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VAPOR GENERATION TECHNIQUES FOR TRACE ELEMENT ANALYSIS

VAPOR GENERATION TECHNIQUES FOR TRACE ELEMENT ANALYSIS FUNDAMENTAL ASPECTS Edited by

ALESSANDRO D’ULIVO CNR, Institute of Chemistry of Organometallic Compounds, Pisa, Italy

RALPH E. STURGEON Inorganic Chemistry Group, Metrology, National Research Council Canada, Ottawa, ON, Canada

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-323-85834-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Kathryn Eryilmaz Editorial Project Manager: Ivy Dawn Torre Production Project Manager: R. Vijay Bharath Cover Designer: Mark Rogers Typeset by MPS Limited, Chennai, India

List of contributors Eduardo Bolea Group of Analytical Spectroscopy and Sensors (GEAS), Institute of Environmental Sciences (IUCA), University of Zaragoza, Zaragoza, Spain Sebastian Burhenn Bochum, Germany

Experimental

Alessandro D’Ulivo CNR, Compounds, Pisa, Italy

Physics

Institute

of

II,

Ruhr-University

Chemistry

of

Bochum,

Organometallic

Jiřı́ Dědina Institute of Analytical Chemistry of the Czech Academy of Sciences, Brno, Czech Republic Alessandro D’Ulivo CNR, Compounds, Pisa, Italy

Institute

of

Chemistry

of

Organometallic

Zuzana Gajdosechova National Research Council Canada, Ottawa, ON, Canada Xiandeng Hou Analytical & Testing Center, Sichuan University, Chengdu, P.R. China; Key Lab of Green Chem & Tech of MOE at College of Chemistry, Sichuan University, Chengdu, P.R. China Jan Kratzer Czech Academy of Sciences, Institute of Analytical Chemistry, Brno, Czech Republic Francisco Laborda Group of Analytical Spectroscopy and Sensors (GEAS), Institute of Environmental Sciences (IUCA), University of Zaragoza, Zaragoza, Spain Xing Liu State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan, P.R. China Yue Liu

College of Chemistry, Tianjin Normal University, Tianjin, P.R. China

Tomáš Matoušek Institute of Analytical Chemistry of the Czech Academy of Sciences, Brno, Czech Republic Stanislav Musil Institute of Analytical Chemistry of the Czech Academy of Sciences, Brno, Czech Republic Enea Pagliano

National Research Council Canada, Ottawa, ON, Canada

Ralph E. Sturgeon Inorganic Chemistry Group, National Research Council Canada, Metrology, Ottawa, ON, Canada Xiaodong Wen

College of Pharmacy, Dali University, Dali, P.R. China

Yafei Zhen Analytical & Testing Center, Sichuan University, Chengdu, P.R. China

ix

x

List of contributors

Chengbin Zheng Key Lab of Green Chem & Tech of MOE at College of Chemistry, Sichuan University, Chengdu, P.R. China Zhenli Zhu State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan, P.R. China Zhirong Zou College of Chemistry and Material Science, Sichuan Normal University, Chengdu, P.R. China; Analytical & Testing Center, Sichuan University, Chengdu, P.R. China

Contents

List of contributors Preface

ix xi

1. Introduction to vapor generation techniques

1

Alessandro D’Ulivo and Ralph E. Sturgeon

1.1 Introduction 1.2 Limitations of current sample introduction and atomization techniques 1.3 Vapor generation techniques 1.4 Favorable features and shortcomings of VGTs 1.5 Overview of book structure and content References

1 2 5 8 10 12

Part I Chemical Vapor Generation

17

2. Chemical vapor generation by aqueous boranes

19

Alessandro D’Ulivo

2.1 Introduction and historical background 2.2 Borane reagents, reaction products, and apparatus 2.3 Processes and mechanisms of chemical vapor generation 2.4 Factors controlling reactivity in chemical vapor generation 2.5 Interferences 2.6 Final remarks, open questions, and future trends References

19 21 32 51 63 73 74

3. Chemical vapor generation of transition and noble metals

91

Stanislav Musil and Tomáš Matoušek

3.1 Introduction and background 3.2 Experimental implementations of chemical vapor generation 3.3 Efficiency of chemical vapor generation 3.4 Detailed discussion of mechanisms and fundamental processes in chemical vapor generation 3.5 Shortcomings with theory, remaining problems, and limitations 3.6 Conclusions and future developments Acknowledgements References

v

91 92 104 108 119 120 122 122

vi

Contents

4. Chemical vapor generation by aqueous phase alkylation

129

Zuzana Gajdosechova and Enea Pagliano

4.1 Introduction 4.2 CVG with tetraalkylborates 4.3 CVG with trialkyloxonium salts 4.4 Metal speciation with Grignard reagents 4.5 Future trends and perspectives References

