摘要:

1996 Winter Conference on Plasma Spectrochemistry Fort Lauderdale Florida January 8 - 73 7996 The 1996 Winter Conference on Plasma Spectrochemistry ninth in a series of biennial meetings sponsored by the /CP information Newsletter features developments in plasma spectrochemical analysis by inductively coupled plasma (ICP) dc plasma (DCP) microwave plasma (MIP) and glow discharge (GDL HCL) sources. The meeting will be held Monday January 8 through Saturday January 13 1996 at the Bonaventure World Conference Center in Fort Lauderdale Florida. Continuing education short courses at introductory and advanced levels will be offered Friday through Sunday January 5 - 7. Spectroscopic instrumentation and accessories will be shown during a three-day exhibition. Objectives and Program The continued growth in popularity of plasma sources for atomization and excitation in atomic spectroscopy and ionization in mass spectrometry and the need to discuss recent developments of these discharges in spectrochemical analysis stimulated the organization of this meeting.The Conference will bring together international scientists experienced in applications instrumentation and theory in an informal setting to examine recent progress in the field. Approximately 500 participants from 25 countries are expected to attend. Approximately 300 papers describing applications fundamentals and instrumental developments with plasma sources are expected to be presented in lecture and poster sessions by more than 200 authors. Symposia organized and chaired by recognized experts will include the following topics 1) Sample introduction and transport phenomena 2) Flow injection spectrochemical analysis 3) Elemental speciation with plasma/chromatographic techniques 4) Plasma instrumentation including chemometrics expert systems on-line analysis software and remote-system automation 5) Sample preparation treatment and automation 6) Excitation mechanisms and plasma phenomena 7) Spectroscopic standards and reference materials 8) Plasma source mass spectrometry 9) Glow discharge atomic and mass spectrometry 10) Applications of stable isotope analyses and 1 1) Laser-assisted plasma spectrometry. Six plenary and 18 invited lectures will highlight advances in these areas. Afternoon poster sessions will feature applications automation and new instrumentation. Five panel discussions will address critical development areas in sample introduction instrumentation elemental speciation plasma source mass spectrometry and novel software and hardware directions.Plenary invited and submitted papers will be published in Fall 1996 after peer review as the official Conference proceedings. Schedule of Activities Preliminary Title and 50-Word Abstract Due for Contributed Papers Exhibitor Booth Reservation and Pre-Registration Deadline Conference Pre-Registration October 13,1995 Hotel Pre-Reservation October 13 1995 Late Pre-Registration Deadline December 8,1995 1996 Winter Conference Short Courses January 5 - 7,1996 1 996 Winter Conference on Plasma Spectrochemistty January 8 - 13,1996 July 3 1995 September 11 1995 Further Information For further information return this form to 1996 Winter Conference on Plasma Spectrochemistry %lCP information Newsletter Department of Chemistry Lederle GRC Towers University of Massachusetts Box 34510 Amherrt MA 01003-4510 USA.ATTN Or. Ramon Barnes Conference Chairman Telephone (41 3) 545-2294 Telefax (41 3) 545-4490. % 0 Send further information. 0 I plan to attend accompanied by 0 I plan to present a paper (D oral 0 poster 0 computer poster). Title 1996 WlNTER CONFERENCE ON PLASMA SPECTROCHEMlSTRY Name Organization Address city Telephone Title State/Country Telefax Date ZIP/Postal Code EMAlLRoyal Society of Chemistry Analytical Division Atomic Spectroscopy Group Eighth Biennial National Atomic Spectroscopy Symposium 8th BNASS University of East Anglia UK 17-20 July 1996 Plenary Lectulers Invited Lecturers call for Papers social Programm workshop Further Detaila Dr S J Hill Professor N Furuta.Professor F Adams Professor J M M e m t and Professor G Hieftje Dr 0 Donard Dr S J Pmy. Dr S Fairweather-Tait Dr A Ellis Dr A G Howard. Dr J Brtnner. Dr J Mmhdl Dr N J Miller-lhli Dr S Tanner and Professor D Littlejohn Contributed oral and poster presentations on recent developments in both p u n and applied atomic spectroscopy - analytical applications theoretical studies or fundamental advances in AAS A m . AFS. inorganic MS and XRF. Thm copies of abstracts must be submitted before 2% February 1996. BNASS has an enviable reputation of being a friendly and dynamic meeting. A number of social events including a Symposium Dinner will form an integral part of the meeting.Immediately prior to the 8th BNASS then will be a Short Course on Sample Pre-treatment and Sample Introduction for Atomic Spectroscopy. 17 July a.m. 1996. Ms Brenda Holliday. Royal Society of Chemistry Thomas Graham House. Science Park Milton Road Cluabridgc Cl34 4WF. UK. T e l M (0)1223 420066; Fax +44 (0)1223 420247; E - d l JAAS@RSC.ORG 1996 WINTER CONFERENCE ON PLASMA SPECTROCHEMISTRY January 843,1996 at the Bonaventure World Conference Center Fort Lauderdale Florida USA Short Course on High-resolution ICP-/US (Sl-3) It is the aim of a short course at the 1996 Winter Conference on Plasma Spectrochemistry (January Sth 7pm) to summarize the state of the art in HR-ICP-MS. The course provides an introduction to ICP-MS with a double focusing magnetic sector mass analyser.This course uffers fundamental background a thorough discussion of analytical features and state of the art information on applications. Different types of double focusing instruments also are considered. Specific topics include fundamental aspects of ICP-MS (physical properties of a double focusing instrument operational characteristics in comparison with quadrupole instruments); analytical characteristics (spectral and non-spectral interferences figures of merit in low and high resolution modes blanks and memory effects HPLC and GC interfaces) and applications (industrial including ultra-pure reagents and alloys environmental geological and biomedical). Course organizers Luc Moens Ghent University Laboratory of Analytical Chemistry Proeftuinstraat 86 8-9000 Ghent Belgium; and Norbert Jakubowski lnstitut fur Spektrochemie und Angewandte Spektroskopie Bunsen Kirchoff-Strasse 11 0-44139 Dortmund Germany. For registration and further information on the Conference and short course please contact Dr. Ramon Barne8 Conference Chairman Department of Chemistry Lederle GRC Tower University of Massachusetts Box 3451 0 Amherst MA 01 003-451 0. Tol 41 3 545 2294; Fax 41 3 545 3757

ISSN:0267-9477    

DOI:10.1039/JA99510BP024

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Journal of Analytical Atomic Spectrometry DIARY OF CONFERENCES AND COURSES 1995 Street Sheffield S1 4CT UK. Fax +44 (0) 114 2768653. developments in the near infrared (NIR) and Raman techniques. The latter has been characterized by new instruments that strongly reduce the Short Course Disposal of Hazardous Waste December 5 Scheele Symposium 1995 on earlier problem with interfering This workshop will focus on Shefield UK Details can be found in J. Anal. At. Spectrom. 1995 10 34N. For further information contact Ms Baldham or Ms Rogers Division of Adult Continuing Education University of Sheffield 196-198 West a renaissance with particular given. ('1 '14 2825391; Modern Applications of Vibrational fluoresence. Spectrometry December 8 Uppsala Sweden Vibrational spectrometry is undergoing pharmaceutical applications on the three techniques FT-IR Raman and NIR.Lectures by the Scheele awardee 1995 professor Patrick Hendra and other experienced spectrometrists as well as graduate students will be Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 59NFor further information contact Christel Anderson The Swedish Academy of Pharmaceutical Sciences P.O. Box 1136 11181 Stockholm Sweden. Telephone +46 8 723 5000. Fax +46 8 20 5511. International Symposium on Environmental Biomonitoring and Specimen Banking December 17-22 Honolulu Hawaii USA Details can be found in J. Anal. At. Spectrom. 1994 9 59N. For further information contact K. S. Subramanian Environmental Health Directorate Health Canada Tunney's Pasture Ottawa Ontario K1A OL2 Canada (phone 613-957-1874; fax 613-941-4545) or G.V. Iyengar Center for Analytical Chemistry Room 235 B 125 National Institute of Standards and Technology Gaithersburg MD 20899 USA (Telephone +1 301 975 6284; Fax + 1 301 921 9847) or M. Morita Division of Chemistry and Physics National Institute for Environmental Studies Japan Environmental Agency Yatabe-Machi Tsukuba Ibaraki 305 Japan (Telephone +81 298 51 6111 ext. 260; Fax +81 298 56 4678). 1996 1996 Winter Conference on Plasma Spectrochemistry January 8-13 Fort Lauderdale Florida USA Details can be found in J. Anal. At. Spectrom. 1994,9,53N. For further information contact Dr. R. Barnes ICP Information Newsletter Department of Chemistry Lederle GRC Towers University of Massachusetts Box 34510 Amherst MA 01003-4510 USA.Telephone + 1 413 545 2294; Telefax + 1 413 545 4490. International Schools and Conferences on X-Ray Analytical Methods January 18-25 Sydney Australia Details can be found in J. Anal. At. Spectrom. 1994,9,47N. For further information contact AXAA '96 Secretariat GPO Box 128 Sydney NSW 2001 Australia. Telephone + 61 2 262 2277; Fax + 61 2 262 2323; Telex AA 17651 1 TRHOST. 8th Sanibel Conference on Mass Spectrometry Metal-Containing Ions and Their Applications in Mass Spectrometry January 20-23 Sanibel Island FL USA For further details contact American Society for Mass Spectrometry 1201 Don Diego Avenue Santa Fe NM 87505 USA. Telephone + 1 505 989 4517; Fax + 1 505 989 1073. Analytica Conference 96 April 23-26 Munich Germany Details can be found in J.Anal. At. Spectrom. 1994,2,69N. For further information contact Messe Miinchen GmbH Messegelande D-80325 Miinchen Germany. Telephone +49 89 51 07 0; Telex 5 212 086 ameg d; Fax +49 89 51 07 177. ASMS Short Course Interpretation of Mass Spectra LC/MS and MS/MS May 11-12 Portland OR USA For further details contact American Society of Mass Spectrometry 1201 Don Diego Avenue Santa Fe NM 87505 USA. Telephone + 1 505 989 4517; Fax + 1 505 989 1073. 44th ASMS Conference on Mass Spectrometry and Allied Topics May 12-17 Portland OR USA For further details contact American Society of Mass Spectrometry 1201 Don Diego Avenue Santa Fe NM 87505 USA. Telephone + 1 505 989 4517; Fax + 1 505 989 1073. Ninth International Symposium on Trace Elements in Man and Animals May 19-24 Ban# Alberta Canada Details can be found in J.Anal. At. Spectrom. 1995,10 58N. For further details contact TEMA-9 The Banff Centre for Conferences P.O. Box 1020 Station 11 Banff Alberta Canada TOL OCO. Telephone + 1 403 762 6308; Fax + 1 403 762 6388 or Dr. Mary L'AbbC. Telephone + 1 613 957 0924; Fax + 1 613 941 6182; EM AIL Mlabbe@HPB.HWC.CA. Total Reflection X-Ray Fluorescence Analysis 10-11 June 1996 Eindhoven Germany 13-14 June 1996 Dortmund Germany The conference will present the state-of- the-art in instrumental and methodological development and demonstrate actual applications. Besides conventional total reflection X-ray fluorescence analysis related methods like X-Ray reflectometry and grazing- emission XRF will be included. The first two days in Eindhoven will focus on surface and thin layer analysis the last two days in Dortmund on micro- and trace analysis.Like the preceeding conferences in Geesthacht (1986 and 1992) Dortmund (1988) Vienna (1990) and Tsukuba (1994) the 6th conference will bring together scientists users instrument manufacturers and all who are interested in the chosen subject. Apart from a few invited lectures contributed papers and posters will constitute the scientific programme. Substantial discussions and exchange of experience are encouraged. For further details contact Gesellschaft Deutscher Chemiker TXRF-Konferenz Postfach 90 04 40 D-60444 Frankfurt Germany. Fax + 49 69 7917 475. Resonance Ionization Spectroscopy June 30-July 5 1996 Pennsylvania USA The Eighth International Symposium on Resonance Ionization Spectroscopy and Its Applications (RIS-96) is an ongoing series of meetings which began in 1980 in Knoxville Tennessee.The conference program will focus on progress in understanding fundamental processes on developments in instrumentation and on new analytical applications. Related disciplines where RIS may play a role in the future will also be featured. The scientific program will consist of keynote and invited lectures as well as contributed oral and poster present a tions. For more information contact Sabrina Glasgow Conference Secretary Department of Chemistry The Pennsylvania State University 184 Materials Research Institute Building University Park PA 16802- 7003 USA. Tel + 1 814 865 0200; Fax +18148630618; Email scg4@psuvm.psu.edu Eighth Biennial National Atomic Spectroscopy Symposium July 17-19 University of East Anglia Norwich UK The Biennial National Atomic Spectroscopy Symposium has an international reputation as the United Kingdom's premier meeting in the field of analytical atomic spectroscopy. The aim of this three day meeting is to 60 N Journal of Analytical Atomic Spectrometry November 1995 Vol.10promote and encourage developments in both fundamental and applied atomic spectroscopy including ICP-MS and XRF by providing a friendly environment where delegates can meet formally and informally to exchange ideas views and results. Plenary lectures given by world renowned spectroscopists provide overviews of important areas of atomic spectroscopy. Invited and submitted lectures as well as posters cover the most recent developments in both pure and applied atomic spetcroscopy.Although the majority of papers tend to focus on analytical applications presentations on theoretical studies or fundamental advances in AA AE AF and XRF are also important components of each BNASS. Preparation and Sample Introduction has been organised for the morning of Wednesday July 27. The course will include aspects of ICP-AES AAS AES XRF AFS and ICP-MS. Separate registration is required. Papers should discuss original unpublished work. Manuscripts of accepted papers will be considered for publication in a special issue JAAS. A Short Course of Sample For further information contact Dr. S. J. Haswell School of Chemistry University of Hull Hull HU6 7RX UK.Telephone + 44 (0)482 465469; Fax + 44 (0)482 466410. 12th Asilomar Conference on Mass Spectrometry Elemental Mass Spectrometry September 20-24 PaciJic Grove CA USA For further details contact American Society of Mass Spectrometry 1201 Don Diego Avenue Santa Fe NM 87505 USA. Telephone + 1 505 989 4517; Fax + 1 505 989 1073. 1997 Seventh International Symposium on Biological and Environmental Reference Materials April 21-25 Antwerp Belgium Details can be found in J. Anal. At. Spectrom. 1995,9,54N. For further details contact Dr J. Pauwels Institute for Reference Materials & Measurements Management of Reference Materials Unit Retieseweg B-2440 Geel Belgium. Telephone + 32 14 571 722; Fax + 32 14 590 406; or Wayne R. Wolf Ph.D Food Composition Laboratory USDA 10300 Baltimore Blvd. Beltsville MD 20705 USA. Telephone + 1 301 504 8927; Fax + 1 301 504 8314 XXX Colloquium Spectroscopicum Internationale September 21st -26t h Melbourne Australia Details can be found in J. Anal. At. Spectrom. 1995,10,58N. For further details contact The Meeting Planners 108 Church Street Hawthorn Victoria 3 122 Australia. Telephone +613 9819 3700; Fax +61 3 9819 5978. Updated information may be obtained from the XXX CSI homepage on the World Wide Web at http://www.latrobe.edu.au/CSIconf/ XXXCSI. html. Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 61 N

ISSN:0267-9477    

DOI:10.1039/JA99510059Nb

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Journal of Analytical Atomic Spectrometry JAAS Editorial Board Chairman. B. L. Sharp (Loughborough UK) A. T. Ellis (Abingdon. UK) R. C. Hutton (Cheshire UK) B. P. Holliday (Cambridge UK) D. Littlejohn (Glasgow UK) S. J. Haswell (Hull UK) H. Crews (Norwich UK) S. J. Hill (Plymouth UK) A. Sanz-Medel (Ovredo Spain) P. D. P. Taylor (Gee/ Belgrum) ~~ ~ JAAS Advisory Board F. C. Adanis (Antwerp Belgrum) G. M. Hieftje (Bloomington IN USA) R. M. Barnes (Amherst MA USA) R. S. Houk (Ames /A. USA) L. Bezur (Budapest Hungary) R. Klockenkamper (Dortmund Germany) M. W. Blades (Vancouver Canada) 6. V. L'vov (St. Petersburg Russia) R. F. Browner (Atlanta GA USA) R. K. Marcus (Clemson SC USA) J L. Burguera (Merida. Venezuela) J. M. Mermet (Vrlleurbanne France) S. Caroli (Rome. Italy) T. Nakahara (Osaka Japan) J.A. Caruso (Crncrnnati OH. USA) Ni Zhe-ming (Beyng China) H. M. Crews (Norwich U K ) J. W. Olesik (Columbus OH USA) A. J. Curtius (Florranopolrs. Brazil) N. Omenetto (lspra Italy) J B. Dawson (Leeds. U K ) C. J. Park (Taelon Korea) M. T. C. de Loos-Vollebregt (Delft The Netherlands) P. J. Potts (Milton Keynes UK) 0. F. X. Donard (Talence France) R. E. Sturgeon (Ottawa Canada) L. Ebdon (Plymouth UK) V. Sychra (Prague Czech Republic) M. S. Epstein (Garthersburg MD USA) P. Van Espen (Antwerp Belgium) Fang Zhao-lun (Shenyang Chrna) R. Van Grieken (Antwerp Belgium) W. Frech (UmeA Sweden) 6. Welz (Uberlrngen. Germany) A. K. Gilmutdinov (Uberlingen Germany) ~~~~ ~~ ~ __ Atomic Spectrometry Updates Editorial Board Chairman *A. T. Ellis (Abingdon UK) J. A.Arnistrong (Edinburgh. UKi 'J. R. Bacon (Aberdeen. UK) R. M. Barnes (Amherst. MA. USA) S . Branch (High Wycombe. U K ) R. Bye (Oslo Norway) J. Carroll (Middlesbrough. UK) M. R. Cave (Keyworth. UK) S. R. N. Chenery (Keyworth. UK) *J. M. Cook (Keyworth. UK) *M. S . Cresser (Aberdeen. UK) H. M. Crews (Norwich. UK) J. S. Crighton (Sunbury-on-Thames U K ) 'J. B. Dawson (Leeds. U K ) J. R. Dean (Newcastle upon Tyne. U K ) *E. H. Evans (Plymouth. UK) J. Fazakas (Budapest Hungary) A. Fisher (Plymouth. UK) L. M. Garden (Middlesbrough. UK) *J. M. Gordon (Cambridge. UK) D. J. Halls (Glasgow UK) K. W. Jackson (Albany. NY USA) R. Jowitt (Middlesbrough. U K ) K. Kitagawa (Nagoya. Japan) J. Kubova (Bratislava Slovak Republrc) *J. Marshall (Mrddlesbrough. UK) * S . J.Hill (P/yf??OutlJ. UK) H. Matusiewicz (Poznan. Poland) A. W. McMahon (Manchester UK) J. M. Mermet (Villeurbanne. France) R. G. Michel (Storrs. CT. USA) *D. L. Miles (Keyworth UK) T. Nakahara (Osaka. Japan) Ni Zhe-ming (Beijing C h m ) P. J. Potts (Milton Keynes UK) W. J. Price (Budlelgh Salterton. U K ) C. J. Rademeyer (Pretoria South Africa) A. Sanz-Medel (Oviedo Spain) *B. L. Sharp (Loughborough UK) I. L. Shuttler (Uberlingen. Germany) S. T. Sparkes (Taunton UK) R. Stephens (Halifax. Canada) J. Stupar (Ljubljana. Slovenia) R. E. Sturgeon (Ottawa. Canada) *A. Taylor (Guildford. UK) G. C. Turk (Gaithersburg MD USA) J. F. Tyson (Amherst. MA USA) P. J. Watkins (London. UK) - B. Welz (Uberlingen Germany) M. White (lspra Italy) J. G. Williams (Egham UK) J. B. Willis (Victoria.Australia) *Members of the ASU Executive Committee ~~ - ._ Managing Editor JAAS Brenda Holliday US Associate Editor JAAS The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK. Telephone i 44 (0) 1223 420066. Fax t 44 (0) 1223 420247. E-mail RSC1 @RSC.ORG (Internet) Production Manager Janice Gordon Publrshrng Staff Sarah Williams Productron Editorral Staff Yasmin Khan Caroline Seeley Ziva Whitelock Roger Young Editorial Secretaries Lesley Turney Fax 81 -3-381 7-1 895. Claire Harris Frances Thomson Advertisements. Advertisement Department The Royal Society of Chemistry Burlington House Piccadilly London W1V OBN UK. Telephone + 44 (0) 171 -287 3091. Fax + 44 (0) 171 -494 11 34. Dr. J. M. Harnly US Department of Agriculture Beltsville Human Nutrition Research Center Beltsville MI? 20705 USA.Telephone + 1 301 -504-8569 Asia-Pacific Associate Editor JAAS Prof. N. Furuta Department of Applied chemistry Faculty of Science and Engineering Chuo University 1-1 3-27 Kasuga Bunkyo-ku Tokyo 11 2 Japan. Telephone 81 -3-381 7-1 906. E-mail nfuruta(aapchem.chem.chuo-u.ac.jp ~~ ~- __ _~ __ __ Information for Authors Full details of how to submit materials for publi- cation 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 publi- cation of original research papers communi- cations and letters concerned with the development and analytical application of atomic spectrometric techniques. The journal is pub- lished twelve times a year including comprehen- sive reviews of specific topics of interest to practising atomic spectroscopists and incorpor- ates the literature reviews which were previously published in Annual Reports on Analytical Atomic Spectroscopy (ARAAS).Manuscripts intended for publication must describe original work related to atomic spectro- metric analysis. Papers on all aspects of the sub- ject will be accepted including fundamental studies novel instrument developments and prac- tical analytical applications. As well as AAS AES and AFS papers will be welcomed on atomic mass spectrometry X-ray fluorescence/emission spectrometry and secondary emission spec- trometry. Papers describing the measurement of molecular species where these relate to the characterization of sources normally used for the production of atoms or are concerned for example with indirect methods of analysis will also be acceptable for publication.Papers describing the development and applications of hybrid techniques (e.g.. GC-coupled AAS and HPLC-ICP) will be particularly welcome. Manuscripts on other subjects of direct interest to atomic spectroscopists including sample prep- aration and dissolution and analyte pre-concen- tration procedures as well as the statistical interpretation and use of atomic spectrometric data will also be acceptable for publication. There is no page charge. The following types of papers will be considered. 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 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 manuscript will be regarded as an undertaking that the same material is not being considered for publication by another journal.Manuscripts (three copies typed in double spacing) should be sent to Brenda Holliday Managing Editor JAAS Dr. J. M. Harnly US Associate Editor JAAS or Prof. N. Furuita Asian- Pacific Editor JAAS. All queries relating to the presentation and sub- mission of papers and any correspondence regarding 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 vra the Editorial Office) would welcome comments suggestions and advice on general policy matters concerning JAAS. Fifty reprints are supplied free of charge. Journal of Amlytrcal Atomc Spectrometry (JAAS) (ISSN 0267-9477) is published monthly 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 Distribution Services Ltd Blackhorse Road.Letchworth Herts SG6 lHN UK Tel t 44 (0) 1462 672555; Telex 825372 Turpin G; Fax t 44 (0) 1462 480947 Turpin Distribution Services Ltd. is wholly owned by The Royal Society of Chemistry 1996 Annual subscription rate EEA f599 00 USA $1136.00 Rest of World f1136 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 11 003 USA Postmaster send address changes to Journal o f Analytcal Atomrc Spectrometry (JAAS) Publications Expediting Inc.200 Meacham Avenue Elmont NY 11 003 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 OThe Royal Society of chemistry 1995 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 publishersJournal of Analytical Atomic Spectrometry JAAS Editorial Board Chairman B. L. Sharp (Loughborough UK) A. T. Ellis (Abingdon UK) B. P. Holliday (Cambridge UK) S. J. Haswell (Hull UK) S. J.Hill (Plymouth UK) R. C. Hutton (Cheshire UK) D. Littlejohn (Glasgow UK) H. Crews (Norwich UK) A. Sanz-Medel (Oviedo Spain) P. D. P. Taylor (Gee/ Belgium) JAAS Advisory Board F. C. Adams (Antwerp Belgium) R. M. Barnes (Amherst MA USA) L. Bezur (Budapest Hungary) M. W. Blades (Vancouver Canada) R. F. Browner (Atlanta GA USA) J. L. Burguera (Merida Venezuela) S. Caroli (Rome Italy) J. A. Caruso (Cincinnati OH USA) H. M. Crews (Norwich UK) A. J. Curtius (Norianopolis Brazil) J. B. Dawson (Leeds UK) M. T. C. de Loos-Vollebregt (Delft The Netherlands) 0. F. X. Donard (Talence France) L. Ebdon (Plymouth UK) M. S. Epstein (Gaithersburg MD USA) Fang Zhao-lun (Shenyang China) W. Frech (Umei Sweden) A. K. Gilmutdinov (Uberlingen Germany) G. M. Hieftje (Bloomington IN USA) R.S. Houk (Ames /A USA) R. Klockenkamper (Dortmund Germany) B. V. Cvov (St. Petersburg Russia) R. K. Marcus (Clemson SC USA) J. M. Mermet (Vilieurbanne France) T. Nakahara (Osaka Japan) Ni Zhe-ming (Beuing China) J. W. Olesik (Columbus OH USA) N. Omenetto (lspra Italy) C. J. Park (Taejon Korea) P. J. Potts (Milton Keynes UK) R. E. Sturgeon (Ottawa Canada) V. Sychra (Prague Czech Republic) P. Van Espen (Antwerp Belgium) R. Van Gr.ieken (Antwerp Belgium) B. Welz (Uberlingen Germany) Atomic Spectrometry Updates Editorial Board Chairman *A. T. Ellis (Abingdon UK) J. A. Armstrong (Edinburgh UK) *J. R. Bacon (Aberdeen UK) R. M. Barnes (Amherst MA USA) S. Branch (High Wycombe U K ) R. Bye (Oslo Norway) J. Carroll (Middlesbrough UK) M. R. Cave (Keyworth UK) S. R. N. Chenery (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. 6. Dawson (Leeds UK) J. R. Dean (Newcastle upon Tyne UK) *E. H. Evans (Plymouth UK) J. Fazakas (Budapest Hungary) A. Fisher (Plymouth UK) L. M. Garden (Middlesbrough UK) *J M. Gordon (Cambridge UK) D. J. Halls (Glasgow UK) *S. J. Hill (Plymouth UK) K. W. Jackson (Albany NY USA) R. Jowitt (Middlesbrough UK) K. Kitagawa (Nagoya Japan) J. Kubova (Bratislava Slovak Republic) *J. Marshall (Middlesbrough UK) H. Matusiewicz (Poznan Poland) A. W. McMahon (Manchester UK) J. M. Mermet (Villeurbanne France) R. G. Michel (Storrs CT USA) *D. L. Miles (Keyworth UK) T. Nakahara (Osaka Japan) Ni Zhe-ming (BeQing China) P. J. Potts (Milton Keynes UK) W.J. Price (Budleigh Salterton OK) C. J. Rademeyer (Pretoria South Africa) A. Sanz-Medel (Oviedo Spain) *B. L. Sharp (Loughborough UK) I. L. Shuttler (Uberlingen Germany) S. T. Sparkes (Taunton UK) R. Stephens (Halifax Canada) J. Stupar (Ljubljana Slovenia) R. E. Sturgeon (Ottawa Canada) *A. Taylor (Guildford UK) G. C. Turk (Gaithersburg MD USA) J. F. Tyson (Amherst MA USA) P. J. Watkins (London UK) B. Welz (Uberlingen Germany) M. White (lspra Italy) J. G. Williams (Egham UK) J. B. Willis (Victoria Australia) *Members of the ASU Executive Committee Managing Editor JAAS Brenda Holliday The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge C64 4WF UK. Telephone +44 (0) 1223 420066. Fax +44 (0) 1223 420247. E-mail RSC1 @RSC.ORG (Internet) Production Manager Janice Gordon Publishing Staff Sarah Williams Production Editorial Staff Yasmin Khan Caroline Seeley Ziva Whitelock Roger Young Editorial Secretaries Lesley Turney Claire Harris Frances Thomson US Associate Editor JAAS Dr.J. M. Harnly US Department of Agriculture Beltsville Human Nutrition Research Center Beltsville MD 20705 USA. Telephone + 1 301 -504-8569 Asia-Pacific Associate Editor JAAS Prof. N. Furuta Department of Applied Chemistry Faculty of Science and Engineering Chuo University 1-1 3-27 Kasuga Bunkyo-ku Tokyo 11 2 Japan. Telephone 81 -3-381 7-1 906. Fax 81 -3-381 7-1 895. E-mail nfuruta@apchem.chem.chuo-u.ac.jp Advertisements Advertisement Department The Royal Society of Chemistry Burlington House Piccadilly London W1 V OBN UK.Telephone + 44 (0) 171 -287 3091. Fax + 44 (0) 171 -494 11 34. Information for Authors Full details of how to submit materials for publi- cation 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 publi- cation of original research papers communi- cations and letters concerned with the development and analytical application of atomic spectrometric techniques. The journal is pub- lished twelve times a year including comprehen- sive reviews of specific topics of interest to practising atomic spectroscopists and incorpor- ates the literature reviews which were previously published in Annual Reports on Analytical Atomic Spectroscopy (ARAAS).Manuscripts intended for publication must describe original work related to atomic spectro- metric analysis. Papers on all aspects of the sub- ject will be accepted including fundamental studies novel instrument developments and prac- tical analytical applications. As well as AAS AES and AFS papers will be welcomed on atomic mass spectrometry X-ray fluorescence/ernission spectrometry and secondary emission spec- trometry. Papers describing the measurement of molecular species where these relate to the characterization of sources normally used for the production of atoms or are concerned for example with indirect methods of analysis will also be acceptable for publication. Papers describing the development and applications of hybrid techniques (e.g.GC-coupled AAS and HPLC-ICP) will be particularly welcome. Manuscripts on other subjects of direct interest to atomic spectroscopists including sample prep- aration and dissolution and analyte pre-concen- tration procedures as well as the statistical interpretation and use of atomic spectrometric data will also be acceptable for publication. There is no page charge. The following types of papers will be considered. Full papers describing original work. Communications which must be on an urgent matter and be of obvious scientific importance. Communications receive priority and are usuaNy published within 2-3 months of receipt. 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All queries relating to the presentation and sub- mission of papers and any correspondence regarding 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 (JAAS) (ISSN 0267-9477) is published monthly 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 Distribution Services Ltd. Blackhorse Road Letchworth Herts. SG6 lHN UK Tel. +44 (0) 1462 672555; Telex 825372 Turpin G; Fax -1-44 (0) 1462 480947. Turpin Distribution Services Ltd. is wholly owned by The Royal Society of Chemistry. 1996 Annual subscription rate EEA f599.00 USA $1 136.00 Rest of World fll36.00. 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 1 1 003. USA Postmaster send address changes to Journal of Analytical Atomic Spectrometry (JAAS) Publications Expediting Inc. 200 Meacham Avenue Elmont NY 11 003. 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. @The Royal Society of Chemistry 1995. 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/JA99510FX061

