摘要:

Proceedingsof the Analytical Division ofThe Chemical SocietyPADSDZ163163163172179191191192CONTENTSReports of MeetingsSummaries of Papers'A Composition of Some Days. , . ''SAC Centenary Celebrations''The Renaissance of Polarography'CorrespondenceRecording of CentenaryAnalytical Division DiaryVolume 12 No 6 Pages 163-1 92 June 197Vol. 12, No. 6 June, 1975PROCEEDINGSANALYTICAL DIVISION OF THE CHEMICAL SOCIETYOF THEOfficers of the Analytical Divisionof the Chemical SocietyPresidentG. W. C. MilnerHon. SecretaryP. G. W. CobbHon. TreasurerJ. K. ForemanSecretaryMiss P. E. HutchinsonHon. Assistant SecretariesD. I. Coomber, O.B.E.; D. W. WilsonEditor. ProceedingsP. C. WestonProceedings is published by The Chemical Society.Editorial: The Director of Publications, The Chemical Society, Burlington House, London, W1 V OBN.Telephone 01 -734 9864.Telex 268001.Subscriptions (non-members) : The Chemical Society, Publications Sales Office, Blackhorse Road, Letch-worth, Herts., SG6 1HN.Non-members can only be supplied with Proceedings as part of a combined subscription with The Analystand A nalytical Abstracts.@ The Chemical Society 1975MEMBERS TIE8BADGE AND HERALDIC SHIELDMembers are reminded that they can still purchase a tie bearing the Coat ofArms of the Society for Analytical Chemistry (above, left) on a backgroundof dark blue, dark green or maroon, a blazer badge depicting the HeraldicBadge of the SAC (above, right), woven in gold and silver on a black back-ground, and a heraldic wall shield depicting the Coat of Arms. A limitedsupply of badges and shields is available.Prices (including VAT and postage) are as follows:Orders should be sent to The Secretary, Analytical Division, The Chem-Cheques should beTie: f 1 .I 0 Badge: f 3.00 Shield: f 4.00ical Society, 9/10 Savile Row, London, WIX IAF.made payable to the Analytical Chemistry Trust Fund

ISSN:0306-1396    

DOI:10.1039/AD97512FX021

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Analytical Division DiaryJULYMonday and Tuesday, 7thboroughAnalytical Division onDevelopment Topics inistry.”and 8th: Lough-“Research andAnalytical Chem-“The Application of a Tunable Dye LaserSystem in AFS,” by A. Goldwasser.“Porous Polyurethane Foams as Extractantsfor Organochlorine Insecticides, PCBs andHeavy Metals from Aqueous Solution,” byP. Musty.“The Determination of Europium in SolidDiketonate Complexes Using an ExcitationEffect,” by B. Maghzian.“Fluorogenic Reactions as Applied to theFluorometric Determination of Some Alkal-oids and Thiopurines,” by A. D. Thomas.“Phosphorimetric Analysis Using a ModifiedFilter Fluorimeter,” by D. L. Phillips.Lecture Theatre J O O l , Edward HerbertBuilding, University of Technology,Lough borough.Monday, 7th-“Automated Polarography,” by S.R. Porter,V. J. Jennings and J. W. Ogleby.“Applications of Electroc%emical Studies ofMetal Cyanide Complexes,” by D. T.Wright.“Some Applications of Hexacyanoferrate(II1)in Thermometric Titrimetry,” by L.Kershaw and L. S. Bark.“The Determination of Polyfunctional Car-boxylic Acids and Phenols, includingVegetable Tannins, by Catalytic Thermo-metric Titrimetry,” by E. J. Greenhow andA. A. Shafti.Gas - Liquid Chromatography,” by S. A.Tarafdar.tions to Pesticide Analysis,” by C. Self.metry,” by J. Hind.“Determination of Some Precious Metals by Saturday and 12* and 13*:LancasterNorth West Region : Summer Meeting.“Liquid Chromatography and its Applica- “Carbon Dating,” by Professor J.C.“Gas Analysis by Quadrupole Mass Spectro-Bevington.The University, Lancaster .The meeting will include a visit to LancasterCollege of Agriculture.Further details can be obtained from theHonorary Secretary of the Region, Mr. G. B.Crump, Thornton Research Centre, ShellResearch Ltd., P.O. Box No. 1, Chester,CH1 3SH.Tuesday, 8th-“Determination of Trace Elements in Soapsand Phosphate Materials by Carbon Furn-ace Atomic-absorption Spectrometry,” byS. I. Pradhan and J. M. Ottaway.Euroanalysis I IAugzlst 25-30, 1975, BzsdapestFor those who have not already made arrange-ments, travel by scheduled air services plusaccommodation in hotels of various categoriescan be obtained at advantageous rates through Copies of the Second Circular are now avail-Barry Martin Travel Ltd., Special Events able and can be obtained from the Secretary,Division, Suite 309/310, Albany House, 324 Analytical Division, 9/10 Savile Row, LondonRegent Street, London W1R 5AA (Tel: 01-636 W1X 1AF. They will not be circulated4563). Application should be made immediatelydirect to the Travel Agent, who will also arrangevisas and Conference registration if required.separately.Printed by Heffers Printers Ltd Cambridge Englan

ISSN:0306-1396    

DOI:10.1039/AD97512BX023

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Vol. 12, No. 6 Proceedings June. 1975 of the Analytical Division of the Chemical Society Reports of Meetings Scottish Region A one-day intensive course on “Atomic- absorption Spectrometry” was held by the Scottish Region starting at 9.15 a.m. on Wednesday, May 28th, 1975, at the University of Strathclyde, Cathedral Street, Glasgow. The Chair was taken by the Chairman of the Region, Dr. J.M. Ottaway, and the following papers were presented : “Principles of Atomic- absorption Spectrometry,” by W. B. Rowston; “Instrument Design,” by C. B. Mullins; “Clinical Analysis,” by I. Dale ; “Agricultural Analysis,” by A. M. Ure; “Metallurgical Analysis,” by J. M. Ottaway ; “Pre-concentra- tion Techniques,” by A. M. Ure; “Recent Instrumental Developments,” by C. B. Mullins ; “Filament and Furnace Atomisers,” by J.M. Ot taway . The meeting included demonstration sessions by Instrumentation Laboratory (UK), Perkin- Elmer, Pye Unicam, Rank Precision Industries, Shandon Southern and Varian Associates. Special Techniques Group An Ordinary Meeting of the Group was held at 2.30 p.m. on Thursday, May 15th, 1975, in College Block, Imperial College, London, S.W.7. The Chair was taken by thevice-chairman of the Group, Dr. P. B. Smith. The subject of the meeting was “New Developments in Molecular Spectroscopy” and the following papers were presented and dis- cussed : “The Detection and Determination of Polynuclear Aromatic Hydrocarbons by Fluor- escence Spectrometry Utilising the Shpol’skii Effect at 77 K,” by G. F. Kirkbright; “Laser Techniques for Pollution Monitoring,” by E.L. Thomas ; “Thin-layer Phosphorimetry,” by J . Miller. Biological Methods Group The Summer Meeting of the Group was held on Friday, May Qth, 1975, and took the form of a visit to the Long Ashton Research Station and National Fruit and Cider Institute, Bristol. Members visited the following departments : fruit juice products, biological control of plant diseases, virology (virus effects on fruit trees), food spoilage and bioassay of growth regulators. The vote of thanks was given by the Chairman of the Group, Mr.F. W. Webb. Particle Size Analysis Group An Ordinary Meeting of the Group was held at 10 a.m. on Thursday, May Mth, 1975, at the Esso Motor Hotel, Dunstable Road, Luton. The Chair was taken by the Chairman of the Group, Dr.W. Cam. The subject of the meeting was “The Coulter Principle” and the following papers were presented and discussed : Introduction by R. N. Martin; “Pulse Response of the Coulter Counter,’’ by B. Scarlett ; “Coincidence Correc- tion in the Coulter Counter,” by P. J. Lloyd; “Application of a Model TA Coulter Counter in Pharmaceutical Development,” by N. A. Orr and J. Spence; “Use of the Coulter Counter on Ultra-filtered Sea Water,” by R. E. Davis; “An Evaluation of a Coulter On-line Monitor,” by M. I. Barnett. The meeting included a visit to Coulter Electronics Ltd. 163

