Joachim Weiss

Handbook of Ion Chromatography

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Joachim Weiss

Handbook of Ion Chromatography

Fourth, Completely Revised and Enlarged Edition

With a Foreword by Oleg Shpigun

Volumes 1, 2 and 3

Author

Dr. Joachim Weiss Thermo Fisher Scientific GmbH Im Steingrund 4-6 63303 Dreieich Germany

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VII

Contents

Volume 1

Foreword XV Preface to the Fourth Edition XVII

1 Introduction 1 1.1 Historical Perspective 1 1.2 Types of Ion Chromatography 4 1.3 The Ion Chromatographic System 5 1.4 Advantages of Ion Chromatography 8 1.5 Selection of Separation and Detection Systems 10

References 12

2 Theory of Chromatography 15 2.1 Chromatographic Terms 15 2.1.1 Asymmetry Factor As 15 2.2 Parameters for Assessing the Quality of a Separation 17 2.2.1 Resolution 17 2.2.2 Selectivity 18 2.2.3 Capacity Factor 18 2.3 Column Efficiency 19 2.4 The Concept of Theoretical Plates (van Deemter

Theory) 21 2.5 van Deemter Curves in Ion Chromatography 25

References 27

3 Anion-Exchange Chromatography (HPIC) 29 3.1 General Remarks 29 3.2 The Ion-Exchange Process 30 3.3 Thermodynamic Aspects 31 3.4 Stationary Phases 36 3.4.1 Polymer-Based Anion Exchangers 37

VIII Contents

3.4.1.1 Poly(styrene-co-divinylbenzene)-Based Polymers 37 3.4.1.2 Polymethacrylate Resins 54 3.4.1.3 Polyvinylalcohol Resins 60 3.4.1.4 Nanobead-Agglomerated Anion Exchangers 63 3.4.1.5 Hyperbranched Condensation Polymers 95 3.4.1.6 Polymeric Monolithic Anion Exchangers 119 3.4.2 Silica-Based Anion Exchangers 126 3.4.3 Other Materials for Anion Separations 130 3.4.3.1 Crown Ether and Cryptand Phases 130 3.4.3.2 Alumina Phases 138 3.5 Eluents for Anion-Exchange Chromatography 142 3.6 Suppressor Systems in Anion-Exchange Chromatography 152 3.6.1 Suppressor Columns 153 3.6.2 Hollow Fiber Suppressors 158 3.6.3 Micromembrane Suppressors 161 3.6.4 Self-Regenerating Suppressors 167 3.6.5 Suppressors with Monolithic Suppression Beds 172 3.6.6 Capillary Suppressors 173 3.7 Anion-Exchange Chromatography of Inorganic Anions 175 3.7.1 Overview 175 3.7.2 General Parameters Affecting Retention 176 3.7.3 Experimental Parameters Affecting Retention when Applying

Suppressor Systems 177 3.7.3.1 Choice of Eluent 177 3.7.3.2 Eluent Concentration and pH Value 193 3.7.3.3 Influence of Organic Solvents 198 3.7.4 Experimental Parameters Affecting Retention when Applying

Nonsuppressed Conductivity Detection 201 3.7.4.1 Choice of Eluent 201 3.7.4.2 System Peaks 210 3.7.4.3 Eluent Concentration and pH Value 213 3.7.5 Polarizable Anions 216 3.8 Anion-Exchange Chromatography of Organic Anions 227 3.8.1 Carboxylic Acids 228 3.8.2 Organophosphates, Organosulfates, and Organosulfonates 244 3.8.3 Phosphonic and Phosphinic Acids 254 3.9 Gradient Elution Techniques in Anion-Exchange Chromatography of

Inorganic and Organic Anions 267 3.9.1 Theoretical Aspects 268 3.9.2 Choice of Eluent 270 3.9.3 Possibilities for Optimizing Concentration Gradients 277 3.9.4 Isoconductive Techniques 278 3.10 Carbohydrates 281 3.10.1 Sugar Alcohols 287 3.10.2 Monosaccharides 290

IX Contents

3.10.3 Oligosaccharides 308 3.10.4 Polysaccharides 314 3.10.5 Carbohydrates Derived from Glycoproteins 320 3.10.5.1 Compositional and Structural Analysis 323 3.10.5.2 Chosen Examples 338 3.11 Amino Acids 341 3.12 Anion-Exchange Chromatography of Proteins 351 3.12.1 Grafted Anion-Exchange Resins 351 3.12.2 Monolithic Anion-Exchange Media 359 3.13 Nucleic Acids 368

References 390

4 Cation-Exchange Chromatography (HPIC) 401 4.1 Stationary Phases 401 4.1.1 Polymer-Based Cation Exchangers 401 4.1.1.1 Poly(styrene-co-divinylbenzene)-Based Polymers 401 4.1.1.2 Grafted Ethylvinylbenzene/Divinylbenzene

Copolymers 404 4.1.1.3 Polymethacrylate and Poly(vinylalcohol) Resins 432 4.1.1.4 Nanobead-Agglomerated Cation Exchangers 434 4.1.2 Silica-Based Cation Exchangers 439 4.2 Eluents in Cation-Exchange Chromatography 448 4.3 Suppressor Systems in Cation-Exchange Chromatography 450 4.3.1 Suppressor Columns 450 4.3.2 Hollow Fiber Suppressors 451 4.3.3 Micromembrane Suppressors 451 4.3.4 Self-Regenerating Suppressors 452 4.3.5 Suppressors with Monolithic Suppression Beds 457 4.3.6 Capillary Suppressors 458 4.4 Cation-Exchange Chromatography of Alkali Metals, Alkaline-Earth

