摘要:

/- one-stop immediate access to all atomic spectrometry literature published since 7 985 including conference papers. jAASbase is a unique database that provides a fully comprehensive up-to-date source of over 23,. 000 analytical atomic spectrometry references. It is designed to meet every atomic spectroscopists information needs - a convenient desktop tool. As a subscriber you will enjoy the following benefits of JAASbase @ Simplicity of use even for the non-specialist @ Economy of effort and expense @ A vast store of references @ Flexibility that fosters thorough searches @ Adaptability - you can add your own data @ Helpdesk and user literature gives added assurance that you can quickly master JAASbase Idealist Software f 21 0.00 $368.00 1994 Subscription Details JAAS Backfile (1 986-93) jAASbase Updates EC €99.00 EC €280.00 EC USA $174.00 USA $490.00 USA (VAT chargeable in the UK) To order JAASbase and for further information please contact Sales and Promotion Department Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF United Kingdom. TeI +44 (0)223 420066.Fax +44 (0)223 423623. ROYAL SOCIETY OF Information Services/- one-stop immediate access to all atomic spectrometry literature published since 7 985 including conference papers. jAASbase is a unique database that provides a fully comprehensive up-to-date source of over 23,. 000 analytical atomic spectrometry references. It is designed to meet every atomic spectroscopists information needs - a convenient desktop tool. As a subscriber you will enjoy the following benefits of JAASbase @ Simplicity of use even for the non-specialist @ Economy of effort and expense @ A vast store of references @ Flexibility that fosters thorough searches @ Adaptability - you can add your own data @ Helpdesk and user literature gives added assurance that you can quickly master JAASbase Idealist Software f 21 0.00 $368.00 1994 Subscription Details JAAS Backfile (1 986-93) jAASbase Updates EC €99.00 EC €280.00 EC USA $174.00 USA $490.00 USA (VAT chargeable in the UK) To order JAASbase and for further information please contact Sales and Promotion Department Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF United Kingdom.TeI +44 (0)223 420066. Fax +44 (0)223 423623. ROYAL SOCIETY OF Information Services/- one-stop immediate access to all atomic spectrometry literature published since 7 985 including conference papers.jAASbase is a unique database that provides a fully comprehensive up-to-date source of over 23,. 000 analytical atomic spectrometry references. It is designed to meet every atomic spectroscopists information needs - a convenient desktop tool. As a subscriber you will enjoy the following benefits of JAASbase @ Simplicity of use even for the non-specialist @ Economy of effort and expense @ A vast store of references @ Flexibility that fosters thorough searches @ Adaptability - you can add your own data @ Helpdesk and user literature gives added assurance that you can quickly master JAASbase Idealist Software f 21 0.00 $368.00 1994 Subscription Details JAAS Backfile (1 986-93) jAASbase Updates EC €99.00 EC €280.00 EC USA $174.00 USA $490.00 USA (VAT chargeable in the UK) To order JAASbase and for further information please contact Sales and Promotion Department Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF United Kingdom.TeI +44 (0)223 420066. Fax +44 (0)223 423623. ROYAL SOCIETY OF Information Services/- one-stop immediate access to all atomic spectrometry literature published since 7 985 including conference papers. jAASbase is a unique database that provides a fully comprehensive up-to-date source of over 23,. 000 analytical atomic spectrometry references. It is designed to meet every atomic spectroscopists information needs - a convenient desktop tool. As a subscriber you will enjoy the following benefits of JAASbase @ Simplicity of use even for the non-specialist @ Economy of effort and expense @ A vast store of references @ Flexibility that fosters thorough searches @ Adaptability - you can add your own data @ Helpdesk and user literature gives added assurance that you can quickly master JAASbase Idealist Software f 21 0.00 $368.00 1994 Subscription Details JAAS Backfile (1 986-93) jAASbase Updates EC €99.00 EC €280.00 EC USA $174.00 USA $490.00 USA (VAT chargeable in the UK) To order JAASbase and for further information please contact Sales and Promotion Department Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF United Kingdom. TeI +44 (0)223 420066. Fax +44 (0)223 423623. ROYAL SOCIETY OF Information Services

ISSN:0267-9477    

DOI:10.1039/JA99409BP011

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 51N Foreword 1994 Winter Conference on Plasma Spectrochemistry San Diego California USA January 10-1 5 1994 Inspired by mild temperatures and sunny skies the participants at the 1994 Winter Conference on Plasma Spectrochemistry mixed the pleasure of a January winter in Southern California with the business of almost 300 oral and poster presen- tations. This eighth in the series of Winter Conferences sponsored by the ICP Information Newsletter attracted significant technical contributions a noteworthy number of participants and an impressive exhibition to the San Diego Princess convention centre on Vacation Island in Mission Bay. The complete programme and presen- tation abstracts have been published along with symposium summaries and personal reflections of session chairper- sons.' The record-breaking conference was well received,2 and many of the authors have selected to contribute to this special issue.The 1994 Winter Con- ference was a success in every respect and we strive to continue this ac- complishment with these conference papers. To summarize some of the conference statistics the meeting set a number of records attendance presentations and distribution. The 520 scientists came from 40 states and 26 countries. Twenty five percent arrived from outside North America. A record number of over 230 participants presented a total of 285 oral or poster papers. The programme con- sisted of 12 symposia three poster sessions and 6 panel discussions. Sample introduction transport phenomena and flow injection topics began the meeting on Monday with a plenary lecture by Rick Browner and invited lectures by John Olesik John Koropchak and Jarda Ruzicka.Two symposia on elemental speciation were highlighted on Tuesday with invited lectures by Les Ebdon Joe Caruso Rita Cornelis and Klaus Heumann. All topics were illustrated in the nearly 60 posters displayed during the late afternoon. Plasma instrumentation and software and sample preparation were the features on Wednesday with a plenary lecture by Wolf Wegscheider and invited lectures by Eric Salin and Skip Kingston. More than 60 posters covered these topics and also laser-assisted spectrochemistry. On Thursday the emphasis shifted to excitation mechanisms and plasma phen- omena.The day began with a plenary lecture by Jim Winefordner followed by invited lectures by Mike Blades Gary Hieftje and Paul Farnsworth. Laser- assisted plasmas spectrometry concluded the oral presentations and over 60 post- ers were presented in the afternoon which included five computer posters on which software and computer results were dem- onstrated. A special panel discussion on teaching spectroscopy with computers was introduced for the first time. Plasma source mass spectrometry fun- damentals instrumentation and appli- cations were featured on Friday and Saturday. Sam Houk and Jim McLaren gave plenary lectures and Dietmar Stuewer Ken Marcus Willard Harrison Gary Horlick and Jose Broekaert gave invited lectures. All of these sessions were stimulating and most participants left with new information and many ideas.However the bay view from the conference centre deck and the mild sunny weather made returning from breaks difficult. Neverthe- less sessions and panel discussions were always well attended. Continuing the conference tradition new instrumentation was the hit of the exhibition. The exhibition comprised 23 companies and organizations and more than 85 people registered as exhibitors. Almost 150 people took 43 short courses prior to the meeting. Of the 30 short course instructors more than two-thirds of them also presented papers. Planning for the 1995 European Winter Conference on Plasma Spectro- chemistry in Cambridge is well under- way. The 1996 Winter Conference is to return to Florida to continue the suc- cesses of the 1982 Orlando and 1990 St. Petersburg meetings. The convention will be held January 8-13 1996 at the World Conference Centre of the Bonaventure Resort & Spa in Fort Lauderdale Florida. Our popular short course pro- gramme will be featured Friday through Sunday January 5-7. Located 20 minutes due West of the Hollywood/Ft. Lauderdale and North of the Miami International airports on the East Coast of Florida the Bonaventure provides a large conference facility numerous rec- reational activities including two golf courses and easy access to Florida's beaches and attractions. Preliminary titles and abstracts are requested by July 3 1995. Please join us. Ramon M. Barnes Department of Chemistry University of Massachusetts M A 01 8003-451 0 USA References 1 ICP I f . Newsl. 1994 19(11) 685; 19( 12) 777; 20( l ) 4; 20(2) 97. 2 Tye C. J. Anal. At. Spectrom. 1994 9 23N.

ISSN:0267-9477    

DOI:10.1039/JA994090051N

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

52N JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Diary of Conferences and Courses 1994 13th International Mass Spectrometry Conference August 29-September 2 Budapest Hungary For further information contact Hung- arian Chemical Society; FO u. 68 H-1027 Budapest Hungary. Telephone 361 201 6883; fax 316 15 61215. EURACHEM Workshop on Evaluation of Measurement Uncertainty in Chemi- cal Analysis September 5-6 Graz University of Technology Austria Details can be found in J. Anal. At. Spectrom. 1994,9 26N. For further information contact Pro- fessor w . Wegscheider Technische Univ- ersitat Graz Technikerstrasse 4 A-8010 Graz Austria. 4th International Conference on Plasma Source Mass Spectrometry September 11-16 A conference sponsored by Finnigan MAT For further information contact Dr.Grenville Holland The Conference Secretary Department of Geological Sciences Science Laboratories South Road Durham City UK DU1 3LE or Dr. Mark Nicho-lls Finnigan MAT Ltd. Paradise Heme1 Hempstead Hertford- shire UK HP2 4TG. Telephone 0442 233555; Fax 0442 233666. EUCMOS XXII XXII European Con- gress on Molecular Spectroscopy September 11-16 Essen Germany Details can be found in J. Anal. At. Spectrom. 1993,8 49N. For further details contact Gesellschaft Deutscher Chemiker Abt. Tagungen P.O. Box 90 04 40 W-6000 Frankfurt 90 Germany. Telephone +49 697917-366; Fax +49 69 7917-475; Telex 4 170 497 gdch d. Geoanalysis 9 4 An International Sym- posium on the Analysis of Geological and Environmental Materials Sep tem ber 18-22 Charlotte Mason Conference Centre Ambleside UK Details can be found in J.Anal. At. Spectrom. 1993 8 49N. For further information contact Mr. D. L. Miles Analytical Geochemistry Group British Geological Survey Kingsley Dunham Centre Keyworth UK NG12 5GG. Telephone 0602 363100; Fax 0602 363200. 7th International Symposium on Environ- mental Radiochemical Analysis September 21-23 Bournemouth UK Dates to Note Synopses of papers January 28 1994. Final date for registration July 15 1994. For further details contact Dr. P. Warwick Department of Chemistry Loughborough University of Tech- nology Loughborough Leicestershire UK LEll 3TU. Telephone 0509 222585 or 0509 222545; Fax 0509 233163. 6th International Colloquium on Solid Sampling With Atomic Spectroscopy October 11-13 Amsterdam The Netherlands Details can be found in J Anal.At. Spectrom. 1993 8 59N. For further information contact Dr. R. F. M. Herber Coronel Laboratory University of Amsterdam Meibergdreef 15 NL-1105 A2 Amsterdam The Netherlands. Third Rio Symposium on Atomic Spectrometry November 6-12 Venezuela Details can be found in J. Anal. At. Spectrom. 1993 8 64N. For further information contact Pro- fessor Jose Alvarado Universidad Simon Bolivar Departamento de Quimica Laboratorio de Absorcion Atomica Apartado postal No. 89000 Caracas 1080-A Venezuela. Fax (0058-2-) 9383221 57 19 13415763355 19621695. Analytica '94-Second National Sym- posium on Analytical Science December 1994 Western Cape South Africa Details can be found in J. Anal. At. Spectrom. 1993 8 60N. For further information contact Dr.I. M. Moodie c/o PO Box 1970 Tygerberg 7505 South Africa. Fax 021-932-4575. 1995 1995 Winter Conference on Plasma Spectrochemistry January 8-13 Cambridge UK For further information contact Janice M. Gordon The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF UK. Telephone +44 (0) 223 420066; fax + 44 (0) 223 420247. Pittcon '95 The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy March 5-10 New Orleans Louisiana USA Details can be found in J. Anal. At. Spectrom. 1994 9 49N. For further information contact The Pittsburgh Conference 300 Penn Center Boulevard Suite 332 Pittsburgh PA 15235-5503 USA. Telephone (41 2) 825-3220; toll free (800) 825-3221; fax (412) 825-3224. Fourth International Conference on Pro- gress in Analytical Chemistry in the Steel and Metals Industry May 16-18 Jean Monnet Building Luxembourg Details can be found in J.Anal. At. Spectrom. 1994 9 50N For details of providing a contribution to the programme or other informationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 53N contact CEC/CETAS Conference R. Jowitt British Steel plc Technical Teesside Laboratories PO Box 11 Grangetown Middlesbrough Cleveland TS6 6UB. Telephone +44 642 467144; fax +44 642 460321 43rd ASMS Conference on Mass Spec- trometry and Allied Topics May 21-25 Atlanta GA USA For further details contact ASMS 1201 Don Diego Avenue Santa Fe NM 87501 USA. Telephone 505 989 4517. Fax 505 989 1073. Vth COMTOX Symposium on Toxi- cology and Clinical Chemistry of Metals July 10-13 University of British Columbia Van- couver British Columbia Canada Details can be found in J.Anal. At. Spectrom. 1994 9 26N. Colloquium Spectroscopicum Inter- nationale (CSI) XXIX August 27-September 1 Leipzig Germany Details can be found in J. Anal. At. Spectrom. 1993 8 50N. Colloquium Spectroscopicum Internation- ale (CSI) XXIX Post Symposium ICP- MS September 1-4 Wer nige r ode/ Hartz Germany Details can be found in J. Anal. At. Spectrom. 1994,9 46N. 1996 1996 Winter Conference on Plasma Spectrochemistry Fort Lauderdale Florida USA January 8-13 Objectives and Programme The continued growth in popularity of plasma sources for atomization and excitation in atomic spectroscopy and ionization in mass spectrometry and the need to discuss recent developments of these discharges in spectrochemical analysis stimulated the organization of this meeting.The Conference will bring together international scientists experi- enced in applications instrumentation and theory in an informal setting to examine recent progress in the field. Approximately 500 participants from 25 countries are expected to attend. Approximately 300 papers describing applications fundamentals and instru- mental developments with plasma sources are expected to be presented in lecture and poster sessions by more than 200 authors. Symposia organized and chaired by recognized experts will include the following topics (1 ) Sample introduction and transport phenomena (2) Flow injection spectrochemical analysis (3) Elemental speciation with plasma/chromatographic techniques (4) Plasma instrumentation including chemometrics expert systems on-line analysis software and remote-system automation (5) Sample preparation treatment and automation (6) Exci- tation mechanisms and plasma phen- omena (7) Spectroscopic standards and reference materials (8) PIasma source mass spectrometry (9) Glow discharge atomic and mass spectrometry (10) Applications of stable isotope analyses and ( 11 ) Laser-assisted plasma spec- trometry.Six plenary and 18 invited lectures will highlight advances in these areas. Afternoon poster sessions will feature applications automation and new instrumentation. Five panel dis- cussions will address critical develop- ment areas in sample introduction instrumentation elemental speciation plasma source mass spectrometry and novel software and hardware directions.Plenary invited and submitted papers will be published in the autumn of 1996 after peer review as the official Confer- ence proceedings. Schedule of Activities Preliminary Title and 50-Word Abstract Due for Contributed Papers July 3 1995 Exhibitor Booth Reservation and Pre- Registration Deadline September 1 1 1995 Conference Pre-Registration October 13 1995 Hotel Pre-Reservation October 13 1995 Late Pre-Registration Deadline De- cember 8 1995 1996 Winter Conference Short Courses January 5-7 1996 1996 Winter Conference on Plasma Spectrochemistry January 8-13 1996 For further information contact Dr R. Barnes ICP Information Newsletter Department of Chemistry Lederle GRC Towers University of Massachusetts Box 34510 Amherst MA 01003-4510 USA. Telephone (413) 545 2294; telefax (413) 545 4490. International Schools and Conferences on X-Ray Analytical Methods January 18-25 Sydney Australia Details can be found in J. Anal. At. Spectrom. 1994 9 47N. For further information contact AXAA '96 Secretariat GPO Box 128 Sydney NSW 2001 Australia. Telephone 61 2 262 2277. Fax 61 2 262 2323. Telex AA 176511 TRHOST.

ISSN:0267-9477    

DOI:10.1039/JA994090052N

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

1995 European Winter Conference on Plasma Spectrochemistry 8-13 January 1995 CAMBRIDGE UK Short Courses A series of short courses of one half day duration will take place on Sunday 8th January. Notes and tuition material will be distributed with each course. Courses 1 and 2 Short Courses on ICP-MS Professor R.S. Houk Ames Laboratory Iowa State University USA Course 1 (AM) Instrumentation and Theory The course will cover fundamental aspects of ICP-MS including:- a) Molecular beam sampling b) Quadrupole and high resolution c) Vacuum technology d) Ion sources e) Detection systems and data hand1 ing f) Sample introduction technologies analys ers Course 2 (PM) Advanced Topics The course will cover more advanced topics on ICP-MS particularly relevant to problem solving. Each topic will be illustrated with relevant applications examples.a) Interferences (spectroscopic and non-spectroscopic and methods of alleviation b) Isotopic analysis c) Chromatographic methods d) Overview of commercial instrumentation Course 3 (PM) Sample Preparation for ICPs Dr S.J. Haswell Hull University UK The course will focus on important aspects of sampling and sample preparation with particular emphasis on ICP measurements. a) Batch methods f o r wet oxidation b) Recent trends in microwave preparation for ICP-MS atomic spectrometry general analytical techniques c) On-line sample preparation d) Extraction methods e) On-line chemical processing f) Miniaturization Course 4 (PM) Speciation Professor O.X. Donard University of Bordeaux France The course will focus on practical aspects of speciation analysis with particular emphasis on ICP and other plasma sampling systems.Sample collection and handling preservation and preparation prior to injection into hyphenated systems using atomic spectrometry and ICP-AES or ICP-MS as detectors will be illustrated with applications from current topical fields . a) Sampling and sample pretreatment b) Separative techniques Differential chemistry Gas liquid ion and SCF c ) Interfacing chromatography techniques to ICPs and other plasma sources and detectors chromatographies Course 5 (AM) Quality Systems in the Laboratory Professor L. Ebdon Dr E.H. Evans University of Plymouth UK The course will discuss how high quality analytical data can be produced in the laboratory that are accurate reliable and adequate f o r the intended purpose.a) Quality assurance principles b) Sampling and sample preparation c) Personnel aspects d) Statistics for quality control e Use of reference materials and f) Equipment and records maintenance g) Audits and accreditation. traceability Course 6 (AM) Sample Presentation for ICPS Dr C McLeod Sheffield Hallam University UK The course is intended as a problem solving workshop and will attempt to rationalise the choice of sampling system for ICP spectrometries by use of practical examples. a) Nebulisation techniques Traditional and high efficiency The role of desolvation Hydride Other vapour techniques e . g . b) Vapour generation Hg oso c) Microsampling systems d) Flow injection e) Laser ablation1995 European Winter Conference on Plasma Spectrochemistry 8-13 January 1995 CAMBRIDGE UK Short Courses A series of short courses of one half day duration will take place on Sunday 8th January. Notes and tuition material will be distributed with each course.Courses 1 and 2 Short Courses on ICP-MS Professor R.S. Houk Ames Laboratory Iowa State University USA Course 1 (AM) Instrumentation and Theory The course will cover fundamental aspects of ICP-MS including:- a) Molecular beam sampling b) Quadrupole and high resolution c) Vacuum technology d) Ion sources e) Detection systems and data hand1 ing f) Sample introduction technologies analys ers Course 2 (PM) Advanced Topics The course will cover more advanced topics on ICP-MS particularly relevant to problem solving. Each topic will be illustrated with relevant applications examples.a) Interferences (spectroscopic and non-spectroscopic and methods of alleviation b) Isotopic analysis c) Chromatographic methods d) Overview of commercial instrumentation Course 3 (PM) Sample Preparation for ICPs Dr S.J. Haswell Hull University UK The course will focus on important aspects of sampling and sample preparation with particular emphasis on ICP measurements. a) Batch methods f o r wet oxidation b) Recent trends in microwave preparation for ICP-MS atomic spectrometry general analytical techniques c) On-line sample preparation d) Extraction methods e) On-line chemical processing f) Miniaturization Course 4 (PM) Speciation Professor O.X. Donard University of Bordeaux France The course will focus on practical aspects of speciation analysis with particular emphasis on ICP and other plasma sampling systems.Sample collection and handling preservation and preparation prior to injection into hyphenated systems using atomic spectrometry and ICP-AES or ICP-MS as detectors will be illustrated with applications from current topical fields . a) Sampling and sample pretreatment b) Separative techniques Differential chemistry Gas liquid ion and SCF c ) Interfacing chromatography techniques to ICPs and other plasma sources and detectors chromatographies Course 5 (AM) Quality Systems in the Laboratory Professor L. Ebdon Dr E.H. Evans University of Plymouth UK The course will discuss how high quality analytical data can be produced in the laboratory that are accurate reliable and adequate f o r the intended purpose. a) Quality assurance principles b) Sampling and sample preparation c) Personnel aspects d) Statistics for quality control e Use of reference materials and f) Equipment and records maintenance g) Audits and accreditation. traceability Course 6 (AM) Sample Presentation for ICPS Dr C McLeod Sheffield Hallam University UK The course is intended as a problem solving workshop and will attempt to rationalise the choice of sampling system for ICP spectrometries by use of practical examples. a) Nebulisation techniques Traditional and high efficiency The role of desolvation Hydride Other vapour techniques e . g . b) Vapour generation Hg oso c) Microsampling systems d) Flow injection e) Laser ablation

