James S. Fritz and Douglas T. Gjerde

Ion Chromatography

Fourth, Completely Revised and Enlarged Edition

InnodataFile Attachment9783527626885.jpg

James S. Fritz andDouglas T. Gjerde

Ion Chromatography

Further Reading

Miller, JM

Chromatography - Concepts and Contrasts 2e

2006

ISBN: 978-0-471-98059-9

McMahon, Gillian

Analytical InstrumentationA Guide to Laboratory, Portable and Miniaturized Instruments

2007

ISBN: 978-0-470-02795-0

Stuart, Barbara H.

Analytical Techniques in Materials Conservation

2007

ISBN: 978-0-470-01280-2

Dean, John R.

Bioavailability, Bioaccessibility and Mobility of EnvironmentalContaminants

2007

ISBN: 978-0-470-31967-3

Wang, Joseph

Analytical Electrochemistry

2007

ISBN: 978-0-470-23187-6

Hahn-Deinstrop, Elke

Applied Thin-Layer Chromatography

2006

ISBN: 978-3-527-31553-6

James S. Fritz and Douglas T. Gjerde

Ion Chromatography

Fourth, Completely Revised and Enlarged Edition

The Authors

Prof. James S. FritzAmes LaboratoryIowa State University332 Wilhelm HallAmes, IA 50011USA

Dr. Douglas T. GjerdePhyNexus, Inc.3670 Charter Park Dr./ Suite ASan José, CA 95136USA

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Printed in the Federal Republic of Germany.Printed on acid-free paper.

ISBN: 978-3-527-32052-3

V

Preface 13

Acknowledgments 15

1 Introduction and Overview 11.1 Introduction 11.2 Historical Development 21.2.1 Early Ion-Exchange Separations 21.2.2 Cation Separations 31.2.3 Separation of Anions 61.2.4 On-line Detection 81.2.5 The Birth of Modern Ion Chromatography 81.2.6 Non-Suppressed-Ion Chromatography 101.3 Principles of Ion Chromatographic Separation and Detection 131.3.1 Requirements for Separation 131.3.2 Experimental Setup 131.3.3 Performing a Separation 141.3.4 Migration of Sample Ions 151.3.5 Detection 171.3.6 Basis for Separation 17

2 Instrumentation 212.1 Components of an Ion Chromatograph (IC) Instrument 212.2 General Considerations 232.3 Eluent 242.4 Pump 262.4.1 Gradient Formation 292.5 Sample Injector 302.6 Columns 312.6.1 Column Hardware 312.6.2 Column Protection 322.6.3 Column Oven 332.6.4 Two-dimensional IC 33

Contents

VI

2.7 Suppressor 332.8 Detector 342.9 Data Acquisition and Calculation of Results 35

3 Resins and Columns 373.1 Introduction 373.2 Polymeric Resins 383.2.1 Substrate and Cross Linking 383.2.2 Microporous Resins 393.2.3 Macroporous Resins 403.2.4 Chemical Functionalization 413.2.5 Resin Capacity 423.3 Resins and Columns for Ion Chromatography 433.3.1 Monolith Columns 433.4 Anion Exchangers 453.4.1 Porous Anion Exchangers 453.4.2 Effect of Functional Group on Selectivity 473.4.3 Effect of Spacer Arm Length 523.4.4 Latex Agglomerated Ion Exchangers 543.4.4.1 Effect of Latex Functional Group on Selectivity 563.5 Cation Exchangers 573.5.1 Sulfonated Resins 573.5.2 Weak-acid Cation Exchangers 613.5.3 Other Types 633.6 Other Resins 633.6.1 Chelating Ion-exchange Resins 633.6.2 Metal Oxides 643.6.3 Multi-purpose Resins 643.6.4 Ion-exchange Disks 65

4 Detectors 694.1 Introduction 694.2 Conductivity Detectors 704.2.1 Conductivity Definitions and Equations 734.2.2 Principles of Cell Operation 744.2.3 Conductance Measurement 754.2.4 Conductivity Hardware and Detector Operation 754.2.5 Contactless Conductivity Detection 764.3 Ultraviolet-Visible (UV–Vis) Detectors 774.3.1 UV–Vis measurement 774.3.2 Direct Spectrophotometric Measurement 784.3.3 Post-column Derivatization 814.3.4 UV–Vis Hardware and Detector Operation 824.4 Fluorescence Detector 834.5 Electrochemical Detectors 85

Contents

VII

4.5.1 Potentiometric Detection 864.5.2 Conductometric Detectors 864.5.3 Amperometric/Coulometric Detection 874.5.4 Pulsed Electrochemical Detection (PED) 894.5.4.1 Pulsed Amperometric Detection (PAD) 914.5.4.2 Integrated Pulsed Amperometric Detection (IPAD) 934.5.4.3 IC–PED 944.5.5 Post-column Derivatization 954.5.6 Electrochemical Hardware and Detector Operation 954.6 Refractive Index Detector 974.7 Evaporative Light Scattering Detector (ELSD) 974.7.1 Nebulizer 984.7.2 Evaporation Chamber 994.7.3 Detection Cell 994.8 Other Detectors 100

