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MS of polymers 2002 montaudo lattimer

MASS
SPECTROMETRY
of POLYMERS
Edited by

Giorgio Montaudo
Robert P. Lattimer

CRC PR E S S
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

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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 microgram or less of material is normally sufficient.
(ii) The molar mass of the material can be obtained directly by measuring 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 underutilized 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 indispensable tool in polymer analysis, and modern MS today complements in
many ways the structural data provided by NMR and IR methods. Contemporary 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 measurement 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 Spectrometry 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
involved degradation 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) instruments. 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 specifications 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

Publications

250

200

150

100

50

0
1999

1997

1995

1993

1991

1989

©2002 CRC Press LLC

1987

FIGURE 1
Polymer mass spectrometry publications.

1985

1983

1981

1979

1977

1975

1973

1971

1969

1967

1965

Year


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 (Chapter 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 Polymers, 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 University 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 interests 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 Rubber 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; Polymer International; Polymer Degradation & Stability; and European Mass Spectrometry. 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


Contributors

Mattanjah S. de Vries University of California, Santa Barbara, California
David M. Hercules Vanderbilt University, Nashville, Tennessee
Heinrich E. Hunziker University of California, Santa Barbara, California
Robert Lattimer Noveon, Inc., Brecksville, Ohio
Giorgio Montaudo University of Catania, Catania, Italy
Maurizio S. Montaudo Istituto per la Chimica e la Tecnologia dei Materiali
Polimerici, Consiglio Nazionale delle Ricerche, Catania, Italy
Hajime Ohtani Nagoya University, Nagoya, Japan
Salvador J. Pastor University of Arkansas, Fayetteville, Arkansas
Michael J. Polce The University of Akron, Akron, Ohio
Laszlo Prokai Univeristy of Florida, Gainesville, Florida
Concetto Puglisi Istituto per la Chimica e la Tecnologia dei Materiali
Polimerici, Consiglio Nazionale delle Ricerche, Catania, Italy
Filippo Samperi Istituto per la Chimica e la Tecnologia dei Materiali
Polimerici, Consiglio Nazionale delle Ricerche, Catania, Italy
Shin Tsuge Nagoya University, Nagoya, Japan
Chrys Wesdemiotis The University of Akron, Akron, Ohio
Charles L. Wilkins University of Arkansas, Fayetteville, Arkansas

©2002 CRC Press LLC


Contents

Preface
Giorgio Montaudo and Robert P. Lattimer

1

Introduction to Mass Spectrometry of Polymers
Michael J. Polce and Chrys Wesdemiotis

2

Polymer Characterization Methods
Giorgio Montaudo and Maurizio S. Montaudo

3

Pyrolysis Gas Chromatography/
Mass Spectrometry (Py-GC/MS)
Shin Tsuge and Hajime Ohtani

4

Electrospray Ionization (ESI-MS) and On-Line Liquid
Chromatography/Mass Spectrometry (LC/MS)
Laszlo Prokai

5

Direct Pyrolysis of Polymers into the Ion Source
of a Mass Spectrometer (DP-MS)
Giorgio Montaudo and Concetto Puglisi

6

Field Ionization (FI-MS) and Field Desorption (FD-MS)
Robert P. Lattimer

7

Fast Atom Bombardment of Polymers (FAB-MS)
Giorgio Montaudo and Filippo Samperi

8

Time-of-Flight Secondary Ion Mass Spectrometry
(TOF-SIMS)
David M. Hercules

9

Laser Fourier Transform Mass Spectrometry (FT-MS)
Salvador J. Pastor and Charles L. Wilkins

©2002 CRC Press LLC


10 Matrix-Assisted Laser Desorption/Ionization Mass
Spectrometry of Polymers (MALDI-MS)
Giorgio Montaudo, Maurizio S. Montaudo, and Filippo Samperi

11

Two-Step Laser Desorption Mass Spectrometry
Mattanjah S. de Vries and Heinrich E. Hunziker

