Spectroscopic Methods of Analysis

302 Pages·1977·6.826 MB·English

by  Gunter Zweig and Joseph Sherma (Eds.)

Volume I
PRINCIPLES, METHODS, AND GENERAL APPLICATIONS
Volume II
INSECTICIDES
Volume III
FUNGICIDES, NEMATICIDES AND SOIL FUMIGANTS,
RODENTICIDES, AND FOOD AND FEED ADDITIVES
Volume IV
HERBICIDES
Volume V
ADDITIONAL PRINCIPLES AND METHODS OF ANALYSIS
Volume VI
GAS CHROMATOGRAPHIC ANALYSIS
Volume VII
THIN-LAYER AND LIQUID CHROMATOGRAPHY AND ANALYSES OF
PESTICIDES OF INTERNATIONAL IMPORTANCE
Volume VIII
GOVERNMENT REGULATIONS, PHEROMONE ANALYSIS,
ADDITIONAL PESTICIDES
Volume IX
SPECTROSCOPIC METHODS OF ANALYSIS
Analytical Methods for
PESTICIDES AND
PLANT GROWTH REGULATORS
Edited by
GUNTER ZWEIG
Office of Pesticide Programs, U.S. Environmental Protection Agency
Washington, D.C.
Volume IX
SPECTROSCOPIC METHODS
OF ANALYSIS
Edited by
G U N T ER Z W E IG and J O S E PH S H E R MA
Office of Pesticide Programs Department of Chemistry
U. S. Environmental Lafayette College
Protection Agency Easton, Pennsylvania
Washington, D. C.
A C A D E M IC P R E SS New York San Francisco London 1977
A SUBSIDIARY OF HARCOURT BRACE JOVANOVICH, PUBLISHERS
COPYRIGHT © 1977, 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 Cataloging in Publication Data
Zweig, Gunter.
Analytical methods for pesticides, plant growth
regulators, and food additives.
Vols. 6- have title: Analytical methods for
pesticides and plant growth regulators.
Includes bibliographies.
CONTENTS: v. 1. Principles, methods, and
general applications.-v. 2. Insecticides,-v. 3.
Fungicides, nematocides and soil fumigants, rodenti-
cides, and food and feed additives, [etc.]
1. Pesticides-Analysis—Collected works.
2. Plant regulators-Analysis-Collected works.
3. Food additives-Analysis-Collected works.
4. Feed additives-Analysis-Collected works.
5. Chemistry, Analytic-Collected works.
I. Sherma, Joseph. II. Title.
SB960.Z9 632'.95 63-16560
ISBN 0-12-784309-4
PRINTED IN THE UNITED STATES OF AMERICA
List of Contributors
Numbers in parentheses refer to the pages on which the authors' contributions begin.
ROBERT J. ARGAUER (101), U. S. Department of Agriculture, Agricultural
Research Service, Agricultural Environmental Quality Institute,
Beltsville, Maryland
N. CYR (51, 75, 137), Department of Chemistry, McGill University, Mon-
treal, Canada
T. CYR (51, 75, 137), National Research Council, Ottawa, Canada
PAUL A. GIANG (153), U. S. Department of Agriculture, Agricultural Re-
search Service, Agricultural Environmental Quality Institute, Belts-
ville, Maryland
R. HAQUE (51, 75, 137), Office of Pesticide Programs, U. S. Environ-
mental Protection Agency, Washington, D. C.
JAMES F. RYAN (1), Gulf South Research Institute, New Orleans,
Louisiana
vii
Preface
Since the publication of Volume I of this treatise in 1963, when
spectrophotometric methods for pesticide analyses were first covered,
this field has progressed so considerably that the editors felt compelled
to publish a single volume devoted solely to the topic of spectroscopic
methods.
Gas-liquid chromatography coupled to mass spectrometry (Chap-
ter 1) has become the optimum method for the identification and confirma-
tion of structure for macro- and microquantities of pesticides. More re-
cently, combination high-pressure liquid chromatography coupled to
mass spectrometry has become practical and appears to be especially
useful for the analysis of heat-labile compounds; this new technique is
briefly discussed in Chapter 1.