ISSN:0306-1396    

DOI:10.1039/AD975120163a

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

172 SAC CENTENARY CELEBRATIONS Proc. Analyt. Div. Chem. SOC. SAC Centenary Celebrations The Centenary Celebrations of the SAC took place at Imperial College, London, from July 16th to July 19th, 1974 and included an extensive programme of lectures on many aspects of analytical chemistry. The following are summaries of two of the papers presented. Summaries of ten other papers presented appeared in the December issue of Proceedings (p.310) and a full report of the Centenary celebrations appeared in the September issue of Proceedings (p. 231). The Centenary Lecture, three Plenary Lectures and thirteen Keynote Lectures appeared in full in a special Centenary Issue of The Analyst in December. Surface Analysis H. J. Cluley The General Electric Company Limited, Hirst Research Centre, Wembley , Middlesex, H A 9 7PP Surface analysis may be taken to comprise the selective examination of that portion of a material which lies at and close to the surface, in order to ascertain its nature, composition and constitution. A need for surface analysis arises from the many technological applicatiohs of materials for which surface properties are of particular importance, for example, when a material is required to emit or reflect radiation, or to catalyse gas-phase reactions.Again, the surface of a material is the location of its interaction with its environment; the chemical and physical nature of the surface of a material, and hence in many instances the manner in which the material functions, may be considerably modified by environmental effects such as particulate contamination, adsorption or deposition from the gas phase, and oxidative or corrosive reactions.In the electrical industry, and particularly in the areas of electronics and semi- conductors, a special interest in surfaces arises from the considerable use of circuitry andJwae , 1975 SAC CENTENARY CELEBRATIONS 173 devices based on thin-film technology.This use creates the need to study not only the thin films’ themselves, but also other factors such as the physical perfection and chemical cleanli- ness of the substrate surfaces on to which the thin films are to be deposited. These and many other technological requirements generate a need to study surfaces with respect to physical texture, constitution or structural effects, and of course chemical compo- sition; it is convenient to consider techniques for surface analysis under these three headings.The examples given to illustrate applications of such techniques are all drawn from work in the author’s own laboratories. Surface Textures Techniques for the study of texture, i e . , microscopy techniques, are briefly compared in Table I. Optical microscopy is still a valuable tool for the study of surface texture, but has the disadvantage of poor depth of field. In addition, its general inability to resolve features much below 1 pm in size (except when some specialised techniques can be used) renders it inapplicable to many present-day surface problems, e.g., the study of semi-conductor devices such as integrated circuits, which have very minute dimensions.TABLE I COMPARISON OF MICROSCOPY TECHNIQUES Optical Scanning Transmission microscopy electron microscopy electron microscopy 0.5 nm Typical resolution 1 CLm 10 nm Depth of field Poor Good Medium Sample preparation Usually easy Easy Requires thin specimens or replicas Such limitations can be overcome by the use of the scanning electron microscope (SEM), in which the surface region under study is scanned by an electron beam; an image of the surface is built up by the signal given by scattered primary electrons, by secondary electrons from the specimen, by the current generated in the specimen, or by other effects.Usually, little or no specimen preparation is involved, except that non-conducting specimens may need to be coated with a very thin (about 10 nm) layer of metal (e.g., by evaporation), in order to prevent charging-up of the specimen and consequent disturbance of the electron beam.As a diagnostic tool in surface studies, the scanning electron microscope offers various advantages ; these include operation over a wide range of magnifications (15-15 OOO x ), resolution down to about 10 nm, large depth of field, and, in particular, the ability to use a variety of “signals” so that information additional to that of surface topography can be obtained.Thus, use of element contrast effects can depict surface regions where a change in composition arises from inclusions, contamination, etc. Use of voltage contrast effects can permit fault location in operating semi-conductor devices.By means of an “element attachment,” the scanning electron microscope can also be used to ascertain the chemical composition of small surface regions (see Techniques for surface composition). Study of surface features that are less than 10 nm in size necessitates the use of the trans- mission electron microscope, which can readily resolve down to 0.5 nm. There is usually, however, a price to be paid in terms of increased effort in the preparation of specimens, in that the specimen or artefact examined needs to be sufficiently thin to permit transmission by the electron beam.For inorganic materials this normally involves either specimen thinning by means of controlled etching procedures, or the preparation of thin replicas that reproduce the contours of the specimen surface.In this latter instance, the structure of the replicating medium imposes a limitation on the resolution attainable, e.g., 4-5 nm with carbon replicas. One common use of the scanning electron microscope is in the study of particle morphology, often at magnifications within the range of optical microscopy. The large depth of field of the scanning electron microscope allows the particles to be examined directly and individual particles to be viewed in order to study such factors as particle size and shape and how these factors change with heating or mechanical treatment, or to discriminate between aggregates and individual particles; the small depth of field of optical microscopy may necessitate the174 SAC CENTENARY CELEBRATIONS Proc.Analyt. Dizi. Chem. SOC. additional step of preparation of a section through the particles to gain comparable informa- tion. The many other uses of the scanning electron microscope, usually at magnifications beyond those of optical microscopy, have included study of fracture surfaces, geometrical and circuit defects in semi-conductor devices, electrical breakdown paths in insulating materials, and defects in plated surfaces for electrical contacts.1 This last example illustrates the use of the scanning electron microscope to observe not only physical imperfections of the surface, but also the presence of surface contaminants, such as may affect contact per- f ormance.The applications of transmission electron microscopy, for example, at magnifications beyond those obtainable with scanning electron microscopy, have included the study of the morphology and size distribution of precipitates in high-alloy steels, of defects in single- crystal materials, surface characteristics of thin films, and surface perfection of substrates for thin films.In this last category, one particular study on glass substrates showed that all the glasses examined, including some normally regarded as “chemically resistant ,” produced weathering growths on the surface after a few days’ exposure to the normal laboratory atmosphere.Although minute, such growths could be of significant size if the glass surfaces were to be used as substrates for thin films below 1 pm in thickness, and this work indicated the undesirability of delay between preparation of a clean glass substrate and subsequent deposition of a thin film.Crystal Structure of Surfaces Useful information on the constitution of surfaces or of deliberately applied surface layers can often be obtained by the use of the crystallographic techniques of X-ray and electron diffraction. Of particular importance is the ability of these techniques to establish the chemical identity of any crystalline species present, for example, an oxide film on a metal can be identified as the oxide and differentiated from the different crystalline species com- prising the metal.Both techniques can be used to obtain information on: crystal structure, orientation and strain effects; crystallite size; single crystal or polycrystalline nature; and the chemical identity of the crystalline species. High-energy electron diffraction, used in .a reflection mode, examines only a very shallow depth of material, typically 5-15 nm, and is therefore suited to the study of very thin surface layers or films.Because of the more penetrative nature of X-radiation, the probing depth of X-ray diffraction is very much greater and typically several micrometres.Thus the two techniques of X-ray and electron diffraction yield similar information, but differ by several orders of magnitude in the depth of the surface layers studied. This illustrates an important point in the choice of techniques for surface studies; any probing technique will derive its signal from a finite depth of material, and the technique chosen for a particular application must be such that the depth of material probed is commensurate with the surface layer of interest.It may be advantageous to use the two diffraction techniques in conjunction, particularly in a surface system where formation of a specific compound is sought and where the degree of reaction may vary at different depths from the surface. One such example concerned the study of a process used to create a surface layer with ohmic properties on semi-conductor silicon, by diffusing in platinum to form a layer several tens of nanometres thick and con- sisting of the 1 : 1 platinum silicide, PtSi.In a fully reacted specimen, PtSi will be detected both in the bulk of the layer and in the outer 5 nm or so by X-ray and electron diffraction, respectively.2 When reaction is not quite complete, PtSi may still be observed in the bulk of the layer by X-ray diffraction, but electron diffraction will reveal the presence of platinum- rich silicides, Pt,Si and Pt,Si, and possibly also unreacted platinum, in the extreme surface layer.The presence of these additional species, if detected in the bulk of the layer by X-ray diffraction, indicates a condition still further removed from that of complete reaction. This example illustrates the use of X-ray and electron diffraction to study a deliberately induced reaction between a surface layer and its substrate ; similarly, compound formation revealed by these techniques may provide evidence of unwanted interactions between surface layer and substrate.The diffraction techniques can be of particular value in studying the nature of deliberately applied surface films, for example, to establish whether such films are single crystal or polycry~talline.~,~ For single-crystal layers grown on a single-crystalJuae, 1975 SAC CENTENARY CELEBRATIONS 175 substrate, X-ray diffraction may be used to assess the crystal perfection of the layer and the orientation relationships between layer and substrate, and hence to guide the crystal grower on the choice of orientation of substrate on which to grow layers of good crystal quality.Techniques for Surface Composition The largest category of techniques available to the surface analyst is that concerned with the chemical composition of surfaces. A representative, but by no means comprehensive, list of such techniques is given in Table 11.Because of differences in the type of information yielded, e.g., detection or determination of elements or compounds and depth of surface layer probed, the surface analyst may need to apply more than one compositional technique to obtain the information necessary to solve a particular problem. This further application involves an important consideration, namely, whether a particular technique is non-destruc- tive or destructive of the specimen under examination.TABLE I1 TECHNIQUES FOR STUDY OF SURFACE COMPOSITION Technique X-ray diffraction Electron diffraction X-ray fluorescence Electron probe SEM + element ESCA Auger spectrxcopy Infrared reflection Pyrolysis gas chromatography Emission spectrography Spark-source mass spectrometry Radioactivation attachment Destructive (D) or non-destructive (ND) ND ND ND ND or D ND ND ND D D D D Depth sampled 0-1-10 pm 5-15 nm 10 pm Information obtainable Chemical identity of } crystalline species Elemental composition Elemental composition Elemental detection Chemical identity Detection of organics Elemental composition Elemental composition Elemental composition The last five techniques listed in Table I1 illustrate analytical tools of general application but which can be applied to studies of surface composition, and which are, in most instances, destructive of the sample.Of special interest to the surface analyst are the intrinsically surface techniques exemplified by those in the upper part of Table 11, all of which can be applied non-destructively, and all of which depend on probing the surface under study by X-ray or electron excitation.This last category of surface compositional techniques will be considered in more detail. Firstly, X-ray and electron diffraction, as noted above, can establish the chemical identity of any crystalline species; in this context these are qualitative techniques.Secondly, X-ray fluorescence, while commonly used to analyse bulk specimens, is in essence a surface technique as only a surface layer is examined, typically a few micrometres in depth. Precise composition of surface layers can be established if suitable means of calibration are available. Again, the fluorescence signal given by a thin surface layer, of a single element or of known composition, provides a rapid, non-destructive means of measuring layer thick- ness when this is appreciably below the “infinite thickness” value; calibration with films of known thickness is necessary. Fluorescent X-radiation as a quantitative signal for elemental composition is also given by the electron probe microanalyser, but the use of electron excitation gives a shallower probing depth, commonly 1-2 