Paraoxon concentration

Methyl parathion and methyl paraoxon concentrations were measured in the condensate from coastal fog in California. Levels ranged from 0.046 to 0.43 pg/L methyl parathion and from 0.039 to 0.49 pg/L methyl paraoxon. The authors noted that the transformation of the methyl parathion to the methyl paraoxon appeared to take place during atmospheric transport of methyl parathion away from the agricultural areas (Schomburg et al. 1991). 

Firstly prepare a stock solution of 1 mM paraoxon in PBS, pH 7.4, and dilute accordingly in PBS to yield standard solutions of concentrations from 1 x 10-8 to 1 x 10-17 M. Serial dilutions must be used to attain the very low levels of paraoxon concentration. Ultra pure water must also be used as the very low levels of pesticide detection mean the effect of impurities in water must be minimised. 

Fig. 24.5. (a) Calibration curve for paraoxon inhibition of acetylcholinesterase-modified CoPC electrodes for paraoxon concentrations between 10-8 and 10 M using a concentration of 2 mM acetylthiocholine chloride (n — 3, S.D. < +11%). Insets show a typical current transient response for an AChE-modified electrode to 2 mM acetylthiocholine chloride, before (b) and after (c) the addition of 1 x 10-17 M paraoxon. 

Amperometric responses, Fig. 24.5, show a calibration curve for paraoxon inhibition of AChE-modified CoPC electrodes for paraoxon concentrations from 1 x 10-17 to 1 x 10 8 M. 

From the slope of the plot of current vs. (current/concentration of ATCh), the Km app for AChE was determined to be 0.66 mM. This biosensor also showed good precision and operational stability for the measurement of ATCh. The relative inhibition of AChE activity was calculated as a function of paraoxon concentration.. The linearity was observed up to 6.9 nM (slope, 14.36%/nM correlation coefficient, 0.9859) to 6.9 nM and the limit of detection of 0.5 nM (0.145 ppb). Moreover, the detection limit for methyl parathion using the present sensor could be expected to be 1.65 nM. Real sample analysis results were in good agreement (90%), which demonstrates the validity of this MWCNTs-SPE modified biosensor to a practical problem. 

Initial parathion concentration was 3.95 X Initial paraoxon concentration was 4.81 X lO M. 

PO)< = Residual Paraoxon concentration at time t in M X 10. Kob = Observed rate constant (Liter Mole Min. i). 

In a study to determine the concentrations of pesticides in air collected during times of peak pesticide use in California, air samples were collected at applications sites and at locations adjacent to the application sites (Baker et al. 1996). Of the samples collected adjacent to the application sites, 50% had levels of methyl parathion greater than the detectable limit of 0.2 ng/m, while 21% had levels of methyl paraoxon... 

Air samples collected in the Sacramento Valley area of California near sites where methyl parathion was heavily used on rice were analyzed by Seiber et al. (1989). Methyl parathion concentrations ranged from 0.2 (minimum detectable level) to 25.67 ng/m depending on the location and time of sampling. Methyl paraoxon, the oxygen analog of methyl parathion, was also detected at a maximum of 3.07 ng/m. The highest concentrations of both compounds were found at sites near locations of heaviest use. 

Table 1. Kinetic parametral for Paraoxon-biocatalytic reaction using the OPH-expressing whole cells. All data wrae based on unit cell concentration (1 mg-dry cell weight ml" ).

Fig. 15.14 The response of the TEo mode to different concentrations of paraoxon and the calibration curve. Reprinted from Ref. 53 with permission. 2008 The Royal Society of Chemistry...

Antibody 15C5 was able to catalyse the hydrolysis of the triester [105] with cat 2.65 x 10 3 min 1 whilst a second antibody from the same immunization programme was later found to hydrolyse the acetylcholinesterase inhibitor Paraoxon [106] with kcat = 1.95 x 10 3min-1 at 25°C (Appendix entry 6.2) (Lavey and Janda, 1996b). Antibody 3H5 showed Michaelis-Menten kinetics and was strongly inhibited by the hapten [104]. It exhibited a linear dependence of the rate of hydrolysis on hydroxide ion concentration, suggesting that 3H5 effects catalysis by transition state stabilization rather than by general acid/base catalysis. 

Figure 2. Kinetic plot of pseudo-first-order rate constant for reaction of paraoxon (kohc) versus concentration of added sodium perborate at 27.5 oc avvarious pH in 0.1 mol dm borate buffer.

Atropine has been found -Ob to reduce markedly the increase in the concentration of total brain acetylcholine In rats later given paraoxon at 0.4 mg/kg. Rats given a dose of paraoxon and than treated with IV lntraperltoneally had brain concentrations of free and total acetylcholine that were essentially the same as those in rats given paraoxon alone, but no tremors or convulsions were observed. These animals survived those given paraoxon alone all died in convulsions within 3-8 min. 

Isocratic conditions have been established for HPLC with ultraviolet detection to determine trace levels of five pesticides frequently used in fruits, e.g., paraoxon, guthion, methyl-parathion, ethyl-parathion, and fenitrothion, in fruits and surface water (95). And HPLC was adapted for the determination of trace concentrations of seven OPP pesticides in drinking water. The analytes concentrated on the precolumn were eluted and separated on a C18 analytical column with an elution gradient program and determined by measuring their UV absorption (48). 

Fig. 4 Chromatograms of a blank apple sample (top) and the same apple sample spiked with 27 of the 31 organophosphorus at the following concentration levels (bottom). 1 = paraoxon methyl 500 2 = malaoxon 6400 3 = paraoxon ethyl 800 4 = metidathion 500 5 = azinphos-methyl 200 6 = phosmet 200 7 = parathion methyl 800 8 = malathion 2000 9 = triazophos 200 10 = fenitrothion 800 11= azinphos ethyl 200 12 = chlorfenvinphos 200 13 = quinalphos 200 14 = parathion-ethyl 400 16 = ethrimphos 1000 17 = diazinon 800 18 = coumaphos 800 19 = fonophos 200 21 = phoxim 400 23 = chlorpyriphos-methyl 500 24 = disulfoton 2000 26 = isofenphos 500 27 = fenchlorphos 500 28 = temephos 400 29 = chlorpyriphos ethyl 200 30 = pyrimiphos-ethyl 200 31 = carbophenothion-ethyl 400. The chromatography was carried out with the Altima column using CH3CN/H20 as mobile phase in gradient elution CHjCN from 28 to 39% in 20 min, and then from 39 to 88% in 40 min. (From Ref. 59.)...

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