ISSN:0267-9477    

DOI:10.1039/JA99409FX053

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

1995 European Winter Conference on Plasma Spectrochemistry 8-13 January 1995 CAMBRIDGE UK Short Courses A series of short courses of one half day duration will take place on Sunday 8th January. Notes and tuition material will be distributed with each course. Courses 1 and 2 Short Courses on ICP-MS Professor R.S. Houk Ames Laboratory Iowa State University USA Course 1 (AM) Instrumentation and Theory The course will cover fundamental aspects of ICP-MS including:- a) Molecular beam sampling b) Quadrupole and high resolution c) Vacuum technology d) Ion sources e) Detection systems and data hand1 ing f) Sample introduction technologies analys ers Course 2 (PM) Advanced Topics The course will cover more advanced topics on ICP-MS particularly relevant to problem solving. Each topic will be illustrated with relevant applications examples.a) Interferences (spectroscopic and non-spectroscopic and methods of alleviation b) Isotopic analysis c) Chromatographic methods d) Overview of commercial instrumentation Course 3 (PM) Sample Preparation for ICPs Dr S.J. Haswell Hull University UK The course will focus on important aspects of sampling and sample preparation with particular emphasis on ICP measurements. a) Batch methods f o r wet oxidation b) Recent trends in microwave preparation for ICP-MS atomic spectrometry general analytical techniques c) On-line sample preparation d) Extraction methods e) On-line chemical processing f) Miniaturization Course 4 (PM) Speciation Professor O.X. Donard University of Bordeaux France The course will focus on practical aspects of speciation analysis with particular emphasis on ICP and other plasma sampling systems.Sample collection and handling preservation and preparation prior to injection into hyphenated systems using atomic spectrometry and ICP-AES or ICP-MS as detectors will be illustrated with applications from current topical fields . a) Sampling and sample pretreatment b) Separative techniques Differential chemistry Gas liquid ion and SCF c ) Interfacing chromatography techniques to ICPs and other plasma sources and detectors chromatographies Course 5 (AM) Quality Systems in the Laboratory Professor L. Ebdon Dr E.H. Evans University of Plymouth UK The course will discuss how high quality analytical data can be produced in the laboratory that are accurate reliable and adequate f o r the intended purpose.a) Quality assurance principles b) Sampling and sample preparation c) Personnel aspects d) Statistics for quality control e Use of reference materials and f) Equipment and records maintenance g) Audits and accreditation. traceability Course 6 (AM) Sample Presentation for ICPS Dr C McLeod Sheffield Hallam University UK The course is intended as a problem solving workshop and will attempt to rationalise the choice of sampling system for ICP spectrometries by use of practical examples. a) Nebulisation techniques Traditional and high efficiency The role of desolvation Hydride Other vapour techniques e . g . b) Vapour generation Hg oso c) Microsampling systems d) Flow injection e) Laser ablation1995 European Winter Conference on Plasma Spectrochemistry 8-13 January 1995 CAMBRIDGE UK Short Courses A series of short courses of one half day duration will take place on Sunday 8th January. Notes and tuition material will be distributed with each course.Courses 1 and 2 Short Courses on ICP-MS Professor R.S. Houk Ames Laboratory Iowa State University USA Course 1 (AM) Instrumentation and Theory The course will cover fundamental aspects of ICP-MS including:- a) Molecular beam sampling b) Quadrupole and high resolution c) Vacuum technology d) Ion sources e) Detection systems and data hand1 ing f) Sample introduction technologies analys ers Course 2 (PM) Advanced Topics The course will cover more advanced topics on ICP-MS particularly relevant to problem solving. Each topic will be illustrated with relevant applications examples.a) Interferences (spectroscopic and non-spectroscopic and methods of alleviation b) Isotopic analysis c) Chromatographic methods d) Overview of commercial instrumentation Course 3 (PM) Sample Preparation for ICPs Dr S.J. Haswell Hull University UK The course will focus on important aspects of sampling and sample preparation with particular emphasis on ICP measurements. a) Batch methods f o r wet oxidation b) Recent trends in microwave preparation for ICP-MS atomic spectrometry general analytical techniques c) On-line sample preparation d) Extraction methods e) On-line chemical processing f) Miniaturization Course 4 (PM) Speciation Professor O.X. Donard University of Bordeaux France The course will focus on practical aspects of speciation analysis with particular emphasis on ICP and other plasma sampling systems.Sample collection and handling preservation and preparation prior to injection into hyphenated systems using atomic spectrometry and ICP-AES or ICP-MS as detectors will be illustrated with applications from current topical fields . a) Sampling and sample pretreatment b) Separative techniques Differential chemistry Gas liquid ion and SCF c ) Interfacing chromatography techniques to ICPs and other plasma sources and detectors chromatographies Course 5 (AM) Quality Systems in the Laboratory Professor L. Ebdon Dr E.H. Evans University of Plymouth UK The course will discuss how high quality analytical data can be produced in the laboratory that are accurate reliable and adequate f o r the intended purpose. a) Quality assurance principles b) Sampling and sample preparation c) Personnel aspects d) Statistics for quality control e Use of reference materials and f) Equipment and records maintenance g) Audits and accreditation. traceability Course 6 (AM) Sample Presentation for ICPS Dr C McLeod Sheffield Hallam University UK The course is intended as a problem solving workshop and will attempt to rationalise the choice of sampling system for ICP spectrometries by use of practical examples. a) Nebulisation techniques Traditional and high efficiency The role of desolvation Hydride Other vapour techniques e . g . b) Vapour generation Hg oso c) Microsampling systems d) Flow injection e) Laser ablation

ISSN:0267-9477    

DOI:10.1039/JA99409BX055

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

899 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Development of a High Liquid Flow Thermospray Sample Introduction System for Inductively Coupled Plasma Atomic Emission - Spectrometry* Invited Lecture J. A. Koropchak and T. S. Conver Department of Chemistry and Biochemistry Southern lllinois University Carbondale IL 62907 -4409 USA This work describes preliminary results of research devoted to the development of thermospray sample introduction systems that are capable of operating with liquid flow rates of 10 ml min-' or more. This development involved the modification of a thermospray power supply to provide about 1 kW of power and longer vaporizers to increase the residence time of the liquid stream within the heated portion of the system. Use of this system with ICP-AES resulted in sensitivity increases of about a factor of two when sample flow rates were increased from 2 to 5 ml min-'.Prospects for further improvement are also described. Keywords thermospray; sample introduction; inductively coupled plasma atomic emission spectrometry; high sample flow Liquid sample introduction (SI) for detection by inductively coupled plasma atomic emission (ICP-AES) or mass spec- trometry (ICP-MS) is most commonly performed using an aerosol technique typically based upon the use of a pneumatic nebulizer. These SI systems are widely used and provide many practical benefits including simple robust operation and resultant analytical measurements that are accurate and pre- cise. At the same time however it is well known that optimized pneumatic SI systems are inherently sample wasteful.Typically only about 1 % of the analyte input at 1 ml min- will actually reach the plasma; the rest goes to waste within the associated spray chamber.'-3 This aspect reduces substantially the flux of analyte input to the plasma and the signal per unit analyte input that might be achieved. For typical analyte flow rates this proportionally worsens the sensitivity and limits of detec- tion (LODs) that might be achieved. For applications where greater sensitivity is required this limitation presents a signifi- cant disadvantage. High performance alternatives to pneumatic SI for ICP spectrometry are typically based on aerosol generation pro- cesses which do not rely on energy from the argon carrier stream.The two primary examples are SI systems based on ultrasonic ( USN)1,2,4 and thermospray (TSP) nebulizer^.^ Hydraulic high pressure nebulization6 and monodisperse dried microparticulate injection7 are more recent additions to the family of aerosol devices which do not rely on energy from an argon gas jet. With TSP the sample is pumped through an electrothermally heated capillary. If the temperature is high enough to vaporize part of the liquid a jet of solvent vapour along with the remaining liquid will exit the capillary resulting in an aerosol. Primary aerosol droplets from TSP nebulizers decrease in size as the degree of vaporization increases and for optimum analytical operation they have been shown to be smaller than those from pneumatic nebulizers.' Further ana- lyte is preconcentrated due to the nature of the thermospray process and TSP aerosols are hot thereby enhancing solvent evaporation and droplet shrinkage.Coupled with desolvation these factors lead to much higher analyte transport than can be obtained with pneumatic SI and values as high as ~ 5 0 % for sample flows of the order of 1 mlmin-1 have been reported.' One aspect of the thermospray systems which significantly affects performance is the capillary diameter with smaller diameters providing higher transport and lower LODS.'?~' It *Presented at the 1994 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 10-15 1994. has also been shown that the higher performance of smaller capillaries can be obtained with only the exit of the capillary being of the smallest diameter.' This allows relatively high sample flow rates (1-2 ml min-l) with small apertures (25-50 pm) for high performance.A recent version of this type of system from our laboratory employs fused silica capillaries which have small diameter apertures laser fused to the exit," allowing relatively high sample flows (1-2 ml min-l) without the high pressures exhibited by long capillaries of small internal diameter." With high performance fused silica aperture ther- mospray (FSApT) systems using 50 pm apertures LOD improvements typically between 15-20 times lower than pneu- matic SI are obtained." Fused silica also provides a more chemically robust wetted element than stainless steel and these systems provide stability and low background with sample matrices common to elemental analysis." Furthermore the fact that aerosol characteristics are tuneable with TSP has been shown to allow reduction of matrix interferences.12 Direct comparison of USN and TSP systems has shown TSP to provide somewhat higher sensitivity lower LODs and lower matrix interference^.'^ A commercial system based on FSApT for ICP-AES was introduced at the 1994 Pittsburgh Conference by Leeman Labs.One reason for the inefficiency of pneumatic sample intro- duction is that the flow of gas used to generate the aerosol also carries the aerosol into the plasma and for optimal ICP operation this flow is limited to about 1 1 min-l. However aerosol quality is also determined by this flow rate which provides the energy for aerosol generation.In general aerosol characteristics for SI into atomic spectrometry instruments are poorer (Le. average droplet sizes are larger) as the gas-to- liquid flow ratio is reduced.14,15 The TSP aerosol generation may be thought of as a pneu- matic process involving the use of solvent vapour as the nebulizing gas.5 A difference between a typical pneumatic nebulizer and TSP lies with the fact that the nebulizing gas with TSP is condensable (typically water). In principle large flows of solvent vapour much larger than those employed with pneumatic nebulizers could be used with thermospray systems as long as sufficient energy for evaporation of the solvent and sufficient means for removing the solvent vapour prior to the ICP are provided. As roughly 1 1 min-' of gas would be generated from 1 ml min- of liquid vaporized liquid flows of the order of 10mlmin-I would provide a gas flow rate ten times higher than the argon flow rates at which conventional pneumatic nebulizers typically operate.Vaporization of these high liquid flows could either be used900 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 with a conventional pneumatic approach (thermojetspray) or directly with high sample flows. In the latter case if high transport efficiency can be maintained with the high sample flows direct improvements in sensitivity can be envisioned generally on the basis of the higher analyte mass flux that would result. For applications which are not typically sample limited (e.g. environmental) this presents a feasible approach to improved LODs.However conventional triac-controlled power supplies and vaporizers for thermospray as used in our laboratory and commonly for LC-MS,I6 are limited to aqueous liquid flow rates of only 1-2 ml min-'. In this paper the development of thermospray power sup- plies and vaporizers capable of vaporizing continuous flows of up to 10 ml min-' of water are described. Further preliminary results for optimizing such systems for direct introduction of samples at these high flow rates including analytical perform- ance data are given. In addition prospects for the application of such vaporizer systems with other aerosol generation approaches are also discussed. Experimental The instrument employed for these studies was a Leeman Labs (Lowell MA USA) ICP 2.5 operated at 1 kW with a coolant flow of 17.51min-l an auxiliary flow of 0.51min-' and a nebulizer flow optimized for compromise multi-element analy- sis with a 10 mg ml-' Ni solution to flow rates ranging from 0.56 to 0.65 1 min-' for sample uptake rates ranging from 2 to 5 ml min-' respectively.The carrier gas flow rate was adjusted with a Tylan (Carson CA USA) Model FC-260 mass flow controller. A Leeman Labs Plasma Spec rapid sequential Cchelle spectrometer was used for wavelength selection. The wavelengths employed for this study were Cu I at 324.754 nm Ni I1 at 231.604 nm Pb I1 at 220.353 nm Se I1 at 196.026 nm and T1 I1 at 190.801 nm. Liquid samples were introduced to the thermospray vapor- izer using a flow injection (FI) mode. The FI system flow was established using either a Dupont (Wilmington DE USA) Model 870 HPLC pump or an Autochrome (Berlin Germany) Model M500 HPLC pump.The de-ionized distilled water (DDW) carrier stream was pumped into an SSI (State College PA USA) pulse dampener and then to a Rheodyne (Cotati CA USA) Model 3725 HPLC injector fitted with a 10ml PEEK sample loop. A 2 pm metal free filter was located in the flow stream between the injector and the thermospray vaporizer. Modifications were made to a standard Vestec (Houston TX USA) thermospray power supply which consists of a triac based control circuit and a transformer that outputs 6 V a.c. to heat the vaporizer probe.16 This standard power supply is capable of providing up to about 150 W of power.16 However our goal was to be able to vaporize up to 10mlmin-1 of water.The power required for this is in the order of 440 W and to account for inefficiencies in power utilization power supplies capable of supplying about 1 kW of power were chosen. In order to increase the power output of the Vestec power supply the first modification made was to replace the original 120 V a.c. input transformer having a 6 V a.c. output with a larger transformer (Square D Company Model 1S43F) having a 12 V a.c. output and capable of delivering 1 kW of power. However the vaporizer resistance affected how much current could be drawn limiting the amount of heat dissipated by the vaporizer. The overall result from this first modification was that only 3-4 ml min-' of water could be vaporized. One way to increase the amount of heat dissipated in the vaporizer would be to raise the vaporizer resistance which was con- sidered impractical.Another way is to increase the voltage applied to the probe which would increase the current drawn thereby increasing the amount of power available. To do this the 120 V ax. input to the transformer was increased to 240 V a.c. by placing a second transformer (Square D company Model 2SlF) in between the output of the triac and the existing 120 V a.c.-12 V a.c. transformer. This second trans- former serves to step the output voltage of the triac from I20 V a.c. to 240 V a.c. By inputting 240 V instead of 120 V the output is increased from 12 V to 24 V resulting in a higher current draw by the vaporizer which increases the amount of power drawn and the heat dissipated by the unit.Fig. 1 compares the conventional and final high power TSP systems. 'The result of this modification was that the power output of the probe was increased to 700 W and the probe could vaporize in excess of 10 ml min-' of water. The thermospray vaporizer used in this study was con- structed from stainless steel. The probe tubing was 30 cm long and had dimensions of 1/16 in 0.d. by 100 pm i.d. To serve as additional heating for the liquid a 1 m length of 500 pm stainless-steel tubing was connected to the entrance end of the TSP probe and was also heated with the TSP power supply (see Fig. 2). This additional tubing was typically coiled as was most of the length of the vaporizer resulting in a compact assembly. Temperature feedback was accomplished by spot welding a J-type thermocouple to the pump end of the 500 pm tubing. Aerosols generated by these high flow thermospray vaporiz- (a) Thermocouple (temperature feedback) To vaporizer 1 To vaporizer 1 5 o w - = t l controller Triac Transformer - Potentiometer 120 V input 6 V output Thermocouple (temperature feedback) To vaporizer Triac controller Potentiometer Tra nsf o rmer 240 V input 24 V output 1 kVA Transformer 120 V input 240 V output 2 kVA Fig.1 Schematic diagrams of thermospray power supplies; (a) standard TSP power supply and (b) modified TSP power supply Length =30 crn 0.d. =A in i.d.= 100 pm t 1 & in Union J a T pow:r lead Control $Length = 1 m thermocouple o.d.=$ in Power lead i.d. =500 pm 1 t Sample from Injector Fig.2 High flow rate thermospray vaporizer; all tubing is stainless steel power leads are 0 gauge copper wire and the thermocouple is a J typeJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 90 1 ers were input to our standard desolvation ~ y s t e m ~ 3 ~ consisting of a cylindrical spray chamber maintained at 150 "C by an Omega (Stamford CT USA) Model CN-9000A temperature controller followed by a Friedrich's condenser chilled to - 5 "C by coolant from a Forma Scientific (Marietta OH USA) Model 2095-2 refrigerated recirculating bath. All solutions were prepared by serial dilution of Leeman Labs Plasma Pure Standard stock solutions using DDW. The carrier stream for these studies was DDW. Results and Discussion Essentially all previous studies have shown that the perform- ance of TSP SI systems is strongly dependent on the operating temperature of the vaporizer.' The operating temperature determines the degree of vaporization aerosol characteristics transport efficiency and LODs obtained with the system.Typically increasing temperatures provide increasing signals until a peak' or plateau" is achieved followed by a steady decline in signal and performance at higher temperatures. The provision of either a peak or plateau signal versus temper- ature profile is indicative of sufficient power availability for the sample flow (composition and flow rate) being input. Observation of profiles where the maximum signal is achieved with the power control maximized can be taken as an indi- cation that there may not be sufficient power being input to the sample flow to optimize the vaporization/aerosol character- istics of the system. Previously the effects of sample flow rate on sensitivity have been studied'?" with conventional TSP systems (ie.conventional vaporizers and power supplies limited to about 150 W input power) and optimum signals were obtained for sample flows between 1.5 and 2.0 ml min-'. Also the operation with higher sample flows would not allow the achievement of a peak or plateau signal versus temperature profile. These observations suggest that insufficient power was input to the flow system to optimally vaporize these higher sample flows with the conventional TSP systems. There are at least two possible reasons for this lack of power the power supply limit is insufficient to optimally vaporize the liquid; and/or the available power is not efficiently coupled into the flow stream. The power supplied by conventional LC-MS power supplies (e.g.150 W with Vestec supplies) would be able to vaporize over 3 ml min-' assuming all of the power is coupled into the flow stream. As some of the power is certainly lost during operation of a TSP vaporizer (e.g. to the air and other components in contact with the vaporizer) it is likely that the previous flow limits observed resulted from lack of available power from the power supply. This. is further suggested by the fact that at higher flow rates RSDs degraded," probably because of lack of regulation by the power supply at full power. To compensate for this lack of power new power supplies capable of providing over 1 kW of power were devised as described under Experimental.However full development of a plateau in the signal with this power supply and a standard 30 cm TSP vaporizer was nat-passible fur sample flm rates above 3 ml min-'. Consequently although sufficient power is available this power was not efficiently coupled into the liquid flow system. One reason for this inefficient power coupling is that higher flow rates result in higher flow velocities and lower residence times for the liquid within the heated capillary. As a result the heat transfer rate through the stainless steel becomes a limiting factor. To overcome this limitation the heated portion of the vaporizer was increased by adding a 1 m length of 500 pm stainless-steel tubing at the entrance to the capillary.Power was now input at the inlet of this tube and the control thermocouple was also located at this point. Using this combi- nation of the higher wattage power supply and the longer vaporizer signal versus temperature profiles as depicted in Fig. 3 for a 4ml min-l sample flow rate were obtained. A d I B D 25 30 35 40 45 50 55 60 Control temperat ure/"C Fig. 3 A copper; B selenium C nickel; D thallium; and E cadmium Signal uersus vaporizer control temperature for high flow TSP clear plateau is observed indicating that the energy input is sufficient to optimize the aerosol characteristics with this flow. In general signals for all elements tested provided similar response curves such that a single temperature could be chosen to reasonably optimize signals for all elements.Similar profiles were obtained for flows up to 5 ml min-l. Optimized operating conditions for flow rates ranging from 2 to 5 ml min-l are indicated in Table 1. As indicated optimal temperatures rose with increasing sample flow rate while optimal carrier gas flow rates declined. The latter feature exemplifies the advantage of SI systems where the aerosol formation process is not controlled by the carrier argon flow rate such as TSP or USN with the capability to optimize the carrier argon flow rate independently of the aerosol formation process. At the same time the pump pressure rose with increasing flow rate as expected. With 5 ml min-' this press- ure began to approach the limits of the pump employed consequently higher flow rates were not studied.However this operating pressure can be reduced by decreasing the length of the capillary region of the vaporizer which provides most of the pressure drop with the current design. Previous results have shown that only the exit of the vaporizer need be of small diameter and most designs of high performance TSP systems (for ICP-AES and LC-MS) have employed apertures at the exit of the capillary to allow relatively high flow operation (1-2 ml min-') with small vaporizer exit diameters (25- 75 J A ~ ) . ' ? ' ~ . ~ ~ Vaporizers employing shorter exit capillaries than the 30 cm of 100 pm tubing used in this study which is the primary source of flow resistance and pressure drop with this vaporizer are currently under development. (Note With such a vaporizer we have recently been able to obtain optimized operation at sample flow rates up to 9 ml min-'.Using the conditions in Table 1 calibration data for a series of elements were obtained. Calibration curves for lead obtained a1 flow rates ranging from 2-5 ml min- have slopes of the lines increasing by over a factor of two in going from 2 to 5mlmin-' or close to the increase in mass input for this change in flow rate. Table 2 provides more detailed data for the effects of flow rate. The data in Table 2 are averages of 3 sets of data collected on separate days. Relative standard deviations (RSDs) for the calibration curves and backgrounds for the 3 sets of data averaged to 3% with a range from 0.5 to 10%. A measure of background noise is given as %RSD in Table 2.These values are based on three replicate measure- ments of a blank solution and are also averages of 3 data sets. All correlation coefficients for the linear calibrations were 20.995. Sensitivity is increased by about a factor of 2 in going from 2 to 5 ml min-l for all of the elements except Cu. At the same time background levels also generally increase. On the other hand flow rate seems to have no effect on background902 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Table 1 High flow thermospray optimum conditions ICP power/kW 1 Argon gas flow rate/l min-' Outer 17.5 Intermediate 0.5 Spray chamber temperature/"C 150 Condenser temperature/"C -5 Sample flow rate/ Argon carrier gas TSP control temperature/"C Pump Pressure/MPa 2 0.645 48 10.4 3 0.600 48 14.0 4 0.570 50 17.6 5 0.555 53 23.5 ml min- ' flow rate/l min-' Table 2 ical parameters High flow thermospray effects of sample flow rate on analyt- Sample flow rate/ml min-' Analytical parameters 2 3 4 5 Pb (220.4 nm)- Slope/counts g pg-' 3.0 x lo5 3.5 x lo5 4.6 x lo5 6.0 x lo5 Background/count s 26620 30751 32736 36409 RSD(%) 1.7 0.6 1.5 0.07 Se (196.0 nm)- Background/counts 9703 10256 10716 11308 RSD(%) 1.5 1.7 1.8 0.8 Slope/counts g pg-' 1.7 x 104 2.0 x 104 2.5 x 104 3.3 x 104 Tl(l90.8 nm)- Slope/counts g pg-l 9.2 x lo3 1.1 x lo4 1.2 x lo4 1.6 x lo4 Background/counts 9070 9511 9696 10590 RSD(%) 1.1 0.5 1.9 1.7 Cu (324.8 nm)- Slope/counts g pg-' 1.7 x lo6 1.8 x lo6 2.0 x lo6 2.1 x lo6 Background/counts 40283 48520 50021 55971 RSD (Yo) 0.5 0.6 0.7 1.1 Ni (231.6 nm)- Background/counts 7443 6849 6674 7282 Slope/counts g pg- ' 1.9 x 105 2.1 x 105 2.6 x 105 3.3 x 105 RSD(%) 1 .o 2.7 2.0 3.3 noise.As indicated earlier higher noise levels with increasing flow can indicate marginal power availability and poor tem- perature regulation. It appears that with this new TSP system this problem is no longer evident. Aerosol plumes produced with the high sample flows were visually larger (e.g. wider and extending further from the vaporizer tip) than those for our previous TSP systems. As a result the spray chamber/desolvation apparatus employed for the current studies may be less than optimum. Future studies will include more detailed investigation of the effects of this portion of the apparatus.Conclusions Signal increases can be obtained which nearly coincide with increases in sample flow rate with the thermospray vaporiz- ation system suggesting that high analyte transport can be obtained with high sample flow rates providing higher analyte mass flux to the ICP. As such this approach may be used to directly increase the sensitivity and likely improve LODs compared to those that are already obtained with TSP SI. Ultimately improvements in LODs of perhaps a factor of 100 compared to pneumatic SI are achievable. Most recently a new high flow vaporizer was designed in this laboratory that allows thermospray generation and optimization at flow rates of up to 9 ml min-'. This new design also shows similar trends as presented in this work and further optimization is continu- ing.Efforts devoted to optimizing the vaporizers (e.g. capillary i.d. capillary length) and desolvation apparatus for this system for best LODs and toward the development of more chemically inert versions comparable to our FSApT systems are also in progress. Furthermore vaporization of these higher liquid flows will also allow the development of pneumatic nebulizers which employ high flows of solvent vapour as the nebulizing gas an approach we call thermojetspray. For example complete vapo- rization of 10 ml min-' of water will result in approximately 10 1 min-' of water vapour. Operation of a pneumatic nebulizer with such a gas flow would provide much higher gas-to-liquid flow ratios than can be obtained with conventional pneumatic nebulizers for ICP-AES.The reason that this ratio and the energy input to aerosol formation can be increased is that with the thermojetspray approach the nebulizing gas is con- densable and can be removed thus avoiding the ICP-imposed gas input limitation. Ideally improved aerosol characteristics and analytical performance would result. Further this approach may allow the use of thermospray-like devices with samples such as slurries that allows self-continuous aspiration and would not require the high sample consumption of the high flow rate approach. The financial support of Chemical Waste Management Inc. and the Hazardous Waste Research and Information Center a Division of the Illinois Department of Energy and Natural Resources (Grant #HWR 92097) and the loan of equipment by Leeman Labs during the completion of this review are greatly appreciated.Assistance from Vestec Inc. in the form of equipment loans and valuable discussions (especially with Marvin Vestal Cal Blakley and John Wilkes) is also greatly appreciated. 1 2 3 4 5 6 7 8 9 10 References Browner R. F. in Inductively Coupled Plasma Emission Spectrometry ed. Boumans P. W. J. M. Wiley New York NY 1987 vol. 2 ch. 8. Gustavsson A. G. T. in Inductively Coupled Plasmas in Analytical Atomic Spectrometry eds. Montaser A. Golightly W. VCH New York 2nd edn. 1992 ch. 15. Koropchak J. A. Spectroscopy 1993. 8 20. Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. Koropchak J. A. and Veber M. Crit. Rev. Anal. Chem. 1992 23 113. Jakubowski N. Feldman I. Stuewer D. and Berndt H. Spectrochim. Acta Part B 1992 47 119. French J. B. Etkin B. and Jong R. Anal. Chem. 1994 66 685. Koropchak J. A. and Winn D. H. Appl. Spectrosc. 1987,41,1311. Koropchak J. A. Aryamanya-Mugisha H. and Winn D. H. J. Anal. At. Spectrom. 1988 3 799. de Loos-Vollebregt M. T. C. Tiggelman J. J. Bank P. C. and Degraeuwe C . J. Anal. Atom. Spectrosc. 1989 4 213.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 903 11 Koropchak J. A. Veber M. and Herries J. Spectrochim. Acta Part B 1992 47 825. 12 Koropchak J. A. Veber M. Conver T. S. and Herries J. Appl. Spectrosc. 1992 46 1525. 13 Koropchak J. A. Coleman G. N. and Conver T. S. 20th Annual Conference of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) Abstract No. 661 Detroit MI USA 1993. Browner R. F. Canals A. and Hernandis V. Spectrochim. Acta Part B 1992 47 659. 14 15 Sharp B. L. J. Anal. At. Spectrom. 1988 3 613. 16 Vestal M. L. and Fergusson G. Anal. Chem. 1985 57 2373. 17 McLean M. A. Vestal M. L. Vestal C. H. Allen M. H. and Field F. A. Proceedings of the 38th American Society for Mass Spectrometry Conference Tucson AZ 1990 p. 1138. Paper 4/00764F Received February 8 1994 Accepted March 29 1994

