The effect of temperature, soil moisture, soil type, adsorption, and formulation on terbufos persistence in soil was studied under laboratory conditions. Temperature appeared to be more important than moisture in influencing the dissipation rate of terbufos. More terbufos was recovered from a granular formulation than a technical formulation one month after incubation. Soil adsorption coefficient and calculated first order rate constant were well correlated. Terbufos was relatively immobile on soil thin layers and in field-located soil columns. Laboratory data agreed reasonably well with measurements of persistence in the field. Terbufos degradation kinetics were qualitatively analyzed and the toxicological significance of studying parent terbufos alone was discussed.
A BSTRA CTThe pyrethroid insecticide bifenthrin was used for control of the squash bug Anasa tristis Deg. on pumpkins. An analytical method has been developed for analysis of bifenthrin residues in pumpkins. The maximum bifenthrin residue found was 0.007 mg kg -'. The bifenthrin residue was also examined by combined gas chromatographylmass spectrometry in the electron impact mode with selected ion-monitoring for detection, 141 Pestic. Sci. 0031-613X/91/$03.50 0 1991 SCI. Printed in Great Britain
A photoionization quadrupole mass spectrometer has been used to study the difference in proton affinities for CH3OH and H2S. Mixtures of CH3OH and H2S are photolyzed in the source of the mass spectrometer with a krypton resonance lamp at 10.63 eV. The only initial reactant ion is H2S+. H2S+ reacts with H2S and CH3OH, under field free conditions, to produce H3S"1" and H(CH2OH)+. The rate constants for these two reactions are measured. The following reaction then proceeds toward equilibrium: HsS+ + CH3OH H2S + CH2OH2+. Further clustering of H(CH,¡OH)+ is observed in competition with the equilibrium. The standard enthalpy change for the equilibrium is found to be -5.1 ± 0.3 kcal/mol and the standard entropy change is found to be -8.0 ± 0.4 cal/deg mol. By comparing this difference in proton affinities with previously determined literature values, the most consistent proton affinity values are 175 kcal/mol for CH:,OH and 170 kcal/mol for H2S.
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