Dissipation rate of different commercial formulations of propamocarb-hydrochloride applied to potatoes using HPLC–DAD

2.1 Chemicals

Certified reference standard of propamocarb-hydrochloride >99% purity was obtained from central agricultural pesticides laboratory (CAPL). Acetonitrile (MeCN) and methanol (MeOH) of HPLC grade were purchased from Merck. Bulk primary secondary amine (PSA) sorbent (Bondesil-PSA, 40 μm) was bought from Subelco. Ammonium acetate (cryst. Extra pure), acetic acid and sodium chloride were purchased from Merck. Magnesium sulfate anhydrous fine powder, trisodium citrate dihydrate (Extra pure) and disodium hydrogencitrate sesquihydrate (Extra pure) were purchased from Merck Ltd.

2.2 Field experiment

For the field experiment, a random block scheme was used with three replications for each test. Propamocarb-hydrochloride was applied with a backpack motorized sprayers with an adjustable nozzle size of 1 mm using the two commercial formulations Previcur-N 72.2% SL and Proplant 72.2% SL. The pesticide application was carried out in Nov. 21th 2008, at the dose recommended by the manufacturers 250 cm³ 100 L⁻¹water. Samples were collected 1 h after application and then after 1, 3, 5, 7, 10 and 15 days. A control sample was also taken at each sampling time. Immediately after collecting the samples, each individual sample was put into plastic bags and transported to the laboratory.

2.3 Standard preparation

Stock solution of propamocarb-hydrochloride was prepared by dissolving 50 mg of the analyte (of accurate weight) in 50 mL MeOH to obtain solution concentration 1 mg/mL (Fig. 1). A working standard solution of 10 μg mL⁻¹ was prepared by appropriately diluting the stock solution with MeOH. Stock solution was stored at −20 ± 2 °C, and working standard solutions were stored in the dark ⩽4 °C when not in use.

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Figure 1

Propamocarb-hydrochloride structural formula.

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2.4 Sample preparation

The samples were homogenized for 5 min at high speed using a food processor (Thermomix, Vorwerk). The homogenate of each sample was then placed into polyethylene 50 mL centrifuge tube and stored frozen at −20 ± 2 °C until further analysis. Samples were extracted according to the procedure described and modified by Lehotay (Lehotay et al., 2010) and validated by Sherif (Abd-Alrahman et al., 2011b). Briefly, 10 g of the homogenized sample was weighed into a 50-mL centrifuge tube. Ten milliliters of 1.0% acidified acetonitrile with acetic acid were added; closed and vigorously shaken for 1 min using a vortex mixer at maximum speed. Afterward, 4 g of anhydrous MgSO₄, 1 g of NaCl, 1 g sodium citrate dihydrate, and 0.5 g disodium hydrogen citrate sesquihydrate were added, and then extracted by shaking vigorously using vortex for 2 min following centrifugation for 10 min at 5000 rpm. An aliquot of 3 mL was transferred from the supernatant to a new clean 5-mL centrifuge tube and cleaned by dispersive solid-phase extraction with 75 mg of PSA and 500 mg of magnesium sulfate. Afterward, centrifugation was carried out at 6000 rpm for 5 min. An aliquot (2 mL) from the supernatant was filtered through a 0.2-ìm PTFE filter (Millipore, USA) and then analyzed by Agilent 1100 HPLC–DAD.

2.5 Apparatus and chromatographic analysis

Pesticide residue analysis was performed with Agilent technologies HP-1100 series high-performance liquid chromatographic system (Agilent Technologies, USA) equipped with a diode array detector and quaternary pump. The separation was performed on a C18 column (150 × 4.6 mm, 5 μm). The mobile phase was (MeOH/Water 70:30 v/v) with a flow rate of 0.8 mL min⁻¹ and detection wavelength of 260 nm Data analysis was performed using Chemistation software.

2.6 Statistical analysis

Data were statistically evaluated by one-way analysis of variance (ANOVA). Determination of the differences among means was carried out by using the least significant differences (LSD) test. All statistical analyses were done using the Statistical Package for social sciences (SPSS 16.0) program.

3.1 Method performance

Control (without pesticide application) samples of potatoes were used for the evaluation of selectivity. The absence of any signal at the retention time of propamocarb-hydrochloride indicated that no matrix compounds are present, which could give false positive signal. The calibration curve of propamocarb-hydrochloride showed a good linearity and strong correlation between concentrations and area in the studied range (0–100 ng mL⁻¹) (r² ⩾ 0.996). A recovery of propamocarb-hydrochloride from potatoes was 87.8 % ranged from 85.5 to 90.1 %, Table 1. Precision was studied by performing repeatability studies, expressed as RSD. Satisfactory precision was obtained for propamocarb-hydrochloride. Repeatability was lower than 7% for all three levels assayed. Similarly, with the examination of the matrix effect, a general tendency was observed toward higher values of RSDs at low spiking concentrations. Instrumental LOD based on S/N of 3:1 and LOQ based on S/N of 10:1 was (0.9 and 2.7 μg kg⁻¹).

3.2 Residual behavior of propamocarb-hydrochloride

Propamocarb-hydrochloride mean residue levels during the sampling period for each application derived from three sub samples are shown in (Table 1 and Fig. 2). Residue levels of propamocarb-hydrochloride were found to be below the MRLs established by the Codex Committee (0.5 mg kg⁻¹) after the application of recommended dose, which were 250 cm³ 100 L⁻¹ water for potatoes, throughout the experimental period (FAO/WHO 2006).

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Figure 2

Dissipation curve of two different formulations of propamocarb-hydrochloride in/on potatoes (a) Previcur-N 72.2% SL and (b) Proplant 72.2% SL.

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The highest residue levels were found in samples taken in the first sampling time 1 h after pesticide application. The highest mean concentration of propamocarb-hydrochloride residue was found in samples treated with Proplant 0.99 mg kg⁻¹, followed by Previcur-N 1.31 mg kg⁻¹, residue levels of propamocarb-hydrochloride had been decreasing in the following period, reaching levels 0.02 and 0.32 mg kg⁻¹ in 10 days after application for Previcur-N and Proplant, respectively.

The results showed different half-life(s) (t_1/2) and PHI for propamocarb-hydrochloride of (2.26, 3) and (6.29, 10) days for Previcur-N and Proplant, respectively. A similar behavior of iprodione and thiacloprid residue levels was observed by (Omirou et al., 2009), Mean residue levels of both pesticides were below the EU established MRLs throughout the experimental period. Also Omirou reported that the highest residue levels in both treatments were determined in samples taken in the first sampling just after pesticide application as in our case. In another study by Beouwer was reported that the dissipation of pesticide deposit was a complex process depending on the various environmental factors like temperature, relative humidity and UV irradiation, metabolism and translocation (pesticide penetration and plant growth), application technique and pesticide formulation (McCrady and Maggard, 1993; Brouwer et al., 1997; Katagi, 2004).