5 Principles of Ion Chromatographic Separations 1055.1 General Considerations 1055.2 Chromatographic Terms 1055.2.1 Retention Factor 1065.3 Selectivity 1095.3.1 Selectivity Coefficients 1105.3.2 Other Ion-exchange Interactions 1125.3.3 Selectivity of Sulfonated Cation-exchange Resin for Metal Cations 1135.3.4 Factors Affecting Selectivity 1205.3.4.1 Polymeric Matrix Effect 1215.3.4.2 Resin Functional Group 1225.3.4.3 Solvation Effects 1235.4 Chromatographic Efficiency 124

6 Anion Chromatography 1316.1 Scope and Conditions for Separation 1316.1.1 Columns 1326.1.2 Separation Conditions 1356.2 Suppressed Anion Chromatography 1386.2.1 Electrolytic Suppressors 1406.2.2 Solid-Phase Reagents, 1990 [7] 1416.2.3 Typical Separations 1426.2.3.1 Isocratic and Gradient Elution 1446.2.3.2 Influence of Organic Solvents 1466.3 Nonsuppressed Ion Chromatography 1476.3.1 Principles 1476.3.2 Explanation of Chromatographic Peaks 1506.3.3 Eluent 1506.3.3.1 General Considerations 150

Contents

6.3.3.2 Salts of Carboxylic Acids 1516.3.3.3 Basic Eluents 1526.3.3.4 Carboxylic Acid Eluents 1536.3.4 System Peaks 1546.3.5 Scope of Anion Separations 1556.3.6 Sensitivity 1556.3.6.1 Conductance of a Sample Peak 1586.4 Coated Columns 1606.5 Optical Absorbance Detection 1636.5.1 Introduction 1636.5.2 Direct UV Absorption 1636.5.3 Indirect Absorbance 1646.6 Detection 1666.7 Pulsed Amperometric Detector (PAD) 1686.8 Evaporative Light Scattering Detector (ELSD) 1706.9 Inductively Coupled Plasma Methods (ICP) 1726.9.1 Atomic Emission Spectroscopy (AES) 1726.9.2 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) 173

7 Cation Chromatography 1757.1 Introduction 1757.2 Columns 1767.2.1 Historical Development 1787.2.2 Phosphonate Columns 1797.2.3 Macrocycle Columns 1817.2.4 Surfactant Columns 1827.3 Separations 1847.3.1 Suppressed-Conductivity Detection 1847.3.2 Non-Suppressed-Conductivity Detection 1877.3.3 Spectrophotometric Detection 1887.4 Effect of Organic Solvents 1917.4.1 Separation of Alkali Metal Ions 1937.5 Separation of Metal Ions with a Complexing Eluent 1957.5.1 Principles 1957.5.2 Separations 1967.5.3 Use of Sample Masking Reagents 1977.5.4 Weak-Acid Ion Exchangers 1987.6 Chelating Ion-Exchange Resins and Chelation Ion

Chromatography 2017.6.1 Fundamentals 2017.6.2 Examples of Metal-Ion Separation 204

8 Ion-Exclusion Chromatography 2078.1 Principles 2078.1.1 Equipment 209

ContentsVIII

8.1.2 Eluents 2098.1.3 Detectors 2108.2 Separation of Organic Acids 2118.2.1 Effect of Alcohol Modifiers 2148.2.2 Separation of Carboxylic Acids on Unfunctionalized Columns 2168.3 Simultaneous Determination of Anions and Cations 2178.4 Conclusions 2208.5 Determination of Carbon Dioxide and Bicarbonate 2228.5.1 Enhancement Column Reactions 2228.6 Separation of Bases 2238.7 Determination of Water 2268.7.1 Determination of Very Low Concentrations of Water by HPLC 2298.8 Separation of Saccharides and Alcohols 2308.8.1 Introduction 2308.8.2 Separation Mechanism and Control of Selectivity 2308.8.3 Detection 2358.8.4 Contamination 235

9 Ion Pair Chromatography 2399.1 Principles 2399.2 Typical Separations 2429.3 Mechanism 246

10 Zwitterion Stationary Phases 25110.1 Introduction 25110.2 Simultaneous Separation of Anions and Cations 25310.3 Separation of Anions 25510.4 Separation of Cations 25610.5 Mechanism 259

11 Capillary Electrophoresis 26311.1 Introduction 26311.1.1 Steps in Analysis 26411.1.1.1 Capillary Pretreatment 26411.1.1.2 Sample Introduction 26411.1.1.3 Sample Run 26511.1.1.4 Detection 26511.2 Principles 26511.2.1 Terms and Relationships 26511.2.2 Zone Broadening 26711.2.3 Sample Injection 26711.3 Electroosmotic Flow (EOF) 26811.3.1 Effect of EOF on Separations 27011.3.2 Control of EOF 27111.4 Separation of Ions 274

Contents IX

11.4.1 Separation of Anions 27411.4.1.1 Separation of Isotopes 27611.4.2 Separation of Cations 27811.4.3 Separations at Low pH 27911.4.4 Capillary Electrophoresis at High Salt Concentration 28011.5 Capillary Electrophoretic Ion Chromatography 28311.5.1 Micellar Electrokinetic Chromatography (MEKC) 28411.5.2 Partial Complexation 28511.5.3 Effect of Ionic Polymers 28711.5.4 Effect of Alkylammonium Salts 29111.5.4.1 Separation Mechanism 29411.6 Summary 294

