1. INTRODUCTION:

With advancement in chemical technology and synthesis of new material, some plant protecting chemicals are designed for the development of agricultural products. They are classified in terms of their origin, use and action namely herbicides, fungicides, insecticides and collectively called as pesticides. Pesticides in fruits and vegetables can be significant channel to human exposure [1]. The persistence nature of these pesticides poses a great threat to the environment, living beings and causes an ecological imbalance [2-4]. The pesticides as such or their residues and degradation products [5-6] can be present in water, soil and vegetables etc.

Constituting an important risk for human health owing to their chronic toxicity [7-11].

Imidacloprid (1-[(6-chloro-3-pyridinyl) methyl]-N-nitro-2-imidazolidinimine) is a nicotine based insecticide.

It is largely used for crop protection as this is highly toxic to many classes of insects. Imidacloprid can be applied as a foliar soil or seed treatment to control many species of sucking and chewing pests [12-14]. Imidacloprid act as an agonist to the postsynaptic nicotinic acetylcholine receptors causing the paralysis and subsequent death of insects [15-16]. This insecticide poses great impacts on some non-targeted organisms. Imidacloprid insecticide has been shown to pose a serious risk to various soil invertebrates and earthworms and affect soil ecosystem adversely. Imidacloprid can easily enter in bodies of water by spray, drift by run of applications [17]. Imidacloprid sorption was found to correlate positively to soil organic matter and mineral clay content, while desorption was lower at lower temperature and low pesticides concentration. This insecticide and its metabolites also bind to particles in sediments that form the floor of fresh water and marine water bodies. Keeping above in view there is a need to find a sensitive and selective method for the extraction of this insecticide [18].

The identification and trace level determination of pesticides and residue becomes a challenging task to the analytical chemists. The enrichment of pesticides via separation and removal demands a high sensitive, selective and precise technique with wide range of applications. Various extraction techniques for the extraction of the insecticides in water, soil and vegetables were used like liquid-liquid partition [19-21], solid phase extraction[22-23] etc. For solid phase extraction various sorbents like C₁₈ [24-26], Silica gel [27] Diatomaceous earth material [28-29], XAD[30], polymers like polystyrenedivinyl benzene[31], activated carbon[32], multi walled carbon nanotubes [33-34] and cyclodextrin[35-38] were used. After extraction various common chromatographic techniques for determination of these insecticides like GC[39-41], HPLC[42-45], UPLC[46-47], capillary electrophoresis[48-49], thin layer chromatography, Spectrophotometry and voltametrically[50] etc. were used. The determination of pesticides especially polar pesticides with GC is difficult because before determination with GC, there is need of derivatisation or hydrolysis of the analytes but no such derivatisation is required in case of HPLC.

So in my work, solid phase extraction of this insecticide was done with β-cyclodextrin polymer from water by using HPLC as an analytical technique.

β-Cyclodextrin (β-CD) is a very stable oligosaccharide that is composed of seven glucose units linked with each other by α-(1,4)-glycosidic linkage. In cavity it can incorporate guest molecules and form supramolecular complexes. When two or more β-Cyclodextrins are covalently linked with each other they are known as the polymers. These β-cyclodextrin polymers have been used for the preconcentration of various analytes. Here, we have developed a relatively simple, rapid, sensitive and selective method for the preconcentration of Imidacloprid by using HPLC attached with UV- detector as an analytical technique. In this NH₃-NH₄Cl buffer solution of pH 9.0 were used for effective extraction of this insecticide. Imidacloprid gets sorbed on β-CDP which can be recovered with the help of eluent for determination.

2. EXPERIMENTAL:

2.1 Materials:

All solvents and water used throughout the experiment were of HPLC grade. Imidacloprid (98%) and 1,4-Butanediol diglycidyl ether were obtained from Sigma Aldrich Company (U.S.A.). β-Cyclodextrin was obtained from SD fine chemical India private limited (Mumbai). Buffer solutions used were hydrochloric acid/ sodium acetate for pH 2.0-3.5, sodium acetate/acetic acid for pH 4.0-6.5, ammonia/ammonium chloride for pH 8-11. Glasswares were washed with chromic acid and soaked in 5% nitric acid and rinsed with double distilled water.

2.2 Apparatus:

HPLC (Waters 515 ) pump equipped with C-18 column (5µm) of 4.6 internal diameter and 21mm length attached with UV-detector. All pH measurements were performed using Digital pH-meter (Toschkon-Toshniwal) with a combined glass electrode. A mechanical shaker, ultrasonicator etc. were used to carry out all the inclusive procedures.

2.3 Procedure

2.3.1 Synthesis of the β-Cyclodextrin polymer (β-CDP)

β-CDP was synthesized by known method [51]. A brief procedure for the synthesis is mentioned here. 20 g of β-CD was dissolved in 50 mL of 20% NaOH. To this 20 mL of 1,4-butanediol diglycidyl ether was added drop wise with continuous stirring on magnetic stirrer. The polymer was formed in 1.5 h and dried at 90 ⁰C. The polymer was ground, sieved and washed with double distilled water 5-6 times. Then, the polymer was dried again at 90°C and kept at room temperature (25°C) in a dessicator.

2.3.2 Preparation of standard solutions

Stock solution was prepared by dissolving 0.025g of Imidacloprid in 25ml of acetonitrile and stored in stained glass stopper bottles at 4^oc. Standard working solutions at 0.01to100 µg/ml of Imidacloprid were prepared daily by appropriate dilutions of aliquots of the stock solution in acetonitrile.