ISSN:0267-9477    

DOI:10.1039/JA9940900899

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 905 Use of an Ultrasonic Nebulizer with Membrane Desolvation for Analysis of Volatile Solvents by Inductively Coupled Plasma Atomic Emission Spectrometry* Robert I. Botto Bayto wn Specialty Products Exxon Research and Engineering Company Bayto wn TX 77522-4255 USA Jim J. Zhu CETAC Technologies Inc. 5600 South 42nd Street Omaha NE 68107 USA The petroleum and chemical industries need a rapid and sensitive technique for the trace analysis of volatile feedstocks intermediates and products including high-purity solvents. Organic solvent loading of the inductively coupled plasma (ICP) adversely effects the application of ICP atomic emission spectrometry to analysis of volatile organic liquids when conventional sample introduction equipment is employed.The use was investigated of a membrane separator as a secondary desolvation device used with an ultrasonic nebulizer (USN) to further reduce organic vapour loading of the plasma and enable pg I-’ determinations to be performed on solvents and petroleum products boiling well below 100 “C. Two prototype membrane desolvator units and the final commercial unit were tested on a variety of materials including hexanes methanol tetrahydrofuran acetone and dichloromethane. Organic vapour removal was efficient enough to permit the analysis of these materials at ‘normal’ sample introduction rates (1-4 ml min-’) and practical operating conditions. Detection limits similar to aqueous solution USN limits were achieved. Owing to the near complete solvent matrix removal provided by the USN-membrane sample introduction system ‘universal calibration’ of the ICP should be possible at certain ICP operating conditions.Experiments were conducted with the goal of achieving a single calibration valid for a variety of volatile organic solvents. Accurate calibration was maintained for several diverse solvent types after applying corrections for solvent nebulization efficiencies relative to the calibration solvent. Keywords Ultrasonic nebulization; inductively coupled plasma atomic emission spectrometry; membrane desolvation; organic solvents; petroleum Today’s commercially available instrumentation for inductively coupled plasma atomic emission spectrometry (ICP-AES) falls short of meeting the needs of the petrochemical industry for trace analysis of volatile organic materials.Rapid sensitive techniques are needed by refineries and chemical plants for the certification of light hydrocarbon feeds for catalytic processing or steam cracking. Elements which are aggressive catalyst poisons or corrosives in the cracking furnace cannot be tolerated even at pg I-’ levels owing to the large volumes of hydrocarbons processed. Shipments of purchased feeds or outgoing products frequently require monitoring for trace contaminants. Increased attention to environmental concerns has heightened demand for trace element monitoring of plant process/waste streams. Petrochemical industry customers have set increasingly demanding specifications on the products they purchase particularly if intended for food-grade or semi- conductor applications.Analytical opportunities abound in the area of ultra-trace analysis of volatile organics by ICP-AES and inductively coupled plasma mass spectrometry (ICP-MS). Examples of potential applications include chelation solvent extraction from natural waters or industrial wastes and liquid chromatography (LC)-ICP-AES or LC-ICP-MS for trace element speciations using volatile mobile phases. A rapid reliable method for the analysis of volatile organics from petrochemical plant pro- cessing is needed for off-line and on-line applications. Ideally the organic molecular type should have no impact on the instrumental operating parameters or the accuracy of the results obtained. Thus a ‘universal calibration’ is required where a single solvent matrix is used to calibrate the instrument for accurate analysis of multiple plant streams or solvent product types.Commercial ICP-AES and ICP-MS technology has recently * Presented at the 1994 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 10-15 1994. begun to address the need for trace element analysis of volatile organic materials. Prior focus has been on heavier oils pet- roleum crudes and lubricants analysed using solvent systems of moderate to low volatility.’ The analysis of volatile organics by ICP-AES or ICP-MS presents considerable operational difficulties as the high organic vapour loading of the plasma tends to destabilize the plasma often to the point of extinc- t i ~ n . ’ ~ Emission from the organic matrix contributes to spec- tral background effects which limit the sensitivity of trace element determination^.^ Various methods have been used to limit solvent vapour load to the plasma including the use of a cooled spray chamber,’ a thermostated condenser between the spray chamber and plasma t o r ~ h ~ ~ a solvent permeable membrane interface between the sample introduction system and the ICP,8 cryogenic desolvation’ and the use of very low sample flow rates’’ or microlitre-size sample injection volumes.ll During the past two years use of an ultrasonic nebulizer (USN) has been investigated for the analysis of volatile organic solvents by ICP-AES.12 The USN makes the analysis of many volatile solvents by ICP-AES practical. The desolvator function of the USN removes the bulk of the organic vapour which would reach the plasma.The use of oxygen admixed with the outer Ar flow partially overcomes residual solvent loading effects and keeps torch parts free of destabilizing carbon deposits. Detection limits for elements in toluene and other solvents having similar volatilities are comparable with the aqueous performance of the USN. Thus the ‘ultrasonic advan- tage’ the factor of 5-10 improvement in sensitivity provided by the USN in the analysis of aqueous solutions has been extended to certain volatile organic solvent systems. The analysis of highly volatile solvents by USN-ICP-AES is still a challenge. Solvents such as hydrocarbons boiling below 90 “C methanol acetone tetrahydrofuran dichloro- methane etc. still produce sufficient vapour carrying through the USN desolvator to overload the ICP.To extend the906 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 ‘ultrasonic advantage’ further to these highly volatile solvents a second stage of desolvation is required. The use was evaluated of a tubular microporous PTFE membrane for removing solvent vapour from the USN thermally desolvated aerosol stream prior to its injection at the base of the plasma torch.13 The device is referred to in this study as a microporous membrane desolvator (MMD). The aim of this study was to investigate the use of the MMD with the USN for the analysis of highly volatile organic solvents by ICP-AES. It was hoped that practical operating conditions could be found for analysing a wide variety of materials which could not be analysed directly by ICP-AES and that detection limits would be comparable to water and less volatile organics.Finally the quest for ‘universal Cali- bration’ of the ICP-AES was pursued for volatile organic hydrocarbons oxygenates and dichloromethane. Experimental Instrumentation The 1976 Model Jarrell-Ash AtomComp 750 was used as previously described by Botto.14 Power was supplied by a 2.5 kW 27 MHz crystal-controlled generator. The USN models used were the CETAC Technologies U-5000 AT and the earlier U-5000 with the ATX-100 automatic tuning upgrade. A schematic view of the MMD is shown in Fig. 1. The 80 cm membrane tube is entirely encased within a heated Pyrex housing which is fitted with an entry and an exit port for gas flow.As the USN desolvated aerosol proceeds through the membrane residual solvent vapour diffuses through < 2 pm pores in the membrane wall. A countercurrent flow of Ar carries the vapour to a vent external to the MMD unit. Approximately 99% of the mass of solvent vapour entering the MMD is removed before the aerosol stream exits to the ICP torch. The interface of the MMD with the USN and ICP torch is shown in Fig. 2. CETAC Technologies provided two prototypes of the MMD plus the final commercial unit MCX-100 for evaluation in this study. The MCX-100 MMD is fitted with two sets of gas flow measurement devices and micrometer valves for con- trolling the flow rates of dry sweep Ar into the membrane sheath and solvent-laden Ar out. The flow rate of aerosol Ar through the USN-MMD is measured and controlled by means of a mass flow controller.The gas system for Ar-02 mixed gas plasma operation has been described.” A 11 polyethylene bottle with the cap perforated for gas entry and exit flows was added to pre-mix outer Ar and O2 prior to entering the torch. A Y connector for withdrawing incidental condensate was placed between the USN desolvator aerosol exit and the MMD aerosol entry connections. All tubing carrying liquids was PTFE other tubing was Tygon or soft rubber. A clean-out kit supplied with the MCX-100 was used to rinse the membrane free of non-volatile oil residue as necessary. A Rainin ‘Rabbit-Plus’ peristaltic pump with Viton pump tubing (0.76 mm i.d. for sample flow 2.29 mm i.d. for USN pumped drain flow) was used for hydrocarbon sample introduc- tion.A gas displacement pumping system was designed for use ~ Microporous PTFE tubing Pyrex housing W Ar with End view solvent va pour Heat cord To ICP Membrane Ar Fig. 1 Diagram of the microporous membrane desolvator (MMD) Membrane desolvator Ar ICP ‘ Drain Ar in USN ‘L-! ‘Samplein Drain Fig. 2 Ultrasonic nebulizer and desolvator fitted with microporous membrane for second stage of desolvation with solvents which attack Viton rubber peristaltic pump tubing (Fig. 3). The peristaltic pump pumps only air into a septum sealed vial containing the solvent sample. The sample flows by displacement into the USN. Reagents and Solutions The solvents and materials analysed in this and the previous study” are presented in Table 1.Spectroscopic grade solvents were used where available. Calibration reference solutions were prepared by diluting Conostan S-21 (Conoco Specialty Products) 100ppm (m/m) or M I 0 21-C (Spex Industries) 500 ppm (m/m) in the appropriate solvent to prepare solutions having 1 pg ml-I of 21 elements. Blanks consisted of the pure solvent only. Calibration standards were prepared minutes before use as some were noted to have limited stability. Calibration standards for propan-2-01 and methanol were prepared by spiking the anhydrous alcohols with multi-element aqueous calibration standards. Blanks having identical water content (10% v/v) were also prepared. Q“‘ Fig. 3 Gas displacement pumping system for solvents which attack and degrade peristaltic pump tubingJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 907 Table 1 Solvents analysed using MMD-USN-ICP-AES Organic solvent Solvents analysed using USN on1yl2- Toluene 2,2,4-Trimethylpentane (isooctane) n-Heptane Propan-2-01 Additional solvents analysed using MMD-USN- 2-PentanoneP-methyl (IBMK) Isoparaffin solvent C yclohexane Butan-2-one (MEK) Aviation fuel Tetrahydrofuran (THF) Hexanes Methanol Acetone Dichloromethane Boiling point/"C 110.6 99.3 98.4 82.4 116.8 80.7 79.6 67 65 56.2 40 98-104 <75-147 66-69 Table 2 Typical operating conditions for MMD-USN-ICP-AES R.f. power Forward Reflected Flow rates Ar outer Ar intermediate Ar aerosol Oxygen outer MMD sweep Ar in MMD Ar/solvent out Sample introduction Temperature setting USN desolvator USN chiller Membrane heater 1.2-1.8 kW 2-8 W automatic control.22 1 min-l 0-1 1 min-' 600-750 ml min - 40-60 ml min 700 ml min-l Maximum 1.2-4 ml min-' 140 "C 160 "C - 15 "C Instrumental Conditions for ICP-AES Typical operating conditions for ICP-AES analysis of volatile solvents (boiling point < 120") are shown in Table 2. The plasma observation height was 16mm above the top of the load coil for all analyses. Spectrometer integration time was 10 s per exposure at peak and background positions. A mini- mum of 2 exposures were acquired per determination. Oxygen was added to the outer Ar flow until the C emission skirting the base and outside of the plasma became invisible to the eye. A faint bullet-shaped area of C2 emission usually remained visible in the initial radiation zone of the plasma.The position of the top of this 'bullet' was 0-5mm above the top of the load coil for all the solvents analysed. Experimental Procedure After igniting the plasma and stabilizing it for a multi-solvent analytical run the first solvent to be analysed was introduced and r.f. power transmission impedance matching was per- formed manually (the automatic impedance matching circuit was not operating sufficiently well for sensitive adjustments). Operating parameters except for impedance matching were left unchanged when changing to the next solvent type. Optimum impedance matching was reestablished while allowing 3-5 min for adequate washout. The position of the C2 'bullet' was noted and analysis proceeded. Detection limits (3sb) were determined from 10 consecutive exposures of the blank following calibration.Instrumental drift was evaluated 1 h after calibration by re-analysing the high calibration refer- ence solution. Membrane efficiency was measured by operating the USN using pure water whilst directing the Ar aerosol stream from the thermal desolvator into a cold trap at - 80 "C. After 30 min the water collected was weighed. The procedure was then repeated with the MMD between the USN and the cold trap. At a membrane sweep Ar flow of 0.71min-' measured efficiencies for water ranged from 99 to 99.7%. Nebulization efficiencies for various solvents were measured by operating the USN for a particular solvent for lOmin afterward measuring the total mass of solvent introduced and the mass collected by the USN pumped drain system.Measured nebulizer efficiencies (aerosol plus vapour) for several diverse solvents ranged from 27 to 37% (Table 3). Measurement precision was approximately & 10% relative. Results and Discussion MMD Optimization Three Ar flow rates can be adjusted independently for MMD optimization the sweep Ar flow in and out of the membrane sheath and the aerosol Ar flow through the USN-MMD to the plasma. However differential pressure across the membrane interface can result in augmentation or reduction of the aerosol Ar flow uia communication with the MMD sweep Ar. Constricting the MMD out flow while maintaining the same in flow results in increased aerosol Ar flow to the plasma. Similarly a restriction downstream of the MMD (a partially fouled plasma torch injector tip for example) will augment the MMD sweep Ar flow at the expense of the aerosol Ar flow.Experiments were conducted with toluene solutions at 1500 W r.f. power and 16mm observation height above the load coil to determine the optimum MMD and aerosol Ar flow rates for MMD-USN-ICP-AES. Fixing the MMD sweep Ar in at 0.71min-' and the aerosol Ar at 0.651min-' the MMD out flow rate was adjusted from 0.451min-' to its maximum unrestricted rate of 0.77 1 min-'. The effects on net intensities of typical Boumans15 'hard' emission lines (Zn I1 206.200 nm Mn I1 257.610 nm) and 'soft' emission lines (Cr I 357.869 nm Cu I 324.754 nm) are shown in Fig. 4. The best detection limits for most elements were measured at the maximum out flow. It is supposed that augmentation of the aerosol Ar flow at restricted flow rates of the MMD Ar out flow was sufficient to cool the plasma emission zone signifi- cantly and/or decrease residence time required for optimum excitation.Another series of experiments fixed the aerosol Ar flow rate at 0.65 lmin-' and varied the MMD in flow rate Table 3 USN nebulization efficiencies for various solvents Solvent Toluene MEK IBMK Dichloromethane Hexanes THF Propan-2-01 Efficiency (YO) 31.7 36.2 30.0 33.3 35.0 30.0 27.8 . E [ I 1 c 2 0.2 0.45 0.57 0.70 0.77 Ar (out) flow rate/l min-' Fig.4 Net spectral line intensities uersus MMD Ar out flow; MMD Ar in flow fixed at 0.7 1 min-'908 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 1399 I E 100 I I (a) - I .- i I I cn ln 243 Y .s 10 cu I e Y D r 7 - zz 0.1 0.40 0.50 0.60 0.70 0.80 Ar (in) flow rate/l min-' Fig.5 Net spectral line intensities versus MMD Ar in flow; MMD Ar out at maximum unrestricted flow rate from 0.4 to 0.8 1 min-l whilst leaving the out flow unrestricted. Very little effect on net intensities was observed as shown in Fig. 5. A flow rate of 0.7 1 min-' was selected for the MMD sweep Ar in for all subsequent work. Similarly an aerosol Ar flow rate of 0.65 1 min-l was found to yield optimum sensitivit- ies for most elements at 1500 W r.f. power. Re-optimization for different solvents and r.f. power settings was performed using the aerosol Ar flow rate. MMD Performance and Benefits Fig. 6 illustrates an important benefit of the MMD for trace analysis of volatile organics. Molecular background emission from C2 obscures trace determinations of Ni using Ni I1 231.60 nm.Other trace element determinations are similarly interfered by C CN C and elevated spectral background derived from the organic matrix. The MMD reduces the matrix-derived background emission and enhances the analyte signal by reducing organic vapour loading of the plasma. Fig. 7 demonstrates that no detrimental washout effects are observed with the use of the MMD. Rinse profiles are similar with or without the MMD and the incremental time for aerosol transport through the MMD is only a few seconds. Use of the MMD with the USN for ICP-AES improves operability and sensitivity for the analysis of volatile organic materials. Fig. 8 compares 3sb detection limits for water and toluene measured using the USN but without the MMD.Note that for toluene 2.0 kW r.f. power and Ar-0 mixed gas plasma operation were required to obtain detection limits t 3 0 I J I I I 230.32 232.88 Wavelengthhm Fig. 6 (a) Molecular background emission and (b) spectra for 1 ppm Ni in toluene (231.60 nm) A with membrane and B without membrane Time scale 2 s per division Fig. 7 Comparison of rinse profiles for A USN and B MMD-USN; sample nebulization times were not equivalent in this test 100 I 0 m 5. 