ISSN:0144-557X    

DOI:10.1039/AP99330FX017

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

May 1993 Analytical Proceedings ANPRDI 30(5) 213-240 (1993) Proceedings of the Analytical Division of The Royal Society of Chemistry CONTENTS 213 Annual General Meeting of the Analytical Division 213 ;Report of Meeting 213 Errata 214 VAM Viewpoint 'Va I i dat i o n of Sa m pl i ng St r ateg i es' 215 Obituary 216 Analytical Viewpoint 'New Time-invariant Definition of Analytical Chemistry Aimed at Improving the Scope of Analytical Abstracts' by Tibor Braun 218 Robert Boyle Valedictory Lecture 'New Understandings Gained from Chemical and Electrochemical Investigation of Redox Reactions of Electron Transfer Metalloproteins and Enzymes: Relevance to Voltammetric (Amperometric) Biosensors' by A. M. Bond 'Light Microscopy: What Determines Image Quality?' by P.J. Evennett 'Microscopy of Dispersions' by J. McMahon 'Measurement of Foam Bubble Sizes' by E. G. Mahers and S. C. Joyce 'Measurement of Fractal Dimension' by Matt G. Reed 'New British Standard for the Determination of Particle Size Distribution Using 227 SUMMARIES OF PAPERS 227 Particle Characterisation by Microscopy and Image Analysis 227 227 228 228 228 Microscope and Image Analysis Methods' by Anthony P. Rood 229 'Towards National Measurement Accreditation Service Accreditation' by K. G. Brocklehurst 229 'Image Analysis. Its Application to the Characterization of Particle Shape' by K. Julian and S. L. Mills 233 AMC Meeting 233 'Analysis of Sulfadimidine in Medicated Animal Feeds' by Ian M. Barwick, Peter Warwick and Neil T.Crosby 235 Equipment News 238 Conferences and Meetings 238 Courses 239 Analytical Division Diary ... 111 ANALYTICAL PROCEEDINGS. MAY 1993. VOL 30 with no risk, no ~ressure, STN Mentor laboratory, a series of instructional software for IBM personal computers, is the best way to learn how to search scientific databases : You will learn in your own office, atyourown . You will be a re le to correct any errors, easily. You will NOT incur charges for online use (though the simulation is so realistic, you‘ll think you’re searching STN I nternationalm). You will choose from several sample databases, derived from CA5, INSPEC, and PHYSICS BRIEFS. Try the interactive approach to retrieva learnin 9 . online information Enquiries from Eire or UK, please telephone The Royal Society of Chemistry at 0223 420066 or telefax 0223 423623. Learn to search scientific databases no bnline charges! STN Mentor Laboratory i s produced by FIZ Karlsruhe for STN International in association with CAS ‘, a division of the American Chemical Society’.

ISSN:0144-557X    

DOI:10.1039/AP99330BX019

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 213 Errata The following corrrections should be Group, which is to be found on p. 182 of Mr. A. P. Hart. Under ‘Members of made to the item on the Annual General the April issue. Under ‘Vice-Chairman’ Committee’ Dr. Price should be replaced Meeting of the Particle Characterisation Dr. C . J . Price should be substituted for by Mr. D. Merrifield.

ISSN:0144-557X    

DOI:10.1039/AP993300213b

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

214 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 Validation of Sampling Strategies It is frequently said by analytical chemists that a result is only as good as the sample analysed. This is clear recognition of the fundamental importance of sample and sampling to the analytical measurement process. Indeed, a poor or inappropriate sample will completely negate any ana- lytical method, no matter how valid or innovative. The recognition that valid sampling underpins all valid analytical measure- ment has led to a project being under- taken within the DTI Valid Analytical Measurement Initiative to look at the problems associated with sampling. This project is being carried out by a consor- tium consisting of the Laboratory of the Government Chemist, UMIST and Cran- field Institute of Technology, with some input from ICI Engineering.The shape of the project was decided through a definition study carried out by Scientific Generics, whereby organiza- tions involved in the many and varied aspects of the sampling process were consulted. The project has been divided into three distinct sub-projects, although there is some degree of overlap and commonality between these sub-projects. The three sub-projects are: 0 Generic sampling principles. The sampling Club for heterogeneous and intrinsically difficult materials. 0 Protocols and guidelines for automated sampling. There is a considerable level of know- ledge and experience throughout the sam- pling community about best sampling practice in individual situations. Much of this knowledge is not written down, and can be applied to a number of situations.This project is addressing the problem of harnessing such knowledge and experi- ence and deriving a number of principles and guidelines which are relevant to a broad range of sample situations. The parameters being considered include phase, analyte concentration, rea- son for analysis, method of analysis, accuracy or precision needed, and eco- nomics of sampling. The guidelines which are produced from this project will be published by the Royal Society of Chemistry. This book will address the sampling parameters outlined above, and guide those involved in sampling to be aware of the influence of these parameters in order that a valid sample, fit for the purpose, is taken. It has often been stated that many sampling guides and standards are statistically based, and often incom- prehensible to both analysts and non- technical sampling staff.This project aims very much towards redressing this and producing literature which can be used by staff at all levels and of all backgrounds who are involved in the sampling process. It is also envisaged that, in collabor- ation with NAMAS, a new NIS (NAMAS Information Sheet) guide on sampling will be produced, based upon the results of the project. A further strand of work within this area is being carried out at UMIST on the development of a hypermedia-based expert system for sampling. This system will use ‘If . . . Then’ logic to guide the user in the development of sampling strategies in a wide spectrum of situations, and is designed to run on an industry standard 486 PC.The sampling club for heterogeneous and intrinsically difficult materials was launched last Autumn with meetings held in London and Manchester. The first meeting of the club proper was held in March, 1993, when the members agreed the initial work programme for the club. The purpose of the club is the transfer of technology between members who have, at first sight, significantly different experi- ences, and problems but, in fact have some similarities in this regard. For exam- ple, a problem in sampling in the nuclear industry may well parallel a problem in the food industry, and the solution is of equal relevance to both parties. The club follows a structured programme which is agreed by members, but which is expected to evolve as new members join.Club members also have access to a database of suppliers of equipment and services relating to sampling. It is also expected that club members will form consortia to carry out research projects into sampling problems of mutual interest. The third sub-project ‘Protocols and Guidelines for Automated Sampling’ addresses the application of automation to sampling processes. Automated Sam- pling systems do provide plenty of poten- tial pitfalls for chemical engineers and analytical chemists alike. Whether a sample is being taken in a chemical plant for routine quality control purposes or in the laboratory to introduce a sample into an instrument such as a gas chromato- graph, the need to validate is the same. Sampling systems need to take into account the chemical and physical environment at the point of sampling, and discrimination effects must be negated if the sample taken is to be representative.Given the wide variation in applications and conditions to which this applies, this is a considerable undertaking. The on-line and process control aspects are being considered separately from laboratory sampling, as the specific problems of these areas differ to some extent. A Technical Index will be produced detailing sampling systems, and giving guidance on the validation of sampling operations. The information will also be incorporated into the hypermedia-based expert system previously mentioned, and the reference text ‘Sampling Systems for Process Analysers’ by Cornish, Jepson and Smurthwaite will be substantially revised and expanded. This project will go a long way towards assuring quality and validity at that most fundamental step in the measurement chain -sampling. This should greatly improve the quality of analytical measure- ments and enable other problems in the measurement process to be highlighted without wondering ‘was the sample not valid?’ Useful Contacts Project Manager: Nick Boley, LGC, Queens Road, Tedd- ington, Middlesex TWll OLY (Tel: 081- 943-731 1). Generic Sampling Principles: Dr. Neil Crosby, LGC (Tel: 081-943-7343). Sampling Club: Ken Can-Brion, Cranfield Institute of Technology, Cranfield, Bedford MK43 OAL (Tel: 0234-754739). Derek Woods, LGC (Tel: 081-943-7432). Automated Sampling and Hypermedia Expert Sys tem : Dr. Paul Thomas, UMIST, Department of Instrumentation and Analytical Science, P.O. Box 88, Manchester M60 1QD (Tel: 061-200-4910).

ISSN:0144-557X    

DOI:10.1039/AP9933000214

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 215 Obituary Dr. Woiciech Vieth Dr. Woiciech Vieth died tragically and unexpectedly as the result of a heart attack on Tuesday, January 12th, the second day of the ‘European Winter Conference on Plasma Spectrochemistry’ in Granada, when he was just preparing his poster for presentation in the early morning. First-aid was immediately at hand from the conference authorities and resuscitation was started in the hospital as soon as possible, but any help was too late. His death was a big shock, not only for all participants of the conference but for all people knowing him and, mainly, for his family. He leaves behind his wife, Elzbieta, and their two children Piotr and Anna, for whom his sudden death was a very big loss. The organizers of the conference and his friends did their very best to help his family in this tragic moment.Many prob- lems were caused by the fact that he died in Spain, a foreign country, having come from the USA and holding only his native Polish citizenship. It took more than a week to overcome all of these problems, but finally the funeral took place in California, his chosen home, where his family is living. Woiciech Vieth was born on May 4th, 1947, in Radom (Poland). He finished his studies at the university in 1970 as one of the best students of his course. In 1974 he started working as a senior engineer in the Structural Research Department at *the Institute of Electronic Materials Techno- logy in Warsaw. Here he was engaged in studying thermal analysis for application in metallographic and crystallization pro- cesses.In 1976 he started lecturing as an ‘Assistant Professor’. In 1977 he was promoted to manager of the mass spectrometric laboratory in the Analytical Division of the Institute, where he came in contact with elemental mass spectrometry for the first time. In his new field of working, he gave proof of considerable talent, becoming an expert in spark source mass spectrometry (SSMS) in a very short time, as can be seen from a number of publications and presentations at conferences of those days. Very soon his main interest was in the development of procedures to improve sensitivity, accuracy and preci- sion in SSMS. With this primary aim, he started a model consideration of pro- cesses taking place in the spark source plasma and in the ion beam of the mass spectrometer.One result of this study was the development of a model calculation which, from a practical point of view, could improve the accuracy by a factor of 3 in the best cases. The results of these investigations were used for a doctoral thesis in 1986. I met him and his family in May, 1987, when he became a guest co-worker of our Institute in a project for the analytical characterization of pure metals. He was accompanied by his wife and his two children from the beginning. My family and I immediately made friends with them, and this was the beginning of a close friendship. In the analytical research work going on in our group he soon found his second love in elemental MS, viz., glow discharge mass spectrometry (GDMS).All his experience from SSMS could be utilized now for this really promising exciting technique, as he felt it, and that was the reason why it took only a very short time before he was fully accepted as a member of our working group with great influence on our work in GDMS. His friendly character and his open-minded nature made this co-operation fruitful and pleas- ing for all persons involved. We got to know him as a co-operative scientist, who worked with high motivation and con- centration for improvement and promo- tion of this new analytical technique. Again, his main interest was to study the physical processes taking place in the glow discharge plasma with the aim of improv- ing sensitivity and accuracy in GDMS on the basis of model calculations.One of his dreams, to work and to live in the USA, was realized by his first employment in California, in 1988, at Charles Evans and Associates in Red- wood City, CA, on the basis of his extraordinary knowledge and experience in solid-state mass spectrometry. Being engaged mainly in commercial analysis of different types of samples by GDMS, he was successful in working out new analyti- cal procedures, not only for conducting but also for semiconducting and even for non-conducting samples. His creativity contributed to a continuous progress in elemental mass spectrometry, expanding this technique to new applications. Although he had a full-time job in routine analysis he nevertheless managed to produce additionally some publications which cover fundamental aspects of GDMS. At the end of his too short life, a respectable scientific achievement is con- nected with his name. More than 20 publications with his authorship can be found in national and international jour- nals of analytical chemistry. We shall miss his presence and his contributions in national and international conferences. His early passing is deeply regretted by his many professional and personal friends, whose condolences are with his family. NORBERT JAKUBOWSKI Institut fur Specktrochemie und angewandte Spectroskopie, Dortmund, Germany