Nuclear magnetic resonance (NMR) spectroscopy and related tech-
niques are helpful in the finite determination of structure. NMR, however,
is not practical for residue analysis, although the use of Fourier trans-
formation has improved the sensitivity of the technique considerably.
Visible and ultraviolet spectrophotometry (Chapter 3), the traditional
instrumental method for pesticide analyses, is still useful for many formu-
lation analyses and has been adapted to the automation of residue anal-
yses of several classes of pesticides (e.g., organophosphates).
Spectrophotofluorometry (Chapter 4) is a highly sensitive technique
for compounds and derivatives that fluoresce when exposed to specific
wavelengths in the ultraviolet. The interfering fluorescent background
from solvent impurities or samples themselves must be removed prior to
analysis or corrected by the selection of optimum excitation and emission
wavelengths.
Infrared spectrometry (IR) (Chapters 5 and 6) is a powerful tool for
the identification of organic molecules. However, despite recent advances
in instrumentation and methodology, IR spectroscopy still suffers from a
lack of sensitivity, and so is not practical for pesticide residue analyses.
We are fortunate in being able to publish what is probably the most com-
prehensive collection of infrared spectra of important pesticides in use
today (Chapter 6).
The editors feel that, although spectral instrumental methods for
pesticide analyses offer unique and selective means of identification, these
techniques do not yet offer sufficient sensitivity to be generally applicable
for routine residue analyses. The exceptions may be some fluorescence
methods and gas chromatography coupled to mass spectrometry. We ex-
ix
X PREFACE
pect, however, that significant advances will be made in improving the
sensitivity of spectral techniques, and that the advances in spectroscopy
will be the subject of subsequent chapters in future volumes of this
treatise.
As in the past, we invite our readers to send us their comments,
suggestions, and corrections.
Gunter Zweig
Joseph Sherma
1
Residue Analysis Applications of
Mass Spectrometry
JAMES F. RYAN
I. INTRODUCTION
The past ten years have witnessed a revolution in pesticide residue
analysis. Improved analytical techniques have pushed detection limits
lower and lower and at the same time have allowed more precise identifi-
cations and confirmations. The development of the gas chromato-
graph-mass spectrometer-computer instrument (GC-MS-COM) is a
major reason for the great improvement in residue analysis. No other ana-
lytical technique or method accommodates the same range of sample
materials, provides the same accuracy of identification, and possesses the
same speed of analysis. The mass spectrometer, especially when used
with a gas chromatograph and a computer, is extraordinarily versatile.
This chapter examines a number of pesticide residue analytical tech-
niques involving mass spectrometers (MS). This is not meant to be a com-
prehensive review of all the MS-residue literature, but rather a guide for
the practicing residue chemist to the mass spectrometric techniques that
have proved useful in this field. This chapter will brieny review modern
GC-MS instrumentation, including sample ionization techniques that
have applications in pesticide residue analysis. Among the latter are the
traditional electron ionization (EI), chemical ionization (CI), field ioniza-
tion (FI) and field desorption (FD), and the recently developed atmo-
spheric pressure ionization (API). In addition, applications to the analysis
of organochlorine pesticides, PCBs, 2,3,7,8-tetrachlorodibenzo-/?-dioxin
(TCDD), carbamates, and organophosphorous pesticides will also be re-
viewed.
There are several relatively recent review articles with a bearing on
pesticide residue analysis. Biros (1971) published a treatise dealing with
the isolation and mass spectral behavior of certain classes of pesticides
and their metabolites. Also, in 1972, Abramson presented applications of
mass spectrometry to trace determinations of environmental toxic mate-
rials.
Damico (1972) has presented an excellent compilation of reference
spectra and electron impact fragmentation pathways of a number of pesti-
1
2 JAMES F. RYAN
cides, their metabolites, and photodecomposition products. McGuire et
al. (1973) have published an Environmental Protection Agency report on
organic pollutant identification utilizing mass spectrometry. Vander
Velde and Ryan (1975) published a short article examining some of the re-
cent applications of alternative ionization techniques to the analysis of
pesticides. Safe and Hutzinger (1973) have published an excellent book
dealing with the mass spectrometric behavior of a number of pesticides
and pollutants.
