RESEARCH
Electron Affinity Detector Used with Chromatograph in Pesticide Analysis Hexabromo derivative of diphenylamine detected in concentrations as low as 0.02 p.p.m. Electron-capturing halogen derivatives of some pesticides, which before had not been detectable using an electron affinity detector in tandem with a gas chromatographic column, have been prepared by Dr. Walter H. Gutenmann and Dr. Donald J. Lisk at Cornell University's pesticide residue labora tory. Some pesticides contain too lit tle chlorine (or else the chlorine is ineffective) to be analyzed by this highly specific and sensitive technique [J. Agr. Food Chem., 11, 301 (1963)]. Diphenylamine. As an example, bromination of diphenylamine, used to retard oxidation in apples (scald con trol), yield a hexabromo derivative which is detected in amounts as low as 0.02 p.p.m. Evaporated acetone ex tract from blended tissue of apples treated with diphenylamine is directly brominated for 30 minutes in the presence of iodine. After evaporation of the excess halogens, the derivative is extracted with hexane and chromatographed. Recovery of the added diphenylamine is good, Dr. Lisk says. Presence of the brominated pesticide is determined using a radium cell as an electron affinity detector. Radium226 in the detector emits beta par ticles (electrons), which are picked up by a stream of nitrogen gas. Pesticide molecules containing halogens collide with the gas molecules and capture the electrons. These would normally have passed to the opposite electrode in the detector. The current is thus decreased in the detector to signal the presence of a pesticide. Although chlorine has a greater electron affinity than bromine, it is frequently more difficult to detect pesticides containing chlorine than it is to detect bromine derivatives. The reason for this, Dr. Lisk says, is that the larger bromine atom on a molecule permits electron-bromine complexes to form which are more stable (longer lifetime) than some electron-chlorine complexes. Therefore, the detector has time to register a decrease in cur rent before electrons break away from 38
C&EN
AUG.
12,
1963
Brominated Derivative is Recovered from Apples Treated with Pesticide Chromatogram shows recovery from extract^ οί .apples lip-irêatecT"' with diphenylamine
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bromine atoms on pesticide molecules. Herbicides belonging to the substituted urea and carbamate groups but containing low percentages (less than 30%) of chlorine have also been detected by Dr. Gutenmann and Dr. Lisk. Chloro IPC (a carbamate herbicide used to control sprouting in potatoes), monuron in grapes, diuron in raspberries, and linuron in carrots—all phenyl-substituted urea herbicideshave been detected* In each case, evaporated crop-acetone extracts are directly hydrolyzed and brominated in acid solutions containing iodine and bromine to give their respective bromi-
nated anilines. After neutralization, these derivatives are extracted with hexane or benzene and directly chromatographed. Diphenyl (used to regulate growth of oranges) is determined as the 4,4'dibromo derivative. Insecticides. The organophosphorus insecticide, guthion, is determined as the methyl ester of 2amino-3,5-dibromobenzoic acid following hydrolysis, bromination, and methylation. Bromination and methylation also yield sensitive derivatives of the herbicides MCP and MCPB (phenoxy acid herbicides used to control broadleaf weeds). Organophosphorus insecticides, usually containing methoxyl or ethoxyl groups, form the corresponding alkyl iodides upon reaction with hydriodic acid (Zeisel alkoxyl reaction). Ethion and malathion, insecticides of the organophosphorus type, are determined after first reacting them with hydriodic acid and chromatographing their respective ethyl and methyl iodides. Sensitivity. Iodine and bromine impart much greater sensitivity than does chlorine when introduced into an organic compound. Although chlorination leads to sensitive derivatives, often more than one product forms which later complicates interpretation of the chromatograms. The Cornell scientists plan to try fluorinations of pesticides, but they expect derivatives to be even less effective than some chlorine-substituted compounds. Methylation has also been used to determine 4-(2,4-DB) and 2,4-D in soils and forage; 2-(2,4,5-trichlorophenoxy) propionic acid (silvex) in water and apples; and 3-amino-2,5dichlorobenzoic acid in tomatoes. These herbicide acids are effective as electron-capturing molecules but must be methylated to pass through the chromatographic column, Dr. Lisk says. A Barber-Colman Model 10 gas chromatograph, a battery-operated radium-226 detector, and 5% silicone grease columns were used in the Cornell studies. Electron affinity spectroscopy is very useful as a routine tool for the determination of halogenated pesticide residues in field-treated samples. The accuracy and sensitivity of the method are "remarkable," Dr. Lisk says. Another advantage of the method is that its specificity allows analysis of the samples without any preliminary cleanup procedures.
