ISSN:0144-557X    

DOI:10.1039/AP99330FX029

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

August 1993 Ana lyt ica I Proceedings ANPRDI 30(8) 321-352 (1 993) Proceedings of the Analytical Division of The Royal Society of Chemistry CONTENTS 321 VAM Viewpoint 322 SAC Gold Medals 322 Reports of Meetings 323 The Royal SQciety of Chemistry Sponsored Awards 1992 323 Analytical Division Honours 324 Analytical Viewpoint 'Requests for Reprints as an Assessment Parameter of Papers Published in Journals of Analytical Chemistry' by Jose L. Guirion and Asuncion Jaime 328 SUMMARIES OF PAPERS 'Isotope Dilution Mass Spectrometry as a Primary Method of Analysis' by P. De Bievre 328 SAC 92 328 'London Chemists and Chemistry, Prior to the Formation of the Chemical Society in 1841' by D. Thorburn Burns 334 The State of Chemistry in 1841 334 'New Directions in Gel Permeation Chromatography' by Barry J.Hunt 'Characterization of Polymer Membranes Using Microelectrodes. Part 1. Diffusion- IimitedEize-exclusion Electrodes' by Amiel M. Farrington and Jonathan M. Slater 338 New Directions in Chromatography 338 341 Research and Development Topics in Analytical Chemistry 341 344 Equipment News 346 Conferences and Meetings 347 Coupes 348 Publications Received 351 Analytical Division Diary iii ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 ROYAL SOCIETY OF CHEMISTRY Bioanalytical Approaches for Drugs including Anti-asthmatics and Metabolites Edited by Eric Reid Guildford Academic Associates 1. D. Wilson ICI Pharmaceutical Division, Macclesfield Methodological Surveys in Biochemistry and Analysis - Volume 22 Series Editor Eric Reid Guildford Academic Associates Bioanalytical Approaches for Drugs, including Anti-asthmatics and Metabolites gives a state-of-the-art account of the subject and focuses on assaying blood and other biological samples, especially drugs which are given in low dosage or which yield metabolites that need subtle investigation.It covers advances in HPLC, SFC, CZE, MS and other detectors, NMR, and automated sample handling, as well as diverse drugs. The book also looks at problems such as analyte lability, stereoselectivity and interferants. This book is the latest volume in the ‘Analysis’ sub-series of Methodological Surveys in Biochemistry and Analysis, which is acknowledged as an integrated reference source, and contains a cumulative analyte index.It will be essential reading for researchers involved in analytical, medicinal, pharmaceutical and bio-organic chemistry. Brief Contents: Metabolite Investigation; Assay Strategies for Various Drugs; Anti-asthmatics and Kindred Agents; Bioanalytically Exploitable Techniques; Cumulative Index of Analytes; General Index. Special Publication No. 11 0 Hardcover ISBN 0 85186 236 5 September 1992 ROYAL CHEMISTRY xiv + 356 pages Price f75.00 To order, Piease write to the: Royal Society of Chemistry, Turpin Distribution Services Limited, Blackhorse Road, Letchworth, Herts SG6 1 HN, United Kingdom, or telephone (0462) 672555 quoting your credit card details. We can now accept AccessNisalMasterCard/Eurocard.Turpin Distribution Services Limited is wholly owned by the Royal Society of Chemistry. For information on other books 8nd joum8is, please write to : Royal Society of Chemistry, Sales and Promotion Department, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. RSC hfembers should obtain members prices and order from : The Membership Affairs Department at the Cambridge address above. COPIES OF CITED ARTICLES information Services The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0144-557X    

DOI:10.1039/AP99330BX031

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 32 1 The Banning of Halogenated Solvents- Implications for Valid Analytical Measurement Halogenated solvents are increasingly being withdrawn from use on environ- mental and health and safety grounds. Many of these materials are capable of depleting stratospheric ozone, causing an increase in the UV-B radiation that reaches the Earth’s surface with poten- tially damaging results in increased inci- dence of skin cancer, cataracts and crop damage. These concerns resulted in the adop- tion, in 1987, of the Montreal Protocol on Substances that Deplete the Ozone Layer. The Protocol, which has since been ratified by over 100 countries, sought to reduce and ultimately end the production and consumption of ozone depleting materials. The original Protocol set up targets for controlling chlorofluorocarbons (CFCs) and Halons.These materials have the highest ozone depleting potentials, together with long atmospheric lifetimes which range from tens to hundreds of years. At the last meeting of the parties to the Protocol in November, 1992, it was agreed to phase out the production of Halons by January 1, 1994, and of CFCs, carbon tetrachloride and l,l,l-trichloro- ethane (methyl chloroform) by January 1, 1996, and to introduce controls for the first time on methyl bromide and hydro- chlorofluorocarbons (HCFCs) . The Euro- pean Commission has adopted a slightly faster phase-out schedule whereby CFCs and carbon tetrachloride will be phased out by January 1, 1995, following an 85% cut in supplies by January 1, 1994.Halons, chlorofluorocarbons and 1,1,1- trichloroethane currently have their lar- gest uses in refrigeration, air condition- ing, solvent cleaning and fire fighting. They are used for foam blowing in the building trade as well as non-flammable solvents in electronics and engineering for metal and component cleaning and degreasing. They are also used in the pharmaceutical and agrochemical industries. The Montreal Protocol and associated EC regulations include provision for ‘essential use’ exemptions from the pro- duction ban. A controlled substance can be considered essential only if it is necess- ary on health and safety grounds, or is critical for the functioning of society. Furthermore, it must be demonstrated that all economically feasible steps have been taken to minimize the use and associated emissions, and that the sub- stance is not available in sufficient quan- tity or quality from banked o r recycled stocks. The essential use provisions for CFCs and carbon tetrachloride may continue until the end of 1999 at the latest.The cessation of production of the controlled substances will also impact on the requirement and hence the economics of production of other solvents. This could result in a decrease in availability of solvents which are not currently subject to controls under the Protocol. In the analytical community the controlled materials are used in a wide range of analytical methods and in organic synthesis, for which their specific solvent properties and spectroscopic characteris- tics make them ideal reagents. Many of the analytical methods likely to be affected have national or internatio- nal regulatory standing.They are used to support United Kingdom and European Community legislation in the areas of health and safety, the protection of the environment and the protection of reve- nue by means of analysis to establish the identity and authenticity of traded mater- ials. Carbon tetrachloride and l,l,l-tri- chloroethane are included as essential reagents in methods under European Community directives controlling veterin- ary drugs and cosmetics and in standards relating to the safety of toys. The solvents are also widely used in validated methods to extract pesticide residues from food- stuffs and in the determination of hydro- carbon oils in tanker ballast waters using International Maritime Organisation methods.The CFCs are used in the determination of mineral oils in soils and waters using the methods of the US Environmental Protection Agency and in several methods for the determination of migration from plastics. Some of the methods are still in draft standard form and are currently under evaluation. In addition to their uses as analytical reagents, and in syntheses, there will be a requirement to retain controlled sub- stances as reference materials for their determination in environmental samples. This will be necessary for as long as these materials remain in the environment, which for some of the materials could be several hundred years after production and use have ended. With the end of the ready availability of the solvents in sight it is necessary to ensure that the analytical community remains able to support all relevant legis- lation.To do this methods are required which are validated and mutually recog- nized both within the EC and worldwide. Validation of analytical methods using alternative solvents and reagents will be a time consuming and expensive process, and to validate against existing methods will require availability of the controlled materials for the foreseeable future. Many groups are investigating alterna- tive solvents and guidance is available from solvent manufacturers on solvent mixtures with similar properties. How- ever, direct replacement of solvents may not prove to be possible. For example, no satisfactory alternatives to the controlled materials have been found for the deter- mination of hydrocarbon oils in waters and soils despite extensive investigation by the relevant standards groups.In such cases alternative technologies, such as supercritical fluid extraction, may be required. Where alternative techniques are proved necessary validation will be expensive in terms of both capital costs and training requirement. In some in- stances, if it is to continue to be possible to provide analysis, continuity of supply will have to be maintained and urgent con- sideration must be given to both reducing the amounts of solvent used and recover- ing and recycling that which is used. Until now the problems of replacing halogenated solvents have been examined on a sectoral basis. However, a more concerted approach by the analytical community is needed to ensure that both regulatory requirements and the objec- tives of the Protocol can be met.As part of the VAM Initiative, the UK Chemical Measurement Advisory Committee (CHEMAC), which advises the LGC on implementation of the VAM Initiative, has started to take a lead in co-ordinating appropriate action, and will be interested to hear proposals for generic work which could be carried out collaboratively. The Chairman of CHEMAC has written to the Department of the Environment request- ing that the continued production of CFCs, carbon tetrachloride and 1 ,l , 1 - trichloroethane for analytical uses be deemed essential. Further information on halogenated322 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 solvents in analytical chemistry can be of CHEMAC from Mr. Peter Bedson at of the LGC is Queens Road, Teddington, obtained from Miss Irena Agater at the the LGC (tel: 081-943-7614). The address Middlesex TWll OLY. LGC (tel: 081-943-7312) and on the work SAC Gold Medals At a reception held on May 18, 1993, the to the presentations orations were deli- twentieth and twenty-first Society for vered by Professor A. Townshend (on Analytical Chemistry Gold Medals were Professor Thorburn Burns) and Professor presented to Professor D. Thorburn J. J . Monaghan (on Professor Games). Burns and Professor D. E. Games. Prior Professor Thorburn Burns receives the twen- tieth Gold Medal from Dr. E. J . Newman (L), President of the Analytical Division At the reception ( L-R): Professor J. J . Monaghan, Professor D. E. Games, Dr. M . P. Games, Dr. E. J . Newman, Mrs. C. Thorburn Burns, Professor D. Thorburn Burns and Professor A . To wnshend Professor Games receives the twenty-first Gold Medal from Dr. Newman

ISSN:0144-557X    

DOI:10.1039/AP9933000321

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

322 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 Reports of Meetings Electroanalytical Group The Annual General Meeting of the Group was held at 1.30 p.m. on Wednes- day, March 17, 1993, at the Linnean Society, Burlington House, London W1. The Chair was taken by the Chairman of the Group, Dr. C. M. G. van den Berg. The following office bearers were elected for the forthcoming year: Chairman- Mr. S. Edwards. Vice-Chairman-Dr. J. M. Thompson. Honorary Secretary - Mr. A. E. Bottom, ABB Kent-Taylor Ltd., Oldends Lane, Stonehouse, Gloucestershire GLlO 3TA. Honorary Treasurer-Dr. B. J. Birch. Honorary Assistant Secretary - Dr . J. P. Hart. Members of Committee-Dr. C. M. G. van den Berg (ex oficio), Dr. J . Burmicz, Mr. J. Comer, Professor A. K. Covington, Dr. S. Dennison (co-opted), Dr.D. Diamond (co-opted), Dr. A. G. Fogg, Dr. P. Fielden and Dr. J. M. Slater. Dr. R. M. Smith and Mr. J. C. Tillman were re-appointed as Honorary Auditors. Joint Pharmaceutical Analysis Group The twenty-third Annual General Meet- ing of the Group was held in the hall of the Royal Pharmaceutical Society of GB at 1 Lambeth High Street, London, on March 25, 1993. In the unavoidable absence of the Chairman, Dr. G. P. R. Carr, the Chair was taken by Immediate Past- chairman, Dr. Alan Mathias. The Honor- ary Secretary, Professor Geoffrey Phil- lips, selected key extracts from the (pre- viously circulated) Report of the 22nd AGM, held on March 19, 1992. The Report was formally adopted. The Chairman confirmed that only three nominations had been received for the three vacancies on the Committee of Management. He declared duly elected Dr. Peter D. Harrowing (Bristol Royal Infirmary) and Dr. David R. Rudd (Glaxo Group Research) but the third nominee, Dr. Alastair Davidson (Medi- cines Testing Laboratory), had subse- quently been appointed by the Royal Pharmaceutical Society as their represen- tative on the Committee. As a conse- quence, Dr. Paul Graham (Fisons Phar- maceuticals) had been invited to fill the third elected member vacancy. The meet- ing endorsed this proposal. Dr. Arthur H. Andrews was appointed Honorary Treasurer. Two nominations had been received for the two posts of Honorary Auditor: the meeting con- firmed the election of Dr. James Page (RSC) and Mrs. Hilary Judd (RPS).