©2002 CRC Press LLC


1
Introduction to Mass Spectrometry
of Polymers

Michael J. Polce and Chrys Wesdemiotis

CONTENTS
1.1 Introduction
1.2 Ionization Methods
1.2.1 Ionization of Volatile Materials
1.2.1.1 Electron Ionization (EI)
1.2.1.2 Chemical Ionization (CI)
1.2.1.3 Field Ionization (FI)
1.2.2 Desorption/Ionization Methods
1.2.2.1 Field Desorption (FD)
1.2.2.2 Secondary Ion Mass Spectrometry (SIMS)
1.2.2.3 Fast Atom Bombardment (FAB) and Liquid
Secondary Ion Mass Spectrometry
(LSIMS)
1.2.2.4 Matrix-Assisted Laser Desorption
Ionization (MALDI)
1.2.3 Spray Ionization Methods
1.2.3.1 Thermospray (TSP)
1.2.3.2 Electrospray Ionization (ESI)
1.3 Mass Analyzers
1.3.1 Scanning Mass Analyzers
1.3.1.1 Quadrupole Mass Filter
1.3.1.2 Quadrupole Ion Trap
1.3.1.3 Magnetic and Electric Sectors
1.3.2 Nonscanning Mass Analyzers
1.3.2.1 Time-of-Flight (TOF) Analyzers
1.3.2.2 Fourier-Transform Ion Cyclotron
Resonance (FTICR)

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1.4

Detectors
1.4.1 Electron Multipliers and Related Devices
1.4.2 Photon Multipliers
1.5 Tandem Mass Spectrometry
Acknowledgments
References

1.1

Introduction

Mass spectral analyses involve the formation of gaseous ions from an analyte
(M) and subsequent measurement of the mass-to-charge ratio (m/z) of these
1
ions. Depending on the ionization method used, the sample is converted to
molecular or quasimolecular ions and their fragments. Molecular ions are
+
generally radical cations (M ˙), formed by electron removal from M; electron

2,3
addition to yield M ˙ is used occasionally for electronegative samples.
Quasimolecular ions may be either positive or negative and arise by adding
+
to M, or subtracting from it, an ion; common examples include [M + H] , [M −

+

H] , [M + Na] , and [M + Cl] . “Soft” ionization methods generate predominantly molecular or quasimolecular ions, whereas “hard” ionization meth1–3
ods also yield fragment ions. The mass spectrometer separates the ions
generated upon ionization according to their mass-to-charge ratio (or a
related property) to give a graph of ion abundance vs. m/z. Mixtures are
often preseparated by gas or liquid chromatography, so that a mass spectrum
can be obtained for each individual component to thereby facilitate sample
2,3
characterization.
The exact m/z value of the molecular or quasimolecular ion reveals the
ion’s elemental composition and, thus, allows for the compositional analysis
1
of the sample under study. If the molecular ions are unstable and decompose
completely, the resulting fragmentation patterns can be used as a fingerprint
1
for the identification of the sample. Fragment ions also provide important
information about the primary structure (i.e., connectivity or sequence) of the
1–3
sample molecules. With soft ionization methods that produce little or no
fragments, fragmentation can be induced by employing tandem mass
4,5
spectrometry (MS/MS).
Mass spectrometry methods have experienced a steadily increasing use in
6
−15
polymer analyses due to their high sensitivity (<10 mol suffice for analysis), selectivity (minor components can be analyzed within a mixture), specificity (exact mass and fragmentation patterns serve as particularly specific
compositional characteristics), and speed (data acquisition possible within
seconds). As mentioned, the analysis of a polymer (or any other sample) by
mass spectrometry presupposes that the polymer can at least partly be converted to gas-phase ions. This chapter briefly reviews the ionization methods
and instrumentation available today for the characterization of synthetic
macromolecules.
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1.2

Ionization Methods

There are three major methods for the preparation of gaseous ions. (i) Volatile
materials are generally ionized by interaction of their vapors with electrons,
ions, or strong electric fields. (ii) Strong electric fields can also ionize nonvolatile materials. In addition, ions from nonvolatile and thermally labile
compounds can be desorbed into the gas phase via bombardment of the
appropriately prepared sample with fast atoms, ions, or laser photons and
via rapid heating. (iii) Alternatively, liquid solutions of the analyte may directly
be converted to gas phase ions via spray techniques. Method (i) can only be
applied to monomers and low-mass oligomers or in conjunction with degradation methods (principally pyrolysis). Methods (ii) and (iii) on the other
hand are amenable to intact polymers. The ensuing sections describe the
specific properties of these ionization methods.

