Interpretation of Mass Spectra of Organic Compounds

700 Pages·1972·11.562 MB·English

by  Mynard Hamming (Eds.)

INTERPRETATION OF
MASS SPECTRA
OF ORGANIC COMPOUNDS
MYNARD C.HAMMING
Continental Oil Company
Research and Development Department
Ponca City, Oklahoma
NORMAN G.FOSTER
Department of Chemistry
Texas Woman's University
Denton, Texas
1972
ACADEMIC PRESS New York and London
COPYRIGHT © 1972, BY ACADEMIC PRESS, INC.
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.
ACADEMIC PRESS, INC.
Ill Fifth Avenue, New York, New York 10003
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1
LIBRARY OF CONGRESS CATALOG CARD NUMBER: 72-87228
PRINTED IN THE UNITED STATES OF AMERICA
PREFACE
The interpretation of mass spectra is inherently a "numbers game." Hence
it should be possible, in theory, to computerize the entire process of inter-
pretation. Some workers have attempted this, and some have even carried the
process further to computerize the operations of the mass spectrometer
from start to finish, i.e., introduce sample, push a button, and accept printout
data and interpretation, all for a specific quantitative analytical situation.
Mass spectrometry deals with ions, both molecular and fragment, and these
are characterized by separation according to their atomic mass unit or mass
number. Thus it is far easier to digest and adjust to these numbers than to infra-
red or nuclear magnetic resonance data which, indeed, deal with "numbers"
but of much less direct meaning to an organic chemist. Mass spectrometry
has grown into a rather extensive field from its beginnings early in this
century, but it is still not "standardized" in the same sense that is true for many
other instrument fields. It continues to grow and expand because of its nu-
merous and apparently endless applications. Its use is essential to both basic and
applied research, and it is part of the sophisticated research "team" developed
today.
Because of the many facets involved in the field, i.e., physics, ion-optics,
electronics, engineering, high-vacuum techniques, chemistry, people with
varied backgrounds become interested in or are assigned to work in mass
spectrometry. To bring all these people to a common ground of knowledge is
an impossibility. Instead, a blending of a group or team of persons with back-
grounds from the above-mentioned areas will usually be achieved. In the
ix
X Preface
process, no one individual is actually a mass spectroscopist, but each becomes
a specialist in a given area. For example, there are literature searchers, com-
puter liaison personnel, electronics maintenance personnel, and theoreticians.
It is interesting to note that as mass spectrometry is applied to a new field, the
new crop of graduate students decides that the field of mass spectrometry was
invented for them and their specifically oriented uses. This volume was
developed with all these people in mind. Data from the instrument may often
be quickly produced, but then problems of interpretation of the data arise.
This treatise was conceived to be an aid in developing facility in the inter-
pretation of organic mass spectra. It was written to assist personnel from the
technician to the intermediate managerial level. Although it may appear too
elementary to our more experienced colleagues, it should prove helpful
to those entering the field regardless of their technical skills and levels of
achievement simply because a broad background is necessary for the working
mass spectroscopist. Many of the items covered will have been neglected or
of little use in the previous technical experience of the novitiate in mass
spectrometry.
The prime aim of this work is to introduce the reader to the subject, survey
basic instrumentation, introduce simple procedures used, and acquaint one
with the existing terminology of mass spectrometry. All of this material is
presented in preparation for the utilization of mass spectral data. Since
fragmentation of the molecular and daughter ions is the key to interpretation
of organic mass spectra, it is covered in some detail. Following this, an intro-
duction to the array format is given which is essential to rapid interpretation
because it provides a visual interpretive peak approach which is simultaneously
compatible to the interpreter and computer. Believing it to be essential that
man control the "idiot" computer, he must have the know-how of all facets of
interpretation in order to utilize the computer to the fullest. The balance of
the book is concerned with the process of instruction so as to meet the require-
ments of eventually turning many of the tedious arithmetic calculations and
testing procedures over to the computer. Example interpretations, calculations,
data-processing procedures, and computer programs are included, even though
it is recognized that the many types of computers available prohibit standard
usage of the programs. The methodology and logic involved are universal,
and hence the examples can stimulate the user to apply these ideas in his own
computer system.