129 130 136 143 144 145

5. Other chemical vapor generation techniques

153

Alessandro D’Ulivo, Yue Liu and Ralph E. Sturgeon

5.1 Introduction 5.2 Chelate formation 5.3 Thermal chemical vapor generation 5.4 Generation of volatile oxides 5.5 Chemical vapor generation of volatile chlorides 5.6 Chemical vapor generation of volatile fluorides 5.7 Chemical vapor generation of volatile bromides 5.8 Chemical vapor generation of volatile carbonyls 5.9 Chemical vapor generation of boron esters 5.10 Chemical vapor generation using SnCl2 5.11 Concluding remarks References

153 154 163 165 169 172 173 173 176 179 179 180

6. Chemical vapor generation in nonaqueous media

191

Xiaodong Wen

6.1 Introduction and background 6.2 Early studies on chemical vapor generation in nonaqueous media 6.3 Experimental implementation of the technique 6.4 Fundamental processes; theory and mechanisms 6.5 Remaining problems, limitations, and shortcomings 6.6 Future developments 6.7 Conclusions References

191 193 194 200 205 205 206 207

Part II Non-Chemical Vapor Generation

211

7. Photo-sono-thermo-chemical vapor generation techniques

213

Ralph E. Sturgeon

7.1 General introduction 7.2 Photochemical vapor generation 7.3 Sonochemical vapor generation 7.4 Thermochemical vapor generation 7.5 Concluding remarks References

213 214 249 252 252 252

Contents

8. Catalysts in photochemical vapor generation

vii 265

Zhirong Zou, Yafei Zhen, Chengbin Zheng and Xiandeng Hou

8.1 Introduction 8.2 Heterogeneous catalysis 8.3 Homogeneous catalysis 8.4 Conclusions Acknowledgments References

265 267 271 275 276 276

9. Plasma-mediated vapor generation techniques

283

Xing Liu and Zhenli Zhu

9.1 General introduction 9.2 Sources for plasma-mediated vapor generation 9.3 Influence of coexisting ions on PMVG 9.4 Analytical performance and applications of PMVG 9.5 Possible mechanisms of PMVG 9.6 Concluding remarks and future trends References

283 284 301 305 307 310 311

10. Electrochemical vapor generation

317

Eduardo Bolea and Francisco Laborda

10.1 Introduction and background to electrochemical vapor generation 10.2 Fundamentals and experimental implementation of ECVG 10.3 Mechanisms of ECVG 10.4 Shortcomings and limitations: interferences in ECVG 10.5 Final remarks and future developments References

317 318 328 330 338 339

Part III Atomization Devices

347

11. Nonplasma devices for atomization and detection of volatile metal species by atomic absorption and fluorescence

349

Jiřı́ Dědina

11.1 Introduction 11.2 Processes taking place in online atomizers 11.3 Online atomization—preliminary considerations 11.4 Online atomizers 11.5 In-atomizer collection—preliminary considerations 11.6 Experimental approaches to in-atomizer collection 11.7 Conclusions and future perspectives Acknowledgments Dedication References

349 351 352 354 380 382 391 392 392 392

viii

Contents

12. Dielectric barrier discharge devices

403

Jan Kratzer and Sebastian Burhenn

12.1 Introduction 12.2 DBD concept and designs 12.3 Plasma chemistry: processes and species 12.4 Analytical applications 12.5 DBD atomizers for AAS 12.6 DBD atomizers for AFS 12.7 DBD excitation for OES 12.8 Analyte preconcentration 12.9 Speciation analysis 12.10 Future perspectives Acknowledgment References

403 404 406 407 409 417 419 427 432 435 437 437

Abbreviations and symbols

443

Index

447

Preface Vapor generation techniques (VGTs) coupled with optical atomic and mass spectrometric detection platforms have been employed for over 50 years to augment analytical performance for the determination and speciation of many elements at trace and ultratrace levels. Since their introduction, the popularity of chemical VGTs has dramatically increased, and many analytical protocols and regulated methodologies employ these derivatization techniques as they simultaneously achieve efficient analyte-matrix separation. Moreover, despite significant recent advances in analytical atomic spectrometry, most notably those in the inorganic mass spectrometry sector, the pursuit of the benefits accrued by interfacing VG techniques as complementary approaches to conventional sample introduction to yield further enhancements in measurement specificity, detection power, and minimization of spectral interferences undoubtedly persists.

Some 25 years have passed since the monograph by Dědina and Tsalev [1] devoted to VG (primarily hydride generation) for trace element analysis was published, comprehensively addressing theoretical and experimental aspects in a critical and in-depth manner. In the interim, new facts and theories have emerged since 1995 which complement/expand those targeted in that treatise, including introduction of several promising new and revolutionary approaches to VG. Only recently, beginning in 2003, dedicated studies of the mechanisms of chemical VG have shed new light on this field with the aim of removing older empirical ideas associated with their use. Almost simultaneously, the introduction of alternative VGTs (photochemical, electrochemical, thermochemical, and sonochemical, chelate, and plasma mediated) provided a further impulse to renew interest and use of these techniques.

