Simultaneous enzymatic kinetic determination of pesticides, carbaryl and phoxim, with the aid of chemometrics

A sensitive and selective enzymatic kinetic method for the simultaneous determination of mixtures of carbaryl and phoxim pesticides was researched and developed. It was based on the inhibitory effect of the pesticides on acetylcholinesterase (AChE), and the use of 5,5′-dithiobis(2-nitrobenzoic) acid (DTNB) as a chromogenic reagent for the thiocholine iodide (TChI) released from the acetylthiocholine iodide (ATChI) substrate. The DTNB-thiocholine reaction was investigated by a spectrophotometric-kinetic approach. The complex rate equation for the formation of the chromogenic product, P, was solved under certain experimental conditions, which enabled the absorbance (A_P, at λ_max = 412 nm) from the mixtures of the two pesticide inhibitors to be directly related to their concentrations provided the absorbance additivity was followed. The spectra were measured for mixtures of carbaryl and phoxim at different concentrations, and at t = 904 s, T = 35 °C, pH = 7.5, c_ATChI = 0.14, and c_AChE = 0.10 mg mL⁻¹. The detection limits of the enzymatic kinetic spectrophotometric procedures for the determination of the carbaryl and phoxim were 4.7 and 0.59 μg L⁻¹, respectively.Calibration models for chemometrics methods, such as principal component regression (PCR), partial least squares (PLS) and radial basis function-artificial neural network (RBF-ANN) were constructed and verified with synthetic samples of the mixtures of the two pesticides. The best performing model was based on the RBF-ANN method yielding at approximately 10 ppb analyte concentrations, %RPE_T (carbaryl = 5.2; phoxim = 6.5), %Recovery (approx.105%) and %RPE_T (6.5). Various spiked town-water samples produced recoveries in the range of 98.8-103% for each pesticide.     

Differential Kinetic Spectrophotometric Determination of Methamidophos and Fenitrothion in Water and Food Samples by Use of Chemometrics

A spectrophotometric method for simultaneous analysis of methamidophos and fenitrothion was proposed by application of chemometrics to the spectral kinetic data, which was based upon the difference in the inhibitory effect of the two pesticides on acetylcholinesterase (AChE) and the use of 5,5′‐dithiobis(2‐nitrobenzoic acid) (DTNB) as a chromogenic reagent for the thiocholine iodide (TChI) released from the acetylthiocholine iodide (ATChI) substrate. The absorbance of the chromogenic product was measured at 412 nm. The different experimental conditions affecting the development and stability of the chromogenic product were carefully studied and optimized. Linear calibration graphs were obtained in the concentration range of 0.5–7.5 ng·mL^?1 and 5–75 ng·mL^?1 for methamidophos and fenitrothion, respectively. Synthetic mixtures of the two pesticides were analysed, and the data obtained processed by chemometrics, such as partial least square (PLS), principal component regression (PCR), back propagation‐artificial neural network (BP‐ANN), radial basis function‐artificial neural network (RBF‐ANN) and principal component‐radial basis function‐artificial neural network (PC‐RBF‐ANN). The results show that the RBF‐ANN gives the lowest prediction errors of the five chemometric methods. Following the validation of the proposed method, it was applied to the determination of the pesticides in several commercial fruit and vegetable samples; and the standard addition method yielded satisfactory recoveries.     

Effect of Different Saline,Alkaline Salts,Fertilizers and Surfactants on the Movement of Some Carbamoyl Group Containing Pesticides in Soils

Abstract

The influence of organic matter, calcium carbonate, flyash, saline and alkaline salts, inorganic fertilizers, surfactants and exchangeable ions on the mobility of five carbamoyl group containing pesticides has been studied using soil thin-layer chromatography. The variations in the R_f, R_B and R_M values of aldicarb, bavistin, carbofuran, dimecron and oxamyl under different treatments are reported and explained on the basis of adsorption and leachability.     

Analytical and kinetic study of the aqueous hydrolysis of four organophosphorus and two carbamate pesticides

The hydrolysis of six selected pesticides has been studied in aqueous solution. Four organophosphorus pesticides (disulfoton, isofenfos, isazofos and profenfos) and two N-methylcarbamate derivatives (oxamyl and ethiofencarb) were selected. Hydrolysis was performed in purified buffered water at different pH in the range 7.0–10.0 (ionic strength?=?2.5?mM, T?=?25°C). At pH?=?8.0, isofenfos and disulfoton (t _1/2?≈?4 years, t _1/2?≈?1 year, resp.) were found to be far more stable than isazofos (t _1/2?≈?5 months), ethiofencarb and profenofos (t _1/2<1 month), themselves more stable than oxamyl (t _1/2?≈?1 day). As expected, a strong dependence on pH was observed for all pesticides: the rate of degradation increased when the pH increased. Degradation products were identified by GC–MS and/or LC–MS. Possible structures are presented in the article.     

