Mass Spectrometry of Polymers

Giorgio Montaudo, Robert P. Lattimer

• Publisher: CRC Press
• Year: 2001
• eBook ISBN: 9781000611519
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Mass Spectrometry (MS) has rapidly become an indispensable tool in polymer analysis, and modern MS today complements in many ways the structural data provided by Nuclear Magnetic Resonance (NMR) and Infrared (IR) methods. Recent advances have sparked a growing interest in this field and established a need for a summary of progress made and results

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)

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 ions.¹ 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 addition to yield M^−· is used occasionally for electronegative samples.^2,3 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 methods also yield fragment ions.^1–3 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 characterization.^2,3

The exact m/z value of the molecular or quasimolecular ion reveals the ion’s elemental composition and, thus, allows for the compositional analysis of the sample under study.¹ If the molecular ions are unstable and decompose completely, the resulting fragmentation patterns can be used as a fingerprint for the identification of the sample.¹ Fragment ions also provide important information about the primary structure (i.e., connectivity or sequence) of the sample molecules.^1–3 With soft ionization methods that produce little or no fragments, fragmentation can be induced by employing tandem mass spectrometry (MS/MS).^4,5

Mass spectrometry methods have experienced a steadily increasing use in polymer analyses⁶ due to their high sensitivity (<10⁻¹⁵ 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.

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)

In this method, the sample is thermally vaporized and approximately 10⁻⁵ 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...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface
• The Editors
• Contributors
• Table of Contents
• 1 Introduction to Mass Spectrometry of Polymers
• 2 Polymer Characterization Methods
• 3 Pyrolysis Gas Chromatography/ Mass Spectrometry (Py-GC/MS)
• 4 Electrospray Ionization (ESI-MS) and On-Line Liquid Chromatography/Mass Spectrometry (LC/MS)
• 5 Direct Pyrolysis of Polymers into the Ion Source of a Mass Spectrometer (DP-MS)
• 6 Field Ionization (FI-MS) and Field Desorption (FD-MS)
• 7 Fast Atom Bombardment of Polymers (FAB-MS)
• 8 Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
• 9 Laser Fourier Transform Mass Spectrometry (FT-MS)
• 10 Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Polymers (MALDI-MS)
• 11 Two-Step Laser Desorption Mass Spectrometry
• Index