pm.The particular virtue of this technique is, of course, its ability to study the surface composition of very small areas, down to 1 pm2. Hence, changes in composition across a surface, arising from diffusion effects, inclusions or contamination, or formation of precipitated species in metals, etc., can be studied in fine spatial detail. Similarly, variations in composition at increasing depths can be studied by examination of a section cut through the material, although the technique is then necessarily destructive.A compar- able facility for microprobe analysis is afforded by a scanning electron microscope with an “element attachment,” in which a dispersive and/or non-dispersive system can be used for resolution and measurement of the fluorescent X-radiation emitted by the sample under the electron beam.There can, however, be advantages in having the microscope facility in the same instrument.176 SAC CENTENARY CELEBRATIONS Proc. Analyt. Div. Chem. SOC. Finally, electron spectroscopy for chemical analysis (ESCA) and Auger spectroscopy must be mentioned as two developing techniques that are potentially of great value to the surface analyst.Electron spectroscopy for chemical analysis involves measurement of the energy of electrons ejected from sample atoms on absorption of incident X-ray photons, while the Auger technique concerns measurement of the energy of electrons emitted by the Auger process from sample atoms irradiated with an electron beam; in both instances, the measured electron energies are diagnostic of the originating atoms.The ejected electrons can emerge from only very shallow surface layers without significant loss of energy, and hence both techniques examine very thin surface layers, typically 0-5-5 nm. The techniques are there- fore particularly suitable for identifying the elements present in very thin surface contami- nants.In addition, because of chemical shift effects, the measured electron energies may be used to deduce the state of chemical combination of the elements detected. The following examples illustrate some applications of these various compositional tech- niques. In connection with the previously mentioned studies of the process for producing ohmic layers on semi-conductor silicon, X-ray fluorescence has been used for non-destructive measurement of the thickness of platinum layers on silicon, over the range 40-160 nm; the calibration graph is linear over this range.The behaviour of platinum-coated tungsten at high temperatures has been studied by means of electron probe microanalysis suppiemented by X-ray diffraction; it was shown that prolonged operation at high temperature could lead to the formation of an interdiffusion region with two distinct zones, the one nearer the platinum comprising a platinum-rich solid solution and the underlying zone containing the intermetallic compound P3W.Use of electron spectroscopy for chemical analysis to study the surface composition of a “chalcogenide glass” containing tellurium, germanium and arsenic, revealed that of these three constituents only tellurium was detectable at the surface, and that the surface tellurium was partly in the oxide form.In many other such ways, the techniques described have been used to study surface composition, and its variation across and at varying depths beneath surfaces, in connection with a wide variety of technological problems.Conclusion The surface analyst now has a wide range of techniques to call on, and can today tackle many problems that would have been unanswerable five or ten years ago. Nevertheless, in choosing means for a particular investigation, the surface analyst may need to consider a number of factors, including not only the type of analytical information required, but also other aspects such as the depth of surface layers to be probed, and whether non-destructive techniques are essential.Such a combination of requirements may often severely restrict the choice of techniques applicable, or even render the problem insoluble. Hence, despite the substantial advances in recent years, there is likely to be a continuing need for new and improved techniques for surface analysis, not least in the certain knowledge that tomorrow’s problems will be even more difficult than today’s.References 1. Richards, B. P., Metal Finish. J., 1971, 17, 208. 2. Richards, B. P., Scobey, I. H., and Wallace, C. A., J . Appl. Crystallogr., 1974, 7, 275. 3. Callaghan, M. P., Patterson, E., Richards, B. P., and Wallace, C. A., J . Cryst. Growth, 1974, 22, 85.4. Falkener, K. R., Wickenden, D. K., Ishenvood, B. J., Richards, B. P., and Scobey, I. H., J . Muter. Sci., 1970, 5, 308. The Atomisation of Metal Oxides using Carbon Furnace Atomic-absorption Spectrometry J. M. Ottaway Department of Pure and Applied Chemistry, University of Strathclyde, Cathedral Street, Glasgow, G1 1XL The improvement in sensitivity obtained by replacing the flame in an atomic-absorption instrument with a carbon furnace or filament device makes this technique suitable for a wide range of new analytical applications. In many instances, direct analysis of a solution of the sample is possible in situations where previously a pre-concentration step was necessaryJune, 1975 SAC CENTENARY CELEBRATIONS 177 to bring the solution concentration within the range of flame absorption analysis.The simplification in analytical methods thus introduced reduces the length of analysis time and avoids operator errors and the possibility of contamination during lengthy chemical pro- cedures. However, the analytical advantages of these techniques can only be realised, and the need for separation procedures avoided, if interferences from the sample matrix are either negligible or can be adequately suppressed.In flames, interferences are minimised by preparing all operating solutions in a chloride medium and this procedure, together with the occasional use of releasing agents, helps to overcome the most prevalent type of chemical interferences, which are generally ascribed to the formation of stable oxides or mixed oxides.In considering the most likely interferences to be found during furnace and filament atomisation, one must take into account the fact that the chemical environment is different from that in a flame, and that the atomisation processes may also be very different. An understanding of these parameters may have an important influence on the choice of the sample solution applied to the furnace or filament atomiser .Although there is a reducing surface of hot carbon or tantalum, there is an almost complete lack of reducing environment inside a carbon furnace tube or above the surface of a filament. When argon or other inert gas is used as the atmosphere, only thermal energy radiated from the walls of the tube is available to dissociate molecules in the vapour phase.Therefore, if the sample is volatilised in molecular form from the surface of the atomiser it may then be difficult to dissociate the molecules into atoms. In flames, high concentrations of radicals and reducing molecules exist and are capable of bringing about rapid reduction or dissociation of molecules in the vapour ph.ase. Despite the reducing environment stable oxide com- pound formation is used to explain many interferences in flames, and it might be expected that these would be more prevalent in a flame atomiser where only thermal energy, of approximately the same temperature as the common flames, is available.It appears, how- ever, that there are fewer oxide type interferences in a furnace and it seems likely that the hot carbon (or tantalum) surface is providing an efficient reducing surface.In common with other workers, we in this department have found that interferences are much more prevalent when solutions prepared in a chloride medium are applied to a furnace or filament atomiser. Interferences are clearly reduced when solutions of salts such as nitrates or sulphates, which break down thermally via oxides, are used, in complete contrast to the behaviour found in flame systems.Evidence is available which suggests that use of chloride solutions results in the voltatilisation of molecular chlorides of many elements and that these are not subsequently atomised in the vapour phase. With many metals the use of nitrate solutions produces metal oxides, which remain on the surface of the atomiser until they are reduced to the metal and released as metal atoms.While it is possible that the metal oxides are decomposed thermally, it seems probable that many metal oxides will be converted into metal atoms by reaction with the graphite tube itself. We have considered the reaction as a possible process for the production of metal atoms (M) and have used thermodynamic data to calculate, for a range of metal oxides, the lowest temperature at which this reaction is thermodynamically favourab1e.l When these temperatures are compared with the lowest temperatures at which the same elements appear in atomic form in the graphite furnace atomiser a good correlation is found to exist for most elements,l which suggests that the above reaction is at least a possible process for the production of metal atoms under conditions in which oxides are formed, i.e., from nitrate or sulphate solutions.The above correlation can only be considered valid if it is assumed that when the appropriate temperature is reached, the rate of reaction of metal oxide with carbon is fast so that there is no delay in the appearance of metal atoms.This assumption is supported by the good agreement found for many elements and Fuller2 has now shown that a study of the kinetics of this process applied to copper solutions provides useful information on the shape of atomic-absorption peaks with respect to time. Many reported applications of carbon furnace or filament atomisers have been concerned with relatively pure aqueous solutions or with organic materials in which the matrix can be removed by ashing prior to atomisation of the metal.The apparent advantages of an oxide178 SAC CENTENARY CELEBRATIONS Proc. Annlyt. Div. Clzenz. SOC. matrix for reproducible and efficient atomisation have prompted us to investigate the analysis of trace constituents of complex inorganic materials by use of sample solutions prepared in oxy-anion media.Dissolution of samples in nitric or perchloric acids should lead to the formation of oxides on thermal decomposition but analysis at the 1 pg g-l level would require that the matrix showed no interference on the atomisation process for ratios of analyte to matrix up to 1 : 106. We have now shown this to be a realistic proposition in a number of applications using a carbon furnace atomiser.Concentrations of lead of 1-100 pg 8-1 in cast iron and steel can be determined without interference from the iron m a t r i ~ . ~ Cast iron and mild steels were dissolved in nitric acid, and stainless-steels in perchloric acid, both acid concentrations being brought to 8 per cent. V/V before application of the solutions to the carbon furnace.Detailed procedures have been given elsewhere ; the relative standard deviation was 3 per cent. at the 25 pg 8-l level and 13 per cent. at the 1.5 pg g-l level and the accuracy of the method was confirmed by comparison of the results with certificate values and values obtained by anodic stripping ~oltammetry.~ Similar procedures have been proposed for the determination of 1-1OOpgg-1 of lead in high-purity copper and copper alloys5 and for similar levels of lead in carbonate rocks.6 While the calcium oxide matrix caused no interference, the copper matrix gave a depression of about 25 per cent.and also produced a background signal during lead atomisation. These effects necessitated the addition of a matching level of high-purity (ASARCO) copper to the standard solutions during the analysis of lead in copper.In both instances a similar precision was obtained and the accuracy of the methods was confirmed by comparison with certificate and other results. The determination of 1-100 pug g-l of soluble and insoluble aluminium in steel has also been carried out. On dissolving the acid-soluble portion of the aluminium in nitric acid, the iron matrix also passes into solution.Aluminium requires a much higher temperature than lead for atomisation in the carbon furnace and a temperature of 2600 "C was used in our work. Some of the iron can be removed by charring the sample, at 1600 "C, in the furnace prior to atomisation but a residual background signal is still obtained during atomisation of aluminium.Efficient background correction is theref ore necessary. The acid-insoluble portion is dissolved by fusion with sodium carbonate - sodium tetraborate and the resulting melt taken up in nitric acid for analysis. No iron matrix is present in this solution and background correction is not necessary. The total aluminium content can be obtained by combining the two solutions for analysis. Results of acceptable accuracy and precision have been obtained and have been published elsewhere.' Recently we have also carried out preliminary investigations into the possibility of adding solid samples of oxide materials directly to the carbon furnace. An interesting application is to the determination of lead in dust produced during the production of sinter. Initial results are encouraging, the relative standard deviation being about 5 per cent. at the 0.0024 per cent. of lead level and there is good agreement with results obtained after dissolution of the samples. It appears to us that the carbon furnace technique of atomisation offers distinct advantages in the analysis, by means of atomic absorption, of trace constituents of complex inorganic materials and further work in this field will no doubt produce many methods of wide applica- tion in industrial analysis. Detailed studies are in progress. References 1. Campbell, W. C., and Ottaway, J. M., Talanta, 1974, 21, 837. 2. Fuller, C. W., Analyst, 1974, 99, 739. 3. Shaw, F., and Ottaway, J. M., Analyst, 1974, 99, 184. 4. Metters, B., and Cooksey. B. G., Analyst, 1974, 99, 457. 5. Shaw, F., and Ottaway, J. M., Atom. Absorption Newsl., 1974, 13, 77. 6. Campbell, W. C., and Ottaway, J. M., Trans. Instn Min. Metall. Sect. B, 1974, 83, 68. 7. Shaw, F., and Ottaway, J. M., Analyst, 1975, 100, 217.