A kinetic spectrophotometric method for the determination of ternary mixtures of reducing sugars with the aid of artificial neural networks and multivariate calibration

A differential spectrophotometric method has been developed for the simultaneous quantitative determination of glucose (GLU), fructose (FRU) and lactose (LAC) in food samples. It relies on the different kinetic rates of the analytes in their oxidative reaction with potassium ferricyanide (K₃Fe(CN)₆) as the oxidant. The reaction data were recorded at the analytical wavelength (420 nm) of the K₃Fe(CN)₆ spectrum. Since the kinetic runs of glucose, fructose and lactose overlap seriously, the condition number was calculated for the data matrix to assist with the optimisation of the experimental conditions. Values of 80 °C and 1.5 mol l⁻¹ were selected for the temperature and concentration of sodium hydroxide (NaOH), respectively. Linear calibration graphs were obtained in the concentration range of 2.96-66.7, 3.21-67.1 and 4.66-101 mg l⁻¹ for glucose, fructose and lactose, respectively. Synthetic mixtures of the three reducing sugar were analysed, and the data obtained were processed by chemometrics methods, such as partial least square (PLS), principal component regression (PCR), classical least square (CLS), back propagation-artificial neural network (BP-ANN) and radial basis function-artificial neural network (RBF-ANN), using the normal and the first-derivative kinetic data. The results show that calibrations based on first-derivative data have advantages for the prediction of the analytes and the RBF-ANN gives the lowest prediction errors of the five chemometrics methods. Following the validation of the proposed method, it was applied for the determination of the three reducing sugars in several commercial food samples; and the standard addition method yielded satisfactory recoveries in all instances.     

Effect of Different Salt Leachates on the movement of some Carbamoyl Groups Containing Pesticides in Soils using Thin-Layer Chromatography

Abstract

The influence of pH, leachates of alkaline and saline salts, inorganic fertilizers and surfactants on the mobility of five carbamoyl group containing pesticides, viz. aldicarb, bavistin, carbofuran, dimecron and oxamyl has been studied using soil thin layer chromatographic technique. The variation in the movement of pesticides under different solvent amendments expressed in terms of Rf, RB and RM values and explained on the basis of adsorption and leachability.     

Simultaneous determination of iron and aluminium by differential kinetic spectrophotometric method and chemometrics

A differential kinetic spectrophotometric method was researched and developed for the simultaneous determination of iron and aluminium in food samples. It was based on the direct reaction kinetics and spectrophotometry of these two metal ions with Chrome Azurol S (CAS) in ethylenediamine-hydrochloric acid buffer (pH 6.3). The results were interpreted with the use of chemometrics. The kinetic runs and the visible spectra of the complex formation reaction were studied between 540 and 750 nm every 30 s over a total period of 285 s. A set of synthetic metal mixture samples was used to build calibrations models. These were based on the spectral and kinetic two-way data matrices, which were processed separately by the radial basis function-artificial neural network (global RBF-ANN) method. The prediction performance of these models was poorer than that from the combined kinetic-spectral three-way array, which was similarly processed by the same method (% relative prediction error (RPE_T) = 5.6). These results demonstrate that improved predictions can be obtained from the data array, which has more information, and that appropriate chemometrics methods can enhance analytical performance of simple techniques such as spectrophotometry.Other chemometrics models were then applied: N-way partial least squares (NPLS), parallel factor analysis (PARAFAC), back propagation-artificial neural network (BP-ANN), single radial basis function-artificial neural network (RBF-ANN), and principal component neural network (PC-RBF-ANN). There was no substantial difference between the methods with the overall %RPE_T range being 5.0-5.8. These two values corresponded to the NPLS and BP-ANN models, respectively. The proposed method was applied for the determination of iron and aluminium in some commercial food samples with satisfactory results.     

Application of response surface methodology for modelling the enzymatic assay of hydrogen peroxide by Emerson–Trinder reaction using 4-iodophenol

In this work, an initial-rate spectrophotometric method and response surface methodology (RSM) were combined for modelling and optimizing the experimental parameters of the enzymatic Emerson–Trinder reaction, for the determination of hydrogen peroxide. This spectrophotometric indicator reaction is currently used in biotechnology for the determination of phenolic compounds (e.g. in industrial samples) and also for determination of various substrates (e.g. in clinical chemistry). Using 4-iodophenol as a hydrogen donor in this reaction, the quality of the generated second-order polynomial response model equation was checked by the kinetic assay of H₂O₂ in real samples (e.g. cosmetic and human pooled serum samples), where their resulting satisfactory analytical characteristics were reported.     