2.3.3 Batch extraction procedure

At room temperature β-CDP and 10.0 ml of buffer solution (pH 9.0) were added to a 250 mL stoppered conical flask. The mixture was allowed to stand for approximately 15 min so that β-CDP could be swollen sufficiently. 10ppm of imidacloprid was added and made up to 150 mL with HPLC grade water. After the mixture was shaken in the mechanical shaker for 40 min, sample solution was filtered and then analyte was eluted with the help of 2.00ml of acetonitrile as eluent. The % uptake was analyzed with the help of HPLC at 278 nm using 80:20 ACN:water as a mobile phase with a flow rate 1.00ml/min. R_t for imidacloprid comes at 2.9 min. Enrichment factor is 75. LOD is 0.45 ng/ml and LOQ is 1.5 ng/ml. RSD is 2.1%.

2.3.4 Sample collection and conditioning

Water samples were collected from the different sources. The water samples were immediately filtered through cellulose membrane filter (0.45 nm pore size), and stored in pre-cleaned polyethylene bottles. After then, pH (9.0) of the sample was adjusted and the preconcentration procedure as described above was applied.

2.3.4 HPLC determination

At room temperature the column of HPLC was operated isocratically using 80:20 (v/v) acetonitrile:water as a mobile phase with a flow rate 1.0ml/min. R_t for fipronil comes at 2.9 min. An aliquot of 20µL was injected into the column. The mobile phase was filtered through a 0.25µm nylon membrane filter. With this mobile phase imidacloprid could be completely separated from the matrix influences at retention time R_t = 2.9min. The concentrations of imidacloprid were calculated by calibration with the peak areas of different standards of imidacloprid used.

HPLC of Imidacloprid

3. RESULTS AND DISCUSSIONS:

3.1 Effect of pH

The sorption of an analyte on the polymer is dependent on the pH of the sample solution due to the hydrogen ions present in the solution. 10ppm of imidacloprid was spiked to a 150ml of sample solution. The pH of this solution was adjusted from 2-11 by using different buffer solutions and then the described preconcentration procedure was applied. As the maximum recovery (≥ 95%) was obtained at pH 9.0 (Fig.1). Therefore the working pH 9.0 was choosen for further optimization.

Fig. 1 Effect of pH on the uptake of Imidacloprid on β-CDP [10ppm of Imidacloprid; 400mg of the sorbent; 150ml sample volume]

3.2 Effect of contact time:

Contact time is another important factor in determining the possibility of application of the β-CDP polymer for the selective and sufficient uptake of imidacloprid. Different shaking time (10 -80min.) were studied for the %age uptake of imidacloprid by β-CDP polymer. The results showed that %age uptake of imidacloprid reach maximum (≥ 95%) at 40 min. (Fig. 2). Therefore, the shaking time of 40 min. was selected as the adsorption equilibrium time.

Fig. 2 Effect of contact time on the uptake of Imidacloprid on β-CDP [10ppm of Imidacloprid; pH 9.0; 400mg of the sorbent; 150ml sample volume]

3.3 Effect of the amount of polymer:

Amount of polymer is an important factor to make method more efficient so that maximum recovery of imidacloprid can be obtained by using minimum amount of polymer. For this different amount of polymer (100mg to 500mg) was investigated by the above described procedure. Results showed that %age uptake increases and after 300mg the increase is very small (Fig. 3) so amount of polymer i.e. 400mg was selected for further.

Fig. 3 Effect of the amount of polymer on the uptake of Imidacloprid on β-CDP [10ppm of Imidacloprid; pH 9.0; contact time 40min. ; 150ml sample volume]

3.4 Effect of the sample volume:

In order to explore the possibility of enriching low concentration of analytes from large volume of solution, the effect of sample volume on the retention of imidacloprid was also investigated. For this purpose, 25, 50, 100, 150, 200, 250, 300, 350 and 400 of sample solutions containing 10ppm of imidacloprid were taken. Quantitative uptakes were obtained for sample volume of ≤200 mL (Fig. 4). Therefore, 150 mL of sample solution was adopted for the preconcentration of analyte from sample solutions.

Fig. 4 Effect of the sample volume on the uptake of Imidacloprid on β-CDP [10ppm of Imidacloprid; pH 9.0; contact time 40min. ; 400mg of sorbent]

3.5 Effect of elution conditions on recovery:

For the elution of imidacloprid from the polymer, different volumes (ranging from 1-5ml) were investigated with 1ml/min. flow rate of eluent. The experimental results showed that quantitative recovery (≥ 95%) was obtained with 2ml (Fig. 5) of the eluent i.e. acetonitrile. Therefore, for eluting imidacloprid 2.0 ml of acetonitrile was used as eluent.

Fig. 5 Effect of the volume of eluent on the uptake of Imidacloprid on β-CDP [10ppm of Imidacloprid; pH 9.0; contact time 40min. ; 400mg of sorbent; 150ml sample volume]

Determination of Imidacloprid in different water samples

Sample	Spiked (ng)	Found (ng)	% Recovery
Tap Water	0.0	30.7	--------
	50.0	78.7	97.5
	40.0	68.9	97.4
R.O. Water	0.0	N.D.	-------
	60.0	58.1	96.8
	85.0	82.1	96.6
Mineral Water	0.0	N.D.	--------
	80.0	77.2	96.5
	90.0	87.1	96.7

CONCLUSION:

The developed preconcentration procedure consists of a simple, reliable and rapid which allows the quantitative recovery of imidacloprid in various water samples. Due to the inclusion capacity of cyclodextrin polymers, imidacloprid can be preconcentrated easily. The preparation of polymer is simple, easy and reliable. The prepared polymer is stable and can be used repeatedly. The method has a good sensitivity, selectivity and accuracy. The method is convenient and can be easily applied to environmental water samples.

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Received on 06.05.2017 Modified on 21.05.2017

Asian J. Research Chem. 2017; 10(3):324-328.

DOI: 10.5958/0974-4150.2017.00054.2