Pesticide Analytical Methodology
A C S S y m p o s i u m Series No. 136 J o h n Harvey, Jr., Editor E.I. Du Pont de Nemours pany
and
Com-
Gunter Z w e i g , Editor U.S. Environmental Protection
Agency Based on a symposium jointly sponsored by the Divisions of Pesticide Chemistry and Analytical Chemistry of the American Chemical Society. The latest analytical techniques for determining small levels of pesticides and their residues Twenty chapters cover recent advances in analytical methodology that enable scientists to assess the toxicological significance of trace levels of pesticides in the environment, in food sources, and in human organs and tissues. CONTENTS The use of HPLC in pesticide metabolism studies, automation of HPLC. evaluation of LC columns, and improvement of mobile-phase selectivity in reversed-phase chromatography are discussed. Recent developments in TLC, especially those applied to the forensic chemistry of pesticide poisonings, and the assessment of human exposure to pesticides are also presented Chemical derivatization techniques, negative ion spectroscopy, and immunochemical technology are included. The final chapters deal with applications of these analytical methods to specific compounds and substrates such as tetrachlorodibenzo-p-diozin, organotin compounds, and airborne pesticides. 406 pages (1980) LC 80-19470
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ternal degrees of freedom of the molecule and results in several competing decomposition reactions. Beuhler et al. then made two suggestions for improving the desorption of intact neutral molecules. T h e first requires paying attention to the nature of the surface itself (12) (and they suggest the use of Teflon), in an effort to reduce the binding energy of the sample to the surface, and making desorption more competitive with decomposition. This has certainly been the thinking of the investigators previously mentioned, who have used glass (1), quartz (5-8), Teflon (9), and Vespel (10, 13). T h e second approach is "rapid heating," which favors the desorption of intact neutral molecules on kinetic grounds. Figure 3, taken from the work of Beuhler et al. (12), illustrates the principle. For nonvolatile molecules one can assume t h a t the energy of activation for the decomposition process is lower t h a n t h a t for the desorption of the intact neutral molecule. In addition, the energy of activation for decomposition will be the rate-determining step for evaporation of the decomposed neutral. Therefore, Arrhenius plots of the relative abundances of the molecular ion and a decomposition ion vs. 1/T will have the appearance shown in Figure 3 for Thyrotropin Releasing Hormone, and will intersect at some value of 1/T. T h e principle behind rapid heating is to get into the temperature region above the intersection before appreciable decomposition has taken place. T h e heating rates being used in this case are approximately 10 °C/s. In addition, Beuhler et al. (12) note the necessity of a data system for this work since individual spectra must be acquired approximately every 2-3 s. This results in about 10 spectra before the sample is consumed. T h e rapid heating approach has been taken u p by a number of investigators. In our own laboratory (10), we have used heating rates similar to those used by Beuhler et al. (12) by inserting a Vespel probe into a hot ion source block, according to the method used by Hansen and Munson (9), and have produced spectra with good molecular ion abundance for guanosine, deoxyguanosine, sucrose, and the glucuronide conjugate of p-nitrophenol. Ultrafast heating rates (up to 1200 °C in 0.1 to 0.2 s) have been used by Daves et al. (14-16) for a n u m b e r of sugars and peptides, a method which he has subsequently termed "flash desorption." This very rapid heating approach also forms t h e principle behind our current work using a pulsed CO2 laser for the desorption of intact neutral molecules, which are then ionized (17) using isobutane. However, it should be noted t h a t laser desorption
ANALYTICAL CHEMISTRY, VOL . 52, NO. 14, DECEMBER
1980
Figure 3. Relative intensity of m/e 363 and m/e 235 ions from TRH plotted or a function of 1/T. Spectrum was obtained by evaporation of TRH from a copper probe surface in a Teflon collision chamber: ( O ) protonated parent molecule of PCA-His-Pro-NH 2 , m/e 363; ( · ) m/e 235 ion formed by loss of pyrrolidinone carboxyl amide from PCA-His-ProNH 2 . Reprinted (with permission) from Ref. 12
methods can also produce ions directly on the probe surface. Both Kistemaker and co-workers (18) and Stoll and Rôllgen (19) describe experiments in which ions are produced by a CO2 laser in an ion source in which the electron beam is t u r n e d off. T h e particular wavelength used (10.6 μτή), the fact t h a t the desorption process is in dependent of wavelength (Kistemaker used a CO2 laser at 10.6 μπι and a N d YAG laser at 1.06 Mm), and recent work by Kistemaker (20) on t h e de pendence of the molecular ion abun dance upon power density, point to this as a thermal process, rather t h a n a photoionization (multiphoton) pro cess. It is therefore closely related to the very rapid heating experiments. Finally, desorption from activated emitters also involves rapid heating, since very high temperatures are reached when t h e emitter current is turned u p (21-23).
Activated Emitter Experiments Activated emitters, of the type com monly used in field desorption, are a logical choice as a sample substrate for the " i n - b e a m " type of desorption we are considering. First of all, they allow the sample to be distributed over an enormous surface area, which greatly reduces the intramolecular forces be-