Supercritical fluid extraction of carbamate pesticides from soils and cereals

Summary A method for the supercritical fluid extraction of carbamate pesticides (propoxur, aminocarb, carbaryl and methiocarb) from soil and cereal samples using CO₂ is proposed. Extractions were at 378 bar and 54 °C. Analytes were determined in the extracts by HPLC with fluorescence detection after post-column derivatization. Recoveries from spiked soil ranged between 39.6 and 91.7%, depending on analyte and soil components. Lowest recoveries were from sandy soils. Aminocarb could not be recovered from any soil using CO₂. Recovery of aminocarb from diatomaceous earth was improved by adding methanol to the extraction cell prior to SFE, but the effect was not observed in soil samples. Recoveries for propoxur and aminocarb from spiked wheat were about 75%, and only between 30–50% for aminocarb from corn and oats, and carbaryl from wheat. Fat was coextracted using CO₂ and retained in the trap together with the analytes, however, appropriate rinsing solvent allowed on-line clean-up of the extract.     

Determination of N-methylcarbamate pesticides using flow injection with photoinduced chemiluminescence detection

A sensitive, economic, rapid and simple method for the determination of four N-methylcarbamate pesticides: methomyl (2.0–80 μg L^?1), aldicarb (5.0–50 μg L^?1), butocarboxim (2.0–60 μg L^?1) and oxamyl (2.0–60 μg L^?1); is reported. It relies on the coupling of photoinduced chemiluminescence (PICL) detection with flow injection (FI) methodology. The automation of FI together with the use of light as a reagent decreased the environmental impact of the analysis. The proposed method was based on the oxidation of these pesticides, previously irradiated on-line with UV light, with cerium(IV), using quinine as a sensitiser. Limits of detection below the legal limits (100 ng L^?1) established by the European Union for drinking waters were obtained without the need of preconcentration steps. A good inter-day reproducibility (1.6–6.4%, n = 5), repeatability (rsd = 2.7 %, n = 25) and high throughput (123 h^?1) were achieved. The method was successfully applied to the determination of methomyl in natural waters with mean recoveries ranging from 90% to 98%.     

Simultaneous analysis of carbamate pesticides in tap and raw water by LC/ESI/MS.

Ten carbamate pesticides including four suspected endocrine disruptors, methomyl, benomyl (carbendazim), aldicarb and carbaryl, were simultaneously analyzed by LC/ESI/MS. The influence of the matrix on the variation of the ion signal intensities of (M + H)+ and adduct ions was investigated. Although the intensities of three oxamyl ions changed depending on the matrix, the variation in the concentration calculation of oxamyl was reduced by using the sum total of the area value of two ions. The limits of the quantitation of ten pesticides without a concentration procedure were from 0.4 - 30 microg/l. The solid-phase recovery rates of ten pesticides spiked into tap water and raw water were in the range of 69-111%. Using this method, the concentrations of the pesticides in tap and raw water sampled at 14 monitoring points in Hyogo Prefecture were determined. Carbendazim in three raw water samples and carbofuran in one of these three samples were detected at low concentrations (less than 0.32 microg/l).     

Spectrophotometric Study on Kinetics of Solvatochromism of Methyl Violet in Aqueous Methanol

The kinetics of the reaction of methyl violet with iodide in aqueous methanol system was studied by spectrophotometric method. The rate of reaction of methyl violet in different alcoholic composition in presence of potassium iodide was observed at pH 4 and 6 at various temperatures (298–318 K). Solvatochromic effect was studied in different percentages of methanol (0–50%). Bathochromic shift was observed with the decrease in polarity of solvent. The color change was attributed to molecule's structure, the delocalization of unit electrical charge causes deepening of color and decrease of delocalization causes fading of color due to reduction of dye. Increase in the rate of reaction was observed with increase in alcoholic content and also affected by potassium iodide salt and increased with increase in concentration of potassium iodide. Energy of activation (E_a) and transition energy (E_T) were calculated with the help of kinetic data. Thermodynamic parameters such as enthalpy change of activation (ΔH*), Gibbs free energy change of activation (ΔG*) and entropy change of activation (ΔS*) were evaluated as a function of concentration of solvent and salt.     

