature decreases towards the ends of the tube from the central atomisation section, as in the standard and high-temperature tube designs, the average vapour temperature is always lower at any time than the tempera- ture of the carbon wall from which atoms are produced. This deviation is more severe in the high-temperature tube due to the increased temperature gradient. Consequently, volatile elements experience low excitation temperatures and exhibit poor detection limits in standard and high-temperature tubes. By reversing the temperature gradient it is possible to pre-heat the vap2ur before atomisation occurs by increasing the temperature of the ends of the tube faster and to a higher equilibrium value than the central atomisation section.Therefore, when atoms are produced either directly from the wall surface or by dissociation of a molecular vapour, they exist in a gas that has a greater temperature than the vaporising surface. The intensity of atomic emission for any element will obviously beDecember, 1979 DESIGN FOR CARBON FURNACE ATOMIC-EMISSION SPECTROMETRY 1149 improved as this temperature difference is increased. The development of a tube operated with a sample cup has allowed this concept to be applied to elements ranging in volatility from cadmium and zinc to copper and chromium. This allows the HGA-72 to be operated in a similar manner to the L’vovlO and Woodriffll atomisers in that atoms and molecules of a number of elements can be introduced into a tube where the vapour is already heated close to its maximum temperature.Further modifications are required, however, to allow efficient atomisation and excitation of involatile elements and elements of high excitation energy, such as arsenic and selenium. At present the CFAES determination of molybdenum, titanium, etc., is best performed with the standard and high-temperature HGA-72 atomiser tubes or by application of rapid furnace heating1’ TABLE VI COMPARISON OF CFAES DETECTION LIMITS FOR ELEMENTS OF VARYING FOR AN HGA-72 ATOMISER VOLATILITY ATOMISED AT MAXIMUM POWER I N DIFFREENT TUBES DESIGNED Values in italics are optimum detection limits.Detection limit/p.g ml-’ Volatile-elements Volatile-elements Tapered Standard High-temperature I 7 -A-pp Element tube with cup tube tube tube tube Lead . . . . . . Gallium . . . . Silver . . . . Tin . . . . . . Manganese . . . . Iron . . . . . . Chromium . . . . Molybdenum . . Titanium . . . . Gadolinium . . . . 0.027 0.000 66 0.000 44 0.030 0.003 1 0.10 0.0063 5.4 0.026 0.008 9 0.063 0.001 5 0.007 3 0.001 3 0.37 18.85 - 2.3 0.036 0.014 0.195 0.001.5 0.009 6 0.000 7 0.084 0.047 9.5 5.37 0.049 0.016 0.24 0.0074 0.025 0.001 6 0.051 0.020 1.32 16.32 1.30 0.033 0.67 0.080 0.16 0.005 0.052 0.017 0.S6 The results presented in this paper indicate that significant improvements in CFAES detection limits can be achieved by alteration of the design and heating characteristics of a standard HGA-72 atomiser tube.Although parts per billion detection limits can now be achieved for a wide range of elements, no single design of commercial atomiser tube is likely to be suitable at present for the optimum atomisation and excitation of all elements. The use of present carbon furnaces with multi-channel spectrometers for simultaneous multi- element analysis would necessitate the adoption of compromise conditions of tube design and temperat~re.~ This may not be a disadvantage in many real analytical situations but would limit the general analytical value of CFAES. The carbon tubes used for all previous published research on CFAES were designed for atomic-absorption measurements. The results indicate that when tubes designed specifically for atomic emission are manufactured, considerable advantages will result if the maximum atom population of all elements can be generated in a vapour already existing a t its maximum temperature. As this is rarely a requirement for atomic-absorption measurements, it is likely that the optimum tube designs for atomic emission and atomic absorption will be different. The optimum tube design for emission may require complete separation of the atomisation and excitation steps in the production of the signal from the analyte. The authors thank The Royal Society for the award of a research grant to J.M.O. for the purchase of the HGA-72 atomiser and the Salters’ Company for the award of a scholarship to D.L.1150 LITTLE JOHN AND OTTAWAY References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. Shaw, F., and Ottaway, J . M., Analyt. Lett., 1975, 8, 911. Littlejohn, D., and Ottaway, J . M., Analyst, 1977, 102, 393. Hutton, R. C., Ottaway, J. M., Rains, T. C., and Epstein, M. S., Analyst, 1977, 102, 429. Ebdon, L., Hutton, R. C., and Ottaway, J . M., Analytica Chim. Acta, 1978, 96, 63. Epstein, M. S., Moody, J. R., Brady, T. J., Rains. T. C., and Barnes, I. L., Analyt. Chem., 1978, 50, Ottaway, J. M., and Shaw, F., Analytica Chim. Acta. 1978, 99, 217. Alder, J . F., Samuel, A. J., and Snook, R. D., Lab. Pract., 1977, 26, 22. Ottaway, J. M., and Shaw, F., Appl. Spectrosc., 1977, 31, 12. Littlejohn, D., and Ottaway, J . M., Analytica Chim. Acta, 1979, 107, 139. L’vov, B. V., “Atomic Absorption Spectrochemical Analysis,” translated by J. H. Dixon, Adam Woodriff, R., Stone, R. W., and Held, A. M., Apibl. Spectrosc., 1968, 22, 408. Robinson, J . W., and Wolcott, D. K., Analytica Chim. Acta, 1975, 74, 43. Ottaway, J. M., and Hutton, R. C., Analyst, 1976,, 101, 683. Littlejohn, D., and Ottaway, J . M., Analyst, 1978, 103, 662. Littlejohn, D., and Ottaway, J. M., Analyst, 1978, 103, 595. Littlejohn, D., and Ottaway, J. M., Can. J . Spectrosc., in the press. Littlejohn, D., and Ottaway, J. M., Analytica Chim. Acta, 1978, 98, 279. 874. Hilger, London, 1970. Received November 22nd, 1978 Accepted July 9th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790401138

出版商:RSC    

年代:1979

数据来源: RSC