ISSN:0144-557X    

DOI:10.1039/AP9933000215

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

216 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 Analytical Viewpoint The following is a member of a continuing series of articles providing either a personal view of part of one discipline in analytical chemistry (its present state, where it may be leading, etc.), or a philosophical look at a topic of relevance to chemists in general or analytical chemists in particular. These contributions need not have been the subject of papers at Analytical Division Meetings. Persons wishing to provide an article for publication in this series are invited to contact the editor of Analytical Proceedings, who will be pleased to receive manuscripts or to discuss outline ideas with prospective authors. New Time-invariant Definition of Analytical Chemistry Aimed at Improving the Scope of Analytical Abstracts Ti bor Braun Department of Inorganic and Analytical Chemistry, L.Eotvos University, P.O. Box 123, 1443 Budapest, Hungary In a recent study’ the question of how true a mirror of the analytical literature the Analytical Abstracts database is was addressed. Analysis has shown that the Analytical Abstracts database is in general terms an appropriate reflection of the field of analytical chemistry as far as topical coverage and journal comprehensiveness is concerned. Some slight distortions, however, have been revealed. The main problem found was the only partial coverage of the contents of so called ‘titled’ analytical chemistry journals by Analytical Abstracts. The expression ‘titled’ analytical chemistry journals has been used to encompass those journals which have analytical chemistry or the name of an analytical sub-field in their title, and are totally and exclusively dedicated to the publication of papers report- ing original research on analytical chemistry.Table 1 presents a ranked list of these journals. The main reason for this distortion is believed to be the difficulty of defining the field of analytical chemistry. In the ‘Guide to Analytical Abstracts’ it is stated that ‘Analytical Abstracts is a monthly journal providing a complete current awareness service covering all aspects of analytical chemistry’. We largely agree with Tyson saying that ‘analytical chemistry, perhaps more than any of the other branches of chemistry, has suffered chronically from an identity crisis. Over the years, many eminent analytical chemists have attempted to provide definitions of the subject.(However) . . . the uncertainty principle of definitions oper- ates to make a scientific discipline appear more diffuse the closer it is examined’.2 In our aforementioned paper’ a selected list of definitions of analytical chemistry was presented. Recently a whole pot- Table 1 Ranking of ‘titled’ analytical chemistry journals* Rank 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Journal J . Chromatogr. Anal. Chem. Anal. Chirn. Acta Anal. Biochem. Fresenius’ J . Anal. Chem. Analyst (London) Bunseki Kagaku J . Assoc. Off. Anal. Chem. Talanta J . Liq. Chromatogr. Anal. Lett. Chromatographia J. Chromatogr. Sci. Chem. Anal. (Warsaw) Analusis J . Anal. Toxicol. J . Anal. At. Spectrom. Int.J. Environ. Anal. Chem. Trends Anal. Chem. Total No. of articles in AA 12 144 5 641 3 816 3 173 2 924 2 625 2 044 1 923 1 709 1538 1 405 1310 848 809 604 598 506 455 364 44 436 No. of articles published (SCIt database) 13 799 7 205 4 321 5 844 3 436 2 820 2 086 2 471 2 112 1 932 1 630 1 789 992 923 787 755 530 680 466 54 578 No. of articles not processed by AA 1655 1564 505 2 671 512 195 42 548 403 394 225 479 144 114 183 157 24 225 102 10 142 Percent of articles not processed by AA 12.0 21.7 11.7 45.7 14.9 6.9 2.0 22.7 19.1 20.4 13.8 26.9 14.5 12.4 23.3 20.9 4.5 33.1 21.9 18.6 * ‘Titled’ analytical chemistry journals not processed by the SCI database (e.g., Fenxi Huaxue and Zh. Anal. Khim.) are not included here. t Science Citation Index, ISI, Philadelphia.ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 Subscription Details JAASbase 1993 Updates f99.00/$218.00 JAASbase Backfile (1987-1992) &230.00/$506.00 Idealist Software f 2 10.00/$462 .OO Six updates will be issued at regular intervals through 1993.217 Special Introductory Offer Take out a subscription to JAASbase Updates, buy the JAASbase Backfile and receive Zdealist absolutely free! Offer available only until July 1993. pourri of excellent definitions was published in a dedicated issue of the Fresenius’ J. Anal. Chern.3-12 Tyson has circum- vented the definition uncertainty problem by adopting a time- invariant definition according to which ‘analytical chemistry is what analytical chemists do’.* We would like to suggest here a definition which approaches the topic from the point of view of chemical information science.According to it ‘analytical chemistry is basically the knowledge included in peer-reviewed primary journals having analytical chemistry or the name of a sub-field of analytical chemistry in their title (i.e., “titled analytical journals”)’. As an explanation, it could be added that there is no more competent medium for evaluating what belongs to the field of analytical chemistry than the editorial advisory boards which are keeping the gatesI3 of the above journals. As in Tyson’s definition where it is quite clear that not everything analytical chemists do is analytical chemistry, it must be emphasized that although everything published in ‘titled’ analytical journals belongs to analytical chemistry, there is additional analytical chemistry published outside the titled analytical journals.An immediate pragmatic application of our definition would be its adoption by the compilers of Analytical Abstracts. This would make Analytical Abstracts a more true mirror of the field it purports to deal with. 1 2 3 4 5 6 7 8 9 10 11 12 13 Note. References Braun, T., Glanzel, W., Maczelka, H., and Zsindely, S., J. Chem. Inf. Comput. Sci., 1993, 33, 164. Tyson, J. F., Analyst, 1992, 115, 881. Cammann, K., Fresenius’J. Anal. Chem., 1992, 343, 812. Valcarcel, M., Fresenius’ J. Anal. Chem., 1992, 343, 814. Zuckerman, A. M., Fresenius’J. Anal. Chem., 1992,343,817. Nan, Zhou, Fresenius’J. Anal. Chem., 1992, 343, 819. Koch, K. H., Fresenius’ J. Anal. Chem., 1992, 343, 821. Perez-Bustamante, J.A, Fresenius’ J. Anal. Chem., 1992, 343, 823. Ortner, H. M., Fresenius’ J . Anal. Chem., 1992, 343, 825. Danzer, K., Fresenius’ J. Anal. Chem., 1992, 343, 827. Green, J. D., Fresenius’J. Anal. Chem., 1992, 343, 829. Stulik, K., and Zyka, J., Fresenius’J. Anal. Chem., 1992,343, 832. Braun, T., and Bujdoso, E., Talanta, 1983,30, 161. The author has been informed that The Royal Society of Chemistry has decided to accept his suggestion and to begin to process ‘titled’ analytical chemistry journals in their entirety. JAASbase A unique database of atomic spectrometry reference information for the practising analyst JAASbase is a new PC-based product from the The database consists of listings of published Royal Society of Chemistry designed to meet every atomic spectrometry papers and conference atomic spectroscopist’s need for a comprehensive, papers, and includes tabulated information relating yet inexpensive source of current analytical atomic to the application of relevant techniques. The spectrometry information. It contains over 20,000 references are easily searched with the database regularly updated references compiled from the manager Idealist which also enables the addition of atomic spectrometry literature. personal data to the database. I 1 1