II. INSTRUMENTATION
The modern gas chromatograph-mass spectrometer-computer
has the following subunits: inlet system, ion source, mass analyzer,
and data acquisition system. In general, there are a number of dis-
tinct members of each subunit. For instance, Fales (1971) outlines at
least thirteen different methods of generating ions from organic molecules.
There are at least four different inlet systems that can be used, and there
are a variety of mass analyzers. However, the instruments that are com-
mercially available, widely used, and of interest to the practicing residue
chemist are relatively few in number. This section will present a brief
overview of GC-MS instrumentation. A more thorough discussion is in
the excellent book by McFadden (1973).
A. Mass Analyzers
The quadrupole and magnetic mass analyzers will be briefly consid-
ered, as these are the most widely used in GC-MS systems.
1. MAGNETIC
The operation of a magnetic analyzer is depicted in Fig. 1. Ions gen-
erated in the source are accelerated toward the magnetic field by a poten-
tial (V). Upon entering the field, the ions are acted upon by an orthogo-
nal force that will cause them to follow a curved flight path. The mass-
to-charge ratio (m/e) of an ion that will traverse the curved flight path
and strike the collector is described by the equation
m/e = H2r2/2V (1)
where H is the magnetic field strength r is the radius of curvature of the
i
flight path, and V is the ion-accelerating voltage. It follows that by varying
either H or V at a fixed radius of curvature, ions of different m/e values
will be brought to focus at the collector. In practice, V usually is held con-
stant while H is varied so that the spectrum of ions is scanned, i.e., each
1. RESIDUE ANALYSIS BY MASS SPECTROMETRY 3
Collector
Ion source
FIG. 1. Ion flight path with magnetic mass analyzer.
ion possessing a different m/e value is individually focused on the col-
lector.
The ability of a mass spectrometer to separate two adjacent ions is
called its resolving power, or resolution. Consider the two ions shown in
Fig. 2. These are said to have a 10% valley resolution, i.e., the height of
the valley is 10% of the overall peak height. Resolution (R) is defined as
the ratio of nominal mass to the actual mass difference of two adjacent
ions.
If one can measure the mass of an ion with enough precision, i.e., to
enough significant places, one can determine the atomic or empirical com-
position of that ion. Such information facilitates structural elucidations
and identification of unknowns. For instance, acetone and «-butane
exhibit a molecular ion at m/e 58. However, because of the mass defect of
the constituent atoms of each ion, i.e., the deviation of the actual atomic
mass from the nominal integer number, the acetone ion has a precise mass
of 58.0417, while the isobutane ion has a mass of 58.0780. Therefore,
H
10% "valley'i i
FIG. 2. Two ions separated by a 10% valley.
4 JAMES F. RYAN
these two ions are said to differ by 36.3 millimass units (mmu). To resolve
the m/e 58 ions of acetone and n-butane, a resolution of 1600 would be re-
quired as shown below, where M is the nominal mass of the two ions in
question, and AM is the mass difference.
R = (2)
This is considered a moderate resolution. Resolution in excess of 8000 is
considered high, since that amount is usually necessary to resolve most
mass doublets. In order to achieve such resolution, double focusing of the
ion beam is usually necessary. A magnet focuses only on the basis of
mass, but an electrostatic analyzer, i.e., a set of curved plates with a volt-
age impressed on them, will focus ions on the basis of their kinetic energy.
Adding such extra focusing reduces the overall number of ions traversing
the mass spectrometer, and thus reduces overall sensitivity. To overcome
such a reduction, the mass range is usually scanned at a slow rate. To
minimize the effects from slow scanning and decreased sensitivity, one
should use only as much resolution as necessary to perform the required
analysis, since the accuracy of a precise mass measurement is indepen-
dent of resolution as long as any mass doublets are separated.
2. QUADRUPOLE
The quadrupole mass analyzer, sometimes referred to as a quadru-
pole filter, has gained wide popularity in the last decade. It consists of
Collector
— ion flight path
entrance
FIG. 3. Quadrupole mass analyzer.