Introduction

In 2014, worldwide paddy (Oryza sativa L.) production was around 741.3 million t (FAO 2015). India has the largest area under rice cultivation and ranked second in production (99.18 million t) in the world (Prabakaran and Sivapragasam 2014). Annual reduction in paddy production accounted to be 2–31% due to heavy infestation of insect pests (Jiang et al. 2010). The use of insecticides is one of the effective techniques for management of insect pests in paddy cultivation. Acetamiprid, ((E)-N1-[(6-chloro-3-pyridyl) methyl]-N2-cyano-N1-methylacetamidine), a neonicotinoid insecticide, is widely used as foliar applications in wide number of crops. In India, it is also recommended for rice, cabbage, okra, chilli and cotton (CIB and RC 2014). This systemic insecticide has a unique mode of action of nicotinic acetylcholine receptor (nAChR) agonists which makes it highly efficient for controlling insect pest such as aphid, whitefly, jassid, thrips, leafminer, beetle, leaf hopper, bug and borer (Anonymous 1998).

Extensive research have been carried out on mobility, adsorption and persistence of acetamiprid in soil (Gupta and Gajbhiye 2007) and in various crops like citrus (Li et al. 2000), okra (Singh and Kulshrestha 2005), gram (Gupta et al. 2005), mustard (Pramanik et al. 2006), chilli (Sanyal et al. 2008), tea (Gupta and Shanker 2008), zucchini (Park et al. 2010) and watermelon (Wu et al. 2012). The method of analysis for quantification of acetamiprid residues in different commodities was described by several scientists using HPLC (Sanyal et al. 2008), gas chromatography (Zhang et al. 2008) and LC-MS (Watanabe et al. 2006; Wu et al. 2012).

However, information on the persistence behaviour of acetamiprid in paddy under tropical climatic condition is scarce. The MRL of acetamiprid in paddy has not been fixed (CAC 2014). Therefore, the aim of the present communication was to validate an easy method for the extraction and quantification of acetamiprid and to study persistence of acetamiprid in paddy and soil under field conditions. This may contribute on the safe use of acetamiprid under the tropical agro-eco-system.

Materials and methods

Field trial

The field experiment was conducted on paddy (variety: Satabdi) at the Regional Research Sub-Station (RRSS), New Alluvial Zone, Bidhan Chandra Krishi Viswavidyalaya, Chakdaha in the district Nadia, West Bengal, following randomized block design (RBD) during February–April, 2015. The meteorological conditions during the study period were recorded: Av. temperature (max 37.61 °C, min 15.58 °C), Av. rainfall (max 3.41 mm, min 0.49 mm) and Av. relative humidity (max 89.63%, min 39.10%). Field soil is Gangetic alluvial (soil order: azonal; physico-chemical characteristics: organic carbon = 8.2 kg ha−1, pH = 6.9, CEC = 13.25 meq 100 g−1 soil, sand = 26.4%, silt = 35.1%, and clay = 38.5%). Acetamiprid was applied in paddy at 10 g a.i. ha−1 (T1), 20 g a.i. ha−1 (T2) and 40 g a.i. ha−1 (T3) maintaining untreated control (T4) throughout the experiment. Each experimental plot was of 20 m2 and each treatment had three replications. Formulation of acetamiprid 20 SP (Coromandel Agrico. Pvt. Ltd., Trade name: Acelon) was applied with knapsack sprayer using 1.5 L water (for three replicated plot under each treatment). Acetamiprid was applied twice, one at the tillering stage and before the flowering stage. The residues of acetamiprid were investigated in paddy leaf, grain, husk, straw and soil.

Sample collection

Representative 250 g of leaf sample, 500 g of paddy grain and 500 g of soil (0–15-cm depth by bucket auger) sample were randomly collected from different spots of each plot at 0 (2 h after application), 1, 3, 5, 7, 10 and 15 days after the last application. Paddy straw, grain and soil were also collected at the time of harvest (30 days after the last application). Each sample was separately homogenized in a mixer grinder (GX7, Bajaj) and stored at −18 °C in plastic packets with proper labelling for a minimum period of time before estimation.

Preparation of standard solution

Accurately weighed 5 (±0.1) mg reference standard of acetamiprid (Sigma Aldrich, 99.9%) was taken in a calibrated 50-mL volumetric flask (certified ‘A’ class) and dissolved in 50 mL acetonitrile. The stock solution was stored in dark place in a refrigerator at 4 °C. The calibration standards of 0.02, 0.05, 0.10, 0.25, 0.50 and 1.0 mg L−1 were prepared by serial dilution from the above stock solutions. Matrix-matched standards were prepared simultaneously by evaporating 2 mL matrix extract and reconstituting with 2 mL of 0.02, 0.05, 0.10, 0.25, 0.50 and 1.0 mg L−1 solvent only calibrations.

