摘要:

Ramon M. Barnes Editor Department of Chemistry LGRC Towers University of Massachusetts Amherst MA 01 003-0035 Telephone (41 3) 545-2294 fax 545-4490 0 bjec t ive The ICP INFORMATION N€WSL€TT€R is a monthly journal published by the Plasma Research Group at the University of Massachusetts and is devoted exclusively to the rapid and impartial dissemination of news and literature information re- lated to the development and applications of plasma sources for spectrochemical analysis. Background ICP stands for inductively coupled plasma discharge which during the past decade has become the leading spectrochemi- cal excitation source for atomic emission spectroscopy. ICP discharges also are applied commercially as an ion source for mass spectrometry and as an atom and ion cell in atomic fluo- rescence spectrometry.The popularity of this source and the need to collect in a single literature reference all of the pertinent data on ICP stimulated the publication of the ICP INFOR- MA TlON NEWSLETTER in 1975. Other popular plasma sources Le. microwave induced plasmas direct current plasmas and glow discharges also are included in the scope of the ICP IN- FORMATION NEWSLETTER. Scope As the only authoritative monthly journal of its type the ICP INFORMATION NEWSLETTER is read in more than 40 coun- tries by scientists actively applying or planning to use the ICP or other types of plasma spectroscopy. For the novice in the field the ICP INFORMATION NEWSLETTER provides a concise and systematic source of information and background material needed for the selection of instrumentation or the development i of methodology.For the experienced scientlst it offers a sin- gle-source reference to current developments and literature. Editorial The ICP INFORMATION NEWSLETTER is edited by Dr. Ramon M. Barnes Professor of Chemistry University of Mas- sachusetts at Amherst with the assistance of a 20-member Board of National Correspondents composed of leading plasma spectroscopists. The Board members from around the world report news viewpoints and developments. Dr. Barnes has been conducting 'plasma research on ICP and other dis- charges since 1968. He also serves as chairman of the Winter Conference on Plasma Spectrochemistry sponsored by the ICP INFORMA TION NEWSLETTER. Regular Features *Original submitted and invited research articles by ICP and *Complete bibliography of all major ICP publications.*Abstracts of all ICP papers presented at mqjor US and inter- *First-hand accounts of world-wide ICP developments. *Special reports on dcp microwave glow discharge and other Calendar and advanced programs of plasma meetings. *Technical translations and reprints of critical foreign-lan- guage ICP papers. *Critical reviews of plasma-related books and software Conference Activities The ICP INFORMATION NEWSLETTER has sponsored seven international meetings on developments in atomic plasma spectrochemical analysis since 1980 in San Juan Orlando San Diego St. Petersburg and Kailua-Kona. Meeting pro- ceedings have appeared as Developments in Atomic Plasma Spectrochemical Analysis (Wiley) Plasma Spectrochemistry and Plasma Spectrochemistry I/-1V (Pergamon Press) as well as in special issues of Spectrochimica Acfa Pad B and Journal of Analytical Atomic Spectrometry. The 1994 Winter Confer- ence on Plasma Spectrochemistry will be held in San Diego California January 10 - 15 1994; its proceedings will be published by Fall 1994.Subscription Information Subscriptions are available for 12 issues on either an annual or volume basis. The first issue of each volume begins in June and the last issue is published in May. For example Volume 18 runsfrom June 1992 through May 1993. Backissuesbeginning with Volume 1 May 1975 also are available. To begin a subscription complete the form below and submit it with prepayment or purchase information. For additional informa- tion please call (41 3) 545-2294 fax (41 3) 545-4490 or contact the Editor.Credit cards accepted. plasma experts. national meetings. plasma progress. To order complete this section and send it to ICP Information Newsletter %Dr. Ramon M. Barnes Depart- ment of Chemistry Lederle GRC Towers University of Massachusetts Amherst MA 01 003-0035 USA. Start a subscription for the following issue 0 Volume(s)- (June 19- - May 19- ) or 0 19 (January - December). Enclosed 0 Prepayment 0 Check or money order QVISA 0 Mastercard Account No. (All 73 or 16 digits) ) or 0 Send invoice. Date Card holder Name Expiration date Card holder Signature . Amount Due $ Mail to Name Organization Address City S ta te/C ou n t ry ZI P/Postalcode Telephone Telexjf ax Note For each credit-card transaction a 4 % service charge will be added reflecting our bank charges.Current subscription rates are $60 (North America) $85 (Europe South America) or $94 (Africa Asia Indian/Pacific Ocean Areas Middle East and Russia). Back issue rates available on request. All payments should be made with US dollars by draft on a US bank by international money order or by credit card. Foreign bank checks are not accepted. 13 Purchase order (No.11 The Royal Society of Chemistry needs well-qualified and reliable freelance workers to write abstracts of journal articles on analytical chemistry. Knowledge of a foreign language is desirable ~ but not essential. The Royal Society Of Chemistry Analytical Division Atomic Spectroscopy Group A one day meeting to be held at Materials Analysis Geological Society Piccadilly London November 18 1992 10.30-1 1 .oO 1 1 .Wl 1.45 Registration and Coffee Prof.H.M. Ortner Technische Hochschule and Analytical Resume Darmstadt ICI Wilton Research Centre Cleveland Industrial speaker to be confirmed Prof. R. J. Donovan Edinburgh University AERE Hanvell University Of Northumbria at Newcastle Tea and close of meeting Materials for ETAAS - a Material Scientists' 1 1.45-12.15 J. Franks Materials Analysis in an Industrial Laboratory 12.15-2.45 12.45-2.W Lunch 2.00-2.45 2.45-3.1 5 Dr C. Pickford Comparing TechniquesPractical Approaches 3.15-3.45 Dr J.R. Dean Future Prospects of Glow Discharge Sources 3.45-4.15 Time Of Flight Mass Spectrometry The registration fee is €30 for members of the Royal Society Of Chemistry $50 for non-members and free for students and retired members. For further details contact Dr M. Thomsen Perkin Elmer Ltd. Post Office Lane,'Beaconsfield HPlO ODL. Tel. 0494 i 679221 and fax 0494 679332. Analytical Chemists/Biochemists Freelance Abstractors wanted = To apply send a CV ok ring for an application form from MIS Z Whitelock Analytical Abstracts The Royal Society of Chemistry Thomas Graham House Milton Road Cambridge CB4 4WF Tel:(0223) 420066 Fax:(0223) 423623

ISSN:0267-9477    

DOI:10.1039/JA99207BP012

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

Journal of Analytical Atomic Spectrometry {Including Atomic Spectrometry Updates) JAAS Editorial Board* Chairman B L Sharp (Loughborough UK) J. Egan (Cambridge UK) J M Mermet (Villeurbanne France) S J. Haswell (Hull UK) D A Hickman (London UK) J Marshall (Middlesbrough UK) D L Miles (Keyworth UK) R D Snook (Manchester UK) "The JAAS Editorial Board reports t o the Analytical Editorial Board Chairman A G Fogg (Loughborough UK) JAAS Advisory Board F C Adams (Antwerp Belgium) R M Barnes (Amherst MA USA) L Bezur (Budapest Hungary) R F Browner (Atlanta GA USA) S Caroli (Rome Italy) A J Curtius (Rio de Janeiro Brazin J B Dawson (Leeds UK) M T C de Loos-Vollebregt (Delft The K Dittrich (Leipzig Germany) L Ebdon (Plymouth UK) M S Epstein (Gaithersburg MD USA) Fang Zhao-lun (Shenyang China) W Frech (Umea Sweden) A L Gray(€gham UK) S Greenfield (Loughborough UK) G M Hieftje (Bloomington IN USA) G Horlick (Edmonton Canada) D Littlejohn (Glasgow UK) B V L'vov (Sf Petersburg Russia) T Nakahara (Osaka Japan) Ni Zhe-ming (€?eying China) N Omenetto (lspra Italy) R E Sturgeon (Ottawa Canada) V Sychra (Prague Czechoslovakia) R Van Grieken (Antwerp Belgium) A Walsh K B (Victoria Australia) B Welz (Uberlingen Germany) T S West (Aberdeen UK) Netherlands) A Sanz-Medel (Oviedo Spain) Atomic Spectrometry Updates Editorial Board Chairmarl "D.L. Miles (Keyworth UK) J. Armstrong (Dumfries UK) J. R. Bacon (Aberdeen UK) C. Barnard (Glasgow UIO R. M. Barn-es (Amherst MA USA) S. Branch (High Wycombe UK) R Bye (Oslo Norway) J. Carroll (Middlesbrough UK) M. R.Cave (Keyworth UK) "J. M. Cook (Keyworth UK) "M. S. Cresser (Aberdeen UK) H. M. Crews (Norwich UK) J. S . Crighton (Sunbury-on-Thames UK) J. R. Dean (Newcastle upon Tyne UK) A. W. McMahon (Harwell UK) J M. Mermet ( Villeurbanne France) R. G. Michel (Srorrs CT USA) T. Nakahara (Osaka Japan) Ni Zhe-ming (Belling China) P. R. Poole (Hamilton New Zealand P. J. Potts (Milton Keynes UIO W. J . Price (Ashburton UK) C. J. Rademeyer (Pretoria South Africa) "M. H. Ramsey (London UK) A. Sanz-Medel (Oviedo Spain) "B. L. Sharp (Loughborough UK) I. L. Shuttler (Uberlingen Germany) S. T. Sparkes (Plymouth UK) R.Stephens (Halifax Canada) J.Stupar (Llub/pna Slovenia) R . E. Sturgeon (Ottawa Canada) A. P. Thorne (London UK) G. C. Turk (Gaithersburg MD USA) J. F Tyson (Amherst MA USA) S .J. Walton (Crawley UK) P Watkins (London UN B. Welz ( Uberlingen Germany) J. Williams (Egham UKI J. B. Willis (Victoria Australia) *J. B. Dawson (Leeds UK) "J. Egan (Cambridge UK) *A. T. Ellis (Oxford UIO J. Fazakas (Bucharest Romania) D. J. Halls (Glasgow UK) "A. Taylor (Guildford UK) "D. A. Hickman (London UK) "S. J. Hill (Plymouth UK) K. W. Jackson (Albany NY USA) R. Jowitt (Middlesbrough UK) K. Kitagawa (Nagoya Japan) J. Kubova f Bratislava Czechoslovakd "J. Marshall (Middlesbrough UK) H. Matusiewicz (Poznan Poland) *Members of the ASU Executive Committee Editor JAAS Judith Egan The Royal Society of Chemistry Dr J M Harnly Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK Telex No 818293 Fax 0223 423623 Beltsville M D 20705 USA E-mail RSCI@UK AC RL GB (JANET) Assistant Editors Brenda Holliday and Ed/tonal Secretary Monique Warner US Associate Editor JAAS US Department of Agriculture Beltsville Human Nutrition Research Center Telephone 0223 420066 BLDG 161 BARC-EAST Telephone 301 -504-8569 Paula O'Riordan Advertisements.Advertisement Department The Royal Society of Chemistry Burlington house Piccadilly London W I V OBN UK Telephone 071-437 8656 Fax 071-494 1134 Information for Authors Full details of how to submit materials for publica tion in JAAS are given in the Instructions to Authors in Issue 1 Separate copies arc available on request The Journal of Analytical Atomic Spectrometry (JAASi is an international journal for the publica- tion of original research papers communications and letters concerned with the development and analytical application of atomic spectrometric techniques The journal is published eight times a year including comprehensive reviews of specific topics of interest to practising atomic spectrosco- pists and incorporates the literature reviews which were previously published in Annual Reports on Analytical Atomic Spectroscopy (ARAAS) Manuscripts inteqded for publication must de- scribe original work related to atomic spectromet- ric anaiysis Papers on all aspects of the subject will be accepted including fundamental studies novel instrument developments and practical ana- lytical applications As well as AAS. AES and AFS papers will be welcomed on atomic mass spec trometry and X-ray fluorescence/emission spec trometry Papers describing the measurement of molecular species where these relate to the char- acterization of sources normally used for the pro duction of atoms or are concerned for example with indirect methods of anaiysis will also be ac- ceptable for publication Papers describing the de velopment and applications of hybrid techniques ( e g GC-coupled AAS and HPLC-ICP) will be par ticularly welcome Manuscripts on other subjects of direct interest to atomic spectroscopists.in- cluding sample preparation aqd dissolution and analyte pre-concentration procedures as wet' as the statistica irterpretation and use of atomic spectrometric data will also be acceptable for pcib- lication There is no page charge The following types of papers will be consid- ered Full papers describing original work Commun/cations which must be on an urgent matter and be of obvious scientific irnportance Communications receive priority and are usually published within 2-3 months of receipt They are Intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems Reviews which must be a critical evaluation of the existing state of knowledge on a particular facet of analytical atomic spectrometry Every paper (except Communications) will be submitted to at least two referees by whose advice the Editorial Board of JAAS will be guided as to its acceptance or rejection Papers that are accepted must not be published elsewhere except by permission Submission ot d manu- script will be regarded as an undertaking that the sahe material is not being considered for publica- tion by another journal Manuscripts (three copies typed In double spacing) should be sent to Judith Egan Editor JAAS or Dr J M Harnly US Associate Editor JAAS All queries relating to the presentation and sub- mission of papers and any correspondence re- garding accepted papers and proofs should be directed to the Editor or US Editor (addresses as above) Members of the JAAS Editorial Board (who may be contacted directly or v/a 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 eight times a year by The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK All orders accompanied with payment should be sent directly t o The Royal Society of Chemistry Turpin Distribution Services Ltd Blackhorse Road Letchworth Herts SG6 1 HN UK Tel +44 (0) 462 672555 Telex 825372 Turpin G Fax +44 (0) 462 480947 Turpin Distribution Services Ltd is wholly owned by The Royal Society of Chemistry 1992 Annual subscription rate EC €347 00 USA $740 00 Canada €408 (excl GST) Rest of World €389 00 Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank Air freight and mailing in the USA by Publications Expediting Inc 200 Meacham Avenue Elmont NY 11003 USA Postmaster send address changes t o Journal of Analytical Atomic Spectmmetry (JAASI Publications Expediting Inc 200 Meacham Avenue Elmont NY 11003 Second class postage paid at Jamaica NY 11431 All other despatches outside the UK by Bulk Airmail within Europe Accelerated Surface Post outside Europe PRINTED IN THE UK Q The Royal Society of Chemistry 1992 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 {Including Atomic Spectrometry Updates) JAAS Editorial Board* Chairman B L Sharp (Loughborough UK) J.Egan (Cambridge UK) J M Mermet (Villeurbanne France) S J.Haswell (Hull UK) D A Hickman (London UK) J Marshall (Middlesbrough UK) D L Miles (Keyworth UK) R D Snook (Manchester UK) "The JAAS Editorial Board reports t o the Analytical Editorial Board Chairman A G Fogg (Loughborough UK) JAAS Advisory Board F C Adams (Antwerp Belgium) R M Barnes (Amherst MA USA) L Bezur (Budapest Hungary) R F Browner (Atlanta GA USA) S Caroli (Rome Italy) A J Curtius (Rio de Janeiro Brazin J B Dawson (Leeds UK) M T C de Loos-Vollebregt (Delft The K Dittrich (Leipzig Germany) L Ebdon (Plymouth UK) M S Epstein (Gaithersburg MD USA) Fang Zhao-lun (Shenyang China) W Frech (Umea Sweden) A L Gray(€gham UK) S Greenfield (Loughborough UK) G M Hieftje (Bloomington IN USA) G Horlick (Edmonton Canada) D Littlejohn (Glasgow UK) B V L'vov (Sf Petersburg Russia) T Nakahara (Osaka Japan) Ni Zhe-ming (€?eying China) N Omenetto (lspra Italy) R E Sturgeon (Ottawa Canada) V Sychra (Prague Czechoslovakia) R Van Grieken (Antwerp Belgium) A Walsh K B (Victoria Australia) B Welz (Uberlingen Germany) T S West (Aberdeen UK) Netherlands) A Sanz-Medel (Oviedo Spain) Atomic Spectrometry Updates Editorial Board Chairmarl "D.L. Miles (Keyworth UK) J. Armstrong (Dumfries UK) J. R. Bacon (Aberdeen UK) C. Barnard (Glasgow UIO R. M. Barn-es (Amherst MA USA) S. Branch (High Wycombe UK) R Bye (Oslo Norway) J. Carroll (Middlesbrough UK) M. R. Cave (Keyworth UK) "J. M. Cook (Keyworth UK) "M. S. Cresser (Aberdeen UK) H. M. Crews (Norwich UK) J. S . Crighton (Sunbury-on-Thames UK) J. R. Dean (Newcastle upon Tyne UK) A. W. McMahon (Harwell UK) J M.Mermet ( Villeurbanne France) R. G. Michel (Srorrs CT USA) T. Nakahara (Osaka Japan) Ni Zhe-ming (Belling China) P. R. Poole (Hamilton New Zealand P. J. Potts (Milton Keynes UIO W. J . Price (Ashburton UK) C. J. Rademeyer (Pretoria South Africa) "M. H. Ramsey (London UK) A. Sanz-Medel (Oviedo Spain) "B. L. Sharp (Loughborough UK) I. L. Shuttler (Uberlingen Germany) S. T. Sparkes (Plymouth UK) R.Stephens (Halifax Canada) J.Stupar (Llub/pna Slovenia) R . E. Sturgeon (Ottawa Canada) A. P. Thorne (London UK) G. C. Turk (Gaithersburg MD USA) J. F Tyson (Amherst MA USA) S . J. Walton (Crawley UK) P Watkins (London UN B. Welz ( Uberlingen Germany) J. Williams (Egham UKI J. B. Willis (Victoria Australia) *J. B. Dawson (Leeds UK) "J. Egan (Cambridge UK) *A.T. Ellis (Oxford UIO J. Fazakas (Bucharest Romania) D. J. Halls (Glasgow UK) "A. Taylor (Guildford UK) "D. A. Hickman (London UK) "S. J. Hill (Plymouth UK) K. W. Jackson (Albany NY USA) R. Jowitt (Middlesbrough UK) K. Kitagawa (Nagoya Japan) J. Kubova f Bratislava Czechoslovakd "J. Marshall (Middlesbrough UK) H. Matusiewicz (Poznan Poland) *Members of the ASU Executive Committee Editor JAAS Judith Egan The Royal Society of Chemistry Dr J M Harnly Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK Telex No 818293 Fax 0223 423623 Beltsville M D 20705 USA E-mail RSCI@UK AC RL GB (JANET) Assistant Editors Brenda Holliday and Ed/tonal Secretary Monique Warner US Associate Editor JAAS US Department of Agriculture Beltsville Human Nutrition Research Center Telephone 0223 420066 BLDG 161 BARC-EAST Telephone 301 -504-8569 Paula O'Riordan Advertisements. Advertisement Department The Royal Society of Chemistry Burlington house Piccadilly London W I V OBN UK Telephone 071-437 8656 Fax 071-494 1134 Information for Authors Full details of how to submit materials for publica tion in JAAS are given in the Instructions to Authors in Issue 1 Separate copies arc available on request The Journal of Analytical Atomic Spectrometry (JAASi is an international journal for the publica- tion of original research papers communications and letters concerned with the development and analytical application of atomic spectrometric techniques The journal is published eight times a year including comprehensive reviews of specific topics of interest to practising atomic spectrosco- pists and incorporates the literature reviews which were previously published in Annual Reports on Analytical Atomic Spectroscopy (ARAAS) Manuscripts inteqded for publication must de- scribe original work related to atomic spectromet- ric anaiysis Papers on all aspects of the subject will be accepted including fundamental studies novel instrument developments and practical ana- lytical applications As well as AAS.AES and AFS papers will be welcomed on atomic mass spec trometry and X-ray fluorescence/emission spec trometry Papers describing the measurement of molecular species where these relate to the char- acterization of sources normally used for the pro duction of atoms or are concerned for example with indirect methods of anaiysis will also be ac- ceptable for publication Papers describing the de velopment and applications of hybrid techniques ( e g GC-coupled AAS and HPLC-ICP) will be par ticularly welcome Manuscripts on other subjects of direct interest to atomic spectroscopists. in- cluding sample preparation aqd dissolution and analyte pre-concentration procedures as wet' as the statistica irterpretation and use of atomic spectrometric data will also be acceptable for pcib- lication There is no page charge The following types of papers will be consid- ered Full papers describing original work Commun/cations which must be on an urgent matter and be of obvious scientific irnportance Communications receive priority and are usually published within 2-3 months of receipt They are Intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems Reviews which must be a critical evaluation of the existing state of knowledge on a particular facet of analytical atomic spectrometry Every paper (except Communications) will be submitted to at least two referees by whose advice the Editorial Board of JAAS will be guided as to its acceptance or rejection Papers that are accepted must not be published elsewhere except by permission Submission ot d manu- script will be regarded as an undertaking that the sahe material is not being considered for publica- tion by another journal Manuscripts (three copies typed In double spacing) should be sent to Judith Egan Editor JAAS or Dr J M Harnly US Associate Editor JAAS All queries relating to the presentation and sub- mission of papers and any correspondence re- garding accepted papers and proofs should be directed to the Editor or US Editor (addresses as above) Members of the JAAS Editorial Board (who may be contacted directly or v/a 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 eight times a year by The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK All orders accompanied with payment should be sent directly t o The Royal Society of Chemistry Turpin Distribution Services Ltd Blackhorse Road Letchworth Herts SG6 1 HN UK Tel +44 (0) 462 672555 Telex 825372 Turpin G Fax +44 (0) 462 480947 Turpin Distribution Services Ltd is wholly owned by The Royal Society of Chemistry 1992 Annual subscription rate EC €347 00 USA $740 00 Canada €408 (excl GST) Rest of World €389 00 Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank Air freight and mailing in the USA by Publications Expediting Inc 200 Meacham Avenue Elmont NY 11003 USA Postmaster send address changes t o Journal of Analytical Atomic Spectmmetry (JAASI Publications Expediting Inc 200 Meacham Avenue Elmont NY 11003 Second class postage paid at Jamaica NY 11431 All other despatches outside the UK by Bulk Airmail within Europe Accelerated Surface Post outside Europe PRINTED IN THE UK Q The Royal Society of Chemistry 1992 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/JA99207FX023