ISSN:0144-557X    

DOI:10.1039/AP9933000322

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 323 Analytical Division Honours At its meeting held on May 12, 1993, the Twenty-first SAC Silver Medal: Dr. Council of the Analytical Division P. R. Fielden (University of Manchester approved the followed recommendations from its Honours Committee. Twenty-sixth Analytical Division Dis- Eighth Robert Boyle Medal in Analyti- tinguished Service Award: Dr. A. G. cal Chemistry: Professor T. Fuginaga Fogg (University of Loughborough). (University of Kyoto, Japan). Twenty-seventh Analytical Division Institute of Science and Technology). Distinguished Service Award: Mr. H. I. Shalgosky (formerly of AEA, Harwell). Schools Lecturer for 1994/5: Professor J. J. Monaghan (ICI Chemicals and Polymers).

ISSN:0144-557X    

DOI:10.1039/AP993300323b

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

324 ANALYTICAL PROCEEDINGS, AUGUST 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 outlines ideas with prospective authors. Requests for Reprints as an Assessment Parameter of Papers Published in Journals of Analytical Chemistry Jose L.Guiiion and Asuncion Jaime E. T.S.I. lndustriales, Universidad Politecnica de Valencia, P.O. Box 22012, 46071 Valencia, Spain The work developed by a scientist is meaningless if it is not communicated to the rest of the scientific community by means of research papers and publications. Nowadays, the scientist tends to publish a great number of papers, not only because of the simple dissemination of scientific knowledge, but also because, in academic circles and to a lesser extent in other institutions, one of the usual criteria for promotion is the number of publications. One question widely debated is whether, in the evaluation of a scientist, not only the number of publications but also the number of citations of a scientific paper should be taken into account.The data must be read prudently when one is trying to establish the true impact of a publication on science. Thus, the scientist should be cautious when interpreting the number of citations as the availability of citation search and the number of publications makes it is easy for the scientist to exaggerate the importance of such data in charting trends and in evaluating colleagues. When publishing a research paper, the intention of the author is that is should be read by a great number of people. Today, modern information technology allows rapid dissemi- nation at many levels and in many languages, especially when the paper is included in abstracts such as Chemical Abstracts or Current Contents. Owing to the fact that abstracts are not always accurate and their usefulness is limited, the scientist should read, whenever possible, an article directly from the original source.However, the reading of the scientific literature in appropriate journals is not always possible because departments or institutions do not have all the journals of their speciality available. A way of solving this deficiency is to request reprints directly from the authors of a given paper or publication. A question then arises as to whether there is some other parameter, apart from citation index, that shows the importance of a publica- tion. We feel, along with M a ~ r a e , ~ that requests for reprints which arise after initial publication could be considered, at least, as a way of assessing the interest engendered by a scientific paper. Of course, we do not suggest this system is an alternative way of quantitatively measuring the value of a publication, but simply that the request for reprints might be taken as a quality factor.This paper deals with the frequency of requests for reprints in a given time, and is also a study of their countries of origin. Finally, a relationship between the citation index and the request for reprints is carried out. We are not aware of any previous papers in the literature developing a similar study. Method To carry out this work it is necessary for published papers to be included in abstract publications such as Chemical Abstracts or Current Contents and to divulge their inclusion. Hence, any material published in local journals or similar publications, which do not have a wide geographical distribution, cannot be used.The papers selected in this work were published by one of the authors in different journals of analytical chemistry in 1985- 1990. On the other hand, in order to compare the results obtained, some papers in photochemistry published by other colleagues were also included in the analysis. Table 1 shows the papers used in this work. The frequency of requests for reprints is calculated in months, using the date of the post-mark or of the request card. The total number of citations has been obtained from the citation index published by SCI, together with Chemical Abstracts and Current Contents. Results and Discussion The usual frequency plot of request for reprints versus time is shown in Fig.1. In general, the points fit a short rising curve Table 1 Papers used in this work Analytical Chemistry - 1 Guiiion, J. L., Talanta, 1985, 32,265. 2 Garcia-Anton, J., and Guiiion, J. L., Analyst, 1985, 110, 1465. 3 Guiiion, J. L., and Garcia-Anton, J., Anal. Chim. Acta, 1985, 177, 225. 4 Garcia-Anton. J., and Guiiion, J. L., Analusis, 1986, 14, 158. 5 Garcia-Anton, J., and Guiiion, J. L., Analyst, 1986, 111, 823. 6 Guiiion, J. L., and Grima, R., Analyst, 1988, 113, 613. 7 Guiiion, J. L., Rev. Metal. (Madrid), 1989, 25, 405. 8 Guiiion, J. L., and Belanche, M., Connaiss. Vigne Vin, 1989, 23, 215. 9 Guiiion, J. L., Monzo, J., Garcia-Anton, J., Ureiia, C., and Costa, J., Fresenius J. Anal. Chem.. 1990, 337, 372. Miranda, M. A., Vargas, F., and Serrano, G.-J., Photochem.Photobiol. B: Biol., 1991, 8, 199. Miranda, M. A., Morera. I., Vargas, F., Gomez-Lechon, M. J., and Castell, J. V., Toxic. in Vitro, 1991, 5, 451. Bosca, F., Miranda, M. A., and Vargas, F., J. Pharm. Sci., 1992, 81, 181. Photochemistry - 10 11 12ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 8 1 c 2 6 2 k 2 4 U Y- al 3 U U 2 0 0 2 4 6 8 Months Fig. 1 Distribution of requests for reprints of papers versus time with a maximum followed by a descending curve concave upwards, flattening as time increases. Notice that the first requests for reprints, one or two, occur in the month following publication of the paper, then the maximum of requests is reached following the publication of the relevant entry in Current Contents. The curve can be interpreted as an increase in the knowledge of the existence of such papers when they are available in the abstracts services, followed by a steady decline.Fig. 2 shows that for some papers, however, the request for reprints does not disappear and the paper can potentially be requested at any time. The presence of new requests could be c Q) 3 5 2 + >. C al 3 U L L 2 8 0 2 4 6 8 10 12 14 16 Months Fig. 2 Distribution of requests for reprints when the paper potentially requested at any time is 325 related to ’ citation in other abstracts, such as Analytical Abstracts. Figs. 3-4 show the cumulative percentage request for reprints versus request time for several papers. When the frequency decays in the way observed in Fig. 1, the resulting curve resembles a good sigmoidal curve, which is similar to the curve obtained for the publication times of papers in several scientific journal^.^^ When the frequency of request for reprints does not disappear, Fig.2, then a flattened sigmoidal curve is obtained. The average time corresponding to 50% of requests is about 2-3 months for those papers presenting a simple distribution and about 4-5 months for papers whose requests continue with time. On the other hand, the last request times are about 6 months and about 1 year, respectively. Table 2 shows the distribution of number of requests for reprints by their countries of origin. For papers in analytical chemistry the total number of countries was 33, almost the same number of countries (36) counted by Braun6 in the scientiometric indicator value for publication productivity between 1978 and 1980.From these 33 countries, 10 countries account for 80% of requests for reprints. Similar results are obtained for papers in photochemistry. It is interesting to note that these countries have the greatest chemistry publication productivity. Table 2 also shows that requests for reprints to the USSR and Japan are lower than 1%. However, these two countries together have an analytical and all chemistry publication productivity of about 16 and 25%, respectively (third and fourth column in Table 2). Conversely, some countries such as Czechoslovakia and Belgium have a high level of requests for reprints, whereas their publication productivity is low. Although the above results are limited to the areas covered by our research, one might wonder if countries such as the USSR and Japan are either not used to requesting reprints or have good reference libraries, whereas countries such as Czechoslo- vakia and Belgium only have good abstract services available.Figs. 5-6 show a graph of concentration of requests for 0 2 4 6 8 Months Fig. 3 Request times of papers until the requests cease. Median time is 2-3 months. Curve A, papers 2 and 12; curve B, papers 9 and 11; curve C, paper 6 in Table 1326 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 1 0 2 4 6 8 10 12 14 16 Months Fig. 4 Request times of papers when the requests do not cease. Median time is 4-5 months. Curve A, papers 5 and 10; curve B, paper 1; curve C, paper 3 in Table 1 reprints versus the number of countries for both research areas.As can be seen, the 50% of requests mark corresponds to a similar number of countries, with average values of 3.6 and 3.2, respectively, even when the number of countries counted for each area is different (33 for analytical chemistry and 15 for photochemistry). Another question worth considering is whether there is some correlation between the total citation index and the interest in a paper measured from the request for reprints, that is, as the number of requests increases so does the citation index. The plot of the cumulative citation index of 9 papers published by the authors versus the number of requests for reprints is shown in Fig. 7. This type of graph is analogous to that obtained by Braun’ in his analysis of the scientific productivity (number of papers) versus the total number of scientists in a country.The points, if plotted in a similar way to Braun’s, could be fitted to a straight line with a poor dispersion (Y = 0.9239). Cumulative index = 1.37 + 0.21 X number of requests Nevertheless, this dotted line divides the papers into two areas. The upper points represent papers with a higher incidence than the incidence corresponding to the linear equation. The lower points plotted far away from the broken line represent papers which are appealing by their title but which are not cited, either because they do not suit exactly the requesting scientist’s work, or because, according to Braun and Table 2 Comparative data for requests for reprints in analytical and all chemistry publication. A, Papers of analytical chemistry; B, papers of photochemistry Requests for World publication reprints (Yo) share (YO) Country A-papers? B-papers$ Analytical’ Chemistry8 USA 17.5 19.5 26.0 22.3 Germany* 16.6 9.7 7.1 8.7 Czechoslovakia 15.0 2.4 2.4 1.7 Poland 8.7 1 3.3 1.7 India 5.5 1 4.3 5.1 Canada 2.4 7.3 3.1 3.1 France 1.6 17 4.6 4.9 Spain 1.6 7.3 4.4 1.9 UK 1.6 2.4 5.7 6.4 USSR <1 t 1 5.5 14.3 Japan tl <1 10.3 11.3 Total 80 75.3 78.2 82.2 Belgium 9.5 9.7 1.4 0.8 * Both East and West Germany.? Total number of countries, 33. $ Total number of countries, 15. 100 80 v) a, 0 U 2 60 8 LC a, .- U 2 40 5 0 20 0 20 40 60 80 100 Cumulative YO of countries Fig. 5 Concentration of requests for reprints in analytical chemistry: 50% of requests arc concentrated in only 3.6 countries.(Total number of countries, 33) 100 80 cn +, a 3 60 + s a .- w $ 40 0 5 20 0 20 40 60 80 100 Cumulative % of countries Fig. 6 Concentration of requests for reprints in photochemistry: 50% of requests are concentrated in only 3.2 countries. (Total number of countries, 15) Bujdoso,’ one does not always understand the psychology of who cites and why. Conclusions The number of requests for a reprint is included as a parameter of the incidence of a published scientific paper. The average time of request for reprints is 2-5 months, The last request time is usually not more than 1 year. The number of requests to a given country is not always related to its chemistry publication productivity. About 3-4 countries account for 50% of requests for reprints, from which the USA and Germany show the highest values.ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 10 I 1 High incidence0 1 6 0 5 Low incidence 327 A tiorrelation between citation index and the number of requests for reprints is proposed. We thank Miguel Angel Miranda for his requests for reprints in photochemistry. 5 Number of reprint requests 6 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 7 Fig. 7 Plot of cumulative citations versus number of requests for papers (Table 1) 9 8 reprints in papers of analytical chemistry. The numbers indicate the References Mor$son, G. H., Anal. Chem., 1983, 55, 1457. Garfield, E., Curr. Contents, 1982, 22(9), 5. Macrae, R . , Int. Analyst, 1988, 2, 2. Braun, T., Bujdoso, E., and Lyon, W. S . , Anal. Chem., 1980,52, 617A. Braun, T., and Bujdoso, E., CRC Crit. Rev. Anal. Chern., 1982, 13(3), 222. Braun, T., Bujdoso, E., and Schubert, A., Literature of Analytical Chemistry: A Scientometric Evaluation, CRC Press, Boca Raton, FL, USA, 1987. Braun, T., Glanzel, W., and Schubert, A,, Trends Anal. Chem., 1989, 8, 281. Braun, T., Glanzel, W., and Schubert, A.. Scientometrics, 1988, 14, 3. Braun, T., Talanta, 1976, 23, 743.

