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STANDARD 7YPE S. 81. Jm JUNIP 7 Potter Street Harlow Circle 001 for further information . Atomic Spectroscopy Group and North East Region of The Royal Society of Chemistry Productivity Enhancement in Atomic Spectroscopy A one day meeting to be held at York University York UK Wednesday February 5,1992 The speakers will include SJ. Haswell M.H. Ramse y C.L.R. Barnard Data Handling Connection Collection and Control G. Hibberd M. Ingham A.N. Eaton J. Marshall The Development of a Continuous Flow Microwave Digestion Method for On-line Sample Preparation Appropriate Precision Matching Analytical Precision Specifications to the Particular Application AA or ICP-Which to Choose? Operational and Quality Control Procedures in a High- ICP-MS-Variations on a Theme Expert Systems for Improving Eficiency in Atomic throughput XRF Laboratory Spectroscopy For further details contact Dr.J. Marshall ICI plc. Wilton Materials Research Centre Room D125 P.O. Box 90 Wilton Middlesbrough Cleveland TS6 8JE UK. Telephone 0642 432029; Fax 0642 437277.Journal of Analytical Atomic Spectrometry (Including Atomic Spectrometry Updates) JAAS Editorial Board" Chairman L. Ebdon (Plymouth UK) J Egan (Cambridge UK) D A Hickman (London UK) J Marshall (Middlesbrough UK) J M Mermet (Villeurbanne France) D L Miles (Keyworth UK) B L. Sharp (Loughborough UK) R D Snook (Manchester UK) "The JAAS Editorial Board reports to the Analytical Editorial Board Chairman A G. Fogg (Loughborough UK) JAAS Advisory Board F C Adams (Antwerp Belgium) R M Barnes (Amherst MA USA) L Bezur (Budapest Hungary) R F Browner (Atlanta GA USA) B V L'vov (Leningrad USSR) S Caroli (Rome Italy) A J Curtius (Rio de Janeiro BraziO N Omenetto (Ispra Italy) L de Galan (Vlaardingen The Netherlands) J B Dawson (Leeds UK) K Dittrich (Leipzig GDR) W Frech (UmeJ Sweden) K Fuwa (Tokyo Japan) A L Gray (Egharn UK) S Greenfield (Loughborough UK) G M Hieftje (Bloomington IN USA) G Horlick (Edmonton Canada) Ni Zhe-ming (Beiiling China) T C Rains (Charleston SC USA) R E Sturgeon (Ottawa Canada) R Van Grieken (Antwerp Belgium) A Walsh K B (Victoria Australia) B Welz (Uberlingen FRG) T S West (Abderdeen UK) Atomic Spectromery Updates Editorial Board Chairman "D L J Armstrong (Dumfries UK) J R Bacon (Aberdeen UK) C Barnard (Glasgow UK) R M Barnes (Amherst MA USA) S Branch (High Wycombe UK) R Bye (Oslo Norway) J Carroll (Middlesbrough UK) M R Cave (Keyworth UK) "J M Cook (Keyworth UK) "M S Cresser (Aberdeen UK) H M Crews (Norwich UK) J S Crighton (Sunbury-on-Thames UK) J R Dean (Newcastle upon Tyne UK) *J B Dawson (Leeds UKI "L Ebdon (Plymouth UK) "J Egan (Cambridge UK) *A T Ellis (Oxford UK) "D J Halls (Glasgow UK) "D A Hickman (London UK) *S J Hill (Plymouth UK) J Fazakas (Bucharest Romania) K W Jackson (Albany NY USA) R Jowitt (Middlesbrough UK) K Kitagawa (Nagoya Japan) "D Littlejohn (Glasgow UK) "J Marshall (Middlesbrough UK) Miles (Keyworth UK) H Matusiewicz (Poznan Poland) J M Mermet (Villeurbanne France) R G Michel (Storrs CT USA) T Nakahara (Osaka Japan) Ni Zhe-ming (Beying China) P R Poole (Hamilton New Zealand) W J Price (Ashburton UK) C J Rademeyer (Pretoria South Africa) M H Ramsey (London UK) A Sanz-Medel (Owedo Spain) I L Shuttler (Uberlingen FRG) S T Sparkes (Plymouth UK) R Stephens (Halifax Canada) J Stupar (Llublpna Yugoslavia) R E Sturgeon (Ottawa Canada) A Taylor (Guildford UK) A P Thorne (London UK) G C Turk (Gaithersburg MD USA) J F Tyson (Amherst MA USA) *A M Ure (Aberdeen UK) S J Walton (Crawley UK) P Watkins (London UK) B Welz (Uberlingen FRG) J Williams (Egham UK) J B Willis (Victoria Australia) "Members of the ASU Executive Committee Editor JAAS Judith Egan The Royal Society of Chemistry Dr J M Harnly Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK Telex No 81 8293 Fax 0223 423623 Assistant Editors Brenda Holliday Editorial Secretary Monique Warner US Associate Editor JAAS US Department of Agriculture Beltsville Human Nutriton Research Center Beltsville MD 20705 USA Telephone 301 -344-2569 Telephone 0223 420066 BLDG 161 BARC-EAST Paula O'Riordan Sheryl Whitewood Advertisements Advertisement Department The Royal Society of chemistry Burlington House Piccadilly London W I V CBN UK.Telephone 071-437 8656 Fax 071-437 8883 Information for Authors Full details of how to submit materials for publica- tion in JAAS are given in the Instructions to Authors in Issue 1 Separate copies are available on request The Journal of Analytical Atomic Spectrometry (JAAS) is an international journal for the publica- tion of original research papers communications and letters concerned with the development and analytical application of atomic spectrometric techniques The journal is published eight times a year including comprehensive reviews of specific topics of interest to practising atomic spectrosco- pists and incorporates the literature reviews which were previously published in Annual Reports on Analytical Atomic Spectroscopy (ARAAS) Manuscripts intended for publication must de- scribe original work related to atomic spectromet- ric analysis Papers on all aspects of the subject will be accepted including fundamental studies novel instrument developments and practical ana- lytical applications As well as AAS AES and AFS papers will be welcomed on atomic mass spec- trometry and X-ray fluorescence/emission spec- trometry Papers describing the measurement of molecular species where these relate to the char- acterization of sources normally used for the pro- duction of atoms or are concerned for example with indirect methods of analysis will also be ac- ceptable for publication Papers describing the de- velopment and applications of hybrid techniques (e g GC-coupled AAS and HPLC- ICP) will be par- ticularly welcome Manuscripts on other subjects of direct interest to atomic spectroscopists in- cluding sample preparation and dissolution and analyte pre-concentration procedures as well as the statistical interpretation and use of atomic spectrometric data will also be acceptable for pub- lication There is no page charge The following types of papers will be consid- ered Full papers describing original work Communications which must be on an urgent matter and be of obvious scientific importance Communications receive priority and are usually published within 2-3 months of receipt They are intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems Reviews which must be a critical evaluation of the existing state of knowledge on a particular facet of analytical atomic spectrometry Every paper (except Communications) will be submitted to at least two referees by whose advice the Editorial Board of JAAS will be guided as to its acceptance or rejection Papers that are accepted must not be published elsewhere except by permission Submission of a manu- script will be regarded as an undertaking that the same material is not being considered for publica- tion by another journal Manuscripts (three copies typed in double spacing) should be sent to Judith Egan Editor JAAS or Dr.J M. Harnly US Assoiia?e Editor JAAS. All queries relating to the presentation and sub- mission of papers and any correspondence re- garding accepted papers and proofs should be directed to the Editor or US Editor (addresses as above) Members of the JAAS Editorial Board (who may be contacted directly or via the Editorial Office) would welcome comments suggestions and advice on general policy matters concerning JAAS Fifty reprints are supplied free of charge Journal of Analytical Atomic Spectrometry (JAASI (ISSN 0267-9477) is published eight times a year by The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF.UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry Turpin Tractions Ltd. Blackhorse Road Letchwarth Herts. SG6 1 HN UK Tel. +44 (0) 462 672555; Telex 825372 Turpin G; Fax +44 (0) 462 480947. Turpin Transactions Ltd. is wholly owned by The Royal Society of Chemistry. 1991 Annual subscription rate EC f309.00 USA $728.00 Rest of World €355.00. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc. 200 Meacham Avenue Elmont NY 11003. USA Postmaster send address changes to Journal of Analytical Atomic Spectrometry (JAAS) Publications Expediting Inc. 200 Meacham Avenue Elmont NY 11003. Second class postage paid at Jamaica NY 11431. All other despatches outside the UK by Bulk Airmail within Europe Accelerated Surface Post outside Europe. PRINTED IN THE UK. 0 The Royal Society of Chemistry 1991. All rights reserved. No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photographic recording or otherwise without the prior permission of the publishers.

ISSN:0267-9477    

DOI:10.1039/JA99106FX001

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

2 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 Atomic Spectrometry Viewpoint Barry L. Sharp Chemistry Department Loughborough University of Technology Loughborough Leicestershire L E 1 1 3TU UK The Hilger Spectroscopy Prize was inaugurated in 1975 by the Atomic Spectroscopy Group of The Royal Society of Chemistry. The Prize is awarded to the young person (under 35 years of age in the year the award is made) who in the opinion of the Committee has made a significant contribution to atomic spectroscopy. It is customary for the recipient to give a lecture on their specialist topic at an ASG meeting within the preceding twelve months. It is timely to review how the first winner of the prize Barry Sharp (B.L.S.) has emerged in the field of atomic spectroscopy in the subsequent 15 years.An interview with Barry Sharp was arranged at the recent 5th Biennial National Atomic Spectroscopy Symposium (BNASS) at Loughborough University of Technology. The interview was conducted by Dr. John Dean (J.D.) (Honorary Secretary) and Dr. John Marshall (J.M.) (the then Vice-chairman). J.D. Can you tell us how you came to be the first recipient of the Hilgel- Spectros- copy Prize and which particular aspects of your work did it recognize? B.L.S. I joined the research group led by Professor Tom West at Imperial College in 1972 and worked on the development of microwave plasma emission spectrom- etry for trace metal analysis and the de- velopment of photon counting systems for atomic spectrometric measurements. Inductively coupled plasma work had just begun in our laboratory and it was evident even then that the ICP source was as near ideal for solution analysis as we were going to get.The microwave plasma was an extremely efficient excitation source for vapours and gases but the poor atomization properties excluded it from consideration as a source for hand- ling solutions. The work on photon count- ing was stimulated by the contemporary enthusiasm for atomic fluorescence spec- trometry (AFS) whose coming appeared to be only the invention of a stable tunable pocket laser away. It is still coming. I received my PhD in 1972 and having joined the staff as a lecturer I made a successful application to the then SRC for equipment to study laser excited atomic fluorescence spectroscopy (LAFS). The SRC believed in doing things by halves and so with half the money I had asked for I purchased a flash lamp pumped tunable dye laser.I had no detection equipment so this I bor- rowed from the physicists and the project proceeded on the days when they did not need their equipment. The intention was to investigate the potential of LAFS as an analytical technique but the laser simply wasn’t reliable enough and so the project developed into a study of saturation and quasi-coherent phenomena in atomic vapours. It was at this time that I re- ceived the Hilger Spectroscopy Prize al- though the laser work was not published until 1976 when it appeared in Spectro- chimica Acta . J.D. You lejl Imperial College in 1975 and went to work at the MacaulayJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 1 VOL.6 Barry Sharp ( L ) with John Carroll Institute for Soil Research (now the Ma- caulay Land Use Research Institute) what prompted this move? B.L.S. Simply that Tom West left Im- perial College to become Director of the Macaulay and offered me a job. J.D. What was your role at the Macaulay? B.L.S. Broadly to develop and implement spectroscopic methods of analysis that might be useful for the analysis of agri- cultural and environmental samples. My first project was to find a suitable method for the field measurement of SO2 one that could be used in remote and inaccessible terrain. One approach was to use an optical radar system i.e. LIDAR (light detection and ranging) in a differen- tial absorption (two wavelength) configuration.I already had the laser system which I had taken with me from Imperial College and so work began to construct a system. It took the efforts of two postgraduate students before we had a working (some days) instrument which was pointed out of one of the upstairs windows of the Institute. As an aside the system had some 64 control routines op- erating its various functions all running on one of the early Intel 8080 micropro- cessor chips which we obtained on a pre- release demonstration board. Once again however the unreliability of the laser particularly when operated in a switched two wavelength mode was unacceptable for a routine instrument. Thus with money running out the project was dropped. J.D. How did your work on nebulizers begin ? B.L.S. This goes back to my time at Im- perial College when one of my postgradu- ate students John Goulter was sponsored by a cement company to investigate the possibility of the direct nebulization of cement powder into an ICP.Needless to say this proved almost impossible to achieve but we had a lot of fun trying and in the process my interest in nebuliz- ers was born. Moving forward to the early 1980s my second major project at the Macaulay was to investigate the pos- sibility of using ICP-AES for the analysis of soil and plant extracts. We did not have sufficient funds to purchase a com- mercial instrument so we bought a source unit and I constructed a system from equipment that we had in the laboratory. The nebulizers available at that time did not seem to be particularly good for our samples and so I decided to make my own and thus my interest was rekindled.The ICP instrument went into routine op- eration and I also became gradually more and more involved in the analytical service operation. The nebulizer work proceeded in fits and starts whenever I had the time to fit it in. An important step at least in terms of the practical con- sequences of the work came in 1985 when in investigating the sources of noise produced by nebulizers which is largely the result of the re-entrainment effect I realized that re-entrainment could be used to improve the gas-liquid interaction in a Babington-type nebulizer. This led to the development and patenting of the Cone- spray nebulizer. The patent was taken out because I was doing some consultancy work with the Royal Aircraft Establish- ment at Farnborough Hampshire UK and it appeared that the device might be useful for fuel spraying in aero-engines this however was not the case.The Conespray consists of a sapphire nozzle with a conical expansion section beyond the nozzle throat. Solution introduced on to the front face of the nozzle adjacent to the conical expansion is drawn down the cone and nebulized from an annular ring close to the nozzle throat. Our laboratory device used a 70 pm nozzle and operated at 250 psig. Although very efficient the high pressure excluded its use on com- mercial ICP systems. Recently however Perkin-Elmer has started marketing a scaled-up version of the Conespray which operates at 30 psig. Although originally developed as a high solids nebulizer the device works equally well with pure solu- tions provided that sufficient sample is pumped to wet the majority of the nozzle throat perimeter In the commercial version this requires about 2 ml min-l but was less than 1 ml min-’ in the origi- nal high-pressure device. This and other developments were described in two reviews which appeared in JAAS in 1988.J.D. This brings us to the present when you have just indicated that you are taking up an appointment at the Univer- sity of Loughborough. What are your plans for the future? B.L.S. I should first say that it was with regret that I decided to leave the Macau- lay but as the Head of the Analytical Division I had become a full time admin- istrator and I wished to return to active re- search.J.M. Will you be concentrating on atomic spectroscopy at Loughborough or will you be involved in a wider range of activ- ities? B.L.S. Inductively coupled plasmas will continue to be my main interest and whether the work involves optical or mass spectrometry sample introduction will be a part of it. I would like to do some more work on atmospheric monitor- ing and perhaps some of the recent devel- opments that have taken place in solid- state lasers might make this a pertinent time to return to this field. J.D. How do you intend to fund these ven- tures? B.L.S. Well sitting next to John Marshall from ICI a potential solution is close at hand. I think that one has to be realistic. As an academic one has to explain one’s ideas to people and hope that they are willing to fund them.Potential sponsors often have their own ideas that they would wish to pursue and hopefully common ground can be found that produces inter- esting research that is academically chal- lenging has practical benefit and provides a good training for our students. J.M. Do you have a view on the academic situation in the UK having been in a quasi-industrial environment? B.L.S. I think that some of my colleagues in academia must think that I am mad coming back to it. Academic research has hardly had a golden period in the UK and it is recognized that it is grossly under- funded. We do have the problem now that when we look at companies like ICI and see what they are doing we cannot afford to train the students in the techniques in which they are interested. I do not see an easy solution to this problem.One ap- proach is for the Universities and Poly- technics to accept that they cannot provide the facilities required and for in- dustry to take a much greater role in train- ing. Academics would continue to teach the basic skills but practical experience would be provided outside the education- al institution. The difficulty with this ap- proach is that unless we get practical hands-on experience the quality of our teaching will suffer. Every teacher at some stage has to give courses in subjects of which they have limited practical ex- perience and one is always aware that the students are receiving a lower quality product. J.M. Does this mean that you see us moving to a situation where more and more postgraduates carry out their research in the laboratories of their sponsors? B.L.S.Unless central government in- creases the level of funding this is inevi- table. A benefit of this is that the postgraduates so trained should at least4 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 have a clearer concept of what the real world is like. However I repeat that if ac- ademics have to do all their research at arms length they will be impoverished as teachers. J.M. I think that there has been a view in Universities that industrial type problems are not really suitable for academic post- graduate research whereas in fact there are quite a few problems that do have a theoretical base that could be examined. B.L.S. The difficulty for industry is that if you have a problem it is not feasible for you to spend the kind of money needed to solve that problem in a University depart- ment.In other words you are not going to buy an ICP-MS or SIMS and give them to a University and say solve the problem for us. This is in part because you recog- nize that they are unlikely to solve that specific problem anyway. Having come from a research institute background it is my view that the initiators usually have a greater understanding of the problem and its context and are therefore best placed to deal with the specifics. What you can hope for is that through your input and other financing that the academic com- munity can acquire the necessary resourc- es and provide a background of science and understanding that you can draw upon.Universities should on the whole concentrate on the science and attempt to establish the boundary conditions for analytical techniques so that their practi- cal implementation is soundly based. J.M. Do you think that atomic spectro- metry has a long-term future? B.L.S. I’m sure that it has but it will evolve and be different from the way we know it now. For example I believe that mass spectrometric detection will ulti- mately replace optical detection to a much greater extent than is currently the case. Our colleagues in the environmental and biological sciences are very impressed with what we can measure using modem techniques but we are not measuring what they require. Primarily they need to know the state of chemical equilibria in situ in the soil solution in the plant membrane inside the cell. Unfortunately atomic spectroscopy is not well placed to deal with these problems. For large molecules immuno-techniques have in some cases provided highly specific reactions that can be monitored by conventional means but there is no parallel for small molecules. Additionally all of our current techniques including chromatography disturb the delicate equilibria that we would like to study. This is the challenge to take analyt- ical chemistry out of the laboratory and to answer the real questions. J.P. The last 1.5 years have obviously been interesting and successful. We would like to thank you for comments and wish you luck in your new position.