Metals, and Amines 460 4.5 Transition Metal Analysis 468 4.5.1 Basic Theory 468 4.5.2 Transition Metal Analysis by Nonsuppressed Conductivity

Detection 472 4.5.3 Transition Metal Analysis with Spectrophotometric

Detection 474 4.5.4 Chelation Ion Chromatography 490 4.6 Analysis of Polyamines 496 4.7 Gradient Techniques in Cation-Exchange Chromatography

of Inorganic and Organic Cations 498 4.8 Cation-Exchange Chromatography of Proteins 505 4.8.1 Grafted Cation-Exchange Resins 507 4.8.2 Monolithic Cation-Exchange Media 523

References 529

X Contents

Volume 2

5 Ion-Exclusion Chromatography (HPICE) 533 5.1 The Ion-Exclusion Process 533 5.2 Stationary Phases 535 5.3 Eluents for Ion-Exclusion Chromatography 542 5.4 Suppressor Systems in Ion-Exclusion Chromatography 543 5.5 Analysis of Inorganic Acids 545 5.6 Analysis of Organic Acids 550 5.7 HPICE/HPIC-Coupling 553 5.8 Analysis of Alcohols and Aldehydes 558 5.9 Amino Acids Analysis 561 5.9.1 Separation of Amino Acids 562 5.9.2 Postcolumn Derivatizations of Amino Acids 570 5.9.3 Sample Preparation 572 5.10 Size-Exclusion Chromatography of Proteins 573

References 580

6 Ion-Pair Chromatography (MPIC) 583 6.1 Survey of Existing Retention Models 584 6.2 Suppressor Systems in Ion-Pair Chromatography 588 6.3 Experimental Parameters that Affect Retention 589 6.3.1 Type and Concentration of Lipophilic Counterions in

the Mobile Phase 590 6.3.2 Type and Concentration of the Organic Modifier 593 6.3.3 Inorganic Additives 596 6.3.4 pH Effects and Temperature Influence 597 6.4 Analysis of Surface-Inactive Ions 599 6.5 Analysis of Surface-Active Ions 615 6.6 Applications of the Ion-Suppression Technique 636 6.7 Applications of Ion Chromatography on Mixed-Mode Stationary

Phases 640 6.7.1 Polymer-Based Mixed-Mode Columns 641 6.7.2 Silica-Based Mixed-Mode Columns 657

References 679

7 Hydrophilic Interaction Liquid Chromatography (HILIC) 683 7.1 Separation Mechanism in Hydrophilic Interaction Liquid

Chromatography 685 7.2 Stationary Phases for HILIC 686 7.2.1 Underivatized Silica 687 7.2.2 Neutral Bonded Phases 690 7.2.3 Charged Bonded Phases 696 7.2.4 Zwitterionic Bonded Phases 698 7.2.5 Mixed-Mode Bonded Phases 701

XIContents

7.3 Factors Affecting Retention in HILIC 707 7.3.1 Organic Solvents 707 7.3.2 Ionic Additives 708 7.3.3 Mobile-Phase pH 709 7.4 Applications 710 7.4.1 Carbohydrates and Amino Acids 710 7.4.2 Organic Acids 713 7.4.3 Hydrophilic Active Pharmaceutical Ingredients 718 7.4.4 Buffer Salts for Biopharmaceutical Applications 720 7.4.5 Nucleobases, Nucleosides, and Nucleotides 721 7.4.6 Melamine and Cyanuric Acid 725

References 728

8 Detection Methods in Ion Chromatography 731 8.1 Electrochemical Detection Methods 731 8.1.1 Conductivity Detection 731 8.1.1.1 Theoretical Principles 732 8.1.1.2 Application Modes of Conductivity Detection 738 8.1.2 Amperometric Detection 743 8.1.2.1 Fundamental Principles of Voltammetry 744 8.1.2.2 Amperometry 748 8.1.3 Charge Detection 771 8.1.3.1 The Charge Signal 774 8.1.3.2 Calibration Behavior in Charge Detection 776 8.1.3.3 Example Applications 778 8.2 Spectrometric Detection Methods 782 8.2.1 UV/Vis Detection 782 8.2.1.1 Direct UV/Vis Detection 782 8.2.1.2 UV/Vis Detection in Combination with Derivatization Techniques 783 8.2.1.3 Indirect UV Detection 799 8.2.2 Fluorescence Detection 803 8.3 Aerosol-Based Detection Methods 810 8.3.1 Evaporative Light Scattering Detection 811 8.3.1.1 Advantages and Limitations of ELSD 814 8.3.1.2 Applications of ELSD 816 8.3.2 Condensation Nucleation Light Scattering Detection 819 8.3.3 Charged Aerosol Detection 822 8.3.3.1 Applications of CAD 829 8.4 Other Detection Methods 844 8.4.1 Refractive Index Detection 844 8.4.2 Radioactivity Monitoring 847 8.4.3 Chemiluminescence Detection 848 8.5 Hyphenated Techniques 853 8.5.1 IC-ICP Coupling 853 8.5.2 IC–MS Coupling 865