ISSN:0306-1396    

DOI:10.1039/AD9751200172

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

June, 1975 THE RENAISSANCE OF POLAROGRAPHY 179 The Renaissance of Polarography The following are summaries of four of the papers presented a t a Meeting of the Analytical Division organised by the Electroanalytical Group held on February 5th, 1975, and reported in the February issue of Proceedings (p, 41). M odern Pola rog rap h ic Techniques G. C. Barker Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Oxfordshire, OX1 1 ORA .Well to the fore as regards analytical importance are square-wave polarography (SW)l and related techniques such as radiofrequency (RF) polarography,2 linear-scan voltammetry3 (LSV) with either a cathode-ray tube or pen recorder presentation, and differential (derivative) and normal pulse polarography.* Advances in the solid-state field have reduced the size and cost of electronic circuits and have made feasible the introduction of polarographs that operate in several modes but that are, in comparison with less versatile earlier instruments employing thermionic valves, inexpensive, compact and very reliable.There are available today solid-state polarographs, based on pulse polarography, which also give conventional or “tast” polarograms.Also, by recourse to anodic or cathodic stripping (with or without simultaneous selection of the differential pulse mode), enhanced sensitivity is obtained for metal cations, for certain organic and inorganic anions and also for some strongly adsorbed organic species (e.g., riboflavin). Such slightly extended pulse polarographs have proved very popular, partly because of their modest ccst and also because of their wide range of application.However, they do not by any means represent the ultimate in polarographic performance and often (but not invariably) greater selectivity and sensitivity can today be obtained essentially by combining one or more a.c. techniques with linear-scan voltammetry. The use of a potential scan that is sychronised with drop growth5 and occurs in the later part of the life of the mercury drop is important as it makes it easy to couple the polarograph to a digital data processor for the purposes of averaging data, to subtract stored averaged blank polarograms and to facilitate the separation of overlapping waves by subtracting waves stored within the digital stores of the processor.One such instrument is the Harwell Multi-mode Polarograph6 (MMP) . Multi - mode Polarograph This instrument can operate in four distinct modes to meet the growing needs of analysts in many scientific and industrial fields. The provision of four modes does not substantially increase the cost or complexity of the instrument as at least two thirds of the circuitry is common to all modes.The instrument offers reliable operation in the corrosive atmosphere of a chemical laboratory while giving a performance and versatility not hitherto achieved in the field of polarography. In all modes, the potential scan lasts for 6 s and starts 4 s after the detachment of the previous mercury drop. The 0-5 or 0.2 V potential scan may be anodic or cathodic and two- or three-electrode operation can be selected.More than one mode can be used simultaneously to give special types of selectivity in an instance in which none of the four basic modes gives the desired type of selectivity or resolution. For example, it might be advantageous to record an RF/SW polarogram7 to lower sensitivity for one of a pair of inter- fering electrode reactions.Further, it may in some instances be advantageous to make use of an integration facility that is available for most of the modes and that makes the instrument more suitable for the study of signals connected with the adsorption and desorption of organic species at the dropping-mercury electrode. Square-wave Mode (255 Hz) Apart from the sychronised potential scan and a significant improvement in the over-all noise factor, this mode resembles earlier square-wave polarographs.The sychronised scan helps to suppress base line variations connected with the capillary response2 and the detection of reversibly reduced species at concentrations below 2 X lo-* M is often a simple matter when the reaction involves two electrons. Resolution is good compared with that obtained by using a d.c.or cathode-ray polarograph and determinations can be carried out in the180 THE RENAISSANCE OF POLAROGRAPHY Proc. Analyt. Div. Chem. SOC. presence of up to a 105-fold greater concentration of a much more easily reduced major constituent. Sensitivity is impaired by irreversibility in the electrode reaction, a loss of a factor of the order of 20 being typical for highly irreversible processes.Sometimes such a large loss in sensitivity can be avoided by an increase in square-wave amplitude (up to 256 mV) . In general, digital processing of square-wave data gives, further sensitivity, enhanced precision and may greatly simplify the analysis of the polarographic data. Radiofrequency (RF) Mode This long known2 mode has hitherto been largely ignored by analysts because of a lack of instrumentation of sufficiently low noise level to realise the potential gain in performance that results when the normal square-wave voltage of the SW mode is replaced by a square- wave modulated high-frequency current.The present RF mode employs a modulated (225 Hz) polarising current of frequency 72 kHz and the normal SW circuits are employed to measure the signals produced by faradaic rectification at the surface of the dropping-mercury electrode.When the RF mode is selected, signals due to the capillary defect2 that produces the capillary response in square-wave polarography tend to vanish. Base line curvature is caused only by features of the dropping-mercury electrode that do not vary from drop to drop and these features do not affect the noise level.Although the measured rectification signal may be smaller than the corresponding square-wave signal, there is often nevertheless a significant improvement in long-term stability of the polarogram when the RF mode is selected and a gain in precision results in low-level determinations. The RF technique, although theoretically complicated, is experimentally very simple.Reducible species often can be detected at levels down to 2 x M if data are averaged over several scans and averaged “blank” recordings are subtracted with the aid of a digital data processor. A sometimes important feature of the RF mode is that it enables distinctions to be made between solutes reduced at similar poten- tials as wave symmetry is dependent on the kinetics of the reaction at the high frequency of the polarising current. This may lead to more certain identification of an organic depolariser. Also, waves of distorted but characteristic shape may be observed with some organic solutes involved in multi-step reactions when a very large polarising current is employed.Square-wave Intermodulation (S WIP) Mode This new mode (as regards instrumentations) relies on the occurrence of intermodulation between odd harmonics of the square-wave frequency when an electrode reaction is in progress on the surface of the dropping-mercury electrode.The SWIP polarogram shows the dependence on potential of the amplitude of a cell current component of twice the square-wave frequency.Sensitivity is very low both for totally irreversible reductions and for very reversible reduc- tions, the maximum sensitivity being observed with species that enter into slightly irreversible reactions at the dropping-mercury electrode. This very unusual type of selectivity can make the SWIP mode ideal for the detection of a minor constituent in the presence of relatively major reactions of the totally irreversible or highly reversible types.For example, zinc(I1) can easily be determined in 0.2 M hydrochloric acid at levels below 10-5 M even when the solution contains as much as 2.5 x 1 0 - 3 ~ of nickel(I1) (see Fig.1). Using the other modes of the instrument (or a pulse polarograph), it may even be difficult to separate the zinc(I1) wave from the signal owing to the reduction of the H,O+ ion and, even if this is not the case, the zinc wave rapidly becomes invisible on adding nickel (11) to the solution, as the results in Fig.1 clearly show. This mode seems to have many potential applications in organic analysis stemming from its novel type of kinetic selectivity. Linear-scan Voltammetric (LSV) Mode This rather pedestrian mode resembles in its performance cathode-ray polarography using a single cell, but the present LSV mode has the unusual advantages that current flowing prior to the start of the scan is automatically compensated and that fairly good semi-automatic compensation for the capacity charging current is provided. Anodic or cathodic stripping (e.g., of denatured DNA) can be carried out using this or any of the other major modes, but it is desirable when high sensitivity is required not to employ wax-impregnated carbon or even glassy carbon for the test electrode.Such stripping mayJune, 1975 THE RENAISSANCE OF POLAROGRAPHY 181 PotentiaVV vs. S.C.E. Fig. 1. SWIP, SW and LSV polarograms for 0.2 M hydrochloric acid containing M Zn(I1) and various amounts of Ni(II), showing the superiority of the SWIP mode for the detection of the Zn(I1) in the presence of two highly irreversible electrode reactions: SWIP, full curves; SW and LSV, broken curves.enhance the sensitivity of a mode by up to at least two orders of magnitude but care has to be taken to avoid erors due to surface contamination and calibration by the method of standard additions is advisable. An up-dated model of the MMP with a choice of square-wave frequencies is being developed.References 1. Barker, G. C., and Jenkins, I. L., Analyst, 1952, 77, 685. 2. Barker, G. C., Andytica Chim. Acta, 1958, 18, 118. 3. Randles, J. E. B., Trans. Faraday SOC., 1948, 44, 334. 4. Barker, G. C . and Gardner, A. W., AERE C/R 2297, H.M. Stationery Office, London, 1959. 5. Barker, G.C., Milner, G. W. C . , and Shalgosky, H. I., “Polarography,” Proceedings of Congress on 6. Barker, G. C., Gardner, A. W.. and Williams, M. J., J . Electroanalyt. Chem., 1973, 42, App. 21-26. 7. Niirnberg, H. W., Meeting of the Society for Analytical Chemistry, London, October loth, 1962. 