ISSN:0267-9477    

DOI:10.1039/JA991060002b

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JASPE2 6(1) 1-92 1 R-68R (1991) February 1991 Journal of Analytical Atomic Spectrometry Including Atomic Spectrometry Updates CONTENTS NEWS AND VIEWS 1 Editorial-Judith Egan 2 Hilger Spectroscopy Prize 2 4 5 7 Conferences and Meetings 8 Papers in Future Issues Atomic Spectrometry Viewpoint-Barry L. Sharp Obituaries-Peter Neil Keliher and Frans J.M.J. Maessen Book Reviews-Julian F. Tyson Andrew Taylor and David J. Halls PAPERS 9 19 25 33 37 41 49 57 73 79 81 89 Determination of Fluorine in Urine and Tap Water by Laser-excited Molecular Fluorescence Spectrometry in a Graphite Tube Furnace With Front-surface Illumination-David J Butcher Richard L Irwin Junichi Takahashi Robert G Michel Application of Platform and Palladium Modification Techniques With Furnace Atomization Plasma Emission Spectrometry-Ralph E Sturgeon Scott N Willie Van T Luong Shier S Berman Comparison of Atomization Mechanisms for Group IIA Elements in Electrothermal Atomic Absorption Spectrometry-Laurie J Prell David L Styris David A Redfield Determination of Lead in Gallium Arsenide by Graphite Furnace Atomic Absorption Spectrometry With an Ammonium Chloride-Chromium(tii) Chloride Chemical Modifier-Ernest Beinrohr Miroslav Rapta Marco Taddia Vanes Poluzzi Determination of Low Levels of Cadmium in Marine and Fresh Water Sediments by Graphite Furnace Atomic Absorption Spectrometry Employing a Reduced Palladium Chloride Modifier and by Inductively Coupled Plasma Atomic Emission Spectrometry-Christopher G Millward Paul D Kluckner Heated Sample Introduction System for the Analysis of Slurries by Inductively Coupled Plasma Atomic Emission Spectrometry-Lyns S Gervais Eric D Salin Line Selection and Determination of Trace Amounts of Elements in Tungsten by Inductively Coupled Placrna Atomic Emission Spectrometry-Martine Carre Olga Diaz de Rodriguez Jean-Michel Mermet Martine Bridenne Yves Marot Comparison of Microwave-induced Plasma Sources-Kimberely A Forbes Edward E Reszke Peter C Uden Ramon M Barnes Assessment of Titanium-Sapphire Lasers and Optical Parametric Oscillators as Sources of Variable Wavelength for Resonant Ionization Mass Spectrometry-Kenneth W D Ledingham Ravi P Singhal CUMULATIVE AUTHOR INDEX INSTRUCTIONS TO AUTHORS JOURNALS OF THE ROYAL SOCIETY OF CHEMISTRY REFEREEING PROCEDURE AND POLICY (1 991 ) ATOMIC SPECTROMETRY UPDATE Ramsey Mark Cave 1R 41 R References Environmental Analysis-Malcolm S.Cresser Janet Armstrong John Dean Michael H. Typeset by Burgess & Son (Abingdol) Ltd FAGE BROS\ Printed in Great Britain by -J Page Bros NorwichThe XXVII Colloquium Spectroscopicum Internationale XXVII CSI will be held in Grieg Hall Bergen Norway June 9-14 1991 IUPAC M - 1991 NORWAY This traditional biennial conference in analytical spectroscopy will once again provide a forum for atomic nuclear arid molecular spectroscopists worldwide to encourage personal contact and the exchange of experience. Participants are invited to submit papers for presentation at the XXVII CSI dealing with the following topics Basic theory and instrumentation of- Atomic spectroscopy (emission absorption fluorescence) Molecular spectroscopy (UV VIS and IR) X-ray spectroscopy Gamma spectroscopy Mass spectrometry (inorganic and organic) Electron spect~oscopy Raman spectroscopy Mossbauer spectroscopy Nuclear magnetic resonance spectrometry Methods of surface analysis and depth profiling Photoacoustic spectroscopy Metals and alloys Geological materials Industrial products Biological samples Food and agricultural products Application of spectroscopy in the analysis of- Special emphasis will be given to trace analysis environmental pollutants and standard reference materials.The scientific programme will consist of both plenary lectures and parallel sessions of oral presentation. Specific times will be reserved for poster sessions. PRE- AND POST-SYMPOSIA In connection with the XXVII CSI the following symposia will be organised Pre-symposie 1. GRAPHITE ATOMISER TECHNIQUES IN ANALYTICAL SPECTROSCOPY June 6-8 1991 Hotel Ullensvang Loofthus Norway. II. CHARACTERISATION OF OIL COMPONENTS USING SPECTROSCOPIC METHODS June 6-8 1991 Hotel Hardangerfjord Q)ystese Norway. 111. MEASUREMENT OF RADIO-NUCLIDES AFTER THE CHERNOBYL ACCIDENT June 6-8 1991 Hotel Solstrand Bergen Norway. Post-symposium- IV. SPECIATION OF ELEMENTS IN ENVIRONMENTAL AND BIOLOGICAL SCIENCES June 17-19 1991 Hotel Alexandra h e n Norway. For further information contact THE SECRETARIAT XXVll CSI HSD Cong ress-Confere nce P.O. Box 1721 Nordnes N-5024 Bergen Norway. Tel. 47-5-31 8414 Telex 42607 hsd n Telefax 47-5-324555 Circle 005 for further information

ISSN:0267-9477    

DOI:10.1039/JA99106BX003

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

4 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 Obituaries Peter Neil Keliher Dr. Peter Neil Keliher Professor of Analytical Chemistry at Villanova Uni- versity died tragically and unexpectedly of a heart attack on July 9th 1990 while on vacation in Freeport the Bahamas. He was 49 years old. He is survived by his wife Bonnie by their two children Mark and Claire and by two sisters Vivian and Joan. At the time of his death Peter was scuba diving with his wife Bonnie both of them highly experienced divers. Peter was a remarkable person in many regards. By birth a New Englander coming from Providence Rhode Island Peter obtained his undergraduate training at St. Michael’s College in Winooski Vermont where he graduated with an A.B. degree in 1962. For his postgraduate training Peter went in search of fresh pas- tures attending Imperial College Univer- sity of London from which he obtained MSc and DIC degrees in 1967 and a PhD in 1969 all in the field of Analytical Chemistry.While at Imperial College Peter worked on a project studying the an- alytical potential of electrogenerated chemiluminescence under the joint guid- ance of Dr. (later Professor) Gordon Kirk- bright and Dr. Bernard Fleet. The Analytical Chemistry group at Imperial College was at that time rapidly achieving prominence under the leadership of Pro- fessor T. s. West FRS and this provided Peter with the vision and opportunity which led him into an academic career. Peter considered his time at Imperial College to be one of the most formative in his life and he maintained strong bonds of professional and personal friendship both with his scientific mentors and also with many student contemporaries from that period. Peter was deeply affected by the premature death of Gordon Kirkbright in 1983 and actively worked to memorialize him through the organization of symposia in his memory and through support of the Gordon Kirkbright Bursary for UK and US postgraduate student travel.Dr. Sue Feng Zhu one of Peter’s recent students was a recipient of such an award. While at Imperial College Peter met and married his wife Bonnie (nke Boys- Korkis) the daughter of a distinguished British surgeon. Peter and Bonnie estab- lished a close caring relationship which touched all those who came into contact with them. An unanticipated addition to the marriage contract was Benjie Bonnie’s childhood horse who accom- panied them to the US and lived out his declining days in great contentment in rural Pennsylvania.Peter’s academic career was spent ex- clusively at Villanova University starting as an Assistant Professor in 1969 with promotions to Associate Professor in 1974 and full Professor in 1979. On his arrival at Villanova Peter focused his at- tention on the then rapidly developing field of atomic spectrometry and quickly established himself in this area working initially with electrodeless discharge lamps and then with the kchelle spectro- meter where he became a pioneer in its use for continuum source atomic absorp- tion spectrometry. Throughout his carreer Peter maintained a strong focus on high resolution atomic spectrometry but branched into other areas including luminescence spectrometry trace pollu- tant analysis and ICP emission spectro- metry.He published a total of 75 papers was the author of several book chapters on atomic spectrometry and co-authored the Analytical Chemistry biennial Funda- mental Review in Atomic Emission Spectrometry from 1984 to 1990. Peter’s advice was frequently sought in editorial matters. He served on the advisory board of Spectrochimica Acta Part B and the Instrumentation Advisory Panel of Ana- lytical Chemistiy. Since 1983 he hadJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 5 been a member of the Editorial Board of ARAAS (Annual Reports on Analytical Atomic Spectrometry) and then the Atomic Spectrometry Updates Board of JAAS.In 1985 he became Editor of the Microchemical Journal and made significant strides during his tenure in re- directing the focus of that journal away from classical wet analysis and towards modem instrumental techniques. Peter was active in a number of profes- sional societies. He was elected treasurer of the Analytical Division of the Ameri- can Chemical Society in 1978 and served two two-year terms in this role. He also served as Alternate Counsellor of the same group. In addition he chaired several key committees of the Society for Applied Spectroscopy. He had a strong commitment to strengthening profession- al activities in analytical chemistry and gave considerable service to both the Fed- eration of Analytical Chemistry and Spectroscopy Societies (FACSS) meeting and the Eastern Analytical Symposium (EAS).He served on the governing board of FACSS for many years as assistant Program Chairman and finally as General Chairman in 1986. As Exhibits Director of the FACSS meeting from 1981 to 1985 he was successful in strengthening the participation of the scientific instru- ment companies and so helped the meeting to re-establish its financial secu- rity. Peter served on the Governing Board of the EAS as Assistant Programme Chairman in 1987 and as Programme Chairman in 1988. Peter had close ties with a number of scientists in the People’s Republic of China was an hono- rary guest Professor at Jilin University in 1988 and served on the Programme Committee for the 1990 Analytical Chemistry Conference held in Changcun.His membership of other professional so- cieties included the American Chemical Society The Royal Society of Chemistry the Society for Applied Spectroscopy the Microchemical Society and the American Association for the Advancement of Science. Throughout his professional career Peter maintained a close and affectionate relationship with all his students both undergraduate and graduate and was always very supportive of their efforts while at university and in later life. In fact one of Peter’s hallmarks and one which will be sorely missed was his strong sense of comradeship which reached ef- fortlessly across barriers of age back- ground and nationality. To have met Peter once even briefly was to be his friend for life. Peter especially enjoyed interna- tional relationships taking much pleasure in visiting friends in their countries and then returning the favour in his own home.A visit to the Keliher home was an occasion to experience wonderful hospi- tality from the whole family which also included various and sundry cats and dogs. While savouring life in many parts of the world Peter was often an astute critic when he felt something could use a little improvement. He once remarked with his strong sense of humor that he was probably one of the few people who had been told ‘if you don’t like it here why don’t you go back where you came from’ on both sides of the Atlantic! Peter Keliher was a great advocate for living life to the full. Nobody who ever met him was unaware of the fact.The many who knew him all around the world will mourn his passing and will miss his lively presence at scientific meetings. He has left his mark firmly on the world of analytical chemistry and we rejoice in the blessings which he imparted to so many lives while he was with us. Richard F. Browner Georgia lnstitute of Technology Atlanta GA USA Frans J. M. J. Maessen Dr. F. J. M. J. Maessen lecturer at the laboratory for Analytical Chemistry of the University of Amsterdam The Neth- erlands unexpectedly passed away at the age of 59 on Thursday November 8th 1990. Frans Maessen served the University since 1961. He started his career there as a technical assistant supervised by Dr. P. W. J. M. Boumans. During this time he developed an interest and expertise in atomic spectrometry.He acquired his bachelors degree in 1969 (cum laude) and his masters degree in 1970 (cum laude) and attained his PhD in 1974 on a thesis entitled ‘Some aspects of spectro- chemical trace analysis with the direct current arc ’ . Since that time he had been the project supervisor of the work on atomic spectrometry in the above mentioned in- stitute. He was an author or co-author of 57 scientific publications. He supervised nine PhD studies on topics in atomic spectrometry ranging from fundamental studies on excitation processes to statis- tical evaluation and HPLC-ICP cou- pling. He was especially known in recent years for his studies on the intro- duction of non-aqueous solutions into the ICP. He had been a member of the Editorial Board of ‘Annual Reports on Analytical Atomic Spectroscopy (ARAAS)’ since 1981 and subsequently when JAAS began publication in 1986 a conscien- tious member of the Atomic Spectrome- try Updates Board. Many scientists will remember his sound scientific work and his friendly personality. H. Poppe University of Amsterdam The Netherlands

ISSN:0267-9477    

DOI:10.1039/JA9910600004

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 5 Book Reviews Flow Injection Atomic Spectroscopy Edited by Luis Burguera. Practical Spec- troscopy (Volume 7). Pp. xii + 353. Marcel Dekker. 1989. Price $125.00. ISBN 0 8247 8059 0. The combination of flow injection (FI) techniques with atomic spectrometric measurement is clearly an area of con- siderable research interest at present. The appearance of a book devoted solely to this combination is indicative of a certain degree of maturity and supports the notion that the flow injection atomic spectrometry (FI-AS) combination has something more to offer than just a slight- ly more precise procedure for discrete nebulization. The scope of the FI-AS combination is well illustrated in the later chapters of this book which cover appli- cations to real analytical problems (in the clinical environmental and agricultural areas) as well as a range of on-line chemi- cal modification to analytes and matrix.The book contains eight contributed chapters by some of the leading workers in the field of FI-AS. A short ‘general in- troduction’ to both analysis by FT and FI- AS by Kent Stewart is followed by an ex- cellent treatment of the ‘theoretical aspects’ by Willem Van der Linden. The first part of this chapter is concerned with topics of relevance to FI in general and following a brief discussion of aspects of impulse response theory relevant to atomic spectrometric detectors concludes with some critical remarks concerning the possible benefits of scaling down which are severely limited by the detector char- acteristics.Jacobus van Staden then deals with the ‘basic components and automa- tion’ before Zhaolun Fang introduces ‘analytical methods and techniques’. This is a comprehensive chapter covering aspects of sample introduction on-line di- lution addition of reagents calibration methods indirect methods hydride and mercury vapour generation sequential and simultaneous determinations. The only topic not covered is liquid-liquid and solid-phase extraction and these6 topics together with continuous precipi- tation are covered by Miguel Valcarcel and Mercedes Gallego in the following chapter. This is also a comprehensive chapter covering most practical aspects of on-line separation and pre- concentration. The first of the two chap- ters on applications contributed by Elias Zagatto Francisco Krug Henrique Ber- gamin and Soren Jorgensen deals with agricultural and environmental analysis.This chapter also contains some discus- sion of basic features of FI-AS and there is some overlap with earlier chapters. The real samples considered are plants waters rocks and soil extracts. Proce- dures relevant to clinical analysis are dis- cussed by Roy Sherwood and Bernard Rocks. This chapter briefly introduces the use of atomic absorption spectrometry (AAS) in clinical analysis before survey- ing the literature for each clinically rele- vant element or pair of elements The authors then discuss the problems asso- ciated with the use of injection valves; the volume required to flush the previous sample out of the loop and connecting tubing and the eventual leakage produced by wear from salt build-up. A discussion of the valveless controlled dispersion ana- lysis follows.The chapter concludes with some specialized applications including FI electrothermal atomization and chro- matography-AAS. The final chapter by Marcela Burguera Jose Burguera and Gilbert Pacey concerns current trends. These authors also try to peer into the future to a certain extent. One of the ap- pendices is a comprehensive listing of FI- AS references. The only real criticism that can be lev- elled at this book is that it took too long to produce. Although the publication date is 1989 the material surveyed for the basis of the book includes papers pub- lished only up to 1986. Not all of the 1986 references are included. In the period 1986-1989 inclusive over 170 publications on FI-AS appeared whereas the total since 1979 is just over 280.Many of the chapters are thus seriously out of date already. Julian F. Tyson University of Massachusetts Amherst MA USA Milk Ongoing Activities and Future Aspects. Annali dell’ Istituto Superiore di Sanita Volume 26 Edited by S. Caroli E. Coni and E. Sabbi- oni. Pp. iv + 109-175. 1990. Price 22.000 Italian Lira; $20. ISSN 002 1 257 1. Few people would agree that breast milk is not the ideal food for young babies or that breast feeding should be discouraged. The high nutritional content of breast milk complemented by protective mater- nal antibodies offers a feeding medium which is far superior (and more conve- nient) than any substitute.Not only is it the most natural of food but it is also subject to complex physiological modifications so that changes in compo- sition are produced to match the require- ments and demands of the growth and development of the infant. It is difficult to concede therefore that breast milk may not be perfect and it could even be harmful. However one recent report showed that breast milk from a mother who had been given gold to treat her arthritis contained substantial amounts of this metal and that some was present in the baby’s blood (Brit. J. Clin. Pharmacol. 1990 29 777). If elements such as gold can be secreted in breast milk then what else might be present to reduce the quality of this food and what are the risks to breast-fed babies when mothers are exposed to other.potentially harmful factors? The reports in this issue of Ann. 1st. Super. Sanita describe the work of groups whose common objectives are to obtain accurate information concerned with the normal content of inorganic and organic components in milk and the impact of maternal exposure from various sources on the composition of breast milk. With just seven short papers the scope of this publication is limited to items that will be of very topical interest. Thus while it is reassuring to discover that radioactivity in samples collected from Italian women in 1986-1987 showed only very small changes in the aftermath of the Chernobyl accident no comparative data relating to other European countries are included. It is the levels of fat-soluble persistent organochlorine pesticides that give the greatest cause for concern and a compre- hensive WHO study of specimens from many countries demonstrates higher than recommended intakes of these com- pounds.It is apparent that were it not for the relatively short period of exposure as- sociated with breast feeding positive in- tervention would be necessary to prevent the appearance of adverse health effects. Therapeutic and environmental expo- sures to antibiotics are considered togeth- er with an account of the normal protein lipid and trace element composition of human milk and infant formulas. While it is concluded still that ‘breast is best’ there is sufficient evidence presented in this short collection to suggest that we should continue to be vigilant in the future.Andrew Taylor Rohens Institute University of Surrey G~ildford UK Handbook of Furnace Atomic Absorp- tion Spectroscopy Asha Varma. Pp. 428. Wolfe Publishing. 1990. Price E156. ISBN 0 8493 3243 5 . JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 Royal Infirmary Glasgow UK I found this handbook disappointing. Dr. Varma had a best-seller with her previous book ‘CRC Handbook of Atomic Absorp- tion Analysis’ published in 1984 and pre- sumably was hoping for a similar success with this book. The handbook contains an introductory section on graphite furnace atomic ab- sorption spectrometry (GFAAS) followed by separate sections on the individual ele- ments listing important information such as wavelength slit-width optimum char and atomization temperatures sensitivity and working range.Each element section concludes with a list of references. After the element sections there follows lists of references to applications under the titles Clinical Analysis Heavy Metals Miscel- laneous Analysis Trace Elements and Wear Metals a strange series of titles under which to classify applications. Ref- erences do not go beyond 1987. The most interesting part is the intro- ductory section which contains some useful information particularly on opti- mizing methods for GFAAS. However it is a serious omission for a book published in 1990 not to mention stabilized temper- ature platform furnace technology or pal- ladium-based modifiers. In buying a book like this one would hope to buy some of the experience of the author in the tech- nique; this does not shine through in this handbook. I could not help comparing this book with another from the same publisher ‘Atomic Absorption Spectrom- etry in Occupational and Environmental Health Practice Volume 11’ by D.L. Tsalev (1984) a bible in our laboratory. The field covered is slightly different but Tsalev’s book is similarly classified into elements with extensive references. Tsalev surveys the literature for each element and uses his experience and intel- lect to highlight the most important prob- lems and the solutions to those problems. Dr. Varma’s book costs 5156. My advice is that most of the contents of the book will be covered adequately in the in- strument manufacturer’s handbook which every user will have already and this will pertain directly to his or her instrument. (N.B. It appears that much of the infor- mation in this book was compiled from manufacturer’s handbooks.) For up-to- date reference lists the reader of JAAS already has Atomic Spectrometry Updates to fulfil hisher needs. In saving you this money I am really trying too hard to become an honorary Scot! David J. Halls

