Degradation carbamate pesticides

Attempts have been made to apply the structure-activity concept (Hansch and Leo 1995) to environmental problems, and this has been successfully applied to the rates of hydrolysis of carbamate pesticides (Wolfe et al. 1978), and of esters of chlorinated carboxylic acids (Paris et al. 1984). This has been extended to correlating rates of biotransformation with the structure of the substrates and has been illustrated with a number of single-stage reactions. Clearly, this approach can be refined with the increased understanding of the structure and function of the relevant degradative enzymes. Some examples illustrate the application of this procedure ... 

Meier, E.R, M.C. Warner, W.H. Dennis, W.F. Randall, and T.A. Miller. 1976. Chemical Degradation of Military Standard Formulations of Organophosphate and Carbamate Pesticides. I. Chemical Hydrolysis of Diazinon. U.S. Army Med. Bioengin. Res. Dev. Lab., Fort Detrick, Frederick, MD. Tech. Rep. 7611. 32 pp. 

Polarographic, thin-layer chromatographic, and spectrometric techniques have been reported for the determination of carbamate pesticides (131-135). These compounds are thermally labile and thus tend to degrade at the elevated temperature of a GC column. Nevertheless, some carbamate insecticides can be determined, directly or indirectly, by GC (44,136-138). The obtaining of thermally stable derivatives has generally been limited to the aryl-V-methylcarbamates. The most difficult carbamate insecticides to chromatograph by GC are the oxime /V-methylcarbamates (139-141). 

Table 6.4 shows first-order rate coefficients and tx/2 values for degradation of a number of pesticides in soils (Rao and Davidson, 1982). The k and t1/2 values calculated from field data are based on the disappearance of the parent compound (solvent extractable). Table 6.4 also includes k and t1/2 values calculated on mineralization (14C02 evolution) and parent-compound disappearance from laboratory studies. The t1/2 values were smaller for field than for laboratory studies. Rao and Davidson (1980) attribute this to the multitude of factors that can affect pesticide disappearance in the field while only one factor is studied in the laboratory. Rao and Davidson (1982) suggested that pesticides be classified into three groups based on values (Table 6.5) nonpersistent (t1/2 < 20 days), moderately persistent (20 < t1/2 < 100 days), and persistent (/1/2 > 100 days). Most chlorinated hydrocarbons are grouped as persistent, while carboxyl-kanoic acid herbicides are nonpersistent. The s-triazines, substituted ureas, and carbamate pesticides are moderately persistent. 

Most soils have a pH ranging from 4 to 9. The degradation of pesticides such as organo-phosphates and carbamates is affected by the pH of the soil. Most organophosphates are hydrolyzed under alkaline conditions, but diazinon is unstable in acid soils. Carbamates such as carbofuran are also hydrolyzed under alkaline conditions. The persistence of neonicotinoids is primarily determined by the pH. Imidacloprid and thiamethoxam are hydrolyzed under alkaline conditions, whereas thiacloprid and acetamiprid are less stable... 

Mueller, H. M. and Stan, H. J., Thermal degradation observed with different injection techniques quantitative estimation by the use of thermolabile carbamate pesticides, J. High Resolut. Chromatogr., 13, 759-763, 1990. 

Aldicarb nitrile (12), a residual degradation product of the carbamate pesticide Aldicarb, can be analysed by GC using a short column and MS detection, with a detection limit of 0.15 ng. The short column is required to avoid thermal degradation of Aldicarb to 12 that could distort the analysis . 

One of the most intensively examined type of pesticides handled under PBl conditions were the chlorinated phenoxy acids and their esters which were determined in water [82-86] and soil samples [83, 86]. Even results of an interlaboratory comparison study of 10 chlorinated phenoxy acids using PBl or TSP ionisation were published by Jones et al. [37] [87]. Statistically significant differences were observed between the interfaces and under these conditions PBl was found to have a better precision than TSP [87]. Betowsld et al. [88] observed thermal degradation induced by residence time in the ion source and the influence of ion source temperature in the ionisation of the chlorinated phenoxy acid derivatives 2,4-D and MCPA. Several authors successfully examined a large number of different carbamate pesticides [67, 89, 90] and their transformation products by PBl-LC-MS [89, 90], by PBl-FIA-MS (flow injection analysis) [67] or by supercritical fluid chromatography (SEC) PBl-interfaced to MS [91],... 

The carbamate pesticides aldicarb, carbaryl and their degradation products, aU under research in photochemical degradation studies, were characterised by TSP-LC-MS and MS/MS. A tentative photodegradation pathway for the different pesticides in water was postulated [256]. 

Carbamates. A fast, sensitive and selective method for the concentration and analysis of 9 N-methylcarbamate pesticides was reported by Volmer et al. [507]. Three different SPME fibres combined with short-column ESI-LC-MS(-i-) and MS/MS were applied. The detection limits observed were 0.3-1.9 pg Signal intensities increasing by a factor of 2-7 were observed [508] using non-volatile buffers in the separation process prior to ESI-MS. After EC removal of the non-volatile buffers was essential. The results obtained by ESI and APCI-LC-MS and MS/MS for the analysis of the eight N-methylcarbamate pesticides and their degradation products were compared with results obtain with the application of TSP or PBI (cf. 15.3.3.1 TSP, carbamates) [108]. ESI-LC-MS and TSP-LC-MS were used for quantitative determination of 10 different carbamate pesticides which showed a broad variety in polarity. ESI-SIM detection limits were typically 10-60 pg which was 10-150 times better than using TSP-MS (cf. 15.3.3.1 TSP, carbamates) [509]. Interfacing a commercial ESI source to an ITMS allowed the determination of carbamates as well as triazines and azo dyes. Identification could be performed either by IT-MS/MS or by ESI-CID [424]. 

The AFT treatment system and the AFT kinetic model were successfully enq)loyed to study the conq)etitive kinetics, the degradation products, and proposed degradation mechanisms of six carbamate pesticides (22) A con5)lete study of the degradation of carbofuran had already been done (25). Based on... 

From the identification of the degradation products by GC-MS, the degradation of die six carbamate pesticides demonstrated attack at the carbamate branch by hydroxyl radicals. In addition, for promecarb and carbaryl, hydroxyl radicals can also initiate attack at the alkyl site. These degradadve steps result in substituted phenols, which may or may not be the final products from AFT treatment. It can be demonstrated by measuring the BODs/COD of a solution of all six carbamates that the biodegradability of die solution is significandy... 

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