12 DNA and RNA Chromatography 29912.1 Introduction 29912.1.1 Importance of DNA and RNA Chromatography 29912.1.2 Organization of this Chapter 30012.2 DNA and RNA Chemical Structure and Properties 30112.3 DNA and RNA Chromatography 30312.3.1 Development of DNA and RNA Chromatography 30312.3.2 Column Properties 30512.3.3 Ion-pairing Reagent and Eluent 30612.4 Temperature Modes of DNA and RNA Chromatography 30712.4.1 Nondenaturing Mode 30712.4.2 Fully Denaturing Mode 30812.4.3 Partially Denaturing Mode 30912.5 Instrumentation 31012.5.1 Effect of Metal Contamination 31012.5.2 The Column Oven 31312.5.3 UV and Fluorescence Detection 31312.5.4 Fragment Collection 31412.6 Applications of DNA Chromatography 31412.6.1 DHPLC 31412.6.2 Nucleic Acid Enzymology 31512.6.2.1 Telomerase Assays 31512.6.2.2 Polynucleotide Kinase Assays 31612.6.2.3 Uracil DNA Glycosylase Assays 31712.7 Applications of RNA Chromatography 31712.7.1 Separation of Messenger RNA from Ribosomal RNA 31812.7.2 Analysis of Transfer RNA 319

13 Sample Pretreatment 32313.1 Dilute and Shoot or Pre-treat the Sample? 32313.2 Particulate and Column-contaminating Matter 32413.3 Preconcentration 325

ContentsX

13.3.1 Collection of Ions from Air 32513.3.2 Preconcentration of Ions in Water 32613.4 Sample Pretreatment 32813.4.1 Anions in Acids 32813.4.2 Neutralization of Strongly Acidic or Basic Samples 32813.4.3 Dialysis Sample Preparation 32913.4.3.1 Passive Dialysis 33013.4.3.2 Donnan (Active) Dialysis 33013.4.4 Isolation of Organic Ions 333

14 Method Development and Validation 33514.1 Choosing a Method 33514.1.1 Define the Problem Carefully 33514.1.2 Experimental Considerations 33614.1.3 Example of Method Development 33814.2 Some Applications of Ion Chromatography 34014.3 Statistical Evaluation of Data 34114.3.1 Common Statistical Terms 34114.3.2 Distribution of Means 34414.3.3 Confidence Intervals 34514.4 Validation of Analytical Procedures 34714.4.1 Analytical Control 349

15 Chemical Speciation 35315.1 Introduction 35315.2 Detection 35515.3 Chromatography 35615.4 Valveless Injection IC 35715.5 Speciation of some Elements 35915.5.1 Chromium 35915.5.2 Iron 36015.5.3 Arsenic 36115.5.4 Tellurium 36215.5.5 Selenium 36315.5.6 Vanadium 36415.5.7 Tin 36415.5.8 Mercury 36515.5.9 Other Metals 365

Index 369

Contents XI

XIII

The first edition of this book in 1982 described the emergence of ion-exchangechromatography as a practical and rapid method of separation and analysis. Theinitial and somewhat restrictive definition of the subject was broadened to 'ionchromatography' in order to describe the efficient chromatographic separation ofanions or cations using any form of automatic detection. In the intervening yearsion chromatography, or IC as it is sometimes called, has undergone impressivedevelopment and has attracted an ever-growing number of users throughout theworld. Over the years, IC has proved to be extremely rugged, sensitive and reli-able. It is the technology of choice to measure ions in such diverse situations asplating baths, drinking water, nuclear power plant water, foodstuffs and so on.Although IC is a well-established technique that is widely used throughout theworld, it continues to enjoy vibrant growth and development. An annual Interna-tional Ion Chromatographic Symposium (IICS) is held at various locations.

This 4th edition of Ion Chromatography has been expanded and extensivelyrevised. Some new features are listed below.

� With some help from William La Course (see Acknowledge-ments), Chapter 4 on Detectors has been expanded and com-pletely rewritten. New features include a strong section onpulsed electrochemical detection and an extensive table ofdyes for tagging for the fluorescent detection of bio ions.

� Chapter 5 (Principles of IC) has been expanded to include adiscussion of factors that affect selectivity and chromato-graphic efficiency.

� Although IC is the preferred method for inorganic ions andsmaller organic ions, the need for chromatographic determi-nation of larger organic and bio ions has been growing rapid-ly, as for example in the pharmaceutical industry. Here, IonPair Chromatography (IPC) is often the preferred technique.A new chapter (Chapter 9) on IPC is now included.

� Chapter 10 on Zwitterion Stationary Phases describes a fasci-nating variation of IC. Pure water can often be used as theeluent to separate sample ions when a zwitterion stationaryphase is employed. Ions elute as cation-anion pairs.

Preface

XIV

� Ions are separated on a truly micro scale on an open capillarybased on differences in their electrophoretic migrationunder the influence of an applied electrical potential (Chap-ter 11). Migration rates of the analyte ions are modified byinteraction with organic ions of opposite charge in the elec-trolyte. Analyte peaks are unusually sharp because the zonebroadening due to mass transfer between two distinct phasesis eliminated.