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JASPEZ 7 ( 6 ) 781-1038 (1992) 'Ill I w September 1992 Journal of Ana lytica I Atomic Spectrometry 1992 WINTER CONFERENCE ON PLASMA SPECTROCHEMISTRY SAN DIEGO CA USA JANUARY 6-1 1 1992 CONTENTS 781 783 791 799 807 81 3 81 9 825 833 839 845 851 859 865 869 877 883 889' 895 899 905 Foreword-Ramon M Barnes Towards the Next Generation of Plasma Source Mass Spectrometers. Plenary Lecture-Gary M H ieftje Drift Diagnostics in Inductively Coupled Plasma Atomic Emission Spectrometry. Plenary Lecture-Martine Noise Characteristics of Aerosols Produced by Inductively Coupled Plasma Nebulizers-Shen Luan Ho-ming Evaluation of an Ultrasonic Nebulizer for Sample Introduction in Inductively Coupled Plasma Atomic Emission Transport Effects With Dribble and Jet Ultrasonic Nebulizers-Matthew A Tarr Guangxuan Zhu Richard F.Carre Emmanuelle Poussel Jean-Michel Mermet Pang Sam C K. Shum R. S Houk Spectrometry-Theresa M. Castillano Nohora P Vela Joseph A Caruso W. Charles Story Browner Performance Characteristics-of an ultrasonic Nebulizer Coupled to a 40.68 MHz Inductivety Coupled Plasma Sealed Inductively Coupled Plasma Atomic Emission Spectrometry. Part 3. Optimization of Experimental Atomic Emission Soectrometer-Isaac B. Brenner Phillipe Bremier Allain Lemarchand Variables-Matthtas J Jahl Ramon M Barnes Analysis of Silane With a Sealed Inductively Coupled Plasma Discharge-Matthtas J Jahl Ramon M Barnes Qualitative Analysis of Arsine by Sealed Inductively Coupled Plasma Atomic Emission Spectrometry-Tracey Determination of the Residual Carbon Content by Inductively Coupled Plasma Atomic Emission Spectrometry Jacksier Ramon M Barnes After Decomposition of Biological Samples-Antoaneta Krushevska Ramon M Barnes Chitra J Amarasiriwaradena Henry Foner Laura Martines Comparison of Sample Decomposition Procedures for the Determination of Zinc in Milk by Inductively Coupled Plasma Atomic Emission Spectrometry-Antoaneta Krushevska Ramon M Barnes Chitra J Amarasiriwaradena Henry Foner Laura Martines Role of Inductively Coupled Plasma Atomic Emission Spectrometry in the Assessment of Reference Values for Trace Elements in Biological Matrices-Sergto Caroli A Alimonti P Delle Femmine F Petrucci 0 Senofonte N Violante A Menditto G Morist A Menotti P Falconieri Multipurpose Flow Injection System.Part 1 . Programmable Dilutions and Standard Additions for Plant Digests Analysis by Inductively Coupled Plasma Atomic Emission Spectrometry-Boaventura Frerre dos Rets Maria Fernanda Gine Francisco Jose Krug Henrique Bergamin Filho Determination of Macro-constituents in Advanced Ceramic Materials by Inductively Coupled Plasma Atomic Emission Spectrometry-Juan C.Farifias Maria F. Barba Determination of Impurities in Lead Zirconate-Titanate Electroceramics by Inductively Coupled Plasma Atomic Emission Spectrometry-Juan C Farifias Maria F Barba Dry-chlorination Inductively Coupled Plasma Mass Spectrometric Method for the Determination of Platinum Group Elements in Rocks-Bruce J Perry Jon C Van Loon D V Speller Determination of Iron and Ten Other Trace Elements in the Open Ocean Seawater Reference Material NASS-3 by Inductively Coupled Plasma Mass Spectrometry-Kunihiko Akatsuka James W McLaren Joseph W Lam Shier S Berman Determination of Impurities in Organometallic Compounds Dissolved in Diethyl Ether by Flow Injection Inductively Coupled Plasma Mass Spectrometry-Steve J Hill James Hartley Les Ebdon Measurements of Inductively Coupled Plasma Temperatures Comparison of N2+ Rotational Temperatures With Optical Pyrometry-lsam Marawi Bradley A.Bielski Joseph A Caruso Frank R Meeks Recycling Nebulization System With a Disposable Spray Chamber for Analysis of Sub-milligram Samples of Geological Materials Using Inductively Coupled Plasma Mass Spectrometry-Zhongxing Chen Henry P Longerich Brian J Fryer continued on inside back cover Typeset by Burgess Tharnes View Abtngdon Oxfordshire Printed in Great Britain by [-) pa ge Bros.Norwlch 0267-9L771199236-291 5 923 929 937 943 951 959 965 971 979 987 993 999 1007 1013 1019 1029 1037 Sample Preparation for Inductively Coupled Plasma Mass Spectrometric Determination of the Zinc-70 to Zinc- 68 Isotope Ratio in Biological Samples-Chitra J Amarastriwardena Antoaneta Kruchevska Henry Foner Mark D Argentine Ramon M Barnes Determination of Traces of Neptunium-237 in Enriched Uranium Solutions Using Inductively Coupled Plasma Mass Spectrometry-Chantal Riglet Olivier Provitina Jean-Luc Dautherrbes Daniel Revy Addition of Molecular Gases to Argon Gas Flows for the Reduction of Polyatomic-ion Interferences in Inductively Coupled Plasma Mass Spectrometry-Jiansheng Wang E Hywel Evans Joseph A Caruso Reduction of the Effects of Concomitant Elements in Inductively Coupled Plasma Mass Spectrometry by Adding Nitrogen to the Plasma Gas-Jane M Craig Diane Beauchemin Quadrupole Versus Magnetic Sector Glow Discharge Mass Spectrometry Comparison of Quantitative Analytical Capabilities-Angelika Raith Wojciech Vieth John C Huneke Robert C Hutton Application of Glow Discharge Mass Spectrometry With Low Mass Resolution for In-depth Analysis of Technical Surface Layers-Norbert Jakubowsi Dietmar Stuewer Sample Preparation of High-purity Titanium for Analysis by Glow Discharge Mass Spectrometry-Duencheng Fang Purnesh Seegopaul Versatile Interface for Gas Chromatographic Detection or Solution Nebulization Analysis by Inductively Coupled Plasma Mass Spectrometry Preliminary Results-Gregory R Peters Diane Beauchemin Determination of Tri- and Tetra-organotin Compounds by Supercritical Fluid Chromatography With Inductively Coupled Plasma Mass Spectrometric Detection-Nohora P Vela Joseph A Caruso Pyrolysis-Gas Chromatographic Atomic Emission Detection for Sediments Coals and Other Petrochemical Precursors-Jeffrey A Seeley Yadi Zeng Peter C Uden Timothy I Eglington lnger Ericson Ultratrace Speciation Analysis of Organolead in Water by Gas Chromatography-Atomic Emission Spectrometry After In-liner Preconcentration-Ryszard Lobinski Freddy C Adams Determination of Halogenated Compounds With Supercritical Fluid Chromatography-Microwave-induced Plasma Mass Spectrometry-Lisa K Olson Joseph A Caruso Application of a Microwave-induced Plasma Atomic Emission Detector for Quantification of Halogenated Compounds by Gas Chromatography-Nada Kovacic Terry L Ramus Factorial Analysis and Response Surface of a Gas Chromatography Microwave-induced Plasma System for the Determination of Halogenated Compounds-Manuel Caetano Rafael E Golding Edgar A Key Comparison of Stripline Source and Enhanced Beenakker Microwave Cavity Designs for Atomic Emission Spectrometry-Mark D Argentine Ramon M Barnes Laser-excited Fluorescence Spectrometry of Phosphorus Monoxide and Phosphorus in an Electrothermal Atomizer Determination of Phosphorus in Plant and Biological Reference Materials and in Nickel Alloys-Zhongwen Liang Robert F Lonardo Junichi Takahashi Robert G Michel Francis R Prelt Jr Applications of Laser-induced Emission Spectral Analysis for Industrial Process and Quality Control-Claus J Lorenzen Christoph Carlhoff Ulrich Hahn Martin Jogwich CUMULATIVE AUTHOR INDEX

ISSN:0267-9477    

DOI:10.1039/JA99207BX025

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 78 1 Foreword 1992 Winter Conference on Plasma Spectrochemistry San Diego California USA January 6-1 1 1992 The ultimate conclusion of a good meeting is to publish the conference presentations as refereed papers in a respected journal collected as the proceedings. In this issue about one- fifth of the total oral and poster presentations from the 1992 Winter Conference on Plasma Spectrochemistry appear after passing the rigors of normal peer review and under- going careful editorial revisions in the short interval since January. These papers comprise a sampling of the confer- ence theme ranging from interfaced chromatography with plasma detection to characteristics of nebulizers plasma source mass spectrometers and numerous applications.They highlight the conference- Formally 204 papers were presented during the six day meeting with six plenary lectures 17 invited lectures 8 1 submitted oral presenta- tions and 100 posters. But where are the other manuscripts? What are we missing? The journal editors worked diligently to contact and encourage authors to submit their manuscripts oq time to expedite the review and editorial stages and to meet the September deadline for this issue. They and the authors deserve our sincerest appreciation and praise. Fortunately another fraction of the Conference presentations will be published either subsequently in this journal or in other forms. Some of the research and developments described at the Conference were not yet ready for manuscript prepara- tion because authors were presenting their latest findings before project completion.Unfortunately in the meantime the impact of the missing presentations will be appreciated only through recollections of attendees or consideration of the conference abstracts. Too bad for both authors and readers alike. The Winter Conference has gained a reputation as the foremost meeting in the field.' Since its beginning in 1980 with 175 spectroscopics gathering in San Juan Puerto Rico the meeting has become an established benchmark for plasma spectrochemistry. This seventh biennial Winter Conference sponsored by the ICP In formation Newsletter continued the concept of presenting state-of-the-art applica- tions developments and research in plasma spectrochemis- try with a single program sequence.The registration total with 450 scientists from 23 countries and 41 states attending was only a few per cent. lower than at the 1990 St. Petersburg meeting. Unlike many larger and smaller conferences concurrent often competitive parallel ses- sions purposely are avoided in the Winter Conference programme. Topics and invited speakers are selected to hold the interest of the audience including both the experienced and novice. However the six day schedule is full especially combined with a weekend of short courses preceding the formal meeting. Listening fatigue is mini- mized by afternoon breaks for poster presentations and an exhibition. In the warm southern California Winter the opportunity for sports relaxation by the pool and excur- sions to the ocean beaches or other San Diego attractions also helps when the rain abates. This format has proven to be successful and popular especially when a participant can hear all the presentations of interest without fear of missing something in another session.Cross fertilization of ideas and approaches is stimulated when research topics some- what remote from ones own professional specialities are described. The programme consisted of 12 popular theme symposia including flow injection spectrochemical analysis; sample introduction and transport phenomena; automation and plasma instrumentation; artificial intelligence chemo- metrics and software for plasma spectrometry; sample preparation and treatment; laser assisted plasma spectro- metry; excitation mechanisms and plasma phenomena; spectroscopic standards reference materials and characteri- zation; plasma source mass spectrometry; glow discharge spectrometry; and chromatographic plasma detection.Highlights of these sessions and six panel discussions along with the final conference programme abstracts and com- mentaries from symposium leaders have already been published.* A report also appeared earlier in this journaL3 Few have questioned whether plasma spectrochemistry has passed its prime. Certainly the inductively coupled plasma (ICP) source is mature and its application in atomic emission spectrometry has been accepted for years. Novel applications new instrumentation improved soft- ware and artful developments keep the research and development field active.The novelty of plasma sources including ICP glow and microwave discharges for mass spectrometry continues and the emphasis on purity and quality is stimulated by the added capabilities and limita- tion of mass spectrometric analysis. Fundamental descrip- tions of the ICP and other plasma sources are progressing slowly as expected since this is the hardest part of science. All-in-all the primary need for a Winter Conference persists and thus the Winter Conference continues. The 1993 European Winter Conference is scheduled for January 10- 15 in Granada Spain. The meeting is chaired by Professor Alfred0 Sanz-Medel and is expected to be as valuable as the previous European Winter Conferences. The proceedings of the 1991 European Winter Conference held in Dortmund Germany apeared re~ently.~. The 1994 Winter Conference again will be held in San Diego CA USA. In addition to the six day programme from January 10- 15 short courses will be offered January 7-9. The deadline for submitting titles and 50-word pre- abstracts is July 2 1993. Furthermore the Sixth Winter Conference on Flow Injection Analysis for the first time will precede the meeting at the same location on January 5-7. Readers enlightened by this Conference proceedings are encouraged to participate in one of these forthcoming Winter Conferences to interact contribute and publish in the next proceedings. Ramon M. Barnes Conference Chairman Amherst MA USA References 1 Beauchemin D. Yves Le Blac J. C. Peters G. R. and Craig J. M. Anal. Chem. 1992 66 442R. 2 ICP Inf Newsl. 1992,17,687-75 1 and 769-823 and ICP Inf Newsl. 1992 18 1-29. 3 Dymott T. C. J. Anal. At. Spectrom. 1992 7 19N. 4 Boumans P. and Broekaert J. Spectrochim. Acta. Par? B 1992 47 1.