In spite of numerous applications, very few efforts have focused on understanding the fundamental processes governing the derivatization reactions associated with the various VGTs. Sources of information, unfortunately, remain mostly fragmented in many papers published across a spectrum of scientific journals. This book, Vapor Generation Techniques for Trace Element Analysis: Fundamental Aspects, is devoted to comprehensive coverage of the fundamental aspects of VGTs, encompassing methodologies ranging from the classical chemical approaches to the most recent frontiers. A thorough overview and critical discussion of the state-of-the-art of knowledge of the mechanisms that control the

xi

xii

Preface

generation of volatile derivatives and their atomization/detection by atomic spectrometry as well as various mechanisms accounting for interference effects is presented in sufficient detail to encompass a thoroughly expanded coverage of the current literature in a single-source collection of material. While serving as an expert source of advice for information relevant to the interpretation of experimental results and guidance for implementation of VGTs in the laboratory, it is not intended to present a comprehensive review of applications, with the exception of those contributing further basic knowledge via adoption of leading-edge technology or shedding light on some essential attribute(s) of the chemistry of the processes involved.

With this objective in mind, the Editors have judiciously identified internationally renowned experts at the forefront of their individual fields of VG and secured contributions which serve to achieve the objectives of this treatise. A homogeneous, coherent approach is followed in each chapter with IUPAC terminology being used throughout, along with a consistent set of definitions and related symbols/abbreviations.

Following a brief introduction to generally assess the state of each technique from a balanced perspective of advantages and shortcomings, a comprehensive discussion of fundamental processes, including interferences and open problems, is presented. Relevant conclusions are supported with prospects for future directions. The book is divided into three sections. Part 1 deals with chemical VG (CVG), encompassing CVG by aqueous boranes and aqueous phase alkylation, CVG in nonaqueous media and that of transition and noble metals, and other classical chemical VGTs. Part 2 covers nonchemical VG, highlighting photo-sono-thermo-chemical processes and the role played by catalysts in such processes, plasma-mediated VG and electrochemical VG. Part 3 is devoted to an in-depth discussion of a variety of dedicated nonplasma atomization devices utilized for the detection of VG species by atomic absorption and fluorescence as well as dielectric barrier discharge (DBD) devices. Although powerful ICP-based optical emission and mass spectrometric instrumentation are frequently coupled with VGTs to enhance detection power or mitigate spectral interferences, these sources, as well as classical flames, are not treated in this volume as they are not specifically dedicated to the detection of volatile VG species in the manner that miniature diffusion flames, quartz tube atomizers and DBD devices have evolved and are used.

The Editors are grateful not only to the contributing authors for sharing their expertise, amicable discussions, hard work, and maintenance of a delivery schedule during a period of COVID-19 challenges but also to the staff of Elsevier for their unwavering support of this project and the production of this volume.

Preface

xiii

Neophytes as well as expert practitioners should find interest in this book as it serves as a comprehensive, single-source overview of recent developments, providing readers with an understanding of the correct implementation and limitations underlying the application of VGTs to everyday analytical problems facing the trace element analyst. As such, the content will appeal to those in research and academic institutions, as well as private sector laboratories and the analytical instrumentation industry.

Alessandro D’Ulivo1 and Ralph E. Sturgeon2 1

2

CNR, Institute of Chemistry of Organometallic Compounds, Pisa, Italy, Inorganic Chemistry Group, Metrology, National Research Council Canada, Ottawa, ON, Canada

Reference [1] J. Dědina, D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, John Wiley & Sons, Inc., Chichester, 1995.

Abbreviations and symbols AAS AB ABC ac-SEGD AFS APDC at_sensit BH BTC CAU cb (CB) CBH CCP CE CF Che-CVG CPE CTAB Cys CQDs CQTA CT CTTS CVG DART DBD DBDTP DCP DDAB DDTC DEDTP DLLME DMA DMAB DMDTC DMF DF ECD ECVG EKE ELCAD ESI ESR/EPR ETAAS

atomic absorption spectrometry ammonia borane [BH3NH3] analyte-borane complex intermediates alternating current-driven solution electrode glow discharge atomic fluorescence spectrometry ammonium pyrrolidine dithiocarbamate (pyrrolidine-1-carbodithioate) sensitivity registered by a particular atomizer/spectrometer system (s/g) hydridoboron species 1,3,5-benzenetricarboxylic acid Christian Albrechts University conduction band sodium cyanotrihydridoborate [NaBH3CN] capacitively coupled plasma capillary electrophoresis continuous flow chelate chemical vapor generation cloud point extraction (rapid and traditional) cetyltrimethylammonium bromide L-cysteine carbon quantum dots conventional quartz tube atomizer cryogenic trap charge transfer-to-solvent chemical vapor generation direct analysis in real time dielectric barrier discharge dibutyldithiophosphate direct current plasma didodecyldimethylammonium bromide diethyldithiocarbamate (N,N-diethylcarbamodithioate) O,O0 -diethyldithiophosphate dispersive liquid liquid microextraction dimethylarsenic acid dimethylamine borane [BH3NH(Me)2] dimethyldithiocarbamate dimethylformamide diffusion flame electron capture detector electrochemical vapor generation electrokinetic extraction electrolyte cathode atmospheric glow discharge electrospray ionization electron spin/paramagnetic resonance electrothermal atomization atomic absorption spectrometry