The type of stimulation found in the various reviews and advances in mass
spectrometry should not be expected in this treatise. Books of the above type
will find their places in the developing mass spectrometry laboratory, just as
many of the other laboratory service and aid books, such as listings of meta-
stable ion processes and isotopic contributions, become a part of the "team"
operation. In a sense, then, this treatise will find its principal use as something
Preface χι
a bit more informative than a short treatise to hand to the newcomer to the
laboratory who wants to "get aboard—join the team."
Portions of the book were developed from lecture and laboratory materials
used in presenting a two-week course in mass spectrometry at the Texas
Woman's University. The suggestions of numerous students involved in these
courses have been incorporated in portions of the book, particularly in
Chapters 2-4. We are indebted to all these students for their comments and
assistance. Quite naturally, the content of a two-week course is considerably
less than is found in this book, but the material contained herein could be
covered in an introductory one-semester course in mass spectrometry. It is
hoped that the volume will find multiple-purpose usage in both the university
and industrial laboratory.
MYNARD C. HAMMING
NORMAN G. FOSTER
ACKNOWLEDGMENTS
In writing a book, authors have different reasons for their efforts. The inspiration or
driving force may come from one or more sources. Among those I (N. G. F.) have recognized
are of the contributions of one man, the late Dr. Elton B. Tucker (formerly of the
American Oil Company, research department), to the field of mass spectrometry and to
my personal life. I was encouraged by Dr. Tucker to return to graduate school to obtain a
Ph.D. degree after nearly five years with the company (1946-1951). In addition, the know-
ledge (acquired for the most part at a later time) of his encouragement o fMr. Harold Wiley
(CEC) during the mass spectrometer installation at Standard Oil Company (Indiana), now
American Oil Company, and his suggestion to form an ASTM-E type committee on mass
spectrometry, plus his efforts in that direction, all indicate the depth of his interest in mass
spectrometry and ASTM. His success as a research executive was well known, but his behind-
the-scenes efforts are probably unknown to many mass spectrometrists.
During the early years of instrument development methods, it should be noted that men
like Dr. Tucker, then head of the Analytical Division, not only utilized all available com-
binations of techniques for separation and identification of organic species in petroleum and
related materials but also provided constant encouragement to the researchers involved in
this work. Thus, although the petroleum companies became known for rather large budgets
for instrument analysis, this persistence was necessary before other research units would begin
to consider expenditures of this magnitude for their analytical interests. This debt is also
owed by modern research teams to some of the European workers, as will be described in
the text.
I (N. G. F.) am indebted to Seymour Meyerson of American Oil Company for encourage-
ment in the form of discussions, particularly of the history of the early days, for suggestions
as to some areas that should be covered in detail, and for review of a chapter of the book.
About five years of contact with "Sy" during my employment at Standard Oil Co. (Indiana)
did a great deal to develop my interest in mass spectroscopy while working in the hydro-
carbon Analysis Group as a separations specialist.
xiii
XIV A cknowledgments
The driving force for the author (M. C. H.) was a desire to provide a book to help those
involved in industrial research mass spectrometry laboratories, who are often expected to
achieve through their own efforts. The brief personal remarks made to me several years ago
by such persons as R. I. Reed, Klaus Biemann, and John Beynon provided a lasting inspi-
ration. A special thanks goes to Fred McLafferty who first guided and inspired me in mass
spectrometry and has retained his interest in my activities. My indebtedness goes to many
other individuals, many of whom are cited in the references. Appreciation is due Continen-
tal Oil Company, Ponca City, Oklahoma for giving approval to publish this book. Indi-
viduals cited are Gerald Perkins, Jr., Harrell T. Ford, and David B. Burrows of the Research
and Development Department. Too often not cited are those many capable people at
Continental Oil Company who in so many ways gave help. Several spectra are from samples
generously supplied by E. J. Eisenbraun and his students at Oklahoma State University.
My wife, Beverly Jane Hamming, developed an understanding of my idiosyncrasies and did
much to encourage me. My daughters, Laura Lee and Heidi Ann, often helped me work
the long hours by just being well behaved.