ISSN:0267-9477    

DOI:10.1039/JA9920700781

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 783 Towards the Next Generation of Plasma Source Mass Spectrometers* Plenary Lecture Gary M. Hieftje Indiana University Department of Chemistry Bloomington IN 4 7405 USA In this paper the present state of plasma source mass spectrometry is reviewed with special emphasis placed on the strengths and weaknesses of currently available systems. Attention is then directed towards basic and applied studies that are underway throughout the world to reduce the remaining shortcomings of the technique. Such efforts include the design and evaluation of novel sources and mass spectrometers modifications in ion- optic and interface systems attempts to understand and overcome isobaric interferences (spectral overlaps from oxides and other polyatomic species) techniques for stabilizing plasma sources and mass spectrometers in an effort to reduce instrumental drift and methods for improving precision both in routine analysis and isotope determination situations. It is argued that plasma source mass spectrometers of the future might be simpler yet more powerful than those now in use.Vacuum pump requirements might be lessened tandem sources will offer new flexibility and capability and simultaneously reading mass spectrometers will speed analyses make interfacing to chromatography devices more practicable and improve precision. Keywords Plasma source mass spectrometry; inductively coupled plasma mass spectrometry; tandem source; instrumentation Plasma source mass spectrometry (PSMS) particularly in the guises of inductively coupled plasma mass spectrometry (ICP-MS) and glow discharge mass spectrometry (GDMS) has become a widely accepted tool for elemental analysis.There are already several hundred commercial ICP-MS instruments alone that are being used in applications that span the spectrum of those customary in elemental analysis. These commercial instruments and those that have been assembled or modified in academic and governmental laboratories have already undergone several stages of evolution and have incorporated newer vacuum-pumping systems ion-optic configurations detectors and interface designs. The most recent commercial offerings provide phenomenal detection limits approaching the parts-per- quadrillion level for one instrument broad linear working ranges excellent performance in a semiquantitative mode high abundance sensitivity and a high degree of user friendliness.It is therefore appropriate to question whether dramatic new developments in PSMS are likely to occur. Will the fundamental studies being carried out in many laboratories throughout the world yield any findings of direct benefit to the practising analyst? Can new plasma sources ion-optic configurations detector arrangements or mass spectrometer alternatives improve performance markedly make the instruments easier to use or more maintenance-free or render them less susceptible to isobaric overlaps and sample-matrix interferences? To answer this question the strengths and weaknesses of existing PSMS systems are considered in an effort to discover where improvement is possible and where it is most needed and a number of current trends and areas of intense study are examined in order to discern the direction that future developments might take.Finally with these considerations in mind it might be possible for us to project how future PSMS designs might look and what they might provide. Sensitivity and Detection Limits in PSMS Low detection limits are the feature of modern PSMS instruments that is most frequently cited as a dominant *Presented at the 1992 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 6- 1 I 1992. strength. It is therefore useful to examine what factors of the instrument performance are responsible for such low detectable levels and what might be done to improve them further. To begin the efficiency of the ICP as an ion source is examined to determine whether it would be appropriate to consider supplanting it with an alternative. A typical ICP employed for mass spectrometry will have the general configuration depicted in Fig.1. Here a pneumatic nebulizer is coupled to a traditional Scott-type spray chamber and the resulting tertiary aerosol is fed into the central tube of an ICP. Ions ultimately generated in the plasma tail flame are then extracted into the first stage of a mass spectrometer interface via a sampling orifice. Although some arrangements might utilize an ultrasonic nebulizer or some other alternative to the pneumatic system and although temperature-regulated spray cham- bers and desolvation systems are frequently employed the following considerations should remain generally valid.10" analy\e atoms s \+/ Sampling cone 1018 argon3 / atoms cm 0 0 1 ppm at 1 cm3 min ' =10"atoms s I 10' analyt atoms cm Fig. 1 Evaluation of the characteristics of a typical sample introduction system shows that most or all of the analyte sent into an ICP finds its way into the first stage of the ICP-MS interface784 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Suppose that a 1 ppm solution of a chosen analyte element is aspirated into the pneumatic nebulizer at a sample- solution flow rate of 1 cm3 min-' and carried into the central tube of the ICP torch by a carrier flow of argon at 1 dm3 rnin-'. If it is also assumed that the aerosol exiting the spray chamber comprises approximately 1 O/o of the solution being aspirated a typical value and if it is posited that only moderate dilution of the resulting atoms takes place in the ICP tail flame it can be calculated that the tail flame will contain approximately 1 x l O9 analyte atoms ~ m - ~ .Further- more it can be shown that the atmospheric-pressure tail flame of the ICP will contain about 1 x loi8 argon atoms ~ m - ~ if the ICP gas temperature is 6000 K. Thus if the initial concentration of the solution of the analyte element is 1 ppm the ratio of analyte atoms to argon atoms in the ICP tail flame is approximately 1 x Moreover because there are approximately 1 x loi5 argon ions in the ICP the ratio of analyte atoms to argon ions will be This same value will pertain to the ratio of analyte ions to argon ions for most elements since ionization for many species is virtually complete. These species contained within the ICP are then ex- tracted into the moderate-pressure (typically 133 Pa) region of the mass spectrometer first stage and form there a supersonic expansion.Straightforward calculations' show that the gas flow through this orifice will consist of approximately 1 x lo2' argon atoms s-l a flow correspond- ing roughly to 3 dm3 min-l at standard temperature and pressure. Because the ratio of analyte species to argon atoms should not be altered by this extraction process this sampled beam will contain a flux of approximately 1 x loi2 analyte atoms (or ions) s-' as indicated in Fig. 1. These simple considerations lead to an extremely signifi- cant conclusion. As shown in Fig.1 a sample flow of 1 cm3 min-l containing 1 ppm of analyte corresponds to a flux of 1 x 1014 analyte atoms s-' entering the pneumatic nebulizer; the 1 O/o efficiency of the nebulizer which has been assumed drops this transport to 1 x 10I2 atoms s-l. In other words virtually every analyte atom that enters the ICP will eventually be extracted into the first stage of the ICP-MS interface. The mass spectrometer and its interface are now consi- dered in order to assess in additional detail where signal losses might arise. As revealed in Fig. 2 the supersonic expansion formed by the sampled plasma gases is skimmed through a second orifice (in the 'skimming cone') with relatively low efficiency. The modelled ICP tail flame at atmospheric pressure (Po) contains about 1 x lo9 analyte atoms and the extracted supersonic beam carries a flux of approximately 1 x 1021 argon atoms s-' and 1 x 10I2 analyte atoms s-l as outlined in Fig.1. However calcula- tions' show that only 1% of the species in that beam enter the second stage of the mass spectrometer. That is the analyte flux into the second stage will consist of approxi- mately 1 x 1O'O analyte atoms (or ions) s-I carried with about 1 x 1019 argon atoms s-' and about 1 x loi6 argon ions s-l. Yet even this moderate ion throughput suffers tremendous loss when it is compared with the final signal levels that are realized in typical ICP-MS instruments. It is common experience that an analyte concentration of 1 ppm will produce a final MS signal of approximately 1 x lo6 counts s-l as listed in Fig.2. Thus if most of the analyte species that are sent into the ICP-MS interface are ionized only about 1 in lo4 of those present in the second stage of the interface is ever detected and only about 1 in every 1 x lo6 in the ICP survives the extraction ion-separation and detection processes. Clearly substantial inefficiencies result from skimming the supersonic expansion and in passing that skimmed volume into the third stage of the interface through the ion optics and mass spectrometer and producing a detectable signal. ..-.+# Space-charge ,.,._ dominated region Sampling cone 1 ppm or 10' analyte atoms cm-3 Po Fig. 2 Illustration of the sources of inefficiency in a typical ICP- MS interface.The greatest losses occur in the skimming of the supersonic expansion (where only 1 ion in 100 is transmitted) and in the third stage of the system (where only 1 ion in 10000 is detected). Adapted from a drawing prepared by D. M. Chambers From these rough calculations several important conclu- sions can be derived. Firstly it seems unlikely that any other atmospheric-pressure ion source will be more efficient than the ICP. Of course substantial gains might be derived through use of more efficient approaches for sample introduction; the model used here figures a 99% loss in the analyte introduced into it. Direct sample insertion laser ablation ultrasonic nebulization and a host of other approaches are superior and the improved detection limits that such techniques provide offer evidence of this.Secondly it must be recognized that a reduced-pressure source might offer analyte throughput efficiencies higher than the ICP. Because a 100-fold loss in analyte throughput occurs at the skimmer orifice an ion source that takes the place of the supersonic expansion in the interface of an ICP mass spectrometer could probably be sampled more effici- ently. Modified GD sources or other reduced-pressure units are already being examined and are discussed in greater detail later. Thirdly it is clear that the most commonly employed mass spectrometer for ICP-MS a quadrupole mass filter is a rather inefficient device.2 Not only does the quadrupole suffer a fairly low transmission efficiency as the foregoing calculations indicate but it is capable of measuring only a single mass at a time.As a result requiring the same number of counts from 50 different elements or isotopes will take 50 times as long as it would on an instrument that measured all masses simultaneously. Lastly the ratio of argon ions to analyte ions in the ICP tail flame must be borne in mind when alternative ion- sampling schemes or mass spectrometric configurations are considered. The calculations described above indicate that even a solution with a relatively high concentration of the analyte element (1 ppm) will produce an analyte ion concentration in the ICP that is only times that of the argon ion number density. Because many ion optic and mass spectrometer arrangements are limited in dynamic range or by space charge (coulombic repulsion) problems,JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL.7 785 they might not be readily applicable to sampling the ions from an ICP. For example it is commonly held that a Paul (r.f.) ion trap can contain no greater than 1 x lo6 ions at a time. If those ions were extracted directly from an operating ICP the trap would probably contain only a single analyte ion even if the initial concentration of the solution were as high as 1 ppm. Given the million-fold loss in analyte ion throughput calculated above it might seem surprising that ICP-MS is capable of providing such low detection limits. If the mass spectrometer and its interface are as inefficient as they appear to be why are ICP-MS detection limits so much lower than those found in ICP emission spectrometry? Again the answer to this question can be found through a simple calculation.By definition the detection limit is that amount or concentration of an analyte that can be reliably distin- guished (at a desired confidence level) from a blank or background signal. Therefore detection limits depend not only on the magnitude of a signal but also on the level of background or blank noise. In turn background noise levels in ICP-MS are far lower than those encountered in ICP emission spectrometry. It was stated above that a solution with a concentration of 1 ppm produces an ICP-MS signal of approximately 1 x lo6 counts s-l. In contrast experience shows that a 1 ppm solution will generate a photocurrent in emission spectrometry of approximately 1 pA or if photon counting is utilized a signal of 40 x lo6 counts s-l.Thus the signal in emission is actually as much as 40 times that in mass spectrometry. However in ICP-MS commonly encoun- tered background count rates are between 1 and 10 counts s-l whereas in emission measurements the back- ground is more of the order of 1 x lo-* A or about 0.4 x 1 O6 counts s-l. This far higher background in emission determinations is caused both by continuum emission from the ICP and also by the fact that photodetectors for emission spectrometry are inherently noisier than ion detectors utilized for MS. This higher level of detector noise derives directly from the fact that photons in the ultraviolet or visible spectral regions are far less energetic than is an ion accelerated onto a detector surface.Because of this energy difference the ‘work function’ that is required in a photon detector must be far lower than that in a detector intended for ions. As a consequence thermally generated noise (t hermionic emis- sion) is a far more serious problem in the photon detector. This unavoidably higher background level in the emission photodetector is exacerbated by continuum emission from the ICP generated by ion-electron recombination and depending upon the spectral region being viewed by molecular emission features. An added complication is that the background level produced by an ICP is not completely stable but fluctuates somewhat because of inherent instabilities in the ICP tail flame in the sample introduction system and by the possible presence of intact or recently evaporated aerosol droplets that enter the tail flame.These considerations lead to important conclusions for both emission and mass spectrometry. Better detection limits in emission spectrometry are likely to be realized only by using a source of lower background emission and to a somewhat lesser extent of greater stability. In contrast gains in the detection capability of PSMS are likely to be achieved only by improving the efficiency of sample utilization or throughput in the mass spectrometer and its interface. Other Shortcomings of PSMS Low detection limits are hardly the only consideration that dictates the use and attractiveness of a technique for Table 1 Key problem areas in PSMS Isobaric overlap (spectral interferences) Sample matrix interferences Long-term instability (drift) Orifice clogging (high salt samples) Cost (compared with ICP emission) Limited precision (~0.5%) Difficulty with transient samples (e.g.FIA) Instrument maintenance elemental analysis. A review of limitations in both ICP-MS and GDMS3 suggests that the most significant shortcomings are those listed in Table 1. Most users would agree that some of the most serious errors in either ICP-MS or GDMS arise from isobaric overlap of atomic spectral peaks with those of polyatomic species. By and large the polyatomic ions of greatest abundance in ICP-MS are oxides whereas the most troublesome overlaps in GDMS are from argides. Other matrix interferences are also troublesome in both techniques. Perhaps the most serious is the so-called ‘mass- dependent interferen~e’,~-~ which affects virtually every element.In brief this type of interference results in a loss of analyte signal which increases in severity as the mass of the interfering element increases. Also the interference is greater for light elements than for heavier ones; hence the term ‘mass-dependent’. Long-term instability is also a problem in most specially constructed and commercial PSMS instruments. Although attempts have been made to stabilize both the plasma source and the mass spectrometer difficulties remain. Some of the drift is no doubt caused by the clogging of sampling and skimmer orifices by sample material. Understandably such clogging is most severe when solution concentrations in ICP-MS are high.Precision is also lower in PSMS than is desirable. Although it is possible to approach levels near 0.5% relative standard deviation (RSD) when an internal standard is employed more common figures are 1-5% RSD. Transient samples also cause difficulty in all current PSMS instruments a result completely of the fact that mass filters are sequentially scanned devices. Whether a quadru- pole mass filter or sector arrangement is used each isotope or element of interest is measured at a time different from others. If a limited measurement interval is available a trade-off must therefore be made between examining a large number of elements or isotopes at reduced sensitivity and precision or restricting a measurement to only a few elements or isotopes in order to improve the signal-to-noise ratio.With a transient sample the luxury does not exist for observing all the spectral peaks of interest for as long as would be desired. Finally improvement is possible in a number of practical areas including instrument costs and maintenance. A mass spectrometer is clearly a more complicated instru- ment in many respects than is an emission spectrometer dictating its higher price and greater level of maintenance. Yet these characteristics are hardly desirable and can be accepted only if other attributes of the method outweigh them. With this narrative as a backdrop let us now review developments that are underway and others which might be on the horizon to overcome the shortcomings of PSMS listed in Table 1. In some cases the most fruitful path for improvement will be fundamental study and characteriza- tion of a particular shortcoming or the characteristics of an instrument that displays it.In other situations the most direct path to improving instrument performance will be to modify the ion source detector ion optics or mass spectrometer.786 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Fundamental Path to PSMS Improvement Because it is 1992 just 500 years after Christopher Columbus landed in the New World it is not inappropriate to reflect on his accomplishments. An interesting parallel exists between the experiences of Columbus and the development of some chemical instrumentation and tech- niques in that they both rely heavily and in some cases excessively on serendipity. While seeking a shorter route to India Columbus stumbled onto a hitherto unrecognized land of immense potential.While searching for ways of obtaining stronger signals users of PSMS sometimes happen upon conditions that lead to reduced inter-element effects and improved precision. Of course neither approach to geographical or scientific discovery is particularly elegant and neither can be guaranteed to succeed. A more satisfactory and certainly satisfying approach would be to identify clearly a desired goal (a new continent reduced interferences or whatever) to explore alternative avenues for attaining the goal and to pursue the approaches in decreasing order of probability of success. In scientific research it is therefore better to clarify the underlying reasons for poor sensitivity inter-element interferences or unacceptable drift and through that understanding to overcome the problems.With this better understanding instrument performance will be more predictable and difficulties that are encountered can be overcome through rational adjustment of operating parameters rather than through empirical optimization. That this fundamental approach can be effective is found in a group of recent p~blications~-~ that were intended to elucidate the origin of mass dependent interference in ICP- MS. The suite of studies was aimed at clarifying the fate of an ion as it leaves the ICP tail flame is extracted through the sampling orifice of an ICP mass spectrometer is skimmed and is transmitted by the ion optics and mass spectrometer.The tools in the investigation included retarding plates placed in the second (1 33 mPa) and third (1.3 mPa) stages of the mass spectrometer interface in order to measure ion kinetic energies and a Langmuir probe which could map the electrostatic characteristics of the supersonic beam extracted into the first stage of the interface. The characteristics and composition of the expansion and the resulting ion beam were then measured and mapped as a number of controlled operating para- meters were varied. Among such parameters were the ICP central-gas flow the aerosol and solvent vapour load in the ICP the pressure in the first vacuum stage and the configuration of the ion optics used in the interface. The studies all verified that the mass-dependent interfer- ence was indeed a result of coulombic repulsion in the dense ion beam that was produced in the interface.However the finding that space charge was greatest in the second stage of the interface was unexpected. In that zone the gas density is sufficiently low that electrostatic repulsion can have a significant effect on ion trajectories yet high enough that charged species are brought into extremely close proximity to each other. Furthermore ion-optical elements that are commonly placed in the mass spectro- meter second stage force ions in the beam even closer to each other where the coulombic effects (space charge) become dramatic. These findings suggest that straightforward solutions to reducing the mass dependent interference in PSMS include relatively simple modifications to the ion optics.In particu- lar for the mass spectrometer used in this laboratory,IO removal of the ion optics in the second stage was sufficient to reduce the interferences to a negligible level. Removing the ion optics in the second stage of the interface had a dramatic effect on ion throughput. To compensate for the resulting loss in efficiency the remain- ing ion optics were retuned slightly and the ‘photon stop’ that is used in most interfaces was removed. Surprisingly removing the photon stop did not increase background levels caused by ‘photon noise’ significantly since the ion detector is placed well off axis and ions are deflected into it by means of two charged plates. In essence the ‘photon stop’ in the mass spectrometer served really as an ‘ion stop’ rather than a photon attenuator.It will be interesting to determine whether similar behaviour is found in commer- cially available and in other laboratory-constructed instru- ments. Regardless of how widely applicable these findings are the point remains clear. By a more thorough understanding of the origins of an interference novel approaches ,might be devised to overcome it. It is therefore important that laboratories throughout the world seek a better understand- ing of why the other shortcomings listed in Table 1 exist and how they might most efficiently be overcome. Improvement Through Instrument Redesign As suggested in the previous section improvements in the performance of PSMS might be achieved by modifying the plasma source the mass spectrometer the ion detector or the PSMS interface.The benefits that might be gained from modifications to the ion source are examined first. Better Ion Sources for PSMS Alleviation of isobaric overlaps from polyatomic species can perhaps be accomplished best by changes in the ion source. Indeed success in this area has already been dramatic and has resulted mostly from the use of mixed-gas plasmas and the adoption of more efficient solvent-removal systems.11J2 For example the addition of nitrogen or other molecular gases serves to raise the thermal temperature of an ICP and to dissociate polyatomic clusters that it would otherwise form. In addition removing the solvent from an aerosol to be sent into an ICP serves to remove the principal source of oxygen responsible for the production of oxide- containing ions. With a cryogenic desolvation device it is possible even to remove volatile substances such as hydrogen chloride and the polyatomic interferences they cause.12 It is possible to tailor the plasma chemistry in additional ways to reduce the severity of polyatomic ion interferences.For example it has been shown that xenon can be added to the central-gas flow of an ICP where it will undergo charge exchange with a number of troublesome polyatomic ions.12 Through this charge-exchange process the polyatomic species are neutralized and are no longer detected. Other source-based methods for reducing isobaric inter- ferences would include the adoptian of modified ICP torches to reduce the entrainment of atmospheric gases.Such torches might include extended outer (coolant) tubes a more linear or laminar outer-gas flow or might be of larger diameter to help isolate the central analyte-contain- ing channel from outside influences. More dramatic alterations in source design might result in even more significant improvements in performance. Especially appealing are the features of a ‘tandem’ s o ~ r c e . ~ J In the tandem arrangement two independent sources are coupled to produce a combination that offers the best characteristics of each. The first source in the pair would be designed to vaporize and atomize the sample directed into it; atoms it produced would then be ionized by the second source. Because the functions of sample atomi- zation and atomic ionization are separated each of the two sources can be independently optimized for its intendedJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY.SEPTEMBER 1992. VOL. 7 787 function. Thus the first source should be able more effectively to atomize the sample fully so polyatomic ions would be in far lower abundance. A tandem source would also lend itself better to feedback stabilization than would a single device. It has already been shown how emission signals from an ICP can be stabilized by use of an emission-based feedback signal.14 A reference element added intentionally to a sample solution provides the feedback signal which in turn can control r.f. power to the ICP in a manner that stabilizes emission signals from it. Although the same technique is being applied currently to stabilize an ICP-MS instrument the concept would be even more effective if applied to a tandem source.Feedback signals from for example both atomic and polyatomic ion channels could be employed to optimize the atomization behaviour of the first source while signals from ions of high ionization energy and from doubly charged ions could be used to enhance and stabilize the performance of the second (ionization) source. Significantly this optimization could be performed automatically under computer control and could be tailored to each individual sample so the resulting mass spectrometric signals would be stronger stabler and more interference-free than is now the case. The two sources in a tandem pair might operate by any of a number of mechanisms; a few are listed in Table 2.Interestingly many of these and other combinations have already been explored for use in tandem sources. Indeed the GD is itself a tandem source with its atomization being performed by a mechanical sputtering phenomenon while ionization occurs either by charge transfer from metastable species (Penning ionization) or by electron impact. Addi- tional details about the tandem source concept can be found e l s e ~ h e r e . ~ J ~ J ~ Hopefully workers will be able to conceive their own tandem source combinations that are even more attractive than those that have already been described in the literature. Because the second (ionization) source in a tandem pair need only ionize atoms directed into it it can be relatively simple. A particularly attractive device is the atmospheric sampling GD described by McLuckey et af.16 Such a system could accept atoms produced by a range of alternative atom sources.In fact because the tandem-source scheme lends itself to modularity any sort of atom generator could be coupled to the ionization source. That is one first source could be employed when it is necessary to atomize conductive solids directly another when liquids are to be analysed another for non-conductive solids one for gase- ous samples another for microsamples one that provides three-dimensional spatial profiling in solids a unit that accepts the effluent from a chromatographic column one to employ flow injection and others. Furthermore it might be possible to employ a first (atomization) source that is modulated in amplitude.13J5 In this arrangement the first (atomization) source would be operated in an alternating fashion at two operating levels one of which is intended to atomize the sample fully and the lower of which is intended merely to fragment the sample.Table 2 Tandem source mechanisms Source I Source 2 (atomization) (excitation/ionization) gas Energykharge transfer Hot high-pressure (metastable species ions etc.) Mechanical Hot surface Chemical reaction Radiative ablation Radiative (sputtering) Electron impact/collision With a fairly fast mass spectrometer attached to the ionization source the tandem pair would thus produce in rapid and alternating sequence an atomic and a fragmenta- tion mass spectrum so the identity and structure of a sample could be determined more unambiguously.Such a combination would be a powerful tool for detection of the effluent from a chromatography column or for identifying the species in which particular chemical elements are contained. As a final point in the consideration of new ion sources for elemental mass spectrometry it must be borne in mind that the characteristics of the ion source are likely to dictate in large measure what the interface to the mass spectro- meter must accomplish and how it must be designed and also what requirements are placed on the mass spectro- meter itself. For example the novel electrospray ion source developed by Blades et al.l7-I9 and modified by Agnes and Horlick20121 operates in an extraordinarily simple and direct fashion. It is necessary merely to apply a high voltage to the solution to be analysed.Charges accumulate on the surface of the solution and lead to its electrostakic disruption. Because many of the charges detach from the surface in the form of elemental ions or polyatomic clusters in which they are included they can be directed into a mass spectrometer for detection. If the interface leading to the mass spectro- meter is suitably designed many of the polyatomic frag- ments are decomposed so a relatively clean atomic mass spectrum can be produced.20*2* Not only does this new approach lead to simplicity in source design and the potential ability to determine the chemical species in which the elements are originally contained but the source can also operate with an extraor- dinarily low gas flow. A low gas flow reduces tremendously vacuum pumping requirements and could conceivably lead to simplified designs of mass spectrometers just as has been the case in the development of mass spectrometric detectors for gas chromatography.Improved Mass Spectrometers for PSMS Benefits will be (and are being) derived from redesign of the mass spectrometer interface and by substituting alternative types of mass spectrometers for those now commonly employed. A clear example of how suitable mass spectro- meter design can overcome some of the problems listed in Table 1 is the use of a high-resolution double focusing mass spectrometer to resolve isobaric overlaps. At a resolving power of approximately 8000 it becomes possible to separate the overlap of ArCl from As; many other useful examples exist.It might be possible to alleviate polyatomic ion interfer- ences in even more subtle ways. Many believe that an abundance of polyatomic species is created in the stagnant zone that lies near the surface of the sampling cone in the PSMS interface. In this relatively cool stagnant region recombination and ion-molecule reactions no doubt take place and might lead to complications in a mass spectrum. Of course if the beam expansion from the sampling orifice were ideal the ion-source gases that entered the sampling orifice would not contain any of the polyatomic species that form in the stagnant boundary layer. In a real interface however the situation might be more similar to that depicted in Fig. 3 in which traces of the polyatomic species near the sampling cone are carried into the interface.Luckily it would seem that such polyatomics would be in greater abundance in the periphery of the expanding plume of ion gases rather than in its centre. Furthermore they might exhibit a different kinetic energy distribution than species skimmed from the centre of the supersonic expan- sion. As a result it might be possible to discriminate against788 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Fig. 3 Schematic illustration of the entrainment of external gases into the supersonic expansion of the mass spectrometer first stage. Polyatomic ions might form in the stagnant zone that lies along the outside surface of the sampling cone and would be carried into the mass spectrometer along with the plasma gases them on the basis of their kinetic energies or possibly through use of a more spatially selective configuration of the ion optical system.In fact recent results from the laboratory of Houk et allZ indicated that a spatially selective ion optical system might be helpful in lessening polyatomic ion interferences. Other approaches to reducing the severity of polyatomic interferences include the use of collisional dissociation either in a moderate-pressure gas cell or with a surface. The first approach has been explored with some success,22 but to this author's knowledge surface-induced dissociation has not yet been examined to aid PSMS. It might be possible also to reduce the deleterious effects of instrument drift by suitable redesign of the mass spectrometer or by adoption of different mass spectrome- tric configurations. Obviously a straightforward method to reduce drift is to stabilize the mass spectrometer itself.Manufacturers already go to some lengths to overcome temperature-induced instability and to minimize instru- mental variation. However these approaches are unlikely to be fully effective since the dominant source of long-term variation in a PSMS instrument is no doubt the plasma source. In such a situation the only reliable ways to overcome drift are either to stabilize the source as documented earlier or to employ internal standardization. In all likelihood both schemes will be necessary since some long-term variation derives from the sample introduction system and from changes in plasma-source throughput whereas others are a consequence of unstable atom pro- duction and atomic ionization.The latter sources of drift will have to be compensated for completely by stabilizing the source; only the former can be compensated for completely by internal standardizati~n.~~ For an internal standard by be fully effective its characteristics must match those of the species to be determined and its variation must be correlated with that of the analyte. Furthermore even if these conditions are both met the analyte peak and that of the internal standard must be measured either simultaneously or in sufficiently rapid sequence that instrumental drift is insignificant during the intervening period. If instrument response varies for any reason between the times when an analyte signal and an internal standard are measured changes in the two signals will not faithfully track each other and complete compensa- tion will not be possible.This behaviour was documented clearly in a study by F ~ r u t a * ~ who examined the precision of isotope ratio measurements as a function of the dwell time on each isotope. A peak hopping approach was employed and the isotope signals of interest were measured as the rate of peak hopping was increased. It was found that precision continu- ously improved until the dwell time per peak was as low as 10 ps beyond which point the precision of the computed isotope ratio was limited by counting statistics. In other words Furuta found that it was necessary to switch back and forth between the two peaks at a rate of 1 x lo5 Hz in order to derive the best precision from the mass spectro- meter. If this situation prevails for other sequential-scanning mass spectrometers they will never be able to provide high precision in a complete multi-element analysis.An operator will have to choose between measuring a spectral peak and its internal standard in sufficiently rapid sequence to obtain high precision or examining several elements but with a greater interval before the internal standard peak can be examined. Because of this greater interval precision will suffer. For these reasons the most attractive desigvs for future atomic mass spectrometer systems are those that offer simultaneous readout of all the isotope peaks of interest. Such systems would not only yield improved precision in accordance with the foregoing arguments but would lend themselves more naturally to the measurement of signals from transient samples such as those produced by laser ablation flow injection and chromatographic systems.Other limitations of current quadrupole mass spectromet- ers also indicate that new directions be taken. These limitations include limited resolution the relative low transmission efficiency described earlier and only moderate stability. Which mass spectrometer alternatives should be consi- dered? Those which are already being explored and which offer some appeal include the Paul ion trap Fourier transform mass spectrometry (FTMS) time-of-flight mass spectrometry (TOFMS) and a sector instrument coupled with a diode-array readout.Preliminary work'with an ion trap has already proved its effectivenes~.~~ However to be attractive from a practical point of view the ion trap will have to be employed with some form of mass-selective pre-filter because of the limited dynamic range mentioned earlier.26 The purpose of the pre- filter would be to exclude argon ions from the trap so its limited dynamic range could be exploited fully. A promis- ing candidate for such a pre-filter would be a quadrupole device operated in a notch filter mode.z7 Even with this preliminary filtering however the appeal of the ion trap might be somewhat limited in that the most abundant analyte species that it contains should be no more than lo6 times the concentration of the least abundant ions. Importantly the ,ion trap unlike the quadrupole mass filter or scanned sector-based mass spectrometer produces a mass spectrum that is derived from ions extracted simultaneously from the source.Thus internal standardiza- tion should be more effective. Furthermore because the ion trap can be scanned at high speed it should be applicable to the measurement of transient samples. An FT mass spectrometer is attractive as a spectrometer for elemental analysis because it provides truly simulta- neous measurement of all masses and offers extraordinarily high r e s o l u t i ~ n . ~ ~ ~ ~ ~ Yet like the ion trap an FTMS offers limited dynamic range and is at present extremely expen- sive. Furthermore if it is to provide high-resolution mass spectra its time of analysis can be relatively long. A TOFMS is appealing for use in elemental MS in part because of its great speed simplicity high transmission efficiency and its ability to extract ions simultaneously from a source.However shortcomings of the TOFMS would seem to be the low duty factors under which it operates its limited resolution and the relatively complex electronics that are ordinarily needed for its operation. However the shortcomings of TOFMS are not as serious as they might initially seem. At first it might seemJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 789 foolhardy to apply TOFMS to a continuously operating ion source such as those commonly employed for atomic spectrometry. After all the TOFMS instrument operates by extracting an extremely brief (typically 20 ns) pulse of ions from a source accelerating them down a flight tube and recording their times of arrival.It is ordinarily not possible therefore to extract another aliquot of ions from the source until the full mass spectrum is recorded a time interval of the order of tens of microseconds. Thus the duty factor of a TOFMS is something of the order of or less. Still a TOFMS offers almost unity transmission efficiency. Fur- thermore it records a complete mass spectrum from each ion pulse extracted from the source; these factors alone would make the TOFMS at least competitive with instru- ments such as quadrupole mass filters and sector-based mass spectrometers. However the TOFMS offers other strengths also. It is extraordinarily fast and also extracts all ions at the same time from an operating source.Thus it should be far more powerful when applied to transient atom sources and should be effective also in reducing the complications of source drift if internal standardization is employed. Moreover as has already been demonstrated experimentally a TOFMS can be operated in a right-angle or in a ‘multiplexed’ c~nfiguration~~ that can raise the duty factor to levels of between 0.1 and 1 .O. In the first of these schemes an ion beam extracted from an operating source forms a supersonic expansion just as is ordinarily found in an ICP-MS interface. Because ions in the expan- sion have only a moderate velocity they fill the expansion volume at only a moderate rate. Once that expansion volume is filled the narrow ion beam can be pulsed in a direction perpendicular to its original route and accelerated in that perpendicular direction down the flight tube.During the time it takes to record the mass spectrum from those ions the initial expansion volume can be refilled with ions that will eventually form the next input pulse to the TOFMS. Importantly this right-angle configuration has the advantage also of accelerating the ions in a direction perpendicular to the initial supersonic expansion. Because the kinetic energy distribution of ions in that perpendicular direction is extremely narrow resolution even in a linear TOFMS is good; a resolving power of approximately 500 has already been demonstrated for a corona discharge source.31 In the second (multiplex) scheme to increase the TOFMS duty factor the flight tube is essentially shared by a number of input pulses simultaneously.Termed ‘Fourier transform TOFMS’,3Z the method can achieve virtually unity duty factor performance from a TOFMS. Another attractive combination for potential use in atomic mass spectrometry couples a sector based mass spectrometer with a diode-array detector. Atomic spectro- metrists are accustomed to using photodiode arrays but for multichannel readout in emission spectrometry. In such applications a trade-off is usually required between the examination of a broad spectral range at only moderate resolution or the measurement of a narrow spectral range but with only a few spectral lines being determined. Of course neither option is completely satisfactory and a number of elegant schemes have been devised in an attempt to overcome the limitation. A linear diode array would seem to be almost ideally suited for atomic mass spectrometry. Moderately priced diode-array formats contain 256 5 12 1024 2048 or 4096 diodes in a row all equally spaced on approximately 25.4 pm centres.Conveniently an atomic mass spectrum con- tains only about 250 atomic mass units of interest and in most applications half-mass resolution is all that is needed. Thus even a 1024 element diode array would enable the measurement of four points over each atomic mass-spectral peak of interest The detector would be simultaneous integrating and could be read out at high speed if a transient sample were being measured. This diode-array approach has long been recognized as viable by people in the mass spectrometry c ~ r n m u n i t y .~ ~ ~ ~ ~ However there are several complications to the approach. Firstly a linear photodiode array such as that generally used in atomic emission spectrometry is not by itself a particularly effective detector for high-energy ions. As a result some kind of ion-to-photon conversion interface is needed. In most arrangements this conversion is a multi- step process which involves colliding the ion of interest with a microchannel plate imaging the resulting pulse of electrons onto the phosphor-coated surface of a fibre optic array and allowing the fibre optic to guide the resulting photons to an individual pixel on the photodiode array. Thus several conversion steps are needed from ions to electrons to photons and back again to an electronic signal in the diode array.In each step losses occur noise is generated and spatial (Le. spectral) resolution can be Sacrificed. Surely improved schemes can be found. A second compromise in the sector-diode array combina- tion is the limited mass range that can ordinarily be covered. Although there are more than enough pixels on an inexpensive linear diode array to cover the atomic mass range of interest the mass display produced by a sector instrument is not linear. Most studies have therefore used arrays to cover only modest mass ranges commonly spanning only a factor of two in atomic mass units (e.g. from mlz to 2mlz). Still novel mass spectrometers sector arrangements multiple diode arrays successive accelerat- ing voltages or switched magnetic fields might be utilized to cover a broader mass range.Conclusion From the foregoing narrative it should be clear that many opportunities exist for overcoming the limitations of PSMS that are listed in Table 1. Fundamental studies that are already underway should serve to characterize more fully how and where polyatomic species are formed in PSMS instruments and how they might be minimized. Such studies might also be able to define better where other sorts of matrix interferences arise and what the sources of long- term instability might be. Similarly the development investigation and adoption of novel ion sources interface configurations ion-optic arrangements and mass spectrometers might aid in over- coming spectral and matrix interferences increase instru- mental stability and precision and make PSMS instruments applicable to samples of different kinds.At the same time cost might be lowered instruments might be reduced in size and their maintenance simplified and the over-all capabili- ties of PSMS enhanced. To achieve these ends future instruments used in PSMS might incorporate a number of the features outlined in Table 3. Perhaps most ,importantly a next-generation system should provide simultaneous measurement of all masses and isotopes of interest in order to enhance precision through isotope dilution or internal standardiza- tion increase sample throughput and make the system more amenable to coupling with an atom source that produces transient signals. Ideally such instruments should be able to operate at either moderate or high resolution depending upon the incidence of spectral interference and the need for high sensitivity.Despite the simplicity of the ICP and GD it would seem that tandem sources have an important role to play in future PSMS instruments. Through proper feedback they could be made more stable and could be configured to reduce the incidence of polyatomic interferences applied to samples of many different kinds by means of a modular design and conceiv-790 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 3 Next-generation PSMS instruments Simultaneous detection of all isotopes Fast Precise Transient sampling Moderate or high resolution Tandem source Stable Fewer polyatomics Flexible (modular) Atomic or molecular Feedback stabilized Reasonable cost ably be employed in a modulated configuration to produce simultaneously atomic and molecular mass spectra.Whether or not a tandem source is employed it is certain that the next generation of PSMS systems should incorpor- ate advanced diagnostics to enhance the characteristics of sample introduction and to stabilize the instrument. Such feedback approaches are becoming increasingly sophisti- cated and powerful and could be incorporated readily even into many current computer-operated units. If these goals are all met it would seem likely that PSMS instruments could dominate the field of atomic spectrome- try. If such were the case economies of production would no doubt yield a reduction in instrumental cost. In turn lower priced instruments would increase the competitive advantage of PSMS over atomic absorption or emission approaches making the cycle repeat itself.It will be interesting to see how far this cycle carries us as we approach the new millenium. 9 Chambers D. M. Ross B. S. and Hieftje G. M. Spectrochim. Acta Part B 1991 46 785. 10 Ross B. S. and Hieftje G. M. Spectrochim. Acta Part B 199 1,46 1263. 1 1 Lam J. W. and McLaren J. W. J. Anal. At. Spectrom. 1990 5 419. 12 Houk R. S. Hu K. and Clemons P. S. paper presented at the Winter Conference on Plasma Spectrochemistry San Diego CA USA January 6-1 1 1992 (paper 1L13). 13 Hieftje G. M. Spectrochim. Acta 1990 44 (Spec. Suppl.) 113. 14 Marks M. A. and Hieftje G. M. Appl. Spectrosc. 1988 42 277. 15 Hieftje G. M. Fresenius’ J.Anal. Chem. 1990 337 528. 16 McLuckey S. A. Glish G. L. Asano K. G. and Grant B. C. Anal. Chem. 1988 60 2221. 17 Blades A. T. Jayaweera P. Ikonomou M. G. and Kebarle P. J. Chem. Phys. 1990 92 5900. 18 Blades A. T. Jayaweera P. Ikonomou M. G. and Kebarle P. Int. J. Mass Spectrom. Ion Proc. 1990 101 325. 19 Blades A. T. Jayaweera P. Ikonomou M. G. and Kebarle P. Int. J. Mass Spectrom. Ion Proc. 1990 102 251. 20 Agnes G. and Horlick G. presented at the 1992 Winter Conference on Plasma Spectrochemistry San Diego CA USA January 6- 1 1 1992 (poster ThP9). 2 1 Horlick G. paper presented at the 1992 Winter Conference on Plasma Spectrochemistry San Diego CA USA January 6-1 1 1992 (paper PL5). 22 Duckworth D. C. and Marcus R. K. Appl. Spectrosc. 1990 44,649. 23 Carre M. Poussel E. and Mermet J.-M. J. Anal. At. Spectrom. 1992 7 791. 24 Furuta N. J. Anal. At. Spectrom. 1991 6 199. 25 Gill C. G. Daigle B. and Blades M. W. Spectrochim. Acta Part B 1991,46 1227. 26 Koppenaal D. W. Barinaga C. J. and Smith M. R. presented at the 1992 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 6-1 1 (paperThP16). 27 Denton M. B. lnt.-J. Mass Spectrom. Ion Phys. 1981,37,241. 28 Barshick C. and Eyler J. J. Am. SOC. MassSpectrom. 1992 in the press. 29 Marcus R. K. Duckworth D. C. Glish G. L. McLuckey S. A. Buchanan M. Wise M. Pochkowski J. M. and Weller This work was Institutes of Health through grant GM 46853 by the National Science Foundation through grant CHE 90-20631 and by the Leco Comorat ion. in part by the References Vickers G. H. PhD Dissertation Indiana University July Brunee C. Int. J. Mass Spectrom. Ion Proc. 1987 76 121. Hieftje G. M. and Norman L. A. Int J. Mass Spectrom. Ion Proc. in the press. Beauchemin D. McLaren J. W. and Berman S. S. Spectro- chim. Acta Part B 1987 42 467. Tan S. H. and Horlick G. J. Anal. At. Spectrom. 1987 2 745. Vickers G. H. Ross B. S. and Hieftje G. M. Appl. Spectrosc. 1989,43 1330. Chambers D. M. Poehlman J. Yang P. and Hieftje G. M. Spectrochim. Acta Part B 1991 46 741. Chambers D. M. and Hieftje G. M. Spectrochim. Acta Part B 1991 46 761. 1989 pp. 64-70. R. R. paper presented at the 1992 Winter Conference on Plasma Spectrochemistry San Diego; CA USA January 6- 1 1 1992 (paper 53). 30 Dodonov A. F. Chernushevich I. V. and Laiko V. V. presented ,at the International Mass Spectrometry Conference Amsterdam The Netherlands August 26-30 199 1 (poster 31 Sin C. H. Lee E. D. and Lee M. L. Anal. Chem. 1991,63 2897. 32 Knorr F. J. Ajami M. and Chatfield D. A. Anal. Chem. 1986 58 690. 33 Boettger H. F. Giffin C. E. and Norris D. D. ACSSymp. Ser. 1979 102 291. 34 Sinha M. P. and Gutnikov G. Anal. Chem. 1991,63 2012. TUA-D20). Paper 2/00625A Received February 5 1992 Accepted May 20 1992