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Journal of Analytical Atomic Spectrometry 1 111111111111111111 1 . ___ ._. . __ JASPE2 lO(11) 59N-61 N 905-1 032 329R-358R (1995) I CONTENTS NEWS PAGES Book Reviews Diary of Conferences and Courses Future Issues 59N 59N 61 N PAPERS Characterization of Ionization and Matrix Suppression in Inductively Coupled Cold’ Plasma Mass Spectrometry Scott D. Tanner Spectroscopic Method for the Determination of the Electron Temperature in Quasi-thermal Air Discharges. Application to an Inductively Coupled Air Plasma at Atmospheric Pressure Anne-Marie Gomes Jean-Philippe Sarrette Lydie Madon Arnaud Epifanie Application of Multi-element Time-resolved Analysis to a Rapid On-line Matrix Separation System for Inductively Coupled Plasma Mass Spectrometry Simon M. Nelms Gillian M. Greenway Robert C.Hutton Computer Simulation of Enclosed Inductively Coupled Plasma Discharges. Part 1. Monatomic Gases Ana Gaillat Ramon M. Barnes Pierre Proulx Maher 1. Boulos Computer Simulation of Enclosed Inductively Coupled Plasma Discharges. Part 2. Molecular Gases Ana Gaillat Ramon M. Barnes Pierre Proulx Maher I. Boulos Determination of Trace Element Impurities in Nuclear Materials by Inductively Coupled Plasma Mass Spectrometry in a Glove-box Zlatan Kopajtic Stefan Rollin Beat Wernli Chantal Hochstrasser Guido Ledergerber Petr Jurcek Determination of Boron Using Mannitol-assisted Electrothermal Vaporization for Sample Introduction in Inductively Coupled Plasma Mass Spectrometry Wen-Ching Wei Chih-Jung Chen Mo-Hsiung Yang Hydride Generation Inductively Coupled Plasma Mass Spectrometric Detection of Lead Compounds Separated by Liquid Chromatography Hueih-Jen Yang Shiuh-Jen Jiang Determination of Antimony by Continuous Hydride Generation Coupled with Non-dispersive Atomic Fluorescence Detection Alessandro O’Ulivo Leonard0 Lampugnani Giovanna Pellegrini Roberto Zamboni Interpretation of the Interference Mechanisms Occurring in the Determination of Antimony(ll1) by Hydride Generation Atomic Absorption Spectrometry Based on Normal Reduction Potentials M.Teresa Martinez-Soria Jesus Sanz Asensio J. Galban Bernal Continuous-flow Microwave-assisted Digestion of Environmental Samples Using Atomic Spectrometric Detection Ralph E. Sturgeon Scott N. Willie Brad A. Methven Joseph W. H. Lam Henryk Matusiewicz Determination of Cadmium at Ultratrace Levels by Cold Vapour Atomic Absorption Spectrometry Guo Xiao-Wei Guo Xu-Ming Noise Studies for Detection Limits for Some Electrothermal Atomic Absorption Determinations and Calculation of the Optimal Detection Limit from One Atomization A.Le Bihan H. Le Garrec J. Y. Cabon Y. Guern Determination of Silicon in Electrolyte Solutions by Electrothermal Atomic Absorption Spectrometry Using Platinum as a Chemical Modifier Masami Fukushima Toshio Ogata Kensaku Haraguchi Kohichi Nakagawa Saburo Ito Masao Sumi Naoto Asami 905 923 929 935 94 1 947 955 963 969 975 98 I 987 993 999 continued on inside back cove1 lllllllllllllllll “1267 - 9 ~ 7 7 ( 1995 1 1 1 1 - A Typeset printed and bound by The Charlesworth Group Huddersfield England. 01484 517077Thermally Stabilized Iridium on an Integrated Carbide-coated Platform as a Permanent Modifier for Hydride-forming Elements in Electrothermal Atomic Absorption Spectrometry Dimiter L.Tsalev Alessandro D’Ulivo Leonard0 Lampugnani Marco Di Marco Roberto Zamboni Direct Determination of Nickel in Heroin and Cocaine by Electrothermal Atomic Absorption Spectrometry Using Deuterium Arc Background Correction Combined with Chemical Modification Pilar Bermejo-Barrera Antonio Moreda- PiAeiro Jorge Moreda-PiAeiro Adela Bermejo-Barrera Direct Coupling of High-performance Liquid Chromatography to Microwave- induced Plasma Atomic Emission Spectrometry via Volatile-species Generation and Its Application to Mercury and Arsenic Speciation Jose M. Costa-Fernandez Florian Lunzer Rosario Pereiro-Garcia Alfredo Sanz-Medel Nerea Bordel-Garcia Assessment of Direct Solid Sample Analysis by Graphite Pellet Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry J. M. Ren R. Rattray Eric D. Salin D. Conrad Gregoire CUMULATIVE AUTHOR INDEX 1003 101 1 1019 1027 1031 ATOMIC SPECTROMETRY UPDATES References 329R

ISSN:0267-9477    

DOI:10.1039/JA99510BX063

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

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Analysis of glass using laser ablation - inductively coupled plasma atomic emission spectrometry. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Anal. Chem. Masaryk Univ. Brno Czech Republic). Parosa R. Reszke E. Ramsza A. Helan V.Novel systems of microwave digestion. 10th Spectroscopic Conference Lanskroun Czech Republic June 14- 16 1995 (Plazmatronika Wroclaw Poland). Cernohorsky T. Dolezal J. Hlavac R. Vyskocilova 0. Optimum conditions for UV-mineralization of solutions. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Environ. Prot. Univ. Pardubice Czech Republic). Matousek T. Dedina J. Mechanism of interferences in selenium hydride atomization in quartz tubes for AAS. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Inst. Anal. Chem. Lab. Trace Element Anal. Acad. Sci. Czech Republic Videnska 1083 142 20 Prague Czech Republic). Docekal B. New approaches in the direct analysis of high purity molybdenum-based materials. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Inst.Anal. Chem. Czech Acad. Sci. Veveri 97 CZ-61142 Brno Czech Republic). Cernohorsky T. Canova L. Analysis of samples with high content of salts by FIA-FAAAS. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Univ. Pardubice Cs. Legii 565,532 10 Pardubice Czech Republic).9 5/C 3 748 951c3749 951c3750 951c3751 951c3752 95lC3753 9 5/c37 54 951c3755 95lC3756 95/c 3 7 57 9 5/C375 8 951c3759 951C3760 Novotny K. Turzikova A. Komarek J. Speciation of copper using Donnan dialysis and FAAS. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Anal. Chem. Masaryk Univ. Brno Czech Republic). Rychlovsky P. Krenzelok M. Volhejnova R. On-line simultaneous sorption preconcentration and determi- nation of Cr(II1) and Cr(1v) with AAS detection.10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Anal. Chem. Charles Univ. Albertov 2030 CZ-128 43 Prague 2 Czech Republic). Krenzelok M. Rychlovsky P. Brzakova S. Separation and determination of organotin compounds by HPLC with AAS detection. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Anal. Chem. Charles Univ. Albertov 2030 CZ-128 43 Prague 2 Czech Republic). Moskalova M. Zemberyova M. Hutta M. Resolution and determination of mercury and methylmercury in environmental samples by trace mercury analyser-TMA 254. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Anal. Chem. Fac. Nat. Sci. Comenius Univ.842 15 Bratislava Czech Republic). Boruvka L. Kristoufkova S. Kozak J. Cd Pb and Zn content in plants grown on heavily contaminated soils. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Dept. Soil Sci. and Geol. Czech Univ. Agric. Prague Czech Republic). Kombercova V. Rejnek J. Novobilsky V. Human hair - information source about loading of human organism by arsenic. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16,1995 (Teacher Training Fac. Univ. J. E. Purkyne 400 96 Usti nad Labem Czech Republic). Fisera M. Rosenberg M. Hladky Z. Kristofikova L. Determination of selenium in yeast by atomic spec- trometry methods. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Inst. Mater. Tech. Univ. Bmo Veslarska 230 637 00 Brno Czech Republic).Korunova V. Selecka A. Determination of selenium and mercury in blood serum in districts Prague-East and Jindrichuv Hradec. 10th Spectroscopic Conference Lanskroun Czech Republic June 14-16 1995 (Inst. Anal. Chem. Lab. Trace Element Anal. Acad. Sci. 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Harrison W. W. Comparison between direct current and radio- frequency glow discharge mass spectrometry for the analysis of oxide-based samples. J. Anal. At. Spectrom. 1995 10 689. (Dept. Chem. Univ. Antwerp (UIA) Universiteitsplein 1 B-2610 Antwerp Belgium). 358 R Journal of Analytical Atomic Spectrometry November 1995 Vol. 10

