Raman Spectroscopy for Chemical Analysis

437 Pages·2000·21.566 MB·English

by  Richard L. McCreery

Raman Spectroscopy for
Chemical Analysis
CHEMICAL ANALYSIS
A SERIES OF MONOGRAPHS OF ANALYTICAL CHEMISTRY AND
ITS APPLICATIONS
Editor
J. D. WINEFORDNER
VOLUME 157
A JOHN WILEY & SONS, INC., PUBLICATION
New York I Chichester I Weinheim I Brisbane I Singapore I Toronto
Raman Spectroscopy for
Ch emic a1 Analysis
RICHARD L. McCREERY
The Ohio State University
Columbus, Ohio
A JOHN WILEY & SONS, INC., PUBLICATION
New York I Chichester i Weinheim I Brisbane I Singapore I Toronto
This book is printed on acid-free paper. @
Copyright 0 2000 by John Wiley & Sons, Inc. All rights reserved.
Published simultaneously in Canada.
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Library of Congress Cataloging-in-PublicationD ata:
McCreery, Richard L.
Raman spectroscopy for chemical analysis / by Richard L. McCreery.
p. cm.-(Chemical analysis ; v. 157)
"A Wiley-interscience publication
Includes index
ISBN 0-471-25287-5 (alk. paper)
I. Raman spectroscopy. 2. Chemistry, Analytic. I. Title. IT. Series.
QC454.R36.M33 2000
543'.08584-dc21 99-08649 1
Printed in the United States of America.
I09 8 7 6 5 4 3 2
CONTENTS
PREFACE ix
ACKNOWLEDGMENTS xiii
LIST OF SYMBOLS xv
CUMULATIVE LISTING OF VOLUMES IN SERIES xix
CHAPTER 1 INTRODUCTION AND SCOPE 1
1.1. History Preceding 1986 I
1.2. Technological Advances 5
1.3. Comparison to FTIR and NIR Absorption 10
1.4. Overview of the Book 12
CHAPTER 2 MAGNITUDE OF RAMAN SCATTERING 15
2.1. Theoretical Overview 1s
2.2. Definition of Raman Cross Section 20
2.3. Magnitude of Raman Cross Sections 24
2.4. Raman Scattering Intensity 30
CHAPTER 3 COLLECTION AND DETECTION OF RAMAN
SCATTERING 35
3.1. Signal Magnitude and Collection Function 35
3.2. Instrumental Variables Comprising the Collection Function 37
3.3. Spectrometer Response Function 41
3.4. Multiplex and Multichannel Spectrometers 43
CHAPTER 4 SIGNAL-TO-NOISE IN RAMAN
SPECTROSCOPY 49
4.1. Definition and Measurement of SNR 49
4.2. Noise Sources 52
V
vi CONTENTS
4.3. Signal-to-Noise Ratio Expressions 61
4.4. SNR Figure of Merit 65
4.5. SNR and Detection Limits 67
4.6. SNR for Multiplex Spectrometers 68
CHAPTER 5 INSTRUMENTATION OVERVIEW AND
SPECTROMETER PERFORMANCE 73
5.1. Major Spectrometer Components 74
5.2. Laser Wavelength 75
5.3. Dispersive vs. Nondispersive Spectrometers 78
5.4. Performance Criteria 79
5.5. Samples for Spectrometer Evaluation 83
CHAPTER 6 SAMPLING MODES IN RAMAN
SPECTROSCOPY 95
6.1. Sampling Overview 95
6.2. Performance Criteria 97
6.3. 180" Backscattering Geometry 99
6.4. 90" Sampling Geometry 114
6.5. Reducing the Laser Power Density at the Sample 118
6.6. Path Length Enhancement 120
6.7. Polarization Measurements 122
CHAPTER 7 LASERS FOR RAMAN SPECTROSCOPY 127
7.1. Overview 127
7.2. Ar+ and Krt Ion Lasers 130
7.3. Helium-Neon Lasers 133
7.4. Neodymium-YAG (Nd:YAG) 134
7.5. Diode Lasers 137
7.6. Laser Wavelength Filtering 142
CHAPTER 8 DISPERSIVE RAMAN SPECTROMETERS 149
8.1. Overview 149
8.2. Dispersive Spectrometer Configurations 155
8.3. Detector Considerations 179
8.4. Single-Channel Detectors 180
8.5. Multichannel Detectors and CCDs 183
CONTENTS vii
8.6. Recording Methods for Dispersive Spectrometers 203
8.7. Examples of Dispersive Raman Applications 215
CHAPTER 9 NONDISPERSIVE RAMAN
SPECTROMETERS 221
9.1. Tunable Bandpass Filters 22 1
9.2. Fourier Transform Raman Spectroscopy 225
9.3. Multichannel Fourier Transform Raman Spectroscopy 240
9.4. Extensions of FT-Raman for Longer Wavelength Operation 245
9.5. FT-Raman Examples 246
CHAPTER 10 CALIBRATION AND VALIDATION 251
10.1. Overview 25 1
10.2. Frequency and Raman Shift Calibration 25 1
10.3. Instrument Response Function Calibration 269
10.4. Absolute Response Calibration 288
10.5. Summary of Calibration and Validation Procedures 289
CHAPTER 11 RAMAN MICROSCOPY AND IMAGfNG 293
1 1.1. Overview of Raman Microscopy 293
11 .2. Single-Point Raman Microspectroscopy 295
11.3. Line Imaging 309
11.4. Two-Dimensional Raman Imaging 316
CHAPTER 12 FIBER-OPTIC RAMAN SAMPLING 333
12.1. Overview of Fiber-optic Sampling 333
12.2. Fiber-optic Basics 334
12.3. Fiber-Spectrometer Interface 337
12.4. Fiber-optic Probes 342
12.5. Comparisons of Fiber-optic Sampling Probes 359
12.6. Waveguide Sampling for Analytical Raman Spectroscopy 364
12.7. Examples of Fiber-optic Sampling 369
CHAPTER 13 RAMAN SPECTROSCOPY OF SURFACES 373
13.1. Overview 373
13.2. Surface Sensitivity 375
...
Vlll CONTENTS
13.3. Sampling Considerations 379
13.4. Surface Raman Spectroscopy without Field Enhancement 382
13.5. Electromagnetic Field Enhancement 390
13.6. Examples of Analytical Applications 409
INDEX 415
PREFACE
This book was inspired by the transition of Raman spectroscopy from a
technically demanding research technique to a useful and practical method
of chemical analysis. There are many fine texts and thousands of scientific
articles on research in Raman spectroscopy, primarily oriented toward under-
standing the Raman effect itself and using Raman scattering to probe molecular