1.2.1

Ionization of Volatile Materials

1.2.1.1 Electron Ionization (EI)
−5
In this method, the sample is thermally vaporized and approximately 10
Torr of its vapors enter the ion source volume where they are ionized by
collision with an electron beam of (typically) 70 eV kinetic energy. Electron
+
ionization can produce intact molecular radical cations, M ˙, by ejection of
1,7
an electron from the sample molecules (Eq. 1.1). This process has a yield
of ∼0.01% and deposits a wide distribution of internal energies to the newly
+
formed molecular ions; as a result, many M ˙ are formed excited enough to
+
+
+
yield a number of fragment ions (Eq. 1.2) via competitive (F 1 , F 2 , F 3 ) and
+
+
+
consecutive (f a , f b , f c ) decompositions.


+



M + e → M ˙ + 2e
+



+

F1 → fa →
+

+

(1.2a)
+

M ˙ → F2 → fb →



+

+

F3 → fc →

(1.1)

(1.2b)
(1.2c)

The EI mass spectrum that results is comprised of the molecular ion and all
fragment ions; the degree of fragmentation can be reduced by lowering the
1,7
electron energy to ≤15 eV. Figure 1.1 shows the EI mass spectra of the
8
photolysis products of poly(ethylene) and poly(propylene). Each spectrum
shows the molecular ions of several hydrocarbon subunits (m/z values
©2002 CRC Press LLC


% TOTAL ION INTENSITY

a

(−CH2 −CH2 −)n
56

10

15 ev
84

5
112
0

15

40

60

80

100

b

120

CH3



% TOTAL ION INTENSITY

15

(−CH−CH2 −)n
10

15 ev

42
84
5

0

126

40

60

80

100

120

m/e
FIGURE 1.1
EI mass spectra using 15 eV ionizing electrons of the laser pyrolysis products of (a) poly(ethylene)
and (b) poly(propylene). (Reprinted from Ref. 8 with permission of John Wiley & Sons)

marked) as well as their fragment ions; their distinctive fragmentation patterns help identify the composition of the original polyolefin.
From functionalized polymers or copolymers, complex mixtures of several
monomers, small oligomers, and other products may arise upon degradation. In such cases, it is advantageous to use GC/MS, which makes it possible
to obtain mass spectra of the single-mixture constituents. The mass spectra
identify the individual components, while the total ion chromatograms
reconstructed from the spectra reveal quantitative compositional information about the polymer, for example, the proportion of oligomers in a random
or block copolymer. GC/MS of pyrolyzed polymers is covered in considerable detail in Chapter 3.
1.2.1.2 Chemical Ionization (CI)
In chemical ionization, gaseous analyte molecules are ionized by ionmolecule reactions with reagent ions, formed by electron ionization from the
9
appropriate reagent gas. The CI ion source is similar to the EI source, but
is operated at a higher pressure (0.1–2 Torr). The chemical ionization process is illustrated for a proton transfer reaction, which is the most common
©2002 CRC Press LLC


2,3,7,9

3

ionization mode.
The sample and a large excess (∼10 fold) of the reagent
gas (RH) are introduced simultaneously into the source. The reagent molecules are ionized by electron impact and react with other reagent molecules
+
to form reactant ions, RH 2 (Eq. 1.3), which protonate the sample (Eq 1.4).
+

+

RH ˙ + RH → RH 2 + R˙

(reagent ion formation)

+

RH 2 + M → RH + MH
+

RH 2 + M → [ M + RH 2 ]

+

+

(1.3)

(proton transfer)

(1.4)

(electrophilic addition)

(1.5)

+

+

+

Typical protonation reagents are CH 5 , (CH3)3C , and NH 4 . Proton transfer
proceeds at the collision rate (every encounter has 100% efficiency) with
exothermic reactions, i.e., when the proton affinity (PA) of M is larger than
9
the PA of RH. The reaction exothermicity (∆PA) ends up as internal energy
+
of MH , which thus can be controlled by the choice of RH. When ∆PA is
small, which is true for reagents of high proton affinity (such as NH3), the
+
internal energy of MH is low and little (if any) fragmentation takes place.
+
In contrast, when ∆PA is large, an appreciable fraction of MH undergoes
fragmentation. Endothermic proton transfer is usually not observed; in such
7,9
a case, electrophilic addition (Eq. 1.5) is much more likely. The large source
+
pressure ensures that RH 2 is thermalized (to avoid endothermic reactions)
and that M is ionized by a chemical reaction (Eqs. 1.4 or 1.5) and not by
electron ionization.
+

+

Ar ˙ + M → Ar + M ˙
+

H 3 + M → 2H 2 + [ M – H ]