8. Barker, G. C., in Hills, G. J., Editor, “Polarography 1964,” Volume 1, Macmillan, London, 1966, p.25. Modern Analytical Chemistry in Industry,” St.Andrews, 1957, p. 199. Voltammetry with the Carbon Wax Based Electrode D. R. Crow Department of Physical Sciences, The Palytechnic, Wolverhampton Of the various indicator electrodes based upon carbon which have been investigated in recent years, that formed by the dispersion of finely divided carbon in a wax matrix has much to recommend it. Unlike electrodes formed by coating or impregnation of graphite rods with wax and other rnaterial~,l-~ and those based on the formation of the carbon-wax electrode (C.W.E.) is easily prepared, does not involve difficulty with occluded oxygen and has a relatively long life.The essential material of the C.W.E. is a dispersion of acetylene black (1 g) in microcrystalline wax (5.5 g).A melt of this composition is introduced into a glass182 THE RENAISSANCE OF POLAROGRAPHY Proc. Annlyt. Div. Chem. SOC. tube of internal diameter about 3 mm to a depth of about 3 cm. After setting, the cylinder of solidified composite is pushed out of the tube to the extent of about 5 mm and a collar of heat- shrinkable tubing is shrunk over the end of the glass tube and the protruding composite. This design prevents the penetration of electrolyte solution between the inner wall of the glass tube and the carbon wax.8 Voltammograms have been obtained with the Beckman Electroscan 30 instrument, usually operating at a potential sweep-rate of 0.3 V min-l.A low-resistance calomel electrode is used as reference while a platinum counter electrode is included in the working cell in order to obviate the need for independent iR corrections to the applied potential.The voltammograms show the typically peaked form expected for solid electrodes under most conditions, the current decreasing rapidly beyond the peak potential to a limiting value controlled more closely by diffusion. In general, the available working potential range of the electrode is from + 1.3 to - 1.3 V, although the nature of the depolariser determines to a large extent the potential of the final current rise.In order to avoid permanent contamination of the electrode surface, it is necessary to strip anodically material that has previously been deposited cathodically and, in the case of many cationic species, it is sufficient simply to return the applied potential to zero for a few seconds in order to achieve this end.The run-to-run reproducibility of peak currents is very good provided that the conditions and mode of operation of the instrumentation are reproduced exactly. Depending on the depolariser, the reproducibility is about IfO.5-2.0 per cent. at M. Both peak and plateau currents are linear functions of depolariser concentration, the line for the latter cutting that for the former at concentrations for which the peaked wave form just disappears (Fig.1). 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ' Concentration/M x lo3 Fig. 1. Plots of peak (A) and plateau (B) currents vmus concentration for the cadmium aquo ion. It is not possible to reproduce physically identical electrodes a t present but, by reference to a standard depolariser solution, it is easy to calculate a conversion factor relating current to concentration from one electrode to another.The size of the faradaic signals in relation to the non-faradaic component makes it possible to perform an analysis, by direct voltammetry, in the region of M with comparative ease. With further refinements, this level can be extended by at least a further order of magnitude. The electrode has further shown its impressive features when used in an amperometric mode.For fixed sweep rate and depolariser concentration, the C. W.E. gives reproducible half-peak potentials (Eplz), some representative values being given in Table I, from which it can be seen that the values are of the order of about 0.2 V more negative than half-wave potentials observed at a dropping-mercury electrode.June, 1975 THE RENAISSANCE OF POLAROGRAPHY 183 Various equations have been proposed, as analogies to the Heyrovskg - IlkoviE equation of classical polarography, to describe the form of the peaked waves at solid electrodes.Solution of the kinetic equations at solid electrodes in terms of the surface concentration of depolariser suggests that a plot of log (i,--i)/i versus E should be linear and of slope un, /0-059 ( n a being the number of electrons transferred in the rate-determining step).Such plots are linear at the C.W.E. but do not have the expected slope. Further, values of n and anYta, can be determined independently from the following relationships due to Matsuda and Ayabeg : for reversible systems and for irreversible systems, a being the transfer coefficient... .. - - (1) ED - Ep12 = 0-048/ana .. .. .. . . (2) E, - ED,3 = 0*057/n .. TABLE I HALF-PEAK POTENTIALS (Ep,J FOR A RANGE OF DEPOLARISERS Depolariser E,[ JV 2’s. S.C. E. Cd2+ - 0.85 Pb2+ - 0.60 +0-15 Hg, Ag:+ 0.00 Co(en) :j3+ - 0.76 NiSCN - 0.97 Ni2+ N - 1.05 T1+ - 0.93 Zn2+ - 1.24 In Table I1 are given values of the slopes from logarithmic analysis and also n and ctna values, calculated from equations (1) and (2), for a number of depolarisers.TABLE I1 COMPARISON OF VALUES OF SLOPES OF LOGARITHMIC ANALYSIS WITH VALUES OF n AND una d log (7) Depolariser dE Cd2+ 27.1 Pb2+ 18-6 Zn2+ 27.1 Ti+ 42-0 Ag+ 40.7 d log (”-’> x 0.059 n an a dE [from eqn.(l)] [from eqn. (2)] 1-6 0.64 0.60 1.1 0.50 0.45 2-5 0.98 0.80 2.4 0.90 0.75 1.6 0.70 0 . 6 ~ Several points of interest arise from Table I1 : (a) n values for Cd2+, Pb2+ and Zn2+, obtained from equation (l), are obviously meaningless, the fractional values indicating a considerable degree of irreversibility. Thus, the values of avta in the last column are more appropriate.For T1+ and Ag+, a much greater degree of reversi- bility is indicated. (b) The slopes of the logarithmic plots for T1+ and Ag+ yield n values of 2-5 and 2.4, respec- tively, if the expected expression for the slope is used! (c) A graph of logarithmic slope versus independently determined a m (or n for almost reversibly reduced ions) is a fairly good straight line of slope 0.02.These observations suggest t,hat the equations to the waves are of the following forms: for for 3RT zp-2 E,,v,. = constant +- nF In (T) . . .. - (3) reversible reductions and EC.,,. = constant + -- .. .. W,F irreversible reductions.184 THE RENAISSANCE OF POLAROGRAPHY Proc. Analyt. Div. Chem. SOC. Photovoltammetric signals have also been investigated with the C.W.E.Irradiation of the carbon-wax electrode with an exposed surface machined to a 45” angle to the incident light is an easier process than that with a D.M.E. The photocurrents observed are of the order of 20 times larger than those observed with a D.M.E., are directly proportional to depolariser concentration and, in these respects, offer considerable analytical advantages in terms of enhanced faradaic signal and a very small non-faradaic signal.The variation of photocurrent with wavelength of light produces a “photopolarographic spectrum” with peaks occurring at the same wavelengths as are observed with the D.M.E. for a given depolariser. This confirms that mercury plays no part in the production of such spectra.10 In summary, the carbon-wax electrode is easily made, shows some unique electrochemical characteristics and is a sensitive analytical probe showing good reproducibility.The increasing development, use and investigation of such solid electrodes is one aspect of the “renaissance of polarography.” I acknowledge with thanks the contributory work of Mr. P. J. Stronach and Dr. J. G. Sharp. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. References Gaylor, V.F., Elving, P. J., and Conrad, A. L., Analyt. Chem., 1953, 25, 1078. Lord, S. S., and Rogers, L. B., Analyt. Chem., 1954, 26, 284. Elving, P. J., and Krivis, A. F., Analyt. Chem., 1958, 30, 1645. Adams, R. N., “Electrochemistry at Solid Electrodes,” Marcel Dekker, New York, 1969. Adams, R. N.. Analyt. Chem., 1958, 30, 1576. Galus, Z., and Adams, R. N., J . Amer.Chem. Soc., 1962, 84, 2061. Adams, R. N., Rev. Polavogr., Kyoto, 1963, 11, 71. Crow, D. R., and Stronach, P. J., J . Electroanalyt. Chem., 1975, 56, 209. Matsuda, H.. and Ayabe, Y., 2. Elektrochem., 1955, 59, 494. Crow, D. R., and Ling, S. L., J . Chem. SOC., Dalton Trans., 1972, 698. Pulse and A.C. Polarographic Techniques for Trace Analysis R. D. Jee University of London, School of Pharmacy, 29-39 Brunswick Square, London, WClN 1AX The renaissance in electroanalytical techniques over the last few years has largely been due to the commercial development of stripping voltammetry and differential pulse polarography.However, a.c. polarographic techniques, which possess many desirable characteristics, have been rather neglected. This is a pity as a.c. methods possess advantages such as low sensi- tivity to oxygen, the determination of reversibly reduced species in the presence of irreversibly reduced species of similar half-wave potential and the possibility of determining surface-active materials.The attractiveness of differential pulse polarography arises from its high sensi- tivity; however, it should not be forgotten that this can be obtained only by using very slow scan rates, so prolonging the time of ana1ysis.l The very nature of pulse techniques makes it difficult to speed up this process.In contrast, a.c. polarographic methods can provide equal or better sensitivities in much shorter times,2 particularly when applied in the fast sweep mode. This mode refers to the procedure in which a complete polarogram is recorded during the life of a single drop of mercury.A new drop is first allowed to grow for a preselected time and tnen the electrode potential rapidly scanned (e.g., 500 mV per 5 s). In principle, any a.c. technique could be applied in the fast sweep mode; however, those techniques which provide some form of discrimination against non-faradaic currents are to be preferred.Barker et aL2 have described a number of sophisticated techniques applied in this way. Two techniques that meet the above requirements and for which simple instru- mentation may be constructed from commercially available units are phase-sensitive funda- mental harmonic and second harmonic a.c. polarography. In the fundamental harmonic technique, the signal applied to the working electrode consists of a small-amplitude low- frequency a.c.voltage superimposed upon a d.c. voltage ramp. The a.c. current flowing through the cell at the same frequency as the applied signal is measured and, in the presence of a reversibly reduced species, a peak-shaped response is obtained. A large background current is also observed but by means of phase-sensitive detection it can be largely eliminated.In the second harmonic technique, the same signal is applied but the current flowing at twice the applied frequency is measured; this form of signal processing has the advantage of givingJune, 19 75 THE RENAISSANCE OF POLAROGRAPHY 185 only a very small background response. These techniques, when applied in the fast sweep mode, give limits of detection of about lo-' M and are therefore ideally suited for trace analy- tical purposes.Fast sweep a.c. polarography at a synchronised dropping-mercury electrode possesses many advantages over conventional polarography and, from a practical point of view, the speed of data acquisition, which allows rapid and easy optimisation of instrumental settings, is probably the most important factor.Polarograms recorded during the life of a single drop of mercury have certain characteristics that differ from those recorded in the conventional manner; some of these characteristics have important analytical consequences. One such property is their distorted shape due to the changing electrode surface