ISSN:0267-9477    

DOI:10.1039/JA9920700783

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 79 1 Drift Diagnostics in Inductively Coupled Plasma Atomic Emission Spectrometry* Plenary Lecture Martine Carre,? Emmanuelle Poussel and Jean-Michel Mermet Laboratoire des Sciences Analytiques University of Lyon I 69622 Villeurbanne Cedex France A simple experiment is described to carry out drift diagnostics in inductively coupled plasma atomic emission spectrometry. It is based on the behaviour of the Ba II 455.403 Zn II 206.200 and Ar I 404.442 nm line intensities as a function of the power the solution uptake rate and the carrier gas flow rate. Additional information on the atomization and ionization conditions is obtained from the Mg II 280.270 nm to Mg I 285.213 nm line intensity ratio. Methods to verify the presence of a drift over a limited period of time are discussed.Several examples illustrate the feasibility of these diagnostics. A conclusion is drawn about the selection of internal standards for drift correction. Keywords Inductively coupled plasma; atomic emission spectrometry; drift Inductively coupled plasma atomic emission spectrometry (IPC-AES) is widely used for routine analysis. Among the figures of merit of an analytical method accuracy and precision are of prime concern. Most users of commercially available ICP-AES systems obtain satisfactory short-term precision. When expressed as the relative standard devia- tion (RSD) of the replicate fluctuations precision is usually less than 0.5% and in the range 0.5-l0/o with and without internal standardization respectively.However the long- term stability e g . over a period of 4 h is often less satisfactory because of drift phenomena. Many causes of drift have been reported in ICP-AES.' They can be classified into three categories (i) change in the energy transfer from the plasma to the sample; (ii) variation in the efficiency of the sample production and transport; and (iii) degradation of the line intensity measurement. Change in the energy transfer can originate in a variation in the forward power a modification of the shape of the coil and a variation in the gas flow rates in particular in the carrier gas flow rate. Change in the sample introduction efficiency can be related to a variation in the carrier gas flow rate or the solution uptake rate a partial blocking of the pneumatic nebulizer and a change in the temperature of the spray chamber and the solution.Degradation of the line intensity measurement is usually linked to either a thermal drift of the optical components of the dispersive system or an opacity of the collimating system. A consequence of drift is the need for frequent time- consuming recalibrations. Where drift is observed between two calibration procedures there is no general rule about the way to correct the data for the change in the calibration graph as drift is not necessarily a simple function of time. Another consequence of drift is observed with sequential ICP systems. If the selection of many elements and several replicates results in a long sequence then a degradation of the RSD is observed when drift occurs between the first and the last replicates.Most publications have dealt with internal standardiza- tion*~~ and computational drift c o r r e ~ t i o n . ~ ~ ~ ~ ~ ~ ~ Lorber et aL4 and Ramsey and Thompson' have described the generalized form of internal reference method (GIRM) and Surprisingly little work has been reported on *Presented at the 1992 Winter Conference on Plasma Spectro- tPresent address Air Liquide Centre de Recherche Claude chemistry San Diego CA USA January 6-1 1 1992. Delorme Les Loges en Josas France. the parameter-related internal standard method (PRISM) respectively. These methods were developed to overcome the difficulty of matching the behaviour of a single internal standard to that of every analytical line. The GIRM was based on the use of four internal standards two atomic lines and two ionic lines whereas the PRISM made use of only two internal standards one atomic line and one ionic line. The influence of four operating parameters power plasma and carrier gas flow rates and solution uptake rate was studied to develop the GIRM whereas only two para- meters power and uptake rate were used for the PRISM.Although significant improvements in stability were ob- tained there was no clear explanation of the origins of the drift. Computational methods can certainly correct for drift to a substantial extent but it seems to be more logical to suppress 'the causes of drift. There is therefore a need for drift diagnostics. The purpose of this work was to describe such diagnostics that can be used on any commercially available sequential ICP system.These diagnostics should permit the ICP user (i) to explain the main origins of the drift; (ii) to provide data on the magnitude of the drift; (iii) to test any improvement; (iv) to make a fair comparison between ICP systems or components; and ( v ) to select adequate internal standards for drift correction. Test Elements for Drift Diagnostics Most of the time drift is related to changes in the energy transfer and in the efficiency of the sample introduction system. Simulation of these changes can be performed by modifying the forward power the carrier gas flow rate and the peristaltic pump rate. The selection of the test elements must consider the influence of these ICP operating para- meters on the line intensities.A line with a low sum of excitation and ionization energies will not be sensitive to a change in the power. The Ba I1 455.403 nm line was therefore selected as the analytical line with the lowest energy sum. In contrast a line such as the Zn I1 206.200 nm line will be very sensitive to the power as its energy sum is one of the highest. However both lines will exhibit similar behaviour when there is a change in the nebulizer efficiency. The Ar I 404.442 nm line was also selected to follow the response of the plasma to changes in the operating parameters. The Mg I1 280.270 nm:Mg I 285.2 13 nm ratio was also used to verify any variation in the atomization and ionization effi~iency.~ This ratio is sensitive to any change in the power and the carrier gas flow rate in particular when792 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL.7 Table 1 Line selection (nm) for drift diagnostics excitation energy (E,,,) ionization energy (Eion) and sum of ionization and excitation energies (Eion +EeXc) expressed in eV Element Eion Eexc Eion+Eexc Ba I1 455.403 5.21 2.72 7.93 Zn I1 206.200 9.39 6.01 15.40 ArI 404.442 MgI 285.213 - 4.35 Mg I1 280.270 7.65 4.42 12.07 - 14.69 - - the value of the ratio is low e.g. less than 6.9 The characteristics of these lines are summarized in Table 1. Usually the concentration was around 1 mg dm-3 for Ba and Mg and 10 mg dm-3 for Zn. Background correction was performed for each line. Influence of the ICP Operating Parameters Three parameters were studied to verify the behaviour of the Ar Zn and Ba test lines (i) the power (change in the energy transfer); (ii) the peristaltic pump rate (variation in the amount of aerosol); and (iii) the carrier gas flow rate (change in both the energy transfer and the amount of aerosol).The experiments were carried out at or near the values that are normally used for routine analysis. Various commercially available ICP systems were used for these experiments and are mentioned in the text. Under conventional operating conditions the behaviour of each of the three test lines against the power was found to be almost independent of the ICP system. A typical result is presented in Fig. I based on the use of a Perkin-Elmer 5500 ICP system. An increase in the power produced an increase of a similar magnitude in both the Ar and Zn line intensities.As mentioned above there was no significant change in the Ba line intensity. A decrease in the peristaltic pump rate led to a decrease of the same magnitude in both the Zn and Ba line intensities irrespective of the ICP system. This can be easily understood as less sample was reaching the plasma. In contrast an increase in the Ar line intensity was observed as a result of a slight increase in the temperature (Fig. 2). A Philips 8060 ICP system was used to obtain the results given in Fig. 2. The negative correlation between the Ar line intensity and the Zn or Ba line intensity as a function of the nebulizer efficiency was also reported by Ivaldi and Slavin.lo It was convenient to summarize the intensity behaviour of each line against the power (P) and the amount of aerosol (Q) using the symbols '+' '-' and '=' (Table 2).The 160 r I 1.1 1.2 1.3 1.4 40 1 .o PowerIkW Fig. 1 Influence of the power on the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities. The signals were normalized to 100 for a power of 1.2 kW. A Perkin-Elmer 5500 ICP system was used for this experiment 300 b 0' I 1 200 400 600 800 1000 Relative variation of the uptake rate 1 I Fig. 2 Influence of the peristaltic pump rate on the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities. The signals were normalized to 100 for the maximum pump rate i.e. 2 cm3 min-'. A Philips PV 8060 ICP system was used for this experiment Table 2 Behaviour of the Ar Zn and Ba lines with a change in the power ( P ) and the amount of aerosol (Q); + indicates an increase - a decrease and = no significant change Parameter Ar Zn Ba + + = - + + + - - P+ P - - - - Q+ Q- symbol '+' indicates that the slope of the intensity as a function of P or Q is positive '-' that the slope is negative and ',=' that there is no significant change near the central value 'of P or Q.Based on the use of these symbols the comparison of the variations of each line intensity with time permits an unambiguous assignment of the parameter that caused the change. For instance a decrease in the Ar and Zn line intensities and no change in the Ba line intensity lead to the symbols - - and = respectively. From Table 2 this set of symbols indicates a change in power with time. More information can be obtained from the variation of the carrier gas flow rate although its influence is more complex than that of the power and the solution uptake rate.In each instance an increase in the carrier gas flow rate leads to a decrease in the Ar line intensity. This is due to a slight cooling of the plasma. The behaviour of the Zn and Ba line intensities depends on some ICP parameters in particular on the observation height. In the instance where the residence time is short i.e. a carrier gas flow rate of 1 dm3 min-' and an injector i.d. of 1.5 mm," the Ba and Zn line intensities exhibit a different spatial distribution as a function of observation height above the load coil. The peak intensity of the Zn line is above that of the Ba line (Fig. 3). An increase in the carrier gas flow rate corresponds not only to a change in the amount of aerosol but also to a shift in the spatial distribution of the two lines.The behaviour of both the Zn and Ba line intensities depends on the observation height used for the experiment. In case A (Fig. 3) the observation height was selected as a com- promise between the various ionic lines. An increase in the carrier gas flow rate leads to a decrease in the Zn line intensity in contrast to an increase in the Ba line intensity. A typical example (Perkin-Elmer 5500 ICP system) is given in Fig. 4. The behaviour of the three lines in case A is summarized in Table 3 in a similar manner to that used in Table 2. In case B (Fig. 3) the observation height was selected to optimize the intensities of low-energy lines such as the Ba I1 455 nm line.Therefore any substantial change in theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 793 I Observation height - Fig. 3 Schematic variation of the Zn I1 206 and Ba I1 455 nm line intensities as a function of the observation height above the load coil. A B and C indicate the positions of the various observation heights used for experiment as described in the text 140 1 40 ' I 1 J 0.75 0.8 0.85 0.9 0.95 Carrier gas flow rate/dm3 min-' Fig. 4 Influence of the carrier gas flow rate on the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities. The observation height corresponds to positions A in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 0.85 dm3 min-I. A Perkin-Elmer 5500 ICP system was used for this experiment Table 3 Behaviour of the Ar Zn and Ba lines with the carrier gas flow rate (D).The observation height corresponds to case A in Fig. 3 Parameter Ar Zn Ba + D+ D- + + - - - carrier gas flow rate always corresponds to a decrease in the Ba line intensity. However this change is negligible for a small variation in the carrier gas flow rate in contrast to the changes in the Ar and Zn line intensities. Both the Ar and Zn line intensities decrease with the carrier gas flow rate. The behaviour of the three lines in case B is summarized in Table 4. An example (Philips PV 8060 ICP system) is given in Fig. 5. Under these conditions the change due to the carrier gas flow rate is similar to that obtained with a variation of the power.In order to separate the two processes it is therefore necessary to use a different observation height such as that used in case A. Table 4 Behaviour of the Ar Zn and Ba lines with the carrier gas flow rate. The observation height corresponds to case B in Fig. 3 Parameter Ar Zn Ba D+ D - + + = - - - - 300 .- * .-n 200 " 0.9 1 .o 1.1 1.2 1.3 Carrier gas flow rate/dm3 min-' Fig. 5 Influence of the carrier gas flow rate on the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities. The observation height corresponds to position B in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 1.05 dm3 min-'. A Philips PV 8060 ICP system was used for this experiment 2oo fi I 1 I I 0.5 0.6 0.7 0.8 0.9 1 Carrier gas flow rate/dm3 min-' Fig. 6 Influence of the carrier gas flow rate on the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities.The observation height corresponds to position C in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 0.75 dm3 min-'. A Spectroflame D ICP system was used for this experiment Table 5 Behaviour of the Ar Zn and Ba lines with the carrier gas flow rate. The observation height corresponds to case C in Fig. 3 Parameter Ar Zn Ba D+ - = + D- + = - In case C (Fig. 3) the observation height was selected to optimize the intensities of high-energy lines such as the Zn I1 206 nm line. In this instance the Zn line intensity remains unchanged for small variations of the carrier gas flow rate and the behaviour of the three lines is summarized in Table 5.An example is given in Fig. 6 (Spectroflame D ICP system). In conclusion the behaviour of the three lines can be predicted for a variation of the power and the peristaltic pump rate. However in the instance of the influence of the carrier gas flow rate it is necessary to carry out experiments as the results will depend on the observation height. Observation of the Presence of Drift Before carrying out time-consuming experiments on drift diagnostics it is preferable to verify the presence of drift possibly over a limited period of time. In the instance where the drift exhibits the same sign e.g. the signal drifts downwards with time the slope of a linear regression of the data can be used successfully. Use of 10-20 replicates794 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL.7 Table 6 Behaviour of the Ar Zn and Ba lines with the carrier gas flow rate. The spatial distributions of the Ba and Ar line intensities are similar Parameter Ar Zn Ba D+ D- + = - - - - - is sufficient to verify that the slope is different from zero. The slope can be expressed as percent. per period of time e.g. per 10 min. Although the slope depends on the number of data points one can estimate that a slope of 1% per time unit corresponds to a 0.3% RSD over the same period of time If the subscripts ‘reg’ (for data after linear regression) ‘exp’ (for experimental data) ‘cor’ (for data corrected for drift) are used for the RSDs of the regression line the experi- mental data and the data corrected for drift respectively then RSDeXp2 = RSD,,,Z + RSD,,; The RSD, value can be easily deduced from eqn.(1). An example is given in Fig. 7. Twenty replicates were used over a period of 14.5 min. The slope of the linear regression and the various RSDs are given in Table 7. It can be seen that the Ar drift is negligible in contrast to that of Zn and Ba. Moreover the signal fluctuations are larger for both Zn and Ba. Taking into account the random distribution of the data it would be difficult to observe such a drift just by inspection of the raw data. As the value of RSD is less than that of RSD,, the influence of the drift on RSD, is negligible over such a short period of time although the magnitude of the drift is large. The conclusion would be different over a period of 4 h. 110 1 0 4 8 12 Tirne/min Fig.7 Variation of the A Ar 1404; B Zn I1 206; and C Ba I1 455 nm line intensities as a function of the replicate number to illustrate the role of short-term fluctuations compared with drift. The total measuring time was 14.5 min. The signals were normalized to 100 for the first replicate. A scale shift of + 5 and - 5 was applied to Ar and Ba respectively. A linear regression is overlapped to each curve to give evidence of the presence of a drift. Slopes are given in Table 7 When a larger number of replicates is used or when the drift exhibits different directions of variation use of the slope of a linear regression is no longer adequate. Visual inspection of the curve of the signal with time is not always helpful to separate the short- and long-term fluctuations.It is therefore necessary for a smoothing procedure to be carried out to minimize the short-term fluctuations. A smoothing procedure is equivalent to an increase in the integration time. Use of a moving average is a simple way of performing such a smoothing.’* If at least 100 data points are available use of the average of nine successive data points is a good compromise. The average of data points 1 to 9 is computed then 2 to 10 and similarly up to the last value. Results can be seen in Fig. 8 for the Zn line intensity. This procedure can be improved by weighting the data. Such a procedure was used13 for background smoothing. For a central value n with a weight of 128 the weight will be 1 1 5 86 52 and 26 for the two (n- 1) and (n+ l) (n-2) and (n + 2) (n - 3) and (n + 3) and (n - 4) and (n + 4) values respectively.The sum is then divided by 686. In Fig. 8 it can be seen that the short-term fluctuations are reduced compared with those observed in the absence of weighting. Visual inspection of the smoothed curves as in Fig. 8 permits the ICP user both to verify the presence of drift and to describe its shape. An alternative to these methods was described in ref. 14. It is based on the use of the RSD of successive values RSD,, and its comparison with the conventional RSD, based on the use of least-squares values. The least-squares standard deviation sexp is L J The SD of successive values is n - I ’1. s,= 1 1 (Xi+! -xJ*/2(n- 1) I l- (3) where n is the number of replicates and 2 is the mean. For a large number of data ssUc is practically independent of the shape of the curve in contrast to sexp. Without a drift sex should be equivalent to s,,,.When sex is greater than ssuc this should indicate the presence of a drift. The reliability of this method depends on the value of RSD, compared with that of RSD,,,. When drift is predominant over the short- term fluctuations (Fig. 9) i.e. RSD,,,>RSD,, the value of RSD, is significantly different from that of RSD, and can be used as evidence of the presence of drift (Table 8). In contrast when the short-term fluctuations predominate over drift (Fig. 7) the value of RSD, is not significantly different from that of RSD, (Table 7). Examples of Drift Diagnostics Thanks to the collaboration of many ICP users a large amount of data has been gathered. It is not possible to provide a comprehensive list of results.The purpose of this ~~ Table 7 Example of observation of drift based on 20 replicates. The slope of a linear regression was used to estimate the drift. The total measuring time was 14.5 min. The RSD subscripts ‘exp’ ‘reg’ and ‘cor’ stand for the experimental data the data after linear regression and the data corrected for drift respectively. The subscript ‘suc’ stands for the RSD obtained with the use of successive data Slope/ Element % per 10 min RSD,,,(%) RSD,(%) RSD,,r(oh) RSD,,,(%) Ar Z n Ba 0.29 1.49 1.21 0.54 0.12 0.53 0.54 2.24 0.66 2.14 2.05 2.63 0.58 2.56 2.1 1JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 795 Table 8 Example of observation of drift based on 20 replicates. The RSD of successive values was used to estimate the drift.The total measuring time was 19.5 min (subscripts as in Table 7) Slope/ Element Oh per 10 rnin RSD,,,(%) RSD,(%) RSD,,,(%) RSD,,,(%) Zn 4.23 2.52 2.48 0.46 0.44 Ba 2.03 1.34 1.21 0.57 0.40 105 1 - m C .$ 100 4 95 0 .- w I a I 0 50 100 150 200 Replicate No. Fig. 8 Variation of the Zn I1 206 nm line intensity as a function of the replicate number. The total measuring time was 100 min. The signal was normalized to 100 for the first replicate. Two smoothing procedures were applied based on the moving average of nine values with (bold line) and without (normal line) a weighting procedure c .p 103 0 .- w - Q a 99 0 5 10 15 20 Ti me/m i n Fig. 9 Variation of the A Zn I1 206; and B Ba I1 455 nm line intensities with time (20 replicates).The signal was normalized to 100 for the first replicate. Short-term fluctuations were small compared with drift 105 .- w .- L Q z 100 C cn fn .- P) .- 95 - P) a 1 I I 0 50 100 150 200 250 Ti me/mi n 90 ' Fig. 10 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 and - 5 was applied to Ar and Ba respectively. The RSDs (O/O) were 0.6 0.45 and 0.75 for Ar Zn and Ba respectively. The warm-up time was 1.5 h I 0 50 100 150 200 2 90 L Time/min D Fig. 11 Variation of the A Ar I 404; B Zn I1 206; and C Ba 11 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 + 3 and -5 was applied to Ba Ar and Zn respectively.The RSDs (Yo) were 1.85 2.1 and 1.4 for Ar Zn and Ba respectively. An increase in the sample introduction efficiency was observed up to 50 min whereas a slight decrease in the energy transfer was observed after 50 rnin section is rather to illustrate the potential of the method by means of selected examples based on the use of the Ar Zn and Ba line intensities for drift diagnostics. Before giving examples of drift it is worth considering results in the absence of drift. Several experiments indicated that the RSDs of the three test lines can be less than 1% over a period of 4 h using commercially available ICP systems. In Fig. 10 an example based on the use of an ICP system without internal standardization is given.A warm-up time of 1.5 h was used and RSDs of 0.6 0.45 and 0.75% were obtained for Ar Zn and Ba respectively. In Fig. 1 1 results are given with RSDs of around 2%; RSDs of 1.85 2.1 and 1.4% were obtained for Ar Zn and Ba respectively. These results are adequate for most applications. Nevertheless a drift was observed which is summarized in Table 9. From Table 2 it can be deduced that a slight increase in the efficiency of sample introduc- tion was observed in the range 0-50 min followed by a slight decrease in the power in the range 50-240 min. Warm-up is a particular case of drift. It is illustrated in Fig. 12 where the warm-up time was around 40 min. The behaviour of the three lines is indicated in Table 10. The experiment was started 10 min after plasma ignition.The warm-up period corresponded to a drastic increase in the energy transfer. After 40 min a slight decrease in the efficiency of the sample introduction system was observed with RSDs of 0.84 1.23 and 0.84% for Ar Zn and Ba respectively. Table 9 Behaviour of the Ar Zn and Ba line intensities over a period of 4 h (Fig. 1 1 ) Timdmin Ar Zn Ba 0-50 - + + - - 50-240 -796 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 113 1 h 0 50 100 150 200 250 Ti me/m i n Fig. 12 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A warm-up time is observed up to 40 min after which a slight decrease in the sample introduction system efficiency is observed where the RSDs (O/O) were 0.84 1.23 and 0.84 for Ar Zn and Ba respectively 115 I A /- I 40 80 120 160 85 ‘ 0 Ti me/mi n Fig.13 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. The drift is explained by a drastic change in the efficiency of the sample introduction system Table 10 Behaviour of the Ar Zn and Ba line intensities over a period of 4 h (Fig. 12) Time/min Ar Zn Ba 0-40 + + = - 40-240 + - The magnitude of drift shown in Fig. 13 is large but can be easily explained. Both the Zn and Ar line intensities exhibit the same decrease in contrast to the Ar line intensity. A drastic degradation of the efficiency of the sample introduction system was the main cause of drift.A more complex case is the overlap of at least two causes of drift (Fig. 14). The behaviour of the three lines is summarized in Table 1 1 . The variation of the Zn line intensity was more important than that of Ar. An increase in the efficiency of energy transfer (Ar+ Zn+ and Ba=) and an increase in the sample introduction system (Ar- Zn+ and Ba+) were observed. Additive effects for Zn and subtractive effects for Ar were therefore observed which explains the stronger effect on the Zn line intensity. Use of the ionic to atomic line ratio of Mg (Fig. 15) clearly indicates an increase in energy transfer during the same time period as the ratio varied from 7.5 to 8.7. This ratio has to be compared with that obtained during a more stable period (Fig.15). 190 C 0 .- 170 .- L m iE 150 0 v) a 130 > .- .- w 5 110 oc B N I A -/- 20 40 60 80 100 I20 140 ’ Time/mi n Fig. 14 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. There was no warm-up time. The drift is explained by the additive effects of an increase in the energy transfer and an increase in the efficiency of the sample introduction system =r” 70 40 80 120 160 Time/min Fig. 15 Variation of the Mg I1 280 nm to Mg I 285 nm line intensity ratio with time. Curve B was obtained at the same time as curves in Fig. 14 whereas curve A was obtained after a 3 h warm-up time. Curve B illustrates the change in the energy transfer 120 110 0 .- * L .- p 100 - m 0 .- go .- c - 80 .-0 50 100 150 200 250 Timelmin Fig.16 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 10 and - 10 was applied to Ba and Ar respectively. The RSDs (O/o) were 4.03,4.14 and 1.07 for Ar Zn and Ba respectively. The large RSDs for Zn and Ar compared with that of Ba give evidence of a problem of power Table 11 Behaviour of the Ar Zn and Ba line intensities over a period of 140 min. Example of the addition of at least two causes of drift (Fig. 14) Time/min Ar Zn Ba 0- 140 + ++ +JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 797 120 I c 100 0 .- c.. L .- 9 80 - (0 C (3 .- a 60 > 40< 20 0 40 80 120 160 200 Ti me/m i n Fig.17 Variation of the A Ar I 404; B Zn I1 206; and C Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 and - 5 was applied to Ba and Ar respectively. The large drift for both Zn and Ba indicates a significant power problem The test elements can also be used to predict a breakdown of the instrument while the ICP is in operation. An example is given in Fig. 16 where the drift was negligible. However the RSDs of both the Ar and Zn lines was large (4.03 and 4.14% respectively) compared with that of Ba ( 1.07%). There was clearly a problem with power. This was con- firmed later (Fig. 17). A drift was observed related to a decrease in the energy transfer. Actually the triode of the high-frequency oscillator failed at the end of the experiment.Conclusions In the instance where the drift has one or two major causes a simple experiment allows the ICP user to suggest explanations for this drift. It is also possible to indicate the magnitude of the drift for the sake of comparison. The only constraint is the need to study the variation of the three lines around the values normally used for routine analysis. It is also worth knowing the cause of drift when an internal standardization procedure is applied. In the in- stance where the drift is assigned to a problem linked with the sample introduction system most elements will exhibit the same behaviour. A single internal standard can there- fore be used to correct for drift and the use of sophisticated computational correction is not necessary.In contrast when the energy transfer is involved in drift the behaviour of the ionic lines will depend on their energy. Moreover the behaviour of the atomic lines with change in the power is rather complex. Consequently a single internal standard will not compensate for drift in every element. Use of computational methods such as GIRM4 or PRISM' can be helpful to solve this problem although the behaviour of atomic lines is not always predictable. The use of optical feedback power regulation has been reported as a means of improving stability.6 The Ar line can be used for this purpose. It was observed that the Ar line intensity can vary with the power and the efficiency of the sample introduction system. Therefore in order to use the Ar line for power regulation the stability of the sample introduction system must be sufficiently good to assume that no drift originates from this part of the system.An efficient way of obtaining this improvement is the use of a thermostated spray chamber. Is Association of a tempera- ture controlled spray chamber with optical feedback power regulation based on the use of an Ar line has been reported16 as providing improvement of a factor of 2 in the RSD reaching an average of 0.15% RSD over a 3 h period. Without correction a long-term stability of better than 1 Yo RSD can be expected over a period of 4 h. A lower value can be obtained with correction. Unfortunately this result cannot be observed for every commercially available ICP system.Our experience is that changes in energy transfer and efficiency of the sample introduction system are the most probable causes of drift. Another current drawback is too long a warm-up time. It is therefore necessary to undertake systematic studies on ICP systems to solve the drift problem which is probably the last challenge in ICP-AES. It is hoped that the approach described in this paper will contribute to a better understanding of drift phenomena. References 1 Ramsey M. H. and Thompson M. Analyst 1984 109 1625. 2 Schmidt G. J. and Slavin W. Anal. Chem. 1982 54 2491. 3 Myers S. A and Tracy D. H. Spectrochim. Acta Part B 1983 38 1227. 4 Lorber A. Goldbart Z. and Eldan M. Anal. Chem. 1984 56 43. 5 Lorber A. and Goldbart Z. Anal. Chim. Acta 1984,161 163. 6 Marks M. A. and Hieftje G. M. Appl. Spectrosc. 1988 42 277. 7 Al-Ammar A. S. Hamid H. A. and Rashid B. H. Spectro- chim. Acta Part B 1990 45 359. 8 Noack S. Stahl Eisen 1990 110 99. 9 Mermet J.-M. Anal. Chim. Acta 1991 250 85. 10 Ivaldi J. C. and Slavin W. J. Anal. At. Spectrum. 1990 5 359. 1 1 Mermet J.-M. Spectrochim. Acta Part B 1989 44 1109. 12 Chemometrics A Textbook eds. Massart D. L. Vandeginste B. G. M. Deming S. N. Michotte Y. and Kaufman L. Elsevier Amsterdam 1988 ch. 6. 13 Boumans P. W. J. M. Ivaldi J. C. and Slavin W. Spectrochim. Acta Part B 199 1 46 64 1. 14 Statistics in Spectroscopy eds. Mark H. and Workman J. Academic Press San Diego 199 1 ch. 7. 15 Vujicic G. and Steffan I. Spectrochim. Acta Part B 1988,43 293. 16 Rupp D. A. Vogel W. Sermin D. F. Routh M. W. and Kinsey W. J. paper presented at the Pittsburgh Conference New York March 5-9 1990 (abstract 765). Paper 2/0086 7J Received February 19 I992 Accepted June I 1992