10 . 4- .- E .- - 8 0 - 1 Q a .- 4- n 0.1 Ag Al Ba Ca Cd Cr Cu Fe Mg Mn MoNa Ni P Pb Sn Ti V Zn Fig. 8 USN 3s detection limit comparison without MMD aqueous detection limits at 1200 W versus toluene at 2000 W assisted by O2 in outer and aerosol Ar flows12 comparable to water.' Fig.9 shows nearly identical USN detection limits obtained for water and toluene at the same r.f. power (1.2 kW) using the MMD. Despite the removal of most of the organic solvent load to the plasma by the MMD addition of a small amount of oxygen to the outer Ar flow still provides the benefit of improved detection limits acquired at lower r.f. power settings (Fig. 10). Oxygen also keeps the plasma torch free of carbon deposits which often form during plasma ignition in the presence of organic vapour. Table 4 contains data for precision and calibration drift obtained for toluene analysis with and without the MMD and 0 in the outer Ar flow. The best performance was obtained with the MMD and with 02. 100 I 0 10 Y 0 I .- .E - 1 C 0 a .- 4- + a 0.1 n 0.01 Ag Al Ba Ca Cd Cr Cu Fe MgMnMo Na Ni P Pb S n Ti V Zn Fig.9 USN 3sb detection limit comparison. Aqueous detection limits at 1200 W versus MMD-USN detection limits for toluene assisted by O2 in outer Ar flowJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 909 100 0 '0 Y 0 I -- Y .- .E - 1 s t 0 0 .- Y - 0.1 0 01 Ag Al Ba Ca Cd Cr Cu Fe Mg MnMoNa Ni P Pb Sn Ti V Zn Fig. 10 0 in outer Ar flow MMD-USN 3s detection limits measured with and without Analysis of Highly Volatile Solvents The MMD-USN sample introduction system permits the analysis of highly volatile organic solvents with excellent sensitivity using practical convenient operating conditions. Detection limits for water and 5 organic solvents three having boiling points below 70°C are shown in Fig.11 (a-e). For most elements detection limits are comparable. Thus the 'ultrasonic advantage' is extendable to highly volatile organic solvents using the MMD. All of the solvents listed in Table 1 were analysed by MMD-USN-ICP-AES using practical sample introduction rates of 1.2-4 ml min-'. Stable operation was maintained at moderate r.f. power (1.2-1.8 kW) with the assistance of a small amount of oxygen in the outer Ar flow. Changing from one solvent type to the next was accomplished in a few minutes whilst the plasma continued to operate. Problems and Peculiarities Several problems and peculiarities were noted in experiment- ation with the MMD for analysis of diverse organic solvents. It has been mentioned that gas displacement pumping was necessary for solvents (MEK THF etc.) which attack Viton peristaltic pump tubing.These solvents also attacked the Tygon tubing in the USN drain system and in the aerosol line between the USN and MMD. Replacing all Tygon tubing carrying either liquid or aerosol with PTFE or PTFE-lined Tygon tubing solved this problem. A test was made to determine whether a switch could be made between direct peristaltic pumping of a solvent which does not attack Viton and gas displacement pumping of the same solvent without the need for ICP-AES recalibration. The same piece of Viton peristaltic pump tubing was used to pump the solvent (toluene) and air for gas displacement (Fig. 3 j. The ICP-AES had been calibrated and stable for several hours. The calibration was checked again and found to be accurate just prior to making the switch.After the switch was made the calibration was found to have remained accurate (Table 5 ) . Thus direct and gas displacement pumping can be used interchangeably for a particular solvent without the need for recalibration. Two elements B and Si expected to be present in both Spex Industries M I 0 21-C and Conostan S-21 multi-element cali- bration reference solutions appeared to be absent when analys- ing the Spex reference solution. The elements are apparently present as chemical species which are volatile and condensable at 140°C in the Spex reference but not in the Conostan reference. When analysing volatile organic solvents for trace elements using the USN or MMD-USN combination it must be kept in mind that only non-volatile forms of the analytes will be determined accurately.Chemical species which are partially or completely volatile at temperatures near the tem- perature of the USN desolvator heater will be condensed and removed to a variable extent in the desolvator condenser resulting in inaccurate or completely misleading results. Volatile analyte forms must be rendered non-volatile by chemical means prior to analysis as in the analysis of tetra- ethyllead in gasoline,l2 Non-volatile oil in sample solutions will cause gradual fouling of the MMD as it deposits in the membrane pore structure. Oil fouling of the MMD produces calibration drift noticeable after several hours of analysing solutions containing 1 % non-volatile (at 160 "Cj oil. The Conostan and Spex multi- element oil reference solutions are prepared in an oil matrix which will slowly foul the MMD.The use of high dilution is recommended to minimize this problem. The membrane is easily rinsed free of oil using a solvent such as toluene and the kit supplied with the commercial unit. The MMD would not be applied easily to the analysis of petroleum crude or heavy oils using lower solvent dilution. However for these sample types excellent sensitivity can be obtained using the USN without the MMD provided that the solvent volatility is moderate to low (xylene or tetralin).16 It is advantageous to run the USN desolvator chiller at as Table 4 Comparison of precision (Is from ten consecutive 10 s exposures) and calibration drift ( I h) for the analysis of toluene by USN-ICP-AES Element A1 Ba Ca Cd Cr cu Fe Mg Mn Mo Na Ni Pb Sn Ti V Zn Ag Average 2000 W with 02 no MMD 1200W no O with MMD 1200 W with 02 with MMD RSD (Yo) 1.9 2.0 2.0 2.0 2.1 2.6 2.1 1.9 1.9 1.9 1.8 2.1 2.1 2.1 1.5 1.8 1.8 2.1 2.0 Drift (YO) + 0.9 + 1.1 + 0.4 - 4.7 - 13 + 4.2 + 3.1 - 6.4 - 5.7 - 5.8 - 7.9 + 7.1 - 12 - 12 - 4.0 - 2.2 - 4.4 - 12 5.9 RSD (Yo) 2.8 2.8 3.1 3.1 2.8 2.6 2.9 2.8 4.8 2.9 3.1 2.8 3.7 3.3 2.9 2.9 2.7 2.9 3.1 Drift (YO) -2.1 - 3.5 - 2.2 - 6.8 - 6.5 - 3.6 - 4.3 - 5.3 - 7.2 - 3.9 - 7.8 0.0 -11 - 8.3 - 13 - 3.8 - 3.3 - 9.5 5.7 RSD (Yo) 1.6 1.8 1.6 1.7 1.8 1.8 1.8 1.8 1.7 1.7 1.9 1.7 2.7 1.7 2.3 1.6 1.8 2.0 1.8 Drift (YO) - 1.9 - 3.7 + 3.3 - 1.1 -4.5 - 3.8 - 3.3 -4.1 - 3.5 - 3.2 - 3.9 - 3.2 - 6.3 + 1.4 - 5.8 - 2.7 - 2.9 - 4.6 3.5910 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 I Al308.22 Ca 315.89 Cd 21 4.44 r r Fe 259.94 Cu 324.75 Mn 257.61 Ag 338.29 50 7 n r Ba 455.40 Ma 279.55 5 0.5 Cr 357.89 Mo 202.03 Na 589.99 1 0.1 0.01 n n V 292.41 Zn 206.20 Ti 334.94 Toluene ,200 p;*-? rn H20 ;%:&$&4 .... /. . . . . .,. Hexane 1600 W lsooctane 1750 W CH2C12 1750 W T'I THF 1500 W MMD-USN 3sb detection limits for various solvents and conditions Fig. 11 Table 5 Equivalence of direct and displacement pumping; concentrations in toluene (pg ml-') Element A1 B Ba Ca Cd Cr c u Fe Mn Mo Na Ni P Pb Si Sn Ti V Zn Ag Mg Direct pumping 12/7/1993 2:13 pm 1.003 1.004 1.004 1.002 0.996 0.993 1.007 1.006 0.997 0.999 0.997 0.998 0.990 1 .000 1.095 0.993 1.000 0.998 1.002 0.998 0.993 Displacement pumping 12/7/1993 7:27 pm 0.991 1.009 0.999 0.999 0.994 0.970 1.010 1.003 0.996 0.997 0.993 1.001 0.994 0.977 1.001 0.959 1.040 0.957 1.014 0.999 0.978 Change (YO) - 1.20 + 0.50 - 0.50 -0.30 - 0.20 - 2.32 + 0.30 - 0.30 -0.10 - 0.20 - 0.40 + 0.30 + 0.40 -2.30 -8.58 - 3.42 + 4.00 -4.11 + 1.20 +0.10 - 1.51 Mean (absolute value) Mean (net) 1.54 - 0.89JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY7 SEPTEMBER 1994 VOL.9 91 1 Table 6 Analysis of organic solvents containing 1 pg ml-' of each element; 1800 W power calibrated using toluene (1 pg ml-' standard and blank) Element Ag A1 Ba Ca Cd Cr c u Fe Mg Mn Mo Na Ni P Pb Sn Ti v Zn Mean Yo RSD Nebulization efficiency measured Matrix effect Density/g ml-' Boiling-pointPC Hexane 1.30 1.38 1.40 1.02 0.92 1.59 1.45 1.12 1.12 1.13 1.10 1.48 1 .00 1.22 0.89 0.89 1.25 1.18 0.89 1.175 18.3 1.104 Moderate 0.6700 69 Isooctane 0.97 1.02 1.02 0.86 0.8 1 1.12 1.05 0.9 1 0.9 1 0.92 0.90 1.09 0.85 0.95 0.80 0.80 0.97 0.93 0.80 0.93 1 10.6 - Slight 0.6919 99.2 Heptane 1.17 1.25 1.26 1.07 1.01 1.37 1.29 1.12 1.13 1.13 1.1 1 1.31 1.05 1.21 0.99 1 .oo 1.20 1.15 0.99 1.148 10.0 - Slight 0.6837 98.4 MEK 1.13 1.18 1.16 1.25 1.26 1.20 1.17 1.23 1.24 1.22 1.23 1.30 1.24 1.20 1.25 1.23 1.21 1.22 1.29 1.221 3.5 1.142 None 0.8054 79.6 THF 0.97 0.97 0.96 0.95 0.94 1 .oo 0.96 0.96 0.95 0.95 0.96 0.99 0.95 0.94 0.92 0.9 1 0.96 0.96 0.96 0.956 2.2 0.946 None 0.8892 67 IBMK 0.87 0.89 0.86 0.93 0.93 0.87 0.87 0.91 0.91 0.90 0.91 0.87 0.91 0.88 0.92 0.90 0.90 0.90 0.94 0.898 2.6 0.946 None 0.7978 116.8 CH,Cl 0.64 0.70 0.70 0.62 0.60 0.76 0.71 0.64 0.65 0.65 0.64 0.76 0.62 0.57 0.58 0.58 0.68 0.66 0.59 0.650 8.7 (1.05)* Slight 1.3266 40 Propan-2-01 0.82 0.80 0.94 0.95 0.71 0.88 1.09 0.88 0.87 0.87 - - - - - - - 0.83 0.93 0.881 10.6 0.877 - 0.7855 82.4 *Accurate measurement was difficult owing to high volatility.Table 7 Analysis of organic solvents (1 pg ml-') corrected for meas- ured nebulizer efficiencies relative to toluene Element Ag A1 Ba Ca Cd Cr c u Fe Mn Mo Na Ni P Pb Sn Ti V Zn Mean Mg Hexane 1.18 1.25 1.27 0.92 0.83 1.44 1.31 1.01 1.01 1.02 1 .oo 1.34 0.9 1 1.10 0.81 0.81 1.13 1.07 0.81 1.064 MEK 0.99 1.03 1.02 1.09 1.10 1.05 1.02 1.08 1.09 1.07 1.08 1.13 1.09 1.05 1.09 1.08 1.06 1.07 1.13 1.069 THF 1.02 1.02 1.01 1 .oo 0.99 1.06 1.01 1.01 1 .oo 1 .00 1.01 1.05 1 .oo 0.99 0.97 0.96 1.01 1.01 1.01 1.007 IBMK 0.92 0.94 0.91 0.98 0.99 0.92 0.92 0.96 0.96 0.95 0.96 0.92 0.96 0.93 0.97 0.95 0.95 0.95 0.99 0.949 Propan-2-01 0.94 0.9 1 1.07 1.08 0.81 1 .oo 1.24 1 .oo 0.99 0.99 - - - - - - - 0.95 1.06 1.003 cold a temperature as possible to maximize the efficiency of desolvation when analysing highly volatile organics using the MMD-USN.The lowest temperature achieved by us was -15°C. This temperature is cold enough to freeze water vapour and certain volatile symmetrical organics such as cyclohexane and benzene. If such are present the USN chiller should be operated as cold as possible without forming ice. The USN becomes plugged within seconds upon aspirating cyclohexane or pure water with the chiller on the maximum cold setting! It is possible to switch from organic to aqueous operation with the chiller temperature below 0 "C.However alcohol must be added to the water solutions as 'antifreeze'. Universal Calibration The concept of a single analytical calibration valid for ICP- AES analysis of multiple aqueous and organic solvent systems was suggested by the observed consistency of calibration factors for elements at 1 pg ml-I concentration in widely different solvent matrices. This is the concept we refer to as 'universal calibration'. There are three requirements for accu- rate universal calibration of the ICP (i) volumetric sample flow to the nebulizer must be constant regardless of sample matrix; (ii) significant matrix effects due to residual solvent loading of the plasma must be absent; and (iii) nebulization efficiencies of the various solvent systems relative to the calibration solvent system must be known or measured.The first requirement can be met by using gas displacement pumping to avoid the volumetric flow changes caused by solvent-softened Viton pump tubing. The high efficiency of solvent matrix removal provided by the MMD-USN combi- nation affords the opportunity to introduce samples to the ICP-AES with matrix components virtually absent. Near com- plete removal of the solvent matrix should provide the means to satisfy the second requirement for universal calibration. Nebulizer efficiencies can be measured although these measurements are difficult to make precisely and are subject to systematic errors. The universal calibration concept was tested by analysing a series of organic solvents containing 1 pg ml-' of 19 elements.The MMD-USN-ICP-AES had been calibrated with toluene solutions. Several initial trials were made to attempt to find ICP-AES operating conditions that adequately mask the effects of residual solvent loading for the most volatile solvents of interest. It was found that higher power and the use of O2 in the outer Ar flow served to minimize matrix effects. Table 6 contains the results of a demonstration run made at 1800 W r.f. power. Observed concentrations clustered about mean values that are significantly higher or lower than the toluene calibration value of 1.00 pg m1-l. Tighter clusters of results (lower values of the relative standard deviation RSD)912 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 can be interpreted to mean little or no matrix influence. Results from the mixed hexanes exhibited the largest amount of matrix interference a plasma cooling effect inferred by the difference between results for ‘hard’ and ‘soft’ line elements (Fig. 4 for comparison). The bias in the means relative to 1.00 is presumed to reflect differences in nebulization efficiencies relative to toluene. Measured nebulization efficiencies relative to toluene are included in Table 6 for all but two of the solvents. The nebulization efficiency value for dichloromethane is presumed to be inaccurate owing to the large portion of mass being transported as vapour. For convenience nebulization efficiencies are given as measured in Table 3.Table 7 shows the results of the analysis of five organic solvents versus toluene calibration with correction made for measured nebulization efficiencies relative to toluene. The means are now close to 1.00 pg ml-’ though still not within the precision of the measurements for three of the solvents. The results for IPA are based on the use of a multi-element aqueous calibration reference and certain of these may reflect bias relative to the organic reference solutions. The determi- nation of nebulization efficiencies has an associated relative precision of f 10%. Therefore the results in Table 7 are consistent with accurate universal calibration of the ICP-AES except where they reflect some residual matrix influence (cf.’ hexanes). Conclusions Use of the MMD with USN-ICP-AES extends the capabilities of the technique for trace element analysis of volatile organic solvents and materials. Highly volatile solvents having boiling points well below 100” can now be analysed with excellent limits of detection obtained.The ICP-AES spectra have reduced molecular/background emission and sensitivities are enhanced. A variety of organic solvents may be analysed with detection limits comparable to USN aqueous detection limits. Thus the ‘ultrasonic advantage’ is extendable to highly volatile organic materials. Future work is needed to extend the universal calibration concept to aqueous and water soluble organic systems. Further work is also needed to explore/extend the applications of MMD-USN-ICP-AES to the analysis of petroleum fuels intermediates and solvent products. The authors express their appreciation to Clifton C. (Skip) Carter for technical assistance and to Gautam N. Shah for encouragement. Mary C . Benham and Kathy Botto expertly prepared the manuscript. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 References Brown R. J. Spectrochim Acta Part B 1983 38 283. Kreuning G. and Maessen F. J. M. J. Spectrochim Acta Part B 1987 42 677. Kreuning G. and Maessen F. J. M. J. Spectrochim Acta Part B 1989,44 367. Tang Y. Q. Du Y. P. Shao J. C. Liu C. Tao W. and Zhu M. H. Spectrochim Acta Part B 1992 47 1353. Hausler D. W. and Taylor L. T. Anal. Chem. 1981 53 1223. Maessen F. J. M. J. Seeverens P. J. H. and Kreuning G. Spectrochim Acta Part B 1984 39 1171. Hill S. J. Hartley J. and Ebdon L. J. Anal. At. Spectrom. 1992 7 23. Backstrom K. and Gustavsson A. Spectrochim Acta Part B 1989 44 1041. Wiederin D. R. Houk R. S. Winge R. K. and D’Silva A. P. Anal. Chem. 1990 62 1155. Nygaard D. D. and Sotera J. J. Appl. Spectrosc. 1987 41 703. Avery T. W. Chakrabarty C. and Thompson J. J. Appl. Spectrosc. 1990 44 1690. Botto R. I. J. Anal. At. Spectrom. 1993 8 51. Zhu J. J. paper presented at PITTCON 93 Abstract 1266 Atlanta GA USA March 1993. Botto R. I. Talanta 1990 37 157. Boumans P. W. J. M. ZCP Znf. Newsl. 1978 4 89. Botto R. I. Spectrochim Acta Part B 1987 42 181. Paper 4/01 41 5D Received March 9 1994 Accepted May 20 1994