ISSN:0267-9477    

DOI:10.1039/JA995100329R

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Characterization of Ionization and Matrix Suppression in Inductively Coupled CCold' Plasma Mass Spectrometry* Journal of Analytical Atomic Spectrometry SCOTT D. TANNER SCIEX 71 Four Valley Drive Concord Ontario Canada L4K 4V8 A parametric study of plasma power and central gas flow was carried out to study the transition from normal analytical conditions to cooler plasma conditions using an inductively coupled plasma mass spectrometer having a balanced load coil. 'Cold plasma' conditions (low power and high central gas flow) permit the determination of K Ca and Fe at trace levels. The effect of changing the position of the ground reference of the load coil was investigated. Trace element ionization is consistent with thermal ionization at low electron density. Ion- molecule chemistry (charge transfer) with NO+ or 02+ may be important at the cooler plasma temperature.Suppression of analyte signals by concomitant matrix elements is partially correlated with the ionization potentials of the matrix element. If the analyte ion signals are normalized to that for NO' the suppression of signals appears to be independent of the matrix element and a modest dependence on the ionization potential of the analyte element is apparent. For high concentrations of elements of low ionization potential an additional or enhanced mechanism of ionization is evident. The onset for this enhanced ionization is sharply defined by a characteristic ionization potential near 6.0 eV. The sensitivity for trace elements does not appear to be affected by this enhanced ionization.The appearance of the enhanced ionization is made evident by a change in the ratio of the NO+ and 02+ signals. Use of cold plasma conditions for the determination of K Ca and Fe in high-purity waters and acids is evident. It appears that the method may also be used for samples having moderate salt content if the analytical protocol includes measurement of the background ions NO+ and 02+. Keywords Inductively coupled plasma; mass spectrometry; secondary discharge; matrix efect ; easily ionized element eflect The appearance of oxide ions of refractory elements was observed in the earliest reports on the performance of induc- tively coupled plasma mass spectrometry (ICP-MS).ly2 With the boundary sampling used these oxide ions could dominate over atomic ions. The appearance of polyatomic ions can compromise the determination of isobaric (having the same mass-to-charge ratio) elements.When the interference appears at the most abundant isotope of the element to be determined the analyst may have to resort to measurements at a less abundant isotope with a concomitant loss of sensitivity. Progress in the reduction of these interferences has been substantial (for example through interface design3 and the use of mixed gas plasmas4) but polyatomic ions associated with primary plasma ions remain a problem. One of the most significant of these interferences is that of ArO" with the dominant isotope of Fe+ at m/z=56. Addition of N2 to the plasma gas can substantially reduce the interference at m/z= 56 but increases the interference at the less abundant isotope 54Fe+ by ArN+.4 In addition the primary plasma ions can interfere directly with the determination of certain elements * Presented at the 1995 European Winter Conference on Plasma Spectrochemistry Cambridge UK January 8-13 1995.(e.g. 40Ar+ with 40Ca+). Even for current instruments that show high abundance sensitivity (a measure of the residual signal at 1 u lower than the mass of a dominant ion which is a limitation in mass spectral resolution) the very large signal for 40Ar+ can interfere with the dominant isotope of K+ at 39 u. In addition the measurement at m/z = 39 is also compli- cated by the presence of 38Ar1H+ and that at the other important isotope of K is interfered with by 40Ar1H+. One solution to this isobaric interference problem is to analyse the sample under high mass spectral resolution (e.g.a magnetic sector mass ~pectrometer).'>~ At a resolution of m/Am=2500 the signal for ArO+ is resolved from that for 56Fe+ at about half peak height. Where the Fe is to be determined at very low trace levels yet higher resolution is required. An alternative approach for certain elements appli- cable to quadrupole-based instruments is to operate the plasma source under conditions of lower power and higher nebulizer flow (and perhaps increased sampling depth and aerosol desolvation). This approach was first reported by Jiang et ~ 1 . ~ for the determination of K isotope ratios. They observed that under these conditions the background mass spectrum changed from one dominated by Arf species to one dominated by NO'.This opened the possibility of determining the K isotope ratios without substantial interference from Ar + or ArH+. The calibration graphs obtained under these 'cooler' plasma conditions were found to show two linear segments intersecting at about 10 mg 1-1 (10 ppm) K. This change in response was ascribed to a change in the dominant plasma ion from NO+ to K+ that is a self-induced matrix effect at about 10ppm. Appearance of a matrix effect at such low concentrations suggests that these plasma conditions may be useful only for analyses of samples having a low salt content. The correlation of the matrix effect with the change of domi- nant plasma ion also inferred that the mechanism of ionization involved ion-molecule chemistry of the plasma ion.This is distinguished from normal plasma conditions for which ana- lyte ionization is dominated by electron impact,8 with ion- molecule chemistry (charge transfer) playing a significant role only in the formation of highly excited ion^.^^^^ The original report on using 'cooler' plasma conditions ended with a caution that such results appeared to be achievable only under con- ditions where a secondary discharge between the plasma and the sampling orifice was suppressed. Subsequently Sakata and Kawabata" recognized the impor- tance of this mode of operation for the determination of K Ca and Fe by ICP-MS. The insertion of a grounded metal shield between the load coil and the torch as suggested by Gray,12 attenuated the secondary discharge.Combining the 'Shield Torch' with conditions of lower plasma power higher nebulizer flow and larger sampling depth led to the suppression of polyatomic argide ions notably ArO+. That report com- pared the spectra with and without the electrostatic shield and mapped the response of certain background and analyte ions as a function of plasma power and carrier flow. Photographs of the region between the sampler and skimmer Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 905demonstrated that the electrostatic shield suppressed the dis- charge that was otherwise present. It was noted that suppres- sion of the secondary discharge was of itself insufficient to attenuate the argide species to useful levels; operation under cooler plasma conditions was also required.Hence two sources for the argide interferences were identified (1) derived from the plasma itself (ascribed to capacitive coupling between the load coil and plasma resulting in ionization of polyatomic argides) and (2) formed within the first stage of the vacuum interface owing to the secondary discharge. Nonose et all3 studied the formation of polyatomic ions within the ICP and in the micro-plasma between the sampler and skimmer both with and without an electrostatic shield of the sort described by Gray' and by Sakata and Kawabata." Polyatomic ions were classified into two groups the first included NO' 0,' and metal monoxide ions and the second included the argide polyatomic ions. It was concluded that the former ions were characteristic of the source plasma (ICP) while the latter were formed in a secondary discharge within the vacuum interface.The dissociation equilibrium for ArX + species was consistent with a plasma temperature of 3500 K which the authors took to indicate equilibration within the vacuum interface. At about the same time Uchida and Itox4 reported that the electrostatic shield discussed above did reduce the secondary discharge but that some evidence of the discharge remained more noticeably at a plasma generator frequency of 40 MHz than at 27 MHz. They suggest that the residual secondary discharge may be responsible for the lower analyte monoxide ion signals observed at the higher frequency. In addition they note that air entrainment at large sampling depths may also lead to the formation of oxide ions of analyte species.Douglas and French'' reported that the intensity of the secondary discharge was reduced for a centre-grounded ('bal- anced') load coil configuration. This configuration may be obtained by various means including physical grounding of the load coil or using a balanced Colpitts oscillator.16 The effects of reducing the secondary discharge were claimed to include reduction in the intensity of doubly ionized species a narrowing of the kinetic energy distribution reduction of ions derived from erosion of the sampling aperture and increased orifice lifetime. This work was undertaken to evaluate the operation of an ICP-MS instrument having a balanced load coil configuration without a physical shield under 'cold' plasma conditions to permit the determination of K Ca and Fe at trace levels.In particular the signals of analyte and plasma background ions were monitored as a function of the plasma conditions (power and central channel gas flow) and of the balance of the load coil. It was furthermore the objective to characterize the effect of concomitant elements on the response of analyte and plasma ions (k the matrix effect). Analyte species which were treated as either trace elements or matrix elements in separate analyt- ical solutions were selected having a range of ionization potential mass and heat of vaporization. It was intended that characterization of the matrix effect would help to delineate the mechanism of ionization in the plasma and the importance of particle vaporization on analyte response.EXPERIMENTAL Instrumentation Experimental results were obtained on a Perkin-Elmer SCIEX Elan 6000 ICP-MS instrument. Since this instrument has not been described in the literature a schematic diagram of the major components is given in Fig. 1 and a functional descrip- tion of these components is provided below. The plasma rf generator is free-running (meaning that the Turbomolecular Turbomolecular Roughing Pump pump pump Fig. 1 Schematic diagram of the Perkin-Elmer SCIEX Elan 6000 ICP-MS system used in this work. The function and operation of the various components are described in the text frequency is automatically varied to maintain tuning) at a nominal frequency of 40 MHz. The three-turn load coil is a component of the Colpitts oscillator a schematic diagram of which is shown in Fig.2. The plasma potential (the dc potential of the plasma resulting from capacitive coupling between the load coil and plasma) is a function of the position of the ground reference along the coil as has been shown by Douglas and French." The load coil is 'balanced' (that is the position of the ground reference along the load coil is optimized) by adjustment of the capacitor plate CP1. Moving CP1 towards CP2 (referred to here as a negative displacement) moves the ground reference down the load coil away from the sampler. Therefore the plasma potential is a function of the position of CP1 and by inference the intensity of a secondary discharge can be minimized (or enhanced) by changing the displacement of CP1. For the experiments involving adjustment of the plasma potential the stand-offs that normally hold CP1 in position were removed along one edge and a threaded insulated rod was attached to this edge.The threaded rod extended through a hole drilled through the generator housing and was held in place with a nut on the outside of the generator. The position of CP1 was adjusted while the plasma was operating by adjusting the length of the threaded rod extending outside the generator housing which therefore moved the edge of CP1 attached to the threaded rod with respect to the fixed plates CP2 and CP3. Since CP1 was fixed in position along one edge this motion caused CP1 to tilt relative to the fixed plates CP2 and CP3 rather than move plane-parallel to CP2 and CP3. The capacitor position was measured as the extension of the threaded rod out of the generator housing.For all other experiments the capacitor was adjusted to give the largest ratio of Na' signal from a 10 pg 1-l (10 ppb) Na solution to background 40Ar' signal measured for a plasma power of 600 W and nebulizer flow of 1.08 1 min-l (ie. the 'cold' plasma conditions described below). This condition was also found to yield the largest Rh' signal for a plasma power of 1200 W OSCILLATOR COLPlrrS rf choke H-l Front (towards LOAD COIL Ground J- reference a s x j - POWER SOURCE Fig. 2 Schematic diagram of the Colpitts oscillator used in this work. Capacitor plate CP1 is adjustable between plates CP2 and CP3 and determines the ground reference of the load coil. Moving CP1 towards CP2 (denoted as a change in the negative direction) moves the ground reference away from the front of the load coil 906 Journal of Analytical Atomic Spectrometry November 1995 VoZ.10and nebulizer flow of 0.77 1 min-' (Le. the 'normal' plasma conditions described below). The sample was delivered at a rate of 1 ml min-l via a peristaltic pump to a cross-flow nebulizer. The double pass spray chamber was mounted outside the torch box and was maintained at ambient room temperature (about 21 "C). The resultant aerosol was not desolvated. For most experiments the entire injector (central carrier) gas flow was passed through the nebulizer (injector flow = nebulizer flow). For experiments in which ion signals were measured as a function of injector flow a constant nebulizer flow of 0.45 1 min-l was passed through the nebulizer (hence maintaining more-or-less uniform nebulization efficiency) and a make-up flow was added to the injector through a T downstream of the spray chamber; hence the total injector flow was the sum of the fixed nebulizer flow and the variable make-up flow.The nebulizer flow rate was metered using the instrument-native flow controller which was cross-calibrated using an external digital flow meter and the make-up flow (when used) was measured using an external digital flow meter. The alumina injector (central gas) tube had an id of 2 mm at its exit to the torch. The plasma (coolant) and auxiliary (intermediate) Ar flows were preset at 15 and l.Olmin-' respectively using the instrument-native flow controllers with a primary Ar input pressure of 50 psi.The sampling depth (distance from the end of the load coil to the sampler orifice) was set to 9mm and was not adjusted. The sampler ( 1.1 mm diameter orifice) and skimmer (0.88 mm diameter orifice) cones were made of nickel. The spacing between the sampler and skimmer was 6.9mm (stan- dard for this instrument). The background pressure in the interface region was about 3 Torr. The high vacuum chamber is differentially pumped by turbomolecular pumps. The higher pressure chamber (about 8 x lov4 Torr) contains the entrance ion optics. These optics consist of a grounded shadow stop at the base of the skimmer cone a cylinder lens and a grounded differential pumping aperture. The shadow stop intercepts unvaporized plasma particles preventing their deposition downstream in the ion optics.It also serves as an on-axis ground potential reference for the extracted plasma. The voltage applied to the cylinder lens is automatically tuned using the system software and is typically ramped in concert with the measured ion mass. The voltage applied to the lens is typically within the range +3 to +1OV and is found to optimize linearly with measured (transmitted) ion mass. To a first approximation the optimum lens voltage is approximately the kinetic energy that the ions gain from the supersonic expansion through the interface;17 hence the optimum linear scanning of lens voltage with meas- ured ion mass. For the 'cold' plasma conditions described below it was found sufficient to maintain the lens voltage at a constant +3 V irrespective of measured ion mass. The energy bandpass of these ion optics is narrow about 3 eV at half-height.18 This bandpass is comparable to the width of the ion energy distributions. Ions having kinetic energies signifi- cantly higher or lower than the applied lens voltage are not efficiently transmitted.Optimum sensitivity is obtained for ions having a small distribution of kinetic energies centred at the applied lens voltage. Ions created in the secondary discharge if present have substantially wider energy distributions and higher kinetic energy than those obtained from the supersonic expansion.15 Therefore these ion optics do not efficiently transmit ions that are generated in a secondary discharge. A further discussion of the ion optics can be found in ref.18. The higher vacuum chamber (about 1.5 x low5 Torr) down- stream of the differential pumping aperture contains the quadrupole mass analyser and the ion detector. The pressure in this chamber is accurately measured using a Bayert-Alpert (hot) ionization gauge tube. It is found that the analyser chamber pressure is a function of the plasma conditions as is to be expected [a cooler plasma is more dense and both the speed of sound (relevant for the flow through the sampler) and the terminal velocity (relevant for the flow through the skim- mer) are lower and so the flow through the interface is increased3]. In fact a reasonable estimate of the relative plasma temperatures for different plasma conditions can be obtained by measuring the relative analyser chamber pressure since the ratio of the zero-corrected (corrected for de-gassing) pressures is very nearly the inverse ratio of the squares of the absolute source plasma temperature^.^^'^ The mass analyser is a quadrupole mass filter with a capacitively coupled ac-only prefilter to enhance ion transport into the mass filter.The ion detector is an ETP Model AF210-Ml8E active film discrete dynode detector. A single ion impacting the surface of the first dynode yields both an analogue and a digital signal each of these having separately adjustable (automatically by the system software) gain factors. Both analogue and digital signals are simultaneously measured and archived. The system software cross-calibrates the dual signals. The digital gain channel is automatically protected against excessive ion current by hard- ware feedback shutdown circuitry.For the experiments reported here the gain of the analogue channel was set to saturate the analogue counter at 2 x lo9 ion counts s-l yield- ing a dynamic range (on the fly in a single scan) of approxi- mately 10'. Solutions The results reported here required the analysis of a number of related solutions. The distilled de-ionized water (DDIW) for background measurements and dilution was prepared in-house by distillation in glass followed by passage through a three- cartridge Millipore de-ionizer. The DDIW was usually pre- pared freshly. The background spectra reported here were obtained for 0.1% nitric acid ('Baker InstraAnalyzed' J. T. Baker Phillipsburg NJ USA) in DDIW.Analytical solutions were prepared as 10 pg I-' (10 ppb) of each analyte by serial dilution with 1 % nitric acid from 1000 mg 1-1 (ppm) standard Li Be B Na Al K Sc Fe Co Zn As Se Rh Pd Cd Sn Sb W T1 and Bi (obtained variously from SPEX Industries Edison NJ USA; Mallinckrodt Paris KY USA; Fisher Scientific Fairlawn NJ USA; and J. T. Baker Solutions). A series of matrix solutions was prepared with an additional 1 3 10 30 100 or 300mgl-' (ppm) of one of the analyte elements in the final dilution. Data are reported here for Li Na Al K Sc Zn Rh Cd T1 and Bi as the matrix elements for a total of 60 matrix solutions plus one 'clean' solution. For the experiments in which the rf generator capacitor position was varied and for those in which ion signals were measured as a function of plasma power and injector flow the analytical solution contained 10 pg 1-l (10 ppb) each of Li Be B Na Mg Sc Co As Rh Ba Ce Tb W Pb and U in 1% nitric acid.Most of these analyte elements were run separately or in groups to ensure that polyatomic interelement interferences did not occur to a significant extent over the plasma conditions studied. Also several of the more abundant isotopes were measured for each element where possible. Thermodynamic data for the elements determined as analyte ions and/or as matrix elements are given in Table 1. The elements cover a wide range of ionization potential atomic mass and heat of vaporization. It might be expected that the element is vaporized in the plasma as a molecule and that atomization follows.It may therefore be more appropriate to consider the heat of vaporization of the molecule. However the identity of the anion is not obvious (nitrate or oxide in nitric acid solution?) and the heats of vaporization of elemental nitrates and oxides do not appear to be readily available. Journal of Analytical Atomic Spectrometry November 1995 VoZ. 10 907Table 1 Thermodynamic properties of elements studied31 Element K Na Li A1 U TI sc Bi Sn Rh c o Fe W B Pd Sb Cd Be Zn Se As Mg Most abundant mass/u 39 23 7 27 238 205 45 209 116 103 24 59 56 184 11 106 121 114 9 64 80 75 Ionization potential/eV 4.339 5.138 5.39 5.984 6.08 6.106 6.54 7.287 7.342 7.46 7.644 7.86 7.87 7.98 8.296 8.33 8.639 8.991 9.32 9.391 9.75 9.81 Heat of vaporization/ kcal mol - 18.88 23.4 32.48 67.9 38.81 80.0 41.1 55.0 127.0 31.5 93.0 84.62 75.0 89.0 46.665 23.86 27.43 14.27 - - - ICP-MS Optimization modes could be obtained by loading the plasma parameter file (including plasma power gas flows and lens voltages) without further operator intervention. In the course of the parametric study presented here the plasma temperature clearly varied substantially as the plasma power and injector flow were adjusted over the wide ranges investigated.Consequently the kinetic energies that the ions gained from the supersonic expansion also varied throughout the course of the parametric experiments. As noted above optimum ion transmission is obtained when the applied lens voltage is comparable to the ion kinetic energy. Therefore the lens voltage should properly have been adjusted for each plasma condition (power and injector flow) set including the determination of the optimum mass-dependent voltage ramp.Some 78 combinations of power and injector flow were studied for each replicate of the parametric study. Even with only one lens voltage to optimize the time required for the experiment would have become prohibitive. Therefore the lens voltage was set to + 3 V throughout the parametric study. This decision holds the ramification that the ion transmission was biased towards plasma conditions yielding ion energies of the order of 3 eV which are cooler than those used under normal analytical conditions. The result is that the parametric curves are biased towards lower power and higher injector flows than they would have been had the ion lens been optimized for each condition.Two standard plasma conditions corresponding to the 'normal' analytical and 'cold plasma' conditions were adopted for most of this work. The plasma and instrumental settings used are RESULTS AND DISCUSSION indicated in Table 2. Initial optimization for the 'cold plasma' conditions was obtained by first setting the plasma power and nebulizer flows setting the lens voltage to + 3 V (fixed) and then adjusting the x-y position (horizontal and vertical pos- ition of the plasma relative to the sampling aperture) for maximum Na + signal and minimum background Ar + signal. The width of the central channel appeared to be more narrow under 'cold plasma' conditions than under 'normal' plasma conditions. These optimum 'cold plasma' operating conditions were confirmed daily.This procedure reproducibly yielded the best analytical conditions for K Ca and Fe and the sensitivity obtained for these elements was reproducible day after day. The appropriate 'cold plasma' conditions typically yielded sensitivities of the order of 1.8 x lo6 counts s-' per ppm for Fe and 30 x lo6 counts s-' per ppm for Li with continuum back- ground signals of less than 5 counts s-l and 40Ar+ (40Ca+) signals for a 0.1% nitric acid solution of less than 3000 counts s - l (less than 1800 counts s-' for DDIW); these ion signals were considered the target for appropriate optimiz- ation. The x-y optimization of the plasma relative to the skimmer as described here required no adjustment when the plasma conditions were returned for 'normal' analytical con- ditions; after set-up the 'cold plasma' and 'normal' operation Table 2 ICP-MS operating conditions Plasma rf power/W Argon gas flow rate/l min-l Plasma (coolant) Auxiliary (intermediate) Nebulizer (central channel) Sampling depth/mm Nebulizer Spray chamber Sample uptake/ml min- ' Desolvation Lens voltage Normal plasma 'Cold' plasma 1200 600 15.0 15.0 1 .o 1 .o 0.77 1.08 9.0 9.0 Cross-flow Scott-type at room temperature 1.0 None Linearly ramped + 3.0 V with ion mass +3.0 V at 7Li+ to +9.0 V at 238U+ (all ion masses) Background Spectra The mass spectrum obtained under 'normal' operating con- ditions for 0.1 YO nitric acid is shown in Fig.3(a). This spectrum was obtained following the matrix study reported below and only a cursory washout of the sample introduction system was attempted; hence some residual signals for the matrix elements Li Na Al Sc and Zn are evident.The dominant ions in the spectrum are O+ and Ar+ at m/z=16 and 40 (and the less abundant Ar+ isotopes at m/z=36 and 38). Other important ions include ArH' and the lower mass ions OH' H20+ or l80+ etc. which presumably derive from the solvent (dilute acid). Of particular note are the large background signals for Ar2+ at m/z=80 and ArO' at m/z=56. The Ar-derived ions Ar' ArH' Ar2+ and ArO+ interfere with the detection of 40Ca 39K 80Se and 56Fe the most abundant isotopes of these elements. For many applications these elements (with the exception of K) can be determined at their less abundant isotopes. In some instances desolvation or mixed gas plasmas can enhance the determination of 56Fe+.However for certain applications such as the analysis of high-purity acids for the semiconductor industry sufficiently low detection limits cannot be obtained under normal analytical conditions. The mass spectrum for the same solution obtained under 'cold plasma' conditions is shown in Fig. 3(b). The dominant background plasma ions are NO' 02+ and H30+ with most Ar-related ions greatly reduced in intensity. The very substan- tial H30 + signal is of itself a strong indication that the plasma is cool. This spectrum differs from that given in ref. 7 with the addition of important peaks in the mass range 16-20u and the persistence of ArH+ at m/z=41. These ions appear to derive from the solvent as no desolvation was used in the present work.The signal remaining at m/z = 56 is thought to be due primarily to contaminant Fe in the water and acid as its magnitude varied with the water distillation batch and with the source and concentration of the acid. This observation of variance in the background at m/z = 56 is the reason the ArO+ signal was not used as a determinant for appropriate 'cold plasma' optimum conditions. The relatively large signal at m/z=80 ascribed to ATz+ seems peculiar since the other 908 Journal of Analytical Atomic Spectrometry November 1995 Vol. 101o'O lo9 108 10' 106 lo5 lo4 lo3 lo2 r '; 10' 5 1 0 0 0 - 104,. 10-5 10' 106 lo5 lo4 lo3 lo2 10' 100 * [ej = 4 x 1 0 ~ ~ ; r = re- = 3500 A A . . . I . 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Mass Fig.3 Spectra of the plasma background ions for a sample containing 0.1% HNOJ obtained under (a) the 'normal' and (b) 'cold' operating conditions given in Table2. The data were obtained using a simul- taneous dual-mode (pulse counting/analogue) detector. The analogue channel was set to saturate at 2 x lo9 counts s-'; the signals for Ar' ArH' and 0' have saturated the analogue channel for the higher power condition. Note that the vertical scales are logarithmic. Other than the plasma power and injector (carrier or nebulizer) gas flow the plasma conditions (including sampling depth sample introduction and load coil configuration) were the same for the two spectra argide ions (except for ArH') have been very substantially attenuated or eliminated. In a separate experiment this ion also showed an apparently unique dependence on spray chamber temperature.Whereas almost all the other ions (background atomic and polyatomic ions as well as analyte atomic ions) showed little variation with spray chamber tem- perature the m/z=80 ion varied by nearly two orders of magnitude over a temperature range from 0 to +44"C decreasing in intensity as the temperature was increased. It is notable also that the relative intensities of NO+ and 02' were a function of the nitric acid concentration with NO' increasing in magnitude at the expense of 02+ as the acid concentration was increased. For nitric acid concentrations below about 0.3% the increase in the NO+ signal is approxi- mately linear with acid concentration.Above this acid strength the NO' signal increases more slowly. Over the range 0.005-4% nitric acid the sum of the NO' and 02' signals remains relatively constant. It is concluded that the major source of NO+ under the experimental conditions was from the nitric acid rather than from air entrainment and that NO' derives from an ion-molecule reaction involving 02'. Finally the level of ionization in the source plasma appears to be 2-3 orders of magnitude lower under 'cold plasma' conditions relative to normal analytical plasma conditions. This con- clusion is drawn from a comparison of the total ion signals measured for the two plasmas which differ by about this magnitude (1O1O uersus 4 x lo7 s-l). With the extracted ion current reduced substantially it is to be expected that space charge effects in the ion optics are much less important.One of the effects of space charge is to reduce ion transmission efficiency,1g920 and so the ratio of the levels of ionization within the ICP is likely to be greater than the measured total ion signal ratio. Another important result of the reduction of the ion current and hence space charge is that space charge- related matrix effects should be much less significant for the 'cold plasma'. The pressure in the analyser chamber under 'normal' analyt- ical conditions was approximately 1.2 x Torr (corrected for zero-flow de-gassing) while under 'cold plasma' conditions this rose to approximately 2.3 x Torr. Taking the gas kinetic temperature of the source plasma under normal con- ditions as 5300K,21922 the ratio of pressures suggests a 'cold plasma' gas kinetic temperature of about 1450 K.This tempera- ture relates to the plasma directly in front of the sampling aperture which is extracted through the sampler. It is substan- tially lower than the dissociation equilibrium temperature for ArX' species determined by Nonose et all3 with a shielded torch configuration. This suggests that under conditions where the secondary discharge is suppressed the argide ions could derive from the source plasma rather than from within the interface. The cooler plasma temperature helps to explain the difference in lens optimization for the two plasma conditions. When the plasma temperature is cooler the mass-dependent kinetic energy gained from the expansion is less and the slope of a plot of kinetic energy uersus ion mass is smaller.22 Since the voltage applied to the lens appears to be comparable to the ion kinetic energy the reduced mass-dependence of the ion kinetic energy resulting from the cooler plasma results in a lower slope of optimum voltage with mass.For the cold plasma conditions reported here the slope was sufficiently small that a static lens voltage (+ 3 V) provided approximately optimum focusing for ions of all masses. Equilibrium ionization efficiencies for the elements have been calculated by H o ~ k ~ ~ assuming an electron density of 1015 cm-3 and ionization and electron temperatures of 7500 K. Polynomial fits of the electronic partition functions over the temperature range 1500-7000 K for most of the elements listed in Table 1 have been tabulated.24 Calculated degrees of ioniz- ation (DOI) using these partition functions extrapolated to the 7500K temperature assumed by H o u ~ ~ ~ are shown in Fig. 4 where the results are plotted against ionization potential.Similar calculations assuming that the ionization and electron temperatures are equilibrated at the gas kinetic temperature of 1450K derived above yield exceedingly low DOIs. The electron density is important in the calculation of equilibrium ion densities according to the Saha equation and reflects the rate of recombination which reduces the degree of ionization. At high electron density ( lo1' ~ m - ~ ) and low temperature 4 5 6 7 8 9 10 Ionization potentiallev Fig. 4 Degree of ionization plotted as a function of ionization poten- tial for two different plasma conditions of electron number density [e- 1 ionization temperature To and electron temperature T,-.The electronic partition functions used were taken from ref. 24 and were strictly valid only for the temperature range 1500-7000 K Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 909(1450 K) the rate of recombination is much greater than the rate of ionization and the DO1 is consequently greatly reduced. Measured values of the electron density corresponding to the 'cold plasma' conditions have not been reported. Conventional optical methods appear to be limited to electron densities above about lOI3 ~ m - ~ . Values of this magnitude have been reported low in the plasma (7 mm above the load coil) for a 1.25 kW Ar ICP at relatively high central gas flow (1.2 1 min-1)?5 A similar value has been reported for a 0.9 kW plasma at a central gas flow of 1.0 1 min-' and 5 mm above the load It may be expected that under 'cold plasma' conditions where the plasma power is substantially less than that used for the above electron density measurements the electron density will be significantly lower.Furthermore the electron and ionization temperatures are probably not equilib- rated with the gas kinetic temperature. For example under the conditions for which the electron density noted above was measured the electron and gas kinetic temperatures were found to be approximately 3500 and 2700 K re~pectively.