(charge exchange)
+

(anion abstraction)


CH3O + M → CH3OH + [M − H]




Cl + M → [M + Cl]

(cation abstraction)

(nucleophilic addition)

(1.6)
(1.7)
(1.8)
(1.9)

Depending on the chemical properties of the analyte, reactions other than
proton transfer and electrophilic addition can be used to produce analyte
molecular or quasimolecular ions. Equations 1.6 through 1.9 exemplify these
alternatives with specific reactant ions, which are particularly effective for
the given reactions.7,9 Overall, negative chemical ionization (Eqs. 1.8 and 1.9)
is used less frequently than positive chemical ionization (Eqs. 1.4 through 1.7).
CI can be used for the analysis of pyrolytic or photolytic degradation
products with or without online chromatographic separation (see Chapters
4 and 5). A variant, namely desorption chemical ionization (DCI) is applicable
to intact low-mass polymers as well. In DCI, the sample is not vaporized before
©2002 CRC Press LLC


Lower temperatures
decomposition favored
lnk

Higher
temperatures
vaporization
favored

Decomposition
Vaporization

1/ T

100

50

373(4;474)
395(5;578)
407(6;682)
420(7;786)
436(8;890)
450(9;994)
472(10;1068)
483(11;1202)
483(12;1306)
488(13;1410)
493(14;1514)
498(15;1618)
512(16;1722)
521(17;1826)
526(18;1930)
532(19;2034)
535(20;2118)
540(21; 2242)
546(22;2346)
552(23;2450)
565(24;2554)
568(25;2658)
575(26;2762)

RELATIVE INTENSITY

FIGURE 1.2
Dependence on temperature of the rate constants of decomposition and vaporization. (Reprinted
from Ref. 10 with permission of the American Chemical Society)

500

1000

1500

2000

2500

3000

MASS (amu)
FIGURE 1.3
Partial DCI spectrum of poly(styrene) using argon as the reagent gas (Eq. 1.6). The solid lines
are n-mer molecular ions, and the dashed lines are fragment ions. The numbers not in parentheses are the evaporation temperatures in K. The first and second numbers in parentheses are
the number of monomer units and the monoisotopic mass, respectively. (Reprinted from Ref. 11
with permission of the American Chemical Society)

entering the CI source but is rapidly heated inside the source. Rapid heating
enhances the probability of sample evaporation vis-à-vis sample decompo10
sition (cf. Figure 1.2); once the sample is in the gaseous state, it is immediately ionized by the surrounding CI reagent ions. A DCI application is
illustrated in Figure 1.3, which reproduces the spectrum of a poly(styrene),
©2002 CRC Press LLC


acquired by rapid evaporation of the polymer from an electrically heated
+
+
rhenium filament. DCI can be combined with K ionization to form [M + K]
adducts; this approach, termed “potassium ion ionization of desorbed
species” (KIDS).
1.2.1.3 Field Ionization (FI)
In FI, gaseous analyte molecules (M) approach a surface of high curvature
that is maintained at a high positive potential, giving rise to a strong electric
7
field near the surface (of the order of 10 V/cm). Under the influence of the
field, quantum tunneling of a valence electron from M to the anode surface
−12
+
+
can take place in about 10 s, creating M ˙. [M + H] may also form with
2,3
polar analytes by hydrogen abstraction from or near the anode. Molecular
ions produced via FI possess lower internal energies than those produced
via EI and, thus, fragment less. This is documented in Figure 1.4 by the EI
vs. FI spectra of poly(ethylene).
The residence times of an ion in the FI and EI sources are approximately
−12
−6
10 and 10 s, respectively. The smaller residence time upon FI eliminates
or reduces the extent of rearrangements; as a result, isomers that produce
7
very similar EI spectra may be distinguishable by their FI spectra.

