Table Of ContentContributors
ARTHUR J. AHEARN P. R. KENNICOTT
E. BERKEY BRUCE E. KNOX
JOEL A. CARTER G. H. MORRISON
R. J. CONZEMIUS JOHN ROBOZ
HARRY FARRAR IV J. R. ROTH
JOCHEN FRANZEN JOHN R. SITES
RICHARD F. HERZOG R. K. SKOGERBOE
W. M. HICKAM H. J. SVEC
RICHARD E. HONIG J. ROGERS WOOLSTON
TRACE ANALYSIS
BY MASS SPECTROMETRY
Edited by
ARTHUR J. AHEARN
Kensington, Maryland
1972
ACADEMIC PRESS New York and London
COPYRIGHT © 1972, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED
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List of Contributors
Numbers in parentheses indicate the pages on which the author's contributions begin.
ARTHUR J. AHEARN (1), 9621 East Bexhill Drive, Kensington, Maryland
E. BERKEY (323), Westinghouse Research Laboratories, Pittsburgh,
Pennsylvania
JOEL A. CARTER (347), Oak Ridge National Laboratory, Oak Ridge,
Tennessee
R. J. CONZEMIUS (135), Ames Laboratory, USAEC, Iowa State University,
Ames, Iowa
HARRY FARRAR IV (239), Atomics International, A Division of North
American Rockwell Corporation, Canoga Park, California
JOCHEN FRANZEN (11), Institute für Spektrochemie und Angewandte
Spektroskopie, Dortmund, Germany
RICHARD F. HERZOG (57), GCA Corporation, GCA Technology Division,
Bedford, Massachusetts
W. M. HICKAM (323), Westinghouse Research Laboratories, Pittsburgh,
Pennsylvania
RICHARD E. HONIG (101), RCA Laboratories, Princeton, New Jersey
P. R. KENNICOTT (179), General Electric Research and Development
Center, Schenectady, New York
BRUCE E. KNOX (423), Materials Research Laboratory and Department of
Material Sciences, The Pennsylvania State University, University
Park, Pennsylvania
xi
xii LIST OF CONTRIBUTORS
G. H. MORRISON (297), Department of Chemistry, Cornell University,
Ithaca, New York
JOHN ROBOZ* (369), Cresskill, New Jersey
J. R. ROTH (297), Department of Chemistry, Cornell University, Ithaca,
New York
JOHN R. SITES (347), Oak Ridge National Laboratory, Oak Ridge,
Tennessee
R. K. SKOGERBOE (401), Department of Chemistry, Colorado State
University, Fort Collins, Colorado
H. J. SVEC (135), Ames Laboratory, USAEC, Iowa State University,
Ames, Iowa
J. ROGERS WOOLSTON (213), RCA Laboratories, Princeton, New Jersey
* Present address: Mount Sinai School of Medicine, New York, New York.
Preface
This book is a review and appraisal of trace analysis of solids and liquids
by mass spectrometric techniques. The ion sources used are principally
but not exclusively electrical discharge type sources. This trace analysis
technique was last comprehensively reviewed in 1966 when it was at a
considerably earlier stage in its development.
The present survey was deliberately made a multi-author effort because
I believe the advantages of diversity outweigh the advantages of uniformity
characteristic of the single author approach.
The first several chapters deal with the basic physics of electrical dis-
charge ion sources, double focusing mass spectrometry, and the measure-
ment of arrays of mass resolved ion beams by electrical detection methods
and with ion sensitive emulsions. There follow chapters dealing first with
the problem of obtaining the chemical composition of the recorded mass
resolved ion sample and then with the problem of relating this composition
to that of the original sample. Several chapters then detail specific tech-
niques for analyzing special samples such as insulators, powders, micro-
samples, biological materials, reactive and low melting point substances,
radioactive materials, and gases in solids. The remaining chapters include
the use of laser ion sources in the analysis of solids and the analysis of
surfaces particularly with sputter ion sources.
Xlll
1
Introductor y Revie w
ARTHUR J. AHEARN*
1.1 Proem 1
1.2 Résumé 2
1.3 Problems and Proposals 4
References 9
1.1 PROEM
Trace analysis of elemental impurities in inorganic materials has become
increasingly important during the past two decades. Many modern ma-
terials such as semiconductors, superconductors, nuclear reactor com-
ponents, and magnetic, thermoelectric, or luminescent materials are
characterized by properties derived from the presence or absence of specific
elements at the parts per million to the parts per billion level.
The need to detect, identify and quantitatively determine impurities and
dopants in modern semiconductors stimulated the development of a new
mass spectrometric technique for such trace analyses. This technique
employes electrical discharge type ion sources. The one most commonly
used is the pulsed radio frequency spark source. Consequently the tech-
nique is loosely designated by the term spark source mass spectrometry.
The following three important features characterize this spark source
mass spectrometric technique. The detection sensitivity is substantially
improved over that of other available methods of wide applicability. For
most elements in most matrices, the detection selectivity is small. Sub-
stantially all stable elements in the periodic table can be recorded simul-
taneously.
* Present address: 9621 East Bexhill Drive, Kensington, Maryland 20795.
1
2 ARTHUR J. AHEARN
The state of the art in spark source mass spectrometry was summarized
by Ahearn ( 1966). Although a number of brief reviews have been published
in the interim, the time seems appropriate for another more extensive
review, summary and assessment of trace analysis by this technique. This
book is largely devoted to mass spectrometric techniques using electrical
discharge type ion sources and the double focusing type mass spectrometers
that such ion sources require. However, two exceptions were made to this
general rule. Because they complement the spark source technique so fruit-
fully, the laser ion source and the sputter ion source are included within
the scope of this book.
Like the 1966 review, the present one represents a multiauthor effort.