ISSN:0144-557X    

DOI:10.1039/AP9933000216

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

218 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 Robert Boyle Valedictory Lecture The following paper is a summary of a lecture given by the Royal Society of Chemistry 150th Anniversary Robert Boyle Fellow in Analytical Chemistry in the Scientific Societies Lecture Theatre, London W1, on November 28th, 1991. New Understandings Gained from Chemical and Electrochemical Investigations of Redox Reactions of Electron Transfer Metalloproteins and Enzymes: Relevance to Voltammetric (Amperometric) Bissensors A. M. Bond* The Royal Society of Chemistry 150th Anniversary Robert Boyle Fellow in Analytical Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX? 3QR Introduction It is a great pleasure to be given the opportunity to present a summary of the Robert Boyle Valedictory Lecture presented in London on November 28th, 1991.This lecture formed the final formal activity associated with the Award of the Robert Boyle Fellowship in Analytical Chemistry by the Royal Society of Chemistry on the occasion of the 150th Anniversary of the Society and, coincidentally, the 300th Anniversary of the death of Robert Boyle. Before presenting the scientific component of my lecture, I would like to report how much I enjoyed my period in 1991 as the Robert Boyle Fellow and also take the opportunity to express again my appreciation to the Trustees of the Royal Society of Chemistry Analytical Chemistry Trust for making the award, Allen Hill and colleagues at the Inorganic Chemistry Laboratory at Oxford University and the Queen's College for acting as gracious hosts for the duration of the award, and to my own University, La Trobe University in Australia, for granting leave to take up the Fellowship.The Valedictory Lecture focused on both fundamental and applied aspects of the redox chemistry of metalloproteins and metalloenzymes. Initially, an overview of the knowledge currently available on electron transfer reactions of proteins and enzymes was presented. Both chemical and electrochemi- cal perspectives were considered and the interrelationships between the two approaches to study the redox chemistry was examined. A summary of this part of the address is given below in Section I. The full paper will be submitted for publication as a collaborative paper with H. A. 0. Hill. In the second stage of the Valedictory Lecture (Section 11), it was postulated that because voltammetric (amperometric) biosensors commonly utilize a combination of chemical and electrochemical reac- tions of proteins or enzymes attached to an electrode surface, the knowledge gained from the fundamental studies should lead to a much greater insight into mechanistic aspects of voltammetric based biosensors. In the applied sense, it was proposed that this knowledge should lead to systematic strategies becoming available, which should enable the per- formance of voltammetric biosensors to be improved in practical applications.The example of a voltammetric biosen- sor given to illustrate fundamental and applied aspects of the subject was the reduction of hydrogen peroxide at a cyto- chrome c peroxidase modified graphite electrode.This work * Permanent address: Department of Chemistry, La Trobe Univer- sity, Bundoora, Victoria 3083, Australia. constitutes a collaborative project which will be published in The Analyst under the authorship of F. A. Armstrong, A. M. Bond, F. Buchi, A. Hamnett, H. A. 0. Hill, A. M. Lannon, 0. C. Lettington and C . G. Zoski. I: Relationships of Chemical and Electrochemical Methods Used to Study the Redox Properties of Metalloproteins and Metalloenzymes As electron transfer reactions within and between metal- containing proteins and enzymes play such an important role in biological energy transduction processes such as photosyn- thesis and respiration, it is not surprising that there has been extensive interest over a long period of time in the study of metalloprotein and metalloenzyme redox chemistry. The process of moving an electron from one site (the donor) to another site (the acceptor) is a seemingly simple process in biological systems.However, the processes are exceedingly complex and many of the reactions involve rapid electron transfer through electron transfer chains over distances in excess of 1OA. Hence, despite the significant advances that have taken place in the understanding of the thermodynamics (free energy) and kinetics (rates and mechanisms) of biologi- cally important electron transfer reactions by ingenious appli- cation of a plethora of experimental approaches and theoreti- cal concepts, it is evident that a great many aspects of the processes still need to be unravelled.' A substantial component of existing knowledge on bidogi- cally important electron transfer reactions is based on the results of investigations of the redox chemistry of simple electron transfer proteins such as the iron-containing cyto- chrome c (Cyt..) proteins and the blue copper-containing plastocyanin (Pc) proteins.These proteins can be isolated from a range of sources in significant amounts and the study of homogeneous reactions with conventional inorganic redox reagents in a buffered aqueous solution phase has given rise to thermodynamically important standard (formal) redox poten- tials for the half-cell reactions: [Cyt.c, Fe"'] + e- [Cyt.c, Fe"] (1) and [PC, Cu"] + e- * [PC, Cu'] (2) where charges on the protein have been omitted for simplicity.Rate data are also available for many intermolecularANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 219 electron transfer reactions between proteins and inorganic redox reagents and the rate laws derived from them have given rise to the postulation of electron transfer reaction mech- anisms. Biologically important redox reactions have also been extensively studied and shown to involve electron transfer processes between different metalloproteins. For example, the reaction between cytochrome c and peroxide, which leads to the biologically harmful peroxide being converted into water, has been demonstrated to involve the formation of a cyto- chrome c-cytochrome c peroxidase complex. A model2 indi- cates that the complex is stabilized by specific salt bridges with the haems in parallel planes with an Fe-Fe distance of about 24 A and an edge-to-edge distance of about 18 A.The use of cytochrome c peroxidase to develop a biosensor to determine hydrogen peroxide will be described in Section 11. For even an elementary intermolecular homogeneous elec- tron transfer reaction, involving a metalloprotein and an inorganic redox reagent or two different metalloproteins, several steps will be required to achieve the transfer of an electron from a protein (designated species A) to the inorganic redox reagent or a different protein (designated species B) in the over-all reaction In many studies the kinetics of the over-all reaction given in eqn. (3) fit the second-order rate law expected for a simple outer sphere bimolecular process, i.e., a second-order rate constant is obtained having the units of 1 mol-' s-'.However, this deceptively simple rate law will almost certainly not be the result of an outer sphere reaction. Rather, it will correspond to a limiting case of a complex reaction scheme. For example, it may be postulated that even in a minimal reaction scheme, the initial step of (i) the diffusion of the reactants A and B is followed sequentially by: (ii) molecular interaction of the two reactants (donor and acceptor complexes) to form a precursor complex [AB] in which an optimum distance for electron transfer may be achieved [e n (4)], (iii) the elementary step of electron transfer to form [Aa;Bx-] [eqn. (5)], and finally (iv) dissociation of [AX+BX- ] [ e qn.(6)] to form the products of the over-all reaction Ax+ and BX-, which then diffuse back into the bulk solution kl A + B C [AB] k- 1 ket k-et A + B AX+ + BX- (3) (4) [AB] C [AX+BX-] ( 5 ) (6) kd k-d [ A ~ + B ~ - ] j( Ax+ + BX- Any of the steps in eqns. (4)-(6) characterized by forward and back homogeneous chemical rate constant k l , k - l , kd and k - d and forward and back electron transfer rate constants ket, k-,t, or indeed diffusion, may be rate limiting. Consequently, confusion can easily arise in unambiguously deconvoluting the elementary electron transfer step [eqn. (5)] characterized by ket and k-,, from the other reactions when, as is often the case, an over-all rate constant, kobs, is the physically measured parameter. Detailed discussion of the issues involved in correctly assigning mechanisms of electron transfer is available in refs.1 and 3. In an attempt to simplify the mechanistic interpretation of the electron transfer step, the synthesis of proteins containing a redox active centre at a fixed and known distance from the metal centre has been undertaken. In a complex of this type, and under ideal circumstances, intramolecular electron transfer between the added redox active site and the redox active metal centre of the metalloprotein can be studied in the absence of diffusion and other complications of the type represented in eqns. (4)-(6), which are present in intermolecu- lar electron transfer reactions. The original procedure' to synthesize modified proteins involved the direct reaction of aquapentaammineruthenium(l1) complex, designated as [aSRu(H20)], with the imidazole of a surface histidine, to give [a5Ru(histidine)], where charges have again been omitted for simplicity in writing the chemical formulae.Subsequently, many forms of modified protein have been prepared for studying biologically relevant electron transfer processes. In interpreting data obtained from intramolecular electron transfer studies on modified proteins, it is assumed that the structure of the surface modified protein is the same as that of the native protein and that the distance and the intervening medium associated with the electron transfer process are both known and can be deliberately varied by altering the site of attachment of the surface redox centre relative to the protein redox centre.Varying the site of attachment also enables orientation and free energy (driving force) effects on the rate of the electron transfer step to be studied. Consequently, modified proteins have become extremely important molecules for studying the mechanisms of biologically important electron transfer reactions, although there are still difficulties in the interpretation of much of the data, which will be considered in detail in a subsequent paper. Electrochemical studies of the redox chemistry of metallo- proteins, while usually reported in isolation to the chemical redox studies, are also very widespread and have also been extensively re~iewed.~ In the electrochemical technique, the electrode provides the source (reduction) or sink (oxidation) for electrons and variation of the applied potential provides the driving force which allows the redox reaction to occur.In a typical electrochemical reaction involving solution-soluble proteins it would be assumed that metalloproteins such as [Cyt.c, Fe"'] or [Pc, Cu"] diffuse towards the electrode surface until they are sufficiently close to the electrode (double layer region) to transfer electrons to the electrode, at which stage the proteins are reduced to [Cyt.c, Fe"] or [Pc, Cu'], etc. The product of the electrode process would then be assumed to diffuse away from the electrode surface and back into the bulk solution. In a mechanism of this type, the charge transfer process can be represented as in eqns. (7a) or (7b) k,, G, k,, a [Cytx, Fe"'] + e- [Cyt.c, Fe"] (74 [PC, CU"] + e- [PC, CU'] (7b) where I$' is the formal potential which ideally would be the standard redox potential or E" value, k, is the heterogeneous charge transfer rate constant at I$' and cx is the charge transfer coefficient .5 The complete electrode process for solution- soluble proteins involves the coupling of the diffusion compo- nent of the experiment with the electrochemically induced electron transfer step.The heterogeneous charge transfer rate constant is related to the driving force of the potential in a way predicted by the Butler-Volmer equation,' whereas k,, for homogeneous chemically based redox reactions is related to the driving force of free energy of the reaction by the Marcus relati~nship.~ Obviously, under some circumstances a correlation may be expected between k, and ket.However, it needs to be recognized that k, is a heterogeneous rate constant for electron transfer across an electrode/solution interface. As k, is a measure of a reaction that necessarily occurs between two phases, k, therefore has the units of m s-' (or cm s-l in much of the electrochemical literature) rather than s-' as is the case for k,, which is a homogeneous rate constant for a reaction in a single phase. For a reaction of the type given in eqn. (7), a close parallel with the intramolecular redox reaction in chemically modified proteins or reaction (5) in a homogeneous redox reaction could be expected provided the diffusion component can be correctly deconvoluted from the electron transfer step in the electrochemical method.Additionally, a correlation of k,220 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 with second-order kobs values would be expected if reactions mimicked a bimolecular outer sphere process, but as noted above, this may be unlikely, as protein redox reactions are more like inner sphere than outer sphere processes. Eqn. (7) and the above discussion refer to the common electrochemical method where diffusion of a solution-soluble protein and electron transfer are coupled. Alternatively, in electrochemical studies it is possible that the metalloprotein may be adsorbed or chemically attached to the electrode surface via, say, a lysine or a histidine group6 and the electrochemical electron transfer reaction studied as in eqn. (8) [Cytec, Fe"']attached -/- e- (E;))attached, (ks)attached, ((Y)attached In this instance, assuming the product of the electron transfer process remains attached to the electrode surface, then an even closer parallel with intramolecular electron transfer reactions of modified proteins may be expected to exist because in neither instance is diffusion from the bulk solution relevant. If the structures of the surface attached and bulk solution forms are similar, then the electrochemical parameters derived for the two types of electrode mechanism should be related to each other and to their chemical redox reaction counterparts.However, unfortunately, the structure of [Cytx, Fellr]attached or any other surface attached protein is often not the same as the solution-soluble native form and if structural changes occur on attachment to the electrode surface eqn.