Associated with the production of every book is a rather large number of people. In the
case of a college professor, this includes the university itself, his colleagues, his graduate
students, supporting personnel at the university, and quite often the members of his family.
The Texas Woman's University generously reduced my (N. G. F.) teaching load by three
hours for several semesters to assist in the preparation of the manuscript. I am grateful
also to Dr. George H. Stewart, Chairman of the Department of Chemistry, and to Dr.
Carlton T. Wendel, also of the Chemistry Department, for taking a portion of my teaching
load during the fall semester of 1970 to permit completion of the work. I am indebted to a
number of my colleagues for helpful discussions and suggestions concerning portions of
the text. In particular, I am indebted to Dr. James A. Hardcastle for the organization of the
items of biological interest in Appendix I, Dr. Robert S. Davidow for suggestions on the
section on the ionizing process, Dr. Lyman R. Caswell for permission to mention parts of
his research, and to Dr. James E. Johnson for the advice on nomenclature and from time-
to-time on mechanisms from the physical-organic chemist's viewpoint. Considerable help
in literature searching and evaluation was given to me by Mrs. Diana Wong-Kiu Shiu
Suvannunt and Miss Julie Pei-Min Liao during their graduate programs. I am indebted to
Dr. George Vose and Mr. Ted Booker of the electron microscope laboratory in the Texas
Woman's University Research Foundation for assistance with and suggestions for the
presentation of Figure 3-1. In addition, Dr. Robert A. Fuerst, Professor of Biology, has been
of great help and assistance in advising me upon a number of editorial-type problems. For
their assistance in literature searching, I am grateful to Mrs. Christine F. Lorenz, Mr. James
W. Lacy, and Mrs. Susan F. Lacy. To my daughters, Karen, Claire, and Francene, I am
indebted for assistance in proofing, and to Karen, for some assistance in the laboratory
concerning data for some of the figures. Last, to my wife, Alice Stover Foster, I am in debt,
not only for tolerating the usual neglect of family that accompanies the work of an author
but also for an assist in the form of a critical grammatical review of over two-thirds of the
manuscript.
We both are indebted to the assistance of many of the manufacturers of equipment for
their kind permission to reproduce some of the figures and tabular material. In particular,
we are indebted to Mr. Ed E. Escher, Mr. Urrie McCleary, and Mr. Charles Johannsen, all
of the CEC Division of Bell and Howell, and to Mr. Gerard Kearns of Picker Nuclear.
We are grateful to Mr. Hunt Payne of Bell and Howell Co. Film Division for technical
assistance concerning spectral papers.
Last, we are indebted to the staff of Academic Press for advice and rapid replies to questions.
CHAPTER 1
INTRODUCTION
I. Scope and Definition
In light of the rapid expansion in mass spectrometry during the last five
years, it is readily apparent that the field is too large to be covered adequately
in a single treatise. Therefore, it is not surprising to note the appearance of
several books dealing with the various specialized areas of mass spectrometry.
(A general bibliography appears following the text.) This treatise will specialize
in the interpretation of organic mass spectra and will introduce the reader
to key references in many of the other major subfields. With the growth in
the use of mass spectrometry in chemistry, it is only natural that a large number
of chemists working in the fields of organic chemistry, biochemistry, and
inorganic chemistry have recently found themselves concerned with the whys
and wherefores of mass spectrometry. The analytical and physical disciplines
have been strongly represented for a number of years. This volume is intended
to aid the organic chemist and biochemist in several ways: We shall (a)
2 1. Introduction
summarize the accepted definitions, concepts, procedures, and working
language of those already in the field; (b) outline the basic concepts of
ionization, fragmentation, and rearrangement of ions as found in mass spectra;
(c) present a summary of the instrumentation, sample handling techniques,
and interpretive procedures currently employed; (d) by means of the array
format of data presentation show how interpretation of complex spectra can
be made easier; and (e) suggest a natural extension into computer interpre-
tation. These aims will be considered mainly for mass spectrometer systems
of low to medium resolution.