ISSN:0267-9477    

DOI:10.1039/JA9940900905

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 913 Mechanism of Volatilization of Tungsten in the Graphite Furnace Investigated by Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry* John P. Byrne Department of Chemistry University of Technology Sydney P.O. Box 123 Broadway New South Wales 2007 Australia Dianne M. Hughes and Chuni L. Chakrabarti Centre for Analytical and Environmental Chemistry Department of Chemistry Carleton University Ottawa Ontario Canada KIS 566 D. Conrad Gregoiret Geological Survey of Canada 601 Booth Street Ottawa Ontario Canada KIA OE8 The mechanism of volatilization of tungsten from a graphite substrate has been investigated using electrother- mal vaporization inductively coupled plasma mass spectrometry (Em-ICP-MS).Vaporization temperatures in the range 800-2700 "C were studied. In this temperature region two distinct vaporization processes occur resulting in two separate ETV-ICP-MS peaks for tungsten. The earlier peak appears at temperatures as low as 850 "C and is attributed to the volatilization of tungsten oxide. At temperatures above 2500 "C a second peak appears when tungsten carbide is vaporized from the graphite surface. Results show that NaCl and NaF chemical modifiers are ineffective in preventing the formation of tungsten carbide. The signal-to- background ratio (S/N) for tungsten varies with vaporization temperature with the optimum S/N and minimum limit of detection (0.51 pg) being obtained at vaporization temperatures around 11 00 "C. Keywords Electrothermal vaporization; inductively coupled plasma mass spectrometry; graphite furnace; tungsten; carbide formation; halide chemical modifiers Electrothermal vaporization (ETV) provides an alternative means of sample introduction for inductively coupled plasma mass spectrometry (ICP-MS).This technique has the advan- tage of small sample size and the possibility of removal of matrix interferences by thermal pre-treatment of the sample prior to analyte vaporization. If these advantages are to be fully exploited a clearer understanding of sample volatilization processes and sample transport to the plasma is necessary. When graphite substrate electrothermal vaporizers are employed the extensive literature from electrothermal atomic absorption spectrometry (ETAAS) provides some useful insight into sample vaporization mechanisms and optimum operating conditions for the ETV.However it should be emphasized that unlike ETAAS where efficient formation of atomic species is paramount in ETV-ICP-MS efficient vaporization of the sample and transport to the plasma is necessary.' The form of the analyte species either molecular or atomic is of less importance. Therefore some of the problems associated with poor atomization efficiencies in ETAAS may not be directly applicable to ETV-ICP-MS. Two of the major causes of reduced sensitivity in ETAAS are (i) loss of volatile molecular species from the furnace prior to atomization and (ii) the formation of refractory oxides and carbides. The former is not a problem in ETV-ICP-MS because any molecular analyte species vaporized from the furnace are transported to the plasma dissociated ionized and detected by the mass spectrometer.This point was illustrated by an ETV- ICP-MS investigation of the vaporization and atomization mechanisms for boron.2 These results showed that the optimum sensitivity for the determination of boron by ETV-ICP-MS is obtained at vaporization temperatures of around 1800 "C. This temperature which is well below the normal atomization tem- perature for boron in ETAAS is high enough to vaporize boron * Presented at the 1994 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 10-15 1994. To whom all correspondence should be addressed. GSC publications No. 14394. as HB02 and BO molecules which are transported to the plasma dissociated and readily detected by the mass spec- trometer.However the problem of carbide formation and the extent to which experience of carbide formation in ETAAS is directly applicable to ETV-ICP-MS has not been extensively studied. For example tungsten which is known to be particu- larly prone to carbide formation is reported3 as not being determinable by ETAAS yet it has been determined both by ETV-ICP-MS and by ICP-AES using sample introduction from a graphite ETV. Park and Hall4 have measured tungsten in geological samples by ETV-ICP-MS using a graphite strip vaporizer and report limits of detection (LODs) of around 5 pg. Kirkbright and Snook5 and Matousek et aL6 report LODs for tungsten of around 0.06 ng for ICP atomic emission spec- trometry (AES) with sample introduction from a graphite electrothermal vaporizer.In the case of ICP-AES LODs for tungsten are significantly improved (x 100) when gaseous halide chemical modifiers such as Freon' and chlorine6 are added to the ETV argon carrier gas. Similar sensitivity enhancements have also been reported7 when 0.25 mol 1-1 NaF was used as a chemical modifier in the determination of Ca Sr Zr and A1 by ICP-AES using a graphite cup direct sample insertion device. These enhancements were attributed to the prevention of carbide formation by the NaF and the preferential volatilization of the analyte in the form of fluorides. It is the purpose of the present study to use ETV-ICP-MS to investigate the mechanism of vaporization of tungsten from a heated graphite substrate and to assess the extent of the problem of carbide formation.The effectiveness of halide solution chemical modifiers in the prevention of carbide forma- tion is also studied. Experimental A Perkin-Elmer SCIEX Elan 5000 ICP mass spectrometer equipped with an HGA-600MS electrothermal vaporizer and a Model AS-60 autosampler was used. Tube wall vaporization was used for all experiments using pyrolitic graphite coated914 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 graphite tubes (Perkin-Elmer part No. 091504). A poly(tetra- fluoroethylene) (PTFE) tube of 80 cm in length and 6 mm i.d. was used to connect the HGA-600MS to the plasma torch. The experimental conditions for both the Elan 5000 and HGA-600MS are given in Table 1. Optimization of plasma and mass spectrometer conditions was accomplished using solution nebulization sample introduction.The operation of the HGA-600MS was completely computer controlled. During the dry and pyrolysis steps of the temperature programme opposing flows of argon gas (300 ml min-') originating from both ends of the graphite tube removed water and other vapours through the dosing hole of the graphite tube. During the high-temperature or vaporization step a graphite probe was pneumatically activated to seal the dosing hole. Once the graphite tube was sealed a valve located at one end of the HGA workhead directed the carrier argon gas flow originating from the far end of the graphite tube directly to the argon plasma at a flow rate of 750 ml min-'. Standards and Reagents High-purity argon gas (99.995% Matheson Gas Products Ottawa Ontario Canada) was used.Solutions were prepared with ultra-pure water obtained from a Milli-Q 2 water purifi- cation system (Millipore Mississauga Ontario Canada). The nitric acid used was analytical grade (Ultrex J. T. Baker Canada Toronto Ontario Canada). The W stock solution (10 ppm) was supplied by Spex Industries Edison NJ USA. For the determination of LODs a 5 ppb W solution was prepared from the stock solution and for all other experiments a 25 ppb W solution was used. The NaCl(O.1 pg p1-') modifier was prepared by dissolving NaCl(99.9% pure Merck AnalaR grade Toronto Ontario Canada) in ultrapure water. Results and Discussion A preliminary experiment was conducted in order to determine the optimum flow rate for the ETV argon carrier gas and to ascertain whether any molecular species such as tungsten oxide or carbide could be detected at various plasma sampling depths.In these experiments 250 pg of W in a 0.01% HN03 solution were vaporized at a temperature of 2700°C. Fig. 1 shows that for ETV-ICP-MS the maximum sensitivity for W is obtained with a nebulizer gas-flow rate of 750 ml min-'. This compares with an optimum flow rate of 1000 ml min-' for solution nebulization. At an r.f. power of 1000 W no Table 1 Instrumental operating conditions ICP mass spectrometer R.f. power/W Coolant Ar flow rate/l min-l Intermediate Ar flow rate/l min-' Carrier Ar flow rate/ml min-' Sampler cone Skimmer cone HGA-600MS electrothermal vaporizer Sample volume/pl Internal Ar flow rate Dry step (10 s ramp) Pyrolysis step Vaporization step Clean-up step (dry and pylolysis steps)/ml min-' Data acquisition Dwell time/ms Scan mode Pnintsjspectral peak Signal measurement Resolution 1000 15.0 900 750 Nickel Platinum 20 300 120°C for 40 s Variable for 10 s Variable for 6 s 2750°C for 5 s 10 Peak hopping 1 Area 0.65 u at 10% maximum lo i l o 8 - I (0 v) + 5 6 - 0 C 0 .- "0 4 - r z m C 0) m .- 2 - 500 600 700 800 900 1000 1100 1200 Flow rate/ml min - ' Fig.1 Variation of ICP-MS signal intensity for lS4Wt as a function of nebulizer flow rate for solution nebulization (SN) signal scale counts s- '; and electrothermal vaporization (ETV) signal scale inte- grated counts significant amount of WOf or WC+ was detected by the mass spectrometer.In all subsequent ETV-ICP-MS experiments the optimum flow rate of 750 ml min-' was used. Vaporization Mechanism for Tungsten Fig. 2 shows the ETV-ICP-MS vaporization curve for W i.e. the variation of integrated signal intensity for ls4W+ as a function of vaporization temperature. In order to interpret the vaporization process for W this curve should be considered in conjunction with the series of ETV-ICP-MS signals shown in Fig. 3. The vaporization curve of Fig. 2 shows that W begins to volatilize from the graphite surface at temperatures around 800°C. The amount of W vaporized increases with increasing temperature up to 1000°C. Since the melting- and boiling- points of W metal are extremely high (m.p. 3410°C b.p. 5660"C),' atomic W could not be volatilized at these low temperatures.However tungsten oxide W03 which would be expected to form when a tungstate solution is dried and heated,' is much more volatile. It has a vapour pressure of 9.1 Torr (1 Torr = 133.322 Pa) at 980 OCl0 and is reported to sublime at around 1100 "C.' Therefore the single peak observed 12 10 0) 3 0 C .- O 6 c c 8 z -. - m C m .P 4 2 0 700 1000 1300 1600 1900 2200 2500 2800 Temperature/"C Fig. 2 ETV-ICP-MS vaporization curve for 250 pg of WJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 915 ( C) 2300' C L - 60 50 40 30 20 I w 5 10 0 c o 2 60 L 2 50 m (A 40 30 20 10 .- t .- 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 Ti m e/s Fig. 3 ETV-ICP-MS signals for 250 pg of W at various vaporization temperatures (a) 1000; (b) 1300; (c) 2300; and (d) 2700°C at a vaporization temperature of 1000°C [Fig.3(u)] could be attributed to the volatilization of WO,. As the vaporization temperature is raised above lOOO"C the ETV-ICP-MS signal intensity rather unexpectedly begins to decrease and then levels off at around 20000 counts when the vaporization temperature reaches 1400 "C (Fig. 2). This decrease can be explained by the onset of tungsten carbide formation which is reported" to commence at about 850 "C and be complete at 1410 "C. As the temperature of the graphite surface rises above 1000°C any residual WO which has not been vaporized will be converted into refractory WC which should not be then released until much higher vaporization temperatures. The amount of WO converted into WC remains approximately constant for vaporization temperatures between 1400 and 2200°C.This is reflected by the flat portion of the vaporization curve in this temperature region and by the fact that the ETV-ICP-MS signal consists of a single peak through- out this temperature range. The peak shape at 1300"C shown in Fig. 3(b) is typical for this temperature range. At a temperature around 2300°C the vaporization curve of Fig. 2 begins to rise as more W is released from the graphite surface. This coincides with the appearance of a small shoulder on the trailing edge of the ETV-ICP-MS signal at 2300°C [Fig. 3(c)]. As the temperature is further increased to 2700°C the vaporization curve rises steeply and a large second peak at between 2 and 8 s now appears in the ETV-ICP-MS signal pulse [Fig.3(d)]. This second peak can be explained by the volatilization of WC at temperatures above 2500 "C. According to Sykes," WC melts to form a eutectic with W2C at 2525 "C and then decomposes at 2600°C to form a W-rich liquid and solid carbon. Thus when W is vaporized from a graphite substrate there are two distinct vaporization processes. At low temperatures WO is volatilized to give a single peak in ETV- ICP-MS; at temperatures above about 2500°C this signal becomes double peaked as WC is released at the higher temperature. Associated with this second and much larger carbide peak is the problem of memory effect. Fig. 3(d) shows a typical blank signal from a re-firing at a temperature of 2700°C. For this blank firing there is no memory effect associated with the early WO peak at between 1 and 2 s but the memory effect from the larger carbide peak between 2 and 8 s amounts to about one third of the signal for 250 pg of W.Double peaks and memory effects pose practical problems for the determination of W by ETV-ICP-MS. Possible strategies to overcome these problems include (i) prevention of formation of carbide by the use of modifiers; and (ii) use of lower vaporization temperatures which release only WO giving a single but smaller peak without memory effects. These two strategies are explored in the remainder of this paper. Effect of Halide Solution Modifiers Use of halide solutions as chemical modifiers have been reported to enhance the sensitivity of refractory elements in ICP-AES. Karanassios et uL7 found that the LODs for a number of elements including Ca Sr and Zr were improved when 0.25 moll-' NaF was added to a graphite cup direct sample insertion device for ICP-AES.These enhancements were attributed to the prevention of carbide formation and the preferential volatilization of these elements as halides. Similar results were reported by Ng and Caruso12 for Zr V and Cr using 7% NH4C1 solution as a chemical modifier. However Park and Hall4 found that for ETV-ICP-MS NH4C1 solution gave no significant enhancement for W and that memory effects still persisted with high-temperature vaporization. In order to evaluate the efficiency of halide solution modifiers in preventing the formation of tungsten carbide 250 pg of W were vaporized in the presence of two different modifiers NaCl and NaF.The amount of modifier used was 0.2 pg and a high vaporization temperature (2700 "C) was chosen. At this tem- perature the formation and release of carbide should be evidenced by the appearance in the ETV-ICP-MS signal of the second peak at between 2 and 8s. The ETV-ICP-MS signals in Fig. 4 show that neither NaCl nor NaF are effective in preventing the formation of tungsten carbide. The large carbide peak persists in the presence of these modifiers and is in fact slightly enhanced as is the earlier peak at between 1 and 2 s. This enhancement can probably be attributed to the 'carrier effect' r e p ~ r t e d ' ~ . ' ~ for a number of elements in ETV- ICP-MS where the addition of halide modifiers improves the transport efficiency of the analyte between the ETV and the plasma.The effect of one of these modifiers NaC1 was investigated for a range of lower vaporization temperatures. Fig. 5 shows that for vaporization temperatures between 1300 and 2300 "C NaCl modifier gives only very marginal enhancements for the ETV-ICP-MS signal for W. The maximum enhancement (about a factor of 2) occurs at a vaporization temperature of 1100 "C. This enhancement factor is comparable with that reportedI3 for a number of elements when similar amounts of 70 I 60 r lv) 50 $ 40 v) w C 0 .- P 30 1- m C m ;7j 20 10 0 2 4 6 8 10 12 Time/s Fig. 4 Effect of halide modifiers on the ETV-ICP-MS signal for W at a vaporization temperature of 2700°C A 250 pg of W; B 250 pg of W + 0.2 pg of NaC1; and C 250 pg of W + 0.2 pg of NaF916 50 45 40 v) 4- 3 35 0 C .0 30 z 1 2 25 01 v) ._ 20 15 10 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 1000 1100 1300 1500 1700 2000 2300 Tern pe ra t u re/"C Fig.5 various vaporization temperatures Effect of NaCl modifier on the integrated signal for W at NaCl modifier were used. Again these enhancements are typical of those for carrier effects in ETV-ICP-MS and cannot be attributed to the suppression of carbide formation. A question that then arises is why the large sensitivity enhancements observed for halide solution modifiers with ICP- AES using sample introduction from heated graphite surfaces does not occur with ETV-ICP-MS. A possible explanation is that the enhancement effect is dependent on the mass of analyte used. Ng and Caruso12 have found that when U was vaporized from a heated graphite cup and determined by ICP-AES the enhancement produced by 7% m/v NH,Cl solution was depen- dent on the mass of U analyte used.The NH4Cl modifier gave about a 10-fold enhancement of emission intensity when a 5 pl sample of 10 ppm of U was used. However when the concen- tration of U was reduced to 100 ppb there was no significant enhancement. In the present ETV-ICP-MS experiments where no significant enhancement by chloride modifier is observed the concentration of W is even lower (25ppb). The same argument might apply to the results of Karnassios et al.7 where 0.25moll-' NaF solution is reported to produce large enhancements for ICP-AES analysis when a graphite cup direct sample insertion device is employed.With this direct insertion device the amount of analyte used was 250ng a 1000-fold greater amount than the 250pg of W used in the present ETV-ICP-MS experiments. For whatever reason it seems that experience with halide solution modifiers in ETV- ICP-AES may not be directly applicable to ETV-ICP-MS where the amount of analyte vaporized from the graphite surface is far smaller. However it is possible that the use of gaseous halide modifiers such as Freon5 or chlorine,6 which have been applied successfully in ETV-ICP-AES may be more effective. This point is yet to be investigated in detail. Optimu 7 Vaporization Conditions for Tungsten Fig. 3(d) .,lustrates the two problems likely to be encountered if high vaporization temperatures are used for the determi- nation of W by ETV-ICP-MS analysis i.e.the occurrence of double peaks and a large memory effect. Both of these problems can be avoided if lower vaporization temperatures are used. At these lower temperatures only tungsten oxide is volatilized giving rise to a single peak which returns to background after about 3 s. The second carbide peak and its concomitant memory effect does not occur at this temperature [see Fig. 3(a) and (b)]. However this strategy results in a lower sensitivity for W since the integrated signal count for tungsten at lower temperatures is less than at 2700°C (Fig. 2). An important 80 1 \ I 60 - 50 40 - - 30 - 20 - v 800 120 1600 2000 2400 2800 Temperature/' C Fig. 6 Signal-to-background ratio for W as a function of vaporization temperature for A 250 pg W + no modifier; and B 250 pg W +0.2 pg NaCl modifier consideration though is the ratio of the W signal-to- background (S/B) blank at various vaporization temperatures.(n Fig. 6 S/B for 250 pg of W as a function of vaporization temperature is plotted in the range 1000-2700°C. The same function is plotted for 250 pg of W in the presence of 0.2 pg of 'NaC1 modifier. These results show that much higher S/B values are obtained at lower vaporization temperatures. In the absence of a modifier this ratio has a maximum value of about 90 at a vaporization temperature of 1000°C. At 2700°C there is a considerable carbide memory effect and the S/B is reduced to around 3 1. In the presence of NaCl modifier the maximum S/B for W still occurs at a low vaporization temperature (llOO°C) but it should be noted that the addition of this modifier has an adverse effect on the S/B at all vaporization temperatures.Since the S/B values for W show considerable variation with temperature the LOD should also show some variation with vaporization temperature. Table2 gives the LOD for W at three different vaporization temperatures. These results show that better LODs are obtained at low vaporization tempera- tures when only W03 is volatilized. The LOD at 2700°C is about a factor of 6 higher. At this high temperature tungsten carbide is also volatilized [see Fig. 3(d)] giving rise to two peaks and much higher blank values from the carbide memory $effect. As a result of these higher and less reproduceable blank values the LOD is degraded at higher vaporization tempera- tures.Hence to achieve optimum LODs low vaporization temperatures should be used; however this may not always be a practical option for samples which contain a high concen- tration of matrix and which require thermal pre-treatment for matrix removal. For example in the determination of W by ETV-ICP-MS in geological materials Park and Hall4 used a 1700 "C ashing step in order to remove the high concentration of sodium matrix introduced during the sample digestion procedure. This was followed by a high-temperature vaporiz- ation step at 3000°C. This procedure resulted in LODs of Table 2 Effect of vaporization temperature on LOD Temperature/"C 1000 1100 2700 Absolute LOD*/pg 0.68 0.51 3.0 _ _ _ _ _ ~ ~ ____ * 3 x standard deviation of blank (n = 5); 10 p1 sampleJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 917 l 2 c Pyrolysis 1500°C Vapor i za t i o n 1 2700'C Y 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Time/s Fig. 7 Temporal behaviour of 184W+ during ashing at 1500 "C (< 16 s) and vaporization at 2700 "C (> 16 s) about 5 pg i.e. about one order of magnitude higher than has been obtained using lower vaporization temperatures. Inclusion of a thermal pre-treatment step also results in premature loss of analyte during ashing. This point is illustrated in Fig. 7. In this experiment 250 pg of W were vaporized at 2700 "C following a 10 s ashing step at 1500 "C. Fig. 7 clearly shows two peaks for W. The first peak at between 4 and 7 s occurs during the ashing step whilst the second peak after 15 s occurs during the 2700°C vaporization stage.These two peaks correspond to those shown in Fig. 3(d) at a vaporization temperature of 2700°C. The first can be attributed to the loss of volatile W03 during the ashing stage whilst the second peak originates from tungsten carbide volatilized during the high-temperature vaporization process. This loss of analyte during ashing will occur if a thermal pre-treatment step at any temperature above 1OOO"C is included in the analytical procedure. Conclusions When W is volatilized from a graphite substrate two distinct vaporization processes occur resulting in two separate peaks in ETV-ICP-MS. At temperatures below 1100 "C WO begins to volatilize giving rise to the earlier peak. The intensity of this peak decreases at vaporization temperatures above 1100 "C as carbide formation occurs.The tungsten carbide formed is then vaporized at temperatures above 2500 "C giving rise to a second and more intense peak in ETV-ICP-MS. Associated with this carbide peak are the problems of memory effects and consequent high blank values. Despite these prob- lems W can be determined effectively by ETV-ICP-MS using a graphite electrothermal vaporizer if the vaporization con- ditions are chosen carefully. The optimum S/B optimum LODs and minimum memory effects are obtained if low vaporization temperatures can be employed. Under these conditions the higher temperature carbide peak does not appear and the memory effects and high blank values resulting from carbide release can be avoided.This study also has implications important to the determi- nation of W by ETAAS. The results obtained show that WC is in fact volatilized at 2700"C yet Slavin3 reported that W cannot be determined by ETAAS. This suggests that any WC that is vaporized is not subsequently dissociated to form W atoms. The high WC bond strength and high vapour pressure of C(g) at this temperature could combine to force the WC(g)$W(g) + C(g) equilibrium toward the left thus effec- tively preventing the dissociation of WC(g) to produce W(g). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 References Gregoire D. C. Lamoureux M. Chakrabarti C. L. Al-Maalawi S. and Byrne J. P. J. Anal. At. Spectrom. 1992,7,579. Byrne J . P. Gregoire D. C. Goltz D. M. and Chakrabarti C. L. Spectrochim. Acta Part B 1994 49 433. Slavin W. Graphite Furnace AAS A Source Book Perkin-Elmer Norwalk CT 1984. Park C. J. and Hall G. E. M. J. Anal. At. Spectrom. 1987,2,473. Kirkbright G. F. and Snook R. D. Anal. Chem. 1979,51 1938. Matousek J. P. Satumba R. T. and Bootes R. A. Spectrochim. Acta. Part B 1989 44 1009. Karanassios V. Abdullah M. and Horlick G. Spectrochim. Acta. Part B 1990 45 119. Handbook of Chemistry and Physics ed. Weast R. C. 55th edn. CRC Press Cleveland OH 1974. Li K. C. and Wang C. Y. Tungsten ACS Monograph No. 94 3rd edn. Reinhold NY 1955. The Oxide Handbook ed. Samsonsov G. V. Plenum Press NY 1973. Sykes W. P. Trans Am. SOC. Steel Treatment 1930 18 968. Ng K. C. and Caruso J. A. Analyst 1983 108 476. Ediger R. D. and Beres S . A Spectrochim. Acta. Part B 1992 47 907. GrCgoire D. C. Al-Maawali S. and Chakrabarti C. L. Spectrochim. Acta. Part B 1992 47 1123. Paper 4/01 0321 Received February 21 1994 Accepted April 15 1994

