Insecticides determination

Ph 0.75-8.0 ppm Combined TLC- nzyme-inhibition procedure. Insecticides determined simultaneously in lettuce ex-... 

Tran-Minh, C. and Pandey, PC. (1990) Insecticide determination with enzyme electrodes using different enzyme immobilization techniques. Biosens. Bioelectron., 5, 461. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

In the normal process ( ), step (J) occurs very rapidly and step (/) is the rate-determining step, whereas in the inhibition process (B), step (3) occurs very slowly, generally over a matter of days, so that it is rate determining. Thus it has been demonstrated with AChE that insecticides, eg, tetraethyl pyrophosphate and mevinphos, engage in first-order reactions with the enzyme the inhibited enzyme is a relatively stable phosphorylated compound containing one mole of phosphoms per mole of enzyme and as a result of the reaction, an equimolar quantity of alcohoHc or acidic product HX is hberated. 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphoms atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphoms, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to m vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian Hver. A typical example is the oxidation of parathion (61) to paraoxon [311-45-5] (110). 

Pesticides. Chlorinated hydrocarbon pesticides (qv) are often found in feed or water consumed by cows (19,20) subsequently, they may appear in the milk, where they are not permitted. Tests for pesticides are seldom carried out in the dairy plant, but are most often done in regulatory or private specialized laboratories. Examining milk for insecticide residues involves extraction of fat, because the insecticide is contained in the fat, partitioning with acetonitrile, cleanup (FlorisH [26686-77-1] column) and concentration, saponification if necessary, and determination by means of paper, thin-layer, microcoulometric gas, or electron capture gas chromatography (see Trace and residue analysis). 

DETERMINATION OF CONCENTRATION OF ORGANOCHLORINE INSECTICIDES IN DRINKING WATER OF SERBIA... 

Determination of the content of organochlorine insecticides in the drinking water is primal for establishing the level of contamination of the water ecosystem and for human health protection. 

These methods are employed for the detection and determination of antibiotics and substances with similar effects, like alkaloids, insecticides, fungicides, mycotoxins, vitamins, bitter principles and saponins [14]. 

It would seem obvious that efforts should be concentrated on following up the many leading observations which have been reported and determining the structure of the substances already isolated. However, re-examination of the biology and chemistry of species which have yielded insecticidal compounds in the past certainly should not be overlooked. Dr. Beckman will show that careful investigation of such a well-known commercial preparation as pyre-thrum extract can reveal the presence of unsuspected and active new constituents Dr. Moore will indicate how some such observations may be turned to profit through synthesis of simple analogs. 

By employing C14-labeled pyrethrum, it has been determined that at 35 °C. the rate of penetration of pyrethrum is more than twice that at 15°C. Cockroaches prostrate at 15 °C. can be returned to normal by transferring them to 35 °C., a process which can be repeated for several hours. Since cockroaches transferred from 35° to 15°C. became prostrate more rapidly than those held continuously at 15°C., the insecticide (or some metabolic toxin) was probably in the vicinity of its site of action at 35 °C. but was ineffective. 

Barnard and colleagues119 reported a powerful absorption band at 1040 cm"1 for sulphoxides, where absorption was proportional to concentration. Oba120 utilized this wavelength for a rapid determination of the synergistic insecticide Sulfox-Cide , 1,2-... 

Absorption in this range is less directly related to the sulphoxide group. Haus and coworkers123 determined the insecticide Sulfox-Cide (see the preceding subsection) by measurements at 288 and 254 nm (maximum and minimum, respectively). 

Self-Test 8.12B Use the data in Table 8.8 to determine at what temperature a 0.050 mol-kg 1 solution of the insecticide malathion, C10H19O6PS2, in camphor will freeze. 

The acyl groups introduced included 4-phenylbenzoyl, phenylacetyl, 4-methoxybenzoyl, acetyl, 2,4-dichlorophenoxyacetyl, and 2,2-dichloropro-pionyl. Introduction of the last pair of acyl groups is important because they are bioactive (insecticides), i.e., the product can be employed in controlled-release formulations [159]. The structures of all these esters were determined by FTIR and NMR spectroscopy, whereas their solution properties, includ-... 

Note Methanolic sodium hydroxide solution can replace potassium hydroxide solution [16] (q.v.). The production of color tones and fluorescence is very dependent on the duration and temperature of heating hence optimal conditions must be determined empirically [16]. Some thiophosphate insecticides do not form fluorescent derivatives [16]. The natural fluorescence of various cumarin derivatives is intensified [1]. 

Groundwater has also been surveyed for methyl parathion. In a study of well water in selected California communities, methyl parathion was not detected (detection limit of 5 ppb) in the 54 wells sampled (Maddy et al. 1982), even though the insecticide had been used in the areas studied for over 15 years. An analysis of 358 wells in Wisconsin produced the same negative results (Krill and Sonzogni 1986). In a sampling of California well water for pesticide residues, no methyl parathion was detected in any of the well water samples (California EPA 1995). In a study to determine the residue levels of pesticides in shallow groundwater of the United States, water samples from 1,012 wells and 22 springs were analyzed. Methyl parathion was not detected in any of the water samples (Kolpin et al. 1998). In a study of water from near-surface aquifers in the Midwest, methyl parathion was not detected in any of the water samples from 94 wells that were analyzed for pesticide levels (Kolpin et al. 1995). 

A recent method to screen the urine for alkyl phosphates as an indicator of exposure to organophosphate insecticides shows that the method can be used to determine environmental exposure to a specific organophosphate pesticide. The method was found to be sensitive, identifying low levels of exposure to insecticides in the environment by quantitation of urinary phosphates (Davies and Peterson 1997). The test is limited in that it is only useful for assessing recent exposure, due to the short half-life of the organophosphate pesticides. 

Bowman BT, Sans WW. 1983. Further water solubility determination of insecticidal compounds. J Environ Sci Health B 18 221-227. 

DePotter M, Muller R, Willems J. 1978. A method for the determination of some organophosphorus insecticides in human serum. Chromatographia 11 220-222. 

Mechanism of action can be an important factor determining selectivity. In the extreme case, one group of organisms has a site of action that is not present in another group. Thus, most of the insecticides that are neurotoxic have very little phytotoxicity indeed, some of them (e.g., the OPs dimethoate, disyston, and demeton-5 -methyl) are good systemic insecticides. Most herbicides that act upon photosynthesis (e.g., triaz-ines and substituted ureas) have very low toxicity to animals (Table 2.7). The resistance of certain strains of insects to insecticides is due to their possessing a mutant form of the site of action, which is insensitive to the pesticide. Examples include certain strains of housefly with knockdown resistance (mutant form of Na+ channel that is insensitive to DDT and pyrethroids) and strains of several species of insects that are resistant to OPs because they have mutant forms of acetylcholinesterase. These... 

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