ContentsXII

8.5.2.1 Electrospray Interface 867 8.5.2.2 IC–MS Applications 875

References 926

9 Quantitative Analysis 935 9.1 General 935 9.2 Analytical Chemical Information Parameters 936 9.3 Determination of Peak Areas 937 9.3.1 Manual Determination of Peak Areas and Peak Heights 938 9.3.2 Electronic Peak Area Determination 940 9.4 Statistical Quantities 944 9.4.1 Mean Value 944 9.4.2 Standard Deviation 945 9.4.3 Scatter and Confidence Interval 945 9.5 Calibration of an Analytical Method (Basic Calibration) 946 9.5.1 Acquisition of the Calibration Function 947 9.5.1.1 Method Characteristic Parameters of a Linear Calibration

Function 948 9.5.1.2 Method Parameters of a Calibration Function of Second

Degree 950 9.5.2 Testing of the Basic Calibration 952 9.5.3 Testing the Precision 953 9.5.3.1 Homogeneity of Variances 953 9.5.3.2 Outlier Tests 953 9.5.4 Calibration Methods 955 9.5.4.1 Area Normalization 955 9.5.4.2 Internal Standard 956 9.5.4.3 External Standard 957 9.5.4.4 Standard Addition 958 9.6 Detection Criteria, Limit of Detection, Limit of Determination 960 9.6.1 Determination of Detection Criteria, Limit of Detection,

and Limit of Determination 961 9.7 The System of Quality Control Cards 964 9.7.1 Types of Quality Control Cards and Their Applications 965

References 972

Volume 3

10 Applications 975 10.1 Ion Chromatography in Environmental Analysis 977 10.1.1 Conventional Water Analysis 977 10.1.2 Analysis of Water Disinfection By-Products 996 10.1.2.1 Analysis of Bromate and Other Oxyhalides 997 10.1.2.2 Analysis of Haloacetic Acids 1021

XIII Contents

10.1.3 Perchlorate Analysis 1024 10.1.4 Analysis of Highly Contaminated Water Samples 1038 10.1.4.1 Seawater Analysis 1041 10.1.4.2 Soil Analysis 1048 10.1.4.3 Air Analysis 1053 10.1.5 Other Environmental Applications 1066 10.2 Ion Chromatography in Power Plant Chemistry 1068 10.2.1 High-Purity Water Analysis 1069 10.2.1.1 Conventional Preconcentration Techniques 1070 10.2.1.2 Large-Volume Direct Injection Techniques 1077 10.2.1.3 RFIC-ESP 1081 10.2.1.4 Capillary Ion Chromatography 1084 10.2.2 Analysis of Conditioned Waters 1085 10.2.3 Cooling Water Analysis 1094 10.2.4 Flue Gas Scrubber Solutions 1106 10.2.5 Analysis of Chemicals 1111 10.3 Ion Chromatography in the Semiconductor Industry 1114 10.3.1 High-Purity Water Analysis 1115 10.3.2 Surface Contaminations 1123 10.3.3 Solvents 1128 10.3.4 Acids, Bases, and Etching Agents 1132 10.3.4.1 Solar Cell Manufacturing 1142 10.3.5 Other Applications 1143 10.4 Ion Chromatography in the Electroplating Industry 1148 10.4.1 Analysis of Inorganic Anions 1149 10.4.2 Analysis of Metal Complexes 1157 10.4.3 Analysis of Organic Acids 1159 10.4.4 Analysis of Inorganic Cations 1161 10.4.5 Analysis of Organic Additives 1162 10.5 Ion Chromatography in the Detergent and Household Product

Industry 1175 10.5.1 Detergents 1175 10.5.2 Household Products 1184 10.6 Ion Chromatography in the Food and Beverage Industry 1194 10.6.1 Beverages 1197 10.6.2 Dairy Products 1224 10.6.3 Meat Processing 1233 10.6.4 Baby Food 1237 10.6.5 Groceries and Luxuries 1254 10.6.6 Sweeteners 1264 10.7 Ion Chromatography in the Pharmaceutical Industry 1271 10.7.1 Counterion Analysis 1272 10.7.2 Analysis of Amines 1283 10.7.3 Analysis of Organic Acids 1291 10.7.4 Antibiotics 1295

ContentsXIV

10.7.5 Ionic Drugs 1311 10.7.6 Assays for Active Pharmaceutical Ingredients and Counterions 1315 10.7.7 Other Pharmaceutical Applications 1318 10.7.8 Biotechnology Applications 1325 10.8 Ion Chromatography in Clinical Chemistry 1336 10.9 Oligosaccharide Analysis of Membrane-Coupled Glycoproteins 1361 10.10 Chemical and Petrochemical Applications 1367 10.10.1 Chemical Applications 1367 10.10.2 Petrochemical Applications 1393 10.10.2.1 Combustion Ion Chromatography 1407 10.10.2.2 Biofuel Applications 1409 10.11 Other Applications 1417 10.12 Sample Preparation and Matrix Problems 1423 10.12.1 Sample Filtration and Preservation 1425 10.12.2 Sample Dilution 1431 10.12.3 Sample Pretreatment Cartridges 1437 10.12.4 Sample Neutralization 1443 10.12.5 Combustion Techniques 1446 10.12.6 Dialysis Techniques 1453 10.12.7 Chemical Modifications of Samples 1455 10.13 Concluding Remarks 1459