~~ The gas kinetic temperature derived from the ratio of pressures is an average temperature for the bulk plasma and other work has shown that the ions may be concentrated in the vicinity of a vaporizing particle.27 Therefore the ionization and electron temperatures may differ significantly from the bulk gas kinetic temperature.Calculated DOIs corresponding to an electron density of only 4 x 10" cm-3 and equilibration of the ioniz- ation and electron temperatures at 3500 K are also presented in Fig. 4. The temperature was chosen assuming local equilib- rium at the ArX' dissociation temperature determined by Nonose et (note that this temperature has been applied to the ICP rather than to the vacuum interface plasma) and the electron density was chosen to yield results which mimic the ionization potential-dependence of the sensitivity data presented below.Under these conditions Fe (ionization poten- tial = 7.87 eV) is approximately 15 % ionized. For the normal plasma the flow through the skimmer is about 1 x lOI9 s - ' . ~ ~ ' For a plasma temperature of 5300 K this corresponds to a sample of about 7.2cm3 s-' from the ICP. If the electron (and ion) density in the ICP is about l O " ~ m - ~ this corresponds to an electron (and ion) flow through the skimmer of about 7 x s-'. The total measured ion current at the detector is of the order of 10" s-' suggesting an over-all transmission efficiency from the skimmer tip to the detector of about Note however that the transmission efficiency for analyte ions is better than this. A fully ionized element of mass 56 u at a concentration in the original nebulized solution of 1 ppm contributes about 1.4 x 10" ions cm-3 to the plasma (assuming a nebulization efficiency of 2% a sample uptake rate of 1 ml min-' a plasma temperature of 5300 K and uniform distribution throughout the central channel at an aerosol gas flow rate of 0.771rnin-').This sample should then yield a flow of about 10" analyte ions s-' through the skimmer. With a sensitivity of about 25 x lo6 counts s-' per ppm (for Fe') this suggests a trans- mission efficiency for Fe' ions of about 3 x and in turn suggests an improvement in transmission efficiency for analyte ions of about 300 relative to that for the background ions (i.e. a corresponding loss mechanism for the background ions). This factor of 300 is consistent with the ion current measure- ments of Gillson et aL2* downstream of the skimmer for which the measured differential U+ current for 0.04 mol U 1-' was the same as the calculated U+ current through the skimmer aperture whereas the total ion current was reduced to 6 pA from the calculated 1500 PA.If the 1 ppm Fe solution is nebulized with the same efficiency into the 'cold plasma' (1.1 1 min-' aerosol gas flow rate and plasma gas kinetic temperature of 1450K) and were 15% ionized (as calculated for an electron density of 4 x 10'O cmV3 and local thermal equilibrium of the ionization and electron temperatures at 3500 K) it would contribute about 6 x lo9 ions cm-3 to the plasma. The sensitivity observed for Fe was 1.8 x lo6 counts s-' per ppm. This corresponds to about 5% of the total ion current measured (4 x lo7 counts s-').If the transmission efficiency for Fe' is the same as for the plasma ions (i.e. if there is no preferential loss of background ions) then the total ion density in the plasma should be about (4 x 107/1.8 x lo6) x 6 x lo9 z 10" ~ m - ~ which is close to the assumed electron density. For the 'cold plasma' the increase in the operating pressure suggests that the flow through the skimmer is doubled to about 2 x loi9 s-'. For an average gas kinetic temperature of 1450K this corresponds to a sample of about 4cm3 s-l from the ICP. For an electron (and ion) density in the ICP of 4 x 1O1O cmP3 this corresponds to an electron (and ion) flow through the skimmer of about 1.6 x 10" s-'. The over-all transmission efficiency is then about 3 x approximately 300 times improved over the normal plasma.This improvement in over-all transmission efficiency may result from reduced space charge effects because the ion current is r e d ~ c e d . ~ ' . ~ ~ The transport efficiency of the analyte Fe' under 'cold plasma' conditions is then approxi- mately the same as in the normal plasma and the ratio of sensitivities (1.8 x lo6 versus 25 x lo6 counts s-' per ppm) is primarily accounted for by the reduced degree of ionization in the 'cold plasma'. These crude estimates of transmission efficiency for the cold plasma clearly depend on the assumed electron density (both for the direct calculations and for the DO1 used). It is worth noting that if the electron density and temperature are this low then the Debye radius at the skimmer is compar- able to the skimmer aperture diameter which may have important ramifications for the dynamics of ion transport through the interface.20 Parametric Study of Plasma Power and Injector Flow The variation of dominant plasma ions and trace analyte ions as a function of injector flow (with a constant 0.45 1 min-' introduced through the nebulizer) at a plasma power or 1200 W is shown in Fig.5. The Ar' signal decreases as the injector flow increases presumably reflecting the decrease in the plasma temperature owing to the translation of the normal analytical zone towards (and past) the sampling orifice. This decrease in plasma temperature is further evidenced by the increase in polyatomic argides (Ar2' ArH' and ArO') with injector flow.At very high injector flows an increase in NO+ and 02' is observed. As noted above the relative intensities of NOf and 02+ are a function of the nitric acid concentration suggesting that the dominant source of the NO' is the acid introduced with the sample. Other data presented here suggest that the change in dominant ion is a result of the cooling of the plasma with the concomitant survival (or production) of neutral molecular species including NO and 02. The various analyte ions optimize at more-or-less the same injector flow nearly independent of ionization potential or mass. There may be a trend favouring higher injector flows for lower mass ions which might reflect mass-dependent radial diffusion within the plasma.30 This effect would result in a higher density of low mass ions slightly earlier in the plasma at higher injector flow.The optimum injector flow observed for analyte ions in Fig. 5(b) is higher than the optimum indicated for 'normal' plasma conditions in Table 2. This is a direct result of having fixed the lens voltage at + 3 V for this experiment as discussed under Experimental. The uniformity of analyte ion optimiz- ation with injector flow shown in Fig. 5(b) is in contrast to the element-dependent optimization observed by Sakata and Kawabata (cJ Fig. 5 of ref. 11 obtained without the ShieldTorch). Comparison of these results indicates that the occurrence of a secondary discharge perturbs the distribution 91 0 Journal of Analytical Atomic Spectrometry November 1995 Vol. 101.2 1 .o 0.8 0.6 0.4 - 2 0.2 5 0.0 0 v) .- .- 0.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Injector fiow/i min-' Fig.5 (a) Variation of plasma background ions with injector (carrier or nebulizer) gas flow at 1200 W plasma power. The injector flow includes 0.45 1 min-' passed through the nebulizer and an adjustable make-up flow added downstream of the spray chamber. (b) Variation of trace elemental ions under the same conditions of ions either within the plasma or during extraction; a low plasma potential has the advantage of a more uniform ioniz- ation region hence more uniform optimization. Corresponding data obtained at 600 W are given in Fig. 6. At this low plasma power there is a very clear separation of Ar-derived plasma ions (at lower injector flows) and NO+ and 02+ (at higher injector flows).The optimization of analyte species now also shows a strong dependence on injector flow with elements having higher ionization potentials or heats of vaporization appearing at lower injector flows and more easily vaporized and ionized elements appearing at higher injector flows. The correlation with plasma temperature is easy to draw but there may be an over-riding dependence for the low ionization potential elements on the concomitant appearance of NO' and 02+ as dominant plasma ions. It is possible then that the separation on the basis of ionization potential may be due to plasma temperature (thermal ionization or electron impact ionization) or it may be due to a change towards dominance of ion-molecule reaction ionization (charge transfer) of the lower ionization potential elements with NO + and 02+ as those ions become important at lower temperature.The W' ion optimizes only at lower injector flows despite having a moderate ionization potential. The heat of vaporiz- ation for W is not available. However the melting-point of W is very high (3650 K3'). It is probable that the hotter plasma conditions at lower injector flows are required to vaporize and atomize this element. Some of the elements show local maxima at both low and high injector flows. The increase in response at low injector flows for Rh+ and Co' species may reflect their relatively high heats of vaporization (the highest of those known for the suite of elements investigated). The Mg' ion also shows a minor local maximum at low injector flow.This might suggest that Mg can be vaporized as either the atom (which has a low heat of vaporization and may appear at .N 1.2 Q - E g 1.0 0.8 0.6 0.4 0.2 0.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 Injector fbwA min-' Fig. 6 (a) Variation of plasma background ions with injector (carrier or nebulizer) gas flow at 600 W plasma power. The injector flow includes 0.45 1 min-l passed through the nebulizer and an adjustable make-up flow added downstream of the spray chamber. (b) Variation of trace elemental ions under the same conditions higher injector flow rates) or as the oxide MgO (which has a high melting-point and may therefore be vaporized only at lower injector flow rates). The appearance of two local maxima (with respect to injector flow) of the same atomic analyte ion may indicate either a change of source of ionization or a change in the source of vaporization and atomization.The intermediate behaviour of Be having a high ionization poten- tial is difficult to explain. These data are comparable to the ShieldTorch data presented in Fig. 6 of ref. 11 with the import- ant addition here of the rise to dominance of NO+ and 02+ at high injector flow. Nonose et ~ 1 . ' ~ also observed the distinc- tion of the argide species from NO' and 02+ on the basis of injector (carrier) gas flow under conditions where the secondary discharge was minimized although that work was performed at higher plasma power. The variation of ion signals with plasma power at an injector flow of 1.10 1 min-' (corresponding to the 'cold plasma'injector flow) is given in Fig.7. The argide ions are important at higher plasma powers and the oxides and molecular ions at lower powers. The Ar2+ ion shows a bimodal optimization appearing at both high and low plasma power perhaps reflecting its source (Ar') at higher power but its persistence owing to less fragmentation at lower power and temperature. Again the more easily vaporized and ionized elements optimize at lower plasma powers corresponding either to lower plasma tempera- tures or to the appearance of NO' as a reactant ion. Elements having a higher ionization potential or vaporization energy appear at higher plasma powers. The behaviour of the analyte ions shown here notably the appearance of W' only at higher power and the optimization of Be+ at intermediate power correlates with their responses to injector flow in Fig.6(b). The decrease in the Ar+ signal as the injector flow is increased or the plasma power is decreased may be due to the cooling of the plasma. The increase of the polyatomic argide ions (ArO' and Ar2') at intermediate injector flow or power Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 91 1500 600 700 800 900 1000 1100 1200 1300 1400 1500 Plasma powerMl Fig.7 (a) Variation of plasma background ions with plasma power at injector (carrier or nebulizer) gas flow of 1.10 I min-l. The entire injector flow passed through the nebulizer and no make-up gas was added. (b) Variation of trace elemental ions under the same conditions may reflect the combined effects of the reduced source ions (0' and Ar') and the persistence of the polyatomic ions resulting from their reaction with Ar at the lower plasma temperature.These effects have opposite temperature depen- dence and could give rise to the appearance of the ionic species at intermediate plasma temperatures. It could be postu- lated that the appearance of NO+ and 02+ at even cooler plasma conditions is a result of the production or persistence of neutral H20 NO or 02 and their subsequent ionization by electron impact. Primary ionization of NO or 0 by electron impact may lead directly to NO+ and 02+. Primary ionization of atoms (H or 0) may be followed by ion-molecule reactions leading to the prominence of NO+ and 0,' accord- ing to H++02+02++H (k=l.l7x 10-9)32 H+ +H20-+H20+ +H (k=8.2 x O+ +H20-+H20+ +O H20+ +02+02+ +H20 H20+ + H20+H30+ +OH (k=2.3 x 10-9)33 ( k z 2 x 10-10)34 (k= 1.7 x 10-9)33 H30+ +NO +X+NO+ + H20 +XH 0,++N0-+NO++O2 ( k = 4 .5 ~ 1 0 - ~ ' ) ~ ~ 0,' +N+NO+ +O (k= 1.2 x where the known rate constants k are given in units of cm3 molecule- s- at room temperature. The reaction forming NO+ from H30+ is a two-step process involving the formation of an intermediate adduct ion and may involve a radical (X = OH 0 or H).37 These reactions have been postulated to be important in flame^^'.^^ for which the flame temperature is of the order of the 'cold plasma' temperature inferred above. The intermediates H,O+ and H30+ are observed as major ions in the 'cold plasma' mass spectrum of Fig. 3(b). Whether NO+ and 0,' are formed by primary electron impact ionization or through subsequent ion-molecule reaction chemistry the very large signal for H30+ which can only be produced by proton transfer indicates that ion-molecule chemistry does play an important role in determining the ionic composition of the 'cold plasma'.Charge transfer ionization of metal atoms by NO+ or 02+ is exothermic for atoms having ionization potentials less than those of NO or O2 (9.26436 and 12.071 eV re~pectively~~). Because these ions are molecular they have multiple internal degrees of freedom (rotation and vibration) that relax the requirement for electronic energy level resonance that charac- terizes atomic ion-atom charge transfer reactions. Therefore NO + and 0,' could behave as relatively indiscriminate charge transfer reactant ions for metal atoms.In flames however the primary source of alkali atomic ions is dissociative charge transfer with H30+ according to4' H30+ +A+A+ +H,O + H This reaction is exothermic for atoms (A) having ionization potentials less than 6.4 eV (being the difference between the ionization potential of H 13.595 eV,31 and the proton affinity of H20 7.22eV41). The importance of this reaction in flames is due in part to the preponderance of H 3 0 + . For most flames NO+ and O,+ are much less abundant and their role in elemental ionization is less well known. By direct analogy with flames it is to be anticipated that ion-molecule chemistry principally charge transfer involving H,O+ NO+ and 0,' can provide an important source of ionization for elemental atoms in the 'cold plasma'.Sensitivity Under normal plasma conditions sensitivity is a reasonably smooth function of analyte mass increasing with mass as shown in Fig. 8 where the sensitivity has been corrected for natural abundance of the isotopes. The ion density in the plasma is expected to be a function of the molarity of the solution although in ICP-MS the sensitivity is often quoted in units of mass/volume. The decrease in sensitivity at low mass is partially due to the enhanced radial diffusion of low mass analyte elements in the plasma,3o but is probably more significantly influenced by space charge effects downstream of the skimmer aperture.19i42 As can be seen in Fig. 8 sensitivity under 'cold plasma' conditions does not appear to show a strong mass dependence.The data of Fig. 8 are presented in Fig. 9 as a function of the ionization potential of the analyte elements. Significant scatter is observed for data obtained under normal plasma conditions reflecting the over-riding 0 50 100 150 200 d z Fig. 8 Sensitivity (corrected to 100% abundance) for trace elements as a function of atomic ion mass-to-charge ratio. Open circles were obtained under normal plasma conditions and filled circles were obtained under 'cold plasma' conditions 91 2 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10(4 x lo1' cmd3) and ionization temperature (3500 K). In fact there is a relatively restricted range of ionization temperature and electron density that is consistent with the observed response curve assuming equilibrium thermal ionization.Assuming that elements having ionization potentials below 6 eV are completely ionized the data of Fig. 9 suggest that an element having an ionization potential of 8 eV is between 5 and 40% ionized and an element with an ionization potential of 9.5 eV is 0.02-0.5% ionized. The ionization temperatures and electron densities that would provide these degrees of ionization can be calculated. These results are presented as the four curves of Fig. 10 where the partition functions of the atom and ion have been assumed to be equal in order to simplify the calculation. The region bounded by these curves indicates the ranges of ionization temperature and electron density which could result in the observed response curve. These ranges can be further restricted by recognizing that the maximum electron density cannot exceed approximately 4 x 10l2 cm-3 (being the ratio of the total ion signals measured for the cold and normal plasmas multiplied by the known electron density of about 10'' cmT3 under normal plasma conditions).Furthermore the electron density cannot be less than about 108cm-3 being the total ion signal measured under cold plasma conditions (4 x lo7 s-l) multiplied by 10 (assuming that the transmission efficiency of the quadrupole is and that this ion flow also represents the minimum number of electrons per second extracted through the skimmer) divided by the neutral gas flow extracted from the plasma through the skimmer (about 4 cm3 s-l as derived earlier). The ionization temperature characterizing the cold plasma response curve is then in the range 2900-4400K (indicated by the shaded region of Fig.10). Again this ionization temperature range is consistent with the dissociation equilibrium tempera- ture for ArX' ions determined by Nonose et all3 It could therefore be argued either that trace element thermal ionization is approximately equilibrated at low charge density or that the mechanism of trace element ionization changes to ion-molecule charge transfer under the cooler plasma conditions. The latter mechanism can explain the bimodal optimization of several of the elements [Figs. 6(b) and 7(b)] on the basis of the change of mode of ionization. 4 5 6 7 8 9 10 Ionization potentiaVeV Fig. 9 Sensitivity (corrected to 100% abundance) for trace elements as a function of ionization potential of the trace element.Open circles were obtained under normal plasma conditions and filled circles were obtained under 'cold plasma' conditions effect of ion mass. Little explicit dependence on ionization potential is observed for normal conditions at least for ioniz- ation potentials <10eV. On the other hand the sensitivity obtained under 'cold plasma' conditions is clearly a function of ionization potential with a significant decrease in sensitivity for ionization potentials above about 8eV. It is notable that the sensitivities for T1 and Bi (high mass) are comparable to those of Li Na and K (low mass) at least within a factor of about four on a molar basis; these elements all have ionization potentials <7.5 eV. The much reduced mass bias (relative to normal conditions) supports the expectation that space charge effects should be less significant for the lower ion current condition. The improved mass bias may also derive in part from reduced radial expansion of the central channel and mass-dependent diffusion in the ICP because the plasma is cooler and sampled earlier relative to the initial radiation zone. It was observed that the sensitivity to trace elements (e.g.Fe Ca K Na and Li) under 'cold plasma' conditions was insensi- tive to the concentration of the acid at least for acid concen- trations below 1 YO. Since the relative proportions of NO' and 02' are a function of the nitric acid concentration this observation suggests that chemical ionization by charge trans- fer with NO+ is not the determinant for sensitivity or might suggest that NO' and 02+ behave equivalently as charge transfer reactant ions.Cooler plasma conditions promote the formation of polya- tomic ions notably the oxide ions of refractory elements. The sensitivity shown for Sc (45 u ionization potential = 6.54 eV) is lower by several orders of magnitude than that expected on the basis of its ionization potential partially because it appears almost exclusively as ScO'. The data given in Figs. 8 and 9 report measurements only for the atomic ion and the elements determined were chosen as being relatively poor oxide-formers. The anomalously low sensitivity to W is probably due to inefficient vaporization as discussed above and noted by others under similar conditions." Oxide or other polyatomic ions of W were not observed at significant intensity.Data given in Figs. 6(b) and 7(b) show that W is efficiently determined only under conditions yielding higher plasma temperatures (higher power and lower injector flow). For elements having ionization potentials below about 6 eV the sensitivities obtained under 'cold plasma' conditions are comparable to those obtained under normal plasma conditions. For elements having ionization potentials between 7 and 8 eV the cooler plasma conditions yield sensitivities that are up to two orders of magnitude lower and this discrepancy increases to 3-4 orders of magnitude above 9 eV. The general form of this response curve was simulated assuming equilibrium ioniz- ation in Fig. 4 for conditions of rather low electron density 4000 c C 3500 5 3000 .- w i Maximum [e-] from ratio of total ion signals I I I I *-*I __- = mi *I- ___- I ' 2500 Electron density/cm* Fig.10 Calculated ionization temperature To, and electron density [e-1 ranges that are consistent with the observed 'cold plasma' response curve assuming thermal equilibrium ionization. The curves indicate the Ton and [e-] which would result in 5 and 40% ionization of an element having an ionization potential (IP) of 8.0 eV and 0.02 and 0.5% ionization of an element having IP = 9.5 eV. The calculations assume that the electron and ionization temperatures are equilibrated. These degrees of ionization bound the observed responses shown in Fig. 9. The minimum and maximum electron densities have been derived as discussed in the text.The shaded range is consistent with the observed response curve. Because the elements having these ionization potentials are generic the electronic partition functions of the atom and ion have been assumed to be equal Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 91 3There does not appear to be a discontinuous change in sensitivity near 6.4 eV and so charge transfer with H30+ does not appear to be a dominant ionization mechanism (at the least it will not apply to elements having ionization potentials greater than 6.4eV). Charge transfer with NO+ and 0,' remains a potential source of ionization. However this ioniz- ation mechanism does not explain the change in sensitivity that occurs near 8 eV which is consistent with thermal ionization.Load Coil Ground Reference and Secondary Discharge The earlier reports on low power/high injector flow oper- ation7*11*13 have all indicated the requirement for reduction or elimination of the secondary discharge to facilitate reduction of the Ar-related background. For the instrument used in this work the plasma potential is maintained at a low level to minimize the secondary discharge under all operating con- ditions by appropriate ground-referencing of the load coil. This ground reference is determined in part by the position of the capacitor plate CP1 relative to CP2 and CP3 in the Colpitts oscillator (Fig. 2). As CP1 is moved towards CP2 the ground reference is moved along the load coil away from the front of the load coil (adjacent to the sampler orifice).Adjusting the capacitor in this manner is expected to increase the plasma potential thereby enhancing the formation of a secondary discharge. The data in Fig. 11 were obtained by adjusting the position of CP1; the position denoted '0' is that used in normal operation to minimize the secondary discharge and provide optimum sensitivity. The plasma dc potential for this position of CP1 inferred from the intercept of ion kinetic energy versus ion mass for a similar ICP-MS system (Elan 5000) is approxi- mately + 3 V.22 For that instrument which shows less discrimi- nation against high energy ions the plasma dc potential was observed to increase non-linearly with a negative displacement of CP1. At a position thought to yield a strong secondary discharge the plasma dc potential was +22 V.In the present work moving the ground reference away from the sampler (in the negative direction) results in an increase in sensitivity for Co but a larger increase in the background signals for Ar' and ArO ' . The position for optimum determination for Fe ' defined as the maximum in the ratio of Fe+ ArO' (approxi- mated as Co' ArO') is close to the '0' position. The relatively monotonic increase in signals with the change in CP1 position to negative values is abruptly discontinuous near the position marked - 10 mm. At this point all the background ion signals dramatically increase with the exception of NO' which decreases markedly in concert with the analyte ion Co' (which is typical for that of atomic elemental ions derived from the -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Capacitor positiodmm Fig.11 Analyte ion (Co') and plasma background ion (Ar+ ArOf NO+ and 02+) signals as a function of the position of the Colpitts oscillator capacitor plate CP1 (see Fig. 2) which determines the position of the ground reference along the plasma load coil sample). The data are interpreted to indicate a gradual increase in the intensity of a secondary discharge (or an equivalent effect) as the capacitor position is adjusted to move the ground reference away from the sampler with a sudden onset of a strong discharge near - 10 mm. The dramatic reduction in the signals for NO+ and Co+ at this point is probably due to a sudden change in ion kinetic energy resulting from the second- ary discharge.13 It was noted above that ion transmission through the ion optics is a strong function of ion kinetic energy with optimum transport for ions having kinetic energies comparable to the applied lens voltage.The reduction in the NO' and Co' signals probably reflects an increase in ion energies above the bandpass of the optics rather than a relative decrease in the ion densities in the plasma. At each position of CP1 an attempt was made to optimize the lens voltage for ion transmission. The optimum voltage was found to remain constant for Co' throughout the range of this experiment while that for Ar' was found to increase (from +3 to about + 6 V at the -9.5 mm position although the results reported were obtained at a constant - 3 V). The optimum lens voltage appears to be approximately equivalent to the ion kinetic energy,18 suggesting that the ion energy for Co' did not change significantly through the range of this experiment while that for Ar' did change somewhat.However the range of voltages available was limited to less than + 17 V. If the ion energy distribution was bimodal (corresponding to ions created in the source plasma and to those created in the secondary discharge) and the ion energies for ions created in the secondary discharge were above + 17 eV then these latter ions would not be effectively transmitted through the ion optics. That is the ion optics employed here discriminate against high energy ions created in a secondary discharge. Therefore the increase in the background ions near the capaci- tor position -10mm together with the decrease at that position for NO+ and Co' suggests that there was a very substantial change in the ion distribution within the sampled plasma strongly favouring argide and other background ions at this position. This effect would probably be more apparent using ion optics that did not discriminate against the higher energy ions.Concomitant Element Matrix Effect The initial publication on cooler plasma operation7 indicated a self-induced matrix suppression effect for K beginning at a concentration of 10mg 1-l (ppm). Since the total ion signal measured under 'cold plasma' conditions is significantly lower than that obtained under normal conditions it can be implied that the ion current through the skimmer tip is also less (perhaps less than the detected ion ratio as indicated above).This suggests that matrix effects resulting from space charge in the ion beam within the ion optics should be less evident. Therefore the appearance of a self-induced matrix effect at rather low concentration might provide some fundamental information regarding the ion dynamics within the plasma source. Reported here are the results of a study of matrix effects under 'cold plasma' conditions intended to provide insight into the mechanism of ionization and ion interaction within the plasma. Mass-dependent matrix effects (depending on the mass of both the analyte and the matrix elements) are expected to arise from space charge effects within the ion beam in the ion optics. Matrix effects that are a function of the ionization potentials of the analyte and matrix elements may be ascribed to ioniz- ation suppression in the plasma.When the sample is introduced as an aerosol the droplets dry to a particle before vaporization and ionization and the size of the dried particle is a function of the salt content of the sample. A matrix effect could then arise owing to incomplete vaporization of the dried particle if 91 4 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10the heat transfer from the plasma is limited and this effect would depend on both the concentration and the heat of vaporization of the matrix element. There remains also the possibility of fractionation in the vaporization process whereby some elements are more readily vaporized than are others even from the same dried particle. Therefore a suite of analyte and matrix elements were selected which covered a range of mass ionization potential and heat of vaporization.Fig. 12 presents the measured ion signal (corrected for isotopic abundance) as a function of the concentration of the elements treated as concomitant elements. The data include results for the 60 different matrix solutions each including only one element at 'high' concentration (covering the range from 1 to 300 mg 1-l) plus the 'clean' 10 ppb standard solution. Since Sc was observed primarily as its monoxide ion the response for ScO' as a function of Sc concentration is given. The ion signal observed is more-or-less a function of the ionization potential of the matrix element. The response curves show two substantially linear regions.In all instances the signal is linear up to an ion signal of about lo7 counts s-'. For matrix elements having ionization potentials below 6.0 eV a second linear region having lower slope appears above about 5 x lo7 counts s-'. The intersection of these linear sections occurs near 3 x lo7 counts s-l which is approximately the ion signal observed for NO' in the blank solution (1% HNO for which NO' is the dominant ion being approximately a factor of four more intense than 02+). For matrix elements having ionization potentials above 6.0eV the slope of the second response region appears to approach zero. Fig. 13 shows normalized ion signals for trace elements as a function of the Rh' signal for solutions containing Rh as the matrix element.No clear evidence of analyte mass-dependence is apparent. In fact the trace elemental ions appear to be suppressed in a rather uniform manner. Nonetheless it is apparent that the use of a single internal standard to correct for the matrix suppression would result in significant error (of the order of a factor of three at best). All the matrices studied showed similar behaviour. The extent of suppression for a given analyte ion is a function of the concomitant matrix element. This is shown in Figs. 14 and 15 where the Co' signal is plotted against matrix concentration and matrix ion signal respectively. If the vaporization of the particle plays an important role in the matrix effect the effect should be a Molarity of matii Fig. 12 Matrix ion signals (corrected to 100% abundance) as a function of concentration of the matrix element.The data presented were obtained for 61 separate solutions comprised of six solutions each of 10 matrix elements plus the 'clean' 10 ppb standard solution. Response curves are indicated by drawing lines through the signals for the seven solutions for each of the matrix elements corresponding to concentrations of 0.01 1,3 10 30,100 and 300 ppm. The data were obtained for the most abundant atomic ion for each matrix element (with the exception of Sc for which ScO+ was measured) and were corrected for 100% natural abundance 100 - P) C .- .Q 10'' .