ISSN:0144-557X    

DOI:10.1039/AP9933000324

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

328 SAC 92 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 The following is a summary of one of the papers presented at the tenth SAC Conference held on September 20-26, 1992, in the University of Reading. The conference incorporated the third Spectroscopy Across the Spectrum Conference and the 150th anniversary celebrations of the Laboratory of the Government Chemist. Summaries of fourteen other papers given at the conference were published in the June issue (p. 244). Isotope Dilution Mass Spectrometry as a Primary Method of Analysis P. De Bievre IRMM Mass Spectrometry, Advisor on Reference Materials, Institute for Reference Materials and Measurements, Commission of the European Communities - JRC, B-2440 Geel, Belgium The need for quantitative determination of an amount of substance (symbol: n ) has given rise to a variety of ingenious tools to realize it.One of these inventions was the balance (Fig. l), which enabled one to compare amounts of substance directly using their weight. When we analyse the characteristics of this comparison process, we observe the following. 1. It is straightforward in comparing weights (which are 2. It is transparent: we understand the comparison process However, when we look a little closer, we note two more points . 3. It disregards the particulate (atomic or molecular) nature of matter (not yet known at that time). 4. It also needs standard weights in order to obtain the same value for the same n weighed at different places (gravity is different at different geographical locations) and we require the determination of n to be independent of its location.The discovery of the particulate nature of matter soon led to the concepts of ‘atomic weight’ and ‘molecular weight’. They allowed workers to express the ‘weights’ in numbers which proportional to masses). fully. were proportional to the number of elementary entities (atoms or molecules). The important ‘insight’ of Loschmidt, Avogad- ro and others, that there existed a constant factor between the macroscopic level, where ‘weighable’ amounts were handled, and the atomic/molecular level, where numbers of atoms lay at the basis of n , was one of the most fascinating products of the human mind. In addition, it was formulated about 200 years before an atom could be observed. We note that the obvious unit for n , a number of elementary entities, could only be arrived at indirectly, i.e., the weighing needed a subsequent ‘correction’ using ‘atomic weights’ (hence the importance of clearly established and generally accepted atomic weights!); the approach could only be used in practice for the determination of ‘weighable’ amounts, i.e., of an extremely large number of atoms; and a new SI quantity (name, amount of substance; symbol, n) with a corresponding SI basic unit (name, mole; symbol, mol) was internationally agreed in 1971.It is a number of entities (atoms/molecules). It would, therefore, be useful to have another instrument than the balance which (a), would be capable of measuring directly numbers of atoms, and ( b ) , could measure n well below ‘weighable’ amounts, an essential condition for trace analysis.We have such a tool: the Isotope Mass Spectrometer. We will now see how it fulfils the above requirements. m Compares amounts of substance ( n ) by comparing weights or masses From early times, weights (or masses) could be precisely compared by a single instrument the balance Recognizing its status, science gave its measurements a base (SI) unit: the kg But the science and technology of chemistry rests on the fact that atoms combine in simple proportions of numbers, so chemists cannot use weights or masses directly to compare amount of substance. They must divide each mass by atomic (or molecular) weights to get what they need. The balance does not take into account the particulate nature of matter. Fig. 1 substance The balance, used in early chemistry to compare amount of Isotope Mass Spectrometer (IMS) In order to understand the IMS in its bare essentials see Fig.2, which shows the essential function which it carries out, viz., comparing two numbers of atoms of different mass (‘isotopes’). When we analyse this comparison process, we observe: firstly, it is Straightforward in comparing substances; and secondly, it is transparent in that we fully understand the comparison process. When we look a little closer, we also note: thirdly, it takes the particulate nature of matter fully into account because it indeed compares numbers of elementary entities (atoms); and fourthly, it determines n in a way which is independent of geographical location. We can therefore conclude that the IMS acts as a balance for numbers of atoms and it takes the particulate nature of matter fully into account (Fig. 3).It sorts out atoms, then counts them. We note, however, that we will have to examine closely how far the quantity we measure (a ratio of numbers of ions detected) is exactly equal to the quantity we want to measure (a ratio of numbers of atoms in a sample). We are concerned by any process in the ion source, collector or amplifier which would disturb a strictly one-to-one relationship between atomsANALYTICAL PROCEEDINGS. AUGUST 1993, VOL 30 r“ -4 compares numbers of atoms of isotopes: RB = - 329 Fig. 2 The (isotope) mass spectrometer (IMS): the modern chemist’s balance to be detected and ions actually measured. In other words, we have to verify whether these processes in the ion source and collector introduce errors in the measurement and, if so, whether they can be sufficiently well identified and quantified so as to yield a completely ‘transparent’ picture of the measurement process.Isotope Dilution Mass Spectrometry If one of the numbers of atoms in Fig. 2 is known, i.e., if it is generated from a sample containing this known number (we define such a sample as a spike), we call the procedure isotope dilution mass spectrometry. In its simplest form (see Fig. 2) it compares an unknown number ( N x ) of atoms of an isotope to a known number ( N y ) of atoms of a non-present but added isotope of the same element through a measurement of a ratio of numbers of atoms ( R B ) of two isotopes: NxlNy = R B (1) Quantity Mass Amount of substance Symbol of quantity m n SI unit mol We’observe that it constitutes a direct approach to determining n as compared with traditional weighing.(The fact that in most instances IDMS measures an induced but carefully controlled change in an isotope abundance ratio, i.e., in a ratio of numbers of atoms, does not detract from the above reasoning and will be commented on later under The General IDMS Equation.) The question noted under Isotope Mass Spectrometer as to how far the quantity we measure is exactly equal to the quantity we want to measure, can now be replaced by the question whether exact proportionality exists between the ratio of numbers of ions coming from unknown sample and spike. This raises the question ‘can errors occur in the IDMS measurement process’ and if so: where are they generated?; how large can they be?; can they be adequately controlled and corrected for most purposes? Such errors will cause an uncertainty in the measurement, even after correction, and will hence contribute to the uncertainty budget of the measurement.This point is addressed in the next section. The Uncertainty Budget of the IDMS Measurement Process An essential condition for a complete and scientific uncertainty statement on a measurement (we call that an ‘orthodox uncertainty’) is a direct and fully understood and controlled measurement process. We have shown that this is the case for the IMS in the IDMS procedure. Another condition for an ‘orthodox’ uncertainty statement is a systematic list of the quantified uncertainties of each step in the measurement process, either by estimating its maximum possible size or by actually determining its size experimentally.This leads to a better ‘total uncertainty’ concept and will avoid the commonly occurring situation in many measurements (illustrated by Fig. 4). In order to estimate the uncertainty of a measurement process, it is necessary to identify the steps where deviations from the measurement model could occur (because of imper- fect realization of the model) or must occur (because of the physics and chemistry involved). In the case of TMS, these are (Fig. 2): the conversion of neutral atoms to ions in the ion source, including the introduction of the sample in the ion source (viscous or molecular flow, or evaporation of a solid); and the use of mass dependent detectors at the collector side of the IMS.Table 1 attempts to identify possible deviations from the Realization of SI unit International standard kilogram in Sevres N A Ratio measurement to (known fraction of) of unknown /kg ‘mol m Physical measurement Small, known corrections only Unknown amount in (fractions of) kg mol Fig. 3 International traceability of measurements of amount of substance and of mass are very similar330 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 T Discrepancy 1 Problem No discrepancy No Droblem 1 Fig. 4 Discrepancy problems can be caused unnecessarily by lack of ‘orthodox’ uncertainty assessment model, to estimate maximum size for these deviations and to indicate how one can correct for these deviations and estimate the extent of the remaining uncertainties after these corrections.An uncertainty ‘budget table’ can be drafted as illustrated in Table 2. The table is conceived such as to be useful in any measurement process and is intended to be a practical tool for any measurement scientist. It also has the characteristic of showing why there are so many ‘disputes’ between experi- menters and statisticians: each of these two can only treat half of the measurement uncertainties; reproducibilities of items (a) and (b) in the table can be handled by statisticians, but the identification of (b) and (c) can only come from experimenters. Reasonable transparency of the IDMS measurement pro- cedure and its uncertainties has now been established and we can proceed to a further important step in the measurement process: the (essential) chemical preparation step for an isotopic measurement.Sample Preparation Step The (chemical) preparation of a sample for any analytical measurement as well as the possible uncertainty generated by it should always be included in the evaluation of the result and in the uncertainty of a measurement. This is very often omitted, thus leading to unrealistically reduced uncertainties (and the problem illustrated in Fig. 4). We do not want to fall into this trap in our treatment of IDMS. It is essential, but often overlooked, that in a well organized IDMS: the ‘spike’ is added to the unknown sample before any (chemical) preparation step of the sample is started; the isotopic homogenization between the isotopic atoms of the unknown sample and spike is carried out (Fig. 5 ) ; the chemical operation to guarantee the isotopic homogenization is per- formed without loss of any substance by carrying it out either in open (acid?) solution if no volatile reaction components can escape which could contain certain isotopes of the element under investigation or in a closed reaction bomb if there is the slightest risk of escape of volatile components containing said isotopes; and the matrix containing the (trace) element to be assayed is thoroughly destroyed by either ‘strong acid’ or ‘closed bomb’ treatment, so as to make the IDMS matrix- independent, an extremely important condition in trace analysis.The end result of the measurement RB