� The reader is introduced to the fascinating world of the chro-matography of large bio ions in Chapter 12. Structural fea-tures, common terms and chromatographic techniques areexplained in a manner that the non-expert can easily under-stand. DNA molecules are extremely large ions relative towhat can normally be separated by a chromatographic pro-cess.

� The new Chapter 13 is devoted to Sample Pretreatment.� Method development, statistical evaluation of data, and vali-

dation and control of analytical procedures are covered in anew chapter (Chapter 14).

As in the previous editions, our goal has been to describe the materials, principlesand methods of ion chromatography in a clear concise style. Whenever possiblethe consequences of varying experimental conditions are considered. Becausecommercial products are constantly changing, the equipment used in ion chroma-tography is described in a somewhat general manner. Our approach to the litera-ture of IC is selective rather than comprehensive. Key references are given togeth-er with the title so that the nature of the reference will be apparent. Our goal is toexplain fundamentals, but also to provide more detailed information in the formof figures and tables.

We have tried to write a book that is enjoyable to read as well as one that isinformative. Although it has been hard work, writing this book has also been astimulating experience. We hope that we have been able to convey this enthu-siasm to our readers.

James S. Fritz, Ames, Iowa October 2008Douglas T. Gjerde, San Jose, California

Preface

XV

In the preparation of our 1st Edition of Ion Chromatography, published in 1982,we received valuable help from many sources, and we have continued to benefitfrom the skill, knowledge and willingness of many individuals over the years.Now with this 4th Edition of the book, we are again pleased to acknowledge somevaluable contributions.

We thank William R. LaCourse, who wrote Section 4.5 on Electrochem-ical Detection. Bill, who is Professor of Analytical Chemistry at the Universityof Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250,lacourse@umbc.edu, is well known for his expertise in pulsed-amperometricdetection, and we are grateful for his willingness to contribute to the sectionwhich deals with this subject.

We also thank Peter Scott, currently a university student, for creating the origi-nal design used on the front cover of this book. Peter also helped with index anddrew several of the figures that appear in the text. Jie Li, Shelly Li and Jeff Sunprovided us with a clearer insight into ion analysis as used in the pharmaceuticalindustry, particularly with regard to the validation of analytical methods. WenziHu, a world leader in ion chromatography using zwitterion stationary phases,kindly supplied reprints of his papers. We also received figures and other informa-tion from several companies who are in the business of providing tools for IonChromatography. They include, in no particular order, Dionex, Hamilton,Metrohm, Zellweger Analytics, Alltech (product line of Grace David) and Transge-nomic.

Our assistants played a major part in preparing and proofing the manuscript.In Ames, Marilyn Kniss did most of the typing, patiently checking the manuscriptfor consistency and clarity of expression. She also handled the correspondence. InSan Jose, Tiffany Nguyen performed additional valuable and necessary work.

We would like to extend our grateful thanks for the love and support of ourrespective families, for they, more than anything else, make life enjoyable, worth-while and meaningful.

James S. Fritz Douglas T. GjerdeAmes, Iowa San Jose, California

Acknowledgments

1

1Introduction and Overview

1.1Introduction

The name ‘ion chromatography’ applies to any modern method for chromato-graphic separation of ions. Normally, such separations are performed on a col-umn packed with a solid ion-exchange material, but if we define chromatographybroadly as a process in which separation occurs by differences in migration, capil-lary electrophoresis may also be included.

Ion chromatography is considered to be an indispensable tool in a modern ana-lytical laboratory. Complex mixtures of anions or cations can usually be separatedand quantitative amounts of the individual ions measured in a relatively shorttime. Higher concentrations of sample ions may require some dilution of thesample before introduction into the ion-chromatographic instrument. ‘Dilute andshoot’ is the motto of many analytical chemists. However, ion chromatography isalso a superb way to determine ions present at concentrations down to at least thelow parts per billion (lg L–1) range. Although the majority of ion-chromatographicapplications have been concerned with inorganic and relatively small organicions, larger organic anions and cations may be determined as well.

As in the three previous editions, our goal has been to describe the materials,principles and methods of ion chromatography in a clear, concise style. The fol-lowing résumé is intended as a kind of road map to guide the reader through thecontents of this book and to highlight some of the changes made in this fourthedition.

In the first chapter we recount some of the historical milestones and brieflycover the most basic principles of ion chromatography, or IC as it is often called.The various components and hardware of IC instruments are described in Chap-ter 2, but it is not our intention to discuss specific commercial instruments. Chap-ter 3 has been updated to include advances in column technology and promisingnew columns, such as monolithic columns. Chapter 4 on detectors has beenexpanded to include new material on the contactless conductivity detector (CCD)and pulsed electrochemical detectors.

Chapter 5 has been completely rewritten and now includes detailed sections onthe factors that influence selectivity and efficiency. An updated and detailed treat-

ment of anion chromatography and cation chromatography is presented in thenext two chapters. Selection of appropriate columns, detectors and eluents is dis-cussed and numerous examples of typical separations are given. Ion exclusionchromatography, Chapter 8, continues to be a popular and useful method for theseparation of hydrophilic sample components, such as carboxylic acids, aminesand carbohydrates.

Ion chromatography is generally defined as an analytical method in whichanions or cations are separated by differences in the rate at which they passthrough a column packed with either an anion- or cation-exchange particles. How-ever, excellent separations of ionic analytes can also be obtained by ion chromatog-raphy on a standard reversed-phase HPLC column. With this technique, separa-tion of cations or anions is achieved by using an aqueous-organic eluent togetherwith an ion-pairing reagent. Ion-pair chromatography, covered in the new Chap-ter 9, is particularly advantageous for separation of organic ions.