Method development

  1. 1.

    Linearity—the linearity of the calibration curve was obtained by plotting the peak area against concentration of the corresponding calibration standards maintained in the concentration range of 0.02–1.0 mg kg−1.

  2. 2.

    Sensitivity—the LOD values of the test compounds were determined by considering three times the average SD of the peak area of all calibration levels in matrix divided by the slope of the calibration equation. LOQ values were determined by considering 10 times the average SD of all calibration levels divided by the slope in matrix.

  3. 3.

    Accuracy—the recovery experiment was conducted by spiking 1.0 mL of acetamiprid standard solutions (0.2–5.0 mg L−1) in different homogenized substrates (paddy leaf, grain, husk, straw and cropped soil) at 0.02, 0.05, 0.10 and 0.50 mg kg−1 with three replications along with a non-spiked blank sample as control.

  4. 4.

    Precision—the precision in terms of intra-day repeatability (relative standard deviation in percentage) and intra-laboratory precision (HorRat) were determined separately, which indicates the acceptability of a method with respect to precision. Horwitz ratio (at 0.05 mg kg−1 level) was calculated in the following way: HorRat = RSD/PRSD, where PRSD is the predicted RSD = 2C−0.15 and C is the concentration expressed as a mass fraction (50 ng g−1 = 50 × 10−9).

  5. 5.

    Matrix effect—matrix effect due to co eluting matrix component were evaluated by post-extraction fortification and comparison with the solvent standard. It was evaluated by following equation: Matrix effect (ME %) = (peak area of matrix standard − peak area of solvent standard/peak area of solvent standard) × 100. In view of the above equation, the negative and positive values of the ME signify matrix-induced suppression and enhancement, respectively.

Extraction and cleanup

Homogenized sample of leaf (5.0 g), grain (10 g), husk (2 g), straw (2 g) and soil (10 g) was taken separately in 50-mL fluorinated ethylene propylene centrifuge tube (Nalgene, Roster, NY). Then, 10 mL water (Merck Milli-Q) was added, vortexed for 1 min and left for 10 min for proper incorporation of water into sample matrices. After that, 10 mL acetonitrile (Rankem, HPLC grade), 6 g Na2SO4 (SRL) and 1.5 g NaCl (Rankem) were added and vortexed well followed by shaking on a rotospin for 15 min at 50 rpm. The sample was centrifuged for 5 min at 5000 rpm. Supernatant liquid (6 mL) was taken to carry out the cleanup procedure. The samples were cleaned-up by dispersive solid phase extraction (d-SPE) with PSA (40 mm Bondesil, Agilent), GCB (UCT) and MgSO4 (SRL). Each of the supernatant aliquot was taken in 2-mL centrifuge tube. For straw, grain, husk and soil, 25 mg PSA and 150 mg MgSO4 were used. For plant samples, 50 mg PSA, 25 mg GCB and 150 mg MgSO4 were used. It was then centrifuged for 5 min at 10,000 rpm. Finally, the clear extracts were filtered with the help of syringe filter (SGE Int. Pvt. Ltd. with 25 mm, 0.22-μm Nylon filter paper) and transferred into the vials for analysis in HPLC.

Instrumentation

The acetamiprid residues were quantified by HPLC (1200 Series Agilent Technologies, USA) consisting of a column (ZORBAX, SB-C18, 5 μm, 4.5 × 250 mm) coupled with UV detector. The mobile phase was methanol (Rankem, HPLC grade) and water (6:4, v/v) and the injection volume was 20 μl. The column temperature was maintained at 30 °C and the flow rate was 1.0 mL min−1 with the run time of 15 min. The λ max value for acetamiprid was 245 nm with retention time 4.0 min.

Data analysis

Residual half-life value was calculated from the first-order kinetics reaction C t  = C 0e−Kt where C t denotes the concentration after time t, C 0 denotes the initial concentration and K denotes the rate constant. This first-order equation on solving resolve into straight line equation log C t  = log C 0 − K/2.303 × t. The fitness of data to first-order kinetics was confirmed by calculation of regression coefficient and determining its statistical significance.