is now physically established within the sample before any sample for the measurement is taken.In other words, the result is ‘frozen’ in the sample. Since it is an isotope abundance ratio, it can be determined on a sample which can be ‘extracted’ in a non- quantitative way: it is sufficient to measure RB in a non- quantitative sample, which is the carrier of the end results Rg. The essential and difficult requirement of quantitativity, sometimes so difficult in classical analytical chemistry, is obviated. Second Order Refinements The key point of the IDMS measurement, the measurement of R g , can be corrected by means of appropriate Isotopic Reference Materials for the (small) isotopic effects described in Table 1, items 1-2, reducing the ‘total uncertainty’ of an IDMS determination to 1 4 .1 YO. In a number of cases, there is an ‘intrinsic’ possibility of calibration of IDMS measurements; if a known (certified) abundance ratio R34 of two other isotopes with masses M3 and M4 is incorporated in the sample to be measured (Fig. 6), and their certified ratio is observed (‘obs’), it can serve to determine correction factors in-situ and in-tempore. These factors can then be used to correct the RB ratio of isotopes with masses M I and M2 in the same measurement (Table 1, item 3). We call this the Internal Ratio Standard (IRS) technique because it uses a known isotope abundance ratio as ‘internal standard’, built into the sample. It allows one to correct for the small systematic errors at the time, and at the place, and under the circumstances of the actual measurement, i.e., under the actual circumstances of their occurrence.This approach opens the possibility of crossing the 0.1% total uncertainty barrier of quantitative assay, even in trace analysis (see Table 1, item 3). Dynamic Range of IDMS: a Road to Accuracy in Trace Analysis So far in this paper we have not paid attention, nor was it important to do so, to the dynamic range of the IDMS measurement procedure. This range becomes particularly important in trace analysis which requires, by definition, a large dynamic range. Trace analysis aims at quantitative determination of trace and ultra-trace amounts, i.e., microgram or nanogram amounts or smaller in gram-size sample (example given in Fig. 7 ) . In Fig. 8 the isotope ratio ( R ) values can be measured on samples down to atoms, which still yield easily measurable ion currents of lo+’ ions s-’ during 100 h at 0.1% ionization efficiency.This number of atoms is roughly equiva- lent to g (of molecular weight 100). Assay of such small amounts (or concentrations) by IDMS requires correspond- ingly small ‘spikes’. Making such small ‘spikes’ can be achieved by metrological dilution of accurately prepared solutions of a Spike Reference Material, allowing the workers to spike the unknown in a roughly 1 : l ‘unknown to spike’ ratio. This procedure effectively means that in the dynamic range intrinsic to trace analysis (again see Fig. 7 ) , the unknown sample is spiked at its own amount (or concentration) level, say at the 8 x g g-’ level for iron in Fig.7. The ‘bridge’ between the large size matrix and the small size sample is constituted by weighing, a process known to have a very large dynamic range. The problem now is that the total (spiked) sample could be too small for the measurement of Rg. The common solution to this problem is the use of a very high sensitivity detector and acceptance of the intrinsic loss of accuracy in order to perform the measurement. If the sample size is too small, no measurement is possible. ‘Overspiking’ a sample with a 102-103 fold larger spike, if a larger isotope ratio mass spectrometer (LIRMAS) capable ofANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 331 Table 1 Attempting to establish the ‘uncertainty’ of IDMS. N x = unknown number of atoms; N y = known number of atoms (spike); RB = abundance ratio of the two isotopic atoms in the ‘blend’ Part of the IDMS measurement process Maximum size of deviation from theoretical model Possible way to correct deviations from the theoretical model (with uncertainties on these corrections) 1.Nx NY d M 2 / M 1 where M , is the mass of the isotope At M = 100 the maximum error is The measurement ~ = RB The ion generation in the ion source concerned Isotopic Reference Materials (RMs) (reduction of the uncertainty by a factor of 10 to - 1 = 0.5% 0.05 yo ) for neighbouring isotopes The ion detection (in the ion collector) 2. The ’standard’ or reference for the comparison: the ‘spike’ 3. Internal ratio calibration in case at least 4 isotopes are available for ‘internal ratio standard’ calibration ratio standard’ Faraday collector: <0.1% Electron multiplier: d M 2 / M 1 maximum ~ 0 .1 % as optimal conditions can be pursued At M = 100 it is possible to reduce the RMs in its metrological preparation above errors by a factor of 10 or more: Building a known isotope abundance ratio into the unknown sample to serve as an ‘internal /--- 1 = 0.01% measuring 102-103 abundance ratios became available, would enable the direct and accurate measurement of the ratio of a small isotope abundance (from the unknown sample) to a large isotope abundance (from the added spike). The net gain, however, is that the resulting blend of unknown sample and spike will yield a 102-103 larger sample for measurement. Based on this reasoning, Finnigan MAT accepted an order from IRMM to develop such an instrument in 1983.It is interesting to know that the instrument was demonstrated to a number of interested customers in 1989-1992, after a long period of difficult developmental problems, and that it enables the measurement of lOS--lO7 abundance ratios (detection limit lo8-10’ ratios): it is designated the Finnigan MAT 262 RPQ mass spectrometer. It is expected that this instrument will considerably expand the use of IDMS in trace analysis down to lower concentrations or smaller sample sizes. The General IDMS Equation In the section on isotope dilution mass spectrometry we indicated that the general definition of IDMS is the measurement of an induced change in an isotope abundance ratio. We will now elaborate on this by going from the simplest IDMS equation NxlNy = RB to the general IDMS equation where R x = the isotope abundance ratio in the unknown sample [two isotopes (the ‘dilution isotopes’) are chosen for this purpose, usually the most abundant ones]; RY = the isotope abundance ratio of the same two isotopes in the known spike; RB = the isotope abundance ratio in the spiked sample (‘blend’).Abundance ratios of other than the two dilution isotopes (or ‘ID isotopes’) appear in the I: terms. These are, in fact, only correction terms in the equations. It is easily seen that the general equation reduces to the simple equation when R x = 03, RY = 0, and other isotopes are absent. Table 2 Proposed format for establishing and presenting the composition of ‘total’ uncertainty for use by laboratories when certifying a Reference Measurement (umpire, referee) or a Reference Material ( a ) ( h ) Y O t......= a Repeatabilily of measurement, as determined from 6 to 10 measurements, each Repeatability of correction factors determined n times separately for each of the known systematic errors, expressed as 2 s with 6 d n < 10 including a separate analytical sample preparation, expressed as 2 s with 6 G n d 10 1st correction factor 2nd correction factor 3rd correction factor nth correction factor Subtotal \’(lst)2 + (2nd)’ + _._. + (nth)’ (c) Estimates o f magnitude of possible (as yet) unknown systematic effects estimated on a 2 s basis 1st correction factor 2nd correction factor 3rd correction factor 11th correction factor Subtotal (1st) + (2nd) + . + (0th) +......+...... k...... +...... = b k...... k...... k...... k...... = c Total = m / F + c‘ +...... --332 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 + 0 Sample Spike Blend Non-quantitative separation and measurement of isotope ratio RB s::;L on small portion of sample Unknown o Known ‘Spike’ number number with of atoms of atoms m m - - 3~ Atoms of (Enriched other isotope chemical of same elements element) Fig. 5 Isotope dilution mass spectrometry 10-2 -I I s 0.2 10-6 t-]- s ------- lo-’ i Fig. 7 Trace analysis is a matter of large dynamic range of the trace element relative to the matrix; trace impurities in Pt containing3.5% of Rh Separation of isotopic ions Solid Gas 100 g - 6 x 1023 atoms - 22.4 I - 3 x 1016 atoms - 1 PI 1 I5P9 u - -Collection (detection) of isotopic ions of mass M1 Fig.6 Intrinsic calibration of the measurement of ion current ratio I , / I2 (ion currents of ions with masses M I and M2) by a known, built-in ‘internal ratio standard’ (I3/& ion currents with masses M3 and M3) The equation also shows that the differences RY - RB and RB - Rx should be large enough to allow a meaningful measurement. There exist, therefore, optimal conditions for IDMS ( e . g . , if any of these differences is zero, no IDMS is possible). These conditions have already been discussed and graphically published for easy use elsewhere. ’ It is important to point out that the general ID equation only consists of ratios of numbers. Firstly, the important ratio that we want to measure: the ratio of an unknown number of atoms of an element to a known number of atoms of the same element in the spike, i.e., an unknown fraction of a mole against a known fraction of a mole; secondly, the isotope abundance ratios of two (same) isotopes in the unknown sample, the spike and the blend.It is equally important to point out that systematic errors which are proportional to R virtually cancel in this equation. Normal ‘error propagation’ (we prefer to say ‘uncertainty propagation’) shows that the uncertainty on NxlNy is for the very major part determined by the uncertainty on RB, just as in the simple equation. 0.1% efficiency -+ 3 x 10’3 ions at 10-11 A - 3 x lo5 s - at 1O-I’ A (-108 ions s-1) - C- 0.1% efficiency - (-108 ions s-7) or -100 h 10-10 10-12 10-14 10-16 10-18 A 109 107 105 103 lo1 ionss-7 ,Electron multiplier, , Ion countina . Fig. 8 How many atoms are available for isotopic measurement? Blank Correction In general, i.e., when samples of natural isotopic composition are determined by IDMS, blank corrections can be similar to those of other methods. In instances where samples with non- natural isotopic composition are determined using natural or non-natural spikes, a more complex procedure has to be used, requiring an extension of the IDMS equation (2):ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 333 where RBL is the isotope abundance ratio in the blank, and NBL the number of atoms in the blank. This equation can be reduced in the two following cases: Conclusions These are as follows. A fully transparent analysis of the measurement process in IDMS is possible; a fully transparent analysis of the uncertainties in the measurement process is possible; IDMS measures directly in SI units for amount of substance; -and if properly carried out, the evaluation of an IDMS result and its associated uncertainty can include the uncertainties introduced in the chemical preparation. IDMS can be used over a very large dynamic range of concentrations because of the large dynamic range of both weighing and the isotope mass spectrometer. IDMS uses the fact that the isotopes are much better ‘representatives’ of an element than the chemical atoms. The help of A. Lamberty2 as well as the conceptual input of H. S. Peiser is gratefully acknowledged. References 1 2 De Bievre, P., and Debus, G. H., Nucl. Instrum. Methods, 1965, 32, 224. Lamberty, A.. and Pauwels, J . , Int. J. Mass Spectrom. Ion Proc., 1991, 104,45.

