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Spectroscopic Methods of Analysis PDF

302 Pages·1977·6.826 MB·English
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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.

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