443

444 EtHg ETV FES FEP FI FIF FIGS FLA-APGD FTIR GC-MS GD GF(A) GF AAS HAC HFA HG HMC HPLC HR HRMS HTFA HTHB IAT IC ICP ICP OES ICP MS ICP-ToF-MS IEP ITEX LC LDLS LE-DBD LEGD LMTC LIF LLE LMWCA LOD LPME LSDBD MC MDF MECA MeHg MIBK MIL MIP MMA MMQTA MWCNT MSPD

Abbreviations and symbols

ethylmercury [CH3CH2Hg1] electrothermal vaporization flame emission spectroscopy fluorinated ethylene propylene flow injection flame-in-flame (atomizer) flame in-gas-shield (atomizer) flowing liquid anode atmospheric pressure glow discharge Fourier transform infrared spectroscopy gas chromatography mass spectrometry glow discharge graphite furnace (atomizer) graphite furnace atomic absorption acetylacetone (pentane-2,4-dione) hexafluoroacetylacetone (1,1,1,5,5,5-hexafluoropentane-2,4-dione) hydride generation hydrido-metal(loid) complex high performance liquid chromatography high resolution high-resolution mass spectrometry trifluoroacetylacetone (1,1,1-trifluoropentane-2,4-dione) hydroxytrihydridoborate [BH3OH2] integrated atom trap ion chromatography inductively coupled plasma inductively coupled plasma optical emission spectrometry inductively coupled plasma mass spectrometry inductively coupled plasma time-of-flight mass spectrometry isoelectric point in-tube extraction liquid chromatography laser-driven light source liquid electrode dielectric barrier discharge liquid electrode glow discharge ligand-to-metal charge transfer laser-induced fluorescence liquid liquid extraction low-molecular-weight carboxylic acid limit of detection liquid-phase microextraction liquid spray DBD multicollector miniature diffusion flame (atomizer) molecular emission cavity analysis methylmercury [CH3Hg1] methylisobutylketone Materials of Institute Lavoisier microwave-induced plasma monomethylarsonic acid multiple microflame quartz T-tube (atomizer) multiwalled carbon nanotube matrix-assisted solid-phase dispersion

Abbreviations and symbols

MS NACVG NCI NDAFS NFDBD NMR OES OH PCARD PCVG PCN PEVG PFA PN PMVG PTFE Py-CVG QA QTA RCPE REE RNS ROS RTIL RVC SA-DLLME SAGD SC SCGD SCVG SDBS SD-SEGD SeMet SHE SIFT MS SN sp ICP MS SPME SPE supply(t) TALIF TBAB TBT TCVG TEM TFDBD TFDDTC THB TMB TMAB TOC ToF MS

445

mass spectrometry chemical vapor generation in nonaqueous solvents negative chemical ionization nondispersive atomic fluorescence spectrometry nebulized film DBD nuclear magnetic resonance optical emission spectroscopy observation height photocatalyst-assisted reduction device (microfluidic-based) photochemical vapor generation porous coordination network plasma electrochemical vapor generation perfluoroalkoxy pneumatic nebulization plasma-mediated vapor generation polytetrafluoroethylene pyrolysis chemical vapor generation quartz atomizer quartz T-tube atomizer rapid cloud point extraction rare-earth element reactive nitrogen species reactive oxygen species room temperature ionic liquid reticulated vitreous carbon surfactant-assisted dispersive liquid liquid microextraction solution anode glow discharge semiconductor solution cathode glow discharge sonochemical vapor generation sodium dodecylbenzenesulfonate single-drop solution electrode glow discharge selenomethionine standard hydrogen electrode selected ion flow tube mass spectrometry solution nebulization single particle mode inductively coupled plasma mass spectrometry solid-phase microextraction solid polymer electrolyte cell with Nafion membrane mass of VS analyte delivered to an atomizer per unit time (g/s) two-photon absorption laser-induced fluorescence tertbutylamine borane [BH3NH2C(Me)3] tributyltin (chloride) thermochemical vapor generation transmission electron microscopy thin-film DBD bis-(trifluoroethyl)-dithiocarbamate [bis(2,2,2-trifluoroethyl)carbamodithioate] tetrahydridoborate, [BH4]2 trimethylborate trimethylamine borane [BH3N(Me)3] total organic carbon time-of-flight mass spectrometry

446 UA-DLLME UARS-CPE UiO UV VIS vb (VB) VG VGT VS XAT XPS

Abbreviations and symbols

ultrasound-assisted dispersive liquid liquid microextraction ultrasound-assisted rapid synergistic cloud point extraction University of Oslo ultraviolet visible spectroscopy valence band vapor generation vapor generation technique volatile species radical halogen atom transfer X-ray photoelectron spectroscopy