ISSN:0144-557X    

DOI:10.1039/AP9933000328

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

334 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 The State of Chemistry in 1841 The following is a summary of part of the paper presented at a Joint Meeting of the South East Region and the Micro & Chemical Methods Group held on December 18th, 1991, in the Linnean Society, Burlington House, London W1. London Chemists and Chemistry, Prior to the Formation of the Chemical Society in 1841 D. Thorburn Burns Department of Analytical Chemistry, The Queen’s University of Belfast, Belfast BT7 5AG The period prior to the foundation of the Chemical Society in 1841 was one of consolidation in contrast to an earlier ‘golden age’ of British chemistry exempli- fied by the work of Priestly, Cavendish and Black. The rise of professionalism and the foundation of Chairs of Chemistry in London account for the foundation of the Society.The contributions of Richard Rigg and William Prout were reviewed earlier.” The account continues with the work and disputes of Richard Phillips and David Boswell Reid, and that of the early London University Professors, namely Edward Turner, Thomas Graham and John Frederic Daniell. The State of Chemistry in Britain Just Prior to 1841 The original intention had been to discuss the ‘then and now of British Analytical Chemistry’, to review techniques and methods new in 1841, together with their replacements or counterparts in 1991. However, a search of the journals for 1841, followed by checking via Walden’s Chronologische Ubersichtstabellen, ’ pro- duced only two interesting novel analyti- cal chemical items, namely Varrentrapp and Will’s method for nitro en2 and Berzelius’ concept of allotropy! Varren- trapp and Will’s method remained the only viable alternative to Dumas’ method for 42 yeas until the advent of Kjeldahl’s method in 1883.47s By the late eighteenth century British chemists were more concerned with the overthrow of the phlogiston theory and the diffusion of the new chemistry of Lavoisier than with creative reseach.The ‘Golden Age of British Chemistry’ repre- sented by Priestly, Cavendish and Black * Anal. Proc., 1993, 30, 272. had passed. By the early nineteenth century the most important centres of chemistry were in mainland Europe, as were the specialized journals for chemistry per se.6 British chemical publi- cation outlets were in general journals such as Phil.Trans. up to 1849, which saw the foundation of The Quarterly Journal of the Chemical Society, renamed the Journal of the Chemical Society in 1862 when it became a monthly p~blication.~ The position with regard to monographs also reflects a decline in new, original, material available for publication. Note, for example, Dalton’s ‘A New System of Chemical Philosophy of 1808,8 which was reprinted unchanged in the second edition of 1842.9 The fourth English edition of Berthollet’s ‘Elements of the Art of Dyeing’ appeared in 1841,’O the original editon being from 1791.” The decline of science, generally, was noted by Charles Babbage in 1830 in his monograph ‘Reflections on the Decline of Science in England’.12 Brewster, in its review,I3 advocated the formation of a British organization similar to the ‘Deutsches Naturforchers Versammlung’.This appeal was well received and contri- buted directly to the formation, in 1831, of the ‘British Association of the Advan- cement of ~ c i e n c e ’ . ’ ~ Chemistry did not become a major, university, subject in its own right in London until 1828, and the appointment of Edward Turner to the Chair of Chemistry at University College. lS Prior to the establishment of this Chair chemistry had to be studied in London via medical schools, series of public lectures and in private laboratories and schools,16 such as those run by Frederick Accum and David Boswell Reid. The build-up of confidence, interest and expertise after the establishment of Chairs in Chemistry in London and the then growing profes- sionalism of science would account for the formation of the Chemical Society of London in 1841.17 The period around the time of the foundation of the Society was one of contrasts, both in chemical competence and of the social behaviour of chemists. The elegant approach and excellent ele- mental analysis data of William Prout for some simple foodstuffs is in complete contrast to the work of Richard Rigg which, coupled with a doubtful eco-mass balance, led him in 1848 to conclude that carbon was not an element but was made by plants! In times of rapid technological change spirited debate is expected, the pamphlet records of the period show many, as for example that between Richard Phillips and David Boswell Reid, to have been conducted at the level of personal invective, behaviour that was in complete contrast with the quiet careful approach of Edward Turner. Richard Phillips and David Boswell Reid: A Pair of Disputatious Chemists Richard Phillips (1778-1851) Phillips was educated as a chemist and druggist under William Allen of Plough Court but received his first instruction in chemistry from Dr.George F ~ r d y c e . ” ~ ~ ~ He was appointed Lecturer in Chemistry at the London Hospital in 1817; soon afterwards he was appointed Professor of Chemistry at the Royal Military College, Sandhurst, and lecturer in Chemistry at Grainger’s School of Medicine. From 1839 he was Chemist Curator at the Museum of Practical Geology, Jermyn Street, a post he held until his death in 1851. He was elected to the Royal Society in 1822, being supported by Daniell amongst others.According to his biogra- pher, Woodward, Phillips was offered the Presidency of the Chemical Society on itsANALYTICAL PROCEEDINGS, AUGUST 1993. VOL 30 335 foundation but declined. He was, how- ever, President 1849-51 .19 He published 70 papers on analytical and mineralogical topics said to be ‘characterised by neat- ness and precision’.18 Many of Phillips papers appeared in Annals of Philosophy and later in Philosophical Magazine, with which it merged. Phillips was co-editor of both journals. Phillips also made significant contribu- tions to pharmaceutical chemistry via his critical reviews of the London Pharmaco- poeia (1811 and 1816) which, although justified, were somewhat resented at the time. He was associated editorially with the editions of 1824,1831,1837 and 1851.His aggressive style of reviewing brought him into dispute with Joseph Hume over Hume‘s paper on the proper- ties of barytes, and later with David Boswell Reid over a book review. The dispute with Hume started with Phillips’ critical remarks in 18142” on Hume’s paper of 1802.2’ Hume’s reply,22 Phillips’ counter replyz3 and Hume’s further reply24 all appeared in Phil. Mag. After the last reply the editor hoped ‘that neither of the gentlemen concerned will continue this dispute any longer, as it is evidently fast sinking into personal invec- tive’. The dispute with Reid will be discussed later. Phillips had two important connections with Michael Faraday,” firstly in 1821 when he persuaded Faraday to take up the subject of electromagnetism.Ever since Oersted’s announcement of the discovery of electromagnetism in the sum- mer of 1820, editors of scientific journals and magazines had been inundated with articles on the subject. Theories to explain the phenomenon multiplied; the net result was confusion. Phillips turned to Faraday to review the experiments and theories of the past months and to separ- ate fact from fiction. Faraday reluctantly agreed to undertake a short historical review, since his attention was focused on problems in chemistry quite remote from electromagnetism. He became enthusi- astic, undertook investigation into the subject and, as we now know, produced the first conversion of electrical into mechanical work. Phillips’ second import- ant connection with Faraday was that he proposed him for election to the Royal Society in 1824.David Boswell Reid (1805-1863) David Reid was educated at the (Edin- burgh) High School and Edinburgh University, where he graduated in 1830.26 Chemistry was his favourite study and he set up a laboratory and taught theoretical and practical chemistry in Edinburgh with great success. He was appointed assistant to Black’s successor, Thomas Charles Hope. After a disagreement with Hope over a Chair of Practical Chemistry he returned to his private courses. Reid was influential in his day in the teaching of Chemi~try.~’ To reduce costs he introduced simple microchemical tech- n i q u e ~ ; ~ ’ , ~ ~ these were similar in many ways to those taught by Belcher and Wilson3’ from 1946 onwards.Reid wrote two popular texts, ‘Elements of Practical Cherni~try’~’ and ‘Rudiments of C h e m i ~ t r y ’ . ~ ~ The latter contained instructions to construct his chemical abacus for facilitating the study of the Atomic Theory of Dalton and Berzelius’ atomic symbols. A vitriolic review3’ of ‘Elements of Practical Chemistry’ brought Reid into dispute with its anonymous author, Richard Phillips. Phillips, one of the journal co-editors, confined his remarks to the chapter on nitric acid. After eight pages he concluded: ‘Mr. Reid is not sufficiently acquainted with science which he has undertaken to teach: and even when he appears to possess some know- ledge of the facts, they are usually very loosely, imperfectly and incorrectly stated . . .’. Reid issued a pamphlet to respond to the criticism in detail.‘An exposure of the misrepresentations in the Phil. Mag. for Dec. 1830 in its attack upon the author’s Elements of Practical Chemistry. ’32 Phil- lips replied in another pamphlet ‘A Letter to Dr. D. B. Reid in answer to his pamphlet . . .’33 (Fig. 1). This did not satisfy Reid, who then issued an exposure of the continued misrepresentation of 81’ RICIl.IRD PRILLIPS, F. R. S. L. k E., kc. Fig. 1 Title-page to Richard Phillips’ pamph- let ‘A Letter to Dr. David Boswell Reid . . .’ (1831) Fig. 2 Title-page to David Boswell Reid’s pamphlet ‘An Exposure of the Continued Misrepresentations by Richard Phillips . . .’ (1831) Richard Phillips34 (Fig. 2). This public row did not appear to harm either career. Reid lectured in Dublin on his teaching methods35 and also gave evidence to the Select Committee on Education in Ireland.36 Reid acquired skills and a re utation in ventilation and in acoustics3 from the design of his own lecture theatre and laboratories. This reputation led him to be the consultant for the ventilation and acoustics of the temporary House of Commons in 1836. Reid moved to Lon- don in 1840 to superintend the heating and ventilation of the new House of Commons building, then in progress. He moved his theory and practical classes from Edinburgh to Duke Street, London. After a disagreement with Mr. Barry, the architect of the new palace of Westmins- ter, much of which was quite public, Reid went in 1855 to New York. He lectured at the Smithsonian Institute and in Boston and maintained his interests in ventila- tion.He died suddenly in 1863, just after appointment as Inspector of Military Hospitals. ? The Early Chairs of Chemistry in London Chairs of Chemistry were slow to appear in comparison with other parts of the United Kingdom. The first in London was that at University College, held by Edward Turner from 1828.336 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 Edward Turner (17961837) Turner was born in Jamaica, the second son of Dutton Smith Turner, a prosperous planter, and Mary Gale Redwar, a creole of English ancestry. 