Chemicals and Reagents
The formulation of quizalofop ethyl (5% EC) was supplied by M/S Dhanuka Agritech Limited, New Delhi. The reference standard of quizalofop ethyl (purity 99.0%) (Fig. 1) was procured from Sigma Aldrich (Humburg, Germany). The HPLC grade solvents - acetonitrile and water were obtained from J.T. Baker (Phillipsburg, New Jersey). The N-propyl ethylene diamine-based primary secondary amine (PSA), graphitized carbon (C18), and florist sorbents were procured from Agilent Technology (Santa Clara, CA). The analytical reagent grade anhydrous magnesium sulfate (MgSO4), and sodium chloride (NaCl) were purchased from Avantor Performance Materials India Limited (Gurugram, India).
The HPLC analysis was performed with an HPLC (1200 series, Agilent, Santa Clara, CA) coupled to a ultra-violet (UV) detector with an isocratic pump. Separation was with an Agilent Eclipse Plus chromatographic C18 column (4.6 mm × 250 mm × 5 µm). The degassed, filtered (0.45 µm Nylon filter), the mobile phase was composed of acetonitrile: water (70:30, v: v) with flow rate at 1 mL min− 1 and column temperature of 20°C. Aliquots (10 µL) were injected directly to identify and quantify Quizalofop ethyl compared to retention time (RT) of external standard peak area.
Briefly, LC–MS/MS analysis was performed with Acquity ultra performance liquid chromatography (UPLC) system (Waters, Milford, MA, USA) and API-3200 triple quadrupole mass spectrometry (MS/MS) system (ABSciex, Vaughan, ON, Canada) coupled with an electrospray ionization (ESI) source operating in positive ion mode.
Preparation of Standard Solutions
A stock solution containing 1g L− 1 of the target herbicide was prepared by dissolving 10 ± 0.1 mg reference standard in 10 mL of acetonitrile and stored at -4°C. All intermediate working solutions (0.04, 0.1, 0.20, 0.4 or 1.0 mg L− 1 were prepared by serial dilution to determine the accuracy, precision, linearity, and sensitivity.
Onion samples were prepared following the QuEChERS method (Anastassiades et al. 2003) and validated as per guidelines of the European Commission (SANTE 2020) for residues determination and dissipation kinetics of quizalofop ethyl. A representative chopped homogenized sample of onion (leaf/bulb) (500g) was smeared thoroughly in a Robot Cuope Blixer 6 v.v (Ridgeland, MS) vertical mixer at 756 ×g speed for 5 min. An aliquot of 10 g was placed in a centrifuge tube (50 mL), to which 10 mL acetonitrile was added and the mix vortexed for 1 min. Afterward, 4 g MgSO4 and 1 g NaCl were added and the mix was immediately vigorously hand shaken and centrifuged in Superspin, Plasto-Craft Centrifuge for 5 min at 8495×g. An aliquot from the upper layer (5 mL) was subjected to clean-up by dispersive solid-phase extraction (d-SPE) using primary secondary amine (PSA) sorbent (250 mg), anhydrous MgSO4 (750 mg), and centrifugation at 8495 ×g speed for 5 min. The final supernatant (1 mL) was filtered through 0.22 µm nylon 66 membrane filter paper (SGE Analytical Science Pvt. Ltd, Mumbai, India) and subjected to HPLC chromatographic analysis at 235 nm absorbance. All experiments were performed in triplicate.
The method validation study was carried out on onion (leaf, immature and mature bulb) and soil samples (not treated with target pesticide) with linearity, the limit of detection (LOD), the limit of quantification (LOQ), accuracy, and precision. The performance of the analytical method was assessed as per the single laboratory validation approach of Kollipara et al. (2011).
Optimization of Chromatographic and Spectrometric Conditions
Optimization of chromatographic conditions of Quizalofop ethyl was done with separation using reverse-phase HPLC columns. Separation and peak shapes were determined with reverse-phase BDS Hypersil C18 column (4.6 mm×250 mm×5 µm) using water: acetonitrile (30:70, v/v) as the mobile phase. Since Quizalofop ethyl produced good absorption at 235 nm, this wavelength was used for detection and quantification.
The final determination of quizalofop-p-ethyl was performed by liquid chromatography tandem mass spectrometry (UPLC-MS/MS) by injecting 10 µL analytes to the system controlled by Analyst 1.5 software for data collection and analysis. Analytes were separated onto a Zorbax ultra aqueous reverse phase C-18 (150 × 3 mm i.d., 3.5 µm particle size) column (Agilent Technologies, USA) maintained at 35°C temperature. The mobile phase was prepared using (A) 5mM ammonium formate in methanol and (B) 5 mM ammonium formate in water. The gradient elution program was as follows: A (95%) B (5%) at the initial time (0 min), A (95%) B (5%) (at 0.96 min), A (50%) B (50%) (at 2.79 min), A (10%) B (90%) (at 5.64 min), A (10%) B (90%) (at 6.55 min), A (95%) B (5%) (at 7.54 min), A (95%) B (5%) (at 8.00 min). The mobile phase flow rate was 0.35 ml min− 1 using binary pump with total run time of 14 min.
Calibration and Linearity
Calibration and linearity were assessed by internal standardization over the range 0.04-1.0 mg kg-1. The matrix-matched standard calibration curves were created at 5 levels (and blank) and injected in duplicates.