ISSN:0267-9477    

DOI:10.1039/JA9910600005

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 7 Conferences and Meetings Achievements and New Directions in Analytical Chemistry Luminescence and Optical Sensors April 9- 10 199 1 London UK This symposium organized by the Ana- lytical Division of the RSC will be part of the 150th Anniversary Annual Chemi- cal Congress to be held at Imperial College London April 8- 1 2 199 1. Sessions will be held on Molecular Fluo- rescence Optical Sensors Atomic Fluorescence Chemiluminescence Fluo- rescence Detection and Immuno- techniques. For further information contact Dr. J. F. Gibson The Royal Society of Chemis- try Burlington House Piccadilly London WlV OBN UK. Third International Conference on Progress in Analytical Chemistry in the Iron and Steel Industry May 14- 16 199 1 Luxembourg The Commission of the European Com- munities and the European Committee for the Study and Application of Analyti- cal Work in the Steel Industry (CETAS) will organize this conference in Luxem- bourg Plateau du Kirchberg Jean Monnet Building rue Alcide De Gasperi.The aim of the conference is to bring to- gether chemists from all over the world working in connection with iron and steel for an exchange of experience covering the different analytical tech- niques used in their laboratories. Ana- lysts working in the iron and steel application sector in affiliated research institutes universities and in governmen- tal control bodies are also encouraged to participate. The conference will include invited lecturers papers and posters. Papers or posters can cover the following possible topics progress in analytical methods; automation including sampling sample preparation and measurement; data and information management and quality assurance concepts; calibration and drift correction procedures including com- puter support; analysis of steel products environmental materials and organics; speciation of elements in steel; analysis of coatings interfaces and surfaces; ana- lysis of raw materials additives and by- products; international standardization of methods and certification of reference materials; and re-cycling and waste man- agement problems.For further information contact the Pre- sidium of CETAS Mrs. M. R. Posch P. 0. Box 1985 NL-1940 Ad Beverwijk The Netherlands. Fourth Surrey Conference on Plasma Source Mass Spectrometry and Second Kingston Conference on Plasma Spec- trometry in the Earth Sciences July 14-18 1991 University of Surrey Guildford UK The Surrey Conference is the fourth bien- nial meeting to be held at the Surrey Uni- versity site.Papers are invited on all aspects of plasma source mass spectrome- try. Following the success of the 1989 meetings the 1991 Surrey Conference will be followed by a second meeting (previously held at Kingston Polytechnic) dedicated to Earth Science applications of plasma spectrometry. The scientific programmes will consist of invited and open lectures discussion sessions and poster events. There will also be full social programmes. Further details can be obtained from the conference organisers Drs. Kym E. Jarvis and John G.Williams NERC ICP- MS Facility Department of Geology RHBNC Egham Surrey TW20 OEX UK or Dr. Ian Jarvis School of Geologi- cal Sciences Kingston Polytechnic Penrhyn Road Kingston on Thames Surrey KT1 2EE UK. IUPAC International Congress on Analytical Sciences August 25-3 1 199 1 Makahuri-Messe Chiba Japan Papers are solicited in the following areas Separation Sciences; Chemical Speciation and Characterization; New Principles Reactions and Techniques; Chemometrics and Robotics; Biochemi- cai/Biomedical; Environmental; High- Tech Materials. Authors wishing to contribute oral or poster presentations should contact The Japan Society of Analytical Chemistry 1-26-2 Nishigo- tanda Shinagawa Tokyo 14 1 Japan (Telefax +8 1-3-5487-2790 BITNET KK9822@ JPNSUT20) as soon as possible.Deadline for submission of abstracts is March 3 1 199 1. Pre- and Post-Symposia will include 199 1 Pacific International Congress on X-ray Analytical Methods August 12-1 6 199 1 Honolulu. Hawaii USA contact Dr. R. Jenkins International Centre for Diffraction Data 1601 Park Lane Swarthmore PA 1908 1 USA; and New Approaches in Trace Element Ana- lysis by Atomic Spectroscopy September 2-4 1991 Kitami Japan contact Profes- sor I. Atsuya Kitami Institute of Techno- logy 165 Kouencho Kitami Hokkaido 090 Japan. 1992 Winter Conference on Plasma Spectrochemistry January 6-1 1 1992 San Diego CA USA The 1992 Winter Conference on Plasma Spectrochemistry will be held Monday January 6 through Saturday January 11 1992 in Southern California. Short courses highlighting special topics will be offered Saturdays January 4 and January 1 1 and Sunday January 5.A three-day exhibition of spectroscopic instrumenta- tion and accessories will also be included. Programme features and symposia will include Automation Expert Systems and Robotics with Plasma Spectroscopy; Chemometric Applications in Plasma Spectrochemistry; Chromatography with Plasma Source Detection; Flow Injection Plasma Spectrometry; Glow Discharge and Low Pressure Plasma Atomic and Mass Spectrometry; Mechanisms and Process in Plasma Sources; Modem Sample Preparation and Calibration Tech- niques; New Instrumentation for Plasma Spectrochemistry; Plasma Source Mass Spectrometry; Process Control Remote and On-line Plasma Analysis; Sample In- troduction Techniques and Phenomena; Spectrochemical Applications of Plasma Sources; and Transform Spectroscopy Interferometry for Plasma Sources. Titles and a 50-word abstract for sub- mitted lecture or poster papers are soli- cited by July 1 1991.Extended conference abstracts are requested by October 7 1991. For further information contact 1992 Winter Conference on Plasma Spectro- chemistry ICP Information Newsletter Department of Chemistry GRC Towers University of Massachusetts Amherst MA 01003-0035 USA AlTN Dr. R. Barnes telephone 413 545 2294; telefax MASS. 413 545 4490 BITNET RBARNES@U- Laboratory Exhibition and Conference NoiTemher 5-7 I99 1 London The 1991 event will be known as the Laboratory Exhibition and Conference and will be held at the New Earls Court 2 Exhibition Centre.The timing of the event has also been changed to Novem- ber. Not only will the new event bring to- gether the world’s leading scientific companies it will also introduce further conferences in addition to the existing Analyticon conference alongside the ex- hibition. Discussions with potential con- ference sponsors are at an advanced stage; the likely outcome includes a con- ference in conjunction with The Chro- matography Society a Clinical Biochemistry meeting in conjunction with the Southern Region of the Associa- tion of Clinical Biochemists and a con- ference organized by The Royal Society of Chemistry. The move to the new venue of Earls Court 2 has enabled the organizers to structure the exhibition into clearly8 JOURNAL OF ANALYTICAL ATOMlC SPECTROMETRY FEBRUARY 1991 VOL.6 defined technology zones Analytical Science Biotechnology Laboratory Automation Environmental Analysis Laboratory Fittings Laboratory Ware and Medical Laboratory Sciences. Further information is available from Evan Steadman Communications Group The Hub Emson Close Saffron Walden Essex CHlO IHL UK. Third International Symposium on Analytical Chemistry in the Explora- tion Mining and Processing of Materi- als August 2-7 1992 Randburg South Africa Interested people are invited to submit titles and abstracts under the general theme ‘The Role of Contemporary Chemi- cal Analysis in Mining and Industrial Technology’. The following topics will be covered Geochemical Exploration; Ex- traction and Beneficiation of Materials Value-added Products; Environmental Aspects; Coal; Metals and Alloys; Rare Earths; Noble and Base Metals; Analytical Assurance and Laboratory Management; Automation and Process Control; and High-technology Materials.Innovation in analytical techniques would be particular- ly welcome. Titles and abstracts (not more than 250 words) should be submitted so as to arrive not later than September 30 1991. Ex- tended abstracts will be required before March 30 1992. All correspondence and submissions must be addressed to The Conference Secretary Mintek Private Bag X30 15 Randburg 2 125 South Africa. SAC 92 September 20-26 1992 Reading An International Conference on Analy- tical Chemistry (SAC 92) organized jointly by the Analytical Division of The Royal Society of Chemistry and the Laboratory of the Government Chemist will be held at the University of Reading. This is the tenth in the series of triennial conferences originally started by the Society for Analytical Chemistry (hence SAC) and on this occasion also cele- brates the 150th anniversary of the found- ing of the Laboratory of the Government Chemist (LGC). The scientific pro- gramme will be organized around plenary invited and contributed papers and posters covering the whole field of analytical chemistry. The langua e of the conference will be English. A e pro- gramme will include workshops where research workers can demonstrate new apparatus and techniques. An opportunity will be made for all participants to visit The Laboratory of the Government Chemist and other scientific establish- ments. Further information is available from The Secretary Analytical Division Royal Society of Chemistry Burlington House Piccadilly London W 1 V OBN UK.

ISSN:0267-9477    

DOI:10.1039/JA9910600007

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

8 JOURNAL OF ANALYTICAL ATOMlC SPECTROMETRY FEBRUARY 1991 VOL. 6 Future Issues will Include - The March issue will contain the follow- ing papers. Contribution of System Components of Dispersion in the Analysis of Micro- volume Samples by Flow Injection Flame Atomic Absorption Spectrometry- Zhaolun Fang Bernhard Welz and Michael Sperling Fluorination and Volatilization of Refrac- tory Elements from a Graphite Furnace for Sample Introduction into an Induc- tively Coupled Plasma Using a FTFE Slurry-Min Huang Zucheng Jiang and Yun’e Zeng Effect of Long-chain Surfactants on Drop Size Distribution Transport Efficiency and Sensitivity in Flame Atomic Absorption Spectrometry With Pneumatic Nebulization - Juan Mora Antonio Canals and Vincente Heman- dis Use of Partial Least Squares Modelling to Compensate for Spectral Interferences in Graphite Furnace Atomic Absorption Spectrometry With Continuum Source Background Correction.Part 1. Determi- nation of Arsenic in Marine Sediments- Douglas C. Baxter Wolfgang Frech and Ingemar Berglund Matrix Interferences Observed With a Thermospray Sample Introduction System for Inductively Coupled Plasma Atomic Emission Spectrometry-M. T. C. de Loos-Vollebregt Runzhong Peng and J. J. Tiggelman Determination of Trace Amounts of Manganese in Water by Laser-enhanced Ionization Spectrometry After Solvent Extraction-A. Miyazaki and H. Tao Determination of Arsenic by Hydride Generation-Inductively Coupled Plasma Mass Spectrometry Using a Tubular Membrane Gas-Liquid Separator- Simon Branch Warren T. Corns Les Ebdon Steve Hill and Peter O’Neill Determination of Low Concentrations of Lithium in Biological Samples Using Electrothermal Atomization Atomic Ab- sorption Spectrometry-B. Sampson Pre-concentration by Coprecipitation.Part 1. Rapid Method for the Determina- tion of Ultra-trace Amounts of Ge in Natural Waters by Hydride Generation- Atomic Emission Spectrometry-Ian D. Brindle Mary E. Brindle Xiao-chun Le and Heng-Wu Chen Direct Determination of Chromium in Estuarine and Coastal Water by Graphite Furnace Atomic Absorption Spectrome- try-4. C. Apte S. D. W. Comber M. J. Gardner and A. M. Gunn Elimination of Copper Interference by Continuous Flow Matrix Isolation in the Determination of Selenium by Flow Injection Hydride Generation Atomic Absorption Spectrometry-Stephen G.OMey Nichola J. Seare Julian F. Tyson and Helen A. B. Kibble Analytical Atomic Spectroscopy Learn- ing from its Past-John B. Dawson Determination of Arsenic in Samples With High Chloride Content by Induc- tively Coupled Plasma Mass Spectro- metry4imon Branch Les Ebdon Mick Ford Michael E. Foulkes and Peter O’Neill Temperature Programmed Static SIMS Study of Phosphate Chemical Modifiers in Electrothermal Atomizers-D. C. Hasell Vahid Majidi and James A. Holcombe Determination of Ultratrace Metals in Sea-water With On-line Pre- concentration and GFAAS Determina- tion-V. Porta 0. Abollino E. Mentas- ti and C. Sarzanini Direct Analysis of Plastics by Laser Abla- tion Inductively Coupled Plasma Mass Spectrometry-John Marshall Jeff Franks Ian Abell and Christopher T. TYe Determination of Ammonium Acetate Extractable Molybdenum in Soil and Aqua Regia Soluble Molybdenum in Soil and Sewage Sludge by Graphite Furnace Atomic Absorption Spectrometry- William H. Rowbottom

