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M ASS S PECTROMETRY of P OLYMERS Edited by Giorgio Montaudo Robert P. Lattimer CRC PR ESS Boca Raton London New York Washington, D.C. Library of Congress Cataloging-in-Publication Data Montaudo, Giorgio. Mass spectrometry of polymers / Giorgio Montaudo, Robert Lattimer. p. cm. Includes bibliographical references and index. ISBN 0-8493-3127-7 (alk. paper) 1.Polymers--Analysis. 2. Mass spectrometry. I. Lattimer, Robert (Robert P.) II. Title. QD139.P6 M66 2001 547′.7046—dc21 2001037684 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. 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Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. Visit the CRC Press Web site at www.crcpress.com © 2002 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-3127-7 Library of Congress Card Number 2001037684 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper Preface Mass spectrometry involves the study of ions in the vapor phase. This analytical method has a number of features and advantages that make it an extremely valuable tool for the identification and structural elucidation of organic molecules—including synthetic polymers: (i) The amount of sample needed is small; for direct analysis, a micro- gram or less of material is normally sufficient. (ii) The molar mass of the material can be obtained directly by meas- uring the mass of the molecular ion or a “quasimolecular ion” containing the intact molecule. (iii) Molecular structures can be elucidated by examining molar masses, ion fragmentation patterns, and atomic compositions determined by mass spectrometry. (iv) Mixtures can be analyzed by using “soft” desorption/ionization methods and hyphenated techniques (such as GC/MS, LC/MS, and MS/MS). Mass spectrometric (MS) methods are routinely used to characterize a wide variety of biopolymers, such as proteins, polysaccharides, and nucleic acids. Nevertheless, despite its advantages, mass spectrometry has been underuti- lized in the past for studying synthetic polymer systems. It is fair to say that, until recently, polymer scientists have been rather unfamiliar with the advances made in the field of mass spectrometry. However, mass spectrometry in recent years has rapidly become an indis- pensable tool in polymer analysis, and modern MS today complements in many ways the structural data provided by NMR and IR methods. Contem- porary MS of polymers is emerging as a revolutionary discipline. It is capable of changing the analytical protocols established for years for the molecular and structural analysis of macromolecules. Some of the most significant applications of modern MS to synthetic polymers are (a) chemical structure and end-group analysis, (b) direct measure- ment of molar mass and molar mass distribution, (c) copolymer composition and sequence distribution, and (d) detection and identification of impurities and additives in polymeric materials. In view of the recent developments in this area, a book such as Mass Spec- trometry of Polymers appears opportune. Even more, in our opinion there is an acute need for a state-of-the-art book that summarizes the progress recently made. No books currently exist that deal systematically with the ©2002 CRC Press LLC whole subject. Therefore we present here an effort to summarize the current status of the use of mass spectrometry in polymer characterization. The Distinctiveness of MS A basic question one might ask is “why pursue mass spectral techniques for analysis of higher-molar mass polymers?”1 After all, a number of “classical” methods are available that have proved very successful at analyzing polymers (e.g., gel permeation chromatography, vapor pressure osmometry, laser light scattering, magnetic resonance, infrared and ultraviolet/visible spectroscopies). In light of this success, what does mass spectrometry have to offer? It turns out that there are important reasons to pursue polymer MS developments other than scientific curiosity and desire for methodological improvements.1 Classical techniques, for example, are always averaging methods; i.e., they measure the average properties of a mixture of oligomers and thus do not examine individual molecules. Furthermore, classical techniques do not normally yield information on the different types of oligomers that may be present, nor do they distinguish and identify impurities and additives in polymer samples. Copolymers and blends will often not be distinguished as to polymer type. Finally, most classical methods do not provide absolute, direct molar-mass distributions for polymers; instead they rely on calibrations made using accepted standards. Mass spectrometry clearly has great potential to examine individual oligomers/components in polymeric systems, and this can add much information to complement and extend the “classical” methods. Historical Background In order to analyze any material by mass spectrometry, the sample must first be vaporized (or desorbed) and ionized in the instrument’s vacuum system. Since polymers are generally nonvolatile, many mass spectral methods have involveddegradation of the polymeric material prior to analysis of the more volatile fragments. Two traditional methods to examine polymers have been flash-pyrolysis GC/MS and direct pyrolysis in the ion source of the instrument. In recent years, however, there has been a marked tendency toward the use of direct MS techniques. While a continued effort to introduce mass spectrometry as a major technique for the structural analysis of polymers has been made over the past three decades, MS analysis did not have a