1.2.2

Desorption/Ionization Methods

1.2.2.1 Field Desorption (FD)
FD and FI have the same ionization mechanism. In FD, the sample is not
vaporized into the gaseous state but deposited directly onto the surface
carrying the strong field (called emitter). Under the strong fields used, no
+
heating or only mild heating of the emitter is needed to desorb M ˙ or [M +
+
H] . Metal salts may be added to the sample to form other types of quasi+
+ 2,3,6a,7
molecular ions, such as [M + Na] or [M + K] .
Field desorption leads
to less excited ions than FI and often gives molecular or quasimolecular ions
6a,7
only, facilitating compositional analyses. FD has been successfully applied
12,13
to polymers with molecular weights up to ca. 10,000 Da;
an example is
14
shown in Figure 1.5. The method is particularly useful for hydrocarbon
polymers with no functional groups which even today are hard to ionize by
any other methods (see Chapter 6).
1.2.2.2 Secondary Ion Mass Spectrometry (SIMS)
This method has traditionally been used for the elemental analysis of surfaces (“dynamic” SIMS). Organic materials can be subjected to SIMS, too, by
depositing them as a thin film on a metal (or other) foil, occasionally together
15,16
+
The sample is bombarded by a primary ion beam (e.g., Ar or
with a salt.
+
Cs ), which leads to the sputtering of secondary ions from the surface. The
+

+
+
latter can be M ˙, M ˙, [M + Ag] (if a silver surface is used), or [M + alkali]
©2002 CRC Press LLC


57

100

85

80

60

a

646

590

534

400

600

800

534

200

449

393

337

281

225

169

20

111

40

100

590

80

b

200

400

600

800

1010

926

842

366

310

422

20

758

40

702

646

478

60

1000

1200

FIGURE 1.4
(a) Electron ionization (70 eV) and (b) field ionization mass spectra of poly(ethylene) 630.
(Courtesy of Dr. Robert P. Lattimer, BF Goodrich Company)

15–17

(if the sample is doped with an alkali metal ion salt).
This SIMS technique is often referred to as “static” or “organic” SIMS and, as a high-energy
17
process, normally causes extensive fragmentation. The structural insight
rendered by SIMS is discussed in detail in Chapter 8.
©2002 CRC Press LLC


11840

100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0

11423
11214

12257
12466
12570
12675

11005

12779
12883
10797

12987

10693
10588

13091
13195
13300
13404

10484
10380

13509
13612

10172
9963

13821
14030

9547

9000

10000

11000

12000
m/z

13000

14000

15000

16000

FIGURE 1.5
Field desorption mass spectrum of poly(styrene) 12500. (Reprinted from Ref. 14 with permission
of John Wiley & Sons)

1.2.2.3

Fast Atom Bombardment (FAB) and Liquid Secondary
Ion Mass Spectrometry (LSIMS)
18
19
FAB and LSIMS are conceptually identical with static SIMS. Now, the
sample is mixed with a viscous liquid of low volatility, such as glycerol,
thioglycerol, 3-nitrobenzylalcohol, or diethanolamine. A droplet of the mixture is bombarded by a fast (keV) beam of ions (LSIMS) or atoms (FAB),
producing ions characteristic of the matrix and the analyte, as shown in
20
+

Figure 1.6. The analyte ions usually are [M + H] , [M − H] , or attachment
ions of M with added or adventitious alkali metal ions. It is believed that
these ions are formed by ion-molecule reactions in the selvedge region (gas
phase region just above the liquid surface of the droplet being bombarded).
Ions that are preformed in solution, such as quaternary ammonium cations
and salt cluster ions, may be directly desorbed into the gas phase.
The liquid matrix provides continuous surface renewal, so that intense
primary beams can be used to produce intense and long-lasting spectra.
Further, the ion source is at ambient temperature, preventing the thermal
degradation of labile compounds. FAB and liquid SIMS are, however, limited
to polar polymers that are miscible with the polar liquid matrices necessary
©2002 CRC Press LLC


+

primary
beam of
bombarding particles

-

+

a
a

a

m

m+

+
-

m

+

m+

+

-

+

m

+

m+

m

m+

ion optics
for mass analysis
of secondary ions

m

a

analyte dissolved
in matrix
FIGURE 1.6
Bombardment of an analyte sample (a) dissolved in a liquid matrix (m) by a primary beam of
atoms or ions (b) to produce sample ions that are characteristic of the analyte. (Reprinted from
Ref. 20 with permission of John Wiley & Sons)