Selectivity and Sensitivity
The selectivity and sensitivity limits of detection and quantification were estimated following the IUPAC approaches which consisted of analyzing the blank sample to establish noise levels and then testing experimentally estimated LOQs for signal/noise ≥ 10.
Accuracy and Precision
Accuracy and precision of the analytical method are defined as the similarity of results obtained by the analytical method to the true value. Accuracy of the method was determined as % recovery by spiking quizalofop ethyl at concentration levels of 0.04, 0.20, and 0.40 mg L− 1 at specification level (1000 mg L− 1). The precision of the method was estimated by repeatability (intra-day precision) and reproducibility (inter-day precision) at 3 concentrations. Repeatability is the intra-day variation obtained at 3 concentration levels and expressed in terms of relative standard deviation (RSD).
The flora Cynodon dactylon (L.), Acrachne racemosa (Roemer & Schultes), Dactyloctenium aegyptium (L.), Boerhaavia diffusa (L.), Parthenium hysterophorus (L.) and Digeria arvensis (L.) were dominant weeds in the experimental field. Associated species of weeds were sampled and identities confirmed by a taxonomist. A field experiment was conducted in a randomized complete block design with red onion, expected bulb weight 30–35 g (variety: Sukhsagar), in 2009 and 2010 at Sonakhali village, Dist. Nadia, West Bengal, India (Bastiaans et al., 2000). The geographical location of the experimental site was 24.11''N latitude and 88.48''E longitude with an altitude of 44.70 m above mean sea level (MSL). The soil was an alluvial sandy loam with the texture of 38.2% clay, 31.8% silt, 21.3% fine sand, and 8.7% coarse sand, low in available nitrogen and medium in available phosphorus and potassium. A plot (50 m2) was used for individual treatment and quizalofop ethyl 5% EC (Targa Super) was applied 30 days after transplanting (DAT) @ 50 and 100 g a.i. ha− 1 at 50% vegetative stage with a volume of 500 L ha− 1. Data were recorded on predominant weed flora, weed density, and dry weight in onion.
Two-kg of randomly selected immature onion leaves from treated plots and the untreated control were collected at 0, 1, 3, 5, 7, 10, and 15 days, immature onion bulbs at 30 days, and mature onion bulbs and soil samples at 55 days after the last treatment. Immediately after collection, samples were placed in polyethylene bags and transported to the laboratory. Samples were thoroughly mixed and 500 g of sub-samples obtained by quartering were stored at -20°C until analysis (Meng et al. 2020).
Observations on Weeds
Weed count was recorded by species from 0.5 × 0.5 m quadrat (a frame, traditionally square, used in ecology and geography to isolate a standard unit of area for the study of the distribution of an item over a large area) from 3 randomly selected places in each plot. The density of grasses, sedges, and broadleaf weeds, and total weeds were recorded at 20, 40, and 60 days after sowing.
Weed Dry Weight
Weeds within frames of the quadrate were shade dried and then dried in a hot-air oven at 80°C for 72 hrs. Dry weights were recorded.
Weed Control Efficiency
WCE = × 100%Weed control efficiency (WCR) was calculated as per the reported procedure (Zia-Ul-Haq et al. 2019).
WCE = Weed control efficiency (per cent);
WDC = Weed biomass (g m− 2) in control plot;
WDT = Weed biomass (g m− 2) in treated plot.
The weed index (WI) was calculated as per the method of Kaur et al. (2020).
X = Yield (kg ha− 1) from minimum weed competition plot;
Y = Yield (kg ha− 1) from the treatment plot for which WI is to be worked out.
Food Safety Evaluation
To evaluate the safety of Quizalofop ethyl in onion the theoretical maximum residue contribution (TMRC) of Quizalofop ethyl was compared to the maximum permissible intake (MPI) at the recommended dose. The TMRC was calculated with an assumption that the onion used for cooking would contain Quizalofop ethyl residue at the highest level applied at the recommended dose (Saha et al. 2014). Per capita consumption of onion (green and bulb, red onion or otherwise) is 35,606.48 mg day− 1, of 1.5 kg of food consumption per day, i.e. 2.37% can be taken as food factor for onion (NHB 2018). No observed effect level (NOEL) of Quizalofop-p-ethyl in mice was < 0.9 mg kg− 1 day− 1. The TMRC was estimated by multiplying the initial residue level quantified at the recommended dose of application with food factor and total intake of food per day. After calculation, TMRC for season-I was 0.088 mg person− 1 day− 1; for the 2nd season, it was 0.071 mg person− 1 day− 1. The MPI was estimated at 0.58 mg person− 1 day− 1.
The estimated daily intake (EDI) of quizalofop ethyl residues was calculated by multiplying the highest residue concentration (mg kg− 1) with the food consumption rate (kg day− 1) divided by the mean body weight of an adult. The long-term risk assessment of intakes compared to pesticide toxicological data was made by calculating the risk quotient (RQ) obtained by dividing the EDI by the relevant acceptable daily intake (ADI). The acceptable risk for long-term human dietary intake of quizalofop ethyl is confirmed when RQ is < 1; if RQ is > 1, it is an unacceptable risk (Song et al. 2020).