great impact upon the polymer community until the past five years or so. During ©2002 CRC Press LLC this period outstanding progress has been made in the application of MS to some crucial problems involving the characterization of synthetic polymers. Developments in two general areas have spurred this progress. Sector and quadrupole mass analyzers, the traditional methods of separation of ions in mass spectrometry, have recently been complemented by the development of powerful Fourier transform (FT-MS) and time-of-flight (TOF-MS) instru- ments. The TOF analyzers are particularly well-suited for detecting higher molar-mass species present in polymers. Parallel to this progress, new ionization methods have been developed that are based on the direct desorption of ions from polymer surfaces. With the introduction of “desorption/ionization” techniques, it has become possible to eject large molecules into the gas phase directly from the sample surface, and thereby mass spectra of intact polymer molecules have been produced. Much progress to date has been made using matrix-assisted laser desorption/ionization (MALDI-MS), which is capable of generating quasimolecular ions in the range of 106 Daltons (Da) and beyond. A brief list of ionization methods is given in Table 1. (One may quibble a bit about the dates given in the table, but we believe these are more or less accurate.) Up until about 1970, the only ionization method in common use was electron impact (EI). Field ionization (FI) was developed in the 1950s, but it was never very popular, and chemical ionization (CI) was just getting started. These three methods (EI, CI, FI) depend upon vaporization of the sample by heating, which pretty much limits polymer applications to small, stable oligomers or to polymer degradation products (formed by pyrolysis or other methods). Field desorption (FD-MS), invented in 1969, was the first “desorption/ionization” method. FD- and FI-MS are often very useful (particularly for analysis of less polar polymers), but they have never been in widespread use. TABLE 1 History of Ionization Methods Electron impact (EI) 1918 Field ionization (FI) 1954 Chemical ionization (CI) 1968 Field desorption (FD) 1969 Desorption chemical ionization (DCI) 1973 252Cf plasma desorption (PD) 1974 Laser desorption (LD) 1975 Static secondary ion mass spectrometry (SSIMS) 1976 Atmospheric pressure chemical ionization (APCI) 1976 Thermospray (TSP) 1978 Electrohydrodynamic ionization (EH) 1978 Fast atom bombardment (FAB) 1982 Potassium ionization of desorbed species (KIDS) 1984 Electrospray ionization (ESI) 1984 Multiphoton ionization (MPI) 1987 Matrix-assisted laser desorption/ionization (MALDI) 1988 ©2002 CRC Press LLC The 1970s and 1980s saw the advent of several new “soft” desorption/ ionization methods, many of which are now well-established in analytical mass spectrometry. The term “desorption/ionization” refers to a method in which the desorption (vaporization) and ionization steps occur essentially simultaneously. MALDI and several other techniques listed in Table 1 have important applications in polymer analysis. One reason for the underutilization of mass spectrometry in polymer analysis lies in the historical development. Magnetic resonance (NMR), infrared (IR), and ultraviolet/visible (UV/vis) spectroscopies have a long history in polymer analysis, while mass spectrometry is a relative newcomer. NMR, IR, and UV/vis techniques of course have the advantage that the polymer does not need to be vaporized prior to analysis. Thus these techniques gained a strong following in the polymer community long before mass spectrometric techniques were developed that could analyze intact macromolecules. In fact, mass spectrometry obtained a rather dubious reputation among many polymer scientists; this skepticism toward polymer MS continued even into the 1990s. The well-known polymer analyst Jack Koenig, in his widely-read book Spectroscopy of Polymers (1992) said: “The majority of the spectroscopic techniques, such as UV and visible or mass spectroscopy, do not meet the spec- ifications of the spectroscopic probe [for polymers].”2 Koenig’s rather skeptical opinion of mass spectrometry for polymer analysis was typical of the viewpoint of many scientists prior to the mid-1990s. Fortunately, the use of mass spectrometry for polymer analysis took on a new dimension at the turn of the century. Figure 1 lists the number of polymer mass spectrometry publications in the CAplus (Chemical Abstracts) database over the years 1965–2000. Up until the mid-1990s there was a steady—but not dramatic—increase in the number of articles. Starting in 1995, however, there has been a marked increase in the number of polymer mass spectrometry reports in the literature. Also the number of symposia and conferences devoted to the subject has grown considerably in the last few years. The major reason for this increase has been the use of MALDI-MS for numerous polymer applications. MALDI is by no means the only mass spectral method that is useful for polymer analysis, but it has provided the impetus to get polymer people interested in what mass spectrometry can do. We find it encouraging that Koenig has included a chapter on mass spectrometry in the second edition of his book (1999).3 At the end of the Mass Spectrometry chapter, Koenig makes these concluding remarks: “Modern MS, particularly with the advent of MALDI, is finally causing polymer chemists to be interested in MS as a structural analysis tool. . . . I expect that in the future MS will join IR and NMR as regular techniques used by polymer chemists.”3 ©2002 