15,38 After attending Bath Grammar School he was appren- ticed to a local country doctor from 1811 to 1814. He spent two years on the wards of the London Hospital before studying medicine at Edinburgh from 1816 to 1819.After an unsatisfactory attempt to set up practice in Bath he went to Paris for further study, where he became attracted to chemistry. From 1821 to 1823 he was a pupil of Stromeyer at Gottingen in mineral analysis and chemistry. He returned to Edinburgh and, like Reid, became an important private teacher of practical chemistry. He also acted as chemical editor for David Brewster's Edinburgh Journal of Science. He was appointed to the new Chair at University College and took up his duties in 1828. He was an excellent lecturer. He pub- lished significantly on mineralogical analysis. Turner was distinguished as the author of the best early nineteenth century text of chemistry39 (Fig. 3), which ran to eight editions, and for his work on atomic weights.Turner placed himself in the delicate position of being umpire between two of the greatest living chemists, namely Thomson and Berzelius, when he investigated the discrepancies between Thomson's and Berzelius' atomic weights, confirming those of Berzelius. ELEblEX'l'S C H E M I S T R Y 1 111 He also noted that the true values were inconsistent with Prout's original hypo- t h e ~ i s . ~ ~ , ~ ~ He was elected FRSE in 1825 and to the Royal Society of London in 1830, the London certificate signatories including Dalton, Henry, Daniell and Prout. Turner's great emphasis upon the analyst as the critical aribiter of theory, as well as his exacting standards, stimulated the later researches of F. Penny and J. S. Stas. Turner died at the young age of 40 after a period of 3 years' ill health.He was succeeded in the University College Chair by Thomas Graham in 1831. Thomas Graham (1805-1869) Thomas Graham42-43 was the son of a prosperous manufacturer who entered Glasgow University in 1819 and studied chemistry under Thomas Thomson. This was against the wishes of his father, who had in mind the Church of Scotland as a career for his son. After graduation in 1826 he worked for 2 years with Thomas Charles Hope in Edinburgh. He returned to Glasgow and taught chemistry pri- vately until he became assistant at the Mechanics Institute, Glasgow. In 1830 he succeeded Andrew Ure as Professor of Chemistry at the Ander- sonian University, Glasgow. During his period at Glasgow he made his most important contributions to inorganic chemistry, which included excellent ana- lytical work via distinguishing between the ortho, meta and para acids of arsenic and phosphorus44 and laid the founda- Fig.3 Turner's 'Elements of Chemistry' (1827) Title-page of the first edition of E. Fig. 4 Title-page of the first edition of 7 Graham's 'Elements of Chemistry' (1842) C: H E 31 I C AL P H I L 0 5; 0 P E \-: Tt!E FORCES T\'HICii COSCCR TO TflC I'RODCCTI* Y C. . A L.. . - 8 CHE\IIC;\L PIi ESOJIES.4. Fig. 5 Title-page of the first edition of J . F. Daniell's 'Introduction to the Study of Chemi- cal Philosophy' (1839) tions of his reputation in physical ~ h e m i s t r y . ~ ~ He was elected FRSE in 1828 and FRS in 1834, his London certificate being signed by Faraday, Phillips, Daniell and Dalton. In 1837 he succeed Edward Turner as Professor of Chemistry at University College.He was then almost fully occu- pied in teaching, writing, consultancy and advising government on matters con- cerned with science. His 'Elements of Chemistry'46 (Fig. 4) was widely used and respected both here and abroad. He participated in the founding of the Chemi- cal Society in 1841 and was its first President. When Dalton died in 1844, Graham was the acknowledged senior British Chemist of his day. The second chair in London was that created in 1831 at King's College, and filled by J . F. Daniell. John Frederick Daniell (1790-1845) Daniell was educated privately in the classics but at an early age showed a fondness for the pursuits of science.47 He was placed in the sugar refining business of a relative, and it was at this stage that he made the acquaintance of William T.Brande, who later signed his FRS certifi- cate in 1814. On marriage, in 1817, he became Director of the Continental Gas Company. Despite his obvious skills in manufacturing enterprises he found teaching and research more congenial to his tastes. His early work was more physical than chemical in nature but heANALYTICAL PROCEEDINGS. AUGUST 1993, VOL 30 337 was nontheless elected the Foundation Professor of Chemistry at Kings College in 1831. He devoted great effort to teaching, developed careful and elaborate lecture demonstrations and was the author of an excellent text, ‘Introduction to the Study of Chemical Philosophy’48 (Fig. 5). His research at Kings became electrochemical in character and he is remembered for the power source which bears his name.He died whilst attending a Council meeting at the Royal Society on March 13, 1845. Conclusions The review of London Chemists, their achievements, disputes and inter-rela- tions via the Royal Society does not fully support Babbage’s gloomy outlook given in the ‘Decline of Science in England’.’‘ Turner’s view of the need for consoli- dation and a period of careful review was expressed when he wrote in 1829 ‘The era of brilliant discovery in chemistry appears to have terminated for the present. The time is arrived for reviewing our stock of information and submitting the principal facts and fundamental doctrines of the science to the severest scrutiny’, was nearer the truth then, and perhaps now.References Walden, P.. Chronologische iibersicht- stobellen, Springer Verlag. Berlin, 1952. Varrentrapp, F., and Will, H., ‘Neue Methode zur Bestimmung des stick- stoffs in organischen Verbindungen’, Ann. Chem. Pharm., 1841,39,257. Berzelius, J.. Jahres-Berichf, 1841. 20. 13. Stephen, W. I., ‘Determination of Nitrogen in Organic Compounds in the Years Before Kjeldahl’s Method’, Anal. Proc., 1984, 21, 215. Thorburri Burns, D., ‘Kjeldahl. the Man, the Method and the Carlsberg Laboratory’, Anal. Proc., 1984,21,210. Ihde, A. J . , The Development of Modern Chemistry, Harper and Row. New York, 1964, (Chemical publica- tions pp. 270-276). Moore, T. S., and Philip, J. C., The Chemical Society 1841-1941, The Chemical Society, London, 1947. Dalton, J . , A New System of Chemical Philosophy, Bickerstaff.Manchester, 1808, 1810, vols. I and 11. Dalton, J., A New System of Chemical 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Philosophy, J. Weale, London, 1842, vols. I and 11. Berthollet. C. L., and Berthollet. A. B., (trans. Ure, A.), Elements of the Art of Dyeing . . . , T. Tegg, London, 1841. Berthollet, C. L., Elements of the Art of Dyeing . . . , Couchman, London, 1791. Babbage, C., ReJEections on the Decline of Science in England and Some of its Causes, R. Clay for B. Fellowes, London, 1830. Brewster, D., Quat. Rev., 1830,53,305. Cardwell, D. S. L., The Organisation of Science in England, Heinemann. Lon- don. 1957, revised 1972, reprinted 1980. Terry, H., ‘Edward Turner MD FRS, (17961837)’. Ann.Sci., 1937, 2, 137. Russell, C. A., Coley, N. G., and Roberts, G. K., Chemists by Profession, Open University, Milton Keynes, 1977. Bud, R. F.. and Roberts, G. K., Science versus Practice, Manchester University Press, Manchester, 1984. ‘Obituary: Richard Phillips’, Proc. Chem. Soc., 1853, 5 , 155. Woodward. B. B., ‘Richard Phillips’, in Dictionary of National Biography, ed. Lee, S . , Smith, Elder and Co., London, 1896, vol. 45, p. 211. Phillips, R., ‘Remarks on Mr. Hume’s paper on Barytes . . .’ Ann. Phil., 1814, 4, 432. Hume, J., ‘Remarks on certain proper- ties of Barytes in its combination with mineral acids . . .’, Phil. Mag., 1802, 14, 357. Hume. J.. ‘Answer to Mr. R. Phillips’s Animadversions . . .’, Ann. Phil., 1815, 5 , 116. Phillips, R., ‘Reply to Mr.Humes Answer. . .’, Ann. Phil., 1815, 5 , 292. Mr. Hume, ‘Reply to Mr. Phillip’s Animadversions’, Ann. Phil., 1815, 5 , 428. Williams, L. P., ‘Michael Faraday’, in Dictionary of ScientiJic Biography, ed. Gillespie, C. C., published by C. Scribner, New York, 1971, vol. 4, p. 527. Relation with Phillips, p. 533. Obituary, Proc. R. SOC. Edinburgh, 1864, 5 , 133. Kennedy, D., ‘Dr. D. B. Reid and the Teaching of Chemistry’, Studies, 1942, 31, 341. Reid. D. B., Elements of Practical Chemistry, MacLachlan and Stewart. Edinburgh, 1830. Reid, D. B.. Rudiments of Chemistry, W. and R. Chambers, Edinburgh. 1836. Belcher, R., and Wilson, C. L.. Quali- tative Inorganic Analysis, Longmans Green, London, 1946. Anon, Phil. Mag., 1830, 8, 449. 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Reid, D.B.. A n exposure of the mis- rep resentaiton of the Philosophical Magazine and Annals of December 1830 and the attack on the author’s Elements of Practical Chemistry, MacLachlan and Stewart, Edinburgh, 1831. Phillips, R.. A letter of Dr. D. B. Reid in answer to his pamphlet ‘An expo- sure. . . ’, s. Highley, London, 1831. Reid, D. B., A n exposure of the con- tinued misrepresentation by Richard Phillips . . ., MacLachlen and Stewart, Edinburgh, 1831. Reid, D. B., ‘On an extension of the study of Physics’, Annu. Rep. Br. Assoc. Dublin Commun., 1835, 126. Evidence before the ‘Select Committee on Education in Ireland’, 1835, Reid, D. B., Theory and Practice of Ventilation, Warming, Lighting and Communication of Sound, Longman, Brown, Green and Longmans, London, 1844. Brock, W. H., ‘Edward Turner’ in Dictionary of Scientific Biography, ed. Gillespie, C. C., published by C. Scribner, New York, 1976, vol. 13, Turner, E., Elements of Chemistry, W. Tait, Edinburgh, 1827. Turner, E., ‘On the Composition of Chloride of Barium’, Phil. Trans., 1829, 119, 291. Turner, E., ‘Experimental Researches on Atomic Weights’, Phil. Trans., 1833, 124, 523. Obituary, Proc. Chem. Soc., 1870, 8, (New Series), 293. Kauffman. G. B., ‘Thomas Graham’ in Dictionary of Scientific Biography, ed. Gillespie, C. C., published by C. Scribner, New York, 1972, vol. 5, p. 492. Graham, T., ‘Researches on the Arse- niates Phosphates and Modifications of Phosphoric Acid’, Phil. Trans.. 1833, 123, 253. Chemical and Physical Researches by Thomas Graham, (Collated and printed for presentation only. Preface and ana- lytical contents by Dr. R. Angus Smith). Printed by Constable, Edinburgh, 1876. Graham, T., Elements of Chemistry, H. Bailliere, London, 1842. Thackray, A., ‘John Frederic Daniell’, in Dictionary of ScientiJic Biography, ed. Gillespie, C. C. Published by C. Scribner, New York, 1971, vol. 3, p. 556. Daniell. J. F., Introduction to the Study of Chemical Philosophy, J . W. Parker, London, 1839 (2nd edn. 1843). p. 397. p. 499.