ISSN:0267-9477    

DOI:10.1039/JA9920700791

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 799 Noise Characteristics of Aerosols Produced by Inductively Coupled Plasma Nebulizers* Shen Luan Ho-ming Pang Sam C. K. Shum and R. S. Houk? Ames Laboratory US Department of Energy and Department of Chemistry Iowa State University Ames IA 50011 USA The noise characteristics of aerosols produced by inductively coupled plasma nebulizers were investigated. A laser beam was scattered by aerosol and detected by a photomultiplier tube and the noise amplitude spectrum of the scattered radiation was measured by a spectrum analyser. Discrete frequency noise in the aerosol generated by a Meinhard nebulizer or a direct injection nebulizer was primarily caused by pulsation in the liquid flow from the pump. By use of a pulse-free pump such as a gas displacement pump or a dual piston pump for liquid chromatography the discrete frequency noise was eliminated.The configuration of the spray chamber affected the level of white noise. A Scott-type spray chamber suppressed white noise while a conical straight- pass spray chamber enhanced white noise relative to the noise seen from the primary aerosol. The noise in the aerosol from a continuous-flow ultrasonic nebulizer had a relatively high 1 / f component. Keywords Noise amplitude spectra; aerosol; nebulizer; inductively coupled plasma mass spectrometry; inductively coupled plasma atomic emission spectrometry The precision detection limits and dynamic range of instrumental measurements including inductively coupled plasma mass spectrometry (ICP-MS) and inductively coup- led plasma atomic emission spectrometry (ICP-AES) are generally influenced by noise in the signals measured.The optimization of experimental variables is often based on reducing the relative amount of noise or increasing the signal-to-noise ratio (S/N). However the S/N provides only an inclusive view of the performance of the system. A more fundamental understanding of noise characteristics re- quires a knowledge of the noise power spectrum (NPS) which may identify the types origins and frequency composition of the noise. Information on noise character- istics sometimes provides a sound rationale for improving the performance of the instrument. The NPS for ICP-MS1-3 and ICP-AES4-15 have been reported and the prevailing types of noise identified as (i) white noise (ii) l/f noise and (iii) interference noise.16 Noise in the ICP from large undissociated wet droplets and undesolvated dry particles has also been characterized exten~ively.~~-~* In ICP spectrometry the precision that can be achieved is often believed to be limited by the sample introduction s y ~ t e m .~ ~ - ~ ~ To improve the stability and the precision of the ICP sample introduction system the sources of noise should be isolated and characterized. Despite the numerous studies of noise behaviour of mass spectrometric or emission signals from the ICP no pub- lished results are known to exist that characterize the noise behaviour of the aerosols themselves in a thorough manner. This is the objective of the present work.Montaser et aLZ7 have described interferometric measurements of light scat- tered from aerosols produced by ultrasonic nebulizers. These results have not yet been published; however the main objective of Montaser's study was to measure droplet sizes and velocities rather than noise behaviour. Experimental A block diagram of the experimental set-up used for noise measurements is shown in Fig. 1. The instrumentation used is summarized in Table 1. The experiments were performed in a darkened room. Vibrations and air flows were minimized. *Presented at the 1992 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 6-1 1 1992. tTo whom correspondence should be addressed. Aerosol 543.5 nm ';$ He-Ne laser analyser Fig. 1 Block diagram of apparatus used for NAS measurements The spectrum analyser actually displayed the square root of the power spectrum sometimes called the amplitude spectrum.The ordinate of all plots was scaled in units of dBV [(l dBV=20 log A where A was the signal amplitude in volts root mean square (RMS)]. Note that this dBV unit was essentially an absolute measure of the noise level whereas the dB unit in our previous work' measured noise (at a specific frequency) relative to the d.c. level. The spectra shown were not normalized to the d.c. level so that the d.c. level (i.e. the scattering signal) observed for the various nebulizers and spray chambers could also be presented. Since the dBV scale is logarithmic the values can readily be converted into relative units (i.e. dB) by simply taking the difference between the noise amplitude at the frequency of interest and the dBV reading at the d.c.level (frequency = 0 Hz). The apparatus shown in Fig. 1 was similar to that used for laser Doppler spectroscopy (LDS) for particle size measure- m e n t ~ . ~ ~ In the present work aerosol droplet size was not measured. Instead noise amplitude spectra (NAS) were taken to characterize the noise sources in the aerosols generated by ICP nebulizers. The noise behaviour of the laser light scattered from the aerosol droplets was assumed to be related to noise processes in the production and transport of the aerosol itself. Spectra measured at several scattering angles had the same basic features as those measured at 15" which was used subsequently because it provided the highest scattering signal.A Meinhard nebulizer (Table 1) was studied because of its common use in ICP spectrometry. Distilled de-ionized water was the only sample used in this work. Different pumps were employed to deliver water to the Meinhard nebulizer as shown in Table 1. Self-priming aspiration (or natural uptake) was also investigated. The direct injection nebulizer (DIN) was similar to those described r e ~ e n t l y . ~ ~ - ~ ~ The internal diameter of the sample800 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 1 Instrumental components Laser Component Manufact ureddescri ption Melles Griot GreNe green He-Ne cylindrical laser head and power supply (Carlsbad CA USA) Focusing lens Iris diaphragm Interference filter PMT Rolyn spherical plano-convex glass lens (Covina CA USA) Edmund precision iris diaphragm (Barrington NJ USA) Melles Griot He-Ne laser line interference filter (Carlsbad) Hamamatsu R 955 side-on type (Japan) PMT power supply Tennelec/Nucleus TC 952 high voltage power supply (Oak Ridge TN USA) (Loveland CO USA) Spectrum analyser Hewlett-Packard Model 3582A X-Y recorder Picoammeter Meinhard nebulizer Direct injection nebulizer Spray chamber Ultrasonic nebulizer Peristaltic pump Syringe pump Single head LC pump Dual head LC pump Houston Omnigraphic Series 2000 Keithley Model 485 autoranging Meinhard Type TR-30-C3 (Austin TX USA) picoammeter (Cleveland OH USA) borosilicate glass concentric nebulizer (Santa Ana CA USA) Construction as described r e ~ e n t l y ~ ~ - ~ ~ Scott-type double-pass spray chamber34 (Precision Glassblowing Englewood CO USA) chamber (Fig.7 of ref. 35 but without any mixing baffle) Continuous-flow similar to the designs of Fassel and Bear,35 Model CPMT transducer (Channel Products Chagrin Falls OH USA) Plasma-Therm Model UNPS- 1 r.f. power supply (Kresson NJ USA) Gilson Minipuls 2 HP-I single channel peristaltic pump (Middleton WI USA) Sage Model 34 1 B single channel syringe pump (Orion Research Boston MA USA) with B-D Plastipak syringe (Becton Dickinson Rutherford NJ USA) Conical straight-pass spray SSI Model 2221) digital HPLC pump (Scientific Systems State College PA USA) with pulse damper Varian 2010 LC pump (Varian Instrument Group Walnut Creek CA USA) with SSI Model LP-21 LO-PULSE pulse damper (Scientific Systems) *Should be free from alias contamination.28 Operating conditions/specifications Wavelength 543.5 nm Power 5 mW Continuous wave Scattering angle 8 15" Aperture 2.5 mm Wavelength 543.5 nm Full width at half maximum 10 f 2 nm Bias voltage -300 V Frequency range d.c.to ~ 2 5 kHz* Sensitivity selects the maximum input level that can be applied to the instrument without overloading Coupling d.c. Passband shape Hanning Averaging mode RMS Number of averages 4 for 1 Hz 16 for 10 Hz 64 for more than 100 Hz 1.d. of sample delivery capillary 30 pm Resonant frequency of transducer 1.36 MHz Incident r.f. power 40 W Reflected r.f. power 0-1 W Liquid flow rate 2.5 cm3 min-' Argon carrier gas flow rate 1.5 dm3 min-I capillary was 30 pm.33 A single head liquid chromatography (LC) pump (Table l ) a dual head LC pump (Table 1 ) and a laboratory-made gas displacement pump29 were used to deliver water to the DIN.Aerosol NAS were measured directly on the aerosol from the Meinhard nebulizer or DIN and after the primary aerosol passed through either a Scott-type double-pass spray chamber or a conical straight-pass spray chamberJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 80 1 (Table 1). Ideally the tertiary aerosol should also be probed at the exit port of the injector tube of the ICP t o r ~ h . ~ ~ . ~ ~ This was not feasible however because the wet aerosol condensed at the cold tip of the injector The aerosol that coalesced on the inside surface of the spray chamber eventually flowed to the chamber drain and then to a waste receptacle.Irregularities in this flow may have caused pressure fluctuations in the chamber as the liquid flow ran into the waste receptacle. In the present work appropriate steps were taken to suppress these pressure pulsation^.^^.^^-^^ Finally noise in the aerosol generated with an ultrasonic nebulizer (USN) (Table 1) was studied. Water was intro- duced into the USN using the dual head LC pump (Table 1). Again NAS were measured on the wet droplets leaving a straight-pass spray chamber. Desolvation was not em- ployed. The spatial position where the aerosol was pro- duced on the face plate of the transducer fluctuated with time which precluded reliable measurements of NAS of the primary aerosol issuing directly from the surface of the USN. Argon was used throughout the present work as nebulizer gas (for the pneumatic nebulizers) and carrier gas (for the USN).The flow rate was controlled with a Matheson 7600 series flowmeter equipped with tube No. 602 (Matheson Gas Products Secaucus NJ USA). The needle valve on this flowmeter was positioned at the outlet. This arrange- ment allowed accurate measurements of flow rate for any back-pressure (or downstream pressure) provided that the gas pressure fed to the flow meter stayed equal to the pressure for which the tube was calibrated. Each nebulization system was oriented horizontally on a movable platform such that the distance between the nebulizer tip or the outlet of the spray chamber and the point of interaction with the laser beam could be altered.Usually the distance was 10.0 mm. The vertical position was also adjusted so that the laser beam could be directed into the centre of the aerosol plume. Results and Discussion Control Experiments A series of experiments were performed without nebulizers to determine whether the measurement system (i.e. laser photomultiplier tube (PMT) and spectrum analyser) contri- buted any significant noise. The NAS were measured from (i) the d.c. current output of a battery connected directly to the input of the spectrum analyser (ii) the output of a flashlight directed onto the PMT (iii) the attenuated output of the laser sent directly onto the PMT and (iv) laser scattering from a piece of abrasive paper with 42 ,um diameter particles detected by the PMT. In each case the signal measured by the picoammeter was - 1 PA which was similar to the magnitude of the scattering signal seen subsequently from aerosols.For each control experiment the white noise was less than - 120 dBV. Some small discrete frequency peaks (< - 1 10 dBV) were also observed. All these ‘instrumental’ noise levels were insignificant compared with the noise levels in the light scattered from the aerosols. Furthermore the background current of the PMT was -3 PA which was also insignificant compared with the usual scattering signals of = 1 PA. Based on these results the apparatus did not contribute appreciable noise compared with that seen from the light scattered by the aerosols. Noise from Aerosol Generated by Meinhard Nebulizer The NAS of scattering from the aerosol generated by a Meinhard nebulizer with liquid delivered by a peristaltic 10 0 -10 -20 -30 -40 -50 2 -60 D 2 -70 \ .- 2 0 2 4 6 8 10 P E 10 - .- s o 0 -10 -20 -30 -40 -50 -60 0 20 40 60 80 100 FrequencyIHz Fig.2 (a) NAS of scattering from the primary aerosol generated by a Meinhard nebulizer with a peristaltic pump; and (6) NAS of the primary aerosol generated by a Meinhard nebulizer with self- priming aspiration (or natural uptake). No spray chamber was used for either (a) or (6). The liquid flow rate was 1.0 dm3 min-I. The argon nebulizer gas flow rate was 1.0 dm3 min-’ pump is shown in Fig. 2(a). Three types of noise are observed white noise ( i e . the asymptote at the higher end of the frequency axis) llfnoise (the gradual decline of noise amplitude as frequency increases from 0 Hz) and interfer- ence noise (the peaks at discrete frequencies).The fre- quency axis in Fig. 2(a) stopped at 10 Hz because no discrete noise peaks were seen at higher frequencies which was the case for all the nebulizers pumps efc. evaluated in this study. In Fig. 2(a) the interference noise peaks at 1.88 and 2.84 Hz were harmonics of the 0.96 Hz fundamental noise. The frequencies of these peaks did not change as the nebulizer gas flow rate and the distance between the nebulizer tip and the point of interaction with the laser beam (i.e. the ‘scattering position’) were changed. However the peak frequencies increased with liquid flow rate (Table 2). At each liquid flow rate studied the fundamental frequency of the pulsation measured from the NAS was essentially the same as the frequency with which a fresh roller touched the compressible pump tubing as the head of the pump rotated.Thus the pulsation in the liquid flow induced the discrete noise peaks observed in the NAS. Similar peaks have been seen in NPS from ICP s i g n a l ~ . ~ ? ~ J ~ Table 2 Pulsation induced by peristaltic pump* Liquid flow rate/ Frequency of Frequency of cm3 min-I pulsation/Hz fundamental noise/Hz 0.84 0.79 0.80 1 .o 0.95 0.96 1.5 1.36 1.36 *All frequencies in this table were measured three timcs cach measurement yielded the same value.802 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 3 Pulsation induced by various pumps* Liquid flow rate/ Frequency of Pump cm3 min- ' fundamental noise/Hz Syringe 0.78 5.2 1.1 7.2 1.6 10.8 Single head LC 0.800 0.32 Single head LC + pulse damper 0.800 0.32t *All frequencies in this table were measured three times each measurement yielded the same value. ?The amplitude of the pulsation was less severe than when the single head LC pump was used alone (without the pulse damper).Also similar peaks at discrete frequencies were seen from NAS taken when the Meinhard nebulizer was fed by the syringe pump or the single head LC pump (Table 3). In each case the frequency of the fundamental noise peak was the same as the frequency of a fluctuation in the liquid flow rate caused by the mechanism that drove the pump.44-46 The amplitude of these noise peaks was lowest (= - 30 dBV) for the syringe pump but such peaks were still present which rebutted the occasional claim that syringe pumps deliver pulseless flow.The dual head LC p ~ m p ~ ~ ~ ~ ~ did not produce detectable interference noise peaks. As shown in Fig. 2(a) harmonic peaks were seen at both odd and even integer multiples of the fundamental fre- quency. The intensity of the harmonic peaks fell off as frequency increased. A similar pattern of harmonic peaks is obtained in the frequency domain when a full-wave rectified sine wave in the time domain is subjected to the Fourier transform process.48 Thus the liquid flow from the peristaltic pump the syringe pump and the single head LC pump is modulated by a full-wave rectified sine wave pattern which is physically reasonable. Fourier transforma- tion of a sawtooth wave also yields this pattern of R (=' -30 -90 1 I I I I I 0 0.2 0.4 0.6 0.8 1.0 Frequency/Hz Fig.3 NAS of DIN aerosol with a single head LC pump (a) without the pulse damper; and (b) with the pulse damper. The liquid flow rate was 50 mm3 min-' harmonics in the frequency domain but the flow output of a pump probably does not have a sawtooth character. Self-priming aspiration (or natural uptake) was also used to deliver liquid. As expected no discrete frequency peaks were present in the noise spectra [Fig. 2(b)]. Comparison of Figs. 2(a) and 2(b) showed that for unknown reasons the white noise level (ie. the baseline asymptote at high frequency) was higher by about 10 dBV when the Meinhard nebulizer was fed by natural uptake. Noise from Aerosol Generated by DIN The NAS from a DIN aerosol using a single head LC pump is shown in Fig.3. Without the pulse damper substantial interference noise was observed [Fig. 3(a)]. The frequency of the fundamental noise was found to be 0.020 Hz which again corresponded to the frequency of the pumpinghefill stroke at the very low liquid flow rate used (50 mm3 min-I). With the pulse damper the interference noise was elimi- nated [(Fig. 3(b)]. Pulse-free pumps including a dual head LC pump (with pulse damper) and a gas displacement pump,29 were investigated with the DIN. As expected no interference noise was present. Only the pulse-free pumps were used for subsequent experiments to suppress noise peaks so that the llfstructure in the NAS could be seen clearly. It is interesting that the NAS obtained with a single head LC pump (with pulse damper) a dual head LC pump (with pulse damper) and a gas displacement pump all gave basically identical llfprofiles and similar white noise levels (measured relative to the d.c.level). Thus the white noise and llfnoise in the NAS were not greatly influenced by the pumps used. Comparison of Noise from Meinhard Nebulizer and DIN The NAS of scattering from the aerosol from the Meinhard nebulizer is compared with that from the DIN in Fig. 4. These two spectra would be fairly similar if they were normalized to the same d.c. level. In each instance the white noise levels were -50 dBV below the d.c. level. The d.c. levels were related to the total scattering signals seen from the two nebulizers. These d.c. levels differed by - 15 dBV because the liquid flow rates and droplet size distribu- tions were different for the two nebulizers.The contribution of white noise to the relative standard deviation (RSD) of a measured signal can be estimated from the white noise level N (in dB) using the following equation:lJO RSDx 1 (1) 10 I I 0 2 -10 -g -20 9 CI .- - a -30 E -40 v) .- O -50 z -60 -70 1 I 1 I 0 20 40 60 80 100 FrequencyIHz Fig. 4 Comparison between the NAS of the aerosol from A Meinhard nebulizer (the liquid flow rate was 1 .O cm3 min-I) and B a DIN (the liquid flow rate was 50 mm3 min-I). A dual head LC pump was used in both instancesJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992. VOL. 7 803 The value of N is measured below (or relative to) the d.c. level. For example in Fig.4 B the white noise level is -65 dBV and the d.c. level is - 15 dBV so N in eqn. (1) is (- 65)-( - 15)= - 50 dB. This subtraction process is equi- valent to normalizing the noise amplitude at the chosen frequency to the d.c. level. In the present study white noise levels of - -48 to - 50 dB were observed for both the Meinhard nebulizer and the DIN. Thus white noise as the sole noise source would be expected to yield RSDs of 0.3-0.4%. These RSDs were comparable to those obtained from DIN experiments in ICP-MS at relatively high signal levels ( x 1 x los-1 x lo6 counts s - ' ) . ~ ~ In contrast this level of white noise is a minor contribution to the total signal instability of ~ 2 % RSD or worse that is typical of many different ICP-MS devices at high signal levels when a Meinhard nebulizer is used.49 -lo fi -30 c .- -50 I 5 % I \ I -70 tz -90 1 0 20 40 60 80 100 FrequencyjHz Fig.5 NAS from aerosol from a Meinhard nebulizer after passage through a Scott-type7 double-pass spray chamber. A dual head LC pump was used. The liquid flow rate was 1 .O cm3 min-I. The argon nebulizer gas flow rate was 1.0 dm3 min-' -40 -50 -60 F 3 -70 P 10 > c1 .- - 5 0 .- $ 0 -10 z -20 -30 -40 -50 -60 -70 t h I I I I 1 0 20 40 60 80 100 F requency/Hz Fig. 6 NAS from aerosols from a Meinhard nebulizer after passage through a conical straight-pass spray chamber. A dual head LC pump was used. The liquid flow rate was 1.0 cm3 min-'. The argon nebulizer gas flow rate was 1.0 dm3 min-I. The argon make-up gas flow rate was (a) 0; and (6) 0.5 dm3 min-I Effect of Spray Chamber on Noise from Aerosol The NAS from aerosol from a Meinhard nebulizer after it passed through a Scott-type double-pass spray chamber with the use of a relatively pulseless pump (the dual head LC pump) is shown in Fig.5. No discrete frequency noise was observed. Note that the white noise level N was x (-82)-(-22)= -60 dB (Fig. 5) which was x 10 dB lower than that present in the primary aerosol (Fig. 4 A). The -60 dB value would correspond to an RSD of -0.1%. Compared with the RSDs of 0.3-0.4% expected for the primary aerosol (as described in the preceeding paragraph) the Scott-type double pass spray chamber apparently improved precision by a factor of 3-4 The NAS from aerosols that were passed through a conical straight-pass spray chamber are shown in Fig.6. Again no discrete peaks were observed. Apparently neither spray chamber contributed noticeable interference noise. Thus the usual audible noise peaks at 200-400 Hz that were seen in either ICP-MS or ICP-AESloJs cannot be blamed on either the nebulizer or the spray chamber. Without argon make-up gas flow (introduced through the spray chamber) the llfnoise in Fig. 6(a) was very large as indicated by the slow decline in noise amplitude as frequency increased. Next an argon make-up gas was added through the drain tube of the spray chambeP at 0.5 dm3 min-l. The total gas flow rate ( i e . the nebulizer gas flow rate plus the make-up gas flow rate) was 1.5 dm3 min-I. Addition of the make-up gas suppressed the llf noise substantially [Fig. 6(b)].Thus the gas flow patterns through a spray chamber can affect the characteristics of noise in the aerosol leaving the chamber. Also it is interesting to note that the white noise level N in Fig. 6(b) is -(-43)-(-8)=-35 dB which is =15 dB higher than that present in the primary aerosol (Fig. 4 A). This indicated that the precision was poorer (by a factor of 5-6) after the aerosol passed through the conical straight-pass spray chamber. This degradation in precision agreed with previous experience with the use of a conical straight-pass spray chamber in ICP-MS experiments with the DIN29 and in other work in ICP-AES.sO The effect of the two spray chambers on llfnoise was illustrated by comparing the NAS from the primary aerosol produced by the Meinhard nebulizer with the NAS from the secondary aerosol produced by the same Meinhard nebu- lizer after passage through either a Scott-type double-pass spray chamber or a conical straight-pass spray chamber.In this instance only each of these spectra was normalized to 0.0 a -0 .- 2 -0.2 E" .- 2 - a -0.4 0 C -0.6 U E .- -0.8 E z -1.0 0 20 40 60 80 100 Frequency/Hz Fig. 7 Normalized NAS of the aerosols from A a Meinhard nebulizer alone; B the same Meinhard nebulizer with a Scott-type double-pass spray chamber; and C the same Meinhard nebulizer with a conical straight-pass spray chamber with make-up gas. Each spectrum is normalized to the same white noise level at - 100 Hz. A dual head LC pump was used and the liquid flow rate was 1.0 cm3 min-'804 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL.7 lo c 0 2 -10 4 -20 F 4- .- - a -30 Em -40 .- % 0 -50 Z -60 1 -70 1 0 20 40 60 80 100 FrequencyIHz Fig. 8 NAS of the aerosol generated by USN (after passage through a straight-pass spray chamber). A dual head LC pump was used. The liquid flow rate was 2.5 cm3 min-l and the argon carrier gas flow rate was 1.5 dm3 mind' the same white noise level (- I .O). As shown in Fig. 7 this normalization process facilitated visual comparison of the extent to which noise amplitude dropped off as frequency increased. Curves B and C were obtained with spray chambers. The Ilfportions of these two curves were similar and lay slightly below that for curve A. In other words the noise dropped off somewhat faster and approached the white noise limit at a lower frequency when either spray chamber was used. Thus use of either spray chamber apparently attenuated 1 lf noise slightly.Such information was hard to discern unless the spectra were normalized to the same white noise level. Noise from Aerosol Generated by USN The NAS of the aerosol generated by a USN with the usual straight-pass spray chamber is shown in Fig. 8.35 The d.c. signal (i.e. the point at 0 Hz) was 13 dBV higher than that for the Meinhard nebulizer at a comparable total gas flow rate [1.5 dm3 min-l Fig. 6(b)]. A higher scattering signal was expected from the USN partly because of its higher uptake rate (2.5 cm3 mine'). It can also be seen from Fig. 8 that a fairly large amount of llfnoise was observed from the USN. The noise amplitude still dropped as frequency increased up to frequencies of at least 100 Hz.The llfnoise profile was worse for the USN (than for the pneumatic nebulizers) regardless of whether or not the NAS were normalized to the d.c. level. The spray chamber for the USN was very similar to the conical straight-pass spray chamber described in the last section. The carrier gas for the USN was added through the same drain port as the make-up gas when the Meinhard nebulizer was used with this type of spray chamber. With the Meinhard nebulizer addition of make-up gas greatly attenuated llfnoise [i.e. compare Figs. 6(a) and 6(b)]. The USN itself was apparently more susceptible to llfnoise than the Meinhard nebulizer because llf noise was still substantial with the USN when the same spray chambers were used for either nebulizer.Discrete frequency noise was not noticeable; however it could have been masked by the llf noise. Conclusions The present study illustrates several points of practical interest for analysis by ICP-MS or ICP-AES (i) interference noise in nebulization is at relatively low frequencies (ie. 0.02-10 Hz) and is caused by pump fluctuations not by either the nebulizer or spray chamber; (ii) the configuration of and gas flow patterns through the spray chamber can affect levels of llfnoise and white noise. In particular the common Scott-type chamber suppresses both llf noise and white noise relative to that in the primary aerosol; (iii) the ultrasonic nebulizer suffers from additional 1 lf noise beyond that expected from the conical spray chamber usually employed; and (iv) the RSD of the scattering signal from the secondary aerosol can be as good as 0.1%.This value is comparable to the best precision commonly achievable for analytical signals from the ICP in cases where special care is taken to optimize precision. Examples include integrated intensity measurements with a thermo- stated spectrometer and Myers-Tracy corrections1 in ICP- AES or isotope ratio measurement by fast peak hopping in ICP-MS. While the sample introduction system can limit precision of spectrochemical measurments with the ICP this need not always be the case. The authors gratefully acknowledge R. Hendrickson of Department of Nuclear Engineering Iowa State University for the loan of the spectrum analyser and S. J. Weeks for the loan of the laser.The authors thank R. K. Winge D. E. Eckels F. G. Smith K. Hu and H.-C. Chang for their helpful discussion and assistance during the course of this work. Ames Laboratory is operated by Iowa State Univer- sity for the US Department of Energy under Contract No. W-7405-Eng-82. This research was supported by the Office of Basic Energy Sciences Division of Chemical Sciences. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 References Crain J. S. Houk R. S. and Eckels D. E. Anal. Chem. 1989 61 606. Furuta N. Monnig C. A. Yang P. and Hieftje G. M. Spectrochim. Acta Part B 1989 44 649. Furuta N. J. Anal. At. Spectrom. 1991 6 199. Walden G. L. Bower J . N. Nikdel S. Bolton D. L. and Winefordner J. D. Spectrochim. Acta Part B 1980 35 535.Belchamber R. M. and Horlick G. Spectrochim. Acta Part B 1982 37 17. Benetti P. Bonelli A. Cambiaghi M. and Frigieri P. Spectrochim. Acta Part B 1982 37 1047. Davies J. and Snook R. D. J. Anal. At. Spectrom. 1986 1 195. Davies J. and Snook R. D. J. Anal. At. Spectrom. 1987 2 27. Antanavichyus R. L. Serapinas P. D. and Shimkus P. P. Opt. Spektrosk. 1987 63 224. Winge R. K. Eckels D. E. DeKalb E. L. and Fassel V. A. J. Anal At. Spectrom. 1988 3 849. Sing R. L. A. and Hubert J . J. Anal. At. Spectrom. 1988 3 835. Montaser A. Ishii I. Tan H. Clifford R. H. and Golightly D. W. Spectrochim. Acta Part B 1989 44 1163. Montaser A. Clifford R. H. Sinex S. A. and Capar S. G. J. Anal. At. Spectrom. 1989 4 499. Goudzwaard M. P. and de Loos-Vollebregt M. T. C. Spectrochim.Acta Part B 1990 45 887. Furuta N. Anal. Sci. 1990 6 683. Ingle J. D. Jr. and Crouch S. R. Spectrochemical Analysis Prentice Hall Englewood Cliffs 1988. Olesik J. W. Smith L. J. and Williamsen E. J. Anal. Chem. 1989 61 2002. Olesik J. W. and Fister J. C. 111 Spectrochim. Acta Part B 1991 46 851. Fister J . C. 111 and Olesik J . W. Spectrochim. Acta Part B 199 1 46 869. Hobbs S. E. and Olesik J . W. Anal. Chem. 1992 64 274. Cicerone M. T. and Farnsworth P. B. Spectrochim. Acta Part B 1989 44 897. Winge R. K. Crain J. S. and Houk R. S. J. Anal. At. Spectrom. 199 1 6 60 1. Browner R. F. and Boorn A. W. Anal. Chem. 1984 56 786A and 875A. McGeorge S. W. and Salin E. D. Appl. Spectrosc. 1985 39 989.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 805 25 Hobbs P.Spillane D. E. M. Snook R. D. and Thorne A. P. J. Anal. At. Spectrom. 1988 3 543. 26 Sneddon J. in Sample Introduction in Atomic Spectroscopy ed. Sneddon J. Elsevier Amsterdam 1990 ch. 1 p. 1. 27 Montaser A. Clifford R. H. and Sohal P. paper presented at the 199 1 European Winter Conference on Plasma Spectroche- mistry Dortmund Germany January 14- 18 199 1. 28 Hewlett-Packard Operating Manual for Model 3582A Spec- trum Analyzer Hewlett-Packard Loveland 1 978. 29 Wiederin D. R. Smith F. G. and Houk R. S. Anal. Chem. 1991 63 219. 30 Wiederin D. R. Smyczek R. E. and Houk R. S. Anal. Chem. 1991,63 1626. 31 Smith F. G. Wiederin D. R. and Houk R. S. Anal. Chim. Acta 1991 248 229. 32 Wiederin D. R. and Houk R. S. Appl. Spectrosc. 1991 45 1408. 33 Shum S.C. K. Neddersen R. and Houk R. S. Anal-vst 1992 117 577. 34 Scott R. H. Fassel V. A. Kniseley R. N. and Nixon D. E. Anal. Chem. 1974 46 75. 35 Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. 36 Hinds W. and Reist P. C. AerosolSci. 1972,3 501 and 5 1 5 . 37 Sharp B. L. J Anal. At. Spectrom. 1988 3 939. 38 Clifford R. H. Ishii I. Montaser A. and Meyer G. A. Anal. Chem. 1990 62 390. 39 Belchamber R. M. and Horlick G. Spectrochim. Acta Part B 1981 36 581. 40 Belchamber R. M. and Horlick G. Spectrochim. Acta Part B 1982 37 1075. 41 Boumans P. W. J. M. and Lux-Steiner M. Ch. Spectrochim. Acta Part B 1982 37 97. 42 Schutyser P. and Janssens E. Spectrochim. Acta Part B 1984 39 737. 43 Schutyser P. and Janssens E. Spectrochim. Acta Part B 1979 34 443. 44 Orion Research Incorporated Sage Model 34IB Syringe Pump Instruction Manual Orion Research Boston 1987. 45 Scientific Systems Incorporated User’s Manual for SSI Model 2220 Digital HPLC Pump Scientific Systems State College 1990. 46 Poole C. F. and Schuette S. A. Contemporary Practice of Chromatography Elsevier Amsterdam 1984. 47 Varian Associates Inc. 2010 Pump/2210 System Operators Manual Varian Instrument Walnut Creek 1984. 48 Sprott J. C. Introduction to Modern Electronics Wiley New York 1981. 49 Jarvis K. E. Gray A. L. and Houk R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry Blackie Glas- gow 1992. 50 Fry R. C. personal communication 1992. 51 Myers S. A. and Tracy D. H. Spectrochim. Acta Part B 1983 38 1227. Paper 2/00184E Received January 9 1992 Accepted July 6 1992