(8) may give rise to an (g)attached Value, which is distinctly different from as derived from eqn. (7). Similarly, (ks)attached and (a)attached values associated with eqn. (8) are not expected to be related to analogous parameters derived from eqn. (7) in any simple manner when the structure of the surface attached protein is distinctly different to that of the solution-soluble form. Obviously, an intermediate type of electrochemical mech- anism as given in eqn. (9) may also apply where electron transfer takes place in the surface confined or attached state, but where mass transport between the bulk solution and the electrode surface occurs by diffusion [Cyt*c, Fe"']bulk * [Cyt.c, Fe'"]attached [Cyt-c, Fe''']attached -k e- F= [Cyt-c, FeI'Iattached ( 9 4 (9b) ( 9 4 [Cyt.c, Fe"]attached * [Cyt-c, Fe"]bulk This latter type of mechanism was proposed by Albery et a1.' and may be the cause of ambiguities in deconvoluting the electrochemical electron transfer rate from diffusion and other steps as measuring k,, noted in chemical redox studies.At the Inaugural Robert Boyle Lecture given at the 150th Anniversary meeting of the Ro al Society of Chemistry, which has been published in full! the many nuances of the voltammetry of proteins and enzymes were described. In this earlier lecture, it was shown that a picture of ver fast rates of heterogeneous electron transfer (k, 2 0.1 cm s- ) is emerging from electrochemical studies of all the simple electron transfer proteins that have been studied in detailg ( e .g . , plastocyanin, cytochrome c, azurin, ferredoxin). This is the logical conclu- sion arising from the fact that current flow normally is detected in the voltammetry of metalloproteins at or near to the reversible potential if the protein is sufficiently pure and the electrode is suitable. A misunderstanding that no current or only a small current at the reversible potential corresponds to slow electron transfer seems to have been made via conclusions reached on the basis of a macroscopic model. Fig. l(a) and (b) contains a collection of examples of the voltammetry of metalloproteins where the current magnitude and wave shape are critically dependent on the nature of the electrode and solution conditions. In several of the examples shown, it is tempting to describe the differences in wave shape Y and current magnitudes as resulting from an increase or decrease in the rate of electron transfer as the solution or electrode surface conditions are modified.In fact, in each example presented in Fig. 1, detailed analysis reveals that if significant current is observed at the reversible potential it is the nature of the mass transport process that is altered and not the rate of electron transfer.' For example, in Fig. l ( a ) , the change from a peak to a sigmoidal shaped voltammetric response represents a decrease in the number of available electroactive sites at the edge plane relative to the basal plane graphite electrode. Fig. l(c) and (d) shows what would happen if a change in k, had actually occurred in the two extreme situations where the entire electrode surface is electroactive [Fig.1(c)] or an array of microscopically small electroactive sites are present [Fig. l(d)]. For homogeneous redox rate data, it has been found that kobs decreases on addition of redox inactive [Cr(NH3)6]3+ to a solution of ferredoxin (Fig. 2) when the reduced form of ferredoxin is oxidized by [CO(NH~)~(C~O~)]+. This effect on the rate data has been explained in terms of competitive binding and electrostatic considerations in which [Cr(NH3)6]3-t inhibits the electron transfer process by competitive binding at the same site as that occupied by [Co(NH3),(C204)]+. It is tempting to explain the changes in the voltammetry of ferredoxin on addition of [Cr(NH3)6I3+ in analogous terms and in some senses this concept can be useful.However, in the voltammetric case, the change from no response to sigmoidal to peak shaped with increasing concentration of [Cr(NH3)6]3+ when negatively charged ferredoxin is reduced at a graphite electrode (Fig. 3) [and vice versa for addition of [Cr(NH3)6I3+ to positively charged cytochrome c as in Fig. l(b) (3,4)] reflects the increasing or decreasing contribution of radial and linear diffusion to the mass transport process and not a change in k,.* With the microscopic model of electron transfer, the interpre- tation of the data is that no measurable current response at potentials near E" for the native form of the protein corresponds to a fully blocked or electroinactive surface. A sigmoidal response corresponds to a partially blocked surface and a peak shaped response to an almost completely electroac- tive surface with relatively few blocked sites.In this model, the presence of [CT(NH~)~]~+ on the electrode surface is assumed to create electroactive sites where reversible electron transfer may take place with ferredoxin, whereas these same sites are assumed to provide a blocked surface for positively charged cytochrome c. Electron transfer metalloproteins are in fact ideally designed to achieve fast rates of electron transfer. The current flowing at potentials near to the reversible potential of the native form could be regarded as the summation of the current flowing from all the fast reaction pathways for the native form of the metalloprotein. In this sense, metalloproteins appear to act like switches under voltammetric conditions with pathways for electron transfer being either extremely fast or extremely slow. However, with the data presented so far, it must be empha- sized that the deliberate choice of only providing results of measurements at or near reversible potentials of the native form of the metalloprotein has actually led to a de fact0 filtering of other electron transfer reaction pathways.In line with this argument, it logically follows that voltammetric studies also should include an examination of potentials well removed from the reversible potential. If this is carried out, additional valuable insights may be gained. For example, Fig. 4 shows that at a gold electrode in NaF electrolyte, reduction of adsorbed cytochrome c occurs at potentials that are several hundred millivolts more negative than the reversible potential of native cytochrome c.Similarly, at alkaline pH values, a well defined response (Fig. 5 ) is observed for reduction of cyto- chrome c at more negative potentials than for the native form, suggesting that the process at more negative potentials results from reduction of a structurally different form of cytochrome c. l4ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 221 I T (a; 0 versus SCE 0 PA a 2 1.0 Y t 0.5 3 F 0 x -1.0 .- 'j -0.5 2 U 1 100 mV H 0 versus SCE (7 1 .o 0.5 r=. I .2 0 1 - -0.5 -1.0 1 0 0.2 0.4 v1/2/(v s - 1 ) 1 / 2 M -0.05 0.0 +0.05 EN Linear limits El /2 Radial limits Lr-y7--] E 3 u I €4 €1 I2 (b) - m .- :t U Ph,Si-PGE Cr(NH,),-PGE PGE, pH 5.0 PGE + $1 z 1 I L u 11 t m - lli u 2.Q 4 0 0 0 Cyt.c L .f J t PG E PGEJ pH 4.0 Cr(NH,),-PGE PG E 2.0 pA I 1 OOHmV 10.25 pA 10.25 pA 10.5 pA 100 mV H 100 mV H 100 mV H I I I 1 I I 0.2 0.1 0.0 -0.1 -0.2 ( E - E")N Fig. 1 Range of shapes actually observed at a scan rate of 30 mV s-' (a,b) and those predicted (c,d) in the voltammetry of metalloproteins dissolved in aqueous media at a pyrolytic graphite electrode (PGE) according to the microscopic model described in detail in refs. 8 and 9. (a) Cytochrome c at (1) a polished edge plane electrode and (2) a freshly cleaved basal plane PGE. Also shown are a plot of peak current for the reduction process at the edge plane electrode versus the square root of the scan rate, d , verifying the predominance of linear diffusion at the polished edge plane surface and a plot of log [(id - i)/i] versus the potential E, verifying the predominance of radial diffusion at the basal plane electrode. Reproduced, with permission, from ref.9d. (b) (1) Cytochrome c at a Ph3Si-modified PGE; (2) as in (1) but at a bare PGE; (3) cytochrome c at a [Cr(NH3)6]3+-modified PGE; (4) as in (3) but at a bare PGE; (5) plastocyanin at a bare PGE at pH 5.0; (6) as in ( 5 ) but at pH 4.0; (7) plastocyanin at a bare PGE; (8) as in (7) but at a [Cr(NH3),I3+-modified PGE. Reproduced, with permission, from ref. 9d. (c) Effect of decreasing the rate of k, on transient cyclic voltammograms with mass transport by linear diffusion. 1, Reversible response or fast k,; 2 4 , progressively decreasing values of k,.See ref. 5 for further details. ( d ) Effect of decreasing the rate of k, on steady-state voltammograms obtained at a single microdisc electrode with mass transport by radial diffusion. 1, Reversible; 2, quasi-reversible; and 3, irreversible cases. Reproduced, with permission, from ref. 10222 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 1.6 I I 1.2 I rn n . 9 0.8 Fig. 2 Decrease of kobs values observed for the oxidation of parsley [Fe2S2]+ ferredoxin by a large excess of [Co(NH3)5(C,O,)]' in the presence of redox inactive [Cr(NH3),I3'. Reproduced, with per- mission, from ref. 11 t +d 2 3 u I -350 rnV versus NHE I 1 Potential - Fig. 3 Cyclic voltammograms at a scan rate of 20 mV s-l and temperature of 25 "C for reduction of pusteuriunum 2[4Fe-4S] ferre- doxin at an edge plane pyrolytic graphite electrode in the presence of increasing concentrations of [Cr(NH&I3'. Reproduced, with per- mission, from ref.12 18 N I Eu e 6 -0.5 0 E N versus SCE Fig. 4 Cyclic voltammograms at a scan rate of 300mVs-' of cytochrome c adsorbed onto a conventionally sized gold disc electrode in KC104 (solid line) and NaF (broken line) electrolytes. Reproduced, with permission, from ref. 13 The above overview was presented to illustrate the substan- tial versatility and commonality, and also some possible ambiguities associated with chemical and electrochemical methods of studying the redox properties of metalloproteins and enzymes. However, t o date, the results of the chemical and electrochemical studies have generally been considered in isolation.The initial aim of this Robert Boyle Valedictory Lecture was to present an overview of data obtained from the t Y 2 3 u -200 0 200 400 NmV versus SHE Fig. 5 Cyclic voltammogram (solid line) at a scan rate of 5 mV s-' and temperature of 25 "C for cytochrome c at pH 9.30 at a 4,4'- bipyridyl modified gold electrode showing the presence of two processes (designated as 1 for the native form and 2 for the base form). Broken line is for the electrolyte without cytochrome c. Reproduced, with permission, from ref. 14a two methodologies in an endeavour to demonstrate how results obtained from the two approaches are in fact mutually beneficial to the understanding of electron transfer reactions in a synergistic sense and to suggest that a great deal may be gained in future studies by concurrently undertaking redox studies by both chemical and electrochemical means rather than solely by one method or the other, as is the currently accepted practice.At the same time, the considerable ambigu- ities that have arisen in both data sets when convoluting factors other than electron transfer from the process were highlighted in the lecture in order to indicate how far we are from a complete understanding of this field even after considering the vast amount of data available for electron transfer reactions of simple metalloproteins. 11. Relationship of the Fundamental Studies to the Development of a Hydrogen Peroxide Biosensor Based on a Cytochrome c Peroxidase Modified Pyrolytic Graphite Electrode Fundamental aspects of both chemical and electrochemical redox chemistry usually need to be brought together in the operation of a voltammetric biosensor. An amalgamation of some of the ideas contained in Part I of the Valedictory Lecture was illustrated in Part I1 using the example of a hydrogen peroxide biosensor based on the reduction of hydrogen peroxide at a pyrolytic graphite electrode modified with cytochrome c peroxidase.The use of electrodes modified with redox active biologically important molecules represents a rapidly expanding area of analytical voltammetry (amperometry) . 15-17 These so-called voltammetric biosensors may achieve remarkable specificity for detection of their biological partners (or other molecules) and enhanced sensitivity by catalysing the analytically required process relative to that which occurs at a bare or unmodified electrode.However, despite the numerous papers and patents available, relatively few biosensors have achieved commercial success primarily because of their inherent instability on storage and in long-term use. It is, therefore, postulated that fundamental studies which provide a rational explanation for the instability may provide a key for improving their per- formance in practical applications. For the voltammetric determination of hydrogen peroxide, the reduction process at carbon electrodes is extremely irreversible and not particularly sensitive. l8?l9 Vacuum depo- sition of gold and palladium can be used to provide anANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 223 inorganic-based catalytic system for amperometric determi- nation of peroxide over the concentration range from 1 X to 1 x mol 1-'.l8 As hydrogen peroxide is an important entity in many biological processes, it is not surprising that electrode surface modification with proteins and enzymes has also been examined as a method of catalysing the reduction of hydrogen pero~ide.'~-'~>'~ The reduction of hydrogen peroxide at an electrode surface is expected to produce water via an over-all two-electron charge transfer process: However, at a bare pyrolytic graphite electrode no reduction of hydrogen peroxide is observed over the available potential range in aqueous media.Armstrong and Lannon2' and Paddock and Bowden*' have shown that the electrochemical reduction of hydrogen peroxide can be catalysed by attaching cytochrome c peroxidase to the pyrolytic graphite electrode surface.Cytochrome c peroxidase is a soluble enzyme of relative molecular mass 34 000 containing a single b-type haem group, for which the catalytic intermediates are reasonably well characterized.22 It has been established in homogeneous redox studies that cytochrome c peroxidase, in its iron(m) form, can rapidly reduce hydrogen peroxide to ate?*-*^ via formation of an oxyferryliron(1v) intermediate, followed by reaction with cytochrome c to regenerate the iron(1n) form of the catalyst. Consequently, when cytochrome c peroxidase is attached to a pyrolytic graphite electrode, the reduction process can be catalysed via an electrochemical analogue of the archetypal biological redox system.20 The enzyme/electrode interface, where transfer of electrons takes place, therefore acts as an electrochemical hydrogen peroxide biosensor.On the basis of existing knowledge, as summarized above, the important aspects of the electrochemistry leading to reduction of hydrogen peroxide can, therefore, be described in terms of three discrete redox steps: (lla) H202 + 2H+ + 2e- -+ 2H20 (10) Fe"' + H202 += {FeIV=O,R+} + H20 (FeIV=O,R+) + e- + FelV=O ( W FeIV=O + e- + 2H+ + Fe"' + H20 ( W H202 + 2H+ + 2e- 2H20 (114 Over-all in which the initial reaction of the Fe"' form of cytochrome c peroxidase with peroxide produces a two-equivalent oxidized intermediate containing ox ferrylhaem(1v) and a protein- bound cation radical (R+).2'h yeast mitochondria, the Fe"' state is regenerated, by two rapid reactions with reduced cytochrome c via the one-electron oxyferryl form.The over-all reaction is highly exergonic. At pH7, the formal (two- electron) reversible reduction potential for hydrogen peroxide is about 1.3 V versus the standard hydrogen electrode (SHE).26 Relevant reversible formal potentials for the inter- mediate couples [eqns. (llb) and (llc)] have not been determined but values can be expected to be of similar magnitude to those measured2' for horseradish peroxidase, i.e., each about 930 mV versus SHE. In contrast, the reversible reduction potential of cytochrome c is 260 mV versus SHE.28 Hence, the potential at which the irreversible electrochemical reduction of hydrogen peroxide is observed at a cytochrome c peroxidase modified graphite electrode at about 600 mV versus SHE is within the range of cytochrome c and cytochrome c peroxidase but is still considerably less positive than the reversible electrode potential expected in the absence of cytochrome c peroxidase. The over-all multi-step reduction process at a graphite electrode is highly irreversible, although the rate-determining process has not been established. Given the gaps in the existing state of knowledge, it is clear that a full theoretical treatment of the voltammetry is not possible.However, a partial description based on some of the principles established in Part I of the lecture can be given. The mechanism by which catalytic reduction takes place must consist of an extensive series of heterogeneous reactions at the electrode surface and homogeneous reactions in the solution phase.However, under some sets of conditions, the current observed for reduction of peroxide appears to attain a diffusion controlled value, as a plot of peak current under conditions of linear sweep voltammetry exhibits a square root dependence on scan rate. Further, the gradient of this linear plot leads to a calculated value of the diffusion coefficient of 7 x 10-6cm2s-1,20 which is in broad agreement with published values for hydrogen peroxide .29 In studies of electrode processes where the overpotential is reduced by surface attachment of a catalyst, a macroscopic model has been used to describe the mass transport-electron transfer process. For example, the diffusion coefficient calcula- tions cited above assume planar diffusion of hydrogen peroxide has occurred at a completely electroactive surface.However, in view of the very low concentration of catalyst used, a microscopic model of the mass transport process would seem to be more appropriate. For catalysis of hydrogen peroxide reduction by cytochrome c peroxidase, the concentration of the catalyst used in the bulk solution is typically less than 1 pmol I-'. This catalyst is then attached to a graphite electrode,20 which has a relatively large area of about 1 cm', so + - I - I I L P 300 500 700 900 300 500 700 900 EImV versus SHE Fig. 6 Linear sweep voltammograms at a scan rate of 8 mV s-l of solutions of H202 (56 pmol I-') in 0.1 mol I-' KCl (pH 7.0) at various times followin addition of cytochrome c peroxidase to a concentration of 0.2 pmol 1- (a) with scans initiated at 1, 30; 2, 178; 3, 325; 4, 750; and 5 , 1080s after addition of cytochrome c peroxidase and to a concentration of 0.5 pmol 1-' ( b ) with scans initiated at 1,60; 2,180; 3, 310; and 4, 710 s after addition of cytochrome c peroxidase ?224 -.t I *. .. - -..