Symbols αT αi εext εin εd εs εt εc or βtrap εr βv βa βi βgen βvol Na Ni Nm NT R(t) Rarea

global efficiency of free atom production (dimensionless) degree of ionization extraction efficiency (in liquid liquid or liquid solid extraction) nebulization efficiency in nebulization system (dimensionless) derivatization efficiency of a VG process (dimensionless) efficiency of transfer of derivatized analyte from the liquid-to-gas phase (dimensionless) efficiency of transfer of volatile derivatized analyte to the detector (dimensionless) efficiency of collection or trapping a VG species (dimensionless) efficiency of release of trapped or preconcentrated VG species (dimensionless) vaporization efficiency in flames and plasmas (dimensionless) atomization efficiency (dimensionless) atom ionization efficiency (dimensionless) overall stage generation efficiency [β gen 5 εd εs εt] (dimensionless) the fraction of trapped analyte which is volatilized (and transported to the observation volume of the detector, dimensionless) number of free atoms (dimensionless) number of ions (dimensionless) number of molecules (dimensionless) total number of atoms (dimensionless) temporal value of a measured signal (dimensionless) integrated value of R(t) (s)

C H A P T E R

1 Introduction to vapor generation techniques Alessandro D’Ulivo1 and Ralph E. Sturgeon2 1

CNR, Institute of Chemistry of Organometallic Compounds, Pisa, Italy, 2 Inorganic Chemistry Group, National Research Council Canada, Metrology, Ottawa, ON, Canada

1.1 Introduction Analytical chemistry increasingly plays an important role in the study and protection of health and the environment, quality control of materials, forensic science, and many other fields in which elemental analysis and speciation are needed, frequently achieved with atomic and molecular spectroscopic procedures. Vapor generation techniques (VGTs), coupled with such detection systems, were introduced more than 50 years ago [1,2] by interfacing early atomic absorption instrumentation with wellknown VG chemical reactions [chemical vapor generation (CVG)], such as the generation of mercury vapor by reduction of mercuric ions with SnCl2, and that of AsH3 from inorganic arsenic using the classic 1836 Marsh test [3]. Subsequent to these first chemical approaches to VG, many other chemical reactions followed for the production of volatile derivatives of elemental species [4], including introduction of novel derivatization methods based on chemical and physical principles different from CVG. The increase in popularity of the different VGTs expanded over the years due to their favorable analytical characteristics, among them ease of automation combined with the possibility of achieving unmatched analytical figures of merit using simple and inexpensive instrumentation. Based on their application and validation, thousands of research papers and many regulated analytical methods arose. Unfortunately, much less attention has been devoted to the elucidation of fundamental

Vapor Generation Techniques for Trace Element Analysis DOI: https://doi.org/10.1016/B978-0-323-85834-2.00001-X

1

© 2022 Elsevier Inc. All rights reserved.

2

1. Introduction to vapor generation techniques

aspects of the mechanisms of generation of the vapor species and their atomization processes in dedicated devices, as well as interference effects taking place at various stages of the analytical procedures.

This book is devoted to a comprehensive coverage of the fundamental aspects of VGTs, presenting the most recent VG approaches that have been added to this field, and discussing the state-of-the-art of knowledge on the mechanisms that control the generation of volatile derivatives and their atomization, as well as various mechanisms accounting for interference effects.

1.2 Limitations of current sample introduction and atomization techniques Problems relating to sample introduction techniques remain a major concern for trace element determination and speciation by analytical atomic spectrometry. Nearly 30 years after the invention of atomic absorption spectroscopy (AAS) by Sir Alan Walsh [5,6] and its commercial acceptance as an analytical technique, Browner and Boorn still considered sample introduction to be the Achilles’ Heel of atomic spectroscopy [7].

An ideal sample introduction technique should provide an efficient and economical means of reproducibly introducing analyte into the atomization/excitation/ionization system (i.e., detection device) while simultaneously preventing/minimizing possible interference from other components of the sample matrix. Nebulizers, predominantly employed for liquid sample introduction into flames and plasmas, possess poor sample utilization efficiencies because the volume fraction of sample successfully introduced into the atomization device, εin, is often lower than B5% (εin , B0.05) [7 10] when operating at sample uptake rates in excess of typically 100 μL min21. Most of the analytes that should have been transferred to the detection device are instead lost to the waste stream. Fig. 1.1 shows the impact of sample uptake rate on the efficiency with which the analyte reaches the detection device (typically a flame or plasma).