References 1461

Index 1485

XV

Foreword

Since the introduction of ion chromatography in 1975, this method has not only developed into the most powerful tool for the determination of inorganic and organic ions of low and high molecular weight but also found its application for the analysis of a wide variety of ionizable organic com­pounds. In the past decade, numerous new technologies introduced in the field of ion chromatography were focused on increasing the speed of analysis, sensitivity, and resolution of separations. For everybody using this method nowadays for either scientific research or routine analysis, it is very impor­tant to have a comprehensive source of information covering all recent devel­opments in this field together with some theoretical background and practical applications. The fourth edition of the Handbook of Ion Chromatog­raphy by Dr. Joachim Weiss completely fulfills this demand. The present edition is considerably updated and expanded, covering all

aspects of ion chromatography and all developments in this field that have been introduced over the past 10 years. This includes the application of hydrophilic interaction and mixed-mode liquid chromatography for the determination of ionic and ionizable compounds, the introduction of novel detection methods and hyphenated techniques, the design of new separation media for capillary and monolithic columns, columns packed with particulate ion exchangers having smaller particle diameters, and mixed-mode stationary phases. A significant part of this new edition is devoted to various applications of

ion chromatography for the determination of ions in a wide range of simple and complex matrices and to practical problems that an analytical chemist might face. It makes this book an indispensable tool for everybody using ion chromatography for everyday routine analysis or doing research work in this field. Since a lot of attention is paid to the determination of biologically rele­vant organic compounds such as amino acids, carbohydrates, proteins, and nucleic acids, this book will also be of great interest and importance for sci­entists and practitioners working in biochemistry and biopharmaceutical industries. There is no doubt that the fourth edition of Handbook of Ion Chromatography

will gain great popularity and wide recognition as an important contribution to

XVI Foreword

analytical science and industry, and I wish the author and the publishing house a lot of success with this outstanding work.

Moscow, October 2015 Prof. Dr. Oleg Shpigun Chemistry Department Lomonosov Moscow State University

XVII

Preface to the Fourth Edition

With the publication of Hamish Small’s legendary paper in Analytical Chemistry in 1975, 2015 marks the 40th anniversary of the introduction of ion chromatog­raphy (IC). Over these four decades, ion chromatography not only became the most dominant method in ion analysis but also developed into a significant chromatographic technique within the field of separation science. While in its earliest embodiments, IC was focused primarily on the analysis of inorganic anions, today IC plays an important role in the analysis of organic and inorganic anions and cations. Although separations of ions by ion-exchange chromatogra­phy prevail, other liquid chromatographic techniques such as ion-exclusion chromatography, reversed-phase liquid chromatography in the ion-suppression mode, and even hydrophilic interaction and mixed-mode liquid chromatography are also used today. Thus, the definition of the term ion chromatography became much broader over the years to be an umbrella term today for all liquid chro­matographic techniques that are suitable for separating and detecting ionic and ionizable species. More than 10 years have passed since the publication of the third edition of

this book in 2004. Although the method of ion chromatography was already well matured and widely accepted at that time, the past decade has seen a number of exciting developments in ion chromatography that are described in this new edition and that further established this analytical technique. Of particular importance are the new hyperbranched condensation polymers developed for anion-exchange chromatography with improved selectivities and chromato­graphic efficiencies. With the introduction of 4 μm ion-exchange packing materi­als, the pathway of method speedup in ion chromatography follows the one used in conventional HPLC by applying UHPLC techniques. In ion chromatography, however, the pathway of using smaller particle sizes and smaller column formats can be followed only to a certain extent due to the limited back pressure toler­ance of metal-free components in the fluidic system of IC instruments. Progress in the design of cation exchangers has been made toward new stationary phases for improved separations of amines. A new section has been devoted to mono­lithic separation materials in anion-exchange chromatography. A considerable effort has been focused on their development, because monolithic media offer the potential benefit of faster analysis while maintaining chromatographic

XVIII Preface to the Fourth Edition

resolution, thus following the trend toward shorter analysis times in an alterna­tive way. In the past, polymer monoliths were used only for the separation of biomolecules such as peptides, proteins, and nucleic acids. Only very recently has progress been made in developing polymer monoliths for the separation of small-molecular weight ions, first in the form of aggregated particle monoliths to avoid PEEK column wall adhesion and finally in the form of nanobead­agglomerated monoliths covalently bonded to the inner column wall. Over the past decade, electrolytic eluent generation (RFICTM) has been

well established as an alternative to manually prepared eluents. RFIC not only facilitates the use of gradient elution techniques in ion chromatography but also provides the user with more consistent data. Electrolytic eluent generation was also a prerequisite for the development of capillary ion chro­matography. As an analytical tool, capillary IC offers several important advantages, including higher mass sensitivity, compatibility with smaller sam­ple volumes, and significant reduction of eluent consumption and associated waste disposal. In addition, the very small flow rates used in capillary IC ena­ble the permanent operation of the system over a long period of time, thus eliminating time-consuming and error-prone steps such as system startup and equilibration as well as manual eluent preparation. The enhanced stabil­ity of capillary IC systems increases laboratory productivity, as fewer calibra­tion sequences are required and the system can quickly be verified for system performance by just running check standards, which is of utmost importance for the pharmaceutical industry (see Section 10.7). Along with the RFIC tech­nology, hydroxide eluents, which are particularly suitable for concentration gradients in anion-exchange chromatography, are increasingly replacing clas­sical carbonate/bicarbonate buffers predominantly used so far. In contrast to carbonate/bicarbonate buffers, which will still be used for relatively simple applications that only require isocratic elution, higher sensitivities are achieved with hydroxide eluents. This trend is supported by an exciting development of hydroxide-selective stationary phases for anion-exchange chromatography. A growing number of applications are based on hyphenation, thus coupling