ISSN:0267-9477    

DOI:10.1039/JA9910600008

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 9 Determination of Fluorine in Urine and Tap Water by Laser-excited Molecular Fluorescence Spectrometry in a Graphite Tube Furnace With Front-surface Illumination David J. Butcher Richard L. Irwin Junichi Takahashit and Robert G. Michel* Department of Chemistry University of Connecticut Storrs CT 06269 USA Fluorine was determined in urine and tap water using laser-excited molecular fluorescence spectrometry (LEMOFS) with an unmodified atomic absorption graphite tube furnace. Molecular fluorescence from magnesium fluoride (excited at 268.94 nm detected at 358.82 nm) was collected using front-surface illumination and detection. A frequency doubled excimer pumped dye laser operating at 500 Hz was used for excitation. This is the first report of the use of front-surface illumination for molecular fluorescence.After optimization of the chemical furnace and laser conditions the detection limit of 0.3 pg of fluorine (as fluoride) was from two to six orders of magnitude better than other methods commonly used for the determination of fluorine and two orders of magnitude more sensitive than the best previously reported LEMOFS detection limit. The linear dynamic range of the technique was five orders of magnitude. Significant interferences from other ions (Na+ H+ CI- Br-) were observed. The sensitivity for the determination of fluorine in a freeze-dried urine standard reference material and in tap water by LEMOFS was sufficiently high to allow the samples of be diluted by a factor of 100 to remove the interferences.Good agreement with certified values was obtained with an analytical precision of 7-1 1 %. Keywords Laser-excited molecular fluorescence spectrometry; electrothermal atomizer; fluorine determination Fluorine is a ubiquitous element that is present in minerals water and many animal tissues and has been shown to be es- sential to human life.' It is routinely added to drinking water as fluoride at a level of 1 mg 1-I. However it is necessary to monitor the fluoride concentration carefully because higher levels of fluoride have been shown to cause bone disorders. A variety of methods have been developed for the determi- nation of fluorine. These include the fluoride ion-selective electrode (ISE) photometric techniques ion chromatography atomic spectroscopy and molecular spectroscopy.The fluoride ISE which was first developed by Frant and ROSS,* is the most widely accepted This method which measures only free fluoride in solution requires the addition of a reagent so- lution called the total ionic strength adjustment buffer (TISAB). This reagent serves to maintain a constant high ionic strength to regulate the pH and to release fluorine from its complexes. For practical analysis the detection limit for fluoride is approximately 0.019 mg 1-I (190 ng) with a sample volume of 10 mi and the linear dynamic range for the ISE is four orders of magnitude. Hallsworth et al.s reported a fluoride ISE method that required a 1 pl solution volume. The solution was confined as a thin layer between the fluoride electrode and a calomel electrode mounted directly below it.An absolute de- tection limit of 10 pg was reported and the linear dynamic range was four orders of magnitude. A variety of photometric techniques have been used for the determination of fluorine.' As with the ISE methods the pho- tometric techniques monitor only free fluoride present in solu- tion. The most sensitive photometric method employs sulphonated alizarin fluorine blue ( potassium 3-[N&-di(car- box ymethyl)aminomethyl]- 1,2-dihydroxyanthraquinone-5-sul- phonate} (AFBS)?' with a detection limit of 10 yg 1-' (10 ng). The major advantage of the AFBS photometric method com- pared with the fluoride ISE is improved reproducibility. Disad- vantages include longer analysis time and less tolerance to interferences. Ion chromatography has been used to separate and measure * Present address Department of Chemistry and Physics Western t On leave from Advanced Technology Institute Ryowa Building 4F $ To whom correspondence should be addressed. Carolina University Cullowhee NC 287234 USA.6- I Kamda Surugadai 3 Chiyoda-Ku Tokyo I0 I Japan. the concentrations of a variety of anions including fluoride with a conductivity detection system.R Fritz et aL9 employed a special anion-exchange column with a potassium hydroxide eluent that did not require a suppressor column. The detection limit for fluoride was 1.5 bg 1-I (150 pg) and the linear dynamic range was four orders of magnitude. Ion chromato- graphy is particularly well suited to water analysis because a variety of anions can be determined.Atomic spectrometry is widely used for the determination of metals and metalloids because of the high sensitivity achieva- ble and ability to detect the total amount of all chemical forms of an element. Compared with metals and metalloid elements little research has been performed on investigations into the use of atomic spectrometry for the determination of fluorine. Dittrich'O has summarized the reasons why fluorine has not been commonly determined by atomic spectrometry. Firstly the most sensitive resonance wavelength is at 95 nm which is far into the vacuum ultraviolet. Secondly fluorine is the most electronegative element and therefore forms stable molecules which are difficult to dissociate. Bond and O'Donnelll' determined fluorine by an indirect atomic absorption spectrometric (AAS) method in an air-coal gas flame.The fluorine concentration was monitored by meas- uring the depression of the magnesium AA signal at 285.2 nm. This technique has a variety of disadvantages including poor sensitivity with a detection limit of 0.2 mg 1-I a short linear dynamic range (one order of magnitude) and suffers severe in- terferences from sulphate and phosphate. Gelhausen and Carnahanl* have recently determined fluorine by atomic emission spectrometry in a helium microwave- induced plasma (He MIP). Using direct ultrasonic nebulization of fluoride solution the detection limit for fluorine was 4 ppm at 685.6 nm. The linear dynamic range was 2.5 orders of magnitude. An alternative method to atomic spectrometry is the deter- mination of fluorine by monitoring the spectroscopic proper- ties of diatomic molecules consisting of fluorine and a metal atom introduced as a reagent in a conventional atom cell."' The most widely studied method has been molecular absorp- tion spectroscopy (MAS) which was first investigated by Tsunoda et ~ 1 .l ~ for the determination of fluorine. They used a deuterium arc to excite aluminium fluoride (AlF) molecules in an air-acetylene flame or above a heated graphite rod. A10 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 detection limit of 24 pg I-' as fluoride was obtained with the flame and 21 pg as fluoride with the graphite rod. The method was successfully applied to the determination of fluorine in an agricultural standard reference material and in organofluorine compounds.More recently Tsunoda et al.I4 investigated the relative sen- sitivities of four alkaline earth monofluorides MgF CaF SrF and BaF. A tungsten lamp was used to excite the molecules vaporized from a graphite rod. Strontium fluoride was shown to be the most sensitive molecule. The relative sensitivities of seven monofluorides have also been studied." Aluminium monofluoride was shown to be the most sensitive with a 1% absorbance signal at 20 pg followed by indium monofluoride 110 pg gallium monofluoride 160 pg strontium fluoride 380 pg calcium monofluoride 500 pg magnesium monofluoride 1.5 ng and lithium monofluoride 182 ng. Tsunoda et a1.16 also demonstrated that a platinum hollow cathode lamp (HCL) could be used to excite AIF at 227.45 nm above a graphite rod.The sensitivity with the platinum HCL and with the continuum source were approximately the same.I6 They used this procedure successfully to determine fluorine in ~ r i n e ~ ~ . ' ~ blood serumt7 and milk.'* Other workers have investigated the determination of fluorine by AlF absorbance measurements with Pt HCL exci- tation in a graphite tube furnace. Venkateswarlu et al." used this method for the determination of fluorine in fluorinated organic molecules. They reported that the organic fluorine had to be converted into inorganic fluorine with a sodium biphenyl reagent in order to obtain accurate results. Itai et al.20 investi- gated the effects of chemical modifers and furnace material upon the determination of fluorine by AIF absorbance.They concluded that magnesium nitrate or barium nitrate were ade- quate chemical modifiers and that glassy carbon and synthetic carbon were the most suitable furnace materials. Gomez et al.2' used AlF absorption to determine fluorine in drinking water and sea-water. The MAS results were in good agreement with results obtained with a fluoride ISE. Dittrich and co-workers22-26 investigated the determination of fluorine by molecular absorption spectrometry employing a wide variety of metallic reagents and deuterium arc excitation in a graphite furnace. The detection limit for fluorine using gallium fluoride (GaF) was 1.6 ng.22 Indium monofluoride and aluminium monofluoride were reported to provide fluorine de- tection limits of 0.8 and 2.2 ng respectively.23 A more recent publication demonstrated that a detection limit of 5 ng could be obtained using MgF for the determination of and a review article2s summarized the MAS results obtained. Dit- trich et a1.26 also developed a method for the determination of fluoride by graphite furnace MAS that included an extraction method to separate and pre-concentrate the fluoride.In order to obtain higher sensitivity Dittrich et ai.27 devel- oped a new technique for the determination of fluorine that in- volves laser-excited molecular fluorescence spectrometry (LEMOFS) in a graphite tube furnace. They employed the molecular fluorescence of magnesium fluoride (MgF) and transverse illumination which requires additional ports in the graphite furnace to allow passage of the laser beam at a right angle to the fluorescence axis [Fig.l(a)]. Detection limits of 10 and 40 pg were obtained using resonance (268.9-268.9 nm) and non-resonance fluorescence transitions (268.9-359.3 nm) respectively. A more modem approach to collect fluorescence from a graphite tube furnace is front-surface illumination,2x which is the collection of fluorescence at 180" to the direction of the laser beam [Fig. I(h)]. A mirror through which a hole is drilled to allow passage of the laser radiation is used to collect ff uorescence along the bore of the tube. There are two primary advantages of front-surface illumination compared with trans- verse illumination. Firstly the probe volume which is the volume illuminated by the laser and observed by the detector is approximately ten-times larger for front-surface illumina- To monochromator t LensT7 To monochromator I P Lens I Fluorescence Probe volume Mi!ror T Fig.1 atomizers ( a ) transverse illumination and ( h ) front-surface illumination Two illumination arrangements of LEMOFS in graphite tube tion. Therefore signals are approximately one order of magni- tude bigger than those using transverse illumination.29 Second- ly front-surface illumination does not require modification of the graphite furnace which means that modem furnace techno- logy that has been developed for the determination of trace elements by atomic absorption spectrometry can be used di- rectly for molecular fluorescence. The determination of fluorine by LEMOFS of magnesium monofluoride (MgF) in a graphite tube furnace with front- surface illumination is described in this paper.This first report of LEMOFS with front-surface illumination demon- strates improved sensitivity compared with previous work. The amounts of reagents the furnace conditions and the laser power were optimized to provide the maximum magnesium monofluoride signal. The effects of a variety of potential in- terferent ions were studied. The sensitivity of this method was sufficiently high to allow the samples to be diluted by a factor of 100 which reduced interferences and maintained detection limits that were still 2-3 orders of magnitude better than conventional methods for the determination of fluorine. The potential of this method for real sample analysis was investi- gated by the analysis of a urine standard reference material and tap water.Experimental Instrumentation A schematic diagram of the equipment used is shown in Fig. 2 and the components are listed in Table 1 . The majority of the work was performed with an excimer pumped dye laser and aJOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 11 I Mirror I Furnace power I Fig. 2 Schematic diagram of the instrumentation for LEMOFS Table 1 Instrumentation Componenthlodel number Manufacturer Primary laser system- Excimer laser/800XR Dye laser/DL- 19P Frequency doubler/5- 12 Secondary laser- system- Tachisto Needham MA USA Molectron Santa Clara CA USA h a d Northvale NJ USA Excimer laser/EMG 104 Dye laser/FL 3002 Frequency doublem37- MSC Lambda Physik Acton MA USA Lambda Physik 1 Lambda Physik Other- components- Boxcar averagedl 65 162 Pre-amplifier/VV IOOBTB Monochromator/H- 10 Graphite furnace/HGA-500 Pyrolytic graphite coated graphite tube/BO 109322 PMTD893QB - 3 50 (Lot NO.11-1 2-68) L’vov platform/l09324 Autosampler/AS-40 AT compatible micro- Computer interfaceJlT280 1 -A Triggering circuitry Data processing software Bandpass filter/B390 (Lot NO. 9-05273-164) computer/System 200 PAR Princeton NJ USA LeCroy Spring Valley NY USA Thorn-EMI Fairfield NJ USA ISA Metuchen NJ. USA Perkin-Elmer Norwalk CT. USA Perkin-Elmer Perkin-Elmer Perkin-Elmer Dell Austin TX USA Data Tanslation Marlboro MA USA Laboratory constructed Asyst Software Rochester NY USA Hoya Tokyo Japan frequency doubler operating at 80 Hz.The maximum power at the MgF excitation wavelength (268.94 nm) was 10 pJ per pulse. Fluorescence was detected at 358.82 nm. A second excimer pumped frequency doubled dye laser system was used to optimize the laser power and to obtain the best detection limit. This laser system was operated at 500 Hz with a maximum power of 200 pJ per pulse. For both laser systems a small portion of the excimer laser beam was used to trigger a boxcar averager that was used to process the fluorescence signal from a PMT. Coumarin 540A ( 1 H,4H-2,3,5,6-tetrahydro-8- (trifluoromethyl)quinolizino[9,9a 1 -gh]coumarin) was em- ployed as the laser dye at a concentration of 16 mmol dm-3 dis- solved in methanol. The frequency doubled output was passed through a telescope to adjust the beam size to 3-5 mm in dia- meter before being passed through the atomizer.The atomizer was a Perkin-Elmer HGA-500 graphite tube furnace equipped with an AS-40 autosampler and a L’vov platform. Fluorescence was collected from the graphite tube furnace using front-surface illumination” [Fig. 1 (h)]. A 2 in diameter plane mirror with a in hole in the centre through which the laser beam passed was positioned in front of the furnace at an angle of 45” with respect to the excitation axis. A 10 cm focal length lens and a 5 cm focal length lens were used to collect the fluorescence. The diameter of both lenses was 5 cm. The longer focal length lens was used after the mirror rather than one of shorter focal length because of the physical problem associated with positioning the mirror between the furnace and the lens. The resultant fluorescence was collected by a gated detec- tion system consisting of a photomultiplier tube (PMT) a pre- amplifier and a boxcar integrator. Data from the boxcar were collected by a computer which plotted the fluorescence signal as a function of atomization time and calculated the integrated fluorescence. A colour bandpass filter Hoya B390 (Hoya Tokyo Japan) was used to discriminate against stray laser ra- diation The maximum transmittance (75%) of this filter was at 390 nm; the full width at half maximum was 140 nm.Detection Limits and Calibration Graphs The detection limits were determined after subtraction of the blank signal by extrapolation of the calibration graphs to a signal level equal to three times the standard deviation of 16 measurements of the blank noise.w The measurement of the blank noise was performed either with the laser tuned to the analytical wavelength (the on-line measurement) or with the laser tuned 0.1 nm away from the analytical wavelength (the off-line measurement).Calibration graphs were established by using 20 p1 aliquots of aqueous standard solutions. Attenuation of the LEMOFS signal was necessary to ensure a linear response of the PMT during the construction of calibration graphs. The attenuation was carried out by inserting calibrated neutral density filters between the two lenses. The attenuation of the laser power was done by inserting calibrated neural density filters before the mirror. Standard Solutions and Samples The water supply at the University of Connecticut contains a significant amount of fluoride.Consequently even after de- ionization of the water with ion-exchange columns the concentration of fluoride in the water was approximately 100 pg 1-I. Therefore the de-ionized water was further purified by passage through a sub-boiling quartz distillation unit (Quartz and Slice Paris France) which reduced the fluoride concentration to 10 pg I-’. The standard fluorine (from NaF) magnesium [from Mg(NO,),] and barium [from Ba(NO,),] solutions were pre- pared daily on a class 100 clean air bench by serial dilution of a 1 g I-’ stock solution. Standard solutions with concentrations less than 1 mg I-’ were stored in polyethylene bottles. The stock solutions were made from high-purity salts (Spex Indus- tries Metuchen NJ USA).Two freeze-dried urine samples were obtained from the Na- tional Institute of Standards and Technology (SRM 267 1 A). These were a low fluoride sample and an elevated fluoride sample. These samples were reconstituted with de-ionized water as directed. Results and Discussion Chemical and Spectroscopic Properties of Magnesium Monofluoride (MgF) Searcy3’ considered the formation of diatomic molecules in a chapter on high temperature inorganic chemistry. High tem- perature was defined as a temperature at which entropy differ- ences have a significant influence in determining the reaction equilibria of interest. Under these high-temperature conditions fluorine was shown to form stable diatomic molecules with all elements in the Periodic Table except for the rare gases.Magnesium monofluoride (MgF) which was first identified12 JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 by Datta,32 has been characterized by the presence of three band systems:33 A211 t X,C+; B2C+ t X2C+; and C2C+ c X2C+; where X2Z+ is the ground state. The A2n t X2Z+ system which is composed of three series of doublets is the most intense system with the highest sensitivity at the 358.82 nm (0,O) vibrational transition.33 The B2C+ t X,Z+ band system has the highest sensitivity at the 268.94 nm (0,O) vibrational transition.33 These two systems have been observed in a direct current (d.c.) arc at 1150 "C3' The C,C+ t X,C+ system is much less intense than the other two with maximum sensitivity at the 234.78 nrnL(O,O) transition.Fuwa and c o - ~ o r k e r s ~ ~ ~ ' ~ employed the 358.82 nm wave- length for the determination of fluorine by MgF molecular ab- sorbance spectrometry (MAS) in a graphite furnace. They reported 1% absorbance at 1.5 ng. Dittrich and co- w o r k e r ~ ~ ~ - ~ ~ . ~ ~ . ~ ~ compared the sensitivity of the 268.94 and 358.82 nm wavelengths for graphite furnace MAS. The former band was approximately twice as sensitive as the latter.25 Both groups reported that MgF was approximately two orders of magnitude less sensitive than AlF by MAS which is the most sensitive molecule for fluorine determination. Dittrich et al.27 determined fluorine by LEMOFS of MgF in a graphite tube furnace with transverse illumination [Fig. l(a)]. They employed MgF rather than AIF because the AIF wavelength (227.45 nm) was inaccessible with their laser system.They obtained a detection limit of 45 pg using a non- resonance transition (excited at 268.94 nm detected at 358.82nm). A detection limit of 1Opg was achieved using resonance fluorescence (excitation and detection at 268.94 nm). For this work MgF was employed for the determination of fluorine by LEMOFS. Aluminium fluoride was not used because the primary laser system could not provide the AIF wavelength. Since performing the bulk of this work it became 8000 4000 .- 2' f feasible to determine aluminium fluoride with the secondary laser system although we did not do this. Excitation was per- formed at the MgF B2C+ t X,C+ (0,O) transition (268.94 nm) and non-resonance fluorescence was detected at the A2n t X2C+ (0,O) transition (358.82 nm).Front-surface illu- mination was employed to improve the sensitivity of the in- strument and to allow the use of unmodified atomic absorption graphite tube furnaces. The non-resonance transi- tion was employed to reduce stray light noise. Chemical Optimization Dittrich and c o - w o r k e r ~ ~ ~ - ~ ~ - ~ ~ summarized procedures to opti- mize the chemical conditions for the determination of non- metals (X) by measuring molecular absorbance of a diatomic molecule (MX) where M is a metallic reagent. The chemical parameters to be optimized included the amount of the metal- lic reagent magnesium; the amount of acid or base required to control the pH; and the amount of chemical modifier barium or strontium.Dittrich and c o - w o r k e r ~ ~ ~ ~ ~ ~ ~ ~ ~ demonstrated that excess of metallic reagent (Mg) should be introduced into the furnace to react with all the analyte non-metal (F). They also proposed that the formation of diatomic molecules in a graphite furnace involves gas-phase combination of the metallic reagent and the analyte. A reaction that competes with the formation of MgF is the formation of magnesium difluoride (MgF,). If the gas phase contains excess of magnesium relative to the fluorine then the equilibrium that controls the system is given by31 (1) Complete conversion of fluorine into MgF is likely by use of excess of magnesium. The results obtained for the optimiza- tion of the MgF signal as a function of the amount of magne- sium introduced (as magnesium nitrate) are summarized in Mg(g) + MgF,(g) + 2MgF(g) 200 loo v 1x104 lxlo-z 1x10' 1x102 1x10' Amount of OH-(as NaOH)/pg Amount of Srlpg Amount of B4pg Fig. 3 Optimization of the reagents for LEMOFS.(a) Effect of magnesium added as Mg(NO,) on the MgF fluroescence signal. Experimental condi- tions NaF (10 ng as F); Ba(N03)? (1.65 pg as Ba); atomization temperature 1800 "C; char temperature 800 'C; and laser power 10 pJ per pulse. ( h ) Effect of hydroxide added as NaOH on the MgF fluroescence signal. Experimental conditions NaF (10 ng as F); Mg(NO,) (20 pg as Mg); atomization temperature 1800 "C; char temperature 800 "C; and laser power 10 yJ per pulse. (c) Effect of barium added as Ba(NO& on the MgF fluorescence signal. Experimental conditions NaF (10 ng as F); Mg(NO,) (20 pg as Mg); atomization temperature 1800 "C; char temperature 800 "C; and laser power 10 pJ per pulse.( d ) Effect of strontium added as Sr(NO& on the MgF fluorescence signal. Experimental conditions NaF (10 ng as F); Mg(N03) (20 pg as Mg); atomization temperature 1800 "C; char temperature 800 'C; and laser power 10 pJ per pulseJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 13 Fig. 3(a). A maximum fluorescence signal was obtained with 20pg of magnesium which is within a factor of two of Dittrich’s optimized amount of lOpg.,’ The reason for the decrease of the MgF signal when more than 20 pg of magne- sium were added to the furnace is not yet known. Work is being done in this laboratory to explore more fully the mecha- nism of MgF formation in a graphite tube furnace.Dittrich’O investigated the effect of pH upon the formation of diatomic molecules and demonstrated that basic conditions enhance the formation of many molecules. This enhancement was attributed to suppression of the high-temperature hydroly- sis of hydrated salts that may be formed during drying of the sample. lo Dittrich and co-workers10~2s~27 also reported that the molecu- lar fluorescence signal can be enhanced by the introduction of an additional metal compound a chemical modifier that forms an insoluble salt with the analyte anion. It was stated that the chemical modifiers provide a nearly simultaneous evaporation of the fluoride and the magnesium. These workers employed Ba(OH) or Sr(OH) which maintained basic conditions to eliminate hydrolysis and allow coincidental evaporation.However Ba(OH) or Sr(OH) were not employed in the present work because it would have been necessary to keep these solutions under an atmosphere of nitrogen in order to prevent precipitation of BaCO or S I - C O ~ . ~ ~ Consequently the effect of pH upon the MgF signal was investigated by the addi- tion of hydroxide (added as sodium hydroxide) and the amount of barium (added as barium nitrate) or strontium (added as strontium nitrate) was optimized separately. The investigation of the effect of sodium hydroxide upon the MgF fluorescence signal is shown in Fig. 3(h). When amounts of hydroxide of less than approximately 1 pg were in- troduced into the furnace no change in the MgF fluroescence signal was observed because MgF does not form a hydrated The addition of large amounts of hydroxide (greater than 10 pg) reduced the MgF signal probably because the hydrox- ide competed with the fluorine for magnesium decreasing the amount of MgF formed in the graphite furnace.Dittrich et ~ 1 . ~ ~ did not investigate the effect of sodium hydroxide upon the MgF signal for LEMOFS probably because a basic compound (barium dihydroxide) was added as the chemical modifier. Either barium nitrate [Ba(NO,),] or strontium nitrate [Sr(NO,),] were used as the chemical modifier rather than the hydroxides because these solutions were stable in air. The ad- dition of barium nitrate caused a significant enhancement in the MgF signal [Fig. 3(c)] with a maximum enhancement of 100 times for the introduction of 1-2 pg of barium.For routine analytical work 1.65 pg of barium were added to the furnace. The addition of larger amounts of barium to the furnace caused suppression of the MgF signal which was probably due to increased formation of BaF relative to MgF. Dittrich et al.27 reported that the optimized amount of barium (added as barium hydroxide) was 13 pg which is a factor of ten higher than the optimized amount of barium nitrate obtained in the present experiments. These data indicate that barium nitrate is a more effective chemical modifier than barium hydroxide. Strontium nitrate was also shown to enhance the MgF signal [Fig. 3(6)] to give a maximum 10-fold increase in signal size with the addition of 10 pg of strontium.The MgF signal was suppressed with the addition of very large amounts of stron- tium probably due to an increase in the formation of SrF rela- tive to MgF. Barium was used as a chemical modifier rather than strontium because a larger increase in signal was obtained with a lower amount of reagent. Dittrich et ~ 1 . ~ ~ did not use strontium as a chemical modifier for LEMOFS. The mechanism for the enhancement of the MgF signal by barium has not been investigated experimentally. Pertinent thermodynamic and physical properties of fluorine barium and magnesium compounds that are likely to be formed in the graphite furnace are given in Table 2 in order to help propose a mechanism for the enhancement induced by barium. Dit- trichIo suggested that the formation of diatomic molecules in- volves gas-phase reaction between the metallic reagent (Mg) and the analyte (F).In the absence of a chemical modifier fluorine is believed to evaporate at a lower atomization tem- perature or temporally sooner in the graphite furnace than magnesium. The observed enhancement in the presence of barium was attributed to more nearly simultaneous evapora- tion of magnesium and fluorine. This enhancement may be caused either (i) the evaporation of fluorine at a higher atomi- zation temperature or temporally sooner in the graphite furnace or (ii) the evaporation of magnesium at a lower atom- ization temperature or temporally later in the graphite furnace. Dittrich et al.,’ suggested that this enhancement is caused by the preferential precipitation of BaF relative to MgF after ad- dition of the reagents to the furnace followed by nearly simul- taneous vaporization of BaF and Mg.Although barium does form an insoluble fluoride the solubility of barium difluoride is greater than the solubility of magnesium difluoride (Table 2). Thus there should be little formation of BaF,(s) in the graphite furnace following addition of the reagents which to some extent invalidates Dittrich’s precipitation hypothesis. Another approach to investigating the signal enhancement by barium is to consider the atomization mechanisms of barium and magnesium in graphite tube furnaces. Although magnesium is a relatively volatile metal with a boiling-point of 1090 “C the recommended atomization temperature for magnesium is 2400 “C.377.38 The mechanism of atomization for magnesium has been investigated because magnesium nitrate is commonly added to graphite furnaces as a chemical Table 2 Physical and thermodynamic properties of barium and magnesium compounds.All data were taken from reference 35 unless indicated otherwise Compound Boiling-point/”C Solubility/g 1-’ HF*/kcal mol-’ HEtkcal mol-I T,,SI”C Ba BaO BaF2 NaF 5aC2 1090 3600 2239 1640 2000 2137 - - - 0.0062 0.076 - 1.2 - 0 -143.8 -263.5 0 - 133.4 -286.9 - 42.0 69.8s - - * HF heat of formation. -t HE heat of evaporation. 