structure and dynamics. These research efforts have reached a high level of
sophistication and have yielded valuable chemical and physical insights, but
they rarely resulted in practical techniques for chemical analysis until approx-
imately 1986. The impediments to broad applications of Raman spectroscopy
to chemical analysis were mainly technological rather than fundamental. The
instrumentation required to observe the weak Raman effect was too cumber-
some and expensive for routine analysis, and interference from fluorescence
precluded application to a broad range of industrial samples. As a result, the
advantages of Raman spectroscopy over more common infrared absorption
techniques were not exploited in analytical problems.
Major technological and scientific innovation in the past 10 to 15 years
has significantly broadened the applicability of Raman spectroscopy, partic-
ularly in chemical analysis. Fourier transform (FT)-Raman, charge-coupled
device (CCD) detectors, compact spectrographs, effective laser rejection filters,
near-infrared lasers, and small computers have contributed to a revolution in
Raman instrumentation and made routine analytical applications possible. An
increase in instrumental sensitivity by factors as large as lo5, plus decreases
in both interferences and noise resulted from this “revolution.” The number
of vendors of Raman spectrometers increased from 3 to 12 over a 10-year
period, and integrated commercial spectrometers led to turnkey operation and
robust reliability.
This book is intended to introduce a student or practitioner of analytical
chemistry to the technical elements and practical benefits of the “Raman revo-
lution.” It is not intended to describe “high-end” Raman techniques such as
nonlinear or time-resolved Raman spectroscopy, nor does it attempt to describe
the many theoretical treatments of Raman scattering. The book emphasizes the
concepts and technology important to applications of Raman spectroscopy in
chemical analysis, with attention to calibration, performance, and sampling
modes. While many recent innovations in analytical Raman spectroscopy are
ix
X PREFACE
technically sophisticated, their objectives are reliability, accuracy, reduction of
interferences, and ease of operation rather than ultimate spectral resolution or
sensitivity. The emphasis of both the theory and instrumentation discussions
in this book is the practical analysis that has resulted from recent technolog-
ical developments. Techniques such as nonlinear Raman (CARS, hyperRaman,
stimulated Raman, etc.), picosecond transient Raman, single-crystal Raman,
gas-phase Raman,and so forth are excluded not because they are unimportant,
but because they currently have limited use in routine chemical analysis.
The audience for this book should include graduate students, practicing
chemists, and Raman spectroscopists who seek information on recent instru-
mentation developments. It is not a comprehensive review but more of a
textbook intended as an introduction to modern Raman spectroscopy. In most
cases, the techniques discussed are available in commercially available spec-
trometers, and the book should be useful to chemists who are implementing
Raman spectroscopy in industrial or academic laboratories. Although a large
number of useful Raman applications involve custom-built instrumentation,
the book emphasizes configurations and components used by current vendors
of integrated Raman spectrometers.
Since commercial spectrometers can be constructed in a variety of config-
urations, instruments from different manufacturers often differ significantly
in applicability and performance. Specific manufacturers are mentioned in
the text to identify a particular approach or optical configuration. Available
commercial units differ widely in performance and are often optimized for
particular sample types. Mention of a manufacturer in the text does not imply
an endorsement but may be useful to the reader in order to appreciate differ-
ences in design objectives. There is no “best” manufacturer or configuration,
but certain designs are more applicable to certain situations, depending on
the sample and analytical objective. It will become obvious that the sample
dictates the choice of spectrometer type, and no single Raman system covers
all possible applications.
The book is divided into roughly three general areas on theory and instru-
mentation. Chapters 1 to 4 cover the origin and magnitude of Raman scattering
and the major factors determining the signalhoise ratio. Chapters 5 to 10
discuss instrumental components and configurations and methods of calibra-
tion. Chapters I1 to 13 address the widely studied specialty areas of Raman
microscopy, fiber-optic sampling, and Raman spectroscopy of surfaces. In
most chapters , many examples of applications to Raman spectroscopy to
analytical problems are provided.
Notes on Conventions
The definitions of several symbols and certain conventions are not used
uniformly in the Raman spectroscopy literature, and some choices were