area with time. The degree of distortion is not large, although it depends upon the scan rate employed, delay time and voltage scanned.The smaller the delay time and voltage scan and the larger the scan time, the greater is the distortion. Even under the worst conditions normally encountered, this distortion is unlikely to cause more than a 10 mV shift in peak potential or a change in half-peak width of more than about 2 mV, while under normal conditions the errors are less than 1 mV.In most instances, the finite response time of the electronics and recorder will introduce a greater degree of distortion. A characteristic that is of concern to the analyst is amplitude distortion. As the ax. polarographic response is proportional to the electrode area, the peak current will depend upon the time during the drop's life at which it occurs.Even with a constant delay time and scan rate, the drop area corresponding to a peak potential will depend upon the initial potential setting, the more positive the initial potential the larger is the observed peak (Fig. 1). Altering the initial potential can easily result in a change of peak current by a factor of 2 or 3, so that when carrying out quantitative measurements, standards and samples must all be measured using the same initial potential, delay time, scan rate and mercury reservoir height.Similarly for this reason, a three-electrode cell is to be preferred to a two-electrode cell. I C Potential + Fig. 1. Effect of initial potential: A, -0-60 V; B, -0.45 V; and C , -0.30 V (all values with respect to Ag- AgCl). Renzil (2 x M) in 0.1 M NaOH, 500 mV voltage scan of 10-s duration.Delay period, 3 s. Fundamental harm- onic a.c., 31-8 Hz. A third characteristic is that a polarogram recorded on the first scan generally gives larger peaks than subsequent scans because of a depletion of electroactive material in the vicinity of the electrode surface (Fig. 2). This effect increases with a decrease in delay time; however, the concentration of electroactive material within the vicinity of the electrode very quickly reaches an equilibrium value such that the second and all subsequent scans are virtually identical.This phenomenon will be of little concern to the analyst as the first few scans are normally used simply to check the correct functioning of the instrument. Electrode materials other than mercury have rarely been applied to pulse and a.c.techniques, the examples that do exist in the literature tending to be confusing as to their usefulness (see Bond3 and references therein). This most probably results from the difficulty in pro- ducing reproducible electrode surfaces and the general decrease in the reversibility of electrode186 THE RENAISSANCE OF POLAROGRAPHY Proc.Artalyt. Div. Chem. SOC. processes at solid electrodes. An electrode material that is at present receiving considerable attention is glassy carbon because of its very low permeability and large positive potential range. When a voltage pulse or step is applied to such an electrode, the resulting background current, however, decays only s l ~ w l y , ~ thus severely limiting the usefulness of pulse or diff- I Cd2+ I Potential --+ Fig.2. Repetitive scans : A, first scan; and B, second and subsequent scans. M) and Cd2+ (lo-* M) in 0.1 M KC1, 500 mV voltage scan of 5-s Fundamental harmonic a.c., 31-8 Hz; A E = Pb2+ duration. Delay period, 1 s. 8 mV. erential pulse techniques with such an electrode. Fig. 3 shows some voltammograms for benzil in 0.1 M sodium hydroxide solution at a glassy carbon electrode, from which the large background current can easily be seen.Increasing the pulse amplitude increases this current. The a.c. techniques also suffer considerably when applied to solid electrodes, although by a suitable choice of technique and such variables as phase angle and frequency the background can be considerably reduced.This is a field that requires considerably more work and that could produce some rewarding results. I F I 4 2 Po t e n t ia I + Fig. 3. Differential pulse voltam- metry a t a glassy carbon electrode. Benzil in 0.1 M NaOH. Bend con- centrations: A, 0; B, 5 x 10-6; C, 1 x and F, 2.5 x M. Initial potential, ' -0-55 V with respect to Ag- AgCl; pulse amplitude, 25 mV; and pulse repetition rate, 1 s .10-5; D. 1.5 x 10-5; E, 2 x 10-5; References 1. Christie, J. H., and Osteryoung, R. A., J . Electvoanalyt. Chem., 1974, 49, 301. 2. Barker, G. C., Gardner, A. W., and Williams, M. J., J . Electroanalyt. Chem., 1973, 42, App. 21. 3. Bond, A. M., Analytica Chim. Acta, 1975, 74, 163. 4. Blaedel, W. J.. and Jenkins, R. A., Analyt. Chem., 1974, 46, 1952.June, 1975 THE RENAISSANCE OF POLAROGRAPHY Some Recent Applications of Polarography to Drug Analysis 187 W.Franklin Smyth Department of Chemistry, Chelsea College, U?ziversity of London, Manresa Road, London, SW3 6 L X A survey of the literature relating to the polarographic analysis of compounds of pharmaceu- tical importance will reveal the frequency of use of the technique in the 1950s, with the publi- cation of authoritative works by Brezina and Zuman,l Volke2 and Z ~ m a n .~ These authors surveyed the polarographic behaviour and assays of the active constituents of many pharma- ceutical preparations in addition to reviewing the application of the technique to biochemistry and medicine. Even with the increase in the number of drug analyses in the 1960s, there was little interest in the application of polarography during this decade, especially in Western Europe and North America.This is not surprising when one considers the emergence of other instrumental methods, such as gas - liquid chromatography, which has the desired selectivity and sensitivity. It is only in the last five years or so that a renewal of interest has been detected in the literature. Apart from traditional d.c.polarography at the dropping-mercury electrode, of drugs with reducible groups, there have been applications of waves due to oxidation pro- cesses at indicator electrodes such as glassy carbon, rotating platinum, silicone rubber based graphite and carbon paste to the formulation analysis of drugs that do not undergo polaro- graphic reduction.This has made the technique applicable to a very wide range of pharma- ceutical preparations. Sensitive polarographic techniques such as differential pulse, cathode- ray and phase-sensitive a.c. polarography have been used in the assay of low levels of drugs and their metabolites in body fluids, principally utilising waves due to reduction processes at the dropping-mercury electrode.These techniques have found application in forensic and pharmacological investigations. Assessment of the Application of Polarography to Drug Analysis Those factors that determine the usefulness or otherwise of the technique in a particular analytical situation can be discussed under the following headings. Testing the Pure Compound for Polarographic Activity The polarographic characteristics of the drug, i.e., wave or peak height and half-wave or peak potential, should be ascertained at the dropping-mercury electrode and at a concen- tration of M in a variety of aqueous supporting electrolytes (0.1 N hydrochloric acid, acetate buffer pH 4.7, phosphate buffer pH 6.8, borate buffer pH 9.2,O-1 N sodium hydroxide solution, 0.1 N tetraalkylammonium perchlorate solution and 0.1 N tetraalkylammonium hydroxide solution).The best defined wave should then be selected and a calibration graph of current veism concentration constructed over a wide concentration range, e.g., 10-2-10-7 M for differential pulse polarography. If linearity is adhered to in the concentration range 10-3-10-4 M then this rangeis ideal for formulation analysis, and similarly the range lo-5-lo-6~ for forensic analysis and less than As the polaro- graphic behaviour of many organic compounds is fundamentally different in non-aqueous media it is useful to examine the drug by polarography in methanol and, say, acetonitrile containing 0.1 M tetraalkylammonium perchlorate as supporting electrolyte.If no suitable wave is found by using the dropping-mercury electrode, then the polarographic behaviour of the drug at a solid electrode such as glassy carbon or rotating platinum should be deter- mined in both aqueous and non-aqeuous media.Polarographic waves produced in non- aqueous media or at solid electrodes are generally only of use in formulation analysis, owing to the high blanks encountered at higher instrument sensitivities.If a suitable wave is produced at the dropping-mercury electrode and in an aqueous support- ing electrolyte, it is possible to follow the intact drug and, in some instances, intact metabolites, in trace analysis, thus avoiding derivative formation procedures or column degradation, which occurs with some gas - liquid chromatographic assays. An example is the analysis of some 1,4-benzodiazepines (I) in body fluids where the benzophenone (11) is prepared by acid hydrolysis prior to the “finish” by means of ultraviolet spectrophotometry, thin-layer chromatography or gas - liquid chromatography. When spectrofluorimetry is used for the M for pharmacological investigations.188 THE RENAISSANCE OF POLAROGRAPHY Proc.AnaZyt. Div. Chem.SOC. finish, a further derivative formation step is introduced by converting the benzophenone (11) to the 9-acridanone (111). Column degradation occurs by the loss of a water molecule from those benzodiazepines where R" = OH on analysis by gas - liquid chromatography. Sen- sitive polarographic methods can be used to determine all the members of this series as intact molecules with the de3ired sensitivity and, in some instances, selectivity.As the reducible azomethine group remains intact in the metabolites, the method can also be used to assay these species4 I II I l l Derivative Formation Various workers have converted drugs that do not have any polarographically reducible or oxidisable groups or for which the polarographic waves are unsuitable for analytical pur- poses, into derivatives that possess well defined polarographic waves.These procedures generally take less than 1 h and as the finish, operated at 25 "C, still avoids decomposition, the method compares favourably with other analytical methods involving derivative for- mation. This method has been applied to formulations for which the polarographic step has been carried out in the reaction mixtures in the absence of interfering reductions of constituents of this mixture. In some instances the reaction mixture acts as a supporting electrolyte.In the analysis of body fluids, it is necessary to separate the derivative from the reaction mixture by means of solvent extraction. Nitrosation and nitration of aromatic nuclei give rise to derivatives that possess compara- tively large and well defined reduction waves, which are ideally suited to quantitative analysis, particularly in alkaline media at pH greater than 10.For example, the phenyl ring in potassium penicillin G has been nitrosated by a method that requires 40 min and that has a detection limit of 1 pg ml-l in the reaction mixture, as determined by differential pulse p~larography.