- E E b z 10-2 10 10' Rh matrix ion signakounts s-' Fig. 13 Ion signals for trace elements (10 ppb each) as a function of Rh ion signal for solutions of different Rh concentration.The analyte ion signals were normalized to their intensities in the lowest Rh concentration 0.01 ppm 10" 1b -5 Matrix molarity 10 -2 i Fig. 14 Ion signal for Co' (added as a trace element at 10 ppb) measured for various matrix solutions as a function of matrix concen- tration. Lines are drawn through the signals obtained for the seven solutions for each of the matrix elements corresponding to concen- trations of 0.01 1 3 10 30 100 and 300 ppm l o 4 10 u lo6 10 10 * Matrix ion signallcounts s-' Fig. 15 Ion signal for Co' (added as a trace element at 10 ppb) measured for various matrix solutions as a function of matrix ion signal (corrected to 100% abundance). The data are the same as those used in Fig. 14 but are plotted here against matrix ion signal function of the matrix concentration (Fig.14) with a more severe suppression for matrix elements with high heats of vaporization (e.g. Rh and Sc). No strong correlation is observed. There is no evidence of a change in suppression for matrix elements having ionization potentials above or below that of the analyte element Co (ionization potential = 7.86 eV). The suppression of the analyte signal does not appear to correlate with the absolute ion signal measured for the various Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 91 5matrix ions (Fig. 15). The best correlation appears to be with ionization potential of the matrix element with lower ioniz- ation potential matrix elements causing more severe suppres- sion.The greater suppressions observed for Bi T1 and Rh appear to be inconsistent and may reflect a mass bias as well (although it is noted that Rh also has a high heat of vaporiz- ation which may also play a role). It remains a possibility that all of these effects are simultaneously important so that the correlation with any one is not good. As with the data of Fig. 13 the trends (or lack thereof) observed for analyte Co' with matrix ion signal were also observed for the other analyte ions. The ion signals measured for Bi' and the major background plasma ions as a function of Bi concentration are given in Fig. 16(u). The matrix ion signal increases linearly with matrix concentration then becomes non-linear and finally shows a plateau at high matrix signal. It is apparent that the matrix ion signal is anti-correlated with the background ion signals and that the non-linearity in the matrix ion response curve appears where the matrix ion signal exceeds the blank NO' signal.This cross-over might imply that NO' acts as the ionization source for Bi (ie. by charge transfer) which in turn suggests that the 'cold plasma' is dominated by ion-molecule chemistry. The similar suppression of the 02' and H20+ signals could indicate that charge transfer from these ions is also important. The H,O+ signal is expected to follow that for H20' since the latter ion is probably the primary source for proton transfer leading to H30+ (charge transfer of H,O+ with Bi is not thermodynamically allowed). Alternatively the common suppression of the background ions may indicate an approach to equilibrium with Bi' being the thermodynamically favoured terminal ion.However chemical equilibrium should result in a large difference in suppression of analyte ion signals 10 10 10 c v) 3 10 a 9 2 109 Molarity of Bi matrix element cn C .- - 108 10 7 106 105 10 -4 lo9 10" Molarity of Na matrix element 10" Fig. 16 (a) Bi' signal and dominant plasma background ion signals as a function of the concentration of Bi as a concomitant matrix element. Bi acts as a 'high ionization potential' matrix element (see text). (b) Na+ signal and dominant plasma background ion signals as a function of the concentration of Na as a concomitant matrix element. Na acts as a 'low ionization potential' matrix element for analyte elements having ionization potentials above or below that of the matrix element; this was not observed in the data of Figs.13-15. Finally the common suppression may indicate enhanced ion-electron recombination if the matrix element induces an increase in the electron density. The data shown in Fig. 16(a) were similar to those obtained for all matrix elements having ionization potentials greater than 6.0 eV. However lower ionization potential elements showed behaviour similar to that shown in Fig. 16(b) for Na as the matrix element. Here the NO' signal behaves as it did for the higher ionization potential elements but the other back- ground ions show first a modest attenuation at moderate matrix concentration (about low3 mol l-') then a recovery as the matrix concentration was increased further. The latter results argue against equilibrium amongst the background ions. For matrix elements having ionization potentials above 6.0 eV the argide ions (Ar' ArH' and ArO') behaved much as did the other trace and background ions. Typical results (obtained for Bi matrix) are given in Fig.17(u) which is analogous to Fig. 16(u). The background argide ions are sup- pressed by the concomitant matrix element although appar- ently not as severely as are NO' and the analyte ions. However for matrix elements having ionization potentials < 6.0 eV the background argide ions are significantly enhanced at high matrix concentration. The data given in Fig. 17(b) are typical of those observed for the low ionization potential matrix elements K Na and Li.For these matrix elements the argide ions are not suppressed at moderate matrix concen- tration; the initial suppression of the ArO' signal is almost certainly due to the suppression of the isobaric Fe+ (Fe was added as an analyte element at 10 ppb). Even for a K S ! Molarity of Bi matrix element .- w 107 u) E - 104 103 los5 10'~ 1 0 ' ~ lo-* Molarity of Na matrix element Fig. 17 Argide ion signals obtained as a function of matrix element concentration obtained in the same experiment as the data of Fig. 16. (a) Bi as a concomitant element. (b) Na as a concomitant element. In both instances the ArO+ signal is dominated by Fe+ added as a trace element at 10 ppb. The initial decay of the ArOf/Fe+ signal in (b) is almost certainly due to the suppression of the Fe' signal and the recovery of the signal is almost certainly due to the enhancement of the ArO' component 91 6 Journal of Analytical Atomic Spectrometry November 1995 Vol.10concentration as low as 30ppm the background signals are significantly enhanced. As noted earlier the lower ionization potential matrix elements also appear to show a second linear response region with a smaller (but non-zero) response factor beyond the concentration yielding a matrix ion signal comparable to the blank NO' signal. It is likely that this second region is a result of an additional or enhanced mechanism of ionization peculiar to matrix elements having low ionization potentials. The transition from 'high ionization potential' behaviour (sup- pression of all background ions) to 'low ionization potential' behaviour (modest suppression and then recovery of back- ground ions other than NO') occurred abruptly near an ionization potential of 6.0 eV; where A1 (ionization potential = 5.984 eV) showed substantially 'low ionization potential' behaviour TI (ionization potential = 6.1 eV) showed strictly 'high ionization potential' behaviour.The dependence has been ascribed to ionization potential rather than mass since 39K behaved strongly as a low ionization potential element and lo3Rh behaved strongly as a high ionization potential element although they differ only by a factor of about 2 in mass. C4%c (ionization potential = 6.54 eV) and 66Zn (ionization poten- tial = 9.391 eV) also appeared to show 'high ionization poten- tial' behaviour although the extent of suppression (about a factor of 3 at the highest concentrations studied) was not sufficient to be certain.] It is significant also that the ion signal obtained at high matrix concentration for some of the elements substantially exceeded the total ion signal for the blank solution.Thermal ionization of high concentrations of low ionization potential elements may substantially increase the electron density in the plasma. For example complete ionization of 30 ppm of K corresponds to an increment of 1.5 x 1014 ions and electrons per second in the plasma (assuming 2% nebuliz- ation efficiency at a sample uptake rate of 1 ml min-'). If these ions and electrons are uniformly distributed within the central channel flow the electron density would be increased by about 10l2 C M - ~ (assuming 1.1 1 min-l and a plasma temperature of 1450K).This is probably a significant enhancement in the electron density. The local plasma temperature could be increased owing to collisional heating involving the electrons accelerated in the rf field and to improved radiative transport. Miller et aE.43 discuss these effects for the dc plasma. The inversion of local electron densities by easily ionized elements (EIE) resonance absorption raises the optical absorption cross- sections for both EIE and Ar. This improves the rate of energy transport and enhances local ohmic heating. Enhancement of the electron density on-axis by the presence of 0.5 mol 1-1 Cs in the nebulized sample has been reported by Caughlin and Blades!4 Of particular interest is the report of Hanselman et u I .~ ~ specifically the data relating to the relatively cool conditions of 1.25 kW plasma power 1.2 1 rnin-l central gas flow with measurements made 7mm above the load coil. An increase in electron density on-axis was observed with the addition of 0.1 mol I-' Cs Li or Ag to the sample while the electron and gas kinetic temperatures were enhanced only for Cs (suppression was observed for Ag and no change for Li). If under 'cold plasma' conditions the plasma temperature is increased by the presence of an EIE this could then enhance thermal ionization and possibly further increase the electron density. This mechanism might explain the additional or enhanced mechanism of ionization in the presence of high concentrations of low ionization potential matrix elements.It does not however explain why most of the background ion signals increase under these conditions but NO' does not. It therefore appears that the ionization mechanism in the 'cold plasma' may involve ion-molecule chemistry in addition to thermal ionization but that there may also be an additional or enhanced mechanism of ionization at high concentrations of low ionization potential elements. It is this enhanced ioniz- ation that is responsible for the second region of linearity in the K-matrix data seen both in this work and in the original work of Jiang et aL7 The concern for analytical use is that the increase in the background signals under these conditions could confound the determination of trace elements (e.g. the determination of Fe in a sample containing a high concen- tration of a low ionization potential concomitant element for which the ArO' background signal is increased).The preceding results show that matrix suppressions are rather severe (occur at low matrix concentration) under 'cold plasma' conditions. Furthermore if the matrix element has a low ionization potential and is at sufficient concentration there is a possibility of increased interference from polyatomic background ions. Finally refractory elements form polyatomic ions (e.g. ScO' from Sc) readily under these plasma conditions and these can also interfere with trace determinations. While the application of 'cold plasma' conditions for the determi- nation of certain elements (e.g. K Ca and Fe) in ultrapure samples is clear it remains to be determined whether there is an analytical protocol that can be established to extend its application to moderate (e.g. 300ppm or less) salt content samples.As was seen in Figs. 14 and 15 an analyte ion signal (Co' in that instance) is suppressed to different extents for different matrix elements and concentrations. However it was noted that the phenomenology of Fig. 13 was duplicated for various matrix elements. As shown now in Fig. 18 the relative suppres- sion for a given analyte element in various matrices is relatively invariant. Normalization of the analyte signal (e.g. Co') to an internal standard (e.g. Rh') minimizes the dependence of the matrix effect on the identity of the matrix element although a matrix effect which is a function of the concentration of the matrix element remains.This is unlike operation under normal plasma conditions where the extent of analyte suppression and the ratio of responses for analyte ions of different masses is strongly dependent on the mass of the matrix element. Although the analyte (Co') and internal standard (Rh+) used in Fig. 18 differ in mass by only a factor of two and have similar ionization potentials the same correlation was observed for any analyte ion using any other as internal standard (provided that there was no contamination of the analyte or internal standard in the matrix solution and there was no isobaric interference). More significantly there appears to be no need to add an internal standard. Fig. 19 gives data comparable to those of Fig. 18 but using the background ion NO' as the internal standard.Clearly NO' is suppressed by the matrix in much the same manner as the analyte ions. This is to be expected if the ionization mechanism is charge transfer 1 0 ' 1 0 2 10 Rh ion SignaVcounts s-' Fig. 18 Correlation of Co' and Rh+ ion signals (both present as trace elements at 10 ppb) in different matrix element solutions at different matrix element concentrations Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 91 7J 105 c 'Y) 104 v) 3 c $ 103 cn v) C .- .- * 102 0 10' l o 4 10' l o 6 10' NO ion signallcounts s-' Fig. 19 Correlation of Co+ signal (Co was added as a trace element at 10 ppb) with the background NO' signal in different matrix element solutions at different matrix element concentrations from NO+ and the matrix effect results from suppression of the reactant ion.This conclusion requires also that charge transfer from the matrix ion is not effective at ionizing trace elements. The latter requirement is likely to hold except for resonant energy levels for which the rate constant for charge transfer between atomic species increases dramatically. The correlations of Fig. 18 and 19 are also consistent with ioniz- ation suppression resulting from enhanced ion-electron recom- bination provided that the matrix element significantly enhances the electron density. It is not clear why the data for the Sc matrix in Fig. 19 are not consistent with the data for the other matrices (it appears that the NO' signal is insufficiently suppressed since in Fig 18 the Co -t signal appears to be appropriately suppressed relative to Rh ). Under normal plasma conditions the matrix effect can be compensated by using multiple internal standards because the matrix suppression is predominantly determined by the masses of the matrix and analyte ions (although the mass-dependence is not linear).Under 'cold plasma' conditions the matrix effect is not strongly correlated with mass. However the analyte ion signal is correlated with the background NO' signal and a plot of one against the other is linear over a wide range of matrix concentrations. This is shown in Fig. 20 for a number of analyte elements in K matrix solutions of various K concentrations. The slopes of the curves differ for the different analyte elements and suggest differing rates of ion production or loss (e.g.ion-electron recombination) relative to those for NO'. The largest slopes are for Co+ Rh' and Pd' which are the elements having the largest heats of vaporization. As implied by the data of Fig. 19 the slopes are independent of the matrix element and are reasonably linear (excepting instances where the matrix standard is contaminated with the trace element). This then allows for determination of the matrix correction factor by measuring the analyte ion and NO+ responses for any acid-matched matrix solution (including an unrelated synthetic acid-matched matrix) and also its acid- matched blank. The concentration of the analyte element [MI is then given by where RFclean is the response factor (e.g.counts s-l per ppb) in a clean standard solution S(M'),ample is the ion signal for the analyte in the sample solution AS(NO+) is the difference in the NO' signals between the sample and the acid-matched - 'v) 2 ' .- cn .Q v) 3 CI 10 10' l o 5 NO ion signallcounts s-' la - E 2 10 10 10 lo-* 10" Normalized NO' signal Fig. 20 Analyte ion signals as a function of K concentration for a series of K matrix solutions. Raw signals are given in (a). The data are replotted in (b) after normalizing the signals to their intensities in the 10 ppb (clean) solution. The data of (b) allow better observation of the variation of slope with analyte element blank and is the ratio of the differences of the analyte ion and NO' signals obtained for any acid- matched matrix and blank solutions.Since the matrix suppres- sion is linear when normalized to the NO' signal the concen- tration of the matrix solution is not important but should be such as to provide substantial signal suppression (e.g. 100 ppm). From a mechanistic point of view it is notable that the trace element signals appear to be suppressed in concert with the NO' signal regardless of the ionization potential of the matrix element. In particular the trace analyte signals do not appear to be enhanced by the presence of a high concentration of a low ionization potential matrix element (cJ the linearity of analyte signals with NO' suppression in Figs. 19 and 20 combined with the apparent insensitivity of the NO + suppres- sion to conditions under which enhanced ionization is observed as shown in Figs.16 and 17). The trace element ion signals appear to follow the NO' signal rather than the other background ions or the EIE matrix signal. It is recommended that NO+ be the reference ion used for matrix correction and not some other background ion. The correlation of analyte ion signal with NO' as shown in Fig. 19 was reproducible for all the elements studied as either trace analyte or concomitant matrix. However correlation with OZ' or another background ion is not as straightfor- ward. For matrix elements having ionization potentials above 6.0eV the slope of the analyte ion versus 0,' response is relatively independent of the matrix element. However for matrix elements having ionization potentials less than 6.0 eV the correlation is not good as shown in Fig.21 in which considerable scatter in the correlation of Co' with 02' is observed for the matrix elements K Na and Li and the A1 91 8 Journal of Analytical Atomic Spectrometry November 1995 Vol. 1010' l o 5 l o 6 10' 0 ion signavcounts s-' Fig.21 Correlation of Co' signal with background 02+ signal for different matrix elements at various concentrations. The scatter observed for K Na and Li matrix solutions is a result of the initial suppression and then enhancement of the 02+ signal as the concen- tration of the matrix elements was increased. The A1 matrix data are distinctly curved reflecting the fact that the 02+ signal was not fully suppressed in these solutions data are significantly non-linear (note that these are the matrix elements having ionization potentials less than 6.0 eV).In fact this scatter arises because the 0,' is first suppressed and then recovers as the matrix concentration is increased. The distinct behaviour for A1 results because the 0,' signal did not recover at high A1 concentration but it also was not as fully suppressed as with higher ionization potential matrix elements. This difference in behaviour between low and high ionization poten- tial matrix elements is exactly analogous to the data presented in Figs. 16 and 17 above and for the same reason. As noted above it appears that there is an additional or enhanced mechanism of ionization for high concentrations of matrix elements having low ionization potentials. This enhanced ionization does not appear to affect the sensitivity to trace level analyte ions but it does affect the background plasma ion signals.Where there is a potential isobaric interference from a background plasma ion the analyst must have a means by which to identify a possible increase in the background signal. One way to do this is to determine the signals for 0,' and NO+ in the sample. Both of these ions are background ions having high signal intensity and low mass and hence are unlikely to suffer serious isobaric interference from the sample (especially since S which might interfere with 02+ has a high ionization potential and is therefore inefficiently ionized). The correlation of these ion signals as a function of matrix concen- tration for various matrix elements is shown in Fig. 22 (this is analogous to Fig.19). For matrix elements having ionization potentials greater than 6.0 eV the O,+ and NO' signals are suppressed more-or-less in unison (slope of 1). However for low ionization potential matrix elements the 0,' signal is not as suppressed as that of NO+. Other background ions includ- ing H20+ H30+ and 0' and the argide ions Ar' ArH' ArO' and AT,+ behave in a manner similar to that of 0,'. It is particularly the response of the argide ions that stimulates our concern since an increase in the ArO' background could be confused with a response for analyte Fe. The analytical protocol allowing recognition of this potential background interference is to measure the 0,' :NO+ ratio in the blank solution and in the sample (these must have the same nitric acid concentration as the NO+ signal is a function of the acid concentration).If the ratio in the sample is similar to that in the blank there is unlikely to be an enhanced background interference. Great accuracy in the measurement of this ratio is probably not required as the background increase is not significant until the ratio changes by a factor of 3 or more. As noted above the matrix element may significantly 107 .- C .- 1 0 5 lo6 10' NO ion signaVcounts s-' i ! l o B Fig. 22 Correlation of the background signals for NO+ and 0,' for different matrix elements at various concentrations. For the low ionization potential matrix elements the 02+ signal suppression does not correlate with the NO+ signal suppression. It is this behaviour for 02+ that is responsible for the scatter shown for the low ionization potential elements in Fig.21 and is taken as an indication of an additional source of ionization under these conditions increase the electron density in the plasma. If the plasma temperature (specifically the ionization and electron tempera- tures) is not concomitantly increased this will result in enhanced ion-electron recombination. Fig. 23 presents the results of calculations of ion densities in the plasma assuming that the matrix effect is one of suppressed ionization owing to enhanced ion-electron recombination. For these calculations the electron density in the absence of a matrix element was assumed to be 4 x 10" cm-3 and the ionization and electron temperatures were assumed to be equilibrated at 3500 K. For each matrix element and for each concentration the electron density was calculated assuming equilibrium thermal ionization of the matrix element (as before assuming 2% nebulization efficiency and uniform dispersal in the central channel at a gas kinetic temperature of 1450 K).The electron density is then a strong function of the concentration and ionization potential of the matrix element. Fig. 23(u) is the analogue of the experimental data given in Fig. 12. The curvature of the response curve at high matrix concentration is predicted although the ion density at which curvature is calculated to occur should be a function of the ionization potential of the matrix element rather than near the NO' signal level for the blank solution. Fig. 23(b) is the analogue of Fig.14 and predicts rather well the order of suppression of the Co' signal with matrix element. The model underestimates the suppression for T1 Bi and Rh perhaps indicating an underlying dependence of the suppression on the mass of the concomitant element which is not accounted for in the model. This mass dependence may point towards the neglect of space charge in the model or may indicate the importance of mass-dependent radial diffusion in the plasma. Fig. 23(c) shows the calculated corre- lation of analyte ion densities with NO+ density as a function of the concentration of K matrix element [cf Fig. 20(b)]. Although the model predicts a non-linear correlation for low ionization potential analyte elements the relative order of the slopes agrees with the experimental data (except for the high ionization potential analyte elements notably Cd and Zn).It is evident that the model also predicts that the relative suppres- sion (that is after normalization to either an internal standard or to NO') is independent of the identity of the matrix element as was observed experimentally in Figs. 18 and 19. The model does not account for the 'enhanced ionization' effect (increase of background ion signals and their lack of correlation with NO+ ) for the low ionization potential matrix elements. The model might be refined by including the effects of radial dispersion and non-uniformity in the plasma (particu- Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 91 910 % .% C U 10 10s 10'~ loe2 Matrix molarity 10 -' 10 -' 10" l o o Calculated normalized NO' density Fig.23 Calculated ion densities as a function of matrix element concentration assuming equiliorium thermal ionization at 3500 K and an electron density in the absence of a matrix element of 4 x 1O1O cmW3. The electron density is increased by an amount given by thermal ionization at 3500 K of the amount of matrix element introduced into the plasma and assuming uniform distribution of the electrons in the central gas flow (1.1 1 min-l) at a gas kinetic temperature of 1450 IS. (a) (b) and (c) are the calculated analogues of the experimental data given in Figs. 12 14 and 20(b) respectively larly in the vicinity of vaporizing particles) and ohmic heating resulting from the increased electron density. Nonetheless the model suggests that thermal ionization may be dominant in the plasma.CONCLUSIONS 'Cold plasma' conditions permit the determination of elements normally interfered with by Ar+ and ArO+ because the conditions suppress the appearance of Ar' and 0'. Without the source ions the polyatomic argide ions are also suppressed. The resultant plasma is dominated by NO+ and 02+ and if no attempt at desolvation is made also the water-derived ions H20+ and H30+. Under the conditions used in this work the relative proportions of NO' and 0,' are determined principally by the nitric acid concentration of the sample. Since the NO' derives from the sample it behaves as a sample species and this has importance for its use as an internal standard. Some polyatomic argide ions persist notably ArH + and Ar2+.The approach is useful only if alternate sources of excitation such as a secondary discharge are eliminated. Sensitivity under 'cold plasma' conditions appears to be primarily a function of ionization potential. The sensitivity decreases markedly above 8 eV and so the technique is most useful for elements having lower ionization potentials. Heat transfer from the plasma to the sample appears to be insufficient to vaporize elements having high heats of' vaporization. Because the cooler plasma conditions favour the formation of molecular ions (notably the oxides of refractory elements) it is important for analytical purposes also to monitor ions at 16 u below the mass of interest to account for isobaric oxide interferences. For example under the conditions used in this work 40CaO+ presents an isobaric interference for 56Fe+ which is approxi- mately 30% of the signal obtained for 40Ca+.Matrix effects under cold plasma conditions are more severe than those obtained under normal analytical conditions. The matrix effect could be consistent with a model of the plasma dominated by ion-molecule chemistry as the reactant ions notably NO+ and 02+ are suppressed by charge transfer to the matrix ion. Alternatively the matrix suppression effect is also consistent with thermal ionization as the ion signals are suppressed by ion-electron recombination enhanced by the increased electron density contributed by the matrix element. However chemical equilibrium is not achieved in the plasma as the suppression of analyte ions is not strongly correlated with ionization potentials of the elements above and below that of the matrix element.Because of the severity of the matrix effect even at low matrix concentrations (in the low ppm range) the approach is most applicable to clean waters and acids with low salt content. However the technique may also be extended to less pristine samples since much of the matrix effect can be corrected for by correlating the analyte ion response with the NO+ ion signal (or to any other sample- derived internal standard). An additional or enhanced mechan- ism of ionization appears for high concentrations of matrix elements having ionization potentials below 6.0 eV. This enhanced ionization is responsible for the change to a smaller but non-zero response factor for the matrix element when the matrix ion signal exceeds the acid-matched blank NO + signal.The additional ionization does not appear to affect the response factor for trace elements. The appearance of enhanced ioniz- ation is significant because it is accompanied by an increase in the background signals of Ar' and polyatomic ions that may interfere with the elements of interest. However the appearance of this interference is indicated by a change in the ratio of NO" to 0,'. An equivalent indicator is the ratio of NO' (or any other sample-derived internal standard) to any background ion other than NO+ (such as H20+ H,O+ or O+ or even Ar' ArH+ or Ar2+). The author is grateful to Professor R. S. Houk (Iowa State University) and Dr. John Olesik (Ohio State University) for very helpful discussions and Peter Muellerchen (SCIEX) for assistance with the Colpitts oscillator experiment.REFERENCES 1 Houk R. S. Fassel V. A. Flesch G. D. Svec H. J. Gray A. L. and Taylor C. E. Anal. Chem. 1980 52 2283. 2 Date A. R. and Gray A. L. Analyst 1981 106 1255. 3 Douglas D. J. and French J. B. J. Anal. At. Spectrom. 1988 3 743. 4 Lam J. W. and McLaren J. W. J. Anal. At. Spectrom. 1990 5 419. 920 Journal of Analytical Atomic Spectrometry November 1995 Vol. 105 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Bradshaw N. Hall E. F. H. and Sanderson N. E. J. Anal. At. Spectrom. 1989 4 801. Moens L. Verrept P. Dams R. Greb U. Jung G. and Laser B. J. Anal. At. Spectrom. 1994 9 1075. Jiang S.-J. Houk R. S. and Stevens M.A. Anal. Chem. 1988 60 1217. Farnsworth P. B. paper presented at the 1994 Winter Conference on Plasma Spectrochemistry San Diego CA January 10-15 1994 paper IL 12. Ogilvie C. M. and Farnsworth P. B. Spectrochim. Acta Part B 1992 47 1389. Farnsworth P. B. and Omenetto N. Spectrochim. Acta Part B Sakata K. and Kawabata K. Spectrochim. Acta Part B 1994 49 1027. Gray A. L. J. Anal. At. Spectrom. 1986 1 247. Nonose N. S. Matsuda N. Fudagawa N. and Kubota M. Spectrochim. Acta Part B 1994 49 955. Uchida H. and Ito T. J. Anal. At. Spectrom. 1994 9 1001. Douglas D. and French J. B. Spectrochim. Acta Part B 1986 41 197. Douglas D. in Inductively Coupled Plasma in Analytical Atomic Spectrometry eds. Montaser A. and Golightly D. W. VCH New York 2nd edn. 1992 ch. 13. Fulford J. E. and Douglas D. J. Appl. Spectrosc. 1986 40 971. Denoyer E. R. Jacques D. Debrah E. and Tanner S. D. At. Spectrosc. 1995 16 1. Tanner S. D. Spectrochim. Acta Part B 1992 47 809. Tanner S. D. Douglas D. J. and French J. B. Appl. Spectrosc. 1994,48 1373. Barnes R. M. Crit. Rev. Anal. Chem. 1978 7 203. Tanner S. D. J. Anal. At. Spectrom. 1993 8 891. Houk R. S. Anal. Chem. 1986 58 97A. de Galan L. Smith R. and Winefordner J. D. Spectrochim. Acta Part B 1968 23 521. Hanselman D. S. Sesi N. N. Huang M. and Hieftje G. M. Spectrochim. Acta Electron. Part B 1994 49 495. van der Mullen J. A. M. Nowak S. van Lammeren A. C . A. P. Schram D. C. and van der Sijde B. Spectrochim. Acta Part B 1988 43 317. Hobbs S. J. and Olesik J. W. Anal. Chem. 1992 64 274. i993,4a 809. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 Gillson G. R. Douglas D. J. Fulford J. E. Halligan K. W. and Tanner S . D. Anal. Chem. 1988 60 1472. Tanner S. D. Cousins L. M. and Douglas D. J. Appl. Spectrosc. 1994 48 1367. Olesik J. W. Dziewatkoski M. P. McGowan G. J. and Thaxton C. paper presented at the 1995 European Winter Conference on Plasma Spectrochemistry Cambridge UK January 8-13 1995 paper 15. Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Cleveland OH 51st edn. 1970 E-74 and D-56-D60. Huntress W. T. Jr. Astrophys. J. (Suppl. Ser.) 1977 33 495. Ferguson E. E. 1973 Atomic Data and Nuclear Data Tables Academic Press New York 1973 vol. 12 p. 159. Fehsenfeld F. C. Schmeltekopf A. L. and Ferguson E. E. J. Chem. Phys. 1967,46 2802. Lindinger W. Fehsenfeld F. C. Schmeltekopf A. L. and Ferguson E. E. J Geophys. Res. 1974,79 4753. Fehsenfeld F. C. Planet. Space Sci. 1977 25 195. Debrou G. B. Goodings J. M. and Bohme D. K. Combust. Flame 1980 39 1. Goodings J. M. Bohme D. K. and Ng C.-W. Combust. Flame 1979 36 27. Huber K. P. and Herzberg G. Molecular Spectra and Molecular Structure I V Constants of Diatomic Molecules Van Nostrand Reinhold New York 1979. Hayhurst A. N. and Telford N. R. Trans. Faraday SOC. 1970 66 2784. Lias S. G. Bartmess J. E. Liebman J. F. Holmes J. L. Levin R. D. and Mallard W. G. ‘Gas-Phase Ion and Neutral Thermochemistry’ J. Phys. Chem. Reference Data 1988 17 (Suppl. l) p. 622. Turner P. J. in Applications of Plasma Source Mass Spectrometry eds. Holland G. and Eaton A. N. Royal Society of Chemistry Cambridge 1991 p. 71. Miller M. H. Eastwood D. and Hendrick M. S. Spectrochim. Acta Part B 1984 39 13. Caughlin B. L. and Blades M. W. Spectrochim. Acta Part B 1985 40 987. Paper 5/02599K Received April 24 1995 Accepted July 12 1995 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 921