Chapter 10 on zwitterion stationary phases is another new addition to thisbook. Separations are generally performed on an HPLC column coated with azwitterion surfactant that contains both positive and negative sites. In some casescations and anions can be separated in a single run using pure water as the elu-ent!

Resolution of the sample peaks obtainable in either liquid or ion chromatogra-phy is limited by two factors. One is due to mechanical pumping, which gives acurved flow profile. The second limiting factor stems from a slow rate of equilibra-tion of solutes between the mobile and stationary phases. Use of very small ion-exchange particles can reduce but never eliminate this source of peak broadening.Capillary electrophoresis (CE), which is covered in Chapter 11, addresses both ofthese issues. No eluent is required for CE; sample ions are separated by differ-ences in their electrophoretic flow rates through open capillary containing nopacking material. The electrophoretic mobilities can be modified by selective in-teractions with electrolyte ions of the opposite charge.

Chapter 12 on Separation of DNA/RNA highlights a trend in IC toward agreater emphasis on analytical separations of bio ions. Sample pretreatment isdiscussed in Chapter 13. Chapter 14 on Method Development and Validationincludes tips for selecting appropriate conditions for an IC analysis, and this finalchapter covers chemical speciation.

1.2Historical Development

1.2.1Early Ion-Exchange Separations

Modern ion chromatography is built on the solid foundation created by extensivework in classical ion-exchange chromatography. Columns containing ion-exchange resins have been used for many years to separate various cations and

1 Introduction and Overview2

anions from one another. Cations are separated on a cation-exchange resin col-umn, and anions on an anion-exchange resin column. The most used types are asfollows:

For example, Na+ and K+ can be separated on a cation-exchange resin (Catex)column with a dilute solution of a strong acid (H+) as the eluent (mobile phase).Introduction of the sample causes Na+ and K+ to be taken up in a band (zone)near the top of the column by ion exchange:

Resin–SO–H+ + Na+, K+ � Resin SO–Na, K+ + H+

Continued elution of the column with an acidic eluent (H+) introduces compe-tition of H+, Na+ and K+ for the exchange sites (–SO3

–), causing the Na+ and K+ tomove down the column. K+ is more strongly retained than Na+, and thus the Na+

zone moves down the column faster than the K+ zone.As originally conceived and carried out for many years, fractions of effluent

were collected from the end of the column and analyzed for Na+ and K+. Then aplot was made of concentration vs fraction number to construct a chromatogram.All this took a long time and made ion-exchange chromatography slow and awk-ward to use. However, it soon was realized that under a given set of conditions allof the Na+ would be in a single fraction of several milliliters and all of the K+

could be recovered in a second fraction of a certain volume. Thus, under pre-deter-mined conditions, each ion to be separated could be collected in a single fractionand then analyzed by spectroscopy, titration, etc., to determine the amount ofeach sample ion.

The ability to collect a single fraction that contains all of the separated sampleion permits the use of step gradients. In this mode, conditions are adjusted sothat an ‘all-or-nothing’ situation prevails. A sample ion either sticks onto the ion-exchange column or it passes quickly through. Conditions are selected so thatonly one ion type will pass through the column while the other sample ions arestrongly retained and form a tight band at the top of the column. Then the eluentis changed so that a second ion is rapidly eluted, while the others remain tightlystuck. Frequently, several gradient steps can be performed to elute different sam-ple ions at each step.

1.2.2Cation Separations

Early studies on the separation of metal cations included separations based onaffinity differences and some specific separation with complexing eluents.

31.2 Historical Development

1 Introduction and Overview

Strelow and his coworkers have published extensive data relating to the selectiv-ity of a sulfonated polystyrene cation exchanger for various cations in acidic solu-tion [1]. The equilibria of cations in hydrochloric, nitric or sulfuric acid solutionswith a cation exchanger involves complexation in some cases as well as competi-tion between H+ and the metal cation for the exchange sites. For example, mercu-ry(II) and cadminium(II) form chloride complexes even in dilute solutions ofhydrochloric acid. Selectivity data in perchloric acid probably give the best indica-tion of true ion-exchange selectivity, because the perchlorate anion has almost nocomplexing properties with metal cations.

In general, cations with a 3+ charge are more strongly retained by a cationexchanger than cations with a 2+ charge, and ions with a 2+ charge are retainedmore strongly than those with a 1+ charge. Fritz and Karraker [2] were able toseparate metal cations into groups according to their charge. Most divalent metalcations were eluted with a 0.1 M solution of ethylenediammonium perchlorate.Bismuth(III) and zirconium(IV) remained quantitatively o n the cation-exchangecolumn. The use of the 2+ ethylenediammonium ion permitted a lower concen-tration to be used than would have been the case with an H+ eluent.

Several inorganic acids exhibit a complexing effect for metal ions. The complex-ing acids include HF, HCI, HBr, HI, HSCN and H2SO4. The complexed metalions are converted into neutral or anionic complexes and are rapidly eluted, whilethe other cations remain on the cation-exchange column.