In the case of plasmas, deployment of expensive low-flow nebulizers operating below 10 μL min21 can achieve some 60% sample utilization efficiency. Operation of conventional nebulizers at mL min21 uptake rates results in generation of high primary droplet densities in spray chambers and their rapid coalescence to yield quite low tertiary aerosol outputs on the order of a few percent [8]. The tertiary droplet sample fraction, capable of being transported to the atomizer, is then desolvated to submicron-sized dry particles which, if typically smaller than 200 nm in diameter, are vaporized with high efficiency (β vB1) in hightemperature plasmas [10 12]. This is not always the case in flames as

Vapor Generation Techniques for Trace Element Analysis

1.2 Limitations of current sample introduction and atomization techniques

3

FIGURE 1.1 Analyte introduction (transport) efficiency, εin, as a function of sample uptake rate when coupled to an unheated double-pass spray chamber: (1) concentric (Meinhard TR-30-A3); (2) concentric (Meinhard HEN); (3) enhanced parallel path (Burgener Research MiraMist). Source: Reproduced with permission from J.W. Olesik, Inductively coupled plasma mass spectrometers, in: H. Holland, K. Turekian (Eds.), Treatise on Geochemistry, vol. 15, second ed., Elsevier, 2014, pp. 309 336, with permission from Elsevier [10].

FIGURE 1.2 Atomization efficiency, β a, for some selected elements in premixed airacetylene flames. Source: Compiled from B.W. Smith, G.E. Parsons, G.E. Bentley, Handbook of Flame Spectroscopy, Springer Science, New York, 1975 [13].

their temperatures are not high enough for the complete vaporization of the resultant dry residues of many refractory species (Fig. 1.2).

Ultimately, chemical and/or thermal processes bring about dissociation of vaporized molecules to yield gaseous free atoms, (the desired target

Vapor Generation Techniques for Trace Element Analysis

4

1. Introduction to vapor generation techniques

for AAS and atomic fluorescence spectrometric detection [AFS]), excited state atoms and ions [for inductively coupled plasma optical emission spectrometric detection (ICP OES)] and ions (for mass spectrometric [MS] detection). It should be noted that the sequential processes of liquid droplet transfer and desolvation followed by vaporization of the solid residue and its atomization, excitation, and ionization are kinetically controlled. All occur within a flowing stream of gases (combustion products in flames and inert support gas with plasmas) having structured temperature fields such that there is a finite time available within which the population of each species attains a steady state. This gives rise to the occurrence of optimal detection regions within the atomizer where local maxima occur for free atoms, excited state species, and ionized analyte. Within the atomizer, the total atom population, NT, is distributed among molecular species, Nm, free atoms (in ground and excited states), Na, and ionized species, Ni. Thus NT 5 Nm 1 Na 1 Ni [14].

The efficiency of the atomization process is defined as β a 5 Na/NT [14], and strongly depends on the element [9,11,14,15], as demonstrated by the data shown in Fig. 1.2, where in Na 1 Ni 5 total atomic species.

For most nonmetallic elements, the efficiency of atomization is lower than many of the metallic elements, sometimes by orders of magnitude.

The combined effects of εin, β v, and β a provide an indication of the global efficiency of free atom production, αT 5 εin β v β a. It is selfevident that αT cannot be greater than the introduction efficiency (αT # εin). In some cases, particularly nonmetals, even assuming β v 5 1, the αT values are very low, and in the part per million range [11,13].

Graphite furnace atomizers (GF), mostly employed in combination with AAS (GF AAS), possess much higher atomization efficiencies than flames [16 18] as well as more favorable sample vaporization characteristics, that is, β v approaches unity (due to greater effective temperatures and typically 103-fold longer residence times during which all processes can proceed). Note that in this case, the effective εin 5 1 for discrete sample introduction. It follows that αT values are also much higher than in flames, typically 0.01 # αT # 1. As a result, despite the small sample volumes used, graphite furnaces yield better detection limits than comparable AAS measurements in flames and optical emission from plasmas, making them sometimes competitive with ICP MS systems [19,20].

For graphite furnaces, the Achilles’ Heel manifests itself in the form of interferences, which can be much more serious than with flame and plasma-based techniques. Many studies have been devoted to this topic and its control by such means as use of matrix modifiers [19], graphite tube surface modifiers [19,21], Zeeman background correction [19,21], and continuum source high-resolution AAS spectrometers [20].

Vapor Generation Techniques for Trace Element Analysis

1.3 Vapor generation techniques

5

1.3 Vapor generation techniques A number of the limitations and shortcomings noted above for liquid sample introduction into flames, plasmas, and graphite atomizers can largely be overcome by the adoption of VGTs and, not surprisingly, Bings et al. concluded that the ideal sample, even for an ICP, would be gaseous [22]. In general, VG is a process wherein nonvolatile (usually ionic, metallic, or organometallic) compounds form volatile or semivolatile species through chemical, physical, or biological processes that may result in their transfer from the condensed phase to the gas phase [23]. In analytical chemistry, VGTs arise from derivatization of the analytical species of interest [solvated atomic ions, oxyanions, oxycations, and simple organometal (-semimetal) species] to create nonpolar or low-polar volatile species, as for example hydrides, alkylated compounds, various complexes with inorganic and organic ligands and other volatile species, which remain to be identified [23].

VGTs can most generally be characterized by two independent major stages: the generation of volatile forms of targeted species (performed in a generator or reactor) and atomization/detection of such species (achieved in an atomizer integrated with an atomic spectrometer).