-. I I ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 5.0 4.5 18.2 3 18.0 17.8 17.8 17.6 a 17.4 17.2 0 1000 2000 0 1000 2000 I 1 5 I \=.I I I 1 0 1000 2000 I 1 ( f ) I 21.2 21.0 a 20.8 20.6 0 1000 2000 0 1000 2000 Time/s Fig. 7 Changes in ellipsometric parameters over the time course of voltammetric scans shown in Fig. 6(a) [(a)-(c)] and 6(6) [(d)-(f)], indicating times at which voltammograms were recorded. The electrode potential was held at +850 mV versus SHE that the area coverage of the graphite electrode, at least in some experiments, may be very low.If this is the situation, the electrochemistry may occur at microscopically small areas over the electrode surface rather than over the entire surface, in a manner analogous to that reported under some conditions for metalloproteins (Section I). Pyrolytic graphite has a highly ordered but anisotropic structure consisting of distinctly different edge and basal plane surfaces. In the presence of oxygen, the relatively reactive edge plane is oxidized and carbon-oxygen functional groups are formed.30 The surface oxidized functional groups impart considerable hydrophilicity and ionic character to the surface and this has been assumed to aid the binding of cytochrome c peroxidase to the edge plane pyrolytic graphite electrode surface.20 If this mechanism of attachment is correct, then the theory for the catalytic reduction of hydrogen peroxide should be based on a microscopic model of diffusion to localized active sites of microscopic dimensions in size, rather than a macro- scopic model of planar diffusion to a fully covered graphite electrode where the entire electrode surface, or at least large parts of the electrode surface, have been considered to be electroactive .Fig. 6(a) shows linear sweep voltammograms recorded at various times following the injection of 0.2 pmol I-' cyto- chrome c peroxidase into a solution of 56 mmol I-' hydrogen peroxide contained in an ellipsometry cell.At short times, a sigmoidal-shaped curve with a small limiting current is obtained, while at longer times a peak-shaped response is found. The ellipsometry of the electrode/solution interface was monitored, while stirring the solution and holding the elec- trode potential at 850 mV. Changes in the ellipsometry were measured as a function of time, and these are shown in Fig. 7(a)-(c). Intervals during which the voltammograms were recorded (in each instance with the stirrer switched off) are indicated. Although changes in the ellipsometric parameters occur over at least two time phases, development of the peak- like voltammetric waveform is complete within the time course of an initial large, dominant change (the first phase). This conclusion was substantiated by the results of several experi- ments in which the concentration of cytochrome c peroxidase was varied.The rate of development of the peak-like waveform (as evaluated from the increase in peak potential) and the rate of changes in the first phase of ellipsometry each increased as the enzyme concentration was raised. Figs. 6(b) and 7(d)-m show voltammograms and ellipsometric changes, respectively, that are obtained following the addition of cytochrome c peroxidase to a concentration of 0.5 pmol 1-'. With the concentration of enzyme as high as 1 pmol l-l, sharp peak-like voltammetry was observed on the first scan and the first, large phase of ellipsometric changes was complete after 2 min. In contrast, with 0.1 pmol 1-' enzyme, sigmoidal-shaped curves are dominant; the development of peak-like voltammetry was slow and incomplete, and the end of the first phase of ellipsometric changes was poorly defined.The transformation from a sigmoidal to peak-like waveform was analysed in terms of a microscopic array model in which catalytic reduction of hydrogen peroxide occurs only at the active sites of enzyme molecules that are adsorbed on the electrode surface. From examination of eqns. ( l l a ) , ( l l b ) and (llc), it is obvious that the process is complex and involves many steps. However, it is known in the limiting case that, assuming all the electrode surface is electroactive and using a model based on linear diffusion, a diffusion coefficient is calculated which is in agreement with that expected for hydrogen peroxide.It is also known that the over-all process is irreversible. Combining all the steps (homogeneous and225 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 0 I lo 0 0 ] I Electrode I Redox active site Redox inactive part of electrode 2 Fig. 8 Illustrations showing (a) a hexagonal array of microelectrodes of radius rt at time t , and spacing dt at time t and the overlap of diffusion layers which takes place (b) as time increases or (c) dt decreases. 6 is the diffusion layer thickness heterogeneous) enables a 'heterogeneous equivalent' descrip- tion of the type developed by Ruzic and Feldberg3'232 to be employed for this irreversible process. In the microscopic array model as applied to the peroxide biosensor, the enzyme molecules behave as if they adsorb as expanding clusters, i.e., generating microelectrodes of increas- ing size.The illustrations in Fig. 8 show an idealized (in this instance hexagonal) microelectrode array [Fig. 8(a)] and a representation of what occurs when the diffusion layers expand from the non-overlapping to overlapping situations as time increases [Fig. 8(b)] or as the inter-electrode spacing (dt) decreases [Fig. 8(c)]. The radius of each microelectrode (rt) and the value of dt are variable. When rt << dt, the diffusion layers generated for each microelectrode are isolated and do not overlap with each other. Under these conditions, and for an appropriate diffusion layer thickness ( 6 ) when radial diffusion is dominant, steady state is readily achieved and under these circumstances, a sigmoidal-shaped rather than a peak-shaped response is predicted.If the linear diffusion model had been employed assuming that the entire electrode surface was electroactive, then a peak- shaped response would have been expected for all times, i.e., the experimentally observed changes in wave shape and potential of the response would not have been predicted. Hence, despite the obvious approximations and uncertainties present in the microscopic model, the hypothesis of a time- dependent array model at least has the virtue of providing a - 2 -4 E .,o a -6 r, = 10 pm r, = 34 prn r, = 50 prn - r, = 65 pm -8 r, = 79 prn -10 1 I I I -400 -200 0 200 400 ( E - E ?,S,)/mV Fig. 9 Simulation showing the development of voltammograms for reduction of hydrogen peroxide with increasing size (radius rt) of a single electroactive cluster of cytochrome c peroxidase molecules on the electrode surface.Simulation parameters: DHzO2 = 1.1 X m2 s-l, cy = 0.5, nHzOZ = 2; the over-all process is assumed to be fully irreversible and to involve reduction of H202. i d i ~ is the steady- state diffusion controlled limiting current obtained when rt = 10 pm qualitative understanding of the voltammetry at both the early and later stages of the experiment. Fig. 9 shows simulation of the development of voltammo- grams at a single site as rt increases. Although there is qualitative similarity, in that there is a trend from sigmoidal to peak-like waveform, the quantitative likeness to experimental data is poor. However, much better agreement between experiment and theory is obtained when the more realistic case is considered, in which rt eventually becomes comparable to d, so that at longer times overlap of diffusion layers occurs, as illustrated in Fig.8. Fig. 10(a) and (b) shows simulations based on an array of microelectrodes with different parameters, using the theory of Gueshi et uZ.33-35 The transformation to a peak- like waveform is now in close agreement with the time- dependent change observed in the experiments. Applying the microscopic theory to the experiment described in Fig. 6(a), it can be calculated that an average effective radius of approxi- mately 4 pm (40000 A) is reached by the time of the first scan (initiated after 30s). As a hemisphere, this would comprise ( 2 d 3 ) x lo9 enzyme molecules of effective diameter approxi- mately 40 A.However, a more plausible situation is that rt corresponds to the radius of an expanding flat disc. This is consistent with the ellipsometry and is more realistic in view of the probability that the only cytochrome c peroxidase mol- ecules contributing to the activity are those that are able to provide unrestricted entry and exit for hydrogen peroxide. The above analysis has focused on the coincidence of the development of peak-like voltammetry with the first and dominant phase of the changes in ellipsometry. No completely convincing explanation for the subsequent further changes is available at present, although it is plausible that secondary adsorption is occurring, i.e., formation of a second layer of enzyme, perhaps over denatured, flattened primary adsorbed molecules.Secondary adsorption is expected to be counter- productive because second-layer enzyme molecules will not be able to receive electrons easily from the electrode and should impede the diffusion of substrate molecules to the more electroactive primary layer. The consequence of this is that the sites for hydrogen peroxide reduction are progressively de- stroyed. In support of this idea, it has been noted that over longer periods, the voltammetric peak broadens with some attenuation, and its position shifts back to more negative potentials. Eventually, a sigmoidal waveform, similar to that observed at very early times, is obtained. In the framework of the microscopic model, the system reverts back to being an array of isolated microelectrodes.In a practical biosensor, which could be used to determine226 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 0 - 40 - 80 - 120 - N 1 6 0 t , , , , 1 k -0.6 -0.4 -0.2 0 0.2 - 240 t I I I I 1 -0.5 -0.3 -0.1 0.1 0.3 ( E - E " ) N Fig. 10 Simulation showing the development of voltammograms for reduction of hydrogen peroxide at an electrode modified with clusters of cytochrome c peroxidase using the array model of Gueshi et (a) Parameters: ap arent k, = 1 x lo-' cm2 s-l, apparent Q: = 0.35, D H ~ o ~ = 1 X 10- cm2 s-l, nHzOz = 2, scan rate = 8 mV s-l, T = 278 K; i is normalized to an area of 1 cm2 and a concentration of 1 x mol l-', dt = 40 pm, r, and 0 (the fraction of surface that is electroinactive given by [(d: - r:)/d:]) are: (1) 00 pm, 0; (2) 33.5 pm, 0.3; (3) 20 pm, 0.75; (4) 12.6pm, 0.905; and ( 5 ) 1.8 pm, 0.988.(b) Parameters as for (a) except apparent 01 = 0.8; d, = 4 pm and r, and 0 are: (1) 00 pm, 0; (2) 2.8 pm, 0.5; (3) 0.69 pm, 0.970; and (4) 0.22 pm, 0.993 9 hydrogen peroxide routinely, it is clear from the above discussion that an optimum surface coverage of active enzyme must be maintained over long periods of time to achieve conditions of linear diffusion and, therefore, the analytically desired time-independent response and linear relationship between peak current and peroxide concentration. The present study reveals that lack of reproducibility found with many enzyme-based voltammetric biosensors when used or stored over long periods of time is likely to be the result of a time- dependent surface state, which leads to variable surface area and mixed modes of diffusion.It is hoped that the understand- ing gained from this study will eventually lead to concepts that will aid the development of reliable voltammetric biosensors, which have both longer storage and useable lieftimes than is the case with most currently reported systems. However, achieving a time-independent surface state with an enzyme modified electrode represents a considerable challenge, and developing voltammetric biosensors from laboratory curiosit- ies to commercially viable devices will probably always require considerable ingenuity. References 1 2 Bowler, B. E., Raphael, A. L., and Gray, H. B., Prog. Inorg. Chem., 1990, 38, 259, and references cited therein. See, for example, Poulos, T., and Finzel, B., in Peptide and Protein Reviews, ed.Heam, M. T. W., Marcel Dekker, New York, 1984, pp. 115-171. 3 See, for example, a Taube, H., Electron Transfer Reactions of Complex Ions in Solution, Academic Press, New York, 1970; Taube. H., Pure Appl. Chem., 1975, 44, 25; Haim, A., Acc. Chem. Res., 1975,8,264; Haim, A., Prog. Inorg. Chem., 1983, 30, 273; 'Marcus, R. A., and Sutin, N., Biochim. Biophys. Acta, 1985, 811, 265. 4 See, for example, Guo, L.-H., and Hill, H. A. O., Adv. Inorg. Chem., 1991, 36, 341, and references cited therein. 5 See, for example, a Bard, A. J., and Faulkner, L. R., Electrochemical Methods, Wiley, New York, 1980; Greef, R., Peat, R., Peter, L. M., Pletcher, D., and Robinson, J., Instrumental Methods in Electrochemistry, Ellis Horwood, Chichester, 1985, and references cited therein.6 Armstrong, F. A., a Struct. Bonding, 1990, 72, 137; Perspect. Bioinorg. Chem., 1991, 1, 141. 7 Albery, W. J., Eddowes, M. J., Hill, H. A. O., and Hillman, A. R., J. Am. Chem. SOC., 1981, 103, 3904. 8 Bond, A. M., Anal. Proc., 1992, 29, 132, and references cited therein. 9 a Bond, A. M., and Buchi, F. N., J. Electroanal. Chem., 1991, 314, 191; Bond, A. M., and Hill, H. A. O., Met. Ions Biol. Syst., 1991, 27, 431; 'Armstrong, F. A., Bond, A. M., Hill, H. A. O., Psalti, I. S. M., and Zoski, C. G., J. Phys. Chem., 1989, 93, 6485; dArmstrong, F. A., Bond, A. M., Hill, H. A. O., Oliver, B. N., and Psalti, I. S. M., J. Am. Chem. SOC., 1989, 111, 9185. 10 Bond, A. M., Oldham, K. B., and Zoski, C. G., Anal. Chim. Acta, 1989, 216, 177. 11 Sykes, A. G., Met. Ions Biol. Syst., 1991, 27, 291. 12 Armstrong, F. A., Cox, P. A., Hill, H. A. O., Lowe, V. J., and Oliver, B. N., J. Electroanal. Chem., 1987, 217, 331. 13 Hinnen, C., Parsons, R., and Niki, K., J. Electroanal. Chem., 1983, 147, 329. 14 a Haladjian, J., Pilard, R., Bianco, P., and Serre, P.-A., J. Electroanal. Chem., 1982, 141, 91; Rodrigues, C. G., Farchione, F., Wedd, A. G., and Bond, A. M., J. Electroanal. Chem., 1987, 218, 251. 15 Wilson and Wilson's Comprehensive Analytical Chemistry, ed. Svehla, G., Elsevier, Amsterdam, 1992, vol. 27. 16 Sensors (A Comprehensive Survey), eds. Gopel, W., Hesse, J . , and Zemel, J. N., VCH, Weinheim, 1992, vols. 2 and 3. 17 Bartlett, P. N., Tebbutt, P., and Whitaker, R. G., Prog. React. Kinet., 1991, 16, 55. 18 Gorton, L., and Svensson, T. J., Mol. Catal., 1986, 30, 49. 19 Higgins, I. J., and Hill, H. A. O., Essays in Biochemistry, 1985, 21, 119. 20 Armstrong, F. A., and Lannon, A. M., J. Am. Chem. SOC., 1987, 109, 7211. 21 Paddock, R. M., and Bowden, E. F., J. Electroanal. Chem., 1989, 260, 487. 22 Yonetani, T., in The Enzymes, ed. Boyer, F. O., Academic Press, New York, 1976, vol. 13, pp. 345-361. 23 Loo, S., and Erman, J. E., Biochemistry, 1975, 14, 3467. 24 Ho, P. S., Hoffman, B. M., Kang, C. H., and Margoliash, E., J. Biol. Chem., 1983, 258,4356. 25 Chance, M., Powers, L., Kumar, C., and Chance, B., Biochemistry, 1986, 25, 1259. 26 Hoare, J. P., in Encyclopedia of Electrochemistry of the Elements, ed., Bard, A. J., 1974, vol. 11, p. 194. 27 Hayashi, Y., and Yamazaki, I., J. Biol. Chem., 1979,254,9101. 28 Taniguchi, I., Funatsu, T., Iseki, M., Yamaguchi, H., and Yasukouchi, K., J. Electroanal. Chem., 1985, 193, 295. 29 Prabhu, V. G., Zarapkar, L. P., and Ohaneshauer, R. G., Electrochim. Acta, 1981, 26, 725. 30 Panzer, R. E., and Elving, P. J., Electrochim. Acta, 1975, 20, 635. 31 RuiiC, I., and Feldberg, S., J. Electroanal. Chem., 1974, 50, 153. 32 RuiiC, I., J. Electroanal. Chem., 1983, 144, 433. 33 Gueshi, T., Tokuda, K., and Matsuda, H., J. Electroanal. Chem., 1979, 89, 247. 34 Gueshi, T., Tokuda, K., and Matsuda, H., J. Electroanal. Chem., 1979, 101, 29. 35 Gueshi, T., Tokuda, K., and Matsuda, H., J. Electroanal. Chem., 1979, 102,41.