4 T,,,. formation temperature. 5 From reference 36. 4 From reference 33.14 JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 modifier. Slavin and c o - w ~ r k e r s ~ ~ . ~ ~ proposed that Mg(N0,)2 is converted into magnesium oxide in the solid phase prior to vaporization.Magnesium oxide is non-volatile (Table 2) with a boiling-point of 3600 "C. A possible reason for the MgF signal enhancement is the conversion of relatively non-volatile magnesium oxide by barium (or barium compounds) into more volatile magnesium metal (boiling-point 1090 "C) causing more coincidental evaporation of magnesium and fluorine. The determination of barium in a graphite tube furnace has been shown to be inhibited by formation of carbide.3x4 Frech et al.39 reported that the tailing observed in barium peaks in graphite furnace atomic absorption may be caused by carbide formation. StyriP proposed that the mechanism involved the reaction BaC,(s) +BaC,(g) +Ba(g). Although we have no experimental evidence to support this perhaps barium carbide formation is involved in the enhanced sensitivity of MgF (2) This mechanism would cause vaporization of magnesium at a lower temperature and temporally sooner in the graphite furnace atomization step.This would cause more coincidental evaporation of magnesium and fluorine and a larger MgF signal. Although these processes may influence the mechanism a rigorous study of all the species present their concentrations and their temporal appearance is necessary to define the chemi- cal reactions involved in the enhancement of MgF by barium. These studies are currently being performed in this laboratory. BaC + 2Mg0 + 2Mg + Ba + 2CO Furnace Optimization The atomization and char temperatures were optimized to obtain the maximum MgF fluorescence signal with the front- surface illumination system.Dittrich ef al.27 reported that an atomization temperature of between 2700 and 3000 "C was re- quired to obtain optimum sensitivity for MgF fluorescence for the determination of fluorine. Fig. 4(a) shows the atomization obtained using optimized amounts of magnesium nitrate (20 pg as magnesium) and barium nitrate (1.65 pg as barium). The fluorescence signal increases with temperature from 1300 to 1800 "C slightly drops off between 1800 and 2100 "C and then is relatively constant up to 2700 "C. The optimum temperature of 1800 "C for the instrument used was approximately lo00 "C lower than Dittrich et al. re- ported for their transverse optical axes. This relatively low atomization temperature tends to support the hypothesis that fairly volatile magnesium and fluorine were vaporized rather than other less volatile species such as magnesium oxide.The atomization temperatures required for LEMOFS would be ex- pected to be the same (within k200 "C) for transverse and front-surface illumination because of previous experience with transverse illumination for laser-excited atomic fluorescence spectrometry (LEAFS).4' Therefore the higher atomization temperature in Dittrich's magnesium fluoride work was probably due to other aspects of the instrumentation and protocol that could not be identified rather than in the use of transverse illumination. Dittrich ef al.27 did not report the use of a charring step in their LEMOFS work. The results obtained for optimization of the charring step using the optimized amounts of Mg (20 pg) and Ba (1.65 pg) and an atomization temperature of 1800 "C are shown in Fig.4(h). The maximum fluorescence signal was obtained with charring temperatures of between 800 and 1 100 "C. The use of temperatures greater than 1 100°C caused a decrease in signal with temperature due to the removal of analyte during the charring step. A charring temperature of 800 "C was used for the remainder of this work. The optimized furnace programme is shown in Table 3. Laser Optimization The laser power was optimized by obtaining a saturation curve 1200 lo00 800 f I 8 01 J I I I I I 6 1200 -1400 1600 1800 2000 2200 2400 2600 2800 i I 0 200 400 600 800 loo0 1200 1400 1600 Char temperature/"C Fig.4 Optimization of furnace parameters for LEMOFS. (a) Effect of atomization temperature on the magnesium fluoride fluorescence signal. Experimental conditions NaF (10 ng as F); Mg(NO,) (20 pg as Mg); Ba(NO,) (1.65 pg as Ba); char temperature 800 "C; and laser power 10 pJ per pulse. (h) Effect of char temperature on the magnesium fluoride fluorescence signal. Experimental conditions NaF (10 ng as F); Mg(NO,) (20 pg as Mg); Ba(NO,) ( 1.65 pg as Ba); atomization temperature 1800 'C; and laser power 10 pJ per pulse Table 3 LEMOFS of MgF Furnace programme for the determination of fluorine by Internal flow*/ Step T /"C Ramp/s Hold time/s ml min-' Dry 200 20 40 300 Char 800 1 30 300 Cool 20 1 10 300 Atomize 1800 O t 3 0 Clean 2700 1 5 300 COO1 20 1 20 300 * Argon.f Maximum power heating (approximately IS00 'C SKI) Table 4 Optimized chemical. furnace and laser conditions for MgF for the determination of fluorine by LEMOFS Chemical conditions Furnace conditions Laser conditions 20 pg of Mg [added as Mg(N0,)2]; 1.65 pg Ba [added as Ba(NO,),] Atomization temperature 1800 "C; char temperature 800 "C Laser power 100 pJ per pulse at 268.94 nm (Fig. 5 ) with the more powerful laser system and the optimum chemical (Mg 20 pg; and Ba 1.65 pg) and furnace conditions (Table 3). Fig. 9 shows that the fluorescence signal was direct- ly proportional to the laser power up to 100 pJ per pulse. The best LEMOFS detection limit was obtained by use of a laser power of I 0 0 p.I per pulse that saturated the MgF transition. A summary of the optimum chemical furnace and laser con-JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL.6 15 1000 I 0.01 ' I I 0.1 1 10 100 lo00 Laser power/@ per pulse Fig. 5 Saturation curve for magnesium fluoride which demonstrates the effect of laser power on the MgF signal. Experimental conditions NaF (10 ng as F); Mg(N0,)2 (20 pg as Mg); Ba(NO,)? (1 -65 pg as Ba); atomization temperature 1800 'C; and char temperature 800 'C 10 1 I 0 1 2 3 t l S Fig. 6 Temporal fluorescence signal obtained for 8 ng of F. Experimen- tal conditions NaF (10 ng as F); Mg(NO,) (20pg as Mg); Ba(NO,) (1.65 pg as Ba); atomization temperature 1800 "C; char temperature 800 "C; and laser power 10 pJ per pulse .- a 2500 (I) E .r 2000 1,l I i i 2G.W 268.25 268.50 268.75 269.00 269.25 Unm Fig. 7 Spectral scan for magnesium fluoride which is a plot of MgF signal w x u s the laser wavelength.Experimental conditions NaF (10 ng as F); Mg(N03)2 (20 pg as Mg); Ba(N03)? (1.65 pg as Ba); atomization temperature 1800 "C; char temperature 800 "C; laser power 10 pJ per pulse at all wavelengths; and approximate laser line width 0.003 nm ditions for the determination of fluorine by LEMOFS using MgF is given in Table 4. A LEMOFS signal of 8 ng of fluoride under these optimized conditions is illustrated in Fig. 6. MgF Excitation Profile Fig. 7 shows an excitation profile for MgF which is a plot of excitation wavelength obtained by tuning the laser versus relative fluorescence signal. The observed bands matched the I I .c 1x10~ C c .- Q 1x10' 8 2 Q) 1x10-' 3 = .- CI - Q K iX10-' L 1 1 I 1 1 lXlO-' 1x10-2 1x10' 1x102 1x10' Amount of fluorinelng Fig. 8 Calibration graph for fluorine by magnesium fluoride fluorescence which is a plot of MgF fluorescence signal versus amount of fluorine (as NaF) added. Experimental conditions NaF (10 ng as F); Mg(NO,) (20 pg as Mg); Ba(NO,) (1.65 pg as Ba); atomization tempera- ture 1800 "C; char temperature 800 "C and laser power 10 pJ per pulse reference values compiled by Rosen,33 at 268.94 268.65 268.38 and 268.13 nm.These four bands corresponded to the (O,O) (1 ,l) (2,2) and (3,3) vibrational transitions respectively associated with the B2Z+ t X,C+ electronic transition. All ana- lytical work was performed with the laser tuned to 268.94 nm which is the peak of the most intense (0,O) vibrational transition. Detection Limits and Linear Dynamic Range A calibration graph for the determination of fluorine by LEMOFS is shown in Fig.8. Using the optimized chemical and furnace conditions and the maximum laser power from the primary laser system (10 pJ per pulse) at a repetition rate of 80 Hz a detection limit of 2 pg of fluorine was obtained. Using the secondary laser system at a repetition rate of 500 Hz the laser power was optimized (100 pJ per pulse) to obtain a detection limit of 0.3 pg. The linear dynamic range (LDR) was five orders of magnitude which is the first LDR reported for LEMOFS. The observed improvement in sensitivity between the two lasers can be predicted from signal to noise ratio considera- t i o n ~ . ~ " ~ ~ The laser power of the secondary laser was ten times larger than the primary laser system which between 10 and 100 pJ would lead to a 10-fold increase in the signal.Owing to increased laser stray light the signal to noise ratio is propor- tional to the square root of the laser power so the detection limit should improve by a factor of 6 0 = 3.2.2R.42 The repeti- tion rate of the secondary laser was five times higher than that of the primary laser system. The signal to noise ratio is propor- tional to the square root of-the repetition rate so an improve- ment in detection limit of 45 = 2.2 was expe~ted.?~.~~ The over- all predicted improvement in the detection limit was therefore 7 which is in good agreement with the experimentally ob- served improvement of 6.6. A summary of all LEMOFS detection limits for the determi- nation of fluorine by MgF fluorescence is given in Table 5.Dit- trich et al." obtained a detection limit of 40 pg using transverse illumination wall atomization and a non-resonance (268.9- 359.4 nm) transition. Using the same illumination geometry and sample introduction the fluorine detection limit was im- proved by a factor of four (10 pg) by use of a resonance (268.94-268.94 nm) transition. In this work with front-surface illumination and non-resonance fluorescence a detection limit of 2 pg was obtained with a laser power of 10 pJ per pulse at a laser repetition rate of 80 Hz. Using the same illumination geo- metry and the same transition the detection limit was improved by a factor of 6.6 (0.3 pg) by the use of a more modem laser system which provided 100 CLJ per pulse at a repetition rate of 500 Hz.The results reported here are 5-20 times more sensitive16 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 ~~ Table 5 Summary of LEMOFS detection limits obtained with MgF fluorescence for the determination of fluorine Detection limit Researchers Illumination Transit ion/nm pg ng ml-I Dittrich ef a/.* Transverse 268.9-268.9 10 0.5 Dittrich ef al.* Transverse 268.9-359 40 2 This work? Front surface3 268.9-359 2 0. I This work? Front surfaces 268.9-359 0.3 0.015 * Taken from reference 27. Experimental conditions Mg(NO& (10 Fg as Mg); Ba(OHI2 (26 pg as Ba); atomization temperature 2700-3000 'C; and charring temperature none. t This work. Experimental conditions Mg(N03)2 (20 pg as Mg); Ba(NO& ( I .65 pg as Ba) atomization temperature 1800 'C and charring temperature 800 'C.$ A laser power of 10 pJ per pulse was used at a repetition rate of 80 Hz. J A laser power of 200 pJ per pulse was used at a repetition rate of 500 Hz. Table 6 Comparison of detection limits obtained by common techniques used for the determination of fluorine Detection limit ~~ Technique Method Pg Pg I-' LEMOFS* MgF 0.3 0.015 MAST AIF 20 1 Spectrophotometryt Sulphonated alizarin 1x104 10 ISES TISAB water 1 . 9 ~ lo5 19 * This work by LEMOFS in a graphite tube furnace with platform atomization t Taken from reference 15 by molecular absorption spectrometry with a platinum $ Taken from reference 6 by spectrophotometry with sulphonated alizarin fluorine 5 Taken from reference 4 with an ISE with an aqueous total ionic strength adjust- and front-surface illumination.hollow cathode lamp and wall atomization. blue reagent. ment buffer. than previously reported LEMOFS results for fluorine. A comparison of the best LEMOFS detection limit obtained (0.3 pg) with other methods for the determination of fluorine is given in Table 6. The best molecular absorption spectrometry detection limit for fluorine is 20 pg obtained by Tsunoda et aE.I5 which is two orders of magnitude less sensitive than the LEMOFS detection limit. A commonly used spectrophoto- metric method involves the formation of a complex between free fluoride ion and sulphonated alizarin fluorine blue. Leonard and Murray6 reported a detection limit of 10 ng which is almost five orders of magnitude worse than the LEMOFS detection limit.The most widely used technique for the determination of free fluoride is the ISE. For routine analy- sis the absolute detection limit for fluoride is 190 ng,4 which is six orders of magnitude worse than the LEMOFS result. These data show that LEMOFS is considerably more sensitive than the methods commonly used for the determination of fluorine. Effects of Other Ions on the MgF Signal Dittrich et al.27 investigated the effects of concomitant salts upon the MAS and LEMOFS signal sizes for MgF. Severe signal depressions were observed in the presence of excess of sodium chloride and bromide at levels 10-1OOO times higher than the level of fluorine. Here the effects of a variety of salts on the MgF fluorescence signal were investigated for the front- surface approach where interferences might be expected to be lower than the transverse approach.Open atomizers such as a graphite tube furnace modified for transverse illumination have been shown to be susceptible to vapour phase interfer- ences in AAS3' and in LEAFS.2n The effect of Na+ added as NaNO on the MgF fluorescence signal is demonstrated in Fig. 9(a). The optimized chemical conditions were used with 10 ng of fluorine at an atomization temperature of 1800 "C. The fluorescence signal was sup- pressed when more than 100 ng of sodium were added to the sample which represented a 10-fold excess on a per mass basis. The suppression was probably due to a reduction in the formation of MgF and an increase in the formation of NaF in the presence of large amounts of sodium. Similar behaviour was observed when Na' was added in the form of NaOH.Fig. 9(h) demonstrates that there was no suppression of the fluorescence signal until a 200-fold excess of sodium relative to fluorine was present. The effect of H' added as nitric acid upon MgF fluorescence was also investigated. Using the optimized chemical and furnace conditions the fluorescence signal de- creased when a 100-fold excess of nitric acid was present [Fig. 9(c)] which suggests that it may be difficult to determine fluorine in samples that have been dissolved by acid digestion unless steps are taken to remove the acid. The influence of anions on LEMOFS was also investigated. Fig. 9(4 is a plot of MgF fluorescence signal versus amount of C1- added as NH,Cl.A suppression of the fluorescence signal was observed when a 100-fold excess of chloride relative to fluoride was present. Similar behaviour was observed when bromide (added as ammonium bromide) was introduced as a 100-fold excess of bromide suppressed the MgF signal [Fig. 9 (e)]. These anions probably suppressed the MgF signal by competing with fluorine for magnesium. As the amount of MgCl or MgBr was increased there was insufficient magne- sium to obtain the maximum amount of MgF. In summary all ions that were investigated caused a sup- pression of the MgF signal when in excess at levels of between 10 and 200 times. The suppressions reported here with front-surface illumination occurred at approximately the same concentration as those reported by Dittrich et a/.*' with transverse illumination. These data indicate that an unmodified graphite furnace was not more effective at reducing vapour phase interferences for LEMOFS than a furnace modified for the use of transverse illumination.The suppression due to cations (Na' H') probably occurred because the suppressing cation competed with magnesium for fluorine which caused a decrease in the level of MgF fluorescence. At this point it is unclear why sodium added as the nitrate caused a greater signal suppression than sodium added as the hydroxide. The anions probably suppressed the MgF fluorescence signal by competing with fluorine for the magnesium. It was concluded from this study that a variety of ions depress the fluorescence signal which indicated that it may be difficult to use LEMOFS in the presence of a sample matrix.Accordingly the technique was tested by performing some real sample analyses. Real Sample Analysis The first sample studied was a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 267 1 a Freeze-Dried Urine. Two samples are provided with this SRM a control level and an elevated level. The concentra-A c c .c 4Ooo - P) g 3000 - 8 2 2000 - 2 5 1000 - > .- * 1 1 tion of fluorine present in the tap water at the University of Connecticut was also determined by LEMOFS. This result was compared with a value obtained with a fluoride ISE. The results obtained for the determination of fluorine in the control level Freeze-Dried Urine sample along with the certified value are given in Table 7.Initially the analysis was carried out by diluting the urine sample ten times. The experi- mental value for fluorine concentration obtained by LEMOFS of 0.259 k 0.044 mg I-' was more than a factor of two lower than the certified value of 0.55 k 0.030 mg I-'. After diluting the sample 30-fold an experimental value of 0.373 mg I-' of fluorine was obtained which was significantly closer to the certified value than the previous result. Finally when the sample was diluted 100 times the fluorine concentration was determined by LEMOFS to be 0.541 f 0.036 mg I-' which was shown to be the same as the certified value by Student's t- test. This study demonstrated that LEMOFS could be used for real sample analysis if the matrix was diluted sufficiently to reduce interferences from ions.This would seem to contradict the interferences studies shown previously which indicated that the mass ratio of the interfering species to the analyte was the important parameter as that ratio is not changed by dilu- tion. However it is possible that at high concentrations of matrix constituents the ratio of interferent to analyte is the governing factor while at low concentrations the absolute amount of the interferent is the governing factor. The results for all of the real sample analyses are summar- ized in Table 8. As was discussed above good agreement with the certified value was obtained by LEMOFS for the control level urine sample by diluting the sample 100 times. Analysis (b) ( C) 800 - 400 200 - 2000 - - I I I Table 7 Determination of fluorine by LEMOFS in NIST SRM 2671a.Freeze-Dried Urine. Molecular fluroescence of MgF excitation at 268.94 nm detection at 359 nm. Experimental conditions Mg(NO,) (20pg as Mg); Ba(NO,) ( I .65 pg as Ba); atomization temperature. 1800 "C; char- ring temperature 800 "C; laser power 10 pJ per pulse. The data are k one standard deviation 5Ooo P) = 1ooo- Q 2.s 3000(1 :f 2000- S E > .- + 1000- K 0 - .- CI Sample Concentration of F/mg I-' RSD '24 2500 2000 1500 lo00 - 500 - 0 ( dl (4 1 I I Certified value 0.550 & 0.030 10-fold dilution 0.259 k 0.044 30-fold diiution 0.373 f 0.053 100-fold dilution 0.541 f 0.036 - 16.9 14.2 6.7 of the elevated level urine sample using a 100-fold dilution by LEMOFS also gave a value which was statistically the same as the certified value.The analytical precision of the LEMOFS measurements was between 7.1 and 10.7%. The concentration of fluorine in the University of Connecti- cut tap water was determined by LEMOFS to be 0.70 k 0.05 mg I-' with an RSD of 7.1%. This result agreed well with a value of 0.77 k 0.09 mg I-' that was obtained with a fluoride ISE. Conclusions Laser-excited molecular fluorescence spectrometry in a graph- ite tube furnace with front-surface illumination was shown to be an extremely sensitive method for the determination of fluorine. Fluorine was determined by the molecular fluorescence of magnesium fluoride in a commercially avail- able atomic absorption graphite furnace. For the first time molecular fluorescence was collected at 180" to the direction of the laser beam from an unmodified atomic absorption graphite furnace using front-surface illumination.Advantages of front-surface illumination include improved sensitivity and the use of modern furnace technology that has been developed for atomic absorption. Barium was shown to enhance the MgF fluorescence signal by two orders of magnitude although the mechanism of this enhancement was uncertain. The detection limit for fluorine was 0.3 pg which is from two to six orders of magnitude more sensitive than commonly used methods. The LDR for LEMOFS was five orders of magnitude which is the first to be reported. The analytical precision was between 7 and 11%. However LEMOFS was shown to be severely af- fected by interferences from other ions which suppressed the fluorescence signal.In spite of these interferences it was pos- sible to show that LEMOFS can be used to determine fluorine accurately in real samples by 100-fold dilution of the sample. The dilution capability can only be employed when using in- strumentation such as that used here. that gives sufficient sensitivity. The LEMOFS technique has considerable potential for the determination of ultratrace amounts of non-metals in samples. This work is continuing in our laboratory to deter- mine the mechanism of the enhancement of the MgF signal by barium and to use LEMOFS as a highly sensitive18 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 Table 8 Results of analysis by LEMOFS of NIST SRM 267 la Free-Dried Urine and University of Connecticut tap water Sample Fluorine concentration/mg I-' LEMOFS Certified value ISE* LEMOFS? RSD % 0.54 k 0.047.4 NIST SRM 267 la Freeze-Dried Urine control level 0.55 k 0.03 - NIST SRM 267 la Freeze-Dried Urine elevated level 5.7 +- 0.3 - 5.6 +_ 0.6 10.7 University of Connecticuttap water - 0.77 k 0.09 0.70 k 0.05 7.I * Fluoride ISE with an acetate total ionic strength adjustment buffer. The data are It one standard deviation. t Molecular fluorescence of MgF excitation at 268.94 nm detection at 359 nm. Experimental conditions Mg(NO& (20 pg as Mg); Ba(NO&( 1.65 pg as Ba); atomization temperature I 8 0 'C charring temperature 800 'C; laser power 10 pJ per pulse. The data are f one standard deviation. method for the determination of other non-metals e.g.chlo- rine bromine and phosphorus. It is possible that the detec- tion limit for fluorine could be improved by using the AIF molecule rather than MgF which would allow further flexibility in dilution of solutions in order to remove interferences. We would like to thank Klaus Dittrich for helpful discussions prior to the start of this work which was supported by the National Institutes of Health under grant number GM 32002. R. G. M. was supported by a Research Career Development Award from the National Institute of Environmental Health Sciences under grant number ESOO130. D. J. B. and R. L. 1. were each supported by State of Connecticut High Technology Graduate Fellowships. References I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Campbell A.D. Pure AppI. Chem. 1987,59,695. Frant M. S. and Ross J. W. Anal. Chem. 1968,40 I 169. Nicholson K. and Duff E. J. Analyst 1981. 106,985. Coetzee J. F. and Martin M. W. Anal. Chem. 1980,52,2412. Hallsworth A. S. Weatherall J. A. and Deutsch D. Anal. Chem. 1976,48 1660. Leonard M. A. and Murray G. T. Analyst 1974,99,645. Deane S . F. Leonard M. A. McKee V. and Svehla G. Analyst 1978,103 1 134. Small H,. Stevens T. S. and Bauman W. C. Anal. Chem. 1975 47 1801. Fritz J. S. Du Val D. L. and Barron R. E. Anal. Chem. 1984 56 1177. Dittrich K. CRC Crit. Rev. Anal. Chem. 1986 16,223. Bond A. M. and O'Donnell T. A. Anal. Chem. 1968,40,560. Gelhausen J. M. and Camahan J. W. Anal. Chem.. 1989,61,674. Tsunoda K. Fujiwara K. and Fuwa K. Anal. Chem. 1977 49 2035.Tsunoda K. Chiba K. Haraguchi H. Chakrabarti C. L. and Fuwa K. Can. J. Spectrosc.. 1982,27,94. Tsunoda K. Haraguchi H. and Fuwa K. Spectrot-him Acta. Part 6 l985,40 1651. Tsunoda K. Chiba K. Haraguchi H. and Fuwa K. Anal. Chem. 1979,51,2059. Chiba K. Tsunoda K. Haraguchi H. and Fuwa K.. Anal. Chem. 1980,52,1582. Takatsu A. Chiba K. Ozaki M. Fuwa K. and Haraguchi H. Spectrochim. Acta Part B 1984,39,365. 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Venkateswarlu P. Winter L. D. Prokop R. A. and Hagen D. F. Anal. Chem. 1983,55,2232. Itai K. Tsunoda H. and Ikeda M. Anal. Chim. Acta 1985 171 293. Gomez M. G. Corvillo M. A. P. and Rica C. C. Analyst 1988 113 1 109. Dittrich K. Anal. Chim. Acta 1978,97,69. Dittrich K. Anal. Chim. Acta. 1979 111 123. Dittrich K. and Vorberg B. Anal. Chim. Acfa 1982 140,237. Dittrich K. Vorberg B. Funk J. and Beyer. V. Spectrochim A m Part 6 1984,39,349. Dittrich K. Shkinev V. M. and Spirakov B. K. Talanta 1985 32 1019. Dittrich K. Hanisch. B. and Stark H. J. Fresenius Z . Anal. Chem. 1986,324,497. Butcher D. J. Dougherty J. P. Preli F. R. Walton A. P. Wei G.-T. Irwin R. L. and Michel R. G. J. Anal. At. Spectrom. 1988,3 1059. Wei G.-T. Dougherty J. P. Preli F. R. Jr. and Michel R. G. J . Anal. At. Speca-om. 1990 5 249. Long G. L. and Winefordner J. D. Anal. Chem. 1983,55,7 13A. Searcy A. W. in Progress in Inorganic Chemistry ed. Cotton F. A. Interscience New York 1963 vol. 3 pp. 49-127. Datta S. Proc. R. Soc. London Ser. A 1921,99,436. DonnPes Spectroscopiques Relatives aux Molecules Diatomiques ed. Rosen B. Pergamon Oxford 1970. Cotton F. A. and Wilkinson G. Ahpanced Inorganic Chemistry A Comprehensive Text John Wiley New York 1980. Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Cleveland OH 57th edn. 1976. Emeleus H. J. Fluorine Chemistry ed. Simons J. H. Academic Press New York 1950 vol. I pp. 1-76. Slavin W. Carnrick G. R. and Manning D. C. Anal. Chem. 1982 54,62 I . Slavin W. Graphite Furnance AAS A Source Book Perkin-Elmer Nonvalk CT 1984. Frech W. Lundberg E. and Cedegren A. frog. Anal. At. Spectrosc. 1985,8,257. Styris D. L. Anal. Chem. 1984,56 1070. Dougherty J. P. Preli F. R. Jr. and Michel R. G. J. Anal. At. Spectrom. 1987,2,429. Bolshov M. A. Zybin A. V. Kolshnikov V. G. and Vasnetsov M. V. Spectrochim Acta Part B 1981.36,545. Paper 0102084B Received May 11th. 1990 Accepted August 30th. I990