ISSN:0267-9477    

DOI:10.1039/JA9940900913

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 919 Vaporization of Acids and Their Effect on Analyte Signal in Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry* D. Conrad Gregoire Geological Survey of Canada Department of Natural Resources Ottawa Ontario Canada KIA OE8 Douglas M. Goltz Marc M. Lamoureux and Chuni L. Chakrabarti Otta wa-Carleton Chemistry Institute Department of Chemistry Carleton University Ottawa Ontario Canada KIS 566 The vaporization properties of HCI and HN03 under various furnace heating conditions were investigated. Drying-step temperatures of 140 "C (50 s) and pyrolysis-step temperatures of 400 "C (1 0 s) were effective in volatilizing most of the chloride from 10 1.11 of 1% v/v HCI however a small amount (40 ng) of acid was retained on the graphite even after pyrolysis at 400 "C. Under the same experimental conditions HNO was completely volatilized from the graphite tube.The effect of a range of concentrations of HCI HNO H2S04 and H3P0 on analyte signals was studied for Co Cu Ag Cs Pb Bi and U. Analyte signals were enhanced by as much as a factor of two in the presence of 1% v/v HN03 and H2S04. Phosphoric acid suppressed analyte signals for Ag and Bi and the use of HCI resulted in relatively small changes in analyte sensitivity. The use of a pyrolysis step in the heating programme reduced the effects associated with acid matrices but at the expense of signal intensity. A mixed modifier-carrier reduced the matrix effects associated with H2S04 and H3P04 and essentially eliminated them for HNO and HCI.Keywords Inductively coupled plasma mass spectrometry; electrothermal vaporization; interferences; acid; chemical modifier Electrothermal vaporization (ETV) as a means of introducing a sample into a plasma has great potential as an analytical tool for ultra-trace analysis. The ETV device was developed to overcome some of the inherent difficulties of solution nebulization in inductively coupled plasma mass spectrometry (ICP-MS). For example the sample transport efficiency using ETV was reported to be about 80% or greater compared with 10% or less for solution nebulization.' The ETV device can also be used to remove the bulk of the water and possibly matrix components through the use of a pyrolysis step in the temperature programme.This makes possible ICP-MS measurements under favourable conditions which include a dry plasma free from solvent 'loading' effects associated with solution nebulization sample introduction.2 have used mass spectrometry to study processes in the graphite furnace under vacuum con- ditions. Findings from these studies were then applied to problems in electrothermal atomic absorption spectrometry (ETAAS). GrCgoire et aL7 have noted that fundamental infor- mation obtained by ETAAS can be readily applied to studies in ETV-ICP-MS. Similarly it has been shown that ETV- ICP-MS can be used for studying numerous processes in the graphite furnace such as interference^,^,^ atomization and vaporization mechanisms.8 Although some studies on matrix effects associated with ETV have been reported only a few have investigated the chemical or physical behaviour of mineral acids in an ETV device and its effects on ICP-MS analyte signals.The effect of acids on 63Cu+ signals from an ICP-MS instrument using a rhenium strip electrothermal vaporizer has been reported by Park et d9 They showed that 1% HC1 HNO HF and H,SO suppressed Cu signals compared with non-acidified Cu solutions. For the acids studied they observed peak broadening with 3% acid which was attributed to unexplained chemical effects. These workers also found that 3% HNO or HF did not reduce the integrated signal whereas 3% HC1 or H,S04 A number of * Presented at the 1994 Winter Conference on Plasma Spectro- GSC contribution 40993 chemistry San Diego CA USA January 10-15 1994.did. Some characteristics of a tungsten furnace have been investigated by Tsukahara and Kubota," who showed that increased HCl concentrations resulted in an increase in the amount of tungsten vaporized. Judicious selection of acids used in ICP-MS aids in avoiding many troublesome spectroscopic interferences. Nitric acid is frequently the acid of choice for solution nebulization ICP-MS determinations because the N+ 0' and H+ species or any polyatomic combination of these are already present in the plasma and originate from water argon and atmospheric gases entrained into the argon plasma. The background spectra of HN03 is relatively simple resulting in fewer spectroscopic interferences compared with HCl which can produce a number of chloride interferences such as the interference 35C1160 + on V (m/z 51).Sulfuric and phosphoric acids are generally avoided since a large number of S and P containing polyatomic species can occur." The success of an ETV-ICP-MS determination is dependent on the complete vaporization of analyte from the graphite tube and the efficient transport of analyte to the argon plasma. The selection of acid as well as the concentration used can affect either analyte vaporization transport or both. The retention of water or acid within the graphite vaporizer even following extended high temperature ( 100-300 "C) evapor- ation is possible and could also have an impact on analyte ETV-ICP-MS signals. This paper reports on the effect of the vaporizer temperature programme on the retention of water HCl and HNO,.Analyte signal pulses for a number of elements are reported for vaporization under compromise multi-element conditions using HCl HNO H,S04 and H3P04 acids as diluents. The use of a mixed chemical modifier to control acid effects is assessed. Experimental A Perkin-Elmer SCIEX Elan Model 5000 ICP-MS instrument equipped with an HGA-600MS electrothermal vaporizer was used. The ETV system was fitted with a Perkin-Elmer AS-60 autosampler. The experimental conditions for both the Elan920 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 5000 and the HGA-600MS are given in Table 1. Standard pyrolytic graphite coated graphite tubes were used for all experiments. Solution nebulization sample introduction was used to optimize the plasma and ion-lens settings of the mass spec- trometer.The ETV device was interfaced to the plasma with an 80 cm length of PTFE tubing (6 mm id.). The flow of argon from the HGA-600MS power supply was regulated with a pneumatic valve and operation of the HGA-600MS was com- pletely computer controlled. Under normal experimental con- ditions during the drying and pyrolysis steps of the temperature programme opposing argon gas flows (300 ml min-') from the ends of the graphite tube remove water and other vapour phase decomposition products through the dosing hole. In this way the matrix components are not introduced into the plasma. During the measurement of analyte signal pulses typically the high-temperature vaporization step a graphite probe seals the dosing hole and the flow of argon is directed from the far end of the graphite tube directly towards the plasma.For studying the temporal vaporization behaviour of HCl and HNO however the dosing hole was sealed during the entire temperature programme in order to observe the amount of ,'Cl I4Nl6O or "N produced from the vaporization of acid during each step in the temperature programme. While this method allowed observation of the temporal behaviour of these ions a quantitative determination of the acid retention after the drying or pyrolysis steps using this method could be misinterpreted. This was due to the larger than expected signals for 35Cl+ '5N'60+ and "N' which arose from the vaporiz- ation of acid that had condensed onto the cooler surfaces such as the transfer line during the drying and pyrolysis heating steps when the dosing hole was sealed. The temporal study of vaporization of HC1 and HNO was accomplished by monitoring the 35Cl+ 14N160+ and "N+ ions and the integrated ion counts were calculated using an 'in-house' Turbo Pascal program using the trapezoidal rule of numerical integration.For studying the effect of acids on analyte signals the integrated signal was reported using the Elan software provided. For all experiments 5-20 pl of sample were pipetted onto the walls of the graphite furnace. For each experiment a minimum of five runs were performed. All measurements were blank subtracted using water as a reagent blank for the acid studies a dry tube for the water studies and NASS-3 sea-water standard as the chemical modifier for experi- ments involving the use of a mixed carrier.The ICP-MS measurements were made using the high-resolution mode (0.7 u Table 1 Instrumental operating conditions and data acquisition parameters at 10% peak height) because of the possible peak overlap of 36Ar+ on the 35Cl+ peak. All solutions were prepared with ultra-pure water obtained from a Milli-Q water purification system (Millipore Mississauga Ontario Canada). The HCl HNO and H2S04 were Ultrex 11 ultrapure-reagent grade (Baker Analyzed J. T. Baker Canada Toronto Ontario). Phosphoric acid (Baker Analyzed) was laboratory-reagent grade. The "N-labelled HNO was purchased from CIL (Cambridge Isotope Laboratories Woburn MA USA) with a concentration of 40% v/v and the isotopic purity was 99% "N. Stock standard solutions were supplied by Spex Industries Edison NJ.The NASS-3 Open Ocean Reference Material for Trace Metals was obtained from the Institute for Environmental Chemistry of the National Research Council of Canada. For the mixed carrier work NASS-3 sea-water was subjected to column chromatography'* to remove any trace elements. A further reduction in trace contaminants was also achieved by diluting the purified NASS-3 sea-water 500-fold with de-ionized water. The carrier gas for all of these experiments was high-purity argon (99.9 %). Results and Discussion Vaporization Properties of Water It is known that water adsorbed on a graphite surface can react during the pyrolysis step to form hydrogen. Interestingly it has also been observed that water is actually difficult to remove from graphite even at elevated temperatures.' Frech and co-workers demonstrated' and Welz reported14 that in an uncoated graphite tube water was actually retained in sufficient amounts even after 15 min at 1200 OC.13*14 These workers also estimated the amount of residual water in a graphite tube following the drying step to be 1 pmol when 1 pl of water was deposited on a graphite tube and dried at 80°C.16 The ICP-MS background spectra have been docu- mented for a dry argon and the intensity of 36ArH+ signal at rn/z=37 has been shown to be indicative of the amount of water present.The temporal behaviour of the vaporization of water from the graphite furnace as monitored by the argon hydride ion is shown in Fig. 1. Corrections for any spectral overlap from the ,'Cl+ ion was accomplished by monitoring the 35Cl -+ ion and applying the appropriate correc- tion.The HGA-600MS was equipped with a pneumatically controlled graphite probe which effectively sealed the dosing hole. The positive pressure of argon within the graphite tube made it unlikely that significant amounts of entrained air or water vapour entered the graphite tube through the dosing hole. inductively coupled plasma mass spectrometer - R.f. power 1000 w Outer argon flow rate Intermediate argon flow rate Aerosol carrier argon flow rate Sampler/skimmer cones platinum 15.0 1 min - 850 ml min - 900 ml min-' Electrothermal vaporizer H GA-600MS - Sample volume Internal argon flow rate Dry step Dry ramp Pyrolysis step Pyrolysis ramp Vaporization step Heating rate 5-10 ~1 300 ml min- 80-140°C (30 S ) 2-20 s 400-1400 "C (10 S) 1-2 s 2400 "C (10 s) 2000 "C s - 2oo I 180 ," 160 2 140 I c 0 m 120 2 3 100 4- .- v) 6 80 .- 60 & 40 v 20 w - m .- Data acquisition - Dwell time 20 ms Scan mode Peak hop transient Points per spectral peak 1 Signal measurement Integrated signal pulse 0 10 20 30 40 50 60 70 80 Time/s Fig.1 Temporal behaviour of water in the electrothermal vaporizer monitored using the 36ArH+ polyatomic ion drying temperature = 140 "C (0-50 s); pyrolysis temperature = 400 "C (50-60 s); and vaporiz- ation temperature = 2400 "C (60-70 s)JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 The temporal behaviour of 36ArH+ in the ETV device confirmed that the bulk of the water from a lop1 sample is removed during the drying step of the temperature programme.A pyrolysis step of 400°C was added however no significant signal for 36ArH + was observed until the high-temperature vaporization step at 60-70 s. The temporal behaviour of 36ArH+ as shown in Fig. 1 provides only a qualitative view of water retained on the graphite surface of the electrothermal vaporizer because some of the 36ArH + signal obtained during the vaporization step originated from water that had condensed on the graphite cones of the ETV device as well as on the transfer line. Retention of water in the ETV device is of some importance in ETV-ICP-MS because of its role in the production of hydride and oxide species and was included as a pretext for the vaporization properties of acids.These hydride and oxide polyatomic ions can interfere with other analytes in a multi- element analytical scheme. For example the production of could interfere with the determination of trace amounts of Fe at m/z=56. The presence of water could also lead to the formation of refractory oxides for some analytes such as the rare earth elements. 4 0 ~ ~ 1 6 0 + Vaporization Properties of Acids Drying and pyrolysis steps in the temperature programme of the ETV investigations can result in the selective volatilization and removal of the bulk of the sample matrix material from the graphite furnace. During the drying step most of the solvent matrix can be driven from the surface of the graphite furnace with the bulk of the remaining portion being removed at higher temperatures during the pyrolysis or vaporization steps.In the case of oxyacids it is believed that these can be retained within the surface of the graphite itself even at temperatures of up to 1000 OC." Pyrolytic graphite coated graphite tubes are less porous and therefore less prone to solvent entrapment than uncoated tubes and thus the retention of acids is greatly reduced. By studying the temporal behaviour of these acids a better understanding can be obtained of how they can affect analyte sensitivity in ICP-MS either by signal suppression (matrix effect or oxide formation) or by signal enhancement (improved analyte volatility and/or trans- port efficiency). Hydrochloric acid (i) Eflect of drying-step temperature. To determine the importance of the temperature of the drying step on the removal of chloride from the graphite furnace 1% v/v HCl was deposited on the walls of a pyrolytic graphite coated graphite tube.The temporal behaviour of HCl during the heating cycle was monitored using the ,'Cl+ ion. Hydrochloric acid was chosen because it is a common sample diluent it is a fairly volatile acid and thus its behaviour is similar to other volatile acids such as HF or possibly HNO,. The 35Cl+ ion was monitored and the counts generated from this ion were proportional to the amount of HC1 present. To investigate the role of the temperature of the drying step on acid retention this temperature was varied from 90 to 160°C and the HCl remaining on the surface during the high-temperature vaporiz- ation step was monitored. A pyrolysis step was not used.The intention of these experiments was to assess the retention of Cl in the ETV device after the drying step qualitatively. The results of these experiments shown in Fig. 2 demonstrate the limits of the drying step in removing chloride from the furnace. Only a slight increase in the integrated ,'Cl+ ion count was obtained as the drying-step temperature was increased from 90 to 160°C with the ion counts reaching a plateau after 120°C. Similarly the number of counts measured during a subsequent vaporization step did not change significantly above 120 "C. The fact that increasing the drying step beyond v) +- 5 150 0 m 125 Q c 100 .- 2 75 -0 Y- - m C .- 50 0 -0 + - 25 e I e B o C - rn Drying step Vaporization step 90 100 110 120 140 160 Drying temperaturePC 92 1 v) 3 + 30 8 rn 0 v -..25 c C 0 20 '4= 2 .- L 0 15 L Y- 0 10 a v) .- 5 b In -0 0 2 I m c C - Fig. 2 Effect of drying temperature on the integrated signal of 35Clf during the drying and vaporization steps using a 1 YO v/v HCI solution drying time = 50 s; and vaporization temperature = 2400 "C ( 10 s) 140°C had no effect on the amount of 35Cl+ removed from the ETV device could also indicate that the acid was actually condensed on cooler regions of the vaporizer. Therefore even with an increased drying temperature or drying time the retained acid was not removed from the ETV device until the high-temperature vaporization step. To determine whether the 35Cl + signal actually originating from chloride condensed or adsorbed on the transfer line rather than from the graphite tube 1% v/v HCl was dried in the graphite furnace in a manner similar to previous experi- ments.Prior to the high-temperature vaporization step the transfer line was disconnected and the ETV device was cleaned by three separate 2400 "C heating cycles for 10 s each. The transfer line was then re-connected and a 10 pl sample of water was deposited on the walls of the ETV device. The vaporizer was then subjected to a drying step and heated to 2400°C during which time the 35Clf intensity was monitored. In this manner any 35Clf residing in the transfer line would be vaporized and detected by the ICP-MS instrument. This experiment showed that approximately 65% of the retained C1 observed was actually originating from the transfer line. Therefore for a 5 pl sample of 1% v/v HC1 which yielded 18 000 counts during the vaporization step approximately 12000 counts would originate from the transfer line.This result is important to those using an ETV design' encorporat- ing a waste vapour venting system located some distance downstream from the vaporizer surface. These devices allow for condensation of water and acids which can easily be volatilized and carried with the analyte to the argon plasma during the high-temperature vaporization step. (ii) Eflect of pyrolysis-step temperature. For these experi- ments a drying-step temperature of 140°C was used. This temperature was reached by constantly increasing the tempera- ture of the ETV device or ramping for 10s until the desired temperature was reached. The drying-step temperature was then held constant for 40 s.Following the drying step the HCl retained in the graphite tube during the pyrolysis step was again monitored using the 35Cl+ ion. Residual chloride remain- ing on the graphite was measured during the high-temperature vaporization step. Three temperatures were chosen for these experiments such that a constant heating rate was maintained under normal ramping conditions. The results of these experi- ments are summarized in Fig. 3 which shows that a greater portion of chloride can be removed when a very high pyrolysis temperature is used. A significant portion of chloride however remained on the graphite even after heating to 1400°C as shown by the number of 35Cl+ ion counts remaining during922 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL.9 400 640 1400 Pyrolysis tempera t u re/"C Fig.3 Effect of pyrolysis temperature on the integrated signal of 35Cl+ during the pyrolysis and vaporization steps sample solution 1 % v/v HC1; drying temperature = 140 "C (50 s); pyrolysis time = 12 s; and vaporization temperature = 2400 "C (10 s) vaporization. It should be noted that a pyrolysis temperature of 1400°C is excessively high and would result in significant analyte losses for many volatile elements such as Cd Zn Ag and Pb for which a pyrolysis temperature of 500°C should never be exceeded unless a chemical modifier is used.lg The effect of the temperature ramp was also investigated for its possible role in the vaporization of chloride from the graphite surface.The temperature ramp time in the drying step was varied from 2 to 20s and from 1 to 10s for the pyrolysis step. These ramp times were chosen as they are typical of those used in ETAAS and ETV-ICP-MS. The temperature ramp of both the drying step and pyrolysis step had no significant effect on the amount of acid retained. The temporal behaviour of the vaporization of HCl during an entire ETV temperature programme (dry to clean-up) was monitored using the 35Cl+ ion and is shown in Fig. 4. A 140 "C drying step was used followed by a pyrolysis step of 1400°C. For this experiment water was also monitored using the 36ArH + ion during the entire temperature programme. Though not shown in Fig. 4 very little chloride from HCl was vaporized until most of the water was removed from the surface of the ETV device which occurred at about 20s into the heating cycle.Following the vaporization step at 2400"C a clean-up step of 2650°C was included during which a chloride signal was still observed. The high-temperature vaporization step and clean-up step have been magnified in Fig. 4 (b) to reveal some details of the analyte pulses in these steps. The chloride pulse in the high-temperature vaporization step is a sharp peak lasting approximately 3-4 s. The more broadly shaped chloride pulse in the clean-up step could be indicative of chloride which is deeply intercalated in the graphite or chloride that has condensed on the transfer line or contact cones. The integrated area of the acid pulse in the clean-up step was close to half the integrated area for acid volatilized during the vaporization step.As pointed out