ISSN:0144-557X    

DOI:10.1039/AP9933000218

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 227 Particle Characterisation by Microscopy and Image Analysis The following are summaries of seven of the papers presented at a Meeting of the Particle Characterisation Group held on September 16, 1992, in The Heath, ICI Chemicals and Polymers Limited, Runcorn, Cheshire. Light Microscopy: What Determines Image Quality? P. J. Evennett Department of Pure and Applied Biology, The University, Leeds LS2 9JT It should be self-evident that any microscope image used for characterization or analysis should be a reliable and adequate representation of the features of interest in the original specimen. However, because of the apparent simplicity of the light microscope, and the fact that it produces some kind of image almost regardless of how badly it may be used, some workers use inadequate, inappropriate or even misleading images.The aim of this paper is to draw attention to the importance of producing a good image with the light micro- scope, to identify some of the fundamental and practical factors involved, and to demonstrate how and why these factors are import ant. It is more than a mere play on words to point out that when using any microscope it is not the specimen which is looked at, but an image of the specimen. This image is formed from information collected by the objective lens, following inter- actions between the specimen and the illuminating radiation, in this instance, light. The prime requirements of a microscope image are that it should contain information about fine detail in the specimen (resolution of this detail), that it should show adequate contrast between features of interest and the background, and that it be presented at a suitable mugn$cution to enable the eye to discern these features.Resolving power and fidelity of the image are influenced by fundamental factors, higher resolving power being provided by objectives of larger numerical aperture and light of shorter wavelength. Contrast may be generated by manipulation of the illuminating and imaging rays. The author is willing to provide further information on microscopy and simple demonstrations of these important phenomena. Microscopy of Dispersions J. McMahon ICI Chemicals and Polymers Ltd., Research and Technology Department, Runcorn, Cheshire WA7 4QD Dispersions are generally two-phase systems in which at least one dimension is in the colloidal range (i.e. , 1-lo00 nm). In such heterogeneous materials the large amount of interfacial free surface energy dictates the macroscopic physical properties. Consequently, small changes at the interface, for example the adsorption or desorption of a surfactant in a solid-liquid dispersion, can have a dramatic effect on bulk properties such as the rheology. Both the dispersed phase and the dispersion medium can be a solid, liquid or gas and the relationship between such parameters as structure, phase size, shape and degree of aggregation to physical properties is of great importance. Determination of dispersion morphology presents challeng- ing problems to the microscopist, particularly when either or both phases are liquids and when the method of preparation for electron microscopy can alter the very property to be deter- mined.For many dispersions optical microscopy gives much non-invasive information. However this can be a disadvantage when quantitative image analysis is required and particularly when the dispersed phase size is close to the resolution limit of the microscope or when the depth of field is too great. The higher resolution of the electron microscope allows, in many instances, better characterization of colloidal disper- sions, but inevitably results in more time-consuming methods. For solid-solid systems a combination of plasma etching and scanning electron microscopy (SEM)' ,2 or microtomy/trans- mission electron microscopy (TEM) is used and these methods are well known.Information on solid-liquid dispersions with TEM and negative or positive staining is easily obtained.' Other techniques for solid-liquid or liquid-liquid dispersions involve rapid freezing in order to stabilize the sample to vacuum conditions. These methods, fully discussed by Robards and S l e t ~ r , ~ are: (i) freeze-drying; (ii) freeze-fracture replica- tion; (iii) cryo-TEM; (iv) cryo-SEM. Each of these techniques has relative merits/demerits, not least of which is the freezing process itself which can dramatically alter the dispersion. Recently, by combining the high resolution obtainable in a field emission SEM with ultra-fine metal coating and an SEM cry0 stage it has been possible to obtain information on colloidal systems which previously would have required freeze- fracture replication.References 1 Hansen, R. H., Pascale, J . V., Benedictus, T. D., and Rentzepis, P. M., J . Polym. Sci., Part A , 1965, 3. 2205.228 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 2 Sawyer, L., and Grubb, D. T., Polymer Microscopy, Chapman and Hall, London, 1987. 3 Robards, A . W., and Sletyr, U. B . , Low Temperature Methodsin Biological Electron Microscopy, Elsevier, Amsterdam, 1985. Measurement of Foam Bubble Sizes E. G. Mahers and S. C. Joyce Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3J W It is extremely difficult to measure liquid foam bubble sizes from an image, which has a large depth of field, due to the semi-transparent nature of the material. One solution might be to use a confocal microscope to section the foam optically, however the bubbles in the foam we were investigating were too large for this technique.This depth of field problem was eliminated by imaging the foam in contact with a transparent, flat surface under conditions of total internal reflection. The resultant image only contained bubbles at the surface and was thus fairly easy to analyse. The surface was viewed at 45 degrees, which introduces a linear geometric distortion that can be easily corrected. This viewing angle also gives an image magnification factor that is a function of position, but which can be usually ignored. The measured bubble size distribution needed to be corrected in order to account for bias towards large bubbles occurring at the surface compared with a random section through the bulk material.’ Bubbles that touched the edges of the image were measured separately and included in the size distribution as ‘censored data’. This method is non-invasive and has allowed measurement of bubble sizes as a function of time or height in a column of foam.Reference 1 De Vries, A. J . , Recl. Trav. Chim. Pays-Bas, 1958, 77, 383. Measurement of Fractal Dimension Matt G. Reed Image Analysis Section, Shell Research Thornton, P.O. Box 1, Chester CHI 3SH ‘Fractals’ are a mathematical model of the symmetry of an object under a change of scale. Examples include the twig and branch structure of a tree or a coastline. In both instances, it is impossible to tell (without a scale bar) whether one is looking at a small- or large-scale picture of the object. The irregularities at the smaller scales have the same appearance as the irregularities at much larger scales.Fractal dimension can therefore be used as a numerical measure of an object’s ‘roughness’ or self-similarity . Fractal dimension is a widely used parameter which has been applied in fields as diverse as medicine, tribology, material science, chemistry, geology and information theory. Shell Research has several years experience in measuring fractal dimension using image analysis methods. An introduction to some of the more straightforward and intuitive aspects of fractal theory has been given. The different methods used at Shell Research Thornton for measuring fractal dimension have been reviewed. Bibliography The Fractal Approach to Heterogeneous Chemistry, ed. Avnir, D ., Wiley, New York, 1989. Mandelbrot, B. B., Fractals Form, Chance and Dimension, Freeman, San Francisco, 1977. Russ, J. C., Computer Assisted Microscopy, Plenum, New York, 1990. New British Distribution Anthony P. Rood Health and Safety Standard for the Determination of Particle Size Using Microscope and Image Analysis Methods Executive, Broad Lane, Sheffield S3 7HQ Work has just been completed on the updating of this standard which was originally drafted in the 1960s. Changes have been far reaching in that the original standard was based on manual sizing and counting using the light microscope, and the standard has now been extended to electron microscopy with image analysis techniques for both areas.Light microscopy is suitable for the measurement of particles in the size range from 3 pm to 1 mm, while scanning electronANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 229 microscopy (SEM) and transmission electron microscopy image analysis for particle measurement is that it retains the (TEM) can be used for smaller sizes down to 2 nm. Image absolute nature of the microscope method while reducing the analysis has removed a considerable amount of hard work and subjectivity and operator fatigue associated with this technique. has greatly improved measurements, but requires that the Details of the new standard, covering both automatic and sample be examined visually before embarking on sizing, to semi-automatic image analysis from the various microscopes avoid artifacts being counted.The main advantage of using have been discussed. Towards National Measurement Accreditation Service Accreditation K. G. Brocklehurst Research and Technology Department, ICI Chemicals and Polymers Ltd., The Heath, Runcorn, Cheshire WA7 400 Over the last few years, ICI Chemicals and Polymers has been committed to improving quality through numerous education programmes. The culmination of this has resulted in a declaration from the Chief Executive that all businesses and functions within the company will, wherever possible, achieve quality assurance under an external accredited scheme. The Analytical and Physical Sciences Group was the first to gain this recognition within the Research and Technology function at Runcorn in September 1992. This group consists of 16 analytical technique areas employing approximately 80 people in sections such as X-ray diffraction, elemental analysis and electron spectroscopy for chemical analysis (ESCA) that provide a service to Chemicals and Polymers in particular and on occasions to other parts of ICI.The steps necessary to gain National Measurement Accredi- tation Service (NAMAS) accreditation in an area such as Analytical and Physical Sciences have been discussed, using the image analysis unit as a typical example. The documen- tation process has been followed through from the writing of the quality manual to the final operating instructions for image analysis. The views and procedures discussed are ways in which ICI wish to operate, and these may differ from other laboratories undertaking the same accreditation.Image Analysis. Its Application to the Characterization of Particle Shape K. Julian and S. L. Mills BICC Cables Limited, Energy Cables Division, Wrexham Technology Centre, Wrexham, Clwyd LL3 9XP This paper presents the initial results of studies conducted to assess the suitability of fractal geometry for describing the textural detail of irregular shaped features. Work is currently ongoing to discriminate between samples of magnesia using macroscopic and microscopic shape descriptors. A previous paper presented at FILPLAS'89' discussed the merits of improving existing empirical test methods used for the evaluation of fillers. A variety of filler materials are used in the manufacture of cable insulations, screens, etc., and these influence the mechanical, electrical and fire survival properties of the finished articles.Reviews of the current literature suggest that characteriz- ation of primary filler properties is restricted to the measure- ment of particle size and surface area. Evaluation of filled components is often reduced to mechanical tests and rheologi- cal assessment of compounds comprising different filler loadings. Yet, if one returns to the early work of the powder technologist, particle size, shape and surface area are con- sidered to be extremely important material properties. They are closely allied, influencing powder behaviour and any subsequent operation or process into which the powder is incorporated. Rapid progress in the powder technology field has led to the development of techniques to enable the determination of particle size and surface area.A number of these methods are employed on a routine basis to characterize the filler materials we use today. The description of particle shape, however, remains unsolved. At the microscopic level of inspection, it is often apparent that striking differences exist between powders when particle or aggregate shapes are compared. The quantifi- cation of such observations would allow a classification of shape to be made which would be superior to the qualitative descriptions presently available. Our earlier presentation cited the use of fractal geometry as a means of describing the particle morphology of filler materials and this paper outlines the work that has been undertaken at BICC to assess the suitability of this approach for the investigation of fillers.The Fractal Dimension The concept of the fractal dimension and with it the discipline of fractal geometry came to prominence following the pioneer- ing work of Mandelbrot. The interested reader is referred to reviews by Fede? and Mandelbrot3 for a theoretical insight into the subject. Fractals can be simply thought of as geometric figures which are extremely irregular in shape, they are unique in that they exhibit a fractional dimension. This situation initially appears at odds with convention as classic geometric forms are not considered to display fractional values of dimension. After all a straight line is represented as one-dimensional, a plane is considered two-dimensional, or are they? An example of a fractal figure is the Triadic Koch curve, part230 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 of which is illustrated schematically in Fig.1. The construction of the curve relies on a scaling process, the curve resulting from the reduction of a line segment of unit length. A defining property of a fractal object is its self-similarity and this can be readily illustrated using the Koch curve as an example. Magnification of a portion of the curve shows that it is composed of small line segments. The curve is considered to be self-similar; small sections of it are said to comprise scaled down versions of the whole. Description of fractal figures is achieved with reference to a fractal (i.e., fractional) dimension, D.For a self-similar figure, its fractal dimension is obtained using the relation: N = (l/j)* (1) where N = the number of copies of the original figure, f = a scaling factor, D = fractal dimension. The initial process of the Koch curve construction divides a line segment into three pieces of equal length. The central piece is removed and replaced by two segments, each of length one third. The resultant figure consists of four segments produced by scaling down the original line segment by one third. This generation process is continued ad infiniturn; the length of the curve increasing by four thirds at each stage of the construction. Using the scaling relation above: Log N = D Log (lln (2) (3) (4) D = Log N/Log (llf) D = Log 4/Log 3 = 1.262 The Triadic Koch curve shows fractional dimension and it is described by a fractal dimension of 1.262.It is more complicated than a line of one dimension as it cannot be indexed with reference to a distance from a fixed point. Alternatively, it does not completely fill the two-dimensional space in which it exists and so cannot be adequately described as two-dimensional. The concepts of scale and self-similarity dictate that fractal figures do not show characteristic measures of length. As one inspects the boundary of a fractal object at increased levels of resolution, finer detail is observed. The actual length that is measured depends on the inspection yardstick adopted and so the perimeter of a fractal object is not a unique defining property. For regular shapes ( e . g . , circles) perimeter is often used as a measure of feature geometry but the question arises as to its validity in the investigation of irregular shaped features.The inadequacies of perimeter measurements can be appre- ciated if the perimeters of a grape and a gooseberry are compared. The true perimeter of the gooseberry will only be realised if the hairs on its surface can be resolved, otherwise the measured value will tend towards that of the grape. Richardson4 was interested in measuring the length of coastlines, the measurement procedure based on a structured walk technique. A pair of dividers are stepped around the coastline and the perimeter estimated from the distance between the divider points and the number of steps required to traverse around the boundary. He showed that the length estimate is dependent on the distance between divider points.Richardson concluded that the perimeter of coastlines behaved as follows: L(G) = AG(l-‘) ( 5 ) Fig. 1 The Triadic Koch Curve where L(G) = length of coastline, G = divider separation, A = a positive constant, C = a constant characteristic of the coast investigated. Mandelbrot’ examined Richardson’s data and presented the perimeter estimates and corresponding yardstick values in the form of logarithmic plots of L(G) versus G . He stated that the data followed a linear relationship and could be explained using the relation: L(G) = AG(l-D) where L(G) = perimeter estimate, G = inspection yardstick. Data fitting yielded a slope of 1 - D where D was defined as the fractal dimension of the coastline.Values of D are observed to vary between one and two; Mandelbrot stated that the magnitude of D reflects the irregularity of the coastline. High D values are associated with increased irregularity and texture. (6) Investigation of Particle Texture Kaye6,’ has adopted the structured walk technique to investi- gate the morphology of powder particles, images obtained from electron microscopy investigations. Studies to determine the suitability of such a technique for the examination of fillers have been instigated at BICC. The manual structured walk approach previously described is not very attractive as it is extremely time consuming and prone to error. Advances in image analysis and processing have allowed the introduction of semi-automated procedures for the determination of fractal dimension.A method adopted at BICC is that of dilation analysis, attributed to Flook.’ Following the capture of images from a scanning electron microscope, features are skeletonized and perimeter estimates obtained at different inspection yardsticks. The data is displayed in the form of a Richardson plot and analysed using linear regression analysis. Initial work in BICC investigated the morphology of latex sphere aggregates. Feature outlines for five aggregates and a circle are illustrated in Fig. 2; Richardson plots are provided in Fig. 3. Attention is drawn to the data presented for the circle: only one linear region is observed in the Richardson plot, the slope of which will reflect the degree of texture present in the feature.For the aggregated latex structures, the data fit is best represented by two linear regions. Kaye has attributed such behaviour to the fact that real life objects are not truly fractal. The perimeter estimates display a dependence on the inspec- tion yardstick used to inspect the features over two different length scales. At small yardstick values the data are fitted to a straight line, the slope of which is referred to as the textural fractal dimension, D,. For larger inspection yardsticks, the data are described by the structural fractal dimension, D,. Comparisons between features are made over similar ranges of inspection yardstick, yardstick and perimeter estimate, data being normalized using the length dimension of each feature. Table 1 lists Dt and D, values for the different shapes examined.The values calculated suggest that all six features are different. Each displays a varying degree of edge texture, described by the magnitude of the fractal dimension D,. It has previously been stated that increased edge texture is associated with high values of D,. For the data presented, the D, values suggest that feature profile six, the circle, displays minimal edge detail, whilst feature five exhibits the most. Visual inspection of the skeletonized images supports these empirical observations and Mandelbrot’s opinion that the magnitude of the fractal dimension reflects the observed profile irregularity. Data fitting over the structural yardstick range showed similar trends but additionally, similarity of profile shape was also reflected in the magnitude of the D, values.Profiles three and five are very similar, elongated in nature and displaying a complex grape-like appearance. Dilation analysis indicated that they have D, values of 1.27 and 1.28, respectively. Profiles two and four are less elongated although the latex spheresANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 n 'i) 7 '3 W U Fig. 2 Feature outlines investigated using dilation analysis remain in a clustered configuration, the features displaying similar D, values of 1.11 and 1.14. Feature one does not show a highly involved arrangement of spheres and this is reflected in the low D, value of 1.03, approaching the value displayed by the circular profile, feature six. Fractals and Their Use in Shape Analysis It is concluded that a method to discriminate between the texture of various particle profiles based on fractal geometry concepts has been successfully implemented at BICC.How- ever, what is the significance of texture discrimination in the investigation of particle shape? Perhaps the way forward is that proposed by Drescher et al. in a recent paper.' The results of our feasibility studies highlight differences between the microstructural features of the latex clusters. What is required is a description of particle shape both on a macroscopic and microscopic scale. The authors have adopted a method based on Medalia's procedure for the generation of dynamic shape factors" to provide a macroscopic description of shape. They suggest that an infinite variety of particle shapes exist and as no single characterization technique is applicable for all situations, an alternative approach is required.The authors measured a number of geometric characteristics (i. e., area equivalent diameter, macroscopic and microscopic shape factors) as part of an investigation of agricultural feedstuffs. All or a combination of these measured parameters were subsequently analysed using cluster and discriminant analysis techniques, allowing compari- sons of different materials to be made. The aim of the technologist is to provide an insight into the effects of particle geometry on powder properties and behaviour. It is hoped that the approach of Drescher et al. will allow the effects of filler 1.3 1.2 1.1 1 0.9 0.8 0.7 I I I I t """\ t 1 0.8 1 I C I 0.7 Il(lllllljll 0.9 0.8 231 0.7 3.5 3 2.5 2 1.5 1 3.5 3 2.5 2 1.5 1 R Fig.3 Richardson plots showing perimeter estimates for different inspection yardsticks: (a) feature 1; (b) feature 2; (c) feature 3; (d) feature 4; (e) feature 5 and (f) feature 6 (as shown in Fig. 2) particle geometry to be investigated and quantitative descrip- tions of particle shape to be made. The aforementioned techniques may in certain circum- stances be prohibited as they are extremely operator intensive. A method to investigate particle texture based upon the observations of Lovejoy" is relatively simple to instigate, data interpretation relying on regression analysis techniques. Love- joy found that the perimeter of rain clouds is proportional to the square root of their area raised to the power D.The parameter D is interpreted as the fractal dimension of the perimeter. A similar approach has been adopted to compare two batches of magnesia. Individual particles are measured using image analysis, projected areas of the features being plotted against perimeter and straight lines fitted to the data using regression analysis. Fig. 4 shows experimental data for magnesia identified as batches 1 and 2. The slopes of the two lines illustrated were found to be significantly different; D values of batches 1 and 2 being 1.09 and 1.20, respectively. It can be inferred from these results that the magnesia particles from batch 2 are more textured, exhibiting a higher degree of complexity. ~~~ ~ Table 1 Fractal dimensions determined by regression analysis of Richardson plot data Feature Dt Ds 6 1.02 1.02 1 1.03 1.03 2 1.04 1.11 4 1.05 1.14 3 1.09 1.27 5 1.14 1.28232 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 5.5 I I I 1 I 1 2 2 2.4 2.6 2.8 3 3.2 3.4 Fig. 4 Area-perimeter relation for magnesia. Batch 1 (O), D = 1.09; batch 2 (a), D = 1.20 Peri meter/t i m Conclusions A technique has been successfully implemented to discriminate between shapes displaying different textural detail using the concept of fractal geometry. The technique is considered applicable to the investigation of filler materials, images of particle profiles obtained using scanning electron microscopy and investigated by image analysis. Work is currently ongoing to discriminate between fillers in terms of textural detail. Shape description using macroscopic dynamic shape factors is being pursued and it is hoped that the results of this work will also be available in the near future. 1 2 3 9 10 11 References Julian, K., and Fordham, 1. L., Mineral Filled Compositions For Cable Use. Proceedings of FILPLAS’89, The 1989 Fillers Conference, The Plastics and Rubber Institute, London, 1989. Feder, J., Fractals, Plenum Press, New York, 1989, pp. 1-40. Mandelbrot, B. B., The Fractal Geometry Of Nature, Freeman, New York, 1983. Richardson, L. F., Gen. Syst. Yearb., 1961, 6 , 139. Mandelbrot, B. B., Science, 1967, 155, 636. Kaye, B. H., Powder Technol., 1978, 21, 1. Kaye, B. H., Part. Charact., 1984, 1, 14. Flook, A. G., in Conference Proceedings of Partech Sym- posium, Drucherei Henrich Schuster, Nuremberg, 1979, pp. Drescher, S., Heidenreich, E., and Muller, G., Part. Part. Syst. Charact., 1990, 7 , 30. Medalia, A. I., Powder Technol., 1970, 4, 117. Lovejoy, S., Science, 1982, 216, 185. 591-599.