ion chromatography with ICP–OES, ICP–MS, and ESI–MS. The advantage of coupling various application forms of ICP with ion chromatography includes the ability to separate and detect metals with different oxidation states. The analytical interest in chemical speciation is based on the fact that the oxida­tion state of an element determines toxicity, environmental behavior, and biological effects. Hyphenation with ESI–MS provides the analyst with mass-selective information. Challenging applications such as the determination of haloacetic acids in water at trace levels by IC–ESI-MS/MS or the identifica­tion and the quantification of metabolites by coupling capillary IC to an Orbitrap MS clearly demonstrate the need for MS hyphenation to achieve the required sensitivity and specificity. The updated and expanded section on hyphenated techniques underlines this importance. Because amino acids, car­bohydrates, proteins, and oligonucleotides are also analyzed by ion-exchange

XIX Preface to the Fourth Edition

chromatography, the respective sections have been updated and expanded in this new edition. In combination with integrated pulsed amperometry as a direct detection method for amino acids and carbohydrates, anion-exchange chromatography revolutionized these two application areas. Thus, ion chro­matography has become almost indispensable for the analysis of low- and high-molecular weight inorganic and organic anions and cations. Since the publication of the third edition, all these developments have made it

necessary to rewrite major parts, so this fourth edition can be confidently regarded as a new text. With the exception of Chapters 2 and 9, every chapter has been renewed or significantly revised. Due to the unmanageable number of existing and newly developed stationary phases for anion- and cation-exchange chromatography, the respective Sections 3.4 and 4.1 on stationary phases have been completely reorganized for better clarity. Chapter 7 on hydrophilic interac­tion liquid chromatography (HILIC) has been added for the first time, because nowadays this technique is also used for separating ionic and ionizable com­pounds. The general structure of this book proved to be of value, and has thus remained unchanged. Sections 3.10.5, 3.12, and 3.13 in Chapter 3 and Section 10.9 in Chapter 10 were originally written by an expert, Dr. Dietrich Hauffe, to whom I expressed my sincere gratitude at that time. However, due to important developments over the past 10 years, these sections have been updated and expanded by myself. In addition, the chapters on detection and applications have been considerably expanded with new material and with numerous practical examples in the form of chromatograms. The ever-growing number of applications in the petrochemical industry, including biofuels, was the reason to devote a separate section to this subject. The objective of this fourth edition is the same as that of the previous three

editions: the author addresses analytical chemists doing research work in this field, who wish to familiarize themselves with this method, as well as practition­ers, who employ these techniques for everyday routine analysis and are looking for a reference book that can help facilitate method development and provide an overview on the existing applications. At this point, I would like to express my sincere gratitude to many of my col­

leagues in all parts of the world, who contributed their experience and knowl­edge to the preparation of this edition. I am particularly grateful to Dr. Kelly Flook (Sunnyvale, USA) for her willingness to review the section on monolithic separation media and for her valuable suggestions, and to Emma Ramirez and Jennifer Ambra (Sunnyvale, USA) for their incredible patience to provide me with electronic versions of the huge number of chromatograms needed for this edition. I am also very grateful to Dr. Andrea Wille (Metrohm, Herisau, Switzerland), Karl-Heinz Jansen (Sykam, Fürstenfeldbruck, Germany), Dr. Maria Ofitserova (Pickering Laboratories, Mountain View, USA), and Volker Nödinger (Tosoh Bioscience GmbH, Stuttgart, Germany) for their unrestricted coopera­tion in providing me with IC methodologies and corresponding chromatograms. Last but not least, I would be grateful for any criticisms or suggestions that could serve to improve future editions of this book.

XX Preface to the Fourth Edition

Finally, many thanks to my wife and children for all their support and amazing amount of understanding and tolerance during the period when I had to spend most of my available spare time at the computer for preparing this new edition.

Niedernhausen Joachim Weiss February 2016

1

1 Introduction

1.1

Historical Perspective

“Chromatography” is the general term for a variety of physicochemical separa­tion techniques, all of which have in common the distribution of a component between a mobile phase and a stationary phase. The various chromatographic techniques are subdivided according to the physical state of these two phases. The discovery of chromatography is attributed to Tswett [1,2], who in 1903

was the first to separate leaf pigments on a polar solid phase and to interpret this process. In the following years, chromatographic applications were limited to the distribution between a solid stationary and a liquid mobile phase (liquid solid chromatography, LSC). In 1938, Izmailov and Schraiber [3] laid the founda­tion for thin-layer chromatography (TLC). Stahl [4,5] refined this method in 1958 and developed it into the technique known today. In their noteworthy paper of 1941, Martin and Synge [6] proposed the concept of theoretical plates, which was adapted from the theory of distillation processes, as a formal mea­surement of the efficiency of the chromatographic process. This approach not only revolutionized the understanding of liquid chromatography but also set the stage for the development of both gas chromatography (GC) and paper chromatography. In 1952, James and Martin [7] published their first paper on gas chromatogra­