ISSN:0144-557X    

DOI:10.1039/AP9933000334

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

338 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 - Solvent New Directions in Chromatography LC delivery system The following is a summary of one of the papers presented at the Analytical Division Symposium of the RSC Annual Chemical Congress, held on April 13th-15thr 1992, in the University of Manchester Institute of Science and Technology. Summaries of five other papers from this symposium were published in the February issue, p. 78. - New Directions in Gel Permeation Chromatography Barry J. Hunt The Polymer Centre, School of Physics and Materials, University of Lancaster, Lancaster LA I 4YA It is now almost 30 years since the technique of gel permeation chromatography (GPC) was developed by Moore' followed by the introduction of the first commercial instrument by Waters Associates.The technique has advanced tremendously since those days, spurred on by the introduction and rapid gain in popularity of high-performance liquid chromatography (HPLC) in the 1970s and 1980s. GPC is now more correctly referred to as size exclusion chromatography (SEC) and is firmly established as a routine technique for characterizing polymers to determine average molar masses and molar mass distributions.* The term SEC will be used throughout this paper. The technique involves passing a polymer solution down a column containing porous gel particles. The polymer mol- ecules are separated according to their size in solution with the larger molecules eluting first and the smaller molecules later. The resultant chromatogram can be used as a fingerprint of the sample or analysed to yield the average molar masses provided the columns have been calibrated with suitable standards of known molar mass.The theory of the permeation process is now well understood3 but calibration still remains a problem. The technique and some of its problems are discussed with an indication as to their solution and where developments might be expected in the futup. A schematic of a modern size exclusion chromatograph is shown in Fig. 1. Each component will be discussed in turn. Solvents These should be pure (HPLC grade or distilled) and filtered before use. Pumps Solvent delivery systems have changed little in principle since the first commercial liquid chromatographs were introduced. Column - I Solvent $I Ll Recorder1 data system Fig.1 Schematic diagram of modern SEC system They vary from relatively cheap single piston pumps to sophisticated twin piston designs. The latter are usually preferred for SEC as it is essential to maintain accurate and reproducible flow rates when using the technique to measure molar masses. In such instances the elution volume (or time) axis is converted to a molar mass axis via a calibration procedure with standards of known molar mass. The elution volume (or time) is proportional to the log(mo1ar mass) and hence small errors in V, can give rise to large errors in the calculated molar masses. It is usual practice to include a low molar mass flow rate marker4 in the sample to enable corrections to be applied in the calculations. Most modern software includes this capability.Unfortunately this procedure only provides an average correction over the total run time (which may be as long as 45 min). Any changes in the flow rate during the course of the run may cancel out in the end but may nevertheless produce errors if they occur during the elution of the polymer peak. This problem has still to be addressed in SEC software. Injector The six-port rotary valve/loop injector is traditionally used in SEC with a loop size up to 200 pl. The design has changed little except for reducing the internal dead volume of the valve. Autoinjectors are occasionally used in routine analyses, in high-temperature SEC and where toxic solvents are used. Columns Column packing materials have seen the most spectacular developments. The orginal Styragel columns were 2 ft long by 0.5 in i.d.packed with cross-linked polystyrene gel with a particle size >37 vm and a broad particle size distribution. Typically, four columns were necessary to achieve reasonable resolution and consequently it could take up to 4 h to elute a sample. In contrast, a modern SEC column (300 x 8 mm i.d.) is packed with 5 pm gel, three such columns giving high resolution separations (50 OOO plates per column is typical) in 40 min. Another major innovation has been the introduction of columns with mixed pore size gels. These give linear Cali- brations and obviate the need to have several columns containing gels with different pore sizes in order to cover the molar mass range of interest. Mixed gel columns are now available to cover high, medium and low molar mass ranges. Very recently narrow-bore (250 x 4.6 mm) and micro-bore (250 x 3.2 mm) SEC columns have become available.These operate at reduced flow rates (0.38 and 0.18 ml min-', respectively) with consequent reduction in solvent consump-ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 339 tion. They are slightly less efficient than conventional columns but give an enhanced signal response. Aqueous SEC2,5 still remains a problem area. A ‘universal’ column which satisfactorily deals with all aqueous polymers has still to be developed. This may be an impossible goal as such a column must be able to deal with a wide range of polymeric species (anionic, cationic, neutral, acidic, basic). Presently available columns are based on hydrophilic packings such as silica, poly(viny1 alcohol), poly(methacry1ate) and chemically modified polystyrene.A variety of aqueous eluents are used and are designed to prevent or minimize adsorption of the sample onto the column. Detectors Detectors for SEC applications can be conveniently divided into three categories: concentration, selective and molar mass. The differential refractive index (RI) detector was the original SEC detector and still remains the most common. It is ‘universal’ and problems of sensitivity and baseline stability have been improved with better optics and thermostatic control. A new type of concentration detector has been introduced in the last few years, variously called the mass or evaporative light scattering detector6 (Fig. 2). The principle of operation involves directing the eluent from the chromato- graphic column into a heated chamber where the liquid is atomized and the solvent evaporated.The involatile sample particles are detected by a light beam. It has several advantages over the RI detector in that it is more sensitive, has a stable baseline, shows no solvent peaks and always produces positive peaks. Apart from these features it can also be used with mixed solvents and solvent gradients in HPLC analyses, making it an attractive alternative to the RI detector for oligomer and low molar mass polymer analysis. In the category of selective detectors the ultraviolet (UV) is the most common. It finds some application in polymer analysis, usually as a second detector in a dual detector system.Solvent stream J Nebulizer- gas N ebu I izer Evaporator Light trap ______--A __--- -T%-( ----- Light trap Section A - A Light source Photomultiplier G;, Fig. 2 Schematic diagram of mass detector The most Significant development in this area has been the introduction of the diode-array detector. This enables multi- wavelength detection with the acquisition of UV-visible spectra in a single run. The resultant matrix of data can be displayed as chromatograms at various wavelengths, as UV spectra of eluted peaks (useful for identification purposes) or as a three-dimensional plot. It has found particular use in copolymer and polymer additive analyses. It was recognized in the early days of SEC that the ideal detector would be one that could directly measure the molar mass of the polymer as it eluted from the column, obviating the necessity for calibration.Devices to measure continually the viscosity or the light scattered by the eluent have been developed. Viscotek manufactures a differential viscometer detector’ based on four capillaries arranged in a Wheatstone bridge configuration (Fig. 3 ) . Transducers monitor pressure changes as the sample passes through the capillaries, the differential arrangement giving increased sensitivity. A refrac- tive index detector is also required to monitor the concen- tration. Absolute molar masses can be calculated without calibration and information on branching may also be obtained. Low-angle laser light-scattering photometers (LALLS) were introduced in the 1970s and their potential as ‘absolute’ SEC detectors was soon realized.’,’ The Chromatix was the first of these” but it never achieved widespread use as an SEC detector because of a lack of comprehensive computer software (it preceded the PC age) and high cost.Recently two new light scattering detectors have been introduced from Wyatt Technology (the ‘DAWN’)” and a Japanese manufac- tured instrument marketed by Polymer Laboratories (PL- LALLS; Fig. 4).‘2 These instruments overcome many of the previous problems and look set to compete with the vis- cometer. Development work is on-going to produce an osmo- Eluent Waste Fig. 3 Schematic diagram of Viscotek differential viscosity detector. Capillaries RI-R4 form the capillary bridge. Pi and A PT are the inlet and differential pressure transducers, respectively.A and B are reservoirs containing sample solution and solvent, respectively, and SA and SB are the corresponding tandem switching valves340 Half \ mirror ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 El 4] Reference detector n \ U Microscope -- Cell window detect0 r lens Fig. 4 Optical diagram of PL-LALS metric d e t e c t ~ r ' ~ to measure osmotic pressure of column eluents continuously and hence obtain M , directly. Dual detector systems have been employed on many occasions in polymer analysis. They have found particular application in the analysis of copolymers where combinations of RI and UV and RI and infrared (IR) have proved very useful.2 The RI detects all components in the sample whilst the UV or IR detector is tuned to a wavelength characteristic of one of the components only.The information obtained from the dual chromatographs enable the chemical composition distribution and the molar mass distribution to be calculated. Thus, any inhomogeneities in the copolymer can be detected. A triple detector comprising refractive index, light scattering and viscometry has recently been developed by the Viscotek Corporation and represents the ultimate in molar mass detection. Data Handling The rapid growth in the availablilty of desk-top microcom- puters in the last 10 years, coupled with their ever-falling prices, has transformed chromatographic techniques. A modern chromatograph is not complete without a PC data handling system. They not only have the capability t o collect and analyse data but to control the operation of the chromatograph.In SEC the time (or volume) axis must be converted to a molar mass axis. This requires the instrument to be calibrated with polymeric standards of known molar mass and narrow molar mass distribution. Unfortunately, these standards are only available for a few common polymers. Various procedures have been employed to convert these calibrations for use with other olymers. The most successful of these was due to Benoit et al. "and plotted the hydrodynamic volume, log(qM) vs. V, rather than logM vs. V,. This is called the Universal Calibration and requires values of K and a (the Mark-Houwink constants) for the polymer of interest. An alternative approach was the use of well characterized broad standard samples €or calibration purposes.This was proposed in the late 1 9 6 0 ~ ~ ~ but it has only been in the last few years, with the advent of high-power PCs, that these pro- cedures have become generally available.'6 The broad standard sample is chromatographed in the normal way, values of M , and M , are entered into the program and the average molar masses of the standard are calculated using the polystyrene calibration. The results are compared with the known values and an iterative procedure is used to adjust the calibration until the calculated results agree with the known values. This calibration can then be used to calculate molar masses of unknown polymers of the same chemical type. The technique is likely to find increasing use as more complex polymers require analysis for which narrow standards are not available.Consequently there will be an increased requirement €or the measurement of absolute molar masses by osmometry and light scattering in order to determine M , and M , of the required broad standards. Special Techniques There has always been a requirement for high-temperature SEC in order to characterize polyolefins such as polyethylene and polypropylene. The first commercial SEC instrument, the Waters 200, was built for this purpose in the 1960s. The sucessor to this instrument, the Model 150C, is still the only commercial instrument available that can operate at tempera- tures up to 150 "C. However another manufacturer is about to introduce instruments which can operate at temperatures up to 200 "C. Materials such as poly(ethy1ene terephthalate) and the various nylons require phenolic solvents such as rn-cresol or o- chlorophenol and temperatures of 100 "C to dissolve the polymers.It is also necessary to operate the columns at these temperatures in order to reduce the viscosity of the solvent and maintain a flow rate of 1 ml min-'. Elevated temperatures are also necessary when using other high viscosity solvents such as N, N-dimethylformamide (DMF), water and dimethyl sulfox- ide (DMSO). The newer, high performance materials such as polyimides, poly(ether ether ketone) and poly(pheny1ene sulfide) require even more aggressive solvents, such as 1- chloronaphthalene, operating at temperatures up to 200 "C. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 References Moore, J. C., Polym. Sci., Part A , 1964, 2, 835. Size Exclusion Chromatography, eds. Hunt, B. J., and Holding. S. R., Blackie, Glasgow, 1989. Yau, W. W., Kirkland, J. J., and Bly, D. D., Modern Size Exclusion Chromatography: Practice of Gel Permeation and Gel Filtration Chromatography, Wiley, New York, 1979. Patel, G. N., and Stejny, J.. J. Appl. Polym. Sci., 1974, 18, 2069. Aqueous Size Exclusion Chromatography, ed. Dubin, P. L., Elsevier, Amsterdam, 1988, vol. 40. Macrae, R., Int. Anal., 1987, 1, 14. Haney. M. A.. Am. Lab., 1985, 17(3), 41 and 17(4), 116. Ouano, A. C., J. Chromatogr., 1976, 118, 303. Kaye. W., and Havlik, A. J., Appl. Opt., 1973, 12, 541. McConnel, M. L., Am. Lab., 1978, 10, 63. Wyatt, P. J., Jackson, C., and Wyatt, G. K., Am. Lab., 1988, 20, May and June. O'Donohue, S. J., Warner, F. P., and Williams, A. G., ISPAC, 4th International Symposium on Polymer Analyses and Charac- terisation, Baltimore, NJ, 1991, 201pp. Yau, W. W., Chemtracts-Macromolecular Chem., 1990, 1, 1. Benoit. H., Rempp, P., and Grubisic, Z., J. Polym. Sci., Part B, 1967, 5 , 753. Balke, S. T., Hamielec, A. E., Leclair. B. P., andPearce. S. L., Ind. Eng. Chem., Prod. Res. Dev., 1969, 8, 54. CALIBER software, Polymer Laboratories Ltd., Church Stretton, Shropshire, UK.