under Experimental this temporal picture of chloride vaporization in the ETV device was obtained by sealing the dosing hole during the entire temperature pro- gramme. During the drying and pyrolysis steps the flow of argon originates from one end of the ETV device to the other instead of both ends and through the dosing hole giving the chloride a possible means of condensing on the transfer line. As a result the chloride peak during the high-temperature vaporization step in Fig. 4 is larger than expected and was caused by chloride that originated from chloride condensed onto the transfer line as well as from the ETV device. Under normal conditions with the dosing hole open for the drying and pyrolysis steps the chloride signal in the high-temperature vaporization step is much lower.Semi-quantitative analysis 7 00 600 500 400 300 200 r 'v) 100 z o 2 cn C .I- m -. t. > 0 20 40 60 80 100 120 80 90 100 Time/s Fig. 4 Temporal behaviour for the vaporization of 1% (v/v) HCI in a pyrolytic graphite coated graphite tube monitored using 35Cl+ drying temperature = 140 "C (0-60 s) pyrolysis temperature = 1400 "C (60-82 s); vaporization temperature = 2400 "C (82-92 s); and clean-up temperature = 2650 "C (92-102 s) for the amount of chloride retained under normal conditions from 20 pl of 1% (v/v) HCl following a 140 "C drying tempera- ture and a 400 "C pyrolysis step revealed that approximately 0.05% of the original amount of acid as chloride was retained under these conditions.This amount of chloride retained corresponds to approximately 1.1 x lop3 pmol of chloride or 40000 pg which is in large excess compared with the amount of analyte vaporized in the ETV device typically from 1 to 50pg. Nitric acid To study the retention of HNO on graphite a suitable ion either mono- or polyatomic must be chosen that is solely derived from HNO,. Mass scans in the spectral region of m/z values of from 14 to 60 were obtained for 1% v/v acid and compared with the background spectrum obtained with de-ionized water. These experiments were similar to those reported in the literature for solution nebulization,20 except in this case the plasma was dry. Choosing a suitable ion was difficult because of the similarity between the mass spectra of the HNO and the mass spectra of the blank which was water.A number of ions were evaluated including 14N160+ I2Ci4N+ 40Ar14N+ 14N'602t and I4N+ but 14N160+ was the only species that gave a satisfactory response for monitoring the vaporization of HNO,. NitricJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 923 acid is a major source of 14N160+ however another important source within the ETV device is the formation of 14N160+ resulting from the vaporization of water which reacts with 14N+ in the plasma. In practice however the 14N160+ ion was found not to be suitable for quantifying the amount of HNO retained on the ETV device because the experiments were difficult to reproduce compared with HCl and were susceptible to high background ion sources such as residual water in the ETV device or water condensed in the transfer line.For this reason "N-labelled HNO was substituted and 15N+ was used as an alternative ion to 14N160+ for monitoring the temporal behaviour of HNO in the ETV device. In this way a direct correlation could be made between the integrated signal of an ion from the acid and the amount of acid remaining in the furnace. At m/z = 15 relatively low background counts were observed which were of the order of approximately 2000 counts s-'. As with previous experiments using HNO a 1% v/v solution was prepared and the temporal behaviour of the HNO was monitored using the 15Nf ion. As shown in Fig. 5 the 15N+ ion appeared to behave in a manner similar to the 35Cl+ ion. During the drying step of the temperature pro- gramme the bulk of the 15Nf was not removed until most of the water had evaporated from the graphite surface. The vaporization of water was observed by monitoring the intensity of the argon hydride (m/z=37) polyatomic ion during the heating programme.As with the chloride ion most of the 15N+ was removed during the drying step however 10-15% of the original "N+ appeared to be retained until the high- temperature vaporization step. The signal for "N' during the vaporization step appeared to be unusually large especially when compared with the chloride signal from 1% HCl. A pyrolysis step was added to remove the residual 15N+ from the ETV device. Neither a 400 nor a 900 "C pyrolysis step had any effect on reducing the intensity of the 15N+ signal during the high-temperature vaporization step indicating that the 15N+ was not originating within the graphite tube.It was indicated under Experimental and explained for the temporal behaviour of chloride from HCl that the temporal behaviour of "N+ was studied with the dosing hole sealed during the entire temperature programme. As with the chloride from HCl when the dosing hole was sealed and the argon flow was directed from one end of the ETV device to the other the vapours of HNO had an opportunity to condense on the transfer line or possibly on the graphite contact cones of the furnace housing resulting in a larger than expected "N+ signal. When the dosing hole was left open during the drying 1200 ~ 1000 1 *0° t I \ 0 10 20 30 40 50 60 Time/s Fig.5 Temporal behaviour for the vaporization of 1 % v/v HNO in a pyrolytic graphite coated graphite tube using a "N-labelled nitric acid solution drying temperature = 120 "C (0-40 s); pyrolysis tempera- ture = 400 "C (40-50 s); and vaporization temperature = 2400 "C (50-60 S) and pyrolysis steps (140 and 400 "C respectively) as would be the normal procedure with the ETV device the flow of argon originates from the ends of the ETV and exits via the dosing hole removing any vaporized HNO from the ETV device. Under these conditions almost all (> 99.97%) of the HNO could be removed from the ETV device during the drying step of the temperature programme. Experiments were performed on both old tubes (>200 firings) as well as new tubes and it was found that tube age was not a significant factor in the retention of HNO in the ETV device.Effect of Acid on Analyte Signals For these experiments the analytes were vaporized at 2400 "C following a 140°C drying step. A pyrolysis step was not used because the objective of these studies was to use experimental conditions that are similar to those reported in the literature and to explore the actual need if any of this heating step. For example under compromise conditions relatively low pyrolysis temperatures such as 200"C have been used to prevent pre- vaporization losses of volatile elements such as Ag or Pb during these heating steps." The implications of using any acid as diluent for ETV-ICP-MS determinations can be import- ant particularly for multi-elemental analysis where compro- mise conditions are required.The effects of 0.05% v/v and 1.0% v/v HNO HC1 H,SO and H,P04 were determined for seven elements Co Cu Ag Cs Pb Bi and U. These elements were chosen both to cover a large atomic mass range of the Periodic Table as well as a range of analyte volatilities. The results of these experiments are summarized in Table 2. For all elements vaporized in the presence of HNO a 1% v/v HNO matrix gave higher integrated signals than the 0.05% HNO matrix. For some elements such as Ag or Pb the effects were fairly pronounced with close to two-fold increases in the integrated signal with increased acid concen- tration. Initially it would appear as though higher concen- trations of HNO would simply mobilize more analyte from the graphite surface however this is probably not the primary cause for the higher analyte signals in higher concentrations of acid.It is possible that a carrier effect with higher acid concentrations is occurring whereby HNO is acting as the physical carrier. Another possibility is the increased degred- ation of the graphite surface in the ETV device with higher acid concentrations which could result in the formation and liberation of soot particles during the high-temperature vapor- ization step. These fine carbon particles could then act as physical carriers themselves or possibly as sources of nucleation for the analyte. For HCl increasing the acid concentration in the sample matrix from 0.05 to 1.0% v/v did not have a significant effect on any of the elements. The integrated signal remained unchanged within experimental error with increased HCl concentration.The integrated signal for most of the elements was fairly similar in either 1% v/v HNO or 1% v/v HCl except for U which was noticeably lower in the HCl matrix. The HC1 matrix could act in two ways that are beneficial to analysis by ETV-ICP-MS. Firstly the HC1 could act as a physical carrier and thereby improve the mass transport of the analyte to the plasma. Alternatively HCl could react with the analytes forming halides which are more volatile than the corresponding oxides making this a potentially desirable acid to use for many analytes. When H,SO was used the integrated signal for most of the elements studied increased significantly with the 1 YO acid compared with HNO and HC1.The effects of increasing the acid concentration of H,S04 were also significant indicating the potential for severe matrix effects. As with HNO when the H2S04 concentration was increased from 0.05 to 1.0% v/v the integrated signal also increased. This was especially true for elements such as Cs and Bi. Phosphoric acid gave data which were unlike those €or the924 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Table 2 Effect of acid concentration on integrated analyte signal pulses (50 pg) Integrated signal/counts c u 7144 6.4 11 417 8.1 10 530 3.6 10 155 4.7 15 720 6.5 21 455 5.6 Ag 12 507 3.8 22 294 9.9 20 060 1 .oo 19 863 3.1 13 497 6.3 18 065 5.0 23 254 9.9 13 225 2.5 c s 163 790 4.5 196 151 3.6 187 167 0.9 187 006 3.6 122 575 5.1 268 796 8.8 188 548 12.6 371 608 9.4 Pb 122 529 7.6 196731 7.7 250 100 0.70 248 335 6.3 129 097 5.7 239 324 7.0 Bi 174 329 8.5 263 292 5.6 212 667 6.1 199 736 4.3 136 053 4.2 368 462 7.4 206 83 1 5.6 192 756 4.0 U 136 369 7.7 225 123 5.4 106 140 6.3 99 589 12.0 116 543 5.3 217413 8.4 335 407 10.1 239 306 6.4 c o 16 275 2.5 19 659 5.6 19 833 1.1 19 616 4.8 25211 6.5 34 887 5.1 45 040 9.7 27 139 10.6 Parameter 0.05% HNO 1.0% HNO RSD* (Yo) (n=5) RSD (Yo) (n=5) 0.05% HCl RSD (Yo) (n=5) 1.0% HCl RSD (Yo) (n=5) 0.05% H,SO4 RSD (Yo) (n=5) 1.0% H2S04 RSD (%) (n=5) 0.05% H,P04 RSD (Yo) (n=5) RSD (%) (n=5) 1.0% H,P04 * RSD relative standard deviation.Table 3 Effect of pyrolysis temperatures on integrated analyte signal pulses (50 pg) Integrated signal/counts Parameter l%HN03 None* RSD (O/o)(n=5) 400 "C RSD (%) (n=5) 900 "C RSD (Yo) (n=5) 1% HCl None RSD (%) (n=5) 400 "C cu c s Pb Bi U 14 790 8.5 13 653 2.6 14 655 5.3 24 201 11.1 12 612 9.1 5502 4.08 268 020 6.1 263 190 2.7 268 030 5.86 235 590 5.1 183 270 8.1 175 660 7.04 163 100 4.3 109 710 6.5 81 765 3.33 171 655 4.8 207415 2.5 217 520 3.26 18 486 12.1 16 674 3.9 16 092 3.4 25 473 5.2 24 227 13 514 8.9 14016 20 336 4.5 19 314 158 210 5.0 156 355 182 330 8.7 125 300 170610 8.4 134 785 120 130 12.2 87 800 RSD (Yo) (n=5) 900 "C RSD (Yo)(n=5) 4.2 26 121 2.0 5.7 15 041 4.1 3.6 15 796 1.9 5.8 127 195 4.7 8.9 44 870 2.9 9.9 25 770 2.9 12.5 82 290 6.3 * No pyrolysis step used.other three acids because as the acid concentration was increased the integrated signal did not increase for all of the elements.Rather Co Ag and U decreased with increased H3PO4 concentration Bi remained unchanged and Cs increased significantly. In comparison with the other acid matrices the integrated signals for most analytes were greater in H,P04 than either HNO or HC1 and similar to the integrated signals obtained in the H,S04 matrix. Both H2S04 and H3P04 have very high boiling-points (> 200 "C) and will remain in the graphite tube especially when a pyrolysis step is not used. Although H,S04 and H3P04 are not commonly used in trace analysis in ETAAS they were included in this study for their potential as matrix interferences in ETV- ICP-MS. Real samples especially biological samples contain both S and P which in acidic media produce H2S04 and H3P04 during the drying and pyrolysis steps.It is possible that the matrix effects observed with these acids were entirely due to the presence of acids in the graphite tube and not to other processes such as those occurring in the plasma. The interaction between these acids and analytes in the ETV device is complex and requires further study. A pyrolysis step was used to study its effect on 1% v/v H N 0 3 and HCl since these acids are the most common ones used as diluents in ETV-ICP-MS. When a pyrolysis step was used a different analytical response was obtained. Two pyrol- ysis temperatures were used 400 and 9OO0C using 1% v/v acids as the matrix. The lower pyrolysis temperature at 400 "C was chosen because it is well below the standard pyrolysis conditions for volatile elements such as Ag.19 Therefore any effect seen would not be associated with analyte losses in the pyrolysis step at 400°C.The higher pyrolysis temperature at 900°C was chosen since temperatures in this range are used for a number of less volatile elements such as Cu or Co. Initially it was postulated that a high-temperature pyrolysis step would remove more acid from the surface of the ETV device and therefore result in lower integrated analyte signals as was illustrated in Table 2. The results of these experiments are summarized in Table 3. Maximum integrated signal inten- sities were achieved when a pyrolysis step was not used with 1% v/v HNO in the matrix. In a 1% v/v HNO matrix increased pyrolysis temperatures had a detrimental effect on the volatile elements such as Ag Bi and Pb.For the other elements there was no change in signal intensity within experimental error with the use of a pyrolysis step except for U which showed only a slight increase in integrated signal intensity with a pyrolysis step. The use of a pyrolysis temperature step for analytes in a 1% HC1 matrix did not result in an increase in the signal intensity for most of the elements studied. For most elements the integrated signals obtained without a pyrolysis step wereJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 925 Table 4 Effect of acid concentration on integrated analyte signal pulses (50 pg) using NASS-3 as a physical carrier Integrated signal/counts Parameter 0.05% HNO 1.0% HN03 RSD ("/)(n=5) RSD (%)(n=5) 0.05% HC1 RSD (%) (n=5) 1.0% HCI RSD (Yo) (n=5) 0.05% H2S04 RSD (%) (n= 5 ) 1.0% H2S04 RSD (YO) (n= 5) 0.05% H3PO4 RSD (Yo) (n=5) 1.0% H3PO4 RSD (YO) (n= 5 ) co 30 382 9.4 28 464 7.9 23 085 2.3 21 648 2.9 40 437 3.42 42 471 5.34 46 581 9.1 27 581 5.6 ~~ c u 10 148 8.1 11 165 3.3 9297 10.4 9409 5.6 29 286 4.1 30 117 4.3 - - - - Ag 29 399 3.3 27 839 4.1 24 848 5.5 19 402 1.3 37 877 7.8 27 204 7.0 31 027 4.6 13 626 2.0 c s 125 492 8.1 115206 3.3 149 903 7.9 144 490 5.4 322 094 3.5 394 135 3.7 181 483 5.9 419 118 4.4 Pb 220 425 5.4 225 277 7.3 256 772 4.7 256 704 6.9 284 412 6.0 381 171 1.6 - - - - Bi 373 365 2.4 366 476 2.8 223 700 4.2 181 660 5.1 300 002 3.1 480 804 6.7 209 078 17.8 389 964 5.0 U 53 410 6.3 48 990 5.4 30 720 8.3 48 030 8.4 58 469 5.3 114 642 6.8 469 163 9.5 188 118 13.6 higher than when a pyrolysis step of 400°C was used.When the pyrolysis temperature was increased to 9OO0C the inte- grated signal decreased further for most elements in 1% HCl and in the case of elements that form volatile halides such as Ag or Pb the effect of using a pyrolysis step was even more dramatic. These results are consistent with the experiments completed using different acid concentrations (Table 2) when a higher concentration of acid resulted in an increase in analyte signal intensity. It is therefore reasonable that the introduction of a pyrolysis step that removes any traces of acid from the ETV device should result in lower integrated signals. These observations indicated that when a pyrolysis step was used the beneficial carrier effects of retained HCl are also removed and hence a lower signal is obtained.Effect of Chemical Modifier-Physical Carrier The addition of a chemical modifier<arrier was evaluated for the role in reducing the matrix effects associated with different acid concentrations. The physical carrier increases the inte- grated signal in ETV-ICP-MS by improving the mass transport of the analyte of interest.2'v22 The physical carrier chosen for this work was NASS-3 sea-water. Purified NASS-3 (see under Experimental) sea-water acts as a mixed modifier in the ETV device since sea-water is essentially a mixture of major ions which include Na K Mg Ca Sr C1 and Br. The NASS-3 sea-water is a readily available high-purity mixed modifier free of trace contaminant elements typical of other modifiers such as Pd.23 The results of the experiments with NASS-3 used as a physical carrier are summarized in Table 4.As before the analytes were vaporized immediately after the drying step and a pyrolysis step was not used. The integrated signals for all of the elements in the presence of NASS-3 were significantly enhanced indicating an improvement in the mass transport of the analytes to the plasma. The NASS-3 also reduced the effects of increased acid concentration for all of the acid matrices. The change in integrated signal when the acid concentration was increased was not nearly as significant when NASS-3 was present in solution. This was observed for all of the acids though less so for H3P04. The reproducibility (relative standard deviation RSD) of the integrated signals was also improved when NASS-3 sea-water chemical modifier- carrier was used.The use of NASS-3 as a chemical modifier in effect introduces into the sample matrix pg amounts of chloride. This excess of chloride (compared with pg amounts of analyte) serves to convert most of the analytes into their chloride form and can interfere or prevent the formation of analyte oxides which are more refractory. These oxides tend to vaporize or be reduced at much higher temperatures compared with the vaporization of chlorides and for some elements such as the rare earth elements oxides are vaporized in the graphite tube at a time when little carrier is present to ensure efficient transport to the argon plasma. Conclusions This work has shown that the use of aqueous external standards for calibration could lead to significant analytical error depending on the composition of the sample matrix.The use of a chemical modifier-carrier in both standard and sample solutions was shown to be essential to the successful application of ETV-ICP-MS to the quantitative analysis of complex mate- rials. Of the acids studied HC1 was least influential in altering analyte sensitivity especially in the presence of NASS-3 as the chemical modifier-carrier. Residual chloride remaining after the drying and pyrolysis steps served as an effective carrier for transporting analyte from the graphite tube to the argon plasma while at the same time prevented the formation of more refractory analyte oxides. The relatively lower sensitivity of analytes except for U in the presence of high concentrations of HNO compared with HC1 can be explained in part by the chemical nature of these acids.In the case of HNO the analytes formed oxides that were reduced to metal atoms during the high-temperature vaporization step making them less volatile compared with their corresponding halides. The behaviour of the less volatile H,PO and H,S04 acids and their effects on analyte signal pulses is complex and could involve both chemical and physical effects. Sample matrices containing large amounts of these acids will probably require detailed study for each analyte determined and could require alternative calibration schemes such as the method of standard additions or isotope dilution. References Carey J. M. and Caruso J. A. Crit. Rev. Anal. Chem. 1992 23 397. Gray A. L. and Date A. R. Analyst 1983 108 1033. Styris D. L. and Kaye J. H. Spectrochim. Acta Part B 1981 36 41. Sturgeon R. E. Michell D. F. and Berman S . S. Anal. Chem. 1983 55 1059. Bass D. A. and Holcombe J. A. Anal. Chem. 1987 59 974. Ham N. S. and McAllister T. Spectrochim. Acta Part B 1988 43 789.7 8 9 10 11 12 13 14 15 16 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Gregoire D. C. Lamoureux M. Chakrabarti C. L. Al-Maawali S. and Byrne J. P. J. Anal. At. Spectrom. 1992 7 579. Byrne J. P. Chakrabarti C. L. Gregoire D. C. Lamoureux M. and Ly T. J. Anal. At. Spectrom. 1992 7 371. Park C. J. Van Loon J. C. Arrowsmith P. and French J. B. Anal. Chem. 1987 59 2191. Tsukahara R. and Kubota M. Spectrochim. Acta Part B 1990 45 779. Evans E. H. and Giglio J. L. J. Anal. At. Spectrom. 1993 8 1. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981 53 2331. Frech W. Persson J. A. and Cedergren A. Prog. Anal. At. Spectrosc. 1980 3 279. Frech W. and Cedergren A. Anal. Chim. Acta 1976 82 93. Welz B. in Atomic Absorption Spectrometry VCH Weinheim 2nd. rev. edn. 1986 p. 203. Frech W. and Cedergren A. Anal. Chim. Acta 1976 82 83. 17 Karanassios V. and Horlick G. Spectrochim. Acta Part B 1989 44 1361. 18 Gregoire D. C. Prog. Anal. At. Spectrosc. 1989 12 433. 19 Slavin W. Graphite Furnace AAS. A Source Book Perkin-Elmer Ridgefield CT 1984 p. 18. 20 Tan S. H. and Horlick G. Appl. Spectrosc. 1986 40 445. 21 Ediger R. A. and Beres S. A. Spectrochim. Acta Part B 1992 47 907. 22 Grkgoire D. C. Al-Maawali S. and Chakrabarti C. L. Spectrochim. Acta Part B 1992 47 1123. 23 Grkgoire D. C. and Sturgeon R. E. Spectrochim. Acta Part B 1993 48 1347. Paper 31071 76F Received December 6 1993 Accepted March 10 1994