The data for hydrochloric acid [3] indicate selective complexing between metalcations and the chloride ion. For example, cadmium(II) has a distribution coeffi-cient of 6.5 in 0.5 M hydrochloric acid, but a D = 101 in 0.5 M perchloric acid.

Calcium(II), which shows no appreciable complexing, has a distribution coeffi-cient of 147 in 0.5 M perchloric acid and 191 in 0.5 M hydrochloric acid. Strelow,Rethemeyer and Bothma [3, 4] also reported data for nitric and sulfuric acids thatshowed complexation in some cases. Mercury(II), bismuth(III), cadmium(II),zinc(II), and lead(II) form bromide complexes and elute in the order given in 0.1to 0.6 M hydrobromic acid [5]. Most other metal cations remain on the column.Aluminum(III), molybdenum(VI), niobium(V), tin(IV), tantalum(V), uranium(VI),tungsten(VI) and zirconium(IV) form anion fluoride complexes and are quicklyeluted from a hydrogen-form cation-exchange column with 0.1 to 0.2 M HF [6].

An eluent containing only 1% hydrogen peroxide in dilute aqueous solutionwill form stable anionic complexes with several metal ions. Fritz and Abbink [7]were able to separate vanadium(IV) or (V) from 25 metal cations, including theseparation of vanadium(V) from 100 times as much iron(III).

Strewlow [8] used hydrogen peroxide and sulfuric acid to separate titanium(IV)from more than 20 cations by cation exchange. Fritz and Dahmer [9] separatedmolybdenum(VI), tungsten(VI), niobium(V) and tantalum(V) as a group fromother metals by adding dilute hydrogen peroxide to the sample solution and pass-ing it through a cation-exchange column.

Most of the eluents listed above are volatile upon heating and do not interferewith colorimetric, titrimetric or other methods for chemical determination of themetal ions separated. For the most part, group separations, rather than separation

4

1.2 Historical Development

of individual metal ions, are obtained, and only a short ion-exchange column isneeded. Another valuable ‘all-or-nothing’ group separation uses an eluent consist-ing of 0.1 M tartaric acid and 0.01 M nitric acid [10]. Antimony(V), molybde-num(VI), tantalum(V), tin(IV) and tungsten(VI) form tartrate complexes in thisacidic medium, but lead(II) and many other metal cations are not complexed andare retained by the cation exchanger. Samples containing tin(IV) must be added tothe column in the tartrate solution.

In a few cases an eluent containing an organic complexing reagent has beenused successfully for the chromatographic separation of several metal ions. Anotable example is the separation of individual rare earth ions with a solution of2-hydroxylisobutyric acid as the eluent [11]. However, such separations necessitatecareful equilibration of the column to maintain a desired pH. Sometimes gradientelution is used, and either the pH or the eluent concentration is changed.

Metal cations usually form complexes with inorganic anions much more read-ily in organic solvents than in water. For example, the pink cobalt(II) cationrequires around 4 M or 5 M aqueous hydrochloric acid to be converted to a bluecobalt(II) chloride anion. In a predominantly acetone solution, the intensely bluecobalt(II) is formed in very dilute hydrochloric acid. Thus, the scope of ion-exchange group separations is increased greatly by carrying out separations in amixture of water and an organic solvent.

Fritz and Rettig [12] showed that zinc(II), iron(III), cobalt(II), copper(II) andmanganese(II) can be separated from each other on a short cation-exchange col-umn with eluents containing a fixed, low concentration of HCl, increasing theacetone concentration from 40% to 95% in steps. Later Strelow et al. [13] pub-lished extensive lists of metal-ion distribution coefficients in water/acetone/hydro-chloric acid systems.

Korkisch and coworkers have studied the effect of ethanol, acetic acid, ethyleneglycol and many other solvents upon the ion-exchange behavior of metal ions insystems containing hydrochloric and other complexing acids [14].

The selectivity of low-capacity cation columns for monovalent ions can beadjusted by the addition of an organic modifier to the eluent. Using a nitric acideluent of pH 2.5, for example, the elution order for monovalent ions is Li+, Na+,NH4

+, K+. Simple amines elute in the order of the carbon number, after NH4+,

with the result that (CH3)NH3+ (methylammonium) can co-elute with potassium.

In most cases, this co-elution is of little significance, because potassium andmethylammonium are not often in the same sample. However, where the analysisof either of these species in the presence of the other is desired, the selectivity canbe modified by the addition of 40% methanol to the eluent [15]. The methanolcauses the potassium to elute later but does not affect the elution time of methyl-ammonium.

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1 Introduction and Overview

1.2.3Separation of Anions

Since most metal ions are cationic, it may sound strange to discuss their separa-tion by anion-exchange chromatography. However, Kraus and Nelson, working atOak Ridge National Laboratory in the USA, found that in aqueous hydrochloricacid solutions a number of metal ions form anionic complexes and are stronglytaken up by anion-exchange resins. For most of the metal ions, a plot of the Dvalue of several thousand is attained. An illustration of such plots for most of themetallic elements in the periodic table was published by Kraus and Nelson in1956 [16].

Separations are generally achieved by adding the sample to an anion-exchangecolumn in rather concentrated hydrochloric acid and eluting the nonsorbed metalions with the same HCl concentration. Then the sorbed metal ions are eluted oneat a time by stepwise reduction of the HCl strength of the eluent. Figure 1.1 illus-trates one of the many practical separations published by Kraus and his coworkers[17].