~ Also, drugs such as glutethimide (IV) ,6 phenobarbital (V)7 and diphenylhydantoin (VI) ,7 which contain phenyl rings, have been nitrated following extraction fiom biological media and the resulting derivatives subjected to polarography.Lauermann6 has applied this method to the determination of glutethimide (IV) in cadaverous material using d.c. polaro- graphy and Brooks et a1.' claim a sensitivity of 1-2 pg ml-l for the polarographic determination of V and VI in the blood of patients who were being administered these drugs.Bearing in mind that nitration of benzenoid compounds can give rise to a mixture of nitro-compounds, Brooks et aZ.7 studied different reaction conditions for the nitration and found that the largest and best defined peaks were produced after reaction for 1 h at 25 "C.H I 'QC2 H5 c€i H5 H I O y 7 C 2 H ' 6 H5 H" 0 H I IV V VI Other examples of derivative formation procedures that have been reported in the literature include the formation of addition compounds between keto-steroids and hydrazine derivatives. The derivatives resulting from these condensation reactions all possess a reducible azomet hine group and in the instances of the 4-nitrophenyl- and 2,4-dinitrophenylhydrazones, large, wellJune, 1975 THE RENAISSANCE OF POLAROGRAPHY 189 defined nitrobenzenoid reduction waves.With these latter derivatives, the reduction potential(s) of the hydrazines are well removed from those of the hydrazones, thus making it possible to measure the derivatives without separation from the reagents.Unfortunately, the 2,4-dinitrophenylhydrazones rapidly hydrolyse back to the corresponding keto-steroids in aqueous supporting electrolytes at trace levels, thus rendering the polarographic assay difficult .* Drugs containing secondary amines and tertiary amines have been converted to N-nitro- saminess and N-oxides,lo respectively, and the resulting reducible compounds used in for- mulation assays.As the polarographic waves of N-nitrosamines are somewhat ill defined owing to a complicated reduction mechanism, they are not recommended for use in trace analysis.ll Rapidity In the absence of separation procedures, a readout can be obtained in about 5-10min with an error of 2-3 per cent., well within the limits specified by the British Pharmacopoeia and adequate for the analysis of body fluids.Automation is possible for quality control work and some applications have already been reported in the literature.12-15 Many pharma- ceutical preparations that contain a reducible or oxidisable active ingredient together with non-electroactive excipients can be assayed in the presence of these excipients whether the latter are soluble or not in the chosen supporting electrolyte.The method of choice is to grind up the formulation in a mortar, mix it with a solvent (methanol or dimethylformamide for free bases and others, and water for the corresponding salts) in which the active con- stituent is soluble and then to dilute an aliquot of the supernatant with appropriate supporting electrolyte prior to polarography. However, for patent medicines with many active con- stituents, together with flavouring and colouring agents, separation prior to electroanalysis is usually a prerequisite.In the early stages of testing for the toxicity of potential pharmaceutical compounds in experimental animals, the compounds are frequently given orally when analytical methods of bioassay are under development, e.g., radiochemical methods.Thus, variations in the gastro-intestinal absorption and behavioural characteristics of these compounds as reflected by plasma levels cannot be monitored. Bearing in mind current awareness of the changes in the use of radioactive compounds and the need for a rapid “cold” method of bioassay, sen- sitive polarographic methods could be more extensively applied at this stage for those com- pounds which are electroactive.Sensitivity The inherent sensitivity of the method is an advantage in that single tablet assays and the determination of small amounts (less than 1 per cent.) of electroactive degradation products can be carried out. In differential pulse polarography, given adequate separation of E, values, these degradation products can be determined irrespective of whether the impurity peak occurs at a more or less negative peak potential than the parent drug.Polarography, under optimum conditions, has a sensitivity comparable to some gas - liquid chromatographic and spectrofluorimetric methods, Le., of the order of 20 ng ml-l, and this fact has been largely unappreciated by those involved in forensic and pharmacological in- vestigations on body fluids.This sensitivity is determined by (a) the background current in the supporting electrolyte due to reduction of trace metal ions, e.g., Pb2+, Zn2+ and Cu2+ (some of these interferences can be removed by the addition of 10-3 M EDTA or by electro- lysis of the supporting electrolyte at a large mercury pool), and (b) by the amount of co- extractable material from body fluids in the samples presented for electroanalysis.This usually results in the reduction peaks not being as smooth as those recorded in purely aqueous media and the peak heights not being as easy to measure. Non-polar solvents, e.g., benzene, extract significantly fewer interfering substances from body fluids and thus comparatively “clean” polarograms are obtained.This result can also be achieved by the addition of salts to the extract, e.g., potassium carbonate, followed by centrifugation. When the indicator electrode is immersed directly in the body fluid or in a mixture of this fluid and supporting electrolyte, the background current is a major interference and the sensitivity is reduced to the microgram per millilitre level, e.g., Jacobsen and Jacobsen15190 THE RENAISSANCE OF POLAROGRAPHY Proc.Analyt. Div. Chewz. Soc. detected 13 pg ml-l of chlordiazepoxide contained in a mixture of equal amounts of serum and 0.1 M sulphuric acid. Evidence was presented that both chlordiazepoxide and its reduction products are adsorbed at the electrode surface, the former more strongly than the serum proteins, thus making it possible to determine this benzodiazepine in the range 0.5-8 pg of nitrazepam per millilitre of serum, without the use of separation procedures.Selectivity The selectivity of the technique is best exploited with those polarographic techniques which produce a peaked response, e.g., a.c., cathode-ray and differential pulse polarography, where species that have E, values that differ by at least 40 mV can be resolved, assuming the re- duction peaks to be sharp and well defined.For example, the active constituents of multi- vitamin preparations possess a wide range of Ep values, thus facilitating the simultaneous determination of individual vitamins in these preparations without the use of separation procedures. Schertel and Sheppard16 simultaneously determined riboflavine, thiamine hydrochloride and nicotinamide in such a preparation by means of cathode-ray polarography.Also, Bieder and Brunel17 have applied rapid a.c. polarography to the study of the urinary excretion of the metabolites of ethionamide (VII) and prothionamide (VIII) in man. In the potentialregion,-0.7 to - 1.3 V, the parent compound and the threemetabolites (the sulphoxide, the 2-carbamoyl-4-pyridine and the 2-isonicotinic acid derivatives) were simultaneously determined.VII, R = C,H, VIII, R = C,H, Similarly, Groves and Smythl* have shown that benzodiazepines (I) with R” = H, R”’ = F and R”” = C1 and with different R’ substituents have marked differences in E, values in alkaline media at pH equal to or greater than 12.This allows for polarographic resolution of metabolites, impossible with any other supporting electrolytes, e.g., for R’ = -CH, CH, OH, E, = -1.16 V; for R‘ = -H, E, = -1.26 V; supporting electrolyte, 1 N sodium hydroxide solution. This is due to the polarographic manifestation of an acid - base equilibrium in the instance of the latter metabolite. Specificity Although polarography can be used to provide information on the functional groups present in the drug and, in some instances, the molecular environment of the functional group, it has so far not been possible to use the technique to establish the identity of the compound as can be done with infrared spectroscopy and with mass spectrometry. It is therefore best used for the assay of drugs in formulations and body fluids, given some idea as to the identity of the compound.Recently, there have been attempts to correlate physico-chemical para- meters, e.g., charge density on atoms in the electroactive groupls and Hammett CT functions,20 with E, or E, values in series of closely related compounds that include the parent drug and metabolites. It is hoped by this means to identify more positively than was previously possible an unknown electroactive metabolite. Much work remains to be done in this field. For a complete evaluation of the applicability of polarography to drug analysis, it is worth- while mentioning that significant financial, and a fair measure of intellectual, interest in the Chemistry Department, Chelsea College, has been shown by the drug industry and other interested non-academic bodies over the past five years, five years in which a renaissance has actually been observed by higher echelons in the same establishment.21 References 1. Brezina, M., and Zuman, P., “Polarography in Medicine, Biochemistry and Pharmacy,” Interscience 2. Volke, J ., “Die Polarographie in der Chemotherapie, Biochemie und Biologie,” Abhandlung der DAW, Berlin, 1964; Jena, 1962. Publishers, New York, 1958.June, 1975 CORRESPONDENCE 191 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. Zuman, P., “Organic Polarographic Analysis,” Pergamon Press, Oxford, 1965. Clifford, J . M., and Smyth, W. F., Analyst, 1974, 99, 241. PAR Application Note AN-111, Princeton Applied Research, Princeton, N. J., 1973. Lauermann, I., Arch. Tox., 1973, 31, 81. Brooks, M. A., de Silva, J. A. F., and Hackman, M. R., Analytica Chim. Acta, 1973, 64, 165. Smyth, M., personal communication, Chelsea College, 1973. Burghardt, H., Dt. ApothZtg., 1968, 108, 1151. Oelschlager, H., and Hoffmann, H., Arch. Pharm., Bed., 1966, 299, 1025. Smyth, W. F., Watkiss, P., Hanley, H. O., and Burmicz, J., Analytica Chim. Acta, to be published. Silvestri, S., Pharm. Acta Helv., 1972, 47, 209. Bargagna Prunai, P., Cinci, A., and Silvestri, S., Farmaco, E d . Prat., 1972, 27, 89. Cullen, L. F., Brindle, M. P., and Papariello, G. J., J . Pharm. Sci., 1973, 62, 1708. Jacobsen, E., and Jacobsen, T. V., Analytica Chim. Acta, 1971, 55, 293. Schertel, M. E., and Sheppard, A. J., J . Pharm. Sci., 1971, 60, 1070. Bieder, A., and Brunel, P., Annls Pharm. Fr., 1971, 29, 461. Groves, J . A., and Smyth. W. F., to be published. Smyth, M. R., Smyth, W. F., and Barrett, J.. to be published. Brooks, M. A., Be1 Bruno, J. J., de Silva, J. A. F., and Hackman, M. R., Analytica Chzm. Acta, in the Dalziel, J . A. W., personal communication, Chelsea College, 1975. press.