ISSN:0267-9477    

DOI:10.1039/JA9951000905

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Spectroscopic Method for the Determination of the Electron Temperature in Quasi-thermal Air Discharges. Application to an Inductively Coupled Air Plasma at Atmospheric Pressure Journal of Analytical Atomic Spectrometry ANNE-MARIE GOMES JEAN-PHILIPPE SARRETTE LYDIE MADON AND ARNAUD EPIFANIE Centre de Physique des Plasmas et de leurs Applications de Toulouse Unit2 associie au CNRS no 277 Universitk Paul Sabatier 11 8 Route de Narbonne 31 062 Toulouse Cedex France A method for the approximation of the electron temperature O in quasi-thermal air discharges based on measurements of the relative emissivities of the weakly resolved OH and NO vibrational bands in the 210-320 nm spectral range is described. The method is very sensitive and particularly suitable between 2000 and 6000 K where intensities emitted by atomic lines cannot be easily distinguished from the molecular vibrational bands.It is shown that departures from thermodynamic equilibrium induced by radiative losses or by a difference between the electron temperature O and the heavy particle temperature Og cannot lead to large systematic errors as long as the ratio X = O 0 stays below 1.2. The uncertainty of the temperature determination is discussed together with an application to an inductively coupled air plasma at atmospheric pressure. Keywords Air; inductively coupled plasma; temperature measurement; non-equilibrium In air plasmas at atmospheric pressure the dissipation of energy by heat conduction is marked by peaks in the thermal conductivity’ corresponding to the dissociation of molecular oxygen near 3500 K and molecular nitrogen near 7000 IS.Hence the temperature of air plasmas produced in direct current (dc) electric arcs or in radiofrequency (rf ) inductively coupled plasmas (ICPs) is relatively low generally under 8000 K.2,3 The energy supplied to the plasma is distributed through a very large number of excited levels atomic levels and the set of vibro-rotational levels of the electronic configur- ations of the various molecular species. The optical spectrum emitted by the plasma is therefore composed of overlapping atomic lines and molecular bands. These radiative energy losses together with the relatively inefficient energy transfer by elastic electron-heavy particle collisions can move the plasma out of local thermodynamic equilibrium (LTE).Generally the plasma can be characterized by two Maxwellian energy distribution functions one for the free electrons and one for the whole set of heavy particles associated with the kinetic temperatures 8 and 8 respectively. The determination of these two temperatures depends on the measurement of population number densities of atomic and molecular excited states. Methods based on relative rotational line intensities emitted by molecule^^-^ lead to a rotational temperature 8 that is generally assimilated to the heavy particle temperature 8,; the molecular spectra from OH NO N and N2+ are generally used. In argon discharges the electron temperature generally assimilated to excitation tem- peratures can be obtained from relative or absolute atomic line intensity measurements through Saha or Boltzmann tem- perature determinations.” In quasi-thermal air plasmas owing to the fast energy transfer between the translational motion of the free electrons and the vibrational motion of the molecules respectively characterized by 8 and 8 these two temperatures are generally assumed to be In the same way the low-lying electronic states of atoms and molecules equilibrate rapidly with the ground electronic state giving excitation temperatures O,, in agreement with 8,.The determination of these parameters cannot be performed easily. We attempted to establish an optical diagnostic method to give the electron temperature 8 which was relatively indifferent to departures from equilibrium. The method is based on a comparison between the calculated equilibrium densities of the NO and OH molecular vibrational or rotational radiative states and their measured emissivities.The previous construction of a collisional-radiative model for the air plasma taking into account radiative and collisional out-of-equilibrium processes for the ground and excited level populations was then neces- sary which allowed us to avoid the more perturbed bands and to quantify the error committed when the equilibrium is assumed. EXPERIMENTAL SET-UP AND EXPERIMENTAL STUDY The characteristics of the ICP and of the optical diagnostic device have been presented elsewhere3>I3 and are summarized in Table 1. The experimental set-up is presented in Fig. 1. The optical spectrum emitted by the (0,O) band of the A2Z-X211 transition in OH (Fig. 2) was obtained with a 100 pm entrance slit-width and with a 350 pm entrance slit-width for the (0 1) vibrational transition of the NO y system (Fig.3). The spectra were Abel transformed; some typical behaviour of the radial variations JA(r) of the NO and OH emissivities for different wavelengths (corresponding to different pixels of the photodi- ode array) is depicted in Fig. 4. The measured NO emissivities normalized by their values on the axis of the discharge JAo(r) = Jn(r)/JA(r = 0) are identical whatever the considered pixel may be. In the same way the radial evolution of the normalized area of the whole band So@) = Jn(r) dA JA(r = 0) dA s is is the same as that obtained for any of the pixels. The normalized radial evolution of the OH emissivities differs for every pixel and most of them show a maximum outside the Journal of Analytical Atomic Spectrometry November 1995 Vol.10 923J Table 1 Characteristics of the experimental set-up Inductively coupled plasma- Power supply Inductor Air-flow rate Torch R. C. Durr rf generator with tuned-line 7-turn water-cooled coil 31 mm long 11.6 1 min-' at atmospheric pressure 20 mm id quartz tube with tangential oscillator 64 MHz 2.2 kW injection Optical set-up for geometrical discharge characterization- Optics 300 mm focal length and 50 mm diameter fused-silica lens (magnification of the set-up 1 1) with a pinhole of 5 mm diameter in the image focal plane (aperture of the light beam 8 = 1") mounting; 640 mm focal length; variable entrance slit-width 1 mm height (studied area of the plasma radial direction 350 pm axial direction 1 mm) 1200 lines mm-' holographic operating in the first order; spectral resolution R = 0.031 nm per pixel (25 pm) in the spectral range 200-350 nm Photodiode array 512 pixels Hamamatsu S2304 with Peltier cooler; pixel size 25 pm width (13 pm sensitive area and 12 pm blind area) and 2.5 mm height Monochromator Jobin-Yvon HR640 Czerny-Turner Grating Detector Control of the mirror stepping motors Microcomputer tube Grating motion contrd _c__ Entrance 1 Detectors cwrtrd and data acquisition slit Diaphragm Image inversion I_ Lens system I Geometrical T scanning masma Water-cooled coil R.F.power supply 1 .o 1 0 -' J Wavelengthlnm 302 304 306 308 310 312 314 316 318 320 Fig.2 Optical spectrum emitted by the air plasma between 304 and 320 nm and observed on the axis of the discharge p 0.9 - s 0.8 - 3 c. .- 0.7 - - .E 0.6 -. E 0.5 -. 0.4 - 2 0.3 - .- &. 2 0.2 0.1 - 0 4 J 226 228 230 232 234 236 238 240 242 Wavelengthhm Fig. 1 Optical and experimental set-up axis the radial position of this maximum namely rref being independent of the pixel. CALCULATION OF THE POPULATION NUMBER DENSITIES OF THE EMITTING LEVELS The total intensity of a line of wavenumber v emitted by molecule a in electronic state T and in the vibro-rotational state characterized by the quantum numbers 21 and J during the transition from level [T 21 J ) to level [T' u' J ' ) is Fig. 3 Optical spectrum emitted by the air plasma between 227 and 242 nm and observed on a different radius of the discharge I rref r - Fig.4 Typical variations of the local emissivities J(r)OH and I The maximum emissivities obtained for OH are used to determine rref. The curve a(r) corresponds to J(r)NO J(rre& expressed14 as a function of the density of the emitting level na[T,,v,J> by where K is a constant and SJ,J and pvv are the Honl-London factors and the band strength respectively. In a non-equilibrium plasma molecular population number densities of vibro-rotational states I Z [ ~ * J > depend on the electron temperature O the heavy particle temperature O the vibrational temperature 8 and the rotational temperature Or. These densities can be calculated from the total population 924 Journal of Analytical Atomic Spectrometry November 1995 Vol.10number density na(O 0,) of species a T and gT are the energy terms and the statistical weight of the electronic state respectively Q?(Oe 0 0,) is the internal total partition function for species a and the vibrational and rotational energies G(v) and F,(J) can be expressed as a function of the usual spectroscopic constants me mexe meye ae Be Pe and De G(v) = CO,(U + f) - O,X,(V + f ) 2 + CO,Y,(U + 4)3 + . . . (3) (4) Fv(J) = BvJ(J + 1) - D,[J(J + 1)12 + . . . with B = Be - ae(v + $) and D = D + Pe(u + 3). narT,,u>(Oe 8 0 0,) of a vibrational state is then written as The molecular population number density ( 5 ) The discrete summation can be assimilated to a continuous one; if F,(J) is limited to the first order the integration gives As a first approximation and in order to interpret the emissivity measurements in terms of electron temperature the equilibrium composition for a monothermal air plasma 0 = 8 = 0 = 8 (LTE) was calculated.The different chemical species taken into account are summarized in Table2. The molecular species made by four atoms or more were not considered in this model because they dissociate at very low temperatures (@ < 2000 K). From the relative concentrations of nitrogen (78%) oxygen (21%) and argon (0.9%) in dry air and for a gas at atmospheric pressure with a 70% moisture content at 300K the population number densities ni of the 30 chemical species present were obtained by resolving the corresponding system of equations. This system is composed of 25 equations representing the ionization or attachment Q.OOE+lO I I 1 YE a.OOE+lO 7.00E+10 $ 6.00E+10 U 5 5.00E+10 f 4.OOE+lO 3.00E+10 .9 8 1.00E+10 a 0.00E+10 .- 4 2*00E+10 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 TemperatureM Fig.5 Calculated population number densities of the main radiative atomic levels or molecular electronic configurations in natural moist air at atmospheric pressure (LTE hypothesis) equilibria (Saha's law) and the dissociation equilibria (Gulberg-Waage's law); and five equations for the constraints imposed on the plasma that is the electrical neutrality the ratio between the total numbers of argon oxygen and hydrogen nuclei and the conservation of the total pressure (Dalton's law). The densities of the vibro-rotational levels and of the total rotational states are deduced from the total densities of each species using eqns.(2) and (6). The results show that below 6000 K radiation is essentially emitted from the NO and OH molecules (Fig. 5); the variations with temperature of the population number densities of the rotational levels emitting the (0-1) vibrational band of the NO y system are very fast and nearly independent in nor- malized values from the J value. The same evolution is obtained for the total population number density of the emitting vibrational level v = 0 (Fig. 6 ) . For the (0-0) transition between electronic levels A and X of OH the intensity emitted by each of the most populated rotational levels 4 < J < 15 (whose population number densi- ties are comparable) shows a maximum between 3970 and 4180 K (Table 3 and Fig.7). For the less populated levels (J > 15) maxima appear for higher temperatures. The sum- mation of all the rotational levels leads for the vibrational state u=O to a maximum at 4090K. Because of the low resolution of the measured spectra at every pixel we have superposition of lines coming from different rotational levels. This temperature was chosen as the reference temperature Qref and the corresponding radius taken as the reference radius rref. In a two-temperature plasma using the usual assumptions 8 = 0 and 8 = 0 the densities of the emitting levels can be obtained from the total densities na(O 0,) through eqn. (2) where the internal total partition function Q?(0 0 0,) can be easily expressed15 as a function of the monothermal internal total partition function Q?(Oe) (7) 0 e 0 Q?(8 O 0,) = Q?(Oe 0,) = - Q p ( 0 ) Table 2 Chemical species considered in the equilibrium model (the species considered in the collisional-radiative model are written in bold) Family Chemical species Family Chemical species Molecular nitrogen Nz Nz+ Molecular hydrogen H2 Molecular oxygen 0 2 o,+ 0 2 - Atomic oxygen 0 o+ 0- Ozone 0 3 Atomic nitrogen N N+ Atomic hydrogen H H+ Nitrogen oxide NO NO' NO- Dinitrogen oxide N 2 0 N20+ N20- Nitrogen dioxide NO2 NOz' NO2- Atomic argon Hydroxyl OH OH' OH- Nitrogen hydride NH Ar Ar' Water %O Journal of Analytical Atomic Spectrometry November 1995 Vol.10 9251 5000 6000 7000 8000 9000 10000 1000 2000 3000 4000 Temperat ure/K Fig. 6 Normalized calculated population number densities for some rotational states and for the ground vibrational state of the NO@) electronic configuration in natural moist air at atmospheric pressure (LTE hypothesis) Fig.7 Normalized calculated population number densities for some rotational states and for the ground vibrational state of the OH(A) electronic configuration in natural moist air at atmospheric pressure (LTE hypothesis) A Y = 0; B 21 = 0 J = 10; C Y = 0 ~r = 15; D Y = 0 J = 20; E Y=O J=30 Table 3 Maximum population number densities and corresponding temperature values for the OH [A u = 0 J ) rotational states J = 0-15 J = 20 and J = 30 (LTE conditions). Last column represents the total value for the OH vibrational state [A Y = 0 ) 4 5 6 J 0 1 2 3 3970 3970 3970 3970 ee/'(K) 3970 3970 3970 nOH(ee)[A,~ = O,J> 1.50 x lo8 4.45 x lo8 7.24 x lo8 9.76 + lo8 1.19 x 109 1.37 x 109 1.50 x 109 10 11 12 13 4040 4060 4090 41 10 J 7 8 9 %/(K) 3970 4020 4020 nOH(ee)[A,u = O J > 1.59 x 109 1.63 x 109 1.63 x 109 1.59 x 109 1.52 x 109 1.43 x 109 1.32 x 109 Total 4180 4420 5500 4090 J 14 15 20 30 Oe/(K) 4130 n ~ H ( e e ) [ ~ u = o,J> 1.19 x 109 1.06 x 109 4.93 x lo8 5.60 x 107 2.42 x 10" ELECTRON TEMPERATURE DETERMINATION Using the number densities calculated in the LTE conditions for the NO state [A v = 0) the ratio of the total vibrational densities nNO(oe)[A,u = O> nNO(oref1LA.u =o> P(8e) = was determined between 2000 and 6000 K (Fig.8). It is then possible to connect the radius r to the temperature 6 making P(Qe)=a(r) where a(r) is the ratio of the local emissivities measured in the discharge for the two positions r and rref JNO (r) JNO (rref) a(r) = ~ The radial evolution of the electron temperature obtained from the experimental Abel inverted proffiles for NO and OH in the rf air plasma between the central windings of the coil is shown in Fig.9. It is compared with the variations in the 926 Journal of Analytical Atomic Spectrometry November 1995 Vol. 101 1 1 1 - . - - - - . . - 10' 1 oo lo-' 1 0-2 1 o9 a lo+ 2 10-5 0 .- c 1 o4 Optically thin plasma (X = 1 .O) Optically thin plasma ( X = 1 .l) 1 o - ~ 1 o-8 1 o4 Optically thin plasma (X = 1.2) 3200 ~- 2000 3000 4000 5000 6000 Electron temperature/)< Fig. 8 Variations in the ratio P(0,) = nNo(8,)[A,U=o> &8ref)[A,o=o) calculated in natural moist air at atmospheric pressure for a LTE plasma and for a monothermal and dithermal optically thin plasma 4800 1 .%I + 3000 ~ I I I I 1 2 3 4 5 0 Radiudmm Fig. 9 Radial variations of the electronic and heavy particle tempera- tures obtained between the third and fourth windings of the coil heavy particle temperature 8 deduced from the shape of the spectra emitted by the (0-1) band of the NO y ~ystem.~ As can be seen the method is very simple and has four major advantages (i) the variation of the ratio p(6,) between 2000 and 6000 K is very fast leading to a great sensitivity in the electron temperature measurement; (ii) there is no need for high resolution; (iii) the method is based on relative intensity measurements and does not require any calibration of the detector; and (iv) departures from equilibrium caused by 100 l 5 O I 1 \ Fig.10 Uncertainty on the determination of O&). l:~Aexp(Oe)~; 2 ANo(Be) with X = 1.0; 3 AN,(8,) with X = 1.1 4 ANo(8,) with X = 1.2 radiation escape or by the non-equipartition of the kinetic energy in the plasma do not really modify the temperature determination. The influence of these effects was studied by Sarrette and c o - w o r k e r ~ ~ ~ * ~ ~ using a collisional-radiative (CR) model through the calculation in a dithermal plasma of the number densities of the emitting levels involved in eqns. (2) and (6). In the CR model more than 90 levels (ground and excited) are taken into account for the 12 chemical species written in bold in Table 2. As yet water vapour has not been included because certain reaction rate coefficients were unavailable and because of the complexity of the resolution of the coupled non-linear system of equations.We have shown that while the number densities of the emitting levels are depopulated the ratio p(6,) is nearly insensitive to departures as long as X = 8 eg remains less than 1.2 as shown in Fig. 8. Sarrette and co-workers observed that in such conditions chemical equilibria move to the highest temperatures. Shifts are generally weak for the total number densities but can be greater for the excited states. In order to analyse the effects of departures from equilibrium on the position of the emissivity maximum for the rotational states of the hydroxyl radical it was assumed that the temperature corresponding to the maxi- mum of the total number density noH was not appreciably modified from its equilibrium value with radiation and for X < 1.2.We calculated in these conditions using eqn. (2) that the positions of the maxima are only shifted for the less populated rotational states (J > 15) with no consequence on the value of the temperature Oref. DISCUSSION The uncertainty on the determination of 8,(r) can be separated into three terms = Aexp + ANO (6,) + AOH (Oe) The first term Aexp(Qe) is related to the measurement of the emissivity J N o ( r ) . We considered a *5% uncertainty on the experimental determination of p(Oe) essentially caused by the Abel transformation. As can be seen in Fig. 10 the absolute Journal of Analytical Atomic Spectrometry November 1995 Vol.10 927value of the uncertainty increases with 8 and does not exceed 60 K below 5000 K. The second term ANO(Oe) is a systematic error connected with the importance of departures from equilibrium. It is obtained from the curves in Fig. 8 and represented in Fig. 10. This error is less than +60K for X g 1.1 and 2500 K < 0 < 5000 K but cannot be ignored for X 3 1.2 and 8 < 2500 K or 8 > 5000 K. In the first case there is a strong underestimation of the electron temperature while the second case leads to its overevaluation. In order to be complete the influence of diffusive processes on p(0,) should have been taken into account. This effect generally ignored cannot be easily evaluated because of the large number of diffusing species in an air plasma.The third term represents the uncertainty on the position rref of the emissivity maximum for OH owing to the accuracy of the geometrical analysis of the plasma and the uncertainty on the corresponding temperature Oref. Although it is possible to decrease the positional uncertainty by an increase in the definition the second point is more difficult to evaluate as long as the hydroxyl radical is not taken into account in the CR model. As previously discussed this can lead to an underestimation of the electron temperature certainly less than 100 K which constitutes the largest part of A(8,) between 2500 and 5000 K. CONCLUSION A very sensitive method for the determination of the electron temperature in an air plasma at atmospheric pressure based on a comparison of the spectra emitted by the NO and OH molecules between 2000 and 6000 K is presented.There is no need for calibration of the detector and it was shown that the method was not affected by departures from thermodynamic equilibrium in the range 2500-5000 K. The authors are greatly indebted to their lamented friend Jean Bacri for kind encouragement valuable contribution and helpful discussions during the course of this work. Jean Belkheir Belhaouari is also thanked for his kind contribution. This work was supported by the ARC CNRS-ECOTECH by ADEME and by Conseil Regional Midi PyrCnCes. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Bacri J. and Raffanel S. Plasma Chem. Plasma Process. 1989 9 133. Laux C. Moreau S. and Kruger C. paper presented at the AIAA 23rd Plasmadynamics and Lasers Conference Nashville TN 1992,2969.Gomes A.-M. Bacri J. Sarrette J.-P. and Salon J. J. Anal. At. Spectrom. 1992 7 1103. Robinson D. J. Quant. Spectrosc. Radiat. Transfer 1964,4 335. Workman J. M. Fleitz P. A. Fannin H. B. Caruso J. A. and Seliskar C. J. Appl. Spectrosc. 1988 42 96. Raeymaekers B. Broekaert J. A. C. and Leis F. Spectrochim. Acta Part B 1988 43 941. Ishii I. and Montaser A. Spectrochirn. Acta Part B 1991 46 1197. Nowak S. van der Mullen J. A. M. and Schram D. C. Spectrochim. Acta Part €3 1988 43 1235. Czemichowski A. J. Phys. D 1987 20 559. Gomes A. M. J. Phys. D 1983 16 357. Park C. paper presented at the A1A.A 22nd Thermophysics Conference 1987 Honolulu HI 1987 J. Thermodynam. Heat Transfer 1989 3 253. Losev S. A. Makarov V. N. Pogosbekyan M. J. Shatalov 0. P. and Nikols’sky V. S. paper presented at the AIAA/ASME 6th Joint Thermophysics and Heat Transfer Conference Colorado Springs CO 1994. Nore D. Gomes A. M. Bacri J. and Cabe J. Spectrochim. Acta Part B 1993 48 1411. Nicholls R. W. and Stewart A. L. in Atomic and Molecular Processes ed. Bates D. R. Academic Press New York 1962 ch. 2 p. 56. Bacri J. Lagreca M. and Medani A. E’hysica 1982 113C 403. Sarrette J. P. Gomes A. M. Bacri J. Laux C. O. and Kruger C. H. J. Quant. Spectrosc. Radiat. Transfer 1995 53 125. Sarrette J. P. Gomes A. M. and Bacri J. J. Quant. Spectrosc. Radiat. Transfer 1995 53 143. Paper 5/02544C Received April 21 1995 A-ccepted June 26 1995 928 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10