ISSN:0267-9477    

DOI:10.1039/JA9920700799

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 807 Evaluation of an Ultrasonic Nebulizer for Sample Introduction in Inductively Coupled Plasma Atomic Emission Spectrometry* Theresa M. Castillano Nohora P. Vela and Joseph A. Carusot Department of Chemistry University of Cincinnati Cincinnati OH 45221 -01 72 USA W. Charles Story Environmental Health Research and Testing 3235 Omni Drive Cincinnati OH 45245 USA A low cost ultrasonic nebulizer has been designed using a forced air cooling method. The performance of this ultrasonic nebulizer in inductively coupled plasma atomic emission spectrometry has been evaluated and compared with a concentric nebulizer and a glass frit nebulizer. Higher signal-to-background ratios have been achieved for the ultrasonic nebulizer compared with the concentric and glass frit nebulizers.Detection limits and sensitivity have been improved by approximately one and two orders of magnitude respectively. In addition the response time of the ultrasonic nebulizer is comparable to those of the other types of nebulizers studied; a relative standard deviation of 3-5% for the ultrasonic nebulizer. Tests were also conducted to determine the effects of a high matrix sample such as synthetic ocean water. Keywords Inductively coupled plasma atomic emission spectrometry; ultrasonic nebulizer; matrix effect Several techniques are available for sample introduction into the inductively coupled plasma (ICP). Among these methods the most commonly used is the direct nebuliza- tion of the sample. Many different types of nebulizers can be used for this purpose including the concentric or cross flow nebulizers the glass-frit nebulizer and the ultrasonic nebulizer.Each type has its own advantages and disadvan- tages. Recent investigations have been conducted to mea- sure the size of the droplets formed by these it was found that the ultrasonic nebulizer generates a broader size distribution of the primary aerosol than the pneumatic nebulizer but mass transport is still much higher owing to greater particle density of the aerosol produced by using the ultrasonic nebulizer. The pneumatic nebulizer has been commonly used because of its simplicity and ease of operation. However transport efficiency is extremely low (about 3-8O/0)~ owing to the very wide range of droplet sizes produced. The glass- frit nebulizer however produces a finer aerosol compared with the pneumatic nebulizer but can suffer from clogging of the fritted glass disk over time.4 The ultrasonic nebulizer also produces a fine aerosol and has high sample transport efficiency which results in better analyte sensitivity and detection limits.In contrast to the glass-frit nebulizer very little clogging occurs with the ultrasonic nebulizer. Another advantage of the ultrasonic nebulizer is that a separate optimization of nebulizer parameters can be carried out since the efficiency of nebulization is independent of the carrier gas flow rate. However the present applications are limited to non-viscous solutions. Ultrasonic nebulizers are available commercially but the high cost of the commercial instrumentation has yet to be justified.Work has been done to develop low-cost ultra- sonic nebulizers5v6 and the performance when coupled with ICP atomic emission spectrometry (ICP-AES) has been e ~ a l u a t e d . ~ - ~ A common feature in the design of most ultrasonic nebulizers is the use of a water-cooled system positioned behind the transducer. However several prob- lems arise with this method of cooling such as corrosion and electrical shorting.1° In an attempt to make a simple reliable and relatively inexpensive ultrasonic nebulizer that can be used for routine analysis in a laboratory an air- cooled system has been designed. Other air-cooled ultra- sonic nebulizers have been described in the literature." An evaluation of the performance of this nebulizer with ICP- AES was carried out in terms of the signal-to-background ratios sensitivity response factor detection limits and response in the presence of a high-salt content matrix.Experimental Design The ultrasonic nebulizer developed in this laboratory is a modification of one designed by Fassel and Bear.12 The first design incorporated a heat sink that was directly attached to the rear of the transducer. The drawbacks of this prelimi- nary design were the instability of the signal and the relatively short lifetime of the transducer. This short lifetime was mainly due to the inefficient cooling of the transducer by the heat sink. Also because the heat sink was attached directly to the back of the transducer the oscillation of the transducer was restricted.In order to compensate for this restriction the transducer had to be operated at a higher power which resulted in a diminished lifetime. In order to improve on the first design a forced-air cooling of the transducer was employed. The current design of the ultrasonic nebulizer is illustrated in Fig. 1. The heat sink was eliminated and two O-rings were used to hold the Sample Air in I c Aerosol *Presented in part at the 1992 Winter Conference on Plasma fTo whom correspondence should be addressed. Spectrochemistry San Diego CA USA January 6-1 1 1992. Fig. 1 I 51' 1 -Carrier gas 1 I 1 Drain i Air out Modified ultrasonic nebulizer with air-cooling system808 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 1 Operating conditions Forward power/kW Gas flow rate/dm3 min-I of Ar Outer Auxiliary Sample uptake rate/cm3 min-I Nebulizer (carrier) gas flow rate/dm3 min-' of Ar Pneumatic Glass frit Ultrasonic Heating chamber temperature/"C Condenser temperature/"C Aerosol chamber temperature/'C Desolvation conditions (for ultrasonic nebulizer) 1 16 1 0.5 0.35 0.20 0.60 160 0 0 transducer in place.The area behind the transducer was enclosed so that air could be circulated in the hollow section. An air flow of approximately 1 dm3 min-l was sufficient for adequate cooling. Instrumentation The plasma instrument used for these experiments was the PlasmaTherm 2500 (PlasmaTherm RTE 73 Kresson NJ USA). The ICP conditions are shown in Table 1. The emission was passed through two focusing lenses to the 1.26 m monochromator (Spex Spectrophotometer Metu- chen NJ USA) with 700 V applied to the photomultiplier tube for detection. Data collection was obtained with a Spex Datamate system.A schematic representation of the ultrasonic nebulizer set-up is shown in Fig. 2. This laboratory-built ultrasonic nebulizer makes use of a cooled spray chamber (University of Cincinnati) cooled to a temperature of about 0 "C. The sample is delivered by means of a peristaltic pump. The solution flows on the surface of the transducer through a capillary tube with a tapered tip that is positioned in such a way that the tip is almost in contact with the surface of the transducer. The transducer used is a Channel Products (Chesterland OH USA) Model CPMT which resonates at a frequency of 1.35 MHz.The dimensions of the transducer are 1.06 in (diameter) and 0.135 mm (thickness). A Wavetek (Indianapolis IN USA) sweep-function generator Model 180 was used as the signal source. The signal was fed into a power amplifier (University of Cincinnati Electronics Shop) which drives the transducer at the resonant fre- quency. A desolvation apparatus is required because of the high transport efficiency of the ultrasonic nebulizer. The heating chamber consists of a quartz tube (0.6 cm i.d. 35 cm long) wrapped with heating tape and powered by a Variac power supply (Fisher Scientific Pittsburgh PA USA). A 20 cm long condenser was used to remove the solute vapour producing a dry aerosol which was carried Dry aerosol to plasma Function t generator 1 Power amplifier 1 Sample Condenser Coolant - in Fig.2 Schematic diagram of the set-up of the ultrasonic nebulizer to the plasma by the nebulizer gas flow. Desolvation conditions are also shown in Table 1. Solutions A 100 ppm multi-element solution containing barium cobalt copper chromium iron magnesium nickel and antimony was prepared in 2% nitric acid from 1000 ppm standard solutions (Fischer Scientific NJ USA). The serial dilution method was used to prepare multi-element solu- tions in concentrations ranging from 10 ppm to 1 ppb. Blanks for each element and concentration were also prepared in an effort to minimize interferences. In order to study matrix effects multi-element solutions containing barium cobalt copper and iron in synthetic ocean water (SOW) were prepared in concentrations rang- ing from 100 ppm to 100 ppb.The composition of the SOW is as described in ref. 13. Results and Discussion Comparison of Different Nebulizers Using the 10 ppm solution for both the pneumatic and glass-frit nebulizers and the 1 ppm solution for the ultrasonic nebulizer signal-to-background ratios were com- pared and the results are shown in Table 2. The values listed for the ultrasonic nebulizer must be multiplied by ten to normalize for the difference in the concentrations. These results illustrate that the signal-to-background ratio for the ultrasonic nebulizer is 20-100 times better than that of the pneumatic and glass-frit nebulizers. A comparison of the sensitivity of the nebulizers studied for each of the elements listed is shown in Table 3.The data ~~ Table 2 Comparison of signal-to-background ratios using a 10 ppm multi-element solution Signal-to-background ratio Element and line Wavelengthhm Pneumatic nebulizer Ba I1 c o I1 Cr I1 c u I1 Fe I1 Mg I Ni I1 Sb I 233.527 236.379 205.552 324.7 54 238.204 285.21 3 22 1.647 206.838 14.5 2.39 1.81 3.48 6.3 1 4.62 0.50 12.8 Glass-frit nebulizer 9.26 1.81 1.29 1.87 2.57 5.94 2.7 1 0.29 Ultrasonic nebulizer* 45.4 15.0 8.98 7.24 34.6 62.0 18.9 1.67 *Values were obtained using a 1 ppm multi-element solution.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 809 Table 3 Comparison of sensitivity Sensitivity (arbitrary units per ppm) Element and line Ba I1 c o I1 Cr I1 c u I Fe I1 Mg I Ni I1 Sb I Pneumatic nebulizer 3.77 x 10-3 9.31 x 10-4 2.25 x 10-4 6 .1 5 ~ 10-3 2.93 x 10-3 1 . 7 7 ~ 9 . 1 4 ~ - Glass-frit nebulizer Ultrasonic nebulizer 2.37 x 10-3 3.23 x 10-4 4 . 5 0 ~ 1 . 3 4 ~ 10-4 9.84 x 10-3 2.35 x 10-3 3 . 1 6 ~ 10-1 1.95 x 10-4 1.25 x lo-' 6 . 1 5 ~ 8 . 1 9 ~ 10-1 4.27 x 10-4 2.85 x lo-* 1.38 x 10-3 1.25 x 10-1 - r2 (for ultrasonic nebulizer) 0.9988 0.9998 0.9994 0.9996 0.9972 1 .oooo 0.9990 0.9998 Table 4 Comparison of detection limits Detection limit* (ppb) Element and line Wavelengthhm Pneumatic nebulizer Glass-frit nebulizer Ultrasonic nebulizer Ba I1 c o I1 Cr I1 c u I Fe I1 Mg I Ni I1 Sb I 233.527 236.379 205.552 324.754 238.204 285.2 13 221.647 206.838 30 135 1150 70 41 1 1 37 - 36 176 246 96 3 54 19 141 - 6 7 4 1 1 2 2 3 200 *Detection limits were calculated using three times the standard deviation divided by the slope of the calibration graph.show an increase in sensitivity by approximately two orders of magnitude using the ultrasonic nebulizer. No value is listed for the sensitivity of the pneumatic and glass-frit nebulizers for antimony because a signal was only seen for the 100 and 10 ppm solutions. Values for r2 (r=regression coefficient) are also shown in Table 3 which indicate that the calibration graphs are linear over at least three orders of magnitude. The detection limits for the three different nebulizers are given in Table 4. As is evident the ultrasonic nebulizer gives detection limits that are approximately 5-20 times better (excluding chromium) than the other two nebulizers studied. The detection limit for chromium however improved by a factor of 200 which is much higher than was reported in previous studies.*l The reason for this is still under investigation.The Mg I 285.2 13 nm line was used to study the response times of the different nebulizers and a 100 ppm solution was used. Once a constant signal was obtained data were collected for 1 min the solution was changed to a 2% nitric acid blank for about 2 min and then back again to the 100 ppm solution within a total time of 5 min. The same procedure was followed for the glass-frit and ultrasonic nebulizers except that a 10 ppm multi-element solution was used for the ultrasonic nebulizer. Representative plots are shown in Fig. 3 for percentage signal intensity versus time. Using the pneumatic nebulizer [Fig. 3(a)] a negative peak was observed before the decrease in the signal indicating the point where the air bubbles formed when the change in solutions reached the nebulizer.However when using the glass-frit nebulizer [Fig. 3(b)] this resulted in a positive peak. There was no indication of this point when using the ultrasonic nebulizer [Fig. 3(c)] as the volume of the air bubble is negligible and might have spread over the total volume of the system. Based on the slopes of the response curves the glass-frit nebulizer showed a slightly longer response time due to the lower nebulizer flow rate used. Although a higher carrier gas flow rate was used for the ultrasonic nebulizer results obtained show a similar response time to that of the pneumatic nebulizer. This could be associated with the desolvation step incorporated when using the ultrasonic nebulizer.Stability tests performed on each of the nebulizers by alternating the 10 ppm mutli-element solution and the blank solution every 5 min for 1 h are illustrated in Fig. 4. The relative standard deviation (RSD) ranges from approx- imately 1-3% for the pneumatic and glass-frit nebulizers [Fig. 4(a) and (b)]. The ultrasonic nebulizer [Fig. 4(c)] loo l-7 I I I loo l-7 50 t 0 1 2 3 4 5 Time/min Fig. 3 nebulizer and (c) ultrasonic nebulizer Response times using (a) pneumatic nebulizer (h) glass-frit810 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 5 Effect of matrix on signal intensity Suppression or Detection limit (ppb) enhancement Element and line Wavelengthhm factor 2% HN03 sow Ba I1 c o I1 c u I Fe I1 233.527 236.379 324.754 2 3 8.204 0.0 1 0.67 I .30 0.68 3 636 14 14 I08 70 23 17 0.12 0.08 0.04 0 7 ( a ) I 1 I I I I 0 10 i 20 30 40 50 Time/mi n 60 Fig.4 Stability test for the (a) pneumatic nebulizer (b) glass-frit nebulizer and ( c ) ultrasonic nebulizer using a 10 ppm multi- element solution and monitoring the Mg 285.2 13 nm line Effect of SOW Matrix on the Performance of the USN The effect of matrix on the analytical signal can be expressed in terms of a suppression or enhancement factor. This factor is the ratio of the analyte signal in an SOW matrix to that in 2% nitric acid solution.13 The results obtained using a 10 ppm multi-element solution containing the four elements in SOW at a sample uptake rate of 0.13 cm3 min-’ are shown in Table 5.Signal suppression occurred for all elements except copper which exhibited an enhancement in the analytical signal. From Table 5 the detection limit for copper is slightly improved while that for barium is severely degraded. This is probably due to the extremely low solubility of barium sulfate especially in the presence of sulfuric acid. The insoluble barium sulfate might have been lost in the sample container or in the desolvation chamber. The other two elements cobalt and iron show no significant effect from the presence of the SOW matrix. The slopes of the calibration graphs generated for copper in SOW and in 2% nitric acid are 0.020 and 0.019 respectively using a sample uptake rate of 0.25 cm3 mind’. This slight increase in sensitivity is consistent with copper having a signal enhancement in an SOW matrix.However the linear range of copper decreased by an order of magnitude in SOW. The same procedure for the stability test was used to determine if the presence of the matrix in solution affects the stability of the signal generated by the ultrasonic nebulizer. The 10 ppm multi-element solution in SOW was used as the analyte solution while the blank consisted of SOW only. The Cu I 324.745 nm line was monitored and the results are given in Fig. 5. The RSD for the six runs ranged from 3 to 8% which shows that the effects due to the presence of the SOW are minimal. Conclusions The simplicity of the design and good performance of the ultrasonic nebulizer makes it a suitable device for sample introduction into the ICP.In addition the laboratory-built yields a slightly higher RSD 2-6%. This is thought to be due to the uneven introduction of the sample onto the transducer. Future work will concentrate on a more efficient means of delivering the sample to the transducer. 0 10 20 30 40 50 60 Timelmin Fig. 5 Stability test for the ultrasonic nebulizer using the I0 ppm Cu solution in an SOW matrixJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 81 1 ultrasonic nebulizer is a very cost effective alternative to commercial ultrasonic nebulizers available at the present time. The total cost of the design is approximately $1000 utilizing the materials available in this laboratory; mainly the desolvation apparatus. The values for analytical figures of merit such as the detection limit and RSD (*/o) in this study are not as low as expected owing to instrument limitations.However the degree of improvement of these values using the ultrasonic nebulizer over the other types of nebulizers is of the same order of magnitude as those previously reported.ll The authors acknowledge the National Institute of Environ- mental Health Science for research grants ES-03221 and ES-04908 and J. Carey and L. Olson for their input and assistance. References Tarr M. A. Zhu G. and Browner R. F. Appl. Spectrosc. 1991 45 1424. Clifford R. H. Ishii I. Meyer G. A. and Montaser A. Anal. Chem. 1990,62 390. Routh M. W. Spectrochim. Acta Part B 1986 41 39. 4 Brotherton T. and Caruso J. A. J. Anal. At. Spectrom. 1987 2 695. 5 Clifford R. H. and Montaser A. Anal. Chem. 1990 62 2745. 6 Jin Q. Zhu C. Brushwyler K. and Hieftje G. M. Appl. Spectrosc. 1990 44 183. 7 Goulden P. D. and Anthony D. H. J. Anal. Chem. 1984,56 2327. 8 Olson K. W. Haas W. J. J. and Fassel V. A. Anal. Chem. 1977 49 632. 9 Taylor C. E. and Floyd T. L. Appl. Spectrosc. 1981,35,408. 10 Denton M. B. Freelin J. M. and Smith T. R. in Sample Introduction in At om ic Spectroscopy A nalyt ical Spectroscopy Library ed. Sneddon J. Elsevier New York 1990 vol. 4 ch. 4. 1 1 Chan S. Zechmann C. A. and Yanak M. M. ICPInf Newsl. 1991 16 436. 12 Fassel V. A. and Bear B. R. Spectrochim. Acta. Part B 1986 41 1089. 13 Wang J. Shen W. L. Sheppard B. S. Evans. E. H. and Caruso J. A. J. Anal. At. Spectrom. 1990 5 445. Paper 2/0 I092E Received March 2 1992 Accepted June 18 1992