Results and discussion

Method validation

The calibration curve showed good linearity with R 2 (Regression coefficient) value ≥0.99 for acetamiprid in solvent as well as in matrix (Fig. 1). The LOD value was set 0.02 mg kg−1. Recovery percentage for the analysis of acetamiprid was found in the range of 67.2–92.0% for leaf, 68.7–93.1% for paddy grain, 68.9–91.7% for husk, 70.0–90.9% for straw and 75.3–94.3% for soil sample (Table 1). The relative standard deviation (RSD) for intra-day repeatability replicate was within the range of 2.2–18.1%. From the recovery at the three higher levels, it was found that all results were in the range of 81.8–94.3% showing good accuracy of the method (EC 2013). But in the case of 0.02 mg kg−1 fortifying level, recovery was ≤80% for all matrices which does not show reliability to the method. Therefore, LOQ was set as 0.05 mg kg−1. The precision of the method in terms of HorRat ratio of the compound in the range between 0.13 and 0.27 was satisfactory (Table 1).

Fig. 1
figure 1

Calibration curve of acetamiprid in different matrix

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Table 1 Result of method validation of acetamiprid in different substrates
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Residue and dissipation of acetamiprid

Data regarding the residues of acetamiprid in paddy leaf and soil are presented in Tables 2 and 3, respectively. The initial residual concentration of acetamiprid was 0.11, 0.23 and 0.99 mg kg−1 in paddy leaf and 0.16, 0.31 and 0.71 mg kg−1 in cropped soil for T1, T2 and T3 doses, respectively. The initial residue of acetamiprid was dissipated by 47–52% in paddy leaf and 50–57% in field soil after 1 day of the last application. In the case of T2 and T3, the dissipation further increased to more than 75% after 3 days. The residual level reached below LOQ (0.05 mg kg−1) after 3 days in T1, 5 days in T2 for both paddy leaf and soil while in T3, it was after 7 days in soil and 10 days in leaf (Tables 2 and 3). No residues of acetamiprid (i.e. below LOQ = 0.05 mg kg−1) was detected in harvested samples of paddy grain, husk, straw and cropped soil collected after 30 days of last application.

Table 2 Dissipation of acetamiprid in paddy leaf for three different doses
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Table 3 Dissipation of acetamiprid in cropped soil for three different doses
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Dissipation kinetics

The residual data for T2 and T3 were subjected to statistical analysis and the linear plot for the first-order reaction kinetics are presented in Figs. 2 and 3 for paddy leaf and field soil, respectively. However, the data for T1 obtained for two intervals only could not be processed statistically. The residual half-life values in paddy plant were 1.5 and 1.8 days for T2 and T3 doses while in soil, the corresponding T1/2 values were 1.3 and 1.4 days (Tables 2 and 3). Comparable half-life values of acetamiprid (1–2 days) have also been reported in other crop plants like mustard (Pramanik et al. 2006), okra (Singh and Kulshrestha 2005) and tea (Gupta and Shanker 2008) while for chilli (Sanyal et al. 2008) and watermelon (Wu et al. 2012), the published values are in the range of 2–5 days. The variation in the persistence nature of acetamiprid in different crops may be due to difference in agro-climatic conditions and the plant matrix. In the absence of any MRL value, the pre-harvest interval (PHI) of acetamiprid in paddy leaf was calculated based on LOQ (0.05 mg kg−1) as 4.0–6.6 days (Table 2).

Fig. 2
figure 2

Dissipation curve for acetamiprid in paddy leaf

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Fig. 3
figure 3

Dissipation curve for acetamiprid in cropped soil

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T1/2 values of acetamiprid obtained in the present study in alluvial soil (1.3–1.4 days) under field condition is comparable with those reported in Beijing and Ji’nan soil located in China (1.2–1.5 days) cropped with watermelon (Wu et al. 2012). However, higher persistence of acetamiprid was observed in Typic inceptisol (T1/2 = 10.0–22.8 days) maintained with various moisture levels under laboratory condition (Pritam et al. 2013).

Thus, the persistence behaviour of acetamiprid is strongly affected by sample matrices and environmental factors. Under ricefield, the degradation of insecticides are governed by both biotic and abiotic (e.g. soil reaction, chemical properties of the insecticide, photodegradation, humidity and temperature) factors (Pritam et al. 2013). The data herein revealed possible contribution of all these environmental factors to the persistence of acetamiprid. Under natural environmental conditions, when all the factors are acting together, it is very likely that the degradation of a particular pesticide will be at a much faster rate. Our results are in full agreement with the abovementioned facts.

Conclusion

Overall, the methods used for extraction, cleanup and estimation of residues were found to be satisfactory qualitatively and quantitatively. The dissipation data along with half-life values in paddy and soil clearly indicated a rapid declining trend of acetamiprid under field conditions. The study indicates that acetamiprid dissipate rapidly in soil than in paddy under field condition. The MRL is yet not fixed by Codex Alimentarius Commission or by the Indian authority FSSAI. We believe that the data from our study will help to understand the safe use of acetamiprid and to fix MRL on rice under Indian context as well.