Whereas the second stage is specific for the given spectroscopic method, the first is common to all VG approaches.

During the generation stage, the liquid sample (slurries have also been processed [24]) can be introduced into the reactor via several means, including from a continuous or flow injection system, as effluent from a chromatographic device (high performance liquid chromatography, ion chromatography, capillary electrophoresis, gas chromatography [GC], etc.) or by discrete injection. Clearly, the efficiency of this process is unity (equivalent to εin 5 1) and requires no further consideration.

As illustrated in Fig. 1.3, it is often desirable to further consider this overall generation stage in terms of two fundamental processes: chemical derivatization/transformation occurring in the reactor, characterized by an efficiency εd, and the phase transfer of the volatile species within a gas liquid separation (GLS) unit which occurs when the liquid reaction mixture is sparged by a (typically) inert gas, characterized by a phase separation efficiency εs, and its concurrent transfer for possible postprocessing characterized by an efficiency εt. A number of approaches can be used for the conversion of analytes to volatile species, including chemical derivatization by suitable chemical reagents (CVG and volatile chelate generation) and radical-mediated derivatization using different physical principles for radical production [photochemical VG (PCVG), sonochemical VG (SCVG), and thermochemical VG (TCVG)], plasmamediated derivatization VG (PMVG), and electrochemical derivatization VG (ECVG). As noted earlier, studies devoted to elucidation of the

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1. Introduction to vapor generation techniques

FIGURE 1.3 Schematic representation of a generic vapor generation technique coupled to an atomic absorption spectrometer illustrating the efficiencies characterizing all major steps, including: derivatization, wherein analytical species are converted to volatile derivatives with a generation efficiency εd, their transfer during sparging from the solution phase into the gas phase with an efficiency εs, and transport to the detection system with a transfer efficiency εt, either directly or after a concentration (trapping) step. If concentration/trapping of the volatile species is conducted followed by their rapid release, these processes are accomplished with a collection efficiency εtrap and subsequent release (volatilization) efficiency εvol. The volatile derivatives are either converted to free atoms, with an atomization efficiency β a, or remain as molecular species, being detected with a suitable spectroscopic technique. For atomic ion detection, ionization efficiency β i 5 Ni/NT is considered.

mechanisms governing the generation of volatile species using different VGTs are relatively scarce compared to those dedicated to their application. The mechanisms of generation are specifically highlighted in this book. Thus, generally with the exception of PMVG, the overall generation stage is achieved with the combined use of two separate devices.

It remains to note that the generation stage may be operated in either of two modes: (1) direct transfer of the generated species to the detection system and (2) analyte collection prior to transport to the detection system. These are briefly considered below.

1. In the direct transfer mode, the volatile species released from the reactor-GLS system has an overall efficiency of β r 5 εd εs. As the volatile species leaving the GLS is conveyed directly to the atomizer/ detection system with an efficiency εt, the overall generation stage efficiency is then the product β gen 5 εd εs εt, representing the fraction of analyte introduced into the generator which is successfully derivatized, released from the liquid reaction medium,

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1.3 Vapor generation techniques

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and transported to an atomizer for detection, the latter having a characteristic atomization efficiency β a.

2. In the collection mode, the released volatile species is first transferred (with an efficiency β gen) to a device which may be an integral part of the generator or the atomizer/detector unit. Traditionally, balloons, cryogenic devices, or redox systems containing a liquid reagent were used to collect either the gaseous components or decompose the species to concentrate it into a small volume of liquid for subsequent processing. More modern approaches now trap the volatile species by directing it to, and subsequently immobilizing it on, a solid support [25], such as by formation of a mercury amalgam on a gold substrate [26] or via thermal decomposition on a heated graphite surface or thin film of reduced noble metal [27], as well as by inducing deposition, typically as a reduced metal or oxide, on various substrates by manipulating the chemistry in the transport gas [28]. Such approaches enable the integration of the generated flux of species, permitting the processing of larger volumes of sample such as to effect a significant enhancement in relative limits of detection since, once trapped, the analyte can be subsequently rapidly desorbed and transported to a suitable atomizer in a short time interval (as for example in a tandem system such as a graphite furnace linked to an ICP MS) to enhance signal-to-noise ratios [29], or the trapping device may be an integral part of the atomizer itself [28,30]. These processes can be characterized by an experimentally determined efficiency representing both that for the trapping step, εtrap, or its convolution with the subsequent release (volatilization) step (εtrap εvol).

The atomization/detection stage is typically based on use of atomic spectrometric methods. The most popular include AAS and AFS [31 35], but a variety of plasma sources as (ICP OES, ICP MS, microwave induced plasma [36 38], capacitively coupled plasma [39,40], direct current plasma [41,42], glow discharge [43 46] and dielectric barrier discharge [DBD] devices [34,35]) have been used. Interfacing with mass spectrometric techniques has primarily been limited to ICP MS, whereas conventional GC MS (and other MS techniques) have essentially been used for species identification and diagnostic studies.