ISSN:0267-9477    

DOI:10.1039/JA9910600009

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 19 Application of Platform and Palladium Modification Techniques With Furnace Atomization Plasma Emission Spectrometry Ralph E. Sturgeon Scott N. Willie Van T. Luong and Shier S. Berman Division of Chemistry National Research Council of Canada Ottawa Ontario K1A OR9 Canada Figures of merit (limit of detection sensitivity and precision) for eight elements (Ag Cd Pb Mn Sn TI As and Se) were obtained using furnace atomization plasma emission spectrometry (FAPES) in combination with atomization of samples from a L’vov platform and the use of a Pd modifier (0.5 pg). A 50 W He plasma was utilized. Precision of replicate integrated signals averaged 2.8% (range 1.743%) for injection by hand of samples at concentrations 13-70-fold above the detection limit.Estimated limits of detection and sensitivities for integrated intensities improved 3-1 0- and 6-1 7-fold respectively for volatile elements (Cd Pb and Ag) compared with sample atomization from the tube wall. Added NaCI up to 24 pg had no effect on the recovery of the integrated signal from Pb when a 100 W plasma was used. Keywords Helium plasma; atomic emission spectrometry; graphite furnace; platform; palladium modifier Furnace atomization plasma emission spectrometry (FAPES) is a novel analytical atomic emission technique based on a combined source wherein the inherent advantages associated with a graphite furnace atomizer (for sample vaporization and atomization) are intimately coupled with those of an atmo- spheric pressure radiofrequency (r.f.) He plasma (for element excitation) in a single convenient unit.’-s Efficient multi- element detection thus becomes feasible using the graphite furnace even for elements having lines of high excitation energy which are normally not accessible through reliance on graphite furnace thermal techniques alone.6 Analytical figures of merit have previously been reported for several elements (Ag Cd Pb Ni Fe Be Bi Cu and P) in a broad effort to characterize the FAPES technique.’ De- tection power in the low pg range is available although preci- sion of replicate signals with sample atomization from the furnace wall has an average relative standard deviation (RSD) of 5%.Additionally analyte emission transients for volatile elements (Cd Pb and Ag) occur early during the initial period of rapid heating and gas expansion from the analytical volume.Smith er al.’ reported on the interference from an easily ionized element (NaCl) on the response for Ag using a FAPES system. A depressive effect was observed when more than 350 ng of Na were present (a 40 000-fold excess over Ag) presumably due to a modification of the excitation characteris- tics of the plasma in the presence of Na. We have also noted interference effects on the determination of Cd and Pb in digested samples of marine sediment and biological tissues,“ although their origin was not investigated. Many matrix effects have been successfully eliminated from graphite furnace atomic absorption spectrometry (GFAAS) when samples are atomized from a L’vov platform7 and chemical modifiers such as reduced Pd are present.x Similar benefits should accrue with FAPES; thus it was the purpose of this study to assess the feasibility of transferring the above GFAAS technology to the FAPES system.Report- ed here are the improved limits of detection (LODs) sensitiv- ity and precision of replicate signal measurement for a number of elements when atomized from a L’vov platform in the presence of reduced Pd as compared with similar figures of merit arising from sample atomization from the tube wall. It is also shown that interference from NaCl on the response from Pb is reduced when these techniques are implemented although they are not as efficiently eliminated as they are in GFAAS. NRCC No. 325 17. Experimental Instrumentation The FAPES system has been fully described previo~sly.*~~~~ Briefly a 13.56 MHz r.f.He plasma was supported inside a Perkin-Elmer Model HGA-2200 graphite furnace. An Ar ex- ternal sheath gas was maintained at a flow-rate of 1 1 min-’ and an He internal plasma gas was set at 200 ml min-I. All signals were digitized with 12 bit resolution and acquired at the rate of 8OOO points per transient (irrespective of the sampling period). In this study grooved pyrolytic graphite coated tubes and solid pyrolytic graphite L’vov platforms (Perkin-Elmer) were used. Reagents Stock solutions (loo0 mg 1-I) were prepared by dissolution of the high-purity metals (Cd Pb Ag Mn Sn and T1) or their salts (As,O and Na,SeO,). Working standards were obtained by dilution of the stock solution with high-purity de-ionized distilled water (DDW) acidified to 1% v/v with sub-boiling quartz distilled HNO (for Cd Pb Ag TI and Mn) or HCl (for Se As and Sn).A 3% m/v solution of high-purity NaCl (Ventron) was prepared in DDW and further purified of trace metals by passage through a column of 8-hydroxyquinoline immobilized on silicasv A 2500 mg I-’ stock solution of Pd chemical modifier was prepared by dissolution of the high- purity metal (Spex Industries) in HNO,. A 250 mg I-’ working solution of Pd was prepared by dilution of the stock with DDW. Procedure All wavelengths (A) were set using the appropriate hollow cathode (Ag Cd Pb Ti Mn and Sn) or electrodeless- discharge lamps (As and Se). The lines selected are summar- ized in Table 1. A nominal spectral bandwidth of 0.08 nm was used for all measurements.In order to compare the results with those from previous studies a 50 W (forward power) He plasma was used. The effect of forward plasma power on the recovery of Pb response in the presence of NaCl was investi- gated for forward powers of up to 100 W. Analytical figures of merit were obtained for each element. Sample volumes of 5 yl were introduced into the furnace by hand using a syringe (Hamilton) equipped with a short section of Teflon tubing. The palladium modifier solution (250 mg 1-I) was co-injected with the sample in the amount of 0.5 pg (2 pl). Atomization conditions were not extensively studied and cannot be considered as truly optimum for any given element.20 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL.6 Table 1 using atomization from the tube wall and no modifier (see reference 5 ) Absolute detection limit. Values in parentheses are those obtained LOD*/pg Excitation Element h/nm energy/eV Peak height Peak area Ag 328.1 As 200.3 Cd 228.8 Mn 279.5 Pb 283.3 Se 196.0 Sn 284.0 TI 377.6 3.78 7.54 5.4 1 4.43 4.37 6.32 4.79 3.28 0.76 ( 1.2) 0.92 (2.0) 0.90 7.2 (21) I03 1700 10 17 0.50 (4.8) 2.0 (6.8) 5.0 7.4 (46) 370 3600 29 37 * LOD based on 30 criterion where o is the estimate of the standard deviation of repetitive measurements of the blank. Samples were charred at 500 "C during which time forward power was applied and the He plasma spontaneously estab- lished. The furnace was allowed to cool to room temperature") and the sample subsequently atomized using the maximum power heating mode of the HGA-2200 controller.Final atomi- zation temperatures (in "C as measured on the furnace power supply meter) selected were 2100 (Cd Pb Ag and Tl) 2500 (Mn and Sn) and 2700 (As and Se). Blank signals recorded for each analyte were obtained by atomizing 5 p1 volumes of DDW containing 1% v/v HNO or HCI along with 2 pl of 250 mg 1-I Pd. Background correction with this system was limited to se- quential measurement of a blank solution and/or an unloaded furnace and plasma cycle. The high time resolution of the de- tection system permitted unambiguous background correction to be implemented although it must be noted that with the simple samples run here no structured background was en- countered. The effect of various masses of NaCl on the response from 5 ng of Pb was investigated.The sample was atomized under various conditions including directly from the tube wall from the wall following the addition of 5 pg of Pd from the plat- form and from the platform following the addition of 5 pg of Pd . Measurements of the time-dependent temperature of the surface of the graphite tube were made using a calibrated Ircon Series 1100 automatic optical pyrometer (Niles) which was focused through the sample introduction hole onto the back wall of the tube. Measurements of the plasma reflected power levels were obtained from the appropriate 1/0 port on the rear of the RFX-600 r.f. power supply (Advanced Energy Indus- tries). This 0-5 V signal was directly compatible with the data acquisition system.Results and Discussion No analytical difficulties were experienced when using the platform with the FAPES system. In general the plasma was easier to establish in this configuration as the centre electrode (r.f. antenna) is in closer proximity to the ground established through the platform (thus a higher field strength resulted). Additionally plasma stability was enhanced at lower input powers. Continuum background intensity however increased 5-1 5-fold (depending on the wavelength). This increase was noted at room temperature and was not due to the image of the incandescent platform overlapping the entrance slits of the spectrometer but rather to the increased plasma power density in the tube as a result of the volume restriction caused by the platform.At high temperatures generally at times beyond the duration of the analyte signal the background intensity was 0 1 2 3 4 5 fh Fig. 1 Time-resolved analyte emission signals for atomization from a platform. Continuous line 0.5 kg Pd modifier; broken line no modifier. (a) As 10 ng; 200.3 nm. (h) Cd 50 pg; 228.8 nm. (c) Pb 500 pg; 283.3 nm. ( d ) Sn 1.25 ng; 284.0 nm higher from the tubes equipped with a platform due to detec- tion of the incandescent image of the platform. Typical emission signals for several of the analytes are shown in Fig. 1. The effect of the Pd modifier is particularly evident for the most volatile elements. In the absence of Pd signals from As and Se were poorly resolved from the back- ground. Signals for Pb Cd and Ag were clearly delayed in time (temperature) relative to those obtained without Pd whereas this time shifting of the signal was less evident for elements such as Mn and Sn. These observations are consis- tent with those reported in the literature dealing with GFAAS.Peaks displayed for As and Cd in Fig. 1 (and that for Se. not shown) exhibit poor signal to noise characteristics in the absence of Pd because at the 500 "C char temperature used here significant amounts of these elements are lost prior to atomization.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 21 ~~~ Table 2 Absolute sensitivity for platform atomization and Pd modifier. Values in parentheses are those obtained using atomization from the tube wall and no modifier (see reference 5 ) Peak height/ Peak area/ Element nA ng-' nA s ng-' Ag As Cd Mn Pb Se Sn TI 3260 (370) 2.3 320 (51) 850 64 (4.7) 0.2 1 110 120 720 ( 120) 0.67 110 (7.4) 250 19 (1.1) 0.13 25 60 Table 3 Precision of replicate measurements.Precision expressed as % RSD for n > 8 at analyte concentrations (+fold in excess of estimated LODs RSD (%) Element Ag As Cd Mn Pb Se Sn T1 Peak height 1.1 (30) 5.2 (100) 2.2 (50) 4.7 (130) 2.1 (70) 5.4 (25) 3.9 (125) 2.8 (30) Peak area 2.0 (50) 4.3 (25) 2.6 (25) 4.2 (25) 2.0 (70) 2.4 (16) 1.7 (40) 3.1 (13) K 75 1 '+\ I 1 I 1 0 20 40 60 80 100 Mass of NaCVpg Fig. 2 Effect of adding NaCl on the recovery of an integrated signal in- tensity from 5 ng of Pb in A the presence and B the absence of 5 pg of Pd modifier. (a) Atomization from the tube wall; (h) atomization from the platform Figures of Merit Table 1 presents the estimated LODs obtained with the present system.For comparative purposes those for Cd Pb and Ag re- ported in a previous studys using only sample atomization from the tube wall are also given. It is clear that there is an en- hancement (3-10-fold) in performance with the platfonn- modifier system for these elements. Peak height LODs are generally superior to those characterizing the signal area because of the rapid evolution of analyte atoms at high temper- ature coupled with their rapid diffusive loss from the analyti- cal volume. Detection limit data for the FAPES system compare favour- ably with GFAAS data (within a factor of 2) with the excep- tion of As and Se." The wavelengths of these two elements are in a region of poor response for the Hamamatsu R446 photomultiplier tube (PMT) used in this study and the emis- sion lines have higher excitation energies than those used for the other elements.At 200nm the radiant sensitivity of the PMT is less than 25% of its maximum. Accounting for this factor alone should make the LOD for As obtained with FAPES comparable to that with GFAAS." The LOD for Se would be similarly enhanced but performance of FAPES would still be more than ten times worse than GFAAS. The lower excitation energy for the Se transition certainly cannot account for its poorer performance relative to As. These two factors however severely limit analytical performance. Line selection in the present study was made on the basis of sensi- tivity rather than any rigorous comparison of signal to noise or detection limit criteria.Non-resonance lines of As Sn and T1 were utilized because sensitivity was 25-fold greater for As at the 200.3 than at the 193.7 nm line 15-fold greater for Sn at the 284.0 than at the 224.6 nm line and 4-fold greater for T1 at the 377.5 than at the 276.8 nm line. Table 2 summarizes the sensitivities of the elements in the terms of absolute response per ng for both peak height (nA) and area (nA s); Table 3 gives the precision of replicate meas- urements. Precision is largely determined by the repeatability of injection of samples into the furnace by hand. As expected over-all precision is slightly better for area measurements. The presence of the platform has on average served to increase the precision of replicate measurements by a factor of 2 over that obtained for a similar suite of elements when atomization occurred from the tube wall.' This effect is most noticeable for the volatile elements with atomization from the wall since these are released during the early period of rapid heating of the furnace and expulsion of the internal gas.Additionally any instability in the plasma which might occur during this period of rapid heating will have less influence on signals for those analytes released at higher temperatures. Effect of NaCl on Response From Lead The effect of increasing the mass of NaCl added on the re- sponse from Pb when atomized from both the platform and the wall in the presence and absence of Pd modifier was investi- gated.It was noted that greater signal recoveries were obtained as the forward power to the plasma was increased. In the pres- ence of 18 Fg of NaCl the peak height signal recovery for Pb was 70%. This increased to 80% at a forward power of 75 W and remained unchanged at 100 W power (it should be noted that quoted forward powers are simply the output levels of the RFX 600 r.f. supply; the amount of power dissipated in the tuning network is unknown even when reflected power levels are near zero). Integrated signal recovery was 70 70 and loo% respectively at these same power levels. As a conse- quence the interference study was conducted with a forward power of 100 W. It is possible that it is the initial temperature of the centre r.f. electrode (before atomization) that deter- mines to some extent the degree of interference encountered.As the forward power is increased there is a concomitant in- crease in the electrode temperature. This might influence the degree of interaction between the sample vapour and the elec- trode surface as discussed in more detail below. Fig. 2 shows the recovery of the integrated signal intensity from 5 ng of Pb in the presence of various masses of added NaCl (corrected for an NaCl blank). The effect of 5 pg of Pd modifier is evident. A depressive interference effect is almost22 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 0 125 100 p 75 2 .- $ 5 0 C - 2E 1 2 3 4 5 ds 2400 2000 1600 1200 g 800 400 0 0 1 2 3 4 5 us Fig. 3 Characteristics of the Pb-NaCI interference system in a 100 W He plasma.(a) A Temperature-time characteristics of the tube wall; 8 reflected power-time characteristics. (h) Lead emission transient. Broken line 5 ng of Pb with 5 pg of Pd; continuous line 5 ng of Pb with 5 pg of Pd and 60 pg of NaCl immediately established in the absence of modifier irrespec- tive of whether or not atomization occurs from the tube wall or the platform. It is not clear from these data whether atomiza- tion from the tube wall or the platform is to be preferred in the presence of small amounts of salt; as the mass of NaCl in- creases beyond 30pg the benefits of atomization from the platform become more obvious. It is equally obvious however that the interference-free range was not increased by the use of a platform although the interference observed at higher masses of NaCl is clearly less with the platform compared with the in- formation obtained from atomization from the wall.Significant depressive interference effects on the signals from several analytes by NaCl have also been reported by Falk et a1.12 using the hollow cathode FANES system. The extent of interference was apparently due to the degree of temporal overlap of the Na C1 and analyte vapour populations in the an- alytical volume. In the present situation the volatilities of NaCl and Pb are very similar and it is expected that there is significant element population overlap with the result that this system can be considered one of the most severe situations likely to arise. Fig. 3 shows the emission signals arising from the atomiza- tion of 5 ng of Pb in 1 % HNO and in the presence of 60 pg of NaCI. In both instances 5 pg of Pd were present as a modifier.Integrated signal recovery is only about 50% in the presence of this amount of NaCl. It is interesting to note the early de- crease in background signal intensity accompanying the rapid release of sample from the platform. This apparent ‘quench- ing’ of the plasma might be due to the momentary formation of a more conductive vapour plume within the analytical volume leading to the formation of a ‘mini-arc’ and the tran- sient shrinkage of the plasma volume actually imaged onto the entrance slit. Alternatively the centre r.f. electrode might be acting as a cooler second surface onto which a portion of the volatilized matrix may condense thereby altering the plasma excitation processes.A short time later this surface layer is re- volatilized as the electrode heats to the temperature of the hotter tube wall by radiation. Such plasma ‘quenching’ was observed only in the presence of large amounts of NaC1. It is clear from Fig. 1 that such effects are absent when only Pd and analyte are atomized. Reflected power losses which occur during the atomization of this sample are also shown in Fig. 3 along with the tempera- ture of the tube wall. The change in reflected power level with temperature is not influenced by the presence of NaCl in the sample but is apparently determined only by the temperature. It is also evident from this figure however that it is not the temperature of the tube wall that accounts for the time depen- dence of the reflected power level.This is clear from the pres- ence of an ‘induction’ period which occurs between establishment of maximum reflected power and steady state tube wall temperature. Thus it appears that it is the tempera- ture of the centre r.f. electrode that influences the magnitude of the reflected power level the latter being heated by radiation from the tube wall. The reason for a rise in reflected power and hence a decrease in the efficiency of the coupling of the r.f. energy into the system is not yet clear. At high tempera- tures however themionic emission of electrons from the graphite surfaces occurst3 which may alter the impedance matching With the r.f. tuner presently used (Heathkit Model 2060A) reflected power levels could not be decreased below 30 W (for 100 W forward power) when the system was at a temperature of 2100 K.Some finite value of resistance is es- tablished in the gas phase which currently cannot be matched to the 50 i2 source output with this tuner. A similar effect has been noted with both the hollow cathode and anode versions of the FANES technique whereby the voltage drop establish- ing the low pressure glow discharge in these devices is significantly decreased owing to the release of thermionic elec- trons from the tube wall as it is heated above 1800-2000 K.I2.l4 At atmospheric pressure the mean free path for electrons in He lies in the pm region.” Thus thermionic electrons released from the centre electrode surface might have a more pro- nounced effect on plasma excitation than those released earlier from the hotter but more distant tube wall. Electron mobility and mean free path are probably fieid dependent and with the present centre electrode configuration will vary with radial po- sition.As a non-uniform radial field distribution is involved and is most intense at the centre electrode electrons generated in this region will contribute the most to the observed analyte emission intensity (assuming excitation by electron impact). Thermionic electrons originating from the surface of the centre electrode are likely to have a greater effect on plasma process- es than those generated some distance away at the tube wall where the field is less intense. It is generally accepted that there are two populations of electrons in plasmas each with a separate temperature.1 2 ~ 1 6 A high temperature group promotes excitation and ionization whereas a lower temperature higher density fraction is involved in collisional de-excitation pro- cesses. Thermionic emission from the tube wall may serve to flood the plasma with a large number of low-energy electrons thereby reducing the analyte emission because of the increased collisional de-excitation rate. The intensity of the He I plasma gas line at 587.6 nm in- creased approximately 5-fold with temperature over the initial ramp heating stage (to 2100 K) of the atomization cycle. This presumably arises because the decrease in plasma particle density accompanying the increased temperature produced a longer mean free path of electron excitation.The intensity of this line subsequently decreased to a level of only about 3- fold greater than the room temperature intensity. In an attempt to re-tune the plasma continuously by hand as the temperature was ramped it was almost possible to eliminateJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 23 this subsequent decrease. Obviously a complete understand- ing of these observations will require fundamental informa- tion on the mechanistics of level populations because in addition to the above there is a substantial increase in the number density of thermionic electrons with increased tem- peratureI3 and these may exert considerable influence on the over-all excitation processes. Conclusions It would appear that all of the advances made in the use of the stabilized temperature platform furnace concept’ currently so important to the field of GFAAS can be conveniently utilized with the FAPES technique.As expected detection limit sensi- tivity and precision significantly improved for all of the vola- tile elements when atomized from a platform in the presence of Pd. It also appears that this approach will be useful in helping to control some matrix interferences which are present when using this technique. The authors thank B. Hutsch Ringsdorff Werke Germany for supplying the centre electrodes. References I Liang D. C. and Blades M. W.. Spectrochim. Acfa. Part B 1989 44,1059. 2 3 4 S 6 7 8 9 10 11 12 13 14 IS 16 Sturgeon R. E. Willie S. N. Luong V. T. and Berman S. S. and Dunn J. G. J. Anal. At. Specworn. 1989,4,669. Smith D. L. Liang D. C. Steel D. and Blades M. W. Specfro- chim. Acta Part B 1990,45,493. Sturgeon R. E. Willie S. N. Luong V. T. and Berman S. S. J. Anal. At. Specrrom. 1990,5,635. Sturgeon R. E. Willie S. N. Luong V. T. and Berman S. S. Anal. Chem. 1990,62,2370. Littlejohn D. Anal. Proc. 1989 26,92. Slavin W. Camick G. R. Manning D. C. and Pruszkowska E. At. Speca-osc. 1983,469. Beach L. M.. Spectroscopy 1987,2,2 1. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981,53,2337. Manning D. C. and Slavin. W. Specrrochim. Ada Part B . 198540 461. Slavin W. in Graphite Furnace AAS a Source Book Perkin-Elmer Norwalk 1984. Falk H. Hoffman E. and Ludke Ch. Prog. Anal. Spectrosc. 1988 11,417. Sturgeon R. E. Berman S. S. and Kashyap S. Anal. Chem. 1980 52 1049. Harnly J. M. Styris D. L. and Ballou N. E. J . Anal. At. Spectrorn. 1990,5 139. Westwood W. D. Prog. S w - Sci. 1976 7 71. de Galan L. Spectrochim. Acfa Part B 1984,39,537. Paper 0i02625E Received June 6th 1990 Accepted October 15th 1990