Enzymatic determination of N-methylcarbamate pesticides at the nanomolar level by the stopped-flow technique

A sensitive and selective kinetic enzymatic method for the determination of N-methylcarbamate pesticides is presented. It is based on their inhibitory effect on electric eel acetylcholinesterase and the use of 5,5'-dithiobis(2-nitrobenzoic) acid (DTNB) as chromogenic reagent for the thiocholine released from the acetylthiocholine iodide substrate. The fast DTNB-thiocholine reaction is monitored photometrically by the stopped-flow technique. Carbaryl, propoxur and carbofuran can be determined at concentrations in the ranges 6.5-120, 2-15 and 0.1-5.0 ng/ml, respectively, by the proposed method. An interference study was also reported.     

Determination of pesticides in soy-based infant formula using liquid chromatography with electrospray ionization tandem mass spectrometry

A sensitive method using liquid chromatography with electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) was developed and validated to quantify and confirm 13 pesticides, including aldicarb sulfoxide, aldicarb sulfone, oxamyl, methomyl, formetanate, 3-hydroxycarbofuran, carbendazim, thiabendazole, aldicarb, propoxur, carbofuran, carbaryl, and methiocarb, in soy-based infant formula. Data acquisition under MS/MS was achieved by applying multiple reaction monitoring of 2 fragment ion transitions to provide a high degree of sensitivity and selectivity for both quantitation and confirmation. Different approaches to constructing calibration curves were compared and discussed to address issues of the extraction efficiency or recovery, and matrix effects. Matrix-matched standard calibration curves with the use of isoprocarb as an internal standard were finally used to achieve the best accuracy of the method. Under most circumstances, recoveries of 13 pesticides, spiked at 5.0, 25.0, and 45.0 microg/kg, were close to 100%. The method detection limits (signal-to-noise ratio > or =3:1; microg/kg) of 13 pesticides were 0.2 for thiabendazole and methiocarb, 0.6 for aldicarb, and 0.1 for the others.     

A New Voltammetric Approach for the Determination of Biomimetic Catalyst Kinetic Constants Based on Substrate Consumption

In the present study, a new voltammetric method is introduced for the determination of kinetic parameters of artificial metalloenzymes (biomimetic complexes) mimicking the catecholase activity towards 3,5-di-tert-butylcatechol (3,5-DTBC) or 2,5-di-tert-butylhydroquinone, and representing a promising alternative to common spectrophotometric method. To demonstrate the effectiveness of the electrochemical approach, two binuclear copper(II) complexes bearing the same pentadentate ligand and different ionic ligands were selected. Apparent kinetic parameters, such as maximum velocity (V_max), Michaelis-Menten constant (K_m), and turnover number (k_cat), obtained by the described method employing differential-pulse voltammetry were in good agreement with the data evaluated from UV-Vis spectrophotometric measurements.     

Multivariate curve-resolution analysis of pesticides in water samples from liquid chromatographic-diode array data

Liquid chromatographic-diode array detection data recorded for aqueous mixtures of 11 pesticides show the combined presence of strongly coeluting peaks, distortions in the time dimension between experimental runs, and the presence of potential interferents not modeled by the calibration phase in certain test samples. Due to the complexity of these phenomena, data were processed by a second-order multivariate algorithm based on multivariate curve resolution and alternating least-squares, which allows one to successfully model both the spectral and retention time behavior for all sample constituents. This led to the accurate quantitation of all analytes in a set of validation samples: aldicarb sulfoxide, oxamyl, aldicarb sulfone, methomyl, 3-hydroxy-carbofuran, aldicarb, propoxur, carbofuran, carbaryl, 1-naphthol and methiocarb. Limits of detection in the range 0.1-2 μg mL⁻¹ were obtained. Additionally, the second-order advantage for several analytes was achieved in samples containing several uncalibrated interferences. The limits of detection for all analytes were decreased by solid phase pre-concentration to values compatible to those officially recommended, i.e., in the order of 5 ng mL⁻¹.     

Spectrophotometric Estimation of Carbaryl Residue in Some Home Garden Vegetables

Abstract

Residues of carbaryl (1-Naphthyl N-methly carbamate) were determined in the fruit and foliage of seven home garden vegetables by a spectrophotometric method. Residues at harvest averaged the following levels; cabbage (head, 0.0 ppm), cucumber (foliage 0.0 ppm; fruit, 0.05 ppm), garden bush beans (fruit, 0.0 ppm), Okra (foliage, 0.0 ppm; fruit, 0.01 ppm), pepper (foliage, 2.72 ppm; fruit, 0.9 ppm), squash (foliage, 6.55 ppm; fruit, 0.0 ppm), and tomato (foliage, 2.07 ppm; green fruit, 0.09 ppm; mature fruit, 0.03 opm). Foliage of the vegetables contained higher concentrations of the insecticide, in most cases, than did the fruits. Even with excessive applications, the carbaryl residues were found to be below the tolerance level established by EPA.     