ISSN:0144-557X    

DOI:10.1039/AP9933000227

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 233 AMC Meeting The following is a paper which was presented to the Analytical Methods Committee's Medicinal Additives in Animal Feeds Sub-committee by Mr. I. M. Barwick at a Meeting held on October 1, 1992, in the Department of Trade and Industry, Victoria Street, London SW1 . Analysis of Sulfadimidine in Medicated Animal Feeds Ian M. Barwick and Peter Warwick Repartmen t of Chemistry, L oug h boro ugh University of Techno logy, Lo ug h bo ro ug h, L eicesters hire LEI? 3TU Neil T. Crosby Laboratory of The Government Chemist, Queen's Road, Teddington, Middlesex TWI I OL Y The incorporation of medicinal additives into animal feeds for prophylaxis or growth promotion is a widespread practice that allows the intensive animal husbandry methods practised today.To safeguard the health of both the consumer and animal, legislation is in place to ensure that the manufacturer's declarations concerning the nature and the quantity of the drug included in medicated feeds are correct. Statutory methods of analysis must possess adequate sensitivity, reproducibility and selectivity. The majority of analytical methods for medicinal additives utilize drug extraction with an organic solvent, usually followed by a clean-up step, before analysis using high- performance liquid chromatography (HPLC) . A common problem is incomplete drug extraction and this is thought to be due to either decomposition of the drug or binding of the drug to feed constituents. The work presented in this paper investigates the recovery of sulfadimidine from pig feeds.Sulfadimidine, SDM [4-amino-N-(4,6-dimethylpyrimidin-2- yl)benzenesulfonamide], is a widely used antimicrobial drug used in pig feeds to treat a condition known as atrophic rhinitis. Analytical recoveries of SDM from feeds have been reported to decrease as the feed ages14 and are especially poor from feeds that have been pelleted. Fig. 1 shows the differences in recoveries from a feed before and after pelleting, with the pelleted feed giving lower recoveries than the non-pelleted feed. In the manufacture of pelleted feeds, the feed is conditioned prior to pelleting by the injection of steam to increase the feed moisture content and temperature. This facilitates efficient pelleting but is thought to accentuate any decompositioddrug-binding tendencies.In addition to pro- cessing differences between pelleted and non-pelleted feeds, 100 g 90 L- 2 > s 80 z v) 70 60 0 10 20 30 40 50 60 Ti rne/da ys Recovery of SDM from meal (A) and pelleted feed (B) (200 Fig. 1 mg kg-7 pelleted feeds also have a higher moisture content than non- pelleted feeds. Experimental The analysis method used in this paper is that described by Conway2 which was later modified by the Analytical Methods C~mrnittee.~ The analysis comprises extraction of SDM from the feed with aqueous acetonitrile, followed by a clean-up step using a solid-phase extraction column before analysis by HPLC. Results and Discussion To investigate the possibility of drug degradation occurring during the conditioning stage, a quantity of SDM powder was placed in a domestic pressure cooker and 'stressed' for 5-15 minutes to simulate the conditioning process.Following removal from the pressure cooker, solutions of SDM were prepared and injected onto the HPLC. Comparison with SDM degradation product standards established that degradation of SDM had not taken place. It was thought that, although stressing SDM in a pressure cooker had not produced any degradation, the increased moisture content in pelleted feeds may increase the rate of hydrolysis of SDM. To investigate this, a saturated solution of SDM in water was prepared. One half of the solution was exposed to light and the other protected from light. Analysis of both solutions using HPLC showed that both solutions were stable over the 62 day period of the study.The SDM concentration remained unchanged and hydrolysis products were not detected. As degradation of SDM was not occurring, irreversible binding of SDM to feed constituents was investigated. Indi- vidual feed components were dosed with SDM and the recovery of the drug from each component measured. The results showed that the recovery of the drug was low from all components and therefore SDM was not binding specifically to a single component. Since drug-feed binding appeared to be occurring, experi- ments were performed to investigate SDM drug recovery as a function of feed moisture content, in an attempt to account for the differences between pelleted and non-pelleted feeds. In the first study, a feed was dosed with SDM, then stored in either a high humidity (water saturated) or a low humidity (silica gel) environment and the recovery of SDM from each of the sets of samples at successive time points determined.The results shown in Fig. 2 indicate that the feed samples stored in the highANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 234 100 80 8 $ 60 > E 40 - 8 2 20 0 I I I 10 20 30 Contact time/days 40 Fig. 2 SDM recovery as a function of feed storage humidity conditions: A, low humidity; B, high humidity humidity environment exhibited the greatest decrease in SDM recoveries with time. Clearly, the higher the moisture content of the feed, the more pronounced was the decrease in SDM recovery. SDM recoveries from the high moisture content feed samples could be improved dramatically by prolonged oven drying at 105°C.SDM recovery increased from 9 to 90% by drying a feed sample for 11 days which had previously been stored in the high humidity environment. A second study investigated the effect of modifying the feed moisture content before dosing with SDM. Feed samples were stored in either high humidity conditions (water saturated), ambient humidity conditions, or low humidity conditions (silica gel) for six days. The moisture contents of the high, low and ambient samples were 18, 6 and 12% d m , respectively. Following dosing with SDM, analysis of each sample produced values of recoveries that were again found to be inversely proportional to the moisture content of the samples as is shown in Fig. 3. Experimental evidence appeared to favour drug-feed bind- ing, and experiments were performed in which the amount of SDM added to uniform amounts of feed was varied.SDM recoveries obtained from the samples indicated that there was a correlation between the amount of drug added and drug recovery. The implication of this was that there appeared to be a limit to the amount of drug that was non-recoverable. Additionally, an experiment was performed where the specific surface area of a feed sample was increased by reducing the particle size of the feed. Recoveries from the smaller particle size sample and a control sample were very similar. This showed that the drug-feed binding was not confined to the surfaces of the feed particles since the feed with the larger 20 1 I I I 0 10 20 30 40 50 Contact time/days Fig. 3 SDM recovery as a function of feed moisture content: A, low humidity; B, ambient humidity; C, high humidity surface area would be expected to show the lower SDM recovery.A supply of SDM labelled with 14C was obtained and used to medicate a sample of commercial feed by addition of a solution of the drug to the feed. 14C-SDM was then recovered from the feed. Additionally, the specific activity of SDM in the extraction solution was measured and showed that incomplete drug extraction had taken place. Drug recovery, when measured by radiochemical detection, gave the same values as those observed by ultraviolet (UV) detection. The use of the 14C labelled SDM showed that there was little loss of activity during the clean-up stages of the analysis. Autoradiographic studies were then performed by filtering off the feed solid after drug extraction and placing small quantities of feed solid on glass microscope slides using adhesive tape .The slides were then placed in contact with autoradiographic film.Following development of the autoradiographic film, correlation between the autoradiographic images of the residual activity on the feed and the feed particles showed that the residual activity was evenly distributed throughout the samples. This implied that SDM was not bound preferentially to any component of the feed. A similar study performed on individual feed component showed that residual activity remained on all samples after drug extraction. In view of the experimental results that feed moisture content was inversely related to the recovery of SDM, and the higher the drug dosing level, the higher the drug recovery, the solubility of the SDM in the water present in the feed was considered.Theoretical calculations were made assuming that a quantity of feed (1 g) contained SDM at a level of 100 pg 8-l; the feed had a moisture content of 12% d m , and the solubility of SDM in water was -4OOpg ~ m - ~ . Calculations indicated that if the water in the feed was assumed to be free, i. e., had not been absorbed by feed particles and could behave as bulk water, then it was possible for 50 pg of SDM to dissolve in the water present in the feed. During some extractions from feeds, low recoveries of 50% of added drug were obtained. If some SDM did dissolve in the water present in the feed, then this portion would be more likely to be extracted than the SDM that remained as a solid. Since this is not the case, an explanation for this is that the dissolved SDM could penetrate further into the feed matrix, possibly into the pores of the constituents.Examination of individual feed components using scanning electron microscopy has shown that all exhibit a degree of porosity. Conclusion The work presented in this paper has shown that the poor recoveries of SDM from medicated feeds can be attributed to binding between the drug and the feed constituents and cannot be attributed to degradation of the drug. The 14C labelled SDM studies have shown that the non-extractable SDM is generally distributed throughout the feed. Experimental evidence en- abled us to put forward a hypothesis in which SDM is dissolved in the water present in the feed and the resulting solution then penetrates deep into the feed matrix, thus preventing the SDM from being extracted. References 1 Holder, C. L., Thompson Jr., H. C., and Bowman, M. C., J. Chromatogr. Sci., 1981, 19, 625. 2 Conway, B., Analyst, 1988, 113, 1397. 3 Analytical Methods Committee, Analyst, 1992, 117, 817. 4 Munns, R. K., and Roybal, E. J., J. Assoc. Off. Anal. Chem., 1982, 65, 4; 1048.