ISSN:0267-9477    

DOI:10.1039/JA9951000923

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Application of Multi-element Time- resolved Analysis to a Rapid On-line Matrix Separation System for Inductively Coupled Plasma Mass Spectrometry SIMON M. NELMS AND GILLIAN M. GREENWAY" University of Hull Cottingham Road Hull North Humberside UK HU6 7RX ROBERT C. HUTTON Fisons Instruments Elemental Analysis Ion Path Road Three Winsford Cheshire UK C W7 3BX A rapid on-line matrix separation system for ICP-MS using multi-element time-resolved analysis was developed for the determination of several trace elements in complex matrix samples. A flow injection manifold was constructed consisting of a mini-column of 8-hydroxyquinoline covalently immobilized on to controlled pore glass. Analytes retained on the column were eluted using 0.1 ml of 2.0 mol I-' nitric acid. Sample volumes of 0.5 ml were analysed yielding a preconcentration factor of 5 in addition to matrix separation.The system was optimized with respect to the variables of buffer concentration buffer pH and eluent acid volume and concentration. Calibrations from both pure water and synthetic sea-water compared well and showed good linearity with correlation coefficients of 0.988-0.999 for a range of analytes. The method showed good within-run reproducibility with precisions (s,) at the 1 ng ml-l level of typically <3%. In general recoveries between 89 and 104% were obtained with the exception of Ni which showed a recovery of 78% under the compromise conditions used. The method was validated by the analysis of estuarine (SLEW-1) and coastal (CASS-2) certified reference materials. Good agreement with the certified values was obtained for both of these materials. Keywords Flow injection time-resolved analysis; inductively coupled plasma mass spectrometry on-line matrix separation immobilized 8-hydroxyquinoline Inductively coupled plasma mass spectrometry (ICP-MS) has rapidly evolved over the past decade to become one of the most sensitive accurate and reliable trace element measure- ment techniques.However the earliest experiments with ICP-MS instrumentation identified that direct analysis of samples with high levels of dissolved solids was not practical owing to rapid blockage of the sampling interface cones and/or torch injector.' The technique is generally regarded as being limited to the analysis of samples having a total dissolved solids (TDS) content of <0.2%.For some analyses this difficulty can be circumvented by dilution of the sample. This approach is not appropriate for samples such as sea-waters because of the very low levels of analyte species present in the undiluted sample. Such samples also contain matrix species which combine with the plasma gas air entrained into the plasma and solvent components leading to polyatomic spectral interferences which can seriously degrade the accuracy of the analysis. Further the presence of easily ionizable matrix elements such as Na may cause ionization suppression because of an increase in the electron density in the plasma.2 A number of procedures have been adopted to alleviate the * To whom correspondence should be addressed. Journal of Analytical Atomic Spectrometry difficulties outlined above particularly involving the separation of the trace analytes from the concentrated matrix using ion- exchange or chelating reagents.Traditionally both solvent exchange3 and batch column separation4 methods have been employed but often these procedures are time consuming and susceptible to contamination. These factors led to the develop- ment of on-line matrix separation using flow injection method- ologies. The first publication describing this approach by Olsen et ~ 1 . ~ used a column containing Chelex-100 resin incorporated into a flow injection manifold to separate pollutant elements from sea-water with subsequent detection by flame atomic absorption spectrometry (AAS). This polymer-based iminodia- cetate (IDA) ion-exchange resin is difficult to use in on-line systems because of the large volume changes that occur when it is converted from a salt form to the acid form.It also requires conditioning steps between each sample analysis which increases the analysis time. Alternative more highly cross-linked IDA resins which are less susceptible to dimen- sional changes have been employed in recent s t ~ d i e s ~ ~ but these also require conditioning between samples. The chelating reagent 8-hydroxyquinoline (8-HQ) has also been exploited in on-line matrix separation When covalently bound to a silica or controlled pore glass (CPG) support this material does not change volume with changing pH or sample composi- tion. It also possesses a reactive chelating surface which is conditioned very rapidly between samples therefore allowing a faster sample throughput.Fang and Welz" reported a rapid low sample consumption matrix separation system based on CPG-immobilized 8-HQ for the determination of heavy metals in sea-water using flame AAS detection. The use of ICP-MS as the detection system for on-line matrix separation is a more recent development. Systems using ICP-MS detection have been described by Beauchemin and Berman" and Bloxham et aL7 In both of these publications the column was incorpor- ated in the flow stream and a separate valve was used to direct the matrix to waste downstream of the column. Fang12 sug- gested incorporation of the column in the loop of a valve to allow direct switching between the sample and eluent streams circumventing the need for an additional valve.With this arrangement some matrix does pass into the detector on switching the valve but if the connecting tubing is sufficiently short the residual matrix is kept to an acceptably low level. In addition this design allows counter-current elution of retained analytes thereby yielding sharper less dispersed peaks. This paper describes a rapid on-line matrix separation system for ICP-MS using a flow injection manifold incorporat- ing a mini-column of CPG-8-HQ for the determination of several trace elements in saline samples. The column of immobi- lized chelate is located in the loop of a manual flow injection (FI) valve. The retained analytes are eluted counter-current to Journal of Analytical Atomic Spectrometry November 1995 VoE.10 929the sample flow to yield improved less dispersed FI peaks. The FI system described is based on fixed-volume injection rather than time-based sampling and gives a fivefold precon- centration in addition to the required matrix separation. The manifold design also offers very low sample consumption. Data from the transient eluted peaks have been collected using the multi-element time-resolved analysis facility supplied with the instrument. Results illustrating the effectiveness of the matrix separation process with respect to the 40Ar23Na inter- ference on 63Cu are presented together with validation of the procedure using coastal and estuarine certified reference materials. EXPERIMENTAL Reagents High-purity de-ionized water (18 MR cm resistivity Elgastat UHQ PS Elga High Wycombe Bucks.UK) was used throughout. Elemental stock solutions (1000 yg ml-' SpectrosoL BDH Poole Dorset UK) were used in the preparation of calibration solutions. The reagents 8-hydroxyquinoline 3-aminopropyltriethoxysilane and p - nitrobenzoyl chloride (Sigma Poole Dorset UK) absolute ethanol (Hayman Witham Essex UK) sodium dithionite (BDH) and concentrated hydrochloric acid (Fisons Loughborough Leicestershire UK) were used in the immobil- ization procedure. Controlled pore glass (CPG 100-125 ym particle size Fluka Gillingham Dorset UK) was used as the immobilization support. Ammonium acetate buffer (Sigma) was prepared from the solid and purified prior to use by passing through a column of Chelex-100 (Sigma) under gravity.Adjustments to pH were made using glacial acetic acid or aqueous ammonia as appropriate. The ammonia solution was prepared by isothermal distillation of concentrated aqueous ammonia (Beecroft and Partners Rotherham UK). Synthetic sea-water was prepared by dissolving sodium chloride sodium sulfate sodium hydrogencarbonate magnesium chloride cal- cium chloride and potassium chloride in water using the procedure described by van Berkel et ~ 1 . ' ~ Samples of the coastal (CASS-2) and estuarine (SLEW-1) certified reference materials (CRMs; National Research Council of Canada Ottawa Canada) were introduced to the manifold without pre-treatment. All analytical work was performed without the use of clean room facilities. Immobilization of 8-Hydroxyquinoline A modified version of the procedure described by Habib14 was used. CPG (1.0 g) was first activated by boiling in 20 ml of 10% v/v nitric acid for 30min.The product was filtered and dried in an oven at 80°C and subsequently silanized by reaction with 10 ml of 10% v/v 3-aminopropyltriethoxysilane in anhydrous toluene for 15min at room temperature. The silanized product was oven dried and reacted with a solution of 10% m/v p-nitrobenzoylchloride in chloroform for 24 h at room temperature. The product was filtered oven dried at 50°C and further treated with a 10% m/v aqueous boiling solution of sodium dithionite for 30min to reduce the nitro group to the amine. The reduced product was again filtered and dried at 50°C. This product was added to hydrochloric acid ( 5 ml 2 and reacted with sodium nitrite (4 ml 2% m/v in water dropwise addition) at 0°C to yield the diazonium salt.Finally the product was rapidly filtered and added to a solution of 8-hydroxyquinoline (20 ml 2% m/v in absolute ethanol). In this step the CPG developed a deep red colour indicating that the diazo compound had been formed and hence the immobilization had been successful. The final product was filtered washed with hydrochloric acid (2 moll-') and water and stored in a vacuum desiccator. Instrumentation ICP-MS measurements were made using a Fisons Instruments PlasmaQuad 2 Plus. The instrument was calibrated and optim- ized prior to operation using a tune solution containing the elements Be Mg Co Y La Eu and Bi at 10ngml-' in a matrix of 5% nitric acid.The transient analyte peaks were monitored using the time-resolved analysis mode. Data acqui- sition and instrument operating parameters are given in Table 1. ICP optical emission spectrometric (OES) measure- ments were made using a Perkin-Elmer Plasma 40 instrument. The instrument operating parameters are given in Table 1. Data output from the ICP was recorded using a chart recorder (BBC Servoscript Croydon Surrey UK). Matrix Separation Manifold The matrix separation manifold is illustrated in Fig. 1. It was designed to facilitate rapid processing to increase the sampling frequency. The immobilized 8-HQ was contained within a glass mini-column (2 cm x 3 mm id.) (Omnifit Cambridge UK) incorporated in the loop of a manual four-way injection valve (Rheodyne Model 5020 Supelco Poole Dorset UK).All the manifold connections were made using 0.8 mm id. PTFE tubing. The reagents were pumped at l.Omlmin-' using a peristaltic pump (Gilson Minipuls 3 Anachem Luton UK) resulting in a matrix separation floiw rate of 2.0 ml min-l and an elution flow rate of 1.0 ml min-I. Table 1 Operating parameters for the ICP and ICP-MS instruments ICP instrument- Aerosol gas flow rate -0.75 1 min-' Intermediate gas flow rate 0.6 I min-l Outer gas flow 12 1 min-' Nebulizer Cross-flow Spray chamber Ryton Emission wavelength for Mn Chart recorder settings Forward power 1350 W Reflected power ow Aerosol gas flow rate Intermediate gas flow rate Outer gas flow rate Nebulizer Spray chamber ICP data acquisition- 257.610 nm 2 V f.s.d. 1 cm min-l ICP-MS instrument- 0.939 1 min-l 1.0 1 min-' 13.0 1 mm-' De Galan type Glass water cooled 10°C Time-resolved analysis peak-jumping parameters- Time per slice 1.00 s Points per peak 3 Detector mode Pulse counting Number of selected isotopes 13 Selected isotopes (ICP-MS)- 48Ti 51V 55Mn 59C0 60Ni 63Cu 64Zn 65Cu Io7Ag 11'3Cd 1151n 140Ce 208pb Eluent loop (0.1 ml) Sample loop (0.5 ml) 1 I A 21' Waste Fig.1 Flow injection manifold 930 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10Matrix Separation Procedure For matrix separation the first valve ( 1) (Fig. 1) served as the sample injection port and the second (2) as the acid eluent port. The third valve (3) allowed matrix species unretained by the column to run to waste during the separation process while aspirating water into the ICP-MS.On elution valve 3 served to direct the eluted analytes into the ICP-MS. The sample and eluent streams flowed in opposite directions through the column thereby yielding sharper eluted peaks. Cross-contamination was minimized by constructing the eluent and sampling streams separately from each other. In addition to on-line matrix separation the manifold also gave a fivefold preconcentration. Determination of Exchange Capacity The capacity of the immobilized 8-HQ was determined for Mn by both a batch and a dynamic method using ICP-OES detection. Batch determination A solution of Mn (20 pg ml-') was prepared in ammonium acetate buffer (0.1 mol dm-3 pH 6.0) and added to 0.05 g of dry immobilized 8-HQ. The mixture was equilibrated with stirring overnight.The solution was filtered and the concen- tration of Mn in the supernatant liquid was measured versus the original concentration. The decrease in the concentration of Mn was used to evaluate the capacity using the equation where C is the capacity (mmol g-') ci and cf are the concen- trations of the metal before and after equilibrium respectively (pg ml-I) M is the relative atomic mass of the metal and v is the volume of solution (ml) equilibrated with a mass m (g) of immobilized chelate. Dynamic determination The flow injection manifold was coupled to an ICP-OES instrument equipped with a chart recorder. Aqueous solutions of Mn were prepared in the range 0-700 pg ml-'. Dry immobi- lized 8-HQ (0.04 g) was packed into a glass mini-column. It was necessary to operate the manifold with an ammonium acetate buffer concentration of 2 moll-' because of the high acidity of the Mn solutions arising from the original stock solution.Nitric acid (2.0 moll-') was used as the eluent. Three repeated analyses were made at each concentration and the results were evaluated in terms of the eluted peak height. The concentration beyond which no further increase in peak height was obtained was deemed to represent the dynamic capacity limit. The dynamic capacity was evaluated using the equation where Cd is the dynamic capacity (mmol g-') c is the concen- tration of the metal at the dynamic capacity limit (pgml-') us is the volume of sample injected (ml) m is the mass of immobilized chelate in the column (8) and M is the relative atomic mass of the metal under study. RESULTS AND DISCUSSION Manifold Design The manifold developed for the project incorporated several design features to simplify and improve the matrix separation procedure.To make the method more rapid and more economi- cal in terms of sample consumption a small sample volume was utilized. To shorten the analysis time further CPG- immobilized 8-HQ was selected as the chelating reagent because column washing and conditioning could be achieved very rapidly between samples compared with iminodiacetate resins. As polymeric-based chelating resins are subject to volume changes between matrix separation and elution the dimen- sionally stable material CPG was selected as the support. This material can also be readily functionalized because of reactive silanol groups on its surface to yield useful insoluble ion-exchangelchelating reagents.The application of counter- current matrix separation and elution yielded sharper transient peaks and avoided compacting the column material at one end of the column thereby reducing the risk of back-pressure still further and ensuring good flow stability. Optimization of the Matrix Separation Procedure The procedure was optimized with respect to the parameters of buffer concentration buffer pH and eluent acid volume and concentration using a univariate approach. Eluent volumes below 0.1 ml yielded badly dispersed peaks giving poor reproducibility and volumes greater than 0.1 ml gave a lower preconcentration factor. For these reasons an eluent volume of 0.1 ml was selected.The complete optimum conditions are given in Table 2. The response of the column to selected analytes with chang- ing buffer pH is shown in Fig. 2. As the optimum buffer pH for matrix separation varies between elements a compromise pH must be selected for multi-element analysis. On the basis of the element responses illustrated in Fig. 2 pH 6.0 was chosen as values below this gave decreased retentions of some analytes. Values above this level gave lower buffer capacities (up to pH 8) and beyond pH 8 gradual hydrolysis of the CPG-based chelating material would occur. The effect of the ammonium acetate buffer concentration on the retention of the selected elements is illustrated in Fig. 3. Samples were prepared by spiking a synthetic sea-water solu- tion with the selected analytes followed by acidification with one drop of concentrated nitric acid.Acidification was per- formed in accordance with the accepted procedure of main- taining the sample integrity with respect to the trace metal content. The results in Fig. 3 show that the buffering process Table 2 Optimum conditions for on-line matrix separation ~~~ Buffer conditions Matrix separation flow rate Elution flow rate Nitric acid eluent concentration Total analysis time 4 min Ammonium acetate 0.05 mol l-' pH 6.0 2.0 ml min-' 1.0 ml min-' 2.0 moll-' (0 F A C E f Y % 2 B a I I I 3.5 4.5 5.5 6.5 7.5 8.5 Fig. 2 D Mn; E Ag; and F Ce Effect of ammonium acetate buffer pH A Pb; B Ni; C Cu; Journal of Analytical Atomic Spectrometry November 1995 Vol.10 93114 r I 12 n C D F G B E l w .A 0 0.02 0.04 0.06 0.08 0.1 0 Buffer concentratiodmol I-' 1 I I I I Fig.3 Effect of ammonium acetate buffer concentration A Ni; B Cu; C Mn; D Ce; E Ag; F V; and G Ti is effective at a minimum concentration of 0.05 moll-l. As the optimum matrix separation pH is more than 41 pH unit away from the buffer pK the buffer capacity is significantly lower than the maximum at pH 4.76.l' With decreasing buffer concentration this leads to a decrease in both the analyte percentage retention and the measurement precision. The anomalous result observed for 48Ti is due to ionogenic retention of 48Ca at low buffer concentrations. In the presence of buffer ionogenic retention of matrix species is reduced. This effect is also slightly evident at zero buffer concentration for 63Cu and 51V because of the formation of 40Ar23Na and 35C1160 in the plasma with residual Na and C1 respectively.The effect on analyte elution of increasing eluent acid concentration is illustrated for selected analytes in Fig. 4. In this study quantitative elution was observed for all the elements investigated at an acid concentration of 2.0 molle1. The acid concentration required for each individual ele- ment increased with increasing formation constant of the element-8-HQ complex. Acid concentrations greater than 2.0 moll-1 were not investigated as these would risk degrading the column and shortening its working life. To determine the effect on the column of repeated exposure to the acid eluent a second column was prepared from the same batch of immobilized 8-HQ and the two columns were compared in terms of element recoveries (Table 3).Recoveries for the selec- ted analytes were evaluated by spiking a synthetic sea-water sample with the selected elements at 10 ng ml-l. Five repeated analyses were made using the optimized manifold conditions I 1 I I 0 0.5 1 .o 1.5 2.0 Acid concentration/moI I-' Fig. 4 Effect of nitric acid eluent concentration A Pb; B Ni; C Cu; D Ag; and E V Table 3 Recovery comparison between aged and fresh columns Column Mn c o Ni c u Ce Aged column (YO) 98 99 72 91 89 Fresh column (YO) 101 104 78 95 95 and the recoveries were evaluated against a 50 ng ml-l multi- element solution prepared in 2.0 nitric acid injected into the manifold with the column by-passed. Recoveries between 89 and 104% were obtained (Table 3).The results showed that the original column performance was not signifi- cantly reduced for the analytes measured after use for approxi- mately 300 h. Dynamic and Batch Column Capacity Evaluations Evaluation of the capacity of the immobilized 8-HQ material was performed using Mn. A batch capacity of 0.086 & 0.009 mmol g - was measured. This compared well with a dynamic capacity measurement of 0.113+_0.003 mmol g-l which is in contrast to previous obser- vations of decreased capacity with dynamic ~peration.~ This illustrates that under the conditions of dynamic operation used in this study the column retains this analyte rapidly. These capacity values are consistent with earlier reported values for this material.4.9i16 The capacities with respect to other elements have not been investigated but on the basis of recovery results it is evident that the capacity of the material is sufficient for a wide range of elements.Evaluation of the Effect of Residual Matrix Species To evaluate the influence of residual matrix on the column following matrix separation and column washing the isotope ratio of 63Cu to 65Cu was evaluated for a spiked pure water sample a spiked sea-water sample and for both CRMs. If residual sodium was present at an unacceptably high level this ratio would be greater than the natural ratio because of the formation of the 40Ar23Na interference. Table 4 illustrates that residual matrix does not Significantly affect the isotope ratio measurement and therefore does not degrade the accuracy of the measurement.The accepted 63Cu 65Cu ratio is greater than the ratios measured in this study.17 This is primarily due to mass discrimination effects arising from the detector dead time (leading to lower count detection at higher count rates) and mass bias due to sequential scanning of the transient analyte peak. Calibration and Analysis of CRMs Using ICP-MS linear calibrations were obtained over the range 0-10ngml-l for analytes in both spiked pure water Table 4 Measured isotope ratio of 63Cu to "Cu for pure and saline water samples Measured 64Cu:65Cu ratio Sample description (n = 5)* Natural 63Cu 65Cu ratio accepted Cu in 5% HNO (10 ng rnl-l) Cu in sea-water (10 ng ml-l) 2.24 2.29 k 0.06 9.77 & 2.18 value" direct aspiration continuous aspiration 5.3 1 & 0.79 2.19 & 0.09 Cu in sea-water 110 ng ml-') injected Cu in 2 moll-' HNO (50 ng mI-l) injected Cu matrix separated from spiked pure Cu matrix separated from spiked sea- CASS-2 Cu ratio (1.5% salinity) SLEW-1 Cu ratio (3.5% salinity) 2.21 k0.05 2.18k0.08 2.19 4 0.06 2.16 k 0.08 water ( 5 ng ml-l) water ( 5 ng rn1-I) matrix separated matrix separated ~- * Values quoted with uncertainty (2 standard deviations 95% confidence limit) except for natural ratio. 932 Journal of Analytical Atomic Spectrometry November 1995 Vol.10Table 5 Comparison between spiked pure and sea-water calibrations Calibrations from spiked pure water- Parameter s,at 5ngml-I (n=5)(%) Correlation coefficient r Sensitivity/counts ng- mi/i05 Detection limit (3s n= 5)/ng ml-' Calibrations from synthetic sea-water- Parameter s at 5 ng ml-l (n=5) (Yo) Correlation coefficient r Sensitivity/counts ng-l m1/105 Detection limit (3s n = 5)/ng ml-' Mn 1.37 0.9999 9.8 0.53 1.77 0.9971 1.17 11.8 c o 1.75 0.9999 9.0 0.0 1 1.57 0.9985 0.01 10.8 c u 3.19 0.9992 1.9 0.30 1.71 0.9985 2.3 0.38 Zn 3.37 0.9884 2.0 0.38 2.68 0.9898 2.5 0.76 Cd 2.9 1 0.9988 1.2 0.02 2.23 0.9989 0.9 0.05 Table 6 Analysis results for the CRMs SLEW-1 and CASS-2 SLEW-1 CASS-2 Element Mn c o Ni c u Zn Cd Pb ~ Found* 12.7 & 0.4 0.05 1 & 0.003 0.738 _+ 0.049 1.68 k0.03 0.90+0.12 0.01 1 * 0.001 0.026 & 0.002 ~ Certified* 13.1 k0.8 0.046 f 0.007 0.743 _+ 0.078 1.76 & 0.09 0.86 & 0.1 5 0.018 k0.003 0.028 & 0.007 Found* 2.18 rfI 0.19 0.032 & 0.002 0.299 f 0.0 15 0.706 & 0.034 1.95 _+ 0.16 0.01 1 & 0.001 0.019 & 0.006 Certified* 1.99 & 0.15 0.025 & 0.006 0.298 If 0.036 0.675 & 0.039 1.97 & 0.12 0.019 & 0.004 0.019 & 0.006 * Concentrations in ng m1-l.Uncertainties expressed as 2 stand- ard deviations of the instrument response to each analyte (95% confidence limit). and spiked synthetic sea-water matrices as described in Table 5. The two calibration sets compared well therefore validating the use of simple pure water calibration solutions for determining analytes in more complex matrices. This procedure utilized much smaller volumes than would be required using a standard additions procedure and was less susceptible to contamination. The matrix separation procedure was validated by the analysis of the two CRMs SLEW-1 and CASS-2 of similar analyte concentration but different salinity. The analytes were quantified by external calibration against acidified multi- element (Mn Cu Zn Ni Co Cd Pb) pure water standards processed through the manifold.Five repeated analyses were made at each concentration and for the CRMs. The calibrations generally showed good linearity with least-squares regression coefficients of 0.997-0.999 being obtained in the concentra- tion ranges 0-20 ng ml-' (Mn) 0-5 ng ml-' (Cu Ni) and 0-0.1 ng ml - ' (Cd Pb Co). Precisions (measured as relative standard deviation s,) for the selected analytes were in the range 0.9-5.5%. Results for the CRM analyses are given in Table 6. For both materials good agreement between the found and certified values was obtained for all the elements measured.Direct aspiration of the saline CRMs for comparison with the matrix separation procedure was not attempted because of the associated problems of cone and injector blockage and signal suppression in the plasma. Injection of saline samples without prior matrix separation was observed to circumvent blockage problems but significant polyatomic interferences (Table 4) and signal suppression remained. It was considered that these problems coupled with the low analyte concen- trations present in the CRMs rendered direct injection inappropriate for this analysis. CONCLUSIONS A rapid on-line matrix separation procedure using a mini- column of CPG-immobilized 8-HQ was developed.The transient peak signals were successfully monitored using time- resolved data acquisition. A sampling frequency of 15 h-' was obtained with typical measurement precisions of s < 3%. Comparable linear calibrations were obtained from spiked pure and sea-water illustrating that external calibration using pure water solutions was suitable for determining the selected analytes in more complex matrices. Residual matrix retained on the column was shown to not affect the accuracy or precision of the analysis by monitoring the effect of the isobaric interference of 40Ar23Na on Cu in terms of the 63Cu 65Cu ratio. Application of the matrix separation manifold to the estuarine CRM SLEW-1 and the coastal CRM CASS-2 yielded results which were in good agreement with the certified values.These results indicate that ultra-trace element analysis of complex matrix samples is feasible in the absence of clean room facilities. The further application of this manifold to analysis of open-ocean sea-water samples would require a greater degree of preconcentration than is necessary for the coastal and estuarine samples analysed in this study. S.M.N. thanks the EPSRC and Fisons Instruments Elemental Analysis for their provision of funding for this project. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Houk R. S. Fassel V. A. Flesch G. D. Svec H. J. Gray A. L. and Taylor C. E. Anal. Chem. 1980,52 2283. Houk R. S. and Olivares J. A. Anal. Chem. 1986 58 20. Shabani M. B. Akagi T. Shimizu H. and Masuda A. Anal. Chem. 1990,62,2709. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981 53 2337. Olsen S. Pessenda L. C. R. Rbiitka J. and Hansen E. H. Analyst 1983 108 905. Hirata S. Kazuto H. and Kumamaru T. Anal. Chim. Actu 1989 221 65. Bloxham M. J. Hill S. J. and Worsfold P. J. J. Anal. At. Spectrom. 1994 9 935. Malamas F. Bengtsson M. and Johansson G. Anal. Chim. Acta. 1984 160 1. Marshall M. A. and Mottola H. A. Anal. Chem. 1985 57 729. Fang 2.-L. and Welz B. J. Anal. At. Spectrom. 1989 4 543. Beauchemin D. and Berman S. S. Anal. Chem. 1989 61 1857. Fang Z.-L. in Flow Injection Separation and Preconcentration VCH Weinheim 1993 ch. 4. Van Berkel W. W. Overbosch A. W. Feenstra G. and Maessen F. J. M. J. J. Anal. At. Spectrom. 1988 3 249. Habib K. A. J. PhD Thesis University of Hull 1991. Perrin D. D. and Dempsey B. in Bufers for pH and Metal Ion Control ed. Albert A. Chapman and Hall 1974 p. 24. Hill J. M. J. Chromatogr. 1973 76 455. Allen L. A. Pang H. Warren A. R. and Houk R. S. J. Anal. At. Spectrom. 1995 10 267. Paper 5/03072B Received May 15,1995 Accepted July 31 1995 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 933

ISSN:0267-9477    

DOI:10.1039/JA9951000929

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Computer Simulation of Enclosed Inductively Coupled Plasma Discharges Part 1 Monatomic Gases* Journal of Analytical Atomic Spectrometry ANA GAILLAT AND RAMON M. BARNES University of Massachusetts at Amherst Chemistry Department Lederle Graduate Research Tower Box 3451 0 Amherst MA 01 003-451 0 USA PIERRE PROULX AND MAHER I. BOULOS Universitk de Sherbrooke Dkparttment de G6nie Chimique Sherbrooke Qutbec Canada J1 K 2R1 An enclosed inductively coupled plasma (EICP) discharge is simulated with argon and neon and the thermodynamic and fluid flow fields are calculated for a sealed discharge. The influence of discharge container geometry and generator power and frequency are examined. Similarities between the model discharges reflect the thermodynamic and transport properties of argon and neon.Neon model discharges are found to be more stable than their argon counterparts. They exhibit higher maximum temperatures and lower wall temperatures than argon at similar operating frequencies. The velocities in the neon discharge simulation are higher than in an equivalent argon discharge. Keywords Enclosed inductively coupled plasma; computer simulation; argon plasma; neon plasma Optimization of the operation of an enclosed inductively coupled plasma (EICP) is a delicate tedious and expensive process when performed Some of the param- eters that should be considered (container geometry generator frequency and power gas pressure etc.) can be changed within only a very narrow experimental range. This does not always allow exhaustive analysis of potentially ideal operational conditions.The same analysis can be performed theoretically by apply- ing modelling techniques. This approach minimizes experimen- tal time and costs while providing in-depth information about the system. Mathematical modelling of plasma discharges has been applied extensively to the analysis of conventional induc- tively coupled plasmas ( ICP)374 and some enclosed plasma discharges5 With appropriate models the parameters of interest can be varied over a wide range and potentially ideal combi- nations of parameters can be evaluated. For this purpose a simulation model was developed for a sealed EICP in argon and neon.6 Some of the most important parameters that affect the optimum operation of an EICP are generator frequency and power discharge container geometry nature of the discharge gas and gas pressure within the discharge container.The effects of the radiofrequency generator power and frequency and discharge container geometry on EICP argon and neon dis- charges are considered in the present study. These monatomic gases were chosen because of the differences in their transport properties. The temperature and velocity fields within the discharges and the maximum axial and wall temperatures are calculated and compared. EXPERIMENTAL Some of the thermodynamic properties of argon and neon (thermal and electrical conductivities) are illustrated in Fig. 1. Thermal conductivity and specific heat functions are similar for both gases. The main difference between the gases is the considerably lower electrical conductivity of neon than argon corresponding to the higher ionization potential of the f ~ r m e r .~ From the analysis of these properties similar plasma behaviour is expected with the exception that the maximum gas tempera- ture in neon should be higher than in argon. The similarity of the thermal conductivity and specific heat functions of argon and neon indicates that both gases will show similar thermal distributions within the discharge. Since the electrical conduc- tivity of neon is lower and the ionization potential (21.56 eV) is higher than for argon (15.76 eV) the neon discharge will therefore have a tendency to maintain higher temperatures in the plasma than argon. The software employed for these simulations HiFILamp was developed from the HiFI (version 2.2) program described 2.50 2.00 1.50 1 .oo 0.50 0.00 1 .oox1 fl pr- m - .- - g 1.oox10' 6 1.00x100' ' ' ' ' ' ' ' I ' ' ' ' ' ' ' ' ' ' ' " ' ' I ' ' ' 300 2300 4300 6300 8300 10300 12300 14300 TemperaturdK ~~ ~ * Presented at the 1994 Winter Conference on Plasma Spectrochemistry San Diego CA USA January 10-15 1994. Fig.1 Thermodynamic properties as a function of temperature for argon and neon (a) thermal conductivity; and (b) electrical conductivity Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 935by Boulos et aL8 Several modifications have been introduced in the program to account for natural convection radiative transfer effects and the enclosed discharge geometry.6 The model based on steady state laminar flow axial symmetry and local thermodynamic equilibrium (LTE) assumptions provides information about the temperature flow and electro- magnetic fields for spherical and cylindrical container con- figurations with no gas flow (sealed mode).Gaillat et aL6 has described in detail the model and the mathematical algorithm used for the solution of its governing equations. The program allows the variation of multiple operational parameters. The present study explores the effect of generator frequency and power. The container shapes have been restricted to single similar volume cylindrical and spherical geometries. The effect of container size is also not presented in this paper since all possible parameter values fall into one of three categories. For very small containers the simulations are non- convergent or oscillatory. If the container volume is above a critical value the container size has negligible influence on the discharge.For extremely large containers a second critical value is reached above which the convergence is compromised and the results are oscillatory unless extremely high powers are used. The smaller critical value for the container diameter is between 45 and 50mm for most gases. The upper critical dimension is in the range 100-12Omm. These observations agree with experimental evidence since discharges contained in very small containers are unstable and those contained in very large containers require higher powers for their operation.' Another parameter that was not considered in the present study is the plasma gas pressure.Restriction of investigations into this parameter is caused by the lack of tabulated thermo- dynamic and transport properties for the discharge gases over a wide range of pressures. For those parameters that influence the EICP discharge but were not investigated here their numerical values remained unchanged throughout the study. The values assigned to these fixed parameters are as follows. (i) The grid accuracy selected for the simulations was set to 'high thus dividing the discharge volume into 40 control volumes in the axial and 20 in the radial direction.6 (ii) Each simulation was allowed to continue to full convergence regardless of the number of iterations 0 0 0 required. The convergence criterion for the three computational residues of the model was set to less than 1%.(iii) The internal gas pressure in the discharge Container was 101.325 kPa. (iv) The discharge container was assumed to have a 60mm diameter in both spherical and cylindrical conditions. For cylindrical discharges the length of the container also was assumed to be 60 mm. The geometrical layout of the discharge containers has been described previously.6 (v) The induction coil consisted of three turns over a 50 mm length centered on the discharge container in both axial and radial directions. The coil diameter was 66 mm. RESULTS AND DISCUSSION Typical temperature axial and radial velocity field profiles in EICP argon and neon discharges obtained from the simulation are shown in Figs. 2 and 3. As expected from the thermo- dynamic and transport properties of the gases neon produces a higher maximum temperature and a smaller volume simulated discharge than argon.Consequently the discharge container wall temperatures for neon are lower than for argon (cJ Figs. 4 and 5). Similar velocity fields exist in both dis- charges and the smaller neon discharge tends to compact the isocontours. The maximum discharge temperature attainable within a neon EICP model decreases as the generator frequency increases (Fig. 4). This behaviour is due to the delocalization and outward shift of the isotherms resulting from a decrease in the pseudo-skin depth as the generator frequency is increa~ed.~ This effect also causes cooling in the axial region and heating of the container walls. The simulation results obtained for argon (Fig.5) are consistent with those obtained for neon. These trends are expected when literature results for the conventional ICP4 are extrapolated to an enclosed discharge. Comparisons between neon and argon EICP simulations in spherical and cylindrical containers also are shown in Figs. 4 and 5. The model suggests a colder discharge exists in the cylindrical than in the spherical container of the same effective volume. This difference results mainly from the greater possibil- ity for recirculation of the discharge gases in the cylindrical 0 0 0 l c m Fig. 2 A cylindrical EICP argon discharge operated at 40 MHz and 700 W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours. The centre of the induction coil windings is represented by a circle; the central axis of the coil is located on the left side of the plots. The high field is at the upper end of the figures 936 Journal of Analytical Atomic Spectrometry November 1995 Vol.10I I 0 0 F.J I 4- 0 I 0 0 0 0 0 1 cm Fie 3 A cylindrical EICP neon discharge at 40 MHz and 700 W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours. Other - parameters as in Fig. 2 10000 I i 8500 z IJ? g 950 - - g 900 850 800 750 700 650 Spherical Cylindrical I 0 10 20 30 40 50 60 70 80 90 100 FrequencyjMHz Fig.4 Variation with frequency of the maximum temperatures in a neon EICP at 750 W (a) maximum discharge temperature; and (b) maximum wall temperature container.6 The cylindrical discharge container walls also are colder than the spherical container walls.Nonetheless the relative positions of the hottest discharge areas in both geo- metric configurations are comparable. These effects appear more conspicuously for argon than neon. For high powers or frequencies the model argon discharge wall temperatures are increased more than in the neon EICP which in turn decreases the stability of the discharge. Changing the container geometry from a spherical to a cylindrical shape produces a discharge removed from the walls and increases the plasma stability. g500r 9000 g 8500 I- 2 8000 7500 A 1150 1 1 1100 ( b ) 1050 1 1000 Spherical A 950 900 850 800 750 700 10 20 30 40 50 60 70 80 90 100 FrequencyIMHz Fig. 5 Variation with frequency of the maximum temperatures in an argon EICP operated at 750 W (a) maximum discharge temperature; and (b) maximum wall temperature This should extend the practical lifetime of the discharge container.Small systematic changes in the maximum gas velocities in both radial and axial directions occur when the generator frequency is varied in the simulation from 10 to 100 MHz (Figs. 6 and 7). Earlier studies of open ICP discharges demon- strated similar trend^.^ The maximum velocities are related Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 937'. LI -2 t ' I I I I I I I I 0.5 I -3 -0.9 = -1.1 -1.3 1 I I I 1 I I I I 10 20 30 40 50 60 70 80 90 100 Frequencyhl Hz Fig. 6 Variation with frequency of the maximum velocities in a neon EICP operated at 750 W (a) maximum axial velocity; and (b) maximum radial velocity for spherical (A) and cylindrical (+ ) discharge containers 1 I 1 I I I I b 9 -0.3 $ -0.5 E .- E -0.9 - - f -Os7 2 $ 1 -1.1 -1.3 1 ' I 1 I 1 1 I I 10 20 30 40 50 60 70 80 90 100 Freq u ency/M Hz Fig.7 Variation with frequency of the maximum velocities in an argon EICP operated at 750W (a) maximum axial velocity; and (b) maximum radial velocity for spherical (A) and cylindrical (+) discharge containers mainly to the maximum gas temperatures present and not to their position within the plasma. As the frequency of the discharge increases the model suggests the axial and radial velocity field isocontours are displaced as is the temperature field towards the container walls.The small asymmetries in the axial velocity fields shown in Fig. 8 Variation with power of the maximum temperatures in a neon EICP operated at 40 MHz (a) maximum discharge temperature; and (b) maximum wall temperature I 9700 9200 5 hE 8700 8200 7700 950 1 Cylindrical 00 1000 1100 1200 1300 1400 1500 PowerMl Fig.9 Variation with power of the maximum temperatures in an argon EICP operated at 40 MHz (a) maximum discharge temperature; and (b) maximum wall temperature 938 Journal of Analytical Atomic Spectrometry November 1995 Vol.102 I -2 h 1.5 r u) \ € 1 x c. .- 8 0.5 - 9) G o .- .- i -0.5 x -l -1.5 1 I I I I I I 1 I 0.5 I 1 L -2 - I 1 I I I I I 1 I 500 600 700 800 900 1000 1100 1200 1300 1400 1500 PowerNV 500 600 700 800 900 1000 1100 1200 1300 1400 1500 PowerNV Fig. 10 Variation with power of the maximum velocities obtained in a neon EICP operated at 40MHz (a) maximum axial velocity; and (b) maximum radial velocity for spherical (A) and cylindrical (+) discharge containers Figs.6 and 7 are due to the presence of small secondary recirculation areas. The noticeable asymmetry in the radial velocity can be explained by considering the field geometry (Fig. 3). The radial velocity field simulation exhibits only one negative velocity region but it presents two nearly equivalent areas of positive velocities. Therefore to keep the recirculation balance the absolute value of the negative velocities has to be approximately twice that of the positive components. The effects of increasing power in the argon and neon model EICP discharges are illustrated in Figs. 8 and 9. As more power is available to the discharge all the discharge tempera- ture fields increase.This is also anticipated from corresponding experience with open ICPS.~ A marked increase in gas velocities follows the increase in generator power because more kinetic energy is made available to the gas at higher radiofrequency powers. The results shown in Figs. 10 and 11 demonstrate parallel trends for argon and neon EICPs. Differences in the absolute values of the fields exist for the two EICP geometric configurations. However the maximum temperatures in neon are influenced less by the container shape than in argon (Figs. 8 and 9). Both gases show a completely analogous behaviour for both container geometries. In all cases the discharges are symmetrical alongside the main axis (ie. the induction coil axis) with only a very small (less than 10%) extension of the discharge towards the high field areas.The simulated effects of frequency and power on a neon discharge are analogous to those for argon even though the gases possess different thermodynamic and transport proper- ties. Therefore argon and neon could be used interchangeably as EICP discharge gases. However the optimum operating conditions for both gases will differ. For all power and fre- quency conditions examined the model suggests that neon Fig. 11 Variation with power of the maximum velocities obtained in an argon EICP operated at 40 MHz (a) maximum axial velocity; and (b) maximum radial velocity for spherical (A) and cylindrical (+) discharge containers produces a hotter and smaller discharge than argon. The wall temperatures generally are lower partly owing to the relatively smaller size of the neon discharge.Within the commonly used ICP operating frequency range (10-100 MHz) the model suggests that both gases behave consistently. No unique frequency exists from the point of view of performance or stability. Neon can be operated at lower frequencies than argon without detriment to the spectroscopic characteristics of the discharge owing to its higher thermo- dynamic temperatures. Thus to produce a required maximum discharge temperature value the model suggests that the operating power necessary for a neon discharge will be lower than for argon. Some of the critical features to consider for a potential discharge support gas are optimum excitation temperature freedom from spectral interferences and conservative use of the discharge container. Typically a compromise among sev- eral of these parameters is needed.If a higher excitation temperature is required the model suggests that changing the discharge gas from argon to neon could be adequate to produce a suitable analytical environment under the frequency and power conditions for which argon would produce a discharge with insufficient thermal energy. Substitution of neon for argon can result in an increase in maximum discharge and axial temperatures without an increase in the wall tempera- tures. The simulated temperature gradients present in both axial and radial directions are more pronounced in neon than in argon resulting in a concentrated discharge removed from the walls of the container.This effect nevertheless does not produce an extreme reduction in the size of the plasma which would make imaging of the analysing optics difficult. Because the wall temperatures in a cylindrically contained discharge remain considerably colder than in an equivalent spherical Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 939container employing a cylindrical instead of a spherical dis- charge container should extend the lifetime of the container without sacrificing spectroscopic performance. Practical EICP discharges in neon krypton and xenon at 350-400 W in a 65,mm spherical container have been generated experimentally to observe their atomic emission spectra from 200 to 900nm.lo Extending the EICP simulation to krypton and xenon requires only the substitution of the appropriate thermodynamic and transport properties.Since these two gases resemble argon in all their thermodynamic and transport properties the results obtained in the simulation of krypton and xenon EICP discharges are fully analogous with those of an argon plasma. Analyte excitation in argon and neon EICP discharges has not been considered in the present investigation although differences in excitation processes described for argon and neon in a radiofrequency boosted pulsed hollow cathode lamp,ll for example can be expected in an EICP. Excited-state populations of atoms and ions in argon and helium12 ICP discharges deviate from LTE. A neon discharge is expected to exhibit non-LTE characteristics between argon and helium since the mass and ionization potential of neon are between those of helium and argon.The model needs to be extended to incorporate non-LTE conditions to describe the argon and neon EICP discharges fully. This research was sponsored by the ICP Information Newsletter. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 Jahl M. J. Jacksier T. and Barnes R. M. J. Anal. At. Spectrorn. 1992 7 653. Jacksier T. and Barnes R. M. J. Anal. At. Spectrom. 1992,7,839. Barnes R. M. and Yang P. Y. Spectrochim. Acta Rev. 1990 13 275. Boulos M. I. and Barnes R. M. in Inductively Coupled Plasma Emission Spectroscopy Part I I ed. Boumans P. W. J. M. John Wiley New York 1987 ch. 9. Paik S. H. and Pfender E. Plasma Chem. Plasma Proc. 1990 10 167. Gaillat A. Barnes R. M. Proulx R. and Boulos M. I. Spectrochim. Acta Part B in the press. Dresvin S . V. Physics and Technology of Low Temperature Plasmas Iowa State University Press IA 1977. Boulos M. I. Mostaghimi J. and Proulx P. High Frequency Induction Plasma UniversitC de Sherbrooke Sherbrooke 1989. Boulos M. I. Pure Appl. Chem. 1985 57 1321. Jacksier T. and Barnes R. M. Appl. Spectrosc. 1994 48 65. Farnsworth P. B. and Walters J. P. Spectrochim. Acta Part B 1982 37 773. Cai M. Montaser A. and Mostaghimi J. Spectrochim. Acta Part B 1993 48 789. Paper 5/02348C Received April 11 1995 Accepted July 17 1995 940 Journal of Analytical Atomic Spectrometry November 1995 Val. 10