ISSN:0267-9477    

DOI:10.1039/JA9910600019

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 25 Comparison of Atomization Mechanisms for Group IIA Elements in - ‘7 r; r-r; fir v IC ( i i f i ’ - ( _ ^ I - _ - -- rr-.r 7 - - i - - Laurie J. Prell* and David L. Styrist Pacific Northwest Laboratory,$ Richland WA 99352 USA David A. Redfield Northwest Nazarene College Nampa ID 83651 USA Atomic absorption and mass spectrometry were used simultaneously to elucidate mechanisms responsible for atomization of Group IIA elements (beryllium magnesium calcium strontium and barium) in pyrolytic graphite furnaces. Gaseous species of these elements deposited as the nitrates and vaporized in 1 atm of nitrogen and in vacuum were analysed in real-time by mass spectrometric sampling. The principal gas phase analyte species observed were carbides oxides and hydroxides.Excluding beryllium the data suggest that Group I IA atomization and carbide formation begin with the dissociative adsorption of the oxides and a perturbation of the surface state associated with the resulting adsorbed elements. Gas phase oxides are formed as a result of associative adsorption and the hydroxides are formed by homogeneous gas phase reactions of the carbides and oxides with water vapour. Keywords Atomization mechanisms; electrothermal atomic absorption spectrometry; mass spectrometry; Group IIA elements Careful investigation of the mechanisms that control the atom- ization of specific elements are providing a valuable database for achieving a more expanded view of fundamental electro- thermal atomization processes.However this approach does not indicate the scope of influence of the controlling mecha- nisms; the question of the validity of extending a given mecha- nism to include other elements is left unanswered. It is more valuable therefore to incorporate a group of elements in a modelling process in order to address this question. The Group IIA elements with the exception of radium are ideal for the purpose of investigating such generalizations within a group. Electrothermal atomic absorption spectromet- ric studies of this group involve primarily intra-group inter- ferences and Atomization mechanisms have been investigated for each of these elements but for the most part they remain to be determined. A systematic study of this group will at the very least provide experimental consistency in the vaporization data.Maessen and Posma4 suggested that beryllium vaporized from the solid metal after it was formed from the reduction of the oxide; however Maessen et al.s questioned these argu- ments. Frech et al.s performed high-temperature equilibrium calculations which indicated that beryllium should remain non-volatile up to 1400 K and analyte losses from beryllium hydroxide formation could occur as low as 1200 K if a rela- tively large volume of water was used. The X-ray diffraction analysis by Runnels et a1.6 found beryllium oxide on the inner furnace surface after heating to high temperatures. They attri- buted this to the beryllium carbide reacting with water while the sample was being transferred into the X-ray diffractometer.The last two research groups proposed that carbide formation competes with atomization. Styris and Redfield’ evaluated atomization mechanisms for beryllium by mass analysing gas phase molecular and atomic species evolved in the furnace. Their results imply atomization by thermal decomposition of adsorbed oxides. Several workers support the idea that magnesium vapour results from volatilization of the oxide followed by dissocia- tion in the vapour Khntor et al.H noted that the appearance temperature of magnesium was independent of the * Present address Lockheed Engineering and Science Company 1050 t To whom correspondence should be addressed. 3 Operated for the US Department of Energy by Battelle Memorial E. Flamingo Road Las Vegas NV 891 19 USA.Institute under contract DE-AC06-76RLO 1830. deposited species chloride or nitrate. They suggested that the free atom signal was due to oxide volatilization and dissocia- tion. Hutton et al.9 monitored oxide and hydroxide molecular emission spectra from the deposited nitrate in the furnace. The appearance of the oxide with the free atom suggested the above mechanism. However the large amounts of magnesium (500 pg) may have perturbed the system by producing changes in the dispersion of deposited analyte and thus contributed to changes in the vaporization scheme. Ohta and Su” atomized magnesium in tungsten tubes obtaining a detection limit 40 times less than that for graphite furnaces. They suggested that carbide formation may be interfering with the formation of the magnesium atom.Little effort has been made to understand calcium atomiza- tion. Hutton et a/.’ used 100 pg of calcium to provide molecu- lar emission spectra and noted the oxide appearing with the free atom and the hydroxide at a high temperature suggesting that free calcium results from volatilization and dissociation of the oxide. These results were disputed by L’vov’* who sug- gested the formation of calcium cyanide or calcium carbide instead of the oxide. Suzuki and Ohta13 used a molybdenum micro-tube to test their own hypothesis that carbide formation slowed atom formation. They observed peak-shape improve- ment when hydrogen gas was introduced. Barium probably the most extensively studied Group IIA element has provoked considerable disagreement concerning atomization mechanisms.StyrisI4 used vacuum vaporization and mass spectrometric analysis to help identify the atomiza- tion mechanisms of barium chloride in both pyrolytic graphite and tantalum furnaces. It was concluded that adsorbed barium oxide is reduced by the surface to give adsorbed barium which desorbs at higher temperatures. Jasim and Barbootiis proposed that barium is volatilized from the condensed phase after the oxide is reduced by the carbon surface. However Nagdaev and BukreevI6 suggested from appearance tempera- ture data that barium oxide dissociates after sublimation. Stur- geon et a[.” agreed with Nagdaev and Bukreev suggesting that the chlorides and oxides are thermally dissociated after va- porization. L’vov et al.ln used semi-empirical estimates of the heat of formation for the carbide” to surmise that the poor sen- sitivity of barium was due to the formation of low volatility carbides that are intercalated in the graphite.Intercalation de- creases the sensitivity for barium because the carbides are un- available for atomization.”’.” StyrisI4 observed gas phase barium carbide in mass spectrometric data. Equilibrium calcu- lations by Frech et al.? show that condensed barium carbide is26 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 only stable over a small temperature range between 1500 and ZOO0 K and that its formation is highly dependent on oxygen partial pressures. The Group IIA element whose atomization mechanism was studied most recently is strontium.22 The coincidence of the strontium atom signal with the molecular emission spectra of the oxide and hydroxide has been attributed to direct volatiliza- tion of strontium oxide (50pg of strontium).Y Nagdaev and Bukreevl‘j used the appearance temperature to conclude that free strontium formed from the sublimation of strontium oxide.L’vov et a1.23 atomized strontium from both tantalum and graphite platforms noting that the increase in the atomic ab- sorption (AA) signal was enhanced by the tantalum platforms. They concluded that the shape of the absorption signal was governed by atom supply and that pulse tailing was due to the formation of carbon shells on analyte micro-particles. Ohta and Su” used tungsten furnaces to test their hypothesis that carbide formation interferes with atom formation; they obtained detec- tion limits 250 times lower than those with graphite furnaces.When atomizing from a molybdenum furnace Suzuki and Ohta13 obtained detection limits of 5 pg and noted that 100-fold amounts of magnesium calcium and sodium did not interfere. Mass analysis of the gas phase molecular and atomic species in the furnace implies that the vaporization of strontium species involves three different types of adsorption sites.22 The ‘type 1 ’ sites involve dissociative adsorption and carbide formation. Associative adsorption occurs at ‘type 2’ sites and ‘type 3’ sites involve dissociative adsorption and free atom release. The above discussions indicate confusion concerning atomi- zation mechanisms for Group IIA elements. Experimental clarification must include information on real-time formations of intermediates that are produced in the furnace.Such infor- mation on atmospheric pressure vaporization is currently only available for beryllium’ and strontium.22 The experiments de- scribed here elucidate the remaining Group IIA element vapor- ization mechanisms by using real-time mass spectrometric analysis of the gaseous species associated with the atomization of these elements from pyrolytic graphite coated graphite fur- naces. The validity of extending to other Group IIA elements the mechanisms involving three types of adsorption sites pre- viously proposed for is therefore evaluated. Experimental Apparatus A mass spectrometry provided the capability for real- time molecular beam sampling and identification of gaseous species generated within pyrolytic graphite coated graphite furnaces (Thermo Jarrell-Ash Franklin MA USA) and heated in 1 atm of nitrogen (99.997%).In a few instances as noted in the text a second mass spectrometry apparatusi4 was employed to provide vacuum-vaporization data for the purpose of deter- mining carbide formation mechanisms. Procedures The dissolved nitrate of a particular Group IIA element was used for each sample Beryllium samples were prepared by dissolving beryllium nitrate (K & K Laboratories Painsville NY USA) in de-ionized water and then diluting to the appro- priate concentrations. Calcium and strontium samples were prepared from lo00 pg 1-I atomic absorption standard nitrate solutions (Spex Edison NJ USA). Magnesium and barium samples were prepared similarly with Specpure (AESAR Sea- brook NH USA) atomic absorption standard nitrate solutions.Solution volumes of either 2 or 42 pl were deposited into the furnace. The larger volume was used in order to decrease surface concentrations of the analytes while keeping the mass of the given analyte constant. Analyte masses are given in Table 1 column 3. The temperature programme for the atomization step for be- ryllium was set to ramp to 2700 K in 1 s and hold for 8 s. A 1 s ramp to 3000 K with a 5 s hold was used for the four other ele- ments. An Ircon 1100 series optical pyrometer (Ircon Niles IL USA) was used to monitor furnace temperatures by observ- ing the central outer portion of the surface of the furnace. The pyrometer output was calibrated against the inner-surface tube temperature to 2600 K with a 0.08 mm diameter W-5% Re W-26% Re thermocouple (Omega Engineering Stamford CT USA).The Extrel quadrupole mass analyser (Extrel Pittsburgh PA USA) was calibrated with water nitrogen oxygen argon carbon dioxide and trichlorotrifluoroethane. Electron energies for ionization were beryllium 30; magnesium 40; calcium 40; strontium 40; and barium 35 eV for the atmospheric pressure vaporization experiments; electron energies of 16 eV were used in the vacuum vaporization experiments. Three beams of a dual beam dual trace and single-sweep oscilloscope provid- ed temporal responses of the pyrometer atomic absorption spectrometer and mass analyser; the mass analyser was set to monitor a single mass during this sweep. The Tektronix 5 110 (Tektronix Beaverton OR USA) oscilloscope was triggered from the ‘read’ output of the furnace power supply and record- ed with a Tektronix C5C oscilloscope camera.Results and Discussion Temperature-dependent profiles of mass spectra (MS) obtained during the atomization step at atmospheric pressure are shown in Fig. 1 (oxides) Fig. 2 (carbides) and Fig. 3 Be 0 loo0 2000 3Ooo TIK Fig. 1 Composite oxide MS temperature profiles for Group IIA elements vaporized at atmospheric pressure. Intensities are plotted relative to the free atom MS signal. Ordinate scales differ from element to element.The mk values are shown in Table 1. Sample volume is 2 p1 0 lo00 2000 3Ooo TiK Fig. 2 Composite carbide MS temperature profiles for Group HA ele- ments vaporized at atmospheric pressure.Intensities are plotted relative to the free atom MS signal. Ordinate scales differ from element to element. The mk values are shown in Table I . Sample volume is 2 plJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 199 I VOL. 6 27 Table 1 Experimental summary and implied reactions Species Be0 (Beoh (BeO) MgO CaO SrO BaO SrO BaO Bell MA Can** sdl Ball Be,C BeC Be& Be$ Mg2C3 LTtt MgC2 LT CaC LT CaC2 LT SrC LT BaC,. LT$$ CaC HT CaC HT BaC HT BaC HT SrC2 HT SK2 LT MgCZ HT§§ Be(02H),SS Mg(OH)2 Ca(OH) Sr(OH) Ba(OH) BeNCN BeCN MgCN CaCN SrCN BaCN mlz 25 50 100 42 56 104 154 I 92 170 9 24 88 138 30 33 42 66 84 50 52 64 100 112 1 62 50 52 64 150 162 112 45 60 74 122 172 49 35 50 66 114 164 - Appearance Large-volume Analyte mass/ng temperature*/K load effect Implied reaction (reaction or reference No.) 20 20 20 50 50 50 90 200 200 10 25 50 50 90 20 20 20 20 25 100 200 50 50 50 90 100 100 50 90 90 50 20 50 50 50 90 20 20 25 50 50 90 2465 2590 2475 430 1650 1040 450 1700 2000 1990 1580 1940 1640 2080 2480 2355 2410 2465 440 410 600 590 420 430 500 1560 2020 2020 2215 2200 1995 1220 400 610 400 400 1750 2575 1725 2080 1730 2280 - NC t - - 0.1 x decrease c<O.1 x decrease Decrease$ - - NC 0 . 3 ~ decrease II 0 . 6 ~ decrease II 0 . 7 ~ decrease II 0 . 8 ~ decrease II - - NC - 5x increase ceO.1 x decrease 15x increase 40x increase Increase 0 . 2 0 ~ decrease 0 . 2 5 ~ decrease 0 . 2 5 ~ decrease 0 . 0 7 ~ decrease Increase 25x increase 20x increase 30x increase 12x increase - - - - - - - - - - (BeO) (ad)+(BeO),(g) + .. . + BeO(g); (reference 7) MO(ad type 2)+MO(g); (reaction 1) SrO(g) + Sr(g) +Sr,O(g); (reference 22) Ba,O,(g) + e- +BaO '+ (g) + O(g) + 2e- BeO(ad)+Be(g) + O(g); (reference 7) MO(ad type 3) + C :M(ad. type 3) + CO(g); T < T2 (reaction 3) M(ad type3) %M(g); T c T (reaction 2) MO(ad type 1) + C(s)+MCz(g) + CO(g); (reaction 4) * Appearnace temperature for a 2 pl volume. t NC = no measurable change. $ Thermal MS profile changes from multiple to a single peak which appears at 1720 K . 0 Ionizer-induced fragment of possibly Ba,02. 1 Direct comparison of the atomic absorption and mass spectrometric signals is not possible. Instrument constants that relate the relative signals to the total amount of particu- I1 Large-volume load effect for atomic metal species based on AA data only.**The 40 u stable isotope is not measurable because of the presence of "Ar. tt LT = low temperature. $3 Appeared only in 42 pI instance. $8 HT = high temperature. lar species in the furnace are not known and all species for a given element have not necessarily been analysed in the 42 WI instance. (hydroxides). The beryllium and strontium spectra which have been reported in references 7 and 22 respectively are includ- ed for completeness. The magnitude of the arbitrary ordinate unit remains constant only for species involving a given element. Molecular spectra are normalized to the full-scale free atom signal. The m/z values used in these experiments and their respective appearance temperatures are shown in Table 1 columns 2 and 4 respectively.For clarity no attempt has been made to show the full complement of species observed for each element; these species are listed in Table 1 column 1. In summary all Group IIA elements exhibit high- temperature (appearance temperature greater than or equal to that of the free atom) gas phase carbides. Magnesium calcium strontium and barium (42 pI volume only) exhibit low-temperature (appearance prior to atomization) gas phase carbides. Low-temperature hydroxides appear in all instances. Oxides appear at temperatures lower than those of the corre- sponding free atoms for all elements except beryllium.28 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 UlgCOH) A t r - Ca(OH) 0 lo00 2000 3000 TK Fig. 3 Composite hydroxide MS temperature profiles for Group IIA ele- ments vaporized at atmospheric pressure.Intensities are plotted relative to the free atom MS signal. Ordinate scales differ from element to element. The mlz values are shown in Table 1 . Sample volume is 2 pl. Since there was no observed beryllium hydroxide in the 2 1.11 instance no spectrum is shown here for Be The following discussions treat separately the oxide carbide and hydroxide species shown in Figs. 1-3; each group is dis- cussed under its respective sub-heading. Mechanisms are dis- cussed in detail and when sufficient data exist thermodynamic evaluations are provided. Thermodynamic data on adsorbed phases of these species are not available but arguments based on knowledge of general adsorption process- es are presented.Among the mechanisms that are implied by the data there exists a set that is consistent with all the data for any one element. If inter-element consistencies between these sets of mechanisms can then be found it may be possible to establish a general mechanism for the entire group. Oxides and Atomization Temperature profiles of the oxides observed for each element are shown in Fig. 1. The Sr,O+ discussed in reference 2 1 and BaO,’ discussed briefly below were also observed but are not shown in Fig. 1 because they were minor species and unique to strontium and barium. Beryllium is unique in Group IIA because it is the only element of the group having an oxide appearing at a tempera- ture greater than that of the free atom. A detailed discussion of the beryllium vaporization mechanisms is given by Styris and Redfield.7 These workers concluded that Be(g) is the result of the thermal dissociation of the adsorbed monoxide and that gaseous polymeric oxides are produced during desorption of the higher polymers.Appearance temperatures of the oxides of magnesium calcium strontium and barium (Fig. 1) precede those of their respective free atoms. Either bulk vaporization desorption or oxide release from direct dissociation of the nitrate are poten- tial mechanisms. The last mechanism does not evidently apply to the beryllium calcium strontium or barium because the appearance temperature of beryllium oxide is greater than that of the free metal; the appearance temperatures of hydroxides and carbides of calcium and strontium are greater than those of the respective oxides; and the barium oxide appearance tem- perature depends on the sample volume.Vaporization does not appear to be the release mechanism for the oxides because melting points*‘ range from 1300 to 2700 K higher than their appearance temperatures. Sublimation is not the mechanism either; all known sublimation pointsZ6 are at least 800 K higher than the appearance temperatures. The results can be ex- plained however by the desorption reaction where M implies Mg Ca Sr and Ba and ‘type 2’ refers to one of the three types of active sites available for oxide adsorption. For a physical description of each type of site in terms of the MO(ad type 2) -+MO(g) ( 1 ) related potential energies the reader is referred to reference 22.However a brief description of the sites is presented below. Two of these sites (‘types 1 and 3’) are postulated to explain how the metal oxide forms free metal atoms at one tempera- ture and metal carbides at a lower temperature (Fig. 2). It is thought that some of the available oxide dissociatively adsorbs on these two sites and the resulting oxygen atom reacts exo- thermically with carbon. Excitation of the lattice from the heat of this reaction will result in the release of the carbide at low temperatures should the lattice vibration modes be appro- priately coupled to the adsorbed species. This evidently occurs at ‘type 1 ’ sites (low-temperature carbides are observed) but not at the ‘type 3’ sites where the metal atom remains until desorbed at higher temperatures.Metal oxide desorption at more elevated temperatures requires the oxide to be associa- tively adsorbed as a molecule on the ‘type 2’ sites where it remains until the thermal energy required for molecular de- sorption is provided. It will be shown later that the above picture of ‘type 3’ sites is too simplistic to account for the ob- served high-temperature carbides and that a quantum descrip- tion of this site is necessary. In summary the involvement of ‘type 1 ’ sites is being invoked for dissociative adsorption and low-temperature carbide formation ‘type 2’ sites for associa- tive adsorption and oxide release and ‘type 3’ sites for dissoci- ative adsorption and free atom release. The broad thermal profile of BaO+ shown in Fig. 1 for the 2 pl samples implies a wide range of desorption energies for barium In this instance dissociative adsorption of the oxide at ‘type 1’ sites may be inhibited by the relatively high energies required for the dissociation of this oxide.Larger amounts of the oxide must then be adsorbed associatively. The ‘type 2’ sites therefore become saturated and the excess of oxide adsorbs on a range of available sites characterized by lower desorption energies. The use of larger volume (42 pl) samples resulted in the appearance of a single-peak barium oxide spectrum at 1720 K and a carbide profile at 500 K (Table 1). This should be com- pared with Figs. 1 and 2 for Iow-volume samples. The change can be accounted for by water-induced surface topology changes providing additional adsorption sites or an increase in the availability of ‘type 1’ and ‘type 2’ sites due to the more dispersed oxide; the ‘type 2’ sites are unsaturated because of this availability. Molecular desorption from ‘type 2’ sites would then be solely responsible for the observed single-peak oxide spectrum.To test the hypothesis of surface-induced topology changes 40p1 of water were desposited on the furnace surface and allowed to dry. A 2 pl volume of barium nitrate solution was then deposited at the same location. The resulting barium oxide spectrum was identical with that of Fig. 1 for the 2 pl volume hence water is not inducing the observed spectrum change. The important implication is that the dispersed low surface concentration of barium associated with large-volume samples increases the probability of barium oxide being disso- ciatively adsorbed at the high activation energy desorption ‘type 1’ sites.There is the possibility that the broad BaO’ spectrum is produced by the barium oxide slowly percolating from the bulk carbon or from near-surface regions. The fact that the barium oxide signal is weak compared with the other Group IIA oxide signals supports this; however the large volume load data are not explained by this possibility. A 170 u mass peak which may correspond to BaO,’ ap- peared at 2000 K (Table 1). It is questionable whether this species originates in the furnace. Certainly BaOz forms readily in air at 770 K from barium?’ or from BaO,*X but de- composes at 1070 K under equilibrium conditions.zH.z9 Further- more the peroxide has not been observed in Knudsen cells with the oxygen partial pressure as high as 101.3 Pa.30 Unfor- tunately furnace-related oxygen partial pressures reported by different workers range from 1 x lo-” to 1 at 2000 K.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL.6 29 Semi-empirical evaluations indicate that oxygen partial pres- sures associated with the gas phase decomposition of MO type compounds range from 0.001 to 100 Pa depending on the amount of It is therefore unlikely that BaO is present in the furnace. Ionizer-induced dissociation of Ba202(g) is probably responsib1e.N It has already been established that strontium is not vapor- ized from the condensed phase but its presence is due to de- sorption from ‘type 3’ sites.Furthermore previous barium vacuum-vaporization datai4 suggest that a desorption mecha- nism is also responsible for free barium. A 100 K decrease in the calcium appearance temperature when the analyte mass increases from 10 to 200 ng is observed. This results from either an nth order of desorption reaction (n > 0)36 or from increased occupancy of lower activa- tion energy desorption sites which increase the rates of de- sorption at a given temperature.’’ It is concluded therefore that magnesium calcium strontium and barium are desorbed and that their desorption can be described by the reaction which proceeds after the oxide molecule dissociatively adsorbs at a ‘type 3’ site. As discussed earlier if the dissociative ad- sorption products react with the surface to form CO(g) as was the situation for ‘type 1’ sites the associated exothermic energy must be poorly coupled to the lattice at the ‘type 3’ location since the Group IIA metal remains adsorbed to rela- tively high temperatures.The adsorbed free analyte species in reaction (2) is one of the products of the dissociative adsorption described by the reaction (3) Assuming equilibrium upper bound values for the Gibbs free energies for reaction (3) were derived using statistical thermo- dynamic procedures described in reference 38. Published rota- tional constants were used to calculate moments of inertia for the metal oxide.26 The maximum centre-of-mass to atom separ- ation was used as the width of the potential well and the atomic radii of the metals were used to approximate the surface to atomic metal separation.The resulting upper bounds for the Gibbs free energies for reaction (3) are -147 kJ mol-I at 1000 K for Mg; -148 kJ mol-’ at 1650 K for Ca; -137 kJ mol-I at lo00 K for Sr; and -138 kJ mol-I at 2000 K for Ba all of which become more negative with increasing temperature. MO(ad type 3) + C(s) -+M(ad type 3) + CO(g) Carbides Magnesium calcium and strontium carbide spectra (Fig. 2) appear at low and high temperatures. It will be shown that the high-temperature carbide formation mechanism proposed in reference 22 to explain the strontium results applies only to strontium. Surface state perturbation arguments will be pre- sented to explain the high-temperature carbides associated with the other elements. Condensed phase reactions are re- sponsible for the low-temperature carbides because molecular gas phase species necessary to form the carbide are not present at 450-600 K.Magnesium carbide exists in two forms at low temperature Mg,C and MgC,. The MgC is not an ionizer-induced dissoci- ation product of Mg,C,. If it were MgC should appear simul- taneously with the MgC,; it does not. Now Mg,C is known to form when an excess of carbon is available;a) hence some of the ‘type 1 ’ sites may be contributing to the excess of carbon. Large-volume (42 pl) solution loads greatly increased the magnitude of low-temperature carbide spectra for magnesium calcium strontium and barium (see Table 1 column 5 ) . In fact large-volume loading was a necessary condition for low- temperature barium carbide formation; this large-volume sample load result is not included in the spectra of Fig.2. These low-temperature carbide enhancements are attributed to increased surface to volume ratios of the oxide crystallites a result of the increased surface dispersion associated with large volumes. The probability of interaction of the oxide molecules with available active sites on the surface therefore increases. As suggested previously for strontium,22 the formation of low-temperature carbides is explainable through the reaction (4) The ‘type 1 ’ sites react with the adatoms from dissociative ad- sorption of the oxide a mechanism that requires participation of two surface defect^;^' the reaction products are MC,(g) and CO(g). This adsorption site type explains the following seem- ingly contradictory results; increases in low-temperature carbide signals with increased solution volume suggest that ‘type 1 ’ sites [reaction (4)] are preferred over other oxide ad- sorption sites i.e.‘type 1 ’ sites involve the greater heat of ad- sorption @,); and the low-temperature carbide appearance suggests a low heat of desorption. This is possible if the adatom products (M,O) of dissociative adsorption chemically bond with ‘type 1 ’ site carbon atoms. The resulting exothermic C-0 reaction supplies the energy necessary to desorb the prod- ucts at low temperatures hence the heat of desorption is effec- tively decreased. The site is envisaged as a surface defect (e.g. an edge dislocation) containing a ‘dangling’ carbon chain and a neighbouring defect with at least one ‘dangling’ carbon.This defect geometry is particularly reasonable for the pyrolytic graphite coated highly textured graphite substrates. A more detailed physical description of these sites is given in Appen- dix 1 of reference 22. When large-volume solution loads were used the low- temperature calcium carbide signal increased significantly (see Table 1 column 5) and appeared at the lower 450 K tempera- ture; the large-volume load disperses the sample and thus de- creases surface concentrations. This increases the probability of calcium oxides finding ‘type 1 ’ sites and adsorbing dissocia- tively. An increased probability for carbide formation follows. It is not clear how the activation energy for desorption dimin- ishes but the decreased appearance temperature implies that it does.As discussed above low-temperature barium carbides were observed only when barium was loaded as a 42pl solution. This is because the BaO dissociation energy is 540 kJ m01-’,~~ which is 3045% greater than the dissociation energies for the oxides of the other elements in the group. It is therefore ener- getically more difficult to adsorb barium oxide dissociatively. Such adsorption can be enhanced however. The more dis- persed barium oxides in the large-volume loadings have higher probabilities of being located in the proximity of sites with high heat of adsorption that exhibit lower crossover energies and hence lower activation energies of adsorption. Dissocia- tive adsorption is enhanced and this enhances the probability of carbide formation.It is perceived that barium oxide mole- cules from low-volume loadings desorb before they receive sufficient energy from the lattice to overcome the dissociative adsorption activation energy barrier. Fig. 2 shows high-temperature strontium carbide appearing several hundred degrees after the free atom. When a calcium interferent was added to strontium both the free atom and carbide appeared at lower temperatures.22 This mutual shifting implies that carbide formation is associated with the free atom. Furthermore high-vacuum vaporization data from both the outer and inner furnace surfaces’* indicate that most of the high-temperature strontium carbide is formed by the heteroge- neous reaction between the free atoms and the graphite surface (i.e.second wall) in the furnace as shown in the reaction MO(ad type 1) + 3C(s) +MC,(g) + CO(g) There is no indication from the vacuum vaporization data that other Group IIA elements participate in similar second-wall re- actions. The high-temperature gas phase carbides therefore evolve directly from the condensed phase a desorption30 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 process which is inconsistent with the low heat of desorption implied by the low-temperature appearance of these same car- bides. This paradox a manifestation of assuming that desorp- tion energies alone are controlling the rate of release of both carbides vanishes should the high-temperature carbide spectra be controlled instead by the carbide formation mechanism de- scribed below.The following mechanism based on solid state band theory is proposed. The carbide formation is initiated through adsor- bate-adsorbent electronic energy configuration changes that occur when surface localized electronic levels (surface states) associated with the adsorbate are perturbed by electron trans- fer between the bulk electronic band structure and the surface For example two surface states are indicated by the two observed high-temperature species (free analyte and carbide) One of these states is the ground state of the ad- sorbed analyte atom at the ‘type 3’ site. The second state which is related to the carbide formation is induced by elec- tron transfer between the above mentioned ground state and the electronic band structure of the ‘type 3’ site.When this occurs the localized adsorbate-adsorbent pair might no longer compose a minimum surface state energy configuration; the ionized adsorbate rapidly relocates to a ‘type 3’ site for example in order to achieve a new ground state configuration. The present data indicate formation of the carbide at the relo- cation position. Note that the two ground state configurations depend on occupancy (with or without the transferred elec- tron). This Franck-Condon type splitting of surface states de- scribed by Morrison,43 provides the ground states from which the observed high-temperature desorption occurs. Evidently the electron exchange between the graphite and strontium surface states occurs at a temperature greater than the appear- ance temperature for free strontium; thus the strontium carbide must form to a large extent by reaction (5).The question of why only strontium and not calcium or barium should be involved in the second-wall reaction now becomes important. A similar question addresses why the strontium desorption energy should exhibit the lower magni- tude. An answer based on Gadzuk’P quantum treatment of surface-induced perturbations of energy level spectra of free adsorbate atoms is presented. The theory that evolved predicts the formation of resonance states (virtual electronic excited states) due to the coupling of free adsorbate atom excited states with the band structure of the adsorbent. The virtual states which control the heat of have been ob- served by Plummer and Young4 for calcium strontium and barium on tungsten by field emission techniques. Plummer and Young suggest from their treatment of the structure in the energy distribution of the field-emitted electrons that the barium adatom ground state is a mixture of the ground state of the free atom and the first two excited states and that these states are shifted and broadened to form three overlapping bands below the Fermi surface.Calcium however has only its free atom ground state and the first excited state that are broad- ened and shifted by the surface interactions hence two virtual states exist below the Fermi surface. Finally strontium field emission data suggest that only a broadened free atom ground state below the Fermi surface accounts for the adsorbed ground state configuration.Similar adsorption behaviour might be expected on pyroly- tic graphite (PG) as non-localized or averaged work functions for PG and tungsten are similar (4.647 and 4.5 eV,4s respective- ly) and the substrate manifests itself through the work function in the theory of Gadzuk4. The more open structure of graphite will of course be influential but the relative behaviours of the adsorbates on the PG and on tungsten should be similar. It is expected that the number of virtual states below the Fermi surface of graphite should be greatest for barium and least for strontium. The strontium would therefore have the lowest heat of adsorption of the three elements and should appear at a lower temperature as observed. The over-all picture associated with high-temperature Group IIA carbides is simply that these carbides form as a result of Franck-Condon splitting of the ground surface states that evolve from the electronic resonance states associated with the adsorbate-bulk adsorbent pair.The weaker resonances expect- ed for strontium indicate an earlier release of strontium and thus a second wall reaction leading to carbide formation. The data suggest that the high-temperature magnesium carbide for- mation mechanism is similar to that of calcium and barium carbides. The beryllium carbides are the only Group IIA carbides whose formation is not explained by the above arguments. Earlier data from mass spectrometric investigations of berylli- um atomization indicate that beryllium carbide formation is closely related to the free oxide instead of the free metal and this formation can be described by the heterogeneous reaction’ xBeO(g) + (x +y )C(s)+Be,,C,(g) + xCO(g) Unlike the adsorption of the other Group IIA oxides only as- sociative adsorption occurs for beryllium oxide.This is evi- dently the reason for the absence of low-temperature beryllium carbides that require ‘type 1 ’ site dissociative adsorption. It is not clear however why beryllium is not adsorbed dissocia- tively. Perhaps the smaller size of the oxide allows such efficient trapping of the molecule that dissociative adsorption does not become energetically favourable. Changes in the magnitudes of products desorbing from each type of site indicate the relative probability of adsorbing a par- ticular species. The relatively large volume-induced increase in the intensity of the low-temperature carbide spectra (Table 1) and the corresponding decrease associated with the high- temperature atomic spectra suggest that ‘type I ’ sites are pre- ferred for oxide adsorption. This implies that the heats of adsorption associated with ‘type 1 ’ sites are of greater magni- tude than those at ‘type 3’ sites.Similarly by comparing rela- tive intensity changes for the gas phase oxide and free metal atom it is seen that the heats of adsorption at ‘type 3’ sites Q (3) are greater than those at ‘type 2’ sites Q(2). It is concluded that heats of metal oxide adsorption [Q(l)] for ‘type 1’ sites are such that Q( 1) > Q(3) > Q(2). This conclusion was made initially for strontium.’’ The present results extend this conclu- sion to magnesium calcium and barium.Hydroxides Temperature profiles associated with hydroxide formation are shown in Fig. 3. The formation of the hydroxides of magne- sium calcium and strontium are coincident with the low- temperature formation of carbides and exhibit signal intensity increases that are coincident with increases in the intensities of their respective carbides. The increases were observed when large-volume loads were used (see Table 1 column 5 ) and when calcium was used to enhance low-temperature strontium carbide formation.’? These observations support a reaction of the type (7) for Group IIA elements4() if this type of reaction is applicable to gas phase carbides. Calcium carbide and hydroxide appeared near 600 K when calcium was loaded as a small-volume sample.However when using large-volume sample loads the carbide and hydroxide appeared at 450 K. The decrease in the calcium hydroxide appearance temperature is perceived to be due to sample volume load promotion of dissociative adsorption as described previously under Carbides. Lowering of the temper- ature at which the carbide forms results in a lowering of the appearance temperature of the hydroxide formed by reaction (7) in the gas phase. The hydroxide of barium exists in spite of the paucity of a low-temperature carbide when low-volume loads are used.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1991 VOL. 6 31 Reaction (7) does not apply but coincidence of the barium oxide (Fig. 1) and the hydroxide (Fig. 3) suggests the reaction Newbury30 observed a linear increase in the barium hydroxide generated in proportion to the amount of water vapour present in Knudsen cells.Beryllium hydroxide was observed by Styris and Redfield’ only when large-volume solution loads were used. In this in- stance a heterogeneous form of reaction (8) explains the for- mation of the hydroxide i.e. a reaction of water vapour with adsorbed instead of gas phase oxide. This is consistent with the nature of beryllium oxide to be adsorbed associatively but not dissociatively as discussed in the latter part of the previous section. The beryllium hydroxide was not formed when a 2 pl load solution was used because either the necessary amount of water vapour was then unavailable or the beryllium oxide was not sufficiently dispersed over the surface.Conclusion Analyses of real-time development of Group IIA gas phase species (except radium) in pyrolytic graphite coated graphite furnaces show that analyte losses occur through formation of gaseous carbides oxides and hydroxides. The data presented here are consistent with the conclusion of earlier workers that carbide formation is a competing mechanism in the atomiza- tion of the species.6.1’-’3.23 The data fail to support the sublima- tion processes proposed by Nagdaev and Bukreev,I6 to explain vaporization of barium and strontium. The mechanism involv- ing three types of adsorption sites previously proposed for strontium,21 was found to be applicable to other Group IIA elements. Atomization of oxides of the Group IJA elements excluding beryllium is the result of dissociative adsorption of the oxide.The free element and high-temperature carbides are a result of Franck-Condon splitting of the adsorbed elements surface states and associated equilibrium established between the re- sulting states. Strontium is desorbed early because it contrib- utes only a single virtual state below the Fermi level of the graphite; a smaller heat of adsorption relative to calcium and barium is indicated. The carbide of strontium is consequently formed by a heterogeneous (second wall) reaction. The oxide desorbs at lower temperatures after being associatively ad- sorbed at ‘type 2’ sites characterized by the lower heat of ad- sorption Q(2). The low-temperature carbides are the result of dissociative adsorption of oxide at the higher heat of adsorp- tion Q(1) (‘type l’) and reaction between the adsorption products and the carbon at the adsorption site.Exothermic energy from this reaction (CO formation) results in the low- temperature release of the carbide into the gas phase. The hy- droxide is formed by the reaction of water vapour with the low-temperature carbides. It may also be the result of the hy- dration of the adsorbed oxide but this is probably a minor path of formation. For those elements in which the oxide precedes free-atom formation the large-volume loads induce a 540-fold increase in carbide and hydroxide precursor losses. Noticeable decreas- es in high-temperature mass and atomic absorption spectro- metric intensities result from these volume-induced precursor losses.It would appear that by increasing the sample volume keeping the analyte mass constant the sample dispersion on the graphite surface is enhanced. This results in an increase of the probability of interaction with higher energy (‘type 1 ’) ad- sorption sites responsible for low-temperature carbide formation. Only the vaporization of beryllium appears to be controlled by different mechanisms. These mechanisms have been dis- cussed in reference 7. The authors express their gratitude to D. R. Ells for his invaluable assistance with the apparatus. This work was sponsored by the Director Office of Energy Research Office of Basic Energy Science Chemical Sciences Division of the US Department of Energy and performed under contract DE- AC06-76RLO 1830.Financial support for David A. Redfield was provided by the Northwest College and University Association for Science Washington State University under contract DE-AM06-76-RLO 2225 with the US Department of Energy. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 References Electrothermal Atomization for Atomic Absorption Spectrometry ed. Fuller C. W. The Chemical Society London 1977 pp. 67-69 74 and 80. Graphite Furnace AAS A Sourcehook ed. Slavin W. Perkin-Elmer Norwalk CT 1984 pp. 79-80 S 9 1 120 and 141. Frech W. Lundberg E. and Cedergren A. frog. Anal. At. Spectrosc. 1985,8,257. Maessen F. J. M. J and Posma F. D. Anal. Chem. 1974,46 1439. Maessen F. J. M. J. Balhe J. and Massee R. Spectrochim.Acta Part B 1978,33,3 1 I . Runnels J. H. Merryfield R. F. and Fischer H. B. Anal. Chem. 1975,47,1258. Styris D. L. and Redfield D. A. Anal. Chem. 1987,59,2897. Kintor T. Laszlo B. and Pungor E. Spectrochim. Acta Part B 1983,38,58 1 . Hutton R. C. Ottaway J. M.. 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' Paper Ol016791 Received April I7th I990 Accepted August 31st 1990

ISSN:0267-9477    

DOI:10.1039/JA9910600025

出版商:RSC    

年代:1991

数据来源: RSC