ISSN:0267-9477    

DOI:10.1039/JA9920700807

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 813 Transport Effects With Dribble and Jet Ultrasonic Nebulizers" Matthew A. Tarr Guangxuan Zhu? and Richard F. Browner$ School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta GA 30332-0400 USA Ultrasonic nebulizers improve analyte transport compared with pneumatic types but require partial or total solvent removal. Aerosol desolvation using a heated chamber followed by a condenser not only reduces the solvent loading in the plasma but also increases the analyte transport relative to transport without desolvation. The influence on emission signals of the interaction of each component in the sample introduction system in terms of analyte and solvent transport losses are reported. Significant analyte losses in the spray chamber and condenser are observed and various approaches to improving analyte transport are presented.Two configurations of sample injection into the ultrasonic nebulizer the dribble and jet type are compared with the jet type showing greater repeatability and improved peak shape for flow injection measurements. Keywords Inductively coupled plasma atomic emission spectrometry; ultrasonic nebulizer; analyte transport; solvent transport The use of ultrasonic nebulizers (USNs) has improved detection capabilities of inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS) primarily through increased analyte transport to the Although USNs have recently become more widely used in atomic spectrometry there are still many fundamental characteristics of these nebulizers that are poorly understood.The aerosol properties of a USN have recently been characterized and shown to produce a broad primary aerosol distribution with significant aerosol vol- ume contained in droplets >300 pm in a diameter.2 However despite these large droplets a significant en- hancement in small droplet generation compared with pneumatic nebulizers (PNs) was also achieved. Increased primary aerosol output at small droplet sizes is not the sole cause for improved detection limits with a USN. The use of a desolvation apparatus can improve detection power irrespective of the nebulizer type used. Zhu et aL3 and Jakubowski et aL4 both reported improved signal intensities and lower background interferences in ICP-MS using pneumatic nebulization with desolvation.With a USN a major increase in analyte transport compared with a PN results from the initial production of a primary aerosol with droplets generally smaller than those from the PN. Aerosol desolvation shifts the primary droplet distribution to smaller sizes3 by evaporation then further increases the analyte transport to the plasma relative to transport without desolvation. Montaser et aL5 have mea- sured detection limits in ICP-AES for a number ofelements comparing PNs used with and without desolvation with USNs used with desolvation. With PNs they report 2-10- fold improvements in limits of detection by using desolva- tion. When comparing normal pneumatic nebulization to ultrasonic nebulization with desolvation they report en- hancement of between 6- and 87-fold.When both systems are compared using desolvation the USN improves detec- tion limits over the PN by a factor of only between 3 and 8. Although the USN still provides significant enahancements these results indicate that the desolvation process itself plays an important role in improving detection limits. This work reports data on solvent and analyte transport the interaction of the heated chamber and condenser on analyte loss processes and the use of USN for flow injection (FI) analysis. *Presented at the 1992 Winter Conference on Plasma Spectro- ?On leave from the Dalian Institute of Chemical Physics Dalian *To whom correspondence should be addressed. chemistry San Diego CA USA January 6- 1 1 1992.People's Republic of China. Experimental A USN was fabricated in-house based on the design of Fassel and Bear.6 A Model CPMT transducer from Channel Products (Chesterland OH USA) was powered by a PlasmaTherm (Vorhees NJ USA) UN:PS- 1 generator operating nominally at 1.35 MHz. The generator was tuned to the resonant frequency of the transducer by maximizing incident power while minimizing reflected power. The power generator had an output range of 0-55 W. In addition a second commercially available USN (CETAC Technologies Omaha NE USA) was also used in these studies. Solvent was introduced to the nebulizers with a high- performance liquid chromatography (HPLC) pump (Con- stametric Model IIG). Samples were introduced via a Rheodyne 7 125 injection valve using various sample loops and directed through polymeric tubing (0.01 in i.d.) and introduced to the nebulizer using one of two methods.In the first method of sample injection termed the dribble USN (DUSN) the tubing [polymeric or poly(tet- rafluoroethylene) 0.04 in i.d.1 is cut at an angle of approximately 45" and placed nearly in contact with the glass surface of the nebulizer. The liquid is then allowed to dribble across the surface of the nebulizer where nebuliza- tion can occur. This method is commonly cited in the literature and is the method used in most commercially available systems. As an alternative sample injection method ajet USN was employed. This method utilizes a length of 50 pm i.d. fused silica tubing (except as noted). The fused silica is positioned approximately 1 cm from the nebulizer surface.At the liquid flow rates used in these studies the back pressure of the HPLC pump causes a stable liquid jet to form at the tip of the fused silica tubing. The liquid jet impacts on the nebulizer surface initially with an interaction diameter only marginally greater than the original jet diameter and then forms a thin film of liquid which is subsequently nebulized through interaction with the acoustical energy of the nebulizer. It is important to note that the liquid jet energy is not adequate to cause direct nebulization through impact with the transducer surface. In the absence of r.f. energy applied to the transducer no detectable aerosol is formed. The nebulizer designed in-house referred to as the Jet USN is illustrated in Fig.l(a). It is possible to convert any conventional USN to jet operation by replacing the large bore sample introduction tubing with 50 pm or smaller i.d. fused silica tubing and using a pump capable of sustaining a stable liquid jet. The commercial unit was operated both in the dribble814 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 (a) Water To condenser f To plasma f Condenser // Air-cooled - USN Samde ' ' introdiction - Spray chamber Fig. 1 Schematic diagrams of USNs used (a) Jet USN; and (b) CETAC USN. Point 'I' indicates intermediate solvent collection point and point 'T' indicates position for measuring aerosol temperature in the CETAC spray chamber and jet modes and is referred to as the CETAC DUSN or CETAC Jet USN respectively.Fig. l(b) illustrates the CETAC USN design. Unless otherwise noted after leaving the spray chamber aerosols were passed first through a heated glass chamber and then through a condenser in order to remove excess solvent. A Perkin-Elmer ICP/6000 ICP atomic emission spectro- meter was used for data collection taking raw data through an analogue-to-digital converter and storing it on an IBM PC/XT computer. Data were collected and stored at 0.5 s intervals. Analyte and solvent transport measurements were car- ried out in order to determine the amounts of analyte and solvent reaching the plasma and the locations of analyte loss. Several different collection methods both direct and indirect were used in order to gain a thorough understand- ing of the mass transport processes and obtain a mass balance for all the sample introduced.Direct collections were performed as previously reported* using solutions of 100 ppm of Cd in 1 O/o HN03 (as) for analyte transport and 1% HN03 (as) for solvent transport. Direct analyte collec- tions were made only after the desolvation system while solvent collections were made both after the spray and after desolvation. Indirect analyte and solvent transport mea- surements were also carried out using the same solutions as for the direct collections. The solvent was collected at the first drain position (spray chamber) and was measured by weighing. The analyte was collected from both drains (spray chamber and condenser) by collecting the liquid from each drain. The glassware associated with each drain was rinsed and the rinse was added to the drain collection.For all analyte collections concentrations were determined by analysis using ICP-MS with blank subtraction. Experiments using supplementary heat in the spray chamber region of the CETAC Jet USN were also con- ducted. The aerosol temperature was measured at location T in Fig. l(b). With no supplementary heat the aerosol temperature in the spray chamber was found to be 45 "C. Heat was applied to achieve aerosol temperatures of 62 and 86 "C in the spray chamber. Height profile measurements for Mn emission were carried out with and without supplementary heat using three different nebulizer gas flow rates (0.68 0.82 and 0.96 dm3 min-l of Ar). Cadmium transport measurements as described above were also carried out with a spray chamber aerosol temperature of 86 "C and a nebulizer gas flow rate of 0.82 dm3 mine'.Filtered de-ionized water Fisher TraceMetal Grade HN03 and Fisher certified atomic absorption reference standard solutions ( 1000 ppm) of Mn and Cd were used in this study. Results and Discussion Visual Observations When both the ultrasonic nebulizers were operated under conditions of greatest operating efficiency corresponding to a power of >45 W for the water-cooled transducer and 25-30 W for the air-cooled transducer nearly all of the introduced sample was nebulized and a pattern of nodes and antinodes could clearly be observed on the surface of the transducers. At the nodal regions droplets of approxi- mately 1 mm in diameter formed on the transducer and remained stationary.The aerosol emanated from the antinodal regions between these stationary droplets. The nodes and antinodes appear to correspond to a standing wave formed on the surface of the transducer with a wavelength determined by measuring the spacing of the nodal distribution of approximately 1 mm. This value agrees fairly well with the calculated value of 1.1 mm for the wavelength of 1.35 MHz waves in distilled water. The nodal pattern on the transducer surface is shown in Fig. 2. The over-all dimensions of the nodal pattern are fairly large compared with the capillary waves observed by Lang.' For the transducer used in this study the theoretical value for capillary waves is 10 ,urn far too small to be observed with the magnification used in Fig.2. Transport Measurements Solvent and analyte transport measurements for the Jet USN the CETAC DUSN as supplied and the CETAC Fig. 2 Photograph of USN during jet nebulization process. Aerosol is slightly visible as a foggy region and the liquid jet is visible as a thin white line travelling vertically from the bottom to the centre of the transducer. Major &ale divisions are in centi- metres and liquid flow rate- 1 cm3 min-'JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 815 Table 1 and condenser= 2 "C Solvent transport for USNs liquid flow rate= 1 cm3 min-'; nebulizer gas flow rate=0.82 dm3 min-I of Ar; heated zone= 140 "C; Nebulizer Collection Transport efficiency Solvent mass No. of (O/O) transport/g min-I measurements 2 Jet USN Point I [Fig.l(a)] no heat 31.2k0.2 Jet USN After desolvator - 0.0 100 2 0.0005 4 Jet USN After desolvator no aerosol - 0.005 1 6 Jet USN Spray chamber drain 27. I * 0.6 CETAC DUSN After desolvator - 0.04 8 f 0.00 1 3 CETAC Jet USN After desolvator - 0.0 180 * 0.0009 3 - (saturated argon) - Table 2 Analyte transport for USNs liquid flow rate= 1 cm3 min-'; heated zone= 140 "C; and condenser=2 "C Transport efficiency Nebulizer Collection Nebulizer gas flow/dm3 min-I of Ar (O/O) No. of measurements Jet USN After desolvator CETAC DUSN After desolvator CETAC Jet USN After desolvator CETAC Jet USN After desolvator CETAC Jet USN After desolvator 0.82 0.82 0.68 0.82 0.96 15.0k0.7 21.1 f 0 . 4 18.4 * 0.6 22.0 f 1 .o 22.7 f 0.5 converted to jet operation are reported in Tables I 2 and 3.Several important observations can be made based on these data. While values as high as 85% for analyte transport have been claimed in the literature,* the present data show a maximum analyte transport of about 22%. This value is based on a sample flow rate of 1 cm3 min-l and is somewhat higher than the 11% analyte transport for a USN at 2.8 cm3 min-' reported by Olson et al.' Based on previous studies,* the more common flow rates for a USN of 2-3 cm3 min-' would probably result in a lower percentage transport but a higher over-all mass transport. Further- more as evidenced by both the analyte and solvent collections of the spray chamber drain approximately 70-80% of the introduced sample is lost in the spray chamber. Although nearly 100% of the sample is nebulized many of the droplets are large and are lost either owing to settling or through collisions with the walls of the spray chamber.Collection of solvent at the intermediate point I [see Fig. l(a)] for the Jet USN with no heat applied indicates a transport of about 30% which is in agreement with the collections of analyte and solvent from the spray chamber drain. At room temperature total solvent transport is approximately equal to aerosol transport and therefore close correspondence of these values is to be expected. Drain collections from the condenser showed that approximately 7% of the sample originally introduced is Table 3 Analyte loss' in spray chamber and condenser drains liquid flow rate=1 cm3 min-'; nebulizer gas flow rate=0.82 dm3 min-' of Ar; heated zone= 140 "C; and condenser= 2 "C Original analyte lost to drain No.of Nebulizer Collection (O/O) measurements Jet USN Spray chamber 76*4 3 Jet USN Condenser drain 7.3 f 0.8 3 drain lost in the condenser. These losses are presumably due to settling and collisional processes. As the supersaturated gas enters the condenser the solvent begins to condense and the dried aerosol particles act as nucleation sites for water condensation; the particles can actually increase in size at this point. Such an increase in size might lead to loss of aerosol particles on the condenser surfaces. In addition the wet inner surface of the condenser might cause changes in the interaction between particles (wet or dry) in the aerosol and the walls of the condenser.The analyte losses in the condenser indicate that the conventional design is not the most efficient for desolvating aerosols. Not only is the analyte transport poor but the system is also limited in its capacity to remove solvent. A condenser is only capable of reducing the solvent loading to its vapour pressure value at the operating temperature of the condenser. Furthermore condensation of water onto the aerosol particles increases the solvent loading beyond the saturation point. In order to illustrate this point the solvent loading for Ar passed through a wet desolvation system is reported in Table 1. This value which should approach the saturation value for Ar at 2 "C accounts for only half of the solvent loading when 1% HNO is nebulized. The remaining solvent is present as condensed water on the aerosol particles.In order to calculate the mass balance the values for analyte transport found by collections from the spray chamber drain and condenser drain and the final aerosol transport can be added and should total 100%. For these studies using the in-house Jet USN design the results are summarized as follows over-all transport 1 5.0 If 0.7%; loss to spray chamber drain 76*4%; loss to condenser drain 7.3fO.8%. The sum of the experimental values was therefore 98 f 6% which accounts for all the analyte within experimental error. Owing to the difficulties of performing direct analyte collections on a wet aerosol no direct measure of the losses Table 4 Analyte transport for CETAC Jet USN with and without supplementary spray chamber heat liquid flow rate= 1 cm3 min-I; nebulizer gas flow rate=0.82 dm3 min-' of Ar; heated zone= 140 "C; condenser=2 "C; and 50 pm capillary Spray chamber Transport efficiency Nebulizer temperature/"C Collection (Yo) No.of measurements CETAC Jet USN 45 (no heat) After desolvator 24.4 0.1 CETAC Jet USN 86 After desolvator 3 2 + 2 2 3816 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 in the heated region could be obtained. However since the mass balance accounted for nearly all the analyte intro- duced it can be assumed that losses in this region are relatively small. This assumption is well founded based on the processes occurring in the heated zone. As the aerosol particles are heated solvent rapidly evaporates thereby reducing the size of the particles.Small particles are much less vulnerable to settling or collisional losses so transport in the heated region is expected to be high. Although the actual values differ all of the transport measurements reported here are in general agreement with the work of Weber et af.,9 using a USN system. These workers report that 93% of the analyte is lost in the spray chamber and 12.8% of the remaining analyte is lost in the desolvator yielding an over-all transport of 5.2% at a liquid flow rate of 2.24 cm3 min-l. Weber et aL9 also suggested I 1 I I I 1 9 10 11 12 13 14 15 0.6 I 0.7 I I I 1 I 1 10 11 12 13 14 15 1.1 I 1 1 .o 1 (c) A 0.9 1 1 I I I I 10 11 12 13 14 15 0.5 ' Height above load coil/mm Fig. 3 Height profile emission characteristics for Mn at 257.610 n m using CETAC Jet USN plasma gas= 14 dm3 min-' of Ar; auxiliary gas=1 dm3 min-' of Ar; power=1.25 kW; heated zone= 140 "C; and condenser=2 "C.Nebulizer gas flows are (a) 0.68 (6) 0.82 and (c) 0.96 dm3 min-' of Ar. Temperatures of aerosol in spray chamber are A 45 (no heat); B 62; and C,,86 "C that condenser losses occur in the system due to condensa- tion of solvent onto dry particles followed by settling or impaction losses. The transport studies obtained in the present work clearly indicate that the major part of the aerosol produced by a USN is lost in the desolvation and transport system. In order to reduce these losses two approaches are possible the production of a primary aerosol that has smaller droplets which are therefore more efficiently transported to the plasma; and design of a desolvation system that efficiently desolvates a large percentage of the aerosol including the larger droplets initially present.For illustrative purposes an experiment was devised to increase analyte transport through the spray chamber. Supplementary heat was applied to the glass spray chamber of the CETAC Jet USN utilizing a 50 pm capillary for sample introduction. Height profiles for Mn emission at three aerosol temperatures and three nebulizer gas flow rates are given in Fig. 3. With no supplementary heat the aerosol initially formed had a temperature of about 45 "C presumably owing to heat transfer from the transducer. By averaging signal enhancements over all the measurement heights for nebulizer gas flows of 0.68 0.82 and 0.96 dm3 min-I the signals increased by 14 29 and 35% respectively. Since the sample is desolvated regardless of the temperature of the spray chamber it is assumed that signal enhancements are mainly the result of increased analyte transport.Furthermore it is important to note that the emission signals show some variation with nebulizer gas flow rate. Although the nebulization process is basically independent of gas flow it is believed that transport is considerably more sensitive to gas flow. In addition it is important to consider the residence time in the plasma which is also a function of gas flow. In order to confirm that the signal enhancements were in fact due to increased transport measurements were made to compare the analyte transport with and without supple- mentary heat at the spray chamber.These transport results (Table 4) indicate an increase in transport of approximately 30% when the spray chamber aerosol is heated to 86 "C. This value is very close to the average emission intensity increase of 29% under comparable conditions [Fig. 3(6)]. Although applying heat to the spray chamber dramatic- ally increases the analyte transport this method has practical drawbacks. Heating the spray chamber causes increased background noise and signal noise due to inter- mittent flash vaporization of droplets that come into contact with the hot glass. Furthermore deposition of the analyte on the walls of the spray chamber can also occur resulting in memory effects. Despite the fact that the method used here is not appropriate for analytical use it does provide evidence that large droplets can be modified to bring about higher transport.With attention paid to the design of the spray chamber heating the aerosol as early as possible above the transducer face might be a viable method for increasing the transport. Owing to the limita- tions in producing small droplets it is probable that with current systems the most effective way of producing higher analyte transport is by improving the transport of larger primary aerosol droplets. Flow Injection Studies The two primary reasons for using a liquid Jet USN are to improve the uniformity of the liquid film formed on the transducer surface; and to minimize the band broadening for low flow rate FI studies and for coupling with micro- HPLC. In order to study band broadening and repeatability effects FI studies were carried out at relatively low flow rates.The use of FI-ICP using a 25 pm capillary Jet USN at a liquid flow rate of 150 mm3 min-' is illustrated inJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 817 5 I 1 0 200 400 600 Time/s Fig. 4 Flow injection ICP emission traces for ( a ) Jet USN (25pm capillary) and (6) DUSN (0.0 1 in i.d. tubing); liquid flow rate= 150 mm3 min-I. Injections (50 mm3) of 10 ppm Mn are indicated by arrows Fig. 4(cr). Such flow rates are too low for the effective operation of the DUSN as illustrated in Fig. 4(b). In Fig. 4 the peaks in each trace are normalized with the first peak height set to unity. Normalization was carried out because of intensity differences between the two nebulizers used.In the case of the Jet USN reasonable repeatability was achieved at 150 mm3 min-l although other flow rates gave somewhat poorer precision. With the DUSN (with 0.00 1 in i.d. tubing) repeatability between peaks became steadily worse as the flow rate was lowered. The instabilities in the DUSN at these flow rates arise owing to the finite gap between the sample tubing and the nebulizer. In order to achieve stable operation there must be a constant and steady supply of solution flowing onto the nebulizer face. As flow rates become low the dribble design cannot provide a continuous flow of sample and instabilities arise. The Jet USN system was not optimized with respect to the total volume of the system for the low flow rates used in these experiments and therefore the data are probably not the best that could be achieved.However a preliminary conclusion is that effective coupling to an ICP at a low flow rate might be possible with minimal band broadening by using an ultrasonic nebulizer with jet sample introduction. Conclusion Data presented here illustrate that typical USNs deliver only about 20% or less of the analyte introduced to the plasma. The percentage transport is dependent on the sample flow rate and generally decreases with increasing flow rate. Improved analytical performance of USNs is a function of both increased aerosol output and the use of desolvation. Although the aerosol particle size distribution in a USN indicates significant amounts of large droplets this aerosol can be effectively utilized for atomic spectro- metry by altering the particle size before losses occur.Such an approach is applicable to other types of nebulizers and can lead to significant improvements in detection limits. The use of a fused silica capillary in place of standard tubing can enhance the performance of USN. Improvement comes from the ease of alignment and the ability to perform low flow experiments such as FI and micro-HPLC. How- ever this technique has certain disadvantages. The usable liquid flow rate range is restricted at high values by back pressure and at low values by the required minimum liquid velocity needed to form a jet. In addition an HPLC pump with minimal pulse fluctuations is required. Clogging may also present a problem for unfiltered solutions especially if very small diameter (< 50 pm) capillaries are used.Finally it is important to pay close attention to all components of a sample introduction system for ICP. As illustrated in this report previous workers have neglected the importance of the desolvation system in enhancing both transport and detection limits. Many assumptions have been made as to transport values without sound experi- mental evidence. Only through close attention to such details can a steady path to improvements in sample introduction for atomic spectrometry be made. This research was supported by the National Science Foundation under Grant No. CHESS-08 183. The authors gratefully acknowledge the following companies for donat- ing or lending equipment used in the study Channel Products; CETAC Technologies; and Perkin-Elmer. M.A.T. acknowledges financial support from an American Chemi- cal Society Analytical Division Fellowship sponsored by Perkin-Elmer. References Olson K. W. Haas W. J. Jr. and Fassel V. A. Anal. Chem. 1977 49 632. Tarr M. A. Zhu G. and Browner R. F. Appl. Spectrosc. 199 1,459 1424. Zhu G. Pan C. and Browner R. F. paper No. 1271 presented at the 1989 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy Atlanta GA USA March 6-10 1989. Jakubowski N. Feldmann I. and Stuewer D. Spectrochim. Acta Part B 1992 47 107. Montaser A. Tan H. Ishii L. Nam S.-H. and Cai M. Anal. Chem. 1991,63 2660. Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. Lang R. J. J. Acoust. SOC. Am. 1962 34 6. Petrucci G. A. and Van Loon J. C. Spectrochim. Acta Part B 1990 45 959. Weber A. P. Keil R. Tobler L. and Baltensperger U. Anal. Chem. 1992,64 672. Paper 2/01 789J Received April 3 1992 Accepted June 12 I992

