CHROMATOGRAPHYAND SEPARATION SCIENCE This is Volume 4 of SEPARATION SCIENCE AND TECHNOLOGY A reference series edited by Satinder Ahuja CHROMATOGRAPHY AND SEPARATION SCIENCE Satinder Ahuja Ahuja Consulting Calabash, North Carolina Amsterdam Boston London New York Oxford Paris San Diego San Francisco Singapore Sydney Tokyo This book is printed on acid-free paper. s1 Copyright ß 2003 Elsevier Science (USA) All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the Publisher. The appearance of the code at the bottom of the first page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of specific clients. 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ACADEMIC PRESS An Elsevier Science Imprint 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.academicpress.com International Standard Book Number: 0-12-044981-1 Printed in the United States of America 02 03 04 05 06 07 08 MM 9 8 7 6 5 4 3 2 1 CONTENTS PREFACE ix 1 Relating Chromatography to Separations I. Defining Separation 2 II. Evolution of Chromatography 3 III. Separations in Everyday Life 6 IV. Basis of Separations 7 V. Modes of Chromatography 13 VI. Unified Separation Science 14 VII. Selectivity and Detectability 15 References 16 Questions for Review 16 2 Simple Separation Methods I. Evaporation 18 II. Precipitation 18 III. Crystallization 19 IV. Filtration 19 V. Membrane Separations 21 VI. Distillation 24 VII. Extraction 26 References 35 Questions for Review 35 v vi CONTENTS 3 Equilibrium Processes in Separations I. Molecular Interactions 38 II. Separation Thermodynamics 38 References 48 Questions for Review 48 4 The Molecular Basis of Separation I. Molecular Interactions 49 II. Solubility Parameter Theory 60 III. Group Interactions 65 References 67 Questions for Review 67 5 MassTransport and Separation I. Types of Diffusion 70 II. Modeling Diffusion 71 III. Chromatographic Implications 72 IV. Diffusion Rates in Various Media 74 V. Mass Transfer 77 References 79 Questions for Review 79 6 Chromatographic Methods I. General Classification of Chromatographic Methods 84 II. Classification Based on Retention Mode 87 III. Classification Based on Sample Introduction 90 IV. Separation Characteristics of Chromatographic Methods 93 V. Basic Chromatographic Theory 95 References 99 Questions for Review 100 7 Paper Chromatography I. Chromatography Paper 102 II. Sample Preparation 103 III. Sample Cleanup 104 IV. Derivatization 105 V. Mobile Phases and Stationary Phases 106 VI. Development of Chromatograms 110 VII. Detection 111 VIII. Quantitation 112 References 112 Questions for Review 112 CONTENTS vii 8 Thin-Layer Chromatography I. Stationary Phases for TLC 114 II. TLC of Enantiomeric Compounds 117 III. Sample Application 119 IV. Mobile Phases 123 V. Development of Chromatograms 124 VI. Detection and Quantitation 125 VII. Applications 126 References 131 Questions for Review 131 9 Gas Chromatography I. Equipment 134 II. Separation Process 135 III. Columns 136 IV. Gas Chromatographic Detectors 142 V. Recording and Analysis 145 VI. Resolution 146 VII. Selection of a Stationary Phase 147 References 152 Questions for Review 152 10 High-Pressure Liquid Chromatography I. Evolution of HPLC 154 II. Advantages over Gas Chromatography (GC) 155 III. Separation Process 157 IV. Retention Parameters in HPLC 160 V. Resolution and Retention Time 163 VI. Equipment 167 VII. Separation Mechanism in HPLC 168 VIII. Stationary Phase Effects 179 IX. A Case Study of Retention Mechanism Investigations 180 X. Molecular Probes/Retention Index 182 XI. Mobile Phase Selection and Optimization 185 References 205 Questions for Review 208 11 Evolving Methods and Method Selection I. Evolving Methodologies 211 II. Selection of a Separation Method 220 III. Chiral Separations 223 IV. Comparison of GC, SFC, HPLC, and CEC for a Selective Separation 239 References 240 Questions for Review 241 Index 243 vii ThisPageIntentionallyLeftBlank PREFACE For more than 25 years, I led a group in Analytical R&D in a major pharmaceutical company (Novartis), and also taught a course on chemical separations at Pace University. I had the pleasure of succeeding Lloyd Snyder, who had taught this course before me. We used Karger, Snyder, and Horvath’s An Introduction to Separation Science (Wiley) as a text for this course. I also taught graduate courses on separations and advanced biochemical analysis at Rutgers University and a course on modern methods of chemical analysis at Polytechnic University. I found that I had to use at least a dozen books to convey the subject of chromatography and separation science to the students. All of those books and more are adequately referenced in this text to enable the reader to seek more details. The books that were found especially useful, besides the one mentioned above, were Pecosk, Shields, Cairns, and McWilliam’s Modern Chemical Analysis (Wiley), Stahl’s, Thin- Layer Chromatography (Springer-Verlag), Snyder and Kirkland’s Introduction to Modern Liquid Chromatography (Wiley), Poole and Poole’s Chromatog- raphy Today (Elsevier), McNair and Miller’s Basic Gas Chromatography (Wiley), Miller’s Separation Methods in Chemical Analysis (Wiley), Giddings’s Dynamics of Chromatography (Marcel Dekker) and Unified Separation Science (Wiley), and books by me, entitled Selectivity and Detectability Optimizations in HPLC (Wiley), Trace and Ultratrace Analysis by HPLC (Wiley), and Chiral Separations by Chromatography (Oxford). I am indebted to the valuable contributions made by these scientists. After several years of dealing with this number of books, it occurred to me to plan a text that would cover this subject of chromatography and separation science in a concise manner and avoid burdening the reader with a lot of details and mathematical equations. This book is designed for students and separation scientists who are not averse to learning a few ix x PREFACE equations dealing with physicochemical processes that influence chromatog- raphy and separations. The main objectives of this book are as follows: . Provide basic information on chromatography and separation science. Describe the relationship between these important fields. Cover how simple extraction or partition processes provide the basis for development of chromatography and separation science. Describe the role of chromatography and separation science in various fields. Discuss the role of chromatography and separation science in development of new methodology. Cover new evolving methods and show how to select an optimum method. Separation can be defined as an operation in which a mixture is divided into at least two fractions having different composition, molecular mass, or stereochemical structure. An actual separation is usually achieved by physical process, although chemical reactions may occasionally be involved in this process. It is important to learn the fundamental physical and chemical phenomena involved in the achievement of separations, as well as those with the development and application of various separation processes. This continually evolving discipline is called separation science. Chromatography derives its name from chroma and graphy, meaning color writing. It is essentially a physical method of separation in which components to be separated are distributed between two phases. Chromatography is a powerful technique concerned with separations of a large variety of complex compounds. The contributions of chromatography to various scientific disciplines and the benefits that chromatography provide to mankind are unparalelled. For example, the progress made in the biological sciences such as biotechnology, clinical pharmacology/therapeutics, and toxicology provides an excellent testimony to the contributions of chromatography. Examples from these and other fields are included throughout this textbook. This book will provide the necessary information to improve the comprehension of separation science. It will show how chromatography relates to separation science and describe how simple separation methods have led to highly useful chromatographic techniques that provide high resolution (Chapters 1 and 2). To provide a better understanding of transport phenomena and thermodynamics, an elementary discussion is included in Chapters 3 to 5. The discussion on chromatography should help improve the under- standing of the principles involved in various modes of chromatography such as paper