ISSN:0267-9477    

DOI:10.1039/JA9940900919

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 927 Determination of Technetium-99 Thorium-230 and Uranium-234 in Soils by Inductively Coupled Plasma Mass Spectrometry Using Flow Injection Preconcentration" Mark Hollenbach James Grohs Stephen Mamich and Marilyn Kroft RUST Geotech Inc. US. Department of Energy Grand Junction Projects Office PO Box 74000 Grand Junction Colorado 87502 USA Eric R. Denoyer The Perkin-Elmer Corporation 767 Main Avenue Norwalk CT 06859 USA A new method is described for the determination of "Tc 230Th and 234U at ultra-trace levels in soils. The method used flow injection (FI) for on-line preconcentration of 'qc 230Th and prior to detection using inductively coupled plasma mass spectrometry (ICP-MS). The FI-ICP-MS method results in greater sensitivity and freedom from interferences compared with direct aspiration into an ICP mass spectrometer.Detection limits are improved by approximately a factor of 10. The FI-ICP-MS method is also faster less labour intensive and generates less laboratory waste than traditional radiochemical methods. The accuracy of the method was tested for '9Tc by comparison to liquid scintillation counting and for 230Th and *%U b analysis of a US Department of Energy reference soil. Detection limits in the soil for 'Tc "'Th and 'MU were 11 mBq g-' (0.02 ng g-') 3.7 mBq g-' (0.005 ng g-') and 0.74 mBq g-' (0.003 ng g-') respectively. Sample preparation analysis protocol and method validation are described. Keywords Technetium-99 thorium-230 and uranium-234 determination; inductively coupled plasma mass spectrometry; flow injection The United States Department of Energy (US DOE) is con- ducting several large environmental restoration programmes to remediate contamination resulting from decades of nuclear weapons production and testing uranium ore processing and nuclear reactor fuel production and reprocessing.Many thou- sands of soil and water samples will be collected and analysed for radionuclides over the next 30 years at a cost of millions of dollars. Traditional radiochemical methods for alpha- and beta-emitting radionuclides require extensive chemical separ- ations long count times and can produce hazardous and/or radioactive laboratory waste. Developing improved analytical methods for radionuclides can result in a significant cost reduction for the US government.Inductively coupled plasma mass spectrometry (ICP-MS) has been used successfully to measure long-lived radionuclides such as '"I 232Th 237Np and 238U.'-3 However ICP-MS used with conventional sample introduction techniques lacks either the sensitivity or the selectivity to measure shorter lived radionuclides at levels important for environmental monitor- ing. Limits of detection (LODs) required for environmental monitoring of radionuclides are based on the radioactivity of the analyte. As the half-life of the analyte gets shorter the number of analyte atoms required to produce a given level of radioactivity gets smaller. The detection power of ICP-MS is limited by the number of analyte atoms present in a sample. Therefore the shorter the half-life of the analyte the lower the ICP-MS LOD must be to detect the analyte at a given level of radioactivity.Technetium-99 is a beta-emitting radionuclide with a maxi- mum beta energy of 0.29 MeV a half-life of 2.12 x lo5 years and a specific activity of 629mBqng-'. It is produced by nuclear fission and has been released into the environment primarily as a result of nuclear fuel repro~essing.~ It is usually determined by radiochemical methods with detection by liquid scintillation counting Thorium-230 and 234U are alpha-emitting radionuclides with half-lives of 8.0 x lo4 and 2.47 x lo5 years and specific activities of 718 and 229 mBq ng-l respectively. Both are members of the 238U decay series. Thorium-230 is commonly found in * Presented at the 1994 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 10-15 1994.uranium mill tailings and is of concern because it decays into 226Ra and then 222Rn which is a radioactive gas that can cause cancer if inhaled. The radioactivity of 234U in soils is equal to that of 238U if both isotopes are present at their natural abundances. Soils that are contaminated by synthetically- enriched U may contain 234U at higher radioactivity levels than 238U. Thorium-230 and 234U are usually determined by alp ha-energy spectrometry. In the present work a flow injection (FI) system is used to separate and concentrate radionuclides by solid-phase extrac- tion Methods are presented for measuring 99T~ 230Th and 234U in soils. Samples were fused with sodium peroxide for 99Tc or digested with a mixture of hydrofluoric nitric and perchloric acids for 230Th and 234U. The sample solution is pumped through a column and the analytes are loaded onto the solid-phase adsorber.The analytes are then eluted directly into the nebulizer of the ICP mass spectrometer. The use of FI results in greater sensitivity and freedom from interferences compared with direct aspiration into an ICP-MS instrument. Detection limits are improved by approximately a factor of 10. The FI-ICP-MS methods have LODs comparable to radiochemical methods but are faster less labour intensive and generate less laboratory waste. Experimental Instrumentation and Apparatus ICP-MS The ICP mass spectrometer used in the study was an Elan 5000 (Perkin-Elmer SCIEX Toronto Canada).The system was fitted with a quartz spray chamber (Precision Glassblowing Parker CO USA) and a Type TR-30-3C con- centric glass nebulizer (J. E. Meinhard Associates Santa Ana CA USA) because the Ryton spray chamber and nebulizer supplied with the instrument are not compatible with the 8 mol I-' nitric acid eluent used for "Tc. Two peristaltic pumps were used (Gilson Medical Electronics Middleton W1 USA) one to pump the spray chamber drain and the other to rinse the nebulizer and spray chamber.928 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Flow injection system AS-90 autosampler ( Perkin-Elmer Corporation Norwalk CT USA) were used. used for the eluent and 0.76 mm i.d. tubing was used with the remote pump for rinsing the nebulizer.laboratory with TEVA. Spec extraction resin for "Tc determi- nation and TRU-Spec resin for 230Th and 234U determination Both a FIAS-200 and a F1AS-400MS F1 'Ystem fitted with an The FIAS mini-columns (Perkin-Elmer) were packed in the Fig. is a schematic diagram Of the F1 showing tubing used to construct the (Eichrom Industries Darien IL USA). The volume of the resin contained in each column was approximately 50 and the tubing arrangement* manifold was made of PolY(tetrafluoroethYlene) (PTFE) the of resin was approximate~y 30 mg. (Upchurch Scientific Oak Harbor WA USA). The tubing that connects the valve to the nebulizer was 0.2mm i.d. and the rest of the manifold tubing was 0.76 mm i.d. The tubing Sample preparation apparatus connections were made with Super Flangeless brand or flange- Iess-style fittings (Upchurch Scientific) because they do not use O-rings that are attacked by strong acid and tubing connec- tions can be made quickly and easily without a flanging tool.Super Flangeless fittings made of poly ether ether ketone were used to connect the tubing to the column because the fittings are designed for connections that need to be made and broken often. The rest of the tubing connections were made with Tefzel flangeless-style fittings. The T-connectors were made of Tefzel (Upchurch Scientific). Flow rates through the FIAS manifold are controlled by selection of pump speed and pump tubing i d . Pump tubing (Perkin-Elmer) with 1.52 mm i.d. was used for pumping the sample and column rinse solution 1.14mm i.d.tubing was ( a ) Remote P-pump l3 AS-90 Sample Waste 4,l Sample Load (Valve position 1) Elan Waste 4,1 Rinse El Inject (Valve position 2) solution Model 242-67 pulverizers equipped with ceramic plates (Bico- Braun International Burbank CA USA) were used to grind soil samples. Cross-flow blenders ( Patterson-Kelly Company East Stroudsburg PA USA) were used to blend the samples. Zirconium crucibles with 55 ml capacity (Fisher Scientific Pittsburgh PA USA) and a Model 51894 oven and hearth plate (Lindberg Watertown WI USA) were used for sample fusions. Fused samples were filtered through a 0.45 pm pore size 47mm diameter type HA membrane filter with a glass filter holder (Millipore Corporation Bedford MA USA) and a filtrator (Fisher Scientific). Mixed-acid digestions were car- ried out in 100ml size Type H P heatable plastic beakers (Nalge Company Rochester NY USA) with 65 mm PTFE covers (Berghof/America Concord CA USA) on Type 2200 hotplates (Barnstead/Thermolyne Corporation Dubuque IA USA).Operating conditions and analysis scheme The ICP-MS and the FIAS system were both under computer control. The operating conditions for the ICP-MS are summar- ized in Table 1. A description of the FIAS control programme is given in Table 2. The baseline-to-baseline width of the resulting elution peak was about 17 s and the signal was integrated across the full width of the peak. Typically one measurement was made per sample resulting in a total analysis time of 6.5 min and 10 ml of sample being consumed. A two- point calibration was used for all analyses.Determination of "Tc During the FIAS program the TEVA-Spec column was rinsed with 0.5 mol 1-1 nitric acid after loading the sample. The analyte was then eluted with 8 moll-' nitric acid. Rhenium Table 1 Instrumental operating conditions ICP-MS Forward power/W Plasma gas flow rate/l min-' Auxiliary gas flow rate/l min-' Nebulizer gas flow rate/l min-' Acquisition parameters (all analyses) Dwell time/ms Scan mode Sweeps per reading MCA channels per spectral peak Resolution/amu 10% peak maximum Signal processing Acquisition parameters (99Tc analyses) Readings per replicate Isotopes measured Internal standard Acquisition parameters (230Th and 234U analyses) Readings per replicate Isotopes measured Internal standards 1000 15 0.8 1 .o 50 1 1 0.8 Peak-hop transient Spectral peaks integrated; signal profile counted 75 99T~ loOMo '"Ru lg7Re '"Re for 99Tc 61 9 233u 234u 229~h 230q-h 232~h160 229Th for 230Th and 233U for 234U Fig.1 Schematic of the FIAS manifold showing (a) the tubing arrangement and (b) the two valve positionsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 929 Table 2 FIAS programme and description of programme steps Speed/rev min - Valve Remote Step Read Time/s Pump 1 Pump 2 position pump Pre-sample - 15 100 0 2 On 1 - 180 25 0 1 On 2 - 30 0 40 1 Off 3 - 3 0 40 2 Off 4 Yes 57 0 40 2 Off 5 - 60 0 40 1 Off 6 - 1 100 0 2 On Programme step Pre-sample 1 2 3 4 5 6 Description of step Sample is pumped at 8 ml min-' through the valve to waste in preparation for loading Sample is pumped through the column to waste at 2.2 ml min-'.Analytes are loaded onto the column Column rinse solution is pumped through the column to waste at 3.5 ml min-' to remove residual sample. This step also improves separ- ation from Ru and Mo for the determination of "Tc Eluent is pumped through the column to the nebulizer at 2 ml min-'. This step is used to delay the start of the read cycle until the analytes approach the nebulizer This is the same as step 3 except that the ICP-MS read cycle is initiated Column rinse solution is pumped through the column to waste at 3.5 mlmin-' to rinse residual eluent and prepare the column for loading of the next sample Pump speed and valve position are set for the autosampler rinse cycle. The autosampler returns to the rinse position for 25 s to rinse the sample inlet line (50 ng 1-') was added to all samples and standards as an internal standard to compensate for any variation in chemical recovery or instrument drift.Rhenium is a very rare element with an average abundance in the earth's crust on the order of 0.01 ng g-1.5 Therefore it was not expected to be in samples at significant concentrations. Determination of 230Th and 234U During the FIAS program the TRU-Spec column was rinsed with 4 moll-' nitric acid after loading the sample. The analyte was then eluted with 0.1 moll-' ammonium oxalate. Thorium-229 and 233U are man-made isotopes with half-lives of 7.34 x lo3 and 1.62 x lo5 years and specific activities of 7880 and 350 mBq ng-' respectively. Thorium-229 and 233U were added to samples and standards at 180 Bq 1-l (22.8 ng 1-') and 18.5 Bq 1-' (52.8 ng 1-I) and used as internal standards for 230Th and 234U respectively.The internal standards are not expected to be in the samples because they are neither naturally occurring nor are they decay products of anything that might be expected to be found in the samples. Reagents De-ionized water (18 MR cm) was used for all dilutions and was prepared using a Milli-Q system (Millipore). Determination of 99Tc The sodium peroxide used for sample fusions was 98% (Aldrich Milwaukee WI USA). The nitric acid used for sample dissolution and preparation of standards was 70% m/m trace metal grade (Fisher Scientific). The nitric acid used to prepare the eluent and column rinse solutions was 70% m/m double sub-boiling quartz distilled (Seastar Chemicals Seattle WA USA).Standard solutions of "Tc used for instrument calibration sample spiking and quality control standards were prepared by dilution of SRM 4288 Technetium-99 (National Institute of Standards and Technology Gaithersburg MD USA). The independent-source 99Tc standard used to verify calibration was prepared by dilution of a 38.5 kBq (61.2 pg) standard (Isotopic Products Laboratories Burbank CA USA). All "Tc standards were prepared in 0.5 mol I-' nitric acid. Standard solutions of Re Mo and Ru were prepared by dilution of 1000 pg ml-' stock solutions (Inorganic Ventures Lakewood NJ USA). (Note Laboratories must have procedures for safe handling of radioactive material and management of radioac- tive waste.Laboratory personnel must be trained to handle radioactive materials safely.) Determination of 230Th and 234U The nitric and perchloric acids were 70% m/m trace metal grade the hydrofluoric acid was 49% m/m analytical-reagent grade and the ammonium oxalate was certified ACS grade (Fisher Scientific). Standard solutions of 230Th used for calibration spiking and quality control standards were prepared by dilution of a 12.4 kBq (17.3 pg) standard (Isotopic Products Laboratories). The 229Th standard used as an internal standard for 230Th was prepared by dilution of a 72.2 kBq (9.16 pg) standard (Isotopic Products Laboratories). Standard solutions of 234U used for calibration spiking and quality control standards were prepared from a 2.36 kBq ml-' (10.3 pg ml-I) standard that was made by dissolving a portion of certified reference material (CRM) U500 Uranium Isotopic Standard (New Brunswick Laboratory Argonne IL USA) in nitric acid.The 233U standard used as an internal standard for 234U was prepared by dilution of a 37.0 kBq (106 pg) standard (Isotopic Products Laboratories). The independent-source standard used to verify calibration of 230Th and 234U was prepared from a 925mBqml-' (1.29 ng ml-' 230Th and 4.04 ng ml-' 234U) standard that was prepared by dissolving a portion of Reference Material No. 101-A Pitchblende Ore-Silica Mixture (New Brunswick Laboratory). Standard solutions of 232Th were prepared by dilution of a 1000 pg ml-1 Th standard (Inorganic Ventures). Preparation of Soil Samples Soil samples were dried in an oven at 103°C for 12-24 h ground to pass through a 325 mesh screen (maximum particle size 45 pm) blended and transferred to polyethylene bottles.Sample dissolution for determination of "Tc A 0.25 g sub-sample was placed in a zirconium crucible. For the spiked sample 50 pl of 7.40 Bq ml-' (11.8 ng m1-l) 99Tc were added to the sample and dried on a hot-plate at 100°C. A 2.25 g portion of sodium peroxide was added to the crucible and mixed with the sample using a small metal spatula. The crucible was placed on a hearth-plate that was then placed in an oven pre-heated to 470 "C for 30 min removed from the oven and allowed to cool to room temperature. The specified oven can hold 24 crucibles. Approximately 45 ml of water were added to the crucible and the fusion mixture was allowed to dissolve for 1 h.A 4ml volume of 70% m/m nitric acid was added to the crucible. The sample solution was diluted to 100 ml with water and filtered through a 0.45 pm membrane filter to remove the small amount of turbidity present in the sample solutions. For the radiochemical 99Tc method 5 g of sample were leached with a mixture of sulfuric acid and potassium persulf- ate. Technetium was extracted from the leachate with tri-n- butyl phosphate (TBP) that was previously equilibrated with sulfuric acid. The TBP was mixed with a scintillation cocktail and the "Tc activity was measured by liquid scintillation counting.6930 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Sample dissolution for determination 0j230Th and 234U A 0.5 g sub-sample was placed in a heat resistant plastic beaker. For the spiked sample 1.54 Bq (2.14 ng) of 230Th and 0.590 Bq (2.58 ng) of 234U were added to the sample.A 10 ml volume of 70% m/m nitric acid 5 ml of 70% m/m perchloric acid and approximately 3 ml of 49% m/m hydrofluoric acid were added. The beaker was covered with a PTFE cover and heated on a hot-plate at approximately 100 "C for 15 min. The covers were removed and the temperature of the hot-plate was raised to approximately 300 "C. Heating was continued until the sample formed a moist bead and the beaker was removed from the hot-plate to cool. (Note Samples containing high concentrations of organic material should not be heated to dryness with perchloric acid because an explosion could result.) A 10 ml volume of water and 1 ml of 70% m/m nitric acid were then added to the beaker and heated on a hot-plate at approximately 150 "C just to boiling.A 12.5 ml portion of 70% m/m nitric acid was added the sample solution was diluted to 50ml with water and any insoluble material was allowed to settle overnight before analysis. Quality Control The following performance check was done daily. A standard containing 100 pg l-' each of Ba Ce Co In Li Mg Pb Rh and T1 was aspirated conventionally in order to verify mass calibration resolution background count rate sensitivity oxide level and the level of doubly-charged species. The instrument was calibrated with a blank and one stan- dard. Calibration was verified by analysing a standard obtained from an independent source.A standard was analysed to verify calibration accuracy at low concentration. Two standards were analysed to verify control of the interferences described below. The first contained the interfering species listed below and the second contained analyte plus the interfering species. Analysis of the interference check standards verified that the interfering species did not give false-positive results and that the analyte could be accurately measured in the presence of the interfering species at the levels tested. The calibration was verified by analysing a standard at half the concentration of the calibration standard and a calibration blank after every ten samples and at the end of the analysis run. One preparation blank and at least one spiked sample were analysed for every 20 samples.Determination of 99 Tc The instrument was calibrated with a 7.40 Bq 1-' (1 1.8 ng 1-') 99Tc standard and a blank. The low concentration verification standard contained 185 mBq I-' (0.294 ng 1-') 99Tc. The first interference check standard contained 100 pg I-' Mo and 25 ngl-' Ru and the second contained lOOpgl-' Mo 25 ng 1-' Ru and 3.70 Bq I-' (5.90 ng 1-') 99Tc. Determination of 230Th and 234U The instrument was calibrated with a 30.9 Bq 1-' (43.0 ng 1-') 230Th and 11.8 Bq 1-' (51.5 ng 1-') 234U standard and a blank. The low concentration verification standard was 309 mBq I-' (0.430 ng 1-I) 230Th and 236 mBq 1-' (0.103 ng 1-') 234U. The first interference check standard contained 0.1 mg 1-l Th and the second contained O.lmgl-' Th and 5.90Bql-' (25.8 ng 1-') 234U.Results and Discussion Sample Preparation The grinding and blending process produced a homogeneous sample with a particle size that was readily attacked by the sodium peroxide fusion or the mixed acid digestion. Determination of 99Tc A sodium peroxide fusion was selected because it is effective for dissolving soils Tc is stable in an alkaline medium and peroxide oxidizes Tc to pertechnetate (Tc04-). The pro- portions of sample flux and nitric acid were selected to optimize sample dissolution yet yield a relatively dilute nitric acid solution. Having Tc in solution as pertechnetate is required for optimum extraction and dilute nitric acid is a suitable matrix for extraction of T c . ~ The small amount of turbidity present in the sample solutions was removed by filtration to prevent it from plugging the extraction column.Determination of 230Th and 234U The mixed-acid digestion is adequate for the determination of Th and U in most soils. If the presence of Th or U in refractory material is a concern a more rigorous digestion such as fusion with lithium metaborate may be desirable. The final sample solutions were prepared to contain 4 moll-' nitric acid because that acid concentration has been reported' to be suitable for the extraction process. Properties of the Extraction Resin The extraction resins used were found to be very durable. The resins are specific for certain analytes and most of the sample matrix constituents pass through the column without being retained. Columns are frequently used for 200 or more analyses. The TEVASpec resin usually has a longer life-time than the TRUSpec resin. Technetium is retained strongly on TEVASpec resin in dilute nitric acid solutions and is retained very weakly in strong nitric acid.7 Rhenium behaves similarly to Tc with TEVA-Spec resin and can act as a surrogate indicating recovery of Tc through the separation.Rhenium is added to samples and standards and 187Re is designated in the analysis pro- gramme as the internal standard for 99Tc. In this way compen- sation for varying recovery through the separation and for instrument drift is achieved in one step. Uranium and Th are retained strongly on-TRUSpec resin in strong nitric acid solutions and are retained very weakly in dilute nitric acid.8 Eluting with 0.1 moll-' ammonium oxalate produces a sharper elution peak than eluting with either dilute nitric acid or water. Use of 229Th and 233U as internal standards for 230Th and 234U is ideal because the chemistry of the internal standards in the separation is the same as that of the analytes.Elution Profiles Fig. 2 shows elution profiles obtained for 99Tc and Re stan- dards. Fig. 3 shows an elution profile obtained for a soil sample that contains 430 mBq g-' (0.684 ng g-') 99Tc. The concen- tration of 99Tc in the sample solution was 1.07Bq1-' (1.70 ng 1-I). Fig. 3 also shows the graph for '"Ru which was monitored to check for the potential interference by 99Ru. A 3min load time was used because the LODs obtained were adequate for this study. Lower LODs could be obtained using longer load times.This was determined by varying the load time of a 7.40 Bq 1-l (11.8 ng 1-') standard. The peak area of the standard increased linearly with respect to load time up to 8 min loading at a rate of 2.2 ml min-'. Fig. 4 shows an elution profile obtained for 229Th 230Th 233U and 234U standards. Fig. 5 shows an elution profile obtained for a soil sample that contains 1.63Bqg-' (2.27 ng g-') 230Th and 1.65 Bq g-' (7.20 ng g-') 234U. The concentrations of 230Th and 234U in the sample solution are 16.3 Bq 1-' (22.7 ng 1-') and 16.5 Bq 1-' (72.0 ng l-') respectively. In all cases the peaks rise sharply and return to the baseline rapidly with minimal tailing.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 931 22 20 r lm 15 m C 3 0 Y m z 3 10 Y .- m 8 Q) C w - E ' I I 1 0 5 10 15 20 Time/s Fig.2 Elution profile for a standard containing 50 ng 1-' of Re and 7.40Bql-'(11.8ngl-') o ~ ~ ~ T c 900 800 7 600 CJY v) c 3 0 > .C 400 C 4- .. Y - 200 0 5 10 15 20 Timels Fig.3 Elution profile for a soil sample containing 429 mBqg-' (0.682 ng g-') of 99Tc Interference Management Determination of99Tc The measurement method could be subject to interferences from 99Ru because 99Tc cannot be distinguished from 99Ru. Ruthenium is a very rare element with an average abundance in the earth's crust of the order of 1 ng g-' of which 99Ru makes up 12.7%. Naturally occurring Ru is not expected to present a problem because it is so scarce and it is separated by the extraction column. Ruthenium-99 is also a fission product produced from the decay of 99Tc by beta particle emission.However 99Ru resulting from 99Tc decay is also expected to be scarce because of the 212000 year half-life of 99Tc and the fact that 99Tc has only been produced from fission for approximately 50 years. High concentrations of Mo could cause an interference if the "'Mo peak is large enough to overlap with mass 99. The magnitude of the problem depends on the abundance sensitivity 0 5 10 15 20 Time/s Fig. 4 Elution profile for a standard containing 180 Bq 1-' (22.8 ng 1-') of 229Th 30.9 Bq 1-' (43.0 ng 1-') of 230Th 18.5 Bq 1-' (52.8 ng 1-I) of 233U and 11.8 Bq I-' (51.5 ng 1-') of 234U I ' I \ \ 0 5 10 15 20 Ti me/s Fig. 5 Elution profile for a soil sample containing 1.63 Bq g-' (2.27 ng g-') of 230Th and 1.65 Bq g-' (7.20 ng g-') of 234U of the ICP mass spectrometer that is used.Newer instruments generally have better abundance sensitivities than older systems (typically greater than 1 x lo6). The efficiency of separating Tc from Ru and Mo was determined with the following experiment. A standard contain- ing lOOpgl-' each of Mo Ru and Re was analysed by conventional aspiration to determine response factors for Mo and Ru relative to Re. A standard containing lOOpgl-' Mo 1 pg 1-l Ru and 50 ng 1-l Re was analysed by the FI-ICP-MS method. Using the peak areas obtained for Mo Ru and Re the previously determined response factors and the known concentration of Re in the standard the concentrations of Mo and Ru were estimated in the eluent. Based on the estimated concentrations in the eluent and the known concentrations in the original standard the separation efficiency was calculated.The separation efficiency varied slightly between columns from 97% to greater than 99.5%. Similar experiments were per-932 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Table 3 Comparison of 99Tc results obtained by FI-ICP-MS and radiochemical methods for the PORT11 soil sample I FI-ICP-MS Radiochemical method6 mBq g-' ng g-' mBq g-' ng 8-l 33 30 40 38 32 36 42 33 43 35 Mean result +95% confidence interval 37-13 35 35 58 31 35 36 34 39 39 0.05 2 0.048 0.060 0.05 1 0.057 0.067 0.053 0.068 0.0515 0.064 0.056 0.056 0.092 0.049 0.056 0.057 0.054 0.062 0.062 52 41 44 56 41 48 41 44 41 44 0.083 0.065 0.070 0.089 0.065 0.076 0.065 0.070 0.065 0.070 0.059 k 0.005 45*4 0.072 +_ 0.006 Table 4 Comparison of 99Tc results obtained by FI-ICP-MS and radiochemical methods for the PORT13 soil sample FI-ICP-MS Radiochemical method 455 407 440 448 418 422 433 414 396 Mean result k 95% confidence interval mBq g-' 427 9 426 426 459 396 422 422 429 426 440 0.723 0.647 0.700 0.71.2 0.665 0.6?1 0.688 0.658 0.630 0.678 k 0.014 0.677 0.677 0.730 0.630 0.67 1 0.671 0.682 0.677 0.700 mBq g-' 426 437 433 429 455 437 41 1 433 440 433 433 * 8 ng g-' 0.677 0.695 0.688 0.682 0.723 0.695 0.653 0.688 0.700 0.688 0.689 k0.013 Table 5 Results obtained for 230Th and 234U in the soil reference material TRM-4 Mean result i- 95% confidence interval Reference values L 95% confidence interval 230Th mBq g-' 1680 1610 1700 1700 1710 1670 1640 1670 1620 1710 1671 L27 1643 i- 10 ng g-' 2.34 2.24 2.37 2.37 2.38 2.33 2.28 2.33 2.26 2.38 2.33 k 0.04 2.29 f 0.01 234u mBq g-' 1690 1700 1620 1640 1640 1620 1650 1670 1690 1650 1657+ 21 1650f9 ng 8-' 7.38 7.42 7.07 7.16 7.16 7.07 7.21 7.29 7.21 7.38 7.24 -t 0.09 7.20 -t 0.04 formed using soil samples spiked with Ru and Mo prior to fusion and comparable results were obtained.The interference check standards described above under Quality Control were analysed at the beginning and end of each analytical run to verify that separations from Mo and Ru are effective. Molybdenum-100 and "'Ru were monitored in each analysis in order to verify the absence of interferences. Determination of 230Th and 234U No significant interferences were observed for the determi- nation of 230Th.Thorium-232 present in samples could interfere with the determination of 234U by formation of ThH which overlaps with 233U the internal standard for 234U. The ratio of ThH to 232Th is approximately 0.01%. Natural Th is essentially 100% 232Th. Any 232Th present in the samples is concentrated by the FI process. Thorium-232 oxide was monitored at mlz248 in each analysis to give an indication of the amount of 232Th present in the samples without exposing the detector to the much higher count rates that would be observed at mlz232. These data were used to flag any potential problems. The 232Th concentrations in the samples analysed for this study were not high enough to cause problems. The concen- tration of 232Th in the interference check standards was selected to demonstrate that the method could tolerate 232Th at the approximate levels found in the samples studied.Several options exist if 232Th is present in samples at high enoughJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 933 concentration to interfere with 233U. The samples could be diluted or a shorter load time could be used however the LOD would be raised. An inter-element correction factor could be used to correct the 233U signal based on the observed 232Th'60 signal. This approach will be investigated in this laboratory in the future. Method Performance Detection limits Detection limits were determined by analysing a blank ten times and multiplying the standard deviation obtained for the ten readings by 3.The LOD for "Tc was 26 mBq 1-l (0.04 ng 1-') in solution or 11 mBq g-' (0.02 ng g-') in the soil. The LOD for "Tc by conventional aspiration ICP-MS was 200 mBq 1-I. The LODs for 230Th and 234U were 37 mBq 1-'(0.05 ng 1-l) and 7.4 mBq 1-l (0.03 ng 1-') respectively. This corresponds to 3.7 mBq g- (0.005 ng g- ') and 0.74 mBq g- ' (0.003 ng g-') in the soil for 230Th and 234U respectively. The LODs for 230Th and 234U by conventional aspiration were 960 mBq 1-' (1.3 ng 1-') and 160 mBq 1-l (0.7 ng 1-') respectively. The LOD for 99Tc by the radiochemical method was 11 mBq g- ' (0.02 ng g-').6 Accuracy and precision of the method for " Tc There are no soil reference materials with known amounts of "Tc available to verify the accuracy of the method. Therefore samples of two soils contaminated with "Tc were obtained and prepared for use as reference materials.Approximately 10 kg of a soil designated PORT1 1 and 3 kg of a soil designated PORT13 were obtained from the Portsmouth Gaseous Diffusion Plant site in Piketon OH USA. The samples were dried and ground as described above under Preparation of Soil Samples blended for 1 d and packaged in 100 g bottles. Ten bottles of each sample were shipped to the Portsmouth Plant laboratory for determination of "Tc by the radiochemi- cal method described under Experimental. The samples were also analysed by the FI-ICP-MS method. The results are given in Tables 3 and 4. The 95% confidence intervals were calculated as described by Natrella' based on the t-distribution by multiplying the appropriate t value by the estimated sample standard deviation and dividing by the square root of the number of measurements.A statistical test for comparing results obtained by different techniques described by Natrella" was used to determine if the mean results obtained by the two methods agreed. The test uses the estimated variances of the data sets and the t statistic to calculate a critical value for the difference in the means. If the absolute value of the difference in the means is not greater than the critical value there is no reason to believe that the means differ. For the PORT11 data the test concluded that the means of the FI-ICP-MS and radiochemical results do differ statistically at the 95% confidence level indicating that a bias exists in at least one of the data sets.This is not of great concern considering the proximity of the results to the LOD of the method and that the agreement of the two means is good enough to satisfy the objectives of the projects that the methods are used to support. The statistical test of the two PORT13 data sets concluded that there is no reason to believe that the mean values differ at the 95% confidence level. Recoveries of pre-digestion spikes were 93.3 and 99.7% for PORT11 and PORT13 respectively by the FI-ICP-MS method. Accuracy and precision of the method for 230Th and 234U Accuracy and precision of the 230Th and 234U method were assessed by analysing a soil reference material designated TRM-4 that was prepared and characterized by the US DOE Grand Junction Projects Office laboratory.'' The results obtained by FI-ICP-MS are given in Table 5.The 95% confi- dence intervals were calculated as described for "Tc. The recommended values were established for the reference soil by alpha-energy spectrometry. The data provided in ref. 11 were used to calculate the 95% confidence intervals for the rec- ommended values and to statistically compare the means of the FI-ICP-MS and alpha-energy spectrometry data sets as described for "Tc. The test concluded that there is no reason to believe that the mean values for either 230Th or 234U obtained by the two techniques differ at the 95% confidence level. Recoveries of pre-digestion spikes were 93.0 and 99.0% for 230Th and 234U respectively for TRM-4 by the ICP-MS method. Conclusions The FI-ICP-MS method described has been used in this laboratory regularly over the past year and has proven to be accurate cost-effective and reliable.The FI approach using solid-phase adsorption on a mini-column is effective only not in improving sensitivity and LODs but also in reducing physical and spectroscopic interferences. The lower cost and higher sample throughput compared with radiochemical methods make the method especially attractive for environmen- tal applications such as those involved in many US DOE environmental restoration and waste management projects. Work by RUST Geotech personnel was performed under DOE contract No. DE-AC04-861D12584 for the U.S. Department of Energy. Radiochemical "Tc analyses were conducted by Billy Short Martin Marietta Utility Services Radiochemistry Division Piketon OH USA. 1 2 3 4 5 6 7 8 9 10 11 References Cox R. J. Pickford C. J. and Thompson M. J. Anal. At. Spectrom. 1992 7 635. Grohs J. F. and Hollenbach M. H. paper presented at the 30th Rocky Mountain Conference Denver CO USA July 31-August Riglet C. Provitina O. Dautheribes J. and Revy D. J. Anal. At. Spectrom. 1992 7 923. Nichols S. Sanders T. W. and Blaine L. M. Sci. Total Environ. Cotton F. A. and Wilkinson G. Advanced Inorganic Chemistry 3rd edn. Interscience New York 1972 p. 990. Short B. W. Piketon OH USA 1993 private communication. Horwitz E. P. and Chiarizia R. unpublished work from the Chemistry Division Argonne National Laboratory Argonne IL USA 1991. Horwitz E. P. Chiarizia R. Dietz M. L. and Diamond H. Anal. Chim. Acta 1993 281 361. Natrella M. G. Experimental Statistics National Bureau of Standards Handbook 91 US Government Printing Office Washington D.C. 1966 pp. 2-1-2-4. Natrella M. G. Experimental Statistics National Bureau of Standards Handbook 91 US Government Printing Office Washington D.C. 1966 pp. 3-26-3-28. Donivan S. and Chessmore R. Uranium Reference Materials publication number UNC/GJ-36(TMC) UNC Geotech Technical Measurements Center US Department of Energy Grand Junction Projects Office Grand Junction CO 1987. 5 1988. 1993 130-131,275. Paper 4/01 1720 Received February 2 1994 Accepted May 18 1994

ISSN:0267-9477    

DOI:10.1039/JA9940900927

出版商:RSC    

年代:1994

数据来源: RSC