Paraoxon detection limits

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... 

O ring. At 20 min inhibition time the detection limits for malathion, parathion methyl and paraoxon were 3, 0.5 and 5pg I respectively. Although these bienzymatic systems look simple, it is difficult to provide optimal conditions for both enzymes. In general the optimum pH, temperature and buffer molarity for different enzymes are different. The experimental conditions are at the levels below the optimum capacity of both enzymes [14], This disadvantage can be minimized by use of a single enzyme system, which is readily inhibited by the pesticide. 

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

In this system, choline formed by acetylcholinesterase is oxidized by choline oxidase and the hydrogen peroxide produced is determined using the luminol/peroxidase CL reaction. The sensor has been used for the analysis of Paraoxon and Aldicarb pesticides, with detection limits of 0.75 pg/L and 4 pg/ L, respectively. Recoveries in the range of 81-108% in contaminated samples of soils and vegetables were obtained. 

The inhibition and the subsequent signal detection were performed in two different solutions. First the pesticide solution was added and then after 10 min (incubation time) the sensor was moved into a new buffer solution where the substrate (5mmoll 1 acetylthiocholine) was injected and the signal measured. This procedure is particularly suitable when a complex matrix, which could pose problems for the direct measurement of thiocholine oxidation, is used. The analytical characteristics of pesticide determination in standard solutions were then evaluated. Detection limits, defined in this work as the concentrations giving an inhibition of 20%, were 30 and 10 ppb for aldicarb and paraoxon, respectively. By increasing the incubation time up to 30 min, an increase in the degree of inhibition could be observed and lower detection limits both for Aldicarb (5 ppb) and Paraoxon (3 ppb) were achieved. 

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. 

Determination of methyl paraoxon can be realized in the range of 10"8 -10 6 mol/L. The detection limit, defined as the concentration of pesticide that produce an inhibition percentage of 10% of the AChE activity correspond to a concentration of 5 10 9 mol/L methyl paraoxon. 

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