The aim of the research was the use of square wave adsorptive stripping voltammetry (SWAdSV) in conjunction with a hanging mercury drop electrode (HMDE) for the determination of nitrothal-isopropyl. It was found that optimal SW technique parameters were frequency, 200 Hz; amplitude, 50 mV; and step potential, 5 mV. Accumulation time and potential were studied to select the optimal conditions in adsorptive stripping voltammetry: 45 s at 0.0 V, respectively. The calibration curve (SWSV) was linear in the nitrothal-isopropyl concentration range from 2.0 × 10−7 to 2.0 × 10−6 mol L−1 with detection limit of 3.46 × 10−8 mol L−1. The repeatability of the method was determined at a nitrothal-isopropyl concentration level equal to 6.0 × 10−7 mol L−1 and expressed as RSD = 5.5% (
The agricultural practices intensively use pesticides, herbicides, fungicides, and other classes of chemical products to achieve maximal productivity. This has resulted in serious impacts on the natural environment, causing an increased level of pollutant residues in water, soil, river sediments, and foodstuffs [
Fungicides are biocides that are usually applied to protect fruits and vegetables against fungi. The increased risk of fungicide residues accumulation can cause serious health problems also through human exposure to their remnants present in the food. Among the fungicides used, nitrothal-isopropyl (diisopropyl 5-nitroisophthalate, NT, Figure
Chemical structure of nitrothal-isopropyl.
The most common methods of NT determination employ gas, liquid or thin-layer chromatography and selective detector [
Up to date there was no voltammetric work dealing with elaboration of electroanalytical method of nitrothal-isopropyl determination. Thus the goal of this work was aimed at the development of a simple and sensitive method for the aforementioned fungicide determination.
All electrochemical measurements were performed with microAutolab potentiostat (EcoChemie, Netherlands) through electrochemical software version GPES 4.9. A three-electrode cell was employed incorporating a hanging mercury drop electrode (AGH University, Cracow), an Ag/AgCl (3.0 M KCl) reference electrode, and a Pt wire as a counter electrode. No special pretreatment of electrochemical station was needed prior to the measurements except degassing the working solution in the voltammetric cell with pure argon (5 N). Mass transport was achieved with a Teflon-coated magnetic stirrer operated by M164 stand (mtm-anko). Measurements of pH were made using a pH-meter (Elmetron, Poland) with a combined glass electrode. All experiments were performed at room temperature 20 ± 1°C.
All chemicals used were of analytical grade. Double distilled demineralized water was exploited throughout experiments. Nitrothal-isopropyl was purchased from Dr. Ehrenstorfer Gmbh (Augsburg, Germany) and used as received. 25 mL of a 1.00 mmol L−1 stock standard solution was prepared by dissolving 7.41 mg of NT in a mixture of ethanol and water (1 : 1, v : v). Solutions with higher dilution were freshly prepared before measurements from the stock standard solution. Britton-Robinson (BR) buffer solutions of different pH values were prepared by the addition of sodium hydroxide solution to a phosphoric, boric, and acetic acid mixture while citrate buffers were composed of sodium citrate in combination with required amount of hydrochloric acid. The final pH was controlled and adjusted using a pH-meter.
10 mL of buffer solution was placed in an electrochemical cell containing a specific amount of analyzed NT standard solution. In order to remove dissolved oxygen degassing was performed before each measurement by passing through an argon stream. Electrochemical measurements of nitrothal-isopropyl were carried out with SWSV and recorded in the potential range from 0.0 to to 2.0 V. The SW voltammetric parameters were as follows: frequency 200 Hz, step potential 5 mV, and amplitude 50 mV with accumulation at 0.0 V for 45 s.
Nitrothal-isopropyl is an electroactive compound and square wave adsorptive stripping voltammograms recorded in its presence show two well-defined reduction signals, first close to −0.1 and second approximately at −0.6 V (Figure
SW voltammograms for
The influence of the supporting electrolyte pH on the electrochemical behavior of nitrothal-isopropyl was evaluated with the peak potential and current analysis. The electrochemical reduction of NT was investigated in the pH range 2.0–12.0 in 0.04 M BR buffer solution (inset in Figure
The optimization of square wave adsorptive stripping voltammetric parameters for nitrothal-isopropyl determination was a crucial step in preparation of electroanalytical method. The results show significant influence of square wave voltammetric parameters on the NT reduction signals (data not shown). The step potential
Cyclic voltammetry was used to study the NT electrochemical behavior. The potential scan was started at pH 2.5 from 0.0 V to the negative direction and reversed at −2.0 V back to the starting potential. As can be seen in Figure
Cyclic voltammograms for 5.0 × 10−6 mol L−1 NT solution in Britton-Robinson buffer (pH = 2.5) at the scan rates (a) 20, (b) 75, (c) 150, (d) 300, and (e) 500 mV s−1.