phy, initiating the rapid development of this analytical technique. High-performance liquid chromatography (HPLC) was derived from the classi­

cal column chromatography and, besides gas chromatography, is one of the most important tools of analytical chemistry today. The technique of HPLC flourished after it became possible to produce columns with packing materials made of very small beads ( 10 μm) and to operate them under high pressure. The develop­ment of HPLC and the theoretical understanding of the separation processes rest on the basic works of Horvath et al. [8], Knox [9], Scott [10], Snyder [11], Guiochon [12], Möckel [13], and others. Ion chromatography (IC) was introduced in 1975 by Small et al. [14] as a new

analytical method. Within a short period of time, ion chromatography evolved

Handbook of Ion Chromatography, Fourth Edition. Joachim Weiss. 2016 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2016 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Introduction

from a new detection scheme for a few selected inorganic anions and cations to a versatile analytical technique for ionic species in general. For a sensitive detec­tion of ions via their electrical conductance, the separator column effluent was passed through a “suppressor” column. This suppressor column chemically reduces the eluent background conductance, while at the same time increasing the electrical conductance of the analyte ions. In 1979, Fritz et al. [15] described an alternative separation and detection

scheme for inorganic anions, in which the separator column is directly coupled to the conductivity cell. As a prerequisite for this chromatographic setup, low-capacity ion-exchange resins must be employed so that low-ionic strength eluents can be used. In addition, the eluent ions should exhibit low equivalent conductances, thus enabling detection of the sample components with reason­able sensitivity. At the end of the 1970s, ion chromatographic techniques began to be used to

analyze organic ions. The requirement for a quantitative analysis of organic acids brought about an ion chromatographic method based on the ion-exclusion pro­cess that was first described by Wheaton and Bauman [16] in 1953. The 1980s witnessed the development of high-efficiency separator columns

with particle diameters between 5 and 8 μm, which resulted in a significant reduc­tion of analysis time. In addition, separation methods based on the ion-pair pro­cess were introduced as an alternative to ion-exchange chromatography because they allow the separation and determination of surface-active anions and cations. Since the beginning of the 1990s, column development has aimed to provide

stationary phases with special selectivities. In inorganic anion analysis, stationary phases were developed that allow the separation of fluoride from the system void and the analysis of the most important mineral acids as well as oxyhalides such as chlorite, chlorate, and bromate in the same chromatographic run [17]. More­over, high-capacity anion exchangers have been developed that enable the analy­sis of, for example, trace anionic impurities in concentrated acids and salinary samples. Problem solutions of this kind are especially important for the semi­conductor industry, seawater analysis, and clinical chemistry. In inorganic cation analysis, simultaneous analysis of alkali and alkaline-earth metals is of vital importance, and can be realized only within an acceptable time frame of less than 15 min by using weak acid cation exchangers [18]. Of increasing impor­tance is the analysis of aliphatic amines, which can be carried out on modern cation exchangers without adding organic solvents to the acid eluent. Since the publication of the third edition in 2004, considerable effort has been

focused on the development of monolithic separation materials for use in ion chromatography. Monolithic media offer the potential benefit of faster analysis or improved resolution with comparable analysis speed, thus following the trend toward shorter analysis times observed in conventional liquid chromatography. While method speedup in conventional liquid chromatography (UHPLC) is achieved by utilizing smaller particle sizes and smaller column formats, this pathway can be followed only to a certain extent in ion chromatography due to the limited back pressure tolerance of metal-free components in the fluidic

3 1.1 Historical Perspective

system of IC instruments. Most research in the area of monolithic sparation media has been devoted to silica-based materials [19], which are not very suitable for ion chromatography, especially for anion separations due to pH limitations. Polymer monoliths, on the other hand, were so far used only for the separation of biomolecules such as peptides, proteins, and nucleotides [20]. Only very recently has progress been made in developing polymer monoliths for the separation of small-molecular weight ions, first in the form of aggregated particle monoliths to avoid PEEK column wall adhesion [21] and finally in the form of nanobead­agglomerated monoliths covalently bonded to the inner column wall [22]. The scope of ion chromatography was considerably enlarged by newly designed

electrochemical and spectrophotometric detectors. A milestone of this develop­ment was the introduction of a pulsed amperometric detector in 1983, allowing a very sensitive detection of carbohydrates, amino acids, and divalent sulfur com­pounds [23,24]. A recent development in the field of electrochemical detection is 3D amperometry. The relationship of 3D amperometry to conventional amper­ometry is in some ways similar to the relationship of diode array detection to single wavelength UV absorbance detection. Three-dimensional amperometry enables the continuous acquisition of current throughout the entire waveform period rather than only during a predefined period within the waveform when current is integrated. The complete data set enables, among other things, post-chromatographic current integration. Because different chemical compounds oxi­dize differently at a given applied oxidation potential, subtle differences in the amount of current generated through a waveform can provide additional informa­tion about the identity and purity of the substances being analyzed. Applications utilizing postcolumn derivatization in combination with photo­