The diversity in experience, point of view and general approach yields an
advantage to the multiauthor endeavor that outweighs the advantage of
uniformity in the single author case. The editor and authors trust that
novitiates in this analytical field will find that this book supplemented
by the 1966 review is useful if not indispensible, and that our fellow workers
in this field will benefit from our efforts. Lastly, technical administrators
should find it helpful when they consider adding this technique to their
analytical facilities.
1.2 RESUME
In the following paragraphs a brief introduction and/or summary of each
chapter is sketched to orient the reader and to aid the busy worker in
skimming.
Chapter 2 treats the physics of electrical discharge ion sources. The
electrical breakdown between electrodes that occurs when, for example,
the pulsed radio frequency voltage reaches the critical value is outlined
and alternative breakdown mechanisms are described. Evaporation and
ion formation processes are outlined. Specific details of electrical discharge
ion sources are presented including recent innovations designed to improve
the performance of this ion source.
One disadvantage of the electrical discharge ion sources described in
Chapter 2 is that the ions formed are spread over a wide range of kinetic
energy. This requires the use of a double focusing mass spectrometer. All
of the commercial mass spectrometers designed for use with electrical dis-
charge ion sources employ a specific design and geometry—the Mattauch-
Herzog geometry.
Chapter 3 treats the transmission of ions through double focusing mass
spectrometers in general and those with Mattauch-Herzog geometry in
1 INTRODUCTOR Y REVIEW 3
particular. The focusing and dispersing properties of deflection fields and
the field combinations to achieve energy focusing and angular focusing
simultaneously are developed. Deviations of the properties of real fields
from those calculated on a simplified first order theory are presented.
Performance criteria and general design are treated as well as data on
specific available instruments. Since this Mattauch-Herzog geometry
dominates Chapter 3 and is basic to substantially all spark source mass
spectrometers of today, the authorship of this chapter is doubly appro-
priate.
After transmission and mass resolution the ion sample from the source is
recorded on the detector. Most of the past as well as the present work is
done using ion-sensitive emulsions as detectors. Chapter 4 treats the
problem of the detection and quantitative measurement of ion beams by
these emulsions. Problems of developing, sensitivity, homogeneity, de-
pendence on ion mass and energy, background as well as advantages and
limitations are covered.
When mass spectra are recorded on an ion sensitive plate a lot of in-
formation is stored therein, but none of it is immediately available and the
quantitative data are only available after considerable measurements.
Moreover, the accuracy and precision of the measurements generally
leave something to be desired. For these reasons electrical detection
techniques are rapidly being adopted. These techniques and typical results
are presented in Chapter 5.
Chapter 6 treats the whole problem of determining the atomic com-
position of the ion sample that is recorded on the detector. Given the
developed ion sensitive plate or the paper record of the electrical detection,
the method of establishing the mass scale is outlined, so that the mass/
charge for any mass line can be established. The element or compound
giving rise to this mass line can usually be identified provided other can-
didates can be systematically eliminated. Semiquantitative trace concen-
tration estimates based on visual comparisons of mass lines—called
"eyeballing" are described. Next the measurements and calculations, to
increase progressively the accuracy and precision to the best currently
attainable, are detailed. Many of the steps essential to the complete
evaluation of mass spectra recorded on ion sensitive emulsions are circum-
vented in the electrical detection technique. A statistical treatment of the
data is also presented in Chapter 6.
Chapter 7 describes in detail computer techniques for the quantitative
determinations of trace concentrations outlined in Chapter 6. Batch,
time-sharing, and dedicated systems will be treated and compared.
One of the advantages of spark source mass spectrometry is that the
4 ARTHUR J. AHEARN
composition of the mass resolved ion sample usually differs from that of
the original solid sample only by a small factor. One of its disadvantages
is that this difference is usually not small enough to be negligible. Chapter
8 treats the various facets of this problem—the magnitude of this relative
sensitivity coefficient SR, the factors contributing to S , the invalidity of
R
calculated SR values and the ensuing standard sample problem.
The problem of producing ions by means of an electrical discharge
between electrodes of metals and semiconductors is fairly straightforward.
Special problems are encountered in analyzing insulators, geological and
biological samples, powders, and microsamples. Techniques for coping with
these problems and typical results are presented in Chapter 9.
Chapter 10 describes special techniques for coping with problems peculiar
to the analysis of low melting and reactive samples like gallium and sodium.
Chapter 11 treats the analysis of radioactive samples with the specific
problems that are associated with such samples.
Chapter 12 covers the analysis of gases in solids.
With any electrical discharge ion source this discharge first samples the
surface of one or both of the electrodes in question. Consequently the initial
mass spectra recorded are characteristic of the surface rather than the
interior of the electrode material. This surface sensing capability along
with the detection sensitivity makes a most useful tool for studying surface
contamination and thin films. Even more powerful techniques are now
available wherein the surface composition of a sample can be studied on a
microscale by analyzing ions sputtered from this surface. These two com-
plementary techniques will be treated in Chapter 13, much of which is
devoted to techniques not using an electrical discharge ion source.
Chapter 14, the last one of this book, is entirely devoted to mass spec-
trometric studies using laser ion sources—hence another exception to the
general scope of this book as stated earlier in this introductory chapter.
Chapter 14 demonstrates that the laser ion source already is a most useful
addition to the mass spectrometric techniques for the study of solids.
1.3 PROBLEM S AND PROPOSAL S
Although the poor accuracy and precision of spark source mass spec-
tromêtry were not a major problem in its early years, accuracy and pre-
cision are now and for some time have been a major problem in this field.
One of the principal factors limiting these criteria is the imprecision in the
measurements of mass spectra recorded on ion sensitive emulsions. This
can be largely circumvented by more adequate recordings and measure-
ments of mass spectra or by recourse to electrical detection.