The above-described cyclic voltammograms, pH effect, and literature survey on the reduction of aromatic nitro compounds [
The dependence between the cathodic peak current and NT concentration was examined using SWAdSV (Figure
Quantitative determination of nitrothal-isopropyl in BR buffer; pH = 2.5 with SWSV. Basic statistic data of the regression line.
Linear concentration range (mol L−1) | 2.00 × 10−7–2.00 × 10−6 |
Slope of calibration graph (A L mol−1) | 4.28 ± 0.08 |
Intercept ( | 0.11 ± 0.01 |
Correlation coefficient | 0.999 |
Number of measurements | 6 |
LOD (mol L−1) | 3.46 × 10−8 |
LOQ (mol L−1) | 1.15 × 10−7 |
SWAdS voltammograms recorded in BR buffer pH 2.5 with increasing nitrothal-isopropyl concentration
The LOD and LOQ values of the method were obtained based on
Recovery and precision of the NT peak currents at various nitrothal-isopropyl concentrations.
Concentration | Precision | Recovery [%] | |
---|---|---|---|
Given | Found | ||
[ | |||
0.200 | 0.202 | 1.4 | 101 |
0.400 | 0.389 | 3.7 | 97.3 |
0.600 | 0.577 | 5.5 | 96.3 |
0.800 | 0.816 | 3.7 | 102 |
1.00 | 1.02 | 3.4 | 103 |
2.00 | 1.99 | 1.8 | 99.5 |
The proposed method selectivity was investigated and evaluated with the addition of heavy metal ions, pesticides, or fungicides. The concentration of each possible interferent was increased from 1.0 × 10−8 through 5.0 × 10−8, 1.0 × 10−7, 5.0 × 10−7, and 1.0 × 10−6 up to 5.0 × 10−6 mol L−1. The recorded voltammograms were compared with the result obtained only in the presence of NT solution at a concentration of 5.0 × 10−7 mol L−1. The presence of cadmium, lead, and cobalt ions did not interfere with NT voltammetric response at any of the investigated concentrations. Dodine, dinotefuran, and cyromazine also had no interference action in the studied concentration range despite presence of their signals at about −1.1 V. Clothianidin precluded nitrothal-isopropyl proper determination only at the highest investigated concentration although its signals were observed from the beginning at −0.6, −0.85, and −1.1 V. Strong adsorption of acibenzolar-S-methyl and metam sodium at the concentration 5.0 × 10−7 mol L−1 and higher at the electrode surface caused severe drop of recorded NT voltammetric signal, although their signals present at −0.5, −1.0, and −0.3, −0.5 V, respectively did not interfere with nitrothal-isopropyl voltammetric signal. These results suggest that in most cases the method is selective and can be used in cases of simple environmental samples without significant deterioration.
Water samples were spiked with nitrothal-isopropyl at the
Results of NT determination in spiked samples with SWAdSV.
Sample | Concentration given [mol L−1] | Concentration found [mol L−1] | CV [%] | Recovery [%] |
---|---|---|---|---|
Tap water | 4.0 × 10−7 | (4.1 ± 0.5) × 10−7 | 4.8 | 102 |
River water (Jasień) | (4.2 ± 0.1) × 10−7 | 2.2 | 104 |
SWAdS voltammograms of nitrothal-isopropyl determination in spiked tap water samples using the standard addition method ((a) sample; (b), (c), and (d) standard additions). Experimental conditions are the same as in Figure
The above-described data clearly demonstrate the possible use of the hanging mercury drop electrode for square wave adsorptive stripping voltammetric determination of nitrothal-isopropyl. Since the proposed methodology is fast and of high precision and accuracy therefore it can be used for NT quantification in water samples with no matrix effects on the measurable response. All the data received using the optimized experimental conditions and voltammetric parameters acknowledged the practical application and viability of the proposed methodology, ensuring a new instrument for quantification of NT in water samples. The use of SWAdSV is usually more efficient than other conventional techniques. The newly developed procedure allows accurate detection of nitrothal-isopropyl and introduces a simple, fast, selective, and highly sensitive methodology. The capability to determine the fungicide content directly from the matrix medium or natural samples without any laborious pretreatment which are usually time-consuming and environmentally unfriendly is one of the main advantages of the method.
The authors declare that they have no competing interests.
The work was financed from resources of the state funds from the Faculty of Chemistry, University of Lodz.