Mass spectrometers and mass spectrographs are instruments that can
analyze substances for their constituent atoms, atomic groups, or molecules
on the basis of a mass-to-charge ratio (m/e) separation of ions formed from
the substance by electron impact or other means. These two types of instru-
ments, while quite distinct, are similar in certain respects. Both have sample
introduction systems, an ion source, a means of separating the ions formed,
and a detector system. Instruments, ion sources, and detectors are discussed
in Chapter 2, and sample introduction systems are covered in Chapter 4.
The term "mass spectrograph" was introduced by Aston in 1919 and refers
to instruments that produce a "mass spectrum" on a photographic plate. Just
six years later, "mass spectrometer" was used by Smythe and Mattauch
(Kiser, 1965a, p. 1). Today the term "mass spectrometer" is applied to those
instruments which produce a mass spectrum by bringing a focused beam of
ions to a fixed collector. The mass-to-charge ratio ion in focus can be varied
by electrical or magnetic field changes. A scan of the mass spectrum is then
produced by varying either of the above fields with time. The ion current is
detected at the collector, amplified electronically, and recorded.
The term "mass spectrometry" is presently used in a loose sense to include
the use of both types of instruments and the studies made with them. Typical
studies include isotope abundances, precise mass determinations, a great
variety of analytical chemical applications involving both qualitative and
quantitative analyses, and appearance potential studies. With such a list, it is
not surprising to find applications to research in all of the physical and
biological sciences.
The objective of this treatise is to furnish the reader with sufficient insight
into many types of instruments, their uses, and usefulness for a particular
problem. With this information, he may better understand the details supplied
in specific references. With the outlined procedures for interpretation of and
guide to computer programs as an aid, the newcomer to the field should be
able to progress rapidly to the point of confidence and competence in his
interpretation of the mass spectra of organic compounds.
//. Historical Developments 3
Π. Historical Developments
The discovery of positively charged electrical "entities" was made before
the turn of the century (Goldstein, 1886). Wien (1898) showed that a beam
of positive ions could be deflected using electrical and magnetic fields. Thomson
(1913) demonstrated that naturally occurring neon consists of two different
atomic weight species (isotopes) with weights of 20 and 22 (g/mole). The term
"isotope" was introduced by Soddy (1913). Kiser (1965a) referred to Thomson
as the Father of Mass Spectrometry and cited his discovery concerning neon
as one of the two most significant mass spectrographic contributions to science,
the second being the contribution of Aston. This second achievement was the
discovery that the various isotopes of the elements do not have integral masses;
they are not simple multiples of a fundamental unit. Aston's fairly elaborate
instrument of 1919 (Aston, 1942) found favor with investigators of isotopes
for precise mass measurements. Dempster (1918) produced a somewhat less
elaborate instrument that was used for the measurement of relative abundances
of isotopes. Dempster's instrument could not be used for precise mass measure-
ments; it was better suited for measuring the relative abundance of the ionic
species present and for studying electron impact processes in gases. It should
be emphasized that these early instruments did not have the benefit of modern
high-vacuum technology and thus were operated at higher pressures than are
used today. Despite this limitation, Thomson was able to detect, by photo-
graphic record, effects which he suggested were due to the dissociation of ions
in flight. These dissociated ions were observed as metastable peaks from the
metastable transitions of the ions (Chapter 3). Also observed by Thomson
were multiple-charged, negative, and fragment ions.
With the electronic developments of the 1920's and 1930's, more sophisti-
cated equipment could be developed. The parallel improvement in vacuum
and electronics technology led eventually to an increasing interest in the field
of mass spectroscopy. The original interest in commercial mass spectrometry
was generated by a group of California engineers headed by Washburn and
with majority backing from Herbert Hoover, Jr. (Washburn, 1970). The
petroleum companies were interested in some form of instrumentation which
could determine the amount of hydrocarbon material present in geological
core samples, etc. While the mass spectrometer as designed could do at least
a portion of this work and was being developed upon this premise, it was
noted that the lighter hydrocarbon substances were much easier to deal with
during analysis. In 1942 wartime pressures for aviation gasoline made mass
spectrometry more essential to the petroleum industry. The first commercial
instrument, built by Consolidated Electrodynamics Corp. (CEC), was delivered
to the Atlantic Refining Corp. This instrument was a 180° Dempster geometry