metric detection opened the field of polyphosphate, polyphosphonate, and tran­sition metal analysis for ion chromatography, thus providing a powerful extension to conventional titrimetric and spectrometric methods. A growing number of applications are based on hyphenation, thus coupling

ion-exchange chromatography with ICP–OES, ICP–MS, or ESI–MS. The advan­tage of coupling the various application forms of ICP with ion chromatography includes the ability to separate and detect metals with different oxidation states. The analytical interest in chemical speciation is based on the fact that the oxida­tion state of an element determines toxicity, environmental behavior, and biolog­ical effects. Hyphenation with ESI–MS provides the analyst with mass-selective information. Depending on the type of MS (single quadrupoles, triple quadru­poles, ion traps, etc.) coupled to IC, molecular weight and/or structural informa­tion can be obtained. The recently published EPA Method 557 [25] for determining haloacetic acids in water at trace levels by IC–ESI-MS/MS, for instance, clearly demonstrates the need for MS hyphenation to achieve the required sensitivity and specificity for challenging applications. These developments made ion chromatography an integral part of both mod­

ern inorganic and organic analyses. Even though ion chromatography is the dominating analytical method for

inorganic and organic ions, ion analyses are also carried out with capillary

4 1 Introduction

electrophoresis (CE) [26], which offers certain advantages when analyzing samples with extremely complex matrices. In terms of detection, spectrometric methods such as UV/Vis and fluorescence detection as well as contactless conductivity detection [27] are commercially available today. Because inorganic anions and cat­ions as well as aliphatic carboxylic acids cannot be detected very sensitively, appli­cations of CE for small ion analysis are rather limited compared to IC, with its universal suppressed conductivity detection being employed in most cases. Dasgupta and Bao [28] and Avdalovic et al. [29] independently succeeded to

miniaturize a conductivity cell and a suppressor device down to the scale required for CE. Since the sensitivity of conductivity detection does not suffer from miniaturization, detection limits achieved for totally dissociated anions and low-molecular weight organics competed well with those of ion chromatography techniques. Even though the works of Dasgupta and Bao, and Avdalovic et al. have never been commercialized, capillary electrophoresis with nonsuppressed conductivity detection can be regarded as a complementary technique for ana­lyzing small ions in simple and complex matrices.

1.2

Types of Ion Chromatography

This book only discusses separation methods that can be summarized under the general term ion chromatography. Modern ion chromatography as an element of liquid chromatography is based on three different separation mechanisms, which also provide the basis for the nomenclature in use.

Ion-Exchange Chromatography (HPIC) This separation method is based on ion-exchange processes occurring between the mobile phase and the ion-exchange groups bonded to the support material. In highly polarizable ions, additional nonionic adsorption processes contribute to the separation mechanism. The stationary phase typically consists of poly­meric resins based on styrene, ethylvinylbenzene, methacrylates, or polyvinyl al­cohols modified with ion-exchange groups. With the exception of polyvinyl alcohols, the resins are usually copolymerized with divinylbenzene for high mechanical and chemical stability. Ion-exchange chromatography is used for the separation of both inorganic and organic anions and cations. Separation of anions is accomplished with quaternary ammonium groups attached to the poly­mer, whereas sulfonate, carboxylate, phosphonate, or mixtures of these groups are used as ion-exchange sites for the separation of cations. Chapters 3 and 4 deal with this type of separation method in greater detail.

Ion-Exclusion Chromatography (HPICE) The separation mechanism in ion-exclusion chromatography is governed by Donnan exclusion, steric exclusion, sorption processes and, depending on the type of separator column, by hydrogen bonding. A high-capacity, totally sulfonated

5 1.3 The Ion Chromatographic System

cation-exchange material based on poly(styrene-co-divinylbenzene) is typically employed as the stationary phase. In case hydrogen bonding should determine selectivity, significant amounts of methacrylate are added to the styrene polymer. Ion-exclusion chromatography is particularly useful for the separation of weak inorganic and organic acids from completely dissociated acids that elute as one peak within the exclusion volume of the column. In combination with suitable detection systems (postcolumn chemistry, RI, ELSD, and Corona CAD), this sepa­ration method is also useful for determining amino acids, aldehydes, and alcohols. A detailed description of this separation method is given in Chapter 5.

Ion-Pair Chromatography (MPIC) The dominating separation mechanism in ion-pair chromatography is adsorp­tion. The stationary phase consists of a neutral porous divinylbenzene resin of low polarity and high specific surface area. Alternatively, chemically bonded octadecyl silica phases with even lower polarity can be used. The selectivity of the separator column is determined by the mobile phase. Besides an organic modifier, an ion-pair reagent is added to the eluent (water, aqueous buffer solu­tion, etc.) depending on the chemical nature of the analytes. Ion-pair chromato­graphy is particularly suited for the separation of surface-active anions and cations, sulfur compounds, and transition metal complexes. A detailed descrip­tion of this separation method is given in Chapter 6.

Alternative Methods In addition to the three classical separation methods mentioned above, reversed-phase liquid chromatography (RPLC) can also be used for the separation of highly polar and ionic species. Long-chain fatty acids, for example, are separated on a chemically bonded octadecyl phase after protonation in the mobile phase with a suitable aqueous buffer solution. This separation mode is known as ion suppression [30]. Chemically bonded aminopropyl phases have also been successfully employed

for the separation of inorganic ions. Leuenberger et al. [31] described the separa­tion of nitrate and bromide in foods on such a phase using a phosphate buffer solution as the eluent. Separations of this kind are limited in terms of their appli­cability, because they can be applied only to UV-absorbing species. Moreover, applications of multidimensional ion chromatography utilizing mixed-

mode phases are very interesting. In those separations, ion-exchange and reversed-phase interactions equally contribute to the retention mechanism of ionic and polar species [32,33]. These alternative techniques are also described in Chapter 6.