Figure 1.1 Separation of metal ions on Dowex 1 × 10 anionexchange resin. (From Ref. [18] with permission.)

In a similar manner, elements that form anionic fluoride complexes can be sep-arated from others and from each other on an anion exchanger by eluting witheluents containing HF plus HCl [18, 19]. Extensive studies of metal ion behavioron anion-exchange columns have also been carried out with eluents containingmixed H2SO4/HF [20, 21].

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1.2 Historical Development

Anion-exchange distribution coefficients for most metallic elements in sulfuricacid solution have been measured [22, 23]. Uranium(VI), thorium(IV), molybde-num(VI) and a few other elements are retained selectively by anion-exchangersfrom solution in approximately 6 M nitric acid [24].

Operating in a predominantly organic solvent greatly improves the ability ofmetal ions to form complexes with halide and pseudo-halide anions. Such com-plexes generally are taken up strongly by an anion-exchange resin. Korkisch, Fritz,Strelow and others have published extensively on anion-exchange separations inpartly nonaqueous solutions. Korkisch and Hazan [25] describe a method to sepa-rate metal ions that form chloride complexes from those that do not. The methoduses an eluent consisting of 90–95% methanol in 0.6 M hydrochloric acid andrequires only a short anion-exchange column. The metal ions studied are eitherretained as a sharp band or quickly pass through the column. Thus, we have an‘all-or-nothing’ situation, and excellent group separations are obtained.

Chromatographic separations of individual ions are also possible, and manyhave been published. An example is shown in Figure 1.2. Ion exchange in non-aqueous and mixed media has been reviewed [26].

Figure 1.2 Separation of nickel(II) and manganese(II) on a6.0 × 2.2 cm column containing Dowex 1 × 8 resin, with partlynonaqueous eluents. (From Ref. [25] with permission.)

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1 Introduction and Overview

Systems containing dimethylsulfoxide, methanol and hydrochloric acid havebeen studied for the anion-exchange behavior of 26 elements [27]. Numerous sep-arations of two- to four-component mixtures of metal ions were carried out withquantitative results.

1.2.4On-line Detection

At this stage in the development of ion-exchange chromatography, separation ofcations or anions was still a slow and laborious process. It was becoming apparentthat widespread use of ion-exchange chromatography as an analytical tool wouldrequire a system that gave fast separations with automatic recording of chromato-grams.

In 1971 an apparatus for ‘forced-flow chromatography’ was described in whichthe eluent was pushed through the analytical column by compressed nitrogen[28]. Detection of eluted ions was by UV-Vis spectrophotometry using a 30 mm ×2 mm flow cell. Iron(III) (10–90 lg) could be separated from most other metalions and measured quantitatively in only 6 min. Forced-flow methods were soondeveloped for the chromatographic separation of a number of other metal ions[29–32]. The chromatograph was modified in 1974 so that a complexing reagentsuch as PAR or Arsenazo could be added to the column effluent via a mixing tee[33]. This made it possible to detect virtually any metal ion that could form ahighly-colored complex. A recorded chromatographic separation of all 30 rareearths was obtained in 1974 [30, 34], with this apparatus. This separation took100 min. Five years later, Elchuk and Cassidy in Canada were able to obtain abetter separation of earths in only 27 min using a similar but improved system[35].

1.2.5The Birth of Modern Ion Chromatography

Liquid column chromatography went ‘high performance’ around 1970 and is nowcommonly referred to as HPLC. Major improvements in speed and efficiencywere obtained by using columns of relatively small bore packed with small spheri-cal particles of uniform diameter, using a pump to provide constant eluent flow,and using automatic detection of the separated sample components. However, ap-plication of this technology for the separation of ions lagged. It was mainly thelack of satisfactory detectors that held up the development of high-performanceion-exchange chromatography. This situation changed dramatically with the pub-lication of a landmark paper in 1975 by Small et al., working at the Dow ChemicalCo. [36]. As the authors put it: ‘It would be desirable to employ some form of con-ductimetric detection as a means of monitoring ionic species in a column effluentsince conductivity is a universal property of ionic species in solution and sinceconductance shows a simple dependence on species concentration. However, theconductivity from the species of interest is generally “swamped out” by that from

8

1.2 Historical Development

the much more abundant eluting electrolyte. We have solved this detection pro-blem by using a combination of resins which strips out or neutralizes the ions ofthe background electrolyte leaving only the species of interest as the major con-ducting species in the effluent. This has enabled us to successfully apply a con-ductivity cell and meter as the detector system.’

This new system, which was given the name ‘ion chromatography,’ enabled theanalyst to quickly separate and measure quantitatively the cations or anions at lowconcentrations in fairly complex samples. A diagram of the system for cation anal-ysis is shown in Figure 1.3. The upper column, called the ‘separator column’, waspacked with polystyrene–2% DVB particles, surface-sulfonated to obtain anexchange capacity of approximately 0.02 mmol g–1. The lower ‘suppressor col-umn’ was packed with anion-exchange resin of high exchange capacity in the hy-droxide form.

Figure 1.3 System for cation analysis by conductimetricchromatography. (From Ref. [36] with permission.)