ISSN:0267-9477    

DOI:10.1039/JA9951000935

出版商:RSC    

年代:1995

数据来源: RSC

摘要:

Computer Simulation of Enclosed Inductively Coupled Plasma Discharges Part 2*. Molecular Gasest Journal of Analytical Atomic Spectrometry ANA GAILLAT AND RAMON M. BARNES University of Massachusetts at Amherst Chemistry Department Lederle Graduate Research Tower Box 34510 Amherst MA 01 003-4510 USA PIERRE PROULX AND MAHER I. BOULOS Universitk de Sherbrooke D6partkment de Gknie Chimique Sherbrooke Qukbec Canada J1 K 2R1 The thermodynamic and fluid flow fields within hydrogen nitrogen and oxygen enclosed inductively coupled plasma (EICP) discharges are calculated using a novel computer simulation. Nitrogen and oxygen discharges give easily convergent results and are more stable than a hydrogen discharge calculation. The maximum calculated plasma temperatures in 700 W 40 MHz EICP discharges are 10 500 K for hydrogen 10 200 K for oxygen 8300 K for argon and 6500 K for nitrogen.The corresponding maximum axial velocities are 48 m s-' for hydrogen 9.2 m S-' for oxygen 1.2 m s-l for argon and 0.36 m s-' for nitrogen. The hydrogen discharge simulation exhibits higher temperature and velocity gradients than oxygen argon or nitrogen. Keywords Enclosed inductively coupled plasma; computer simulation; hydrogen plasma; nitrogen plasma; oxygen plasma 16.0 14.0 L 12.0 7 10.0 .t > 8.0 .- c 3 6.0 F E FE - (D 4.0 0 2.0 0.0 300 2300 4300 6300 8300 10300 12300 14300 Temperature/K The development of a practical enclosed inductively coupled plasma (EICP) discharge enables the formation and appli- cation of atmospheric pressure discharges with gases typically not employed for conventional inductively coupled plasma (ICP) sources. For example the atomic spectra of chlorine,' neon krypton and xenon2 from EICP discharges have been reported recently.Optimization of these laboratory discharges can be aided by computer simulations and a comparison of argon and neon EICP simulation was illustrated in Part 1 of this ~eries.~ In the present paper the EICP computer simulation has been extended to molecular gases including hydrogen nitrogen and oxygen. Diatomic gas transport and thermo- dynamic properties differ significantly from those of monatomic gases principally because of the dissociation of the molecule into atoms. Consequently the computer simulation requires modification to accommodate the calculations relevant to molecular gases.Computer models of conventional ICP discharges have included heli~m,~ argon nitrogen and oxygen,' but reports on a hydrogen ICP simulation are scarce.6 No experimental hydrogen ICP spectrochemical discharge has been described. In one comparison the maximum calculated temperatures were 10400 K for argon 8950 K for oxygen 7510 K for nitrogen and 6830 K for hydrogen.6 Hydrogen oxygen and most diatomic molecules dissociate at fairly low temperatures compared with nitrogen and their thermodynamic and transport properties differ significantly. Dissociation of hydrogen and oxygen causes a maximum in their thermal conductivity and heat capacity at 3000-4000 K (Figs. 1 and 2). The maximum for nitrogen is about 7000 K. Consequently hydrogen and oxygen behave fairly differently to nitrogen or monatomic gases in plasma discharges and the * For Part 1 of this series see ref.3. t Presented at the 1994 Winter Conference on Plasma Spectrochemistry San Diego CA USA January 10-15 1994. Fig. 1 Thermal conductivity as a function of temperature for argon hydrogen nitrogen and oxygen 2 % 1 . 0 ~ 1 0 ~ .= 2 . 0 ~ 1 0 ~ 300 2300 4300 6300 8300 10300 12300 14300 I I / 0 6.0~10~ g 8.0~10~ ? 4 . 0 ~ 1 0 ~ o.ox1oo Temperature/K Fig. 2 Heat capacity as a function of temperature for argon hydrogen nitrogen and oxygen size of the discharge should be small and the maximum temperature high. However the electrical conductivities of argon and oxygen are higher below 7000K than those of hydrogen and nitrogen. In earlier simulations substantial differences were found in Joule heating and temperature fields for hydrogen and oxygen ICP discharges although their dissociation behaviour is similar.6 The goal of the present investigation is to examine the properties of these molec- ular gas discharges by computer simulation of the EICP configuration.EXPERIMENTAL The EICP model and its mathematical algorithm used for the solution of the governing equations have been described by Journal of Analytical Atomic Spectrometry November 1995 Vol. 10 941Gaillat et ~ 1 . ~ The model is based on steady state laminar flow axial symmetry and local thermodynamic equilibrium assumptions (LTE) and provides information about the tem- perature flow and electromagnetic fields for spherical and cylindrical discharge configurations in the sealed (no flow) mode.The program HiFILamp also includes natural convec- tion and radiative transfer effects as well as the enclosed characteristics of the discharges. The version of HiFILamp used for these simulations consists of the option of extra high density grids. In this configuration the total control volume is divided into 40 x 80 cells. The modelling of EICP diatomic gas discharges presents some practical problems which arise from the same properties that make these plasmas useful. For hydrogen and oxygen discharges the characteristic sharp plasma temperature gradi- ents require use of extra high density grids (40x80) for the computer ~imulation.~ With only high density grids (40 x 20) non-convergent or unstable results are obtained.* For mon- atomic gases (argon helium neon etc.) and high dissociation temperature diatomic gases such as nitrogen the use of extra high density grids (40 x 80) in the computer simulation does not provide sufficient advantage to outweigh the increased processing time required.When hydrogen and oxygen enclosed plasmas are simulated with extra high density calculation grids (40 x 80) convergent and stable solutions are obtained for a wide range of power and frequency operating conditions for cylindrical and spheri- cal geometric configurations. However only one set of results is presented here for a 60 mm spherical container at 700 W and 40 MHz. Of all the operating parameters that can be studied with this program only the gas properties are changed in this investigation.The values of the geometrical parameters are kept constant and all fixed parameters values assigned have been described in Part l.3 The processing time for a fully convergent simulation when using extra high density grids for hydrogen and oxygen is approximately 2 h with a Dell 486/33 MHz personal computer. Under similar convergence requirements a standard high density simulation for argon and nitrogen takes approximately 1 h with the same computer. RESULTS AND DISCUSSION One objective of the present study was to establish accurate and stable simulation results for hydrogen and oxygen EICP discharges. Molecular gases exhibit maxima in their thermal conductivity and heat capacity as a function of temperature at their dissociation temperature^^,'^ as shown in Figs.1 and 2. This characteristic defines the behaviour of a discharge by controlling the shape and size of the highest temperature areas within the discharges.' Thermal conductivity and specific heat functions for hydro- gen nitrogen and oxygen are similar. The feature that domi- nates each of these curves is the peak in the distribution of these properties with respect to temperature resulting from the dissociation of the gases. The main difference amongst these three gases consists of the location of the dissociation maxima. While hydrogen and oxygen properties peak at temperatures well below the operating temperature of an EICP nitrogen shows a maximum at much higher temperatures. From the analysis of the properties of these gases similar behaviour is expected for hydrogen and oxygen which will differ markedly from those of nitrogen or argon.Argon hydrogen nitrogen and oxygen EICP spherical dis- charges generated with the same generator power (700 W) and frequency (40 MHz) are compared in Fig. 3. The 6000 K iso- therm for the four discharges represents their different shapes and sizes. The model suggests that hydrogen produces the (2 f ) 0 0 0 Fig. 3 Position and shape of the 6000 K isotherm for argon hydrogen nitrogen and oxygen EICP discharges operated at 40 MHz and 700 W in a spherical container. Results obtained by using the values of the fixed parameters presented in ref. 3. The centre of the induction coil windings are represented by the circles most compact discharge followed by oxygen nitrogen and argon.In contrast to the results of Yu and Girshick,' the hydrogen and oxygen discharges are hottest (10490 and 10 170 K respectively) followed by argon (8320 K) and nitro- gen (6460 K). The high temperatures and the shape of the hydrogen and oxygen plasmas are potentially useful as ETCP support gases. The small discharge size will keep the container walls relatively cool and increase the lifetime of the containers. The temperature axial and radial velocity fields for hydro- gen oxygen nitrogen and argon EICPs at 700 W and 40 MHz are illustrated in Figs. 4-7 respectively. The radial and axial temperatures at different locations in the induction coil are plotted in Figs. 8-13. The differences in the thermodynamic properties of the gases are responsible for the diversity in the temperature and velocity fields for these EICP discharges. These properties characterize the discharge by controlling the shape and size of the highest temperature areas (Fig.3). These observations are supported by a systematic study of the influence of these properties on the discharge fields which has been presented recently.'' The plasmas produced with hydrogen and oxygen in the simulation are hotter and more compact than those calculated for nitrogen and argon. This effect is apparent in both axial and radial directions (Figs. 8 and 9). Although the dissociation energies for hydrogen nitrogen and oxygen are high (435.72 948.93 and 497.48 kJ mol-l respectively) the temperature at which the dissociation occurs controls the properties of the discharge. The temperatures at which hydrogen nitrogen and oxygen reach 50% dissociation are 4285 7075 and 3759K respectively.Hydrogen and oxygen produce discharges that are cooler in the periphery where the dissociation processes principally occur. In the central regions of the plasma where most of the gas molecules are already dissociated the tempera- tures reached by the hydrogen and oxygen discharges are considerably higher than in an argon or nitrogen EICP. These two effects result in a model discharge with a very hot and highly localized core and a cooler outside area (Figs. 10 and 11). The hydrogen discharge is smaller and hotter because its transport properties (e.g. heat capacity) are higher than those 942 Journal of Analytical Atomic Spectrometry November 1995 Vol.100 0 Fig.4 A cylindrical EICP hydrogen discharge operated at 40MHz and 700 W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours 3 3 0 0 0 0 0 0 Fig. 5 A cylindrical EICP oxygen discharge operated at 40 MHz and 700 W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours of oxygen. These predictions are significantly different from the conclusions reached by Yu and Girshick,' who calculated much cooler hydrogen than oxygen discharges. The simulated oxygen EICP results are however consistent with earlier models of and experimental data for conventional oxygen ICP discharge^.^,^^ For the nitrogen EICP isotherms throughout the simulated discharge are more homogeneously distributed than in the other diatomic gases (Fig.12) because the maximum discharge temperature is lower than the maximum of the nitrogen dissociation temperature. Although nitrogen dissociation requires more energy at high temperatures than hydrogen and oxygen the nitrogen discharge maintains a uniform radial temperature as does the argon EICP. These results also parallel earlier models of conventional nitrogen ICP discharges.13 Since argon is monatomic the discharge produced extends uniformly almost to the container wall (Fig. 13). The homo- geneous temperature distribution calculated for the EICP is very similar to that observed for a conventional argon ICP without a central flow.14 Axial and radial velocity fields increase as the computed plasma isotherms shrink and the maximum discharge tempera- ture increases.Almost a factor of five distinguishes the maxi- mum axial flow rates between the gases. The maximum axial velocities are 47.7 m s-l for hydrogen 9.22 m s-l for oxygen 1.17 m s-' for argon and 0.36 m s-' for nitrogen. Radial velocities also increase in the same order. Very fast gas flow in the hydrogen discharge is predicted with high radial flow into the side of the discharge in the centre of the induction coil. This high axial and radial velocity rather than the small hot discharge could be the limiting characteristic that prevents the easy generation of a practical hydrogen ICP discharge. The response of the model discharge to changes in the generator power and frequency is shown in Figs. 14 and 15 respectively. The effects of increasing power in all the EICP discharges studied are as expected.As more energy is available to the discharge all the discharge temperature fields increase. This is also anticipated from corresponding experience with open ICPs.' In addition the temperature field isocontours are displaced towards the container walls with the increase of the Journal of Analytical Atomic Spectrometry November 1995 VoZ. 10 943. . 0 0 0 3 3 0 -0.05 2 0.15 0 Fig.6 A cylindrical EICP nitrogen discharge operated at 40 MHz and 700W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours pj -0.6 3 3 0 0 0 c -0.2 - ‘0 - /I Fig. 7 A cylindrical EICP argon discharge operated at 40 MHz and 700 W (a) temperature; (b) axial velocity; and (c) radial velocity isocontours 12000 l2O0O - 10000 0 -f C .i 8000 g 6000 - a 3 4000 I- 3 2000 10000 8000 s 8j CI 2 6000 .- - c.5 4000 0 2000 t I I I I 5 I 10 15 20 25 30 rlrnm Fig.8 Radial temperature profile at axial grid point 40 (middle of the coil) for argon hydrogen nitrogen and oxygen cylindrical EICP discharges operated at 40 MHz and 700 W I I I I I L I 20 30 40 50 60 10 z /mm Fig. 9 Axial temperature profile at the coil axis; for argon hydrogen nitrogen and oxygen cylindrical EICP dischargeis operated at 40 MHz and 700 W 944 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10l2OO0 9 % 5000 t- 4000 3000 2000 1000 Grid int 60 400: 2000 r /rnm Fig. 10 Radial temperature profiles of a 40 MHz and 700 W hydrogen cylindrical EICP discharge at grid point 20 (beginning of the coil) grid point 40 (middle of the coil) and grid point 60 (end of the coil) - 1 12000 1 10000 8000 % 6000 L 4000 2000 I I I I I 5 10 15 20 25 30 rlmm Fig.11 Radial temperature profiles of a 40 MHz and 700 W oxygen cylindrical EICP discharge at grid point 20 (beginning of the coil) grid point 40 (middle of the coil) and grid point 60 (end of the coil) 7000 Grid-point 40 6000 5000 5 4000 3000 2000 1000 I 5 10 15 20 25 30 rlrnrn Fig. 12 Radial temperature profiles of a 40 MHz and 700 W nitrogen cylindrical EICP discharge at grid point 20 (beginning of the coil) grid point 40 (middle of the coil) and grid point 60 (end of the coil) generator frequency in good agreement with literature and experimental data. Within the commonly used ICP operating frequency range (10-100 MHz) the model suggests that all the gases behave consistently. No unique frequency exists from the point of view of performance or stability. If the spectroanalytical application requires a high- temperature high-velocity plasma the model suggests that hydrogen and oxygen are the preferred gases. This conclusion corresponds to earlier observations for conventional ICP dis- charge~.~~'~ A plasma supported by either hydrogen or oxygen 7000 6000 1 Gridmid60 \ ' \\ I 0 I I I I I 5 10 15 20 25 30 I r /mrn Fig.13 Radial temperature profiles of a 40 MHz and 700 W argon cylindrical EICP discharge at grid point 20 (beginning of the coil) grid point 40 (middle of the coil) and grid point 60 (end of the coil) 12900 11900 10900 y 9900 8900 7900 6900 5900 1350 r 750 500 700 900 1100 1300 1500 1700 1900 PoweriW Fig.14 Effect of generator power on (a) the maximum discharge temperature and (b) the maximum wall temperature for argon hydro- gen nitrogen and oxygen cylindrical EICP discharges operated at 40 MHz will displace the hot discharge from the container walls minimize thermal failure and improve the operating lifetime of the container.l69l7 However hydrogen can diffuse through hot quartz walls in practical EICP discharges containing hydrogen and the discharge lifetime in a sealed EICP is finite." Thus an oxygen EICP could be easier to operate for extended periods.Nitrogen and argon EICP simulations predicted homo- geneous axial and radial temperature distributions. The dis- charges closely approach the container walls potentially jeopardizing the integrity of the quartz.13 These results agree with qualitative experimental data available for EICP discharges.Molecular gas EICP simulations provide data for nitrogen and oxygen discharges consistent with conventional ICP discharge experimental and computed results. Molecular gases produce discharges smaller than those supported by argon Journal of Analytical Atomic Spectrometry November 1995 VoZ. 10 94512000 (a 11 11000 10000 y 9000 t-!! 8000 7000 1 N I z E ' I I ~ 1 ' 600 10 20 30 40 50 60 70 80 90 100 FrequencyIMHz Fig. 15 Effect of generator frequency on (a) the maximum discharge temperature and (b) the maximum wall temperature for argon hydro- gen nitrogen and oxygen cylindrical EICP discharges operated at 700 w with the oxygen plasma supporting higher temperatures than both argon and nitr~gen.~ New results have been computed for hydrogen that appear self-consistent with its properties and other molecular gases.An extra high density grid is necessary and effective for hydrogen and oxygen calculations. The results of this model for argon nitrogen and oxygen correspond closely to earlier results from models and experimental measurements for conventional ICP discharges. The predicted hydrogen EICP discharge is therefore also expected to rep- resent accurately the hydrogen ICP discharge. Verification however requires that a practical hydrogen ICP be produced experimentally. Furthermore the direct application of the EICP simulation to other diatomic gases (e.g. chlorine and hydrogen chloride) is expected.The effects of frequency and power on model hydrogen nitrogen and oxygen discharges are analogous with those for argon although the gases possess different thermodynamic and transport properties. For all power and frequency conditions examined hydrogen produces the hottest and smallest simu- lated discharge followed by oxygen argon and nitrogen. The wall temperatures in a model hydrogen discharge generally are lower owing to the relatively smaller size of the discharge. This effect is less pronounced with oxygen although the simulated wall temperatures are still considerably lower than those present in an argon plasma. An important practical limitation to the application of an argon EICP even at relatively low generator power is the relatively high wall temperatures which can reach the softening point of quartz.Therefore unless the discharge is sustained at very low frequen- cies the operating range for the power of an argon plasma will be considerably smaller than that of a molecular gas. In contrast the simulated hydrogen EICP model results agree with the experimental observation that with the addition of hydrogen to an EICP discharge the discharge is reduced and moved away from the container walls. Thus in hydrogen admixtures the lifetime of practical EICP containers is extended. This research was sponsored by the ICP InJorrnation Newsletter. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Jacksier T. and Barnes R. M. Appl. Spectrosc. 1994 48 65. Jacksier T. and Barnes R. M. Appl. Spectrosc.1994 48 385. Gaillat A. Barnes R. M. Proulx P. and Boulos M. I. J. Anal. At. Spectrom. 1995 10 935. Cai M. Montaser A. and Mostaghimi J. Spectrochim. Acta Part B 1993 48 789. Barnes R. M. and Yang P. Y. Spectrochim. Acta Rev. 1990 13 275. Yu W. and Girshick S. L. Proceedings cf the 9th International Symposium on Plasma Chemistry Pugnochiuso Italy September Gaillat A. Barnes R. M. Proulx P. and Boulos M. I. Spectrochim. Acta Part B in the press. Boulos M. I. Mostaghimi J. and Proulx P. High Frequency Induction Plasma UniversitC de Sherbrooke Sherbrooke 1989. Dresvin S . V. Physics and Technology of Low Temperature Plasmas Iowa State University Press IA 1977. Kurtz A. and Mentel J. J. Phys. D Appl. Phys. 1984 17 1343. Gaillat A. M. and Barnes R. M. paper presented at the 1995 European Winter Conference on Plasma Spectrochemistry Cambridge UK January 1995 paper M1. Yang P. and Barnes R. M. Spectrochiwr. Acta Part B 1989 44 1093. Barnes R. M. Kovacic N. and Meyer G. A. Spectrochim. h a Part B 1985 40 907. Barnes R. M. and Schleicher R. G. Spectrochim. Acta Part B 1985 40 81. Boulos M. I. and Barnes R. M. in Inductively Coupled Plasma Emission Spectroscopy Part 11 ed. Boumans P. W. J. M. John Wiley New York 1987 ch. 9. Jacksier T. and Barnes R. M. J. Anal. At. Spectrom. 1992,7,839. Jahl M. J. Jacksier T. and Barnes R. M. J. Anal. At. Spectrom. 1992 7 653. Jacksier T. and Barnes R. M. J Anal. At. Spectrom. 1994,9,1299. 1989 V O ~ . 1 pp. 31-36. NOTE-Ref. 3 is to Part 1 of this series. Paper 5 J02349A Received April 11 1995 Accepted July 17 1995 946 Journal of Analytical Atomic Spectrometry November 1995 Vol. 10

ISSN:0267-9477    

DOI:10.1039/JA9951000941

出版商:RSC    

年代:1995

数据来源: RSC