ISSN:0306-1396    

DOI:10.1039/AD9751200179

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

June, 1975 CORRESPONDENCE 191 Recording of Centenary Video Recordings of the Opening Ceremony of the SAC Centenary Celebrations The first part of the Opening Ceremony at the Royal Institution, London, on July 16th, 1974, was recorded on videotape. The recording includes the President’s introduction, the open- ing address by Sir Alan Hodgkin and the presentation of greetings by fraternal delegates.Copies of this recording have been made on the following systems : 1-in Ampex (low band), recorded on Ampex 5803 (also suitable for Ampex 7003 and 5103) &in Sony reels, CV 2100 ACE(H) $-in Philips Cassettes, N 1500. The recording is in black and white, and neither the picture nor the sound is of high quality. I t may, however, serve to remind members of an impressive occasion. The recording lasts for about 1 hour, and may be borrowed by Regions and Groups of the Division, free of charge, from the Secretary of the Ana- lytical Division. The Region or Group which borrows a recording should specify the system required, and will be responsible for the safety and speedy return of the tape. Please note that the tapes are rather heavy for despatch by post ; if possible, arrangements should be made to collect them from the Secretary’s office.

ISSN:0306-1396    

DOI:10.1039/AD975120191b

出版商:RSC    

年代:1975

数据来源: RSC