The Ion Chromatographic System

The basic components of an ion chromatograph are shown schematically in Figure 1.1. It resembles the setup of conventional HPLC systems.

1.3

6 1 Introduction

Figure 1.1 Basic components of an ion chromatograph.

A pump delivers the mobile phase through the chromatographic system. In general, dual-piston pumps are employed. A pulse-free flow of the eluent is nec­essary for employing sensitive conductivity, UV/Vis, and amperometric detec­tors. Therefore, a sophisticated electronic circuitry (sometimes in combination with pulse dampeners) is used to reduce residual pulsation as much as possible. The sample is injected into the system via a valve injector, as schematically

shown in Figure 1.2. A three-way valve is required, with two ports being con­nected to the sample loop. Sample loading is carried out at atmospheric pres­sure. After switching the injection valve, the sample is transported to the separator column by the mobile phase. Typical injection volumes are between 5 and 100 μL, but smaller and larger injection volumes are used for capillary-scale ion chromatography and large-volume direct injections, respectively. The most important part of the chromatographic system is the separator col­

umn. The choice of a suitable stationary phase (see Section 1.5) and the chro­matographic conditions determine the quality of the analysis. The column tubes are manufactured from inert materials such as PEEK (polyether ether ketone). In general, separation is achieved at room temperature. Only in very few cases – for example, for the analysis of long-chain fatty acids – an elevated temperature is required to improve analyte solubility. An elevated column temperature is also recommended for the analysis of inorganic and organic cations on weak acid cation exchangers for selectivity reasons. Very rarely column temperatures below ambient are used to avoid analyte degradation. The analytes are detected and quantified by a detection system. The perform­

ance of any detector is evaluated according to the following criteria:

7 1.3 The Ion Chromatographic System

Figure 1.2 Schematic representation of a loop injector.

Sensitivity Linearity Resolution (detector cell volume) Noise (detection limit)

The most commonly employed detector in ion chromatography is the conduc­tivity detector, which is used with or without a suppressor system. The main function of the suppressor system as part of the detection unit is to chemically reduce the high background conductivity of the electrolytes in the eluent and to convert the sample ions into a more conductive form. In addition to conductiv­ity detectors, UV/Vis, amperometric, charge, fluorescence, and MS detectors are used, all of which are described in detail in Chapter 8. Chromatographic signals are displayed in the form of a chromatogram. Quan­

titative results are obtained by evaluating peak areas or peak heights, both of which are proportional to the analyte concentration over a wide range. In the past, this was performed using digital integrators that were connected directly to the analog signal output of the detector. Due to the lack of GLP/GLAP conform­ity, digital integrators are hardly used anymore. Modern detectors feature USB ports that enable the connection to a personal computer or a host computer with a suitable chromatography software. Computers also take over control functions, thus allowing fully automated operation of the chromatographic system. Because corrosive eluents such as diluted acids and bases are often used in ion

chromatography, all parts of the chromatographic system being exposed to these

8 1 Introduction

liquids should be made of inert, metal-free materials. Conventional HPLC sys­tems with tubings and pump heads made of stainless steel are only partially suited for ion chromatography, because even stainless steel is eventually cor­roded by aggressive eluents. Considerable contamination problems would result, because metal ions exhibit a high affinity toward the stationary phase of ion exchangers, leading to a significant loss of separation efficiency. Moreover, metal parts in the chromatographic fluid path would make the analysis of analytes such as orthophosphate, complexing agents, and transition metals more difficult.

1.4

Advantages of Ion Chromatography

The determination of ionic species in solution is a classical analytical problem with a variety of solutions. Whereas in the field of cation analysis both fast and sensitive analytical methods (AAS, ICP, polarography, and others) have been available for a long time, there was a lack of corresponding, highly sensitive methods for anion analysis before ion chromatography was introduced in the mid-1970s. Conventional wet-chemical methods such as titration, photometry, gravimetry, turbidimetry, and colorimetry are all labor-intensive, time-consum­ing, and occasionally troublesome. In contrast, ion chromatography offers the following advantages:

Speed Sensitivity Selectivity Simultaneous detection Stability of the separator columns

Speed

The time necessary to perform an analysis becomes an increasingly important aspect, because enhanced manufacturing costs for high-quality products and additional environmental efforts have led to a significant increase in the number of samples to be analyzed. With the introduction of high-efficiency separator columns for ion-exchange,

ion-exclusion, and ion-pair chromatography in recent years, the average analysis time could be reduced to about 10 min. Today, a baseline-resolved separation of the seven most important inorganic anions [34] requires less than 5 min. Quanti­tative results are obtained in a fraction of the time previously required for tradi­tional wet-chemical methods, thus increasing sample throughput.

Sensitivity

The introduction of microprocessor technology, in combination with modern high-efficiency stationary phases, makes it a routine task to detect ions in the lowest microgram/Liter concentration range without preconcentration. The