for these ionization methods (see Chapter 7). FAB and LSIMS have extensively
been applied to low-molecular-weight polyglycols and related compounds
21–23
(<5,000 Da).
Figure 1.7 shows the mass spectrum of a poly(ethylene
+
glycol) with added NaBr; quasimolecular ions ([M + Na] ) and fragments
from H2O loss can readily be identified. Many other fragments appear at
low m/z where matrix ions and matrix cluster ions can also contribute; for
this reason, fragmentation of FAB and LSIMS generated ions is often sought
22,23
through MS/MS experiments.
1.2.2.4 Matrix-Assisted Laser Desorption Ionization (MALDI)
24
MALDI is the newest and most promising desorption method for synthetic
25
macromolecules. The polymer is dissolved in the appropriate solvent and
mixed with a solution of the matrix to achieve a molar ratio of analyte to matrix
of 1:100–1:50,000. A solution of an auxiliary ionization agent (e.g., a metal
ion salt) may be added and a small droplet (≤1 µL) of the resulting mixture
26
are loaded onto a target surface (Figure 1.8). As the solvent evaporates, a
solid solution of the sample (and the auxiliary agent) in the matrix is obtained,
which is bombarded by laser light. The matrix must have a strong absorption
at the wavelength emitted by the laser; normally pulsed UV (N2, 337 nm)
and IR (CO2, 10.6 µm) lasers are employed. Upon irradiation of the crystalline
sample mixture, intact protonated, deprotonated, or metal ion attached mol24–27
ecules are desorbed for m/z analysis.
MALDI is extremely sensitive, with the total amount of sample deposited
onto the target being in the pico- to femtomole range. Polymers up to about
6
28
10 Da can be ionized by this method (Figure 1.9). Up to approximately
50,000 Da, singly charged ions are formed exclusively or predominantly,
while at higher molecular weights multiply charged ions are usually coproduced in considerable abundance. The high dilution of the analyte in the
©2002 CRC Press LLC


133

100.0

50.0

156

100
100.0

177

142
98 112

186

150

243

199 214

200

2.5X

287
259 271

250

300
569

525

331

303

375

347

350

613

481

657

437

50.0
393

419

400

463

450

507

551

500

595

550

639

600

683

650

40.0

20.0

701
745
789
833
727

700

750

800

850
m/z

877

921

900

965

950

FIGURE 1.7
FAB mass spectrum of poly(ethylene glycol) 600. (Reprinted from Ref. 21 with permission of
Elsevier Science)

matrix prohibits analyte-analyte interactions, which could lead to the forma7
tion of analyte clusters, thereby complicating molecular-weight assignments.
The MALDI matrices are usually organic compounds. In UV-MALDI, which
25
is most widely used for synthetic polymers, the matrix is an aromatic organic
compound carrying oxo, hydroxyl, and/or carboxyl groups; commonly selected
matrices are 2,5-dihydroxybenzoic acid (DHB), 2-(4-hydroxyphenylazo)benzoic acid (HABA), α-cyano-4-hydroxycinnamic acid (α CHCA), trans-3indoleacrylic acid (IAA), dithranol, and all-trans retinoic acid (Figure 1.10).
The macromolecules are not energized directly upon irradiation; the light is
rather absorbed by the matrix which is ionized and dissociated. This process
breaks down the crystalline structure of the matrix, changing it to a supercompressed gas, in which charge transfer reactions with the analyte mole+
27
cules can take place (mainly H or metal ion transfer). As the gas expands,
it transports entrapped analyte ions and molecules from the surface into
the gas phase where, at the selvedge, further charge transfer reactions to
neutral analyte molecules are possible. Collisions within the expanding gas
©2002 CRC Press LLC


30,000 V

sample
and
matrix

photon beam

H+
+
+

+
+

+

+ +
+ +

H+
H+
ions desorbed from matrix

FIGURE 1.8
MALDI source. (Reprinted from Ref. 26 with permission of Academic Press)

(“matrix plume”) dissipate most of the internal energy of the analyte ions
formed. The sequence of these desorption/ionization events is schematically
27
summarized in Figure 1.11.
MALDI is today the ionization method of choice for the analysis of the
compositions, end groups, and molecular weight distributions of intact synthetic polymers. The promise and limitations (particularly in reproducing
actual molecular weight distributions) of MALDI, which have been the subject of vigorous debate in the literature, are presented in more detail in
Chapter 10. Here, MALDI’s capabilities are exemplified by Figure 1.12, which
shows the mass spectrum of a poly(ethylene glycol) that was derivatized
29
+
with the drug acetaminophen. The exact m/z values of the [M + Na] ions
observed confirm that the polyglycol carries the drug labels at both ends, as
depicted below.
PEG
O

H2
C

C
H3C

O

O
C
H2

C

CH3

n

O

acetaminophen

Further, only one distribution is observed, consistent with the absence of
mono- or underivatized PEG.
©2002 CRC Press LLC


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