CRC Press LLC 350 300 250 ns 200 o ati c bli 150 u P 100 50 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 6 6 6 7 7 7 7 7 8 8 8 8 8 9 9 9 9 9 5 7 9 1 3 5 7 9 1 3 5 7 9 1 3 5 7 9 Year FIGURE 1 Polymer mass spectrometry publications. ©2002 CRC Press LLC Book Organization and Scope The book consists of two introductory chapters followed by nine chapters on applications. Since it is relatively new to polymer science, mass spectrometry needs to be introduced in some detail, and this is done in Chapter 1. On the other hand, many analytical chemists will need an introduction to polymer characterization methods, and this is done in Chapter 2. The rest of the chapters cover in detail the most relevant applications of mass spectrometry to the analysis of polymers. Because of the low volatility of polymeric materials, many mass spectral methods for polymers have involved pyrolysis (or thermal degradation), and this topic is covered in Chapter 3 (pyrolysis-GC/MS), Chapter 5 (direct pyrolysis-MS), and Chapter 6 (pyrolysis-FI/FD-MS). Chemical degradation methods are discussed in connection with fast atom bombardment analysis (Chapter 7). For synthetic polymers, the most popular desorption/ionization method has been matrix-assisted laser desorption/ionization (MALDI-MS, Chapter 10). Several other techniques have important applications in polymer analysis. The more widely used methods are covered in this book: electrospray (Chap- ter 4), field ionization/desorption (Chapter 6), fast atom bombardment (Chapter 7), secondary ion mass spectrometry (Chapter 8), and laser desorption (Chapters 9 and 11). The present book is designed to be practical in nature. That is, the individual chapters are not intended to be exhaustive reviews in a particular field. Instead, they introduce the subject and describe typical applications in a tutorial manner, with pertinent references from the literature. We trust that the book will be useful to both novices and experienced practitioners in polymer MS. G. Montaudo Catania, Italy R. P. Lattimer Brecksville, Ohio References 1. Schulten, H.-R. and Lattimer, R. P., Applications of Mass Spectrometry to Poly- mers,Mass Spectrom. Rev., 3, 231, 1984. 2. Koenig, J. L., Spectroscopy of Polymers, American Chemical Society, Washington, DC, 1992. 3. Koenig, J. L., Spectroscopy of Polymers:Second Edition, Elsevier, Amsterdam, 1999. ©2002 CRC Press LLC The Editors Robert Lattimer,B.S., Ph.D., is a Senior Research Associate at Noveon, Inc. (formerly a division of the BF Goodrich Co.) in Brecksville, Ohio. He has been supervisor of mass spectrometry since 1974. Dr. Lattimer has a B.S. in chemistry from the University of Missouri and a Ph.D. in physical chemistry from the University of Kansas. He was a postdoctoral associate at the Uni- versity of Michigan prior to coming to BF Goodrich/Noveon. Dr. Lattimer is an internationally recognized authority in the analytical characterization and degradation of polymeric materials. His research inter- ests include mechanisms of crosslinking and pyrolysis of polymers, and the mass spectrometric analysis of polymeric systems. He is Editor of the Journal of Analytical and Applied Pyrolysis and a past Associate Editor of Rubber Chemistry and Technology. Dr. Lattimer is past Chairman of the Gordon Research Conference on Analytical Pyrolysis, and he received the ACS Rub- ber Division’s Sparks-Thomas Award in 1990. He has won two Rubber Division Best Paper Awards, as well as three Honorable Mentions. Dr. Lattimer is a member of the American Chemical Society and its Rubber, Polymer, and Analytical Divisions. He is a past Councilor and Chairman of the Akron Section ACS. He is a member and past Vice President of the American Society for Mass Spectrometry. Dr. Lattimer lives in Hudson, Ohio, with his wife Mary and two sons, Scott and Paul. Giorgio Montaudo, Ph.D. is a Professor of industrial chemistry at the Department of Chemistry, University of Catania, Italy and Director of the Institute for Chemistry & Technology of Polymeric Materials of the National Council of Research of Italy, Catania. Dr. Montaudo received a Ph.D. in chemistry from the University of Catania. He was a postdoctoral associate at the Polytechnic Institute of Brooklyn (1966) and at the University of Michigan (1967-68 and 1971) and he was a Humboldt Foundation Fellow, 1973 at Mainz University. Dr. Montaudo has been active in the field of the synthesis, degradation, and characterization of polymeric materials. A major section of his activity has been dedicated to develop mass spectrometry of polymers as analytical and structural tools for the analysis of polymers. He is the author of more than 300 publications in international journals and chapters in books. Dr. Montaudo serves on the Editorial Board of Journal of Analytical & Applied Pyrolysis;Macromolecules; Macromolecular Chemistry & Physics;Poly- mer International; Polymer Degradation & Stability; and European Mass Spec- trometry. He is a past member of the Editorial Board of Journal of Polymer ©2002 CRC Press LLC Science, and Trends in Polymer Science. He received the Award of the Italian Chemical Industry, Milan 1990. His participation in over 120 international invited lectures includes: Charles M. McKnight Lecture, April 1998, The University of Akron; Visiting Professor, May-July 1980, Mainz University; Visiting Professor, March-September 1988, University of Cincinnati; Visiting Professor, September-November 1995, Universitè Pierre & Marie Curie Paris. Dr. Montaudo lives in Catania, Italy, with his wife Paola. He has a son, Maurizio, and a daughter, Matilde. ©2002 CRC Press LLC