A practical method for the separation of anions was described in the samepaper [36]. This endeavor necessitated the development of a new low-capacityanion-exchange resin. It had been known for some time that cation- and anion-exchange resins have a marked tendency to clump together. Using this principle,a satisfactory anion-exchange material of low capacity was prepared by coatingsurface-sulfonated cation exchanger

In the original scheme for the separation of anions a mixture of sodium hydrox-ide and sodium phenate was used in the eluent. The suppressor column waspacked with a cation-exchange resin of high capacity. The suppressor column con-

9

1 Introduction and Overview

verted the eluent ions to water plus phenol, while the sample anions A– were con-verted to the highly conducting pair H+A–.

An instrument called the ‘ion chromatograph’ was offered commercially by thenewly organized Dionex Co. and became an immediate success. The new technol-ogy made it possible to separate and determine both cations and most anions, butthe ability to determine anions at low ppm concentrations had the greater impact.Many cations could already be determined by various spectral methods and byreasonably good chromatographic methods, but prior to the advent of ion chroma-tography there was no general analytical method for anions, especially at very lowconcentrations. Once the scientific world became aware that anions in fairly com-plex mixtures could be easily separated and quantified, even at low ppm concen-trations, the use of ion chromatography exploded. A powerful new analytical tech-nique had again facilitated scientific endeavors that were previously impractical.

1.2.6Non-Suppressed-Ion Chromatography

A major disadvantage of the original Ion Chromatograph was that it required theuse of a large suppressor column that contributed to peak broadening and re-quired frequent regeneration. The eluent for anion separations had to be a base,and anions of very weak acids could not be detected because their acidic formafter suppression was too weakly conducting.

In 1979 a synthetic method was described for producing anion-exchange resinsof very low exchange capacity [37]. A porous polymeric resin was chloromethylatedunder mild conditions and then alkylated with trimethylamine to form ionic qua-ternary ammonium groups. The exchange capacity could be varied from 0.2 to1.5 meq g–1 by controlling the time and temperature of the chloromethylation.This drastically lower exchange capacity permitted the use of much lower eluentconcentrations than had previously been possible.

In 1979, Gjerde, Fritz and Schmuckler described a simple system for anionchromatography with eluents containing anions of very low conductivity such asbenzoate or phthalate [38]. Anions were separated on a column containing macro-porous anion-exchange particles of very low exchange capacity: 0.07, 0.04 or0.007 mmol g–1. The eluent was an aqueous solution of the sodium or potassiumsalt of an organic anion that had a significantly lower equivalent conductancethan the anions to be separated. In this method some of the eluent anion isreplaced by a sample anion of significantly higher conductance as the sample ionis eluted from the column and passes through the conductivity detector. Becauseof the low resin capacity, an eluent containing only ca. 10–4 M of an organic acidsalt, such as benzoate or phthalate, could be used. The eluent conductance was solow that no suppressor column was needed, and the separated sample ions couldbe detected with a simple conductivity detector. Numerous anion separations weredemonstrated, and in some instances detection limits below 1 ppm were obtained.This method was initially called ‘single-column ion chromatography’ and later‘non-suppressed-ion chromatography.’ An additional paper on anion chromatog-

10

1.2 Historical Development

raphy [39] was published in 1980. A chromatographic separation of halide ions isshown in Figure 1.4.

Figure 1.4 Separation of 4.8 ppm of fluoride, 5.1 ppm ofchloride and 26.0 ppm of bromide on XAD-1, 0.04 mequivg–1; eluent is 0.65 mM potassium benzoate, pH 4.6.(From Ref. [39] with permission.)

The non-suppressed method for anion chromatography was followed quickly in1980 by a similar method for cations [40]. This method also introduced the con-cept of indirect conductivity detection. A 1 × 10–3 M solution of nitric acid wasused as the eluent in conjunction with a sulfonated cation-exchange column oflow exchange capacity. In this method, a baseline of relatively high conductance isestablished when the column is equilibrated with the acidic eluent. After introduc-tion of a sample mixture, such as Na+ and K+, and continued elution with theeluent, the sample cations are gradually resolved into zones in which some of thehighly conducting H+ (equiv. conductance = 350 S cm2 equiv–1) is exchanged for asample cation of much lower conductance. A sharp ‘peak’ of lower conductance isobtained for each sample cation.

A mixture of Li+, Na+, NH4+, K+, Rb+ and Cs+ was separated in less than 10 min

with a blend of 0.17 meq g–1 and unfunctionalized cation-exchange resins with1.25 × 10–4 nitric acid as the eluent (Figure 1.5). Although separation of divalentcations with nitric acid was not practical, a fast separation of magnesium and cal-cium in tap water was obtained with a 1 × 10–3 M ethylene diammonium nitrateeluent.

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1 Introduction and Overview

Figure 1.5 Separation of alkali metal cations on ammoniumcation exchange column with a conductivity detector. Column:350 × 2.0 mm. Packed with 0.059 mequiv g–1 cation exchangeresin.

Development of the ion-chromatographic methods that use a conductivitydetector was accompanied by a significant increase in chromatographic efficiency.The ion-exchange materials were of much smaller and more uniform size and thepacking efficiency of the column was also improved. The changes that occurredwere not unlike those in partition chromatography when it went from ‘liquidchromatography’ to ‘high-performance liquid chromatography’ (HPLC).

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