ISSN:0144-557X    

DOI:10.1039/AP9933000338

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYTICAL PROCE,EDINGS, AUGUST 1993, VOL 30 341 Research and Development Topics in Analytical Chemistry The following is a summary of one of the posters presented at a Meeting of the Analytical Division held on July 7-8, 1992, in the University of Birmingham. Summaries of eighteen other papers and posters given at the Meeting were published in the March issue, p. 129. Characterization of Polymer Membranes Using Microelectrodes Part 1 Diff usion-limited/Size-exclusion Electrodes Amiel M. Farrington and Jonathan M. Slater Analytical Science Group, Birkbeck College, University of London, Gordon House, 29 Gordon Square, London WCIH UPP There are a wide variety of electrode materials available for use in electroanalytical techniques. For most applications these electrodes must be stable, ionically conductive and electro- chemically active.However, if the electrode material does not satisfy all these criteria it can be modified, for example, with polymer film coatings. Such modifications result in electrodes with different properties to those of conventional electrodes,' for example electron-transfer reactions of dissolved substances can be facilitated and the polymer membrane may act as a selective transport barrier. For passive membranes the redox reactions occur at the standard potentials quoted for liquid electrolytes, but with much smaller diffusion-limited currents since transport through the membrane is slower than through a liquid. Preparation of polymer membranes may be via one of two methods: ( a ) traditional/solvent-cast membranes; ( b ) electro- chemically formed membranes.The first of these techniques involves the coatirig of the electrodes with polymer solution. The solvent is then allowed to evaporate leaving a thin film of polymer on the electrode. Methods of coating the electrode include dip coating, spray coating and spin coating. It is also possible to use a modified technique and attach polymer films onto the electrode which have been previously cast on a glass slide.2 Ion-selective electrodes are formed by this method. Using the second technique3 membranes are grown on the electrode surface by either galvanostatic or potentiostatic polymerization of the electroactive monomers. Membranes formed by this method can be either conducting4 or insulating.s Electrochemical methods have numerous advantages over solvent-cast methods of membrane preparation.The resulting thickness of the polymer membrane can be controlled via the charge supplied. Uniform films are produced provided the preparation conditions are controlled. Extremely thin insulat- ing films can be prepared electrochemically (typically less than 50 nm); these layers act mainly as filters by size or charge exclusion.6 Sandwiches of different polymers may be used to produce electrode films, each layer being chosen for its unique chargehize-exclusion properties. Investigations of membrane modified electrodes have been traditionally carried out using macroelectrodes. However, the unique characteristics of micro electrode^^ (i. e. , electrodes with at least one dimension <50 pm) make them ideal candidates for further membrane research.The diffusion geometries of macro- and microelectrodes are very different. Macroelec- trodes are characterized by predominantly planar diffusion, edge effects being of little importance, whilst the smaller electrode area of microelectrodes leads to radial diffusion, arising predominantly from edge effects. The diffusion layer of a microdisc electrode (which is assumed to radiate from about three times the diameter of the electrode, i.e., 30 pm for a 10 pm diameter electrode) may be greater or less than the membrane thickness. The thinnest membranes are prepared by electrochemical methods, on average around 3-5 pm, whereas solvent-cast methods of preparation yield films of around 50- 100 pm. The diffusion profile of the microelectrode is therefore confined within the solvent-cast type membrane, but extends outside the much thinner electrochemically prepared films.This investigation compares the behaviour of platinum macroelectrodes and microelectrodes, modified with poly(phe- nol) membranes. These poly(pheno1) modified electrodes were used for the electrochemical measurement of oxygen, ferro- cene and ferrocenemonocarboxylic acid. Experimental Reagents Poly(pheno1) films were prepared by electropolymerization of phenol (50 mmol dmP3, Hopkin and Williams, AnalaR) in an aqueous solution with tetraethylammonium perchlorate (0.1 mol dm-3, Fluka). Oxygen measurements were carried out in an aqueous solution of potassium nitrate (0.2 mol dm-3, BDH, AnalaR). The electrochemistry of ferrocene ( 5 mmol dmP3, Aldrich) and ferrocenemonocarboxylic acid ( 5 mmol dmP3, Sigma) was studied in acetonitrile solutions (Romil, Spectra pure) with sodium perchlorate (50 mmol dm-3, Fluka) as the supporting electrolyte.Electrochemical Measurements A three-electrode configuration, consisting of a saturated calomel reference electrode, a glassy carbon auxiliary elec- trode and a platinum working electrode, was used in conjunc- tion with either an Amel 433 potentiostat or a Metrohm 612 scanner and 61 1 detector, for macroelectrode studies. Micro- electrode studies were carried out in a two-electrode con- figuration, consisting of a saturated calomel reference elec- trode and an EG&G platinum microelectrode with a Metrohm 612 scanner and a Keithley 485 picoammeter.Results and Discussion Polymerization of poly(pheno1) was carried out by cycling the working electrode between 0 and 1 V, at a scan rate of342 ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 50 mV s-l (Fig. 1). Poly(pheno1) is an insulating polymer; therefore, electropolymerization occurs only on the first cycle, thus no current flows on the second and subsequent scans. The effect of pH on the electropolymerization of phenol was studied for both macro- and microelectrodes and results are shown for the microelectrode system, although both systems produced the same effect (Fig. 1). Larger currents are observed at lower pHs. This is to be expected since the lifetime of the phenol radical is reduced at low pHs. The measurement of oxygen by poly(pheno1) modified platinum electrodes was investigated by linear scan voltam- metry betweenoand -1.2 V, at ascanrateof50 mV s-I.With macroelectrodes oxygen was observed to pass through the poly(pheno1) membrane on the first scan only (Fig. 2). Second and subsequent linear scans of the solution exhibit reduced currents. A complete explanation cannot be formulated without further experimentation, but a number of factors are thought to be responsible. These include: the diffusion limitation of oxygen reduction; bulk consumption of the oxygen at the electrode surface; or entrapped oxygen in the poly(pheno1) membrane. A much reduced current was obtained in a nitrogen purged solution (Fig. 2), confirming that the reaction measured is oxygen reduction. Figs. 3 and 4 show the currents produced from similar experiments on a platinum microelectrode.Currents obtained were 20 times less on a poly(pheno1) modified electrode than those observed on an unmodified microelectrode. The second and subsequent scans Electrode potentialN 0.2 0.4 0.6 0.8 1.0 1.2 1.4 t I I I I I I I - I \ Fig. 1 The first scan of the cyclic voltammogram of phenol (50 mmol dmP3) in tetraethylammonium perchlorate (0.1 mol dm-3) buffered to A, pH 4; B, pH 7; and C, pH 9 at a scan rate of 50 mV s-', on a platinum microelectrode; D, second and subsequent scans at pH 7 0.10 -0.15 -0.40 -0.65 -0.90 -1.15 -1.40 Electrode potentialN Fig. 2 Linear scan of potassium nitrate (0.2 mol dm-') at a poly(pheno1) modified platinum macroelectrode: A, atmospheric oxygen level; B, as A , for the second and subsequent scans; and C, for a nitrogen purged solution, at a scan rate of 50 mV s-' were identical with the first, indicating that the reaction is not limited by diffusion of oxygen through the membrane.Ferrocene compounds comprise much larger molecules than oxygen. The electrochemistry of ferrocene and ferrocene- monocarboxylic acid was investigated with both poly(pheno1) modified and unmodified platinum macro- and microelectrodes by cycling between 0 and 1 V, at a scan rate of 100 mV s-'. Ferrocenemonocarboxylic acid was studied in conjunction with ferrocene to investigate any charge-exclusion properties the poly(pheno1) membrane may exhibit. There was no apparent difference in the results obtained for both of the ferrocene compounds and hence only the ferrocenemonocarboxylic acid results are shown.The macroelectrode results suggest that ferrocene and ferrocenemonocarboxylic acid are unable to permeate the poly(pheno1) membrane (Fig. 5). However, -0.2 0 0.2 0.4 0.6 0.8 1.0 1.2 Electrode potentialN Fig. 3 Linear scan of potassium nitrate (0.2 mol dm-3) at a platinum microelectrode: A, in an oxygen purged solution; and B, in an atmospheric oxygen level solution, at a scan rate of 50 mV s-' I I 1 I -0.2 0 0.2 0.4 0.6 0.8 Electrode potentialN Fig. 4 Linear scan of potassium nitrate at a poly(pheno1) modified microelectrode: A, in an oxygen purged solution; B , in an atmospheric oxygen level solution; and C, in a nitrogen purged solution, at a scan rate of 50 mV s-' Electrode potentialN -0.4 -0.2 0 0.2 0.4 0.6 0.8 1.0 Fig.5 Cyclic voltammogram of ferrocenemonocarboxylic acid (5 mmol dm-3) in sodium perchlorate (50 mmol dm-'): A, on a platinum macroelectrode; and B, on the same electrode modified with a poly(pheno1) membrane, at a scan rate of 100 mV s-'ANALYTICAL PROCEEDINGS, AUGUST 1993, VOL 30 343 this does not appear to be true for a microelectrode (Fig. 6 ) . This may be an artefact of the potential since it may not be able to overcome the high film resistance in a macro- electrode of the insulating poly(pheno1) layer. In a microelec- trode the resistance offered by the poly(pheno1) layer in relation to the electrode resistance is small; therefore, the Electrode potentialN 0 0.2 0.4 0.6 0.8 1 .o 7 I I I 10.2 nA Fig. 6 Cyclic voltammogram of ferrocenemonocarboxylic acid ( 5 mmol dmP3) in sodium perchlorate (50 mmol dm-'): A, on a platinum microelectrode; and B, on the same electrode modified with a poly(pheno1) membrane, at a scan rate of 100 mV s-' true electrocfiemistry of the ferrocene compounds may be observed. Conclusion The results obtained suggest that oxygen, ferrocene and ferrocenemonocarboxylic acid are all able to permeate the poly(pheno1) membrane, although the diffusion is slower than through a liquid. More work is required to determine the full kinetic picture, as is work on larger molecules to determine the size-exclusion properties of the film and this is currently in progress. References 1 Ryan, M. D., and Chambers. J. Q., Anal. Chem., 1992,64,79R. 2 Moody, G. J., Oke, R. B., and Thomas, J. D. R., Analyst, 1970, 95, 910. 3 Imisides, M. D.. John, R., Riley, P. J., and Wallace, G. G., Electroanalysis, 1991, 3, 879. 4 Simonet, J . , and Rault-Berthelot, J., Prog. Solid State Chem., 1991, 21, 1. 5 McCarley, R. L.. J . Electrochem. Soc., 1990, 137, 218c. 6 Wang, J., Chen. S-P., and Lin. M. S., J. Electroanaf. Chem., 1989, 273, 231. 7 Wightman, R. M., Science, 1988, 240, 415.

ISSN:0144-557X    

DOI:10.1039/AP9933000341

出版商:RSC    

年代:1993

数据来源: RSC