Evaluation of pesticide residue in grape juices and the effect of natural antioxidants on their degradation rate

Various studies have been drawn toward the beneficial properties of fruit juices because they have several components, such as phenols, vitamins, and flavonoids, with antioxidant effects. However, fruit juices can also contain residues of pesticides used as standard pest control methods in crops. Many of these pesticides are degraded through oxidative mechanisms, and their persistence in juices can be enhanced by antioxidants. This study covers the degradation of four pesticides, aldicarb, demeton-S-methyl, fenamiphos, and methiocarb, to their respective sulfoxide and sulfone in grape juices, water (pH 3.5) and water (pH 3.5) with quercetin (one of the most important flavonoids of grape) added in an attempt to establish whether the presence of antioxidants can affect the degradation rate of pesticides. For this purpose, a multiresidue method based on solid-phase extraction (SPE) was developed for the simultaneous determination of these pesticides and their metabolites in commercial juices. The extraction procedure was carried out in C₁₈ columns. The subsequent elution of pesticides was performed with dichloromethane prior to the determination by liquid chromatography-tandem mass spectrometry (LC-MS/MS), using two precursor-product ion transitions. Average recoveries for all the pesticides studied were higher than 80%, with relative standard deviations lower than 15% in the concentration range 0.005–0.05 μg/mL, and the quantification limits achieved ranged from 0.1 to 4.6 μg/L. The results demonstrated that degradation was slower in fruit juices and aqueous solutions with quercetin than in water. Several commercial grape juices were also analyzed to establish the levels of these pesticides. Methiocarb, fenamiphos, and demeton-S-methyl were found at low levels in some samples.     

Application of chemometric methods to the simultaneous kinetic spectrophotometric determination of iodate and periodate based on consecutive reactions

A kinetic spectrophotometric method for the simultaneous determination of iodate and periodate in mixtures was proposed. The method is established on the different kinetic behaviours of the analytes which react with starch–iodide in the presence of sodium chloride in sulfuric acid medium. The kinetic data were collected from 260 to 900 nm every 10 nm, within a time range of 0–180 s at 1 s interval, and the absorbance collected at 291, 354 and 585 nm, respectively, increased linearly with the concentration between 0.1–1.2 mg L^− 1 for both iodate and periodate. The mechanism investigation revealed that the iodate/periodate–iodide–starch system is a consecutive reaction. Subsequently, the mathematical model for the quantitative kinetic determination based on the consecutive reactions by utilizing chemometric methods was deduced, and the simultaneous determination of synthetic mixtures of iodate and periodate was then applied. Kinetic data collected at 291, 354 and 585 nm, were processed by chemometric methods, such as classical least square (CLS), principal component regression (PCR), partial least square (PLS), back-propagation artificial neural network (BP-ANN), radial basis function–artificial neural network (RBF-ANN) and principle component–radial basis function–artificial neural network (PC-RBF-ANN). The results showed that calibration model with the data collected at 354 nm had some advantages for the prediction of the analytes as compared with the ones of other two wavelengths, and the PLS and PC-RBF-ANN gave the lower prediction errors than other chemometric methods. The proposed method was applied to the simultaneous determination of iodate and periodate in several real samples; and the standard addition method yielded satisfactory recoveries in all instances.     

Application of Alizarine Derivatives as Chromogenic Reagents for the Spectrophotometric Determination of Some Sulfa Drugs

A spectrophotometric procedure for the determination of six pharmaceutical sulfonamides containing a primary aromatic amino group using alizarine (I), alizarine blue (II), alizarine red (III), and quinalizarine (IV), that offers advantages of simplicity, rapidity, sensitivity, precision, and stability, has been developed. The proposed method is based on the formation of colored products with the chromogenic reagents (I-IV). A linear correlation was found between absorbance at λ_max and the concentration. For more accurate results, Ringbom optimum concentration ranges are evaluated. The molar absorptivities (ε), absorptivities (a), and Sandell sensitivities for all sulfonamides derivatives with the chromogenic reagents tested were calculated. The procedures developed are applied for bulk sulfa drugs and some of their dosage forms.