ISSN:0267-9477    

DOI:10.1039/JA9920700813

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 819 Performance Characteristics of an Ultrasonic Nebulizer Coupled to a 40.68 MHz Inductively Coupled Plasma Atomic Emission Spectrometer* Isaac B. Brenner Geological Survey of Israel 30 Malkhe Israel Street Jerusalem 9550 1 Israel Phillipe Bremier and Allain Lemarchand Jobin-Yvon 16-18 Rue du Canal Longjumeau Cedex 91 163 France Analytical performance characteristics [limits of detection (LODs) memory effects long and short-time reproducibility effect of high salt content and accuracy] of an air-cooled ultrasonic nebulizer coupled to a 40.68 MHz r.f. inductively coupled plasma (ICP) were evaluated. Approximately 1 0-fold enhancements in the LODs only slightly affected by ICP power were observed with a simultaneous sequential polychromator.Limits of detection were somewhat better than those quoted in the literature for 27 MHz r.f. ICP generators. The present configuration allowed the determination of trace elements at the pg dm-3 and sub-pg dm-3 concentration level in pristine and waste waters. The use of a Trassy-Mermet sheath gas assembly and a 3 mm injector tube allowed the direct analysis of saline solutions containing trace elements at the pg dm-3 level using matrix-matched calibration protocols. With this configuration memory effects were minimum and the long- and short-term variations could be compensated for with the use of internal references. As a result relative standard deviations are improved and the need for frequent system readjustment and recalibration was eliminated.A comparison of the data for standard reference and spiked materials indicated that the accuracy was satisfactory. Keywords Sample introduction; inductively coupled plasma; trace element analysis of water; ultrasonic nebulization The most common devices for sample introduction into an inductively coupled plasma (ICP) are pneumatic nebu- lizers. With the use of these robust devices ICP atomic emission spectrometry (AES) has become the accepted technique for routine multi-element analysis. However the limits of detection (LODs) for several important analytes in particular those that have toxicological significance are inadequate for the analysis of surface and sub-surface waters industrial eMuents and related environmental ma- terials owing to the low efficiency of aerosol transport.Improved LODs can be obtained by using a 40.68 MHz generator1V2 with on-line column preconcentration and flow injection techniques3y4 and hydride generati~n.~ However the desired LODs are still inadequate or the techniques are element-selective and tedious. Thus the growing need for enhanced LODS has resulted in revived interest in the ultrasonic nebulizer (USN). The application of the USN for sample introduction in ICP-AES was described in the late 1 9 7 0 ~ . ~ - ~ A water-cooled USN was used successfully for the analysis of effluents and sludges8 and of sea-water after column precon~entration.~ However the device was not routinely applied for the analysis of solutions containing even moderately high concentrations of acid and salt since droplet size interfer- ence effects related to liquid viscosity and surface tension were identified accompanied by unsatisfactory system reliability.Chemical interferences in the desolvation sys- tem ('desolvation interference effects') and in the plasma were also suggested to explain the decline in element inten~ities.~J~ The latter processes were explored in an in- depth evaluation of an improved version of the original Ames nebulizer.I0 Several unpublished reports described the application of a commercially available air-cooled USN (CETAC Omaha NA USA) for various analytical disciplines. Prelimi- nary data were also published for the analysis of low-salt solution^.^^ In this paper performance characteristics are *Presented at the 1992 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 6-1 1 1992.given for the air-cooled system which differed from that described by Olsen et al. and other investigators.6-8 Experimental Ultrasonic Nebulization System The instrumentation (CETAC U-5000) differs from previ- ous USN designs and features an air-cooled transducer assembly for heat dissipation and regulation of the trans- ducer power and the heating and cooling temperatures of the desolvation system. These modifications result in longer transducer life improved analytical repeatability and gen- eral system reliability. The detailed operation procedure is described in the operation manuals and only a few points will be highlighted here. In order to prevent transducer damage and ensure that a minimum layer of about 1 mm for the production of acoustic waves occurs all solutions were continuously pumped at approximately 3 cm3 min-I.Regular pumping was also necessary in order to avoid uneven production of the aerosol cloud. It is now recognized that the limiting factor for enhanced LODs with the USN is the high amounts of water vapour introduced into the plasma resulting in considerable energy consumption.14 In the present system water vapour was removed by employing a desolvation system consisting of a temperature controlled heated cell followed by a water- cooled condenser to obtain dry aerosol particles. The USN spray chamber included a steep drain for rapid gravita- tional removal of solution droplets a tangential argon gas inlet for continuous sweeping of the transducer face plate and an auxiliary rinse for high-salt solutions.The last facility was employed when very high salt concentrations were aspirated. Tube lengths were reduced as much as possible and the system placed in close proximity to the towh housing in order to minimize dead volumes and wash- out time. The time for the desolvation system to attain a tempera- ture of 140 "C was about 5 min. The period of time needed for the refrigeration system to attain a temperature of from - 5 to - 10 "C was approximately 90 min. A further period of about 15 min was needed for stabilization of the USN820 ALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 1 Instrumentation JY 50 Polyscan Gratings Spectral range/nm Dispersiodnm mm-1 Resolu tion/nm Slit dimensions/pm Optical transfer Generator In st rumen t control JY 36 Grating Spectral rangelnm Resolution Slitdpm Photomultiplier tube SIM-SEQ 0.5 m polychromator N flushed 2 dm3 min-l Concave holographic 3600 grooves mm-I 0.55 0.028 Entrance 20 exit 50 Plasma focused on entrance slit via an N flushed extension tube Jobin-Yvon 40.68 MHz maximum power rating 2.5 kW JY (ISA) Spectralink electronics IBM PC Sequential 0.65 m N flushed 2 dm3 min-' Plane holographic 2400 grooves mm-l double order 160-800 ( 1 st order) 160-3 10 (2nd order) Variable depending on slit dimensions and order 0.006 nm mm-I maximum Entrance 30 exit 25 Hamamatsu R300 for the UV R 106 for < 190 nm 160-4 10 Table 2 ICP and USN operating conditions ICP- Power/kW Torch Observation height Coolant gas flow rate/ Aerosol carrier flow rate/ Sheath gas flow rate/ dm3 min-' dm3 min-l cm3 min-' USN- Instrument Sample delivery rate/dm3 mind' Desolva tion Heat ing/"C CoolingPC Integration time/s Washout time/s 0.7-1 Jobin-Yvon Ryton demountable torch with aerosol injector of alumina 3 mm diameter 12 mm above the upper coil 12 0.8 0.2 and 0.6 for alkalis CETAC U-5000 including desolvation system (heater and cooler systems) 2.5 peristaltic 140 -5 to -10 Polychromator 5- 10 and monochromator 1 30-80 itself.It was observed that when an excess of liquid accumulated in the spray chamber condensation occurred in the tube leading to the torch. This effect was minimized by continuous peristaltic pumping. Failure to regulate this level resulted in intensity drifts.The drain peristaltic pump rate only slightly exceeded that of the sample. Spectrometer and ICP Operating Conditions The main body of data presented here was obtained with a Jobin-Yvon (Longjumeau France) JY 50 Polyscan Sim-Seq multichannel spectrometer. This instrument has a facility which allows the utilization of spectral lines k2.2 nm on either side of the fixed spectral ~hanne1.l~ Using the 20 channel instrument additional spectral lines were mea- sured with adequate LODs. Additional data were obtained using a JY 36 high-resolution sequential system.16 This spectrometer has a variable resolution facility consisting of computer controlled variable exit and entrance slits. The main details of this configuration are listed in Table 1 and the operating conditions in Table 2.It was observed that when high-salt solutions (> 1%) were injected continuously into a USN the narrow torch injectors were clogged within a short period of time. Consequently a JY Ryton demountable torch with a wide injector (3 mm) was employed. A Trassy-Mermet sheath gas tube was also installed between the spray chamber and torch base. This facility permitted optimization of the plasma for maximum LODs and reduction of the memory effects due to salt deposition on the inner wall of the inje~t0r.l~ Conditions for multi-element simultaneous analysis were determined by adjusting the so-called normal analytical zone using a 100 mg dm-3 Y solution. The LODs of the alkali elements were enhanced by increasing the sheath gas flow to 0.6 dm3 min-I.I8 Calibration Pristine water Multi-element calibrations were performed using aqueous solutions.A 5% HN03 (Suprapur Merck Darmstadt Germany) solution was used as the low standard. A multi- element composite Spex QC-19 (Spex Industries Edison NJ USA) was diluted to form a graded series of standards varying from 10-100 pg dm-3. The concomitant elements were also present in these standards and facilitated multi- element analysis for the major and minor elements. High-salt solution In order to compensate for interference effects in the nebulizer and in the ICP due to high concentrations of concomitants in high-salt solutions matrix-matched cali- brations were performed. For the analysis of water contain- ing 0.1-1 Yo NaCl calibration standards were prepared using Suprapur NaCl spiked with Spex QC-19 standard to produce standards varying from 10 to 500 pg dm-'.All operations were performed in a class 100 work hood with ultrapure (Millipore Bedford MA USA) de-ionized water. For the analysis of LiBOz fusion solutions portions of aJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 82 1 5 0.5% LiB02 (Johnson Matthey London UK) solution were spiked to produce analyte concentrations in the pg dm-3- mg dm-3 range. a' 0 Results General Obervations The USN produced a large amount of aerosol over an extended period of time of at least 12 h. Plasma extinction and blockage did not occur in the presence of 1% NaC1. ! Limits of Detection Limits of detection were determined at several ICP power levels. The data in Figs. 1-3 indicate that the majority of lines used were not significantly affected by changes in power in the range 1-0.6 kW.However several LODs improved slightly with decreasing power and some of the 'hard lines' e.g. P S and Se were somewhat degraded with decreasing power (Figs. 1 and 2). A multi-element solution containing 100 pg dm-3 of analytes was then employed to determine LODs ( 2 4 using both simultaneous and sequen- tial multi-element detection at a power of 0.7 kW. These are compared with those obtained with a conventional Mein- hard nebulizer (denoted CONV in Table 3) and the JY 40.68 MHz generator. In comparison with LODs obtained by pneumatic nebulization those obtained by USN are about ten times better. (Enhan F is the LOD enhancement factor defined as the ratio between the LODs obtained by conventional nebulization and those obtained by USN.) Thus many types of solutions containing low salt contents can be analysed without the need of preconcentration.In Fig. 4 the USN LODs obtained in the present study are compared with the values reported by other investiga- tors.*J 1*12 The data indicate that a considerable improve- ment has been achieved with the present design. It is also evident that some of the LODs obtained in the present study are significantly enhanced. This could be attributed to the use of the high frequency 40.68 MHz r.f. generator as reported by Mermet and co-workers.1*2 An important observation is that the low LOD of 4 pg dm-3 for the Na I 330.2 nm spectral line using these conditions eliminated the need for additional alkali detec- tors in the near-infrared range.In Table 3 the results obtained when several spectral lines were accessed using the polyscan device i.e. lines located within the 2.2 nm interval of the fixed slits are shown. In these cases the LODs were not degraded. The sub-pg dm-3 limit for A1 permits the analysis of biological fluids for this important element and the enhanced LODs for the hydride- and vapour- forming elements As Se Sb T1 and Hg eliminated the 0 ; 1000 800 600 Power/W Fig. 1 B; D Se; E Pb; and F Zn Influence of ICP power on the USN LODs for A P; B S; C L E PowertW Fig. 2 Influence of ICP power on the USN LODs for A Al; B Ca; C Cr; D Mg; and E Cu 1.6 I p 0.8 \ n B \ - 1000 800 600 PowertW Fig. 3 Influence of ICP power on the USN LODs for A Ni; B Co; C Cd; D Ba; E Fe; and F Mn I This work Refs.11 and 12 [7 Ref. 8 I n 1 E 2 7J cn 0 0 -J 0.1 0.01 I 1 I I I I I 1 1 I 1 1 I 1 1 1 1 1 1 I I 1 I Ag As Ba Be Cd Co Cu Mg Mo Pb Se Zn Al B Be Ca Cd Cr Fe Mn Ni Sb TI Element Fig. 4 Comparison of the air-cooled USN LODs with data from the literature refs. 8 1 1 and 12; all values + 2 a need to apply hydride generation which requires the installation of a different sample introduction system and utilization of time-consuming chemical procedures to con- dition the samples and the standards (reduction of high to lower valency states). An additional advantage of the USN procedure is the ability to perform a multi-element analysis of these elements whereas with hydride generation this is not feasible owing to the individual chemical treatments required to optimize analyte responses.A reasonable explanation for the amount of the enhance- ment is not clear from the present data. A relationship with excitation and ionization potentials was not observed. It was however noticed that the LODs for Hg and B were822 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 3 LODs in aqueous media by USN (20) measured in mg dm-3 Element Wavelengthhm Ag A1 Al As As B Ba Ba Be Ca Cd Cd Cd c o c o Cr Cr c u Fe Fe Hg Mg Mg Mn Na Ni Ni P Pb S Sb Se Se Se Si Sr TI V Zn Ag I 328.0 A1 I 308.21 5 Al I 396.15 As I 189.0 As I 193.69 B I 249.77 Ba I1 233.527 Ba I1 493.44 Be I1 31 3.042 Ca I1 317.933 Cd I1 214.438 Cd I1 226.502 Cd I1 228.802 Co I1 228.616 Co I1 237.862 Cr I1 205.55 Cr I1 267.716 Cu 1324.754 Fe I1 238.204 Fe I1 259.940 Hg I 194.22 Mg I1 279.553 Mg I 285.213 Mn I1 257.610 Na I 330.2 Ni I1 216.556 Ni I1 231.604 P I 178.287 Pb 11 220.353 S I 180.734 Sb 1206.833 Se I 196.026 Se 1203.985 Se 1206.83 Si 1288.158 Sr I1 216.59 TI I 190.864 V I1 310.230 Zn I1 213.856 LOD/pg dm-3 This work 0.3 0.2 0.1 0.7 0.4 0.1$ 0.1 0.18 0.02 0.2 0.2 0.15 0.1 0.2 0.2 0.2 0.1 0.05 0.1 0.05 1.5$ 0.04 0.06 0.02 4 0.3 0.2 3.5 1 0.5 1 2.3 5 1 0.4 0.4 0.5 0.5 0.09 CONV* 1 10 I 15 8 1 0.1 2.39 0.1 2 0.6 0.65 0.4 1 5 1 0.8 0.9 1 0.8 4 0.07 0.7 0.15 100 2.7 3 10 5 20 7.4 10 1159 3009 10 10 8.3§ 2.5 0.5 Enhan F t 50 10 21.4 20 10 1 12.8 5 10 3 4.3 4 5 25 5 8 18 10 16 3.3 3.7 1.7 11.6 7.5 25 9 15 2.9 5 40 7.4 4.3 23 300 25 20.8 20 5 5.6 *LODs (20) using a Meinhard concentric nebulizer.?Enhancement factor; LOD conventiona1:LOD USN. #In several cases higher values were obtained. §Values quoted from ref. 19. significantly reduced in several cases. This intensity incline for B and Hg was attributed to analyte loss in the desolvation system as a result of volatilization during aerosol dehydration.1° In acid media B forms volatile species that can be redeposited. This interesting phenome- non requires further investigation. In the presence of high concentrations of NaCl (1% m/m) LODs were degraded by factors varying approximately from 2 to 10 (Table 4). Despite this decrease numerous trace elements can be determined in brines and similar high-salt solutions (polluted sea-water and mineralized brines) by performing a preliminary dilution.Background Correction For the analysis of pristine water it was observed that background differences between standards and samples were usually insignificant and that compensation was only rarely required However for saline solutions where Ca and Mg concentrations were high background compensa- tion was required. In the case of matrix-matched calibration systems background correction was applied when neces- Table 4 LODs of USN for solutions containing high NaCl concentrations. Blank solution prepared from ultrapure NaCl (Suprapur Merck). Spectral lines listed in Table 3 LOD/pg dm-3 Element 1 O/o NaCl B 0.5 Cd 0.8 Ba 0.6 c o 0.5 Mn 0.2 Fe 0.15 Cr 0.8 A1 1.4 c u 0.6 Aqueous 0.07 0.2 0.1 0.2 0.02 0.1 0.2 0.2 0.05 sary. Regions of background compensation were selected by scrutinizing spectral profiles of the solutions.The positions selected were similar to those listed by Brenner and Eldad.20,21 Wash-out Times The wash-out time was determined by nebulizing an acid blank (2% HN03) followed by a 0.5% NaCl solution containing several trace elements at the mg dm-3 level. The wash-out time was defined as the time required for the system to evacuate the saline aerosols and produce 1% of the original element concentration. The data illustrated in Fig. 5 indicated that this level was attained after about 80 s. It should be noted that this period included nebulization of dead volume for approximately 10 s. This is considered to be adequate for the USN. For pure aqueous solutions the wash-out period amounted to about 40 s.Small contamina- tions of the elements of interest in the blank solutions were taken into account. The wash-out times reported here are similar to those observed by Chan and co-workers.11J2 The influence of tailing-off effects was significant only when trace concentrations were determined consecutively with solutions containing very high concentrations of the ele- ments to be determined. Spurious emission spikes were not observed. In the case of trace concentrations of Hg and B in aqueous solution the wash-out times were about 60 s. In order to take full advantage of the linear range of 4 orders of magnitude an additional wash-out period of 15-30 s was needed. However this situation is uncommon when trace metals are to be determined. 10000 0.1 ' ' I I I 0 20 40 60 80 100 Time/s Fig.5 Wash-out times of the USN for a 0.5% NaCl solution for A Na; B Ba; C Ni; and D MnJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 823 Table 5 Comparison of long-term precision of the USN with and without an internal reference (Co); 0.1% NaCl containing a multi- element spike of 100 pg dm-j Without internal reference With internal reference Mean/ SD/ Mean/ SD/ Element pg dm-3 pg dm-) RSD (Yo) pg dm-3 pg dm-j RSD (O/O) Zn Pb Cd Ni Ba co Mn Fe Cr 101.7 100.8 99.1 98.9 99.5 100.3 101.5 97.9 99.4 0.9 0.8 0.9 1.1 0.8 0.7 0.8 1.2 0.7 0.9 0.8 0.9 1.1 0.8 0.7 0.8 1.2 0.7 100.9 101.4 100.2 98.6 99.2 101.2 99.9 99.2 - 0.5 0.6 0.5 0.6 0.4 0.7 0.7 0.3 - 0.5 0.6 0.5 0.7 0.4 0.7 0.7 0.3 - Table 6 Data for SLRS-1 and NIST SRM 1643b by USN; all data in pg dm-j except where indicated SLRS- 1 NIST SRM 1643b Element Determined Recommended* Determined Recommended* RSD (Yo) A1 As B Ba Be Cd c o Cr c u Fe Mn Mo Ni Pb Se srs V Zn CaS 9 MgS § NaS 9 K4 9 18 180 - 20.7 - - - 0.3 3.5 1.6 0.9 1.8 32 - - 135 0.87 2.1 26.4 5.8 9.9 0.97 23 NDt 22.2 - - - - 0.4 3.6 31.5 1.8 0.8 1.I - - 136 0.66 1.34 25.1 5.99 10.4 1.3 15 58 47 42.4 18.5 23 29.5 22.4 20.3 87 28 89 49 19 12.5 224 43 60 29.9 6.9 - - *From refs. 22 and 23. TND = not detected. $Value obtained by extrapolation using a 500 pg dm-3 calibration standard. §Values in mg dm-3. ND 49 ND 44 19 20 26 19 21.9 99 28 85 49 24 10 227 45 66 1.2 1.4 - - 2 5 5 1.8 2 1.4 2.2 2.6 1.5 1.8 1.8 2.2 1.6 10 8 1.2 1.4 1.9 30 8 - - Precision The short-term precision in a pure aqueous solution was approximately 1% at the p g dm-3 level and < 1% at the mg dme3 level.The long-term precision of the USN for a solution containing 0.1 O/o NaCl (m/v) and a 100 pg dm-3 multi-element spike was determined for an analysis period of 90 min. The data listed in Table 5 indicate that the relative standard deviations (RSDs) for the ten measure- ments were approximately 1 %. These values were improved by a factor of about 2 with the use of the internal reference method. Under these conditions it can be concluded that both the short- and long-term variations of the USN are similar to those obtained with a conventional pneumatic nebulizer. Accuracy The accuracy of the USN was evaluated by replicate analysis of a National Institute of Standards and Techno- logy (NIST) Standard Reference Material (SRM) 1643b Trace Elements in Water (n= 10) and National Research Council of Canada (NRCC) SLRS- 1 Riverine Water ( n = 4).These samples represent pristine waters of low-salt content. The data provided in Table 6 indicate that bias between the USN and the recommended value^^^^^^ is absent. With few exceptions the percentage deviation [(USN value - recommended value)/( recommended value)] x 100 varied from < 1 to 15%. The precisions for the SLRS-1 data are similar to those for SRM 1643b. Noteworthy is the capability of determining As and Se directly. In order to determine the accuracy of analysis of high-salt solutions 0.5% m/v NaCl and LiB02 solutions containing 5-500 pg dm-3 multi-element spikes were analysed using a matrix-matched procedure.The results are listed in Table 7. At the 10 pg dm-3 concentration level the trace element recoveries varied from about 80 to 120%. At higher analyte concentrations recoveries were in the 95- 105% range and even better indicating that Mo Cr Zn Cd Ni Co Mn Fe V Cu and Pb can be determined with satisfactory accuracy and precision in the presence of relatively high-salt concen- trations. In the case of Pb a background correction was824 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 7 Spiked data for 0.5% NaCI and 0.5% LiBOl solutions; calibration with matrix-matched procedure 0.5% NaCl 0.5% LiBOz Addition of spike/Pg dm-3 Addition of spike/pg dm-3 Element Cd c u Cr Fe Mn Mo Pb 10 11.3 9 .6 12.4 9.75 9 . 2 11.8 5 10 20 7.7 11.7 19.8 50 52 47 51.7 50.5 48.5 49 30 31.1 100 98 103 102 101 99 102 40 50 36.6 50.9 Element Cd c u Cr Fe Mn Mo Sr Ti ~ 10 11.4 13 8.5 12 9.6 9.2 10.3 1 1 100 98 102 102 101 108 110 99 97.5 500 496 506 492 490 49 5 5 10 503 500 applied. Recent data obtained in our laboratories indicate that the use of Sc as an internal reference results in a considerable improvement in the precision and accuracy when solutions containing 1000-5000 mg dm-3 of total salt are aspirated. In geoanalysis samples are frequently decomposed using an LiBOz fusion (0.5 g of sample and 1-2 g of fusion reagent) and dissolved in 200-500 dm3 of dilute acid usually HN03. However the final volume of 500 cm3 precludes trace element determination employing conven- tional ICP-AES because of high dilution factors.The USN data for the LiBOZ spike demonstrate that trace elements can be determined in this type of solution. Dynamic Range In environmental analysis it is essential that the dynamic range exceeds the expected range of concentrations which can vary unexpectedly. In the present application Ca Na K and other elements in the mg dm-3 concentration range were determined in NIST SRM 1643b and SLRS-1 using a two-point calibration consisting of an acid blank and a multi-element standard of 500 pg dm-3. Thus satisfactory values were obtained even when the concentration ex- ceeded the upper calibrator. As a result of the 4-fold linear range of calibration the number of standards was reduced. Conclusions The data demonstrate that the USN coupled to an ICP-AES can potentially be used for the determination of pg dm-3 and mg dm-3 concentration levels allowing the direct analysis of low-salt pristine water biological fluids and environmental geological and related materials.In the case of high LOD enhancement the use of the USN overcomes the need to analyse the samples by electrothermal atomic absorption spectrometry and hydride generation. More- over the alkali elements can be determined using less sensitive spectral lines in the ultraviolet and the ultravio- let-visible regions of the spectrum. The large dynamic range of calibration allows the determination of minor elements including the alkalis in the mg dm-3 range. High-salt solutions can be analysed on a routine basis using the sheath gas attachment and the wide injector tube which prevent salt accumulation and eventual blockage.However a matrix-matched calibration technique was required to compensate for interference effects. References 1 Capelle B. Mermet J.-M. and Robin J. Appl. Spectrosc. 1982 36 102. 2 Marichy M. Mermet M. Murillo M. Poussel E. and Mermet J.-M. J. Anal. At. Spectrom. 1989 4 209. 3 Hartenstein S. T. Ruzicka J. and Christian G. D. Anal. Chem. 1985 57 21. 4 Ruzicka J. Anal. Chem. 1983 55 1041A. 5 Huang B. Zeng X. Zhang Z. and Liu J. Spectrochim. Acta Part B 1988 43 38 1 . 6 Olsen K. W. Haas W. J. and Fassel V. A. Anal. Chem. 1977 49 632. 7 Boumans P. W. J. M. and De Boer F. J. Spectrochim. Acta Part B 1975 30 309. 8 Taylor C. E. and Floyd T. L. Appl. Spectrosc.198 1,35,408. 9 Berman S. S. McLaren J. W. and Willie S. N. Anal. Chem. 1980 52 488. 10 Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. 1 1 Chan S. K. and Zajac J. P. paper presented at the 21st Annual Conference Canadian Mineral Analysts Ontario Canada September 26-29 1989. 12 Chan S. K. Zechmann C. A. and Yanak M. M. Analysis of Water and Wastes Using ICP-AES With Ultrasonic Nebuliza- lion CETAC Report 1990. 13 Freeden K. J. Am. Lab. (Fairfield Conn.) 1990 December 24 23. 14 Tang Y. Q. and Trassy C. Spectrochim. Acta Part B 1986 41 143. 15 Myers R. Spectroscopy 1989 4 16.. 16 Brenner I. B. and Lemarchand A. paper presented at the XXVII Colloquium Spectroscopicum Internationale 199 1 Bergen Norway June 9-14 1991. 17 Murillo M. and Mermet J.-M. Spectrochim. Acta Part B 1987 42 1151. 18 Brenner I. B. and Myers R. paper presented at the 1989 Pittsburgh Conference Atlanta GA USA March 6-10 1989 paper 301. 19 Winge R. K. Fassel V. A. Peterson V. J. and Floyd M. An Atlas of Spectral Information. Physical Sciences Data 20 Elsevier 1985. 20 Brenner I. B. and Eldad H. ICP Inf Newsl. 1984 10 451. 21 Brenner I. B. and Eldad H. ICP In$ Newsl. 1986 12 243. 22 U.S. Dept. of Commerce National Institute of Standards and Technology Certificate of Analysis for I643b Trace Elements in Water Gaithersburg MD USA. 23 National Research Council Canada Marine Analytical Chemistry Standards Program Certificate of Analysis for SLRS-1 Riverine Water Halifax Nova Scotia Canada. Paper 2/002 I IF Received January 9 1992 Accepted May 14 I992

ISSN:0267-9477    

DOI:10.1039/JA9920700819

出版商:RSC    

年代:1992

数据来源: RSC