ISSN:0144-557X    

DOI:10.1039/AP9933000233

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 235 EQUIPMENT NEWS Emission Spectrometer The Optima 3000 compact, computer-con- trolled, inductively coupled plasma optical emission spectrometer features a new detec- tor designed specifically for plasma emission spectroscopy and a patented optical system, which permit simultaneous measurement of spectral background and each analyte line. Sixty elements can be measured in less than one minute at multiple wavelengths, with no loss of precision or sensitivity. The Optima 3000 includes over 5000 emission lines so that alternate interference-free lines can be selected for superior results. Perkin-Elmer Ltd., Post Office Lane, Bea- consfield, Buckinghamshire HF9 1QA. Raman System The Spex Raman 500 is an easy-to-use single grating Raman system that can supply spectra in seconds.It is ideal for almost any molecu- lar application, semiconductor evaluation, ce- ramic and composite study and polymer identification. It can be used for both micro and macro work. For microanalysis un- matched spatial resolution is achieved with the 1482 confocal microscope. For macro work the UVISIR Illuminator provides a host of sample schemes. Instruments S.A. (UK) Ltd., 2-4 Wigton Gardens, Stanmore, Middlesex HA7 IBG. greater than the incremental improvements the whole industry has made in the last ten years. It also offers 10 times faster analysis than any other system currently available, with an unprecedented resolution of 133 eV at manganese with an acquisition rate of 10 OOO counts s-'; this is a ten-fold increase for the same resolution over the nearest com- petitor.Oxford Instruments, Analytical Systems Division, Halifax Road, High Wycombe, Buckinghamshire HP12 3SE. Microanalysis Detector The Kevex SuperDry is the world's first en- ergy dispersive microanalysis detector requir- ing no liquid nitrogen. It features a compact series of solid-state heat extraction devices which utilise the Peltier cooling principle. GPC Systems Two GPC systems are announced: the PL- GPC 1 10 temperature controlled system for use up to 110 "C and a unique system for use up to 210 "C. The PL-GPCllO is fully micro- processor-controlled and incorporates a re- fractive index detector, injection valve and columns in the air circulation oven, for total thermal uniformity. The solvent delivery sys- tem (50 ~1 to 5 ml min-') includes on-line degassing and high precision flow for con- ventional and narrow bore GPC columns.The PL-GPC210 is an automated complete system designed for GPC/SEC up to 210 "C. Polyolefins, for which temperatures in excess of 150 "C can be an advantage, are more ac- curately analysed with the PL-GPC2 10's high sensitivity RI detector. Detector for X-ray Analysis The Link GEM high purity germanium detec- tor offers guaranteed resolution of 115 eV at manganese and 65 eV at fluorene, a single- step function increase in performance that is CONCEPT GD glow discharge mass spectrometer The risk of detector damage is eliminated, as is the chore of having to fill a dewar during holiday periods. Fisons Instruments, Crawley, West Sussex RHlO 2BR.Glow Discharge Mass Spectrometer Based on the successful CONCEPT mass spectrometer, the CONCEPT GD is a glow discharge mass spectrometry system consist- ing of a mass spectrometer, a glow discharge ion source and a comprehensive MACH 3 data acquisition and instrument control sys- tem. Designed for the quantitative analysis of solids, it is principally intended for the bulk analysis of samples from 100% down to ppb level components with the minimum of sam- ple preparation. Kratos Analytical Ltd., Barton Dock Road, Urmston, Manchester M3 1 2LD. Polymer Laboratories Ltd., Essex Road, Church Stretton, Shropshire SY6 6AX. Nitrogen Detector for GPC A new, highly stable nitrogen detector is an- nounced for the makers' 610 Series gas chro- matograph. It is used in the determination of organically bound nitrogen in a number of key environmental applications, as well as in drug-related analyses.Specially designed for the Unicam 6 series of gas chromatographs (4600, 610), it can also be used with older chromatographs in the makers' range. CB12PX. Unicam Ltd., York Street, Cambridge Solid Phase Extraction Columns236 ANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 A range of SPE columns in PTFE with 16 ging with download facility. Tne range Of- Water neatment System different pre-tested proprietary branded fered includes S ~ ~ X - S for co7 H2S9 so2, The H2F2 unit removes particles, colouring, chemistries utilize new 'High Flow' pure No23 c12, HCN with others to follow. A bra- dissolved organic material and hydrocarbons PTFE frits.The combination of a PTFE col- chure is available. (including solvents) viruses and 99.9% of all umn and PTFE frits results in near zero ex- MSA (Britain) Ltd.7 East Shawhead, bacteria. It operates without the addition of tractables from column and frits. The Coatbridge ML5 4TD- any chemicals; indeed it actually removes the columns are available in 1, 3, 6 and 12 ml taste and odour of any chemicals that are pre- sizes with bed sizes ranging from 50 to 2000 Titer Blocks sent, including chlorine. A three-stage system mg Single, 96-well titer blocks, with 1 ml capac- combining dual filters with an ultraviolet HPLC Technology Ltd., Wellington ity per well capped, and 1.2 ml uncapped, are sterilizer, it is easily Plumbed into an existing House, Waterloo Street West, Macclesfield, announced.Available in non-sterile polysty- water SUPPIY and is simple to maintain. Cheshire SK11 6PJ. rene or sterilizible polypropylene, the blocks k h n c e d Engineering Services, Ultra- are compatible with single, 8 and 12 tip violet Technology Division, Newham Road, pipettors; and the Brandel-range of Receptor Hook, Basingstoke RG27 9LS. makes for easy storage and incubation, is Solid Phase Extraction Application Preparation System for GC Binders. An 8 x 12 flexible cover strip, which and GC-MS Development Kits The new HP 7686 Prepstation system pro- available as an optional extra. Of benchtop GC and GC-MS 'Ys- before injection on to the column. Offering greater ease of use and lower costs than man- ual sample preparation methods, it brings in- creased productivity to the analytical laboratory.Semat Technical (UK) Ltd., 1 Executive Application Development Kits offer a cost- preparation Park, Hatfield Road, St. Albms, Hertford- effective way to evaluate related media when with automation Of shire AL1 ~ T A . optimizing separation conditions in any ap- plication involving solid phase extraction. Conductivity Meter They are especially useful when reverse A new conductivity meter which will with- phase chemistries are being used, where dif- stand the rigours of factory and field testing ferent degrees of carbon loading and end Cap- Hewlett-Packard S*A*, I5O route d' Nant- is fully waterproof and has tactile-effect rub- ping can be used to achieve specific results. d'Avril, CH-1217 Meyrin 2, ber operating keys.A multi-range instrument Each kit contains 60 solid phase extraction with a wide measurement spectrum in four devices, five each of twelve solid phase ex- Capillary Electrophoresis System ranges from 0.0 CIS to 19.99 mS, it offers an tl'action media, ranging from ClS chemistries Incorporating a highly sensitive diode-may accuracy Of 1% Of full Scale. Temperature to ion exchange- They are with detector with a wide linear detection range Compensation iS fully automatic and a tern- either and HP Extended Light Path Capillaries, the perature coefficient control enables accurate format typically used in conjunction with a HP 3DCE system, designed and developed in measurements to be carried out on solutions syringe. Europe, offers a significant advance Over ex- With Coefficients different from Water.Whatman Scientific Ltd., Whatman isting CE instruments in the range of corn- pounds amenable to separation. AS well as Team Valley Estate, Gateshead, Tyne and Kent ME16 OLS- biological macromolecules, the system han- Wear NE11 ONS. dles amino-acids, chiral drugs, pesticides, in- Centrifugal Evaporator organic ions, organic acids, oligonucleotides Buffer Solutions The PTFE Gyrovap GL is a fully acid resis- and DNA restriction fragments, whole cells I u p ~ c traceable buffer solutions with a p~ tant centrifugal evaporator featuring full mi- and virus particles and many other substances guarantee certificate are available. n e new croprocessor control of temperature, time and of interest to pharmaceutical and bioscience certificate guarantees a p~ value of fi.010 vacuum ventilation With a 10 method mem- laboratories.d'Avril, CH-1217 Meyrin 2, Switzerland. led pH meter (resolution 0.001 pH) calibrated Banbury, Oxfordshire 7RG* type devices Or the Palintest Ltd., Palintest House, Kingsway, House, St. Leonard's Road, 20/20 Maidstone, for the makers' buffers. The pH values are ory- Hewlett-Packard S.A., 150 route du Nant- obtained through the use of a quality control- against precision ampoule buffers of pH v- A* Howe and Ltd., Beaumont Food Analysis System The UK's most advanced PC-based labora- tory system, the Specimen Control System (SCS), has been developed in a collaboration between the Luton Public Health Laboratory and UK software house Microft Technology Ltd. It is currently installed at Public Health Laboratory Service Laboratories in Luton, Oxford and Poole.It records the results of all tests performed on milk, food and water sam- ples examined in the microbiology depart- ments at Luton, Oxford and Poole; additionally in the Luton laboratory it logs the findings of examinations performed on Datient mecimens. 6.865 and pH 7.410.-The exact values-of these buffers are determined using the Stand- ard Hydrogen Electrode Apparatus. Radiometer Ltd., The Manor, Manor Royal, Crawley, West Sussex RHlO 2PY. Combined pH Measurement and Pro- portional and Integral Control The 4500 Series of pH transmitters has been extended with the introduction of a new ver- sion which combines pH measurement with P&I control. It offers three types of control output.By selecting the appropriate code number 1-6, it can be programmed for pulse frequency, pulse width or analogue control. Chamber Furnace A family of high-capacity chamber furnaces is announced, the modular design of which tailors the configuration of the furnaces to the needs of the user. Chamber dimensions can be tailored to suit the application, at a range of maximum operating temperatures up to 1800 "C. The chamber is constructed using high-grade ceramic fibre modules and has an aperture lining of alumina bricks with a sili- con carbide or, in 1800 "C models, high alu- mina hearth. Pyro Therm Furnaces, Unit 3, Halcyon House, 20 Goward Street, Market Harbor- ough, Leicestershire LE16 9AF. MicrLft Technology Ltd. (telephone 081- The mode of Operation can be set for both direct and reverse acting, thereby enabling Peristaltic Pumps both acid and alkali dosing to be achieved.The LSM range of peristaltic pumps offers up 948- 8255). The instrument is available either for wall to six dispensing ranges from 0.025 to 1850 Pocket Toxic Gas Monitor mounting (Model 4535) or for panel mount- ml per unit dispensed unit, accurate to better The MicroMAC is available in two models: ing (Model 4545). than &0.5%. They are supplied as a single an Alarm version offering Real Time, Short ABB Kent-Taylor Ltd., Howard Road, unit or as a fully automated dispensing sys- and Long Term exposure values, and a Do- Eaton Socon, St. Neots, Huntingdon, Cam- tem. The units are manual or with microproc- simeter version offering additional datalog- bridgeshire PE 19 3EU.essor control which is fully compatible withANALYTICAL PROCEEDINGS, MAY 1993, VOL 30 237 existing hardware such as personal comput- ers, printers and analysis equipment. The pumps are self-priming and there is no con- tamination of product because the liquid is in contact only with the silicone tubing. Vestec Ltd., Felstead Road, Longmead In- dustrial Estate, Epsom, Surrey KT19 9BB. Circulators and Baths The new DC-Line circulators and baths fea- ture completely digital electronics, user- friendly operation using microprocessor control, safety features exceeding all legal re- quirements, Real Time Temperature Adjust- ment for QC procedures according to IS0 9000 standards, a Fault Identification System displaying the reason for any malfunction, CFC-free refrigeration units, a temperature range extending from -20 up to 200 “C de- pending on the model, and an RS232C inter- face for remote control with the top model DC5 of the new program.HAAKE Mess-Technik GmbH u. Co., Dieselstrasse 4, D-7500 Karlsruhe 41, Ger- many. Microscope The TMX 1000 Explorer AFM complete Atomic Force Microscope system is an- nounced. The TMX Explorer can be placed direct on the sample to be analysed, allowing AFM imaging of samples of any size. It can be used to image samples in air or while sub- merged in liquids. Ease of operation is pro- vided by an optical microscope with 200x magnification for viewing the probe and sam- ple, and by computer controlled probe ap- proach allowing the tip to be automatically moved into imaging position. The system can be used for scans from atomic resolution to areas as large as 150 x 150 pm.TopoMetrix, 1505 Wyatt Drive, Santa Clara, CA 95054, USA. Network Servers The new HP ChemServer 4900 series family of network servers brings together informa- tion from a variety of analytical data systems from HP and other vendors, allowing labora- tories to make optimal use of their computers and data handling devices. As a result, chem- ists can share and put to immediate use infor- mation from a number of instrumental analytical techniques. Hewlett-Packard S.A., 150 route du Nant- d’ Avril, CH-1217 Meyrin 2, Switzerland. Xylene Replacement MICROCLEAR is an odourless biodegrad- able replacement for xylene in histology. It has been designed as a paraffin cleaning agent. Compared with currently used xylenes and toluene it is non-toxic, fume-free and non skin irritating.Extensive testing has shown excellent results using both immunoperoxi- dase and chemical staining procedures. MI- CROCLEAR should be used with MICROCOVER mountant for coverslipping. J. T. Baker UK, Wyvols Court, Swallow- field, nr Reading, Berkshire RG7 IPY. LITERATURE Literature is available on the CADAS 5O/CADAS 30 fully automatic spectro- photometers, which have been developed to provide a broad spectrum of high quality water analyses for the experienced operator and the interested beginner. Dr. Bruno Lange (UK) Ltd., P.O. Box 93, Camberley, Surrey GU 15 1 DU. Two application notes and a technical note are announced: ‘Hot Stage & Polarized FT-IR Microspectroscopy of Polypropylene Single Fiber’ describes methods to determine the crystallinity and molecular orientation of sin- gle polypropylene fibres; ‘m-IR Spectros- copy of Polymers and Additives Using Product Validation Software Techniques’ de- scribes methods for quantitative determina- tion of vinyl acetate and butylhydroxytoluene in polyethylene, and for validation of an acrylic polymer in comparison with other manufacturing lots; and ‘A Flexible, Modular FT-IR Spectrometer for the Analytical and Research Laboratory’ describes some of the primary design considerations for an FT-IR spectrometer.Perkin-Elmer Ltd., Post Office Lane, Bea- A catalogue lists a comprehensive range of accessories and upgrades for the makers’ range of mass spectrometry products: PRO- FILE and CONCEPT. Kratos Analytical Ltd., Barton Dock Road, Urmston, Manchester M3 1 2LD.The new HP Environmental Catalogue de- scribes over 40 internationally used environ- mental analysis methods. Hewlett-Packard S.A., 150 route du Nant- d’Avril, CH-1217 Meyrin 2, Switzerland. The Supelco Reporter, Vol. XI, No. 4, fea- tures a SUPELCOSIL LC-DABS HPLC col- umn for identifying and quantifying, in a single analysis, a wide range of amino acids in protein hydrolysates and physiological flu- ids. Among the other topics discussed are Re- zorian A- 16 1 analytical cartridges for selectively removing hydrophobic substances from biological sample solutions. Supelchem UK Ltd., Shire Hill, Saffron Walden, Essex CB 11 3AZ. Among the products featured in a new cata- logue focusing on biotechnology are the SCHl Cryo-Thermo Store and SCH2 Cryo- Store electronically controlled baths for stor- ing samples below -20 “C. Also included are 5 hybridization ovens and the Gene-TECH fast refrigerated temperature cycler, a range of shaking water-baths, 3-D sample rockers, and STC-1 rotators. Stuart Scientific Co. Ltd., Holmethorpe Avenue, Holmethorpe Industrial Estate, Red- hill, Surrey RHl 2NB. A brochure describes the System 1/02 multi- channel analyser which measures both mois- ture and oxygen concentration. Panametrics Ltd., Unit Two, Villiers Court, 40 Upper Mulgrave Road, Cheam, Surrey SM2 7AJ. Fisons announce the inclusion of the new Fistreem Calypso Water Still range in their 1993 Apparatus Catalogue. Fisons Scientific Equipment, Bishop Meadow Road, Loughborough, Leicester- consfield, Buckinghamshire HP9 1QA. shire LE11 ORG.

ISSN:0144-557X    

DOI:10.1039/AP9933000235

出版商:RSC    

年代:1993

数据来源: RSC