Interestingly, molecular spectroscopy, such as fourier tranform infrared spectroscopy [47 49] and molecular emission cavity analysis [50 52] have also been employed, for which no analyte atomization is required.

Plasma source methods inherently entail general-purpose atomizers optimal for efficient atomization of generated volatile species. By contrast, those for AAS (and partially also for AFS) are usually not ideal for the atomization of volatile species. As such, dedicated atomizers, such as quartz tube atomizers (QTA) [33], graphite and metal atomizers [33],

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1. Introduction to vapor generation techniques

miniature flames [33] and DBD devices [34,35], have most commonly been employed in combination with AAS or AFS detection.

As evident from earlier discussion, the atomization/detection stage may comprise either of two modes (Fig. 1.3): (1) direct atomization and (2) in-atomizer collection.

1. With direct atomization, the volatile species are immediately introduced into the detection volume of the spectrometer wherein atomization occurs. All of the above-mentioned atomizers/detectors are capable of operation in this mode. Clearly, the atomization efficiency, β a, is the fraction of analyte transported into the atomizer which is converted to detectable species (free atoms). It is evident that this would be the most significant consideration if AAS detection is used, but other spectroscopies necessitate the consideration of additional subsequent excitation and ionization efficiencies.

2. With in-atomizer collection, the volatile species are trapped in the atomizer (with an efficiency εtrap) concurrent with its generation, phase separation, and transfer. It is then volatilized (with an efficiency εvol) and atomized (with an efficiency β a). This mode can be performed only with some dedicated atomizers employed for AAS or AFS detection, namely QTA, graphite and metal atomizers, as well as DBDs, or with tandem systems such as an electrothermal vaporizer coupled with a plasma source for OES or MS detection.

1.4 Favorable features and shortcomings of VGTs There is a fundamental desire to utilize VGTs as they generally confer several significant advantages for analyses, including: (1) efficient matrix separation, which often leads to a reduction of interferences and better detection limits; (2) high transport (introduction) efficiency of analyte into the atomic spectroscopic detectors; (3) high selectivity (in some cases) to permit differentiation of chemical species of a particular element; and (4) enabling the use of gas phase separation methods (i.e., GC) for speciation of inorganic as well as organometal (-semimetal) species of some elements.

Noteworthy is that there is the possibility for further postmanipulation of the gaseous products of derivatization, subjecting them either to preconcentration in a cold trap, chemical trap, chromatographic column, or to in-atomizer collection, as noted above. This methodology leads to favorably enhanced relative detection capabilities.

As concerns improvements in sensitivity and detection power that can be achieved with respect to conventional spectroscopic methods employing liquid nebulization or direct sampling with a GF, these will

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be dependent on the specific VGT employed and on the analyte. In this case, the critical step entails derivatization, for which the conversion efficiency of the analyte to a volatile species may be much less that quantitative, despite β v and β a remaining quantitative. In principle, β v does not have to be specifically accounted for with VGTs since it is frequently a gaseous molecular compound that is introduced to the detector (atomization cell) in the absence of any solvent, thus consuming no “vaporization” energy from the atomization device. Moreover, β a may be enhanced with VGTs since simple molecules are typically formed having low atomization energies. Such conditions may generate more favorable values of αT compared to conventional sample introduction techniques using flames, plasmas, and GF atomizers.

Generally, all VGTs present similar favorable features and potential shortcomings. Their common limitations remain interference effects caused by concomitant species present in sample solutions. Interferences typically manifest themselves by perturbing εd, εs, or β a parameters. A classification scheme for interferences was first postulated by Dědina [53] for hydride generation. In general, such a strategy still maintains some validity and can thus also be of relevance to other VGTs. Liquid phase interferences affect the apparent derivatization efficiency (εd) of the volatile species in that it may initially be formed in the reaction mixture with lower efficiency (compared to the calibrator sample), or generated more slowly (kinetic effect), or quantitatively formed in the reaction mixture but the gas liquid phase transfer (εs) occurs with reduced efficiency.

Gas phase interferences may also occur and can be further classified as transport interferences, collection interferences, and atomization interferences.

Transport interference effects may arise due to a loss of (unstable) volatile analyte species along conduit lines or to a perturbation of their transport kinetics. Collection interferences can take place either in the collection mode of generation and/or during in-atomizer trapping mode of the atomization/detection stage through alteration of trapping (εtrap) and volatilization (εvol) efficiencies characteristic of various devices (i.e., trapping in a GC column, cryogenic collection device, QTA, graphite atomizer, DBD).

Atomization interferences affecting β a encompass those typically present in specific atomizers dedicated to the atomization of volatile species (QTA, miniature flames, and barrier discharge devices). Such interferences are not problematic when using plasma source atomic spectrometric methods.

In general, all types of interferences can manifest their effects directly during the analytical run (direct interferences) as well as during successive runs (memory effect interferences).

Vapor Generation Techniques for Trace Element Analysis